GaN nanowires sputtered with Ag shell layers

We have demonstrated the fabrication of GaN-core/Ag shell nanowires and investigated their annealing effects. Scanning electron microscopy has revealed that the thermal annealing facilitated the surface-roughening of the heteronanowires. TEM investigations indicated that the thermal annealing has changed the shell morphology from continuous Ag layer to the discrete Au nanoparticles or islands. X-ray diffraction suggested that the thermal annealing has enhanced the crystallinity of Ag shell. Photoluminescence measurements revealed that the Ag-sputtering has induced an UV peak, in addition to the GaN-associated emission peaks. Thermal annealing has further changing the overall shape of the PL spectrum and we have discussed the possible emission mechanisms.     

Tailoring the photoluminescence of MgO nanowires using the Ag shell layers and nanoparticles

We fabricated MgO/Ag core-shell nanowires, where core nanowires were coated with a conformal layer of Ag via sputtering. By subsequent thermal annealing, we generated Ag nanoparticles. Photoluminescence (PL) studies revealed that Ag-shell coating decreased the intensity of blue emission, whereas that of UV emission remained almost invariant. The subsequent annealing has induced an additional green emission band. We discuss here the possible mechanism by which the PL properties have been tailored by Ag-layer coating and by forming nanoparticles. This approach can be applied to tailor and improve the optical properties of different composite nanostructures.     

Fabrication of GaN nanowalls and nanowires using surface charge lithography

We demonstrate the possibility for controlled nanostructuring of GaN by focused-ion-beam treatment with subsequent photoelectrochemical (PEC) etching. The proposed maskless approach based on direct writing of surface negative charge that shields the material against PEC etching allows fabrication of GaN nanowalls and nanowires with lateral dimensions as small as 100 nm. The results obtained show that the occurrence of undercut etching inherent to gallium nitride PEC etching depends on the depletion length in doped GaN material, it being nearly fully suppressed in the structures below a critical size of about 200 nm for the investigated GaN layer of doping concentration of 1.7 × 10¹⁷ cm^− 3.     

Photoluminescence polarization in strained GaN/AlGaN core/shell nanowires

The optical polarization properties of GaN/AlGaN core/shell nanowire (NW) heterostructures have been investigated using polarization resolved micro-photoluminescence (μ-PL) and interpreted in terms of a strain-dependent 6 × 6 k·p theoretical model. The NW heterostructures were fabricated in two steps: the Si-doped n-type c-axis GaN NW cores were grown by molecular beam epitaxy (MBE) and then epitaxially overgrown using halide vapor phase epitaxy (HVPE) to form Mg-doped AlGaN shells. The emission of the uncoated strain-free GaN NW core is found to be polarized perpendicular to the c-axis, while the GaN core compressively strained by the AlGaN shell exhibits a polarization parallel to the NW c-axis. The luminescence of the AlGaN shell is weakly polarized perpendicular to the c-axis due to the tensile axial strain in the shell.     

Nanoscale effects on heterojunction electron gases in GaN/AlGaN core/shell nanowires

The electronic properties of heterojunction electron gases formed in GaN/AlGaN core/shell nanowires with hexagonal and triangular cross sections are studied theoretically. We show that at nanoscale dimensions, the nonpolar hexagonal system exhibits degenerate quasi-one-dimensional electron gases at the hexagon corners, which transition to a core-centered electron gas at lower doping. In contrast, polar triangular core/shell nanowires show either a nondegenerate electron gas on the polar face or a single quasi-one-dimensional electron gas at the corner opposite the polar face, depending on the termination of the polar face. More generally, our results indicate that electron gases in closed nanoscale systems are qualitatively different from their bulk counterparts.     

调制掺杂AlxGa1-xN/GaN异质结中二维电子气的光致发光

研究了调制掺杂ＡｌｘＧａ１－ｘＮ／ＧａＮ异质结中与二维电子气（２ＤＥＧ）有关的光致发光,发现温度４０Ｋ时Ａｌ０．２２Ｇａ０．７８Ｎ／ＧａＮ异质结中２ＤＥＧ与光激发空穴复合形成的发光峰位于３．４４８ｅＶ,低于ＧａＮ自由激子峰４５ｍｅＶ。由于ＡｌｘＧａ１－ｘＮ／ＧａＮ界面极强的压电极化场的影响,光激发空穴很快扩散进ＧａＮ平带区,导致２ＤＥＧ与光激发空穴复合几率很低,在ＧａＮ中接近Ａｌ０．２２Ｇａ０．７     

Crystal structure transfer in core/shell nanowires

Structure engineering is an emerging tool to control opto-electronic properties of semiconductors. Recently, control of crystal structure and the formation of a twinning superlattice have been shown for III-V nanowires. This level of control has not been obtained for Si nanowires, the most relevant material for the semiconductor industry. Here, we present an approach, in which a designed twinning superlattice with the zinc blende crystal structure or the wurtzite crystal structure is transferred from a gallium phosphide core wire to an epitaxially grown silicon shell. These materials have a difference in lattice constants of only 0.4%, which allows for structure transfer without introducing extra defects. The twinning superlattices, periodicity, and shell thickness can be tuned with great precision. Arrays of free-standing Si nanotubes are obtained by a selective wet-chemical etch of the core wire.     

