短硫鏈分子對殼核硒化鎘/硫化鋅量子點光學性質之研究

Abstract

本論文主要藉由由光譜及光學量測，討論含硫基的短硫鏈分子對殼核硒化鎘/硫化鋅量子點表面的影響和效用。由於量子點的表面對體積的比例極高，因此量子點的表面電子分佈結構會決定其物理性質及光學性質。此篇論文，我們使用一些硫醇分子，包括β-巰基乙醇 （BME） ，3 - 巰基丙酸（MPA）和1 - 丙硫醇（NPM）去測試分子和殼核量子點表面缺陷的交互作用。其中分成兩個研究的時間點 : 加入硫醇分子立即量測，以及加入分子後，過24小時再量測。我們主要只用超快吸收光譜、上轉換螢光光譜、溫度相關光致發光光譜和穩態螢光光譜來釐清硫醇分子對量子點的作用和機制。由實驗結果，可以知道硫醇分子 (thiol) 先由硫基孤對電子對 (sulfur lone-pair) 靠近量子點表面，以微弱的配位鍵形式 (coordination-type) 靠在量子點表面並減少表面缺陷，使量子點的表面鈍化 (surface passivation)，螢光強度增強 (PL enhancement) . 長時間後，硫醇分子會轉為硫醇基(thiolate)，分子改由較強的共價鍵形式 (covalent-type) 鍵結在量子點表面，但同時也產生新的缺陷，致使電洞由核內轉移出來，使螢光減弱 (PL quench)。此篇使用的硫醇分子對量子點有不同的影響，可能是抗氧化力、硫醇分子解離能力，或者是否有其餘孤對電子對作用等因素。最後，我們發現只要量子點表面附近有足夠的電子雲，就能使量子點表面鈍化。這是硫醇分子常被用來進行量子點配位體交換 (ligand exchange) 的主要原因。

The optical property analysis of short-chain thiol-containing molecules capping on CdSe/ZnS quantum dots (QDs) is reported. The electronic and optical properties of QDs are closely related with the highly surface-to-volume ratio and their surface electronic structure. Here we used core/shell QDs with some thiol molecules including β-Mercaptoethanol(BME), 3-Mercaptopropionic acid (MPA), and 1-propanethiol(NPM) to examine the effect of QD surface-thiol interaction of defect sites in short (immediately) and long (24 hr.) aging time. A comprehensive study of ultrafast spectroscopy, up conversion fluorescence, and temperature-dependent photoluminescence was used to clarify the function of thiols on QDs surface. We found the thiol molecules interact with QD only by weak coordination-type bonds through the sulfur lone-pair electrons. The thiol molecules can passivate the surface of QDs by preventing core electron from defect sites on the surface thus enhance PL intensity. While the strong covalent-type bonds are formed as thiol turn to thiolate through the long time incubation, new hole traps would be produced thus PL intensity quench. Thiol-containing molecules under investigation show different performance, which attribute to anti-oxidation, dissociation ability and second-order oxygen of the thiols. Finally, we found that the surface passivation occurs as long as the QDs are surrounded with negative charges since thiol-/-dithiol molecules have sulfur lone-pair electrons. That is the main reason why thiol-molecules usually apply to exchange ligands on the QDs surface.