Controlled CVD growth of Cu-Sb alloy nanostructures

Sb based alloy nanostructures have attracted much attention due to their many promising applications, e.g. as battery electrodes, thermoelectric materials and magnetic semiconductors. In many cases, these applications require controlled growth of Sb based alloys with desired sizes and shapes to achieve enhanced performance. Here, we report a flexible catalyst-free chemical vapor deposition (CVD) process to prepare Cu-Sb nanostructures with tunable shapes (e.g. nanowires and nanoparticles) by transporting Sb vapor to react with copper foils, which also serve as the substrate. By simply controlling the substrate temperature and distance, various Sb-Cu alloy nanostructures, e.g. Cu(11)Sb(3) nanowires (NWs), Cu(2)Sb nanoparticles (NPs), or pure Sb nanoplates, were obtained. We also found that the growth of Cu(11)Sb(3) NWs in such a catalyst-free CVD process was dependent on the substrate surface roughness. For example, smooth Cu foils could not lead to the growth of Cu(11)Sb(3) nanowires while roughening these smooth Cu foils with rough sand papers could result in the growth of Cu(11)Sb(3) nanowires. The effects of gas flow rate on the size and morphology of the Cu-Sb alloy nanostructures were also investigated. Such a flexible growth strategy could be of practical interest as the growth of some Sb based alloy nanostructures by CVD may not be easy due to the large difference between the condensation temperature of Sb and the other element, e.g. Cu or Co.     

Growth of p-type Si nanotubes by catalytic plasma treatments

We have demonstrated a new technique to transform bulk materials into one-dimensional nanostructures. We have shown that p-type Si nanotubes (SiNTs) can be grown by a simple dual RF plasma treatment of p-type Si substrates at 500?°C. These SiNTs have diameters of ～50-80?nm with tubular wall thickness of ～10-15?nm. The use of Cu vapor and reactive plasma has enabled the growth of these SiNTs instead of Si nanowires.     

工艺条件对氧化硅纳米结构的影响

通过金属催化化学气相沉积法,采用四氯化硅作为硅源合成了棒状氧化硅纳米结构.对产物进行了场发射扫描电镜、透射电镜及附带X射线能谱仪的表征测试.生长工艺条件包括沉积位置、反应时间、氩气冲洗次数和基板对产物纳米结构的影响进行了探讨,其中前三者分别影响气相硅源的浓度、获取硅源的量和残余氧的浓度,而基板的成分和表面粗糙度对纳米结构的生长影响显著.     

Novel C/Cu sheath/core nanostructures synthesized via low-temperature MOCVD

C/Cu sheath/core nanocable arrays were mass-produced on various substructures, such as Si, SiO(2), Cu or glass, by using a one-step low-temperature metal-organic chemical vapor deposition. The novel nanostructures consist of a faceted Cu nanowire core with six side surfaces and four top surfaces, and a sheath of carbon. The as-synthesized nanocables are demonstrated excellent oxidization resistance and field emission properties, and are expected to be excellent candidates as nano-interconnectors, or nanocables, in electronic devices and nano-emitters for field emissions.     

Germanium nanowire epitaxy: shape and orientation control

Epitaxial growth of nanowires along the 111 directions was obtained on Ge(111), Ge(110), Ge(001), and heteroepitaxial Ge on Si(001) substrates at temperatures of 350 degrees C or less by gold-nanoparticle-catalyzed chemical vapor deposition. On Ge(111), the growth was mostly vertical. In addition to 111 growth, 110 growth was observed on Ge(001) and Ge(110) substrates. Tapering was avoided by the use of the two-temperature growth procedure, reported earlier by Greytak et al.     

In Situ Synthesis of CuO and Cu Nanostructures with Promising Electrochemical and Wettability Properties

A strategy is presented for the in situ synthesis of single crystalline CuO nanorods and 3D CuO nanostructures, ultra‐long Cu nanowires and Cu nanoparticles at relatively low temperature onto various substrates (Si, SiO₂, ITO, FTO, porous nickel, carbon cotton, etc.) by one‐step thermal heating of copper foam in static air and inert gas, respectively. The density, particle sizes and morphologies of the synthesized nanostructures can be effectively controlled by simply tailoring the experimental parameters. A compressive stress based and subsequent structural rearrangements mechanism is proposed to explain the formation of the nanostructures. The as‐prepared CuO nanostructures demonstrate promising electrochemical properties as the anode materials in lithium‐ion batteries and also reversible wettability. Moreover, this strategy can be used to conveniently integrate these nanostructures with other nanostructures (ZnO nanorods, Co₃O₄ nanowires and nanowalls, TiO₂ nanotubes, and Si nanowires) to achieve various hybrid hierarchical (CuO‐ZnO, CuO‐Co₃O₄, CuO‐TiO₂, CuO‐Si) nanocomposites with promising properties. This strategy has the potential to provide the nano society with a general way to achieve a variety of nanostructures.     

