Group III-nitride nanowires

Group III-nitride nanowires have attracted a lot of research interest in the past decade. They contain both the intrinsic properties of III-nitride materials and some unique properties induced by the nanowire structures. This article reviews the growth methods to obtain III-nitride nanowires, and discusses the pros and cons of both top-down and bottom-up approaches, with detailed discussions on different epitaxy methods. The most widely used catalyst-induced epitaxy and extrinsic particle free epitaxy to grow III-nitride nanowires are compared. The properties of those nanowires make them promising candidates for a broad range of applications, including optoelectronic, electronic and electromechanical devices, which are also presented, with a focus on the current challenges and recent progresses.

This review was submitted as part of the 2016 Materials Literature Review Prize of the Institute of Materials, Minerals and Mining run by the Editorial Board of MST. Sponsorship of the prize by TWI Ltd is gratefully acknowledged.     

Texture of CoSi2 films on Si(111), (110) and (001) substrates

Synchrotron radiation was used to study the texture of polycrystalline CoSi₂ films that were formed by a solid-state reaction between a 30 nm Co film and Si(111), (110) and (001) substrates. All films were strongly textured, and several texture components were identified. We discuss the simultaneous occurrence of axiotaxy (i.e. alignment of lattice planes across the interface) and several different types of epitaxy in each of the films. Comparison of the different texture components observed on the three substrate orientations suggests a strong preference for the alignment of CoSi₂{110} planes in the film with Si{110} planes in the substrate, and twinning around Si[111] directions.     

Formation mechanism of silicide nanoparticles by induction thermal plasmas

The condensation mechanism of metal mixture in thermal plasmas was investigated experimentally and numerically to prepare nanoparticles of silicon base intermetallic compounds. Silicon powder premixed with metal powder (Mo, Ti, Co, Fe, Cr, or Mn) was introduced into the plasma. The nanoparticles were prepared on condition that metal vapor was quickly quenched by the water-cooled copper coil. The nucleation rate expression was used for the estimation of critical saturation ratio. The nucleation temperature of the metal almost corresponds to the melting temperature, while silicon has wide liquid range between the nucleation and melting temperature, resulting in better preparation of silicide. For Mo–Si system, nucleation position of Mo is different from that of Si. Therefore, quenching position has strong effect on the particle composition of molybdenum silicide nanoparticles.     

Formation mechanism of silicide nanoparticles by induction thermal plasmas

The condensation mechanism of metal mixture in thermal plasmas was investigated experimentally and numerically to prepare nanoparticles of silicon base intermetallic compounds. Silicon powder premixed with metal powder (Mo, Ti, Co, Fe, Cr, or Mn) was introduced into the plasma. The nanoparticles were prepared on condition that metal vapor was quickly quenched by the water-cooled copper coil. The nucleation rate expression was used for the estimation of critical saturation ratio. The nucleation temperature of the metal almost corresponds to the melting temperature, while silicon has wide liquid range between the nucleation and melting temperature, resulting in better preparation of silicide. For Mo–Si system, nucleation position of Mo is different from that of Si. Therefore, quenching position has strong effect on the particle composition of molybdenum silicide nanoparticles.     

A precursor nanowire templated route to CdS nanowires

We report the synthesis of the Cd-cysteine precursor nanowires using CdCl₂·2.5H₂O, L-cysteine and ethanolamine in the solvent of water at room temperature. The average diameter and lengths of Cd-cysteine precursor nanowires are 200 nm and several hundred microns, respectively. The precursor nanowires are used as the source materials for cadmium and sulfur and the template for the subsequent preparation of CdS nanowires using ethylene glycol as the solvent by a solvothermal method at 200 °C. The formation process of CdS nanowires is discussed. The samples are characterized using X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, energy dispersive spectroscopy and Fourier transform infrared spectroscopy.     

