Physical Fundamentals of Nanomaterials [1 ed.] 9780124104174

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Physical Fundamentals of Nanomaterials

Physical Fundamentals of Nanomaterials

Bangwei Zhang Hunan University

William Andrew is an imprint of Elsevier The Boulevard, Langford Lane, Kidlington, Oxford, OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright r 2018 Chemical Industry Press. Published by Elsevier Inc., under an exclusive license with Chemical Industry Press. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress ISBN: 978-0-12-410417-4 For Information on all William Andrew publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Glyn Jones Acquisition Editor: Glyn Jones Editorial Project Manager: Naomi Robertson Production Project Manager: Anusha Sambamoorthy Cover Designer: Greg Harris Typeset by MPS Limited, Chennai, India

Foreword Since the beginning of 21st century nanomaterials have always been hotspots of scientific research. This book, with the author’s many years of research and based on the latest original paper, systematically introduces the development of nanomaterials physics, including the main preparation methods of nanomaterials, nanostructure and formation mechanism of materials, in particular, nanomaterials’ mechanics, thermodynamics, light, electricity, magnetism, and other physical properties. Instead of categories, this book introduces various nanomaterials, and elaborates and discusses their common problems, so that have a better understanding readers from a physical perspective on nanomaterials. Author’s analyses of nanomaterials physics theories, book reviews, and the technical progress points are the highlights. This book uniquely stresses the double-edged nature of nanomaterials. This book is to be engaged in nanomaterials research by technical personnel, and can also be made available to institutions of higher education learning in physics, materials physics, materials chemistry, materials science teachers, students of science and engineering, and other professional reference, also for those stakeholders of reference books interested in nanotechnology development.

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Preface Nanomaterials nanomaterials were discovered in the beginning of 21st century and since then humans have entered the era of nanomaterials. This development can be attributed mainly to the advancement of technical and scientific knowledge, among various other reasons, which will be discussed and analyzed in more detail in the first chapter. If adequate attention is paid to nanomaterials and nanotechnology, and to its advantages and disadvantages, to keep it on the right track, it will bring enormous prosperity to the mankind. Nanomaterials, and nanotechnology as a whole, include many elements. A physical base is required to understand and interpret the relevant phenomena of nanomaterials from a physical perspective, to extend the understanding of instructions from various aspects, particularly in-depth research and extensive applications of nanomaterial and nanotechnology, and to provide the basis for knowledge. Therefore, physical base plays an important role in nanomaterials. Since mid-1980s, the author’s research group has carried out a number of research experiments on nanomaterials. The methods used included chemical reduction, mechanical alloying, and uniform precipitation. The research contents included determination of structural, thermal, and magnetic properties in addition to the preparation of nanosized alloy materials. During preparation, conception, and in the process of writing the book, the following two rules were taken into account. First was to ensure the book can reflect the current status and level of academic development, including the history of nanotechnology. To effectively achieve this, it was necessary to have sufficient information from original papers. Author’s group accumulated several materials over the years of their study, but it was still far from adequate to write a book. Thanks to internet, information access has become very fast and easy. Nowadays, nanomaterials are developing so fast and the original papers are so vast in number that it is hard to grasp them completely and without failure, particularly in case of popular fields and subjects. The author has used the most recent and updated information on the subject. Pursuit of further details reflect the current situation and development of nanomaterials and invite the readers to comment. Second was the question of how to write? For many years, I always told myself and my students that in order to write, research papers or books, you have to put yourself in the position of the readers, and conceive and write from their point of views so that it not only reflects results of your research work, but is easier for readers to read and understand. Many books and papers involve issues of context, and not only are read at a glance but can also inspire the views or comments from the reader. There are a small number of exceptions, not so easy to speculate on its intention. During the course of writing this book, the author read the Nobel Prize speech of Nobel Laureate Professor Smalley, one of the discoverer of C60, published in the Mod. Phys. Rev. in 1997. He mentioned in his Nobel Prize speech that he would like to call it as “Discovering the Fullerenes” instead of “The discovery of Fullerenes” for the reason that the discovery was far from complete and scientists around the world are still working on it. He made an honest statement. What an aweinspiring and honest scientist! The physics Professor Huang Kun, who was invited to submit a report on semiconductor superlattice to China Physics Society on the 60 anniversary of the opening ceremony, very clearly

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explained in a few sentences what a superlattice is. People who are not familiar with the term, as well as scholars, were convinced and could grasp the understanding of superlattice structure. This, of course, is a respected scholar. I wish this book to achieve what I have always insisted and I welcome readers’ comments. Intention of writing this book is to present the most important and commonly used theories and methods for the preparation of nanomaterials, structure and formation mechanism of nanomaterials, and physical performance and physical theories of descriptions of nanomaterials. In nutshell, the nanomaterial physics content, coupled with the necessary context of situation, is presented to the readers. The purpose of the book is to give readers the main content about development of nanomaterials to have a clear and comprehensive knowledge, and to be able to increase interest in nanomaterials, so as to help the future generation of scientist in the development of nanomaterials. The author will be glad to accomplish this goal. Application of nanomaterials is important, but they have not been involved beyond the physical basis of nanomaterials range. Negative effects of nanomaterials are also presented to generate the full attention and focus. Although just a little space in the introduction, this book very prominently highlights the double-edged nature of nanomaterials to warn and protect the global village, and to progress in the right direction. The author is grateful to his wife for the tremendous help which enabled author to devote to writing. Without her contribution, it was difficult to complete the writing. Hunan University Library and Information Network Center supplied the literatures and internet connection. Author also received encouragement and support from friends, and would like to express his sincere appreciations. In this book, all pictures, except a small portion, comes from the author or other published authors, the remaining references are permeated by the original copyright holders (institutes, publishing houses, magazines and papers, and web information). The details are listed in the table. The author thanks them here. Bangwei Zhang Mountain Yuelu March, 2008

Preface to the English Version of “Physical Fundamentals of Nanomaterials” Nanomaterials have passed through various stages including a very long budding stage, an initial preparation phase, a rapid development stage, and then eventually entered into the industrial and commercial practical stage by the turn of the century. When I prepared to write this book in Chinese, nanomaterials had just entered into the industrial and commercial practical stage, i.e., the so-called “Nano-hot” period. After studying and analyzing the whole history of the development of substance civilization of human society, I found that a few specific materials have decided the division of such development, and only those specific materials can be used as standard of such division. In this way, till date, the developmental stages of substance civilization of human society have gone through the Stone Age, Bronze Age, Iron Age, Steel/Cement Age, and the Silicon Age based on the dividing standard (specific materials) proposed by the author. Furthermore, I believe that since the beginning of the new century, human society has now entered the Nanomaterials Age. This six-age systems theory on the division of the substance civilization of human society is discussed briefly in the beginning of the book. However, the theory is just a judgment and detailed reasons and explanations are required to conform it. Through 2011 12, I have published three papers on the subject, two in cooperation with Yan Yinjian, Interdisciplinary Description of Complex Systems 10(2) (2012) 114 126; Arts and Social Sciences Journal, 2011: ASSJ-28; and Journal of South China University of Technology (Social Science Edition) 13(6) (2011) 101 109, in Chinese. However, more research is needed to explain the thesis and for the people to understand it more clearly. After the Chinese version of this book was published by Chemical Industry Press, the editor, Mrs. Gang Wu wrote an email to me “inviting you to fill out the Proposal for English book” in October 20, 2011, and that Elsevier was interested to publish the English version of the book. I filled the proposal quickly. The problem was who could act as the translator(s). I had no energy to do that myself, because I just started to write an English Monograph for Elsevier, and it would need several years to finish. Mrs. Wu and I consulted that we, everyone, struggled to find the right translator(s). There was no result for over one year. In January 2013, Dr. Zhang Heng visited me when he returned to China from the University of Connecticut, United States. We were familiar to each other since we worked together for years in a teaching and researching group at the Hunan University, China, before he went to the University of Connecticut. We discussed the problem, and I asked him if he could do this work, and he immediately agreed to. When he returned to the United States on his way through Beijing, he met Mrs. Gang Wu at Beijing Airport, China. They spoke about the publication of the English version of the book with a sincere and dedicated attitude. There are 316 pages in the Chinese text of the book, so the translation is quite a heavy work. Dr. Heng therefore invited Drs. Wu Lijun and Meng Qingping, Brookhaven National Lab, Long Island, New York, United States, and Dr. Tan Zhaoshen to do the translation. Dr. Tan worked for more than 20 years in the research and development of material synthesis, processing,

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characterization, crystalline and microstructure analysis, and their applications in energy conversion and storage. He is now residing in Conroe with his wife Gunan, northern Houston, TX, United States. Even though they are all specialists and familiar with the material sciences and nanomaterials, it is still a big challenge to them. They are all middle-aged scholars, have busy careers and heavy workload, have to take care of their families, and have a lot of other things to do. This translation work is totally an extra task for them. However, they did an excellent job in making the English version of the book available to the English readers all over the world. As the author of this book, I had no contribution to the English version, and I have no words to express my gratitude, but just felt very happy and grateful to Dr. Heng and all of the translators. Without their hard work, this English version could not have been published. I also wish to express my sincere thanks to them. Of course, Mrs. Gang Wu and the Chemical Industry Press, the editors Simon Tian, Glyn Jones, Naomi Robertson in the Elsevier Press and the Elsevier Press also made great contributions to the publication of the English version. I am very grateful to them for their excellent work. Bangwei Zhang Hunan University, Changsha, Hunan, China November 21, 2017