SmB6单晶纳米结构的可控制备及场发射特性研究

作为一种典型的近藤拓扑绝缘体, 近年来六硼化钐(SmB₆)材料受到了凝聚态物理和材料科学领域研究者的广泛关注。与块体材料相比, SmB₆纳米材料由于具有更大的比表面积而拥有更为丰富的表面电子态, 因此被认为是一个研究表面量子效应和物理机制的理想平台。由于场发射电流主要来源于纳米材料的表面态, 所以研究SmB₆纳米材料的场发射特性可以为研究其表面量子特性提供有益的参考。本研究利用化学气相沉积法, 通过控制实验条件在硅衬底上分别实现了SmB₆纳米带和纳米线薄膜的生长。研究结果表明: 所制备的SmB₆纳米线和纳米带分别为沿着[100]和[110]方向生长的立方单晶结构。场发射特性的测试结果发现: SmB₆纳米带薄膜的开启电场为3.24 V/μm, 最大电流密度达到了466.16 μA/cm ², 其场发射性能要优于纳米线薄膜。同时考虑到SmB₆拥有很低的电子亲和势、高电导率和丰富的表面电子态, 所以若可以进一步提高其场发射特性, 那么很可能在冷阴极电子源领域有潜在应用。     

Nanostructure formation of Cu/Si(100) thin film induced by ion beam bombardment

A simple method for fabricating self-organized Cu nano-dots on Si(100) substrate by low energy Ar⁺ ion beam bombardment of a Cu thin film at room temperature over a large area is demonstrated. The morphological evolution has been investigated using scanning electron microscopy and atomic force microscopy. It was found that nano-ripple patterns formed on a Cu grain surface on a 110 nm thick polycrystalline Cu thin film under normal ion incidence. Uniformly distributed Cu nano-dots were obtained by bombardment of 55 nm thick nano-crystalline Cu thin films. The formation mechanism of the Cu nanostructures was discussed with the aid of numerical simulations using a modified damped Kuramoto–Sivashinsky equation.     

Investigation of thermal stability of Mo thin-films as the buffer layer and various Cu metallization as interconnection materials for thin film transistor-liquid crystal display applications

Cu/Mo/Si multi-layer structures were fabricated to investigate diffusion behaviors and thermal stability between Cu and Mo. Physical vapor deposition (PVD), chemical vapor deposition, electroplating and electrolessplating were used to grow 100 nm thick Cu films as interconnection materials, and radio-frequency sputtering system was introduced to grow 37.5 nm thick Mo films as a buffer layer. All Cu/Mo/Si multi-layer specimens were annealed at 350 to 700 °C for 30 min. When the annealing temperature was over 600 °C, the Cu diffused through Mo into Si, and the Cu₃Si phase and Mo-Si intermetallic compounds formed at the Mo/Si interface. The diffusion mechanism is the grain boundary diffusion. The results indicate that Cu film deposited by PVD had best crystallinity, lower roughness, large adhesive energy and resistivity. The values of the resistivity, diffusion activity energy and large adhesive energy are 5.47 μΩ-cm, 0.948 eV and 2.46 N/m, respectively.     

PECVD下基底温度对SiC薄膜形态、成分及生长速度的影响

在单晶Si和多晶Cu基底表面上使用等离子体增强化学气相沉积(PECVD)方法沉积了SiC薄膜. 通过高分辨透射电子显微镜(HRTEM)、X射线光电子能谱仪(XPS)及扫描电子显微镜(SEM)研究基底温度对SiC薄膜成分、结构及生长速度的影响规律。结果表明: 在60~500℃基底温度下制备的SiC薄膜均为非晶态薄膜, 薄膜的生长速度随基底温度的升高而线性降低, 并且在相同沉积条件下, 薄膜在Si基底上的生长速度要高于Cu基底。此外, 薄膜中的硅碳原子比随基底温度的升高而降低, 当基底温度控制在350℃左右时, 可以获得硅碳比为1:1较理想的SiC薄膜。     

