Low-temperature growth and ethanol-sensing characteristics of quasi-one-dimensional ZnO nanostructures

ZnO nanorods, nanobelts, nanowires, and tetrapod nanowires were synthesized via thermal evaporation of Zn powder at temperatures in the range 550-600 °C under flow of Ar or Ar/O₂ as carrier gas. Uniform ZnO nanowires with diameter 15-25 nm and tetrapod nanowires with diameter 30-50 nm were obtained by strictly controlling the evaporation process. Our experimental results revealed that the concentration of O₂ in the carrier gas was a key factor to control the morphology of ZnO nanostructures. The gas sensors fabricated from quasi-one-dimensional (Q1D) ZnO nanostructures exhibited a good performance. The sensor response to 500 ppm ethanol was up to about 5.3 at the operating temperature 300 °C. Both response and recovery times were less than 20 s. The gas-sensing mechanism of the ZnO nanostructures is also discussed and their potential application is indicated accordingly.     

Fabrication, morphology, and photoluminescence properties of GaN nanowires and nanorods by ammoniating Ga2O3/V films on Si(1 1 1)

GaN nanowires and nanorods have been successfully synthesized on Si(1 1 1) substrates by magnetron sputtering through ammoniating Ga₂O₃/V films at 900 °C in a quartz tube. X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) spectrum were carried out to characterize the structure, morphology, and photoluminescence properties of GaN sample. The results show that the GaN nanowires and nanorods with pure hexagonal wurtzite structure have good emission properties. The growth direction of nanostructures is perpendicular to the fringes of (1 0 1) plane. The growth mechanism is also briefly discussed.     

The influence of hydrogen on the growth of gallium catalyzed silicon oxide nanowires

In this paper, we report that amorphous silicon oxide nanowires can be grown in a large quantity by chemical vapor deposition with molten gallium as the catalyst in a flow of mixture of SiH₄, H₂ and N₂ at 600 °C. Meanwhile, when we grow these nanowires under the same conditions but without H₂, octopus-like silicon oxide nanostructures are obtained. The reasons and mechanisms for the growth of these nanowires and nanostructures are discussed. Blue light emission is observed from SiO_x nanowires, which can be attributed to defect centers of high oxygen deficiency. These SiO_x nanowires may find applications in nanodevices and reinforcing composites.     

Large-scale synthesis of ZnO nanowires using a low-temperature chemical route and their photoluminescence properties

Single-crystalline zinc oxide (ZnO) nanowires were synthesized from zinc powder and H₂O through a simple chemical route at 730 °C in Ar atmosphere. The potential exists for bulk synthesis of ZnO nanowires at temperatures significantly less than the 200–300 °C of thermal evaporation methods reported formerly. Scanning electron microscopy and transmission electron microscopy observations reveal that the ZnO nanowires are structurally uniform, have lengths up to several hundreds of micrometers and diameters of about 40–60 nm and crystallize in a hexagonal structure. The growth of ZnO nanowires is controlled by the vapor–solid crystal-growth mechanism. Photoluminescence measurements show that the ZnO nanowires have a strong near-band ultraviolet emission at 380 nm and a green light emission at 520 nm caused by oxygen vacancies. PACS 81.05.Ys; 78.55.Et     

Synthesis and characterization of GaN nanowires by ammoniating Ga2O3/Cr thin films deposited on Si(1 1 1) substrates

GaN nanowires have been successfully synthesized on Si(1 1 1) substrates by magnetron sputtering through ammoniating Ga₂O₃/Cr thin films at 950 °C. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), FT-IR spectrophotometer, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (TEM), and photoluminescence (PL) spectrum were carried out to characterize the microstructure, morphology, and optical properties of GaN samples. The results demonstrate that the nanowires are single-crystal GaN with hexagonal wurtzite structure and high-quality crystalline, have the size of 30-80 nm in diameter and several tens of microns in length with good emission properties. The growth direction of GaN nanowires is perpendicular to the fringe of (1 0 1) plane. The growth mechanism of GaN nanowires is also discussed in detail.     

