Vertically aligned cadmium chalcogenide nanowire arrays on muscovite mica: a demonstration of epitaxial growth strategy

We report a strategy for achieving epitaxial, vertically aligned cadmium chalcogenide (CdS, CdSe, and CdTe) nanowire arrays utilizing van der Waals epitaxy with (001) muscovite mica substrate. The nanowires, grown from a vapor transport process, exhibited diameter uniformity throughout their length, sharp interface to the substrate, and positive correlation between diameter and length with preferential growth direction of [0001] for the monocrystalline wurtzite CdS and CdSe nanowires, but of [111] for zinc blende CdTe nanowires, which also featured abundant twinning boundaries. Self-catalytic vapor-liquid-solid mechanism with hydrogen-assisted thermal evaporation is proposed to intepret the observations. Optical absorption from the as-grown CdSe nanowire arrays on mica at 10 K revealed intense first-order exciton absorption and its longitudinal optical phonon replica. A small Stokes shift (～1.3 meV) was identified, suggesting the high quality of the nanowires. This study demonstrated the generality of van der Waals epitaxy for the growth of nanowire arrays and their potential applications in optical and energy related devices.     

Morphology control of layer-structured gallium selenide nanowires

Layer-structured group III chalcogenides have highly anisotropic properties and are attractive materials for stable photocathodes and battery electrodes. We report the controlled synthesis and characterization of layer-structured GaSe nanowires via a catalyst-assisted vapor-liquid-solid (VLS) growth mechanism during GaSe powder evaporation. GaSe nanowires consist of Se-Ga-Ga-Se layers stacked together via van der Waals interactions to form belt-shaped nanowires with a growth direction along the [11-20], width along the [1-100], and height along the [0001] direction. Nanobelts exhibit a variety of morphologies including straight, zigzag, and saw-tooth shapes. These morphologies are realized by controlling the growth temperature and time so that the actual catalysts have a chemical composition of Au, Au-Ga alloy, or Ga. The participation of Ga in the VLS catalyst is important for achieving different morphologies of GaSe. In addition, GaSe nanotubes are also prepared by a slow growth process.     

Surfactant effect of Au on Ga adsorption on GaAs(0001) surface

Basing on the density-functional theory, we have investigated the atomistic and electronic structures of Ga adsorption on GaAs(0001) surface with pre-absorbed Au monolayer for the understanding of the surfactant effect of Au on the growth of GaAs nanowires. The results show that the deposited Au layer enhances significantly the stability of the Ga adatom on substrate compared to the direct adsorption of Ga on GaAs(0001) surface. The reason is that more electrons of the Ga 6p levels are transferred toward surface bands of substrate because of mediation of the Au layer. It is revealed that Au plays a catalyst role to assist the adsorption of Ga on GaAs(0001) surface. These results offer valuable information in the epitaxial growth of semiconductor nanowires.     

Fabrication and photoluminescence of high-quality ternary CdSSe nanowires and nanoribbons

Ternary alloy CdSSe nanowires and nanoribbons were successfully grown through a one-step thermal evaporation route using Au as a catalyst. The nanostructures obtained are uniform in diameter, and have smooth surfaces. High-resolution transmission electron microscopy, energy dispersive x-ray spectra and x-ray diffraction showed that both the nanowires and the nanoribbons have high-quality single-crystalline nature, and their compositions can be determined as CdS(0.6)Se(0.4) and CdS(0.3)Se(0.7), respectively. The mechanisms of formation of these two different nanostructures were discussed. The photoluminescence measurements showed very strong band-edge emission for both samples, which further demonstrates the single-crystal nature of the as-obtained CdSSe alloys. This finding may be extended for fabricating other composition-tunable 1D?ternary alloy nanostructures.     

Catalyst incorporation at defects during nanowire growth

Scanning and transmission electron microscopy was used to correlate the structure of planar defects with the prevalence of Au catalyst atom incorporation in Si nanowires. Site-specific high-resolution imaging along orthogonal zone axes, enabled by advances in focused ion beam cross sectioning, reveals substantial incorporation of catalyst atoms at grain boundaries in <110> oriented nanowires. In contrast, (111) stacking faults that generate new polytypes in <112> oriented nanowires do not show preferential catalyst incorporation. Tomographic reconstruction of the catalyst-nanowire interface is used to suggest criteria for the stability of planar defects that trap impurity atoms in catalyst-mediated nanowires.     

