V含量对ZnO薄膜结构及光学特性的影响

0引言氧化锌是一种六角纤锌矿结构的直接宽带隙半导体,其室温下禁带宽度为3.37eV。它具有多种优良的物理性能,在声表面波[1,2]、透明电极[3,4]、光电材料[5]、蓝光器件[6]等方面都有较大的应用潜力。氧化锌价格低廉,不仅能制成良好的半导体和压电薄膜,亦能够制成良好的透明导电薄膜。理论计算表明[7],氧化锌掺杂V、Cr、Fe、Co、N i元素能够产生自旋极化,形成高于室温的稀磁性透明半导体,是下一代微电子和光电子领域自旋电子学器件有重要价值的材料之一。根据理论计算,V掺杂的ZnO膜具有最高的居里温度。V yatkin实验小组[8]用钒离子注入…     

First-principles studies of vanadia-titania catalysts: beyond the monolayer

Periodic density functional calculations have been used to investigate the structure and stability of epitaxial vanadium oxide films grown on the TiO(2)(001) anatase surface. The formation energy of films of V(2)O(5) stoichiometry, initially low, is found to rapidly increase with the film thickness, at variance to what is obtained for reduced pseudomorphic VO(2) films. This is in tune with results of oxygen-assisted molecular beam epitaxy. The oxidation of thick, viz. >2 monolayers (ML), VO(2) films yields a c(2 x 2) reconstructed surface, in agreement with low energy electron diffraction. These films are composed by partially reduced inner V atoms in a distorted-octahedral environment, and by isolated surface dioxovanadium centers exhibiting a distorted trigonal-bipyramidal coordination. Single scattering simulations of X-ray photoelectron diffraction patterns have also been performed, taking both 2- and 3-ML surface surface-oxidized films as models. Results are in fair agreement with experiments referring to films grown in oxidizing conditions, which suggests that coherent vanadia ultrathin films could be formed in vanadia-titania catalysts. The electronic structure of the films has been finally studied, finding that the terminal oxygens carried by the surface dioxovanadium species have a strong nucleophilic character, which makes them potential active centers for selective oxidation catalysis.     

Amperometric titrations of cobalt(II) with hexacyanoferrate(III) in ammonium citrate and glycine media

An amperometric titration of cobalt(II) with hexacyanoferrate(III) in aqueous ammonium citrate or aqueous glycine solution at pH 9.8 or pH 8.0 respectively, is reported. Cobalt concentrations of 2-30 mg/l were successfully determined. In citrate solutions cerium(III) and iron(III) interfered, and in glycine solutions, copper(II) and vanadium(V).     

A new oxidimetric reagent: potassium dichromate in a strong phosphoric acid medium--VII. Photometric titration of vanadium(IV) and of cerium(III) alone and in mixtures with iroN(II)

Vanadium(IV) can be accurately titrated with potassium dichromate in media containing phosphoric acid of 3-12M concentration: the change in absorption of vanadium(IV) is followed in the region 660 mmicro using a red filter. It is more convenient to carry out the titration in 3M phosphoric acid because at higher concentrations chloride, nitrate, cerium(III) and manganese(II) may interfere. Photoelcetric titration is more convenient than potentiometric because the former can be made in a 3M phosphoric acid medium, whereas the latter is possible only in 12M phosphoric acid. The simultaneous differential photometric titration of iron(II) and vanadium(IV) is also possible. Conditions have been found for the photometric titration of cerium(III) and of cerium(III) plus iron(II). The titration is carried out (at 450 mmicro or with a blue filter) in about 10.5M phosphoric acid. Application of the method to a cerium mineral is considered.     

A Probe of Valence and Conduction Band Electronic Structure of Lead Oxide Films for Photodetectors

We investigate how the electronic structure of amorphous lead oxide (a-PbO) films deposited on ITO substrate is changed after annealing at various temperatures. Both experimental soft X-ray spectroscopic and density functional theory (DFT) based computational techniques are used to explore the electronic structure of this material. X-ray emission, resonant X-ray inelastic scattering, and X-ray absorption spectroscopic techniques are employed to directly probe the valence and conduction bands. We discover that the films are very stable and remain amorphous when exposed to temperatures below 300 °C. An amorphous-to-polycrystalline (α-PbO phase) transformation occurs during annealing at 400 °C. At 500 °C, an alpha to beta phase change is observed. These structural modifications are accompanied by the band gap value changing from 1.4±0.2 eV to 2.0±0.2 eV upon annealing at 400 °C and to 2.6±0.2 eV upon annealing at 500 °C. A difference between surface and bulk structural properties is found for all samples annealed at 500 °C and above; these samples also exhibit an unexpected suppression of O : 2p density of states (DOS) near the bottom of the conduction band, whereas additional electronic states appear well within the valence band. This study provides a significant step forward to understanding the electronic properties of two polymorphic forms of PbO needed for optimization of this material for use in X-ray sensors.     

