Fabrication of reliable semiconductor nanowires by controlling crystalline structure

One-dimensional SnO(2) nanomaterials with wide bandgap characteristics are attractive for flexible and/or transparent displays and high-performance nano-electronics. In this study, the crystallinity of SnO(2) nanowires was regulated by controlling their growth temperatures. Moreover, the correlation of the crystallinity of nanowires with optical and electrical characteristics was analyzed. When SnO(2) nanowires were grown at temperatures below 900?°C, they showed various growth directions and abnormal discontinuity in their crystal structures. On the other hand, most nanowires grown at 950?°C exhibited a regular growth trend in the direction of [100]. In addition, the low temperature photoluminescence measurement revealed that the higher growth temperatures of nanowires gradually decreased the 500 nm peak rather than the 620 nm peak. The former peak is derived from the surface defect related to the shallow energy level and affects nanowire surface states. Owing to crystallinity and defects, the threshold voltage range (maximum-minimum) of SnO(2) nanowire transistors was 1.5 V at 850?°C, 1.1 V at 900?°C, and 0.5 V at 950?°C, with dispersion characteristics dramatically decreased. This study successfully demonstrated the effects of nanowire crystallinity on optical and electrical characteristics. It also suggested that the optical and electrical characteristics of nanowire transistors could be regulated by controlling their growth temperatures in the course of producing SnO(2) nanowires.     

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.     

Enhanced gas sensing performance of SnO2/α-MoO3 heterostructure nanobelts

Extremely high sensitivity and low working temperature of gas sensors are realized from SnO(2)/α-MoO(3) heterostructure nanobelts. Their sensitivity against 500 ppm ethanol is up to 67.76 at the working temperature of 300?°C, which is higher than that of bare α-MoO(3) and SnO(2) nanostructures. Also the working temperature can be lowered down to 120?°C. Such behaviors are attributed to the variation of the junction barrier at the SnO(2)/α-MoO(3) interface. The present results imply that heterostructured 1D nanomaterials may yield gas sensors with improved characteristics, and can be applied to a wide range of gas sensors.     

Tuning the morphologies of SiC nanowires via the control of growth temperature, and their photoluminescence properties

Single crystalline SiC nanowires were synthesized by a catalyst free vapor deposition method using elemental silicon and graphite carbon as the starting materials. The phase, morphology, crystal structure, and defects of the products were characterized by x-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy. Within a 6?h reaction time, the morphology of the SiC nanowires can be tuned to cylinder, hexagonal prism, or bamboo shape by simply altering the reaction temperature from 1470?°C, 1550?°C to 1630?°C, respectively. The photoluminescence of these differently shaped SiC nanowires was measured and is discussed. Based on the characterization results, the vapor-solid growth mechanisms for the multi-shaped SiC nanowires are proposed by taking into account the possible reactions between intermediate gas phases, the reaction steps, and the surface energy minimization.     

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.     

Nanostructured SnO(2) films prepared from evaporated Sn and their application as gas sensors

This paper describes the morphology, stoichiometry, microstructure and gas sensing properties of nanoclustered SnO(x) thin films prepared by Sn evaporation followed by a rheotaxial growth and thermal oxidation process. Electron microscopy was used to investigate, in detail, the evolution of the films as the oxidation temperature was increased. The results showed that the contact angle, perpendicular height, volume and microstructure of the clusters all changed significantly as a result of the thermal oxidation processes. Electron diffraction and x-ray photoelectron spectroscopy measurements revealed that after oxidation at a temperature of 600?°C, the Sn clusters were fully transformed into porous three-dimensional polycrystalline SnO(2) clusters. On the basis of these results, a prototype SnO(2) sensor was fabricated and sensing measurements were performed with H(2) and NO(2) gases. At operating temperatures of 150-200?°C the film produced measurable responses to concentrations of H(2) as low as 600?ppm and NO(2) as low as 500?ppb.     

