Optical properties of heavily doped GaAs nanowires and electroluminescent nanowire structures

We present GaAs electroluminescent nanowire structures fabricated by metal organic vapor phase epitaxy. Electroluminescent structures were realized in both axial pn-junctions in single GaAs nanowires and free-standing nanowire arrays with a pn-junction formed between nanowires and substrate, respectively. The electroluminescence emission peak from single nanowire pn-junctions at 10 K was registered at an energy of around 1.32 eV and shifted to 1.4 eV with an increasing current. The line is attributed to the recombination in the compensated region present in the nanowire due to the memory effect of the vapor-liquid-solid growth mechanism. Arrayed nanowire electroluminescent structures with a pn-junction formed between nanowires and substrate demonstrated at 5 K a strong electroluminescence peak at 1.488 eV and two shoulder peaks at 1.455 and 1.519 eV. The main emission line was attributed to the recombination in the p-doped GaAs. The other two lines correspond to the tunneling-assisted photon emission and band-edge recombination in the abrupt junction, respectively. Electroluminescence spectra are compared with the micro-photoluminescence spectra taken along the single p-, n- and single nanowire pn-junctions to find the origin of the electroluminescence peaks, the distribution of doping species and the sharpness of the junctions.     

Synthesis and characterization of cadmium telluride nanowire

CdTe nanowires with controlled composition were cathodically electrodeposited using track-etched polycarbonate membrane as scaffolds and their material and electrical properties were systematically investigated. As-deposited CdTe nanowires show nanocrystalline cubic phase structures with grain sizes of up to 60 nm. The dark-field images of nanowires reveal that the crystallinity of nanowires was greatly improved from nanocrystalline to a few single crystals within nanowires upon annealing at 200?°C for 6?h in a reducing environment (5%?H(2)+95%?N(2)). For electrical characterization, a single CdTe nanowire was assembled across microfabricated gold electrodes using the drop-casting method. In addition to an increase in grain size, the electrical resistivity of an annealed single nanowire (a few 10(5)?Ω?cm) was one order of magnitude greater than in an as-deposited nanowire, indicating that crystallinity of nanowires improved and defects within nanowires were reduced during annealing. By controlling the dopants levels (e.g.?Te content of nanowires), the resistivity of nanowires was varied from 10(4) to 10(0)?Ω?cm. Current-voltage (I-V) characteristics of nanowires indicated the presence of Schottky barriers at both ends of the Au/CdTe interface. Temperature-dependent I-V measurements show that the electron transport mode was determined by a thermally activated component at T>-50?°C and a temperature-independent component below -50?°C. Under optical illumination, the single CdTe nanowire exhibited enhanced conductance.     

Designing and building nanowires: directed nanocrystal self-assembly into radically branched and zigzag PbS nanowires

Lead sulfide nanowires with controllable optoelectronic properties would be promising building blocks for various applications. Here, we report the hot colloidal synthesis of radically branched and zigzag nanowires through self-attachment of star-shaped and octahedral nanocrystals in the presence of multiple surfactants. We obtained high-quality single-crystal nanowires with uniform diameter along the entire length, and the size of the nanowire can be tuned by tailoring the reaction parameters. This slow oriented attachment provides a better understanding of the intricacies of this complex nanocrystal assembly process. Meanwhile, these self-assembled nanowire structures have appealing lateral conformations with narrow side arms or highly faceted edges, where strong quantum confinement can occur. Consequently, the single-crystal nanowire structures exhibit strong photoluminescence in the near-infrared region with a large blue-shift compared to the bulk material.     

Optical trapping and integration of semiconductor nanowire assemblies in water

Semiconductor nanowires have received much attention owing to their potential use as building blocks of miniaturized electrical, nanofluidic and optical devices. Although chemical nanowire synthesis procedures have matured and now yield nanowires with specific compositions and growth directions, the use of these materials in scientific, biomedical and microelectronic applications is greatly restricted owing to a lack of methods to assemble nanowires into complex heterostructures with high spatial and angular precision. Here we show that an infrared single-beam optical trap can be used to individually trap, transfer and assemble high-aspect-ratio semiconductor nanowires into arbitrary structures in a fluid environment. Nanowires with diameters as small as 20 nm and aspect ratios of more than 100 can be trapped and transported in three dimensions, enabling the construction of nanowire architectures that may function as active photonic devices. Moreover, nanowire structures can now be assembled in physiological environments, offering new forms of chemical, mechanical and optical stimulation of living cells.     

