Handbook of Porous Silicon [2014 ed.] 9783319045085

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_38-1 # Springer International Publishing Switzerland 2014

Biocompatibility of Porous Silicon Suet P. Low* and Nicolas H. Voelcker Mawson Institute, University of South Australia, Mawson Lakes, SA, Australia

Abstract The biocompatibility of porous silicon is critical to its potential biomedical uses, both in vivo within the human body for therapy and diagnostics, and in vitro for biosensing and biofiltration. Published data from cell culture and in vivo studies are reviewed, and a number of emerging applications for bioactive or biodegradable silicon are discussed.

Biocompatibility The term “biocompatibility” is defined as “the ability of a material to perform with an appropriate host response in a specific situation” (Williams 2008). A biocompatible material can be inert, where it would not induce a host immune response and have little or no toxic properties. A biocompatible material can also be bioactive, initiating a controlled physiological response. For porous silicon, bioactive properties were initially suggested based on the observation that hydroxyapatite (HA) crystals grow on microporous silicon films. HA has implications for bone tissue implants and bone tissue engineering (Canham 1995). An extension of this work showed that an applied cathodic current was able to further promote calcification on the surface (Canham et al. 1996). More recently, Moxon et al. showed another example of bioactive porous silicon where the material promoted neuron viability when inserted into rat brains as a potential neuronal biosensor, whereas planar silicon showed significantly fewer viable neurons surrounding the implant site (Moxon et al. 2007).

Biodegradability A comprehensive review on pSi biodegradability is covered in chapter “▶ Biodegradability of Porous Silicon”. We discuss this here as the degradation rate, and products can influence its biocompatibility in biomedical applications. Porous silicon is instable in aqueous solutions and degrades into orthosilicic acid (Si(OH)4) (Allongue et al. 1993) as a result of oxidative hydrolysis (Scheme 1). Silicic acid is a nontoxic small molecule and the common form of bioavailable silicon in the human body (Carlisle 1972, 1982). Silicic acid does not accumulate within the human body and has been shown to be absorbed readily by the gastrointestinal tract of humans and is rapidly excreted via the urinary pathway (Reffit et al. 1999). Although silicic acid at concentrations of 2 mM has been reported to be cytotoxic to fibroblasts and macrophages (Tanaka et al. 1994), high concentrations of silicic acid up to 100 mM have been tested in vitro on cells with no apparent affect on their viability (Mayne et al. 2000). The rate of dissolution can be controlled by the porosity of porous silicon (Anderson et al. 2003) and by its surface chemistry (Canham et al. 1999, 2000). Silicon with medium *Email: peng.low@flinders.edu.au Page 1 of 13

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_38-1 # Springer International Publishing Switzerland 2014

Scheme 1 Proposed mechanism for porous silicon degradation in aqueous solutions, adapted from Allongue et al. (1993). (a) A Si-H-terminated surface immersed in H2O. (b) The Si-H bond undergoes hydrolytic attack and is converted to Si-OH and produces a hydrogen molecule. (c) The Si-OH at the surface polarizes and weakens the Si-Si backbonds, which are then attacked by H2O, producing HSi(OH)3. (d) In solution, the HSi(OH)3 molecule is quickly converted to Si(OH)4 releasing a second hydrogen molecule

porosity (62 % porosity) shows slow degradation, whereas higher porosity silicon (>80 % porosity) showed exponential release of silicic acid over time (Anderson et al. 2003). Surface modification has been applied to the porous silicon surface to impart protection against hydrolytic attack and has the dual role of being able to change the surface chemistry (Low et al. 2006). By applying different surface modifications, porous silicon degradation rates can be tuned anywhere from minutes to months (Godin et al. 2010). This makes porous silicon as an ideal transient material for localized drug delivery or cell delivery purposes. The degradation rate of porous silicon increases with increasing pH (Anderson et al. 2003), and the local tissue pH therefore has to be taken into consideration when designing porous silicon for a certain biomaterial application. Different methods for surface modification and subsequent effect on cells are covered in chapters “▶ Functional Coatings of Porous Silicon” and “▶ Silicon-Carbon Bond Formation for Porous Silicon”. In brief, the functional groups presented on the surface of porous silicon allow for the attachment of biological factors and proteins in culture medium, which in turn influence cell attachment. Several in vitro culture studies have shown that surface modification of the porous silicon surface can modulate cell attachment and growth (Low et al. 2006; Yang et al. 2010). Neuroblastoma (Low et al. 2006; Yang et al. 2010; Gentile et al. 2012; Khung et al. 2006), human embryonic kidney cells (Sweetman et al. 2011), B50 cells (Mayne et al. 2000; Bayliss et al. 1997, 1999), and primary mesenchymal cells (Clements et al. 2011; Noval et al. 2012) are a few cell types that have been successfully cultured on porous silicon surfaces.

Cytotoxicity As discussed above, the degradation products of porous silicon have been shown to be relatively harmless and have opened the use of this material in biological environments. The interaction of porous silicon and cells is covered in chapter “▶ Assessment Framework”. Cell culture on porous silicon, but silicic acid is not the only degradation product that may induce cytotoxicity. It has been recently demonstrated that porous silicon is capable of producing reactive oxygen species (ROS) (Belyakov et al. 2007; Kovalev et al. 2004). ROS have important physiological roles such as signalling molecules to regulate cell proliferation, apoptosis, and differentiation (Finkel and Holbrook 2000). ROS generation by porous silicon is directly related to the surface chemistry (Kovalev et al. 2002, 2004), and therefore, porous silicon particles are more susceptible to generating ROS. Recent investigations have demonstrated that untreated particles generate ROS at

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_38-1 # Springer International Publishing Switzerland 2014

concentrations that lead to cell death, whereas simple surface stabilization with oxidation was able to mitigate this effect (Low et al. 2010; Santos et al. 2010). The size of porous silicon particles is also an important factor. Particles smaller than 3 mm have been demonstrated to be cytotoxic to monocytes (Ainslie et al. 2008); loss in Caco-2 cell metabolic activity was seen with particles between 1.2 and 25 mm in size (Santos et al. 2010). In contrast, particles below 500 nm were demonstrated to be nontoxic to lymphoma cells (De Angelis et al. 2010), and particles smaller than 1 mm did not cause any cytotoxic effects with macrophage and endothelial cells (Godin et al. 2012).

Fate of Porous Silicon Particles in the Body The retention of porous silicon in the body has recently been shown to be transient. Intravenous injection of porous silicon nanoparticles into mice leads to its accumulation within the liver and the spleen, demonstrating rapid removal from the circulatory system (Bimbo et al. 2010; Park et al. 2009). Another study intravenously injected oxidized and aminosilane-functionalized porous silicon microparticles into mice. The study revealed that surface chemistry and charge affected microparticle distribution within the body (Tanaka et al. 2010a). In this study, porous silicon microparticles after intravenous administration were also found to accumulate within the liver and spleen. The enzyme lactate dehydrogenase (LDH) is often used as an indicator of tissue damage, and this study found LDH levels were only increased after multiple administrations of the particles and cytokine levels remained stable. There was no difference in LDH levels between particles with different surface chemistries. Other studies have shown that intravenously administered particles that accumulated within the liver and spleen degraded over a period of 4 weeks, and cells within these organs retained their normal morphology (Park et al. 2009). Tanaka et al. showed in a mouse model that injected porous silicon particles loaded with siRNA accumulated primarily within the liver and spleen (Tanaka et al. 2010b). Clearance or degradation of the particles within these organs occurred within 3 weeks for the spleen but significantly longer in the liver, indicating that degradation kinetics of the porous silicon particles was organ dependent. No tissue injury or inflammatory cytokines were detected for the organs investigated, and the morphology of the cells within these organs remained unchanged. This study again demonstrated that porous silicon did not cause any adverse tissue effects when injected into the body. For drug delivery applications, ingestion of porous silicon is of particular interest. Porous silicon is reasonably stable at low pH (Anglin et al. 2008), showing degradation kinetics that are suitable for drug delivery to the intestine. An investigation into the distribution of ingested porous silicon microparticles showed that they passed through the gastrointestinal tract without signs of uptake of particles within the lining of the gastrointestinal tract (Bimbo et al. 2010). Another study utilized 18 F-radiolabeled thermally hydrocarbonized porous silicon particles to investigate the accumulation and retention of ingested particles in a rat model. Once again, the particles were stable in stomach acid, remained in the GI tract, and did not cross the intestinal cell layer (Sarparanta et al. 2011). Another article by the same authors attached the protein hydrophobin class II to the thermally carbonized particles. This conveyed mucoadhesive properties to the particles, allowing them to attach to gastric cells. The particles were stable in gastric fluid, and in vivo experiments showed that the particles were retained within the stomach cavity for a period of time. However, upon entering the intestinal tract, the particles lost their adhesive properties and were quickly expelled (Sarparanta et al. 2012). Particle size can also be used to control the distribution throughout the body and can be a form of targeted drug delivery. Porous silicon particles larger than 519 nm in diameter are unable to Page 3 of 13

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_38-1 # Springer International Publishing Switzerland 2014

Fig. 1 Thermally oxidized and amino-silanized porous silicon membranes containing cultured limbal cells were implanted under the conjunctiva of rats. (a) Images of the implant site after 0, 3, 6, and 9 weeks showing gradual dissolution of the membrane. (b) Histological analysis of the implant site with the porous silicon (PS). Small amount of inflammatory cells (IC) are found and the formation of a fibrous capsule around the porous silicon membrane (F) (Low et al. 2009)

cross the placenta into a fetus and can help prevent fetal exposure to administered drugs (Refuerzo et al. 2011).

In Vivo Behavior of Porous Silicon Implants A range of in vitro cell culture studies have been carried on porous silicon to demonstrate the suitability of this material as a support for mammalian cells. Implants, on the other hand, can experience a variety of tissue and host immune responses, such as generalized cytotoxic effects, microvascularization, and hypersensitivity to the implant. To date, there have been limited numbers of studies on the effects of porous silicon in vivo. In 2000, Rosengren et al. implanted unmodified porous silicon into the abdominal wall of rats with flat silicon and titanium as controls. An inflammatory response was observed with a resultant fibrous capsule forming around the implant with minimal cell death at the cell-porous silicon interface, and it was noted that the tissue response was similar for porous silicon, flat silicon, and titanium (Rosengren et al. 2000). Fibrous encapsulation of an implant is a common tissue response to a foreign body (Ratner and Bryant 2004). The factor determining the outcome of the implantation is the thickness of the capsule and the degree of inflammation around the capsule, often measured by the frequency of inflammatory cells such as macrophages and foreign body giant cells. Excessive capsule thickness or inflammation can cause pain or discomfort around an implant and can ultimately result in implant rejection. For active pSi implants, such as devices for drug delivery, this encapsulation may also influence the rate of drug release. With the aim of developing a drug delivery vehicle, hydrosilylated and thermally oxidized porous silicon microparticles were injected into the vitreous of rabbit eyes (Cheng et al. 2008). It was noted that the particles settled into the inferior vitreous cavity over a few days. Degradation of the hydrosilylated particles took considerable time (>4 months) in comparison to untreated particles which degraded within a period of 3–4 weeks. No adverse effects were observed in ocular tissues including the retina and the lens. Furthermore, normal ocular pressure was maintained. Implants of porous silicon membranes under the rat conjunctiva demonstrated similar results (Fig. 1). A small inflammatory response was initially observed, but histological examination of the Page 4 of 13

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_38-1 # Springer International Publishing Switzerland 2014

implant site showed that a thin fibrous capsule formed around the porous silicon membrane with only a small fraction of inflammatory cells surrounding the implant site (Low et al. 2009). The capsule around was significantly thinner than the fibrous capsule surrounding the surgical sutures used to hold the membranes in place. Tissue erosion and vascularization were absent, indicating that porous silicon was highly biocompatible within tissues of the eye. Outside of the eye, the bioactive properties of pSi implants were investigated in contact to nerve tissue. Porous silicon films on bulk silicon supports were implanted into the sciatic nerve of a rat. Nerve tissue could hence grow on the porous region or the flat region. The authors observed that the formed fibrous capsule formation was significantly thinner on the porous silicon region in comparison to the flat silicon region. They postulated that the porous nature allowed for the implant to anchor strongly to the tissue and thus prevent sheer forces that may influence the formation of fibrous capsules. They also determined that a greater percentage of axons formed on the porous silicon, further highlighting the bioactivity of porous silicon in terms of promoting neural cell formation (Johansson et al. 2009).

Toward In Vitro and In Vivo Biosensors Porous silicon is rousing interest in the biosensor community because of several unique intrinsic material properties. First and foremost are the optical properties which include photoluminescence, thin-film reflectance, and photonic effects (Jane et al. 2009). Second, the material has a high surface area (allowing higher binding density as compared to flat surfaces), the ability to introduce size exclusion layers (filtering out undesired molecules), and a well-developed surface chemistry with a range of options for bioreceptor immobilization. The photoluminescence properties of porous silicon have been exploited since the initial discovery of this effect (Canham 1990). Quenching of the photoluminescence signal has been utilized to detect proteins and enzyme activity (Letant et al. 2004), selectively capture streptavidin biomolecules (Letant et al. 2003), selectively detect myoglobin from a serum solution (Starodub et al. 1999), and capture DNA (Chan et al. 2000). However, the simplest form of a porous silicon biosensor utilizes thin-film interference effects, resulting in a characteristic Fabry-Perot fringe pattern. Changes in the position of the fringe pattern indicate the binding or loss of molecules within the porous layer (Brecht and Gauglitz 1995). This technique has been utilized to detect down to femtomolar concentrations of proteins and DNA binding to the porous silicon surface (Lin et al. 1997; Steinem et al. 2004; Szili et al. 2011). Porous silicon structures with alternating layers of low and high porosity show 1D photonic effects with sharp stop bands. Depending on the interface between the layers, these structures are termed Bragg mirrors or rugate filters (Pavesi and Dubos 1997). The binding to or release of molecules from the porous layer leads to shifts in the spectral peak (Guillermain et al. 2007). The photonic properties of porous silicon have been used by the Sailor group for the in situ monitoring of cell viability. This concept has been coined the “smart Petri dish.” A light source is aimed at an incident angle which is reflected away from the detector. Cells attached to the porous silicon surface scatter some of the light back to the detector, leading to a small spectral peak. Change in cell morphology as a result of cell death leads to an increase in light scattering and therefore an increased detector signal. This allows the label free and in situ monitoring of cell viability without the need for adding dyes into the cell culture medium (Schwartz et al. 2006). The described optical effects could also be used for implanted biosensors which combine the aspects of biocompatibility and biodegradability with the optical effects which are retained upon implantation. Monitoring of a sensor implanted underneath the skin can be accomplished by merely Page 5 of 13

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_38-1 # Springer International Publishing Switzerland 2014

irradiating the sensor with a light source and collecting the reflected spectra. A drawback is the fouling of the sensor when placed into a complex biological environment which will interfere with the sensor readout. The Gooding group utilized hydrosilylation and subsequent conjugation of oligoethylene oxide (OEG) moieties to produce a non-fouling layer, which effectively prevented the adhesion of proteins while still maintaining reflectivity, even when placed into human blood plasma (Kilian et al. 2007). This feat bodes well for the possibility of in situ monitoring in biological fluids and in vivo.

Porous Silicon for Tissue Engineering The biocompatible, bioactive, and biodegradable properties of porous silicon render this material a suitable scaffold for tissue reengineering. For example, with relevance to neural engineering, it has been demonstrated that porous silicon is able to support the growth of neuronal cells that still maintain their action potential capabilities (Ben-Tabou de Leon et al. 2004). Another study has found that porous silicon with an average pore diameter of 300 nm is able to guide axonal growth (Johansson et al. 2005, 2008) suggesting a potential application for porous silicon in nerve regeneration. The early studies identifying bioactive porous silicon in the ability to form HA crystals have generated interest in its use as a bone matrix alternative. Porous silicon-substituted HA structures implanted into the femur displayed better bone integration around the implant compared to a nonporous HA implant (Porter et al. 2006). This was attributed to the porous silicon degradation, revealing voids for bone ingrowth and allowing dynamic remodeling. The biodegradable nature of porous silicon makes it an ideal carrier for cell therapy applications. Here, stem cells on a suitable carrier are implanted into a host, for example, in order to regenerate tissue function. After delivery of the cells into the host, it is desirable that the carrier be degradable in vivo. As a potential treatment for ocular surface disease, human limbal stem cells isolated from the cornea have been expanded on porous silicon membranes as carriers and used to demonstrate cell outgrowth from membranes in an animal model. The stem cell migrated from the porous silicon membrane into the surrounding tissue, and histological analysis of the porous silicon membranes after 8 weeks showed low inflammatory response and absence of vascularization of the implant and significant implant degradation (Low et al. 2009).

Localized Drug Delivery Porous silicon has also been shown to be an effective platform in sustained drug delivery exploiting the large drug loading capacity that stems from the large internal surface. A study looked at five different orally received drugs and their compatibility with a porous silicon delivery vehicle. The drugs were loaded into thermally carbonized and thermally oxidized porous silicon particles. Watersoluble drugs usually display fast drug release profiles, while poorly soluble drugs show slow release kinetics (Salonen et al. 2005). Porous silicon particles were able to moderate the drug release kinetics, on the one hand reducing the release rate of water-soluble drugs while on the other hand enhancing release kinetics for poorly soluble drugs (Wang et al. 2009). Coupled with its stability at low pH (Anglin et al. 2008), porous silicon makes an ideal carrier for oral drug delivery into the small intestine. A tenfold increase in permeation of insulin across intestinal cell layers was achieved when delivered with porous silicon particles over traditional soluble permeation solutions (Foraker Page 6 of 13

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_38-1 # Springer International Publishing Switzerland 2014

Fig. 2 Pseudocolored SEM images of dendritic cells at low (top) and high (below) magnification. (a) Cells only; (b) dendritic cells phagocytosing porous silicon particles and (c) porous silicon particles loaded with liposaccharide antigen being taken up by the dendritic cells (Meraz et al. 2012)

et al. 2003), suggesting that apart from moderating drug release, porous silicon can enhance drug absorption by the body. Another significant advantage of using porous silicon particles is to be loaded with a high payload of drug so that a single dose of the drug delivery system suffices for continuous therapeutic effects, avoiding repeated drug administrations. Here, amino-functionalized porous silicon particles have been used to deliver siRNA to successfully silence a gene for an oncoprotein in vivo where the silencing effect was maintained for several weeks, whereas traditional applications required multiple administrations (Tanaka et al. 2010b). The porous silicon particles have also been used for sustained peptide delivery in rats. In this case, surface modification was used to modulate peptide release from the particles. Thermally carbonized, thermally oxidized, and undecylenic acid-conjugated thermally carbonized particles were loaded with a peptide. They found that as peptide release was partially related to the degradation kinetics of pSi, the thermally oxidized particles, which degraded the fastest, had the greatest release over a 2-week period both in vitro and in vivo, whereas the more stable thermally carbonized particles released less peptide (Kovalainen et al. 2012). This was also demonstrated with a ghrelin antagonist; sustained release was achieved with this peptide over 17 h when loaded into pSi particles, and without the particles, the peptide lost its activity within 4 h (Kilpeläinen et al. 2009). These studies demonstrate the advantages of using pSi as a carrier vehicle for protein delivery. Bioactivity of proteins can be preserved, and the lifetime of a loaded protein can be extended, leading to a sustained drug delivery profile.

Vaccine Development A further application for biocompatible porous silicon relates to vaccination using antigen-loaded particles. Porous silicon particles were conjugated to antigens that specifically target the toll-like receptors on dendritic cells (DC). This stimulated phagocytosis by dendritic cells, maturing the cells to become antigen-presenting cells (Fig. 2) (Meraz et al. 2012). The activated DCs increased

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_38-1 # Springer International Publishing Switzerland 2014

proinflammatory cytokines IL-1b, TNF-a, and IL-6, and when the activated DCs were injected into mice, they migrated into the lymphatic system where they activated T cells by upregulation of cell surface receptors and presenting the antigen along with major histocompatibility (MHC), all of which play a role in mediating an active immune response. This study demonstrated effective stimulation of the immune system with antigen-loaded porous silicon which is highly relevant to the development of vaccines for various diseases.

Summary Since the discovery that porous silicon can stimulate the formation of HA crystals in simulated body fluid, the use of porous silicon in biomaterial applications has soared. Apart from bioactivity, properties such as in vitro and in vivo biocompatibility, biodegradability, high surface area, tunability of pore size and porosity, and finally ease of surface modification have contributed to this increasing interest. These properties open exciting avenues for neural, ocular, and bone tissue engineering and also for drug and vaccine delivery. Combining the biocompatibility with the material’s optical properties of porous silicon enables diagnostic applications such as smart tissue cultureware and implantable biosensors.

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_38-1 # Springer International Publishing Switzerland 2014

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_38-1 # Springer International Publishing Switzerland 2014

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_38-1 # Springer International Publishing Switzerland 2014

Porter AE, Buckland T, Hing K, Best SM, Bonfield W (2006) The structure of the bond between bone and porous silicon-substituted hydroxyapatite bioceramic implants. J Biomed Mater Res A 78A(1):25–33 Ratner BD, Bryant SJ (2004) Biomaterials: where we have been and where we are going. Annu Rev Biomed Eng 6:41–75 Reffit DM, Jugdaohsingh R, Thompson RP, Powell JJ (1999) Silicic acid: its gastrointestinal uptake and urinary excretion in man and effects on aluminium excretion. J Inorg Biochem 76:141–147 Refuerzo JS, Godin B, Bishop K, Srinivasan S, Shah SK, Amra S, Ramin SM, Ferrari M (2011) Size of the nanovectors determines the transplacental passage in pregnancy: study in rats. Am J Obstet Gynecol 204(6):546.e5–546.e9 Rosengren A, Wallman L, Bengtsson M, Laurell T, Danielsen N, Bjursten LM (2000) Tissue reactions to porous silicon: a comparative biomaterial study. Phys Status Solidi A 182:527–531 Salonen J, Laitinen L, Kaukonen AM, Tuura J, Björkqvist M, Heikkilä T, Vähä-Heikkilä K, Hirvonen J, Lehto VP (2005) Mesoporous silicon microparticles for oral drug delivery: loading and release of five model drugs. J Control Release 108:362–374 Santos HA, Riikonen J, Salonen J, Mäkilä E, Heikkilä T, Laaksonen T, Peltonen L, Lehto V-P, Hirvonen J (2010) In vitro cytotoxicity of porous silicon microparticles: effect of the particle concentration surface chemistry and size. Acta Biomater 6(7):2721–2731 Sarparanta M, Mäkilä E, Heikkilä T, Salonen J, Kukk E, Lehto V-P, Santos HA, Hirvonen J, Airaksinen AJ (2011) 18F-Labeled modified porous silicon particles for investigation of drug delivery carrier distribution in vivo with positron emission tomography. Mol Pharm 8(5):1799–1806 Sarparanta MP, Bimbo LM, Mäkilä EM, Salonen JJ, Laaksonen PH, Helariutta AMK, Linder MB, Hirvonen JT, Laaksonen TJ, Santos HA, Airaksinen AJ (2012) The mucoadhesive and gastroretentive properties of hydrophobin-coated porous silicon nanoparticle oral drug delivery systems. Biomaterials 33(11):3353–3362 Schwartz MP, Derfus AM, Alvarez SD, Bhatia SN, Sailor MJ (2006) The smart Petri dish: a nanostructured photonic crystal for real-time monitoring of living cells. Langmuir 22(16):7084–7090 Starodub VM, Fedorenko LL, Sisetskiy AP, Starodub NF (1999) Control of myoglobin level in a solution by an immune sensor based on the photoluminescence of porous silicon. Sens Actuators B 58:409–414 Steinem C, Janshoff A, Lin VS-Y, Volcker HE, Ghadiri MR (2004) DNA hybridization-enhanced porous silicon corrosion: mechanistic investigations and prospect for optical interferometric biosensing. Tetrahedron 60:11259–11267 Sweetman MJ, Harding FJ, Graney SD, Voelcker NH (2011) Effect of oligoethylene glycol moieties in porous silicon surface functionalisation on protein adsorption and cell attachment. Appl Surf Sci 257(15):6768–6774 Szili EJ, Jane A, Low SP, Sweetman M, Macardle P, Kumar S, Smart RSC, Voelcker NH (2011) Interferometric porous silicon transducers using an enzymatically amplified optical signal. Sens Actuators B 160(1):341–348 Tanaka T, Tanigawa T, Nose T, Imai S, Hayashi Y (1994) In vitro cytotoxicity of silicic acid in comparison with that of selenious acid. J Trace Elem Exp Med 7(3):101–111 Tanaka T, Godin B, Bhavane R, Nieves-Alicea R, Gu J, Liu X, Chiappini C, Fakhoury JR, Amra S, Ewing A, Li Q, Fidler IJ, Ferrari M (2010a) In vivo evaluation of safety of nanoporous silicon carriers following single and multiple dose intravenous administrations in mice. Int J Pharm 402:190–197 Page 11 of 13

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_38-1 # Springer International Publishing Switzerland 2014

Tanaka T, Mangala LS, Vivas-Mejia PE, Nieves-Alicea R, Mann AP, Mora E, Han H-D, Shahzad MMK, Liu X, Bhavane R, Gu J, Fakhoury JR, Chiappini C, Lu C, Matsuo K, Godin B, Stone RL, Nick AM, Lopez-Berestein G, Sood AK, Ferrari M (2010b) Sustained small interfering RNA delivery by mesoporous silicon particles. Cancer Res 70(9):3687–3696 Wang F, Hui H, Barnes TJ, Barnett C, Prestidge CA (2009) Oxidized mesoporous silicon microparticles for improved oral delivery of poorly soluble drugs. Mol Pharm 7(1):227–236 Williams DF (2008) On the mechanisms of biocompatibility. Biomaterials 29(20):2941–2953 Yang C-Y, Huang L-Y, Shen T-L, Yeh JA (2010) Cell adhesion, morphology and biochemistry on nano-topographic oxidised silicon surfaces. Eur Cell Mater 20:415–430

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_38-1 # Springer International Publishing Switzerland 2014

Index Terms: Biocompatibility 1 Biosensors 5 Cytotoxicity 2 Drug delivery 6 Lactate dehydrogenase (LDH) 3 Porous silicon 1 Silicic acid 1 Tissue reengineering 6 Vaccination 7

Page 13 of 13

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_39-1 # Her Majesty the Queen in Right of United Kingdom 2014

Biodegradability of Porous Silicon Qurrat Shabir* pSiMedica Ltd, Malvern, Worcester, UK

Abstract In the biomaterials field there is an increasing interest in medically biodegradable materials. The medical biodegradability of mesoporous silicon is now established both in vitro and in vivo. The review highlights the techniques used to date to characterize this phenomenon, the degradation kinetics, and the various factors that can influence the kinetics of dissolution into orthosilicic acid.

Introduction There is growing interest and acceptance in replacing permanent prostheses by temporary ones in the human body. These would in effect help the body to heal itself and require biomaterials to have “biodegradability” within physiological environments (Ratner et al. 2004). Currently four different terms are found in the literature to signify that a material or device will eventually disappear after having been introduced into a living organism: biodegradation, bioerosion, bioabsorption, and bioresorption (Ratner et al. 2004). Unfortunately no agreed distinctions exist; we will use the biodegradability term here. Figure 1 shows examples of biodegradable materials and their uses. The in vitro discovery in 1995 that high porosity mesoporous silicon (pSi) can be rapidly biodegradable, unlike solid crystalline silicon (Canham 1995), and subsequent in vivo demonstrations of biodegradability and biocompatibility (Bowditch et al. 1999; Ji-Ho et al. 2009; Sarparanta et al. 2012; 2014) have been very significant in this regard. The huge surface area (e.g., 100–500 cm2/cm3) of pSi coupled with its nanostructured skeleton promotes solubility in water and biological media. An increasing level of research is being conducted on the use of both porous silicon and silica materials in drug and nutrient delivery (see handbook chapters “Drug delivery with porous silicon” and “Porous silicon and functional foods”). A biodegradable porous matrix offers the dual advantages of sustained release at target sites in the body and gradual biological elimination after administration (Ahuja and Pathak 2009). The performance of porous silicon in this regard should be compared with that of biodegradable polymers that can “microencapsulate” drugs (Park et al. 2005) and mesoporous biodegradable silicas that can also entrap them (Finnie et al. 2009). This review discusses the hydrolysis mechanism underlying mesoporous silicon biodegradability; the factors affecting typical kinetics of that biodegradability, together with techniques used to date to tune those kinetics and timescales achieved.

