磁控溅射法可控制备有序碲纳米线阵列

采用磁控溅射法在较低基底温度下(200 ℃)制备了有序碲纳米线阵列, 并利用X射线衍射、扫描电镜和透射电镜对所制备薄膜进行了相、形貌和微观结构分析。结果表明, 所制备的纳米线阵列由单晶碲纳米线组成, 单根碲纳米线具有针状形貌, 并沿[101]晶向生长, 平均直径和长度分别为100 nm和1 μm。氩气压力和基底温度均对碲纳米线阵列的形成具有重要影响, 以平衡碲原子沿[101]晶向和(101)晶面方向的扩散和生长。提出了碲纳米线阵列的生长机制, 包括吸附、结合、成核和生长等过程。     

Ultrafast growth of single-crystalline Si nanowires

Silicon nanowires (SiNWs) have been catalytically synthesized by heat treatment of Si nanopowder at 980 °C. The SiNWs comprise crystalline Si nanoparticles interconnected with metal catalyst. The formation mechanism of nanowires generally depends on the presence of Fe catalysts in the synthesis process of solid–liquid–solid (SLS). Although gas phase of vapor–liquid–solid (VLS) method can be used to produce various of different nanowire materials, growth model based on the SLS mechanism by heat treatment is more ascendant for providing ultrafast growth of single-crystalline Si nanowires and controlling the diameter of them easily. The growth of single-crystalline SiNWs and morphology were discussed.     

The influences of technological conditions and Au cluster islands on morphology of Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> nanowires grown by VLS method on GaAs substrate

The synthesis of semiconductor nanowires is more and more interested to the applications for building blocks of the innovative nano-sized devices and circuits, but the research and fabrication of these nanowires are also holding a number of difficulties and challenges. Among many different kinds of semiconductor nanowires, Ga₂O₃ is increasingly grown for many promising applications in nano-device production, namely nanowire LED and Laser. So far there are many synthesizing methods of semiconductor nanowires, among them the vapor–liquid–solid (VLS) method is simple, cheap and popular. However, when we use the VLS method for nanowire growth, various technological problems exist. This paper aims at investigating some influences of the growth technological conditions and Au metal catalyst on the morphology of Ga₂O₃ nanowire grown by VLS on GaAs substrate. The main considering factors include the different growing temperatures and times, the effects of Au diffusion, Au droplets formation, Au cluster islands formation, and gas volume of the growing tube/ampoule at the 10⁻¹ torr low air pressure. The obtained experimental results regarding the structural properties of nanowires under these effects investigated by scanning electron microscopy, field emission scanning electron microscopy, high angle annular dark field and bright field, scanning transmission electron microscopy, energy-dispersive X-ray techniques, and focus ion beam are presented and discussed.     

Hydrothermal synthesis of one-dimensional single-crystalline rutile TiO2 nanostructures and their application in dye-sensitized solar cells

One-dimensional (1D) TiO2 nanowire arrays are fabricated on transparent conducting substrates via a low temperature hydrothermal route for application in dye-sensitized solar cells (DSSCs). The as-prepared sample on fluorine-doped tin oxide (FTO) substrate is found to be single-crystalline rutile TiO2 structures from X-ray and electron diffractions. The length and diameter of the nanowires depend mainly on the growth time and temperature. With increasing the reaction time, the growth rate becomes slower and the interface adhesion between the growth nanowires and the substrate becomes weaker. In the same time the adjacent nanowires aggregate to larger the apparent diameter of the nanowire making the gaps among the nanowires to disappear at last. The nanowires exhibit flower-like morphology on the non-conducting surface of FTO substrate. By using TiO2 nanowire arrays with 2 microm long on FTO substrate as the photoanode in DSSCs, an overall light conversion efficiency of 1.58% is achieved with an open circuit voltage of 0.714 V, a short circuit current density of 4.68 mA cm(-2), and a fill factor of 0.472.     

反应源温度对ZnO纳米线阵列定向性及发光性能的影响

以Au薄膜为催化剂、ZnO与碳混合粉末为反应源,采用碳热还原法在单晶Si衬底上制备了ZnO纳米线阵列.通过扫描电子显微镜( SEM)、X射线衍射仪(XRD)、荧光分光光度计对样品的表征,研究了反应源温度对ZnO纳米线阵列的定向性和光致发光性能的影响.样品在源温度920℃条件下沿(002)方向择优生长,定向性最好,温度过低不利于ZnO纳米线阵列密集生长,而温度过高导致Zn原子二次蒸发,因而也不利于纳米线阵列的定向和择优生长;样品在源温度880℃有最强的近紫外带边发射,表明温度过高和过低都不利于ZnO晶体结构的优化;由于ZnO纳米线在缺氧氛围下生长,氧空位是缺陷存在的主要形式,因此所有样品都有较强的绿光发射.温度升高导致纳米线生长速度提高而增加了氧空位缺陷数量,从而使样品绿峰强度增强并在源温度920℃时达最大值,但温度的进一步升高可导致ZnO纳米线表面Zn元素的蒸发而降低氧空位缺陷的数量,从而抑制绿峰强度.     

