Synthesis and gas sensor properties of flower-like 3D ZnO microstructures

Flower-like ZnO 3D microstructures composed of nanorods have been successfully prepared via a facile hydrothermal method using p-nitrobenzoic acid as the structure-directing agent. The structures and morphologies of the final products have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscope (HRTEM). The possible mechanism for the synthesis of the flower-like ZnO microstructures has been proposed primarily. The gas sensitivity of the flower-like ZnO microstructures has been studied to a series of organic vapors at different operation temperatures and vapor concentrations. The results show that the flower-like ZnO microstructures composed of nanorods have good gas sensor properties to ethanol.     

A task specific basic ionic liquid for synthesis of flower-like ZnO by hydrothermal method

Flower-like ZnO morphology, with different shapes, have been successfully synthesized via a novel and environment-friendly hydrothermal method using zinc acetate and a task specific dicationic dibasic ionic liquid, [mmpim]₂[OH]₂, which plays an important role in fabrication of ZnO structure. The structure and morphology of the product were characterized by X-ray diffraction and scanning electron microscopy, which show different flower-like morphologies. Photoluminescence spectrum of the product exhibits a strong ultraviolet emission at 391 nm and two weak blue-green emissions at about 450 and 500 nm.     

Technology of ZnO nanofibers based devices

In the presented work, the possibility of fabrication of ZnO single- and multi-nanofiber structures using a standard microelectronic device technology were studied. An innovative fabrication step, namely, selective wet chemical nanofibers etching through a photoresist mask, was used to define the active area, along with mesa etch in the Si/SiO₂ substrate. Test structures in the configuration of a resistor and Schottky diode with chemically active electrospun ZnO nanofibers were prepared. The Ti/Au ohmic and Pt Schottky contacts were fabricated using a lift-off photolithography process. Optical and scanning electron microscopy studies were done to characterize ZnO nanofibers and topography of contacts. The measurements made for electrical characterization showed linear I–V dependence and saturation of the current for single ZnO nanofiber structures.     

Hydrothermal synthesis of hexagonal ZnO clusters

Hydrothermal process was applied to synthesize zinc oxide nanocrystals. X-ray powder diffraction and scanning electron microscopy were used to analyze the crystal structure and surface morphology. XRD pattern analysis showed that the ZnO clusters are single hexagonal phase of wurtzite structure (space group P63 mc) with no impurity of Zn and Zn(OH)₂. Also, SEM images revealed that the size of a single ZnO crystal is between 200-500 nm in diameter and 2-5 μm in length. The influence of potassium iodide (KI) as a surfactant on the crystallinity of ZnO has been investigated.     

Hydrogen peroxide-assisted hydrothermal synthesis of hierarchical CuO flower-like nanostructures

Hierarchical CuO nanostructures were synthesized through a hydrogen peroxide-assisted hydrothermal route in which Cu(OH)₂ was the copper source. The CuO nanostructures were composed of numerous nanobelts that radiated from the center of the nanostructure and formed a flower-like shape with a diameter of 5-10 μm. The nanobelts had lengths of 2.5-5 μm and widths of 150-200 nm. The H₂O₂ concentration directly influenced the product morphology. As the concentration of H₂O₂ increased, the length and width of the nanobelts increased and the quantity of the nanobelts decreased. The possible formation mechanism of hierarchical CuO flower-like nanostructures was presented.     

Growth and optical and field emission properties of flower-like ZnO nanostructures with hexagonal crown

Flower-like zinc oxide (ZnO) nanostructures with hexagonal crown were synthesized on a Si substrate by direct thermal evaporation of zinc power at a low temperature of 600 °C and atmospheric pressure. Field emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Raman spectroscopy and photoluminescence were applied to study the structural characteristics and optical properties of the product. The result indicated that the flower-like product with many slender branches and hexagonal crowns at the ends were single-crystalline wurtzite structures and were preferentially oriented in the <001> direction. The photoluminescence spectrum demonstrated a strong UV emission band at about 386 nm and a green emission band at 516 nm. The field emission of the product showed a turn-on field of 3.0 V/µm at a current density of 0.1 μA/cm², while the emission current density reached about 1 mA/cm² at an applied field of 5.9 V/μm.     

Nonvolatile memory devices based on ZnO/polyimide nanocomposite sandwiched between two C60 layers

The memory effects of a three-layer nonvolatile memory device Al/C₆₀/ZnO nanoparticles embedded in a polyimide (PI) layer/C₆₀/p-Si were investigated by using capacitance-voltage (C-V) measurements. Transmission electron microscopy and selected area electron diffraction pattern measurements showed that ZnO nanocrystals were formed inside the PI layer. The insertion of C₆₀ layer improved the charge trap state density in the ZnO nanoparticle. The density was estimated by the flatband voltage shift in the C-V hysteresis, which increases with the max sweep voltage. Possible operating mechanisms corresponding to the charging and discharging process in the structure are proposed on the basis of the C-V results.     