Self-assembly of highly oriented palladium-based nanowires on mica surface

The self-assembly of various nanostructures is recently attracting a great deal of research attention. In this paper, we demonstrate that a palladium chloride aqueous solution, mixed with a proper ammonia solution, can produce Tetra-amminepalladous chloride (Pd(NH₃)₄Cl₂·H₂O) nanowires. These nanowires can spontaneously form the two-dimensional hexagon-oriented Pd(NH₃)₄Cl₂·H₂O arrays on mica surfaces. We can control the length and height of these nanowires by adjusting their deposit time on the mica substrate. This method can be potentially used in making sensors or in making templates to wire and position nanodevices.     

Photoconductive and polarization properties of individual CdTe nanowires

Metal-Cd_0.42Te_0.58-metal nanowires were electrodeposited into the pores of anodized aluminum oxide (AAO) membranes, and the polarization sensitive photoconductance was analyzed for individual nanostructures. Non-linear I-V curves were observed, and the short-circuit current density, open-circuit voltage, and fill factor were determined. These nanowires exhibited a power conversion efficiency of 0.56%, which is higher than some comparable nanomaterials of greater complexity.     

Multi-component co-condensation model of Ti-based boride/silicide nanoparticle growth in induction thermal plasmas

Numerical analysis is conducted for the titanium-based boride and silicide nanoparticle synthesis using an induction thermal plasma including the material evaporation process and the nanoparticle growth process with nucleation and co-condensation. Both systems present the nano-scaled particle size distributions. Ti-B system shows the smaller particle diameter, sharper distribution, larger particle number density, and wider range of the composition than Ti-Si system. Ti-Si system provides a narrower range of the silicon content due to the simultaneous co-condensation of titanium and silicon. Finally the correlation between the particle size and the nonmetal content of the synthesized nanoparticles is presented on a chart.     

Microwave-assisted preparation of calcium sulfate nanowires

We have successfully developed a new synthetic route for the rapid preparation of calcium sulfate nanowires by thermal transformation of calcium dodecyl sulfate (CDS) in organic solvents of ethylene glycol (EG) and N,N-dimethylformamide (DMF). The products are characterized by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM), and determined to be single-phase CaSO₄·0.5H₂O consisting of single-crystalline nanowires with aspect ratio up to about 62. In this method, the different types of organic solvents used have no obvious influences on the morphology, phase, and formation time of the product. The microwave heating can remarkably shorten the reaction time compared with conventional heating methods.     

Thermoelectric properties of p-type PbSe nanowires

The thermoelectric properties of individual solution-phase synthesized p-type PbSe nanowires have been examined. The nanowires showed near degenerately doped charge carrier concentrations. Compared to the bulk, the PbSe nanowires exhibited a similar Seebeck coefficient and a significant reduction in thermal conductivity in the temperature range 20 K to 300 K. Thermal annealing of the PbSe nanowires allowed their thermoelectric properties to be controllably tuned by increasing their carrier concentration or hole mobility. After optimal annealing, single PbSe nanowires exhibited a thermoelectric figure of merit (ZT) of 0.12 at room temperature.      

Large-scale synthesis of defect-free silver nanowires by electrodeless deposition

We report a new method that generates high-density, long aspect ratio and large-area silver nanowire arrays within the pores of anodic aluminum oxide (AAO) membrane by reducing silver nitrate with ethylene glycol. The freestanding and defect-free silver nanowires can be obtained at high yields. In a typical procedure, the as-prepared silver nanowires have a mean diameter of ∼ 53 nm. It is found out that both temperature and reaction time are important factors in determining the morphology and aspect ratios of nanowires. Lower temperature and longer time are favorable to form polycrystalline silver nanowires with high uniformity and aspect ratios.     