Investigation of indium oxide as a self-catalyst in ZnO/ZnInO heterostructure nanowires growth

We investigated the self-catalytic role of indium oxide in the growth process of ZnO/ZnInO heterostructure nanowires on Si(111). The prepared nanowires had hexagonal cross sections and were tapered with tip diameters of 90 ± 5 nm and base diameters of 230 ± 5 nm. Energy dispersive X-ray and field emission Auger spectroscopies indicated that the grown nanowires were heterostructures of ZnO and ZnInO. Analysis of the early growth process revealed that indium may play a self-catalytic role. Therefore, the vapor-liquid-solid mechanism is likely to be responsible for growth of ZnO/ZnInO nanowires. X-ray diffraction and room temperature photoluminescence (PL) data demonstrated that the presence of indium results in a decrease in nanowires' crystallinity. These wires produced a large PL emission peak in the ultraviolet (UV) region and a smaller peak in the green region of the electromagnetic spectrum. The UV peak of the ZnO/ZnInO nanowires is blue-shifted with respect to that of pure ZnO nanowires.     

GaP/GaN核壳型纳米复合材料的制备

核壳型纳米复合材料由于具有优异的性质,近年来是纳米复合材料重要研究内容之一,本文采用了两步溶剂热的方法合成了具有核壳结构的GaP/GaN纳米复合材料.     

Synthesis of GaN nanowires through Ga2O3 films' reaction with ammonia

GaN nanowires were synthesized by ammoniating Ga₂O₃ films on Ti layers deposited on Si (111) substrates at 950 °C. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transformed infrared spectroscopy (FTIR) and high-resolution transmission electron microscopy (HRTEM). The XRD, FTIR and HRTEM studies showed that these nanowires were hexagonal GaN single crystals. SEM observation demonstrated that these GaN nanorods with diameters ranging from 50 nm to 100 nm and lengths up to several micrometers intervene with each other on the substrate.     

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.     

Studies on the synthesis of CdCO3 nanowires and porous CdO powder

In the present study, we demonstrate the self transformation of aqueous cadmium acetate into CdCO₃ nanowires through hydrothermal reaction. The reaction temperature and the volume ratio of water to ethanol were found to be crucial for the formation of CdCO₃ nanowires. The nanowires are of single crystal in nature having width ∼ 17-30 nm as observed from selected area electron diffraction (SAED) pattern and transmission electron microscopic (TEM) results. The major weight loss found in thermogravimetric analysis (TGA) corresponds to the formation of CdO and CO₂. The powder X-ray diffraction (PXRD) patterns of CdCO₃ and CdO are respectively indexed to pure rhombohedral and cubic phases. The photoluminescence (PL) spectrum of CdO exhibits an emission peak at 483 nm due to the transition between the valence and conduction bands.     

Fabrication of GaN nanowires and nanorods catalyzed with tantalum

Single-crystalline GaN nanowires and nanorods have been fabricated through ammoniating Ga₂O₃ films catalyzed with tantalum (Ta) by RF magnetron sputtering, and microstructure, morphology and optical properties were investigated in particular. The results indicate that the nanowires have a hexagonal wurtzite structure with size about 50 nm in diameter and more than ten microns in length, however, the nanorods are rod-like structures with smooth surface and 100–300 nm in diameter. The growth direction of these nanostructures are perpendicular to the (100) crystal plane. The photoluminescence spectrum at room temperature exhibits a strong UV light emission band centered at 364 nm.     

Characteristics of SiOx-cored composite nanowires with Ag shell layers

This paper reports the preparation of Ag-coated SiO_x nanowires and analyzes changes in the structural properties and photoluminescence (PL) spectra induced by a thermal annealing process. The thermal induced changes in the sample morphology, generating Ag nanoparticles on the core of SiO_x nanowires and crystalline structures were affected, facilitating the generation of the Ag₂SiO₃ phase. The overall shape of the PL spectrum was changed significantly by both the Ag-coating and the subsequent thermal annealing. Possible emission mechanisms were discussed. This study gives insight into the annealing process regarding various coaxial one-dimensional materials, particularly with metal shell layers.     

