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.     

Correlation between the performance and microstructure of Ti/Al/Ti/Au Ohmic contacts to p-type silicon nanowires

Understanding the electrical and microstructural aspects of contact formation at nanoscale is essential for the realization of low-resistance metallization suitable for the next generation of nanowire based devices. In this study, we present detailed electrical and microstructural characteristics of Ti/Al/Ti/Au metal contacts to p-type Si nanowires (SiNWs) annealed at various temperatures. Focused ion beam cross-sectioning techniques and scanning transmission electron microscopy (STEM) were used to determine the microstructure of the source/drain metal contacts of working SiNW field-effect transistors (FETs) annealed for 30 s in the 450-850?°C temperature range in inert atmosphere. Formation of titanium silicides is observed at the metal/semiconductor interface after the 750?°C anneal. Extensive Si out-diffusion from the nanowire after the 750?°C anneal led to Kirkendall void formation. Annealing at 850?°C led to almost complete out-diffusion of Si from the nanowire core. Devices with 550?°C annealed contacts had linear electrical characteristics; whereas the devices annealed at 750?°C had the best characteristics in terms of linearity, symmetric behavior, and yield. Devices annealed at 850?°C had poor yield, which can be directly attributed to the microstructure of the contact region observed in STEM.     

Selective growth of Ge nanowires by low-temperature thermal evaporation

High-quality single-crystalline Ge nanowires with electrical properties comparable to those of bulk Ge have been synthesized by vapor-liquid-solid growth using Au growth seeds on SiO(2)/Si(100) substrates and evaporation from solid Ge powder in a low-temperature process at crucible temperatures down to 700?°C. High nanowire growth rates at these low source temperatures have been identified as being due to sublimation of GeO from substantial amounts of GeO(2) on the powder. The Ge nanowire synthesis from GeO is highly selective at our substrate temperatures (420-500?°C), i.e., occurs only on Au vapor-liquid-solid growth seeds. For growth of nanowires of 10-20?μm length on Au particles, an upper bound of 0.5?nm Ge deposition was determined in areas of bare SiO(2)/Si substrate without Au nanoparticles.     

Novel fabrication of an SnO(2) nanowire gas sensor with high sensitivity

We fabricated a nanowire-based gas sensor using a simple method of growing SnO(2) nanowires bridging the gap between two pre-patterned Au catalysts, in which the electrical contacts to the nanowires are self-assembled during the synthesis of the nanowires. The gas sensing capability of this network-structured gas sensor was demonstrated using a diluted NO(2). The sensitivity, as a function of temperature, was highest at 200?°C and was determined to be 18 and 180 when the NO(2) concentration was 0.5 and 5?ppm, respectively. Our sensor showed higher sensitivity compared to different types of sensors including SnO(2) powder-based thin films, SnO(2) coating on carbon nanotubes or single/multiple SnO(2) nanobelts. The enhanced sensitivity was attributed to the additional modulation of the sensor resistance due to the potential barrier at nanowire/nanowire junctions as well as the surface depletion region of each nanowire.     

The template-free synthesis of square-shaped SnO(2) nanowires: the temperature effect and acetone gas sensors

Square-shaped single-crystalline SnO(2) nanowires and their sphere-like hierarchical structures were synthesized successfully with a template-free hydrothermal approach. It was found that an intermediate phase-Na(2)Sn(OH)(6)-is first produced because it is slow to dissolve in ethanol/water media. The intermediate phase gradually decomposes and converts into SnO(2) at temperatures higher than 200?°C. The reaction temperature also affects the microstructure of SnO(2) nanomaterials. Uniform square-shaped SnO(2) nanowires, which form sphere-like hierarchical structures in 100% structure yield, can be produced at 285?°C on a large scale. The diameter of the nanowires shows a decrease accompanying the increase of the reaction temperature. The temperature effect could be a result of the faster and oriented growth of SnO(2) nanowires along their [Formula: see text] direction at higher temperature. Chemical sensors constructed with square-shaped SnO(2) nanowires exhibit excellent stability, good sensitivity and selectivity, as well as a quick response and short recovery times under exposure to acetone gas in practical applications.     

