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Copyright © 2012 by American Scientific Publishers All rights reserved. Printed in the United States of America Science of Advanced Materials Vol. 4, pp. 401–406, 2012 (www.aspbs.com/sam) One-Dimensional Nanostructures by Pulsed Laser Ablation Rusen Yang Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN 55455 ABSTRACT CONTENTS 1. Introduction . . . . . . . . . . . . . . . . 2. Pulsed Laser Ablation Process . . . . 3. Growth Mechanisms of Nanowires . 3.1. Vapor–Liquid–Solid Process . . 3.2. Oxide-Assistant Growth Process 3.3. Vapor-Solid Growth . . . . . . . . 4. Summary . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . References and Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 401 401 402 402 404 404 405 405 405 1. INTRODUCTION One-dimensional (1D) semiconductor nanostructures are of particular interest because of their unique role for the fundamental study as well as their potential applications in nanoscale electronic and optoelectronic devices. To date, various new techniques have been developed to grow 1D nanostructures, such as thermal chemical vapor deposition (CVD),1 molecular beam epitaxy (MBE)2 and chemical beam epitaxy (CBE).3 Among various techniques to synthesize nanomaterials, the laser ablation of solid targets is of particular interest because bulk-quantity nanowires can be readily obtained directly from solid source materials and this growth method is applicable in synthesizing nanowires containing complex chemical compositions. Since Theodore Maiman made the first ruby laser in 1960,4 pulsed laser ablation (PLA) was soon developed and attracted wide interests because of its great potential in material processing. Pulsed laser was firstly employed for the preparation of thin films in 19655 and then for hole piercing, micro-and nanomachining, and surface cleaning.6 In the past decade, PLA has been intensively investigated for the synthesis of nanostructures. The nanomaterial synthesis with PLA can take place in two distinct conditions: one occurs in a vacuum or gaseous environment, and the other occurs in liquid. The nanomaterials from the solution normally have spherical or roughly spherical shape, except when particles aggregate into fractal structure. Spherical or roughly spherical nanomaterials have been the major materials from the vacuum or diluted gas. Interestingly, 1D nanostructures have also been discovered from laser ablation of solid materials in gaseous environment. Considering the unique property and application brought by those one-dimensional nanostructures, we will discuss different growth processes of 1D materials with PLA technique. Starting with a general description of the PLA method, we will discuss respectively the vapor–liquid–solid (VLS) process, oxide-assistant growth (OAG), and vapor–solid (VS) growth, and finally conclude with a summary. 2. PULSED LASER ABLATION PROCESS Email: [email protected] Received: 28 April 2011 Accepted: 19 May 2011 Sci. Adv. Mater. 2012, Vol. 4, No. 3/4 The laser ablation of solid targets is an effective technique to produce bulk-quantity nanomaterials directly from the 1947-2935/2012/4/401/006 doi:10.1166/sam.2012.1296 401 REVIEW One-dimensional (1D) nanostructures represent a unique system for investigating phenomena at the nanoscale and are also expected to play a critical role as both interconnects and functional components in the fabrication of nanoscale electronic devices. Pulsed laser can provide extremely intense energy in a small spot and it can ablate virtually any materials. Various nanostructures have been investigated and fabricated with pulsed laser ablation, and this technique is not limited by the type by of materials or crystal structures. This article focuses Delivered Ingenta to: on 1D nanostructures and presents anUNIVERSITY overview of theOF common growth MINNESOTAmechanism of nanowires by pulsed