Growth of nickel silicides in Si and Si/SiOx core/shell nanowires

We exploited the oxide shell structure to explore the structure confinement effect on the nickel silicide growth in one-dimensional nanowire template. The oxide confinement structure is similar to the contact structure (via hole) in the thin film system or nanodevices passivated by oxide or nitride film. Silicon nanowires in direct contact with nickel pads transform into two phases of nickel silicides, Ni31Si12 and NiSi2, after one-step annealing at 550 °C. In a bare Si nanowire during the annealing process, NiSi2 grows initially through the nanowire, followed by the transformation of NiSi2 into the nickel-rich phase, Ni31Si12 starting from near the nickel pad. Ni31Si12 is also observed under the nickel pads. Although the same phase transformations of Si to nickel silicides are observed in nanowires with oxide confinement structure, the growth rate of nickel silicides, Ni31Si12 and NiSi2, is retarded dramatically. With increasing oxide thickness from 5 to 50 nm, the retarding effect of the Ni31Si12 growth and the annihilation of Ni2Si into the oxide confined-Si is clearly observed. Ni31Si12 and Ni2Si phases are limited to grow into the Si/SiOx core-shell nanowire as the shell thickness reaches 50 nm. It is experimental evidence that phase transformation is influenced by the stressed structure at nanoscale.     

Synthesis and luminescence properties of ZnS:Mn/ZnS core/shell nanorod structures

Mn-doped ZnS nanorods synthesized by solvothermal method were successfully coated with ZnS shells of various thicknesses. The powder X-ray diffraction (XRD) measurements showed the ZnS:Mn nanorods were wurtzite structure with preferential orientation along c-axis. Transmission electron microscopy images (TEM) revealed that the ZnS shells formed from small particles, growing along a-axis orientation, which was proved by the XRD measurements. Room temperature photoluminescence (PL) spectra showed that the intensity of Mn emission first increased and then decreased with the thickening of the ZnS shells. The effects of ZnS shells on the luminescence properties of ZnS:Mn nanorods is discussed.     

Fabrication of horizontally grown silicon nanowires using a thin aluminum film as a catalyst

We present a new method for the fabrication of horizontal silicon nanowires for application in nanoelectronic devices. A web of horizontally connected silicon nanowires is grown on a silicon substrate using a thin aluminum film as a catalyst. A thin layer of oxide is thermally grown on a silicon substrate. The oxide layer is then selectively etched using photolithography. A thin layer of aluminum is thermally evaporated on the substrate with the patterned oxide layer. When the sample is annealed above the eutectic temperature, we show that the silicon gets deposited along the grain boundaries of aluminum in the form of thin nanowires. We show that this phenomenon is due to the high solubility of silicon in aluminum at high temperatures.The surface morphology was analyzed using Scanning Electron Microscopy (SEM). The compositional analysis was done using Energy Dispersive X-ray spectroscopy (EDX).     

Electrical breakdown and nanogap formation of indium oxide core/shell heterostructure nanowires

We report the electrical breakdown behavior and subsequent nanogap formation of In(2)O(3)/InO(x) core/shell heterostructure nanowires with substrate-supported and suspended structures. The radial heterostructure nanowires, composed of crystalline In(2)O(3) cores and amorphous In-rich shells, are grown by chemical vapor deposition. As the nanowires broke down, they exhibited two distinct current drops in the current-voltage characteristics. The tips of the broken nanowires were found to have a cone or a volcano shape depending on the width of the nanowire. The shape, the size, and the position of the nanogap depend strongly on the device structure and the nanowire dimensions. The substrate-supported and the suspended devices exhibit distinct breakdown behavior which can be explained by the diffusive thermal transport model. The breakdown temperature of the nanowire is estimated to be about 450?K, close to the melting temperature of indium. We demonstrated the usefulness of this technique by successful fabrication of working pentacene field-effect transistors.     

Ni-silicide growth kinetics in Si and Si/SiO2 core/shell nanowires

A systematic study of the kinetics of axial Ni silicidation of as-grown and oxidized Si nanowires (SiNWs) with different crystallographic orientations and core diameters ranging from ～ 10 to 100 nm is presented. For temperatures between 300 and 440?°C the length of the total axial silicide intrusion varies with the square root of time, which provides clear evidence that the rate limiting step is diffusion of Ni through the growing silicide phase(s). A retardation of Ni-silicide formation for oxidized SiNWs is found, indicative of a stress induced lowering of the diffusion coefficients. Extrapolated growth constants indicate that the Ni flux through the silicided NW is dominated by surface diffusion, which is consistent with an inverse square root dependence of the silicide length on the NW diameter as observed for (111) orientated SiNWs. In situ TEM silicidation experiments show that NiSi(2) is the first forming phase for as-grown and oxidized SiNWs. The silicide-SiNW interface is thereby atomically abrupt and typically planar. Ni-rich silicide phases subsequently nucleate close to the Ni reservoir, which for as-grown SiNWs can lead to a complete channel break-off for prolonged silicidation due to significant volume expansion and morphological changes.