Showing papers by "Peidong Yang published in 2003"

One‐Dimensional Nanostructures: Synthesis, Characterization, and Applications

Abstract: This article provides a comprehensive review of current research activities that concentrate on one-dimensional (1D) nanostructures—wires, rods, belts, and tubes—whose lateral dimensions fall anywhere in the range of 1 to 100 nm. We devote the most attention to 1D nanostructures 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 optical, 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.

Low-temperature wafer-scale production of ZnO nanowire arrays.

Abstract: Since the first report of ultraviolet lasing from ZnO nanowires, substantial effort has been devoted to the development of synthetic methodologies for one-dimensional ZnO nanostructures. Among the various techniques described in the literature, evaporation and condensation processes are favored for their simplicity and high-quality products, but these gas-phase approaches generally require economically prohibitive temperatures of 800–900 8C. Despite recent MOCVD schemes that reduced the deposition temperature to 450 8C by using organometallic zinc precursors, the commercial potential of gas-phase-grown ZnO nanowires remains constrained by the expensive and/or insulating (for example, Al2O3) substrates required for oriented growth, as well as the size and cost of the vapor deposition systems. A low-temperature, large-scale, and versatile synthetic process is needed before ZnO nanowire arrays find realistic applications in solar energy conversion, light emission, and other promising areas. Solution approaches to ZnO nanowires are appealing because of their low growth temperatures and good potential for scale-up. In this regard, Vayssieres et al. developed a hydrothermal process for producing arrays of ZnO microrods and nanorods on conducting glass substrates at 95 8C. Recently, a seeded growth process was used to make helical ZnO rods and columns at a similar temperature. Here we expand on these synthetic methods to produce homogeneous and dense arrays of ZnO nanowires that can be grown on arbitrary substrates under mild aqueous conditions. We present data for arrays on four-inch (ca. 10 cm) silicon wafers and two-inch plastic substrates, which demonstrate the ease of commercial scale-up. The simple two-step procedure yields oriented nanowire films with the largest surface area yet reported for nanowire arrays. The growth process ensures that a majority of the nanowires in the array are in direct contact with the substrate and provide a continuous pathway for carrier transport, an important feature for future electronic devices based on these materials. Well-aligned ZnO nanowire arrays were grown using a simple two-step process. In the first step, ZnO nanocrystals (5–10 nm in diameter) were spin-cast several times onto a four-inch Si(100) wafer to form a 50–200-nm thick film of crystal seeds. Between coatings, the wafer was annealed at 150 8C to ensure particle adhesion to the wafer surface. The ZnO nanocrystals were prepared according to the method of Pacholski. A NaOH solution in methanol (0.03m) was added slowly to a solution of zinc acetate dihydrate (0.01m) in methanol at 60 8C and stirred for two hours. The resulting nanoparticles are spherical and stable for at least two weeks in solution. After uniformly coating the silicon wafer with ZnO nanocrystals, hydrothermal ZnO growth was carried out by suspending the wafer upside-down in an open crystallizing dish filled with an aqueous solution of zinc nitrate hydrate (0.025m) and methenamine or diethylenetriamine (0.025m) at 90 8C. Reaction times spanned from 0.5 to 6 h. The wafer was then removed from solution, rinsed with deionized water, and dried. A field-emission scanning electron microscope (FESEM) was used to examine the morphology of the nanowire array across the entire wafer, while single nanowires were characterized by transmission electron microscopy (TEM). Nanowire crystallinity and growth direction were analyzed by X-ray diffraction and electron diffraction techniques. SEM images taken of several four-inch samples showed that the entire wafer was coated with a highly uniform and densely packed array of ZnO nanowires (Figure 1). X-ray diffraction (not shown) gave a wurtzite ZnO pattern with an enhanced (002) peak resulting from the vertical orientation of the nanowires. A typical synthesis (1.5 h) yielded wires with diameters ranging between 40–80 nm and lengths of 1.5–2 mm.