laser ablation. We first introduce a general IP scheme of the pulsed laser ablation in a tube furnace for the : 128.101.142.152 nanowire growth. Subsequently, we discuss Fri, various growth mechanisms 01 Jun 2012 20:00:48 involved to generate 1D nanostructures from pulsed laser ablation, including vapor–liquid–solid growth, oxide-assisted growth, and vapor–solid growth. Defects can also play an important role and they have been observed in the nanowires from different growth process. KEYWORDS: Laser Ablation, Nanostructure, Nanowire, VLS, VS. REVIEW One-Dimensional Nanostructures by Pulsed Laser Ablation Yang solid source materials, and it has been employed for the well control over the growth temperature at the substrate, synthesis of nanostructures such as fullerenes7 and carflowing gas type and speed, and pressure during the whole bon nanotubes8 Pulsed laser can achieve extremely high growth process. Furnace by itself can be used to grow temperature within extremely short time and it has many nanomaterials through a vapor deposition process,15 while advantages in fabricating nanostructures. First, due to the pulsed laser is another unique and localized heating source highly intense energy of the laser spot, almost any matewith extremely high temperature within extremely short rial can be ablated for the synthesis purpose. In addition, time. The integration of PLA with furnace has been widely PLA can generally allow better control over stoichiometry employed to grow different nanomaterials. PLA can allow of the deposited materials, which will benefit the growth better control over stoichiometry of the deposited materiof complex materials. Introducing the pulsed laser into the als, which will benefit the growth of complex functional tube furnace has enabled the growth of many high-quality materials. The solid target for the laser ablation contains nanomaterials, which cannot be easily achieved otherwise. the material, or component materials, for the targeting Nanomaterials can be synthesized with ultra-short nanowires. It can also contain impurity metals as the cat(picosecond or femtosecond) pulse,9 10 nanosecond alyst for the VLS growth. The target is normally located pulse,11 12 or even continuous-wave conditions,13 14 in the middle of the furnace, and a substrate or collecting although the ablation process is very complicated and colder finer is adjacent to the target. Under the laser ablacan be different. When the laser is focused on the solid tion the target material will be vaporized and deposited on Delivered by Ingenta to: or colder finger. With careful control of the target, the intense laser radiation can cause the matethe substrate UNIVERSITY OF MINNESOTA rials ejection and plasma plume from the solid target. parameters for the laser (pulse energy, wavelength, etc.) : 128.101.142.152 The plasma plume will eventually condense IP to form thin and the parameters for the furnace (pressure, temperature, Fri, 01 Junand 2012 carrier 20:00:48 film or nanostructures. The generation, transformation, gas, etc.), researcher can synthesize novel nanocondensation of the plasma plume play important roles structures with excellent controllability. in nanomaterials preparation. The nanomaterial growth is determined by both laser parameters (wavelength, pulse 3. GROWTH MECHANISMS length, and fluence) and properties of the ambient medium OF NANOWIRES that can be controlled with the furnace. When the pulse is ultra-short, the laser energy is mainly 3.1. Vapor–Liquid–Solid Process transferred to the target instantaneously and the plasma Since the VLS growth mechanism was proposed by Wagforms right after the laser pulse. The plasma will expands ner and Ellis in 1964 for silicon whiskers,16 this process without any further heating process and condense with has been widely adopted to explain the growth of nanoa short life. Otherwise, the later part of the incident