單一硒化鎘/硫化鋅量子點與二氧化鈦奈米粒子界面之電子轉移行為與連接分子長度的關係

Abstract

本實驗中，我們藉由共聚焦顯微鏡結合TCSPC系統，針對單一硒化鎘/硫化鋅量子點進行單分子實驗的觀測；藉由記錄螢光軌跡得到其螢光生命期，量測其藉由雙官能基分子鍵結至二氧化鈦薄膜後的電子傳遞速率。量子點和二氧化鈦薄膜之間的距離可以藉由不同碳鏈長度的連接分子控制。藉由有機分子將量子點和二氧化鈦連結之後，可以提供一個模組系統，探討光致電子轉移的機制。了解這些機制對於量子點敏化太陽能電池的發展以及效率的提升將有很大的幫助。 我們利用448 nm波長的脈衝雷射進行單分子實驗，發現量子點的螢光生命期隨著連接分子的長度縮短而降低，進一步由螢光生命期推導出電子傳遞的速率常數後可以發現，在連接分子碳數為3、6、11時，速率常數分別為2.8x107、1.9x107 、3.5x106 s-1，量子產率分別為67.4、45.8、7.8%。實驗數據結果顯示，電子轉移的速率常數隨著連接分子的碳鏈增長有下降的趨勢。 我們進一步利用Marcus發表的電子轉移模型去計算量子點和二氧化鈦之間的電子耦合常數，在連接分子長度為1.5、6.2、13.8 Ȧ 時，電子耦合常數分別為4.07、3.82、1.58 cm-1，如我們所預期的，連接分子的長度增長之後減弱了電子供體與受體間電子耦合的強度。 接著我們利用具芳香性的雙官能基分子連接量子點與二氧化鈦，發現其電子傳遞速度比預期中更加快速，我們推測可能是連接分子的π軌域強化了電子施體和電子受體間的電子耦合強度，導致雖然距離增長，電子傳遞速率卻變快的現象。 另外我們以不同直徑大小的量子點，藉相同長度的雙官能基分子連接到二氧化鈦薄膜上，發現在有連接分子的存在下電子傳遞速度仍舊隨著粒徑下降而上升。

Single molecule fluorescence spectroscopy (SMS) which combined home-made confocal microscopy and time-correlated single photon counting (TCSPC) system was used to record the fluorescence trajectories and to calculate lifetimes of single semiconductor quantum dot (QD) at the single-molecule level. Electron transfer (ET) rate from single QD to TiO2 film via bifunctional linkers were obtained. CdSe/ZnS QDs were tethered to TiO2 film through bifunctional mercaptoalkanoic acid (MAAs), the interparticle distance of which is controlled by the linker length. The fabricated QD-TiO2 heterodimers provide a model system for the single-molecule exploration of photoinduced electron transfer between QDs and charge acceptors, which is essential process in quantum dot-sensitized solar cells (QDSSCs). The trajectories and lifetimes were obtained by SMS with an excitation of 448 nm pulsed laser. The differences of lifetimes provide strong evidence for chain-length-dependent ET efficiency. ET rate constants were determined to be 2.8x107, 1.9x107, and 3.5x106 s-1 for the chain-length of 1.5, 6.2 and 13.8 Ȧ, the quantum yield were also determined to be 67.4, 45.8 and 7.8 %. The results showed that ET rate constants decreased with length of linkers increased. We further calculate electron coupling elements (HDA) between QDs and TiO2 using the semiclassical Marcus expression. The HDA were determined to be 4.07, 3.82 and 1.58 cm-1 of three different chain lengths, and be close to our prediction. We further tethered QDs by linkers with aromatic spacer. The results suggest that the presence of π orbitals accelerates ET rates between QDs and TiO2. We attributed the rapid ET rates to enhancement of electronic coupling strength. Finally, we examined the size-effect on QD-MAA-TiO2 systems. The results suggest that the size-dependence of ET dynamics is still valid regardless of the existence of linkers.