电场条件下氧化锌结晶特性及极化产物的拉曼光谱分析

开展高压电场调控纳米材料结构形貌和性能研究在功能材料领域具有重要的理论和实际意义.本文在高压电场条件下合成了氧化锌纳米粉体,并对粉末试片进行了后期电场极化处理,研究了电场对氧化锌的结构形貌、点缺陷、拉曼光谱的影响.以X射线衍射仪(XRD)、扫描电镜(SEM)和拉曼光谱仪对产物的结构形貌、拉曼位移、缺陷分布等进行了表征.结果表明,高压电场条件下氧化锌的完全晶化时间和温度比未施加电场时明显延长和升高,直流电场能够显著促进前驱物中氧化锌的形核,并降低晶化速度.不同电场强度下氧化锌具有不同的显微形貌.纳米氧化锌粉末试片在直流电场中极化后,其阴极面和阳极面的拉曼光谱表现出明显的差异.有明显漏电电流的情况下,阳极面在1050 cm~(–1)处的二级光学声子模A_1(LO)的强度显著提高,且拉曼强度I_1=438 cm~(–1)和I_2=1050 cm~(–1)的比值与极化电场的场强呈线性关系.当调转试片正反面进行二次极化时,原来在阳极面尖锐的1050 cm~(–1)峰经过阴极极化而消失.阳极面1050 cm~(–1)拉曼峰的锐化与氧化锌晶粒内的缺陷重新分布和双肖脱基势垒有关.     

Preparation and properties of ZnO nanostructures by electrochemical anodization method

ZnO thin films with diverse nanostructures, including nanodot, nanowire and nanoflower, have been fabricated on zinc foils by a simple and rapid electrochemical anodization method. The ZnO thin films reveal very strong visible emission that is ascribed to the transition between V_OZn_i and valence band. Under the dc or ac electric field, the electroinduced surface wettability conversion from the superhydrophobic to hydrophilic state was observed and the generation of surface defective sites on ZnO films under electric field was used to explain the transition mechanism. This work provides a simple and rapid method for synthesizing different ZnO nanostructures in large scale, and electric field can be used to modulate the wettability of ZnO nanostructures.     

介电力显微镜探测氧化锌纳米线表面吸附分子对电导率的影响

利用场效应晶体管器件和介电力显微镜来研究氧化锌纳米线表面吸附分子对其电导率的影响. 相比于空气中,ZnO纳米线场效应晶体管器件在氮气中电导率更高,介电力显微镜得的介电信号也是在氮气中更大. 影响ZnO纳米线电导率变化的主要原因是表面吸附分子数量的变化,而并不是电极与材料之间接触性质的变化.     

Oscillations of light absorption in 2D macroporous silicon structures with surface nanocoatings

We investigated the near-IR light absorption oscillations in 2D macroporous silicon structures with microporous silicon layers and CdTe, ZnO surface nanocrystals. The electro-optical effect was taken into account within the strong electric field approximation. Well-separated oscillations were observed in the spectral ranges of the surface bonds of macroporous silicon structures with surface nanocrystals. The model of the resonant electron scattering on impurity states in electric field of heterojunction “silicon-nanocoating” on macropore surface as well as realization of Wannier-Stark effect on the randomly distributed surface bonds were considered. The Wannier-Stark ladders are not broken by impurities because of the longer scattering lifetime as compared with the period of electron oscillations in an external electric field, in all spectral regions considered for macroporous silicon structures with CdTe and ZnO surface nanocrystals.     

电化学方法制备ZnO纳米颗粒掺杂类金刚石薄膜及其场发射性能研究

采用液相电化学沉积技术制备了ZnO纳米颗粒掺杂的类金刚石(DLC)薄膜, 研究了ZnO纳米颗粒掺杂对DLC薄膜场发射性能的影响. 利用X射线光电子能谱、透射电子显微镜、Raman光谱以及原子力显微镜分别对薄膜的化学组成、 微观结构和表面形貌进行了表征. 结果表明: 薄膜中的ZnO纳米颗粒具有纤锌矿结构, 其含量随着电解液中Zn源的增加而增加. ZnO纳米颗粒掺杂增强了DLC薄膜的石墨化和表面粗糙度. 场发射测试表明, ZnO纳米颗粒掺杂能提高DLC薄膜的场发射性能, 其中Zn与Zn+C的原子比为10.3%的样品在外加电场强度为20.7 V/μm时电流密度达到了1 mA/cm². 薄膜场发射性能的提高归因于ZnO掺杂引起的表面粗糙度和DLC薄膜石墨化程度的增加.     