Electrical properties of individual CoPt/Pt multilayer nanowires characterized by in situ SEM nanomanipulators

Here we report for the first time accurate and comprehensive measurements of electrical properties of individual CoPt/Pt multilayer nanowires both with periodic and non-periodic layer structures. A remarkably high failure current density of 1.69 × 10(12) A m(-2) for the periodic MNW and a similar 1.76 × 10(12) A m(-2) for the non-homogeneous MNW has been measured. The resistance of both types of multilayer nanowire structures are well fitted by a series resistance model, determining the separate resistance contribution of the component layers and magnetic/nonmagnetic interfaces for a single multilayer nanowire. The field-dependent interface resistance of both samples is calculated, 13.2 Ω for periodic layer structures and 4.84 Ω for non-periodic layer structures. The clear physical picture of the resistance distribution within individual multilayer nanowires is then determined. The accurate electrical testing of magnetic multilayer nanowires provides basic and necessary electrical parameters for their usage as building blocks or interconnects in nanoelectronics and nanosensors.     

Vertically Aligned GaAs Nanowires on Graphite and Few-Layer Graphene: Generic Model and Epitaxial Growth

By utilizing the reduced contact area of nanowires, we show that epitaxial growth of a broad range of semiconductors on graphene can in principle be achieved. A generic atomic model is presented which describes the epitaxial growth configurations applicable to all conventional semiconductor materials. The model is experimentally verified by demonstrating the growth of vertically aligned GaAs nanowires on graphite and few-layer graphene by the self-catalyzed vapor-liquid-solid technique using molecular beam epitaxy. A two-temperature growth strategy was used to increase the nanowire density. Due to the self-catalyzed growth technique used, the nanowires were found to have a regular hexagonal cross-sectional shape, and are uniform in length and diameter. Electron microscopy studies reveal an epitaxial relationship of the grown nanowires with the underlying graphitic substrates. Two relative orientations of the nanowire side-facets were observed, which is well explained by the proposed atomic model. A prototype of a single GaAs nanowire photodetector demonstrates a high-quality material. With GaAs being a model system, as well as a very useful material for various optoelectronic applications, we anticipate this particular GaAs nanowire/graphene hybrid to be promising for flexible and low-cost solar cells.     

Single crystalline PtSi nanowires, PtSi/Si/PtSi nanowire heterostructures, and nanodevices

We report the formation of PtSi nanowires, PtSi/Si/PtSi nanowire heterostructures, and nanodevices from such heterostructures. Scanning electron microscopy studies show that silicon nanowires can be converted into PtSi nanowires through controlled reactions between lithographically defined platinum pads and silicon nanowires. High-resolution transmission electron microscopy studies show that PtSi/Si/PtSi heterostructure has an atomically sharp interface with epitaxial relationships of Si[110]//PtSi[010] and Si(111)//PtSi(101). Electrical measurements show that the pure PtSi nanowires have low resistivities approximately 28.6 microOmega.cm and high breakdown current densities>1x10(8) A/cm2. Furthermore, using single crystal PtSi/Si/PtSi nanowire heterostructures with atomically sharp interfaces, we have fabricated high-performance nanoscale field-effect transistors from intrinsic silicon nanowires, in which the source and drain contacts are defined by the metallic PtSi nanowire regions, and the gate length is defined by the Si nanowire region. Electrical measurements show nearly perfect p-channel enhancement mode transistor behavior with a normalized transconductance of 0.3 mS/microm, field-effect hole mobility of 168 cm2/V.s, and on/off ratio>10(7), demonstrating the best performing device from intrinsic silicon nanowires.     