Mechanism of self-assembled growth of ordered GaAs nanowire arrays by metalorganic vapor phase epitaxy on GaAs vicinal substrates

We report the self-catalyzed growth of GaAs nanowire arrays by metalorganic vapor phase epitaxy (MOVPE) on GaAs vicinal substrates. The effect of substrate misorientation on the nanowire growth and the influence of growth parameters such as temperature and input V/III ratio have been studied in detail. Variation in the nanowire growth mechanism and consequential changes in the nanowire growth morphology were observed. A VLS growth mechanism with negligible effect of the vicinal surface gave rise to randomly distributed droplet-terminated GaAs nanowires at 400?°C and multiprong root-grown GaAs nanowire clusters at 500?°C with low V/III ratio. The substrate misorientation effect was dominant at 500?°C with higher V/III ratio, in which case the combined effect of the vicinal surface and the self-catalyzed Ga droplets assisted the realization of self-assembled and crystallographically oriented epitaxial nanowire arrays through the vapor-solid mechanism.     

Control of GaAs nanowire morphology and crystal structure

The morphology and crystal structure of Au-seeded GaAs nanowires (NWs) grown by molecular beam epitaxy were investigated as a function of the temperature, V/III flux ratio, and Ga flux. Low and intermediate growth temperatures of 400 and 500?°C resulted in a strongly tapered morphology, with stacking faults occurring at an average rate of 0.1?nm(-1). NWs with uniform diameter and the occurrence of crystal defects reduced by more than an order of magnitude were achieved at 600?°C, a V/III flux ratio of 2.3, and a Ga impingement rate on the surface of 0.07?nm?s(-1). Comparison of nanowire densities on the various post-growth surfaces suggests a possible incubation time between the moment the Ga shutter is opened and when nanowire growth is initiated. Increasing the flux ratio favored uniform sidewall growth, making the process suitable for the fabrication of core-shell structures.     

Device fabrication with solid-liquid-solid grown silicon nanowires

High quality, single-crystal silicon nanowires were successfully grown from silicon wafers with a nickel catalyst by utilizing a solid-liquid-solid (SLS) mechanism. The nanowires were composed of a crystalline silicon core with an average diameter of 10?nm and a thick outer oxide layer of between 20 and 30?nm at a growth temperature of 1000?°C. When utilizing the SLS growth mechanism, the diameter of the silicon nanowire is dependent solely upon the growth temperature, and has no relation to either the size or the shape of the catalyst. The characteristics of the silicon nanowires are highly dependent upon the properties of the silicon substrate, such as the crystal phase of silicon itself, as well as the doping type. The possibility of doping of silicon nanowires grown via the SLS mechanism without any external dopant source was demonstrated by measuring the electrical properties of a silicon nanowire field effect transistor.     

In(Ga)As quantum dot formation on group-III assisted catalyst-free InGaAs nanowires

Growth of GaAs and In(x)Ga(1-x)As nanowires by the group-III assisted molecular beam epitaxy growth method on (001)GaAs/SiO(2) substrates is studied in dependence on growth temperature, with the objective of maximizing the indium incorporation. Nanowire growth was achieved for growth temperatures as low as 550?°C. The incorporation of indium was studied by low temperature micro-photoluminescence spectroscopy, Raman spectroscopy and electron energy loss spectroscopy. The results show that the incorporation of indium achieved by lowering the growth temperature does not have the effect of increasing the indium concentration in the bulk of the nanowire, which is limited to 3-5%. For growth temperatures below 575?°C, indium rich regions form at the surface of the nanowires as a consequence of the radial growth. This results in the formation of quantum dots, which exhibit spectrally narrow luminescence.     

Temperature effects on electrical transport in semiconducting nanoporous carbon nanowires

In this paper we report on the effect of temperature on the electrical conductivity of amorphous and nanoporous (pores size around 0.5?nm) carbon nanowires. Poly(furfuryl alcohol) nanowires with diameter varying from 150 to 250?nm were synthesized by a template-based technique and upon pyrolysis yielded amorphous carbon nanowires with nanosized pores in them. We observed significant (as high as 700%) decrease in electrical resistance when the nanowire surface temperature was increased from room temperature to 160?°C. On the basis of the experimental and microscopy evidence, we infer a thermally activated carrier transport mechanism to be the primary electrical transport mechanism, at elevated temperatures, in these semiconducting, amorphous, and nanoporous carbon nanowires.     

Synthesis, characterization and opto-electrical properties of ternary Zn2SnO4 nanowires

Ternary oxides have the potential to display better electrical and optical properties than the commonly fabricated binary oxides. In our experiments, Zn(2)SnO(4) (ZTO) nanowires were synthesized via thermal evaporation and vapor phase transport. The opto-electrical performance of the nanowires was investigated. An individual ZTO nanowire field-effect transistor was successfully fabricated for the first time and shows an on-off ratio of 10(4) and transconductance of 20.6 nS, which demonstrates the promising electronic performance of ZTO nanowire in an electrical device. Field emission experiments on ZTO nanowire film also indicate their potential application as a field emission electron source.     