Mechanism of Biodegradation and Degradation Products In vitro degradation studies of porous silicon have shown release of orthosilicic acid from both anodized films and microparticles using molybdate blue assays or ICP analysis (Anderson *Email: [email protected] Page 1 of 7

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_39-1 # Her Majesty the Queen in Right of United Kingdom 2014 POLYMERS

PLGA/PLA Grafts, sutures, implants, depots

CERAMICS

Hydroxyapatite/Tri calcium phosphate Orthopaedic devices &tissue engineering scaffolds

METALS

SEMICONDUCTORS

Mg / Fe alloys

Mesoporous Silicon

Coronary Stents Paediatric implants

Brachytherapy Tuneable drug delivery

Fig. 1 Biodegradable materials of different classes and their medical uses

Fig. 2 In vitro biodegradation of an 83 % porosity mesoporous silicon membrane

et al. 2000; Anglin et al. 2008; Chiappini et al. 2010). Mesoporous silicon membranes/microparticles in simulated body fluids change color, becoming transparent as biodegradation proceeds to completion (Fig. 2). These results are more evident as the released silicic acid forms a blue-colored complex with molybdenum blue and the color intensity increases with time (Fig. 3). Both in vitro (Canham 1995) and in vivo studies (Bowditch et al. 1999; see Fig. 4) have used electron microscopy to reveal mesoporous film corrosion and disappearance. The first in vivo study (Bowditch et al. 1999) of implanted discs used a combination of electron microscopy and monitoring of disc weights (2014). Additional characterization methods to monitor corrosion in nonmedical environments are discussed in the handbook chapter “Corrosion behavior of porous silicon.” Porous silicon in aqueous conditions undergoes hydrolysis to form orthosilicic acid and the reaction is catalyzed by OH-; hence the rate of dissolution increases with increasing pH. Dissolution of unoxidized silicon can be described with a simplified two-step process: Si þ 2H2 O ! SiO2 þ 2H2 SiO2 þ 2H2 O ! Si ðOHÞ4 The oxidative first step involves electronic carrier (hole) injection and is dependent on both electronic bandgap and doping of the semiconductor. Complete hydrolysis of the oxide phase then generates orthosilicic acid, which is the natural bioavailable form of silicon, freely diffusible in human tissues, and readily excreted via the kidneys (Jugdaohsingh et al. 2002; Refitt et al. 1999). The biocompatibility of porous silicon is reviewed in detail elsewhere in this handbook (“Biocompatibility of porous silicon”) so is not discussed here.

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_39-1 # Her Majesty the Queen in Right of United Kingdom 2014

Fig. 3 Molybdenum blue assay for orthosilicic acid released from mesoporous silicon membranes at different time points

Fig. 4 In vivo biodegradation of a 30 % porosity mesoporous silicon layer in the subcutaneous site of the guinea pig. Plan view HREM images of porosified silicon disc surfaces (a) prior to implantation, (b) after 4 weeks in vivo, and (c) after 12 weeks in vivo

Kinetics of Degradation The kinetics of biodegradation is affected by physical parameters like degree of crystallinity, porosity, surface area, and pore size distribution. A striking example is the difference in solubility between amorphous and polycrystalline silicon (Shabir et al. 2011). The kinetics is also tunable by pore wall surface chemistry which affects wettability by body fluid and resistance to initial hydrolysis. Mesoporous silicon is often manufactured by electrochemical etching techniques (see handbook chapter “Routes of formation for porous silicon”) resulting in hydrogen-terminated surfaces (Si-Hx). For drug delivery applications, a less reactive surface is crucial, and the hydrogen termination of the freshly etched pSi is normally replaced (Li et al. 2009). By converting the reactive groups into a more stable oxidized, hydrosilylated, or (hydro) carbonized form, the pSi surface can be modified in terms of hydrophilicity and resistance to hydrolysis (Canham et al. 1999). Such changes in pore wall chemistry have been shown to significantly change biodegradation kinetics, as summarized in Table 1.

Page 3 of 7

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_39-1 # Her Majesty the Queen in Right of United Kingdom 2014

Table 1 Biodegradation kinetics with differing pSi structures, surface chemistries, and biological environments Surface chemistry Native oxide (autoclaved) Native oxide Thermal oxidation Hydrosilylation Silicon native oxide PEGylation

pSi structure and physical parameters Low porosity layer on n + Si discs (30 % porosity and 30 mm thickness) Multilayer microparticles (~67 % porosity) Multilayer microparticles (~67 % porosity) Multilayer microparticles (~67 % porosity) Microparticles (40 nm APD) Microparticles (40 nm APD)

Silicon native oxide Rapid thermal oxidation (800C) Silicon native oxide

Biological fluid/ body site Blood plasma (subcutaneous site)

Degradation kinetics >3 months halflife

Reference Bowditch et al. (1999) Canham, (2014)

Vitreous humor (eye) 1 week half-life

Cheng et al. (2008)

Vitreous humor (eye) 5 weeks half-life

Cheng et al. (2008)

Vitreous humor (eye) 16 weeks half-life

Cheng et al. (2008)

Phosphate buffered saline Phosphate buffered saline Phosphate buffered saline Phosphate buffered saline Phosphate buffered saline

100 % after 2 days Godin et al. (2010) 100 % after 3–4 days 100 % after 4 h

Godin et al. (2010) Park et al. (2009)

3 h half-life

Hon et al. (2012)

80 % after 96 h

Tzur-Balter et al. (2013)

Nanoparticles (126 nm diameter, 7.5 nm APD) Nanoparticles (80–120 nm diameter, ~5 nm APD) Microparticles (20–50 mm, 446 m2/g, 1.5 ml/g, 10.7 nm APD) Hydrosilylation Microparticles (20–50 mm, (dodecyl groups) 367 m2/g, 0.84 ml/g, 7.6 nm APD) Composites with Films and microparticles PLLA

Phosphate buffered saline

~5 % after 300 h

Tzur-Balter et al. (2013)

Phosphate buffered saline

McInnes et al. (2009, 2012)

Composites with Microparticles polycaprolactone

Simulated body fluids

Difficult to quantify but slow kinetics Difficult to quantify but slow kinetics

Henstock et al. (2014)

The hydrolysis of silica-based surfaces is also strongly pH dependent (Iler 1979). Comparison of hydrolysis rates at pH 2 and 9 shows an increase in excess of three orders of magnitude in the alkali fluid. Significant differences are therefore expected and observed between physiological environments of varying pH. Examples of relevance to medical uses are the low pH condition inside lysozymes (Gu et al. 2012) and the low pH microenvironment in polymer-pSi composites due to polymer biodegradation products (Henstock et al. 2014; McInnes et al. 2009, 2012). Another is the widely varying stabilities observed in foodstuffs and beverages for oral consumption (Canham 2007). The implications of the latter case are discussed in the handbook chapter “Porous silicon and functional foods.”

Conclusions There has been growing interest in development of nanostructured porous silicon-based medical therapy over the past few years. Porous silicon dissolves in body fluids into orthosilicic acid, a benign bone nutrient bioavailable from the diet. To make pSi more compatible with loaded

Page 4 of 7

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_39-1 # Her Majesty the Queen in Right of United Kingdom 2014

drugs and nutrients, various strategies are used to make the nanostructured surfaces less reactive, resulting in slower biodegradation in body fluids. As expected, nanoparticles completely biodegrade much faster than microparticles and films of similar morphology. Composites of biodegradable polymers and porous silicon are likely to exhibit much slower biodegradation rates of the semiconductor component. There is much potential to tailor the silicon surfaces in terms of chemistry, pore size, pore structure, and porosity making it a versatile carrier system for controlled release applications.

References Ahuja G, Pathak K (2009) Porous carriers for controlled/modulated drug delivery. Indian J Pharm Sci 71(6):599–607 Anderson SHC, Elliott H, Wallis DJ, Canham LT, Powell JJ (2000) Dissolution of different forms of partially porous silicon wafers under simulated physiological conditions. Phys Stat Solidi (a) 197:331–335 Anglin EJ, Cheng L, Freeman WR, Sailor MJ (2008) Porous silicon in drug delivery devices and materials. Adv Drug Deliv Rev 60(11):1266–1277 Bowditch AP, Waters K, Gale H, Rice P, Scott EAM, Canham LT, Reeves CL, Loni A, Cox TI (1999) In-vivo assessment of tissue compatibility and calcification of bulk and porous silicon. Mat Res Soc Symp Proc 536:149–154 Canham LT (1995) Bioactive silicon structure fabrication through nanoetching techniques. Adv Mater 7:1033–1037 Canham LT (2007) Nanoscale semiconducting silicon as a nutritional food additive. Nanotechnology 18:185704 Canham LT (2014) Porous silicon for medical use: from conception to clinical use, Chap 1. In: Santos HA (ed) Biomedical uses of porous silicon. Woodhead publishing, UK. pp 3–20 Canham LT, Reeves CL, Newey JP, Houlton MR, Cox TI, Buriak JM, Stewart MP (1999) Derivatized mesoporous silicon with dramatically improved stability in simulated human blood plasma. Adv Mater 11(18):1505–1507 Cheng L, Anglin E, Cunin F, Kim D, Sailor MJ, Falkenstein I, Tammewar A, Freeman WR (2008) Intravitreal properties of porous silicon photonic crystals: a potential self-reporting intraocular drug-delivery vehicle. Br J Ophthalmol 92(5):705–711 Chiappini C, Liu X, Fakhoury JR, Ferrari M (2010) Biodegradable porous silicon barcode nanowires with defined geometry. Adv Funct Mater 20(14):2231–2239 Finnie KS, Waller DJ, Perret FL, Krause-Heuer AM, Lin HQ, Hanna JV, Barbe CJ (2009) Biodegradability of sol–gel silica microparticles for drug delivery. J Sol Gel Sci Technol 49:12–18 Godin B, Gu J, Serda RE, Bhavane R, Tasciotti E, Chiapinni C, Lu X, Tanaka T, Decuzzi P, Ferrari M (2010) Tailoring the degradation kinetics of mesoporous silicon through PEGylation. J Biomed Mater Res 94(4):1236–1243 Gu L, Ruff LE, Qin Z, Corr M, Hedrick SM, Sailor MJ (2012) Multivalent porous silicon nanoparticles enhance the immune activation potency of agonistic CD40 antibody. Adv Mater. doi:10.1002/adma.201200776 Henstock JR, Ruktanonchai UR, Canham LT, Anderson SI (2014) Porous silicon confers bioactivity to polycaprolactone composites in vitro. J Mater Sci 25(4):1087–1097 Hon NK, Shaposhnik Z, Diebold ED, Tamanoi F, Jalali B (2012) Tailoring the biodegradability of porous silicon nanoparticles. J Biomed Mater Res 100(12):3416–3421 Page 5 of 7

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_39-1 # Her Majesty the Queen in Right of United Kingdom 2014

Iler RK (1979) Chemistry of silica: solubility, polymerization, colloid and surface properties and biochemistry. Wiley, New York Jugdaohsingh R, Anderson SH, Tucker KL, Elliott H, Kiel DP, Thompson RP, Powell JJ (2002) Dietary silicon intake and absorption. Am J Clin Nutr 75(5):887–893 Li HL, Zhu Y, Xu D, Wan Y, Xia L, Zhao X (2009) Vapour-phase silanization of oxidised porous silicon for stabilizing composition and photoluminescence. J Appl Phys 105:114–307 McInnes SJP, Thissen H, Choudbury NR, Voelcker NH (2009) New biodegradable materials produced by ring opening polymerisation of poly(L-lactide) on porous silicon substrates. J Coll Interf Sci 332:336–344 McInnes SJ, Irani Y, Williams KA, Voelcker NH (2012) Controlled drug delivery from composites of nanostructured porous silicon and poly(L-lactide). Nanomedicine 7(7):995–1016 Park JH, Ye M, Park K (2005) Biodegradable polymers for microencapsulation of drugs. Molecules 10:146–161 Park J-H, Gu L, von Maltzahn G, Ruoslahti E, Bhatia SN, Sailor MJ (2009) Biodegradable luminescent porous silicon nanoparticles for in vivo applications. Nat Mater 8(4):331–336 Ratner BD, Hoffman AS, Schoen FJ, Lemons JE (eds) (2004). Biomaterials science: an introduction to materials in medicine, 2nd edn. Elsevier, US. p 851 Refitt DM, Jugdaosingh R, Thompson RPH, Powell JJ (1999) Silicic acid: its gastrointestinal uptake and urinary excretion in man and effects on aluminium excretion. J Inorg Biochem 76:141–147 Sarparanta M, Bimbo LM, Rytkonen J, Makila E, Laaksonen TJ, Laaksonen P, Nyman M, Salonen J, Linder MB, Hirvonen J, Santos HA, Airaksinen AJ (2012) Intravenous delivery of hydrophobinfunctionalized porous silicon nanoparticles: stability, plasma protein adsorption and biodistribution. Mol Pharm 9:654–663 Shabir Q, Pokale A, Loni A, Johnson DR, Canham LT, Fenollosa R, Tymczenko M, Rodríguez I, Meseguer F, Cros A (2011) Medically biodegradable hydrogenated amorphous silicon microspheres. Silicon 2011:173–176 Tzur-Balter A, Rubinskia A, Segal E (2013) Designing porous silicon-based microparticles as carriers for controlled delivery of mitoxantrone dihydrochloride. J Mater Res 28(2):231–239

Page 6 of 7

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_39-1 # Her Majesty the Queen in Right of United Kingdom 2014

Index Terms: Biocompatibility 1 Biodegradability 1 Biodegradable materials 1–2 Biodegradation kinetics 3 In-vitro biodegradation 1–2 In-vivo biodegradation 2–3 Mesoporous silicon 1

Page 7 of 7

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_50-1 # Springer International Publishing Switzerland 2014

Cell Culture on Porous Silicon Nicolas Voelcker* and Suet P. Low Mawson Institute, University of South Australia, Adelaide, SA, Australia

Abstract Cell culture is a powerful in vitro characterization technique to optimize the properties of a biomaterial for in vivo biomedical use by conversely revealing potential sources of cytotoxicity. A comprehensive literature survey of the range of cell types cultured on porous silicon is given, together with a discussion of how surface chemistry, topography, and porosity gradients affect cell behavior.

Introduction Cell culture is often utilized to determine the biocompatibility of materials and precedes or even replaces in vivo animal and human testing. The behaviour of cells such as attachment, proliferation, morphological changes, metabolic changes, cytotoxicity, protein expression, and RNA expression are all important factors that have to be taken into account when designing a biomaterial (Freshney 2005; Masters 2000). In this regard, many materials are being investigated for their suitability for the culture of cells or even to study cellular interactions. Porous silicon is a popular choice for biosensor, bio-microelectromechanical systems (bioMEMS), biomaterials and tissue engineering applications. The porous structure, degradability, electrical conductivity, overall biocompatibility (see chapter “▶ Biocompatibility of Porous Silicon”), and ease of surface modification make this a fascinating platform to investigate cell culture interactions. For example, porous silicon disks are being developed for delivery of therapeutic ocular cells (Fig. 1).

Early Studies A variety of mammalian cells have been successfully cultured onto porous silicon surfaces. The first publications on this topic by Bayliss et al. demonstrated that attachment of Chinese hamster ovary (CHO) cells proceeded on porous silicon surfaces to a similar extent as on bulk silicon (Bayliss et al. 1997a, b). This was also confirmed with the neuronal cell line B50 (Bayliss et al. 2000). Cell viability in these studies was determined using two colorimetric assays, the MTT based on enzymatic reduction of a tetrazolium salt to a purple formazan and the neutral red uptake assay. B50 and CHO cells were cultured on bulk silicon, porous silicon, glass, and polycrystalline silicon. Both viability assays suggested that the neuronal cells showed preference for porous silicon above the other surfaces, while CHO cells showed the lowest viability on the porous silicon surface (Bayliss et al. 1999, 2000). The surfaces of the porous silicon used in these early studies were not modified post-etching, and it was not until a study utilized porous silicon surfaces with an oxide layer for cell culture that surface chemistry was found to play a crucial factor (Chin et al. 2001). Rat *Email: [email protected] Page 1 of 15

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_50-1 # Springer International Publishing Switzerland 2014

Fig. 1 Cells being cultured on porous silicon particles that have been compressed into a disk form as part of a cell delivery platform (Low 2008)

hepatocytes were cultured onto ozone-oxidized porous silicon that was further modified by fetal bovine serum and collagen type I coating. Here, the hepatocytes showed a preference for the collagen-coated surface (Chin et al. 2001). Viability assays such as MTT, XTT, MTS, or Alamar Blue are commonly used to determine the suitability of a material as a support for the attachment and growth of cells. These assays are based on the reduction of the tetrazolium dyes by cellular enzymes to formazan dyes with characteristic color. In 2006, it has come to light that porous silicon, even with an oxide layer, interferes with these assays by reducing tetrazolium dyes (Laaksonen et al. 2007; Low et al. 2006). Passivating the surface against hydrolytic attack reduces but does not completely remove the interfering behavior (Laaksonen et al. 2007). Dye uptake viability assays such as neutral red which make use of the ability of viable cells to incorporate the dye in the lysosomes were found to be not compatible with porous silicon either, since the neutral red dye can also ingress into the porous layer (Low et al. 2006). These findings suggest that viability assays for cells in contact with porous silicon need to be carefully evaluated for compatibility.

Surface Modification Surface modification of porous silicon has been used to protect the surface against hydrolytic attack in aqueous medium and stabilize or slow down surface degradation. It can also be used to promote or prevent mammalian cell adhesion (Low et al. 2006; Faucheux et al. 2004). The changes in surface chemistry have long been known to affect the attachment and proliferation of anchorage-dependent mammalian cells on materials featuring otherwise almost identical topography, where cell attachment can be inhibited on very hydrophobic or hydrophilic surfaces (Groth and Altankov 1996; Yanagisawa et al. 1989). This has been mainly attributed to the amount of serum proteins (containing attachment factors) that is pre-adsorbed to the surface (Faucheux et al. 2004), which in turn can mediate cell attachment (Webb et al. 2000). Freshly etched porous silicon (Si–H) is rather hydrophobic, whereas ozone-oxidized surface (Si–OH) is very hydrophilic. Attachment of proteins in cell culture medium has been known to bind to moderately hydrophilic surfaces, leading to greater cell attachment on those surfaces (Webb et al. 1998). Water contact angles, qualitatively describing surface wettability for freshly etched and surface-modified porous silicon surfaces, are shown in the table below (Table 1). Arguably, the simplest method to stabilize the porous silicon surface is oxidation. A popular technique is to use ozone to rapidly generate a Si–OH capped surface with a thin oxide layer. Alternatively, thermal treatment in air (400–800  C) is used to generate thicker oxide layers (Pap et al. 2004). Surface hydroxyl groups can be further reacted with silanes, which can further stabilize the surface against hydrolytic attack, as well as provide a means of attaching functional groups to the Page 2 of 15

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_50-1 # Springer International Publishing Switzerland 2014

Table 1 Sessile drop water contact angle measurements for unmodified and surface-modified porous silicon etched under the same conditions (Low et al. 2006) Surface modification of porous silicon Freshly etched Amino silanized Collagen coated Polyethylene glycol silanized Fetal bovine serum coated Ozone oxidized

Contact angle >99 56 32 26 10 300  C are dehydrated, with a predominance of Si–O–Si bonding and few Si–OH species (Zangooie et al. 1998). Although the dissolution reaction may be thermodynamically favored for all oxide types, the kinetics of dissolution are more important. In addition to the dependence on pH and thermal history of the sample, porosity and feature size play roles in determining the rate of dissolution of porous Si and its oxides in aqueous media. The rate of dissolution of porous Si at physiologic pH generally increases with increasing sample porosity (Anderson et al. 2003), and the rate of dissolution tends to decrease with increasing feature size in the order microporous >> mesoporous > macroporous (Sailor 2012). As mentioned above, silicon oxides will dissolve in strong base following Eq. 10. The product shown in Eq. 10 is the species [SiO2(OH)2]2, the doubly deprotonated form of silicic acid, Si(OH)4. While it is the dominant species that exists in highly basic (pH > 12) water (pKa of [SiO(OH)3] ¼ 12), in neutral or acidic solution, silicic acid exists in its fully protonated form, Si(OH)4 (pKa of Si(OH)4 ¼ 10) (Greenwood and Earnshaw 1984). Thus at neutral, physiologic pH the dissolution of SiO2 proceeds as given in Eq. 15. SiO2 þ 2 H2 O ! SiðOHÞ4ðaqÞ

(15)

When the solution concentration of Si(OH)4 is sufficiently large, the reaction of Eq. 15 runs in reverse, and silicic acid condenses back into a solid (Eq. 16). In the course of this condensation reaction, various “polysilicic acids” with the general formula [SiOx(OH)42x]n, where 2 <  < 0, are present in solution (Greenwood and Earnshaw 1984). In neutral or acidic solutions, these oligomers can condense to the point of precipitation. Page 8 of 24

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_37-1 # Springer International Publishing Switzerland 2014

SiðOHÞ4ðaqÞ ! SiO2 þ 2 H2 O

(16)

The reaction of Eq. 16 is the chemistry that occurs during the “sol–gel” process, used to prepare colloids, films, or monoliths of porous silica from solution precursors (Brinker and Scherer 1990). This reaction explains why elemental silicon does not corrode appreciably at pH values 500  C, significant carbonization occurs (Salonen et al. 2004), leading to black films with a sooty appearance (Ruminski et al. 2010). This more extensive pyrolysis removes the residual hydrogen from the hydrocarbon layer, and the high carbon content material is referred to as “thermally carbonized porous Si,” or TCPSi. The acetylene precursor used in these reactions can be replaced with an organic polymer (polyfurfuryl alcohol) to generate similar materials (Kelly et al. 2011). Although less well defined than material prepared by the hydrosilylation route, the pyrolyzed porous Si formulations are chemically stable (Salonen et al. 2004; Bimbo et al. 2010; Jalkanen et al. 2009; Limnell et al. 2007; Tsang et al. 2012), leading to strong interest in these formulations for bioimplant, drug delivery (Salonen et al. 2005a, b, 2008; Lehto et al. 2005; Limnell et al. 2006; Heikkila et al. 2007; Kaukonen et al. 2007), and biosensor (Ruminski et al. 2010; Jalkanen et al. 2009; Tsang et al. 2012; Salonen et al. 2006; Bjorkqvist et al. 2004b, 2005) applications. H Si Si

Si

Si Si

H

H

H +H Si

C

C

H

485 °C

C

C

Si C

C

H

H Si C

C

ð22Þ

C

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_37-1 # Springer International Publishing Switzerland 2014

Modification strategies that generate functional nanostructures from pyrolyzed, carboncontaining porous Si have been demonstrated (Jalkanen et al. 2009; Salonen et al. 2006; Bjorkqvist et al. 2004b, 2005; Sciacca et al. 2010). The radical coupling method of Iijima (Iijima and Kamiya 2008), developed to functionalize carbon fibers and diamond-like carbon, has been found to work on hydrocarbonized porous Si as well (Sciacca et al. 2010). For hydrocarbonized porous Si, the reaction involves a radical initiator (benzoyl peroxide) and a dicarboxylic acid linker (Eq. 23). The chemistry allows permeation of aqueous solutions and attachment of specific biological capture probes.

ð23Þ

Porous Silicon Nitrides After silicon oxide, silicon nitride is probably the most important modification of silicon in the semiconductor industry. A common etch stop and dielectric layer used in integrated circuit manufacture, the fracture toughness and durability of silicon nitride (Si3N4) has also spurred its use in high performance parts such as bearings and gas turbine blades. The larger index of refraction of Si3N4 relative to SiO2 (2.0 vs. 1.5, respectively) makes silicon nitride an attractive alternative to silicon oxide for optical devices made from porous Si. The industrial processes used to prepare silicon nitride involve direct reaction of Si with N2 at high temperature (for the bulk material) and CVD or plasma-enhanced CVD deposition from N2, NH3, and silane precursors (for thin films). Similarly, porous Si can be nitrided by heating in NH3 or N2 ambients (Bjorkqvist et al. 2003; Morazzani et al. 1996) or by plasma-assisted CVD. A typical thermal preparation involves heating in pure N2 at 1,100  C for 12 min (Bjorkqvist et al. 2003). Such high temperatures tend to decrease surface area, pore diameter, and pore volume in the resulting material (Bjorkqvist et al. 2003). A low temperature (600  C) process has been developed using a rapid thermal processor in a pure N2 ambient (James et al. 2010). Although the lower temperature preserves the pore structure, it introduces a significant quantity of silicon oxide (Lai et al. 2011). The stability of porous Si samples modified with silicon nitride is improved compared with as-formed porous Si, though it is comparable to samples prepared with a thermally grown silicon oxide (Bjorkqvist et al. 2003; James et al. 2009, 2010; Lai et al. 2011).

Attachment of Biomolecules to Functionalized Porous Si Biologically active molecules have been covalently attached to porous Si surfaces for a variety of medically relevant applications: antibodies have been attached to porous Si to impart molecular

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_37-1 # Springer International Publishing Switzerland 2014

selectivity for biosensing (Serda et al. 2010; Tinsley-Bown et al. 2000; Gu et al. 2012; Lowe et al. 2010; Andrew et al. 2010; Wu et al. 2009; Rossi et al. 2007; Meskini et al. 2007; Bonanno and DeLouise 2007; Park et al. 2006; Starodub et al. 1996); therapeutics have been attached to porous Si for slow release drug delivery (Wu et al. 2008, 2011a, b; Chhablani et al. 2013; Sailor and Park 2012); and sugars and polyethers have been attached to porous Si as antifouling coatings in bioimplants (Schwartz et al. 2005; Kilian et al. 2007a, b; Godin et al. 2010). The strategy generally involves a linker molecule that connects the biologically active species to the porous Si surface. Many suitable surface attachment chemistries for such linkers have been described in this chapter. To attach a biological species to an immobilized linker, an activating step is usually employed to allow formation of a covalent bond between the species and the linker. The book Bioconjugate Techniques by Hermanson (1996) is a leading reference on the chemical reactions that can be employed to form these bonds. The most common approach for porous Si involves the use of carbodiimide coupling reagents such as 1-ethyl-3-[3-dimethylaminopropyl]carbodiimide hydrochloride, or EDC. EDC is employed to couple a primary amine (NH2) group to a carboxyl group, forming an amide bond. Thus the chemistry is amenable to a porous Si surface that has been modified with carboxyl (CO2H) or amine (NH2) species as long as the target molecule has a corresponding amine or carboxyl group. One example is given in Eq. 24 (Wu et al. 2008).

ð24Þ

Comparison of Si–C vs. Si–O Chemistries For situations where the functionalized material must eventually dissolve, such as in vivo drug delivery, chemistries involving Si–O bonds represent an attractive alternative to Si–C chemistries. The time needed for highly porous SiO2 to dissolve in aqueous media can be on the order of hours to days – appropriate for many short-term drug delivery applications. In contrast, the Si–C chemistries of porous Si generally display significantly longer degradation times, which can be as long as several months or even years (Canham et al. 1999, 2000; Cheng et al. 2008). This is particularly true of the pyrolysis-derived (carbonized and hydrocarbonized) materials (Eq. 22) (Salonen et al. 2008). The well-defined degradation mechanism for porous SiO2 generates silicic acid (Si(OH)4) as the end product (Eq. 15), which is a naturally occurring species that is present in body tissues at a mean concentration of ~5 ppm (Van Dyck et al. 2000). In contrast, samples prepared by pyrolysis of organic materials may contain polycyclic aromatics or other potentially toxic species that could be difficult to identify or characterize. For both the thermal oxide and carbon pyrolysis reactions, the degree of stability in aqueous media can be tuned by longer thermolysis times or higher temperatures.