基于梳状分等级结构ZnO纳米带的快速响应VOC气体传感器

通过化学气相沉积法(CVD)合成出梳状分等级结构的ZnO纳米带,使用场发射扫描电子显微镜(SEM)和X射线衍射仪(XRD)对材料组成和结构进行了分析。利用这种材料制备了厚膜型管式气敏元件,并采用静态配气测试系统进行了气敏性能测试。测试结果表明,工作温度大约为225℃时,这种结构的材料对有机挥发性气体(volatile organic compounds,VOC)具有极快的响应和恢复速度,响应时间为2s,恢复时间为3s。最后分析了材料结构对气敏性能的影响。     

以PAMAM树形分子为模板剂纳米氧化锌的制备及其光催化性能研究

以3.5代端酯基PAMAM树型分子为模板,成功制备了颗粒均匀的纳米氧化锌.采用TEM、XRD及UV-vis等手段对合成样品的形貌和结构进行表征.初步探讨了纳米ZnO形成的可能模板机理.测试其在紫外光作用下光降解罗丹明B的性能,结果表明,该法制备的纳米ZnO具有很高的光催化活性.     

Multifunctional ZnO:V thin films deposited by rf-magnetron sputtering from aerogel nanopowder target material

ZnO:V thin films have been grown onto suprasil substrates by rf-magnetron sputtering at room temperature using nanocrystalline powder synthesized by modified sol-gel method. In our approach the water for hydrolysis used in the synthesis of nanopowder was slowly released by esterification reaction followed by a thermal drying in ethyl alcohol at 250 °C. The effects of V concentration on structural, electrical, morphological and optical properties were studied. The as-deposited films with a thickness of about 0.4 μm were polycrystalline with a hexagonal wurtzite structure and preferentially oriented in the (002) crystallographic direction. The films present high optical transmittance in the visible range of approximately 90%, carrier concentration of about 10²⁰ cm^− 3 and electrical resistivity of 10^− 3 Ω cm at room temperature. In the as-prepared state the films also present ferromagnetic properties attributed to the presence of vanadium based secondary phases.     

Detection of Defect‐Induced Magnetism in Low‐Dimensional ZnO Structures by Magnetophotocurrent

The detection of defect‐induced magnetic order in single low‐dimensional oxide structures is in general difficult because of the relatively small yield of magnetically ordered regions. In this work, the effect of an external magnetic field on the transient photocurrent measured after light irradiation on different ZnO samples at room temperature is studied. It has been found that a magnetic field produces a change in the relaxation rate of the transient photocurrent only in magnetically ordered ZnO samples. This rate can decrease or increase with field, depending on whether the magnetically ordered region is in the bulk or only at the surface of the ZnO sample. The phenomenon reported here is of importance for the development of magneto‐optical low‐dimensional oxides devices and provides a new guideline for the detection of magnetic order in low‐dimensional magnetic semiconductors.     

High-temperature-dependent optical properties of ZnO film on sapphire substrate

In order to fabricate fiber-optic temperature sensors based on ZnO film, it is important to study the temperature-dependent optical properties of this material. In this work, we deposited ZnO films on c-plane (0001) sapphire substrate at 250 °C. Atomic force microscope and X-ray diffraction measurements show the smooth surface and high orientation along [0001] of ZnO film, respectively. The high-temperature-dependent optical properties of ZnO film were measured by ultraviolet-visible transmission with temperatures ranging from room-temperature to 300 °C and analyzed by theoretically fitting the optical absorption edge curve. It is observed that the band gap energy red shifts nonlinearly from 3.345 to 3.153 eV with increasing temperature. The sharp absorption edge of ZnO films after annealing at 300 °C is almost consistent with that of the as-deposited sample, indicating an excellent thermal stability and the potential application in fiber-optic temperature sensors.     

Growth of ZnO quantum dots on Si nano ripples

Substrates with nano-scale ripples are excellent templates for the deposition of semiconductor nanostructures. We have prepared quasi periodical nano-scale ripples on Si (100) substrates with spatial wavelength λ from ~ 70 to ~ 150 nm by ion beam sputtering. ZnO QDs with diameters from 17 to ~ 30 nm and heights from 2 to ~ 4 nm have been successfully deposited by reactive ion beam sputter deposition. The QD size and distribution were found to be dependent both on growth conditions and spatial wavelength of the nano-scale ripple. On substrates with λ ~ 150 nm, ZnO QDs were distributed evenly across the wafer, while on substrates with λ ~ 70 nm, ZnO QDs were preferentially located along the crest of the nano-scale ripples. As the QD height decreases from ~ 4 to 2 nm, room temperature photoluminescence UV emission energy blue shifts by 80 meV. Possible sources of the blue shift are presented.     

