Showing papers by "Peidong Yang published in 2005"
TL;DR: This work introduces a version of the dye-sensitized cell in which the traditional nanoparticle film is replaced by a dense array of oriented, crystalline ZnO nanowires, which features a surface area up to one-fifth as large as a nanoparticle cell.
Abstract: Excitonic solar cells1—including organic, hybrid organic–inorganic and dye-sensitized cells (DSCs)—are promising devices for inexpensive, large-scale solar energy conversion. The DSC is currently the most efficient2 and stable3 excitonic photocell. Central to this device is a thick nanoparticle film that provides a large surface area for the adsorption of light-harvesting molecules. However, nanoparticle DSCs rely on trap-limited diffusion for electron transport, a slow mechanism that can limit device efficiency, especially at longer wavelengths. Here we introduce a version of the dye-sensitized cell in which the traditional nanoparticle film is replaced by a dense array of oriented, crystalline ZnO nanowires. The nanowire anode is synthesized by mild aqueous chemistry and features a surface area up to one-fifth as large as a nanoparticle cell. The direct electrical pathways provided by the nanowires ensure the rapid collection of carriers generated throughout the device, and a full Sun efficiency of 1.5% is demonstrated, limited primarily by the surface area of the nanowire array.
5,308 citations
TL;DR: A method for growing vertical ZnO nanowire arrays on arbitrary substrates using either gas-phase or solution-phase approaches is presented and the nanorod arrays made in solution have a rod diameter, length, density, and orientation desirable for use in ordered Nanorod-polymer solar cells.
Abstract: A method for growing vertical ZnO nanowire arrays on arbitrary substrates using either gas-phase or solution-phase approaches is presented. A ∼10 nm-thick layer of textured ZnO nanocrystals with their c axes normal to the substrate is formed by the decomposition of zinc acetate at 200−350 °C to provide nucleation sites for vertical nanowire growth. The nanorod arrays made in solution have a rod diameter, length, density, and orientation desirable for use in ordered nanorod−polymer solar cells.
1,437 citations
TL;DR: By controlling various aspects of nanowire growth, these methods will enable their efficient and economical incorporation into devices to enable their practical integration into devices.
Abstract: Silicon nanowires were synthesized, in a controlled manner, for their practical integration into devices. Gold colloids were used for nanowire synthesis by the vapor−liquid−solid growth mechanism. Using SiCl4 as the precursor gas in a chemical vapor deposition system, nanowire arrays were grown vertically aligned with respect to the substrate. By manipulating the colloid deposition on the substrate, highly controlled growth of aligned silicon nanowires was achieved. Nanowire arrays were synthesized with narrow size distributions dictated by the seeding colloids and with average diameters down to 39 nm. The density of wire growth was successfully varied from ∼0.1−1.8 wires/μm2. Patterned deposition of the colloids led to confinement of the vertical nanowire growth to selected regions. In addition, Si nanowires were grown directly into microchannels to demonstrate the flexibility of the deposition technique. By controlling various aspects of nanowire growth, these methods will enable their efficient and eco...
674 citations
TL;DR: The results illustrate the efficacy of field-effect control in nanofluidics, which could have broad implications on integrated nanof LU circuits for manipulation of ions and biomolecules in sub-femtoliter volumes.
Abstract: We report a nanofluidic transistor based on a metal-oxide-solution (MOSol) system that is similar to a metal-oxide-semiconductor field-effect transistor (MOSFET). Using a combination of fluorescence and electrical measurements, we demonstrate that gate voltage modulates the concentration of ions and molecules in the channel and controls the ionic conductance. Our results illustrate the efficacy of field-effect control in nanofluidics, which could have broad implications on integrated nanofluidic circuits for manipulation of ions and biomolecules in sub-femtoliter volumes.
624 citations
TL;DR: A dynamic gate effect that seems to involve mobile surface charges and causes hysteresis in the transconductance, among other effects is described.