Thermal conductivity of individual silicon nanowires

Abstract: The thermal conductivities of individual single crystalline intrinsic Si nanowires with diameters of 22, 37, 56, and 115 nm were measured using a microfabricated suspended device over a temperature range of 20–320 K. Although the nanowires had well-defined crystalline order, the thermal conductivity observed was more than two orders of magnitude lower than the bulk value. The strong diameter dependence of thermal conductivity in nanowires was ascribed to the increased phonon-boundary scattering and possible phonon spectrum modification.

Langmuir-Blodgett Silver Nanowire Monolayers for Molecular Sensing Using Surface-Enhanced Raman Spectroscopy

Abstract: Langmuir−Blodgett technique was used to assemble monolayers (with areas over 20 cm2) of aligned silver nanowires that are ∼50 nm in diameter and 2−3 μm in length. These nanowires possess pentagonal cross-sections and pyramidal tips. They are close-packed and are aligned parallel to each other. The resulting nanowire monolayers serve as excellent substrates for surface-enhanced Raman spectroscopy (SERS) with large electromagnetic field enhancement factors (2 × 105 for thiol and 2,4-dinitrotoluene, and 2 × 109 for Rhodamine 6G) and can readily be used in ultrasensitive, molecule-specific sensing utilizing vibrational signatures.

Single-crystal gallium nitride nanotubes.

Abstract: Since the discovery of carbon nanotubes in 1991 (ref. 1), there have been significant research efforts to synthesize nanometre-scale tubular forms of various solids. The formation of tubular nanostructure generally requires a layered or anisotropic crystal structure. There are reports of nanotubes made from silica, alumina, silicon and metals that do not have a layered crystal structure; they are synthesized by using carbon nanotubes and porous membranes as templates, or by thin-film rolling. These nanotubes, however, are either amorphous, polycrystalline or exist only in ultrahigh vacuum. The growth of single-crystal semiconductor hollow nanotubes would be advantageous in potential nanoscale electronics, optoelectronics and biochemical-sensing applications. Here we report an 'epitaxial casting' approach for the synthesis of single-crystal GaN nanotubes with inner diameters of 30-200 nm and wall thicknesses of 5-50 nm. Hexagonal ZnO nanowires were used as templates for the epitaxial overgrowth of thin GaN layers in a chemical vapour deposition system. The ZnO nanowire templates were subsequently removed by thermal reduction and evaporation, resulting in ordered arrays of GaN nanotubes on the substrates. This templating process should be applicable to many other semiconductor systems.

Optical Cavity Effects in ZnO Nanowire Lasers and Waveguides

Abstract: Wide band gap semiconductor nanostructures with near-cylindrical geometry and large dielectric constants exhibit two-dimensional ultraviolet and visible photonic confinement (i.e., waveguiding). Combined with optical gain and suitable resonant feedback, the waveguiding behavior facilitates highly directional lasing at room temperature in controlled-growth nanowires. We have characterized the nanowire emission in detail with high-resolution optical microscopy. The waveguiding behavior of individual zinc oxide (ZnO) nanowires depends on the wavelength of the emitted light and the directional coupling of the photoluminescence (PL) to the emission dipoles of the nanowire. Polarization studies reveal two distinct regimes of PL characterized by coupling to either guided (bound) or radiation modes of the waveguide, the extent of which depends on wire dimensions. Pumping with high pulse energy engenders the transition from spontaneous to stimulated emission, and analysis of the polarization, line width, and line spacing of the laser radiation facilitates identification of the transverse and longitudinal cavity modes and their gain properties. Interpretation of the lasing spectra as a function of pump fluence, with consideration of ZnO material properties and ultrafast excitation dynamics, demonstrates a transition from exciton (fluence 1 IJ/cm 2 ) and gain saturation behavior (fluence > 3 IJ/cm 2 ) modified by the constraints of the nanoscale cylindrical cavity.

Dendritic nanowire ultraviolet laser array.

Abstract: Self-organized dendritic crystal growth is explored to assemble uniform semiconductor nanowires into highly ordered one-dimensional microscale arrays that resemble comb structures. The individual ZnO nanowires have uniform diameters ranging from 10 to 300 nm. They are evenly spaced on a stem with a regular periodicity of 0.1-2 micrometer. Under optical excitation, each individual ZnO nanowire serves as a Fabry-Perot optical cavity, and together they form a highly ordered nanowire ultraviolet laser array.

