Synthesis and optical properties of ZnTe single-crystalline nanowires

Single-crystalline ZnTe nanowires with the zincblende structure have been synthesized on silicon (Si) substrates via a vapor phase transport method. The ZnTe (99.99%) powders were used as the source, and 10 nm-thick thermal evaporated gold (Au) film was used as the catalyst. The as-prepared ZnTe nanowires have diameters of 30-80 nm and lengths of more than 10 microm. The products were analyzed by X-ray diffraction, field emission scanning electron microscopy, and high-resolution transmission electron microscopy. Optical properties of these nanowires were investigated by room-temperature Raman scattering spectrum and temperature-dependent photoluminescence measurements. The results show that the as-prepared ZnTe nanowires are of high crystal quality.     

Structural Stability and Half Metallic Features of Zn0.5Cr0.5S under Pressure

Spin-polarized first-principle was performed to study the structural stability and the electronic states of Cr doped ZnS with the Cr component of 50% in zincblende （ZB）, wurtzite （W） and rocksalt （RS） structures under pressure. The results show that the zincblende and wurtzite structures become unstable under low pressures of about 4.68 and 9.61 GPa, respectively, but the rocksalt structure can be maintained up to an extremely high pressure of about 32.92 GPa. Both zincblende and wurtzite Zno.sCro.5S display half metallic features under pressure, while rocksalt Zno.sCro.sS exhibits metallic feature. The half metallic features can be ascribed to the stronger interactions between S-3p and Cr-3d states and the metallic feature is due to the higher crystal symmetry of rocksalt Zn0.5Cr0.5S. These results can provide helpful guidance for Cr doped ZnS to be used in spintronic devices.     

Wurtzite-zincblende superlattices in InAs nanowires using a supply interruption method

Crystal phase control in single III-V semiconductor nanowires has emerged recently as an important challenge and possible complement to conventional bandgap engineering in single material systems. Here we investigate a supply interruption method for precise crystal phase control in single nanowires. The nanowires are grown by metalorganic vapor phase epitaxy using gold particles as seeds and are analyzed by transmission electron microscopy. It is observed that wurtzite segments with controlled length and position can be inserted on demand into a pure InAs zincblende nanowire. The interface between wurtzite and zincblende segments can be made atomically sharp and the segments can be made only a few bilayers in thickness. The growth mechanisms, applicability and limitations of the technique are presented and discussed.     

Surface faceting dependence of thermal transport in silicon nanowires

Surface faceting on sidewalls is ubiquitously observed during crystal growth of semiconductor nanowires. However, predicting the thermal transport characteristics of faceted nanowires relevant to thermoelectric applications remains challenging. Here, direct molecular dynamics simulations show that thermal conductivity is considerably reduced in crystalline <111> Si nanowires with periodic sawtooth faceting compared to nanowires of same size with smooth sidewalls. It is discovered that surface phonon scattering is particularly high with {100} facets, but less pronounced with {113} facets and remarkably low with {111} facets, which suggests a new means to optimize phonon dynamics for nanoscale thermoelectric devices. This anomaly is reconciled by showing that the contribution of each facet to surface phonons is due to diffuse scattering rather than to backward scattering. It is further shown that this property is not changed by addition of an amorphous shell to the crystalline core, similar to the structure of experimental nanowires.     

Charge-order fluctuations in one-dimensional silicides

Metallic nanowires are of great interest as interconnects in nanoelectronic devices. They also represent important systems for understanding the complexity of electronic interactions and conductivity in one dimension. We have fabricated exceptionally long and uniform YSi(2) nanowires through self-assembly of yttrium atoms on Si(001). The wire widths are quantized in odd multiples of the Si substrate lattice constant. The thinnest wires represent one of the closest realizations of the isolated Peierls chain, exhibiting van Hove type singularities in the one-dimensional density of states and charge-order fluctuations below 150 K. The structure of the wire was determined through a detailed comparison of scanning tunnelling microscopy data and first-principles calculations. Quantized width variations along the thinnest wires produce built-in Schottky junctions, the electronic properties of which are governed by the finite size and temperature scaling of the charge-ordering correlation. This illustrates how a collective phenomenon such as charge ordering might be exploited in nanoelectronic devices.     