Ag-assisted CBE growth of ordered InSb nanowire arrays

We present growth studies of InSb nanowires grown directly on [Formula: see text] and [Formula: see text] substrates. The nanowires were synthesized in a chemical beam epitaxy (CBE) system and are of cubic zinc blende structure. To initiate nanowire nucleation we used lithographically positioned silver (Ag) seed particles. Up to 87% of the nanowires nucleate at the lithographically pre-defined positions. Transmission electron microscopy (TEM) investigations furthermore showed that, typically, a parasitic InSb thin film forms on the substrates. This thin film is more pronounced for InSb((111)B) substrates than for InAs((111)B) substrates, where it is completely absent at low growth temperatures. Thus, using InAs((111)B) substrates and growth temperatures below 360?°C free-standing InSb nanowires can be synthesized.     

Diameter control of tungsten oxide nanowires as grown by thermal evaporation

Tungsten oxide nanowires with controllable diameter were synthesized on Si substrates by thermal evaporation of tungsten trioxide powder in a tube furnace. Depending on the temperature of the source (900-1000?°C), tungsten oxide W(18)O(49) nanowires with diameters ranging from 10 to 100?nm are obtained with high yield. The exponential dependence of the nanowire diameter on the source temperature leads to an energy of about 2.0?eV. The growth process is discussed; it is believed to be a kinetic effect.     

Growth of InAs/InP core-shell nanowires with various pure crystal structures

We have studied the epitaxial growth of an InP shell on various pure InAs core nanowire crystal structures by metal-organic vapor phase epitaxy. The InP shell is grown on wurtzite (WZ), zinc-blende (ZB), and {111}- and {110}-type faceted ZB twin-plane superlattice (TSL) structures by tuning the InP shell growth parameters and controlling the shell thickness. The growth results, particularly on the WZ nanowires, show that homogeneous InP shell growth is promoted at relatively high temperatures (～500?°C), but that the InAs nanowires decompose under the applied conditions. In order to protect the InAs core nanowires from decomposition, a short protective InP segment is first grown axially at lower temperatures (420-460?°C), before commencing the radial growth at a higher temperature. Further studies revealed that the InP radial growth rate is significantly higher on the ZB and TSL nanowires compared to WZ counterparts, and shows a strong anisotropy in polar directions. As a result, thin shells were obtained during low temperature InP growth on ZB structures, while a higher temperature was used to obtain uniform thick shells. In addition, a schematic growth model is suggested to explain the basic processes occurring during the shell growth on the TSL crystal structures.     

Low-temperature fabrication of high-performance metal oxide thin-film electronics via combustion processing

The development of large-area, low-cost electronics for flat-panel displays, sensor arrays, and flexible circuitry depends heavily on high-throughput fabrication processes and a choice of materials with appropriate performance characteristics. For different applications, high charge carrier mobility, high electrical conductivity, large dielectric constants, mechanical flexibility or optical transparency may be required. Although thin films of metal oxides could potentially meet all of these needs, at present they are deposited using slow and equipment-intensive techniques such as sputtering. Recently, solution processing schemes with high throughput have been developed, but these require high annealing temperatures (T(anneal)>400?°C), which are incompatible with flexible polymeric substrates. Here we report combustion processing as a new general route to solution growth of diverse electronic metal oxide films (In(2)O(3), a-Zn-Sn-O, a-In-Zn-O, ITO) at temperatures as low as 200?°C. We show that this method can be implemented to fabricate high-performance, optically transparent transistors on flexible plastic substrates.     

TaSi2 nanowires: A potential field emitter and interconnect

TaSi2 nanowires have been synthesized on a Si substrate by annealing NiSi2 films at 950 degrees C in an ambient containing Ta vapor. The nanowires could be grown up to 13 microm in length. Field-emission measurements show that the turn-on field is low at 4-4.5 V/microm and the threshold field is down to 6 V/microm with the field enhancement factor as high as 1800. The metallic TaSi2 nanowires exhibit excellent electrical properties with a remarkable high failure current density of 3 x 10(8) A cm(-2). In addition, effects of annealing temperatures and capability of metal silicide mediation layer on the growth of nanowires are addressed. This simple approach promises future applications in nanoelectronics and nano-optoelectronics.     