Morphogenesis of One‐Dimensional ZnO Nano‐ and Microcrystals

Abstract: timescales of hundreds of seconds (0.00 lm 2 s ±1). It is interesting that in the neutral MEA experiment, in which some of the surface amine groups are protonated, the diffusion coefficient was slightly lower than in the other cases. A surprising result was obtained when measuring the diffusion coefficients of the three sizes of MESA particles on the neutral surface derivatized with hexadecanethiol (C 16 H 33 SH). Comparing D values to those found on the MESA surface, we found near agreement for all but the small-diameter particles (6 lm ” 90 nm), which reproducibly stuck to the substrate immediately on contact. For larger particles, the long-range elec-trostatic force (presumably between the negatively charged sulfonate groups and an image charge in the substrate) is expected to dominate, [22] but for smaller ones the short-range van der Waals force is more important. Apparently, the two are closely balanced for the C 16 H 33 SH functionalized surface with rods of the size investigated here. While the scaling of these interactions is not completely understood at present, these initial studies provide some guidance as to the conditions that are desirable for nanorod assembly experiments. The observation of pH-dependent diffusion may, for example, be useful for affixing particles to specific areas or in specific conformations on basic surfaces. More importantly, the particle tracking method described here provides a simple and convenient method for quantifying the surface diffusion of non-spherical particles under arbitrary conditions. Experimental Whatman Al 2 O 3 filter membranes that contain 300±350 nm diameter internal pores were used as a template material. For 90 nm internal pore diameter, Al 2 O 3 membranes were prepared in-house by the electrochemical anodization of an Al plate [23]. In both cases, one face of the membranes was coated with approximately 150 nm of thermally evaporated Ag. More Ag was electrodepos-ited (Silver 1024, Technic, Inc.) directly onto the evaporated Ag in order to close any open pores. This Ag layer was then used as the back contact in the electrochemical cell, and more Ag was deposited, further filling-in the pores. The membrane and cell were rinsed with deionized H 2 O, and Au solution was added (Orotemp, Technic, Inc.). Plating was stopped with the desired rod length was reached. The Ag backing was removed by dissolving in 2 mL of 50 % HNO 3 , and the Al 2 O 3 template was dissolved …

Metalorganic Chemical Vapor Deposition Route to GaN Nanowires with Triangular Cross Sections

Abstract: High-quality gallium nitride nanowires have been synthesized via metal-initiated metalorganic chemical vapor deposition for the first time. Excellent substrate coverage was observed for wires prepared on silicon, c-plane, and a-plane sapphire substrates. The wires were formed via the vapor−liquid−solid mechanism with gold, iron, or nickel as growth initiators and were found to have widths of 15-200 nm. Transmission electron microscopy confirmed that the wires were single-crystalline and were oriented predominantly along the [210] or [110] direction. Wires growing along the [210] orientation were found to have triangular cross-sections. Transport measurements confirmed that the wires were n-type and had electron mobilities of ∼65 cm2/V·s. Photoluminescence measurements showed band edge emission at 3.35 eV (at 5 K), with a marked absence of low-energy emission from impurity defects.

Thermal conductivity of Si/SiGe superlattice nanowires

Abstract: The thermal conductivities of individual single crystalline Si/SiGe superlattice nanowires with diameters of 58 and 83 nm were measured over a temperature range from 20 to 320 K. The observed thermal conductivity shows similar temperature dependence as that of two-dimensional Si/SiGe superlattice films. Comparison with the thermal conductivity data of intrinsic Si nanowires suggests that alloy scattering of phonons in the Si–Ge segments is the dominant scattering mechanism in these superlattice nanowires. In addition, boundary scattering also contributes to thermal conductivity reduction.