Manufacturing aspects of oxide nanowires

Exploring the mass manufacturing aspects of nanostructures can enable the transition from laboratory-based research into a commercial product. Among the several one-dimensional nanostructures, oxide nanomaterials have a wide variety of applications including energy harvesting, photonics and biosensing applications. In this article, mass manufacturing aspects of bottom-up grown silica nanowires on silicon (Si) by metal thin film catalysis have been detailed. The investigation reports on (a) a growth model derived from studying nanowire nucleation as a function of heating time, (b) nanowire growth rate estimation via weight differential of the Si substrate before and after growth, and (c) reusability of the Si substrate for nanowire growth.Silica nanowires were found to grow on Pd coated Si substrate in an open tube furnace at 1100 °C with Ar as a carrier gas and a Si support wafer. Nanowires nucleated following a combination of Vapor Liquid Solid (VLS) and Oxide Assisted Growth (OAG) mechanisms conducive for mass manufacturing. The role of SiO vapor was found to be critical in the growth of the wires. Further, five distinct growth regimes were identified while estimating the growth rate. Experimental observations indicated the non-reusability of the Si substrate after one time growth due to depletion of catalyst.     

Low temperature and self-catalytic growth of tetragonal SnO nanobranch

Branched nanostructures of SnO with a single crystalline tetragonal structure in the < 110> longitudinal direction were vertically grown on silicon substrates using a low-temperature vapor transport process. The growth mechanism of SnO nanobranches was revealed as a multi-step process of V-L-S(vapor-liquid-solid) growth; initial condensation and coalescence of Sn vapors on the substrate formed a SnO buffer layer and was followed by the self-catalytic V-L-S growth of SnO nanotrunks. Secondly, additional condensation of Sn vapors on the (110) surface of the nanotrunk led to epitaxial growth of nanobranches in the < 110> direction by the same self-catalytic V-L-S process.     

Substrate considerations for graphene synthesis on thin copper films

Chemical vapor deposition on copper substrates is a primary technique for synthesis of high quality graphene films over large areas. While well-developed processes are in place for catalytic growth of graphene on bulk copper substrates, chemical vapor deposition of graphene on thin films could provide a means for simplified device processing through the elimination of the layer transfer process. Recently, it was demonstrated that transfer-free growth and processing is possible on SiO(2). However, the Cu/SiO(2)/Si material system must be stable at high temperatures for high quality transfer-free graphene. This study identifies the presence of interdiffusion at the Cu/SiO(2) interface and investigates the influence of metal (Ni, Cr, W) and insulating (Si(3)N(4), Al(2)O(3), HfO(2)) diffusion barrier layers on Cu-SiO(2) interdiffusion, as well as graphene structural quality. Regardless of barrier choice, we find the presence of Cu diffusion into the silicon substrate as well as the presence of Cu-Si-O domains on the surface of the copper film. As a result, we investigate the choice of a sapphire substrate and present evidence that it is a robust substrate for synthesis and processing of high quality, transfer-free graphene.     

Carbon-assisted synthesis of nanowires and related nanostructures of MgO

MgO nanowires and related nanostructures have been prepared by carbon-assisted synthesis, starting from polycrystalline MgO or Mg without the use of metal catalysts. The study has been carried out with different sources of carbon, all of them yielding the nanostructures with some differences. It has been possible to obtain nanotrees and other interesting nanostructures by this method. It has also been possible to obtain aligned MgO nanowires by carbon-assisted synthesis over Au-coated Si substrates. A vapor-solid mechanism of one-dimensional growth seems to be operative in the reactions carried out in bulk, but a vapor-liquid-solid mechanism applies when Si substrates are used.     

One-step synthesis of novel snowflake-like Si-O/Si-C nanostructures on 3D graphene/Cu foam by chemical vapor deposition

The recent development of synthesis processes for three-dimensional (3D) graphene-based structures has tended to focus on continuous improvement of porous nanostructures, doping modification during thin-film fabrication, and mechanisms for building 3D architectures. Here, we synthesized novel snowflake-like Si-O/Si-C nanostructures on 3D graphene/Cu foam by one-step low-pressure chemical vapor deposition (CVD). Through systematic micromorphological characterization, it was determined that the formation mechanism of the nanostructures involved the melting of the Cu foam surface and the subsequent condensation of the resulting vapor, 3D growth of graphene through catalysis in the presence of Cu, and finally, nucleation of the Si-O/Si-C nanostructure in the carbon-rich atmosphere. Thus, by tuning the growth temperature and duration, it should be possible to control the nucleation and evolution of such snowflake-like nanostructures with precision. Electrochemical measurements indicated that the snowflake-like nanostructures showed excellent performance as a material for energy storage. The highest specific capacitance of the Si-O/Si-C nanostructures was ～963.2 mF/cm² at a scan rate of 1 mV/s. Further, even after 20,000 sequential cycles, the electrode retained 94.4% of its capacitance.