Direct growth of oxide nanowires on CuOx thin film

Oxide nanowires were directly grown on a CuO(x) thin film deposited by plasma-enhanced atomic layer deposition without additional metal catalysts. Oxide nanowires would exhibit metal-catalyst-free growth on the CuO(x) thin film with oxide materials diffused on the top. Through a focused ion beam and transmission electron microscopy, we could verify that SnO(2) and ZnO nanowires were grown as single-crystalline structures just above the CuO(x) thin film. Bottom-gate structural SnO(2) and ZnO nanowire transistors exhibited mobilities of 135.2 and 237.6 cm(2) V(-1) s(-1), respectively. We anticipate that a variety of large-area and high-density oxide nanowires can be grown at low cost by using the CuO(x) thin film.     

Near-ultraviolet zinc oxide nanowire sensor using low temperature hydrothermal growth

Two near-ultraviolet (UV) sensors based on solution-grown zinc oxide (ZnO) nanowires (NWs) which are only sensitive to photo-excitation at or below 400?nm wavelength have been fabricated and characterized. Both devices keep all processing steps, including nanowire growth, under 100?°C for compatibility with a wide variety of substrates. The first device type uses a single optical lithography step process to allow simultaneous in?situ horizontal NW growth from solution and creation of symmetric ohmic contacts to the nanowires. The second device type uses a two-mask optical lithography process to create asymmetric ohmic and Schottky contacts. For the symmetric ohmic contacts, at a voltage bias of 1?V across the device, we observed a 29-fold increase in current in comparison to dark current when the NWs were photo-excited by a 400?nm light-emitting diode (LED) at 0.15?mW?cm(-2) with a relaxation time constant (τ) ranging from 50 to 555?s. For the asymmetric ohmic and Schottky contacts under 400?nm excitation, τ is measured between 0.5 and 1.4?s over varying time internals, which is ～2 orders of magnitude faster than the devices using symmetric ohmic contacts.     

Template assisted electrochemical growth of cobalt nanowires: influence of deposition conditions on structural, optical and magnetic properties

The influence of electrodeposition potential, pH, composition and temperature of the electrolytic bath on the structure of cobalt nanowires arrays electrodeposited into anodic aluminum oxide (AAO) porous membranes is reported. XRD, SEM, and TEM analysis were employed to characterize structural (crystal phase, crystallographic texture, and grain size), and morphological nanowire properties. It was confirmed that at pH 2 the electrodeposition potential has not influence on the preferred crystallographic orientation of the electrochemically grown Co nanowires. At pH 4 the electrodeposition potential controls the growth of cobalt nanowires along some preferential crystallographic planes. The electrolytic pH bath modulates the fcc or hcp phase exhibited by the cobalt nanowires. Single crystalline nanowires with a hcp phase strongly oriented along the (2021) crystallographic plane were obtained at pH 4 and at -1.1 V (vs. Ag/AgCl), a result not previously reported. High electrolytic bath temperatures contributed to improve the single crystalline character of the cobalt nanowires. The presence of chloride anion in the electrolytic bath also influenced on the structural properties of the resulting cobalt nanowires, improving their crystallinity. The optical reflectance of the samples shows a structure in the UV-blue region that can be assigned to the two-dimensional morphology arising in the shape of the almost parallel nanowires. Magnetic measurements showed that different electrodeposition potentials and electrolytic bath pH lead to different magnetic anisotropies on the nanowire array samples.     

UV light emitting transparent conducting tin-doped indium oxide (ITO) nanowires

Multifunctional single crystalline tin-doped indium oxide (ITO) nanowires with tuned Sn doping levels are synthesized via a vapor transport method. The Sn concentration in the nanowires can reach 6.4 at.% at a synthesis temperature of 840?°C, significantly exceeding the Sn solubility in ITO bulks grown at comparable temperatures, which we attribute to the unique feature of the vapor-liquid-solid growth. As a promising transparent conducting oxide nanomaterial, layers of these ITO nanowires exhibit a sheet resistance as low as 6.4 Ω/[Symbol: see text] and measurements on individual nanowires give a resistivity of 2.4 × 10(-4) Ω cm with an electron density up to 2.6 × 10(20) cm(-3), while the optical transmittance in the visible regime can reach ～ 80%. Under the ultraviolet excitation the ITO nanowire samples emit blue light, which can be ascribed to transitions related to defect levels. Furthermore, a room temperature ultraviolet light emission is observed in these ITO nanowires for the first time, and the exciton-related radiative process is identified by using temperature-dependent photoluminescence measurements.