Interplay between crystal phase purity and radial growth in InP nanowires

The interplay between crystal phase purity and radial growth in InP nanowires is investigated. By modifying the growth rate and V/III ratio, regions of high or low stacking fault density can be controllably introduced into wurtzite nanowires. It is found that regions with high stacking fault density encourage radial growth. Through careful choice of growth conditions pure wurtzite InP nanowires are then grown which exhibit narrow 4.2?K photoluminescence linewidths of 3.7?meV at 1.490?meV, and no evidence of emission related to stacking faults or zincblende insertions.     

Determination of transport properties in chromium disilicide nanowires via combined thermoelectric and structural characterizations

The Seebeck coefficient, electrical conductivity, and thermal conductivity of individual chromium disilicide nanowires were characterized using a suspended microdevice and correlated with the crystal structure and growth direction obtained by transmission electron microscopy on the same nanowires. The obtained thermoelectric figure of merit of the nanowires was comparable to the bulk values. We show that combined Seebeck coefficient and electrical conductivity measurements provide an effective approach to probing the Fermi Level, carrier concentration and mobility in nanowires.     

Large-scale solution-phase growth of Cu-doped ZnO nanowire networks

Film-like networks of Cu-doped (0.8-2.5 at.%) ZnO nanowires were successfully synthesized through a facile solution process at a low temperature (<100 degrees C). The pH value of solution plays a key role in controlling the density and quality of the Cu-doped ZnO nanowires and the dopant concentration of ZnO nanowires was controlled by adjusting the Cu2+/Zn2+ concentration ratio during the synthesis. The structural study showed that the as-prepared Cu-doped ZnO nanowires with a narrow diameter range of 20-30 nm were single crystal and grew along [0001] direction. Photoluminescence and electrical conductivity measurements showed that Cu doping can lead to a redshift in bandgap energy and an increase in the resistivity of ZnO. The thermal annealing of the as-grown nanowires at a low temperature (300 degrees C) decreased the defect-related emission within the visible range and increased the electrical conductivity. The high-quality ZnO nanowire network with controlled doping will enable further application to flexible and transparent electronics.     

Magnetic properties of single-crystalline CoSi nanowires

We have observed unusual ferromagnetic properties in single-crystalline CoSi nanowire ensemble, in marked contrast to the diamagnetic CoSi in bulk. High-density freestanding CoSi nanowires with B20 crystal structure are synthesized by a vapor-transport-based method. The reaction of cobalt chloride precursor with a Si substrate produces high-aspect-ratio CoSi nanowires. The high-resolution transmission electron microscopy and electron diffraction studies reveal superlattice structure in CoSi nanowires with twice the lattice parameter of simple cubic CoSi lattice. The zero-field-cooled and field-cooled (ZFC-FC) measurements from the nanowire ensemble show freezing of the disordered surface spins at low temperatures. The magnetoresistance (MR) measurements of single nanowire devices show a negative MR whose magnitude gets larger at lower temperatures.     

Self-catalyzed growth of GaAs nanowires on cleaved Si by molecular beam epitaxy

Self-assembled GaAs nanowires have been grown on Si by molecular beam epitaxy without the use of any outside metal catalyst. The growth occurs on Si facets obtained by the cleavage of Si(100) substrates. The growth has been obtained with or without Ga pre-deposition. In both cases two kinds of nanowires have been obtained. The wires of the first type clearly present a Ga droplet at their free end and have a lattice structure that is wurtzite for wide regions beneath the Ga droplet. The second type, in contrast, ends with pyramidally shaped GaAs and has a crystal lattice that is mainly zincblende with only a few and small wurtzite regions, if any. The Ga-ended nanowires are longer than the others and thinner on average. The experimental findings suggest that the two types of nanowires grow after different growth processes.     

