Direct evidence for electron beam irradiation-induced phenomena in nanowired ZnO thin films

In-situ electron beam induced microstructural transformation experiments, leading to porosity in nanowires of ZnO, have been performed under a TEM operated at an electron accelerating voltage of 200 kV. For this purpose, nanowired (diameter: 20 to 80 nm) films of ZnO with thickness ~ 100 to 120 nm, were grown via metal-catalyst free-vapor phase mechanism. The evolved porosity (pore size about 2 to 20 nm) in nanowires, under electron beam irradiation, has been attributed to different bond-breaking phenomena at molecular Zn-O. Such nanoporous objects of ZnO are beneficial for various optical and sensing devices.     

Field emission characteristics of CuO nanowires by hydrogen plasma treatment

CuO nanowires were formed on copper-coated silicon substrates by a wet chemical process, immersing them in a hot alkaline solution. The effect of hydrogen plasma treatment on the field emission characteristics of the CuO nanowires was investigated. The results showed that hydrogen plasma treatment enhanced the field emission characteristics of the CuO nanowires showing a decrease in turn-on voltage as well as an increase of field enhancement factor. It is believed that hydrogen plasma treatment plays an important role in the improvement of field emission characteristics of CuO emitters.     

Synthesis of AgBr/ZnO nanocomposite with visible light-driven photocatalytic activity

AgBr/ZnO nanocomposite was synthesized via chemical precipitation from pure ZnO nanowires, AgNO₃, and NaBr. Inductively coupled plasma optical emission spectroscopy, X-ray diffraction, and high resolution transmission electron microscopy results confirmed the forming of AgBr/ZnO nanocomposite. High resolution transmission electron microscopy results of the as-synthesized AgBr/ZnO nanocomposite revealed that AgBr nanoparticles were attached to the surface of ZnO nanowires. UV-vis diffuse reflectance spectra of both pure ZnO and AgBr/ZnO nanocomposite displayed a band gap edge at about 350-380 nm. However, compared with pure ZnO, an additional broad tail from approximately 400 nm to 700 nm appeared in the UV-vis diffuse reflectance spectrum of AgBr/ZnO nanocomposite. The photocatalytic studies indicated that the as-synthesized AgBr/ZnO nanocomposite was a kind of promising photocatalyst in remediation of water polluted by some chemically stable azo dyes under visible light.     

Effects of surface modification of the individual ZnO nanowire with oxygen plasma treatment

This study examined the effects of an oxygen plasma treatment on the properties of ZnO nanowires with diameters of 80 nm using a single nanowire field effect transistor. After the oxygen plasma treatment, the carrier concentration and mobility of individual ZnO nanowires decreased with a substantial positive shift in the threshold voltage. The shifting was accounted to the surface modification, resulted to the improved gas sensitivity under hydrogen gas exposure and an enhanced photocurrent response time in ultraviolet illumination. The plausible surface mechanisms responsible for these significant changes after the surface modification were suggested by considering the surface analysis and electrical transport mechanism.     

Electrically pumped waveguide lasing from ZnO nanowires

Ultraviolet semiconductor lasers are widely used for applications in photonics, information storage, biology and medical therapeutics. Although the performance of gallium nitride ultraviolet lasers has improved significantly over the past decade, demand for lower costs, higher powers and shorter wavelengths has motivated interest in zinc oxide (ZnO), which has a wide direct bandgap and a large exciton binding energy. ZnO-based random lasing has been demonstrated with both optical and electrical pumping, but random lasers suffer from reduced output powers, unstable emission spectra and beam divergence. Here, we demonstrate electrically pumped Fabry-Perot type waveguide lasing from laser diodes that consist of Sb-doped p-type ZnO nanowires and n-type ZnO thin films. The diodes exhibit highly stable lasing at room temperature, and can be modelled with finite-difference time-domain methods.     