Controlling heterojunction abruptness in VLS-grown semiconductor nanowires via in situ catalyst alloying

For advanced device applications, increasing the compositional abruptness of axial heterostructured and modulation doped nanowires is critical for optimizing performance. For nanowires grown from metal catalysts, the transition region width is dictated by the solute solubility within the catalyst. For example, as a result of the relatively high solubility of Si and Ge in liquid Au for vapor-liquid-solid (VLS) grown nanowires, the transition region width between an axial Si-Ge heterojunction is typically on the order of the nanowire diameter. When the solute solubility in the catalyst is lowered, the heterojunction width can be made sharper. Here we show for the first time the systematic increase in interface sharpness between axial Ge-Si heterojunction nanowires grown by the VLS growth method using a Au-Ga alloy catalyst. Through in situ tailoring of the catalyst composition using trimethylgallium, the Ge-Si heterojunction width is systematically controlled by tuning the semiconductor solubility within a metal Au-Ga alloy catalyst. The present approach of alloying to control solute solubilities in the liquid catalyst may be extended to increasing the sharpness of axial dopant profiles, for example, in Si-Ge pn-heterojunction nanowires which is important for such applications as nanowire tunnel field effect transistors or in Si pn-junction nanowires.     

Control of growth orientation and shape for epitaxially grown In2O3 nanowires on a-plane sapphire

Vertically aligned indium oxide nanowires were grown on a-plane sapphire substrate by the method of catalyst-assisted carbothermal reduction. The morphology and crystal structure of the nanowires are determined by X-ray diffraction, transmission electron microscopy and field-emission scanning electron microscopy. Two types of In₂O₃ nanowires were found by controlling the growth conditions. The nanowires with a hexagonal cross-section were shown to grow in [1 1 1] direction, whereas those with a square cross-section grow in [0 0 1] direction. In addition to the temperature effects, the concept of supersaturation in Au catalyst is proposed to explain the formation of these two types of nanowires. Besides, tapering, which is explained with the interplay between the vapor-liquid-solid and vapor-solid growth mechanisms, is observed in the nanowires.     

Synthesis and optical properties of CdS nanowires by a simple chemical deposition

CdS nanowires were prepared by a paired cell method, using highly ordered porous anodic alumina (PAA) membranes as templates. The morphology, structure, and composition of these nanowires were characterized by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), energy dispersive X-ray spectroscopy (EDS), and selected area electron diffraction (SAED) pattern. SEM images indicated that nanowires arrays are highly ordered and coincide with the contours of PAA. The EDS analysis, combined with HRTEM images and SEAD patterns, confirmed the formation of CdS. The formation and growth mechanism of CdS nanowires, as well as the optical properties, were also analyzed in details. The results indicated that the formation and growth of CdS nanowires experience from initial nuclei, nanoparticles, linear pearl-chain structures to final nanowires, attributed to electrostatic adsorption and ions diffusion. A strong emission with a maximum around 430 nm was observed from the synthesized CdS nanowires, which was attributed to the recombination of trapped electron/hole pairs. The absorption edge in UV–Vis spectrum of CdS/PAA nanoarrays showed a blue shift due to the quantum confinement effects. The technique can be used to fabricate other compound nanowires and newly light-emitting semiconductor materials.     

Solvothermal route to CdS nanocrystals

We developed a facile solvothermal route to various CdS nanocrystals in diethylenetriamine. Well-crystalline CdS nanowires and flower-like CdS nanocrystals were obtained at temperatures 220°C and 200°C, respectively. The nanowires have smooth surfaces and are grown along polar [0?0?0?1] direction. The flower-like CdS nanocrystals have hierarchical architectures composed of fine nanowires with lengths up to 10?µm. The effects of temperature on the morphology and structure of the final products are also investigated. The experimental observations indicates that high temperature facilitates fabricate CdS nanocrystals with regular morphology and excellent crystallisation. Moreover, it is found that organic solvent diethylenetriamine plays an important role in the growth of CdS nanocrystals. Diethylenetriamine can bind Cd²⁺ ions to form intermediate complex, resulting in final CdS nanocrystals with regular morphologies.     

