水熱法合成氧化鋅奈米結構及其光電特性研究

Abstract

本研究以氯化鋅(ZnCl2)和六亞甲基四胺(HMTA)作為前驅物，進行水熱法合成氧化鋅奈米結構，探討溶液濃度、成長溫度與成長時間等參數對氧化鋅奈米結構所造成之影響，藉此可有效合成出氧化鋅奈米管、奈米線與準直性奈米柱陣列。並以多步驟水熱法成長，可以有效增加長寬比(aspect ratio)，一步驟成長之長寬比為22.5，五步驟成長之長寬比增加至57，每增加一次成長皆使長度成一定比例增加，並保持奈米柱之外觀完整。PL光譜則用來研究氧化鋅奈米結構之光學性質，可由紫外光波段訊號推算出氧化鋅奈米結構之能隙約為3.37 eV與氧化鋅塊材能隙相當；可見光波段之缺陷發光會隨著表面積貢獻增加而便增強，且隨著更換溶液次數增加的OH分子團，缺陷發光波鋒具紅移之現象，經波段分析可得知黃光(~580 nm)波段之缺陷發光與OH分子團相關。 其次，利用水熱法成長之氧化鋅奈米線進行單根奈米線電傳輸性質量測，奈米線線徑與電阻率呈現反比的關係，並且氧化鋅單根奈米線場效電晶體(FET)相較於薄膜電晶體有較好的電子遷移率。最後，更運用於單根奈米線與奈米柱陣列紫外光感測器，採以極小之偏壓進行量測(小於0.005 V)，可降低能源消耗並且得到極快速之反應時間(小於1 s)並改善持續光導效應(persistent photoconductive)，致使感測器的開關更為快速且明顯。

ZnO nanostructures were synthesized via hydrothermal method which was using ZnCl2 and HMTA mixed solution as the precursor. The parameters , such as solution concentration, growth temperature, and growth time, greatly influenced the morphology of nanostructures. The nanotubes(NTs), nanowires(NWs), and well-aligned nanorods(NRs) could effectively be fabricated by controlling the growth condition. By multiple-step growth, not only the length but the aspect ratio were increased with steps of growth, and the shape of NRs kept integrity. Furthermore, PL spectra which had near-bnad-emission(~3.37 eV) and defect-related emission showed the optical properties of ZnO nanostructures. The defected-related emission intensity was greatly enhanced as the more and more area of ZnO contributed. The amount of OH group, which increased as the steps of growth increased, was closely related to yellow emission (~580 nm) and caused the red shift phenomenon. In addition, the electrical transport property of single NW was investigated. It was found that the diameters of NWs had a inverse proportion with resistivity and the electron mobility of single NW FET was higher than thin film FET. Here we fabricated two types of UV photodetector including single NW device and well-aligned NRs device, both of which demonstrated a very small voltage bias (less than 0.005 V). With the lower energy consumption and the weaker persistent photoconductive (PPC) effect, our UV photodetector was found to exhibit short response time and high sensitivity.