Acknowledgment and Authorization Details for Figures Used in the Book There are totally 335 figures in this book. Besides a small number of figures cited from the author’s published papers or prepared by the author himself, most were cited from the original papers of academic journals that were hosted by academic associations or publishing press, some were cited from online web articles and materials of some academic magazines, and a few were cited from the published books. The author has contacted these associations, publishing press, journals or article authors and obtained their authorization to use these figures. Besides the figures indicated by cited references in this book, particular acknowledgement is expressed here, I deeply appreciate them to let me use their figures in this book. The following are the details of figure authorization. As the references of each figure have been listed in the book text, only the associations, publishing press, journal, or the name of article author who authorized the figure to be used, as well as the figure number in the book have been presented here. To be simple, only the numbering of each figure in each chapter is listed, but not the chapter. For example, Figure 6.8 in Chapter 6 will be expressed as Chapter 6: 8. In addition, please exonerate me from that titles of all authors, such as Professor or Doctor, would not be mentioned. Elsevier: A total of 136 figures were cited. Chapter 2: 18, 21 26, 33, 34, 37 42, 44, 45. Chapter 3: 3 5, 7 9, 15(a) 19, 22, 24. Chapter 4: 3 7, 9 11, 16 19, 21 27, 2938, 39(b), 40 50. Chapter 5: 19, 21. Chapter 6: 1, 3 16(a), 17 20, 24, 26, 27. Chapter 7: 6, 7, 9, 15, 17, 21, 22, 25, 27, 29 31. Chapter 8: 3 7, 19. Chapter 9: 7 10, 12, 31. Chapter 10: 1 4, 8, 13, 16(a), 23 (a), 24(b), 27 29, 31, 39, 41, 43, 44. APS: A total of 46 figures were cited. Chapter 2: 1. Chapter 5: 11, 12. Chapter 7: 1, 2, 5, 11, 18, 20, 23, 28. Chapter 8: 1, 2, 8, 9, 16, 17, 26 28. Chapter 9: 5, 11, 13, 14, 18, 24 26, 28, 32, 33. Chapter 10: 6, 9 12, 19, 26, 32 36, 38, 40, 42. AIP: A total of 49 figures were cited. Chapter 2: 3 5, 11, 20. Chapter 3: 28, 30, 31, 35. Chapter 4: 2, 8, 20. Chapter 5: 13, 23, 24. Chapter 7: 10, 12 14, 19, 24, 26. Chapter 8: 14, 15, 18, 20, 21, 29 32. Chapter 9: 1 4, 6, 15, 16, 21 23, 29(a), 30. Chapter 10: 14, 16(b), 20, 22, 23(b), 30. IOP: Nine figures were cited. Chapter 2: 27, 28, 32, 35, 36. Chapter 3: 26. Chapter 8: 33, 34. Chapter 10: 5. ACS: Nine figures were cited. Chapter 3: 12. Chapter 5: 7 9, 22. Chapter 8: 10 13. RSC: One figure was cited. Chapter 3: 2. JpJAP: One figure was cited. Chapter 3: 21. Springer: Nine figures were cited. Chapter 1: 5. Chapter 2: 2, 12, 17. Chapter 4: 14, 15. Chapter 5: 20. Chapter 7: 8. Chapter 10: 7. IEEE: Four figures were cited. Chapter 10: 15, 17, 21, 24(a). Science: A total of 13 figures were cited. Chapter 2: 6, 15, 16. Chapter 3: 36, 38. Chapter 5: 2 6. Chapter 6: 22, 23. Chapter 7: 16. The following figures were cited not from above literatures, but online network, or information. The author’s name and figure number were listed.

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Chapter 1: C K Ober: 1. P Holister: 2. T Boake: 3(b). W Locke: 3(a). Chapter 2: A Gutsch: 7. S Girshick: 8 10. J R Bleeke and R F Frey: 13. F C Eversteijn: 19. Feng Li: 30. Nanofilm Technologies International: 43. Chapter 3: K Mauritz: 14. Fuzhai Cui: 15(b). Yuanming Zhang: 23. A Inoue: 32. C N R Rao: 34. D L Carroll: 37. Chapter 4: U Ko¨ster: 28. R Z Valiev: 39(a). Chapter 5: B N Dev: 10. F S Bates: 14, 18. Z G Wang: 15. R Sharma: 16, 17. Chapter 6: C Koch: 2. Chapter 7: Q Jiang: 3, 4. Chapter 9: G Chen: 20.

CHAPTER

INTRODUCTION

1

CHAPTER OUTLINE 1.1 Nanomaterial Age .............................................................................................................................1 1.2 What Are Nanomaterials? ..................................................................................................................3 1.3 History of Nanomaterial Development .................................................................................................5 1.3.1 Germination Stage........................................................................................................ 5 1.3.2 Preliminary Preparation Stage ....................................................................................... 7 1.3.3 Rapid-Development Stage ............................................................................................. 8 1.3.4 Industrial and Commercial Application Stage ................................................................ 10 1.4 Importance of Nanomaterials .......................................................................................................... 11 1.4.1 Nanotechnology Programs of Leading Countries ............................................................ 11 1.4.2 Nanotechnology Investment Among Leading Countries................................................... 11 1.4.3 Analysis of the Importance of Nanotechnology .............................................................. 13 1.5 Potential Problems of Nanomaterials............................................................................................... 14 1.6 Purpose of This Book: Fundamentals of Nanomaterial Physics .......................................................... 17 References ........................................................................................................................................... 18

1.1 NANOMATERIAL AGE Humans learned to use fire and stone tools in the earliest stages of their development. The use of tools and equipment moved humanity from barbarism to civilization, and from passive use of nature to active improvement of nature. Humans thus created a brilliant civilization worldwide. The use of tools and equipment has played an indispensable role in developing human initiative and conquering nature. Manufacturing tools and devices is impossible without materials. Hence, materials represent cornerstones of human social development and modern civilization. There would be no development or progress for the human society or its prosperous civilizations and economies without material development. Throughout human history, development and application of materials have represented milestones in social civilization and economic progress. A new class of materials and their applications tends to cause such major changes in human society that people name eras after the materials that define them. Naming the human era from the materials used has been well recognized to the Old Stone, New Stone, Bronze, and Iron Ages but remains discrepancy to the modern history. Fig. 1.1 shows one of Physical Fundamentals of Nanomaterials. DOI: https://doi.org/10.1016/B978-0-12-410417-4.00001-0 Copyright © 2018 Chemical Industry Press. Published by Elsevier Inc., under an exclusive license with Chemical Industry Press.

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the options for eras within the history of human development. Polymers, concrete/steel, and silicon overlap in the Information Age, which begins in today’s era and continues beyond, although it is not named after a material. The international community recognizes materials, energy sources, and information technology as three pillars of modern civilization. The materials used in these three pillars are the basis for advancement in energy and information, as all tools, devices, and systems are manufactured from materials. Previous ages were named after the materials most representative of them. What material would be appropriate to use in naming the current age? Nanomaterials. Thus, our era should be named the Nanomaterial Age. Naming historical eras of human development after the materials that most represent them not only embodies the irreplaceable and fundamental role of materials in human development but also faithfully reflects historical reality. As early as the Spring and Autumn Warring States periods within China more than 2000 years ago, our ancestors gradually mastered the technology required to manipulate iron alloys with higher melting points to achieve better hardness and strengths than that provided by bronze (an alloy of copper, tin, aluminum, and other elements). Iron farm tools, hand tools, and various weapons were widely used and contributed significantly to the development of this civilization. China created a splendid ancient civilization. However, the tendency to rust and brittleness of iron produced severe constraints on social development. Scientific and technical personnel in the newly capitalist world gradually mastered steel by controlling the amount of carbon alloyed with the iron. Steel significantly improved upon the performance and lifetime of iron to stimulate progress and social development. Upon the discovery and application of cement, a new and unprecedented prosperous industrial society emerged. Industrial society allowed people to create a variety of appliances and systems, including cars, trains, aircrafts, and skyscrapers using steel and concrete. However, the era is not named for the objects constructed within it, but rather after the materials that were used to make them.

Stone age (~35,000 years) Bronze age (~1800 years) Iron age (~3300 years)

Polymer age (~50 years) Concrete/Steel age (~60 years) Silicon materials age (~35 years) Information age (~15 years)

5000

4000 3000 2000

1000 BC

0

1000 1900 1960 1990 2010 AD

FIGURE 1.1 One of the options for eras within the history of human development.

1.2 WHAT ARE NANOMATERIALS?

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In the 1950s, silicon research and development led to the discovery and application of transistors and integrated circuits (ICs). This led to the development and widespread use of computers, televisions, and a large number of household electrical appliances. This era of human history was marked by the emerging development of silicon. This era was more active and energetic than that of steel and concrete. Thus, the iron and steel industries are sometimes known as sunset industries. That said, steel continues to play an important role in the world, particularly via recent development of microstructural control of ultrafine grained steel to make “super steel,” which increases the strength of carbon steel to 400 500 MPa. By using this super steel, the 300-m-tall Eiffel Tower could be built to a height of 1500 m. In addition, the theoretical strength of iron is 13,734 MPa, while actual strengths achieved via development and utilization of the metal are only of 1/5th 1/10th of this value. There is significant room to further increase the strength of steel. Thus, even though this material belongs to a so-called sunset industry, research and development are still worthwhile. However, in general and global terms, iron and steel are not as prestigious in the Silicon Era as they once were. Today, the Silicon Era is being replaced by the era of nanomaterials. This change in eras and related improvements in human society will not cause sadness for even the most nostalgic person, as they will produce a higher level of development and further improvements to quality of life. Attaching the names of famous items or some kind of comprehensive abstraction to historic eras of human development, instead of the most represented materials, conflicts with historical facts and leads to misdirection. The computer is an important product of the Silicon Era. Systems have become increasingly fast as new generations of technology have been developed. Critical computer parts include processors and chips, wherein transistor speed and performance are determined by the pace of miniaturization [1]. The rate of transistor miniaturization relies on Si quality, performance enhancements, and improvements in manufacturing technologies. Overall, computer performance relies on Si and process technology. Without Si, the computers that are so common today would not be possible. Since its establishment in the 1960s, the Intel Corporation took advantage of a progressive, in-depth study of Si materials and transistor miniaturization and further developed this into generation after generation of processors. Even though Intel is not the leading computer manufacturer, it has been a leading company in the field for decades. Bell Labs invented the transistor and was intended to be a significant organization but lost its way in the 1960s and is no longer an industry leader. Today, in the post Silicon Era, naming it the information age would not communicate the types of information products that signify the time and would intentionally ignore development and research in nanomaterials. In fact, some countries have applied this type of bias and focused only on development of and investment in information products, reducing or ignoring investments in nanomaterial research and development. Therefore, referring the current time to as the age of nanomaterials is reasonable.