Abnormal blueshift of UV emission in single-crystalline ZnO nanowires

Single-crystalline ZnO nanowires (NWs) were synthesized by a facile vapor transport method. The good orientation and high crystal quality were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and high-resolution transmission electron microscope (HRTEM) measurements. Excitation-power-dependence photoluminescence spectra of ZnO NWs show that the UV emission displayed an evident blueshift with increasing excitation power and the corresponding energy shift might be as large as 10 meV. This anomalous phenomenon correlates to the band bending level caused by the surface built-in electric field due to the existence of substantial oxygen vacancies. By increasing the excitation power, the enhanced neutralization effect near the surface will reduce the built-in electric field and lead to a reduction of band bending which triggers the blueshift of the UV emission.     

Metal‐nanoparticle‐coating‐induced enhancement and weakening of resonant Raman scattering in ZnO: effect of surface electric field

Effects of Ag and Ti nanoparticle coatings on resonant Raman scattering in various ZnO thin films are presented. The longitudinal optical (LO) phonons, irrespective of the ZnO quality, exhibit an enhancement and a weakening by the Ag and Ti nanoparticle coatings, respectively. The enhancement (weakening) is always accompanied by a reduced (an increased) intensity ratio of the second to first‐order LO phonons, which can be associated with changes in the electron‐phonon coupling strength in the probed area of ZnO. Angle‐resolved X‐ray photoelectron spectroscopy provides evidence for the bending of the surface energy bands and their changes induced by the metal coatings. The effect of metal nanoparticle coatings on the Raman scattering of ZnO is thus attributed to the changes in the surface electric field. Copyright © 2011 John Wiley & Sons, Ltd.     

Enhanced Field Emission from Vertical ZnO Nanoneedles on Micropyramids

Vertical ZnO nanoneedles with sharp tips are secondarily grown on tips of primarily grown ZnO micropyramids by a vapour transport process. The field emission （FE） properties exhibit a lower turn-on electric field and a higher field enhancement factor as compared with vertical ZnO microrods. This result indicates that ZnO nanoneedles have good optimum shapes for FE due to electron accumulation at sharp tips.     

纳秒脉冲电压同轴电场下有机玻璃和尼龙的闪络特性

 利用基于SOS的SPG200高压脉冲功率装置研究了有机玻璃(PMMA)和尼龙1010在变压器油中的闪络特性。研究结果表明：闪络电压随内电极直径增加近似指数增加，随闪络距离延长近似线性增加，相应的击穿时延也近似线性增加。受实验条件的影响，有机玻璃和尼龙1010的闪络性能略有差异。同轴径向电场降落集中在内电极表面附近一定径向距离内，同轴电场下的闪络电压主要由内电极表面电场与径向电场分布的均匀程度决定。增加内电极直径和延长闪络距离都可以提高闪络电压，而增加内电极直径的效果更显著。     

Visible-light photocatalysis of ZnO deposited on nanoporous Au

ZnO deposited on nanoporous Au showed photocatalytic decomposition toward methyl orange under visible light, unlike ZnO sputtered on flat Au without a nanoporous structure. First-principles calculations suggested that the surface lattice disorder in nanoporous Au induced a band gap narrowing and a large built-in electric field in the adjacent ZnO, resulting in the visible-light photocatalytic response.     

Effect of quantum confinement and influence of extra charge on the electric field gradient in ZnO

By means of electron-nuclear double resonance (ENDOR), it is shown that the Al impurity, which acts as a shallow donor in ZnO, leads to a significant reduction of the electric field gradient in ZnO single crystals. In ZnO quantum dots, however, the gradient on the Al sites remains virtually unchanged. When the Zn²⁺ ion is substituted by Mn²⁺ in a ZnO single crystal, the electric field gradient slightly increases (by about 20%). Therefore, the Mn²⁺ ions can be used as probes to monitor the electric field gradients in ZnO crystals.     

