單一膠體半導體及金屬奈米結構的光學性質

Abstract

在這個論文中，我們利用單分子檢測技術來研究奈米材料的螢光性質，包括膠體的硒化鎘相關的量子點及金的奈米團簇。對於硒化鎘╱硫化鋅量子點而言，在表面加上HDA分子後，其螢光閃爍的行為會有明顯的改變，亮暗期的比例及亮期內所有輻射出的光子數分別增加到2.8和13倍。此外螢光衰減呈現出接近單一指數衰減的曲線並且有著較長的生命期。這些實驗結果表示HDA提供了更高程度的表面鈍化。另外，我們也研究了在不同高壓下硒化鎘╱硫化鋅量子點的振動與電子能態。在PL及Raman的光譜中，壓力引起的晶格常數呈現二次方的變化可以明顯的觀察到直到約7Gpa。這樣二次方的關係和理論預期是相符合的。 對於碲硒化鎘╱硫化鋅量子點而言，我們利用群體及單一量子點的螢光量測來研究光引起的螢光增強現象。群體量子點在空氣和真空中都可以發現螢光增強的現象，但有著不同的增加因子。從單一量子點來看，在光照之後有一些單一量子點螢光會增強。對於這些光照螢光增加的單一量子點而言，在其螢光閃爍行為中發現有著較長的亮期及亮期中有著較高的量子效率，且其亮期的螢光強度呈現著不同強度的分佈。根據群體與單一量子點的螢光量測，我們假設螢光增強的來源為光照引起的表面鈍化以及形成了中性核╱帶電表面層量子點兩者的貢獻。 最後我們也研究了金奈米團簇的螢光性質，尤其是針對單一金奈米團簇，我們發現在溶液中的群體金奈米團簇會有光引起螢光增加現象，但在空氣中則呈現著光漂白的現象。從單一金奈米團簇來看，螢光閃爍的行為可以被觀察到，且其亮暗期的分佈也呈現著和硒化鎘量子點類似的Power-law的行為，此外單一金奈米團簇的螢光衰減呈現以~7 ns生命期的單一指數衰減曲線。

In this thesis, fluorescence properties of nanometer-sized materials, including colloidal CdSe related quantum dots (QDs) and gold nanoclusters (NCs) were investigated based on single-molecule detection techniques. For CdSe/ZnS QDs, the blinking behavior can be dramatically modified by additional hexadecylamine (HDA) ligands. The on/off time ratio and the total number of emitted photons within the on-time duration can be enhanced up to 2.8 and 13-fold, respectively. Further, fluorescence decay exhibits more close to single exponential decay profiles and has longer measured lifetime. These results suggested that HDA ligands can provide the higher degree of surface passivation. Moreover, we also studied the vibrational and electronic states of CdSe/ZnS QDs under various high pressures. Pressure-induced quadratic lattice variations can be observed explicitly from both the PL and Raman spectra up to ~7 GPa. This quadratic relationship is consistent with the theoretical prediction. For CdSeTe/ZnS QDs, Photoinduced fluorescence enhancement (PFE) was studied by means of both ensemble and single-QD fluorescence measurements. Ensemble fluorescence intensity can be increased either in air or in vacuum, but with different enhanced factors. At the single-QD levels, fluorescence intensity was increased for some of individual QDs upon illumination. For these PFE individual QDs, relatively long on-times, high quantum yields within the on-times, and multi-levels on-states was found in fluorescence blinking time traces. According to ensemble and single-QD fluorescence measurements, we proposed that the origin of PFE phenomenon is attributed to the both contributions of surface passivation by photo-induced charged carriers and formation of the neutral core/charged shell QD states. Finally, fluorescence properties of gold NCs were investigated, in particular, down to single-NC level. Photo-induced fluorescence enhancement was found for ensemble gold NCs in solution, but ensemble gold NCs at ambient environment exhibits photo-bleaching phenomenon. At the single-NC levels, fluorescence blinking was observed and the distribution of on/off-times also follows the power-law similar to CdSe QDs. Moreover, the fluorescence decay profiles for single NCs show mono-exponential decay behavior with lifetime of ~7 ns.