Large scale synthesis of highly pure single crystalline tellurium nanowires by thermal evaporation method

Single crystalline tellurium nanowires were successfully synthesized in large scale by a facile approach of vaporizing tellurium metal and condensing the vapor in an inert atmosphere onto a Si substrate. Tellurium was evaporated by heating at 300 degrees C at 1 torr and condensed on the Si substrate at 100-150 degrees C, in the downstream of argon (Ar) gas at a flow rate of 25 sccm for 30 min. The as-synthesized nanowires have diameters between 100-300 nm and lengths up to several micrometers. The single crystalline nanowires grew in a preferred [0001] direction. The obtained nanowires were highly pure as only tellurium metal was used in the vaporization process, and no other reagent, surfactant, or template were used for the growth. This low temperature and high-yield approach to the tellurium nanowires synthesis may facilitate its industrial production for various applications.     

A novel method for preparing vertically grown single-crystalline gold nanowires

A surfactant-free, template-less and seed-less method, namely the thermal-assisted photoreduction (TAP) process, has been developed to synthesize vertically grown Au nanowires (30-80?nm in diameter and about 2?μm in length) on the surface of thin film titanium dioxide (TiO(2)), which is locally excited by blackbody radiation. The Au nanowires thus produced are single-crystalline with a preferred [Formula: see text] growth direction. The electrical behavior investigated using a nanomanipulation device indicates that the Au nanowires possess an excellent electrical resistivity of about 3.49 × 10(-8)?Ω?m.     

Ferromagnetism driven by oxygen vacancies in SnO2 nanowires

Room temperature ferromagnetism has been observed in SnO2 nanowires synthesized by a chemical vapor deposition using Au layers as catalyst. The nanowires are homogeneous and single-crystalline grown along the [101] direction, with diameters ranging from 25 to 100 nm and length greater than 20 microm. The special magnetization reaches 0.114 emu/g for the nanowires with diameter of approximately 25 nm and reduces with increasing diameters. Branched SnO2 nanowires were prepared via a two-step vapor-liquid-solid approach, and an enhanced magnetization was obtained. To the contrary, the nanowires annealed at 1300 degrees C in air were completely transformed into the particles and exhibit weakened magnetization. These results demonstrate that the ferromagnetic properties of the samples depend on the surface-to-volume ratio of nanowires. With a combined study of photoluminescence, our results reveal that the oxygen vacancies at the surface of nanowires contribute to the ferromagnetism of SnO2 nanowires. This argument is further confirmed by a sequential annealing in a rich-oxygen atmosphere, then in a low vacuum condition.     

A generic approach for embedded catalyst-supported vertically aligned nanowire growth

We demonstrate a general approach for growing vertically aligned, single-crystalline nanowires of any material on arbitrary substrates by using plasma-sputtered Au/Pd thin films as a catalyst through the vapor-liquid-solid process. The high-energy sputtered Au/Pd atoms form a reactive interface with the substrate forming nanoclusters which get embedded in the substrate, thus providing mechanical stability for vertically aligned nanowire growth. We demonstrate that our approach for vertically aligned nanowire growth is generic and can be extended to various complex substrates such as conducting indium tin oxide.     

One-dimensional cadmium sulfide (CdS) nanostructures by the solvothermal process: Controlling crystal structure and morphology aided by different solvents

Cadmium sulfide (CdS) nanowires and nanorods with different aspect ratios were successfully synthesized by the solvothermal method aided with various solvents, namely ethylenediamine, ethanolamine and triethylene tetraamine. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) analyses revealed that, highly pure CdS nanostructures were crystallized with different structures and preferable growth orientations depending on solvent nature. Field emission electron microscope (FE-SEM) images showed that the aspect ratio of CdS nanostructures depends upon the dielectric constant and boiling temperature of solvents. CdS nanostructures with the highest aspect ratio in the form of nanowire were obtained using ethylenediamine, whereas CdS nanorods were produced in the presence of ethanolamine and triethylene tetraamine solvents. The absorption edge of CdS nanowires and nanorods showed a blue shift compared with that of bulk CdS due to an increase in their band gap energies.     

MS(M=Cd或Zn)纳米粒子负载TiO_2纳米线的制备及可见光催化性能

以具有层状结构的钛酸盐纳米线、CdCl2或ZnSO4及硫代乙酰胺为原料,采用两步水热合成法制备了高温稳定的六方相CdS或低温稳定的立方相ZnS纳米粒子负载的TiO2纳米线复合材料。首先,CdCl2或ZnSO4与钛酸盐纳米线在水热条件下进行离子交换制得含Cd2+或Zn2+的钛酸盐纳米线;然后,在硫代乙酰胺溶液中于160℃下直接处理含Cd2+或Zn2+的钛酸盐纳米线就可获得负载有硫化物纳米粒子的TiO2复合纳米线。它们在酸浸后,TiO2纳米线的表面仍存在少量硫化物纳米粒子。通过测试酸浸后样品、纯TiO2纳米线和商用P25对亚甲基蓝水溶液的可见光催化降解实验结果证实,含CdS纳米粒子样品的光催化活性最高。     