Self-Organized GaN Quantum Wire UV Lasers

Abstract: Quantum wire lasers are generally fabricated through complex overgrowth processes with molecular beam epitaxy. The material systems of such overgrown quantum wires have been limited to Al−Ga−As−P, which leads to emission largely in the visible region. We describe a simple, one-step chemical vapor deposition process for making quantum wire lasers based on the Al−Ga−N system. A novel quantum-wire-in-optical-fiber (Qwof) nanostructure was obtained as a result of spontaneous Al−Ga−N phase separation at the nanometer scale in one dimension. The simultaneous excitonic and photonic confinement within these coaxial Qwof nanostructures leads to the first GaN-based quantum wire UV lasers with a relatively low threshold.

Fabrication of silica nanotube arrays from vertical silicon nanowire templates.

Abstract: A simple thermal oxidation-etching process was developed to translate vertical silicon nanowire arrays into silica nanotube arrays. The obtained nanotubes perfectly retain the orientation of original silicon nanowire arrays. The inner tube diameter ranges from 10 to 200 nm. High-temperature oxidation produces relative thick, rigid, and pinhole-free walls that are made of condensed silica. This method could be useful for fabrication of single nanotube sensors and nanofluidic systems.

Nanotechnology: Wires on water

Abstract: A centuries-old technique for transporting timber is the inspiration for a new method of assembling nanowires into large-scale, ordered patterns that could form the basis of a new generation of electronic devices.

ZnO Nanoribbon Microcavity Lasers

Abstract: Exploration of one-dimensional semiconductor nanostruc-tures has led to great progress in the areas of optoelectronics in the past few years. [1] Nanolasers, [2±4] waveguides, [5] frequency converters (second or third harmonic generators), [6] photoconductive optical switches, [7] and sensors [8,9] have been developed based on oxide nanowires. The single crystalline nature of these nanowires makes them ideal candidates for probing size-dependent and dimensionality-controlled physical phenomena. In particular, the transverse nanoscale and longitudinal microscale dimensions (i.e., large aspect ratio) as well as well-defined faceting nature of such nanostructures enable the observation of unique optical confinement and mi-crocavity effects. [2±4] Previously, hexagonal cylindrical ZnO nanowires have been examined as a laser gain medium. These nanocylinders indeed can serve as miniaturized Fabry± Perot optical cavities in the ultraviolet (UV) region with high gain and quality factor. [10] Based on classical waveguide theory , different transverse optical modes can be sustained within waveguides of different cross-sections. [11] It is thus fundamentally interesting to examine the optical cavity effects within nanowires with cross-sections other than hexagonal. Herein, we examine the lasing phenomenon from ZnO nanoribbons with pseudo-rectangular cross-section. Cavity-length dependent optical mode analysis reveals different cavity effects from those of hexagonal nanocylinders. ZnO, being environmentally benign and having a large bandgap (3.37 eV) and exciton binding energy (60 meV), has been considered as a promising candidate for UV light-emitting diodes and laser diodes. It has also displayed an astonishing series of nanostructures with different morphologies. Among many others, the hexagonal nanocylinders, [2±4] nano-ribbons, [5,13] tetrapods, [12] and comb-like nanowire arrays [14] are highly interesting for their fundamental significance in revealing microcavity effects as well as near-field optical coupling phenomena. The ZnO nanoribbons in this work were synthesized using two methodologies. One is a carbon thermal reduction process at 900 C. [12] The other is the high temperature (1350 C) approach developed by Wang and co-workers. [13] The two methods involve different growth mechanisms. The low temperature process utilized Au as the growth initiator. The observation of Au nanoparticles on the nanoribbon tips in the transmission electron microscope (TEM, Figure 1B) suggests that this is a vapor±liquid±solid (VLS) growth process. In contrast , the high-temperature approach is a vapor±solid (VS) growth process without any foreign metal initiation. Regardless of these different growth mechanisms, the growth directions and side facets of the produced nanoribbons are identical for the two sets of the samples. The length of the …

Watching GaN Nanowires Grow

Abstract: We report real-time high temperature transmission electron microscopy observations of the growth of GaN nanowires via a self-catalytic vapor-liquid-solid (VLS) mechanism. High temperature thermal decomposition of GaN in a vacuum yields nanoscale Ga liquid droplets and gallium/nitrogen vapor species for the subsequent GaN nanowire nucleation and growth. This is the first direct observation of self-catalytic growth of nanowires via the VLS mechanism and suggests new strategies for synthesizing electronically pure single-crystalline semiconductor nanowires.