Effect of Ga doping on micro/structural, electrical and optical properties of pulsed laser deposited ZnO thin films

Undoped and Ga doped ZnO thin films (1% GZO, 3% GZO and 5% GZO) were grown on c-Al₂O₃ substrates using the 1, 3 and 5 at. wt.% Ga doped ZnO targets by pulsed laser deposition. X-ray diffraction studies revealed that highly c-axis oriented, single phase, undoped and Ga doped ZnO thin films with wurtzite structure were deposited. Micro-Raman scattering analysis showed that Ga doping introduces defects in the host lattice. The E₂^High mode of ZnO in Ga doped ZnO thin film was observed to shift to higher wavenumber indicating the presence of residual compressive stress. Appearance of the normally Raman inactive B₁ modes (B₁^Low, 2B₁^Low and B₁^High) due to breaking of local translational symmetry, also indicated that defects were introduced into the host lattice due to Ga incorporation. Band gap of the Ga doped ZnO thin films was observed to shift to higher energy with the increase in doping concentration and is explicated by the Burstein-Moss effect. Electrical resistivity measurements of the undoped and GZO thin films in the temperature range 50 to 300 K revealed the metal to semiconductor transition for 3 and 5% GZO thin films.     

Enhanced ultraviolet sensitivity of zinc oxide nanoparticle photoconductors by surface passivation

Zinc oxide nanoparticles were created by a top-down wet-chemical etching process and then coated with polyvinyl-alcohol (PVA), exhibiting sizes ranging from 10 to 120 nm with an average size approximately 80 nm. The PVA layer provides surface passivation of zinc oxide nanoparticles. As a result of PVA coating, enhancement in ultraviolet emission and suppression of parasitic green emission is observed. Photoconductors fabricated using the PVA coated zinc oxide nanoparticles exhibited a ratio of ultraviolet photo-generated current to dark current as high as 4.5 × 10⁴, 5 times better than that of the devices fabricated using uncoated ZnO nanoparticles.     

Characterization of ZnO Based Varistor Derived from Nano ZnO Powders and Ultrafine Dopants

Nanosized ZnO powders were prepared with a two-step precipitation method. The average size of ZnO particles was about 80 nm and their size distribution was narrow. Combining with ultrafine additive powders, ZnO base varistor was produced via an oxide mixing route. ZnO varistor derived from normal reagent grade starting materials was investigated for comparison purpose. Outstanding microstructure of the ZnO varistor derived from nanosize ZnO powders and ultrafine dopants was obtained: uniform distribution of fine ZnO grains (less than 3 microns), grain boundary and the dopant position. Higher varistor voltage (U=492 V/mm) and nonlinear coefficient (α=56.2) as well as lower leakage current (TL=1.5 μuA) were achieved. The better electrical properties were attributed to the uniform microstructure, which in turn led to stable and uniform potential barriers. Also this improved technique is more feasible for producing ZnO nanopowders and resulting varistor in large scales.     

Properties of ZnO:Al films deposited on polycarbonate substrate

Transparent conducting aluminum-doped zinc oxide (ZnO:Al) films have been prepared on polycarbonate (PC) substrates by pulsed laser deposition technique at low substrate temperature (room-100 °C); Nd-YAG laser with wavelength of 1064 nm was used as laser source. The experiments were performed at various oxygen pressures (3 pa, 5 pa, and 7 Pa). In order to study the influence of the process parameters on the deposited (ZnO:Al) films, X-ray diffraction and atomic force microscopy were applied to characterize the structure and surface morphology of the deposited (ZnO:Al) films. Polycrystalline ZnO:Al films having a preferred orientation with the c-axis perpendicular to the substrate were deposited with a strong single violet emission centering about 377–379 nm without any accompanying deep level emission. The average transmittances exceed 85% in the visible spectrum for 300 nm thick films deposited on polycarbonate.     

Catalyst-free synthesis of ZnO nanowire arrays on zinc substrate by low temperature thermal oxidation

High density ZnO nanowire arrays were successfully synthesized without catalyst by direct oxidation of zinc substrate in air below 400 °C, lower than the melting point of zinc metal. The as-grown ZnO nanowires are single crystalline with a Wurzite structure extending in <110> direction. The diameters of the ZnO nanowires range from 20 to 150 nm and the lengths from several to over ten micrometers. Room temperature photoluminescence measurements reveal an intense ultraviolet emission at 397 nm. The present work highlights the promise of the low temperature, direct oxidation process in the high-yield synthesis of high quality semiconductor nanowire arrays for nano-devices.