structures from different methods. The impurity metal is laser pulse continuously irradiates the plasma plume and normally induced intentionally to serve as catalyst in liqenhances the excitation and ionization of the species in uid state to define and guide the growth of nanowires.17 the plume. Meanwhile, it also irradiates the solid target to ablate more species into the plume. Consequently, the However, some source materials can decompose under plume can expand further with a longer life. The interface laser ablation and form liquid droplets to serve as the between the ejected plasma plume and the ambient gas catalyst assisting the growth of nanowire. Such VLS provides a path for possible chemical reactions and heat growth without impurity metal is also called self-catalytic exchange. Plasma will cool down and condense, resulting or catalyst-free growth.18 19 The laser-assisted VLS proin the nanostructures either on a substrate or in the cool cess has produced nanowires from elementary materials ambient gas. (Si,17 Ge,17 and B20 ), binary materials (In2 O3 ,21 SnO2 ,22 Figure 1 is a schematic PLA setup for the growth of ZnO,23 SiOx ,24 Zn3 P2 ,25 GaN,26 GaAs,27 and MgO28 ), nanomaterials in a tube furnace. Tube furnace can achieve ternary materials (GaAs06 P04 , InAs05 P05 , CdSx Se1−x ,29 Rusen Yang received his Ph.D. degree in Materials Science and Engineering from Georgia Institute of Technology in 2007, where he continued as Post Doctoral Associate. He joined Mechanical Engineering at the University of Minnesota-Twin Cities as an assistant professor in 2010. He has discovered novel nanostructures, such as ZnO, SnO2 , Zn3 P2 , and investigated their application potentials. His most recent work on energy harvester based on piezoelectric nanomaterials made significant contribution in the field of renewable energy. 402 Sci. Adv. Mater., 4, 401–406, 2012 Yang One-Dimensional Nanostructures by Pulsed Laser Ablation Sci. Adv. Mater., 4, 401–406, 2012 403 REVIEW is a widely accepted model for the nanowire growth, but the exact form of vapor species and their interaction with the catalyst particle require further investigation. Although the exact role or chemical composition of vapor species and their interaction with the liquid droplet are not fully understood, we can still develop certain guide lines to choose the metal as well as the growth condition. The VLS growth method takes advantage of the metal catalyst in liquid state to initiate and guide the growth of the nanowire. Consequently, the selection of the metal material and the growth temperature is critical. The liquid Fig. 1. Schematic setup for the nanowire growth in a tube furnace with metal should be able to dissolve the nanowire component the laser ablation of a solid target. element. At the same time, they cannot react and form more stable solid phase than the desired nanowire phase. In and indium-tin-oxide (ITO),18 30 and even more complex other words, the metal catalyst should be physically active materials.31 In addition, this process has been used for but chemically stable, which also explains that the noble impurity-doped nanowire32–35 as well as block-by-block metal like Au has been successfully used for the growth of superlattice nanowires.36 Delivered bymany Ingenta to: nanomaterials. In addition, the growth condition can The VLS growth process is schematically shown in OF UNIVERSITY MINNESOTA also be estimated with the equilibrium phase diagram for Figure 2(a). The target is composed of mainly theIPsource : 128.101.142.152 the elementary material17 or pseudobinary phase diagram37 in the material for the targeting nanostructure (materialFri, Mn 01 Jun 2012 20:00:48 for more complex materials, as shown schematically in Fig. 2) and a small fraction of the catalyst metal (material Figure 2(b). For instance, the phase diagram of Si-Au takes Mm in the Fig. 2). The laser ablation of the target