Diameter-dependent composition of vapor-liquid-solid grown Si(1-x)Ge(x) nanowires

Diameter-dependent compositions of Si(1-x)Ge(x) nanowires grown by a vapor-liquid-solid mechanism using SiH(4) and GeH(4) precursors are studied by transmission electron microscopy and X-ray energy dispersive spectroscopy. For the growth conditions studied, the Ge concentration in Si(1-x)Ge(x) nanowires shows a strong dependence on nanowire diameter, with the Ge concentration decreasing with decreasing nanowire diameter below approximately 50 nm. The size-dependent nature of Ge concentration in Si(1-x)Ge(x) NWs is strongly suggestive of Gibbs-Thomson effects and highlights another important phenomenon in nanowire growth.     

Tunable electronic transport characteristics of surface-architecture-controlled ZnO nanowire field effect transistors

Surface-architecture-controlled ZnO nanowires were grown using a vapor transport method on various ZnO buffer film coated c-plane sapphire substrates with or without Au catalysts. The ZnO nanowires that were grown showed two different types of geometric properties: corrugated ZnO nanowires having a relatively smaller diameter and a strong deep-level emission photoluminescence (PL) peak and smooth ZnO nanowires having a relatively larger diameter and a weak deep-level emission PL peak. The surface morphology and size-dependent tunable electronic transport properties of the ZnO nanowires were characterized using a nanowire field effect transistor (FET) device structure. The FETs made from smooth ZnO nanowires with a larger diameter exhibited negative threshold voltages, indicating n-channel depletion-mode behavior, whereas those made from corrugated ZnO nanowires with a smaller diameter had positive threshold voltages, indicating n-channel enhancement-mode behavior.     

Room temperature photoluminescence properties of CuO nanowire arrays

Cupric oxide (CuO) nanowire arrays were synthesized by a simple and cost-effective thermal oxidation method. Optical properties of CuO nanowires (NWs) have been investigated through photoluminescence (PL) measurements at room temperature. CuO NWs appear several defects and vacancies related emission bands which usually appear only at low temperatures. Blue–green light emission with peaks at around 403, 474 and 489 nm is obtained from CuO NWs. In addition, the PL studies of CuO at room temperature reveal three emission peaks at around 713, 735 and 758 nm in the red and near infrared region. The study is attributed to understand the optical properties of CuO NWs and the design of CuO-based photo-electronic devices.     

Germanium nanowires with 3-nm-diameter prepared by low temperature vapour-liquid-solid chemical vapour deposition

We report the growth of germanium nanowires (Ge NWs) with single-step temperature method via vapour-liquid-solid (VLS) mechanism in the low pressure chemical vapour deposition (CVD) reactor at 300 degrees C, 280 degrees C, and 260 degrees C. The catalyst used in our experiment was Au nanoparticles with equivalent thicknesses of 0.1 nm (average diameter approximately 3 nm), 0.3 nm (average diameter approximately 4 nm), 1 nm (average diameter approximately 6 nm), and 3 nm (average diameter approximately 14 nm). The Gibbs-Thomson effect was used to explain our experimental results. The Ge NWs grown at 300 degrees C tend to have tapered structure while the Ge NWs grown at 280 degrees C and 260 degrees C tend to have straight structure. Tapering was caused by the uncatalysed deposition of Ge atoms via CVD mechanism on the sidewalls of nanowire and significantly minimised at lower temperature. We observed that the growth at lower temperature yielded Ge NWs with smaller diameter and also observed that the diameter and length of Ge NWs increases with the size of Au nanoparticles for all growth temperatures. For the same size of Au nanoparticles, Ge NWs tend to be longer with a decrease in temperature. The Ge NWs grown at 260 degrees C from 0.1-nm-thick Au had diameter as small as approximately 3 nm, offering an opportunity to fabricate high-performance p-type ballistic Ge NW transistor, to realise nanowire solar cell with higher efficiency, and also to observe the quantum confinement effect.     