Pristine semiconducting [110] silicon nanowires

We report results of ab initio calculations on silicon nanowires oriented along the [110] direction and show for the first time that these pristine silicon nanowires are indirect band gap semiconductors. The nanowires have bulk Si core and are bounded by two (100) and two (110) planes in lateral directions. The (100) planes are atomically reconstructed with dimerization in a manner similar to the (100) surface of bulk Si but the dimer arrays are perpendicular to each other on the two (100) planes. An interesting consequence of surface reconstruction is the possibility of polytypism in thicker nanowires. We discuss its effects on the electronic structure. These findings could have important implications for the use of silicon nanowires in nanoscale devices as experimentally [110] nanowires have been found to grow preferentially in the small diameter range.     

GaN纳米线材料的特性和制备技术

GaN是一种具有优越热稳定性和化学性质的宽禁带半导体材料，这种材料及相关器件可以工作在高温、高辐射等恶劣环境中，并可用于大功率微波器件．最近几年，由于GaN蓝光二极管的成功研制，使GaN成为了化合物半导体领域中最热门的研究课题．简要介绍了GaN纳米线材料的制备技术；综述了GaN纳米线材料的制备结果和特性．用CVD法研制的GaN纳米线的直径已经达到5～12nm，长度达到几百个微米．纳米线具有GaN的六方纤锌矿结构，其PL谱具有宽的发射峰，谱峰中心在420nm．GaN纳米线已经在肖特基二极管的研制中得到应用．     

Probing the wurtzite conduction band structure using state filling in highly doped InP nanowires

We have grown InP nanowires doped with hydrogen sulfide, which exhibit sulfur concentrations of up to 1.4%. The highest doped nanowires show a pure wurtzite crystal structure, in contrast to bulk InP which has the zinc blende structure. The nanowires display photoluminescence which is strongly blue shifted compared with the band gap, well into the visible range. We find evidence of a second conduction band minimum at the gamma point about 0.23 eV above the band edge, in excellent agreement with recent theoretical predictions. Electrical measurements show high conductivity and breakdown currents of 10(7) A/cm(2).     

Thermal conductivity modeling of core-shell and tubular nanowires

The heteroepitaxial growth of crystalline core-shell nanostructures of a variety of materials has become possible in recent years, allowing the realization of various novel nanoscale electronic and optoelectronic devices. The increased surface or interface area will decrease the thermal conductivity of such nanostructures and impose challenges for the thermal management of such devices. In the meantime, the decreased thermal conductivity might benefit the thermoelectric conversion efficiency. In this paper, we present modeling results on the lattice thermal conductivity of core-shell and tubular nanowires along the wire axis direction using the phonon Boltzmann equation. We report the dependence of the thermal conductivity on the surface conditions and the core-shell geometry for silicon core-germanium shell and tubular silicon nanowires at room temperature. The results show that the effective thermal conductivity changes not only with the composition of the constituents but also with the radius of the nanowires and nanopores due to the nature of the ballistic phonon transport. The results in this work have implications for the design and operation of a variety of nanoelectronic devices, optoelectronic devices, and thermoelectric materials and devices.     

Doped ZnO nanowires obtained by thermal annealing

Doped ZnO nanowires were prepared in a very simple and inexpensive thermal annealing method using ZnSe nanowires as a precursor. As doped, P doped, and As/P codoped ZnO nanowires were obtained in this method. X-ray diffraction shows that the zincblende ZnSe nanowires were converted to doped wurtzite ZnO nanowires. The incorporation of the dopants was confirmed by energy dispersive X-ray spectroscopy. The doping concentration could be adjusted by changing the annealing temperature and duration. Scanning electron microscopy indicated that the morphology of the ZnSe nanowires was essentially retained after the annealing and doping process. Photoluminescence spectroscopy also verified the incorporation of the dopants into the nanowires.     