Temperature conditions for GaAs nanowire formation by Au-assisted molecular beam epitaxy

Molecular beam epitaxial growth of GaAs nanowires using Au particles as a catalyst was investigated. Prior to the growth during annealing, Au alloyed with Ga coming from the GaAs substrate, and melted. Phase transitions of the resulting particles were observed in?situ by reflection high-energy electron diffraction (RHEED). The temperature domain in which GaAs nanowire growth is possible was determined. The lower limit of this domain (320?°C) is close to the observed catalyst solidification temperature. Below this temperature, the catalyst is buried by GaAs growth. Above the higher limit (620?°C), the catalyst segregates on the surface with no significant nanowire formation. Inside this domain, the influence of growth temperature on the nanowire morphology and crystalline structure was investigated in detail by scanning electron microscopy and transmission electron microscopy. The correlation of the nanowire morphology with the RHEED patterns observed during the growth was established. Wurtzite GaAs was found to be the dominant crystal structure of the wires.     

Synthesis of the tube-brush-shaped SiC nanowire array on carbon fiber and its photoluminescence properties

Tube-brush-shaped nanostructure of SiC nanowires was synthesized on polyacrylonitrile-based carbon fibers. The morphology and microstructure of the nanowires were characterized by X-ray powder diffraction, field emission scanning electron microscopy and high-resolution transmission electron microscopy. A quasi-periodically twin structure with (111) plane as the boundary along the SiC nanowires was observed. The vapor-solid growth mechanism of the SiC nanowire brush is also discussed. Moreover, some separated blue-shifted photoluminescence peaks around 469 nm were measured. The separated blue-shifted emission peaks are attributed to the quantum confinement of nanoscaled twin segments along each nanowire rather than the apparent diameters of the nanowires. The SiC nanowire brushes hopefully can find potential applications in nanotechnology.     

GaN nanowires: CVD synthesis and properties

The growth of GaN nanowires from Ga and NH₃ sources in the flow of Ar carrier gas using a chemical vapor deposition (CVD) system was systematically studied. The substrates used were Si(111) and Si(100). Fabricated nanowires were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX). We investigated the influence of growth temperature, catalyst used, Ga amount, and the ratio of Ar and NH₃ flow rates on the morphology and properties of GaN nanowires. We found that the best results were obtained for a growth temperature of 950 °C. Optimal catalysts were Au and metallic Ni, while the use of nickel nitrate was found to lead to formation of SiO_x nanowire bunches in addition to GaN nanowires. For the optimal temperature and catalyst used, the influence of the Ga to N ratio on the nanowire growth was studied. It was found that different types of nanostructures are observed in relatively Ga-rich and in relatively N-rich conditions. Growth mechanisms of different types of nanowires, including the stacked-cone nanowires and the microscale structures formed by lateral growth under N-rich conditions, are discussed.     

A study of the growth mechanism of CVD-grown ZnO nanowires

ZnO nanowires were grown by CVD process using both pure Zn powder and a mixture of ZnO and graphite powders as the Zn source, and the key factors controlling nanowire growth were identified. In both processes, the partial pressure of zinc vapor determines the prevailing growth morphology and is sensitive to the growth conditions. In the case of Zn powder as the source, the predominant growth mechanism is driven by self-catalyzed growth on the Si substrate, and in the case of a mixture of ZnO and graphite used as the source, the formation of ZnO nanowires is controlled by the vapor–liquid-solid mechanism, where the gold particles serve as catalyst.     

Measurement of Local Si‐Nanowire Growth Kinetics Using In situ Transmission Electron Microscopy of Heated Cantilevers

A technique to study nanowire growth processes on locally heated microcantilevers in situ in a transmission electron microscope has been developed. The in situ observations allow the characterization of the nucleation process of silicon wires, as well as the measurement of growth rates of individual nanowires and the ability to observe the formation of nanowire bridges between separate cantilevers to form a complete nanowire device. How well the nanowires can be nucleated controllably on typical cantilever sidewalls is examined, and the measurements of nanowire growth rates are used to calibrate the cantilever‐heater parameters used in finite‐element models of cantilever heating profiles, useful for optimization of the design of devices requiring local growth.     

