Shape control of cadmium hydroxides (Cd(OH)2) sensitive to pH quenching depth and massive production of CdSe nanocrystals by their chemical transformation

This study demonstrates that the structure of cadmium hydroxides (Cd(OH)(2)) precipitated from a basic cadmium nitrate solution can be finely controlled by adjusting the pH of the precursor solution. The synthesis process involves only pouring the saturated solution into pure water to quench the pH and the total process is finished within 30 s. At a shallow pH quenching, the unstable nanoparticles turned into microparticles via a ripening process. Cd(OH)(2) was precipitated in the form of one-dimensional nanowires and then two-dimensional plates as the pH quenching was increased. At a large pH quenching, porous aggregates of Cd(OH)(2) were obtained due to the too fast precipitation. The ultrafine Cd(OH)(2) nanowires were readily transformed into CdSe chain-like nanocrystals. The transformation was quick and gave 100% yield, facilitating massive production of CdSe nanocrystals in an aqueous condition. The Cd(OH)(2) nanowires were directly grown on Si nanowires and transformed into CdSe chain-like nanocrystals, decorating the surface of each Si nanowire.     

A novel and high yield synthesis of CdSe nanowires

CdSe nanowires (CdSe-NWs) at large scale were obtained through a simple and clean method. The reaction was carried out without complexing agents which often facilitate the preferential nanostructure growth. In this work, CdSe-NWs with lineal and zigzag shape around 40 and 130 nm in diameter with a wurtzite-type structure were synthesized. Quantitative EDX analysis indicated the stoichiometric formation of CdSe, while both HRTEM and HAADF-STEM analysis ruled out the presence of other phases such as CdO or nanotubes formation, respectively. Diameter and length of the nanowires varied with the reaction time and temperature. This synthesis method makes the nanostructures purification process easier and also is non-toxic and its high conversion make it an efficient method for obtaining CdSe-NWs.     

Si nanowires synthesized with Cu catalyst

The metal copper which is a newly developed interconnecting material for integrated circuit (IC) has been used as the catalyst to catalyze the formation of the Si nanowires in high temperature tube furnace. The growth direction of the straight Si nanowires is <111> and the polyhedron η″-Cu₃Si alloy is on the tip of the Si nanowires. The synthesis temperature of the Si nanowires is 500 °C. Such a low temperature implies that the vapor-solid (VS) should be the growth method. The cheap Cu catalyst is favorable for the mass synthesis of Si nanowires.     

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.     

The influence of surface oxide on the growth of metal/semiconductor nanowires

We report the critical effects of oxide on the growth of nanostructures through silicide formation. Under an in situ ultrahigh vacuum transmission electron microscope, it is observed from the conversion of Si nanowires into the metallic PtSi grains epitaxially through controlled reactions between lithographically defined Pt pads and Si nanowires. With oxide, instead of contact area, single crystal PtSi grains start forming either near the center between two adjacent pads or from the ends of Si nanowires, resulting in the heterostructure formation of Si/PtSi/Si. Without oxide, transformation from Si into PtSi begins at the contact area between them, resulting in the heterostructure formation of PtSi/Si/PtSi. The nanowire heterostructures have an atomically sharp interface with epitaxial relationships of Si(20-2)//PtSi(10-1) and Si[111]//PtSi[111]. Additionally, it has been observed that the existence of oxide significantly affects not only the growth position but also the growth behavior and the growth rate by two orders of magnitude. Molecular dynamics simulations have been performed to support our experimental results and the proposed growth mechanisms. In addition to fundamental science, the significance of the study matters for future processing techniques in nanotechnology and related applications as well.     

Electrodeposition and characterization of CdSe semiconducting nanowires

In this paper, we present our work on the electrodeposited CdSe semiconducting nanowires. Using a low cost and low temperature approach by electrochemistry, CdSe nanowires were successfully grown using polycarbonate template. Depending on the host pore dimension of the substrate, wire diameter can be varied from 400 nm down to 30 nm and wire length from a few microns to tens microns. The as-deposited nanowires exhibit predominantly metastable zinc blende (ZB) structure but after the heat treatment they become wurtzite (W) structure. A combination of different characterization techniques, such as X-ray diffraction, SEM, TEM-HRTEM and EDXS, was used to investigate the growth morphology, crystalline structure and defects in the nanowires. The luminescent properties of CdSe nanowires have also been studied by means of photoluminescence.     

Nitrogen-doped tungsten oxide nanowires: low-temperature synthesis on Si, and electrical, optical, and field-emission properties

Very dense and uniformly distributed nitrogen-doped tungsten oxide (WO(3)) nanowires were synthesized successfully on a 4-inch Si(100) wafer at low temperature. The nanowires were of lengths extending up to 5 mum and diameters ranging from 25 to 35 nm. The highest aspect ratio was estimated to be about 200. An emission peak at 470 nm was found by photoluminescence measurement at room temperature. The suggested growth mechanism of the nanowires is vapor-solid growth, in which gaseous ammonia plays a significant role to reduce the formation temperature. The approach has proved to be a reliable way to produce nitrogen-doped WO(3) nanowires on Si in large quantities. The direct fabrication of WO(3)-based nanodevices on Si has been demonstrated.     