1.2 WHAT ARE NANOMATERIALS? A nanometer is a unit of measurement of geometric dimensions. One nanometer is 1029 m. How small is this? Fig. 1.2 shows lengths ranging from 1 m to 0.1 nm. The figure shows many familiar items so that we can consider them in a concrete manner. A cat is approximately 0.3 m tall; a bee

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100 m

Pin head 1–2 mm

Hair diameter ~50 µm

Red/white blood cell 2–5 µm

M i c r o w o r l d

N a n o w o r l d

DNA ~2 nm width

1m

10–1 m

0.1 m 100 mm

10–2

Cat 0.01 m ~0.3 m 1 cm 10 mm

m

10–3 m

1 mm

10–4 m

0.1 mm 100 µm

10–5 m

0.01 mm 10 µm

10–6 m

1 µm Wave length of visible light

10–7 m

0.1 µm 100 nm

10–8 m

0.01 µm 10 nm

10–9 m

10–10 m

1 nm

Bee ~15 mm MEMS 10–100 µm width

Protein of blood cell Pollen

InAs qunatitative dot

0.1 nm Silicon atom

FIGURE 1.2 Sizes of certain objects, including nanoscale objects.

is about 15 mm in size; the head of a pin takes up 1 2 mm; a microelectromechanical system (MEMS) is built on the scale of 10 100 µm; the diameter of a human hair is 50 µm; a pollen grain is about 10 µm in size; red and white blood cells are about 2 5 µm; the wavelengths of visible light range are from about 0.4 to 0.7 µm; the atomic radii of Au and Si are 0.144 and 0.117 nm, respectively [2]; the size of an indium arsenide quantum dot is 10 nm; and deoxyribonucleic acid

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is about 2 nm. In this figure, the region from 1 to 100 µm is known as microworld, and the range from 1 to 100 nm is called the nanoworld. The earliest definition of nanotechnology was presented by Taniguchi in 1974 [3], who stated that nanotechnology is a very high-accuracy, high-fineness area of product technology, with accuracy that can reach 1 nm. United Kingdom (UK) nanomaterials expert Baoer Hualun [4] stated that nanotechnology is science and technology that is used to fabricate novel materials and microdevices on the scale of thousands of molecules or atoms. One of the most accepted definitions of nanoscale science and technology is one published by the United States (US) Nanoscale Science, Engineering and Technology (NSET) group in 2000. It defines nanoscale science and technology as research and technology development at the atomic, molecular or macromolecular levels, in the length scale of approximately 1 100 nm range, to provide a fundamental understanding of phenomena and materials at the nanoscale, to create and use structures, devices, and systems that have novel properties and functions because of their small and/or intermediate size. Nanomaterials are the foundation of nanotechnology. We can provide a definition for nanomaterials: all materials with dimensions in the range of 1 100 nm that provide new features and functionalities because of their small dimensions. The current literature regarding the definition of nanotechnology is not particularly uniform.

1.3 HISTORY OF NANOMATERIAL DEVELOPMENT This book concerns only artificial nanomaterials and does not discuss natural nanomaterials. The historical development of nanomaterials can be divided into four phases: ancient to 1959, the embryonic stage of development; 1960 90, the initial preparation phase; 1991 2000, rapid development; and post-2001, commercial and industrial application.

1.3.1 GERMINATION STAGE That humans prepared and applied nanomaterials via simple methods can be traced back to ancient civilizations such as those in China. For example, the pigment carbon black was the source of China ink in four treasures of Chinese culture. In October 1982, archeologists at the late Yangshao culture site at Dadi Bay of Wuying village, Qinan County, Gansu province unearthed a house foundation that contained drawings that were painted approximately 5000 years ago. The painting on the ground near the rear wall was drawn with black pigment. Preliminary identification by the Gansu Provincial Museum Antiquities conservation lab shows that the pigment is carbon black. Approximately 5000 years ago, our ancestors knew how to manufacture carbon black and use it to produce pigment. About 1800 years ago, Eastern Han Dynasty Cao Zhi (192 232 BC) wrote a poem that created an explicit record of the use of nanomaterials. His sixth Yuefu poem says “China ink comes from pine smoke, writing brush comes from the rabbit fur; ancients feel bird trail, the text has been changed.” This indicates how ancient people harvested carbon from smoke by burning pine and then mixed it with resin to make ink. Li Tinggui in Southern Tang Dynasty created the famous inkstick produced at Huizhou of Anhui Province. The inkstick produced at Huizhou within Anhui Province was considered the best during the Song Dynasty of Pan Gu, Ming Dynasty of Cheng Junfang, and the Qing Dynasty of Hu Kenwen, Cao Sugong, Wang Jinsheng, and Wang

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Jiean. Carbon black pigments and inks were used proudly in China for thousands of years and are examples of early manufacturing and use of nanomaterials. Since the start of the 20th century, carbon black has been mixed with rubber to increase the strength and wear resistance of the latter. Since there is significant demand for carbon black that can be used in pigments, leading countries produce large amounts of the material. Carbon black is a nanomaterial, and the current-related ASTM standards (N110 N990) define carbon black grades by nanoparticle size. The N110 standard defines an average carbon black particle size of approximately 15 nm. In 1951, the German scientist Kanzig observed BaTiO3 particles sized between 10 and 100 nm in the microregion of polarity [5]. This shows that people had begun to experiment with nanomaterials. During this long period, awareness of nanomaterials was quite shallow, and their use was sporadic. One very important event within this period signaled a conscious desire to explore nanomaterial preparation—a classic report by the famous theoretical physicist and Nobel Award winner Feynman on December 9, 1959 at a US Physics Meeting held at the California Institute of Technology (CIT). Feynman stated, “there is plenty of room at the bottom” [6]. In this famous speech and article made with a wide range of references, Feynman offered a predictive review of several aspects of nanomaterials and related technologies. First, he asked why we can’t save the 24-volume British Encyclopedia on a tip of a pin. In his view, this could be achievable. He even calculated that the encyclopedia would have to become only 25,000 times smaller. How does one achieve this? Feynman’s vision was that small machines could be used to manufacture even smaller machines, etc., until molecular machines are manufactured. This is often referred to as the topdown approach. Feynman made clear that we might be able to arrange atoms according to our own needs. The ability to do this would be a great achievement! Feynman expected to invent better electron microscopes with the ability to see individual atoms and expressed wonder at the potential to view individual atoms clearly. Feynman also said that the people of the 1960s would be blamed by the year 2000 if no one carried out additional serious research within the field of nanoscale science. However, the historical record shows that most mainstream scientists were skeptical of Feynman’s warning. This situation did not change significantly until the early 1980s. In 1981, Massachusetts Institute of Technology (MIT) Professor Drexler inherited Feynman’s ideas and continued to promote the study of nanotechnology [7]; however, this was not recognized by the mainstream scientific community until the early 1990s. Feynman’s initiative had been shelved for nearly 30 years. Zyvex Corporation lead researcher Merkle wrote the “It’s impossible” article, a detailed analysis of why Feynman’s initiative on nanotechnology was not accepted by scientists and reached an incorrect conclusion. As a theoretical physicist working with less-developed technology in 1959, Feynman was able to make profound forecasts regarding nanotechnology. Today, Feynman’s vision has been achieved. In 1981, Binnig and Rohrer of IBM Zurich Research Labs invented the scanning tunneling electron microscope (STM), followed by the atomic force microscope in 1986, which could both see and manipulate atoms on a metal surface. In 1989, IBM’s Foster used an STM to directly manipulate 35 Xe atoms, successfully writing the letters “IBM” on an Ni substrate. Direct human manipulation of atoms was a great discovery. Some of Feynman’s other ideas such as molecular machines are still being implemented. Moreover, Feynman’s dream of storing the Encyclopedia Britannica on the tip of a pin via manipulation of atoms is entirely possible. Some even estimate that the entire

1.3 HISTORY OF NANOMATERIAL DEVELOPMENT

7

collection of the US Library of Congress can be stored within an Si chip 0.3 m in size. The physical theories and predictions made by this outstanding physicist have once again proven to be both correct and powerful.

1.3.2 PRELIMINARY PREPARATION STAGE After a long germination period, the initiative created by Feynman in 1959 as a call for advances in nanomaterials stumbled into its second stage of development. This stage was characterized by a limited following in the mainstream scientific community. Only a few scientists studied the subject in a fragmented, decentralized, and slow manner. Thus, this should be viewed as a preparation period. This preparatory stage took place over the course of 30 years, but we will briefly describe only the most important events that took place within it. In 1961, Japanese scientist Kubo conducted a theoretical study of quantum size effects associated with metal nanoparticles [8]. With the decrease of the number of atoms in the particle, the electron energy levels near the Fermi level split from their former continuous state to discrete states, with the average spacing between the electron levels inversely proportional to the number of electrons in the particle. When level spacing is greater than heat energy, magnetic energy, static power, photon energy, and condensed energy from the superconducting state, so-called quantum effects such as the famous Kubo effect are produced. These effects are different from those experienced by macro-sized objects. Kubo theory plays a role in promoting experimental research on nanoparticles. In 1970, Benjamin invented the mechanical alloying (MA) method of alloy powder preparation [9]. MA is performed by inducing a solid-state reaction between elemental powders via hard ball milling and is also referred to as the ball-milling method in the literature. Nanomaterial research grew after the start of the 1980s. In that decade, the most prominent work was published by the Gleiter group [10] regarding the preparation of metal nanopowders that could then be compressed into bulk materials and by the Smalley Group [11] on the discovery of C60. Work by the Gleiter Group that was published in 1986 produced a breakthrough in nanomaterials research. In 1985, Kroto, Smalley, Curl, et al. prepared carbon atom clusters by using a laser to heat and vaporize graphite electrodes in toluene. Mass spectrometry found lines from C60 and a few from C70. The study also found that C60 has a closed structure made from 60 carbon atoms. It includes 12 pentagons and 20 hexagons and is structured like a football. C60 is known in the literature as a Bucky ball, and the related series of materials are known as Fullerenes. This name is borrowed from that of Richard Buckminster Fuller (1895 1983), an architect who invented “high-energy aggregate geometry,” which has been used to build polyhedral vaults. Moreover, Kroto published an article with the architect’s name in the title: “C60: Buckminsterfullerene.” Fig. 1.3A shows the structure of C60, and (B) shows the US Pavilion in the 1967 Canada Montreal World Exposition Fair. The building is 60 m tall and the two structures shown in Fig. 1.3A and B are quite similar. Because Kroto, Smalley, and Curl first discovered C60 and determined its structure, they won the Nobel Prize in Chemistry in 1996. In the mid-1980s, Denmark van Wonterghem and others at the Technical University of Denmark as well as their UK collaborators [12] prepared amorphous alloy nanoparticles via chemical reduction. This is a very economical method of nanopowder preparation.

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CHAPTER 1 INTRODUCTION

FIGURE 1.3 C60 structure (A) and the US Pavilion in the 1967 Canada Montreal World’s Exposition Fair (B).