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Sailor MJ, Park JH (2012) Hybrid nanoparticles for detection and treatment of cancer. Adv Mater 24:3779–3802 Sakka T, Tsuboi T, Ogata YH, Mabuchi M (2000) Raman scattering from metal-deposited porous silicon. J Porous Mater 7:397–400 Salhi B, Gelloz B, Koshida N, Patriarche G, Boukherroub R (2007) Synthesis and photoluminescence properties of silicon nanowires treated by high-pressure water vapor annealing. Phys Status Solidi A-Appl Mater 204:1302–1306 Salonen J, Lehto VP, Bjorkqvist M, Laine E, Niinisto L (2000) Studies of thermally-carbonized porous silicon surfaces. Phys Status Solidi A-Appl Res 182:123–126 Salonen J, Laine E, Niinisto L (2002) Thermal carbonization of porous silicon surface by acetylene. J Appl Phys 91:456–461 Salonen J, Bjorkqvist M, Laine E, Niinisto L (2004) Stabilization of porous silicon surface by thermal decomposition of acetylene. Appl Surf Sci 225:389–394 Salonen J, Laitinen L, Kaukonen AM, Tuura J, Bjorkqvist M, Heikkila T, Vaha-Heikkila K, Hirvonen J, Lehto VP (2005a) Mesoporous silicon microparticles for oral drug delivery: loading and release of five model drugs. J Control Release 108:362–374 Salonen J, Paski J, Vaha-Heikkila K, Heikkila T, Bjorkqvist M, Lehto VP (2005b) Determination of drug load in porous silicon microparticles by calorimetry. Phys Status Solidi A-Appl Mater 202:1629–1633 Salonen J, Tuura J, Bjorkqvist M, Lehto VP (2006) Sub-ppm trace moisture detection with a simple thermally carbonized porous silicon sensor. Sens Actuator B-Chem 114:423–426 Salonen J, Kaukonen AM, Hirvonen J, Lehto VP (2008) Mesoporous silicon in drug delivery applications. J Pharm Sci 97:632–653 Sasano J, Schmuki P, Sakka T, Ogata YH (2003a) Laser-assisted nickel deposition onto porous silicon. Phys Status Solidi A-Appl Res 197:46–50 Sasano J, Murota R, Yamauchi Y, Sakka T, Ogata YH (2003b) Re-dissolution of copper deposited onto porous silicon in immersion plating. J Electroanal Chem 559:125–130 Sasano J, Schmuki P, Sakka T, Ogata YH (2004) Laser-assisted maskless Cu patterning on porous silicon. Electrochem Solid State Lett 7:G98–G101 Sasano J, Schmuki P, Sakka T, Ogata YH (2005) Maskless patterning of various kinds of metals onto porous silicon. Phys Status Solidi A-Appl Mater Sci 202:1571–1575 Schwartz MP, Cunin F, Cheung RW, Sailor MJ (2005) Chemical modification of silicon surfaces for biological applications. Phys Status Solidi A-Appl Mater 202:1380–1384 Sciacca B, Alvarez SD, Geobaldo F, Sailor MJ (2010) Bioconjugate functionalization of thermally carbonized porous silicon using a radical coupling reaction. Dalton Trans 39:10847–10853 Serda RE, Mack A, Pulikkathara M, Zaske AM, Chiappini C, Fakhoury JR, Webb D, Godin B, Conyers JL, Liu XW, Bankson JA, Ferrari M (2010) Cellular association and assembly of a multistage delivery system. Small 6:1329–1340 Sieval AB, Demirel AL, Nissink JWM, Linford MR, van der Maas JH, de Jeu WH, Zuilhof H, Sudholter EJR (1998) Highly stable Si–C linked functionalized monolayers on the silicon (100) surface. Langmuir 14:1759–1768 Sieval AB, Linke R, Zuilhof H, Sudholter EJR (2000) High-quality alkyl monolayers on silicon surfaces. Adv Mater 12:1457–1460 Song JH, Sailor MJ (1998a) Dimethyl sulfoxide as a mild oxidizing agent for porous silicon and its effect on photoluminescence. Inorg Chem 37:3355–3360

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Song JH, Sailor MJ (1998b) Functionalization of nanocrystalline porous silicon surfaces with aryllithium reagents: formation of silicon–carbon bonds by cleavage of silicon–silicon bonds. J Am Chem Soc 120:2376–2381 Song JH, Sailor MJ (1999) Chemical modification of crystalline porous silicon surfaces. Comment Inorg Chem 21:69–84 Starodub NF, Fedorenko LL, Starodub VM, Dikij SP, Svechnikov SV (1996) Use of the silicon crystals photoluminescence to control immunocomplex formation. Sens Actuators B 35:44–47 Stefano LD, Oliviero G, Amato J, Borbone N, Piccialli G, Mayol L, Rendina I, Terracciano M, Rea I (2013) Aminosilane functionalizations of mesoporous oxidized silicon for oligonucleotide synthesis and detection. J R Soc Interface 10:20130160 Steiner P, Kozlowski F, Lang W (1995a) Electroluminescence from porous Si after metal deposition into the pores. Thin Solid Films 255:49–51 Steiner P, Kozlowski F, Lang W (1995b) Depositing metals into porous Si- the impact on luminescence. Materials Research Society, Boston, pp 665–670 Stewart MP, Buriak JM (1998) Photopatterned hydrosilylation on porous silicon. Angew Chem Int Ed Engl 37:3257–3260 Stewart MP, Buriak JM (2001) Exciton-mediated hydrosilylation on photoluminescent nanocrystalline silicon. J Am Chem Soc 123:7821–7830 Sweryda-Krawiek B, Chandler-Henderson RR, Coffer JL, Rho YG, Pinizzotto RF (1996) A comparison of porous silicon and silicon nanocrystallite photoluminescence quenching with amines. J Phys Chem 100:13776–13780 Terry J, Linford MR, Wigren C, Cao R, Pianetta P, Chidsey CED (1997) Determination of the bonding of alkyl monolayers to the Si(111) surface using chemical-shift, scanned-energy photoelectron diffraction. Appl Phys Lett 71:1056–1058 Terry J, Linford MR, Wigren C, Cao RY, Pianetta P, Chidsey CED (1999) Alkyl-terminated Si(111) surfaces: a high-resolution, core level photoelectron spectroscopy study. J Appl Phys 85:213–221 Thomas JC, Pacholski C, Sailor MJ (2006) Delivery of nanogram payloads using magnetic porous silicon microcarriers. Lab Chip 6:782–787 Thompson CM, Nieuwoudt M, Ruminski AM, Sailor MJ, Miskelly GM (2010) Electrochemical preparation of pore wall modification gradients across thin porous silicon layers. Langmuir 26:7598–7603 Tinsley-Bown AM, Canham LT, Hollings M, Anderson MH, Reeves CL, Cox TI, Nicklin S, Squirrell DJ, Perkins E, Hutchinson A, Sailor MJ, Wun A (2000) Tuning the pore size and surface chemistry of porous silicon for immunoassays. Phys Status Solidi A-Appl Res 182:547–553 Tsang CK, Kelly TL, Sailor MJ, Li YY (2012) Highly stable porous silicon carbon composites as label-free optical biosensors. ACS Nano 6:10546–10554 Tsuboi T, Sakka T, Ogata YH (1998) Metal deposition into a porous silicon layer by immersion plating: influence of halogen ions. J Appl Phys 83:4501–4506 Van Dyck K, Robberecht H, Van Cauwenbergh R, Van Vlaslaer V, Deelstra H (2000) Indication of silicon essentiality in humans – serum concentrations in Belgian children and adults, including pregnant women. Biol Trace Elem Res 77:25–32 Voelcker NH, Alfonso I, Ghadiri MR (2008) Catalyzed oxidative corrosion of porous silicon used as an optical transducer for ligand-receptor interactions. Chem Biochem 9:1776–1786 Wu EC, Park J-H, Park J, Segal E, Cunin F, Sailor MJ (2008) Oxidation-triggered release of fluorescent molecules or drugs from mesoporous Si microparticles. ACS Nano 2:2401–2409 Wu C-C, Alvarez SD, Rang CU, Chao L, Sailor MJ (2009) Label-free optical detection of bacteria on a 1-D photonic crystal of porous silicon. Proc SPIE 7167:71670Z-71670Z–71610 Page 23 of 24

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Wu EC, Andrew JS, Buyanin A, Kinsella JM, Sailor MJ (2011a) Suitability of porous silicon microparticles for the long-term delivery of redox-active therapeutics. Chem Commun 47:5699–5701 Wu EC, Andrew JS, Cheng L, Freeman WR, Pearson L, Sailor MJ (2011b) Real-time monitoring of sustained drug release using the optical properties of porous silicon photonic crystal particles. Biomaterials 32:1957–1966 Yang YJ, Meng GW (2010) Ag dendritic nanostructures for rapid detection of polychlorinated biphenyls based on surface-enhanced Raman scattering effect. J Appl Phys 107:044315 Zangooie S, Bjorklund R, Arwin H (1998) Protein adsorption in thermally oxidized porous silicon layers. Thin Solid Films 313–314:825–830 Zhang XG (2004) Morphology and formation mechanisms of porous silicon. J Electrochem Soc 151:C69–C80

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_55-1 # Springer International Publishing Switzerland 2014

Colloidal Lithography Luca Boarinoa* and Michele Lausb a Nanofacility Piemonte, Istituto Nazionale di Ricerca Metrologica, Torino, Italy b Dipartimento di Scienze e Innovazione Tecnologica (DISIT), Università del Piemonte Orientale “A. Avogadro”, INSTM, UdR Alessandria, Alessandria, Italy

Abstract This chapter is a visual guide to the numerous approaches to nanolithography nanofabrication on large area based on supramolecular self-assembly. A short history of this recent scientific and technological field, an outline of the most-cited methods of self-assembly, and tables reporting different nanofabrication methods are reported. Indications on requirements, advantages, and drawbacks of the various approaches are also listed in the table. Thanks to the recently developed metal-assisted catalytic etching (MACE), the colloidal patterns can be easily propagated to silicon and other semiconductors opening a wide field of morphology, nanostructures, and applications.

Introduction Micro- and nanospheres were used in the past for phase separation processes, cosmetics, electronics, calibration, microfluidics, environment applications, biotechnology, and life science applications including, among others, immunoassays, cell isolation, biocatalysis, and nucleic acid technology. In recent years, thanks to the pioneering work of [1, 2], micro- and nanospheres as well as nano-objects were employed as building blocks to form self-assembled masks suitable for large area nanolithography and nanostructuration of thin films and surfaces. This novel technique, colloidal lithography, combines the supramolecular self-assembly of single nano-objects, with thin film deposition, reactive ion etching, epitaxial growth, and metal-assisted chemical etching. The term “polymer colloid” [3] refers to a dispersion of polymeric spheres in water of nonaqueous solutions with diameters ranging from hundreds of nanometers to several micrometers. The first material employed for nanosphere lithography was polystyrene, but a fine tuning of the polymerization conditions allows for the synthesis of different monodisperse polymer colloids such as poly(methyl methacrylate) and other polyacrylates. The synthetic techniques for the preparation of colloids are known as heterophase polymerizations [4] and include a variety of different processes including suspension, dispersion, emulsion, as well as seeded emulsion [5] and nanoemulsion polymerizations [6, 7]. Colloidal systems like silica and polystyrene nanospheres are available in commerce as aqueous solutions typ. 5–10 % in volume, with a good control of size and surface characteristics and functionalization.

Self-Assembly of Nanospheres The word “self-assembly” means to build or putting together, without an external contribution, small building blocks into a periodic structure in which these individual elements are arranged into regular

*Email: [email protected] Page 1 of 9

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_55-1 # Springer International Publishing Switzerland 2014

registry. This process is one of the primordial phenomena happening in nature during the parallel assembly of organic and inorganic structures at different scales. Despite this remote origin, the interest of scientists towards this mechanism is rather recent, mainly driven by the fundamental studies of Langmuir and Blodgett on the closed packed arrangements of amphiphilic molecules on liquids and solids [8]. In 1946, Bigelow and coworkers [9] observed the dense packing of monolayers of long chain alkylamines on platinum surfaces. In these systems, even if it was not recognized at that time, self-assembly was clearly the core mechanism. More recently, in 1983, [10] introduced the concept of self-assembled monolayers with close packing chemisorbed alkanethiolate on gold surfaces. Today, it is clear that many driving forces are able to lead to the self-assembly of atoms, molecules, polymers, particles, and nano-objects. These forces include ionic, covalent, metallic, hydrogen, and coordination bonds but also weak interactions, like van der Waals, p-p and hydrophobic, colloidal and capillary, convective and shear, magnetic, electrical, Casimir, and optical forces. Nevertheless, at the base of the self-assembly, there is the self-recognition mechanism that was fundamentally summarized by the medieval alchemists in the sentence “similia similibus solvuntur.”

Supramolecular Self-Assembly: Formation of 2D Arrays of Colloidal Spheres Monodispersed colloidal spheres can be self-assembled into ordered 2D arrays on solid supports or in thin films of liquids using a number of strategies [11–14]. Table 1 shows the main methods, requirements, advantages and drawbacks, and reference to literature. At the present stage of development, all of these methods are only capable of generating colloidal arrays built up by small domains, and the largest single domain usually contains fewer than 10,000 colloidal spheres [15]. These methods form 2D hexagonal arrays in which the spheres are in Table 1 Supramolecular self-assembly: main methods, characteristics, and reference to literature Method Air-liquid interface, Langmuir Blodgett Slow evaporation

Spinning

Requirements Spreading agent (ethanol, methanol), control of particle surface charge, control of the particle immersion, addition of surfactant for packing, control of water Ph Piranha functionalization of substrates, use of different solvents like F-oil or mercury Substrate treatment with wetting agent or surfactant, Piranha functionalization

Electrophoretic Solid electrodes, polarization 50–100 V/cm Capillarity Optical forces

Capillary setup, micrometric slits Laser-based optical standing wave pattern

Advantages Large ordered areas, applicable to any surface, no need of surface functionalization Good packing, large ordered domains

Drawbacks Only floating nanospheres

Applicable only to idrophilic surfaces, presence of double layers Good packing, reduced Applicable only to double layers, large hydrophilic surfaces, ordered domains, highly no order under industrializable 50 nm Assembly of small metal Complexity of setup, particles (under 50 nm) limited ordered domains Complexity of setup Arbitrary 2D or 3D patterns Complexity of setup

References [20–28]

[29]; [13]; [30–32]; [15] [33]; [2]

[34–38]

[39] [40, 41]

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_55-1 # Springer International Publishing Switzerland 2014

Table 2 Widely used techniques of colloidal lithography and relative literature references Scheme

Building block Material Carboxylate Silicon polystyrene nanospheres

Method RIE

Reference [47]

Polystyrene nanospheres

Silicon dioxide, silicon

DRIE

[48, 49]

Polystyrene nanospheres

Graphene

RIE

[50]

Polystyrene nanospheres

Metals, silicon

Thermal [1]; [51, evaporation, tilted 52, 53] thermal evaporation, e-gun, sputtering

Polystyrene nanospheres

Silver, silicon

Metal-assisted etching

[54, 55, 56]

Polystyrene nanospheres, CdSe QDots

Glass

Evaporation templating

[57]

(continued)

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_55-1 # Springer International Publishing Switzerland 2014

Table 2 (continued) Scheme

Building block Material Polystyrene GaN nanospheres Si

Method MBE

Reference [58] [59]

Polystyrene nanospheres

GaN, ZnO

Vapor-liquid-solid

[60]

Polystyrene nanospheres

Si, SiO2, glass, thin metal films, Au, Si, Ni, Co, Py, others

RIE, thermal [61, 62] evaporation, e-gun, sputtering, ion milling

Polystyrene N-type silicon nanospheres, , SiO2 Fe, Ni, carbon nanotubes

Plasma-enhanced hot filament chemical vapor deposition

Silica nanospheres

PS, PMMA, other polymers

Heating of substrate [66] above polymer Tg (glass transition temperature)

Polystyrene and silica nanospheres

Silicon, SiO2, optical photoresist

UV exposure, UV laser ablation

[63–65]

[67, 68]

physical contact. To vary independently the lattice constant and the particle size, a chemical reduction by dry etching is necessary. The self-assembly of nanospheres is generally limited to 100–80 nm. Under this limit, in fact, the synthetic processes usually produce broad polydispersions, leading to highly defective packed arrays. Remarkable exceptions are constituted by core-shell particles prepared by seeding or self-seeding technique [16, 17] and silica particles prepared using TEOS and basic amino acids [18, 19].

Nanostructuration of Thin Films and Surfaces by Colloidal Lithography Once the self-assembly is obtained, and the array of spheres packed, a quantity of methods have been applied for the propagation of the colloidal mask to the substrate. This variety of method can be roughly listed in the following categories: shadow evaporation, growth, dry etching, and ion milling through the interstitial sites, wet etching, and metal-assisted etching. In Table 2, some of the mostused techniques of propagation of colloidal masks are shown. Under 50 nm, diblock copolymers are

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_55-1 # Springer International Publishing Switzerland 2014

reaching such a high scientific and industrial consideration to be cited in the ITRS as a possible alternative to optical lithography for the 22 nm nodes and for the future [42–46].

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21. Horvolgyi Z, Mate M, Zrinyi M (1994) On the universal growth of two-dimensional aggregates of hydrophobed glass beads formed at the (aqueous solution of electrolyte) – air interfaces. Coll Surf A: Physicochem Eng Asp 84:207 22. Wickman HH, Korley JN (1998) Colloid crystal self-organization and dynamics at the air/water interface. Nature 393:445 23. Kondo M, Shinozaki K, Bergstrom L, Mizutani N (1995) Preparation of colloidal monolayers of alkoxylated silica particles at the air-liquid interface. Langmuir 11:394 24. Deckman HW, Dunsmuir JH, Gruner SM (1989) Microfabrication cellular phosphors. J Vac Sci Technol B7:1832 25. Lenzmann F, Li K, Kitai AH, Stover HDH (1994) Thin-film micropatterning using polymer microspheres. Chem Mater 6:156 26. Fulda KU, Tieke B (1994) Langmuir films of monodisperse 0.5 μm spherical polymer particles with a hydrophobic core and a hydrophilic shell. Adv Mater 6:288 27. Burmeister F, Schafle C, Matthes T, Bohmisch M, Boneberg J, Leiderer P (1997) Colloid monolayers as versatile lithographic masks. Langmuir 13:2983 28. Siroktin E, Apweiler JD, Ogrin FY (2010) Macroscopic ordering of polystyrene carboxylatemodified nanospheres self-assembled at the water-air interface. Langmuir 26(13):10677 29. Denkov ND, Velev OD, Kralchevsky PA, Ivanov IB, Yoshimura H, Nagayama K (1992) Mechanism of formation of two-dimensional crystals from latex particles on substrates. Langmuir 8:3183 30. Dimitrov AS, Nagayama K (1996) Continuous convective assembling of fine particles into twodimensional arrays on solid surfaces. Langmuir 12:1303 31. Velev OD, Denkov ND, Paunov VN, Kralchevsky PA, Nagayama K (1993) Direct measurement of lateral capillary forces. Langmuir 9:3702 32. Dimitrov AS, Miwa T, Nagayama K (1999) A comparison between the optical properties of amorphous and crystalline monolayers of silica particles. Langmuir 15:5257 33. Deckman HW, Dunsmuir JH, Garoff S, McHenry JA, Peiffer DG (1988) Macromolecular selforganized assemblies. J Vac Sci Technol B6:333 34. Richetti P, Prost J, Barois P (1984) Two-dimensional aggregation and crystallization of a colloidal suspension of latex spheres. J Phys Chem 45:L1137 35. Trau M, Saville DA, Aksay IA (1996) Field-induced layering of colloidal crystals. Science 272:706 36. Trau M, Saville DA, Aksay IA (1997) Assembly of colloidal crystals at electrode interfaces. Langmuir 13:6375 37. Yeh SR, Seul M, Shraiman BI (1997) Assembly of ordered colloidal aggregrates by electricfield-induced fluid flow. Nature 386:57 38. Solomentsev Y, Bohmer M, Anderson JL (1997) Particle clustering and pattern formation during electrophoretic deposition: a hydrodynamic model. Langmuir 13:6058 39. Sun Z, Yang B (2006) Fabricating colloidal crystals and construction of ordered nanostructures. Nanoscale Res Lett 1:46 40. Misawa H, Sasaki K, Koshioka M, Kitamura N, Masuhara H (1992) Multibeam laser manipulation and fixation of microparticles. Appl Phys Lett 60:310 41. Mio C, Marr MDW (1999) Tailored surfaces using optically manipulated colloidal particles. Langmuir 15:8565 42. International Technology Roadmap for Semiconductors (2009) Edition, metrology. http://www. itrs.net/links/2009ITRS/2009Chapters_2009Tables/2009_Metrology.pdf

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43. Cheng JY, Mayes AM, Ross CA (2004) Nanostructure engineering by templated self-assembly of block copolymers. Nat Mater 3:823 44. Darling SB (2007) Directing the self-assembly of block copolymers. Progr Polym Sci 32(10):1152 45. Daga VK, Schwartz EL, Chandler CM, Lee J-K, Lin Y, Ober CK, Watkins JJ (2011) Photoinduced ordering of block copolymers. Nano Lett 11:1153 46. Andreozzi A, Lamagna L, Seguini G, Fanciulli M, Schamm-Chardon S, Castro C, Perego M (2011) The fabrication of tunable nanoporous oxide surfaces by block copolymer lithography and atomic layer deposition. Nanotechnology 22:335303 47. Chau CF, Melvin T (2008) The fabrication of macroporous polysilicon by nanosphere lithography. J Micromech Microeng 18:064012 48. Cheung CL, Nikolic RJ, Reinhardt CE, Wang TF (2006) Fabrication of nanopillars by nanosphere lithography. Nanotechnology 17:1339 49. Li W, Xu L, Zhao WM, Sun P, Huang XF, Chen KJ (2007) Fabrication of large-scale periodic silicon nanopillar arrays for 2D nanomold using modified nanosphere lithography. Appl Surf Sci 253:9035 50. Liu L, Zhang Y, Wang W, Gu C, Bai X, Wang E (2011) Nanosphere lithography for the fabrication of ultranarrow graphene nanoribbons and on-chip bandgap tuning of graphene. Adv Mater 23:1246 51. Haynes CL, Van Duyne RP (2003) Dichroic optical properties of extended nanostructures fabricated using angle-resolved nanosphere lithography. Nano Lett 3(7):939–943 52. Kosiorek A, Kandulski W, Chudzinski P, Kempa K, Giersig M (2004) Shadow nanosphere lithography: simulation and experiment. Nano Lett 4(7):1359–1363 53. Patzig C, Rauschenbach B, Fuhrmann B, Leipner HS (2008) Glancing angle sputter deposited nanostructures on rotating substrates: experiments and simulations. J Appl Phys 103:024313 54. Huang Z, Fang H, Zhu J (2007) Fabrication of silicon nanowire arrays with controlled diameter, length, and density. Adv Mater 19:744–748 55. Boarino L, Destro M, Borini S, Chiodoni A, Bellotti F, Amato G (2009) Self-catalytic etching of silicon: from nanowires to regular mesopores. Physica Status Solidi A Appl Mater Sci 206(6):1250 56. Boarino L, Imbraguglio D, Enrico E, De Leo N, Celegato F, Tiberto P, Pugno N, Amato G (2011) Fabrication of ordered silicon nanopillars and nanowires by self-assembly and metal-assisted etching. Physica Status Solidi A Appl Mater Sci 208(6):1412 57. Chen J, Liao W, Chen X, Yang T, Wark SE, Son DH, Batteas JD, Cremer PS (2009) Spatially selective optical tuning of quantum dot thin film luminescence. ACS Nano 3(1):173 58. Bengoechea-Encabo A, Albert S, Sanchez-Garcia MA, Lòpez LL, Estradé S, Rebled JM, Peirò F, Nataf G, de Mierry P, Zuniga-Perez J, Calleja E (2012) Ordered Gan/Ingan nanorods arrays grown by molecular beam epitaxy for phosphor-free white light emission. J Cryst Growth 353:1 59. Fuhrmann B, Leipner HS, Höche H (2005) Ordered arrays of silicon nanowires produced by nanosphere lithography and molecular beam epitaxy. Nano Lett 5(12):2524 60. Fan HJ, Fuhrmann B, Scholz R, Syrowatka F, Dadgar A, Krost A, Zacharias M (2006) Wellordered ZnO nanowire arrays on GaN substrate fabricated via nanosphere lithography. J Cryst Growth 287:34 61. Tiberto P, Boarino L, Celegato F, Coïsson M, De Leo N, Vinai F, Allia P (2010) Magnetic and magnetotransport properties of arrays of nanostructured antidots obtained by self-assembling polystyrene nanosphere lithography. J Appl Phys 107(9):09B502-1 Page 7 of 9

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_55-1 # Springer International Publishing Switzerland 2014

62. Tiberto P, Boarino L, Celegato F, Coisson M, Enrico E, De Leo N, Vinai F, Allia P (2011) Synthesis of Ni80Fe2O and Co nanodot arrays by self-assembling of polystyrene nanospheres: magnetic and microstructural properties. J Nanopart Res 13:4211 63. Huang ZP, Carnahan DL (2003) Growth of large periodic arrays of carbon nanotubes. Appl Phys Lett 82(3):460 64. Xu T, Miao J, Ashraf M, Lin N, Chollet F (2009) Synthesis of regular nano-pitched carbon nanotube array by using nanosphere lithography for interconnect applications. Mat Lett 63:867 65. Park KH, Lee S, Koh KH, Lacerda R, Teo KBK, Milne WI (2005) Advanced nanosphere lithography for the areal-density variation of periodic arrays of vertically aligned carbon nanofibers. J Appl Phys 97:024311-1 66. Jang SG, Choi DG, Heo CJ, Lee SY, Yang SM (2008) Nanoscopic ordered voids and metal caps by controlled trapping of colloidal particles at polymeric film surfaces. Adv Mater 20:4862 67. Wu W, Memis OG, Mohseni H (2007) A deep sub-wavelength process for the formation of highly uniform arrays of nanoholes and nanopillars. Nanotechnology 18:485302–485305 48530 68. Grojo D, Boarino L, De Leo N, Rocci R, Panzarasa G, Delaporte P, Laus M, Sparnacci K (2012) Size scaling of mesoporous silica membranes produced by nanosphere mediated laser ablation. Nanotechnology 23:485305-14

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_55-1 # Springer International Publishing Switzerland 2014

Index Terms: 2D arrays of colloidal spheres formation 2 Colloidal lithography 4 Nanostructuration of thin films and surfaces 4 Polymer colloid 1 Self-assembly of nanospheres 1 Supramolecular self-assembly methods 2

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_27-1 # Springer International Publishing Switzerland 2014

Color of Porous Silicon Leigh Canham* pSiMedica Ltd., Malvern Hills Science Park, Malvern/Worcester, UK

Abstract The visual color of a material is often not important for many applications but can be crucial for those that involve consumer acceptance and branded products. Solid silicon is gray, but porous silicon can have varied colors depending on its physical form and pore contents. Silicon chip-based layers can exhibit vivid colors, tunable across the visible spectrum through their lowered refractive index and optical interference with the underlying bulk silicon. Highly columnar morphologies, referred to as “black silicon,” include highly porous forms. Even white silicon is possible via photonic crystals. Polydisperse mesoporous silicon microparticle powders are typically dark brown through light tan, depending on bandgap widening, particle size, and the level of oxidation, which is useful for matching skin tone in cosmetic products, but disadvantageous with various foodstuffs, beverages, and oral care products. The color of such powders can be better tuned chemically by the impregnation of specific food nutrients that themselves have vivid colors. Some such natural pigments can themselves benefit with improved fading resistance as a result of UV protection via oxidized porous silicon impregnation.

Introduction The color of the semiconductor in your computer chip or the color of your implanted biomaterial or pharmaceutical tablet is of little consequence. The color of your toothpaste, face cream, beverage, or food is another matter. Consumers are used to the manufactured brands of brilliant white toothpaste with blue, red, or green stripes. Brown is not a popular color for bathroom products (see the handbook review “▶ Porous Silicon for Oral Hygiene and Cosmetics”). There are brown and even black popular foodstuffs – think of bread, peanuts, cereals, chocolate, coffee, and marmite (see handbook chapter “▶ Porous Silicon and Functional Foods”). However, these are in the minority, and once again the consumer associates specific foods with specific colors. In this review we detail how the color of mesoporous silicon can be tuned, like many other properties (see handbook chapter “▶ Tunable Properties of Porous Silicon”). This has been achieved by both control of the physical structure of silicon at the nanoscale and chemical means. The “physical color” of porous silicon films, membrane flakes, and photonic crystals is much more easily tuned than those of milled microparticle powders. The latter display various shades of brown, rather than the gray of solid silicon, and this has to date been an obstacle for applications in certain highvolume consumer products.

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_27-1 # Springer International Publishing Switzerland 2014

Fig. 1 Varied “physical colors” from ultrathin single layers of stain-etched mesoporous silicon (LHS- A. Loni unpublished 2009) and anodized mesoporous silicon photonic crystals (RHS – Gooding Group Univ. New South Wales, Australia http://www.rsc.org/Publishing/ChemTech/Volume/ 2009/02/biosensors.asp)

Fig. 2 Reversible color changes due to ethanol wetting (LHS) and evaporation from mesopores (RHS)

Mesoporous Silicon Layers, Multilayers, and Photonic Crystals The very first studies of mesoporous silicon noted the different colors of anodized and so-called stain films (Uhlir 1956; Turner 1958; Archer 1960). Uhlir referred to his surfaces as having a “matte black, brown or red deposit” (Uhlir 1956). Turner commented that “several orders of interference colors can be seen as the film thickens” (Turner 1958). The first use of such colored silicon in the late 1950s was in p-n junction delineation (Iles and Coppen 1958; Whoriskey 1958; Robbins 1962). Porous silicon, with its lower refractive index than solid silicon, induces optical interference effects as etched films on wafers. A colorimetric analysis for layer thicknesses below 500 nm, at quantified porosities, showed that interference color directly related to the optical thickness of anodized singlelayer structures (Lazarouk et al. 1997). Figure 1 shows examples for stain-etched p + wafers. The visual color of a given layer can be further changed by plasmonic effects of deposited metal nanoparticles (Lublow et al. 2012) or through controlled oxidation of the silicon skeleton. Many natural organisms have evolved to optimize their visual appearance using layers of modulated refractive index – so-called structural color (Xu and Gao 2013). They can have not only very vivid colors but also change their color in response to external stimuli. Increasingly, we are learning to utilize such “biomimetic” designs in synthetic structures to achieve specific functionalities such as colorimetric sensors (Wang and Zhang 2013). The color palettes achieved via porous silicon photonic crystals are impressive and include white (see Fig. 1 and Mangaiyarkarasi et al. 2008). For detail on the optics, the reader is referred to the reviews of Wehrspoon and Schilling (2003), Sailor (2012), Pacholski (2013), and Agarwal (photonic crystal review in this handbook). Of particular interest from a sensing perspective is that the color of such structures can vary reversibly, or non-reversibly, with substances entering and leaving the mesopores, as first demonstrated by Arwin and co-workers (Bjorkland 1996). Figure 2 shows the reversible effect of a volatile solvent like ethanol entering the pores. In this regard, mesoporous silicon can truly be described as a “silicon chameleon,” not only for its tunable color of electroluminescence under bias (Canham

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_27-1 # Springer International Publishing Switzerland 2014

Fig. 3 Golden silicon flakes. Mesoporous multilayer structures removed from the substrate after anodization

Fig. 4 Physical colors of dark gray solid silicon powder and brown porous silicon powders. The brown hue of mesoporous silicon is tunable via porosity, microparticle size, and oxide content (Loni A (2008) Unpublished data. Intrinsiq Materials Ltd)

1993) but also its changing color upon pore filling (Fig. 2), analyte binding, or chemical reaction (Bonanno and DeLoiuse 2010). Multilayers of mesoporous silicon of modulated porosity, when detached from the wafer, have the typical appearance of metal-based “glitter” or even “gold leaf” (Fig. 3) and yet are pure silicon. Unlike conventional glitter, they can be loaded for sustained release of actives like fragrance, thereby having a dual cosmetic role (see handbook chapter “▶ Porous Silicon for Oral Hygiene and Cosmetics”).

