一維氧化鋅奈米結構之製備及其應用

Abstract

一維奈米材料由於其卓越的物理與化學特性使其成為近幾年重要之研究領域，在許多一維材料中，氧化鋅由於獨特的光、電及壓電特性而具有廣泛的應用性，成為近年來受矚目的研究方向，於本論文中，我們加入新的製程方法或技術改善一維氧化鋅奈米結構之應用。實驗中，利用掃描式電子顯微鏡(SEM)和穿透式電子顯微鏡(TEM)研究材料之微結構，並使用發光光譜儀(PL)與電流-電壓量測系統分析其光性與電性。本論文依氧化鋅奈米結構之應用與實驗設計分成四部份。第一部分，我們所提出的一個簡單的方法製備氧化鋅奈米尖錐陣列，使奈米尖錐於25-100 ℃的環境中具備有低的啟動電場、高的場效增益因子與穩定的場發射特性。而優異的場發射特性主要來自於降低氧化鋅發射子本身的氧空缺濃度與小角度的尖錐角度，此簡單的製程方法展現了極大的潛力應用於場發射元件與發光元件上。第二部份，我們所提出的一個有趣的方法來製作氧化鋅-氧化錫核-殼奈米線結構應用於氣體感測器上，此氧化鋅-氧化錫核-殼奈米線具備高的氫氣感測特性，如在溫度250 ℃時通入200 ppm的氫氣其敏感度高達89%，而高敏感性主要來自於氧化錫膜對於氫氣的完全反應所造成，我們認為利用兩階段成長的方式所製備的氧化鋅-氧化錫核-殼奈米線具備在氣體感測使用的潛力。第三部份，成功的製備垂直佳且分布均勻的氧化鋅奈米柱於低成本、可撓的聚對苯二甲酸乙二酯(PET)基板上，該元件能夠重複且穩定操作於強度25-70 μW/cm2的紫外光的照射且具備有快的反應時間與回覆時間。此外，元件具備機械可撓性、高可靠度與不同層級的光反應，因而具備有極大的潛力應用於紫外光感測器上。第四部份，成功的製備緻密且均勻的的鎵摻雜氧化鋅奈米柱薄膜/金/鈦/二氧化矽/p-型矽基板的電阻式記憶體元件，此種記憶體元件具備可逆與穩定的操作於開/關狀態間且具備有超過一百次的穩定度特性，而元件的轉態機制主要跟鎵摻雜氧化鋅奈米柱與柱之間的氧空位細絲形成與斷裂有關。結果顯示，此一緻密的鎵摻雜氧化鋅奈米柱薄膜在電阻式記憶體的研究中具備有極高的應用價值。

One-dimensional nanostructers are a new class of advanced materials that have been receiving a lot of research interest in the last decade due to their superior physical and chemical properties. Among one-dimensional materials, zinc oxide (ZnO) is one of the most important materials and has attracted much interest in recent years, due to its unique optical, electrical, and piezoelectric properties and versatile applications. In this dissertation, we propose new methods or technologies to improve the applications of 1D ZnO nanostructure. Macrostructure of the samples was characterized by scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). The optical and electrical properties were investigated by photoluminescence (PL) and current–voltage (I–V) characterization. The main focus of this dissertation can be divided into four parts. In first part, we demonstrate a simple method to fabricate ZnO nanotip array, which exhibit low turn-on field, high field enhancement factor and stable field emission properties at 25-100 ℃. The good field emission properties are attributed to reduced oxygen vacancy concentration and small tip angle of ZnO emitters, which shows good potential for developing field emission and light emiting devices. In the second part, we provide another interesting route of fabricating ZnO-SnO2 core-shell nanowires for gas sensor applications. The ZnO-SnO2 core-shell nanowires exhibited good hydrogen sensor performance, such as the sensitivity is up to 89% against 200 ppm hydrogen at 250℃. Such high sensitivity was believed to be controlled by the nanoscale SnO2 layer, which was determined from pinch-off and fully conductive state. The ZnO-SnO2 core-shell nanostructures made by two-step chemical growth have high potential for gas sensor application. In the third part, vertical well-aligned and uniform ZnO nanorods were successfully prepared on low cost and flexible PET polymer substrate by aqueous solution method under various growth conditions. The photocurrents can be repeatly and reproducibly switched by modulating UV exposure with power densities of 25-70 μW/cm2. The fast response time (100 sec) and rapid recovery time (120 sec) are achieved in UV turn-on/off switching measurements. Owing to the mechanical flexibility, nondestructive properties, high reliability and multilevel photoresponse, the well-aligned ZnO nanorods grown on transparent and flexible PET polymer substrates have high potential for UV photodetector applications. In the fourth part, vertically well-aligned and uniform Ga¬-doped ZnO (GZO) nanorod thin films were successfully grown on Au/Ti/SiO2/p-Si substrates, which used to make resistive switching memory devices. Such memory devices can be reversibly switched between ON and OFF states, with a stable resistance ratio of 10 times, narrow dispersion of ON and OFF voltages, and good endurance performance of over 100 cycles. The resistive switching mechanism in this design is related to the formation and rupture of conducting filaments consisting of oxygen vacancies, occurred at interfaces between GZO nanorods (grain boundaries). Results show that the resulting compact GZO nanorod thin films have a high potential for resistive memory applications.