Growth of β-gallium oxide nanostructures by the thermal annealing of compacted gallium nitride powder

Various β-gallium oxide (β-Ga₂O₃) nanostructures such as nanowire, nanobelt, nanosheet, and nanocolumn were synthesized by the thermal annealing of compacted gallium nitride (GaN) powder in flowing nitrogen. We suggest that Ga₂O₃ vapor might be formed by the reaction of oxygen with the gaseous Ga formed by GaN decomposition. The Ga₂O₃ vapor diffuses into voids derived by compacting GaN powder and is supersaturated there, resulting in the growth of Ga₂O₃ nanostructures via the vapor–solid (VS) mechanism. Ga₂O₃ plate-like hillocks and nanostructures were also grown on the surface of a c-plane sapphire placed on the GaN pellet.     

The influence of growth temperatures on the characteristics of GaN nanowires

This paper presents the investigation of the properties of GaN nanowires synthesized from Ni-catalyzed chemical vapour deposition method under various growth temperatures. The influence of the growth temperatures on the morphological, structural and optical characteristics of the synthesized GaN nanowires was investigated in this work. Field-emission scanning electron microscopy images revealed that the 950 °C was the optimal growth temperature for synthesizing uniform, straight and smooth morphology of GaN nanowires. X-ray diffraction results demonstrated that the synthesized low dimensional GaN structures have the hexagonal wurtzite structure. Ultraviolet and blue emissions were detected from photoluminescence measurements. In addition, phonon replicas with the energy separation of 90 meV have been observed at the lower energy of the blue emission region in photoluminescence spectra.     

Synthesis and characterization of GaN nanowires

In this work, GaN nanowires were fabricated on Si substrates coated with NiCl₂ thin films using chemical vapor deposition (CVD) method by evaporating Ga₂O₃ powder at 1100 °C in ammonia gas flow. X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscope (HRTEM) and photoluminescence (PL) spectrum are used to characterize the samples. The results demonstrate that the nanowires are single-crystal GaN with hexagonal wurtzite structure. The growth mechanism of GaN nanowires is also discussed.     

Effect of source temperature on the morphology and photoluminescence properties of ZnO nanostructures

ZnO nanostructures have been synthesized by heating a mixture of ZnO/graphite powders using the thermal evaporation and vapor transport on Si(1 0 0) substrates without any catalyst and at atmospheric argon pressure. The influence of the source temperature on the morphology and luminescence properties of ZnO nanostructures has been investigated. ZnO nanowires, nanoflowres and nanotetrapods have been formed upon the Si(1 0 0) substrates at different source temperatures ranging from 1100 to 1200 °C. Room temperature photoluminescence (PL) spectra showed increase green emission intensity as the source temperature was decreased and ZnO nanowires had the strongest intensity of UV emission compared with other nanostructures. In addition, the growth mechanism of the ZnO nanostructures is discussed based on the reaction conditions.     

Formation of thermally stable low-resistance Ti/W/Au ohmic contacts on n-type GaN

A Ti(12 nm)/W(20 nm)/Au(50 nm) metallization scheme has been investigated for obtaining thermally stable low-resistance ohmic contacts to n-type GaN (4.0×10¹⁸ cm^-3). It is shown that the current–voltage (I–V) characteristics of the samples are abnormally dependent on the annealing temperature. For example, the samples that were annealed at temperatures below 750 °C for 1 min in a N₂ ambient show rectifying behavior. However, annealing the samples at temperatures in excess of 850 °C results in linear I–V characteristics. The contact produces a specific contact resistance as low as 8.4×10^-6 Ω cm² when annealed at 900 °C. It is further shown that the contacts are fairly thermally stable even after annealing at 900 °C; annealing the samples at 900 °C for 30 min causes insignificant degradation of the electrical and structural properties. Based on glancing angle X-ray diffraction and Auger electron microscopy results, the abnormal temperature dependence of the ohmic behavior is described and discussed. PACS 72.80.Ey; 73.40.Cg; 73.20.At; 79.60Bm; 73.40.Gk     

Fabrication and characterization of CuO-core/TiO2-shell one-dimensional nanostructures