Surfactant mediated assembly of gold nanowires on surfaces

The potential of surfactant interactions to direct both the placement and orientation of gold nanowires onto surfaces from solution has been investigated. Gold nanowires were synthesized by template electrodeposition in porous aluminum oxide membranes. Their assembly onto surfaces was controlled by functionalizing the nanowires and surfaces with self-assembled monolayers of thiol based surfactants. Nanowires were assembled from solution onto patterned functional surfaces, and after excess solvent had evaporated the arrangement of nanowires on the surface was observed. A variety of assembly techniques, based upon wettability, electrostatic, or chemical interactions have been studied. Nanowire assembly onto surfaces with patterned wettability resulted in the placement of nanowires on hydrophilic regions with a specific orientation. Hydrogen bonding and carboxylate salt attachment of mercaptoundecanoic acid functionalized nanowires to reactive regions of patterned surfaces has been demonstrated, with unbound wires removed by washing. Similarly, electrostatic interactions between charged nanowires and surfaces have been demonstrated to preferentially assemble nanowires onto oppositely charged surface regions. Although selective attachment of nanowires to reactive surface regions was achieved by both chemical and electrostatic assembly techniques, these methods did not control the orientation of assembled nanowires.     

Photoresponse of hydrothermally grown lateral ZnO nanowires

In this paper, a simple self-assembled lateral growth of ZnO nanowires (NWs) photodetector has been synthesized by a hydrothermal method at a temperature as low as 85 °C. The ZnO NWs exhibit single-crystalline wurtzite with elongated c-axis and can be selectively lateral self-assembled around the edges of ZnO seeding layer. The current of ZnO NWs is sensitive to the variation of ambient pressures, i.e. 4.47 μA was decreased to 1.48 μA with 5 V-bias as 1.1 × 10^− 6 Torr changed to 760 Torr, accordingly. Moreover, the current-voltage characteristics of ZnO NWs photodetectors can be evidently distinguished by UV illumination (i.e. λ = 325 nm). The photocurrent of ZnO NWs with UV illumination is twice larger than dark current while the voltage biased at 5 V. Consequently, this faster photoresponse convinces that the hydrothermally grown lateral ZnO NWs devices have a fairly good for the fabrication of UV photodetectors.     

Synthesis of vertical arrays of ultra long ZnO nanowires on noncrystalline substrates

Vertically aligned arrays of ultralong ZnO nanowires were synthesized on SiO₂ substrates with carbothermal vapor phase transport method with Au seeding layer. High density of vertically aligned ZnO nanowires with lengths from a few to ∼300 μm could be grown by controlling growth conditions. Supply of high concentration of Zn vapor and control of the ratio between Zn vapor and oxygen are found to have the most significant effects on the growth of long ZnO nanowires in the vapor-solid growth mechanism. The nanowires are of high crystalline quality as confirmed by various structural, compositional, and luminescent measurements. Luminescent and electrical properties of ZnO nanowires with different growth conditions were also investigated.     

A review on germanium nanowires

Ge nanowires exhibit wide application potential in the fields of nanoscale devices due to their excellently optical and electrical properties. This article reviews the recent progress and patents of Ge nanowires. The recent progress and patents for the synthesis of Ge nanowires using chemical vapor deposition, laser ablation, thermal evaporation, template method and supercritical fluid-liquid-solid method are demonstrated. Amorphous germanium oxide layer and defects existing in Ge nanowires result in poor Ohmic contact between Ge nanowires and electrodes. Therefore, Ge nanowires should be passivated in order to deposit connecting electrodes before applied in nanoelectronic devices. The experimental progress and patents on the application of Ge nanowires as field effect transistors, lithium batteries, photoresistors, memory cell and fluid sensors are discussed. Finally, the future development of Ge nanowires for the synthesis and practical application is also discussed.     