两步化学沉积法制备ZnO薄膜及其场发射特性

在低温常压条件下,以ITO玻璃为衬底,采用电化学法与湿化学法结合的两步化学沉积法制备了团簇状ZnO薄膜。利用XRD,SEM分析了薄膜结构和表面形貌,并采用二极管结构在高真空条件下对薄膜进行了场发射性能测试。稳定发射后,开启电场为3.0 V/μm。当电场为5.8 V/μm时,电流密度为583.3 μA/cm²。研究表明:两步化学沉积法低温制备ZnO薄膜是可行的,且该薄膜具有良好的场发射性能。     

Nanoscale structural investigation of Zn_(1-x)Mg_xO alloy films on polar and nonpolar ZnO substrates with different Mg contents

Zn_(1-x)Mg_xO alloy films are important deep ultraviolet photoelectric materials.In this work,we used plasma-assisted molecular beam epitaxy to prepare Zn_(1-x)Mg_xO films with different magnesium contents on polar(0001) and nonpolar(1010) ZnO substrates.The nanoscale structural features of the grown alloy films as well as the interfaces were investigated.It was observed that the cubic phases of the alloy films emerged when the Mg content reached 20% and 37% for the alloy films grown on the(0001) and(1010) ZnO substrates,respectively.High-resolution transmission electron microscopy images revealed cubic phases without visible hexagonal phases for the alloy films with more than 70% magnesium,and the cubic phases exhibited three-fold and two-fold rotations for the alloy films on the polar(0001) and nonpolar(1010) ZnO substrates,respectively.This work aims to provide references for monitoring the Zn_(1-x)Mg_xO film structure with respect to different substrate orientations.     

一维纳米氧化锌自驱动紫外探测器的构建与性能研究

本文通过化学气相沉积法制备了ZnO纳米材料, 利用扫描电镜、光致发光谱、X衍射光谱及拉曼光谱等方法对制备的材料进行了表征. 基于制备的单根ZnO线分别构建了三种不同结构的紫外探测器件: Ag-ZnO-Ag肖特基型、PEDOT:PSS/n-ZnO结型和p-Si/n-ZnO结型紫外探测器, 并对器件的性能进行了研究. 结果表明: 三种不同结构的器件都表现出良好的整流特性, 对紫外线均有明显的光响应; 在零偏压下, 都有明显的自驱动特性. 三种器件中, p-Si/n-ZnO型紫外探测器性能最为优异: 在零偏压下, 暗电流约在1.2×10^-3 nA, 光电流在5.4 nA左右, 光暗电流比为4.5×10³, 上升和下降时间分别为0.7 s和1 s. 通过三类器件性能比较, 表明无机p-Si更适合与ZnO构建pn结型自驱动紫外探测器.     

五边形截面的Ag纳米线局域表面等离子体共振模式

五边形截面的单晶Ag纳米线对ZnO量子点荧光具有增强的现象. 为解释这一现象, 利用时域有限差分法对五边形截面的Ag纳米线的局域表面等离子体共振模式进行了理论模拟. 结果表明, 五边形截面的Ag纳米线在紫外区域存在两个消光峰, 分别由Ag纳米线的横向偶极共振(340 nm)和四极共振(375 nm)引起; 这两个消光峰与ZnO量子点荧光增强峰相一致, 而且随着Ag纳米线的半径增大而红移; 消光峰对应的共振模式取决于Ag纳米线的截面形状; 根据Ag纳米线电场增强倍数与激发光波长变化关系曲线可知, 最大增强电场位于五边形截面的顶点处, 而边线处电场增强较小. 理论模拟的结果较好地解释了Ag纳米线/ZnO量子点体系的荧光增强现象, 也为Ag纳米线在提高半导体材料发光效率、生物探测等方面的应用提供有益的参考.     

