摘要
在本論文中，我們把金濺鍍在基板上當催化劑，透過vapor-liquid-solid (VLS) 機制在1000℃左右長成氧化鋅的奈米結構(如:奈米線、奈米柱與奈米薄片)。再以掃描式電子顯微鏡(SEM)檢視其奈米結構的形貌。所用的基板材料有a面的藍寶石(sapphire)、(100)與(111)面的矽。實驗結果可發現，長在矽上的為方向雜亂的氧化鋅奈米線或奈米薄片；而長在sapphire上的為c方向或a方向的奈米線。有些樣品甚至可長成幾乎都是c方向的奈米線(即直立的奈米柱)。 我們以氦鎘雷射(325 nm)為激發光源的光激發螢光光譜(photoluminescence)儀測得氧化鋅奈米線與奈米薄片的發光有兩個波段。其一為紫外光波段，發光強度最大值約位於380 nm處，為激子(exiton)躍遷所發的光；其二為綠光波段，為晶體缺陷所造成的束縛態發光或稱之為深層發光。在室溫下，奈米薄片比其他的奈米結構擁有更好的發光特性，亦即它所發紫外光的強度較綠光強，(意味著缺陷較少)。 此外，我們發現氧化鋅奈米柱所發的螢光有偏極化現象，其紫外光波段與綠光波段的發光強度與角度呈平方餘弦函數的關係。其中，紫外光波段在c軸方向上(即奈米柱的成長方向)的發光強度最強；而綠光在此方向上發光強度最弱。亦即，兩者的平方餘弦函數在相位上差了90度。 由此可推論紫外光波段的偏振現象與奈米柱一維結構的非勻向性有關。而綠光的偏極化原因可推論為綠光產生之處大多位於晶體表面的缺陷。 此紫外光與綠光偏極化方向垂直的現象提供了一個簡單的方法來證實氧化鋅奈米線的缺陷發光大部分發生在晶體表面。
並列摘要
In this thesis, ZnO nanostructures (such as nanowires, nanorods and nanosheets) were grown at about 1000℃ by using gold as catalyst via Vapor-Liquid-Solid (VLS) mechanism. These ZnO nanostructures were examined by a field-emission scanning electron microscope (SEM). The substrates used were sapphire with the a plane and silicon with the (100) and (111) plane. The experimental result shows that none-oriented ZnO nanowires or nanosheets were grown on silicon substrates, and nanowires along the c axis or the three a axes of ZnO crystal were grown on the a plane sapphire substrates. Some samples even possess that most of the orientation of nanowires is along the c axis (i.e. standing nanorods). Photoluminescence (PL) spectra of ZnO nanowires and nanosheets were measured using a He-Cd laser (325 nm) as the excitation source. Two emission bands were observed. One is the ultraviolet (UV) band with peak position at about 380 nm caused by the transition of excitons, and the other is the green band commonly referred to as a deep-level or trap-state emission caused by the defects of the crystal. At room temperature, nanosheets shows better optical property than nanowires or nanorods, that is to say, nanosheets possess larger intensity of the UV band than that of the green band. In addition, we found that the PL emission of ZnO nanorods is polarized. The intensity of both the UV emission and the green emission varies with the polarization angle by the relation of square cosine function. The intensity of UV emission has its maximum along the c-axis of ZnO crystal. However, intensity of green emission has its minimum in this direction. Namely, two intensity curves are 90o out-of-phase. The polarization of the UV band is attributed to the anisotropy of the one dimensional structure of nanorods. However, the polarization of the green emission can deduce an interesting result that the green emission mostly occurs at the surface defects of the nanorods. The property that the UV emission and the green emission are polarized out of phase provides a simple way to verify that the defect emission occurs mostly at the surface defects of ZnO nanorods.