NbMoTiVSi_x refractory high entropy alloys strengthened by forming BCC phase and silicide eutectic structure

In order to improve mechanical properties of refractory high entropy alloys,silicide was introduced and NbMoTiVSi_x(x=0,0.1,0.2,0.3,and 0.4,molar ratio) refractory high entropy alloys are prepared by vacuum arc melting.Phase composition,micro structure evolution and mechanical properties were systematically studied.Results show that the silicide phase is formed in the alloys with addition of silicon,and the volume fraction of silicide increases from 0 to 8.3 % with increasing of silicon.Microstructure observation shows that the morphology of dendrite changes from columnar to near equiaxed,eutectic structure is formed at grain boundaries and composed of secondary BCC phase and silicide phase.The average length of the primary and second dendrites decreases with the increasing of silicon.Whereas,the ratio of eutectic structure increases from 0 to 19.8 % with the increment of silicon.The refinement of microstructure is caused by heterogeneous nucleation from the silicide.Compressive tests show that the yield and ultimate strength of the alloys increases from 1141.5 MPa to 2093.1 MPa and from 1700.1 MPa to 2374.7 MPa with increasing silicon content.The fracture strain decreases from 24.7 %-11.0 %.Fracture mechanism is changed from ductile fracture to ductile and brittle mixed fracture.The improvement of the strength is caused by grain bounda ry strengthening,which includes more boundaries around primary BCC phase and eutectic structure in grain boundary,both of them is resulted from the formation of silicide.     

Enhancement of green emission from Sn-doped ZnO nanowires

Oxygen vacancies play a crucial role in the emission characteristics of oxide nanomaterials. In this study, we found that the green emission intensity of ZnO nanowires can be enhanced through a Sn-doping concentration which increases the number of oxygen vacancies. Undoped ZnO nanowires showed blue emission at 380 nm, but as the concentration of Sn was increased, the green emission peak at around 500 nm, which is attributed to oxygen vacancies, showed drastic enhancement. On the basis of XPS compositional analysis, it was confirmed that the green luminescence intensity was closely related to the number of oxygen vacancies in Sn-doped ZnO nanowires. These results pave the way to a greater understanding of tunable light emission from nanowires, which could be a key technology for next-generation display devices, including flexible and transparent displays.     

Superplasticity in intermetallic NiAl nanowires via atomistic simulations

A novel superplastic deformation in an intermetallic B2-NiAl nanowire of cross-sectional dimensions of ∼ 20 Å with failure strain as high as ∼ 700% at 700 K temperature is reported. The minimum temperature under which the superplasticity has been observed is around 0.36 T_m, which is much lower than 0.5 T_m (T_m = melting temperature i.e. 1911 K for bulk B2-NiAl). Superplasticity is observed due to transformation from crystalline phase to amorphous phase after yielding of the nanowire.     

Rapid mass production of ZnO nanowires by a modified carbothermal reduction method

ZnO nanowires are one of the most important nanomaterials for future optoelectronic nanodevices. However, the challenge of producing great quantities of ZnO nanowires has hindered the wide application and commercialization of ZnO nanowires. Here, we employ a modified carbothermal reduction method to synthesize great quantities of ZnO nanowires within 5 min. The synthesis was carried out in a quartz tube furnace at atmospheric pressure without using any catalysts. A nitrogen/air mixed gas was used as a reactive and carrier gas. About 0.85 g ZnO nanowires were obtained using 1 g ZnO and 1 g graphite powder as source materials. The prepared ZnO nanowires exhibited a hexagonal wurtzite crystal structure. The average length and diameter of the prepared ZnO nanowires were about 1.8 μm and 35 nm, respectively. Optical absorption and photoluminescence properties of the prepared ZnO nanowires have also been investigated.     

Stress-induced martensitic phase transformation in Cu-Zr nanowires

A novel stress-induced martensitic phase transformation in an initial <100>/{100} B2-CuZr nanowire is reported for the first time in this letter. Such behavior is observed in a nanowire with cross-sectional dimensions of 19.44 × 19.44 Å² over a temperature range of 100-400 K and at a strain rate of 1 × 10⁹ s^− 1 using atomistic simulations. Phase transformation from an initial B2 phase to a BCT (Body-Centered-Tetragonal) phase is observed via nucleation and propagation of {100} twinning plane under high strain rate tensile deformation.