本文關(guān)鍵詞：一維氧化鎢納米材料無催化劑生長的原位透射電鏡研究　出處：《浙江大學》2017年博士論文　論文類型：學位論文

  更多相關(guān)文章： 原位透射電鏡 氣-固生長 W_(18)O_(49) 納米線 納米管 生長機理

【摘要】：作為一維納米材料研究的基礎(chǔ),以納米線和納米管為首的一維納米材料的生長機理研究一直是研究熱點。氣相生長法因能簡單高效地制備幾乎所有無機物的一維納米結(jié)構(gòu)的優(yōu)點,受到眾多科研工作者的關(guān)注。近年來,研究者借助原位透射電子顯微鏡技術(shù)(TEM,transmission electron microscopy),在納米線和納米管催化生長的機理研究方面取得了顯著的進展,催化生長熱力學和動力學體系的建立日趨完善。與此相對應(yīng)是,納米線和納米管無催化劑生長的機理目前還很不明朗,主要原因是相關(guān)的原位電鏡研究還很缺乏。鑒于上述局面,本論文選擇半導體氧化鎢中廣受關(guān)注的W18O49一維納米結(jié)構(gòu)作為研究對象,在環(huán)境透射電鏡中采用熱氧化金屬鎢的方法成功地無催化生長出W18O49的納米線和納米管結(jié)構(gòu),結(jié)合原位電鏡觀察、晶體平衡理論和第一性原理計算來研究無催化劑納米線和納米管的生長機理,本論文的創(chuàng)新點如下:(1)采用熱氧化法在電鏡中成功實現(xiàn)了 W18O49納米線的無催化劑生長,并通過設(shè)計熱氧化鉬網(wǎng)負載鎢片的實驗直接證實了納米線的氣-固(VS,vapor-solid)生長機理;原位高分辨TEM觀察發(fā)現(xiàn)納米線的頂部生長是按照二維形核生長的方式實現(xiàn)的,即納米線通過頂部平臺的臺階形核與徑向流動生長的方式逐分子層生長,并且在每一分子層生長過程中伴隨著晶格原子周期性地從頂部邊緣擴散至新臺階邊處;基于BFDH(Bravais-Friedel-Donnay-Harker)原理的晶體形狀預測表明W18O49晶體在[010]方向明顯長于其他方向,第一性原理計算表明在適量O2氣氛下W18O49的(010)面是能量最高的面,在這個面上的二維形核勢壘最小,因而納米線會沿著[010]方向擇優(yōu)生長。(2)原位研究了 W18O49納米線的生長長度與時間的關(guān)系,發(fā)現(xiàn)納米線的生長分為兩個明顯不同的過程,初期的非線性快速生長和后期的線性緩慢生長;原位研究了各種條件包括O2壓強、溫度、側(cè)壁臺階和電子束流密度等的變化對納米線生長的實時影響;采用自相關(guān)分析來研究納米線相鄰兩分子層生長的形核過程的相關(guān)性問題,發(fā)現(xiàn)VS生長中的相鄰分子層形核相互獨立的置信度很高,這一結(jié)果與VLS(vapor-liquid-solid)中的不同,原因在于VS生長中的形核直接受氣源壓力影響,而VLS生長中的形核受催化劑液滴中的飽和度影響。(3)我們發(fā)現(xiàn)在電鏡中O2壓強較大的區(qū)域熱氧化鎢絲會出現(xiàn)W18O49管狀結(jié)構(gòu)生長,采用高分辨透射電鏡技術(shù)原位觀察納米線的頂部生長動態(tài)過程,發(fā)現(xiàn)這些納米管的生長是通過側(cè)壁臺階外延生長的方式來實現(xiàn)的;根據(jù)原位觀察與理論分析,我們提出納米管的側(cè)壁外延生長機理,即在高飽和度條件下納米線的側(cè)壁臺階生長很活躍,高溫下在側(cè)壁臺階生長過程中晶體為了熱耗散更快,會通過表面原子擴散進行形狀調(diào)整以形成管狀結(jié)構(gòu)。
[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.
【學位授予單位】：浙江大學
【學位級別】：博士
【學位授予年份】：2017
【分類號】：TB383.1;O643.36

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