Abstract: ZnO nanowire field-effect transistors (FETs) were fabricated and studied in vacuum and a variety of ambient gases from 5 to 300 K. In air, these n-type nanowire transistors have among the highest mobilities yet reported for ZnO FETs (Ie ) 13 ( 5c m 2 V -1 s -1 ), with carrier concentrations averaging 5.2 ( 2.5 10 17 cm -3 and on-off current ratios ranging from 10 5 to 10 7 . Four probe measurements show that the resistivity of the Ti/Au-ZnO contacts is 0.002-0.02 ?‚cm. The performance characteristics of the nanowire transistors are intimately tied to the presence and nature of adsorbed surface species. In addition, we describe a dynamic gate effect that seems to involve mobile surface charges and causes hysteresis in the transconductance, among other effects.
580 citations
TL;DR: The reaction was shown to be structure insensitive over a series of Pt/SBA-15 materials with particle sizes between 1.7 and 3.6 nm, suggesting that coordinatively unsaturated metal atoms present in small particles are more active for C2H6 hydrogenolysis than the low index planes that dominate in large particles.
Abstract: Platinum nanoparticles in the size range of 1.7-7.1 nm were produced by alcohol reduction methods. A polymer (poly(vinylpyrrolidone), PVP) was used to stabilize the particles by capping them in aqueous solution. The particles were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). TEM investigations demonstrate that the particles have a narrow size distribution. Mesoporous SBA-15 silica with 9-nm pores was synthesized by a hydrothermal process and used as a catalyst support. After incorporation into mesoporous SBA-15 silica using low-power sonication, the catalysts were calcined to remove the stabilizing polymer from the nanoparticle surface and reduced by H2. Pt particle sizes determined from selective gas adsorption measurements are larger than those determined by bulk techniques such as XRD and TEM. Roomtemperature ethylene hydrogenation was chosen as a model reaction to probe the activity of the Pt/SBA-15 materials. The reaction was shown to be structure insensitive over a series of Pt/SBA-15 materials with particle sizes between 1.7 and 3.6 nm. The hydrogenolysis of ethane on Pt particles from 1.7 to 7.1 nm was weakly structure sensitive with smaller particles demonstrating higher specific activity. Turnover rates for ethane hydrogenolysis increased monotonically with increasing metal dispersion, suggesting that coordinatively unsaturated metal atoms present in small particles are more active for C 2H6 hydrogenolysis than the low index planes that dominate in large particles. An explanation for the structure sensitivity is suggested, and the potential applications of these novel supported nanocatalysts for further studies of structure -activity and structure-selectivity relationships are discussed.
543 citations
TL;DR: It is found that addition of silver ion enhances the crystal growth rate along 100, and essentially determines the shape and surface structure of the Pt nanocrystals.
Abstract: We report the synthesis of monodisperse Pt nanocrystals with three different shapescubes, cuboctahedra, and octahedra, selectively, with similar sizes of 9−10 nm by a modified polyol process. We found that addition of silver ion enhances the crystal growth rate along 〈100〉, and essentially determines the shape and surface structure of the Pt nanocrystals. After the reaction, the silver species can be easily removed by repetitive precipitation giving pure Pt nanoparticles. Two-dimensional arrays of the Pt nanocrystals were assembled by using the Langmuir−Blodgett (LB) method. The particles were evenly distributed on the entire substrate, and their surface coverage and density can be precisely controlled by tuning the surface pressure. The resulting Pt LB layers are potential candidates for 2-D model catalysts as a result of their high surface area and the structural uniformity of the metal nanocrystals.
520 citations
TL;DR: Well-aligned stripe patterns with tunable orientation, thickness and periodicity at the micrometre scale were obtained by transferring nanoparticles from a floating film onto a substrate in a dip-coating fashion.
Abstract: Significant advancement has been made in nanoparticle research, with synthetic techniques extending over a wide range of materials with good control over particle size and shape1,2,3,4,5,6. A grand challenge is assembling and positioning the nanoparticles in desired locations to construct complex, higher-order functional structures. Controlled positioning of nanoparticles has been achieved in pre-defined templates fabricated by top–down approaches7,8. A self-assembly method, however, is highly desirable because of its simplicity and compatibility with heterogeneous integration processes. Here we report on the spontaneous formation of ordered gold and silver nanoparticle stripe patterns on dewetting a dilute film of polymer-coated nanoparticles floating on a water surface. Well-aligned stripe patterns with tunable orientation, thickness and periodicity at the micrometre scale were obtained by transferring nanoparticles from a floating film onto a substrate in a dip-coating fashion. This facile technique opens up a new avenue for lithography-free patterning of nanoparticle arrays for various applications including, for example, multiplexed surface-enhanced Raman substrates and templated fabrication of higher-order nanostructures.
