一維奈米結構之二氧化錫的光導特性

Abstract

我們主要研究了二氧化錫一維奈米線 (nanowire) 的光導特性。二氧化錫(SnO2) 在奈米尺寸下擁有較高的表面積對體積比 (surface-to-volume ratio)，大幅增加了對光的靈敏度及吸收率。然而表面複雜的能態卻會對光導造成巨大的影響。 我們將會報告由氣液固相製程所生成的SnO2奈米線在紫外光波段擁有高效率的光轉電的能力。而以往所討論主宰SnO2光導機制都只著重在表面氧分子吸附的電洞攫取，而忽略了因為能帶彎曲導致的電子電洞對分離所貢獻的光電流。而在真空時，由於氧分子吸附能力的降低，大大增長了電子電洞對存活的時間，進而大幅增加了光導率。 我們也參考了利用金屬粒子修飾增加SnO2奈米線偵測活躍氣體靈敏度的模型來增加光導率。利用金屬粒子在SnO2奈米線上形成蕭基屏障 (Schottky barrier)增加能帶彎曲導致電子電洞對分離的效果更為顯著，有效增長電子電洞的生命期(lifetime) 進而增加SnO2奈米線的光導率。

In this thesis, the photocurrent of one-dimensional tin dioxide nanowires has been carefully measured. The mechanism responsible for photocurrent is explicated here and an alternative route to enhance photocurrent is investigated, too. They are presented as follows. I. High photocurrent gain in SnO2 nanowires Unlike previous reports, the responsible mechanism of photocurrent in metal oxide nanostructures, always emphasize on the carrier trapping of charged oxygen molecules. Here we point out that space charges induced by surface defects does play a significant role, especially, for nanostructured materials with a large surface to volume ratio. The huge photoresponse gain with a value of about one hundred thousand detected in vacuum represents an outstanding example to illustrate out proposed mechanism. The study shown here provides a useful guideline to achieve photodetectors based on nanostructured materials with high sensitivity. II. Photocurrent enhancement of SnO2 nanowires through Au-nanoparticles decoration It is found the sensitivity of photoresponse of SnO2 nanowires can be enhanced by metallic decoration. The underlying mechanism is attributed to the formation of the Schottky junction on the nanowires surface in the vicinity of metallic nanoparticles. The increment in the barrier height and width of space charge region due to the existence of Schottky junctions increases the surface electric field and enhances the spatial separation effect, which then prolongs the lifetime of photoinduced electron and consequently increases the gain. The results shown here provide an alternative way for enhancing the photoresponse of semiconductor nanostructures, which should be useful for creating highly sensitive photodetectors.