Mesoporous Silicon Membranes and Powders To achieve reasonable quantities (e.g., 100–1 kg) of porous silicon powder, thick (e.g., 100–200 mm) mesoporous membranes can be detached from large-diameter wafers using the electrochemical “liftoff” technique (see handbook chapters “▶ Porous Silicon Formation by Anodization” and “▶ Porous Silicon Membranes”) and converted to powders via mechanical means (see handbook chapter “▶ Milling of Porous Silicon Microparticles”). Both the membranes and powders are typically brown in color (see Fig. 4). For powders with irregular acicular-shaped microparticles, the color is now dominated by optical absorption and scattering, rather than interference. The brown hue gets lighter; the higher the porosity, the smaller the particle size and the higher the oxide content. Large batches of multicolor porous silicon powders with defined particle size distributions (e.g., d50 in the range of 1–25 mm) have not been achieved via “structural color.” Instead more progress has been made using pigment impregnation. Figure 5 shows examples of the three primary colors achieved using food-grade additives. Of particular interest are hydrophobic and light-sensitive nutrients such as lycopene and beta-carotene that have vivid colors. They can impart a desired Page 3 of 7

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_27-1 # Springer International Publishing Switzerland 2014

Fig. 5 Varied “chemical colors” from mesoporous silicon powders, oxidized (800 C 3 h air) and then impregnated with nutrients or food additives (Canham LT (2009) Unpublished data. Intrinsiq Materials Ltd UK). The light brown color was tuned to red using carmine, yellow using curcumin, and blue using a commercial food dye formulation (E133, E122). Solvent loading was used with 1–2 wt% pigment

hue to mesoporous structures (Canham et al. 2010), in return be UV protected (Canham and Aston 2012; Pavlikov et al. 2012), and potentially have their bioavailability improved as well when ingested (Canham 2007). Control over silicon particle shape, porosity, and polydispersity could provide structural control of color of powders in the future. Of relevance here are the so-called silicon colloids made by “bottom-up” routes. Porous silicon microspheres of 0.5–5 5 mm diameter scattered yellow, orange, and red colors when under white light illumination (Fenollosa et al. 2010).

Black Silicon Porous silicon films of one particular color have recently shown potential in a number of specific application areas. The so-called black silicon has been etched into a morphology that almost completely suppresses optical reflectivity over a very broad spectral range (Koynov et al. 2006). Its visual appearance however does not directly impact most of the uses currently under development. The first optically black silicon structures were probably made by anodization as early as the 1950–1960s. Koltun studied films generated at lower current densities than Uhlir and Turner (Uhlir 1956; Turner 1958; Koltun 1964). He described their persistent black color being due to a “high degree of dispersion” of silicon. Interestingly, he also recorded reflectivity from his “photocells,” but focused on the infrared rather than the visible range (Koltun 1964). Sometimes also referred to as silicon “nanograss” because of its columnar morphology, black silicon also became an unwanted by-product of reactive ion etching (Jansen et al. 1995) but has subsequently been deliberately realized using pulsed laser ablation and metal-assisted etching (Her et al. 1998; Koynov et al. 2006; Chen et al. 2011). It is now being widely investigated for its low reflectivity in solar cell applications (Ma et al. 2006; Branz et al. 2009; Yuan et al. 2009; Koynov et al. 2011; Oh et al. 2012): photodetectors (Su et al. 2013) and photoelectrodes (Ao et al. 2012). Other Page 4 of 7

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_27-1 # Springer International Publishing Switzerland 2014

Fig. 6 Color matching: porous silicon and instant coffee for burst aroma release. The jar on the left contains a commercial instant coffee/mesoporous silicon blend after boiling water addition. The jar on the right shows the mesoporous silicon component trapped in the meniscus of boiling water

potential uses are in microsystems (Roumanie et al. 2008), sensing (Gervinskas et al. 2013), its tunable wetting (Dorrer and Ruhe 2007; Zhang et al. 2013), and its antibacterial properties (Ivanova et al. 2013).

Color Matching Manipulation of color in silicon-based devices and nanostructures can be for a myriad of technical reasons (Cao et al. 2010; Doan and Sailor 1992; Seo et al. 2011; Kuznetsov et al. 2012). Highlighted here are very different applications where the intrinsic color of porous silicon is important for consumer acceptance. The example chosen in Fig. 6 is from the beverage industry (see handbook chapter “▶ Porous Silicon and Functional Food”). Instant coffee is an example of a beverage, which in both dried powder and liquid form has the same color as mesoporous silicon powder. Six properties of mesoporous silicon are important here for it to be utilized: its ability to entrap coffee aroma under ambient storage and thermally release it into the headspace above boiling water, its ability to stay afloat on boiling water due to its hydrophobicity and low density, its taste and mouthfeel, its low oral toxicity, and, last but not least, its low cost and an appropriate color.

References Ao X, Tong X, Kim DS, Zhang L, Knez M, Muller F, He S, Schmidt V (2012) Black silicon with controllable macropore array for enhanced photoelectrochemical performance. Appl Phys Lett 101:111–901 Archer RJ (1960) Stain films on silicon. J Phys Chem Solid 14:104–110 Bjorklund RB, Zangooie S, Arwin H (1996) Colour changes in thin porous silicon films caused by vapor exposure. Appl Phys Lett 69(20):3001 Bonanno LM, DeLoiuse LA (2010) Integration of a chemically responsive hydrogel into a porous silicon photonic sensor for visual colorimetric readout. Adv Funct Mater 20(4):573–578 Page 5 of 7

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_27-1 # Springer International Publishing Switzerland 2014

Branz HM, Yost VE, Ward S, Jones KM, To B, Stradins P (2009) Nanostructured black silicon and the optical reflectance of graded-density surfaces. Appl Phys Lett 94:231121 Canham LT (1993) The silicon chameleon. Nature 365:695. doi:10.1038/365695a0 Canham LT (2007) Nanoscale semiconducting silicon as a nutritional food additive. Nanotechnology 18:185704, 6 pages Canham LT, Aston R (2012) Method of protecting skin from UV radiation using a dermatological composition having porous silicon. US Patent 8128912 B2 Canham LT, Loni A, Godfrey A (2010) Colouring techniques. International Patent WO/2010/ 038065 Cao L, Fan P, Barnard ES, Brown AM, Brongersma ML (2010) Tuning the colour of silicon nanostructures. Nano Lett 10(7):2649–2654 Chen T, Si J, Hou X, Kanehira S, Miura K, Hirao K (2011) Luminescence of black silicon fabricated by high-repetition rate femtosecond laser pulses. J Appl Phys 100:073–106 Doan V, Sailor MJ (1992) Luminescent color image generation on porous silicon. Science 256(5065):1791–1792 Dorrer C, Ruhe J (2007) Wetting of silicon nanograss: from superhydrophilic to superhydrophobic surfaces. Adv Mater 20(1):159–163 Fenollosa R, Ramiro-Manzano F, Tymczenko M, Meseguer F (2010) Porous silicon microspheres: synthesis, characterization and application to photonic microcavities. J Mater Chem 20:5210–5214 Gervinskas G et al (2013) Surface-enhanced Raman scattering sensing on black silicon. Ann Phys 525(12):907–914 Her TH, Finlay RJ, Wu C, Deliwala S, Mazur E (1998) Microstructuring of silicon with femtosecond laser pulses. Appl Phys Lett 73:1673 Iles PA, Coppen PJ (1958) On the delineation of p-n junctions in silicon. J Appl Phys 29:1514 Ivanova EP et al (2013) Bactericidal activity of black silicon. Nat Commun 4:2838, 7 pages Jansen H, De Boer M, Legtenberg R, Elwenspoek M (1995) The black silicon method: a universal method for determining the parameter setting of a fluorine-based reactive ion etcher in deep silicon trench etching with profile control. J Micromech Microeng 5:115–120 Koltun MM (1964) Nature of film on surface of silicon photocell during anodic etching. Russ J Phys Chem 38(3):381–383 Koynov S, Brandt MS, Stutzmann M (2006) Black non-reflecting silicon surface for solar cells. Appl Phys Lett 88:203107 Koynov S, Brandt MS, Stutzmann M (2011) Black thin film silicon. J Appl Phys 110:043–537 Kuznetsov AI, Miroshnichenko AE, Fu YH, Zhang J-B, Luk’yanchuk B (2012) Magnetic light. Sci Rep 2:492 Lazarouk S, Jaguiro P, Katsouba S, Maiello G, La Monica S, Masini G, Proverbio E, Farrari A (1997) Visual determination of thickness and porosity of porous silicon layers. Thin Solid Films 297:97–101 Lublow M, Kubala S, Veyan J-F, Chabal YJ (2012) Colored porous silicon as support for Plasmonic nanoparticles. J Appl Phys 111:084–302 Ma LL, Zhou YC, Jiang N, Lu X, Shao J, Lu W, Ge J, Ding XM, Hou XY (2006) Wide-band “black silicon” based on porous silicon. Appl Phys Lett 88:171–907 Mangaiyarkarasi D, Breese MBH, Ow YS (2008) Fabrication of three dimensional porous silicon distribution Bragg reflectors. Appl Phys Lett 93:221–905 Oh J, Yuan HC, Branz HM (2012) An 18.2 % efficient black silicon solar cell achieved through control of carrier recombination in nanostructures. Nat Nanotechnol 7:743–748 Page 6 of 7

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Pacholski C (2013) Photonic crystal sensors based on porous silicon. Sensors 13:4694–4713 Pavlikov AV, Lartsev AV, Gayduchenko IA, Timoshenko VY (2012) Optical properties of materials based on oxidized porous silicon and their applications for UV protection. Microelectron Eng 90:96–98 Robbins H (1962) Junction delineation in silicon. J Electrochem Soc 109(1):63–64 Roumanie M et al (2008) Enhancing surface activity in silicon microreactors: use of black silicon and alumina as catalyst supports for chemical and biological applications. Chem Eng J 135: S317–S326 Sailor MJ (2012) Chapter 5.3 Optical reflectance measurements. In: Porous silicon in practice. Wiley VCH, Weinheim Seo K, Wober M, Steinvurzel P, Schonbrun E, Dan Y, Ellenbogen T, Crozier KB (2011) Multicolored vertical silicon nanowires. Nano Lett 11:1851–1856 Su Y et al (2013) High responsivity MSM black silicon photodetector. Mater Sci Semicon Proc 16(3):619–624 Turner DR (1958) Electropolishing silicon in hydrofluoric acid solutions. J Electrochem Soc 105(7):402–408 Uhlir A (1956) Electrolytic shaping of germanium and silicon. Bell Sys Tech J 35:333–347 Wang H, Zhang KQ (2013) Photonic crystal structures with tunable structure color as colorimetric sensors. Sensors 13:4192–4213 Wehrspoon RB, Schilling J (2003) A model system for photonic crystals: macroporous silicon. Phys Stat Solidi A 197(3):673–687 Whoriskey PJ (1958) Two chemical stains for making p-n junctions in silicon. J Appl Phys 29:867 Xu J, Gao Z (2013) Biomimetic photonic structures with tunable structural colors. J Colloid Interface Sci 406:1–17 Yuan HC, Yost VE, Page MR, Stradins P, Meier DL, Branz HM (2009) Efficient black silicon solar cell with a density graded nanoporous surface: optical properties, performance limitations and design rules. Appl Phys Lett 95:123501, 3 pages Zhang T, Zhang P, Li S, Li W, Wu Z, Jiang Y (2013) Black silicon with self-cleaning surface prepared by wetting process. Nanoscale Res Lett 8:351, 5 pages

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_29-1 # Springer International Publishing Switzerland 2014

Diamagnetic Behavior of Porous Silicon Klemens Rumpf* and Petra Granitzer Instítute of Physics, Karl-Franzens-University, Graz, Austria

Abstract After a brief introduction to diamagnetism, the magnetic properties of silicon are briefly outlined. The magnetic behavior of silicon consists of a diamagnetic and a paramagnetic term, whereas the diamagnetism predominates. Furthermore, various types of porous silicon like as-etched and oxidized porous silicon are discussed, and the dependence of the diamagnetism on the surface treatment and thus on the paramagnetic defects is outlined. Nanostructuring of silicon results in a modification of the magnetic behavior with reduced diamagnetic contribution, and a further posttreatment of the samples leads to a smaller diamagnetic susceptibility.

Introduction Diamagnetism is present in all matter, and this diamagnetic behavior is caused by the modification of the motion of the electrons by an applied magnetic field resulting in a magnetic moment which is antiparallel to the generating field. The susceptibility w of a diamagnetic material is negative (w
1) and normally varies only slightly with temperature. Ionic and covalent-bonded crystals, materials with complete filled shells, offer a diamagnetic behavior: the so-called Lamor or Langevin diamagnetism. Due to an even number of electrons, no net magnetic moment is present and such materials do not exhibit permanent magnetic dipoles. In such substances diamagnetism arises in an external field due to the modification of the motion of orbital electrons in the atoms to shield the external field. In systems with free electrons, the so-called Landau diamagnetism is present which is caused by an orbital motion. In semiconductors and metals, free electrons can contribute to the diamagnetic behavior because the electrons will perceive a magnetic force caused by the external field and thus modify their motion (Cullity and Graham 2008). In the case of semiconductors, the holes can also deliver a diamagnetic contribution. Corresponding to Lenz’ law the occurring local magnetic moment counteracts the applied magnetic field. In semiconductors the density of the conduction electrons is much lower than in metals and temperature dependent. Usually in semiconductors the diamagnetism from the lattice predominates. Diamagnetic materials are not related to a magnetic moment on the atomic scale without magnetic field, in contrast to para- and ferromagnetic substances. In more than half of the elements of the periodic table, paramagnetism dominates and thus the susceptibility offers a positive value. The susceptibility of diamagnetic materials is in general very small; thus, these materials are treated as nonmagnetic. The susceptibility of these materials is dependent neither on the magnetic field nor on the temperature, and thus, the magnetization is linear with the field and independent of temperature. Table 1 shows the negative susceptibility values of selected materials.

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_29-1 # Springer International Publishing Switzerland 2014

Table 1 List of selected diamagnetic materials with their molar susceptibility values (wm ¼ w ∙ V m ¼ w∙M R ) (LandoltBörnstein 1986) Name Bismuth Copper Carbon Gallium Germanium Gold Indium Silver Silicon Silicon dioxide Sulfur

Formula Bi Cu C Ga Ge Au In Ag Si SiO2 S

Molar susceptibility wm (106 cm3 mol1) 280 5.46 3.96 21.6 11.6 27.97 10.2 20.7 3.65 29.6 15.4

Beside electrical investigations of semiconductors, the examination of the magnetic properties has also been of importance to get knowledge of the electronic structure. Information about the band structure as well as the configuration of trapping levels could be obtained (Sonder and Stevens 1958).

Diamagnetism of Silicon and Porous Silicon The literature susceptibility value for bulk silicon is given as w ¼ 4.2 106 (Blakemore 1985) and for SiO2 w ¼ 16.3 106 (Schenck 1996). Silicon is a weakly diamagnetic material, where the susceptibility depends on the doping. Measurements of the magnetic susceptibility of semiconductors show that the lattice term, the conduction electrons, as well as electrons trapped on donor atoms contribute to the susceptibility (Sonder and Stevens 1958). The diamagnetic contribution of the lattice term is due to the core electrons. The contribution of the conduction electrons decreases with decreasing temperature. At low temperatures electrons are trapped by donors which results in a superposed “Curie paramagnetism,” so the magnetic behavior of doped silicon is composed of the diamagnetism due to core electrons as well as conduction electrons and the paramagnetic term arising from unionized donors (Sonder and Stevens 1958). The Curie paramagnetism at low temperatures arising from trapped electrons on donor atoms shows a deviation from the Curie law with increasing doping density due to interactions between neighboring donors (Helms and Poindexter 1994). Measurements of the magnetic susceptibility of p-type Si (5  1015 carriers) show a small temperature dependence which arises from the paramagnetic component (Candea et al. 1977). The paramagnetic behavior will be discussed in the handbook chapter “▶ Paramagnetic and Superparamagnetic Silicon Nanocomposites.” Magnetization measurements of a highly n-doped silicon wafer (1019 cm3) give a clear diamagnetic signal (w ¼ 1.2 106) which can be seen in Fig. 1. Furthermore, for comparison the magnetization curves of porous silicon in the mesoporous range prepared from the same substrate are shown. The sample has been prepared by anodization in a 10 wt% hydrofluoric acid solution with a current density of 40 mA/cm2. The resulting porosity of the porous layer was about 70 %. On the one hand as-etched porous silicon offering a hydrogen-terminated surface and on the other hand

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_29-1 # Springer International Publishing Switzerland 2014

Fig. 1 Comparison of the magnetization curves of bare n+ silicon (full line), as-etched porous silicon prepared from the same wafer (dotted line), and aged porous silicon offering a native oxide layer (dashed line). All three samples show a diamagnetic behavior, whereas the estimated susceptibility varies between 1.2  106 (Si), 7.5  107 (as-etched), and 2.6  107 (aged). The measurements have been performed at T ¼ 4.2 K. The diamagnetic behavior of porous silicon decreases if the sample is oxidized (in contrast to hydrogen terminated) which might be caused by the occurrence of dangling bonds

porous silicon aged in ambient air and thus exhibiting a native oxide layer are measured. In the case of the porous silicon (mesoporous morphology) samples, the susceptibility is lowered from w ¼ 7.5 107 to 2.6 107 which might be due to the surface modification caused by the etching process and the concomitant occurrence of dangling bonds (Matsuoka et al. 2012) at the surface and Si/SiO2 interface. These paramagnetic defects are mainly located at regions with high mechanical stress (Wehrspohn et al. 2000) and lead to a decrease of the occurring diamagnetic behavior. In comparison hydrogenated amorphous silicon offers a susceptibility of about w ¼ 1.0 106 (Baugh et al. 2001). With increasing porosity these effects become more and more important because the surfacevolume ratio increases. Porous silicon samples with low porosity (below 30 %) show a similar susceptibility to the bulk silicon substrate.

Conclusion The magnetic behavior of silicon consists of a diamagnetic and a paramagnetic term, wherein the diamagnetism predominates. The susceptibility arises from three contributions, the core electrons, conduction electrons, and trapped electrons. The paramagnetic term of the susceptibility depends on the temperature and on the doping density of the substrate. Nanostructuring of silicon results in a modification of the magnetic behavior with reduced diamagnetic contribution. Further posttreatment of the porous silicon, e.g., oxidation, leads to a smaller diamagnetic susceptibility. Porous silicon exhibits predominantly diamagnetic behavior wherein the paramagnetic term is modified by additional occurring dangling bonds and thus the magnetic properties also depend on the porosity of the material. In general silicon as well as porous silicon offers a diamagnetic behavior which is small compared to materials such as bismuth, gold, or copper (see Table 1). Furthermore,

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_29-1 # Springer International Publishing Switzerland 2014

strong diamagnetism is associated with superconductivity in relation with silicon quantum well nanostructures (Bagraev et al. 2006), which has not been observed in the case of porous silicon.

References Bagraev NT, Gehlhoff W, Klyachkin LE, Malyarenko AM, Romanov VV, Rykov SA (2006) Superconductivity in silicon nanostructures. Physica C 437–438:21 Baugh J, Han D, Kleinhammes A, Wu Y (2001) Magnetic susceptibility and microstructure of hydrogenated amorphous silicon measured by nuclear magnetic resonance on a single thin film. Appl Phys Lett 78:466 Blakemore JS (1985) Solid state physics, 2nd edn. Cambridge University Press, Cambridge Candea RM, Gee CM, Hudgens SJ, Kastner M (1977) Temperature dependence of the diamagnetic and dielectric susceptibility of silicon. Phys Rev B 16:2657 Cullity BD, Graham CD (2008) Introduction to magnetic materials. Wiley, Hoboken Helms CR, Poindexter EH (1994) The silicon-silicon dioxide system: its microstructure and imperfections. Rep Prog Phys 57:791 Landolt-Börnstein (1986) Zahlenwerte und Funktionen aus Naturwissenschaft und Technik, Band 16, Diamagnetische Suszeptibilität, Hrg.: K.-H. Hellwege, A.M. Hellwege, Springer Matsuoka T, Vlasenko LS, Vlasenko MP, Sekiguchi T, Itoh KM (2012) Identification of a paramagnetic recombination center in silicon/silicon-dioxide interface. Appl Phys Lett 100:152107 Schenck JF (1996) The role of magnetic susceptibility in magnetic resonance imaging: MRI magnetic compatibility of the first and second kinds. Med Phys 23:815 Sonder E, Stevens DK (1958) Magnetic properties of n-type silicon. Phys Rev 110:1027 Wehrspohn RB, Deane SC, French ID, Gale I, Hewett J, Powell MJ, Robertson J (2000) Relative importance of the Si–Si bond and Si–H bond for the stability of amorphous silicon thin film transistors. J Appl Phys 87:144

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_29-1 # Springer International Publishing Switzerland 2014

Index Terms: Curie-paramagnetism 2 Diamagnetic materials 2 Diamagnetism 1–2 of silicon and porous silicon 2 Lamor diamagnetism 1 Landau diamagnetism 1 Langevin diamagnetism 1

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_91-1 # Springer International Publishing Switzerland 2014

Drug Delivery with Porous Silicon Jarno Salonen* Department of Physics and Astronomy, University of Turku, Turku, Finland

Abstract Biodegradable porous silicon is under preclinical assessment for a range of drug delivery applications. Studies to date on oral, subcutaneous, intravenous, and intravitreal modes of delivery are reviewed. Both the merits of this nanostructured carrier technology and some existing challenges are briefly discussed.

Introduction Any chemical substance used for medical therapy can be considered a drug. Today many thousands of synthetic drugs are available and drug delivery is a multibillion euro market with hundreds of companies developing formulations that improve efficacy and safety (Ranade and Cannon 2011). The use of nanotechnology and nanostructured materials is also being increasingly investigated in this field (Kumar et al. 2013). The first report about bioactivity in 1995 (Canham 1995) made PSi an attractive choice when considering the utilization of it in biomedical applications. In the following 2 years, several reports related to biocompatibility of PSi (see chapter “▶ Biocompatibility of Porous Silicon”) were published, e.g., the paper describing bioresorbable property of PSi (Canham et al. 1996a, b, 1997; Canham 1997; Canham and Reeves 1996). The tunable biocompatibility of PSi makes it a very fascinating material. While the medium porosity Si (p < 60 %) is bioactive, high porosity Si is bioresorbable and its dissolution rate in body fluids, excluding gastric fluid, is dependent on the porosity, varying from hours to days. That is an important advantage considering drug delivery applications, where the dissolution of a carrier material is an essential factor and strongly dependent on the administration route. The tunable pore size and volume together with a number of methods to modify the surface chemistry are also important factors for drug delivery applications. These factors enable high drug loads and tunable release properties. In addition, the possibility to produce all kinds of particle sizes from large microparticles to nanoparticles below 100 nm makes PSi applicable to all the drug administration routes from enteral to parenteral (Fig. 1). This is very promising because of the increasing number of new challenges the pharmaceutical industry is currently facing, in order to increase bioavailability of new drug candidates (Ranade and Cannon 2011; Lehto et al. 2013).

Oral Delivery Although there were biomedical studies of PSi published in the late 1990s and early 2000s (Bayliss et al. 1999; Desai et al. 1999), the first paper about PSi in oral drug delivery was published in 2003 *Email: jarno.salonen@utu.fi Page 1 of 11

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_91-1 # Springer International Publishing Switzerland 2014

Fig. 1 Illustration of the most commonly used drug administration routes. Bold text entries represent the routes where PSi has been utilized to date

(Foraker et al. 2003). In the paper, Foraker et al. used microfabricated porous silicon particles to enhance insulin permeability across Caco-2 cell monolayer, which is commonly used in vitro model of the human small intestinal mucosa to predict the absorption of orally administrated drugs. They observed that the flux of insulin across the cell monolayer was approximately 50-fold compared with liquid formulations and nearly 10-fold higher compared with liquid formulations with permeation enhancer, if insulin was loaded in PSi. In 1994, Anglin et al. showed for the first time that the surface chemistry has a clear effect on the release rate of drug loaded in PSi (Anglin et al. 2004). They used a steroid dexamethasone as a model drug. The mechanism of drug release is thought to involve a combination of leaching and matrix dissolution. A year later, Salonen et al. published a paper in which they studied the effect of PSi on oral delivery of five different model drugs (Salonen et al. 2005). They found that the release rate of a loaded drug was dependent on the characteristic dissolution behavior of the drug. When the dissolution rate of the unloaded drug was high, the loading of the drug in the PSi microparticles caused slightly delayed release. However, with clearly poorly soluble drugs, the loading remarkably improved dissolution. They also found that pH dependency of the dissolution was reduced when the drug was loaded into the PSi. In addition, it was possible to load a relatively high amount of drugs, up to 45 wt%, into PSi. These observations were interesting because many potential drug candidates cannot be delivered orally due to their poor pharmacokinetics. This includes the poor solubility and dissolution of the drug in the intestinal lumen, poor permeation properties in the gastrointestinal (GI) tract, as well as high intestinal or hepatic first-pass metabolism. It is estimated that more than 95 % of new drug molecules suffer from these kinds of problems in bioavailability (Brayden 2003). The improved solubility and dissolution behavior is based on the fact that the formation of crystalline material is restricted by the confined space of the pores. It is well known that in their amorphous state, many drugs exhibit higher dissolution rates than their crystalline counterparts, especially when solubility is limited by high lattice energies (Yu 2001). The dissolution rates from the porous materials will also be improved by their very high surface areas. Despite the improved solubility behavior of loaded drugs observed in vitro, it is not obvious to gain an improved bioavailability also in vivo. There are also other obstacles to overcome before a drug reaches the systemic circulation than solubility issues, such as permeation across a gut wall and first-pass metabolism. In 2010, Wang et al. reported important results about in vitro–in vivo correlation (IVIVC) (Wang et al. 2010a). They observed that all the pharmacokinetic parameters of Page 2 of 11

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_91-1 # Springer International Publishing Switzerland 2014

Fig. 2 PYY3-36 plasma concentrations up to 96 h after subcutaneous delivery in mice with three different PSi nanocarriers: thermally oxidized (TOPSi), thermally hydrocarbonized (THCPSi), and undecylenic acid treated THCPSi (UnTHCPSi) (Kovalainen et al. 2013) (Reprinted with permission from American Chemical Society)

indomethacin were significantly improved when PSi was used as a drug carrier. Using a fasted rat model, they found out that the maximum plasma concentration was 2.6-fold when PSi was used and the time to reach it reduced from 2.75 h down to 0.56 h. In addition, the bioavailability of indomethacin increased from 53.54 % up to 100 %. All the parameters were better compared to commercially available formulation (Indocid) and the IVIVC was found to be high (level A).

Subcutaneous Delivery There are a number of different peptides with promising therapeutic effects, but which cannot be used practically. Peptide and protein delivery is a rather challenging task and typically conventional drug delivery systems are not useful. In general, bioavailability of orally administered peptides is very poor, and because peptides have a short duration of action in vivo due to their rapid degradation and elimination from the blood circulation, the treatment may require several daily injections to keep the plasma concentration at an effective therapeutic level. Although there are several different systems developed for subcutaneous delivery, the main problem, in the case of peptides, is that they are very fragile biomolecules and cannot stand strong solvents, high temperatures, or harsh environments at all. Drug loading in PSi is a relatively gentle process which can be done at room temperature using mild solvents, with achievement of high payloads. In 2007, Prestidge et al. showed that a protein can be loaded and released from PSi (Prestidge et al. 2007), and in 2009, Kilpeläinen et al. showed in vivo that the loaded peptide was pharmacologically active after the loading (Kilpelainen et al. 2009). In the following years, more results about subcutaneous administration of different peptides were published (Kilpelainen et al. 2011; Kovalainen et al. 2012, 2013). In addition to the remaining pharmacological activity, some other important findings were published recently. For example, a prolonged release of a peptide YY (PYY) up to several days from PSi nanoparticles and clearly enhanced bioavailability are both promising results for future development of PSi based peptide delivery systems (Fig. 2) (Kovalainen et al. 2013).

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_91-1 # Springer International Publishing Switzerland 2014

Intravenous Delivery The highly sophisticated immune system of the human body makes intravenous delivery of inorganic particles problematic. Despite the challenges related to the intravenous route, the high hopes for rapidly developing nanotechnology have initiated an intensive research of targeted drug delivery. One of the main aims of this research is focused on the development of novel cancer therapies and this has been the major research field of PSi nanoparticles too. Due to the versatile properties of PSi, it can be used as a simple drug carrier but also as a multifunctional material for therapeutic and imaging purposes. Tasciotti et al. introduced this kind of multifunctional multistage delivery system in 2008 (Tasciotti et al. 2008). They used PSi as a first-stage carrier in which the second-stage nanoparticles, quantum dots or single-walled carbon nanotubes, were loaded. In 2009, Park et al. reported the first in vivo results of PSi nanoparticles (Park et al. 2009). They used PSi intrinsic photoluminescence properties for near-infrared monitoring the nanoparticles in vivo. With a dextran coating, the particles manage to avoid rapid clearance and the blood circulation time of nanoparticles increased significantly. Even more importantly, they observed a passive accumulation of nanoparticles in the tumor tissue (enhanced permeability and retention effect, EPR). In 2012, also Godin et al. observed enhanced accumulation in the tumor tissue, although they used large discoidal particles instead of nanoparticles (Godin et al. 2012). In addition to passive accumulation, PSi can be used in active targeting also. Several other groups have published their work on actively targeted delivery recently (Secret et al. 2013; Mann et al. 2011; Shen et al. 2013; Rytkönen et al. 2012). Unfortunately, in the other in vivo studies only rapid clearance of the PSi nanoparticles in the spleen or liver has been observed without any controlled accumulation or homing, although many different surface chemistries have been tested (Kovalainen et al. 2013; Tanaka et al. 2010a; Bimbo et al. 2010; Sarparanta et al. 2012; Rytkönen et al. 2012). However, a promising release of siRNA from particles cleared to spleen and liver for at least 3 weeks has been reported (Tanaka et al. 2010b). The results indicate that particles may be able to release their payload even after they have been captured in the spleen or liver. This may be used as a new potential delivery system for compounds which suffer from fast renal clearance or need to be delivered to metabolic tissues (e.g., type IB prodrugs).