432 citations
TL;DR: Inorganic nanotubes were successfully integrated with microfluidic systems to create nanofluidic devices for single DNA molecule sensing and a transition from current decrease to current enhancement during translocation was observed on changing the buffer concentration, suggesting interplay between electrostatic charge and geometric blockage effects.
Abstract: Inorganic nanotubes were successfully integrated with microfluidic systems to create nanofluidic devices for single DNA molecule sensing. Inorganic nanotubes are unique in their high aspect ratio and exhibit translocation characteristics in which the DNA is fully stretched. Transient changes of ionic current indicate DNA translocation events. A transition from current decrease to current enhancement during translocation was observed on changing the buffer concentration, suggesting interplay between electrostatic charge and geometric blockage effects. These inorganic nanotube nanofluidic devices represent a new platform for the study of single biomolecule translocation with the potential for integration into nanofluidic circuits.
316 citations
TL;DR: One-dimensional semiconductor subwavelength optical elements are capable of efficiently guiding light through liquid media, suggesting a role for such materials in microfluidics-based biosensing applications.
Abstract: This article focuses on one-dimensional (1D) semiconductor subwavelength optical elements and assesses their potential use as active and passive components in photonic devices. An updated overview of their optical properties, including spontaneous emission, ultrafast carrier dynamics, cavity resonance feedback (lasing), photodetection, and waveguiding, is provided. The ability to physically manipulate these structures on surfaces to form simple networks and assemblies is the first step toward integrating chemically synthesized nanomaterials into photonic circuitry. These high index semiconductor nanowires are capable of efficiently guiding light through liquid media, suggesting a role for such materials in microfluidics-based biosensing applications.
283 citations
TL;DR: In this paper, the vapor-liquid-solid crystal growth mechanism was briefly introduced for the general synthesis of nanowires of different compositions, sizes, and orientation, and the implication of these heterojunction nanometrics in light-emission and energy conversion was discussed.
Abstract: The following article is based on the Outstanding Young Investigator Award presentation given by Peidong Yang of the University of California, Berkeley, on April 14, 2004, at the Materials Research Society Spring Meeting in San Francisco.Yang was cited for “innovative synthesis of a broad range of nanowires and nanowireheterostructure materials, and the discovery of optically induced lasing in individual nanowire devices.” One-dimensional nanostructures are of both fundamental and technological interest.They not only exhibit interesting electronic and optical properties associated with their low dimensionality and the quantum confinement effect, but they also represent critical components in potential nanoscale devices. In this article, the vapor–liquid–solid crystal growth mechanism will be briefly introduced for the general synthesis of nanowires of different compositions, sizes, and orientation. Unique properties, including light-emission and thermoelectricity, will be discussed. In addition to the recent extensive studies on “single-component” nanowires, of increasing importance is incorporating different interfaces and controlling doping profiles within individual single-crystalline nanowires. Epitaxial growth plays a significant role in fabricating such nanowire heterostructures. Recent research on superlattice nanowires and other nanostructures with horizontal junctions will be presented. The implication of these heterojunction nanowires in light-emission and energy conversion will be discussed. Ways to assemble these one-dimensional nanostructures will also be presented.
TL;DR: Well-ordered mesoporous alumina materials with high surface area and a narrow pore size distribution were synthesized using a sol-gel based self assembly technique.
TL;DR: It is found that nanoribbons function efficiently as waveguides in liquid media and provide a unique means for probing molecules in solution or in proximity to the waveguide surface and presage the use of nanowire waveguide in microfluidics and biology.