1.3.3 RAPID-DEVELOPMENT STAGE Through development and preparation, nanomaterials gradually established a reputation in the scientific community, winning the attention of more and more scientists. Thus, nanomaterials entered their rapid-development stage. In the last few years of the 20th century, some have referred to this as “nanofever.” The rapid development of nanomaterials both firmly established their position and identified possible applications. The rapid development associated with this stage can be supported by the total number of papers published and patents issued. Fig. 1.4 shows the total annual distribution of nanotechnology papers worldwide from 1991 to 2000. The rates of publication are growing quickly. There are no statistics available for the time period before 1990, which suggests that such publications were rare. Fig. 1.5 shows the distribution of nanotechnology patents among 14 leading industrialized countries between 1976 and 2002, as determined by the group led by Roco of the US National Science Foundation (NSF) [13]. This survey report is informative because it includes distribution by country and by subject and also identifies patents by large companies in the United States. The trend is similar to that shown in Fig. 1.4. Prior to 1990, very few patents appear worldwide. Since 1990, and especially after 1995, the rate of patent issuance increases dramatically. During this time period, scientists conducted a wide range of research, identification, and exploration activities regarding nanomaterial performance and applications. Because there are so many research results, we can only discuss a few of them. One important signal that an academic field has become established is its ability to support subject-specific international academic meetings. In July 1990, Baltimore held the first International Conference on Nano Science and Technology in conjunction with the fifth International STM

1.3 HISTORY OF NANOMATERIAL DEVELOPMENT

9

FIGURE 1.4 Nanotechnology papers published worldwide by year from 1991 to 2000.

FIGURE 1.5 Annual distribution of patents issued between 1976 and 2002 within 14 major industrial countries [13].

Conference. This conference presented “nanomaterial science” and “nanobiological” studies as named subjects. This clearly marks the official birth of nanomaterials science as a discipline. In 1991, Japanese scientist Iijima discovered carbon nanotubes (CNTs) using high-resolution electron microscopy [14]. The nanotubes Iijima discovered were multiwalled carbon nanotubes, which present spiral structures along their cylindrical axes. Two years later, Iijima [15] and Bethune et al. [16] independently observed single-walled carbon nanotubes (SWCNT). Thus, the study of CNTs began. In 1996, Sandia National Laboratory announced that they had created IC-controlled smart MEMS. The scientists embedded electric motors into thin, etched channels, creating an Si chip only 1 mm2 with entire MEMS embedded. In 1997, Australia’s Cornell and others [17] created biometric sensors by combining a biological discrimination mechanism with physical conversion technology. In 1998, the Dekker Group [18] and Martel et al. [19] developed a carbon nanotube field emission transistor. In November 1999, Yale University announced on the Internet that a research coalition led by Reed of Rice University had developed molecular-scale storage for the first time. In August 1999,

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CHAPTER 1 INTRODUCTION

Chou of Princeton University [20] found that one can directly form arbitrarily shaped polymer microstructures without use of a set of common etch platemaking technology, including exposing, chemical developer, and etching photoresist. Such kind of lithography induced self-assembly technology plays an important role in polymer-based electronic and optoelectronic devices. On January 21, 2000, US President Bill Clinton made a famous speech on nanotechnology for the Nanomaterials Physics Foundation. During this speech at the CIT, he stressed that the United States would implement its National Nanotechnology Innovation (NNI) program. US authorities in science and technology had prepared this for many years, with the goal of enabling the United States to be a global science and technology leader. The plan was submitted to Congress in Autumn of 2000 and approved in November. Thus, key legislation was created to help the United States lead the field of nanotechnology. During this period, China made significant progress in nanomaterials and nanotechnology. In 1993, the Chinese Academy of Sciences Beijing Vacuum Physics Laboratory successfully wrote the word “China” by manipulating atoms. This marks the entry of China into the field of nanotechnology. In the first half of 1999, Beijing University assembled SWCNTs standing on metal surfaces for the first time in history and developed the world’s finest STM probes.

1.3.4 INDUSTRIAL AND COMMERCIAL APPLICATION STAGE Basic research on nanomaterials began to take place in quantity at the beginning of the 21st century. Since then, nanomaterials have found their use in industrial and commercial applications. Not all nanomaterial applications have been explored, but gradual progress is taking place. Of course, because of so-called nanofever promotion, some people believe that nanomaterials will be everywhere soon, and various products have been advertised as “nano” regardless of whether they contain nanomaterials. This misuse of terminology has nothing to do with science. The Chairman Roco of the NSET Subcommittee of the United State National Science and Technology Council (several US nanotechnology files come from this individual) published statements in 2002 that argue that industrial and commercial nanotechnology applications can be divided into four stages. Phase I began in 2001 and was referred to as the passive nanostructure phase. It focused on applications of nanomaterials as coatings, nanoparticles, nanostructured materials, polymers, and ceramic materials. Typically, these are not active nanomaterial applications. Phase II started in 2005 and was referred to as the active nanostructure stage. It included nanomaterial applications such as transistors, amplifiers, actuators, and applications of adaptive mechanisms. Thus, active nanomaterial applications were employed. Phase III started in 2010 and was referred to as the three-dimensional nanosystem stage. It involves nonuniform nanostructures and self-assembly of nanomaterials. Phase IV is expected to start in 2020 and was referred to as the molecular nanosystem stage. It involves nanomaterials used biological simulations and design of new applications for nonuniform nanomolecular systems. If Roco’s prediction is correct, the most comprehensive nanomaterial applications will not appear until 20 years later.

1.4 IMPORTANCE OF NANOMATERIALS

11

1.4 IMPORTANCE OF NANOMATERIALS The importance of nanomaterials can be seen from advances in national and regional awareness of nanotechnology and large investments in the field.

1.4.1 NANOTECHNOLOGY PROGRAMS OF LEADING COUNTRIES The “National Nanotechnology Initiative” proposed by the United States became active in 2000 but actually began as early as 1996. Sector discussion, argumentation, project directions, and implementation plans were publicly announced in January 2000 by President Clinton after several years of development. This is attributed to the senior US scientific sector’s highly attention and repeal stressing to nanomaterials and technology in many years. For example, the US NSF Director, who also served as the Presidential science adviser, Dr. Neal Lane, stated at a congressional hearing in April 1998, “If I was asked an area of science and engineering that will most likely produce the breakthrough of tomorrow, I would point to nanoscale science and engineering.” In March 1998, the President’s previous adviser of science and technology, Dr. Gibbons, claimed nanotechnology as one of five key technologies for the 21st century economic development. In fact, a US NNI paper was prepared with a subtitle “to lead the next industrial revolution.” However, this subtitle was removed before it was submitted in July 2000. On January 16, 2003, the US Senate passed the “21st Century Nanotechnology Research and Development Exhibition Act.” On May 9, 2003, they adopted a “2003 Nanotechnology Research and Development Act” as well. All these illustrate a strong US emphasis on nanotechnology. Either at nearly the same time or sometime later, the world’s leading countries and regions followed the United States to create programs for nanoscience and nanotechnology development at the national or regional levels.

1.4.2 NANOTECHNOLOGY INVESTMENT AMONG LEADING COUNTRIES Since 1997, the United States has made significant nanotechnology investments to ensure that it has a leading position in nanoscience and technology. As shown in Table 1.1 [21], this investment has increased 11 times between 1997 and 2006. In addition to the United States, many world’s major countries and groups of countries, including the European Union (EU), Japan, and China, have invested in nanotechnology. Data on these investments are presented in Table 1.1 [21]. Such investments have increased year over year since 1997. However, as shown in Fig. 1.6, investment growth slowed before 2000. However, the pace accelerated after 2000, amply demonstrating that the world was focused on nanoscience and technology at the beginning of the new century. Nanotechnology has been important to China since the mid-1980s. The government’s science and technology administration established the “Climb Program” and other related important projects. Subsequently, investments in nanoscience and technology from the Ministry of Science and Technology and other relevant departments led to joint issuance of a national program for nanotechnology development (2001 10). This requested forming the NNI system step by step within

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Table 1.1 Research and Development Investment in Nanotechnology Among Major Industrialized Countries and Country Group [21] ($100 Million) Country/ Region USA EU Japan Othersa Total Relative to 1997 (%)

1997 1.16 1.26 1.20 0.7 4.32 100

1998 1.90 1.51 1.35 0.83 5.59 129

1999 2.55 1.79 1.57 0.96 6.87 159

2000 2.7 2.0 2.45 1.1 8.25 191

2001 4.65 2.25 4.65 3.8 15.35 355

2002 6.97 4.0 7.2 5.5 23.67 547

2003 8.62 6.5 8.0 8.0 31.12 720

2004 9.89 9.60 9.0 9.0 37.49 866

2005 12.0 10.50 9.5 10.0 42.0 972

2006 13.03

2007 b

12.78c

a Other countries and regions include Australia, Canada, countries within Eastern Europe, Israel, China, South Korea, and Singapore. b For 2006, the US NNI demanded nanotechnology investments of $1054 million. The table shows the real estimated value. c This is the 2007 USA NNI requested total nanotechnology investment, which should increase by 21.3% relative to the amount provided in 2006.

FIGURE 1.6 Nanotechnology investments in major industrialized countries [21].

the “Tenth Five” period. After a demonstration period in 2001, the “Basic research in nanotechnology” program was formally activated in 2002. On March 22, 2002, a “National Nanoscience Center” was unveiled at the Institute of Chemistry. In March 2003, authorities announced that the state would invest about $30 million to establish the National Nanoscience Centers via the Chinese Academy of Sciences, while Chinese Academy of Sciences nanotechnology center, Peking University and Tsinghua University applied as the initial launch organization. As China is not a developed country, there is a large difference in nanotechnology investment in China compared with in the developed countries.

