摘要
钴氧化物是一种重要的过渡金属氧化物,常温常压下主要以岩盐型结构的氧化亚钴(CoO)和尖晶石型结构的四氧化三钴(Co_3O_4)两种形式存在。由于钴的3d电子壳层的部分填充特性,钴及钴的氧化物的表现出许多独特的性能,在催化、磁记录、高温超导、热敏、压敏、热吸收层、磁记录介质、电致变色薄膜等方面有重要的应用前景。不过,至今为止,有关钴氧化物的研究主要涉及它的电子输运特性、介电特性、超电容性能和电阻开关特性等方面,对于钴氧化物特别是薄膜的一些基本的物理特性如能带结构、禁带宽度和光电性能等方面的研究不多,而且数据比较分散。本文主要研究如何控制工艺参数获得单一相的CoO和Co_3O_4薄膜,在此基础上测量这两种钴氧化物薄膜的光电性能。
采用脉冲激光沉积(PLD)的方法,以金属钴为靶材,通过改变衬底温度、沉积时间、反应气体氧气的流量等工艺参数,先后在玻璃衬底上成功制备了纯的CoO薄膜以及Co_3O_4薄膜。通过X射线衍射(XRD)、X射线光电子能谱(XPS)、原子力显微镜(AFM)研究了沉积CoO和Co_3O_4薄膜的制备工艺。X射线衍射(XRD)测试表明CoO薄膜具有高度的(111)择优取向,400℃-500℃为CoO薄膜最佳的结晶温度。X射线衍射(XRD)、X射线光电子能谱(XPS)测试相互证明了钴氧化物薄膜随氧流量的增加发生了相变。当氧气流量增大到15sccm时,钴氧化物薄膜开始由CoO相转变为Co_3O_4。通过紫外-可见光谱(UV-Vis)测定CoO和Co_3O_4薄膜的透射光谱来确定透明薄膜的折射率和厚度,并可以通过透射率折算出吸收系数,来判定CoO和Co_3O_4薄膜带隙类型和光学禁带宽度。结果表明两种钴氧化物均为间接能带结构,且光学禁带宽度分别为0.82eV和1.21eV。同时进一步成功的在重掺硅片衬底上制备了CoO薄膜以及Co_3O_4薄膜,通过Ⅰ-Ⅴ的测量获得了Si/CoO/Cu结构的击穿电压为7.5V,Si/Co_3O_4/Cu结构的击穿电压为5.6V。
Among the transition metal oxides, cobalt oxide is one of the most studied oxides due to its importance for various scientific technologies. There are two known stable oxides of cobalt; CoO and Co_3O_4. both being cubic in structure, the latter having a wider range of stability. Of these, Co_3O_4 belongs to the normal spinel crystal structure of the type AB_2O_4. The monoxide CoO crystallizes in the rock salt structure. Cobalt oxide has many industrial applications, such as solar selective absorber, catalyst in the hydrocracking process of crude fuels, pigment for glasses and ceramics and catalyst for oxygen evolution and oxygen reduction reaction. It is also widely used as perpendicular magnetic recording medium, magneto-optical recording medium, sensors electrochemical anodes and newly invented application in supercapacitors. Cobalt oxides also show electrochromic properties and are potential materials for non-emitting displays, smart windows, and thermal control for space vehicles. They are attractive in solar cells as selective absorbers and corrosion protective coatings. In this paper, we report the details of acomparative study of cobalt oxide structural analysis, phase formation, surface morphology, and optical properties of thin films deposited by PLD using Co target in oxygen reactive atmosphere.
CoO and Co_3O_4 thin films have been deposited on glass substrates at different temperature and with various oxygen flow rate by reactive pulsed laser deposition techniques (PLD). Co (99.9%) was used as target in oxygen reactive atmosphere; Nd-YAG laser with wavelength of 1064 nm was used as laser source. High purity metallic Co disk was used as the target material, and high purity oxygen was used as the reactive gas. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV-visible spectrometry were applied to characterize the crystalline phases of the deposited films. Results show that when the oxygen flow rate is less than 15sccm, the deposited films is (111)-oriented rocksalt CoO, while films deposited at oxygen flow rate greater than 15sccm, the deposited films are spinel Co_3O_4. The indirect band structures of both the CoO and Co_3O_4 phases are proved from UV-Vis absorbance spectra, with bandgaps of 0.82eV for CoO and 1.21eV for Co_3O_4, respectively.
引文
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