Abstract: The manipulation of photons in structures smaller than the wavelength of light is central to the development of nanoscale integrated photonic systems for computing, communications, and sensing. We assemble small groups of freestanding, chemically synthesized nanoribbons and nanowires into model structures that illustrate how light is exchanged between subwavelength cavities made of three different semiconductors. The coupling strength of the optical linkages formed when nanowires are brought into contact depends both on their volume of interaction and angle of intersection. With simple coupling schemes, lasing nanowires can launch coherent pulses of light through ribbon waveguides that are up to a millimeter in length. Also, interwire coupling losses are low enough to allow light to propagate across several right-angle bends in a grid of crossed ribbons. The fraction of the guided wave traveling outside the wire/ribbon cavities is used to link nanowires through space and to separate colors within multiribbon networks. In addition, we find that nanoribbons function efficiently as waveguides in liquid media and provide a unique means for probing molecules in solution or in proximity to the waveguide surface. Our results lay the spadework for photonic devices based on assemblies of active and passive nanowire elements and presage the use of nanowire waveguides in microfluidics and biology.
TL;DR: This overgrowth study provides insights on how different metal ions could be reduced preferentially on different Au nanorod surfaces, so that the composition, aspect ratio, shape, and facet of the resulting nanostructures can be rationally tuned.
Abstract: Electrochemically prepared Au nanorods were used as seeds for the overgrowth of thin shells of gold, silver, and palladium by using a mild reducing agent, ascorbic acid, in the presence of surfactants at ambient condition. The unique crystal facets of the starting nanorods results in anisotropic crystal overgrowth. The overgrowth rates along different crystallographical directions can be further regulated by adding foreign ions or by using different metal reduction methods. This overgrowth study provides insights on how different metal ions could be reduced preferentially on different Au nanorod surfaces, so that the composition, aspect ratio, shape, and facet of the resulting nanostructures can be rationally tuned. These surfactant-stabilized bimetallic Au(core)M(shell) (M=Au, Ag, Pd) nanorod colloids might serve as better substrates in surface-enhanced Raman spectroscopy as well as exhibiting enhanced catalytic properties.
TL;DR: In this paper, the precursor formamide 1 was employed instead of the monomer 2 because isocyanides are relatively unstable in air at room temperature, and the oxidation of the polymer was performed in a mixture of solvents to ensure solubility of reagents and products throughout the process.
Abstract: The precursor formamide 1 was employed instead of the monomer 2 because isocyanides are relatively unstable in air at room temperature. [19] a) [20] Molecular modeling shows that (in common with the phenyl benzo-ate compounds previously described [17]) an angle exists between the planar phenoxy and TTF ring systems. [21] The oxidation of the polymer was performed in a mixture of solvents to ensure solubility of reagents and products throughout the process. The CD spectra of the polymer in THF and in CH 2 Cl 2 are essentially identical, but the Cotton effects observed in CH 2 Cl 2 :MeCN 7:3 are somewhat different, presumably owing to aggregation, as seen in other polymeric systems. See a) B.1 mol of TTF unit, taking the molecular weight of the monomer. The Fe III salt was spectrophotometri-cally titrated as ferric acetate in water prior to use; see: D. [23] For references on calculated and experimental UV-vis absorption of cation radicals and dications of TTF derivatives, see: a) Silicon nanowires are attractive building blocks for nano-scale electronic systems due to their compatibility with existing semiconductor technology. Studies have focused on their synthesis, [1±5] with considerable advances made in the control of structures, [6] electrical, [7,8] and thermal [9] properties. For practical applications, different strategies have been explored to fabricate nanowire-based devices. The pick-and-place approach [7,8,9] has succeeded in making individual devices such
TL;DR: Single-crystalline diluted magnetic semiconductor GaN:Mn nanowires with controlled Mn concentrations have been successfully synthesized and incorporated into devices.
Abstract: Single-crystalline diluted magnetic semiconductor GaN:Mn nanowires with controlled Mn concentrations have been successfully synthesized and incorporated into devices. These nanowires exhibit Curie temperatures above room temperature, magnetoresistances near room temperature, and spin-dependent transport. The nanowires are used as building blocks for the fabrication of GaN:Mn/n-SiC based light-emitting diodes.
TL;DR: These observations reflect the interplay between the competing effects of charge and size of streptavidin on nanochannel conductance.
