Nanocables and Nanojunctions 1588830624

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Encyclopedia of Nanoscience and Nanotechnology

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Nanocables and Nanojunctions Yuegang Zhang Intel Corporation, Santa Clara, California, USA

Weiqiang Han University of California, Berkeley, California, USA

Gang Gu Molecular Nanosystems Inc., Palo Alto, California, USA

CONTENTS 1. Introduction 2. Composite Nanotubes 3. Nanocables 4. Nanowire Superlattices 5. Nanojunctions Glossary References

1. INTRODUCTION Nanostructures have unique properties due to the quantum confinement effects arising from the reduced dimensions. One-dimensional nanostructures have received more and more attention in recent years following the discovery of carbon nanotubes [1, 2] and related nanowires [3]. Carbon nanotubes are structures made by rolling up graphite sheets into seamless cylinders with diameters in nanometer scale. The structural change from graphite sheets into carbon nanotubes introduces quantum confinement along the direction of the tube circumference. This confinement allows electron transport only along the direction of tube axis, which makes carbon nanotube a good one-dimensional electron system. It is interesting to note that the combination of the quantum confinement effect and the electronic structure of graphite can change the properties of carbon nanotubes more than other materials: A single carbon nanotube can be metallic or semiconductor depending on its diameter or its chirality—a notation on how the nanotube is rolled up [4]. This property gives us great hope to realize intramolecular devices by engineering the atomic structure of carbon

ISBN: 1-58883-062-4/$35.00 Copyright © 2004 by American Scientific Publishers All rights of reproduction in any form reserved.

nanotubes and fusing them together into one molecule. Several theoretical and experimental investigations have shown the junction of a metallic tube and a semiconducting tube could work as a Schottky diode [5–7]. Although this kind of intramolecular junction is of great interest in molecular devices, it is difficult to find a practical method to fabricate the junctions in a controllable way. Comparing with carbon nanotubes, the properties of non-carbon nanotubes and nanowires are normally easier to control because they are primarily determined by their chemical composition instead of geometry. For example, BN is insulator independent of its chirality and diameter, and silicon nanowires are always semiconductors like their bulk counterpart. Similar quantum confinement effects, however, would exist for all these different one-dimensional structures. Construction of nanoscale heterostructures by combining these one-dimensional objects will provide great potential for future quantum devices. Although a rich variety of one-dimensional nanostructures has been discovered in recent years, a general technology to build the one-dimensional heterostructure has not been well established. Unlike the two-dimensional heterostructures such as thin-film semiconductor superlattices that can be precisely engineered by techniques such as molecular beam epitaxy (MBE), phase formation in one-dimensional nanostructures is not straightforward and is hard to control by simple physical deposition. Nevertheless, some very exciting progress has been made in synthesizing heterogeneous nanotubes, nanowires, and their junctions. And some have already show promises in real applications. For example, the method of making carbon nanotube and titanium carbide junction has been used to improve the contact in nanotube field-effect transistors [8, 9]. The heterogeneous one-dimensional nanostructures composed of nanotubes and nanowires can generally be divided

Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 6: Pages (61–76)

62 into two categories according to the direction of nanowire axes relative to the interfaces that separate different materials in the heterogeneous nanostructures. In the first category, the axes of the nanowires and nanotubes are parallel to the interface planes. The phase separation is in the radial direction. Most nanotubes or nanowires in this category have homogeneous phase distribution along the whole length. The nanostructures in this category are also called composite nanotubes or nanowires (different from nanotube or nanowire composite which normally refers to a material made by dispersing nanotubes or nanowires into a matrix material). A representative structure of this category is a “nanocable” that has a core nanowire sheathed by one or more layers of different materials [10]. Composite nanotubes and carbon nanotubes with a second material filled into their hollow cores also belong to this category. In the second category, the axes of the nanotubes and nanowires are normal to the heterointerfaces. This category includes heterojunctions connecting nanotubes and solid nanowires [8, 11] or nanowire superlattices that are composed of alternating semiconductor nanowire segments of different chemical composition [12–14]. Because of the small cross section of nanotubes and nanowires, the junctions formed from them are in nanometer scale. They are therefore called “nanojunctions.” The nanojunctions represent a new kind of structure that could have great potential applications in nanoscale electronics and optoelectronics. This chapter will give a review of the progress on fabrication and characterization of the heterogeneous one-dimensional nanostructures of both categories.