1.4 IMPORTANCE OF NANOMATERIALS

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1.4.3 ANALYSIS OF THE IMPORTANCE OF NANOTECHNOLOGY Why should so much importance be attached to nanotechnology worldwide? It is driven by human understanding of nature and the physical world. For thousands of years, but in particular via research and production activities that have occurred over the past few centuries, human knowledge of macroscopic and microscopic aspects of nature and the physical world have produced significant advances. Many mysteries have been uncovered, and some theories can successfully explain natural phenomena. As the human understanding of the macro- and microphysical world has become deeper, it has become clear that macro- and microvisions of the material world are not complete. There is a gap between the areas of 1 µm and 1 nm, because the area above 1 µm is macro, whereas dimension of less than 1 nm is considered microsized. The largest known atom in the periodic table is Cs, with an atomic radius of 0.262 nm. Scientists have discovered that objects within this size range exhibit quantum coherence phenomena. Thus, they can be referred to as mesoscopic. The range from 1 to 100 nm is the nanoscale area that has been defined above, while the range from 100 to 1000 nm is referred to by physicists as the mesoscopic region. The mesoscopic region includes atomic clusters and submicron objects. This book is focused on discussion of nanomaterials; mesoscopic issues are not discussed in depth. Because nanoscience is not yet fully developed, there is a significant amount of work remaining for scientists to do. For our understanding of the nanoscale world to become deeper, new discoveries, principles, and theories must be developed. Thus, the wonders of the nanoscale world will be shown to humanity, beyond what has been revealed by existing nanotechnology research. Some of the needs of the next industrial revolution follow. Today’s industry is fairly well developed and capable of producing a wide range of products to satisfy the needs of the public. However, there is no doubt that industry still has many problems and needs people who can innovate and develop solutions to these problems. In many cases, product quality is not high and product selection is too small. Labor productivity is relatively low and consumes too many resources. There is significant harm to the environment, and sustainable development practices are not always followed. Thus, there is a need for deeper knowledge of nature and consumer desires. Early indications and research results suggest that nanomaterials and technologies are likely to drive the next industrial revolution. The US NNI forecasted that nanotechnology would produce more than $1 trillion of global gross domestic product per year from 2010 to 2015. In the next 10 years, the annual output from nanomaterials and related processing steps will reach 340 billion dollars. Within 10 15 years, nanotechnology will produce semiconductor-related industrial output of 300 billion dollars annually, and half of new medical products will rely on nanotechnology, resulting in an annual impact of 180 billion dollars. By this time, nanocatalytic oil and chemical processing will produce 100 billion dollars annually, and industries related to air will produce 70 billion dollars per year of nanotechnology-related output. The measurement and simulation industry will expand by 22 billion dollars per year. Another benefit of nanotechnology is to improve the environment and the quality of life, increasing the human life. Due to insufficient knowledge, policy mistakes, or various other ideological deviations during the previous process of industrialization, various environmental problems have appeared. Examples include the appearance of a hole in the ozone layer, global climate change, soil and water pollution, desertification, depletion of minerals, and declines in natural

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resources. All these problems must be resolved. Only in this way can resources be controlled at a reasonable level or new alternatives created to ensure the sustainable development of human society. One can imagine healthier agricultural developments that provide enough nutritious food for everyone, and medication to protect people from disease, or to help them to heal rapidly. Preliminary results suggest that nanotechnology will play an important role in this regard. Roco anticipated that nanotechnology development and applications would increase energy output by 45 billion dollars per year over the next 10 years, while reducing lighting energy costs by about 10% ($100 billion), and also increasing the health care product output by 31 billion dollars. Finally, nanotechnology can help us to fight terrorism and support the need for increased national and human security. After the 9/11 terrorist incident, fighting a global war on terror became a top priority. Nanotechnology may make a difference in rapid detection of harmful substances, terrorist containment, and protection of counter-terrorism officers and soldiers. In this regard, the United States is ahead. In March 2002, the US military and MIT formed a new Institute for Soldier Nanotechnology (ISN) and signed a 5-year “Super soldier” plan [22]. The military provided a total of $50 million of funding for the ISN, and various industrial partners provided an additional $40 million. The ISN developed nanomaterials and devices to ensure that future combat soldiers are less vulnerable to enemy attacks and injuries, as well as to environmental threats, thus greatly improving their ability to survive. Its fundamental idea is that of creating the 21st century of clothing for soldiers that combines technology with light weight and volume and is comfortable to wear. This type of clothing will be made from molecular materials that are almost invisible, soft, and as light as paper but with the ability to become as hard as armor if a soldier breaks a leg. The goal is to replace the 40-pound (18 kg) bulletproof clothing currently worn by the US soldiers. The ISN has proposed that the weight of equipment carried by current US soldiers could decrease from 125 145 (57 66 kg) to 45 pounds (20 kg).

1.5 POTENTIAL PROBLEMS OF NANOMATERIALS History has shown that science and technology can be double-edged swords. There is no doubt that they bring humanity significant benefits. However, science and technology can cause confusion and other problems, and even hazards and disasters. Automobiles have brought significant benefits and helped human civilization to progress, but burning gasoline generates exhaust, air pollution, and climate change. Refrigerators and air conditioners provide convenience and comfort, but the use of Freon causes the holes in the ozone layer, resulting in increased solar ultraviolet radiation and increased risk of skin disease. Development of fission and other nuclear technologies has deepened human understanding of the subatomic world, provided cheap nuclear power, and created convenient ways to diagnose diseases. However, nuclear fission and nuclear technology are the basis for the atomic bomb. Likewise, many medicines serve the needs of patients but have side effects. The petrochemical industry produces plastic products and films that bring enormous convenience and lifestyle benefits, but the growth in use and mass production of nondegradable plastics has polluted the environment in a manner that is very difficult to control. Development of cloning technology will undoubtedly change the world but has serious ethical problems and the potential for unpredictable disasters if used for human cloning. Of course, the adverse effects of science on

1.5 POTENTIAL PROBLEMS OF NANOMATERIALS

15

technology can occur in different ways, some of which are small. Moreover, serious problems are more likely to occur when people apply the results of scientific research without appropriate research or preparation. In these cases, one might not know what they need to control or prevent. With any kind of new technology, specific studies should be carried out before large-scale implementation. This way, precautions can be taken against aspects of science and technology that might adversely affect people or the environment. Based on the above discussion, it is expected that people will ensure that nanoscience and technology serve the human interest without harm. Below, we will describe the potential risks of nanotechnology with the hope of avoiding repetition of the mistakes of the past. The 25th annual meeting of the American Chemistry Society was held in New Orleans, Louisiana from March 23 to 27, 2003 with participation of about 11,000 chemical and materials scientists. Three groups of scientists at the meeting reported on the preliminary toxicity tests performed on metals, ceramics, and organic nanothin films [23]. Lam of National Aeronautics and Space Administration Johnson Space Center and Warheit of DuPont groups reported that nanotubes damage lung tissue in mice. Both these research teams discovered that CNTs induce granuloma, which is necrosis complex of nanoparticles and living tissue. However, the Warheit research group reported that the inflammation abated in 3 months later. Both sets of researchers warned that concerns about nanotube toxicity could not be resolved until conclusions were made about how animal leg tissue interacts with the airborne particles. Nobel Laureate Smalley from Rice University forecast that SWCNTs could be sold by the ton if a cheap mass production method were devised. In such a situation, there could be significant exposure to harm. Other academic studies have examined the toxicities of carbon-based nanomaterials. For example, Magrez published research results related to the cytotoxicities of carbon-based nanomaterials in 2006 [24]. CNTs are not the only materials that have generated warnings. Nanoparticles made from Teflon (i.e., polytetrafluoroethylene (PTFE)) are even more dangerous. Toxicologist Oberdo¨rster of the School of Medicine and Dentistry of New York Rochester University reported that most tested animals died within 4 h of exposure to air contaminated with 20-nm PTFE nanoparticles for 15 min; however, exposure to air with 130 nm PTFE particles (the size of a bacterium) had no pathogenic effects. Oberdo¨rster reminds us that small particles may have different chemical properties from larger ones, and that it is important to conduct additional comparative studies to better understand these phenomena. In addition, histological studies have shown that macrophages have trouble removing small particles from foreign matter. In another study, Oberdo¨rster found that inhaling magnesium and carbon-13 isotope nanoparticles affects a rat’s sense of smell and then its entire brain. In January 2004, Derfus et al. of the University of California San Diego [25] published a toxicity study on CdSe semiconductor 1,2-dimethylhydra-zine. Under certain conditions, this material can be very poisonous to liver cells, causing cell death when the collapse of the CdSe lattice releases free Cd21 ions. They also found that coating this chemical with a layer of a material such as ZnS can significantly reduce, but not eliminate, toxicity. It is noteworthy that several previous studies have concluded that CdSe quantum dots are compatible with cells. Several researchers have reminded us that preliminary research on nanoparticle toxicity does not have a large impact. However, scientists believe that further research is necessary. Rice University Centre for Biological and Environmental Nanotechnology Director Colvin said, “There is issue of risk with every new technology. It would be silly to think we have nothing to consider.”

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CHAPTER 1 INTRODUCTION

Research into nanoparticle toxicity must continue, Colvin said, “I would much rather face this now than after nanotechnology becomes widespread.” In June 2003, the government of the UK asked the Royal Society and Royal Academy of Engineering Science and Technology to conduct independent research on the benefits and potential problems of nanotechnology. The purpose of this research was to determine which issues might become relevant now and in the future regarding the use of nanotechnology, and their impacts on the environment, health and safety, ethics, and society, as well as to investigate other sources of uncertainty. The institutions also assessed the potential for regulation of nanoscience and technology development. The results of this research will be published online. The US Congress has held several recent hearings on the potential problems related to nanotechnology. US nanotechnology industry groups such as the Nano-Business Alliance established a Health and Environmental Issues Task Force in July 2003 to investigate the impact of nanotechnology on health and the environment. The goal of this effort is to reduce concern among the government and consumers that nanotechnology could adversely impact the environment. The association includes both nanotechnology researchers and nanoindustry/business personnel. In the short term, this group will collect information on research achievements for publication on its website to help nanotechnology companies to avoid mistakes when they increase or decrease their nanomanufacturing businesses. The association’s long-term plan is to produce nanotechnology standards and to help relevant organizations to coordinate their efforts with other countries. In contrast, the Action Group on Erosion, Technology, and Concentration (ETC Group), a Canadian environmental organization, views developments such as genetically modified crops and nanotechnology (especially self-assembly and self-replicating materials) as unsafe. They fear that nanotechnology-based materials that are released into the environment will have a negative impact on people. In January 2003, they published an 80-page article on their website to “catalyze to widespread public debate on the social a view to stimulate wider public debate of the societal impacts of technology.” After reviewing the information above, it is clear that that nanomaterials and nanotechnology provide hope for humanity, and that they will make human society even more brilliant tomorrow. This drives scientists work hard to develop technology, and governments offer good plan, great support, and large amount investment. Preliminary findings show that nanomaterials may present some problems, and that nanotechnology is a double-edged sword [26,27]. Therefore, critical attention from the public is entirely understandable. The best strategy is not only to study the benefits of nanomaterials but also to investigate the risk of harmful impacts on people and the environment. Such studies should be implemented before large-scale nanomaterial applications rather than after harm has already taken place. It is important to ensure that the potentially harmful side of nanotechnology does not impact humanity. Strategies that seek to damage the reputation of nanotechnology due its possible hazards are certainly not desirable. Toxic technology is everywhere in the world. For example, nearly every CD contains arsenic, but, as Professor Colvin has said, “it would be almost amazing if all materials were as safe as water.” The researcher indicated that it would be wrong to categorize nanoparticles as either safe or terribly risky. Each item must be analyzed specifically. Smalley, a Nobel Laureate in Chemistry, put it in a more amusing way: “in the end, we haven’t [wanted] you to eat nanomaterials.”