Dosage spectrophotometrique de differents metaux (Mn,Ce, V,Ni, Fe) au moyen de la formaldoxime

Formaldoxime forms highly coloured complexes (? = 4000–18000) with manganese, cerium, vanadium, nickel and iron in alkaline media. On the basis of a study of reaction mechanisms and of the optimum conditions for the formation of the different complexes, new spectrophotometric methods are described for determination of cerium and iron, as well as improved methods for determination of manganese, nickel and vanadium. The use of formaldoxime for Mn, Ce and V is particularly beneficial in trace analysis and the methods suggested are specific.     

Vanadium oxide surface studies

The vanadium oxides can exist in a range of single and mixed valencies with a large variety of structures. The large diversity of physical and chemical properties that they can thus possess make them technologically important and a rich ground for basic research. Here we assess the present status of the microscopic understanding of the physico-chemical properties of vanadium oxide surfaces. The discussion is restricted to atomically well-defined systems as probed by surface techniques. Following a brief review of the properties of the bulk oxides the electronic and geometric structure of their clean single crystal surfaces and adsorption studies, probing their chemical reactivity, are considered. The review then focuses on the growth and the surface properties of vanadium oxide thin films. This is partitioned into films grown on oxide substrates and those on metal substrates. The interest in the former derives from their importance as supported metal oxide catalysts and the need to understand the two-dimensional overlayer of the so-called “monolayer” catalyst. On the single crystal metal substrates thin oxide layers with high structural order and interesting properties can be prepared. Particular attention is given to ultrathin vanadium oxide layers, so-called nano-layers, where novel phases, stabilised by the substrate, form.     

Titrimetric determination of vanadium(IV) with cerium (IV)sulphate at room temperature using ferroin as indicator

A procedure has been developed for the visual titrimetric determination of vanadium(IV) with cerium(IV) sulphate, using ferroin is redox indicator. The method has been extended to the determination of iron(II) and vanadium(IV) in mixtures.     

Study of the first nucleation steps of thin films by XPS inelastic peak shape analysis

The initial steps in the formation of thin films have been investigated by analysis of the peak shape (both inelastic background and elastic contributions) of X‐ray photoelectron spectra. Surface coverage and averaged height of the deposited particles have been estimated for several overlayers (nanometre range) after successive deposition cycles. This study has permitted the assessment of the type of nucleation and growth mechanisms of the films. The experiments have been carried out in situ in the preparation chamber of an XPS spectrometer. To check the performance of the method, several materials (i.e. cerium oxide, vanadium oxide and cadmium sulfide) have been deposited on different substrates using a variety of preparation procedures (i.e. thermal evaporation, ion beam assisted deposition and plasma enhanced chemical vapour deposition). It is shown that the first deposited nuclei of the films are usually formed by three‐dimensional particles whose heights and degree of surface coverage depend on the chemical characteristics of the growing thin film and substrate materials, as well as the deposition procedure. It is concluded that XPS peak shape analysis can be satisfactorily used as a general method to characterize morphologically the first nanometric moieties that nucleate a thin film. Copyright © 2006 John Wiley & Sons, Ltd.     

Sequential titration of iron(II) and vanadium(IV) mixtures with cerium(IV) sulphate, with ferroins as indicators

The titration of vanadium(IV) with cerium(IV) sulphate, with nitroferroin as indicator, is proposed. Unlike ferroin, the indicator does not need a catalyst in this system. By suitable choice of experimental conditions iron(II) can be titrated first to a ferroin end-point and then vanadium(IV) to a nitroferroin end-point.     

A New Class of Homogeneous Visible‐Light Photocatalysts: Molecular Cerium Vanadium Oxide Clusters

The first systematic access to molecular cerium vanadium oxides is presented. A family of structurally related, di‐cerium‐functionalized vanadium oxide clusters and their use as visible‐light‐driven photooxidation catalysts is reported. Comparative analyses show that photocatalytic activity is controlled by the cluster architecture. Increased photoreactivity of the cerium vanadium oxides in the visible range compared with nonfunctionalized vanadates is observed. Based on the recent discovery of the first molecular cerium vanadate cluster, (nBu₄N)₂[(Ce(dmso)₃)₂V₁₂O₃₃Cl] ? 2 DMSO ( 1 ), two new di‐cerium‐containing vanadium oxide clusters [(Ce(dmso)₄)₂V₁₁O₃₀Cl] ? DMSO ( 2 ) and [(Ce(nmp)₄)₂V₁₂O₃₂Cl] ? NMP ? Me₂CO ( 3 ; NMP=N‐methyl‐2‐pyrrolidone) were obtained by using a novel fragmentation and reassembly route. Pentagonal building units {(V)M₅} (M=V, Ce) reminiscent of “Müller‐type” pentagons are observed in 2 and 3 . Compounds 1 – 3 feature high visible‐light photooxidative activity, and quantum efficiencies >10 % for indigo photooxidation are observed. Photocatalytic performance increases in the order 1 < 3 < 2 . Mechanistic studies show that the irradiation wavelength and the presence of oxygen strongly affect photoreactivity. Initial findings suggest that the photooxidation mechanism proceeds by intermediate formation of hydroxyl radicals. The findings open new avenues for the bottom‐up design of sunlight‐driven photocatalysts.     

