REVIEW One-Dimensional Nanostructures: Synthesis, Characterization, and Applications** By Younan Xia,* Peidong Yang,* Yugang Sun, Yiying Wu, Brian Mayers, Byron Gates, Yadong Yin, Franklin Kim, and Haoquan Yan This article provides a comprehensive review of current research activities that concentrate on one-dimensional (1D) nanostructuresÐwires, rods, belts, and tubesÐ whose lateral dimensions fall anywhere in the range of 1 to 100 nm. We devote the most attention to 1D nanostruc- tures that have been synthesized in relatively copious quantities using chemical methods. We begin this article with an overview of synthetic strategies that have been exploited to achieve 1D growth. We then elaborate on these approaches in the following four sections: i) anisotropic growth dictated by the crystallographic structure of a solid material; ii) anisotropic growth confined and directed by various templates; iii) anisotropic growth kinetically controlled by supersaturation or through the use of an appropriate capping reagent; and iv) new concepts not yet fully demonstrated, but with long-term potential in generating 1D nanostructures. Following is a discussion of techniques for generating various types of important heterostructured nanowires. By the end of this article, we highlight a range of unique properties (e.g., thermal, mechanical, electronic, optoelectronic, optical, nonlinear opti- cal, and field emission) associated with different types of 1D nanostructures. We also briefly discuss a number of methods potentially useful for assembling 1D nanostructures into functional devices based on crossbar junctions, and complex architectures such as 2D and 3D periodic lattices. We conclude this review with personal perspectives on the directions towards which future research on this new class of nanostructured materials might be directed. 1. Introduction technology. There are a large number of opportunities that might be realized by making new types of nanostructures, NanostructuresÐstructures that are defined as having at or simply by down-sizing existing microstructures into the least one dimension between 1 and 100 nmÐhave received 1±100 nm regime. The most successful example is provided steadily growing interests as a result of their peculiar and by microelectronics, where ªsmallerº has meant greater per- fascinating properties, and applications superior to their formance ever since the invention of integrated circuits: bulk counterparts.[1±3] The ability to generate such minus- more components per chip, faster operation, lower cost, and cule structures is essential to much of modern science and less power consumption.[4] Miniaturization may also repre- sent the trend in a range of other technologies. In informa- ± tion storage, for example, there are many active efforts to [*] Prof. Y. Xia, Dr. Y. Sun, B. Mayers, Dr. B. Gates, Dr. Y. Yin develop magnetic and optical storage components with criti- Department of Chemistry, University of Washington cal dimensions as small as tens of nanometers.[5] It is also Seattle, WA 98195 (USA) E-mail: [email protected] clear that a wealth of interesting and new phenomena are Prof. P. Yang, Dr. Y. Wu, F. Kim, H. Yan associated with nanometer-sized structures, with the best Department of Chemistry, University of California established examples including size-dependent excitation or Berkeley, CA 94720 (USA) [6] [7] E-mail: [email protected] emission, quantized (or ballistic) conductance, Coulomb [8] [**] The UW and UCB groups contributed equally to this review article. This blockade (or single-electron tunneling, SET), and metal± work has been supported in part by AFOSR (UW); ONR (UW); DOE insulator transition.[9] It is generally accepted that quantum (UCB); NSF (DMR-9983893 at UW, DMR-0092086 and CTS-0 103 609 at confinement of electrons by the potential wells of nanome- UCB); Alfred P. Sloan Foundation (UW and UCB); Camille and Henry Dreyfus Foundation (UW and UCB); David and Lucile Packard Founda- ter-sized structures may provide one of the most powerful tion (UW); Beckman Foundation (UCB); Research Corporation (UCB); (and yet versatile) means to control the electrical, optical, and the 3M Company (UCB). B. M., B. G., and Y. Y. thank the Center for Nanotechnology at the UW for two IGERT Fellowships supported by magnetic, and thermoelectric properties of a solid-state NSF (DGE-9 987 620) and one fellowship award. functional material. Adv. Mater. 2003, 15, No. 5, March 4 Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 0935-9648/03/0503-0353 $ 17.50+.50/0 353 Y. Xia et al./One-Dimensional Nanostructures Two-dimensional (2D) nanostructures (or quantum reduction (or quantum confinement). They are also expected REVIEW wells)[10] have been extensively studied by the semiconductor to play an important role as both interconnects and functional community because they can be conveniently prepared using units in fabricating electronic, optoelectronic, electrochemi- techniques such as molecular beam epitaxy (MBE).