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.     

聚丙烯酰胺辅助溶剂热法合成CdSe纳米线

通过溶剂热法, 在180℃, 利用聚丙烯酰胺辅助合成了直径约为20nm, 长度为几百纳米到几微米的CdSe纳米线. 通过XRD、TEM、HRTEM表征了产物的结构和形貌, 并且讨论了反应时间对产物形貌的影响以及聚合物辅助纳米线生长的机制. 通过紫外-可见光谱和光致发光光谱研究了纳米线的光学性能, 在660nm处有一明显的吸收峰, 与体相材料相比具有明显的量子尺寸效应.     

Investigation of initial growth of ZnO nanowires and their growth mechanism

ZnO nanowires were synthesized on Si substrates by a simple metal vapor deposition method without any catalysts. The initial growth and the growth mechanism of the ZnO nanowires were studied using scanning and transmission electron microscopy. We found that the ZnO nanowires grew on the Si substrate via a self-seeding vapor-solid mechanism. The growth process of the ZnO nanowires consisted of four steps: self-seeding, one-dimensional epitaxial growth of the nanowires on the seeds by a base-growth mode, further acceleration of nanowire growth with additional seeding, and active formation of the nanowires.     

Dynamics of dysprosium silicide nanostructures on Si(001) and (111) surfaces

The growth and coarsening dynamics of dysprosium silicide nanostructures are observed in real-time using photoelectron emission microscopy. The annealing of a thin Dy film to temperatures in the range of 700–1050 °C results in the formation of epitaxial rectangular silicide islands and nanowires on Si(001) and triangular and hexagonal silicide islands on Si(111). During continuous annealing, individual islands are observed to coarsen via Ostwald ripening at different rates as a consequence of local variations in the size and relative location of the surrounding islands on the surface. A subsequent deposition of Dy onto the Si(001) surface at 1050 °C leads to the growth of the preexisting islands and to the formation of silicide nanowires at temperatures above where nanowire growth typically occurs. Immediately after the deposition is terminated, the nanowires begin to decay from the ends, apparently transferring atoms to the more stable rectangular islands. On Si(111), a low continuous flux of Dy at 1050 °C leads to the growth of kinked and jagged island structures, which ultimately form into nearly equilateral triangular shapes.     

Si-CdSSe core/shell nanowires with continuously tunable light emission

Uniform Si-CdSSe core/shell nanowires were controllably synthesized by a multisource thermal evaporation route. Both the silicon core and the alloyed CdSSe shell are of high-quality and single crystalline. The silicon core is grown via the gold-catalyzed VLS route with a silicon wafer piece at the high temperature zone as the source. These preferentially grown Si nanowires further serve as templates for the afterward depositions of CdSSe shells using CdS/CdSe powders at the low temperature zone of the furnace as sources. The composition/band gap of the shells can be continuously modulated by the S/Se ratio of the evaporation sources, making these prepared heterostructures have strong and spectral position/color largely tunable light emission at the visible region. These kind of structures may have potential applications in multicolor nanoscaled light-emitting devices. This flexible growth route will also be applicable for controllable synthesis of other Si wire containing heterostructures.     

Road Map for the Controlled Synthesis of CdSe Nanowires,Nanobelts, and Nanosaws—A Step Towards Nanomanufacturing

One‐dimensional (1D) nanostructures of CdSe have been found to exhibit morphologies of nanowires, nanobelts, and nanosaws, but their synthesis is by trial and error. To meet the needs of large‐scale, controlled, and designed synthesis of nanostructures, it is imperative to systematically find experimental conditions under which the desired nanostructures are synthesized reproducibly, in large quantity, and with controlled morphology. This article reports the first systematic study on the growth of 1D CdSe nanostructures by a vapor–liquid–solid (VLS) process by varying a wide range of experimental conditions. Over 150 experiments have been conducted to investigate the morphology dependence of three different types of nanostructures: nanowires, nanobelts, and nanosaws, over various substrate temperatures and pressures. The results of this work yield a road map for the controlled growth of 1D CdSe nanostructures. This research serves as a guidance and “menu” for scaling up of the synthesis of CdSe nanostructures. This is a key step towards the controlled synthesis of nanostructures to meet the needs of many industrial applications of nanomanufacturing.     

In-situ TEM observation of repeating events of nucleation in epitaxial growth of nano CoSi2 in nanowires of Si

The formation of CoSi and CoSi2 in Si nanowires at 700 and 800 degrees C, respectively, by point contact reactions between nanodots of Co and nanowires of Si have been investigated in situ in a ultrahigh vacuum high-resolution transmission electron microscope. The CoSi2 has undergone an axial epitaxial growth in the Si nanowire and a stepwise growth mode was found. We observed that the stepwise growth occurs repeatedly in the form of an atomic step sweeping across the CoSi2/Si interface. It appears that the growth of a new step or a new silicide layer requires an independent event of nucleation. We are able to resolve the nucleation stage and the growth stage of each layer of the epitaxial growth in video images. In the nucleation stage, the incubation period is measured, which is much longer than the period needed to grow the layer across the silicide/Si interface. So the epitaxial growth consists of a repeating nucleation and a rapid stepwise growth across the epitaxial interface. This is a general behavior of epitaxial growth in nanowires. The axial heterostructure of CoSi2/Si/CoSi2 with sharp epitaxial interfaces has been obtained. A discussion of the kinetics of supply limited and source-limited reaction in nanowire case by point contact reaction is given. The heterostructures are promising as high performance transistors based on intrinsic Si nanowires.     