Intravitreal Delivery Continuously increasing life expectancy and prevalence of diabetes in developed countries have increased an incidence of retinal diseases like age-related macular degeneration and diabetic retinopathy rapidly. Unfortunately, intravitreal delivery is a challenging task. There are obstacles like the blood–retina barrier and the tight junctions of the retinal pigment epithelium which lower the effectiveness of dosed drugs and complicate the administration (Cheng et al. 2008). Since the introduction of anti-VEGF medications, the need of intravitreal injections has significantly increased. Due to the short half-life of most injectable intravitreal drugs, frequent administrations are necessary. To overcome these problems, new delivery systems are continuously studied to obtain prolonged sustained release of drugs and reduce the frequency of injections. Despite having promising relevant properties, the use of PSi in intravitreal drug delivery has not been extensively studied. Low et al. implanted PSi membranes into rat eye at superior, temporal, and inferior locations and the rats were observed over a period of 9 weeks until the pieces were no longer visible with a microscope (Low et al. 2009). Implanted PSi did not erode the underlying or overlying Page 4 of 11

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_91-1 # Springer International Publishing Switzerland 2014

tissue, did not cause a marked accumulation of inflammatory cells, and did not become vascularized. The results indicated that PSi is nontoxic, noninflammatory, and biodegradable material and suitable for intravitreal drug delivery. Chhablani et al. showed that in oxidized PSi covalently loaded daunorubicin demonstrated sustained intravitreal release for 3 months without any evidence of toxicity, while physisorbed daunorubicin was released within 2 weeks and localized retinal toxicity were observed due to high daunorubicin concentration. Wu et al. added a new functionality to PSi drug delivery system (Wu et al. 2011). They used 1-dimensional porous silicon photonic crystal in intravitreal delivery. The reflectance spectrum of the crystal (i.e., color) changed from red to green as daunorubicin was releasing enabling real-time monitoring of the drug release process. These types of multifunctional delivery systems based on versatile properties of PSi are expected to be published much more in future.

Additional Functionalities of Porous Silicon Delivery System In the previous chapter, two examples of multifunctional drug delivery systems applying PSi luminescent and photonic properties were described, but there are many other possibilities to include additional functionalities to the delivery systems. One of the most studied applications is PSi-polymer composite structures in which PSi have a dual role as a hydroxyapatite growth activator and drug carrier (Mukherjee et al. 2006; Fan et al. 2009, 2011). The composite structures are described in detail in chapter “▶ Polymer-Porous Silicon Composites.” Due to the easily modified surface chemistry, PSi enables attachment of all kind of active species on its surface. However, in certain case, simple capping of PSi may lead to desired functionality. This approach has been used to obtain pH-triggered delivery systems, in which pH-sensitive protein or polymer is used to trigger the release of loaded drug (Perelman et al. 2008; McInnes et al. 2012). Another approach to the triggered drug release is to use thermoresponsive polymers. Vasani et al. grafted poly-N-isopropylacrylamide on PSi and demonstrated that modulation of temperature significantly alters the release of anticancer drug camptothecin (Vasani et al. 2011). There are some other strategies also to obtain triggered release, like oxidation triggered release and electrically enhanced erosion of PSi. The latter is relatively rarely studied applications, but promising results considering, e.g., chronotherapeutic applications have been reported (Leong et al. 2007).

Summary There are a number of potential drug administration routes in which PSi can be used (Table 1). Some of the routes are not even explored yet. In biomedical applications, in addition to the nontoxic behavior of the carrier material and bioresorbability, it is important that the degradation products are also nontoxic. PSi degradates into monomeric silicic acid, which is the most natural form of silicon and an important nutrient for humans (Jugdaohsingh et al. 2004). Despite the very positive results published about PSi, there still exists one big stumbling block which hinders the commercialization. This is the cost of PSi production for drug delivery applications. The cost of silicon wafers used in microelectronics is high because of the strict demands for high purity. These demands are well beyond that what would be needed for biomedical applications, and although it is possible to produce low-cost PSi from metallurgical grade silicon (Loni et al. 2011; Chadwick et al. 2012), it is not yet clear if this type of PSi can be used for biomedical purposes. Quite recently, a new promising approach to produce low-cost PSi has been started to explore, namely, Page 5 of 11

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_91-1 # Springer International Publishing Switzerland 2014

Table 1 Examples of drug delivery studies using PSi as a drug carrier material Administration route In vivo Oral

Study type Dissolution

Oral

Dissolution

Ibuprofen

Oral

Dissolution

Furosemide

Oral

Permeability

Doxorubicin Gramicidin A, papain

Parenteral –

Release Loading, release

Ghrelin antagonist, melanotan II Peptide YY

Subcutaneous

X

Subcutaneous

X

Griseofulvin

Oral

Indomethacin

Oral

Ethionamide

Oral

Dissolution, permeability

Triclosan

Parenteral

Daunorubicin

Intravitreal

siRNA Mitoxantrone

Intravenous Intravenous

Release, antibacterial activity Dissolution Sustained release, monitoring Gene silencing Sustained release Release Sustained release

Ketoconazole

Several

Compound Furosemide, griseofulvin, ibuprofen Ranitidine, antipyrine

X

X

Effect Improved dissolution rate Sustained release, no effect Effect of surface chemistry Enhanced permeation Sustained release Sustained release

Pharmacological activity Bioavailability

Sustained release

Permeability

Enhanced permeation Enhanced bioavailability Enhanced permeation and metabolic activity Sustained release

Bioavailability

Release, antifungal activity

Sustained release

Sustained release

References (Salonen et al. 2005) (Salonen et al. 2005) (Limnell et al. 2006) (Kaukonen et al. 2007) (Vaccari et al. 2006) (Prestidge et al. 2007, 2008) (Kilpelainen et al. 2009, 2011) (Kovalainen et al. 2012, 2013) (Bimbo et al. 2011) (Wang et al. 2010a) (Vale et al. 2012)

(Wang et al. 2010b) (Wu et al. 2011) (Tanaka et al. 2010b) (Tzur-Balter et al. 2013) (Tang et al. 2013)

magnesium-induced reduction of silicon dioxide to silicon (Bao et al. 2007 and chapter “▶ Porous Silicon Formation by Porous Silica Reduction”), which may open a way to produce low-cost PSi with an environmentally friendly process (Batchelor et al. 2012).

References Anglin EJ, Schwartz MP, Ng VP, Perelman LA, Sailor MJ (2004) Engineering the chemistry and nanostructure of porous silicon Fabry-Pérot films for loading and release of a steroid. Langmuir 20(25):11264–11269 Bao Z, Weatherspoon MR, Shian S, Cai Y, Graham PD, Allan SM, Ahmad G, Dickerson MB, Church BC, Kang Z, Abernathy Iii HW, Summers CJ, Liu M, Sandhage KH (2007) Chemical reduction of three-dimensional silica micro-assemblies into microporous silicon replicas. Nature 446(7132):172–175, www.nature.com/nature/journal/v446/n7132/suppinfo/nature05570_S1.html

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_91-1 # Springer International Publishing Switzerland 2014

Batchelor L, Loni A, Canham LT, Hasan M, Coffer JL (2012) Manufacture of mesoporous silicon from living plants and agricultural waste: an environmentally friendly and scalable process. Silicon 4(4):259–266. doi:10.1007/s12633-012-9129-8 Bayliss SC, Heald R, Fletcher DI, Buckberry LD (1999) The culture of mammalian cells on nanostructured silicon. Adv Mater 11(4):318–321 Bimbo LM, Sarparanta M, Santos HA, Airaksinen AJ, Makila E, Laaksonen T, Peltonen L, Lehto VP, Hirvonen J, Salonen J (2010) Biocompatibility of thermally hydrocarbonized porous silicon nanoparticles and their biodistribution in rats. ACS Nano 4(6):3023–3032. doi:10.1021/ nn901657w Bimbo LM, Makila E, Laaksonen T, Lehto VP, Salonen J, Hirvonen J, Santos HA (2011) Drug permeation across intestinal epithelial cells using porous silicon nanoparticles. Biomaterials 32(10):2625–2633. doi:10.1016/j.biomaterials.2010.12.011 Brayden DJ (2003) Controlled release technologies for drug delivery. Drug Discov Today 8:976–978 Canham LT (1995) Bioactive silicon structure fabrication through nanoetching techniques. Adv Mater 7(12):1033–1037 Canham LT (1997) Biomedical applications of porous silicon. In: Canham LT (ed) Properties of porous silicon, vol 18, EMIS datareviews. Short Run Press, London, pp 12–22 Canham LT, Reeves CL (1996) Apatite nucleation on low porosity silicon in acellular simulated body fluids. In: Cotell CM, Meyer AE, Gorbatkin SM, Grobe GL (eds) Thin films and surfaces for bioactivity and biomedical applications, vol 414, Materials research society symposium proceedings. Materials Research Society, Pittsburgh, pp 189–194 Canham LT, Newey JP, Reeves CL, Houlton MR, Loni A, Simmons AJ, Cox TI (1996a) The effects of DC electric currents on the in-vitro calcification of bioactive Si wafers. Adv Mater 8(10):847–849 Canham LT, Reeves CL, King DO, Branfield PJ, Crabb JG, Ward MCL (1996b) Bioactive polycrystalline silicon. Adv Mater 8(10):850–852 Canham LT, Reeves CL, Loni A, Houlton MR, Newey JP, Simons AJ, Cox TI (1997) Calcium phosphate nucleation on porous silicon: factors influencing kinetics in acellular simulated body fluids. Thin Solid Films 297(1–2):304–307 Chadwick EG, Beloshapkin S, Tanner DA (2012) Microstructural characterisation of metallurgical grade porous silicon nanosponge particles. J Mater Sci 47(5):2396–2404. doi:10.1007/s10853011-6060-0 Cheng L, Anglin E, Cunin F, Kim D, Sailor MJ, Falkenstein I, Tammewar A, Freeman WR (2008) Intravitreal properties of porous silicon photonic crystals: a potential self-reporting intraocular drugdelivery vehicle. Br J Ophthalmol 92(5):705–711. doi:10.1136/bjo.2007.133587 Desai T, Hansford D, Kulinsky L, Nashat A, Rasi G, Tu J, Wang Y, Zhang M, Ferrari M (1999) Nanopore technology for biomedical applications. Biomed Microdevices 2:11–40 Fan DM, Loni A, Canham LT, Coffer JL (2009) Location-dependent controlled release kinetics of model hydrophobic compounds from mesoporous silicon/biopolymer composite fibers. Phys Status Solidi A-Appl Mater Sci 206(6):1322–1325. doi:10.1002/pssa.200881118 Fan DM, Akkaraju GR, Couch EF, Canham LT, Coffer JL (2011) The role of nanostructured mesoporous silicon in discriminating in vitro calcification for electrospun composite tissue engineering scaffolds. Nanoscale 3(2):354–361. doi:10.1039/c0nr00550a Foraker AB, Walczak RJ, Cohen MH, Boiarski TA, Grove CF, Swaan PW (2003) Microfabricated porous silicon particles enhance paracellular delivery of insulin across intestinal Caco-2 cell monolayers. Pharm Res 20(1):110–116 Page 7 of 11

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Godin B, Chiappini C, Srinivasan S, Alexander JF, Yokoi K, Ferrari M, Decuzzi P, Liu XW (2012) Discoidal porous silicon particles: fabrication and biodistribution in breast cancer bearing mice. Adv Funct Mater 22(20):4225–4235. doi:10.1002/adfm.201200869 Jugdaohsingh R, Tucker KL, Qiao N, Cupples LA, Kiel DP, Powell JJ (2004) Dietary silicon intake is positively associated with bone mineral density in men and premenopausal women of the Framingham Offspring cohort. J Bone Miner Res 19(2):297–307. doi:10.1359/jbmr.0301225 Kaukonen AM, Laitinen L, Salonen J, Tuura J, Heikkila T, Limnell T, Hirvonen J, Lehto V-P (2007) Enhanced in vitro permeation of furosemide loaded into thermally carbonized mesoporous silicon (TCPSi) microparticles. Eur J Pharm Biopharm 66(3):348–356 Kilpelainen M, Riikonen J, Vlasova MA, Huotari A, Lehto VP, Salonen J, Herzig KH, Jarvinen K (2009) In vivo delivery of a peptide, Ghrelin antagonist, with mesoporous silicon microparticles. J Control Release 137(2):166–170. doi:10.1016/j.jconrel.2009.03.017 Kilpelainen M, Monkare J, Vlasova MA, Riikonen J, Lehto VP, Salonen J, Jarvinen K, Herzig KH (2011) Nanostructured porous silicon microparticles enable sustained peptide (Melanotan II) delivery. Eur J Pharm Biopharm 77(1):20–25. doi:10.1016/j.ejpb.2010.10.004 Kovalainen M, Monkare J, Makila E, Salonen J, Lehto VP, Herzig KH, Jarvinen K (2012) Mesoporous silicon (PSi) for sustained peptide delivery: effect of PSi microparticle surface chemistry on peptide YY3-36 release. Pharm Res 29(3):837–846. doi:10.1007/s11095-0110611-6 Kovalainen M, Mönkäre J, Kaasalainen M, Riikonen J, Lehto V-P, Salonen J, Herzig K-H, Järvinen K (2013) Development of porous silicon nanocarriers for parenteral peptide delivery. Mol Pharm. doi:10.1021/mp300494p Kumar A, Mansour HM, Friedman A, Blough ER (2013) Nanomedicine in drug delivery. Taylor & Francis, Boca Raton Lehto V-P, Salonen J, Santos H, Riikonen J (2013) Nanostructured silicon-based materials as a drug delivery system for water-insoluble drugs. In: Drug delivery strategies for poorly water-soluble drugs. Wiley, pp 477–508. doi:10.1002/9781118444726.ch15 Leong WY, Loni A, Canham LT (2007) Electrically enhanced erosion of porous Si material in electrolyte by pH modulation and its application in chronotherapy. Phys Status Solidi A-Appl Mater Sci 204(5):1486–1490. doi:10.1002/pssa.200674400 Limnell T, Riikonen J, Salonen J, Kaukonen AM, Laitinen L, Hirvonen J, Lehto VP (2006) The effect of different surface treatment and pore size on the dissolution of ibuprofen from mesoporous silicon particles. Eur J Pharm Sci 28:S34 Loni A, Barwick D, Batchelor L, Tunbridge J, Han Y, Li ZY, Canham LT (2011) Extremely high surface area metallurgical-grade porous silicon powder prepared by metal-assisted etching. Electrochem Solid State Lett 14(5):K25–K27. doi:10.1149/1.3548513 Low SP, Voelcker NH, Canham LT, Williams KA (2009) The biocompatibility of porous silicon in tissues of the eye. Biomaterials 30(15):2873–2880. doi:10.1016/j.biomaterials.2009.02.008 Mann AP, Tanaka T, Somasunderam A, Liu XW, Gorenstein DG, Ferrari M (2011) E-selectintargeted porous silicon particle for nanoparticle delivery to the bone marrow. Adv Mater 23(36): H278. doi:10.1002/adma.201101541 McInnes SJP, Szili EJ, Al-Bataineh SA, Xu JJ, Alf ME, Gleason KK, Short RD, Voelcker NH (2012) Combination of iCVD and porous silicon for the development of a controlled drug delivery system. ACS Appl Mater Interfaces 4(7):3566–3574. doi:10.1021/am300621k Mukherjee P, Whitehead MA, Senter RA, Fan DM, Coffer JL, Canham LT (2006) Biorelevant mesoporous silicon/polymer composites: directed assembly, disassembly, and controlled release. Biomed Microdevices 8(1):9–15. doi:10.1007/s10544-006-6377-7 Page 8 of 11

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Park JH, Gu L, von Maltzahn G, Ruoslahti E, Bhatia SN, Sailor MJ (2009) Biodegradable luminescent porous silicon nanoparticles for in vivo applications. Nat Mater 8(4):331–336. doi:10.1038/nmat2398 Perelman LA, Pacholski C, Li YY, VanNieuwenhz MS, Sailor MJ (2008) pH-triggered release of vancomycin protein-capped porous silicon films from. Nanomedicine 3(1):31–43. doi:10.2217/ 17435889.3.1.31 Prestidge CA, Barnes TJ, Mierczynska-Vasilev A, Skinner W, Peddie F, Barnett C (2007) Loading and release of a model protein from porous silicon powders. Phys Status Solidi A-Appl Mater Sci 204(10):3361–3366 Prestidge CA, Barnes TJ, Mierczynska-Vasilevl A, Kempson I, Peddiel F, Barnett C (2008) Peptide and protein loading into porous silicon wafers. Phys Status Solidi A-Appl Mater Sci 205(2):311–315. doi:10.1002/pssa.200723113 Ranade VV, Cannon JB (2011) Drug delivery systems, 3rd edn. CRC Press, Grayslake Rytkönen J, Miettinen R, Kaasalainen M, Lehto V-P, Salonen J, Närvänen A (2012) Functionalization of mesoporous silicon nanoparticles for targeting and bioimaging purposes. J Nanomater Article ID 896562. doi:10.1155/2012/896562 Salonen J, Laitinen L, Kaukonen AM, Tuura J, Bjorkqvist M, Heikkila T, Vaha-Heikkila K, Hirvonen J, Lehto VP (2005) Mesoporous silicon microparticles for oral drug delivery: loading and release of five model drugs. J Control Release 108(2–3):362–374 Sarparanta M, Bimbo LM, Rytkonen J, Makila E, Laaksonen TJ, Laaksonen P, Nyman M, Salonen J, Linder MB, Hirvonen J, Santos HA, Airaksinen AJ (2012) Intravenous delivery of hydrophobinfunctionalized porous silicon nanoparticles: stability, plasma protein adsorption and biodistribution. Mol Pharm 9(3):654–663. doi:10.1021/mp200611d Secret E, Smith K, Dubljevic V, Moore E, Macardle P, Delalat B, Rogers ML, Johns TG, Durand JO, Cunin F, Voelcker NH (2013) Antibody-functionalized porous silicon nanoparticles for vectorisation of hydrophobic drugs. Adv Healthc Mater 2(5):718–727. doi:10.1002/ adhm.201200335 Shen HF, Rodriguez-Aguayo C, Xu R, Gonzalez-Villasana V, Mai JH, Huang Y, Zhang GD, Guo XJ, Bai LT, Qin GT, Deng XY, Li QP, Erm DR, Aslan B, Liu XW, Sakamoto J, Chavez-Reyes A, Han HD, Sood AK, Ferrari M, Lopez-Berestein G (2013) Enhancing chemotherapy response with sustained EphA2 silencing using multistage vector delivery. Clin Cancer Res 19(7):1806–1815. doi:10.1158/1078-0432.ccr-12-2764 Tanaka T, Godin B, Bhavane R, Nieves-Alicea R, Gu J, Liu X, Chiappini C, Fakhoury JR, Amra S, Ewing A, Li Q, Fidler IJ, Ferrari M (2010a) In vivo evaluation of safety of nanoporous silicon carriers following single and multiple dose intravenous administrations in mice. Int J Pharm 402(1–2):190–197. doi:10.1016/j.ijpharm.2010.09.015 Tanaka T, Mangala LS, Vivas-Mejia PE, Nieves-Alicea R, Mann AP, Mora E, Han HD, Shahzad MMK, Liu XW, Bhavane R, Gu JH, Fakhoury JR, Chiappini C, Lu CH, Matsuo K, Godin B, Stone RL, Nick AM, Lopez-Berestein G, Sood AK, Ferrari M (2010b) Sustained small interfering RNA delivery by mesoporous silicon particles. Cancer Res 70(9):3687–3696. doi:10.1158/00085472.can-09-3931 Tang L, Saharay A, Fleischer W, Hartman PS, Loni A, Canham LT, Coffer JL (2013) Sustained antifungal activity from a ketoconazole-loaded nanostructured mesoporous silicon platform. Silicon 5(3):213–217. doi:10.1007/s12633-013-9143-5

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_91-1 # Springer International Publishing Switzerland 2014

Tasciotti E, Liu XW, Bhavane R, Plant K, Leonard AD, Price BK, Cheng MMC, Decuzzi P, Tour JM, Robertson F, Ferrari M (2008) Mesoporous silicon particles as a multistage delivery system for imaging and therapeutic applications. Nat Nanotechnol 3(3):151–157. doi:10.1038/ nnano.2008.34 Tzur-Balter A, Gilert A, Massad-Ivanir N, Segal E (2013) Engineering porous silicon nanostructures as tunable carriers for mitoxantrone dihydrochloride. Acta Biomater 9(4):6208–6217. doi:10.1016/j.actbio.2012.12.010 Vaccari L, Canton D, Zaffaroni N, Villa R, Tormen M, di Fabrizio E (2006) Porous silicon as drug carrier for controlled delivery of doxorubicin anticancer agent. Microelectron Eng 83(4–9):1598–1601. doi:10.1016/j.mee.2006.01.113 Vale N, Makila E, Salonen J, Gomes P, Hirvonen J, Santos HA (2012) New times, new trends for ethionamide: in vitro evaluation of drug-loaded thermally carbonized porous silicon microparticles. Eur J Pharm Biopharm 81(2):314–323. doi:10.1016/j.ejpb.2012.02.017 Vasani RB, McInnes SJP, Cole MA, Jani AMM, Ellis AV, Voelcker NH (2011) Stimulus-responsiveness and drug release from porous silicon films ATRP-grafted with poly(N-isopropylacrylamide). Langmuir 27(12):7843–7853. doi:10.1021/la200551g Wang F, Hui H, Barnes TJ, Barnett C, Prestidge CA (2010a) Oxidized mesoporous silicon microparticles for improved oral delivery of poorly soluble drugs. Mol Pharm 7(1):227–236. doi:10.1021/mp900221e Wang MJ, Coffer JL, Dorraj K, Hartman PS, Loni A, Canham LT (2010b) Sustained antibacterial activity from triclosan-loaded nanostructured mesoporous silicon. Mol Pharm 7(6):2232–2239. doi:10.1021/mp100227m Wu EC, Andrew JS, Cheng LY, Freeman WR, Pearson L, Sailor MJ (2011) Real-time monitoring of sustained drug release using the optical properties of porous silicon photonic crystal particles. Biomaterials 32(7):1957–1966. doi:10.1016/j.biomaterials.2010.11.013 Yu L (2001) Amorphous pharmaceutical solids: preparation, characterization and stabilization. Adv Drug Deliv Rev 48(1):27–42. doi:10.1016/s0169-409x(01)00098-9

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_91-1 # Springer International Publishing Switzerland 2014

Index Terms: Administration route 1 Bioactive 1 Biocompatibility 1 Biodegradable porous silicon 1 Bioresorbable property 1 Composite structures 5 Degradation products 5 Drug delivery 1 In vitro drug 2 In vivo drug 2 Intravenous delivery 4 Intravitreal delivery 4 Microparticles 1 Nanoparticles 1 Oral delivery 2 Peptide 3 pH-triggered delivery systems 5 Poorly soluble drugs 2 Protein 3 Silicic acid 5 Subcutaneous delivery 3

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_57-1 # Springer International Publishing Switzerland 2014

Drying Techniques Applied to Porous Silicon Leigh Canham* pSiMedica Ltd., Malvern Hills Science Park, Malvern, Worcester, UK

Abstract Wet-etched mesoporous silicon is normally dried in air, but this limits the range of porosities and surface areas achievable, due to capillary force-induced collapse of the silicon skeleton. The various alternative drying techniques are reviewed with particular attention paid to supercritical drying, a powerful technique applicable to all physical forms of porous silicon.

Introduction When porous silicon is fabricated via any route that uses liquids (see handbook chapter “▶ Routes of Formation for Porous Silicon”), it requires careful drying. Drying stresses due to liquid surface tension are primarily responsible for the changes that occur. Drying science and technology has at least two dedicated journals, emphasizing its importance in materials science in general and nanomaterials in particular (Wang et al. 2005; Pakowski 2007). The seminal work of Scherer in the drying of highly porous silica from gels (Scherer 1990) clarified the interplay of fluid flow and differential shrinkage of the solid network that can occur as a result of capillary stresses. This chapter first describes the degradation of porous silicon films during air-drying that has been revealed and then surveys the different methods of drying applied to date. Most attention is given to pentane drying and critical point (“supercritical”) drying. The former technique is easiest to implement and can significantly reduce but not eliminate drying-induced changes. The latter technique has been shown to be the most powerful technique for mesoporous silicon and is also now utilized by the silicon micromachining community.

Air-Drying An electrochemically etched mesoporous silicon film on a wafer can visually have good macroscale uniformity when still wet (see handbook chapter “▶ Colour of Porous Silicon”), but become crazed and sometimes even disintegrates as it dries in air. Normal air-drying of films leads to progressively increasing and often dramatic shrinkage, cracking, and peeling of the layer as the porosity and/or thickness is increased (see Fig. 1 example). This is caused by the buildup of capillary forces that arise from pore liquid evaporation. The maximum amplitude of the associated stresses occurs when the menisci enter the pores. A variety of different drying-induced morphologies can result (Wang et al. 2009; Xu and Wang 2009; Skryshevsky et al. 2011; Wang et al. 2013). Bellet and co-workers have shown that the Laplace equation can be used to quantify those stresses which are directly proportional to the surface tension of the fluid and inversely proportional to the radius of the

*Email: [email protected] Page 1 of 7

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_57-1 # Springer International Publishing Switzerland 2014

Fig. 1 Shrinkage and crack development in a highly porous silicon film during air-drying, as revealed by sequential wide-field photoluminescence imaging (Mason et al. 2002)

meniscus and hence increase with decreasing pore size (Belmont et al. 1996; Bellet 1997; Bellet and Canham 1998). Whether or not a given porous silicon structure is degraded by those stresses depends on its mechanical properties (see handbook chapter “▶ Mechanical Properties of Porous Silicon”). Experimentally, for layers of fixed porosity and pore size distribution, the onset of cracking and macroscopic deformation has been shown to be strongly related to layer thickness. Below a critical value hcrit., cracking of the air-dried film can be completely absent, as monitored by electron microscopy. Drying-induced damage is thus much more prevalent for thick films and for microporous and mesoporous films than for macroporous ones. Air-drying effects on morphology and properties have received continuous study (Gruning and Yelon 1995; Amato et al. 1996; DiFrancia et al. 2000; Lerondel et al. 2000; Chamard et al. 2001; Lei et al. 2006; Qiu et al. 2008; Wang et al. 2009; Gaev and Rekhviashvili 2012). In order to maintain the delicate nanostructures created by wet etching, a number of drying techniques have been investigated and these are surveyed in the next section.

Survey of Drying Techniques A variety of strategies have been investigated to minimize fracture and shrinkage of highly porous materials during drying or at least improve reproducibility of material properties. Table 1 lists various drying techniques available and indicates those studies that used specific drying techniques on porous silicon structures to date. The one technique listed which does not appear to have been used yet with mesoporous silicon is microwave drying, although it has been utilized, for example, with mesoporous silica (Bhagat et al. 2008). Vacuum drying, derivatization with subcritical drying, slow controlled drying, microwave drying, and spray drying do not avoid capillary forces; their merits lie elsewhere. Pentane drying is a facile technique to significantly increase the critical thickness h crit for layer cracking. Here water in the pores is replaced first by ethanol and then pentane before drying in air, since pentane has a very low surface tension. It reduces capillary forces but does not eliminate them. Page 2 of 7

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_57-1 # Springer International Publishing Switzerland 2014

Table 1 Drying techniques of relevance to highly porous silicon Drying technique Atmospheric drying Vacuum drying

Basis of technique Pore fluid evaporates into air Transfer of wet structure into evacuated chamber Freeze-drying (lyophilization) Pore fluid is frozen then sublimed in evacuated chamber Supercritical drying (critical Pore fluid is removed after point drying) conversion into a supercritical state

Advantage Convenience Avoids exposure to air (oxygen) Industrial process in widespread use

Use with porous silicon The vast majority of studies Koizumi et al. 1996

Avoids capillary stresses Industrial process

Canham et al. 1994

Derivatization and subcritical Chemical surface drying modification prior to atmospheric drying Pentane drying Pore fluid is replaced by liquid pentane which is then allowed to evaporate in air Slow controlled drying Evaporation rate is controlled and lowered Microwave drying Evaporation rate is increased

Mechanical strengthening of network Minimizes capillary stresses

Spray drying

Very useful for nanocomposite particle formation or microencapsulation

Amato and Brunetto 1996, Amato et al. 1997

Frohnhoff et al. 1995 Von Behren et al. 1997 Xu et al. 1998 Kolasinski et al. 2000 Chang et al. 2011 Linsmeier et al. 1997

Bellet 1997 Oton et al. 2002

More reproducibility of Pellegrini et al. 1995 capillary stress effects Industrial process Lower drying times Industrial process in Jung et al. 2013 widespread use

Two established methods of avoiding the liquid/vapor interface during drying are illustrated in Fig. 2, namely, freeze-drying and supercritical drying. Freeze-drying has proved successful in foodstuff preparation but has not worked as well with mesoporous inorganic materials. We therefore look in more detail at supercritical drying.