2. COMPOSITE NANOTUBES A composite nanotube refers to a multiwalled nanotube (MWNT) that is composed of shells with more than one kind of chemical composition. There is a big difference between a composite nanotube and compound nanotubes. A composite nanotube has distinguishable phase separation between cylindrical shells that constitute the tube, while all the shells in a compound nanotube consist of a single compound phase. Examples of compound nanotubes are nanotubes of W2 S [15], Mo2 S [15, 16], BN [17–20]. Composite nanotubes synthesized today are normally a mixture of carbon shells and shells of compound materials such as BN. B-C-N nanotubes are widely studied because theoretical calculation predicts that their electrical properties are controlled by their chemical composition instead of the subtle geometrical parameters [21, 22]. Similar to their pure carbon and compound counterpart, composite nanotubes are normally produced by arc-discharge and laser ablation method. The main tool used for characterizing composite nanotube is a high-resolution scanning transmission electron microscope (STEM) equipped with a parallel electron energy-loss spectrometer (PEELS) [23, 24]. The STEMPEELS system has a spatial resolution in subnanometer scale which other analytical microscopes cannot compete with and thus can be used to map the spatial distribution of elements in nanostructures. A linear scan of focused electron beam across an object and measurement of the elemental concentration at each step during scanning is called

Nanocables and Nanojunctions

elemental profiling, which is a powerful technique to determine the structure of one-dimensional nanostructures. Earlier arc-discharge experiments for synthesizing B-C-N nanotubes used anodes made of a mixture of B, N, and C [25–27]. The obtained products contained a majority of carbon and only a very low concentration of boron and nitrogen. Elemental analysis of the nanotubes in the products showed inhomogeneous distribution of B, N, and C in the radial direction with normally B and N rich at outer surface layer [25, 27]. The B:N ratio was close to 1:1. The very low concentration of B and N in the sample suggests these elements are doped into the lattice of carbon nanotubes. There are several possibilities for how these elements are distributed in the tubes. One is that they form homogeneous Bx Cy Nz compound shells that wrap the carbon nanotube cores. Another scheme is that BN exists in domains in the outer carbon shells. The weak signal of the B and N in STEM-PEELS, however, failed to give a conclusive evidence of intershell phase separation. B-C-N nanotubes produced by laser ablation using a target made of a compressed mixture of BN and C powders gave a similar result as arc-discharge [24]. The results obtained by STEM-PEELS show all nanotubes are C rich although the BN and C ratio in the target is 1:1. A lot of BN crystallites were found in co-products. The tubes with a few walls are normally composed of pure carbon. Those with more walls are normally not uniform in diameter along their axis as well as the B and N distribution. The distribution of B and N is rich at the outer surface layers similar to the B-C-N nanotubes produced by earlier arc-discharge method [25, 27]. Again, it is unable to be determined whether the B and N are homogeneously doped into graphite lattice (true ternary Bx Cy Nz phase) or they are mixed binary phases such as BN or BCx embedded in the graphite outer layers (subcomposite tubes). The laser ablated B-C-N nanotubes not only have similar morphologies and element distribution as those in earlier arc-discharged products, they should also share a similar growth mechanism since both methods employ a local hightemperature process to vaporize graphite and BN. From the feature that all obtained composite nanotubes have carbon cores, a two-step growth model has been proposed and is shown in Figure 1 [24]. The first step is the nucleation and growth of pure multiwalled carbon nanotubes. This step occurs in the high-temperature plasma region where B and N can easily diffuse out of the graphite lattice and form more favorable BN crystallites of planar structure. Carbon nanotubes formed in the process are all multiwalled because the presence of B and N atoms in this step could introduce defects on the graphitic wall that are preferable nucleation

Figure 1. Growth model of heterogeneous B-N-C composite nanotubes. Reprinted with permission from [24], Y. Zhang et al., Chem. Phys. Lett. 279, 264 (1997). © 1997, Elsevier Science.