SnO2 Nanoribbons as NO2 Sensors: Insights from First Principles Calculations

Abstract: SnO2 nanoribbons with exposed (1 0 1) and (0 1 0) surfaces have recently been demonstrated to be highly effective NO2 sensors even at room temperature. The sensing mechanism is examined here through first principles density functional theory (DFT) calculations. We show that the most stable adsorbed species involve an unexpected NO3 group doubly bonded to Sn centers. Significant electron transfer to the adatoms explains an orders-of-magnitude drop in electrical conductance. X-ray absorption spectroscopy indicates predominantly NO3 species on the surface, and computed binding energies are consistent with adsorbate stability up to 700 K. Nanoribbon responses to O2 and CO sensing are also investigated.

Low‐Temperature Wafer‐Scale Production of ZnO Nanowire Arrays.

Abstract: Since the first report of ultraviolet lasing from ZnO nanowires, substantial effort has been devoted to the development of synthetic methodologies for one-dimensional ZnO nanostructures. Among the various techniques described in the literature, evaporation and condensation processes are favored for their simplicity and high-quality products, but these gas-phase approaches generally require economically prohibitive temperatures of 800–900 8C. Despite recent MOCVD schemes that reduced the deposition temperature to 450 8C by using organometallic zinc precursors, the commercial potential of gas-phase-grown ZnO nanowires remains constrained by the expensive and/or insulating (for example, Al2O3) substrates required for oriented growth, as well as the size and cost of the vapor deposition systems. A low-temperature, large-scale, and versatile synthetic process is needed before ZnO nanowire arrays find realistic applications in solar energy conversion, light emission, and other promising areas. Solution approaches to ZnO nanowires are appealing because of their low growth temperatures and good potential for scale-up. In this regard, Vayssieres et al. developed a hydrothermal process for producing arrays of ZnO microrods and nanorods on conducting glass substrates at 95 8C. Recently, a seeded growth process was used to make helical ZnO rods and columns at a similar temperature. Here we expand on these synthetic methods to produce homogeneous and dense arrays of ZnO nanowires that can be grown on arbitrary substrates under mild aqueous conditions. We present data for arrays on four-inch (ca. 10 cm) silicon wafers and two-inch plastic substrates, which demonstrate the ease of commercial scale-up. The simple two-step procedure yields oriented nanowire films with the largest surface area yet reported for nanowire arrays. The growth process ensures that a majority of the nanowires in the array are in direct contact with the substrate and provide a continuous pathway for carrier transport, an important feature for future electronic devices based on these materials. Well-aligned ZnO nanowire arrays were grown using a simple two-step process. In the first step, ZnO nanocrystals (5–10 nm in diameter) were spin-cast several times onto a four-inch Si(100) wafer to form a 50–200-nm thick film of crystal seeds. Between coatings, the wafer was annealed at 150 8C to ensure particle adhesion to the wafer surface. The ZnO nanocrystals were prepared according to the method of Pacholski. A NaOH solution in methanol (0.03m) was added slowly to a solution of zinc acetate dihydrate (0.01m) in methanol at 60 8C and stirred for two hours. The resulting nanoparticles are spherical and stable for at least two weeks in solution. After uniformly coating the silicon wafer with ZnO nanocrystals, hydrothermal ZnO growth was carried out by suspending the wafer upside-down in an open crystallizing dish filled with an aqueous solution of zinc nitrate hydrate (0.025m) and methenamine or diethylenetriamine (0.025m) at 90 8C. Reaction times spanned from 0.5 to 6 h. The wafer was then removed from solution, rinsed with deionized water, and dried. A field-emission scanning electron microscope (FESEM) was used to examine the morphology of the nanowire array across the entire wafer, while single nanowires were characterized by transmission electron microscopy (TEM). Nanowire crystallinity and growth direction were analyzed by X-ray diffraction and electron diffraction techniques. SEM images taken of several four-inch samples showed that the entire wafer was coated with a highly uniform and densely packed array of ZnO nanowires (Figure 1). X-ray diffraction (not shown) gave a wurtzite ZnO pattern with an enhanced (002) peak resulting from the vertical orientation of the nanowires. A typical synthesis (1.5 h) yielded wires with diameters ranging between 40–80 nm and lengths of 1.5–2 mm.