Molecular dynamics simulations of lattice thermal conductivity and spectral phonon mean free path of PbTe: Bulk and nanostructures

In this work, molecular dynamics (MD) simulations are performed to predict the lattice thermal conductivity of PbTe bulk and nanowires. The thermal conductivity of PbTe bulk is first studied in the temperature range 300-800 K. Excellent agreement with experiments is found in the entire temperature range when a small vacancy concentration is taken into consideration. By studying various configurations of vacancies, it is found that the thermal conductivity in PbTe bulk is more sensitive to the concentration rather than the type and distribution of vacancies. Spectral phonon relaxation times and mean free paths in PbTe bulk are obtained using the spectral energy density (SED) approach. It is revealed that the majority of thermal conductivity in PbTe is contributed by acoustic phonon modes with mean free paths below 100 nm. The spectral results at elevated temperatures indicate molecular scale feature sizes (less than 10 nm) are needed to achieve low thermal conductivity for PbTe. Simulations on PbTe nanowires with diameters up to 12 nm show moderate reduction in thermal conductivity as compared to bulk, depending on diameter, surface conditions and temperature.     

Synthesis, two-dimensional assembly, and surface pressure-induced coalescence of ultranarrow PbS nanowires

Ultranarrow (1.8 nm) PbS nanowires are synthesized in a single step, under benchtop conditions at relatively low temperature (90 degrees C). The nanowires exhibit a nearly perfect crystal lattice, high width uniformity, and tight side-by-side registry. Two-dimensional (2D) assembly over large areas (>15 microm2) is achieved using the Langmuir Blodgett method. The wire width can be readily controlled in the range 1.8-10 nm by a surface pressure-induced coalescence reaction, as monitored by transmission electron microscopy and Raman spectroscopy. The fluorescence of the 2D assembly shows strong polarization dependence along the long axis of the wires, making the system potentially suitable for orientation-sensitive devices.     

Robust Room-Temperature Ferromagnetism with Giant Anisotropy in Nd-Doped ZnO Nanowire Arrays

As an important class of spintronic material, ferromagnetic oxide semiconductors are characterized with both charge and spin degrees of freedom, but they often show weak magnetism and small coercivity, which limit their applications. In this work, we synthesized Nd-doped ZnO nanowire arrays which exhibit stable room temperature ferromagnetism with a large saturation magnetic moment of 4.1 μ(B)/Nd as well as a high coercivity of 780 Oe, indicating giant magnetic anisotropy. First-principles calculations reveal that the remarkable magnetic properties in Nd-doped ZnO nanowires can be ascribed to the intricate interplay between the spin moments and the Nd-derived orbital moments. Our complementary experimental and theoretical results suggest that these magnetic oxide nanowires obtained by the bottom-up synthesis are promising as nanoscale building blocks in spintronic devices.     

Prediction of ultra-high aspect ratio nanowires from self-assembly

We employ a combination of ab initio total energy calculations and classical molecular dynamics (MD) simulations to investigate the possible self-assembly of nanoscale objects into ultrahigh aspect ratio chains and wires. The ab initio calculations provide key information regarding selective chemical functionalization for end-to-end attraction and the subtle interplay of the energy landscape, which is then used to fit classical potentials. MD simulations are carried out to predict short-time dynamical properties of assembly as a function of synthesis conditions, including solvent, chemical functionalization, temperature, and concentration. Our results suggest an efficient technique for bringing nanoscale objects together to form ultrahigh aspect ratio nanowires with high-quality alignment. We show that the electronic structure of the resulting nanowires depends strongly on the end functionalization.     

Bidirectional growth of indium phosphide nanowires

We present a bidirectional growth mode of InP nanowires grown by selective-area metalorganic vapor-phase epitaxy (SA-MOVPE). We studied the effect of the supply ratio of DEZn ([DEZn]) on InP grown structure morphology and crystal structures during the SA-MOVPE. Two growth regimes were observed in the investigated range of the [DEZn] on an InP(111)B substrate. At low [DEZn], grown structures formed tripod structures featuring three nanowires branched toward the [111]A directions. At high [DEZn], we obtained hexagonal pillar-type structures vertically grown on the (111)B substrate. These results show that the growth direction changes from [111]A to [111]B as [DEZn] is increased. We propose a growth mechanism based on the correlation between the incident facet of rotational twins and the shapes of the grown structures. Our results bring us one step closer to controlling the direction of nanowires on a Si substrate that has a nonpolar nature. They can also be applied to the development of InP nanowire devices.