Growth of germanium nanowires on silicon(111) substrates by molecular beam epitaxy

Heteroepitaxial growth of Ge nanowires was carried out on Si(111) substrates by MBE. Au seeds were used as precursor for the VLS growth of the nanowires. Even if the Au droplets do not act as catalyst for the dissociation of gas, they are local preferential areas where the energetic barrier of Ge nucleation is lowered compare to the remaining non activated surface. Two sets of Au seeds were used as precursors for the VLS process. The first set have an average diameter of 125 nm and the second of 25 nm. In-situ RHEED monitoring showed a Au wetting layer between these seeds before the nanowires growth as well as at the end of the Ge nanowires growth. It means that the wetting layer acted as a surfactant from the Si(111) surface to the Ge grown layer between the nanowires. Analysis of SEM images brought the fact that the diffusion of gold from the droplets on the surface and the sidewalls of the nanowires via the Ostwald ripening is a key parameter of the growth of the nanowires.     

Catalyst-nanostructure interaction and growth of ZnS nanobelts

Details of the vapour-liquid-solid Au droplet catalysed growth of ZnS nanobelts are elucidated in this work. The inclination of the Au droplet after solidification shows that it is indeed in the liquid state during nanobelt growth. Numerous stacking faults are observed when (0001) wurtzite is the side surface of the nanobelt. Compressive stress at the droplet-nanobelt-atmosphere triple interface is the cause of the stacking faults. Sawteeth-like structures are observed on the Zn-terminated polar (0001) side surface only. These surfaces are chemically active, while S-terminated [Formula: see text] surfaces and non-polar surfaces are not. On these active surfaces, autocatalysed vapour-solid growth leads to the formation of the observed sawteeth.     

Catalytic action of gold and copper crystals in the growth of carbon nanotubes

Multi-wall carbon nanotubes are grown in a chemical vapor deposition process by using bulk gold and copper substrates as catalysts. Nanotube growth starts from a nanometer-sized roughness on the metal surfaces and occurs in a mechanism where the catalyst particle is either at the tip (Au) or root (Cu) of the growing nanotube. Whereas Au leads to nanotubes with good structural perfection, nanotubes grown from Cu show a higher density of defects. High-resolution transmission electron microscopy shows the bonding between Au and carbon at the metal-nanotube interface whereas no bonds between Cu and carbon occur. Highly mobile Au or Cu atoms adsorb at the growing edge of a carbon nanotube from where diffusion along the nanotube wall can lead to the formation of Au or Cu nanowires inside the central hollow of carbon nanotubes.     

Vertical single-crystal ZnO nanowires grown on ZnO:Ga/glass templates

Vertical single-crystal ZnO nanowires with uniform diameter and uniform length were selectively grown on ZnO:Ga/glass templates at 600/spl deg/C by a self-catalyzed vapor-liquid-solid process without any metal catalyst. It was found that the ZnO nanowires are grown preferred oriented in the [002] direction with a small X-ray diffraction full-width half-maximum. Photoluminescence, field-emission scanning electron microscopy, and high-resolution transmission electron microscopy measurements also confirmed good crystal quality of our ZnO nanowires. Field emitters using these ZnO nanowires were also fabricated. It was found that threshold field of the fabricated field emitters was 14 V//spl mu/m. With an applied electric field of 24 V//spl mu/m, it was found that the emission current density was around 0.1 mA/cm/sup 2/.     

Preparation and photovoltaic properties of the composite based on CdS nanorod arrays and PCPDTBSe

Vertically aligned CdS nanorod arrays (NRA) were synthesized through facile one-step and template-free hydrothermal approach directly on indium tin oxide (ITO)-coated glass substrates. A low band gap polymer, namely Poly[4,4-bis(2-ethylhexyl)cyclopenta[2,1-b:3,4-b’]dithiophene-2,6-diyl-alt-2,1,3-benzoselenadiazole-4,7-diyl] (PCPDTBSe) was synthesized by the Stille coupling reaction. An organic/inorganic hybrid solar cell prototype device was fabricated based on the as-prepared CdS NRA and PCPDTBSe (ITO/CdS arrays/PCPDTBSe/Au), showing a power conversion efficiency of 0.10 %.