Abstract: Conductance characteristics of nanofluidic channels (nanochannels) fall into two regimes: at low ionic concentrations, conductance is governed by surface charge while at high ionic concentrations it is determined by nanochannel geometry and bulk ionic concentration. We used aminosilane chemistry and streptavidin-biotin binding to study the effects of surface reactions on nanochannel conductance at different ionic concentrations. Immobilization of small molecules such as aminosilane or biotin mainly changes surface charge, affecting conductance only in the low concentration regime. However, streptavidin not only modifies surface charge but also occludes part of the channel, resulting in observable conductance changes in both regimes. Our observations reflect the interplay between the competing effects of charge and size of streptavidin on nanochannel conductance.
TL;DR: In this paper, the authors demonstrate the potential of the nanowire-in-trench fabrication approach for the integration of VLS grown nanostructures into functional nanomechanical devices.
Abstract: Atomic force microscopy has been used to characterize the mechanical elasticity of Si nanowires synthesized by the vapor-liquid-solid method. The nanowires are horizontally grown between the two facing Si(111) sidewalls of microtrenches prefabricated on a Si(110) substrate, resulting in suspended single and double clamped nanowire-in-trench structures. The deflection of the nanowires is induced and measured by the controlled application of normal forces with the microscope tip. The observed reversibility of the nanowire deflections and the agreement between the measured deflection profiles and the theoretical behavior of single and double clamped elastic beams demonstrate the overall beamlike mechanical behavior and the mechanical rigidity of the clamping ends of the nanowire-in-trench structures. These results demonstrate the potential of the nanowire-in-trench fabrication approach for the integration of VLS grown nanostructures into functional nanomechanical devices.
TL;DR: Transient study reveals the kinetics of field effect modulation is controlled by ion-exchange step, which has potential implications in subfemtoliter analytical technology and large-scale nanofluidic integration.
Abstract: We report the integration of inorganic nanotubes into metal-oxide-solution field effect transistors (FETs) which exhibit rapid field effect modulation of ionic conductance. Surface functionalization, analogous to doping in semiconductors, can switch the nanofluidic transistors from p-type to ambipolar and n-type field effect transistors. Transient study reveals the kinetics of field effect modulation is controlled by ion-exchange step. Nanofluidic FETs have potential implications in subfemtoliter analytical technology and large-scale nanofluidic integration.
TL;DR: It is shown that these modes are largely dipolar in nature and rely on short-range EM coupling between nanowires and the observed dependence on polarization direction confirms prior theoretical predictions that large electromagnetic fields are localized in the interstitials between adjacent Nanowires.
Abstract: Regular-shaped metal nanocrystals and their ensembles can serve as ideal substrates for studying surface-enhanced Raman scattering (SERS). We synthesized well-defined silver nanowires for a systematic study of SERS signal with respect to polarization and structural ordering. The observed dependence on polarization direction confirms prior theoretical predictions that large electromagnetic (EM) fields are localized in the interstitials between adjacent nanowires. We show that these modes are largely dipolar in nature and rely on short-range EM coupling between nanowires.
TL;DR: The size of the gold clusters is shown to have a good correlation with the microemulsion parameters, which in turn controls thesize of the synthesized nanowires.
Abstract: Galvanic displacement processes are employed in water-in-oil microemulsions to deposit gold nanoclusters selectively on Si surfaces and sidewalls. The gold clusters then serve as catalysts to achieve selective growth of vertically and laterally aligned Si nanowire arrays by chemical vapor deposition via the vapor-liquid-solid growth mechanism. The size of the gold clusters is shown to have a good correlation with the microemulsion parameters, which in turn controls the size of the synthesized nanowires.
TL;DR: Thermal stability of facetted Pt nanocrystals on amorphous silica support films was investigated using in situ transmission electron microscopy and temperature-induced wetting of Pt nanoparticles on silica surface can be attributed to the interfacial mixing of Pt and SiO(2) and the resulting negative interface energy.