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Copper metallization of hydroxyl-modified amorphous Si:C:H films

X-ray photoelectron spectroscopy (XPS) was used to examine the initial stages of copper deposited by Physical vapor deposition (PVD), or sputter deposition, interacting with amorphous silicon:carbon:hydrogen (a-Si:C:H) films and hydroxyl modified amorphous silicon:carbon:hydrogen (a-Si:C:H/OH) films under Ultra-high vacuum (UHV) conditions. Amorphous-Si:C:H films were formed by condensing vinyltrimethylsilane (VTMS) on a titanium substrate (temperature ≤90 K) followed by electron beam bombardment (500 eV), and annealing to 300 K in UHV. Amorphous Si:C:H/OH films were formed by condensing H₂O on the condensed VTMS multilayer (≤90 K) followed by electron beam bombardment (500 eV) and annealing to 300 K in UHV. The stoichiometry of the unmodified and modified a-Si:C:H films was determined by XPS to be C_4.5:Si and C_4.3:O_0.44:Si, respectively. XPS measurements of PVD Cu on the modified film at 300 K indicate initial conformal growth with Cu(I) and Cu(0) formation at the Cu/Si:C:H/OH film interface. At higher Cu coverages, only Cu(0) was observed. In contrast, 3-dimensional island formation (Volmer–Weber growth) of Cu(0) was observed on the unmodified film. Annealing both the modified and unmodified films up to 800 K in UHV produced no significant change in the Cu(3p)/Cu(2p_3/2) intensity ratio, indicating negligible Cu diffusion through the film into the titanium substrate below 800 K.     

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.     

Silver nanoisland induced synthesis of ZnO nanostructures by vapor phase transport

ZnO nanostructures including nanorod and nanotower were synthesized on Ag nanoisland coated Si substrate by thermal evaporation and vapor phase transport at atmospheric pressure. The as-prepared ZnO nanorods and nanotowers were single crystal growing along [0001] direction. The growth of ZnO nanostructures strongly depended on the surface morphology of the nanoisland Ag film deposited by electroless nanoelectrochemistry. The growth mechanism of the ZnO nanostructures was proposed on the basis of experimental data. A strong room-temperature photoluminescence in ZnO nanostructures has been demonstrated. The growth technique would be of particular interest for direct integration in the current silicon-technology-based optoelectronic devices.     

The shape control of ZnO based nanostructures

Tetrapod-shape ZnO nanostructures are formed on Si substrates by vapor phase transportation method. The effects of two important growth parameters, growth temperature and VI/II ratio, are investigated. The growth temperature is varied in the range from 600 degrees C to 900 degrees C to control the vapor pressure of group II-element and the formation process of nanostructures. VI/II ratio was changed by adjusting the flux of carrier gas which affects indirectly the supplying rate of group VI-element. From the scanning electron microscopy (SEM), systematic variation of shape including cluster, rod, wire and tetrapod was observed. ZnO tetrapods, formed at 800 degrees C under the carrier gas flux of 0.5 cc/mm2 min, show considerably uniform shape with 100 nm thick and 1-1.5 microm long legs. Also stoichiometric composition (O/Zn - 1) was observed without any second phase structures. While, the decrease of growth temperature and the increase of carrier gas flux, results in the irregular shaped nanostructures with non-stoichiometric composition. The excellent luminescence properties, strong excitonic UV emission at 3.25 eV without deep level emission, indicate that the high crystalline quality tetrapod structures can be formed at the optimized growth conditions.     

Single-crystal SnO(2) nanoshuttles: shape-controlled synthesis, perfect flexibility and high-performance field emission

Vertically aligned single-crystal SnO(2) nanoshuttle arrays with uniform morphology and a relatively high aspect ratio were synthesized by a simple hot-wall chemical vapor deposition (CVD) method. It was found that regulating the growth temperature gradient could change the shape of the SnO(2) nanostructure from nanoshuttles to nanochisels and nanoneedles, and a self-catalyzing growth process was responsible for tunable morphologies of SnO(2) nanostructures. The as-synthesized SnO(2) nanoshuttles showed ultrahigh flexibility and strong toughness with a large elastic strain of ～ 6.2, which is much higher than reported for Si and ZnO nanowire as well as most crystalline metallic materials. The field emitter fabricated using SnO(2) nanoshuttle arrays has a low turn-on electric field of around 0.6 V μm(-1), and a high field emission current density of above 10 mA cm(-2), which is comparable with the highest emission current density of carbon nanotube and nanowire field emitters.