Sacrificial template method of fabricating a nanotube

Abstract: Methods of fabricating uniform nanotubes are described in which nanotubes were synthesized as sheaths over nanowire templates, such as using a chemical vapor deposition process. For example, single-crystalline zinc oxide (ZnO) nanowires are utilized as templates over which gallium nitride (GaN) is epitaxially grown. The ZnO templates are then removed, such as by thermal reduction and evaporation. The completed single-crystalline GaN nanotubes preferably have inner diameters ranging from 30 nm to 200 nm, and wall thicknesses between 5 and 50 nm. Transmission electron microscopy studies show that the resultant nanotubes are single-crystalline with a wurtzite structure, and are oriented along the direction. The present invention exemplifies single-crystalline nanotubes of materials with a non-layered crystal structure. Similar “epitaxial-casting” approaches could be used to produce arrays and single-crystalline nanotubes of other solid materials and semiconductors. Furthermore, the fabrication of multi-sheath nanotubes are described as well as nanotubes having multiple longitudinal segments.

One-Dimensional Nanostructures: Synthesis, Characterization, and Applications

Abstract: This article provides a comprehensive review of current research activities that concentrate on one-dimensional (1D) nanostructures—wires, rods, belts, and tubes—whose lateral dimensions fall anywhere in the range of 1 to 100 nm. We devote the most attention to 1D nanostructures 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 optical, 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.

Single nanowire waveguides and lasers

Abstract: Quasi one-dimensional nanostructures are unique probes of cavity quantum electrodynamics because they are capable of exhibiting photonic and/or electronic confinement in two dimensions. The near-cylindrical geometry and sharp end facets of zinc oxide (ZnO) nanowires enable the realization of active nanoscale optical cavities that exhibit UV/blue photoluminescence (PL) waveguiding and lasing action at room temperature under appropriate optical pumping conditions. Study of individual nanostructures is crucial for isolating geometry-dependent effects, and here it is achieved through both near- and far-field microscopies. The polarization of the emitted PL or lasing from individual nanostructures characterizes the coupling of the spontaneous emission to cavity modes, depending both on the wavelength of the emitted light and the nature of the emitting species (i.e., excitons and intrinsic defects in various charge states). In addition, the spectral evolution of the lasing/PL as a function of the pump fluence indicates both exciton and electron-hole plasma dynamics. Variations of size, geometry, and material on the prototypical cylindrical ZnO nanowire lead to further observation of unique photonic and/or carrier confinement effects in novel nanostructures.

Functional bimorph composite nanotapes and methods of fabrication

Abstract: A two-layer nanotape that includes a nanoribbon substrate and an oxide that is epitaxially deposited on a flat surface of the nanoribbon substrate is described. A method for making the nanotape that includes providing plural substrates and placing the substrates in a quartz tube is also described. The oxide is deposited on the substrate using a pulsed laser ablation deposition process. The nanoribbons can be made from materials such as SnO2, ZnO, MgO, AI2O3, Si, GaN, or CdS. Also, the sintered oxide target can be made from materials such as TiO2, transition metal doped TiO2 (e.g., Co0.05Ti0.95O2), BaTiO3, ZnO, transition metal doped ZnO (e.g., Mn0.1Zn0.9O and Ni0.1Zn0.9O), LaMnO3, BaTiO3, PbTiO3, Yba2Cu3Oz, or SrCu2O2 and other p-type oxides. Additionally, temperature sensitive nanoribbon/metal bilayers and their method of fabrication by thermal evaporation are described. Metals such as Cu, Au, Ti, AI, Pt, Ni and others can be deposited on top of the nanoribbon surface. Such devices bend significantly as a function of temperature and are suitable as, for example, thermally activated nanoscale actuators.

