二氧化錫的光電性質研究 – 光致發光及電致發光

Abstract

為尋找環境友善、成份簡單單一且低成本之白光螢光材料，我們選擇錫之最穩定氧化物—二氧化錫為研究主題。二氧化錫為一寬能隙材料。過去文獻中所報告其能帶在3.6~4 eV之間，因此二氧化錫被視為一有效之光致發光系統。錫有二價與四價之離子，具有價數轉換的空間；錫及其氧化物具有豐富之物理化學性質，產生在光電及半導體上許多可變化的空間。更重要的是錫與二氧化錫無毒而安全。因此我們選擇二氧化錫為研究對象。 我們分別以直接氧化法，及未曾報導過的高壓水熱法，歧化反應法，重複高溫水合法製備各種型態與缺陷各異之樣品，以Ｘ光測定其結構為二氧化錫，以SEM觀察其形態與結構，而當這些樣品以波長325奈米之紫外光激發時，產生橘光、黃光及高強度白光等一系列之光致發光現象。另外，以直流及交流電壓驅動時，可觀察到電致發光現象。 在其發光機制的方面，過去文獻多僅推測其為氧缺陷所形成之表面發光中心，我們首次提出了低配位數錫原子—氧缺陷模型，並以時間相關密度泛函進行理論來進行計算，並與樣品之光譜進行比對，找到三種能與光譜相符特定的結構。 研究結果直接指出成份簡單、無毒且經濟之二氧化錫可作為紫外線激發之螢光材料；特別是白光，以二氧化錫為單一材料所發之白光尚為市場之所未見。此外研究結果並指出在電致發光方面二氧化錫為一具潛力之材料。

The study is aiming to produce white luminescence from economical, environment friendly material with simple methods. SnO2 as a transparent conducting oxide (TCO) has its reported band-gap value between 3.6 ~ 4 eV, therefore it is expected to be an effective photoluminescence system. Meanwhile, the two oxidation states of Sn — Sn(IV) and Sn(II) have enabled the possibilities of transitions between them. The abundant physical and chemical properties of Sn and its oxide have opened a wide space in semiconductor and optoelectronics. Furthermore, Sn and SnO2 are non-toxic. These are the reasons that SnO2 is chosen as the object of study. SnO2 of different properties are prepared by direct firing, high-pressure water treatment, dismutation reaction and repeated water-assisted annealing. The samples differ in morphology and defects. X-ray measurements have settled the structure as cassiterite (tetragonal SnO2). Microstructures are presented under SEM observation. As the samples are excited by 325nm UV, series of photoluminescence of orange, yellow and high-intensity white are observed. On the other hand, electroluminescence is also observed while driven by DC and AC electric field. The mechanism of photoluminescence of SnO2 in literatures was simply attributed to the oxygen vacancies located at the surface. We have first proposed a model of low-coordinated Sn with oxygen vacancies. By inspecting the result of TD-DFT calculations, 3 specific structures are found matching the spectrum. In conclusion, it is indicated in the study that simple-formulated, non-toxic and low cost material – SnO2 could serve as the phosphor material under UV excitation. Especially, the white photoluminescence produced by single-component SnO2 is not yet seen in the commercial market of phosphor. It is also indicated that SnO2 is a potential material in electroluminescence applications.