一维氧化钨纳米材料无催化剂生长的原位透射电镜研究
本文关键词:一维氧化钨纳米材料无催化剂生长的原位透射电镜研究 出处:《浙江大学》2017年博士论文 论文类型:学位论文
更多相关文章: 原位透射电镜 气-固生长 W_(18)O_(49) 纳米线 纳米管 生长机理
【摘要】:作为一维纳米材料研究的基础,以纳米线和纳米管为首的一维纳米材料的生长机理研究一直是研究热点。气相生长法因能简单高效地制备几乎所有无机物的一维纳米结构的优点,受到众多科研工作者的关注。近年来,研究者借助原位透射电子显微镜技术(TEM,transmission electron microscopy),在纳米线和纳米管催化生长的机理研究方面取得了显著的进展,催化生长热力学和动力学体系的建立日趋完善。与此相对应是,纳米线和纳米管无催化剂生长的机理目前还很不明朗,主要原因是相关的原位电镜研究还很缺乏。鉴于上述局面,本论文选择半导体氧化钨中广受关注的W18O49一维纳米结构作为研究对象,在环境透射电镜中采用热氧化金属钨的方法成功地无催化生长出W18O49的纳米线和纳米管结构,结合原位电镜观察、晶体平衡理论和第一性原理计算来研究无催化剂纳米线和纳米管的生长机理,本论文的创新点如下:(1)采用热氧化法在电镜中成功实现了 W18O49纳米线的无催化剂生长,并通过设计热氧化钼网负载钨片的实验直接证实了纳米线的气-固(VS,vapor-solid)生长机理;原位高分辨TEM观察发现纳米线的顶部生长是按照二维形核生长的方式实现的,即纳米线通过顶部平台的台阶形核与径向流动生长的方式逐分子层生长,并且在每一分子层生长过程中伴随着晶格原子周期性地从顶部边缘扩散至新台阶边处;基于BFDH(Bravais-Friedel-Donnay-Harker)原理的晶体形状预测表明W18O49晶体在[010]方向明显长于其他方向,第一性原理计算表明在适量O2气氛下W18O49的(010)面是能量最高的面,在这个面上的二维形核势垒最小,因而纳米线会沿着[010]方向择优生长。(2)原位研究了 W18O49纳米线的生长长度与时间的关系,发现纳米线的生长分为两个明显不同的过程,初期的非线性快速生长和后期的线性缓慢生长;原位研究了各种条件包括O2压强、温度、侧壁台阶和电子束流密度等的变化对纳米线生长的实时影响;采用自相关分析来研究纳米线相邻两分子层生长的形核过程的相关性问题,发现VS生长中的相邻分子层形核相互独立的置信度很高,这一结果与VLS(vapor-liquid-solid)中的不同,原因在于VS生长中的形核直接受气源压力影响,而VLS生长中的形核受催化剂液滴中的饱和度影响。(3)我们发现在电镜中O2压强较大的区域热氧化钨丝会出现W18O49管状结构生长,采用高分辨透射电镜技术原位观察纳米线的顶部生长动态过程,发现这些纳米管的生长是通过侧壁台阶外延生长的方式来实现的;根据原位观察与理论分析,我们提出纳米管的侧壁外延生长机理,即在高饱和度条件下纳米线的侧壁台阶生长很活跃,高温下在侧壁台阶生长过程中晶体为了热耗散更快,会通过表面原子扩散进行形状调整以形成管状结构。
[Abstract]:As the basis of one-dimensional nanomaterials, the research on the growth mechanism of one-dimensional nanomaterials, headed by nanowires and nanotubes, has always been the focus of research. Gas phase growth method to prepare almost all one-dimensional nanostructures of inorganic material has the advantages of simple and efficient system, by many scientific researchers. In recent years, with the help of in situ transmission electron microscopy (TEM, transmission electron microscopy), researchers have made significant progress in the research of nanowire and nanotube catalytic growth mechanism, and the establishment of catalytic growth thermodynamics and kinetics system is becoming more and more perfect. In contrast, the mechanism of nanowires and nanotubes without catalyst growth is still unclear, mainly due to the lack of related in situ electron microscopy. In view of the above situation, this thesis chooses the popular W18O49 one-dimensional nano tungsten oxide semiconductor structure as the research object, in the environment of the transmission electron microscope by using the method of thermal oxidation of metallic tungsten successfully without the catalytic growth of W18O49 nanowires and nanotubes, calculation of growth mechanism of nanowires and nanotubes without catalyst combined with in situ electrical microscopic observation, crystal equilibrium theory and first principle, the innovation of this paper are as follows: (1) by the thermal oxidation method in the electron microscope in the successful implementation of the catalyst free growth of W18O49 nanowires, and the design of thermal oxidation of molybdenum tungsten film load experiments demonstrated nanowires gas-solid (VS, vapor-solid) growth mechanism; in situ high resolution TEM observed at the top of the nanowire growth by two-dimensional nucleation and growth way, namely nanowires by step nucleation and radial flow of the top platform The growth by molecular layer growth, and in every layer growth process with the lattice atoms periodically from the top edge of the diffusion to new step edge; based on BFDH (Bravais-Friedel-Donnay-Harker) crystal shape principle prediction show that W18O49 crystal in the [010] direction was significantly longer than that in the other direction, the results show that in the amount of O2 atmosphere W18O49 the first principle (010) is the highest energy surface, in the surface of the two-dimensional nucleation barrier minimum, thus nanowires along the preferential growth direction of [010]. (2) the relationship between the growth and the length of time in situ study of W18O49 nanowires, found the nanowire growth is divided into two distinct processes, linear initial nonlinear fast growth and slow growth period; in situ study of a variety of conditions including real-time effects of O2 pressure and temperature, the side wall of the steps and the electron beam the density on the growth of nanowires; the correlation between autocorrelation analysis of adjacent nanowires two molecular nucleation layer growth, VS growth in the adjacent molecular nucleation layer independent of the degree of confidence is very high, this result with VLS (vapor-liquid-solid) in different, because the growth of VS the nucleation is directly influenced by air pressure effect, while VLS growth in the nucleation saturation effect of catalyst in a liquid droplet. (3) we found in the electron microscope in regional thermal oxidation of tungsten O2 pressure will appear larger growth of W18O49 tubular structure, dynamic growth process by the top line high resolution transmission electron microscopy observation of nano technology in situ, found that the growth of these nanotubes is grown by epitaxial sidewall step approach to achieve; according to the analysis of in situ observation and theory. We propose the extension of side wall nanotube growth mechanism, namely the sidewall step in the nanowire growth under the condition of high saturation is very active, high temperature in the side wall in the process of crystal growth step to heat dissipation faster, will adjust the shape to form a tubular structure by surface diffusion.
【学位授予单位】:浙江大学
【学位级别】:博士
【学位授予年份】:2017
【分类号】:TB383.1;O643.36
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