单层和少层二硫化钼的制备及Co掺杂ZnO薄膜室温铁磁性的研究

发布时间：2018-09-15 06:43

【摘要】：本文的主要内容为单层和少层二硫化钼的制备以及Co掺杂ZnO薄膜室温铁磁性的研究。其中第一章讲述了二维材料的基本概念,二硫化钼的结构、性质、应用,以及稀磁半导体的简单介绍和目前稀磁半导体磁性机理的主要理论,并阐述了本文的选题目的与意义;第二章介绍了二硫化钼层数与晶粒大小的表征方法,以及常见的二硫化钼制备方法;第三章展示了1-4层二硫化钼的机械剥离法制备和单层二硫化钼薄膜的化学气相沉积法制备,并探索二硫化钥薄膜的生长过程以及影响二硫化钼薄膜生长的因素;第四章讲述了在不同退火条件下Co掺杂ZnO薄膜的磁性和形貌结构的变化,以及可能的磁性起源;第五章为总结与展望。第三章和第四章的具体内容如下:第三章:我们首先采用机械剥离法在三氧化二铝衬底上制备单层、双层、三层和四层二硫化钼,接下来我们又以硫和三氧化钼为原材料,通过化学气相沉积法在三氧化二铝衬底上制备得到了单层二硫化钼薄膜。我们发现温区中硫蒸汽的浓度对于单层二硫化钼薄膜的生长起决定性作用,硫蒸汽的浓度过高或过低时都不利于单层二硫化钼薄膜的生长。实验中可以通过调节硫源的位置和氩气的流量来控制温区中硫蒸汽的浓度。当硫源置于气流下游,氩气流量为50 sccm时,适合单层二硫化钼薄膜的生长,而且光学性质测试表明所制备的二硫化钼薄膜是大面积均匀连续的单层膜,其质量与机械剥离的单层二硫化钼的质量相当。我们还发现当硫源置于气流下游,氩气流量为20 sccm时,可以在SiO2/Si衬底上直接制备MoS2/MoO2微板块而不需要金属氧化物粉末的帮助。最后我通过分析温区中不同位置上的产物,指出硫与三氧化钼的反应是一分步反应,首先三氧化钼被硫还原成亚氧化物MoO3-x,然后亚氧化物MoO3-x再被硫化为二硫化钼,硫在整个反应过程中既是还原剂又是硫化剂。第四章:本章主要证明Co掺杂ZnO薄膜室温铁磁性的起源是与多种因素有关的。我们通过磁控溅射法在单晶硅衬底上沉积Zn0.95Co0.05O薄膜,并使薄膜在H2(5%)/N2(95%)的气氛中不同的温度下退火,我们发现退火后薄膜由顺磁性变为铁磁性,其饱和磁矩随着退火温度的升高而逐渐增加,在700℃达到最大值,并且当退火温度为620℃和650℃时,薄膜饱和磁矩的大小会突然增大一个量级。巧合的是,当退火温度为620℃,薄膜开始被刻蚀;而当退火温度为650℃,薄膜开始变为非晶态。我们认为,氧空位的产生是薄膜由顺磁性变为铁磁性的原因,而由于氢气刻蚀而增强的晶界效应和非晶态物质的产生分别导致了薄膜饱和磁矩的两次突然增大。随后,我们把氢气退火后的薄膜在氧气气氛中退火四次,发现薄膜重新结晶,除了生成纤锌矿结构ZnO的六方晶粒外,还有少量闪锌矿结构ZnO的立方晶粒。薄膜的饱和磁矩在经过第一次氧气退火处理后首先减小一个数量级,而在经过第二次氧气退火处理后又恢复到原来初始时的大小,并且基本上不随第三次和第四次氧气退火处理而变化。我们认为,氧空位浓度的减少,非晶态物质的消失,晶界效应的减弱,导致了薄膜的饱和磁矩在第一次氧气退火处理后减小,而铁磁性的Co掺杂闪锌矿结构的ZnO晶粒的生成是薄膜饱和磁矩在第二次氧气退火后恢复的原因。
[Abstract]:The main contents of this paper are the preparation of single-layer and few-layer molybdenum disulfide and the study of room temperature ferromagnetism of Co-doped ZnO thin films. In the first chapter, the basic concepts of two-dimensional materials, the structure, properties and applications of molybdenum disulfide, as well as the brief introduction of diluted magnetic semiconductors and the main theory of magnetic mechanism of diluted magnetic semiconductors are described. In the second chapter, the characterization methods of the number of layers and grain size of molybdenum disulfide and the common preparation methods of molybdenum disulfide are introduced; in the third chapter, the mechanical peeling method of 1-4 layers of molybdenum disulfide and the chemical vapor deposition method of single layer of molybdenum disulfide film are presented, and the growth process of key disulfide film is explored. Chapter 4 describes the changes of magnetic properties and morphological structure of Co-doped ZnO films under different annealing conditions, and the possible origin of magnetic properties. Chapter 5 is a summary and prospect. Chapter 3 and Chapter 4 are as follows: Chapter 3: We first adopt the mechanical peeling method in aluminum oxide lining. Monolayers, double layers, three layers and four layers of molybdenum disulfide were prepared on the substrates of aluminum oxide by chemical vapor deposition (CVD). The concentration of sulfur vapor in the temperature range can be controlled by adjusting the position of the sulfur source and the flow rate of argon. When the sulfur source is placed downstream of the gas stream and the flow rate of argon is 50 sccm, the sulfur vapor is suitable for the growth of monolayer molybdenum disulfide films and the optical properties can be tested. It is also found that MoS2/MoO2 microplates can be prepared directly on SiO2/Si substrates without the help of metal oxide powders when the sulfur source is placed downstream of the gas stream and the argon flow rate is 20 sccm. Finally, by analyzing the products at different locations in the temperature range, it is pointed out that the reaction of sulfur with molybdenum trioxide is a one-step reaction. First molybdenum trioxide is reduced to MoO3-x by sulfur, and then MoO3-x is sulfided to molybdenum disulfide. Sulfur is both a reducing agent and a vulcanizing agent in the whole reaction process. The origin of room temperature ferromagnetism of bright Co doped ZnO thin films is related to many factors. We deposited Zn0.95Co0.05O thin films on silicon substrates by magnetron sputtering and annealed the films at different temperatures in H2(5%)/N2(95%) atmosphere. We found that the films changed from paramagnetism to ferromagnetism after annealing, and the saturation magnetic moment changed with annealing temperature. The saturation magnetic moment of the films increases by an order of magnitude when the annealing temperature is 620 and 650. Coincidentally, when the annealing temperature is 620, the films begin to be etched, and when the annealing temperature is 650, the films begin to become amorphous. It is the reason why the films change from paramagnetism to ferromagnetism, and the grain boundary effect enhanced by hydrogen etching and the formation of amorphous materials lead to two sudden increases in the saturation magnetic moment of the films, respectively. The saturated magnetic moment of ZnO thin films decreases by an order of magnitude after the first oxygen annealing treatment, and then returns to the original size after the second oxygen annealing treatment, and basically does not change with the third and fourth oxygen annealing treatment. We believe that the decrease of oxygen vacancy concentration, the disappearance of amorphous substances and the decrease of grain boundary effect lead to the decrease of saturation magnetic moment of the films after the first oxygen annealing, while the formation of ZnO grains with ferromagnetic Co-doped sphalerite structure is the reason for the recovery of saturation magnetic moment of the films after the second oxygen annealing.
【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TQ136.12;TB383.2

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