We have fabricated CuO-core/TiO₂-shell one-dimensional nanostructures by coating the CuO nanowires with MOCVD-TiO₂. The structure of the core/shell nanowires has been investigated by using scanning electron microscopy, transmission electron microscopy, and X-ray diffraction analysis techniques. The CuO-cores and the TiO₂-shells of the as-synthesized nanowires have been found to have crystalline monoclinic CuO and crystalline tetragonal anatase TiO₂ structures, respectively. The CuO-core/TiO₂-shell nanowires are winding and has rougher surface, whereas the CuO nanowires are straight and have smoother surface.Influence of the substrate temperature and the growth time on the structure such as the morphology, size, and crystallographic orientation of CuO nanowires synthesized by thermal oxidation of Cu foils have also been investigated. All the nanowires have only the CuO phase synthesized at 600 °C, whereas those synthesized at 400 °C have both CuO and Cu₂O phases. The highest density of CuO nanowires with long thin straight morphologies can be obtained at 600 °C. In addition, the growth mechanism of the CuO nanowires has been discussed.     

In situ scanning electron microscope electrical characterization of GaN nanowire nanodiodes using tungsten and tungsten/gallium nanoprobes

A lithography-free technique for measuring the electrical properties of n-type GaN nanowires has been investigated using nanoprobes mounted in a scanning electron microscope (SEM). Schottky contacts were made to the nanowires using tungsten nanoprobes, while gallium droplets placed in situ at the end of tungsten nanoprobes were found to be capable of providing Ohmic contacts to GaN nanowires. Schottky nanodiodes were fabricated based on single n-type nanowires, and measured current–voltage (I–V) results suggest that the Schottky nanodiodes deviate from ideal diodes mainly due to their nanoscopic contact area. Additionally, the effect of the SEM electron beam on the I–V characteristics was investigated and was found to impact the transport properties of the Schottky nanodiodes, possibly due to an increase in carrier density in the nanodiodes.     

Effect of intrinsic lattice defects on excess currents of gallium arsenide tunnel diodes

Gallium arsenide tunnel diodes were irradiated with electrons of energy E = 2.0 MeV at room temperature. Secondary hump structures were observed in the region of 0.45 V and 0.65–1.1 V forward bias. The variation in the excess currents of diodes subjected to electronic bombardment is described on the basis of models including three levels; E_B + 0.25 eV, E_B + 0.55 eV, and E_C–0.5 eV. Annealing of radiation-induced defects is shown to take place in two stages at temperatures of 140–180°C and 200–240°C. Switching and memory effects are observed when the current-voltage characteristics (CVC) of tunnel diodes are measured at liquid-nitrogen temperatures in darkness.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 88–92, September, 1976.     

Large-scale crystalline GaN nanowires synthesized through a chemical vapor deposition method

Large-scale, high-density gallium nitride nanowires were successfully synthesized by the direct reaction of gallium and ammonia using gold as initiator. The as-synthesized product was characterized by XRD, SEM, TEM, SAED, and EDS. The results showed that the product is hexagonal wurtzite GaN with high purity. The nanowires have diameters in the range of 60–100 nm and are a few tens of micrometers in length. A remarkable feature is that catalyst particles were observed at the ends of the nanowires, indicating that the growth process can be controlled by the vapor–liquid–solid mechanism. The present results revealed that gold is an effective and advantageous catalyst for the growth of GaN nanowires. PACS 81.05.Ea; 81.10.Bk; 68.65.-k     

Two-step synthesis of one-dimensional single crystalline GaN nanowires

Straight and smooth GaN nanowires were synthesized on quartz substrates through the direct reaction of Ga₂O₃ thin films with flowing ammonia in a horizontal oven without using a template or catalyst. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), transmission electron microscopy (TEM) and photoluminescence (PL) were used to characterize the samples. The straight and smooth cylindrical nanostructures are high quality single crystalline hexagonal wurtzite GaN nanowires with diameters ranging from 5 to 30 nm and lengths up to 20 μm. The near-band-edge emission peak located at 367 nm was observed at room temperature.     

Epitaxy of AlxGa1−xSb films from a molten metal.

Conclusions Crystallization from a solution of antimony in a gallium-aluminum melt has been used to produce epitaxial Al_xGa_1–xSb films with x=0 to 0.4; isothermal (T = 450, 500, 550°C) and isoconcentration (0; 1; 2.4 at. % Al) sections have been drawn for the liquidus surface on the composition plane. X-ray microspectral analysis has been used to examine the composition of the Al_xGa_1–xSb films in relation to aluminum content in the melt, as well as the distribution of Al and Ga over the epitaxial film. These films had a perfect structure. The results enable one to determine the solution composition needed to grow Al_xGa_1–xSb epitaxial films at 450–550°C on gallium antimonide substrates, and it is shown to be possible to make heterostructures in the Al_xGa_1–xSb-GaSb system.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 16, No. 9, pp. 146–148, September.We are indebted to Zh. I. Alferov and V. I. Shveikin for attention and support in this work.     