Synthesis, morphology and compositional evolution of silicon nanowires directly grown on SnO(2) substrates

We here propose an all-in situ method for growing vapor-liquid-solid (VLS) silicon nanowires (SiNWs) directly on SnO(2) substrates in a plasma-enhanced chemical vapor deposition system. The tin catalysts are formed by a well-controlled H(2) plasma treatment of the SnO(2) layer. The lowest temperature for the tin-catalyzed VLS SiNWs growth in a silane plasma is ～250?°C. The effects of substrate temperature and H(2) dilution of silane on the morphology and compositional evolution of the SiNWs were systematically investigated. The catalyst content in the SiNWs can be effectively controlled by the deposition temperature. Moreover, enhanced absorption (down to ～1.1?eV) is achieved due to the strong light trapping and anti-reflection effects in the straight and long tapered SiNWs.     

Rutile structured SnO2 nanowires synthesized with metal catalyst by thermal evaporation method

One-dimensional (1-D) nanostructures such as tubes, rods, wires, and belts have attracted considerable research activities owing to their strong application potential as components for nanosize electronic or optoelectronic devices utilizing superior optical and electrical properties. Characterizing the mechanical properties of nanostructure is of great importance for their applications in electronics, optoelectronics, sensors, actuators. Wide-bandgap SnO2 semiconducting material (Eg = 3.6 eV at room temperature) is one of the attractive candidates for optoelectronic devices operating at room temperature, gas sensors, and transparent conducting electrodes. The synthesis and gas sensing properties of semiconducting SnO2 nanomaterials have became one of important research issues since the first synthesis of SnO2 nanobelts. Considering the important application of SnO2 in sensors, these structures are not only ideal systems for fundamental understanding at the nanoscale level, but they also have potential applications as nanoscale sensors, resonator, and transducers. The structured SnO2 nanorods have been grown on silicon substrates with Au catalytic layer by thermal evporation process over 800 degrees C. The resulting sample is characterized and analyzed by X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and energy-dispersive X-ray spectroscopy (EDS). The morphology and structural properties of SnO2 nanowires were measured by scanning electron microscopy and high-resolution transmission electron microscopy. The mean diameter of the SnO2 nanorods grown on Au coated silicon (100) substrate is approximately 80 nm. In addition, X-ray diffraction measurements show that SnO2 nanorods have a rutile structure. The formation of SnO2 nanowires has been attributed to the vapor-liquid-solid (VLS) growth mechanisms depending on the processing conditions. We investigated the growth behavior of the SnO2 nanowires by variation of the growth conditions such as gas partial pressure and temperature.     

Effects of calcination temperature on microstructures and photocatalytic activity of titanate nanotube films prepared by an EPD method

Titanate nanotube films are fabricated on F-doped SnO(2)-coated glass substrates via an electrophoretic deposition method using hydrothermally prepared titanate nanotubes as precursors. The effects of calcination temperature on the microstructures and photoactivity of as-prepared titanate nanotube films are investigated and discussed. The results indicate that the intercalated sodium ions (Na(+)) in the as-prepared titanate nanotubes are easily removed during the electrophoretic deposition. The phase transformation of titanate to anatase and diffusion of Na(+) ions from glass substrates into films occur at 400?°C. With increasing calcination temperature, the crystallization of anatase enhances and sodium content in the films increases. At 500?°C, the tubular structure still holds and the films show the highest photocatalytic activity probably due to their good crystallization, large specific surface areas and tubular structures.     

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.     