produces the form shown in Figure 2(b), with Mn replaced by Si the plasma plume containing mainly Mn with a small fracand Mm replaced by Au. In comparison, the phase diagram tion of Mm . The plume quickly cools down and liquid for the binary and ternary materials can be very complex. droplets containing Mm and Mn start to form. The supersatHowever, this complexity can be significantly reduced by uration due to the continuous addition of Mn into the liquid a pseudobinary phase diagram for the catalyst material and will result in the precipitation of Mn and the formation of the targeting nanomaterial. For instance, the pseudobinary the nanowires, as shown in Figure 2(a). The VLS process phase diagram of Au-GaAs exhibit Au-Ga-As liquid as well as GaAs solid,38 which will take a similar form as shown in the Figure 2(b). Both cases exhibit a region with both the liquid phase and the Mn (Si or GaAs)-rich solid phase. Consequently, Au can be used for the catalyst to grow Si or GaAs at the proper temperature, which is also confirmed experimentally17 37 39 Following the similar process, the metal catalyst as well as the growth temperature can be estimated for other nanomaterials with the corresponding phase diagram. Figure 2(c) shows a ZnSe nanowire terminated with a Au particle at about 800  C. ZnSe vapor generated by laser ablation was transported by the carrier gas and then dissolved in the Au film to form nanometer-sized liquid droplets. When the concentration of ZnSe in the droplet become supersaturated at the substrate temperature, ZnSe crystal will precipitate, leading to the growth of ZnSe nanowires. At the end of the growth the remaining droplet solidifies and forms a particle at the tip of the nanowire, as shown in Figure 2(c). The selected area electron diffraction (SAED) pattern confirmed the single crystalline structure of the ZnSe nanowire growing along 001 Fig. 2. Nanowire growth from VLS process. (a) Phase diagram used to guide the selection of metal catalyst and growth temperature. direction. (b) Schematic illustration of the nanowire formation from VLS process. Without introducing impurity metal, laser ablation a (c) Bright-field TEM image of single ZnSe nanowire with a Au tip, with (In 2 O3 )09 (SnO2 )01 target produces droplets with high the inset showing the [100] zone axis SAED pattern. Reprinted with pertin content that initiate the self-catalytically growth of mission from [48], Y. Jiang et al., J. Phys. Chem. B 108, 2784 (2004). ITO nanowires through the VLS process.18 All the ITO © 2004, American Chemical Society. One-Dimensional Nanostructures by Pulsed Laser Ablation nanowires are terminated with the solid catalyst, which is a common phenomenon for the nanowires from the VLS process. REVIEW 3.2. Oxide-Assistant Growth Process Yang very reactive clusters may easily coalesce. The subsequent reconstruction and O migration from the center to the surface result in the crystalline Si core that serves as the nucleus and precursor for the Si nanowires, as shown schematically in Figure 3(a). The Silicon nucleus surrounded by silicon oxide is also confirmed with the transmission electron microscopy (TEM) image of the product at the early stage of the growth, as shown in Figure 3(c). The nanowire nuclei that having their fast growth direction (112 for silicon in this case) normal to the substrate surface undergo faster growth and forms nanowires. Otherwise, the nucleation with undesirable orientation may stop growing or form Si nanoparticle chains due to the renucleation, as shown in the Figures 3(b) and (d). It’s widely accepted that metal can initiate and guide the growth of the nanowires through the VLS process. In addition, oxide has also been shown to assist the growth of nanowires, following a so-called oxide-assisted growth (OAG).40 41 OAG process is a very versatile method and has been used to grow nanowires from elemental materials (Si,42 Ge),43 binary materials (GaAs,11 GaP,11 GaN),11 and even more complex materials (yttrium–barium–copper– oxygen, YBCO).31 Metal is not necessary for OAG process and no metal 3.3. Vapor-Solid Growth droplet is observed at the tip of the nanowire either. In by Ingenta to: place of the metal catalysts, oxide materialsDelivered are purposely In the absence of metal or oxide catalysts, nanowires have UNIVERSITY OF MINNESOTA added to the target with the material of interests. The also been grown directly from vapor, which is normally : 128.101.142.152 growth of the nanowire can be significantly IP enhanced by referred as a vapor–solid (VS) process. In this process, 42 2012 the 20:00:48 the oxide. For instance, mixtures of Si andFri, SiO01 GaAs laser ablation of the target containing the materials of 2 , Jun and Ga2 O3 ,44 are used for the growth Si nanowire and interest produces the vapor that condenses and forms the GaAs nanowire respectively. nanowire directly without any catalyst. Nanowires have The OAG process can be discussed with the growth been grown through VS process from elemental materiof silicon nanowire as an example.45 Silicon oxide in the als (boron),47 binary materials (ZnSe,48 ZnO,49 Al2 O3 ),50 plasma plume is very important for the nucleation and and more complex materials (P -doped Zn1−x Mgx O).51 growth of the nanowire. The density-functional calculaDifferent nanomaterials have been proposed to follow tions indicates the energetically favorable configurations different growth process, such as anisotropic growth, of silicon monoxide clusters (SiO)n for n > 5 contain a defect-induced growth (e.g., through screw dislocation), or sp3 Si core surrounded by a silicon oxide sheath.46 Those self-catalytic growth. Fig. 3. Nanowire growth from OAG process. (a) Schematic illustration and (c) corresponding TEM image of Si nanoparticles precipitated from the decomposition of SiO matrix. (b) Schematic illustration and (d) corresponding TEM image of the fast growth and formation of nanowire from the nanoparticles in a preferred orientation. Reprinted with permission from [45], S. T. Lee et al., J. Mater. Res. 14 , 4503 (1999). © 1999, Springer. 404 Sci. Adv. Mater., 4, 401–406, 2012 Yang One-Dimensional Nanostructures by Pulsed Laser Ablation Fig. 4. Nanowire growth from VS process. (a) TEM image of a single ZnSe nanoribbon and (b) corresponding high-resolution TEM image with the inset showing the [100] zone axis SAED pattern. Reprinted with permission from [48], Y. Jiang et al., J. Phys. Chem. B 108, 2784 (2004). © 2004, American Chemical Society. (c) TEM image of a typical boron nanowire with the inset SAED pattern showing some amorphous halo rings. Reprinted by Ingenta with permission from [47] X. M. Meng et al., Chem. Phys. Delivered Lett. 370, 825 (2003). © 2003, to: Elsevier. UNIVERSITY OF MINNESOTA Sci. Adv. Mater., 4, 401–406, 2012 405 REVIEW IP : 128.101.142.152 From the similar or even the same growth condition, from the target, which indicates a vapor-form agent and Fri, 01proJun 2012 20:00:48 material-transporting process. In other words, boron vapor different nanostructures can be formed from different is produced from the laser ablation and transferred with cesses. For example, the laser ablation of ZnSe target can the carrier gas. Because the temperature decreases at the produce ZnSe nanowire on a Au-coated substrate through downstream, boron nanoclusters form and serve as the a VLS process and ZnSe nanoribbon on bare substrate stable sites for the rapid adhesion of additional boron surface through a VS process.48 Following the aforemenmolecules and eventually result in the formation of boron tioned VLS process, the nanowires grow from the Au parnanowires. ticles, as indicated in Figure 2(c). In order to minimize the 48 interface energy, ZnSe would crystallize with its (001) close-packed plane aligned with the solid-liquid interface, 