MBE法生长ZnO纳米线阵列的结构和光学性能

在氧等离子体辅助的MBE系统中, 以1 nm厚的Au薄膜为催化剂, 基于气?液?固(VLS)机制实现了低温ZnO纳米线阵列在Si(111)衬底表面的生长. 通过场发射扫描电子显微镜(FE-SEM)可以观察到, ZnO纳米线阵列垂直生长在衬底上, 直径为20~30 nm. X射线衍射(XRD)和高分辨透射电镜(HRTEM)结果表明: ZnO纳米线为六方纤锌矿结构, 具有沿c轴方向的择优取向. 光致发光(PL)谱显示在380 nm附近有强烈ZnO本征发射峰, 475~650 nm可见光区域有较强的缺陷导致的发射峰.     

Ultralong zinc-blende ZnS nanowires grown on polar C face of 6H-SiC substrates at low temperatures by metalorganic chemical vapor deposition

Ultralong ZnS nanowires with high purity were grown on Au-coated polar C face of 6H-SiC substrates via metalorganic chemical vapor deposition at low temperatures. The ZnS nanowires have zinc-blende structure and the length is up to tens of micrometers. HRTEM investigations show that the nanowires are well crystalline single crystal grown along [1 1 1] and free of bulk defects. However, sparse straight and curved nanowires with poor crystalline nature are randomly grown on the Au-coated Si face of 6H-SiC substrates. We deduce that the growth of ZnS is related to the substrates and C face can enhance Au-catalytic VLS growth. The CL spectra of an individual nanowire grown on C and Si face reveal different optical properties. Intrinsic sulfur and zinc vacancies are the main reasons for the 458.1 nm and 459.2 nm blue emission detected in the nanowire grown on C face and Si face, respectively. Nevertheless, an unusual green emission at 565.1 nm is observed in the poor crystalline nanowire grown on Si face, which originates from the bulk defects.     

Fabry–Perot Microcavity Modes in Single GaP/GaNP Core/Shell Nanowires

Semiconductor nanowires (NWs) are attracting increasing interest as nanobuilding blocks for optoelectronics and photonics. A novel material system that is highly suitable for these applications are GaNP NWs. In this article, we show that individual GaP/GaNP core/shell nanowires (NWs) grown by molecular beam epitaxy on Si substrates can act as Fabry‐Perot (FP) microcavities. This conclusion is based on results of microphotoluminescence (μ‐PL) measurements performed on individual NWs, which reveal periodic undulations of the PL intensity that follow an expected pattern of FP cavity modes. The cavity is concluded to be formed along the NW axis with the end facets acting as reflecting mirrors. The formation of the FP modes is shown to be facilitated by an increasing index contrast with the surrounding media. Spectral dependence of the group refractive index is also determined for the studied NWs. The observation of the FP microcavity modes in the GaP/GaNP core/shell NWs can be considered as a first step toward achieving lasing in this quasidirect bandgap semiconductor in the NW geometry.     

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     

Fabrication of vertical ZnO nanowires on silicon (100) with epitaxial ZnO buffer layer

Vertical ZnO nanowires were successfully grown on epitaxial ZnO (002) buffer layer/Si (100) substrate. The nanowire growth process was controlled by surface morphology and orientation of the epitaxial ZnO buffer layer, which was deposited by radio-frequency (rf) sputtering. The copper catalyzed the vapor-liquid-solid growth of ZnO nanowires with diameter of approximately 30 nm and length of approximately 5.0 microm. The perfect wurtzite epitaxial structure (HCP structure) of the ZnO (0002) nanowires synthesized on ZnO (002) buffer layer/Si (100) substrate results in excellent optical characteristics such as strong UV emission at 380 nm with potential use in nano-optical and nano-electronic devices.     

Structural and optical properties of glancing angle deposited TiO2 nanowires array

TiO2 nanowires (NWs) have been synthesized by glancing angle deposition technique using e-beam evaporator. The average length 490 nm and diameter 80 nm of NWs were examined by field emission-scanning electron microscopy. Transmission electron microscopy emphasized that the NWs were widely dispersed at the top. X-ray diffraction has been carried out on the TiO2 thin film (TF) and NW array. A small blue shift of 0.03 eV was observed in Photoluminescence (PL) main band emission for TiO2 NW as compared to TiO2 TF. The high temperature annealing at 980 degrees C partially removed the oxygen vacancy from the sample, which was investigated by PL and optical absorption measurements.     

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.     