Nanowire Arrays for Thermoelectric Devices

Abstract: This study reports on the fabrication and characterization of two prototype thermoelectric devices constructed of either silicon (Si) or bismuth telluride (Bi2 Te3 ) nanowire arrays. The growth mechanisms and fabrication procedures of the Si and Bi2 Te3 devices are different as described in this paper. To characterize the thermoelectric device components, current-voltage (I-V) characteristics were first used to estimate their performance. For the Si device, the I-V characteristics suggest ohmic contacts at the metal-semiconductor junction. For the Bi2 Te3 device, the I-V characteristics curve showed a rectifying contact. Either low doping of the Bi2Te3 or surface contamination, i.e. native oxide, may cause the rectifying contact. The reversible Peltier effects occurring within the Si device were analyzed using a micro-thermocouple. Results indicated possible limitations of using Si nanowire arrays for the thermoelectric device.Copyright © 2003 by ASME

Single‐Crystal Gallium Nitride Nanotubes.

Abstract: Since the discovery of carbon nanotubes in 1991 (ref. 1), there have been significant research efforts to synthesize nanometre-scale tubular forms of various solids. The formation of tubular nanostructure generally requires a layered or anisotropic crystal structure. There are reports of nanotubes made from silica, alumina, silicon and metals that do not have a layered crystal structure; they are synthesized by using carbon nanotubes and porous membranes as templates, or by thin-film rolling. These nanotubes, however, are either amorphous, polycrystalline or exist only in ultrahigh vacuum. The growth of single-crystal semiconductor hollow nanotubes would be advantageous in potential nanoscale electronics, optoelectronics and biochemical-sensing applications. Here we report an 'epitaxial casting' approach for the synthesis of single-crystal GaN nanotubes with inner diameters of 30-200 nm and wall thicknesses of 5-50 nm. Hexagonal ZnO nanowires were used as templates for the epitaxial overgrowth of thin GaN layers in a chemical vapour deposition system. The ZnO nanowire templates were subsequently removed by thermal reduction and evaporation, resulting in ordered arrays of GaN nanotubes on the substrates. This templating process should be applicable to many other semiconductor systems.

Photochemical Synthesis of Gold Nanorods.

Abstract: Gold nanorods have been synthesized by photochemically reducing gold ions within a micellar solution. The aspect ratio of the rods can be controlled with the addition of silver ions. This process reported here is highly promising for producing uniform nanorods, and more importantly it will be useful in resolving the growth mechanism of anisotropic metal nanoparticles due to its simplicity and the relatively slow growth rate of the nanorods.

Design and Fabrication of Silica Nanotube Arrays and Nanofluidic Devices

Abstract: Silica nanotube arrays have been fabricated from vertical silicon nanowire templates prepared by Vapor-Liquid-Solid (VLS) epitaxial growth. The silicon nonowire arrays are uniformly oxidized by a dry oxidation process in a tube furnace heated to 1000 °C and filled with pure O2 , which gives SiO2 sheaths with continuous silicon wire cores inside. A dry etch process with XeF2 as etchant removes the silicon cores, thus silica nanotube arrays are obtained. The resulting silica nanotubes are more than 10 μm long with an inner diameter range from 10 to 200 nm. A nonofluidic device has been fabricated based on individual silica nanotubes. The nanotube is drop-casted onto a silica substrate from a solution and after the solution is dried out, a 200 nm thick chrome layer is sputter deposited. A 3 μm wide Cr gate structure is patterned across the silica nanotube. Photoresist is patterned to define the hydrophilic (silica) and hydrophobic (photoresist) regions. The hydrophilic regions form a reservoir at each side of the nanotube, thus giving us a nanofluidic device. Ion current flow through the nanotube has been measured in 1 M KCl solution and the measured current matches the theoretical estimation reasonably well.Copyright © 2003 by ASME