1.6 FUNDAMENTALS OF NANOMATERIAL PHYSICS

17

1.6 PURPOSE OF THIS BOOK: FUNDAMENTALS OF NANOMATERIAL PHYSICS Right now, there is no unanimous method for nanomaterial classifications in the literature, as they can be categorized from various perspectives. Nanomaterials can be categorized by dimensions and by their structures. Thus, they can be divided into zero-dimensional nanomaterials (quantum points and nanopowders), one-dimensional nanomaterials (quantum wires, nanotubes, and nanorods), two-dimensional nanomaterials (quantum traps and nanofilms), three-dimensional nanomaterials (block-type nanomaterials), nanocomposites, and nanostructured materials. The materials science approach allows us to divide nanomaterials into metal nanomaterials, nanooxides, and ceramics, and nanopolymers, as well as organic and inorganic nanocomposites. Based on their properties and applications, nanomaterials can be divided into nanoelectronics, nanomagnetic materials, nanobiomaterials/nanoenvironmental protection materials, nanomedicines, nanobuilding materials, and nanomilitary materials. This book focuses on the basic physics of nanostructured materials. Therefore, it does not traverse the above nanomaterial classifications one by one. Rather, a variety of common problems and the nanomaterials that are related to them are elaborated and discussed. Contents include principles of preparation, nanomaterial structures and formation mechanisms, as well as nanomaterial properties. However, the focus of this text is on nanomaterials physics. The introduction includes a description of nanomaterials and a brief overview of their development, importance, and place in history. It also includes descriptions of nanomaterial research developments as well as problems and hazards that may exist. It begins by making clear that humanity has entered the Nanomaterial Era, and that the Nanomaterial Era just started. The preparation principle and method, structures, and formation mechanisms of nanomaterials includes a variety of comprehensive physical, chemical, and mechanical preparation method, structure, and formation mechanism of nanomaterials, including nanopowders, nanowires, nanotubes, nanorods, nanofilms, and nanoblocks. Structures and formation mechanisms are discussed together with the preparation method and principle except nanoblock materials. Self-assembly is discussed in its own chapter. In addition to the fundamental physical problems for the preparation, formation mechanisms, and structures of nanomaterials, nanomaterials physics is reflected in the various physical properties (mechanical, thermal, electrical, magnetic, and optical) of nanomaterials. A variety of physical phenomena related to nanomaterials are discussed, along with the physical theories that describe them. This includes basic physical effects and magnetic/optical properties, as well as theories of thermal conductivity, deformation and fracture, electron transport, etc. This book focuses on phenomena and principles related to nanomaterials and contrasts them to their nonnano counterparts. It also compares experimental results and materials science principles to discuss nanomaterial characteristics. However, it describes various performance-related nanomaterials physics theories qualitatively, rather than using theoretical or mathematical formulas and equations. This organizing of this book is quite different from other published nanobooks; it can be counted as an attempt.

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CHAPTER 1 INTRODUCTION

REFERENCES [1] Zhang B. J Hunan Univ (Soc Sci ed) 2006;20(5):112. [2] Smithells CJ, editor. 5th ed. Metals reference book, 100. London and Boston: Butterworths; 1976. p. 102. [3] Taniguchi N. On the basic concept of ‘NanoTechnology’. In: Proc Intl. Conf. Prod. Eng. Tokyo, Part II, Japan Society of Precision Engineering; 1974. [4] Zheng X. Nanotechnology—the science and technology new star of 21st century—the interview of Paul Walun, nanomaterial expert of Oxford University interview with nano-materials expert at Oxford University, Paul Warren. Science and Technology Daily December 20, 2000. [5] Kanzig WH. Phys Acta 1951;24:175. [6] Feynman R. Eng Sci 1960;23:22. [7] Drexler KE. Proc Natl Acad Sci USA 1981;78:5275. [8] Kubo R. J Phys Soc Jpn 1962;17:975. [9] Benjamin JS. Mater Trans 1970;1:2943. [10] Birringer R, Gleiter H. Trans Jap Ins Met 1986;27:43. [11] Kroto HW, Heath JR, O’Brien SC, et al. Nature 1985;318(No. 6042):162. [12] van Wonterghem J, Mørup S, Charles SW, et al. Phys Rev Lett 1985;55:410. [13] Huang Z, Chen H, Yip A, et al. J Nanoparticle Res 2003;5(3/4):333. [14] Iijima S. Nature 1991;354:56. [15] Iijima S, Ichihashi T. Nature 1993;363:603. [16] Bethune S, Bethune DS, Klang CH, et al. Nature 1993;363:605. [17] Cornell BA, Braach-Maksvytis VLB, King LG, et al. Nature 1997;387:580. [18] Tans SJ, Verschueren ARM, Dekker C. Nature 1998;393:49. [19] Martel R, Schmidt T, Shea HR, et al. J Appl Phys Lett 1998;73:2447. [20] Chou SY, Zhuang L, Guo L. Appl Phys Lett 1999;75(7):1004. [21] Nanoscale Science, Engineering, and Technology Subcommittee Committee on Technology, National Science and Technology Council. The national nanotechnology initiative, environmental, health, and safety research needs for engineered nanoscale materials; Sept 2006. [22] Zhang B. Modern Weapons 2006;12:17. [23] Service RF. Science 2003;300(5617):243. [24] Magrez A, Kasas S, Salicio V, et al. Nano Lett 2006;6(6):1121. [25] Derfus AM, Chan WCW, Bhatia SN. Nano Lett 2004;4(1):11. [26] Zhang B. Nanosci Technol 2006;3(6):61. [27] Zhang B. Sci Technol Dialectics 2008;25(1):98.

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PRINCIPLES, METHODS, FORMATION MECHANISMS, AND STRUCTURES OF NANOMATERIALS PREPARED VIA GAS-PHASE PROCESSES

2

CHAPTER OUTLINE 2.1 Principles of Physical Vapor Deposition .......................................................................................... 20 2.1.1 Nucleation ................................................................................................................. 21 2.1.2 Growth ...................................................................................................................... 22 2.2 Physical Vapor Deposition .............................................................................................................. 26 2.2.1 Electrical Resistance Heating Method .......................................................................... 26 2.2.2 Plasma Heating Method .............................................................................................. 29 2.2.3 Laser Heating Method................................................................................................. 31 2.3 Chemical Vapor Deposition ............................................................................................................. 38 2.3.1 CVD Thermodynamics and Kinetics .............................................................................. 39 2.3.2 CVD Process Technology for Nanomaterial Preparation................................................... 42 2.3.3 Catalytic CVD and CNT Preparation .............................................................................. 48 2.4 Filtered Cathodic Vacuum Arc Deposition ........................................................................................ 58 2.4.1 Magnetic Filtration and FCVA Devices .......................................................................... 59 2.4.2 Examples of Filtered Cathodic Vacuum Deposition Films................................................ 60 2.5 Comparison of Various Vapor Deposition Methods............................................................................ 65 References ........................................................................................................................................... 66

Most common nanomaterials are manufactured rather than naturally occurring. Therefore, the first problem that one encounters is that of nanomaterial preparation. There are many types of nanomaterials, but there is no uniform method of classifying them, and the methods used to make these nanomaterials vary. How can the preparation methods be characterized? At present, there are no uniform classification standards. Nanomaterial synthesis is described and discussed in three chapters of this book—the three chapters address gas, liquid, and solid-phase preparation, respectively. Organizing the preparation of nanomaterials by dimension would produce a scattered, repetitive discussion because individual Physical Fundamentals of Nanomaterials. DOI: https://doi.org/10.1016/B978-0-12-410417-4.00002-2 Copyright © 2018 Chemical Industry Press. Published by Elsevier Inc., under an exclusive license with Chemical Industry Press.

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CHAPTER 2 NANOMATERIALS PREPARED VIA GAS-PHASE PROCESSES

preparation methods such as sol
gel coagulation can be used to synthesize nanomaterials with zero, one, and two dimensions. The state of matter used in classification refers to the state in which preparation takes place, rather than the states of the reagents. For example, one can use liquids or solids to start a gas-phase preparation, but they will first convert to gas phase, and then, the relevant reactions occur in the gas phase. Self-assembly will be presented in a separate chapter because of its special nature and importance. This chapter discusses the theories, methods, syntheses, and structures involved in gas-phase nanomaterial synthesis. Regardless of the state of the initial raw material, the common feature of the gas-phase syntheses is that the reagents involved become gases or vapors before producing nanoparticles via nucleation and growth. If an apparatus uses a solid substrate, gaseous atoms or ions impinging on a substrate can cool to form nanowires or nanofilms via nucleation and growth. The nanomaterial sizes, shapes, and compositions produced by gas-phase syntheses can be controlled by manipulating process parameters such as pressure, temperature, and distance between an evaporation source and the substrate. This method has attracted increasing interest. We cannot discuss all available gas-phase preparation methods but focus on a few that are typical or canonical. Many so-called new preparation methods are actually the improvements of existing and popular methods. Gas-phase syntheses can be divided into physical vapor deposition (PVD), chemical vapor deposition (CVD), and methods that involve both physical processes and chemical reactions.