Supercritical Drying Of the techniques listed in Table 1, supercritical drying appears the most powerful for achieving very high porosity or very high surface area silicon structures (see handbook chapter “▶ Pore Volume (Porosity) in Porous Silicon”). The fluid used is normally carbon dioxide since its critical temperature (304.1 K) is conveniently low (see Fig. 3). The HF-based electrolyte used in, for example, anodization is first removed from the pores by extensive ethanol rinsing, and then the alcohol is replaced by liquid carbon dioxide. This is then compressed and heated in an autoclave beyond its critical point. After further flushing to remove residual ethanol, it is then converted to a gas through pressure reduction at constant temperature. The overall phase diagram path is shown by the red arrow in Fig. 2.

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_57-1 # Springer International Publishing Switzerland 2014

Fig. 2 The phase transitions involved in water-air, ethanol-air, or pentane-air-drying (green arrow); freeze-drying (blue arrow); and supercritical drying (red arrow) of wet porous silicon. For air-drying, the liquid L in the pores immediately becomes a gas G via evaporation. For freeze-drying, it is first solidified (S) and then becomes a gas via sublimation around the triple point. For supercritical drying, it is taken around the supercritical point to become a gas

10,000 solid supercritical fluid

pressure P (bsi)

1,000

liquid 100 critical point

10 triple point

gas

1 200

250

300

350

400

temperature T (K)

Fig. 3 The phase diagram of carbon dioxide, the most popular supercritical solvent

Supercritical drying of porous silicon films on wafers has been particularly successful (see references in Table 1) and should be applicable to other physical forms such as membranes, microparticles, nanowire arrays, and nanoparticles. Of particular relevance in this regard is that the silicon MEMS community also utilizes this technique to avoid micromachined structure stiction, to clean silicon surfaces, and to improve device yield (Kim et al. 1998; Jafri et al. 1999; Namatsu et al. 1999). There are now commercially available systems for both silicon powder and wafer batch processing (Fig. 4). Supercritical fluids could also be utilized in the purification of porous silicon (Koynov et al. 2011).

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_57-1 # Springer International Publishing Switzerland 2014

Fig. 4 Supercritical dryers, (a) benchtop system (Quorum Technologies Ltd, UK) for processing stain-etched porous Si powders, (b) benchtop system (Quorum Technologies Ltd, UK) for processing an anodized 150 mm diameter silicon wafer, (c) cleanroom-compatible automated multiwafer system (Tousimis Corp., USA)

Concluding Remarks The uniformity and properties of wet-etched mesoporous silicon can be adversely affected by how it is dried. Air-drying can lead to substantial loss of porosity and surface area. In a number of applications, the benefits of porous silicon scale with its surface area, pore volume, or a combination of the two. In drug delivery, for example, small molecule API payload can be increased by increasing pore volume; large biomolecule payload can rely on large internal surface areas accessible by large pores. Supercritical drying, rather than air-drying, adds expense to manufacture but is likely to be important for material optimization in many high-value applications.

References Amato G, Bullara V, Brunetto N, Boarino L (1996) Drying of porous silicon: a Raman, electron microscopy and photoluminescence study. Thin Solid Films 276(1–2):204–207 Amato G, Brunetto N (1996) Porous silicon via freeze drying. Mater Lett 26(6):295–298 Amato G, Brunetto N, Parisini A (1997) Characterization of freeze-dried porous silicon. Thin Solid Films 297(1–2):73–78 Bellet D (1997) Chapter 1.5: Drying of porous silicon. In: Canham LT (ed) Properties of porous silicon. IEE, London, pp 38–43 Bellet D, Canham LT (1998) Controlled drying: the key to better quality porous semiconductors. Adv Mater 10(6):487–490 Belmont O, Bellet D, Brechet Y (1996) Study of the cracking of highly porous p+ type silicon during drying. J Appl Phys 79:7588 Bhagat SD, Kim YH, Yi G, Ahn YS, Yeo JG, Choi YT (2008) Mesoporous silica powders with high specific surface area by microwave drying of hydrogels: a facile synthesis. Micro Meso Mater 108:333–339 Canham LT, Cullis AG, Pickering C, Dosser OD, Cox TI, Lynch TP (1994) Luminescent silicon aerocrystal networks prepared by anodisation and supercritical drying. Nature 368:133–135 Chamard V, Pichat C, Dolino G (2001) Rinsing and drying studies of porous silicon by high resolution X-ray diffraction. Solid State Commun 118(3):135–139

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_57-1 # Springer International Publishing Switzerland 2014

Chang SW, Chuang VP, Boles ST, Ross CA, Thompson CV (2011) Densely packed arrays of ultrahigh aspect ratio silicon nanowires fabricated using block co-polymer lithography and metal assisted etching. Adv Funct Mater 19(15):2495–2500 DiFrancia G, Ferrara V, Lancellotti L, Quercia L (2000) Stress measurement technique to monitor porous silicon processing. J Porous Mater 7:319–321 Frohnhoff S, Arens-Fischer R, Heinrich T, Fricke J, Amtzen M, Theiss W (1995) Characterization of supercritical dried porous silicon. Thin Solid Films 255(1–2):115–118 Gaev DS, Rekhviashvili SS (2012) Kinetics of crack formation in porous silicon. Semiconductors 46(2):137–140 Gruning U, Yelon A (1995) Capillary and Van der Waals forces and mechanical stability of porous silicon. Thin Solid Films 256(1–2):135–138 Jafri IH, Busta H, Walsh ST (1999) Critical point drying and cleaning for MEMS technology. Proc SPIE 3880. doi:10.1117/12.359371 Jung DS, Hwang TH, Park SB, Choi JW (2013) Spray drying method for large scale and high performance silicon negative electrodes in Li ion batteries. Nano Lett 13(5):2092–2097 Kim CJ, Kim JY, Sridharan B (1998) Comparative evaluation of drying techniques for surface micromachining. Sens Actuat A64:17–26 Koizumi T, Obata K, Tezuka Y, Shin S, Koshida N, Suda Y (1996) Effects of oxidation on electronic states and photoluminescence properties of porous silicon. Jpn J Appl Phys 35:L803–L806 Kolasinski KW, Barnard JC, Ganguly S, Koker L, Wellner A, Aindow M, Palmer RE, Field CN, Hamley PA, Poliakoff M (2000) On the role of the pore filling medium in photoluminescence from photochemically etched porous silicon. J Appl Phys 88(5):2472–2479 Koynov S, Pereira RN, Crnolatac I, Kovalev D, Huygens A, Chirvony V, Stutzmann M, deWitte P (2011) Purification of nanoporous silicon for biomedical applications. Adv Eng Mater 13(6): B225–B233 Lei ZK, Kang YL, Cen H, Hu M (2006) Variability on Raman shift to stress coefficient of porous silicon. Chin Phys Lett 23(6):1623–1626 Lerondel G, Amato G, Porisini A, Boarino L (2000) Porous silicon nanocracking. Mater Sci Eng B 69(70):161–166 Linsmeier J, Wust K, Schenk H, Hilpert U, Ossau W, Fricke J, Arens-Fischer R (1997) Chemical surface modification of porous silicon with tetraethoxysilane. Thin Solid Films 297:26–30 Mason MD, Sirbuly DJ, Buratto SK (2002) Correlation between bulk morphology and luminescence in porous silicon investigated by pore collapse resulting from drying. Thin Solid Films 406:151–158 Namatsu H, Yamazaki K, Kurihara K (1999) Supercritical drying for nanostructure fabrication without pattern collapse. Microelectron Eng 46(1–4):129–132 Oton CJ et al (2002) Scattering rings in optically anisotropic porous silicon. Appl Phys Lett 81(26):4919–4921 Pakowski Z (2007) Modern methods of drying nanomaterials. Dry Porous Mater 66:19–27 Pellegrini V, Fuso F, Lorenzi G, Allegrini M, Diligenti A, Nannini A, Pennelli G (1995) Improved optical emission of porous silicon with different postanodization processes. Appl Phys Lett 67:1084 Qiu W, Kang YL, Li Q, Lei ZK, Qin QH (2008) Experimental analysis for the effect of dynamic capillarity on stress transformation in porous silicon. Appl Phys Lett 92:041906 Scherer GW (1990) Theory of drying. J Am Ceram Soc 73(1):3–14 Skryshevsky VA, Vorobey G, Jamois C, Munguia J, Lysenko V (2011) Drying induced selfformation of semi-ordered nano-porous silicon micro-hairs. Phys Status Solidi C8(6):1805–1807 Page 6 of 7

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_57-1 # Springer International Publishing Switzerland 2014

Von Behren J, Chimowitz EH, Fauchet PM (1997) Critical behaviour and the processing of nanoscale porous materials. Adv Mater 9:921 Wang B, Zhang W, Mujumdar AS, Huang L (2005) Progress in drying technology for nanomaterials. Drying Technol 23(1–2):7–32 Wang F, Song S, Zhang J (2009) Surface texturing of porous silicon with capillary stress and its superhydrophobicity. Chem Commun 28:4239–4241 Wang D, Ji R, Albrecht A, Schaaf P (2013) Ordered arrays of nanoporous silicon nanopillars and silicon nanopillars with nanoporous shells. Nanoscale Res Lett 8(42):1–9 Xu SH, Wang LW (2009) Porous silicon microtube structures induced by anisotropic strain. J Appl Phys 106:073516 Xu D, Guo G, Gui L, Tang Y, Zhang B, Qin G (1998) Preparation and characterisation of freestanding porous silicon films with high porosities. Electrochem Solid State Lett 1(5):227–229

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_52-2 # Springer International Publishing Switzerland 2014

Effects of Irradiation on Porous Silicon Roberto Koropecki* and Roberto Arce Grupo de Semiconductores Nanoestructurados, Instituto de Física del Litoral, IFIS Litoral (UNL-CONICET), Santa Fe, Argentina

Abstract Besides the well-known effect of photoluminescence, the impinging of photons and other kinds of particles such as electrons, ions, and muons on porous silicon produces important effects. Some of these effects can modify the structure and properties of the material, distorting the interpretation of data based on the use of irradiation. Some of the irradiation effects are useful in different applications such as photodynamic therapy or display applications. This work is a review of the effects of irradiation on porous silicon.

Introduction Electrons and ion beams, as well as photons with different energies, are usually employed as probe in characterization techniques for the study of porous silicon (PS) properties. The interaction between the different beams and PS is also present in a variety of sensing devices and filters. These devices usually exploit the energy exchange between the incident beam and the porous silicon. As a result, some structural or electronic changes occur, which may be reversible, metastable, or irreversible. So it is important to know these effects in order to properly design the devices and also to properly interpret the results of the analysis techniques. A general textbook on the effects of radiation in solids can be found in reference (Sickafus et al. 2007). In this review, we deal with the effects of irradiation on porous silicon.

Characterization Techniques Using Irradiation The luminescence of porous silicon (Canham 1990) discovered by Canham in 1990 was responsible for much of the rise and progress of silicon photonics. Originally, the focus was on the potential use of its electroluminescence in optoelectronic devices, but now it is mainly used for sensing purposes, in chemical sensors and biosensors, and also in characterization techniques for the study of electronic defects and recombination processes. The luminescence can be excited by irradiation with photons having different energies (Cullis et al. 1997; Gardelis et al. 1996; Dalba et al. 1998, 1999; Sham et al. 2000; Pettifer et al. 1995), with multiphoton excitation (Diener et al. 1995), or with electrons (Maurice et al. 1995; Biaggi-Labiosa et al. 2008; Timoshenko et al. 1996; Cullis et al. 1994; Hummel et al. 1995). Besides the well-known UV–VIS–NIR absorption spectroscopy and the FTIR spectroscopy, a variety of spectroscopic techniques employed in PS characterization also use photons. Among them we can mention the following: Raman spectroscopy, ultraviolet

*Email: roberto.koropecki@ifis.santafe-conicet.gov.ar *Email: [email protected] Page 1 of 14

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_52-2 # Springer International Publishing Switzerland 2014

photoemission spectroscopy (UPS) (Aprelev et al. 1997), X-ray absorption spectroscopies (such as XAS, EXAFS, and XANES) (Dalba et al. 1998, 1999; Zhang et al. 2000; Van Buuren et al. 1994; Schuppler et al. 1995), X-ray photoelectron spectroscopy (XPS) (Frohnhoff et al. 1995; DimovaMalinovska et al. 1995; Thönissen et al. 1996; Kanungo et al. 2009; Matsumoto et al. 1997; Voss 1997; Buzaneva et al. 2000; Debarge et al. 1998; Jarvis et al. 2012; Kanungo et al. 2010; Zanoni et al. 1999; Harraz et al. 2008; Bolotov et al. 2012), and X-ray diffraction (XRD) (Zanoni et al. 1999; Lee et al. 2005). Other spectroscopies employed in porous silicon study use electron beams. This is the case of Auger electron spectroscopy (AES) (Hummel et al. 1995; Schuppler et al. 1995; Harraz et al. 2008; Bolotov et al. 2012; Lee et al. 2005; Thompson et al. 1998; Pavesi et al. 1994; Wise et al. 1996; Kostishko et al. 2004; Bedikjan and Danesh 1997; Xiong et al. 2001; Galiy et al. 1998; Baranauskas et al. 1999; Jin et al. 2006), electron energy loss spectroscopy (EELS) (Cullis et al. 1994; Berbezier et al. 1996; Vasin et al. 2011; Martín-Palma et al. 2006; Song et al. 2000), electron microprobe spectroscopy (EDX) (Matsumoto et al. 1997; Zanoni et al. 1999; Bolotov et al. 2012; Lee et al. 2005), and low-energy electron diffraction spectroscopy (LEEDS) (Li et al. 2000). Electron beams are used also in the morphological characterization of PS, as in the case of scanning electron microscopy (SEM) (Zanoni et al. 1999; Bolotov et al. 2012; Berbezier et al. 1996; Vasin et al. 2011; Chiboub et al. 2010; Gorbanyuk et al. 2006), transmission electron microscopy (TEM) (Lee et al. 2005; Martín-Palma et al. 2006; Song et al. 2000; Li et al. 2000; Chiboub et al. 2010), and reflection electron diffraction. PS has also been investigated by using ion spectroscopies, such as elastic recoil detection analysis (ERDA) (Jarvis et al. 2012), Rutherford backscattering spectroscopy (RBS) (Jarvis et al. 2012), secondary ion mass spectroscopy (SIMS) (Aprelev et al. 1997; Kanungo et al. 2010; Zanoni et al. 1999; Thompson et al. 1998; Kempson et al. 2010; Torchinskaya et al. 1997, 1998; Torchynska et al. 1999; Kleps et al. 1997, 1998; Banerjee et al. 2008; Ćwil et al. 2006; Fried et al. 1999), and desorption/ionization on porous silicon mass spectrometry (DIOSMS) (Wei et al. 1999; Li et al. 2005). Subatomic particles as positive muons have been irradiated on porous silicon to perform muon spin spectroscopy (mSR) (Harris et al. 1997) and positrons to perform positron and positronium annihilation spectroscopies (PAS) (Itoh et al. 1996; Knights et al. 1995; Suzuki et al. 1994). The different characterization techniques, and the link with the different chapters of this handbook, are resumed in Table 1. In the following, we will review the main consequences produced by different irradiation processes.

Table 1 Characterization techniques associated to the different classes of probe beams Associated characterization techniques Raman, UPS, XAS, EXAFS, XANES, IR, PL, XR Electrons AES, EELS, EDX, RHEED CL, SEM, TEM, etc. Ions (H, He, C, O, P, ERDA, RBS, SIMS, DIOSSi, N, etc.) MS Subatomic particles PAS, mSR (positrons, muons)

Class of probe beam Photons

Related handbook chapters ▶ Characterization of Porous Silicon by Infrared Spectroscopy, ▶ Photoluminescence of Porous Silicon, and ▶ X-ray Diffraction in Porous Silicon ▶ Characterization Challenges with Porous Silicon ▶ Microscopy of Porous Silicon ▶ Chemical Characterization of Porous Silicon ▶ Characterization Challenges with Porous Silicon

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_52-2 # Springer International Publishing Switzerland 2014

Fig. 1 General scheme of different effects of irradiation

Irradiation Effects Figure 1 shows a general scheme of different effects occurring in porous silicon under irradiation.

Photoirradiation Effects Many properties can be altered by irradiating porous silicon with photons. Photooxidation (Frohnhoff et al. 1995; Salonen et al. 1999; Fu et al. 1993; Tamura and Adachi 2009; Koropecki et al. 2004a, b, 2006, 2007; Aouida et al. 2006; Kovalev et al. 2004; Zhang et al. 1995), generation of electronic defects associated to dangling bond (Koropecki et al. 2007; Aouida et al. 2006; Arce et al. 2006) or weak bonds, and photo-diffusion or loss of hydrogen from the PS surface (Koropecki et al. 2007; Aouida et al. 2006; Collins et al. 1992; Frello and Veje 1997) are some of the photoinduced effects. Photoinduced changes manifest themselves in effects such as changes in energy or intensity of the photoluminescence spectra (Koropecki et al. 2004a, b, 2006, 2007; Aouida et al. 2006; Arce et al. 2006; Collins et al. 1992; Frello and Veje 1997; Xu and Adachi 2010; Fauchet 1996; Mandal et al. 2004; Choi et al. 1995), changes in the electron paramagnetic resonance signal (Koropecki et al. 2004a, 2007; Aouida et al. 2006; Collins et al. 1992; Mandal et al. 2004; Timoshenko 2009a) mainly associated with dangling bonds, changes in the effective dielectric function, changes in hydrophilicity (Tamura and Adachi 2009), and generation of singlet oxygen (Kovalev et al. 2002, 2004, 2005; Timoshenko 2009a; Gross et al. 2003; Loponov et al. 2010; Fujii et al. 2005, 2007; Gongalsky et al. 2011; Pikulev et al. 2006; Konstantinova et al. 2007; Lee et al. 2007; Timoshenko 2009b), among others. Depending on the preparation conditions, on the PL excitation characteristics, and on the PL spectral region, the luminescence intensity increases or decays during the photon exposure or multiple peaks with different behaviors (Koropecki et al. 2004b; Fauchet 1996; Choi et al. 1995; El Houichet et al. 1997). The enhancement and spectral shift of the PL spectra were reported for samples irradiated with 60Co g rays (Bhave et al. 1997). The behavior of the photoinduced evolution may depend on the atmosphere in which the porous silicon is embedded (Salonen and Laine 1996) and on the preparation conditions (Koropecki et al. 2007; Aouida et al. 2006; Collins et al. 1992).

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_52-2 # Springer International Publishing Switzerland 2014

It has been reported that irradiation with 60Co g rays also produces enhancement of the PL and radiation-induced recrystallization (Bhave et al. 1997). On the other side, there are reports of the degradation of the photoluminescence signal induced by g ray irradiation (Astrova et al. 1995; Agekyan et al. 1999). Many authors report on the photoinduced degradation of PL in porous silicon. The nature of the degradation mechanisms is complex and involves different processes (Cullis et al. 1997). Tischler et al. report that there are physicochemical effects (Tischler et al. 1992) related with photooxidation which are mostly irreversible and partially recovered by immersion (Tischler et al. 1992) in hydrofluoric acid. Illumination under nonoxidizing atmosphere produces a little – although not negligible – degradation effect (Koropecki et al. 2007; Collins et al. 1992). Photoinduced loss of hydrogen from fresh PS in vacuum has been reported (Koropecki et al. 2007). Non-radiative recombination paths are created and associated to unsaturated dangling bonds (DB), which can be responsible for the luminescence decay. A photoinduced increase of the DB density has been measured by electron paramagnetic resonance (EPR) both for exposed (Koropecki et al. 2006, 2007; Collins et al. 1992; Tischler et al. 1992) and for nonexposed samples to an oxidizing atmosphere. Electron beam-induced hydrogen effusion was also reported with a consequent increase of the DB density and decay of the cathodoluminescence (Ruano et al. 2011). Thermal effects might be responsible for hydrogen loss. However, there are experimental evidences showing that the kinetics of the photoinduced decay of luminescence and the decay of the cathodoluminescence as well as the kinetics of hydrogen desorption during electron beam irradiation are not compatible with thermal effects or direct exchange of energy between the radiation and the hydrogen atoms. Instead, these kinetics are compatible with short-lived highenergy fluctuations (SLEFs) occurring during bimolecular recombination of carriers (Koropecki et al. 2007; Arce et al. 2006; Ruano et al. 2011). Whatever the excitation be, which creates electron hole pairs, the densities of carriers within the bands will be the result of the balance between generation and recombination. The main recombination processes are non-radiative and occur mediated by defects located in the hydrogenated surface layer of the nanostructure. The model considers that during SLEFs some structural changes occur, involving diffusion of hydrogen atoms and creation of dangling bonds, which act as recombination centers. As a result, the carrier densities in the conduction and valence band change, and so the bimolecular recombination rate. This process is self-limited resulting in a decay of the luminescence following approximately a power law (Koropecki et al. 2006, 2007; Arce et al. 2006; Ruano et al. 2011) with exponent approximately
1/3. The cumulative density of dangling bonds created during this process also follows a power law (Koropecki et al. 2006) with exponent 1/3. In agreement, hydrogen release with a rate which follows a power law (Ruano et al. 2011) with exponent approximately
2/3 also occurs. Samples prepared under high level of illumination, starting from high-resistivity silicon either n type (Arce et al. 2006) or p type (Koropecki et al. 2006), show this behavior. However, there is experimental evidence that this is only one of the irradiation-induced effects. In fact, there is radiation-induced evolution of the luminescence related to photoinduced oxidation of nanostructures. It has been reported that samples prepared in darkness or under low illumination level using high-resistivity silicon have greater DB densities than samples prepared under illumination (Koropecki et al. 2007). The PL spectra of these samples show the increase of a high-energy band at the expenses of the decrease of a low-energy band (Koropecki et al. 2004a, b, 2006, 2007; Arce et al. 2006). There are experimental evidences supporting that these effects are related to oxidation of the nanostructure and that also take place for samples prepared under high level of illumination, although the intensity of the luminescence associated to this effect is negligible compared to that associated to the one associated to the photoinduced quenching (Koropecki et al. 2007). For samples prepared under Page 4 of 14

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_52-2 # Springer International Publishing Switzerland 2014

high level of illumination, the PL photoinduced quenching in vacuum shows the same time dependence as in air atmosphere, although the latter is faster than the former. One of the photoinduced effects of porous silicon exposed to molecular oxygen is the generation of reactive species. Molecular oxygen has the triplet state 3S as ground state and two singlet excited states 1D and 1S. The “singlet oxygen” 1O2 is very reactive and has a lot of applications, mainly in photodynamic therapy of cancer. Direct optical excitations of O2 to its singlet excited states are forbidden by selection rules of spin and parity conservation. However, 1O2 can be effectively created in chemical reactions or under electrical discharges or by a process of photosensitization in which a photoexcited molecule transfers its energy and spins to a triplet ground state O2 molecule, which is then promoted to its singlet state (Timoshenko 2009a). Porous silicon is an efficient photosensitizer due to its large specific surface combined with the long radiative lifetime of excitons confined in nanocrystalline silicon (Kovalev et al. 2002; Gross et al. 2003; Fujii et al. 2005). See handbook chapter “▶ Porous Silicon in Photodynamic and Photothermal Therapy” for more details of its use. Although direct photoinduced transitions in free molecular oxygen from the triplet to the singlet states are forbidden by selection rules, these transitions occur due to perturbations. Absorption and emission bands related to 3S and 1D transitions, which occur at 7,882 cm
1 and 13,121 cm
1, have been observed in the upper atmosphere spectrum (Kearns 1971). The radiative lifetimes of 1D and 1S states are 2.7  103 s and 7.1 s (Kearns 1971; Huie and Neta 2002). The 1S state quenches very rapidly to the 1D state (Timoshenko 2009b). As an example, the estimated lifetime in water is about 10
11 s so that it is not relevant in physiology. The 1D state has larger lifetimes; it goes from 2 ms in water to 1 ms in CFCl3, so that it has enough time to interact with other species in the solution (Huie and Neta 2002). The photo-generation of singlet oxygen by photosensitization usually involves photoexcitation of a light-absorbing substance, usually a die-like organic molecule, named photosensitizer. The excitation of this “energy donor” to its first excited singlet state initiates the energy transfer process to the “energy acceptor” nonabsorbent oxygen triplet. Some of the energetic relaxation processes of the singlet excited state of the photosensitizing result in the transference of energy and spin to the energy acceptor by a Förster dipole–dipole process (Timoshenko 2009b) or an intersystem crossing process involving the decay of the donor to its lower energy triplet state which interacts with the oxygen to produce singlet oxygen (Timoshenko 2009b; Huie and Neta 2002; Derosa and Crutchley 2002). Due to the indirect nature of silicon, the computed lifetime of confined excitons in the nanostructure of porous silicon at room temperature is as long as 10–100 ms for the triplet state, similar to the singlet configuration (Cullis et al. 1997; Timoshenko 2009a). Therefore, excitons in porous silicon have an electronic configuration similar to that of dye molecules. The size distribution and consequently the energy gap of the nanocrystalline structures allow the energy match of the singlet–triplet splitting energy of 3O2 molecules. This characteristic, combined with the large specific surface of porous silicon, makes it an excellent photosensitizer (Gross et al. 2003). The quantum yield of generation of 1O2 in porous silicon, measured by chemical trapping, was (Xiao et al. 2011) 0.10  0.02 in ethanol and 0.17  0.01 in H2O. Although the generation rates are lower and less effective than those obtained with organic molecules employed in photodynamic therapy, porous silicon overcomes toxicity and degradation problems which are the main drawbacks of treatment with these organic molecules (Xiao et al. 2011). There is an extensive list of works related to the generation of singlet oxygen using porous silicon as a photosensitizer energy donor (Kovalev et al. 2002, 2004, 2005; Timoshenko 2009a, b; Gross et al. 2003; Loponov et al. 2010; Fujii et al. 2004, 2005, 2007; Gongalsky et al. 2011; Pikulev et al. 2006; Konstantinova et al. 2007, 2011; Lee et al. 2007; Xiao et al. 2011; Heitmann et al. 2004). There are also reports of the combination of porous silicon and anilines as photosensitizers (Parkhutik et al. 2007). Page 5 of 14

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_52-2 # Springer International Publishing Switzerland 2014

Ion Irradiation Effects The effects of porous silicon irradiation with ions have been investigated extensively. There are a large variety of studies of the effects of ion bombardment on the photoluminescence spectrum of PS. Table 2 shows the effects of different bombarding species. The experiments include ions of different types and energies such as oxygen ions with 14 MeV (Bhave et al. 1996, 1997, 1999), as well as H+, He2+, and Ne2+ with 35–109 keV (Jacobsohn et al. 2006), 30 keV He+ (Baratta et al. 2004), and 10 keV C+ (Liu et al. 2001). The suppression and subsequent recovery of the PL by 800 eVAr and N ion irradiation has been reported (Du et al. 2008). Similar behavior has been published for 35 keV He+ and 42 keV H+ ion irradiation (Jacobsohn et al. 2005, 2006) and for 250 keV Ne ion irradiation (Barbour et al. 1992). Similarly, there are studies of the effect on the PL of low-energy (10 eV) oxygen ions from a plasma generated by electron cyclotron resonance (O’Keeffe et al. 1996). The effects of plasma fluorination from CF4 (Freon 14) gas on PS photoluminescence were studied (Pan et al. 2004). The effects of irradiation with oxygen ions were compared with that of silicon ions (Bhave et al. 1999). Au+7 (Sehrawat et al. 2004) and Ni ion (Mehta et al. 1996) effects in the PL spectra of porous silicon samples have been also studied. Irradiation with ions was used on monocrystalline substrates prior to anodization, in order to manage the porous silicon properties. This allows technique of 3D micromachining of silicon Table 2 Effects of different bombarding species on porous silicon Bombarding species Si7+ Electrons O ions H+, He+, Ne2+

Energy/characteristics 10 MeV 1–6 MeV 14 MeV 35–109 keV

He+ C+

30 keV 10 keV

Ar, N ions O ions

800 eV 800 eV

Ne O ions CF4 Au7+

250 keV 10 eV (plasma) (plasma) 100 MeV

Ni ions He ions

85 MeV 2 MeV (prior to porosification) 5–24 MeV (prior to porosification) 2 MeV (prior to porosification) 2 MeV (previous to porosification)

Si ions He ions Protons

Effects Increasing PL Stabilization Increasing PL Fluence-dependent damage PL reduction, recovery in air Fluence-dependent PL reduction Red PL emission reduction Blue PL emission rising PL suppression, recovery in air PL spectrum shape changes Subsequent PL decrease in air PL reduction PL enhancement, fatigue suppression Gap expansion, dielectric function reduction PL enhancement, stabilization PL degradation for high doses PL suppression Defect creation in bulk Si, allowing 3D patterning Defect creation in bulk Si, allowing 3D patterning Defect creation in bulk Si, allowing tuning of PS properties Control of reflectance in porous silicon microcavities

References Bhave et al. (1996, 1997) Bhave et al. (1999) Jacobsohn et al. (2005, 2006) Baratta et al. (2004) Liu et al. (2001) Du et al. (2008) Du et al. (2008) Barbour et al. (1992) O’Keeffe et al. (1996) Pan et al. (2004) Sehrawat et al. (2004) Mehta et al. (1996) Teo et al. (2004) Punzón-Quijorna et al. (2012) Teo et al. (2007) Mangaiyarkarasi et al. (2007)

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_52-2 # Springer International Publishing Switzerland 2014

(Teo et al. 2004; Punzón-Quijorna et al. 2012). Tunable color contrast of the emission of PS has been achieved by varying the dose of He+ ion irradiation (Teo et al. 2007). Images with different reflected colors have been also obtained by irradiation of optical microcavities with 2 MeV protons (Mangaiyarkarasi et al. 2007). Helium ions from an electron cyclotron resonance source were used to study the effects on the irradiation prior to anodization on the PL spectrum of porous silicon, comparing them with the post-anodization effects (Yamauchi et al. 2002). Irradiation of ZnO-filled PS with 120 MeV Au heavy ions was reported to produce a decrease in the photoluminescence associated with PS simultaneously with an increase in the ZnO deep level related photoluminescence (Singh et al. 2009). It has been reported (Huang 1997) that positron irradiation produces a large blueshift (~126 nm), followed by a two-peak generation in the PL spectrum of PS. Although the exposure time employed was large, the result is important not only because it is a potential tool for the porous silicon band structure engineering but also to prevent modifications in positron annihilation spectroscopic experiments, as it is a characterization technique for porous silicon (Suzuki et al. 1994; Itoh et al. 1993; Dannefaer et al. 1996; Biasini et al. 2000). Muon states have been also implanted in porous silicon in order to perform mSR spectroscopy (Harris et al. 1997). In general, the PL efficiency has been reported to increase or decrease, depending on the interplay between altered oxidation level and defect creation produced by irradiation with ions or photons. Other properties, such as the PS electrical transport (Gokarna et al. 1999) and the electron-induced secondary electron emission spectra from PS surface (Ruano et al. 2009a, b), have been demonstrated to be modified by ion bombardment.