Mechanism of self-assembled growth of ordered GaAs nanowire arrays by metalorganic vapor phase epitaxy on GaAs vicinal substrates

We report the self-catalyzed growth of GaAs nanowire arrays by metalorganic vapor phase epitaxy (MOVPE) on GaAs vicinal substrates. The effect of substrate misorientation on the nanowire growth and the influence of growth parameters such as temperature and input V/III ratio have been studied in detail. Variation in the nanowire growth mechanism and consequential changes in the nanowire growth morphology were observed. A VLS growth mechanism with negligible effect of the vicinal surface gave rise to randomly distributed droplet-terminated GaAs nanowires at 400?°C and multiprong root-grown GaAs nanowire clusters at 500?°C with low V/III ratio. The substrate misorientation effect was dominant at 500?°C with higher V/III ratio, in which case the combined effect of the vicinal surface and the self-catalyzed Ga droplets assisted the realization of self-assembled and crystallographically oriented epitaxial nanowire arrays through the vapor-solid mechanism.     

A cathodoluminescence study of the influence of the seed particle preparation method on the optical properties of GaAs nanowires

Cathodoluminescence at 8?K is used to compare the optical properties of AlGaAs-capped GaAs nanowires, grown by metal-organic vapour phase epitaxy and seeded by gold particles prepared by different methods. Six different methods were used to fabricate and deposit gold seed particles onto GaAs substrates: colloid particles, aerosol particles and particles defined by electron beam lithography. The nanowires were grown with and without an in?situ annealing step prior to the nanowire growth. The morphology showed no significant differences between the nanowires. The emissions from ensembles of nanowires have the same peak position, irrespective of seed particle type. Without the in?situ annealing step prior to the nanowire growth, there are significant differences in the emission intensity and emission patterns from nanowires grown from different seed particles. When an in?situ annealing step is included, all the resulting nanowires show identical optical emission intensity and emission patterns. This shows the importance of using an in?situ annealing step prior to growth. This study demonstrates that different preparation methods for gold seed particles can be used to produce GaAs nanowires with highly similar optical properties. The choice of particle preparation method to be used can therefore be based on availability and cost.     

Silicidation of silicon nanowires by platinum

The solid-state reaction between platinum and silicon nanowires grown by the vapor-liquid-solid technique was studied. The reaction product PtSi is an attractive candidate for contacts to p-type silicon nanowires due to the low barrier height of PtSi contacts to p-type Si in the planar geometry, and the formation of PtSi was the motivation for our study. Silicidation was carried out by annealing Pt on Si nanowires from 250 to 700 degrees C, and the reaction products were characterized by transmission electron microscopy. Strikingly different morphologies of the reacted nanowires were observed depending on the annealing temperature, platinum film thickness, silicon nanowire diameter, and level of unintentional oxygen contamination in the annealing furnace. Conversion to PtSi was successfully realized by annealing above 400 degrees C in purified N2 gas. A uniform morphology was achieved for nanowires with an appropriate combination of Si nanowire diameter and Pt film thickness to form PtSi without excess Pt or Si. Similar to the planar silicidation process, oxygen affects the nanowire silicidation process greatly.     

Synthesis of Co3O4 nanowire arrays supported on Ni foam for removal of volatile organic compounds

Crystalline Co3O4 nanowire arrays freely supported on Ni foam are successfully synthesized using a template-free method. The effects of reaction time, concentration of reactants, and temperature on the morphology of the nanowires are studied. The results indicate that uniform Co3O4 nanowires could be synthesized at 90 degrees C, and a transformation of the samples' morphology from nanoparticles to nanowires to microrods is observed by controlling the concentration of the reactants. The well-ordered nanowires synthesized under the selected reaction conditions are composed of spinel Co3O4 with diameters of 500-580 nm and lengths of 6-8 microm. These nanowires show good catalytic activity for the ozone catalytic oxidation of toluene.     

The characteristic of the ZnO nanowire morphology grown by the hydrothermal method on various surface-treated seed layers

ZnO nanowires with various density and diameters were obtained by altering the wetting properties of aluminum doped ZnO (AZO) seed layers. AZO seed layers deposited on glass substrates were treated by inductor coupled plasmas (ICP) or atmospheric pressure plasma (APP) systems in order to control wettability before ZnO nanowire growth. Morphology of ZnO nanowires grown by the hydrothermal method was influenced by wettability of AZO seed layers. Our investigation shows that the diameter of ZnO nanowire increases and the length of ZnO nanowire decreases when the contact angle on the AZO seed layer is decreased. As a result, we confirmed that the morphology of ZnO nanowires could be easily controlled by surface treatment of seed layers with the plasma system.     