2.1 PRINCIPLES OF PHYSICAL VAPOR DEPOSITION Gas-phase condensation to produce solid powders is similar to vapor deposition to produce liquid droplets, followed by liquid crystallization to form solid particles. Increases and decreases in energy within the system drive the accumulation of atoms to form stable nuclei. The nuclei then grow and condense to form particles under the influence of continued atomic collisions. Thermodynamic theory indicates that the Gibbs free energy G is minimized in stable, isothermal, and isobaric systems. At atmospheric pressure, the difference between the Gibbs free energy (G) and the free energy (F) is negligible. Therefore, decreases in the free energy F of the system provide the driving force for phase transformation. When a stable new nucleus β-phase is established in a metastable α-phase, the β-phase is the most stable, and the free energy difference is negative. Thus, the free energy of the system decreases. At the same time, since the structures of the old and new phases are different, differing interfaces must be created between the two phases, and the free energy of the system increases. One can develop a new expression for the critical radii of the nuclei by considering these two extremes. Therefore, the focus of the problem lies in properly deriving the energies of these expressions. Since nucleation and growth are dynamic processes, at least two issues must be considered. If a critical nucleus becomes a stable new phase, this must occur after gaining at least one atom or molecule after the atoms or molecules in the old phase collide to overcome the thermal barrier. A new, stable nucleus can then form based upon the frequency of collision with atoms or molecules. The critical nucleus must grow to reach a new stable phase, while thermal fluctuations continually

2.1 PRINCIPLES OF PHYSICAL VAPOR DEPOSITION

21

generate new critical nuclei. Thus, the actual number of stable nuclei is far smaller than the number of critical nuclei calculated under balanced conditions. A factor must be added to account for this difference. Hence, homogeneous nucleation of phase transitions, nucleation rates, and nuclear growth rates have been research topics of interest since classical nucleation theory was put forth in the 1920s [1
3], including in 2003 [4] and 2006 [5,6]. These subjects continue to be actively researched.

2.1.1 NUCLEATION There have been various improvements to classical nucleation theory. In this discussion, we compare these improved theories to the classical theory and discuss how to choose between them. In 1997, Ford [7] modified classical nucleation theory based on kinetic theory and statistical mechanics, and by taking capillary effects into account. The classical nucleation rate, Jcla, is expressed as Jcla 5



 SPvs εðlÞ εði Þ exp 2 ; VZβ i  exp kT kT kT

(2.1)

where S  Pv/Pvs is the supersaturation, Pv is the vapor pressure, Pvs is the saturated vapor pressure, V is the volume of the system, β i is the nuclear growth parameter (a flux of atomic or molecular nuclei multiplied by the surface area of i atomic nuclei),  is the critical status, i represents the number of atoms in a critical nucleus, ε(i) is the nucleation power required for a nucleus containing i atoms, ε(l) is the nucleation function of the monomer, k is the Boltzmann parameter, T is the temperature, and Z is the Zeldovich factor [3]. The latter can be expressed by using
Z5 2

 2  1=2 1 @ Ωs ðiÞ ; 2πkT @i2 i5i

(2.2)

where Ωs(i) is the grand potential. Its relationship with the nucleation power is εðiÞ 5 Ωs ðiÞ 2 ikT lnS

(2.3)

Based on the above classical nucleation relationship and the statistical mechanics of the nucleation rate, the classical nucleation rate can be expressed as !  2=3 2σ 1=2 S2 P2vs υ1 σA0 icla ; Jcla 5 V exp 2 3kT mπ ðkTÞ2 

(2.4)

where m is the quantity of atoms or molecules, σ is the surface tension, A is the surface area of the nucleus, and υ1 the nuclear volume. Ford’s modification of nucleation theory considers capillary effects. The nucleation rate Jcap is expressed as follows using statistical mechanics: 

2σ Jcap 5 mπ

1=2

2=3

SPvs υl σA0 icla 2 υc ðkTÞ 3kT

!

;

(2.5)

where υc is the scale volume. Fig. 2.1 compares the nucleation rate results calculated using Eq. (2.5) derived from the classical nucleation theory Eq. (2.4), and Ford’s modification is considered capillary effect to experimental data. Data indicate that the nucleation rates of water, butanol,

CHAPTER 2 NANOMATERIALS PREPARED VIA GAS-PHASE PROCESSES

J(theory)/J(experimental)

22

105

100

10–5

Water (revised model) Water (classical theory) n-Butanol (revised model) n-Butanol (classical theory) n-Pentanol (revised model) n-Pentanol (classical theory)

220

240

260

280

Temperature (K)

FIGURE 2.1 Relationships between vapor nucleation rates produced from theoretical calculations and experimentally, as a function of temperature, for several substances [7] (experimental values from [8
10]).

and pentanol produced by Ford’s modification are approximately 104
106 times larger than those calculated using classical theory. Neither classical nucleation theory nor Ford’s modifications are in good agreement with the experimental results. A number of other scientists have improved upon classical nucleation theory. Approaches have included the use of density functional theory [11], the Monte-Carlo method [12], and molecular dynamics [13]. These studies have helped us to deepen our understanding of nucleation. However, since the nucleation rate is sensitive to experimental conditions such as temperature, pressure, impurities, and mutual interactions between atoms, there remains much to be done regarding their prediction [4,14].

2.1.2 GROWTH The large numbers of particle (or droplet) nuclei produced via vapor condensation will grow. According to classical phase transition theory, growth process is the nuclei growing up via atoms or molecular through gas
solid (or gas
liquid) interface under the phase transformation driving force. The speed of growth depends on the frequency with which atoms jump across the phase boundary, as well as their mean free paths. However, there is another growth process that occurs primarily via cohesion of the stable core. Flagan shows in Fig. 2.2 that nuclei merged together first, and then, the clustered particles reunited and grew up [14]. Granqvist and Buhrman [15] prepared Al, Mg, Zn, Sn, Cr, Fe, Co, Ni, Cu, Ga, etc., multiple kinds of metal nanoparticles under Ar, and found that growth occurred via particle condensation. When metal nanoparticles are smaller than 20 nm, electron microscopy shows that they are nearly all spherical. Larger particles tend to have shapes that are defined by their crystal

2.1 PRINCIPLES OF PHYSICAL VAPOR DEPOSITION

23

FIGURE 2.2 Schematic of gas-phase nanoparticle growth [14].

FIGURE 2.3 Al nanoparticle size distribution [15]. [Points are experimental test results, and the curve is the frequency distribution from the calculation of x (Wx 5 11.05 nm). The total area under the curve is used for normalization, and the total number of particles is 518. Two σ values are used. Arrows indicate the position of the intermediate particle size x.]

structures. The authors carried out a detailed particle size distribution study which found that particle diameters occur in a logarithmic Gaussian distribution. The experimental points in Fig. 2.3 show typical Al nanoparticle size distribution results. Measured particle sizes are taken from the middle of the histogram. The relationship equation similar as merged and gartered droplets for nanoparticles was obtained by using Central Limit Theorem statistical model.

24

CHAPTER 2 NANOMATERIALS PREPARED VIA GAS-PHASE PROCESSES

Particle distribution expressions tend to treat particle growth as a series of discrete events. That is, they consider only growth via particle collisions, so that any individual step (j 5 1, 2, etc.) can be studied. Assuming that the original particle volume distribution is F0(v), which indicates that all particles are sized # v, these particles will merge to grow. If one considers only events where two particles merge over certain period of time, the volume distribution after the jth step is ð

Fj ðvÞ 5

Gf ðv; uÞd½Fj21 ðuÞ

(2.6)

u

where u represents a particle merged with another particle to form a new particle with volume v, and Gj (v,u) is a function that need not be specified. The theory assumes that the volume change after each merger is represented by the portion of the merged volume that is present due to mixing. This portion of the volume can be written as v 2 u 5 εj v;

(2.7)

where {εj } is a set of independent random variables that bear no relationship to the volume. That is, only v/u appears in the equation, so a new function can be defined: 0

Gj ðv; uÞ  Gj

v u

:

(2.8)

A well-known partition function theorem can be obtained by introducing this equation into Eq. (2.6). Vi 5 Tj Vj21 ; 0

0





(2.9)

where Vj and Tj are the relative variables of the partition functions Fj ðvÞ and Gj  Gj v=u , respectively. After combining these n times, we produce n

Vn 5 V0 L Tj ;

(2.10)

j51

where V0 indicates the original volume. Thus, Eq. (2.10) can be rewritten as ln

n X Vn 5 Tj; V0 j51

(2.11)

where {Tj } is a set of independent random variables with the same probability distribution. Based on the central limit theorem from mathematical statistics [16], the size distribution of logarithm lnaf;ðVn =V0 Þ is an asymmetric Gaussian distribution. For spherical particles, the Gaussian distribution of volumes can be written as fLN ðxÞ 5

1 ð2πÞ1=2

  ðln x2ln xÞ2 ; exp 2 2 ln2 σ ln σ

(2.12)

where fLN ðxÞ is a log-normal distribution function (LNDF), x is the particle diameter, x is the statistical average particle size, and σ is the standard deviation. The definitions of x and σ are listed as the following equations, respectively: ln x 5

X

n lnxi i i

 X 21 ni i

(2.13)

2.1 PRINCIPLES OF PHYSICAL VAPOR DEPOSITION

" ln σ 5

X

ni ðln xi 2ln xi Þ2

i

X

25

!21 #1=2 ni

;

(2.14)

i

where ni is the particle that falls at the center of columnar distribution in the range of xi . The geometric standard deviation is a dimensionless quantity and is always  than 1. P greater By using the theoretical frequency distributions of x [17], i ni Δx=x fLN ðxÞ, where Δx is the histogram interval size, drawing to x and choosing two different types of geometric values for the standard deviation σ, one can produce two curves as shown in Fig. 2.3. The result is clearly in line with the experimental data, which indicates that the LNDF statistical model based on the central limit theorem is able to describe the nanoparticle size distributions for the nanoparticles synthesized via evaporation in inert gases. This shows theoretically that growth during nanoparticle preparation is achieved primarily via merging of stable nuclei, rather than absorption of individual atoms at the solid interface. This is because merger growth is a precondition for use of the LNDF. Please note that the LNDF distribution is mentioned several times in subsequent chapters. From Eq. (2.12), it can be demonstrated that the error function of the total number of particles whose size is smaller than x can be expressed as fLNðxÞ 5

1 1 ln ðx=xÞ 1 erf pffiffiffiffi : 2 2 2 ln σ

(2.15)

Plotting the equation fLN (x) against x on logarithmic graph paper produces a straight line and represents a logarithmic probability plot. It can be used to derive constant values of x and σ for characterization of the LNDF distribution. Granqvist and Buhrman [15] compared their various experimental nanoparticle results with the theoretical results using logarithmic probability. They found that the experimental points fall on the theoretically defined line in a manner that is much better than that shown in Fig. 2.3. This also illustrates the effectiveness of the above theoretical model. Mapping the experimental values of x and σ produces Fig. 2.4. This figure also includes experimental data from nanoparticles prepared via vapor coacervation by other authors [15]. Based on the figure, it can be shown that 1.36 # σ ,1.60. The only exception stems from Ga particles, which exhibit a large geometric standard deviation. Granqvist and Buhrman experimentally measured the impacts of vapor pressure and inert gas selection on nanoparticle size. They found that the particle size increases linearly with the vapor pressure and that the heavier the inert gas atoms, the more significant is the linear increase in particle size. The reason for this is that the greater the vapor pressure, or the heavier the inert gas atoms, the free path of the evaporated metal atoms will be limited more effectively, and this thus reduces the nucleation speed with which nuclei are deposited promptly. These factors cause nuclei to collide, producing increased opportunity for growth and larger nanoparticles. In 2003, Chiu et al. [18] used the same vapor deposition method in Ar to prepare smooth, pearl-like (8-hydroxyquinoline) aluminum nanoparticles. Upon fixing the evaporation temperature T and the distance between the source and the evaporation cooling plate d, they found that nanoparticle sizes increased linearly with the Ar gas pressure P. However, this linear relationship was imperfect. If T and P were kept constant, the nanoparticle size increased linearly with the distance d, although not perfect linear relationship (see Figure 3 in Ref. [18]). These results can be explained by the discussion presented above.