Conclusions Summarizing, photoinduced and ion or electron beam effects include oxidation, hydrogen release, dangling bond creation, and creation of reactive molecular species. Ion bombardment can in addition introduce a variety of point defects in the silicon skeleton. Highly focused beams, such as used in micro-Raman, can cause thermally induced hydrogen exodiffusion, sintering, defect creation, etc. An awareness of potential irradiation effects is important in both the characterization and processing of porous silicon.

References Agekyan VF, Stepanov YA, Emtsev VV, Lebedev AA, Poloskin DS, Remenyuk AD (1999) Effect of g irradiation on the photoluminescence kinetics of porous silicon. Semiconductors 33:1315–1317 Aouida S, Saadoun M, Ben Saad K, Bessaïs B (2006) UV photooxidation induced structural and photoluminescence behaviors in vapor-etching based porous silicon. Mater Sci Eng C 26:495–499 Aprelev AM, Lisachenko AA, Laiho R, Pavlov A, Pavlova Y (1997) UV (hn ¼ 8. 43 eV) photoelectron spectroscopy of porous silicon near Fermi level. Thin Solid Films 297:142–144 Arce RD, Koropecki RR, Olmos G, Gennaro AM, Schmidt JA (2006) Photoinduced phenomena in nanostructured porous silicon. Thin Solid Films 510:169–174

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_52-2 # Springer International Publishing Switzerland 2014

Astrova EV, Emisev VV, Lebedev AA, Poloskin DI, Remenyuk AD, Rud YV, Khartsiev VE (1995) Degradation of the photoluminescence of porous silicon caused by 60Co g radiation. Semiconductors 29:674–676 Banerjee M, Bontempi E, Tyagi AK, Basu S, Saha H (2008) Surface analysis of thermally annealed porous silicon. Appl Surf Sci 254(6):1837–1841 Baranauskas V, Bin Li B, Peterlevitz AC, Tosin MC, Durrant SF (1999) Structure and properties of diamond films deposited on porous silicon. Thin Solid Films 355–356:233–238 Baratta GA, Strazzulla G, Compagnini G, Longo P (2004) Raman and photoluminescence study of ion beam irradiated porous silicon: a case for the astrophysical extended red emission? Appl Surf Sci 226(1–3):57–61 Barbour JC, Dimos D, Guilinger TR, Kelly MJ (1992) Control of photoluminescence from porous silicon. Nanotechnology 3:202–204 Bedikjan L, Danesh P (1997) RF plasma treatment of porous silicon. J Non Cryst Solids 220:261–266 Berbezier I, Martin JM, Bernardi C, Derrien J (1996) EELS investigation of luminescent nanoporous p-type silicon. Appl Surf Sci 102:417–422 Bhave TM, Bhoraskar SV, Kulkarni S, Bhoraskar VN (1996) Improvement in the photoluminescence efficiency of porous silicon using high-energy silicon ion irradiation. J Phys D Appl Phys 29:462–465 Bhave TM, Bhoraskar SV, Singh P, Bhoraskar VN (1997) Radiation induced recrystallisation and enhancement in photoluminescence from porous silicon. Nucl Instrum Meth Phys Res B 132:409–417 Bhave TM, Hullavarad SS, Bhoraskar SV, Hegde SG, Kanjilal D (1999) FTIR studies of swift silicon and oxygen ion irradiated porous silicon. Nucl Instrum Methods Phys Res B 156(1–4):121–124 Biaggi-Labiosa A, Fonseca LF, Resto O, Balberg I (2008) Tuning the cathodoluminescence of porous silicon films. J Lumin 128(3):321–327 Biasini M, Ferro G, Monge MA, Di Francia G, La Ferrara V (2000) Study of the structure of porous silicon via positron annihilation experiments. J Phys Condens Matter 12:5961–5970 Bolotov VV, Korusenko PM, Nesov SN, Povoroznyuk SN, Roslikov VE, Kurdyukova EA, Sten’kin YA, Shelyagin RV, Knyazev EV, Kan VE, Ponomareva IV (2012) Nanocomposite por-Si/SnOx layers formation for gas microsensors. Mater Sci Eng B 177:1–7 Buzaneva E, Gorchinsky A, Popova G, Veblaya T, Zankovych S, Boiko Y, Zolotarenko P, Pogorelov V, Bukalo V, Benilov A, Lazarouk S, Beyliss S, Starovoitov A, Senkevich A (2000) Photophysical properties of nano Si/SiO x composites in Al/composite/mono Si structures for green light emitting and photodetector-Schottky diodes. Mater Sci Semiconduct Process 3:529–537 Canham LT (1990) Silicon quantum wire array fabrication by electrochemical and chemical dissolution of wafers. Appl Phys Lett 57(10):1046 Chiboub N, Boukherroub R, Gabouze N, Moulay S, Naar N, Lamouri S, Sam S (2010) Covalent grafting of polyaniline onto aniline-terminated porous silicon. Opt Mater 32:748–752 Choi S-H, Chung H, Shin G-S (1995) Conditions of luminescence degradation or enhancement in porous silicon. Solid State Commun 95(6):341–345 Collins RT, Tischler MA, Stathis JH (1992) Photoinduced hydrogen loss from porous silicon. Appl Phys Lett 61(14):1649–1651 Cullis AG, Canham LT, Williams GM, Smith PW, Dosser OD (1994) Correlation of the structural and optical properties of luminescent, highly oxidized porous silicon. J Appl Phys 75(1):493 Page 8 of 14

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Cullis AG, Canham LT, Calcott PDJ (1997) The structural and luminescence properties of porous silicon. Appl Phys Rev 82:909–965 Ćwil M, Konarski P, Pająk M, Bieniek T, Kosiński A, Kaczorek K (2006) RuO2/SiO2/Si and SiO2/ porous Si/Si interfaces analysed by SIMS. Appl Surf Sci 252:7058–7061 Dalba G, Daldosso N, Fornasini P, Graziola R, Grisenti R, Rocca F (1998) X-ray absorption spectroscopy on light emitting porous silicon by XEOL and TEY. J Non Cryst Solids 232–234:370–376 Dalba G, Daldosso N, Diop D, Fornasini P, Grisenti R, Rocca F (1999) Local order in light emitting porous silicon studied by XEOL and TEY. J Lumin 80:103–107 Dannefaer S, Kerr D, Craigen D, Bretagnon T, Taliercio T, Foucaran A (1996) A positron annihilation investigation of porous silicon. J Appl Phys 79(12):9110–9117 Debarge L, Stoquert JP, Slaoui A, Stalmans L, Poortmans J (1998) Rapid thermal oxidation of porous silicon for surface passivation. Mater Sci Semiconduct Process 1:281–285 Derosa MC, Crutchley RJ (2002) Photosensitized singlet oxygen and its applications. Coord Chem Rev 233–234:351–371 Diener J, Ben-Chorin M, Kovalev DI, Ganichev SD, Koch F (1995) Light from porous silicon by multiphoton vibronic excitation. Phys Rev B 52(12):R8617–R8620 Dimova-Malinovska D, Sendova-Vassileva M, Marinova TS, Krastev V, Kamenova M, Tzenov N (1995) Correlation between the photoluminescence porous silicon and chemical bonding in porous silicon. Thin Solid Films 255:191–195 Du XW, Jin Y, Zhao NQ, Fu YS, Kulinich SA (2008) Controlling surface states and photoluminescence of porous silicon by low-energy-ion irradiation. Appl Surf Sci 254(8):2479–2482 El Houichet H, Oueslati M, Bessaïs B, Ezzaouia H (1997) Photoluminescence enhancement and degradation in porous silicon: evidence for nonconventional photoinduced defects. J Lumin 71:77–82 Fauchet PM (1996) Photoluminescence and electroluminescence from porous silicon. J Lumin 70:294–309 Frello T, Veje E (1997) Time-varying phenomena in the photoelectric properties of porous silicon. J Appl Phys 81(10):6978–6985 Fried M, Polgár O, Lohner T, Strehlke S, Levy-Clement C (1999) Comparative study of the oxidation of thin porous silicon layers studied by reflectometry, spectroscopic ellipsometry and secondary ion mass spectroscopy. J Lumin 80:147–152 Frohnhoff S, Arens-Fischer R, Heinrich T, Fricke J, Arntzen M, Theiss W (1995) Characterization of supercritically dried porous silicon. Thin Solid Films 255:115–118 Fu JS, Mao JC, Wu E, Jia YQ, Zhang BR, Zhang LZ, Qin GG, Wui GS, Zhang YH (1993) Gammarays irradiation: an effective method for improving light emission stability of porous silicon. Appl Phys Lett 63:1830–1832 Fujii M, Minobe S, Usui M, Hayashi S, Gross E, Diener J, Kovalev D (2004) Generation of singlet oxygen at room temperature mediated by energy transfer from photoexcited porous Si. Phys Rev B 70(8):085311 1–085311 5 Fujii M, Usui M, Hayash S, Gross E, Kovalev D, K€ unzner N, Diener J, Timoshenko VY (2005) Singlet oxygen formation by porous Si in solution. Phys Status Solidi (a) 202(8):1385–1389 Fujii M, Nishimura N, Fumon H, Hayashi S, Akamatsu K, Tsuruoka T, Shimada M, Katayama H, Kovalev D, Goller B (2007) Photosensitization of oxygen molecules by surface-modified hydrophilic porous Si. Eur Phys J D 43(1–3):193–196 Galiy PV, Lesiv TI, Monastyrskii LS, Nenchuk TM, Olenych IB (1998) Surface investigations of nanostructured porous silicon. Thin Solid Films 318:113–116 Page 9 of 14

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_52-2 # Springer International Publishing Switzerland 2014

Gardelis S, Bangert U, Hamilton B, Pettifer RF, Hill DA, Keyse R, Teehan D (1996) Chemical nature of the luminescent centre in fresh and aged porous silicon layers. Appl Surf Sci 102:408–412 Gokarna A, Bhave TM, Bhoraskar SV, Kanjilal D (1999) Effect of swift high energy phosphorous ions on the optical and electrical properties of porous silicon. Nucl Instrum Methods Phys Res B 156:100–104 Gongalsky MB, Kharin AY, Zagorodskikh SA, Osminkina LA, Timoshenko VY (2011) Photosensitized generation of singlet oxygen in porous silicon studied by simultaneous measurements of luminescence of nanocrystals and oxygen molecules. J Appl Phys 110(1):013707 Gorbanyuk TI, Evtukh AA, Litovchenko VG, Solnsev VS, Pakhlov EM (2006) Porous silicon microstructure and composition characterization depending on the formation conditions. Thin Solid Films 495:134–138 Gross E, Kovalev D, K€unzner N, Diener J, Koch F, Timoshenko VY, Fujii M (2003) Spectrally resolved electronic energy transfer from silicon nanocrystals to molecular oxygen mediated by direct electron exchange. Phys Rev B 68(11):115405 1–115405 11 Harraz FA, Salem MS, Sakka T, Ogata YH (2008) Hybrid nanostructure of polypyrrole and porous silicon prepared by galvanostatic technique. Electrochim Acta 53:3734–3740 Harris PJ, Bayliss SC, Canham LT, Cottrell S (1997) Implanted muon states in porous silicon. Thin Solid Films 297:84–87 Heitmann J, M€uller F, Yi L, Zacharias M, Kovalev D, Eichhorn F (2004) Excitons in Si nanocrystals: confinement and migration effects. Phys Rev B 69(19):195309-1–195309-7 Huang YM (1997) Positron irradiation: a technique for modifying the photoluminescent structures of porous silicon. Appl Phys Lett 71(26):3850–3852 Huie RE, Neta P (2002) Chemistry of reactive oxygen species. In: Gilbert DL, Colton CA (eds) Reactive oxygen species in biological systems: an interdisciplinary approach. Kluwer, New York, pp 33–73 Hummel RE, Ludwig M, Chang SS, LaTorre G (1995) Comparison of anodically etched porous silicon with spark-processed silicon. Thin Solid Films 255:219–223 Itoh Y, Murakami H, Kinoshita A (1993) Positron annihilation in porous silicon. Appl Phys Lett 63(20):2798–2799 Itoh Y, Murakami H, Kinoshita A (1996) Positron/positronium annihilation in low dimensional silicon materials. Appl Surf Sci 102:423–426 Jacobsohn LG, Cooke DW, Bennett BL, Muenchausen RE, Nastasi M (2005) The role of the chemical nature of implanted species on quenching and recovery of photoluminescence in ion-irradiated porous silicon. J Appl Phys 98(7):076108 1–076108 3 Jacobsohn LG, Bennett BL, Cooke DW, Muenchausen RE, Nastasi M (2006) Ion irradiation of porous silicon: the role of surface states. Nucl Instrum Methods Phys Res B 242(1–2):164–166 Jarvis KL, Barnes TJ, Prestidge CA (2012) Surface chemistry of porous silicon and implications for drug encapsulation and delivery applications. Adv Colloid Interface Sci 175:25–38 Jin J-H, Min NK, Hong S-I (2006) Poly(3-methylthiophene)-based porous silicon substrates as a urea-sensitive electrode. Appl Surf Sci 252:7397–7406 Kanungo J, Maji S, Saha H, Basu S (2009) Stable aluminium ohmic contact to surface modified porous silicon. Solid-State Electron 53(6):663–668 Kanungo J, Maji S, Mandal AK, Sen S, Bontempi E, Balamurugan AK, Tyagi AK, Uvdal K, Sinha S, Saha H, Basu S (2010) Surface treatment of nanoporous silicon with noble metal ions and characterizations. Appl Surf Sci 256(13):4231–4240 Page 10 of 14

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_52-2 # Springer International Publishing Switzerland 2014

Kearns DR (1971) Physical and chemical properties of singlet molecular oxygen. Chem Rev 71(4):395–427 Kempson IM, Barnes TJ, Prestidge CA (2010) Use of TOF-SIMS to study adsorption and loading behavior of methylene blue and papain in a nano-porous silicon layer. J Am Soc Mass Spectrom 21:254–260 Kleps I, Nicolaescu D, Lungu C, Musa G, Bostan C, Caccavale F (1997) Porous silicon field emitters for display applications. Appl Surf Sci 111:228–232 Kleps I, Nicolaescu D, Garcia N, Serena P, Gil A, Zlatkin A (1998) Investigation of porous silicon morphology for electron emission applications. Ultramicroscopy 73:237–245 Knights AP, Kowalski G, Saleh AS, Towner A, Patel MI, Rice-Evans PC, Moore M, Gledhill GA, Nossarzewska-Orlowska E, Brzozowski A (1995) Positron annihilation spectroscopy applied to porous silicon films. J Appl Phys 78:4411–4415 Konstantinova EA, Demin VA, Timoshenko VY, Kashkarov PK (2007) EPR diagnostics of the photosensitized generation of singlet oxygen on the surface of silicon nanocrystals. JETP Lett 85(1):59–62 Konstantinova EA, Demin VA, Kashkarov PK (2011) Photoelectron and photosensitization properties of silicon nanocrystal ensembles. In: Masuda Y (ed) Nanocrystal. InTech, Rijeka, pp 313–348 Koropecki R, Arce RD, Schmidt J (2004a) Photo-oxidation effects in porous silicon luminescence. Phys Rev B 69:205317 1–205317 6 Koropecki RR, Arce RD, Schmidt JA (2004b) Infrared studies combined with hydrogen effusion experiments on nanostructured porous silicon. J Non Cryst Solids 338–340(1):159–162 Koropecki RR, Arce RD, Gennaro AM, Spies C, Schmidt JA (2006) Kinetics of the photoinduced evolution of the nanostructured porous silicon photoluminescence. J Non Cryst Solids 352(9–20):1163–1166 Koropecki RR, Arce R, Spies C, Gennaro AM, Schmidt JA (2007) Role of hydrogen in the photoinduced evolution of porous silicon luminescence. Phys Status Solidi C 4(6):2150–2154 Kostishko BM, Nagornov YS, Appolonov SV (2004) The modification of the properties of n-type conductivity porous silicon by argon ion irradiation. Vacuum 73:105–108 Kovalev D, Gross E, K€unzner N, Koch F, Timoshenko VY, Fujii M (2002) Resonant electronic energy transfer from excitons confined in silicon nanocrystals to oxygen molecules. PhysRev Lett 89(13):137401 1–137401 4 Kovalev D, Gross E, Diener J, Timoshenko VY, Fujii M (2004) Photodegradation of porous silicon induced by photogenerated singlet oxygen molecules. Appl Phys Lett 85(16):3590–3592 Kovalev D, Gross E, Diener J, Timoshenko V, Fujii M (2005) Photoluminescence fatigue effect in luminescent porous silicon induced by photosensitized molecular oxygen. Phys Status Solidi (c) 2(9):3188–3192 Lee K-W, Park D-K, Kim Y-Y, Shin H-J (2005) Investigation of the interface region between a porous silicon layer and a silicon substrate. Thin Solid Films 478:183–187 Lee C, Kim H, Cho Y, Lee WI (2007) The properties of porous silicon as a therapeutic agent via the new photodynamic therapy. J Mater Chem 17(25):2648–2653 Li W, Zhao D, Haneman D (2000) Low-energy electron diffraction from heated porous silicon surfaces. Surf Sci 448:40–48 Li Q, Ricardo A, Benner SA, Winefordner JD, Powell DH (2005) Desorption/ionization on porous silicon mass spectrometry studies on pentose-borate complexes. Anal Chem 77:4503–4508 Liu W, Zhang M, Lin C, Zeng Z, Wang L, Chu PK (2001) Intense blue-light emission from carbonplasma-implanted porous silicon. Appl Phys Lett 78(1):37–39 Page 11 of 14

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_52-2 # Springer International Publishing Switzerland 2014

Loponov K, Goller B, Moskalenko A, Kovalev D, Lapkin A (2010) Efficiency of porous silicon photosensitizer in the singlet oxygen-mediated oxidation of organic compounds. J Photochem Photobio A: Chem 211(1):74–77 Mandal NP, Sharma A, Agarwal SC (2004) Arresting photodegradation of porous silicon by a polymer coating. Solid State Commun 129(3):183–186 Mangaiyarkarasi D, Breese MBH, Sheng OY, Blackwood DJ (2007) Porous silicon microcavities fabricated using ion irradiation. Nucl Instrum Methods Phys Res B 260(1):445–449 Martín-Palma RJ, Pascual L, Landa-Cánovas AR, Herrero P, Martínez-Duart JM (2006) HRTEM analysis of the nanostructure of porous silicon. Mater Sci Eng C 26:830–834 Matsumoto T, Masumoto Y, Nakashima S, Mimura H, Koshida N (1997) Coupling effect of surface vibration and quantum confinement carriers in porous silicon. Appl Surf Sci 113–114:140–144 Maurice J-L, Rivière A, Alapini A, Lévy-Clément C (1995) Electron beam irradiation of n-type porous silicon obtained by photoelectrochemical etching. Appl Phys Lett 66(13):1665 Mehta BR, Sahay MK, Malhotra LK, Avasthi DK, Soni RK (1996) High energy heavy ion induced changes in the photoluminescence and chemical composition of porous silicon. Thin Solid Films 289(1–2):95–98 O’Keeffe P, Komuro S, Morikawa T, Aoyagi Y (1996) Oxygen plasma induced enhancement and fatigue-suppression of the photoluminescence from porous Si. J Non Cryst Solids 198–200:969–972 Pan LK, Ee YK, Sun CQ, Yu GQ, Zhang QY, Tay BK (2004) Band-gap expansion, core-level shift, and dielectric suppression of porous silicon passivated by plasma fluorination. J Vac Sci Technol B 22(2):583–587 Parkhutik V, Chirvony V, Matveyeva E (2007) Optical properties of porphyrin molecules immobilized in nano-porous silicon. Biomol Eng 24(1):71–73 Pavesi L, Ceschini M, Mariotto G, Zanghellini E, Bisi O, Anderle M, Calliari L, Fedrizzi M, Fedrizzi L (1994) Spectroscopic investigation of electroluminescent porous silicon. J Appl Phys 75(2):1118–1126 Pettifer RF, Glanfield A, Gardelis S, Hamilton B, Dawson P, Smith AD (1995) X-Ray excited optical luminescence (XEOL) study of porous silicon. Phys B: Condens Matter 208–209:484–486 Pikulev VB, Kuznetsov SN, Saren AA, Gardin YE, Gurtov VA (2006) Energy transfer under photoexcitation of porous silicon-fullerene nanocomposite in oxygen-containing ambient. Techn Phys Lett 32(2):129–131 Punzón-Quijorna E, Torres-Costa V, Manso-Silván M, Martín-Palma RJ, Climent-Font A (2012) MeV Si ion beam implantation as an effective patterning tool for the localized formation of porous silicon. Nucl Instrum Methods Phys Res 282:25–28 Ruano GD, Ferron J, Koropecki RR (2009a) Reversible ion induced modification of consequent secondary electron emission in porous silicon. Open Surf Sci J 1:46 Ruano GD, Ferrón J, Koropecki RR (2009b) Secondary electron emission in nanostructured porous silicon. J Phys Conf Ser 167:012006 Ruano GD, Ferron J, Arce RD, Koropecki RR (2011) Kinetics of electron induced desorption of hydrogen in nanostructured porous silicon. Phys Status Solidi (a) 208(6):1453–1457 Salonen J, Laine E (1996) The quenching and recovery of photoluminescence in porous silicon. J Appl Phys 80(10):5984–5985 Salonen J, Lehto V-P, Laine E (1999) Photo-oxidation studies of porous silicon using a microcalorimetric method. J Appl Phys 86(10):5888

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_52-2 # Springer International Publishing Switzerland 2014

Schuppler S, Friedman SL, Marcus MA, Adler DL, Xie Y-H, Ross FM (1995) Size, shape, and composition of luminescent species in oxidized Si nanocrystals and H-passivated porous Si. Phys Rev B 52(7):4910–4925 Sehrawat K, Singh F, Singh BP, Mehra RM (2004) Ion beam modification of porous silicon using high energy Au+ 7 ions and its impact on photoluminescence spectra. J Lumin 106(1):21–29 Sham TK, Sammynaiken R, Zhu YJ, Zhang P, Coulthard I, Naftel SJ (2000) X-ray excited optical luminescence (XEOL): a potential tool for OELD studies. Thin Solid Films 363(1–2):318–321 Sickafus KE, Kotomin EA, Uberuaga BP (eds) (2007) Radiation effects in solids, vol 235. Springer, Dordrecht Singh RG, Singh F, Sulania I, Kanjilal D, Sehrawat K, Agarwal V, Mehra RM (2009) Electronic excitations induced modifications of structural and optical properties of ZnO-porous silicon nanocomposites. Nucl Instrum Methods Phys Res B 267(14):2399–2402 Song M, Fukuda Y, Furuya K (2000) Local chemical states and microstructure of photoluminescent porous silicon studied by means of EELS and TEM. Micron 31:429–434 Suzuki R, Mikado T, Ohgaki H, Chiwaki M, Yamazaki T, Kobayashi Y (1994) Positron-lifetime study on porous silicon with a monoenergetic pulsed positron beam. Phys Rev B 49:17484–17487 Tamura T, Adachi S (2009) Photo-oxidation effects of light-emitting porous Si. J Appl Phys 105:113518 1–113518 7 Teo EJ, Tavernier EP, Breese MBH, Bettiol AA, Watt F, Liu MH, Blackwood DJ (2004) Three-dimensional micromachining of silicon using a nuclear microprobe. Nucl Instrum Methods Phys Res B 222(3–4):513–517 Teo EJ, Breese MBH, Bettiol AA, Champeaux FJT, Wijesinghe TLSL, Blackwood DJ (2007) Tunable colour emission from patterned porous silicon using ion beam writing. Nucl Instrum Methods Phys Res B 260(1):378–383 Thompson WH, Yamani Z, AbuHassan L, Gurdal O, Nayfeh M (1998) The effect of ultrathin oxides on luminescent silicon nanocrystallites. Appl Phys Lett 73(6):841–843 Thönissen M, Berger MG, Arens-Fischer R, Gl€ uck O, Kr€ uger M, L€ uth H (1996) Illuminationassisted formation of porous silicon. Thin Solid Films 276:21–24 Timoshenko V (2009a) Light-induced generation of singlet oxygen in porous silicon. In: Baraton M-I (ed) Sensors for environment, health and security. Springer, Dordrecht, pp 125–139 Timoshenko V (2009b) Singlet oxygen generation and detection for biomedical applications. In: Baraton M-I (ed) Sensors for environment health and security. Springer, Dordrecht, pp 295–309 Timoshenko VY, Gareeva AR, Kashkarov PK, Petrov VI, Sieber I, Dittrich T (1996) Stable and efficient cathodo- and photoluminescence from ultrathin porous silicon layers. Thin Solid Films 276:287–289 Tischler MA, Collins RT, Stathis JH, Tsang JC (1992) Luminescence degradation in porous silicon. Appl Phys Lett 60(5):639–641 Torchinskaya TV, Baran NP, Korsunskaya NE, Dzhumaev BR, Khomenkova LY, Sheinkman MK (1997) Photoluminescence and EPR studies of porous silicon. J Lumin 72–74:400–402 Torchinskaya TV, Korsunskaya NE, Khomenkova LY, Sheinkman MK, Baran NP, Misiuk A, Surma B, Dzhumaev B (1998) Two ways of porous Si photoluminescence excitation. Mater Sci Eng B 51:162–165 Torchynska TV, Sheinkman MK, Korsunskaya NE, Khomenkovan LY, Bulakh BM, Dzhumaev BR, Many A, Goldstein Y, Savir E (1999) OH-related emitting centers in interface layer of porous silicon. Phys B Condens Matter 273–274:955–958

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_52-2 # Springer International Publishing Switzerland 2014

Van Buuren T, Tiedje T, Patitsas SN (1994) Effect of thermal annealing on the conduction- and valence-band quantum shifts in porous silicon. Phys Rev B 50(4):2719–2722 Vasin AV, Muto S, Ishikawa Y, Salonen J, Nazarov AN, Lysenko VS (2011) Attribution of white-light emitting centers with carbonized surface in nano-structured SiO2:C layers. Thin Solid Films 519:4008–4011 Voss J (1997) The scanning soft X-ray microscope at Hasylab: imaging and spectroscopy of photoelectrons, photoluminescence, desorbed ions, reflected, scattered and transmitted light. J Electron Spectrosc Relat Phenom 84(1–3):29–44 Wei J, Buriak JM, Siuzdak G (1999) Desorption - ionization mass spectrometry on porous silicon. Nature 399:243–246 Wise M, Sneh O, Okada LA, George SM (1996) Reaction kinetics of H20 with chlorinated Si(111)
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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_28-1 # Springer International Publishing Switzerland 2014

Electrical Transport in Porous Silicon Sanjay K. Ram* Department of Physics and Astronomy, Aarhus University, Aarhus C, Denmark Interdisciplinary Nanoscience Center – iNANO, Aarhus University, Aarhus C, Denmark

Abstract The future development of porous silicon (PS)-based optoelectronic devices depends on a proper understanding of electrical transport properties of the PS material. Electrical transport in PS is influenced not only by each step of processing and fabrication methods but also by the properties of the initial base substrate. This chapter endeavors to chronologically document how the knowledge base on the nature of carrier transport in PS and the factors governing the electrical properties has evolved over the past years. The topics covered include the proposed electrical transport models including those based on effective medium theories, studies on contacts, studies on physical factors influencing electrical transport, anisotropy in electrical transport, and attempts to classify the PS material.