Bundled tungsten oxide nanowires under thermal processing

Ultra-thin W(18)O(49) nanowires were initially obtained by a simple solvothermal method using tungsten chloride and cyclohexanol as precursors. Thermal processing of the resulting bundled nanowires has been carried out in air in a tube furnace. The morphology and phase transformation behavior of the as-synthesized nanowires as a function of annealing temperature have been characterized by x-ray diffraction and electron microscopy. The nanostructured bundles underwent a series of morphological evolution with increased annealing temperature, becoming straighter, larger in diameter, and smaller in aspect ratio, eventually becoming irregular particles with size up to 5?μm. At 500?°C, the monoclinic W(18)O(49) was completely transformed to monoclinic WO(3) phase, which remains stable at high processing temperature. After thermal processing at 400?°C and 450?°C, the specific surface areas of the resulting nanowires dropped to 110?m(2)?g(-1) and 66?m(2)?g(-1) respectively, compared with that of 151?m(2)?g(-1) for the as-prepared sample. This study may shed light on the understanding of the geometrical and structural evolution occurring in nanowires whose working environment may involve severe temperature variations.     

Selective synthesis and superconductivity of In-Sn intermetallic nanowires sheathed in carbon nanotubes

We demonstrate a simple and reproducible technique to synthesize crystalline and superconducting In-Sn intermetallic nanowires sheathed in carbon nanotubes (CNTs). The method is based on the catalytic reaction of C(2)H(2) over a mixture of both SnO(2) and In(2)O(3) particles. Importantly, tetragonal β-In(3)Sn and hexagonal γ-InSn(4) nanowires with diameters of less than 100?nm are selectively synthesized at different SnO(2) to In(2)O(3) weight ratios. CNTs may serve as cylindrical nanocontainers for continuous growth of liquid-phased In(1-x)Sn(x) nanowires during growth process as well as for their solidification into In-Sn intermetallic nanowires during the cooling process. Microscopic and spectroscopic analyses clearly reveal evidence of a core-shell structure of the CNT-sheathed In-Sn intermetallic nanowires. Magnetization measurements show that the superconducting In-Sn nanowires have a critical magnetic field higher than the value of their bulk intermetallic compounds. Our method can be adopted to the nanofabrication of analogous binary and ternary alloys.     

Fabrication of TiO2 nanotubes by atomic layer deposition and their photocatalytic and photoelectrochemical applications

The formation of TiO(2) nanotubes was conducted by atomic layer deposition (ALD) with tris-(8-hydroxyquinoline) gallium (GaQ(3)) nanowires as a template at different substrate temperatures, 50, 100, and 200?°C. TiO(2) nanotubes were formed only at 50 and 100?°C. Although a higher growth rate at 50?°C was observed, nanotubes with better uniformity, conformality, and less residual chloride were obtained at 100?°C because of a different formation mechanism. A photocatalysis test of TiO(2) nanotubes prepared by different cycle numbers at 100?°C was conducted. It showed that TiO(2) nanotubes prepared by 400 cycles of ALD and treated at 700?°C for 1 h to form anatase phase had the best photocatalytic performance. Compared with P-25, the nanotubes showed higher photocatalytic degradation of rhodamine B and water splitting efficiency.     

Growth and optical properties of sallow-like hierarchical SnO2 nanostructures

Novel self-organized hierarchical SnO2 nanostructures have been successfully prepared by vapor phase transport with the assistance of a stainless-steel grid at 950 degrees C. Scanning electron microscopy shows that the synthesized product displays interesting sallow-like morphology, in which numerous secondary branches (beak-like nanowires) are grown randomly around the main stems (microwires). Transmission electron microscopy analysis indicates that the branches grow along a direction of [100] and the beak is formed with the growth direction switching to [110]. A room temperature photoluminescence spectrum of the present SnO2 nanostructures shows a strong emission at 572 and 604 nm(-1).     

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.     