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sites for additional layers and the foreign atoms could also serve as the bridging atoms in lip-lip growth model [28]. The second step is partial coating of B-C-N layers. The growth of additional walls could also initiate from the defects on the carbon nanotube wall. The B and N diffusion process is reduced by lower temperature when the nanotube moves out of the plasma center. The inert gas used in the arcdischarge and laser ablation processes provides an efficient heat exchange medium for cooling down the species and works as a barrier to confine the expansion of the plasma and sustain a high density of multielement vapor for doping BN in nanotubes. The two-step model, however, is not adequate to explain the formation of all B-C-N nanotube structures. One exception is the sandwiched C-BN-C nanotubes, where carbon is rich both in core shells and in outer shells [23]. The growth mechanism for such sandwiched nanotubes is still not clear. One possible model is a simultaneous growth of multiphase shells through a lip-lip-interaction [28]. Another possibility is that the tube flies through three zones in the discharge chamber during its layer-by-layer growth, and the three zones have different concentrations of active elements and thus form sandwiched structure. The sandwiched C-BN-C nanotubes were produced by arc-discharge in nitrogen atmosphere using a HfB anode and a carbon cathode [23]. The use of non-carbon anode increases the B and N concentration in the product because the carbon cathode is only slightly evaporated in the arcdischarge. The high concentration of B and N enables more precise determination of local chemical composition using a STEM-PEELS. Figure 2 shows a high-resolution electron

A D

micrograph of such a B-C-N MWNT and its elemental profile measured by scanning electron beam across the tube [23]. The high-resolution image cannot give any information about phase separation because all possible phases in B-C-N system, BN, BC2 N, BC3 , have very similar crystal structure with graphite, and thus give the same image in the transmission electron microscope. The elemental profile, however, can give detailed phase information of all shells assuming each shell is composed of a single phase. In a cross-section elemental profile of single-phase cylindrical structure, maximum only occurs at the position that corresponds to the inner edge parallel to the probing electron beam. This is because this position has the largest equivalent thickness through which electron beam transmits and loses its energy. The elemental profile of a multiwalled nanotube can be simulated by adding up the profiles of all the shells with each shell as a single-phase cylinder. For a single-phase MWNT, there should be only two maxima in the profile because the probe size of the STEM, 0.5–1 nm, cannot resolve adjacent individual shells in a MWNT. Figure 2B shows four maxima in the C profile and two maxima in the B and N profiles. The positions of the two maxima in the B and N profiles coincide with two minima of the C profile. The result can be interpreted as a C-BN-C three-layered tubular structure as modeled in Figure 2D. Simulated elemental profiles (Fig. 2C) from the model agree with the experimental results very well. The analysis of the C-BN-C nanotubes has provided the first clear evidence of intershell phase separation (with atomic-scale sharp interfaces) in composite nanotubes. More recently, some other composite nanotubes have been reported. One example is the TiO2 -SiO2 composite nanotubes synthesized by sol–gel template method presented in [29]. The coaxial structure is made by sequential formation of amorphous SiO2 nanotubes in the pores of anodic alumina and TiO2 nanotubes within the SiO2 nanotubes. The composite nanotubes have a relatively large diameter of 200–250 nm. Another type of novel composite nanotubes are produced by growth of NbS2 nanotubes on carbon nanotube templates. High-resolution transmission electron microscopy and energy dispersive X-ray analysis revealed that the uniform well-crystallized NbS2 is nucleated and grown from an intermediate phase of NbO2 which is unevenly wrapped on the carbon nanotube templates. A multipoint nuclei site growth mechanism has been proposed to account for nanotube formation [30].

3. NANOCABLES B

C

A coaxial nanocable is generally defined as a multilayered structure with a core nanowire coaxially sheathed by one or more shells of different materials. Nanocables include filled nanotubes and core-shell nanowire heterostructures.

3.1. Filling Carbon Nanotubes

Figure 2. Phase separation in a C-BN-C composite nanotube. Reprinted with permission from [23], K. Suenaga et al., Science 278, 633 (1997). © 1997, American Association for the Advancement of Science.

The discovery of carbon nanotubes has stimulated scientists for a range of potential applications [1], including the filling of hollow carbon nanotubes with chosen materials. Three main approaches have been used for filling carbon nanotubes with both crystalline and noncrystalline