Strain-controlled growth of nanowires within thin-film cracks

There is continued interest in finding quicker and simpler ways to fabricate nanowires, even though research groups have been investigating possibilities for the past decade. There are two reasons for this interest: first, nanowires have unusual properties-for example, they show quantum-mechanical confinement effects, they have a very high surface-to-volume ratio, enabling them to be used as sensors, and they have the ability to connect to individual molecules. Second, no simple method has yet been found to fabricate nanowires over large areas in arbitrary material combinations. Here we describe an approach to the generation of well-defined nanowire network structures on almost any solid material, up to macroscopic sample sizes. We form the nanowires within cracks in a thin film. Such cracks have a number of properties that make them attractive as templates for nanowire formation: they are straight, scalable down to nanometre size, and can be aligned (by using microstructure to give crack alignment via strain). We demonstrate the production of nanowires with diameter <16 nm, both singly and as networks; we have also produced aligned patterns of nanowires, and nanowires with individual contacts.     

Toward optimized light utilization in nanowire arrays using scalable nanosphere lithography and selected area growth

Vertically aligned, catalyst-free semiconducting nanowires hold great potential for photovoltaic applications, in which achieving scalable synthesis and optimized optical absorption simultaneously is critical. Here, we report combining nanosphere lithography (NSL) and selected area metal-organic chemical vapor deposition (SA-MOCVD) for the first time for scalable synthesis of vertically aligned gallium arsenide nanowire arrays, and surprisingly, we show that such nanowire arrays with patterning defects due to NSL can be as good as highly ordered nanowire arrays in terms of optical absorption and reflection. Wafer-scale patterning for nanowire synthesis was done using a polystyrene nanosphere template as a mask. Nanowires grown from substrates patterned by NSL show similar structural features to those patterned using electron beam lithography (EBL). Reflection of photons from the NSL-patterned nanowire array was used as a measure of the effect of defects present in the structure. Experimentally, we show that GaAs nanowires as short as 130 nm show reflection of <10% over the visible range of the solar spectrum. Our results indicate that a highly ordered nanowire structure is not necessary: despite the "defects" present in NSL-patterned nanowire arrays, their optical performance is similar to "defect-free" structures patterned by more costly, time-consuming EBL methods. Our scalable approach for synthesis of vertical semiconducting nanowires can have application in high-throughput and low-cost optoelectronic devices, including solar cells.     

Controllable positioning and alignment of silver nanowires by tunable hydrodynamic focusing

Assembly and alignment of nanowires or nanotubes are critical steps for integrating functional nanodevices by the bottom-up strategy. However, it is still challenging to manipulate either an array of nanowires or individual nanowires in a controllable manner. Here we present a simple but versatile method of positioning and aligning nanowires by hydrodynamic focusing that functions as 'hydro-tweezers'. By adjusting the flow duration and flow rates of the sheath flows and sample flow, the density, width and position of the nanowire arrays, as building blocks of nanodevices, can be readily tuned in the hydrodynamic focusing process. This approach exhibits great potentials in the assembly of an array of functional nanodevices. With this method, multiple nanowire arrays can be positioned and aligned on predefined locations. Further focusing the sample flow, nanowires flow in single file. Thus single nanowires can also be lined up and located to desired positions.     

Defect-driven synthesis of self-assembled single crystal titanium nanowires via electrochemistry

One-dimensional single crystal nanostructures have garnered much attention, from their low-dimensional physics to their technological uses, due to their unique properties and potential applications, from sensors to interconnects. There is an increasing interest in metallic titanium nanowires, yet their single crystal form has not been actualized. Vapor-liquid-solid (VLS) and template-assisted top-down methods are common means for nanowire synthesis; however, each has limitations with respect to nanowire composition and crystallinity. Here we show a simple electrochemical method to generate single crystal titanium nanowires on monocrystalline NiTi substrates. This work is a significant advance in addressing the challenge of growing single crystal titanium nanowires, which had been precluded by titanium's reactivity. Nanowires grew non-parallel to the surface and in a periodic arrangement along specific substrate directions; this behavior is attributed to a defect-driven mechanism. This synthesis technique ushers in new and rapid routes for single crystal metallic nanostructures, which have considerable implications for nanoscale electronics.     