[11] Thanks cal, and electromechanical devices with nanoscale dimensions. to the efforts from many research groups, significant progress In comparison with quantum dots and wells, the advancement has also been made with regard to zero-dimensional (0D) of 1D nanostructures has been slow until very recently, as hin- nanostructures (or quantum dots)[12] in the past two decades. dered by the difficulties associated with the synthesis and fab- For example, a rich variety of chemical methods have already rication of these nanostructures with well-controlled dimen- been developed for generating quantum dots with well-con- sions, morphology, phase purity, and chemical composition. trolled dimensions and from a broad range of materials.[13] Although 1D nanostructures can now be fabricated (in the With quantum dots as a model system, a lot of interesting setting of a research laboratory) using a number of advanced chemistry and physics has been learned by studying the evolu- nanolithographic techniques,[22] such as electron-beam tion of their fundamental properties with size.[14] Using quan- (e-beam) or focused-ion-beam (FIB) writing,[23] proximal- tum dots as active components, various types of nanoscale probe patterning,[24] and X-ray or extreme-UV lithography,[25] devices have also been fabricated as prototypes in many further development of these techniques into practical routes research laboratories. Notable examples include quantum-dot to large quantities of 1D nanostructures from a diversified lasers,[15] single-electron transistors,[16] memory units,[17] sen- range of materials, rapidly, and at reasonably low costs, still sors,[18] optical detectors,[19] and light-emitting diodes requires great ingenuity. In contrast, unconventional methods (LEDs).[20] For most of these applications, it is believed that based on chemical synthesis might provide an alternative and the dimension of an individual quantum dot may represent intriguing strategy for generating 1D nanostructures in terms the ultimate limit to the miniaturization of currently existing of material diversity, cost, throughput, and the potential for functional devices. high-volume production.[26] Recently, one-dimensional (1D) nanostructures such as This article reviews current research activities that center wires, rods, belts, and tubes have also become the focus of on nanometer-sized wires, rods, belts, and tubes. Since carbon intensive research owing to their unique applications in meso- nanotubes (CNTs) have already been reviewed by a number scopic physics and fabrication of nanoscale devices.[21] It is of authors,[27] we intend to exclude them from the scope of generally accepted that 1D nanostructures provide a good sys- this article. The main text of this article is organized into eight tem to investigate the dependence of electrical and thermal sections: The next section (Section 2) explicitly discusses sev- transport or mechanical properties on dimensionality and size eral concepts related to the growth of nanowires, as well as Younan Xia was born in Jiangsu, China, in 1965. He received a B.S. degree in chemical physics from the University of Science and Technology of China (USTC) in 1987, and then worked as a graduate student for four years at the Fujian Institute of Research on the Structure of Matter, Academia Sinica. He came to the United States in 1991, received a M.S. degree in inorganic chemistry from the University of Pennsylvania (with Prof. A. G. MacDiarmid) in 1993, and a Ph.D. degree in physical chemistry from Harvard University (with Prof. G. M. Whitesides) in 1996. He has been an Assistant Professor of Chemistry at the University of Washington in Seattle since 1997, and was promoted to the rank of tenured Associate Professor in 2001. His research interests include nanostructured materials, self-assembly, microfabrication, surface modification, conducting polymers, microfluidic and microanalytical systems, and novel devices for optics, optoelectronics, and display. Peidong Yang was born in Jiangsu, China, in 1971. He received a B.S. degree in chemistry from the University of Science and Technology of China (USTC) in 1993. He came to the United States in 1993, received a Ph.D. degree in inorganic chemistry from Harvard University (with Prof. Charles Lieber) in 1997. He then spent two years as a postdoctoral fellow in Prof. Galen Stucky's lab at University of California, Santa Barbara. He has been an Assistant Professor of Chemistry at the University of California, Berkeley since 1999. His research interests