A high sensitivity gas sensor for formaldehyde based on CdO and In(2)O(3) doped nanocrystalline SnO(2)

The gas-sensing characteristics of In(2)O(3) and CdO doped nanocrystalline SnO(2) compounds for formaldehyde were investigated in this study. The phases of the resulting materials and the morphologies of the sensing layers were characterized by x-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Indirect-heating sensors using SnO(2)-In(2)O(3)-CdO compounds as sensitive materials were fabricated on an alumina tube with Au electrodes and platinum wires. All measurements were performed at several operating temperatures from 100 to 180?°C. Good gas-sensing responses to formaldehyde have been found for all the prepared samples. It is shown that the sensors exhibited high sensitivity at low operating temperature (133?°C), making them promising candidates for practical detectors for formaldehyde.     

Fluorine-doped tin oxide films grown by pulsed direct current magnetron sputtering with an Sn target

Fluorine-doped tin oxide (FTO) films have been deposited by pulsed DC magnetron sputtering with an Sn target. Various ratios of CF4/O2 gas were injected to enhance the optical and electrical properties of the films. The extinction coefficient was lower than 1.5×10(-3) in the range from 400 to 800?nm when the CF4O2 ratio was 0.375. The resistivity of fluorine-doped SnO2 films (1.63×10(-3)?Ω?cm) deposited at 300?°C was 27.9 times smaller than that of undoped SnO2 (4.55×10(-2)?Ω?cm). Finally, an FTO film was consecutively deposited for protecting the oxidation of indium tin oxide films. The resistivity of the double-layered film was 2.68×10(-4)?Ω?cm, which increased by less than 39% at a 450?°C annealing temperature for 1?h in air.     

Brush-shaped ZnO heteronanorods synthesized using thermal-assisted pulsed laser deposition

Brush-shaped ZnO heteronanostructures were synthesized using a newly designed thermal-assisted pulsed laser deposition (T-PLD) system that combines the advantages of pulsed laser deposition (PLD) and a hot furnace system. Branched ZnO nanostructures were successfully grown onto CVD-grown backbone nanowires by T-PLD. Although ZnO growth at 300 °C resulted in core-shell structures, brush-shaped hierarchical nanostructures were formed at 500-600 °C. Materials properties were studied via photoluminescence (PL), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterizations. The enhanced photocurrent of a SnO(2)-ZnO heterostructures device by irradiation with 365 nm wavelength ultraviolet (UV) light was also investigated by the current-voltage characteristics.     

Magnetic stability of SnO₂ nanosheets

Room temperature ferromagnetism has been observed in freshly synthesized and post-annealed SnO? nanosheets. The results of x-ray diffraction and x-ray photoelectron spectroscopy reveal that the newly synthesized samples and those annealed at 400?°C under either an O? or Ar atmosphere possess rutile structure and no other impurity phases are observed. The fitting results of the O 1s and Sn 3d spectra from SnO? samples annealed at 400?°C under an O? or Ar atmosphere both indicate that oxygen vacancies inevitably exist in these samples. It is found that the saturation magnetization of all the annealed samples does not feature mono-dependence on oxygen vacancies, whereas an Sn vacancy related origin seems more plausible to account for variations in the magnetization of samples studied. This finding corresponds to first-principle calculation results from our previous work. Furthermore, the Curie temperature of SnO? nanosheets was estimated to be around 300?°C, rendering it a very good option for the next generation of spintronics.     

Direct growth of core-shell SiC-SiO(2) nanowires and field emission characteristics

A simple, direct synthesis method was used to grow core-shell SiC-SiO(2) nanowires by heating NiO-catalysed silicon substrates. A carbothermal reduction of WO(3) provided a reductive environment and carbon source to synthesize crystalline SiC nanowires covered with SiO(2) sheaths at the growth temperature of 1000-1100?°C. Transmission electron microscopy showed that the SiC core was 15-25?nm in diameter and the SiO(2) shell layer was an average of 20?nm in thickness. The thickness of the SiO(2) shell layer could be controlled using hydrofluoric acid (HF) etching. Field emission results of core-shell SiC-SiO(2) and bare SiC nanowires showed that the SiC nanowires coated with an optimum SiO(2) thickness (10?nm) have a higher field emission current than the bare SiC nanowires.