64 materials. The first approach involves inserting the selected materials, either physically or chemically, into the opened carbon nanotubes [31, 32]. The as-produced carbon nanotubes, however, normally have closed caps at their ends. Filling of nanotubes, therefore, always relies on a method to open the ends of the nanotubes. Oxidation and acid treatment are two efficient routes for opening the tips of carbon nanotubes [33–35]. Open nanotubes can be filled via capillary action using either a low melting or low surface tension material (i.e., material with a surface tension lower than the threshold value of 100–200 millinewtons per meter) [36]. The first filling experiment reported by Ajayan et al. used an oxidation method to simultaneously open and fill multiwalled carbon nanotubes (MWCNTs) [31]. Metallic Pb was first deposited onto samples containing MWCNTs produced by arc-discharge method. The samples were then heated in air to about 400
C which is above the melting point of Pb. High-resolution electron microscopy (HREM) images taken after the heat treatment show that MWCNTs are opened at their tips and filled with solid materials (Fig. 3). Using this approach, metal (Pd, Ag, Au, Bi), metal oxides (oxides of Mo, Sn, Ni, U, Co, Fe, Nd, Sm, Eu, La, Ce, Y, Cd), metal chlorides (AuCl, UCl4 , FeBiO3 , HReO4 , MoO2 , and SnO, have all been successfully filled into carbon nanotubes [33, 37–46]. This approach is also useful for filling single-walled carbon nanotubes (SWCNTs or SWNTs) with halides, oxides, carbides, and metals [47–63]. One good example is KI filled SWCNT prepared by heating KI with SWCNT inside an evacuated silica ampoule at the temperature of 954 K for 2 hr. Figure 4 shows a phase image reconstructed from a 20-member focal series of a 1.6-nm-diameter SWCNT containing a KI single crystal [49]. In cross section, the filled crystal can be regarded as a single KI unit cell viewed along the 110 direction of the parent rock salt structure, with the 001 direction parallel to the tube axis. The crystal is well resolved in regions 1 and 3 of the restoration, whereas in region 2, the crystal is rotated about

a

b

c

Figure 3. HREM images of MWNTs filled with Pb compound (dark contrast). Reprinted with permission from [31], P. M. Ajayan and S. Iijima, Nature 361, 333 (1993). © 1993, Macmillan Magazines Ltd.

Nanocables and Nanojunctions

Figure 4. Phase image showing a 110 projection of KI incorporated within a 1.6-nm-diameter SWCNT, reconstructed from a focal serials of 20 images. The upper left inset shows an enlargement of region 1. The lower right inset shows the surface plot of the region 1. Reprinted with permission from [49], R. R. Meyer et al., Science 289, 1324 (2000). © 2000, American Association for the Advancement of Science.

its axis. In the 110 projection, each white spot corresponds to a column of pure I or pure K that is exactly one, two, or three atoms in thickness, as can be seen in the enlargement of region 1 (upper left inset). The different phase shifts arising from different atomic columns are visible when region 1 is displayed as a surface plot (lower right inset) [49]. SWCNT filled with polystyrene and styrene-isoprene is also reported [64]. In 1998, a new type of self-assembled hybrid structures consisting of fullerene arrays inside SWCNTs, so-called “peapods,” were reported [65, 66]. Recent progress in the high yield synthesis opens up a huge possibility for the peapod research. Various types of fullerenes and endohedral metallofullerenes, such as C60 , Gd@C82 , Sm@C82 , La@C82 , La2 @C82 , La2 @C80 , Erx Sc3−x @C80 , Dy@C82 , can now be fully inserted inside SWCNTs [67–80]. Potential applications of peapods range from data storage to quantum cascade laser, quantum computing [81, 82], field-effect transistor [75], and hydrogen storage [83]. The second approach is to simultaneously encapsulate the materials during the formation of carbon nanotubes.

Nanocables and Nanojunctions

65

Such encapsulation has been observed in MWCNTs synthesized by arc-discharge [84–95], electrolysis [96–98], fieldanode activation [99], laser ablation [100], chemical vapor deposition [101–103], and pyrolysis [104] with metals, oxides, carbide, nitrides, chlorides, and sulfides. Among these processes, arc-discharge produces the best-graphitized sheathing MWCNTs, which is also true in synthesizing hollow carbon nanotubes. Arc-discharge is also more versatile for encapsulation of different elements. In this process, a graphite anode is drilled and filled with a mixture of graphite and the chosen element powders. One then proceeds with the conventional Krätschmer–Huffman arc deposition technique. Encapsulation abilities of more than 41 elements have been systematically investigated [39, 85, 87, 89]. This approach is also useful for filling single-walled carbon nanotubes [105, 106]. The third approach is a two-step process. First, nanowires are synthesized and then used as templates to deposit graphite layers on their surface [107, 108]. Zhang et al. deposited thick graphitic layers on the surface of Si nanowires by using a hot filament chemical vapor deposition method. When the temperature was as high as 1000
C, Si cores tended to transform into SiC cores [107]. Han et al. reported the synthesis of GaN nanorods coated with graphitic carbon layers (usually less than 5), including single carbon layers, on preproduced GaN nanorods using a conventional thermal chemical vapor deposition method. The graphitic carbon layers conform to the shape of the GaN nanorods [108].