Growth of straight silicon nanowires on amorphous substrates with uniform diameter, length, orientation, and location using nanopatterned host-mediated catalyst

We report a new approach, termed "growth by nanopatterned host-medicated catalyst" (NHC growth), to solve nonuniformities of Si nanowires (NWs) grown on amorphous substrates. Rather than pure metal catalyst, the NHC uses a mixture of metal catalyst with the material to be grown (i.e., Si), nanopatterns them into desired locations and anneals them. The Si host ensures one catalyst-dot per-growth-site, prevents catalyst-dot break-up, and crystallizes catalyst-dot (hence orientating NWs). The growth results straight silicon NWs on SiO2 with uniform length and diameter (4% deviation), predetermined locations, preferred orientation, one-wire per-growth-site, and high density; all are 10-100 times better than conventional growth.     

Selective area growth of GaN nanowires using metalorganic chemical vapor deposition on nano-patterned Si(111) formed by the etching of nano-sized Au droplets

We report on the selective area growth of GaN nanowires (NWs) on nano-patterned Si(111) substrates by metalorganic chemical vapor deposition. The nano-patterns were fabricated by the oxidation of Si followed by the etching process of Au nano-droplets. The size of formed nano-pattern on Si(111) substrate was corresponding to the size of Au nano-droplet, and the diameter of GaN NWs grown was similar to the diameter of fabricated nano-pattern. The interesting phenomenon of using the nano-patterned Si(111) substrates is the formation of very clear substrate surface even after the growth of GaN NWs. However, in the case of GaN NWs grown using Au nano-droplets, there was several nanoparticles including GaN bulk grains on the Si(111) substrates. The smooth surface morphology of nano-patterned Si(111) substrates was attributed to the presence of SiO₂ layer which prevents the formation of unnecessary GaN particles during the GaN NW growth. Therefore, we believe that nano-patterning method of Si(111) which was obtained by the oxidation of Si(111) substrate and subsequent Au etching process can be utilized to grow high-quality GaN NWs and its related nano-device applications.     

Fabrication and characterization of flower-like CuO-ZnO heterostructure nanowire arrays by photochemical deposition

A simple two step solution-based method was applied to fabricate CuO-ZnO heterostructured nanowire (NW) arrays. First, ZnO nanowires were grown on a Si substrate using the ammonia solution hydrothermal reaction. Afterwards, flower-like CuO crystals were photochemically deposited on the tip of the ZnO NWs, using ultraviolet (UV) light (312 nm wavelength) irradiation at room temperature. The morphology of the CuO was controlled by reaction time, density of ZnO NWs, and concentration of the solution. Because the deposited CuO is p-type and has narrow band gap properties, CuO-ZnO heterostructured NWs exhibited a stable p-n junction property and good ability to absorb visible light. Through investigation of UV light-triggered reaction phenomena, we found that the production of OH(-) from the photocatalytic process on the surface of ZnO NWs plays a critical role in the CuO deposition mechanism.     

Strain accommodation in Ga-assisted GaAs nanowires grown on silicon (111)

We study the mechanism of lattice parameter accommodation and the structure of GaAs nanowires (NWs) grown on Si(111) substrates using the Ga-assisted growth mode in molecular beam epitaxy. These nanowires grow preferentially in the zincblende structure, but contain inclusions of wurtzite at the base. By means of grazing incidence x-ray diffraction and high-resolution transmission electron microscopy of the NW-substrate interface, we show that the lattice mismatch between the NW and the substrate is released immediately after the beginning of NW growth through the inclusion of misfit dislocations, and no pseudomorphic growth is obtained for NW diameters down to 10 nm. NWs with a diameter above 100 nm exhibit a rough interface towards the substrate, preventing complete plastic relaxation. Consequently, these NWs exhibit a residual compressive strain at their bottom. In contrast, NWs with a diameter of 50 nm and below are completely relaxed because the interface is smooth.     