4. SUMMARY leading to growth along the 001 direction. In comparison, no particle is present in any of the ZnSe nanoribbons, PLA can maintain a good stoichiometry of complex comwhose growth can be understood in terms of vapor–solid positions due to its intense energy that is not available in process. The ZnSe nanoribbons tended to nucleate on the other vapor deposition method. We reviewed the develbare substrate surface and grow along 120 direction, opment of various laser assisted methodologies for the as shown in Figures 4(a) and (b). The growth of ZnSe growth of 1D nanomaterials in the gaseous environment. nanoribbon most likely originates from the anisotropic The classical VLS process developed in the 1960s is still growth kinetics along different crystallographic directions applicable to many nanowires from PLA. When the taras a result of the particular growth conditions. The growth get can decompose and form liquid droplet a self-catalytic of nanoribbons is normally related to two factors. The surgrowth can happen. Oxides are found to be effective to face energy determines the preferential surface that will initiate and enhance the growth through an OAG progrow. The growth kinetics determines the final structure. In cess. In the absence of liquid or oxide, nanowires can other words, the crystal planes with lower surface energy grow through a VS process, although the detailed mechtend to be flat and grow larger, forming the enclosure anism needs further investigation. In addition, defects side surfaces of the nanoribbons. The incoming molecules can also play an important role during the formation of tend to diffuse towards the rougher growth front, resulting nanowires and they have been observed in the nanowires in the fast nanoribbon growth along this direction. This grown from VLS process,17 OAG process,42 as well as VS anisotropic growth may also explain the formation of many process.23 The study of the growth mechanism will extend 1 52 other nanomaterials. our fundamental understanding of the phenomena in the VS process is also proposed for the growth of amornanoscale and provide certain prediction on developing phous boron nanowire in Figure 4(c) from laser ablation novel nanostructures. of a boron target.47 The electron energy loss spectroscopy (EELS) analysis confirmed the boron composition and Acknowledgments: The author is truly grateful for the absence of any other element or impurities. The the financial support from the Department of Mechanical SAED confirmed the amorphous structure of the nanowire. Engineering and the College of Science and Engineering of the University of Minnesota. The boron nanowire was found on the substrate far One-Dimensional Nanostructures by Pulsed Laser Ablation Yang References and Notes 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. REVIEW 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. M. H. Hong, D. K. T. Ng, L. S. Tan, Y. Zhou, and G. X. Chen, J. Alloy. Compd. 449, 250 (2008). Z. W. Pan, Z. R. Dai, and Z. L. Wang, Science 291, 1947 (2001). 27. X. W. Zhao, A. J. Hauser, T. R. Lemberger, and F. Y. Yang, NanZ. H. Wu, X. Y. Mei, D. Kim, M. Blumin, and H. E. Ruda, Appl. otechnology 18, 485608 (2007). Phys. Lett. 81, 5177 (2002). 28. T. Yanagida, A. Marcu, K. Nagashima, H. Tanaka, and T. Kawai, M. T. Bjork, B. J. Ohlsson, T. Sass, A. I. Persson, C. Thelander, J. Appl. Phys. 102, 016102 (2007). M. H. Magnusson, K. Deppert, L. R. Wallenberg, and L. Samuelson, 29. Y. J. Choi, I. S. Hwang, J. H. Park, S. Nahm, and J. G. Park, NanAppl. Phys. Lett. 80, 1058 (2002). otechnology 17, 3775 (2006). I. H. Maiman, Nature 187, 493 (1960). 30. R. Savu and E. Joanni, Scripta. Mater. 55, 979 (2006). H. M. Smith and A. F. Turner, Appl. Optics 4, 147 (1965). 