Effect of surface morphology of metallic zinc films deposited by ion beam sputter deposition on the formation of ZnO nanowires

Metallic zinc film with various surface roughnesses was deposited on Si (100) substrates by ion beam sputter deposition utilizing beam energies at 8, 12 and 16 keV. The surface roughness of the metallic zinc film increased as ion beam energy increased and was found to act as a crucial factor for the formation of ZnO nanowires by subsequent thermal oxidation. ZnO nanowires with diameters of ∼30 nm and average length of ∼1 μm were obtained from 12 to 16 keV ion beam deposited samples while no ZnO nanowires were found on 8 keV ion beam deposited samples. Photoluminescence study of ZnO nanowires exhibits a strong UV emission at 377.2 nm (3.287 eV) with a full-width at half maximum of 95.0 meV and negligible defect related deep level emission. The ZnO nanowires are grown along the [110] direction and the growth mechanism is likely due to a solid state based-up diffusion process. Field-emission measurement shows a turn-on field of 7.9 MV/m and a field enhancement factor β of 691 is achieved.     

Effect of surface morphology of metallic zinc films deposited by ion beam sputter deposition on the formation of ZnO nanowires

Metallic zinc film with various surface roughnesses was deposited on Si (100) substrates by ion beam sputter deposition utilizing beam energies at 8, 12 and 16 keV. The surface roughness of the metallic zinc film increased as ion beam energy increased and was found to act as a crucial factor for the formation of ZnO nanowires by subsequent thermal oxidation. ZnO nanowires with diameters of ∼30 nm and average length of ∼1 μm were obtained from 12 to 16 keV ion beam deposited samples while no ZnO nanowires were found on 8 keV ion beam deposited samples. Photoluminescence study of ZnO nanowires exhibits a strong UV emission at 377.2 nm (3.287 eV) with a full-width at half maximum of 95.0 meV and negligible defect related deep level emission. The ZnO nanowires are grown along the [110] direction and the growth mechanism is likely due to a solid state based-up diffusion process. Field-emission measurement shows a turn-on field of 7.9 MV/m and a field enhancement factor β of 691 is achieved.     

MBE法生长ZnO纳米线阵列的结构和光学性能

在氧等离子体辅助的MBE系统中, 以1 nm厚的Au薄膜为催化剂, 基于气?液?固(VLS)机制实现了低温ZnO纳米线阵列在Si(111)衬底表面的生长. 通过场发射扫描电子显微镜(FE-SEM)可以观察到, ZnO纳米线阵列垂直生长在衬底上, 直径为20~30 nm. X射线衍射(XRD)和高分辨透射电镜(HRTEM)结果表明: ZnO纳米线为六方纤锌矿结构, 具有沿c轴方向的择优取向. 光致发光(PL)谱显示在380 nm附近有强烈ZnO本征发射峰, 475~650 nm可见光区域有较强的缺陷导致的发射峰.     

Field emission from zinc oxide nanostructures and its degradation

Arrays of zinc oxide (ZnO) nanowires and nanobelts were synthesized by the thermal evaporation of mixed powders of ZnO and graphite. Neither catalyst nor vacuum environment was involved in the fabrication. For comparison, the ZnO nanowires were grown on a pre-deposited transitional ZnO film on a brass substrate and the ZnO nanobelts were grown directly on a Si substrate. Their field emission properties were systematically measured. Current density of 10 μA/cm² was achieved at the fields of 5.7 and 6.2 V/μm from the nanowires and nanobelts, respectively. Also, the emission sites were found to distribute uniformly on the whole cathode. In the preliminary test on the stability, the ZnO nanobelts, which were sharp at the tip but wide at the root, exhibited better robustness than the ZnO nanowires. The post-test scanning electron microscopy (SEM) observation showed that the degradation of their field emission capability resulted from the breaking of the nanowires, which was tentatively attributed to the resistive heating during the field emission. In contrast, the shedding of the ZnO from the substrate was not so serious as imagined.     

Field electron emission from hydrogen plasma treated nano-ZnO thin films

A nano-Zno films are deposited on the Mo film/ceramic substrates by using the electron beam vapor deposition technique. Then a hydrogen plasma treated method is used to improve the characteristics of ZnO thin films by microwave plasma chemical vapor deposition system. Effects of process parameters on morphologies and structures of the ZnO thin films are detected and analysed by field emission scanning electron microscopy, X-ray diffraction spectrum and energy dispersive spectrum. The experimental result indicates that the hydrogen plasma treated techniques can essentially reduce the surface resistance and improve the field emission current density of the nano-ZnO thin films. For the hydrogen plasma treated sample, its field emission current density can increased more than three times at 2.2 V/microm electric field condition.     