Multi-quantum-well nanowire heterostructures for wavelength-controlled lasers

Rational design and synthesis of nanowires with increasingly complex structures can yield enhanced and/or novel electronic and photonic functions. For example, Ge/Si core/shell nanowires have exhibited substantially higher performance as field-effect transistors and low-temperature quantum devices compared with homogeneous materials, and nano-roughened Si nanowires were recently shown to have an unusually high thermoelectric figure of merit. Here, we report the first multi-quantum-well (MQW) core/shell nanowire heterostructures based on well-defined III-nitride materials that enable lasing over a broad range of wavelengths at room temperature. Transmission electron microscopy studies show that the triangular GaN nanowire cores enable epitaxial and dislocation-free growth of highly uniform (InGaN/GaN)n quantum wells with n=3, 13 and 26 and InGaN well thicknesses of 1-3 nm. Optical excitation of individual MQW nanowire structures yielded lasing with InGaN quantum-well composition-dependent emission from 365 to 494 nm, and threshold dependent on quantum well number, n. Our work demonstrates a new level of complexity in nanowire structures, which potentially can yield free-standing injection nanolasers.     

Photomodulated rayleigh scattering of single semiconductor nanowires: probing electronic band structure

The internal electronic structures of single semiconductor nanowires can be resolved using photomodulated Rayleigh scattering spectroscopy. The Rayleigh scattering from semiconductor nanowires is strongly polarization sensitive which allows a nearly background-free method for detecting only the light that is scattered from a single nanowire. While the Rayleigh scattering efficiency from a semiconductor nanowire depends on the dielectric contrast, it is relatively featureless as a function of energy. However, if the nanowire is photomodulated using a second pump laser beam, the internal electronic structure can be resolved with extremely high signal-to-noise and spectral resolution. The photomodulated Rayleigh scattering spectra can be understood theoretically as a first derivative of the scattering efficiency that results from a modulation of the band gap and depends sensitively on the nanowire diameter. Fits to spectral lineshapes provide both the band structure and the diameter of individual GaAs and InP nanowires under investigation.     

Large thermoelectric figure-of-merits from SiGe nanowires by simultaneously measuring electrical and thermal transport properties

The strongly correlated thermoelectric properties have been a major hurdle for high-performance thermoelectric energy conversion. One possible approach to avoid such correlation is to suppress phonon transport by scattering at the surface of confined nanowire structures. However, phonon characteristic lengths are broad in crystalline solids, which makes nanowires insufficient to fully suppress heat transport. Here, we employed Si-Ge alloy as well as nanowire structures to maximize the depletion of heat-carrying phonons. This results in a thermal conductivity as low as ～1.2 W/m-K at 450 K, showing a large thermoelectric figure-of-merit (ZT) of ～0.46 compared with those of SiGe bulks and even ZT over 2 at 800 K theoretically. All thermoelectric properties were "simultaneously" measured from the same nanowires to facilitate accurate ZT measurements. The surface-boundary scattering is prominent when the nanowire diameter is over ～100 nm, whereas alloying plays a more important role in suppressing phonon transport for smaller ones.     

Direct synthesis of high-density lead sulfide nanowires on metal thin films towards efficient infrared light conversion

We report chemical-vapor-deposition (CVD) synthesis of high-density lead sulfide (PbS) nanowire arrays and nano pine trees directly on Ti thin films, and the fabrication of photovoltaic devices based upon the PbS nanowires. The as-grown nanowire arrays are largely vertically aligned to the substrates and are uniformly distributed over a relatively large area. Field effect transistors incorporating single PbS nanowires show p-type conduction and high mobilities. These catalytic metal thin films also serve as photocarrier collection electrodes and greatly facilitate device integration. For the first time, we have fabricated Schottky junction photovoltaic devices incorporating PbS nanowires, which demonstrate the capability of converting near-infrared light to electricity. The PbS nanowire devices are stable in air and their external quantum efficiency shows no significant decrease over a period of 3?months in air. We have also compared the photocurrent direction and quantum efficiencies of photovoltaic devices made with different metal electrodes, and the results are explained by band bending at the Schottky junction. Our research shows that PbS nanowires are promising building blocks for collecting near-infrared solar energy.     

