碲化鎘膠體量子點之能量共振轉移

Abstract

我們利用光激螢光及時間解析光頻譜技術來分析單一種尺寸(5.1 奈米)與混合兩種尺寸(3.4 和 5.1 奈米)的水溶性碲化鎘量子點之Förster能量共振轉移。

在單一種尺寸樣品中，我們觀察到量子點薄膜之光激螢光能量相較於水溶液有一個紅移的現象。我們也利用時間解析光譜分析載子的複合機制，發現載子的生命期會隨著不同量子點尺寸而改變。綜合以上討論可證實載子以非輻射性的方式將能量從小的量子點轉移到大的量子點。並且發現能量紅移與樣品的尺寸分佈有關。

我們將兩種不同尺寸的量子點混和。從光激螢光光譜上可以觀察到小量子點的螢光強度受到抑制，伴隨著大量子點的螢光強度增強，這是由於小量子點的放射光譜與大量子點的吸收光譜之間的重疊增加，導致量子點之間的偶極交互作用力增強，造成明顯且有效率的能量轉移。

最後，我們發現對於能量轉移的效率而言，小量子點與大量子點的混和濃度比例會是一個很重要的影響因素。當小量子點與大量子點的混和比例越大，大量子點充分地被小量子點包圍，提升小量子點的能量轉移效率。小量子點的數量持續增加，大量子點的比例相對減少，此時小量子點的載子複合機制將不受到大量子點的影響。

The dynamics of Förster resonance energy transfer (FRET) in mono-dispersed (5.1 nm) and mixed-size (50 % 3.4 nm and 50 % 5.1 nm) water-soluble CdTe quantum dots (QDs) were studied by using photoluminescence (PL) and time-resolved PL (TRPL) spectroscopy.

A significant PL energy red-shift was observed for the mono-dispersed CdTe colloidal QDs films when compared with that of the mono-dispersed CdTe colloidal QDs solutions. The energy dependent TRPL spectra directly reveal the energy transfer from small to large QDs. We found that the amount of PL energy red-shift is independent of CdTe QDs’ size but depends on the PL line-width.

To study the effect the spectral overlap of donor emission and acceptor absorption on the FRET efficiency, a mixture of two different sizes CdTe QDs (50 % 3.4 nm and 50 % 5.1 nm) was studied. The quenching/enhancement of PL intensity and lifetime of donors/acceptors are evidences of FRET effect in mixed-size water-soluble CdTe QDs.

Furthermore, the donor to acceptor concentration ratio in the mixed-size CdTe QDs films is an important parameter to influence the FRET efficiency. As the ratio is increased, the FRET rate and the efficiency is firstly enhanced and then decreased. It is due to the fact that acceptors are surrounded by more and more donors in the beginning. However, the relative amount of available acceptors is decreased if the ratio is further increased.