反应源温度对ZnO纳米线阵列定向性及发光性能的影响

以Au薄膜为催化剂、ZnO与碳混合粉末为反应源,采用碳热还原法在单晶Si衬底上制备了ZnO纳米线阵列.通过扫描电子显微镜( SEM)、X射线衍射仪(XRD)、荧光分光光度计对样品的表征,研究了反应源温度对ZnO纳米线阵列的定向性和光致发光性能的影响.样品在源温度920℃条件下沿(002)方向择优生长,定向性最好,温度过低不利于ZnO纳米线阵列密集生长,而温度过高导致Zn原子二次蒸发,因而也不利于纳米线阵列的定向和择优生长;样品在源温度880℃有最强的近紫外带边发射,表明温度过高和过低都不利于ZnO晶体结构的优化;由于ZnO纳米线在缺氧氛围下生长,氧空位是缺陷存在的主要形式,因此所有样品都有较强的绿光发射.温度升高导致纳米线生长速度提高而增加了氧空位缺陷数量,从而使样品绿峰强度增强并在源温度920℃时达最大值,但温度的进一步升高可导致ZnO纳米线表面Zn元素的蒸发而降低氧空位缺陷的数量,从而抑制绿峰强度.     

Effect of chemically reactive species on properties of ZnO nanowires exposed to oxygen and hydrogen plasma

We present a systematic study on the effect of oxygen and hydrogen plasma-generated reactive species on the properties of ZnO nanowires. Upon exposure to oxygen plasma, the electrical conductivity of an individual ZnO nanowire decreased with substantial changes in the surface chemistry, indicating a decrease in the number of donor-like defects and an increase in the number of electron-trapping species. In contrast, an individual ZnO nanowire exposed to hydrogen plasma showed a drastic increase in conductivity up to two orders of magnitude due to the incorporated hydrogen acting as a shallow donor inside the ZnO nanowires without a sputtering process.     

ZnO纳米线的气相沉积法自催化生长及发光性能研究

介绍了一种通过气相沉积法自催化生长氧化锌纳米线的方法。氧化锌纳米线的生长方向为〈001〉,其尺寸随反应温度的升高而增大。光致发光分析表明绿光发射强度随氧化锌纳米线尺寸而变化。当氧化锌纳米线直径小至5～10nm时,由于量子效应而表现出非常强的绿光发射。     

氢化及沉积温度对掺铝ZnO透明导电薄膜性能影响

采用射频磁控溅射方法在玻璃衬底上制备了掺铝ZnO透明导电薄膜(AZO)。为了降低AZO薄膜的电阻率, 采用在溅射气氛中通入一定比例H₂的方法对AZO薄膜进行氢化处理, 并研究了溅射气氛中H₂含量及衬底温度对AZO薄膜氢化效果的影响。结果表明: 在低温条件下, 氢化处理能有效降低AZO薄膜的电阻率; 在衬底温度为100℃的低温条件下, 通过调节溅射气氛中H₂的比例, 制备了电阻率为6.0×10^-4 Ω·cm的高质量氢化AZO薄膜, 该电阻值低于同等条件下未氢化AZO薄膜电阻值的1/3; 但随着衬底温度的升高, 氢化处理对薄膜电学性能的改善效果逐渐减弱。     

Growth and characterization of indium phosphide nanowires on transparent conductive ZnO:Al films

The epitaxial growth of indium phosphide nanowires (InP NWs) on transparent conductive aluminum-doped zinc oxide (ZnO:Al) thin films is proposed and demonstrated. ZnO:Al thin films were prepared on quartz substrates by radio frequency magnetron sputtering, then InP NWs were grown on them by plasma enhanced metal organic chemical vapor deposition with gold catalyst. Microstructure and optical properties of InP nanowires on ZnO:Al thin films were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectric spectroscopy (XPS), photoluminescence and Raman spectroscopy at room temperature. SEM shows that randomly oriented and intersecting InP nanowires were grown to form a network on ZnO:Al thin films. Both wurtzite (WZ) and zincblende (ZB) structures coexist in the random orientation InP NWs on ZnO:Al thin film had been proved by XRD analysis. XPS result indicates Zn diffusion exists in the InP NWs on ZnO:Al. The photoluminescence spectra of InP nanowires with Zn diffusion present an emission at 915 nm. Zn diffusion also bring effect on Raman spectra of InP NWs, leading to more Raman-shift and larger relative intensity ratio of TO/LO.     