3.2. Filling Inorganic Nanotubes Analogous to filled carbon nanotubes, inorganic nanotubes, such as BN [17], Bx Cy Nz [25], and sulfide nanotubes [15], have also been filled with different materials. This category of nanocable is usually synthesized through a simultaneous phase separation process during the synthesis of nanotubes and nanowires. The first sophisticated coaxial nanocable structure with insulated shells was produced via laser ablation of a composite target containing BN, graphite, and a small amount of SiO and Li3 N at high temperature [10]. Figure 5 shows the transmission electron microscope (TEM) images of the fabricated coaxial nanocables. They are normally a few tens of micrometers long and 10–100 nm in their diameters. The diameters are quite uniform throughout the whole length of the nanocable (Fig. 5A). High-resolution TEM images in Figure 5B and C indicate that the core of the nanowire has a crystalline phase whose electron diffraction pattern (left bottom inset of Fig. 5B) and lattice image (right bottom inset of Fig. 5C) fit well with that of cubic SiC. The intermediate layer shows an amorphous phase. The outermost layer consists of graphite multiwalls. A detailed elemental profile analysis across the nanocable by STEM-PEELS reveals the Si:C ratio in the core wire and Si:O ratio in the intermediate layer is 1:1 and 1:2, respectively. The outer layer of the nanocable is composed of C and BN with signature of intershell phase separation similar to that of composite nanotubes described in the previous section. The solid phases in the core and intermediate layer region have been

Figure 5. HREM images and selected-area electron diffraction pattern of SiC-SiO2 -(BN)x Cy coaxial nanocables. Reprinted with permission from [10], Y. Zhang et al., Science 281, 973 (1998). © 1998, American Association for the Advancement of Science.

further confirmed by comparing the Si L-edge fine structure with those of SiC and SiO2 reference spectra (Fig. 6). The characterization therefore gives a well-defined composite nanowire structure with SiC-SiO2 -(BN)x Cy multilayers forming a semiconductor-insulator-(semi)conductor coaxial nanocable. BN nanotubes filled with SiC were synthesized by using multiwall carbon nanotubes as templates [109]. This method combines both carbon nanotubes (CNTs)-substitution reaction [110] and confined reaction [111, 112]. Through the CNT-substitution reaction, CNTs react with boron oxide vapor in the presence of nitrogen gas to form BN-NTs, whose diameters and lengths are similar to those of the starting CNTs. The formation of the SiC filling proceeds by the penetration of SiO vapor into the cavity of the nanotubes. The subsequent reaction of SiO vapor with the inner carbon layers or volatile carbon mono-oxide in the interior forms SiC nanowires. The entire length of the nanotubes can be filled. SiC filled (BN)x Cy nanotubes, which are a metal(C)-insulator(BN)-semiconductor (SiC) (MIS)

66 A

Nanocables and Nanojunctions B

Figure 6. (A) A schematic illustration of a coaxial nanocable. (B) Si-L fine structure obtained from a nanocable with electron probe positions as indicated by arrows in (A). Spectrum c is obtained by subtracting a from b, representing a contribution from the core of the nanocable. Reference spectra from pure SiO2 and SiC phases are displayed for comparison. Reprinted with permission from [10], Y. Zhang et al., Science 281, 973 (1998). © 1998, American Association for the Advancement of Science.

structure, also form in the product. Using the same methods, BN and (BN)x Cy nanotubes filled with boron carbide [113] and (BN)x Cy nanotubes filled with iron boride [114] have been prepared. Other similar structures, such as Fe filled C-BN [115] and BN nanotubes [116], Fe-Ni alloy filled BN nanotubes [116], Mo filled BN nanotubes [117], Al2 O3 filled BN nanotubes [118], GaN filled BN nanotubes [119], BN nanotubes filled with SiC and SiN [120], and ZrO2 filled BN nanotubes [121] have also been reported. C60 filled BN nanotubes have been theoretically studied and experimentally achieved [122, 123]. Hofmann et al. found Ni17 S18 filled MoS2 nanotubes combined with sulfide nanowires when they checked nanostructures on processed, sulfur contaminated Mo grids. Carbon is found both in the outer walls and in the core of the wire [124].