Statistical analysis of the breaking processes of Ni nanowires

We have performed a massive statistical analysis on the breaking behaviour of Ni nanowires using molecular dynamic simulations. Three stretching directions, five initial nanowire sizes and two temperatures have been studied. We have constructed minimum cross-section histograms and analysed for the first time the role played by monomers and dimers. The shape of such histograms and the absolute number of monomers and dimers strongly depend on the stretching direction and the initial size of the nanowire. In particular, the statistical behaviour of the breakage final stages of narrow nanowires strongly differs from the behaviour obtained for large nanowires. We have analysed the structure around monomers and dimers. Their most probable local configurations differ from those usually appearing in static electron transport calculations. Their non-local environments show disordered regions along the nanowire if the stretching direction is [100] or [110]. Additionally, we have found that, at room temperature, [100] and [110] stretching directions favour the appearance of non-crystalline staggered pentagonal structures. These pentagonal Ni nanowires are reported in this work for the first time. This set of results suggests that experimental Ni conducting histograms could show a strong dependence on the orientation and temperature.     

Microcavity effects and optically pumped lasing in single conjugated polymer nanowires

Conjugated polymers have chemically tuneable opto-electronic properties and are easily processed, making them attractive materials for photonics applications. Conjugated polymer lasers, in a variety of resonator geometries such as microcavity, micro-ring, distributed feedback and photonic bandgap structures, have been fabricated using a range of coating and imprinting techniques. Currently, one-dimensional nanowires are emerging as promising candidates for integrated, subwavelength active and passive photonic devices. We report the first observation of optically pumped lasing in single conjugated polymer nanowires. The waveguide and resonator properties of each wire are characterized in the far optical field at room temperature. The end faces of the nanowire are optically flat and the nanowire acts as a cylindrical optical cavity, exhibiting axial Fabry-Pérot mode structure in the emission spectrum. Above a threshold incident pump energy, the emission spectrum collapses to a single, sharp peak with an instrument-limited line width that is characteristic of single-mode excitonic laser action.     

Fabrication of large number density platinum nanowire arrays by size reduction lithography and nanoimprint lithography

Large number density Pt nanowires with typical dimensions of 12 microm x 20 nm x 5 nm (length x width x height) are fabricated on planar oxide supports. First sub-20 nm single crystalline silicon nanowires are fabricated by size reduction lithography, and then the Si nanowire pattern is replicated to produce a large number of Pt nanowires by nanoimprint lithography. The width and height of the Pt nanowires are uniform and are controlled with nanometer precision. The nanowire number density is 4 x 10(4) cm(-1), resulting in a Pt surface area larger than 2 cm(2) on a 5 x 5 cm(2) oxide substrate. Bimodal nanowires with different width have been generated by using a Pt shadow deposition technique. Using this technique, alternating 10 and 19 nm wide nanowires are produced.     

Formation of hybrid structures: copper oxide nanocrystals templated on ultralong copper nanowires for open network sensing at room temperature

A facile large-scale synthesis approach for producing intrinsically p-type nanowires with uniform coverage of nanocrystals to form a highly interconnected porous nanowire network is of great demand for p-type sensing. Here, we have demonstrated synthesis of a very high aspect ratio (10(2)-10(5)) open network of interconnected hybrid nanocrystals-nanowire copper and copper oxide nanomaterials. The copper nanowire scaffold is employed to realize a porous and highly interconnected network of hybrid metal-metal oxide nanocrystal-nanowire structures. The structural and composition tunability of the hybrid nanomaterials is demonstrated. The hybrid copper-copper oxide nanowires exhibit enhanced gas/light sensing properties without any operating temperature. This may be attributed to enhanced medium diffusion due to the porous network of highly interconnected nanocrystal-nanowire structures.     

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.