Abstract: Thermal stability of facetted Pt nanocrystals on amorphous silica support films was investigated using in situ transmission electron microscopy in a temperature range between 25 and 800 °C. The particles started to change their shapes at ∼350 °C. Above 500 °C, the particles spread on the support film with increasing temperature, rather than becoming more spherical. Such temperature-induced wetting of Pt nanoparticles on silica surface can be attributed to the interfacial mixing of Pt and SiO2 and the resulting negative interface energy.
TL;DR: The sensitivity of the confocal microscope combined with a high-resolution piezoelectric stage enables analysis of the crystalline phase and crystallographic orientation of an individual nanowire with an excellent spatial and spectral resolution in a short acquisition time.
Abstract: Polarized Raman spectra and corresponding Raman scattering intensity images of an isolated gallium nitride nanowire with a diameter of 170 nm are presented. The sensitivity of the confocal microscope combined with a high-resolution piezoelectric stage enables analysis of the crystalline phase and crystallographic orientation of an individual nanowire with an excellent spatial and spectral resolution in a short acquisition time.
TL;DR: Atom Transfer Radical Polymerization (ATRP) was used to grow methacrylate polymer chains from the surface of silicon/silica core/shell nanowires, and the resulting structures were characterized by SEM, TEM, and EELS.
Abstract: As a way to control the surface properties of nanowires and nanotubes, we present a method for growing polymer from the surface of silicon/silica core/shell nanowires. After modification of nanowire surfaces with polymer initiators, Atom Transfer Radical Polymerization (ATRP) was used to grow methacrylate polymer chains from the surface. The resulting structures were characterized by SEM, TEM, and EELS. After etching the silicon cores, the resulting polymer-coated nanotubes will have hydrophilic silica cores with hydrophobic polymer shells.
TL;DR: In this article, the new dimeric complex (tBuO)2Ti({mu}-O2Si[OSi(OtBu)3]2)]2 (1), prepared via silanolysis of Ti(IV)/SiO2, is a structural and spectroscopic (NMR, FT-IR, UV-vis, XPS) model for Ti(III) and SiO2.
Abstract: The new dimeric complex [(tBuO)2Ti({mu}-O2Si[OSi(OtBu)3]2)]2 (1), prepared via silanolysis of Ti(OtBu)4 with (HO)2Si[OSi(OtBu)3]2, is a structural and spectroscopic (NMR, FT-IR, UV-vis, XPS) model for Ti(IV)/SiO2. The molecular complex was used to prepare titanium-containing silica materials through both the thermolytic molecular precursor method (yielding TiO2{center_dot}3SiO2) or by grafting 1 onto mesoporous SBA-15 silica. Grafting 1 onto SBA-15 yields mostly isolated Ti(IV) sites, as evidenced by DRUV-vis and photoluminescence spectroscopies. The resulting materials were found to be active and highly selective in the epoxidation of cyclohexene, yielding up to 71% of cyclohexene oxide based on oxidant (cumene hydroperoxide) after 2 h at 65 C in toluene.
TL;DR: It is shown that bilayer nanoribbons-made here of tin dioxide and copper-are convenient structures for observing as-made interfaces as they respond to changing temperature in a transmission electron microscope (TEM) and is well suited to observing interfacial phenomena driven thermally or by the application of mechanical, electrical, or magnetic forces.
Abstract: Solid-solid interfacial processes greatly affect the performance of electronic and composite materials, but probing the dynamics of buried interfaces is challenging and often involves lengthy or invasive sample preparation. We show that bilayer nanoribbons-made here of tin dioxide and copper-are convenient structures for observing as-made interfaces as they respond to changing temperature in a transmission electron microscope (TEM). At low temperatures (<200 degrees C), differential thermal expansion causes the bilayers to bend when heated or cooled, with the motion determined by the extent of Cu-SnO(2) epitaxy. At higher temperatures, we are able to watch-in real time and with nanometer resolution-a progression of grain growth, interdiffusion, island formation, solid-state chemical reactions, and melting. This novel TEM geometry is readily applicable to other nanoribbon/coating combinations and is well suited to observing interfacial phenomena driven thermally or by the application of mechanical, electrical, or magnetic forces.
TL;DR: In this article, the authors add a missing reference to the article published in phys. stat. sol. (b) 241, 2775 (2004), referring to one of their own earlier papers in Nano Lett. 3, 1063 (2003).