26

CHAPTER 2 NANOMATERIALS PREPARED VIA GAS-PHASE PROCESSES

FIGURE 2.4 The x 2 σ schematic of a log-normal distribution of sizes of nanoparticles prepared via vapor condensation [15].

2.2 PHYSICAL VAPOR DEPOSITION PVD includes only physical processes that occur with no chemical reactions. In PVD, a certain pressure of inert gas is introduced into a container under vacuum, and any of a variety of methods can be used to make raw materials (mostly solid) either transition to the gas phase or melt into liquids and subsequently evaporate to form vapors. Nanomaterials are then formed via nucleation and growth. In high-vacuum containers, evaporated gas atoms or molecules hit the cold wall or the base plate, forming nanomembranes or nanowires. This is similar to vacuum evaporation coating. If the container is filled with a low-pressure inert gas, the vaporized atoms will collide with gas molecules and each other, cooling and condensing to form nanopowders in the gas phase. Various heating methods can be used for gas-phase deposition, including electrical resistance, high-frequency induction, plasma, electric explosion, and electron beam heating, as well as electric arc discharge, molecular beam epitaxy (MBE), and ion sputtering. In this chapter, we discuss only the resistance, plasma, and laser heating methods.

2.2.1 ELECTRICAL RESISTANCE HEATING METHOD In electrical resistance heating, the high temperatures required for evaporation in an inert gas are produced using an electrically resistive heating element. This causes materials such as metal or oxide to evaporate. Grain sizes can be controlled by manipulating the heating temperature, inert gas selection, and pressure, as well as the distance between the evaporation source and the cooling plate on which the nanoparticles collect. Resistive heating elements can be made from metal wires or graphite. Materials are placed in a crucible for evaporation.

2.2 PHYSICAL VAPOR DEPOSITION

27

The inert gas evaporation method was first proposed by Pfund [19], Burger and van Cittert [20] in the 1930s and greatly improved by Wada [21] and Uyeda [22] of Nagoya University in Japan. Preparation of nanoparticles in this manner produces uniform particle size distributions that are easy to control and clean particle surfaces. Thus, this method has attracted extensive attention, and many researchers have updated and improved upon it, developing various related equipment. However, this method has some disadvantages. The most serious of these is the difficulty of ensuring that particles remain dispersed and do not interact with each other. When the particles interact, it is difficult to avoid the possibility of electronic tunneling between them although an oxide layer was formed on the particle surface by inducing oxygen during preparation. Of course, particles can be separated via appropriate preventive procedures. Fig. 2.5 shows resistive heating inert gas evaporation and condensing equipment used by Granqvist and Buhrman [15] to prepare various nanoparticles in 1976. The heating element is made from graphite. The pressure, heating temperature, and distance between the cooling panels and evaporation source can be adjusted. Temperatures are measured using a Pt
PtRh thermocouple or optical pyrometer. This nanopowder preparation method works only to synthesize a few grams of material at a time and does not offer a high yield. Thus, it is used primarily for laboratory work. In 2001, Zhong Lin Wang’s group at the Georgia Institute of Technology in the United States used thermal evaporation of an oxide powder to produce a variety of semiconducting oxide nanobelts without the use of a catalyst [23]. This is an example of the PVD method. They used a horizontal alumina tube furnace with oxide powder placed in the portion of the tube that would be in the center of the oven. With this apparatus, the evaporation time and furnace tube temperature and pressure could be controlled. Cooling water pipe Pressure meter

Optical pyrometer Gas flow throttle

Glass cylinder vat Cu cooling sheet L-shape rod Oxygen cylinder

Small valve Large cap Crucible with metals Pt-Rh thermal couple

Ar

O2

Graphite heating unit To diffusion pump

FIGURE 2.5 Nanoparticle synthesis equipment that uses a graphite heating element to evaporate raw materials and condense particles in inert gases [15].

28

CHAPTER 2 NANOMATERIALS PREPARED VIA GAS-PHASE PROCESSES

Nanoscale ZnO strips are prepared from ZnO as shown in Fig. 2.6. They have uniform widths that typically range from 50 to 300 nm. With thicknesses of 10
30 nm, their ratio of width to thickness is approximately 5
10. The lengths of these strips can range from dozens to hundreds of micrometers, or they can be as long as a few millimeters. No nanoparticles were found at the heads or ends of the strips. Transmission electronic microscopy (TEM) indicates the presence of a rectangular section, as shown in Fig. 2.6D. However, further observation indicates that the strip is corrugated. This is due to bending that produces strain. Energy-dispersive X-ray spectroscopy and X-ray diffraction (XRD) analyses show that the ZnO nanobelts have hexagonal wurtzite structures with lattice constants that are identical to those of ZnO (a 5 0.3249 nm and c 5 0.5206 nm). Highresolution transmission electron microscopy (HRTEM) and electron diffraction analysis show that the ZnO nanobelts are single crystals that grow along two directions. One of the two directions is [0 0 0 1], where neither defects nor dislocations are found. The crystals also grow along the [0 1 1 0] direction, where they exhibit no dislocations, but stacking faults are present parallel to the

FIGURE 2.6 TEM and HRTEM images of ZnO nanostrips. Parts (A)
(C) show the geometric shapes of the strips. Part (D) is a cross-section view. Part (E) shows strip growth along the [0 1 0] direction and indicates the presence of stacking faults. Part (F) shows an HRTEM image of (E) in box 1. The surface is perfect; noncrystalline diffraction in the surface above occurs due to use of an amorphous carbon film as the sample support during TEM analysis; (G) HRTEM image of (E) in box 2 [23]. HRTEM, high-resolution transmission electron microscopy; TEM, transmission electronic microscopy.

2.2 PHYSICAL VAPOR DEPOSITION

29

axis throughout entire nanostrips. This is shown in Fig. 2.6E and G. Fig. 2.6F shows that the nanosurface is clean, atoms are sharply visible, and there is no overlay of amorphous material. SnO2 and In2O3 nanostrips can be synthesized via evaporation of their respective powders in a manner similar to that used with ZnO. In addition, CdO nanostrips have been made via evaporation of CdO powder. However, their shapes are not as regular as those of ZnO, SnO2, and In2O3 strips. This synthesis produced SnO2, rather than SnO strips, upon evaporating SnO powder at 1000 C. Thus, the Sn21 in the SnO powder becomes the Sn41 detected in the SnO2 nanostrips during preparation. The reason for this valence change is not clear. The mechanism of powder nanostrip formation via evaporation may be one in which the vapor directly transforms into a solid phase. The nanostrip formation via powder evaporation may be the vapor directly transforms to solid, named vapor
solid (VS) mechanism. It means the evaporated powder that falls into high-temperature region after arriving on the cold substrate undergoes nucleation and growth to form nanostrip structure. Gyorgy et al. [24,25] used alumina crucibles to evaporate Cu and Ni, resulting in Cu/Ni nanocomposite films prepared at deposition rates of 0.1
0.6 nm/s under (3
10) 3 1028 Torr (1 Torr 5 133.32 Pa) of pressure. Mica substrates with temperatures between 250 and 300 C were used. The thin film thickness was between 0.6 and 6 nm. The researchers primarily studied magnetic property changes in these nanomembranes.

2.2.2 PLASMA HEATING METHOD There are many ways to generate plasmas. One can produce plasmas via electrode gas discharge, microwave, laser, high-energy particle beam, or electrodeless radiofrequency (RF) methods. The thermal plasma temperature is locally balanced. That is, the ion and electron temperatures are nearly the same. The electron density in a hot plasma can reach 1021
1026/m3. An RF discharge can be generated via either capacitive or inductive coupling. The most frequently used method is generation of a high-intensity plasma arc, followed by an inductively coupled high-frequency discharge. Because it does not involve chemical changes, thermal plasma PVD is a form of PVD. The graphite electrodes used in arc discharges are thermally stable but quickly become corrupted and begin to evaporate in plasma, thus staining the synthesis products. To avoid contamination, a plasma can be produced via a cold microwave or thermal RF, instead of electrode. However, a cold plasma requires less pressure and greatly limits the particle temperature and yield. Thus, inductively coupled plasma devices are used much more commonly. Because no electrode makes contact with the gas, the products have low levels of impurities. Fig. 2.7 is a sketch of a high-frequency plasma reactor [26]. The section on the right side is a treatment device that removes exhaust gas and wastewater. Frequencies of 3
4 MHz are typically used under normal pressure, with a power level of 40 kW and efficiency of 30%
40%. The local plasma temperature is 10,000 K. The resulting nanoparticles are collected using a dust filter, and the exhaust gas is cleaned using an alkaline or acidic solution. Plasmas of up to 6 N m3/h can be generated in the N2, Ar, or even air. Solid, liquid, or gaseous precursors can be delivered to the plasma. Their residence time in the hot zone is less than 1 s, and their cooling rates can reach 106
109K/s. Thus, a precursor can be physically transformed into nanoparticles. Alloy or compound nanoparticles can be synthesized if a combination of solid particle types is added to a plasma. Clearly, making alloy or compound nanoparticles from several types of precursor particles requires that all particles be completely evaporated. Experiments have shown that

30

CHAPTER 2 NANOMATERIALS PREPARED VIA GAS-PHASE PROCESSES

FIGURE 2.7 High-frequency plasma reactor [26]. Nanoparticle source

High yield deposition device

High resolution deposition device

Beam mass spectrometer detector Splitter

Substrate 2 Plasma gas

Reactant

Electronic meter

P ≈ 1Torr Nozzle

P ≈ 1 atm P