Titration of vanadium(IV) with cerium(IV) sulphate, with ferroin as indicator, in aqueous acetone as facile reaction medium An analytical and mechanistic study

A new procedure for the titration of vanadium(IV) with cerium(IV) sulphate, with ferroin as indicator, in aqueous acetone medium is reported. A differential determination of iron(II) and vanadium(IV) is also possible. A probable mechanism for the fast ferriin-vanadium(IV) reaction in acetone medium is given.     

Optical Switching in VO2 Thin Films

Vanadium dioxide thin films have been deposited from vanadium alkoxides VO(OR)3. An amorphous film is formed that transforms into crystalline VO2 upon heating at 500°C under a reducing atmosphere. Optically transparent VO2 thin films are then obtained that exhibit both electrical and optical switching around 70°C. The switching temperature together with the shape of the hysteresis loop can be modified by doping VO2 films with foreign cations. Doped MxVO2 (M = W6+, Nb5+, Ti4+, Cr3+ or Al3+) thin films have been prepared under the same conditions by mixing the vanadium alkoxide and a metal salt in an alcoholic solution. The switching temperature decreases when the film is doped with high-valent cations (W6+) and increases with low-valent cations (Al3+, Cr3+). The transition temperature first decreases and then increases when TiIV is added to the VO2 film while the width of the hysteresis loop is significantly reduced.     

Rh-V alloy formation in Rh-VOx thin films after high-temperature reduction studied by electron microscopy

Rh nanoparticles (mean size 10 and 15 nm), prepared by epitaxial growth on NaCl surfaces, were covered with layers of crystalline vanadium oxide (mean thickness 1.5 and 25 nm) by reactive deposition in 10(-2) mbar O2. The 1.5 nm film was further stabilized with a coating layer of 25 nm amorphous alumina. The so-obtained Rh/vanadia films, containing vanadium in the V3+ and V2+ state, were treated in 1 bar O2 at 673 K for 1 h and thereafter reduced in 1 bar H2 at increased temperatures, particularly between 723 and 873 K. The structural and morphological changes were followed by (high-resolution) transmission electron microscopy and selected area diffraction. Oxidation at 673 K transforms the purely vanadia-supported samples into Rh/V2O5, while in the alumina-supported films containing only small amounts of VOx, the formation of topotactic V2O3 is observed. The formation of Rh-V alloys during the subsequent reduction is strongly determined by the intimate contact and the structural and orientational relationship between Rh particles and the surrounding VOx phase. Reduction above 473 K transforms the support into substoichiometric vanadium oxides of composition VO and V2O. Analysis of high-resolution images and diffraction patterns reveals the presence of different alloy phases after reduction with increasing T (from 573 up to 823 K). In the alumina-supported film (low V/Rh ratio) the epitaxial alignment between the Rh particles and the surrounding V2O3 phase apparently favours the primary formation of defined alloys of type V3Rh and VRh3, followed by VRh at higher temperature. On the contrary, mainly V3Rh5 is formed in the purely VOx-supported Rh/films, due to different epitaxial relations in the initial state. Possible pathways of alloy formation are discussed.     

纳米二氧化钒薄膜的制备及红外光学性能

采用双离子束溅射方法在Si3N4/SiO2/Si基底表面沉积氧化钒薄膜, 在氮气气氛下热处理获得二氧化钒薄膜. 利用X射线衍射(XRD)、扫描电子显微镜(SEM)和X射线光电子能谱(XPS)研究了热处理温度对氧化钒薄膜晶体结构、表面形貌和组分的影响, 利用傅里叶变换红外光谱(FT-IR)对二氧化钒薄膜的红外透射性能进行了测试分析. 结果表明, 所制备的氧化钒薄膜以非晶态V2O5和四方金红石结构VO2为主, 经400 ℃、2 h热处理后获得了(011)择优取向的单斜金红石结构纳米VO2薄膜, 提高热处理温度至450 ℃, 纳米结构VO2薄膜的晶粒尺寸减小. FT-IR结果显示,纳米VO2薄膜透射率对比因子超过0.99, 高温关闭状态下透射率接近0. 小晶粒尺寸纳米VO2薄膜更适合在热光开关器件领域应用.     