Transport modulation in Ge/Si core/shell nanowires through controlled synthesis of doped Si shells

Appropriately controlling the properties of the Si shell in Ge/Si core/shell nanowires permits not only passivation of the Ge surface states, but also introduces new interface phenomena, thereby enabling novel nanoelectronics concepts. Here, we report a rational synthesis of Ge/Si core/shell nanowires with doped Si shells. We demonstrate that the morphology and thickness of Si shells can be controlled for different dopant types by tuning the growth parameters during synthesis. We also present distinctly different electrical characteristics that arise from nanowire field-effect transistors fabricated using the synthesized Ge/Si core/shell nanowires with different shell morphologies. Furthermore, a clear transition in the modification of device characteristics is observed for crystalline shell nanowires following removal of the shell using a unique trimming process of successive native oxide formation/etching. Our results demonstrate that the preferred transport path through the nanowire structure can be modulated by appropriately tuning the growth conditions.     

Real-time observation of tubule formation from amorphous carbon nanowires under high-bias Joule heating

The tubule formation process from amorphous carbon nanowires under high-bias-caused Joule heating was observed in real time in a high-resolution transmission electron microscope. The crystallization of the amorphous carbon nanowires occurred in two distinct ways: the formation of tubular graphitic basal planes parallel to the nanowire axis on the surface and the formation of nano-onions in the interior of the nanowire. The tubule formation mechanism is a process of solid-state atom diffusion at high temperatures. Energetically, the tubule formation is caused by the exceptionally low surface energy of the (0002) plane of graphite. Higher input power to the amorphous nanowires generally leads to improved graphitization and, in turn, to increased conductance. The results suggest that nanotube formation in the arc-discharge growth process may involve the formation and crystallization of amorphous carbon.     

The catalyst-free synthesis of large-area tungsten oxide nanowire arrays on ITO substrate and field emission properties

Tungsten oxide nanowire arrays have been grown on indium tin oxide coated glass substrate using tungsten trioxide powders as source by thermal evaporation approach without any catalysts. When the O₂/Ar flow rate ratio was 1/100, large-scale, high-density and uniformly distributed tungsten oxide nanowire arrays were obtained. The morphology and structure properties of the tungsten oxide nanowires were characterized by scanning electron microscopy, X-ray diffraction, and transmission electron microscopy. The influences of the oxygen concentration on the growth, density, shape and structure of the nanowires were investigated. The possible growth mechanism which governs the various types of nanowire arrays as the O₂/Ar flow rate ratios changed is also discussed. Field emission properties of tungsten oxide nanowire arrays were studied at a poor vacuum condition. The remarkable performance reveals that the tungsten oxide nanowire arrays can be served as a good candidate for commercial application as electron emitters.     

Effect of Thiolated Ligands in Au Nanowire Synthesis

Thiolated ligands are seldom used as morphology‐directing reagent in the synthesis of Au nanostructures due to their low selectivity toward the different facets. Recently, we developed a thiolated ligands‐induced synthesis of nanowires where the selective Au deposition only occurs at the ligand‐deficient Au–substrate interface. Herein, the structural effect of thiolated ligands in this active surface growth is systematically investigated. It is revealed that their ability of rendering surface is closely related to the molecular structure. Ligands with aromatic backbones are capable of inducing nanowire formation, whereas those with aliphatic backbones cannot, likely because the former can pack better at short time scale of the rapid growth. The substituents of the ligands are critical for the colloidal stability of the final structure. It is further demonstrated that aromatic and aliphatic ligands could be mixed to turn on the continual lateral growth, leading to nanowires with tapered ends. The ligand generality in this growth mode also allows the creation of superhydrophobic surface, with the nanowire forest providing the nanoscale surface roughness and the hydrophobic ligand offering the surface property. These applications of the thiolated ligands in the nanosynthesis open a new approach for controlled synthesis of Au‐based nanostructures with various morphologies and properties.     

Three-dimensional morphology of GaP-GaAs nanowires revealed by transmission electron microscopy tomography

We have investigated the morphology of heterostructured GaP-GaAs nanowires grown by metal-organic vapor-phase epitaxy as a function of growth temperature and V/III precursor ratio. The study of heterostructured nanowires with transmission electron microscopy tomography allowed the three-dimensional morphology to be resolved, and discrimination between the effect of axial (core) and radial (shell) growth on the morphology. A temperature- and precursor-dependent structure diagram for the GaP nanowire core morphology and the evolution of the different types of side facets during GaAs and GaP shell growth were constituted.