外电场极化对纳米氧化锌拉曼活性及气敏性能的影响

采用沉淀法制备了纳米氧化锌粒子,着重对其进行了不同条件(电场强度、极化温度)下的外电场极化处理.以X射线衍射仪和拉曼光谱仪对产物的结构、拉曼位移等进行了表征.测试了氧化锌极化产物在乙醇、丙酮气体中的气敏性能,研究了外电场效应对纳米氧化锌拉曼光谱和气敏性能的影响机制.结果表明,纳米氧化锌样品在外电场中存在着极化电场强度和温度的阈值,当电场强度和温度分别大于9375 V·cm~(-1)和150℃时,纳米氧化锌试片出现明显的漏电现象,极化效应显著降低并消失.在电场强度和温度阈值范围内,外电场极化作用能够导致氧化锌437 cm~(-1)处的拉曼特征峰强度明显降低.随外电场强度和极化温度增加,纳米氧化锌元件在丙酮气体中的灵敏度逐渐升高,在乙醇气体中的灵敏度逐渐降低,即外电场极化可以有效调控纳米氧化锌的气敏选择性.     

Formation of aligned ZnO nanorods on self-grown ZnO template and its enhanced field emission characteristics

We report a novel method for producing aligned ZnO nanorods (ANR) on self-grown ZnO template in a single step process involving growth of ZnO by vapor transport, followed by quenching of growing ZnO flux in liquid nitrogen. In the present study Zn powder turns into ZnO sheet under oxygen flow at ∼900 °C and bottom surface of the sheet acts as template for the growth of ANR. It is revealed from XRD and EDAX analysis that the bottom of the sheet is Zn rich region and acts as self catalyst for the growth of ANR. The grown nanorods have length up to several tens of micrometers with diameters ranging from ∼100 to 150 nm. Microstructural analysis of ANR indicates the fractal like configuration. The field emission properties have been investigated for ANR with fractal geometry using the ANR on self-grown ZnO template as a cathode directly. The turn-on electric field required to draw current density of ∼1.0 μA/cm² has been found to be ∼0.98 V/μm. The field enhancement factor based on Fowler-Nordheim (F-N) plot was found to be ∼7815 for ANR. The fractal geometry of ANR has been shown to be advantageous for achieving improved field emission features. The present investigations of synthesis involving formation of ANR over self-grown ZnO template, together with fractal configuration of the as-synthesized ANR, are first of their type.     

Optical anisotropy of ZnO nanocrystals on sapphire by thermoreflectance spectroscopy

The band-edge excitonic transitions of vertically aligned and tilted ZnO nanorods on sapphire have been characterized using thermoreflectance (TR) measurements in the temperature range between 30 and 300 K. The TR spectra of the nanorods with largely {0001} planes show considerable difference in energy and line shape with respect to those of the other sample with largely side planes of {1010}. The TR result at each temperature clearly indicates the band-edge excitons (A, B, and C) measured from the largely {0001} planes of the rods are lower in energy with respect to those obtained from the other sample dominated by the side planes of {1010}. Optical anisotropy in the transition amplitudes of the TR spectra for the vertically aligned and tilted ZnO nanorods is observed. The TR is shown to be very sensitive to the detection of the ZnO nanorods' alignment.     

Investigation of the fill factor of dye-sensitized solar cell based on ZnO nanowire arrays

The fill factor of dye-sensitized solar cells based on the ZnO nanowire array is very low, which is usually ascribed to a rapid charge recombination. In this article, the influence on the fill factor of ZnO nanowire array cell is investigated and discussed by comparing dark current and decay rate of open circuit potential of the ZnO nanowire array cell with those of the ZnO nanoparticle cell, TiO₂ nanoparticle cell and TiO₂-coated ZnO nanowire array cell. The results demonstrate that the low fill factor of the ZnO nanowire array cell is largely caused by a rapid decrease of electron injection efficiency rather than a rapid charge recombination, which is decided by the absorption nature of Ru-complexed dye molecules on ZnO surface and repellency of radial electric field. The fill factor of the ZnO nanowire array cell can be improved by coating ZnO nanowires with a wide band gap semiconductor material or metal oxide insulator.     

Confined Franz–Keldysh effect in ZnO quantum dots

Within the framework of the effective mass approximation, the confined Franz–Keldysh effect is investigated theoretically in a cylindrical ZnO quantum dot (QD). Numerical results show that the application of an electric field can decrease the strength and the threshold energy of the optical absorption coefficient in ZnO QD. There are additional oscillations in the absorption above the effective band gap, which are due to the Franz–Keldysh effect which occurs in the presence of the electric field. Our results also show that the electric field has a more obviously influence on the optical absorption in cylindrical ZnO QD with larger dot height.