Growth of GaAs Nanowires on Si Substrates Using a Molecular Beam Epitaxy

Au-catalyzed GaAs nanowires were grown on Si substrates by vapor-liquid-solid growth method using a molecular beam epitaxy (MBE). The MBE growth could produce controlled crystalline orientation and uniform diameter along the wire axis of the GaAs nanowires by adjusting growth conditions including growth temperature and V/III flux ratio. Growths of GaAslang001rang as well as GaAslang111rang nanowires were observed by transmission electron microscopy and scanning electron microscopy. Epitaxially grown GaAslang111rang nanowires on a Si(111) substrate were verified through x-ray diffraction out-of-plane 2thetas/omega-scans. A strong room-temperature photoluminescence (PL) was observed from the epitaxially grown GaAslang111rang nanowires on a Si(100) substrate. Results of low-temperature (10 K) PL measurements and current-sensing atomic force microscopy indicated that the GaAs nanowires on a Si substrate were unintentionally doped with Si     

Epitaxial growth of silicon nanowires using an aluminium catalyst

Silicon nanowires have been identified as important components for future electronic and sensor nanodevices. So far gold has dominated as the catalyst for growing Si nanowires via the vapour-liquid-solid (VLS) mechanism. Unfortunately, gold traps electrons and holes in Si and poses a serious contamination problem for Si complementary metal oxide semiconductor (CMOS) processing. Although there are some reports on the use of non-gold catalysts for Si nanowire growth, either the growth requires high temperatures and/or the catalysts are not compatible with CMOS requirements. From a technological standpoint, a much more attractive catalyst material would be aluminium, as it is a standard metal in Si process lines. Here we report for the first time the epitaxial growth of Al-catalysed Si nanowires and suggest that growth proceeds via a vapour-solid-solid (VSS) rather than a VLS mechanism. It is also found that the tapering of the nanowires can be strongly reduced by lowering the growth temperature.     

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.     

Silicon nanowire growth on glass substrates using hot wire chemical vapor deposition

We report here, the first observation of silicon nanowire growth via the VLS route at 400 °C using the HWCVD technique with gold (Au) as catalyst. The supersaturation of the alloy droplet, due to a large flux of atomic silicon generated due to efficient dissociation of the silane over the hot wire, leads to the precipitation of Si nanowires. The hot wire process plays a dual role in the entire nanowire growth. Firstly, the atomic hydrogen generated from the hot wire leads to the formation of the metal nanoclusters. Secondly, it offers a continuous supply of silicon atoms enabling efficient diffusion of Si into the Si-Au eutectic alloy leading to the growth of dense silicon nanowires as observed in the SEM. The Raman and TEM data show that the Si nanowires are amorphous in nature. Precise tuning of the hot wire CVD process parameters gives rise to a high density of silicon nanowires having diameters as small as 50 nm and lengths of about a few microns.     

Stranski-Krastanow growth of germanium on silicon nanowires

There have been extensive studies of germanium (Ge) grown on planar silicon (Si) substrates by the Stranski-Krastanow (S-K) mechanism. In this study, we present S-K growth of Ge on Si nanowires. The Si nanowires were grown at 500 degrees C by a vapor-liquid-solid (VLS) method, using silane (SiH4) as the gaseous precursor. By switching the gas source from SiH4 to germane (GeH4) during the growth and maintaining the growth conditions, epitaxial Ge islands deposited on the outer surface of the initially formed Si nanowires. Transmission electron microscopy (TEM), scanning TEM, and energy-dispersive X-ray spectroscopy techniques were utilized to identify the thin wetting layer and the three-dimensional Ge islands formed around the Si core nanowires. Cross-sectional TEM verified the surface faceting of the Si core nanowires as well as the Ge islands.     

Modelling the formation of high aspect CdSe quantum wires: axial-growth versus oriented-attachment mechanisms

Following the recent low temperature synthesis of high quality and single crystal CdSe quantum nanowires, we have used a thermodynamic model to investigate the plausibility of axial-growth and oriented-attachment formation mechanisms. Using surface energies for clean and alkylamine-passivated CdSe surfaces reported elsewhere by Manna et al (2005 J. Phys. Chem. B 109 6183), we have compared equilibrium and metastable shapes of CdSe nanowires as a function of aspect ratio and axial orientation for different degrees of surface passivation. In general, the theoretical results support the oriented-attachment of low aspect quantum dots or nanorods, followed by coalescence to form high aspect [Formula: see text] quantum wires.