The effect of the aminating temperature on the synthesis of gallium nitride nanowires doped with magnesium

GaN nanowires doped with Mg have been synthesized at different temperature through ammoniating the magnetron-sputtered Ga₂O₃/Au layered films deposited on Si substrates. X-ray diffraction (XRD), Scanning electron microscope (SEM), high-resolution TEM (HRTEM) equipped with an energy-dispersive X-ray (EDX) spectrometer and photoluminescence (PL) were used to analyze the structure, morphology, composition and optical properties of the as-synthesized sample. The results show that the ammoniating temperature has a great impact on the properties of GaN. The optimally ammoniating temperature of Ga₂O₃/Au layer is 900 C for the growth of GaN nanowires(NWs). The band gap emission (358 nm) relative to that (370 nm) of undoped GaN NWs has an apparent blueshift, which can be ascribed to the doping of Mg. Finally, the growth mechanism is also briefly discussed.     

One-dimensional gallium nitride micro/nanostructures synthesized by a space-confined growth technique

Hexagonal GaN prismatic sub-micro rods and cone nanowires have been synthesized in a large scale by a novel and controllable space-confined growth method. The as-synthesized GaN products are highly crystalline with a wurtzite structure. The prismatic rods have lengths of 15∼20 μm and diameters of 400∼500 nm enclosed by hexagonal smooth side surfaces and a pyramidal end. And the cone nanowires have average diameters of 150∼200 nm and lengths up to several tens of μm with a cone tip. The photoluminescence (PL) studies reveal prominent band-gap UV emission properties of GaN products and narrow FWHM, indicating the excellent luminescent performance and high crystal quality. For field emission characteristic of GaN nanowires, the turn-on field is about 9.5 V/μm and the current density reaches 1.0 mA/cm² at an electric field of 18 V/ μm. The contrast experiments indicate a novel growth control can be achieved by using a graphite tube as reaction vessel. The sealed graphite tube combined with metallic initiator is greatly responsible for large-scale and uniform preparation of GaN prismatic rods and cone nanowires. Highly symmetric GaN hexagonal micropyramids are grown on a bare Si substrate. The growth mechanism and the control function of the graphite tube are demonstrated. These low-dimensional structures not only enrich semiconducting GaN family, but also are good building blocks for optoelectronic devices. PACS 81.10.Bk; 81.07.-b; 81.05.Ea     

Electromagnetic radiation and variations in the conductivity of muscovite specimens during dehydration

The authors give experimental results on the measurement of electromagnetic radiation and electrical conductivity of muscovite specimens during heating or cooling in the temperature range from 20 to 600°C. It was established that at some temperatures there were variations in the electrical conductivity with the attendant onset of pulsed electromagnetic radiation in the radio-frequency range from 247.5 kHz to 35.8 MHz. The oscillations in the conductivity and the maximum intensity of electromagnetic radiation coincide with the boiling points of weakly bound water at about 100°C, the electrical activity of centers on the surface of micas (20–450°C), the temperatures of crystallization of muscovite in a hydrothermal process from 480 to 160°C, and phase transitions at 550–590°C. The electromagnetic radiation is probably due to fluctuations in the defects in the mineral under thermal excitation and adhesion processes as a result of dehydration of the muscovite.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 27–33.     

Rotational hysteresis loss and anisotropy in yttrium garnet ferrite single crystals

The rotational hysteresis loss in Y₃Fe₅O₁₂ single crystals is found to increase with the field up to 2.5–3 kOe at all temperatures between –183 and 250 ° C, this being followed by a fall, a further peak at 8–10 kOe, and a slow fall, with some residual effect even at 20 kOe. This loss is sensitive to heat treatment; annealing for 2 hr at 275 ° C greatly reduces the loss at –183 ° C and halves the main (second) peak at 20 ° C. These anomalies do not occur in ferrites with the spinel structure. The first anistropy constant is slightly reduced at all temperatures as a result of annealing.