Introduction An important attraction of porous silicon (PS) has been its customizable morphology which can be tailored to change its optoelectronic properties to suit the required application. In case of luminescence, an important property of PS, morphology can be modified to tune the intensity and the peak position of luminescence over a wide range of wavelengths (Marsh 2002). However, the versatile microstructural nature of porous silicon that imparts to it these exciting possibilities is also the main hindrance in the studies of its electrical properties. The inhomogeneous microstructure of PS impedes the comparability between studies of different laboratories (Foll et al. 2002; Bisi et al. 2000). Even within a sample of the material, the complex and inhomogeneous microstructure and crystallites (Islam et al. 2001; Islam et al. 2005) can result in lack of uniformity in the observed electrical properties (Dutta et al. 2002). In the past decades, electrical transport properties of PS have been found to be dependent not only on its microstructure (Kocka et al. 1996a) but also on several factors like the base c-Si selection (Zimin and Bragin 2004), anodization method (Dutta et al. 2002), metal contact formation (Simons et al. 1995; Martin-Palma et al. 2002), annealing (Zimin and Komarov 1998), electrical measurement method (Boarino et al. 2009), etc. In addition, the electrical properties are strongly influenced by external factors such as ambient atmosphere (Zhang et al. 1995) and residual electrolyte (Parkutik 1996). Thus, the understanding of the transport properties of carriers through such a disordered system is challenging. The early research on the electrical transport properties of porous silicon carried out in the 1980s revealed that the resistivity of the porous silicon layer was a few orders of magnitude higher than the original substrate (Beale et al. 1985). While quantum confinement model has been successfully used to explain the luminescence properties of PS, applying it to explain the transport properties of PS has *Email: [email protected] *Email: [email protected] Page 1 of 15

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_28-1 # Springer International Publishing Switzerland 2014

been difficult and less unequivocal. In this chapter, we present an overview of the electrical properties of porous silicon based on the literature published in the last three decades.

Electrical Transport Characteristics and Mechanisms: General Overview The earliest reports on the electrical properties of PS were about the high electrical resistivity in this material. Electrical resistivity in PS is 5 orders of magnitude higher than in intrinsic Si due to the depletion of free carriers. One reason behind the depletion is the widening of energy gap due to quantum confinement and decreased thermal generation of free carriers. The depletion of free carriers can also occur due to their trapping, which occurs during the preparation of PS either because the binding energy of dopant impurities are increased or because of the formation of surface states. Even after etching, the concentration of dopants remains unchanged, although the dopants are in neutral state (Bisi et al. 2000). The electrical conductivity of PS can be dependent on voltage and/or temperature. The commonly observed thermally activated temperature dependence of DC conductivity behavior in PS is expressed by a relation: sðT Þ ¼ s0 expðEa =k B T Þ,

(1)

where activation energy (Ea) and conductivity prefactor (s0) are material property used to explain transport mechanism. In most reports, the value of Ea has been found to be  0.5 eV. This value is half of the energy of the bandgap deduced by luminescence and also comparable to the Ea in intrinsic Si 0.5 eV, suggesting that mechanisms other than quantum confinement are in play as well. All these indicate the disordered nature of the PS skeleton and its local crystalline structure as they are expected to have strong geometrical effects on the conductivity (Bisi et al. 2000). Thus, there has been little consensus on any predominant transport mechanism in PS. This is evident from Table 1 which shows the reported studies on the electrical transport characteristics of PS and the different conduction mechanisms proposed by various workers to explain the observed transport behaviors. In the electrical measurements of PS samples, an important factor is the direction of the current flow, that is, whether the measurement is done in sandwich configuration or in coplanar configuration. Another important consideration is whether the PS material is freestanding or is still supported by the mother substrate.

Contact Phenomena in Porous Silicon The characteristics like linear (ohmic), symmetric (quasi-linear), or nonlinear (rectifying or Schottky types) behavior of I–V characteristics in PS have been reported depending on the type of metal contact (Simons et al. 1995), measuring (Diligenti et al. 1996a), and device configuration (coplanar or sandwich) (Kanungo et al. 2010), whether the PS is freestanding or attached with base substrate, microstructure (Dutta et al. 2002), and thickness of PS layer (Ben-Chorin et al. 1994; Balagurov et al. 2000). Here the term microstructure includes the effect of all the processing methods and the initial base substrate’s properties. An ohmic contact on PS like in any semiconductor material is crucial for the development of a device. Rectifying behavior can originate due to either unstable contact or work-function value of the metal chosen, or it can be due to both. For example, according to the work function of Al, it should provide ohmic contact to PS, but instead it shows a rectifying

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_28-1 # Springer International Publishing Switzerland 2014

Table 1 Summary of electrical transport models described for porous silicon Year of study Reference 1993 Mares et al. (1993) 1993 Koyama et al. (1993) 1994 Ben-Chorin et al. (1994) 1995 Ben-Chorin et al. (1995a) 1995 Lehmann et al. (1995) 1995 Schwarz et al. (1995) 1996 Kocka et al. (1996a, b) 1996 Lee et al. (1996) 1996 1998

Diligenti et al.(1996a) Ray et al. (1998)

1998

Mathur et al. (1998)

1998

Hamilton et al. (1998) Balberg (2000)

2000

PS physical form

Al/PS/Al

Electrical transport model Tunneling between thermally vibrating surface states (Berthelot-type conduction) Band transport

Nanoporous

Al/PS/c-Si/Al

Poole-Frenkel process

Nanoporous

Al/PS/c-Si/Al

Hopping at Fermi level

Mesoporous film (n+) Mesoporous membrane Nanoporous membrane Nanoporous membrane Mesoporous membrane

Al/PS/c-Si/Al

Surface trap dominated

Al/PS coplanar (CP) Au/PS/Au Al/PS (CP)

Transport in nearly extended states and process similar to multiple trapping model Two transport channels: hopping at transport edge and thermionic emission across energy barriers Quantum confinement model

4 probe CP

Tunneling between the Si nanocrystals (Si-NCs)

Al/PS/c-Si/Al

Thermionic emission across spatially varying bandgaps Tunneling of carriers to localized states near band edges, at high temperatures and variable range hopping near Ef at low temperature Coulomb blockade

Macroporous Al/PS/cSi/Al

2001

Mikrajuddin et al. (2001)

2002

Axelrod Nanoporous et al. (2002) Dutta et al. (2002)

2002 2004 2007 2008 2009

PS device form AuCa/PS/cSi/Al

Forsch Nanoporous et al. (2004) membrane Islam et al. (2007) Nanoporous Bouaicha et al. (2008) Islam et al. (2009) Nanoporous

Two transport channel via nanocrystal network and purely amorphous phase Activated conduction in moderate temperature, at higher temperatures, conductivity obeys VogelTammann-Fulcher law Al/PS/c-Si/Al Thermally activated hopping within a fractal network of nanocrystallites Al/PS/c-Si/Ag-Al Generalized effective medium approximation (EMA) using porosity (Po) and uniformity of PS Al/PS (CP) and Poole-Frenkel transport Al/PS/Al Al/PS/c-Si/Al Carrier generation-recombination in depletion region formed on PS side Al/PS/c-Si/Al EMA by considering Si nanoparticles with size distribution embedded in SiO2 and vacuum Al/PS (CP) Mott hopping and Efros-Shklovskii hopping at low temperature

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_28-1 # Springer International Publishing Switzerland 2014

behavior. This is due to the existence of large density of surface states on unmodified PS which create barrier against the current flow. Therefore, methods or techniques like annealing, oxidation, derivatization by organic groups and polymer, nitridation, halogenations, and metal (like Cu, Ag, In, etc.)-induced modification of the porous silicon surface are used to stabilize PS by passivation of defect states in order to obtain a stable and reliable electrical contact to PS (Dutta et al. 2002; Zimin and Komarov 1998; Zimin and Bragin 1999). This topic is the focus of the dedicated chapter “▶ Ohmic and Rectifying Contacts to Porous Silicon” elsewhere in this handbook. Transition from nonlinear (rectifying) to linear (ohmic) behavior of Al contacts on PS depending on postdeposition treatments has been reported by many workers (Dutta et al. 2002; Zimin and Bragin 1999). In some cases, the range of linear part extends only up to few volts, while a broad range of voltages have also been reported for linear part of I–V characteristics. Surface oxidation has been considered a very efficient way of passivating the defects and improving the stability of PS to a large extent. Although it helps in coplanar configuration to obtain ohmic contacts, in sandwich design the contacts may still show improved rectifying behavior because the Fermi level (Ef) gets pinned at the PS/c-Si interface where a large number of volume traps and interface states exist (Islam et al. 2007). Zimin et al. (1995) reported a coplanar Al ohmic contact to n-type PS having a contact resistivity of the order of 103 to 102 O cm for low porosity and low resistivity samples, but they observed a rectifying behavior for high resistive PS, both n- and p-type. Martin-Palma et al. (1999) reported the same for the Al/PS/c-Si/Al sandwich structure, where rectifying behavior can be seen even after prolonged exposure to the atmosphere. Use of noble metals (Pd, Pt, and Ru) (Kanungo et al. 2009) or their ions are also reported to modify and stabilize the surface of PS in order to obtain reproducible metal contacts thereafter.

Characteristics of Current Versus Voltage (I–V) Behavior in Porous Silicon Diode There are two types of diode structures in PS that exist in the literature, structure-1: metal1/PS/c-Si/ metal2 and structure-2: metal1/PS/metal2, where the former is the most widely investigated device structure. The structure-1 leads to three different junction formations at the interfaces of (1) metal1/ PS, (2) PS/c-Si, and (3) c-Si/metal2. The I–V characteristics usually show asymmetric behavior due to current rectification. Typically the junction, c-Si/metal2 is made to be ohmic with negligible series resistance, so that it plays no role in I–V characteristics of the diode. The effect of metal1 in the junction of metal1/PS on I–V behavior is not very clear as discussed in section C. There has been much contention in assigning the possible contribution of metal1/PS and PS/c-Si interfaces to the electrical properties of PS diode structures. In some cases (Pulsford et al. 1994; Giebel and Pavesi 1995), no dependence of the type of metal1 was found, suggesting that the interface of PS/c-Si instead of metal1/PS may be responsible for rectification (Pulsford et al. 1994). This led to a suggestion that the transport of carriers within the PS layer thickness and across the PS/c-Si heterojunction governs the device characteristics (Ben-Chorin et al. 1995b). The I–V characteristics in PS have been contradictorily attributed to both the bulk (PS) properties and junction properties in the reports (Ben-Chorin et al. 1994; Zimin 2000). Some authors have attributed the electrical behavior of thin PS structures to junctions under both reverse and forward bias condition (Zimin 2000; Futagi et al. 1993; Pavesi et al. 1994; Bhattacharya et al. 2000), while others have contributed the electrical behavior under reverse bias conditions to junctions and that under forward bias conditions to bulk conduction mechanisms (Ben-Chorin et al. 1995b). The electrical response of a c-Si/PS heterojunction is determined by the interplay of band edge offsets and density of defect Page 4 of 15

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_28-1 # Springer International Publishing Switzerland 2014

states in the PS layer. Therefore, another possibility is that a large density of states associated with mid-gap defects present in PS pins the Ef and significant band bending and depletion occur only inside silicon rather than at the metal/PS interface (Ben-Chorin et al. 1995b; Pulsford et al. 1993). The thickness of PS layer is also one of the factors that influence the contact behavior. As thin PS layer show rectifying characteristics while thick PS layer show an almost symmetric one, Ben-Chorin (Ben-Chorin et al. 1994) proposed that the rectifying barrier is at the interface between PS and the doped substrate. It was also suggested by them (Ben-Chorin et al. 1994) that the usual diode structure formed by a metal contact, a PS layer, and a doped substrate can be visualized as a series combination of a voltage-dependent resistance and a rectifying barrier. Their study shows that the temperature- and voltage-dependent conductivity relationship follows Poole-Frenkel (PF)-type conduction, where transport mechanism in high fields involves field-enhanced thermal excitation from Coulombic traps: pffiffiffiffiffiffiffiffiffiffiffiffiffi F =F 0 ðT Þ sðF, T Þ ¼ s0 e e Ea kT



(2)

pffiffiffiffiffiffiffiffiffiffiffiffi kT =q F 0 ðT Þ ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffi q =p 0 r In this type of conduction, charge carriers are thermally excited from traps to some transport band, and barrier energy reduction due to the electric field enhances conduction. Charges will move along special paths with the lowest barrier energies, those which offer the lowest resistance (Ben-Chorin et al. 1994). Further, based on their observations that the conductivity scales correctly with the thickness, they ruled out the possibility of space charge-limited current (SCLC) type of conduction (Ben-Chorin et al. 1995a). In another report, a power law behavior, J / Vn/dm with exponent, n ¼ 2 typical of SCLC was observed (Peng et al. 1996). They argued that the total current of the device is dominated by carrier transport in the high-resistivity PS layer which can be modeled as sandwiched between two conducting materials and the band bending at the metal1/PS interface is so small that the Schottky junction can be neglected. Superlinear behavior of current with rise in voltage (n < 2) (Koyama and Koshida 1993) was also observed by Kocka et al. (1996a, b; Fejfar et al. 1995); however, behavior was linear at lower voltages. In addition, Ea obtained from temperature dependence of transport was also found to be field dependent, where Ea decreases at high voltages as expected from the PF mechanism. However, the initial rise in Ea at low voltages (Kocka et al. 1996a) is explained either by assuming a parallel combination of two transport paths with different Ea (through and over the barrier between nanocrystals) of which one is voltage dependent (e.g., PF-type) or by assuming a series combination of Schottky contact and of SCLC controlled by traps inside the nanocrystals. The contacts dominate at low V and the SCLC at high V. Within this model the Ea represents the thermal generation of carriers from the Fermi level to the transport paths. The influence of temperature on the I–V characteristics of PS/c-Si structures was reported by Ref (Theodoropoulou et al. 2004), which also elucidated the mechanisms dominating under different bias conditions. According to this study, for the reverse bias condition, the ohmic bulk resistance dominates at first, and junctional resistance comes into play at a later time. On the other hand, under forward bias conditions, ohmic bulk conduction dominates, but with time gives way to PF-type conduction in the bulk. The time at which the change of mechanisms occurs is a function of temperature, with the change occurring later in lower temperatures. Based on the analyses of reverse I–V characteristics, Islam et al. (2007) proposed that the characteristics of PS/c-Si heterojunctions are found to behave like the Schottky junctions where where F is electric field and

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_28-1 # Springer International Publishing Switzerland 2014

transport is governed by carrier generation-recombination in the depletion region formed on the PS side. Fermi level of c-Si gets pinned to the defect levels at the interface resulting in ln(I) / V1/2. In addition, there are several reports which attributed observed I–V behavior of metal1/PS diodes to the formation of a Schottky barrier between PS and the metal1 (Canham 1997). In a reported case (Ray et al. 1998) of low value of ideality factor, n  1 with low series resistance in the low voltage range, it was proposed that PS/c-Si junction characteristics are controlled by carrier diffusion in the PS and the observed rectification of electron current is due to the barrier between c-Si and PS. However, in most of the cases, high values of ideality factor, n (>5) with either high (Ben-Chorin et al. 1995b; Futagi et al. 1993; Pulsford et al. 1993) or low values of series resistance due to PS layer (Pulsford et al. 1993), suggest most of the applied voltage does not drop on the barrier, but rather on the PS layer, which impedes a determination of reliable junction parameters in PS/c-Si heterojunctions using the conventional analysis of forward I–V characteristics (Koshida and Koyama 1992; Pulsford et al. 1993). According to the report (Koshida and Koyama 1992), tunneling of carriers dominates at high voltages.

Temperature-Dependent Conduction Behavior in Porous Silicon Most of the observations of temperature-dependent electrical conduction in PS indicate that the transport of electrons is thermally activated, with Ea in the range of 0.3–0.7 eV (Koyama and Koshida 1993; Ben-Chorin et al. 1994; Kocka et al. 1996b; Lee et al. 1996; Fejfar et al. 1995; Lubianiker et al. 1996; Lubianiker and Balberg 1997). The measured activation energies can be related to activation of carrier over mobility edges or to typical energy barriers for carrier hopping. In the former, the activation energy indicates the energy difference between the Fermi energy and the mobility edge. In the latter, the activation energy is related to a typical barrier height separating neighboring localized states (Bisi et al. 2000). In addition, disorder-induced localization of free carrier affects the free carrier motion in a way similar to amorphous silicon. However, in many cases, it has also been observed that there is a distinguishable change in the electrical transport behavior for high and low temperature regions usually at some critical temperature (Islam et al. 2009; Zimin 2006; Mikrajuddin and Shi 2000). The Ea is found to have relatively high value in the relatively high temperature region to a low value in the low temperature region (Ben-Chorin et al. 1995a; Islam et al. 2009; Fejfar et al. 1995; Lubianiker and Balberg 1997). However, in some cases, the conductivity is almost temperature independent below 200 K (Koyama and Koshida 1993; Ben-Chorin et al. 1995a). The critical temperature around which the electrical transport behavior changes is found to depend on the size of Si nanocrystals, porosity, their size distribution (Zimin 2006), and their microstructures (Zimin 2006). According to Ben-Chorin et al. (1994), PS is like a disordered assembly of three dimensional quantum wells with large bandgaps of the nanocrystallites (NCs). The transport in PS mainly involves dangling bonds (surface states) near Ef and hopping of carriers takes place in these surface states in the entire temperature range. The thermally generated carriers at the mobility edge do not take any part in the transport, making the bending of bands at the metal1/PS interface irrelevant. On the other hand, Kocka et al. (1996b) assumed that the transport in PS takes two paths – one through the surface grain barrier and the other over the surface grain barrier. This mechanism is field dependent in accordance with PF-type effect. In 2000, Balberg (2000) classified the electrical transport in PS by reanalyzing experimental data collected from various published papers about the temperature dependence of DC dark conductivity, according to Meyer-Neldel rule (MNR) Page 6 of 15

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_28-1 # Springer International Publishing Switzerland 2014

lnðs0 Þ ¼ BMN þ E a =E MN

(3)

where BMN and EMN are MNR parameters. The relationship between conductivity prefactor s0 and activation energy Ea as per Eq. 1 of various PS samples shows two well-separated straight lines suggesting two different mechanisms. One of the lines is similar to amorphous Si, and most of the PS data falling on this line belong to data of high temperature thermally activated region, suggesting extended state transport. The other line, which accumulates most of the data from low temperature regions, represents intercrystallite hopping-tunneling conduction. It must be noted that most measurements in the group similar to a-Si behavior were taken from nanosize porous layers, while the second group obeying the MNR in low temperature regions were low porosity samples. This means two different conduction mechanisms take place in different “parts” of the samples. The amount each of them contributes to the conductivity depends on the sample and the measurement temperature. Nevertheless, most of the currently available models suggest that the temperature dependence of electrical conductivity obeys the Arrhenius relationship with a single activation energy for the entire range of temperatures (Ben-Chorin et al. 1994; Pulsford et al. 1994; Fejfar et al. 1995; Lubianiker and Balberg 1997; Diligenti et al. 1996b). However, according to Mikrajuddin et al. (2001), Arrhenius-type activated conduction behavior may be exhibited in moderate temperature region if both continuous networks of blocked and unblocked sites appear in a PS layer. At higher temperatures, continuous networks of unblocked sites and blocked sites occupying discrete positions can be found in PS site, and conductivity obeys Vogel-Tammann-Fulcher (VTF) law, which can be expressed as s ¼ s0 T 1 exp½B=ðT  T 0 Þ,

(4)

where s0, B, and T0 are constants. Mikrajuddin et al. (2001) derived the VTF behavior by using mean-field approximation of Ising model and used the model to fit published experimental data to validate it. However, at low temperatures carrier conduction is often found to be assisted by variable range hopping (VRH): lnðsÞ / ðT x =T Þm

(5)

where m gives the information about type of carrier conduction and depends on dimensionality (mainly arising from crystallite sizes) and the temperature region. Tx is a constant for the material and often “x” is replaced with the name of transport mechanisms. So far Mott hopping transport where a T-0.25 (m ¼ 0.25) dependence is followed at low temperatures was considered for most of the varieties of PS: nanoporous (Islam et al. 2009), mesoporous (Zimin 2006), and macroporous (Mathur et al. 1998; Ben-Chorin et al. 1995a). Berthelot-type conduction (m ¼ 1) was also reported and attributed to the fractal structure of PS (Mares et al. 1993; Mehra et al. 1998), and variable range hopping has been observed in porousamorphous Si of different compositions (Yakimov et al. 1995, 1996). Islam et al. (2009) critically analyzed their temperature-dependent conductivity data over a wide range of temperatures for the applicability of various transport mechanisms. Their findings suggest extended state conduction for T > 300 K, Berthelot-type conduction when 180 < T < 280 K, Mott hopping in the range 140 < T < 180 K, and Efros-Shklovskii hopping for T < 120 K. A clear cross over from Mott to EfrosShklovskii VRH transport is observed at low temperatures. It is interesting to note that Berthelot-

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_28-1 # Springer International Publishing Switzerland 2014

type conduction in Ref. Islam et al. (2009) was also observed in the similar range of temperature (190–270 K) as in Ref. Mares et al. (1993). Similarly, the temperature range of Mott hopping transport observed in various types of PS is 140–180 K for nanoporous (Islam et al. 2009), 110–200 K for mesoporous (Zimin 2006), and 100–150 K for macroporous (Mathur et al. 1998).

Anisotropy in Electrical Transport of Porous Silicon The cubic lattice structure of c-Si makes it an isotropic optical medium. However, nanostructuring of c-Si by porosification of low-symmetry Si surfaces or the formation of micrometer-sized Si periodic structures converts the resulting c-Si into a strongly birefringent material (see handbook chapter “▶ Optical Birefringence of Porous Silicon”). Anisotropy in electrical properties of PS was first demonstrated by Forsh et al. in a mesoporous system obtained by (110) c-Si (Forsh et al. 2004). They observed substantial decrease in the conductivity measured in parallel to the surface, i.e., (001) crystallographic direction (s||), compared to the conductivity in sandwiched configuration, (110) crystallographic direction (s⊥). The conductivity in both directions follows exponential dependence on V1/2 and s⊥ > > s|| at any voltage. PF mechanism was used to explain the dependence of the conductivity on the electric field in the PS. According to Forsh et al. (Forsh et al. 2004, 2005), an increase in thermal emission of carriers across the potential barriers at the boundaries of NCs can be due to the electric field-induced enhancement of thermal ionization of impurity atoms and reduction in fluctuations of the potential profile (barriers at boundaries of NCs). Previously, the PF mechanism of conduction was observed in sandwiched configuration PS device prepared from (100)-oriented p-type c-Si wafers (Ben-Chorin et al. 1994). The temperature-dependent conduction shows the widening of the gap between the conductivities in both directions at lower temperatures, which means the Ea in (110) direction (Ea)⊥ is smaller than Ea in (001) direction (Ea)||. Apparently, the material has a certain distribution of potential barriers by height. As the length of the percolation path (constituted by Si-NCs) in the perpendicular (110) direction is shorter than that in the parallel (001) direction owing to the shape anisotropy of NCs, the average height of potential barriers in the perpendicular direction will also be lower than that in the parallel direction. This will lead to higher values of s⊥ and lower values of its (Ea)⊥ compared to s||. Earlier work on both freestanding nanosized PS and mesoporous anodized from (100) c-Si show almost isotropic behavior (Kocka et al. 1996a). It was argued that the transport is controlled by the more or less homogeneous (isotropic) “tissue” part of PS, in which c-Si “islands” are embedded. However, in 2006 Borini et al. demonstrated anisotropic behavior in the conductivity of (100) mesoporous by measuring temperature-dependent conductivity of their sample using two different electrode configurations (Boarino et al. 2009; Borini et al. 2006). The authors observed that the electronic transport parallel to the sample surface (s||) is strongly inhibited at room temperature but not along the perpendicular direction (s⊥). This behavior was well correlated with the typical microstructure of the mesoporous where, due to the presence of branched columnar morphology, the s|| pathways are poorly interconnected, with several bottlenecks in which potential barriers are built up. Thus, the transport is strongly inhibited in the longitudinal (parallel to the sample surface direction), while in the transverse direction (perpendicular to the sample surface) the bottlenecks can be easily bypassed following the alternative pathways available. It was also shown (Borini et al. 2006) that such electrical anisotropy can be reversibly removed by heating the samples (increasing temperature from 20  C to 100  C) when s|| increases almost six orders of magnitude equaling s⊥. The rise of temperature allows the charge carriers to overcome the nanoconstrictions (Coulomb blockade due to charges trapped in the nanoconstrictions), opening the longitudinal Page 8 of 15

Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_28-1 # Springer International Publishing Switzerland 2014

percolative pathways. The increase in temperature can remove the Coulomb blockade of a fraction of NCs, until the percolation threshold is reached and exceeded.

Conductivity Versus Porosity in Porous Silicon Effective Medium Theory Approach Among many other microstructural parameters, porosity is one such physical parameter, which is generally used to describe the degree of porous nature of a PS layer. Porosity has been well researched with the fabrication methods and environment. Therefore, if this physical parameter could be correlated with electrical conductivity of the PS using any analytical way, it could serve an important role in tailoring the microstructure to obtain desired device properties. However, not much work has been done to explore this correlation. Effective medium approximation (EMA) as proposed by Bruggeman was used to some extent to determine a correlation between the effective conductivity of the PS layer and porosity of the layer: n X si  seff vi ¼0 si þ 2seff i

(6)

According to Dutta et al., up to certain low values of porosity, this theory worked well but failed to explain the effective conductivities of mid to highly porous layer (Dutta et al. 2002). The major challenge in the pore shape is the possibilities of different pore branching formations during PS growth (Saha et al. 1998). PS layers with “identical porosity” might have different surface-tovolume ratios, leading to different effective conductivities. They followed generalized EMA (GEMA), which accounts for the general form of the shape of the inclusions, where they included the extent of pore branching by “uniformity factor” (Saha et al. 1998): n X i

1=t

1=t

si  seff vi ¼0 ’p 1=t 1=t si þ 2seff 1  ’p

(7)

where ’p is the percolation volume fraction and t is a nonlinearity correction factor. So for spherical inclusions with ’p ¼ 2/3 and t ¼ 1, the GEMA reduces to Bruggeman’s EMA. The theoretically calculated values match well with the experimental values up to the porosity range 70 %. But beyond this range of porosity, the calculated value underestimates the effective conductivity. Similar problem of mismatch between calculated and experimental results of effective conductivity was also observed by Bouaicha et al. in the porosity range exceeding 65 % (Bouaicha et al. 2006): vox

sQD=QW  seff sox  seff sv  seff þ vv þ vQD=QW ¼0 sox þ 2seff sv þ 2seff sQD=QW þ 2seff

(8)

where DE

sQD=QW ¼ sSi e4kT

(9)

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_28-1 # Springer International Publishing Switzerland 2014

They assumed in their theoretical calculation that the nanoporous silicon is formed by three phases: vacuum, oxide, and c-Si nanocrystallites (quantum dots (QD) for nanoporous or quantum wire (QW) for mesoporous structure) having the same mean-size dimension. The contribution of the latter phase in the total electrical conductivity was developed analytically by using the quantum confinement theory. This assumption worked well when the porosity was within 30–65 %, and beyond that theoretical values were too low compared to the experimental ones. However, for large porosities (greater than 65 %), where the PS structure exhibit visible luminescence, they could successfully obtain a perfect agreement between the theory and the experiment for all porosities when they considered that the base medium is vacuum in which silicon crystallites are incorporated (Khardani et al. 2006). This means that for the case of high porosities, the role of porosity is substituted by the quantum dot volume fraction in the fitting procedure. They successfully extended this work to obtain a good correlation between effective conductivity of mesoporous Si material and their corresponding porosity. In 2008, however, they further modified their work by considering the Si-NCs as being formed by multiple-sized crystalline dots (John and Singh 1994) embedded in silicon dioxide and vacuum (Bouaicha et al. 2008): N X sox  seff sv  seff sQDi  seff þ vv þ vQDi ¼0 vox sox þ 2seff sv þ 2seff sQDi þ 2seff i¼1

(10)

As a result, they obtained a good agreement between theory and experiment for all porosities. In this case (Eq. 9), all values of DE are considered including those < DE0. This avoids the tendency of the medium to be an insulator for higher porosities unlike what happens when the PS medium is considered to have three phases with single mean-sized QD.

Percolation Theory Approach A different approach to correlate porous silicon conductivity with material porosity was described in Ref. Aroutiounian and Ghulinyan (2003). In this work, the conductivity was shown to be mainly crystalline for porosities much lower than the percolation threshold at 57 %, while a fractal behavior was observed at porosities near percolation threshold. For higher values of porosities, the conductivity was described as a quasi-one-dimensional hopping. The report concluded that in PS with increasing porosity, at lower temperatures, the dimension of the channels of electrical current flow decrease from 3 to 1, as described by the Mott law for amorphous semiconductors. However, the model results described in this work show some deviation from the experimental results. In spite of workers having presented models to fit their experimental data for a range of porosity, none of the works have attempted to fit the data of others with their models. If these models were tested to fit a wider range of published data, one could hope to find a more comprehensive model that could find wider application to make porosity a useful parameter in predicting the electrical behavior of PS.

Attempts on Classification of Electrical Properties of Porous Silicon The study of a heterogeneous material as PS could benefit greatly from a classification system that would allow a more systematic understanding and correlation of its properties. A wide variety of PS has been classified into some broad microstructural groups based on porosity. However, the same agreement has not been reached in correlating electrical properties of these PS groups to parameters

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Handbook of Porous Silicon DOI 10.1007/978-3-319-04508-5_28-1 # Springer International Publishing Switzerland 2014

related to porosity of the material. This is because electrical current which flows through the Si network depends largely on the size of Si structure and its surroundings and is not directly linked to pore size. In 1997 a comprehensive review of the properties of porous silicon was published by Canham that included a classification proposed by Ben-Chorin (Canham 1997). Ben-Chorin classified PS into two broad and distinct classes, “low porosity” and “nanosized porous Si” material, and explained the plausible electrical transport behavior of these two groups. The “low porosity” material is prepared from highly doped c-Si wafers (resistivity < 1 Ocm), and quantum confinement does not play any role in transport. The nanosized PS is prepared from low-doped c-Si wafers and mostly under illumination. Crystallite size in such material is usually 100 nm (Klobes et al. 2006). For mesoporous silicon with porosity less than 50 % (equivalent pore size