Titration of vanadium(IV) with cerium(IV) sulphate, with ferroin as indicator, in aqueous alcohol as a facile reaction medium

A new procedure for the titration of vanadium(IV) with cerium(IV) sulphate, with ferroin as indicator, in aqueous alcohol medium, is reported. The visual titration gives accurate results but potentiometric titration fails in this medium; this failure is attributed to the sluggish indicator electrode behaviour. Experimental conditions for preliminary quantitative photochemical reduction of vanadium(V) with alcohol have been established.     

In situ investigations on organic foam films using neutron and synchrotron radiation

We report on small-angle neutron scattering (SANS) and X-ray scattering (SAXS) investigations of foam films stabilized by sodium dodecyl sulfate. Previous measurements on dry foams (Axelos, M. A. V.; Boue, B. Langmuir 2003, 19, 6598) have shown the presence of spikes in the two-dimensional scattering data which suggest that the incident beam is reflected on some film surfaces. The latter interpretation is confirmed by new neutron studies performed on ordered ("bamboo") foams which allow selection of single films. In the first case, we show that the spikes of the scattered intensity can be obtained by reflection on two parts of the foam, namely, the films and the Plateau borders. With synchrotron radiation, first observations of distinct interference fringes have allowed an accurate determination of the film thickness. A comparison with X-ray and neutron data is made, opening a general discussion about the capabilities of small-angle scattering techniques for studying the microscopic properties of foam films.     

Determination of vanadium in refractory metals, steel, cast iron, alloys and silicates by extraction of an NBPHA complex from a sulphuric-hydrofluoric acid medium

A method for determining up to 0.15% of vanadium in high-purity niobium and tantalum metals, cast iron, steel, non-ferrous alloys and silicates is described. The proposed method is based on the extraction of a red vanadium(V)-N-benzoyl-N-phenylhydroxylamine complex into chloroform from a sulphuric-hydrofluoric acid medium containing excess of ammonium persulphate as oxidant. The molar absorptivity of the complex is 428 l.mole(-1).mm(-5) at 475 nm, the wavelength of maximum absorption. Interference from chromium(VI) and cerium(IV) is eliminated by reduction with iron(II). Common ions, including large amounts of titanium, zirconium, molybdenum and tungsten, do not interfere.     

Precipitation-redispersion of cerium oxide nanoparticles with poly(acrylic acid): toward stable dispersions

We exploit a precipitation-redispersion mechanism for complexation of short chain polyelectrolytes with cerium oxide nanoparticles to extend their stability ranges. As synthesized, cerium oxide sols at pH 1.4 consist of monodisperse cationic nanocrystalline particles having a hydrodynamic diameter of 10 nm and a molecular weight of 400 000 g mol(-1). We show that short chain uncharged poly(acrylic acid) at low pH when added to a cerium oxide sols leads to macroscopic precipitation. As the pH is increased, the solution spontaneously redisperses into a clear solution of single particles with an anionic poly(acrylic acid) corona. The structure and dynamics of cerium oxide nanosols and their hybrid polymer-inorganic complexes in solution are investigated by static and dynamic light scattering, X-ray scattering, and chemical analysis. Quantitative analysis of the redispersed sol gives rise to an estimate of 40-50 polymer chains per particle for stable suspension. This amount represents 20% of the mass of the polymer-nanoparticle complexes. This complexation adds utility to the otherwise unstable cerium oxide dispersions by extending the range of stability of the sols in terms of pH, ionic strength, and concentration.     

Vanadium oxides on aluminum oxide supports. 1. Surface termination and reducibility of vanadia films on alpha-Al2O3(0001)

Using density functional theory and statistical thermodynamics, we obtained the phase diagram of thin VnOm films of varying thickness (approximately 2-6 A, 1-6 vanadium layers) supported on alpha-Al2O3(0001). Depending on the temperature, oxygen pressure, and vanadium concentration, films with different thickness and termination may form. In ultrahigh vacuum (UHV), at room temperature and for low vanadium concentrations, an ultrathin (1 x 1) O=V-terminated film is most stable. As more vanadium is supplied, the thickest possible films form. Their structures and terminations correspond to previous findings for the (0001) surface of bulk V2O3 [Kresse et al., Surf. Sci. 2004, 555, 118]. The presence of surface vanadyl (O=V) groups is a prevalent feature. They are stable up to at least 800 K in UHV. Vanadyl oxygen atoms induce a V(2p) core-level shift of about 2 eV on the surface V atoms. The reducibility of the supported films is characterized by the energy of oxygen defect formation. For the stable structures, the results vary between 4.11 and 3.59 eV per 1/2O2. In contrast, oxygen removal from the V2O5(001) surface is much easier (1.93 eV). This provides a possible explanation for the lower catalytic activity of vanadium oxides supported on alumina compared to that of crystalline vanadia particles.