Abstract: The authors add a missing reference to the article published in phys. stat. sol. (b) 241, 2775 (2004), referring to one of their own earlier papers in Nano Lett. 3, 1063 (2003). (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Patent•
06 Jan 2005
TL;DR: In this paper, a method for controlling the crystallographic growth direction and geometric and physical characteristics of nanowires using a metal-organic chemical vapor deposition and substrate selection is presented, which is entirely compatible with existing GaN thin-film technology and should enable the realization of a new generation of GaN nanowire devices and systems.
Abstract: A method for controlling the crystallographic growth direction and geometric and physical characteristics of nanowires using a metal-organic chemical vapor deposition and substrate selection. As an illustration of the method, epitaxial growth of wurtzite gallium nitride on (100) y -LiAIO2 and (111) MgO single crystal substrates resulted in the selective growth of nanowires in the orthogonal [110] and [001] directions, respectively. Triangular and hexagonal cross sections were observed as a result of substrate-induced constraints of lattice parameter matching and symmetry registry. These nanowire arrays exhibit a systematic difference in their temperature dependent band-edge emission resulting from the different size, shape, and anisotropic polarity of the nanostructures. Scaling of the synthetic process is entirely compatible with existing GaN thin-film technology and should enable the realization of a new generation of GaN nanowire devices and systems.
TL;DR: In this paper, the authors focus on one-dimensional (1D) semiconductor subwavelength optical elements and assess their potential use as active and passive components in photonic devices, including spontaneous emission, ultrafast carrier dynamics, cavity resonance feedback (lasing), photodetection, and waveguiding.
Abstract: This article focuses on one-dimensional (1D) semiconductor subwavelength optical elements and assesses their potential use as active and passive components in photonic devices. An updated overview of their optical properties, including spontaneous emission, ultrafast carrier dynamics, cavity resonance feedback (lasing), photodetection, and waveguiding, is provided. The ability to physically manipulate these structures on surfaces to form simple networks and assemblies is the first step toward integrating chemically synthesized nanomaterials into photonic circuitry. These high index semiconductor nanowires are capable of efficiently guiding light through liquid media, suggesting a role for such materials in microfluidics-based biosensing applications.
01 Jan 2005
TL;DR: In this article, the top-down and bottom-top approaches were combined to achieve lateral growth of silicon nanowires with controlled size and density, and the beam-like mechanical behavior of these nanostructures were demonstrated by measuring linear elastic deflections of the nanowire under normal forces applied by the microscope tip.
Abstract: Nano-electromechanical systems (NEMS) are emerging as processes are being developed for fabricating structures in the nanometer dimensional range. While offering vastly expanded capabilities such as unprecedented sensitivity to force or to added mass, NEMS present engineers with unprecedented challenges in materials processing, device design, fabrication and integration. Two approaches are utilized for accessing the nanometer domain, the top-down (derived from standard microfabrication paradigm of thin-film deposition, lithography and etching) and bottom-top (synthetic approach). By merging the top-down and bottom-top approaches, we have been able to achieve lateral growth of silicon nanowires with controlled size and density. In one approach, galvanic displacement is combined with microemulsion techniques to selectively deposit catalyst nanoparticles on Si, followed by vapor-liquid-solid growth of Si nanowires [1]. In the second approach, size-selected gold clusters are deposited followed by the VLS growth [2]. Combining these with microfabricated trenches exposing two opposite vertical Si(111) sidewalls, we have obtained singly and doubly clamped beam-like suspended nanostructures, with controlled diameter, density and lengths (Fig. 1). These are important steps towards the realization of a number of NEMS-based devices, including mechanical resonant sensors, nanoseparation devices and thermoelectric generation. Atomic force microscopy is employed to characterize the mechanical elasticity of these suspended Si nanowires [3]. Our experiments provide direct evidence of the mechanical rigidity of the clamping ends of these self-assembled nanowire-in-trench structures. We have also demonstrated the beam-like mechanical behavior of these nanostructures by measuring linear elastic deflections of the nanowires under normal forces applied by the microscope tip and then comparing the results with the theoretical behavior of singly and doubly clamped elastic beams.