Large-scale solution-phase growth of Cu-doped ZnO nanowire networks

Film-like networks of Cu-doped (0.8-2.5 at.%) ZnO nanowires were successfully synthesized through a facile solution process at a low temperature (<100 degrees C). The pH value of solution plays a key role in controlling the density and quality of the Cu-doped ZnO nanowires and the dopant concentration of ZnO nanowires was controlled by adjusting the Cu2+/Zn2+ concentration ratio during the synthesis. The structural study showed that the as-prepared Cu-doped ZnO nanowires with a narrow diameter range of 20-30 nm were single crystal and grew along [0001] direction. Photoluminescence and electrical conductivity measurements showed that Cu doping can lead to a redshift in bandgap energy and an increase in the resistivity of ZnO. The thermal annealing of the as-grown nanowires at a low temperature (300 degrees C) decreased the defect-related emission within the visible range and increased the electrical conductivity. The high-quality ZnO nanowire network with controlled doping will enable further application to flexible and transparent electronics.     

Electron field emission studies on ZnO nanowires

The zinc oxide (ZnO) nanowires with different morphology and diameter were synthesized on silicon (100) substrates by heating pure zinc powder at low temperatures of 450 °C and 480 °C. Scanning electron microscopy (SEM) was used to analyze the morphology and diameter of samples. The electron field emission properties between different morphology of ZnO nanowires samples were compared. A low turn-on field at 3.6 V/μm was observed from nanorods due to better alignment, and a strong emission current density of 3.6 mA/cm² at electronic field 9.0V/μm was obtained from needle-like nanowires sample. The emission stability of ZnO samples is also presented.     

Thickness optimized nanocrystalline ZnO-coated silicon nanowires for cold cathode application

Silicon nanowire is an important material for the potential use as a cold cathode, but there are some bottlenecks like oxidation of the surface during field emission thereby degradation of its performance. To compete with carbon based nanostructures in this field the performance of Si nanowires as field emitter should be improved. Here, we report a simple technique for the significant improvement of field emission properties of Si nanowires by ZnO nanoparticle coating. Boron-doped p-type Si wafers were chemically etched to synthesize vertically aligned silicon nanowires and they were coated with different thicknesses of ZnO layer by radio frequency magnetron sputtering technique. The nanostructured thin films were studied by X-ray photoelectron spectroscopy for compositional and valence states information while their morphological information was obtained by a field emission scanning electron microscope and a high resolution transmission electron microscope. The field assisted electron emission performance of Si nanowires significantly improved for the thickness optimized ZnO coating. The photoluminescence spectra showed a peak at ~558 nm assigned to surface defect states of ZnO and the field emission from Si nanowires coated with ZnO for different times were correlated with the surface defect structures. The mechanism of such improvement is also discussed.     

Low-temperature fabrication of bunch-shaped ZnO nanowires using a sodium hydroxide aqueous solution

Bunch-shaped ZnO nanowires film was successfully fabricated by the forced-hydrolysis-initiated-nucleation of anhydrous zinc acetate in an aqueous solution of zinc acetate and sodium hydroxide at low temperature. X-ray diffraction and a field emission scanning electron microscope clarified their formation mechanism and morphology development. The morphology was controllable by adjusting the solution temperature and deposition time. ZnO nanowires obtained at 65 degrees C for 6 h have a high aspect ratio of about 106. The smaller diameter with higher aspect ratio of ZnO nanowires, the easier the formation of bunch shapes by the capillary force during the drying process. This fabrication technique indicated that bunched ZnO film was prepared at low cost, and fittable to low heat-resistance substrates such as a polymer substarte.     

Growth of ZnO nanowires in hollow-type magnetron O2/Ar RF plasma

In this study we investigate how the sputtering of zinc from a zinc target is influenced by the properties of O₂/Ar plasma when the discharge parameters such as gas pressure ratio and target bias voltage are changed. We also investigate plasma conditions for the formation of ZnO nanowires that are created and deposited on the substrate. We found that a plasma condition with strong optical emission from Zn neutrals is an important factor for the deposition of ZnO nanowires. Both the growth of nanowires and the emission intensity from Zn strongly depend on the partial pressure of oxygen.