The growth mechanism for silicon oxide nanowires synthesized from an Au nanoparticle/polyimide/Si thin film stack

During pyrolysis of polyimide (PI) thin film, amorphous silicon oxide nanowires (SiO(x)NWs) were produced on a large scale through heat treatment of an Au nanoparticle/PI/Si thin film stack at 1000?°C. It was shown that carbonization of the PI film preceded the nucleation of the SiO(x)NWs. The formation of the SiO(x)NWs was sustained by the oxygen derived from carbonization of the polyimide thin film while Si was provided from the substrate. Au nanoparticles promoted the SiO(x)NW growth by inducing localized melting of the Si substrate and by catalyzing the nanowire growth.     

Diameter-dependent or independent: toward a mechanistic understanding of the vapor-liquid-solid si nanowire growth rate

Si nanowires have received continued increased attention because they keep the promise of monolithic integration of high-performance semiconductors with new functionality into existing silicon technology. Most Si nanowires are grown by vapor-liquid-solid mechanism, and despite many years of study, this growth mechanism remains under lively debate. For instance, contradictory results have been reported on the effect of diameter size on nanowire growth rate. Here, we developed a universal kinetic model of Si nanowire growth based on surface diffusion which takes into account adatom diffusion from the sidewall and substrate surface into the liquid droplet as well as the Gibbs-Thomson effect. Our analysis shows that the diameter independence for Si nanowires is affected by the interplay between the Gibbs-Thomson effect and the surface diffusion, whereas the diameter dependence is mainly influenced by the Gibbs-Thomson effect. The results based on the proposed model are in good agreement with experimental data.     

Selective-area catalyst-free MBE growth of GaN nanowires using a patterned oxide layer

GaN nanowires (NWs) were grown selectively in holes of a patterned silicon oxide mask, by rf-plasma-assisted molecular beam epitaxy (PAMBE), without any metal catalyst. The oxide was deposited on a thin AlN buffer layer previously grown on a Si(111) substrate. Regular arrays of holes in the oxide layer were obtained using standard e-beam lithography. The selectivity of growth has been studied varying the substrate temperature, gallium beam equivalent pressure and patterning layout. Adjusting the growth parameters, GaN NWs can be selectively grown in the holes of the patterned oxide with complete suppression of the parasitic growth in between the holes. The occupation probability of a hole with a single or multiple NWs depends strongly on its diameter. The selectively grown GaN NWs have one common crystallographic orientation with respect to the Si(111) substrate via the AlN buffer layer, as proven by x-ray diffraction (XRD) measurements. Based on the experimental data, we present a schematic model of the GaN NW formation in which a GaN pedestal is initially grown in the hole.     

Full surface embedding of gold clusters on silicon nanowires for efficient capture and photothermal therapy of circulating tumor cells

We report on rapid thermal chemical vapor deposition growth of silicon nanowires (Si NWs) that contain a high density of gold nanoclusters (Au NCs) with a uniform coverage over the entire length of the nanowire sidewalls. The Au NC-coated Si NWs with an antibody-coated surface obtain the unique capability to capture breast cancer cells at twice the highest efficiency currently achievable (~88% at 40 min cell incubation time) from a nanostructured substrate. We also found that irradiation of breast cancer cells captured on Au NC-coated Si NWs with a near-infrared light resulted in a high mortality rate of these cancer cells, raising a fine prospect for simultaneous capture and plasmonic photothermal therapy for circulating tumor cells.     

Effect of carbon tetrabromide on the morphology of GaAs nanowires

Carbon is a commonly used p-type dopant in planar III-V semiconductors, however its use in nanowire (NW) growth has been much less reported. In this work we show that the morphology of gold assisted GaAs NWs can be strongly modified by the presence of CBr(4) vapor during growth by metalorganic vapor phase epitaxy. GaAs NWs were grown under conditions which result in strong tapering and lateral growth at low growth temperatures by the use of triethylgallium (TEGa) instead of the more usual precursor, trimethylgallium (TMGa). Under these conditions, NWs grown in the presence of CBr(4) exhibit higher axial and lower radial growth rates, and negligible tapering compared with NWs grown in the absence of CBr(4) under the same conditions. We attribute this primarily to the suppression of the 2d growth rate by CBr(4), which enhances the axial growth rate of the nanowires. NWs grown with CBr(4) show stacking-fault-free zincblende structure, while the NWs grown without CBr(4) show a high density of stacking faults. This work underlines the striking effects which precursor chemistry can have on nanowire morphology.