31. Y. F. Zhang, Y. H. Tang, X. F. Duan, Y. Zhang, C. S. Lee, N. Wang, V. Kovalenko, SPIE-Int. Soc. Opt. Eng. 5449, 424 (2004). I. Bello, and S. T. Lee, Chem. Phys. Lett. 323, 180 (2000). H. W. Kroto, J. R. Heath, S. C. O’Brien, R. F. Curl, and R. E. 32. S. Y. Lee, Y. W. Song, and S. Lee, J. Cryst. Growth 310, 4612 Smalley, Nature 318, 162 (1985). (2008). A. Thess, R. Lee, P. Nikolaev, H. J. Dai, P. Petit, J. Robert, C. H. Xu, 33. S. Y. Lee and Y. W. Song, Thin Solid Films 518, 1323 (2009). Y. H. Lee, S. G. Kim, A. G. Rinzler, D. T. Colbert, G. E. Scuseria, 34. S. Y. Lee, Y. W. Song, and K. Kim, Thin Solid Films 518, 1318 D. Tomanek, J. E. Fischer, and R. E. Smalley, Science 273, 483 (2009). (1996). 35. B. Eisenhawer, D. Zhang, R. Clavel, A. Berger, J. Michler, and Y. F. Zhang, R. E. Russo, and S. S. Mao, Appl. Phys. Lett. S. Christiansen, Nanotechnology 22, 075706 (2011). 87, 133115 (2005). 36. Y. Y. Wu, R. Fan, and P. D. Yang, Nano Lett. 2, 83 (2002). T. Q. Jia, H. X. Chen, M. Huang, X. J. Wu, F. L. Zhao, M. Baba, 37. X. F. Duan and C. M. Lieber, Adv. Mater 12, 298 (2000). M. Suzuki, H. Kuroda, J. R. Qiu, R. X. Li, and Z. Z. Xu, Appl. Phys. 38. M. B. Delivered by Ingenta to:Panish, J. Electrochem Soc. 114, 516 (1967). Lett. 89, 101116 (2006). 39. M. L. Taheri, B. W. Reed, T. B. LaGrange, and N. D. Browning, W. S. Shi, Y. F. Zheng, N. Wang, C. S. UNIVERSITY Lee, and S. T. Lee, Adv. OF MINNESOTA Small 4, 2187 (2008). Mater. 13, 591 (2001). IP : 128.101.142.152 40. S. T. Lee, N. Wang, Y. F. Zhang, and Y. H. Tang, Mrs. Bull 24, 36 Y. Jiang, X. M. Meng, J. Liu, Z. R. Hong, C. S. Lee, and S. T. Lee, Fri, 01 Jun 2012 20:00:48 (1999). Adv. Mater. 15, 1195 (2003). 41. R. Q. Zhang, Y. Lifshitz, and S. T. Lee, Adv. Mater. 15, 635 W. K. Maser, E. Munoz, A. M. Benito, M. T. Martinez, G. F. de la (2003). Fuente, Y. Maniette, E. Anglaret, and J. L. Sauvajol, Chem. Phys. 42. N. Wang, Y. F. Zhang, Y. H. Tang, C. S. Lee, and S. T. Lee, Appl. Lett. 292, 587 (1998). Phys. Lett. 73, 3902 (1998). E. Munoz, W. K. Maser, A. M. Benito, M. T. Martinez, G. F. de la 43. Y. F. Zhang, Y. H. Tang, N. Wang, C. S. Lee, I. Bello, and S. T. Fuente, A. Righi, J. L. Sauvajol, E. Anglaret, and Y. Maniette, Appl. Lee, Phys. Rev. B 61, 4518 (2000). Phys. a-Mater. 70, 145 (2000). 44. W. S. Shi, Y. F. Zheng, N. Wang, C. S. Lee, and S. T. Lee, Appl. R. S. Yang and Z. L. Wang, Philos. Mag. 87, 2097 (2007). Phys. Lett. 78, 3304 (2001). R. S. Wagner and W. C. Ellis, Appl. Phys. Lett. 4, 89 (1964). 45. S. T. Lee, Y. F. Zhang, N. Wang, Y. H. Tang, I. Bello, C. S. Lee, A. M. Morales and C. M. Lieber, Science 279, 208 (1998). and Y. W. Chung, J. Mater. Res. 14, 4503 (1999). R. Savu and E. Joanni, J. Mater. Sci. 43, 609 (2008). 46. R. Q. Zhang, M. W. Zhao, and S. T. Lee, Phys. Rev. Lett. 93, 095503 Y. Sun, G. M. Fuge, and M. N. R. Ashfold, Chem. Phys. Lett. 396, (2004). 21 (2004). 47. X. M. Meng, J. Q. Hu, Y. Jiang, C. S. Lee, and S. T. Lee, Chem. Y. J. Zhang, H. Ago, M. Yumura, S. Ohshima, K. Uchida, Phys. Lett. 370, 825 (2003). T. Komatsu, and S. Iijima, Chem. Phys. Lett. 385, 177 (2004). 48. Y. Jiang, X. M. Meng, W. C. Yiu, J. Liu, J. X. Ding, C. S. Lee, and C. Li, D. H. Zhang, S. Han, X. L. Liu, T. Tang, and C. W. Zhou, S. T. Lee, J. Phys. Chem. B 108, 2784 (2004). Adv. Mater. 15, 143 (2003). 49. T. Okada, R. Q. Guo, J. Nishimura, M. Matsumoto, and Z. Q. Liu, D. H. Zhang, S. Han, C. Li, T. Tang, W. Jin, X. L. Liu, D. Nakamura, Appl. Phys. a-Mater. 93, 843 (2008). B. Lei, and C. W. Zhou, Adv. Mater. 15, 1754 (2003). 50. J. Proost and S. Van Boxel, J. Mater. Chem. 14, 3058 I. Amarilio-Burshtein, S. Tamir, and Y. Lifshitz, Appl. Phys. Lett. (2004). 96, 103104 (2010). 51. S. S. Lin, J. I. Hong, J. H. Song, Y. Zhu, H. P. He, Z. Xu, Y. G. Y. Lifshitz, I. Aharonovich, and S. Tamir, Nanotechnology Wei, Y. Ding, R. L. Snyder, and Z. L. Wang, Nano Lett. 9, 3877 19, 065608 (2008). (2009). R. S. Yang, Y. L. Chueh, J. R. Morber, R. Snyder, L. J. Chou, and Z. L. Wang, Nano Lett. 7, 269 (2007). 52. Z. L. Wang, Adv Mater. 15, 432 (2003). 406 Sci. Adv. Mater., 4, 401–406, 2012