Very recently, controlled growth of core-shell and multishell heterostructures of silicon-germanium systems has been approached using a chemical vapor deposition system [143]. This work is based upon control of radial versus axis growth (Fig. 7). Metal nanoparticles could act as a catalyst to direct axis growth by a vapor-liquid-solid growth process (Fig. 7a). Axial growth proceeds when reactant activation and addition occurs at the catalyst and not on the nanowire surface (Fig. 7b). By altering conditions to favor radial growth on the nanowire surface, it is possible to drive conformal shell growth (Fig. 7c). In this work, the radial growth is turned on by moving the growth substrate downstream to favor uncatalyzed surface growth. Subsequent introduction of different reactants and/or dopants produces multiple shell structures (Fig. 7d). By this method, core-shell structures such as intrinsic silicon (i-Si)/boron doped (p-type) silicon, i-Si/SiOx /p-Si, i-Ge/p-Si, Si-Ge, SiGe-Si, and p-Si/i-Ge/SiOx /p-Ge core-multishell nanowire structures have been synthesized [143]. TEM images and elemental mapping show the Si-Ge core-shell structure with a sharp interface (Fig. 8a). Figure 8b shows that the Ge shell is fully crystallized. Figure 8c shows a cross-section elemental mapping of a Si-Ge-Si core-double-shell nanowire. Another interesting experiment in this work is to measure the I–V curve of p-Si/i-Ge/SiOx /p-Ge core-multishell nanowires, which can be used as field-effect transistors. The source, drain, and gate contacts were made by selective etching and metal deposition onto the inner i-Ge shell and outer p-Ge shell, respectively. Another controllable example is the growth of CdSe/ CdS/ZnS core-shell nanorods [144]. Short CdSe nanorods (aspect ratios range from 2:1 to 10:1) were first grown by sol–gel processes [145] and then put into the precursor solution for the growth of CdS/ZnS graded shells at low temperature (160
C) [144]. Interfacial segregation is used to

Au

a

b Core

Cross section

3.3. Core-Shell Nanowire Heterostructures One-dimensional core-shell nanowire heterostructures are another kind of nanocable with radial heterostructures. After high-temperature reaction processes, many semiconductors and metal nanowires are found to have a thin oxide shell, forming a core-shell nanowire. Such configurations are Si/SiOx [125], SiC/SiOx [126–128], Ge/ GeO2 [129], GaN/GaOx [130], and Zn/ZnO [131]. Other similar structures, such as Si3 N4 /Si/SiO2 [132], B/SiO2 [133], Ge/SiO2 [134], Ag/SiOx [135], CdS/SiO2 [136], Zr/ZrO2 [137], La(OH)3 /Ni(OH)2 [138], NiFe/Cu [139], Au/Ag [140], and Cu2 S/Au [141], have also been synthesized using chemical vapor deposition, laser ablation, electrodeposition, and redox deposition techniques. In addition to the inorganic core-shell nanowires, semiconductor/polymer coreshell nanowires, such as CdSe/poly(vinyl acetate), have been produced using low-temperature solution methods [142].

c Shell

d Shell

Figure 7. Synthesis of core-shell nanowires by chemical vapor deposition. (a) Gaseous reactants (red) catalytically decompose on the surface of Au nanocluster leading to nucleation and direct nanowire growth. (b) One-dimensional growth is maintained as reactant decomposition on the Au catalyst is strongly preferred. (c) Synthetic conditions are altered to induce homogeneous reactant decomposition on the nanowire surface, leading to a thin, uniform shell (blue). (d) Multishells are grown by repeated modulation of reactants. Reprinted with permission from [143], L. J. Lauhon et al., Nature 420, 57 (2002). © 2002, Macmillan Magazines Ltd.

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4. NANOWIRE SUPERLATTICES

a

b

–– 1/ (422) 3

–– 1/ (224) 3 –

1/ (242) 3

c

Figure 8. Si-Ge and Si-Ge-Si core-shell nanowires. (a) Cross-section elemental mapping indicating a 21-nm-diameter Si core (blue circles), 10-nm Ge core-shell (red circles), and