Size-Dependence of Interfacial Electron Transfer from Single CdSe/ZnS Quantum Dots to TiO2 film

藉由結合共聚焦顯微鏡及TCSPC系統的單分子光譜，在單分子級的解析度下觀察單一半導體之量子點的螢光軌跡與生命期得以實現。本次實驗中，直徑分別為3.6、4.6及6.4 nm的CdSe/ZnS量子點將以迴旋塗布的方式塗布在玻璃或二氧化鈦薄膜表面，並利用波長為375 nm的脈衝雷射激發量子點，及透過單分子光譜來觀察單一量子點在不同表面上的螢光軌跡與生命期，藉由其變化來探討量子點與二氧化鈦薄膜界面之間的電子轉移之動力學現象。量子點吸附在玻璃與二氧化鈦薄膜表面的螢光生命期之差距，表現出與量子點的粒徑具有相關性，並且展現出隨粒徑增加，而生命期的差距減小的趨勢。透過生命期的差距，我們估計出粒徑為3.6及4.6 nm的量子點與二氧化鈦界面之間的電子轉移速率常數，其值分別為(1.5±1.4)×107 及 (6.8±8.1)×106 s-1，以及其電子轉移的量子產率分別為22.6及13.3 %。但由於粒徑為6.4 nm的量子點之生命期差距太小，無法透過這個方法估計出電子轉移速率常數。在觀察量子點的閃爍行為時，發現其on-state及off-state的機率密度分布為non-exponential decay。並利用truncated-power law 對on-state的機率密度分布做模擬計算，藉由計算得到的參數以及Marcus ET model，我們更進一步地用理論計算的方式計算出三種不同粒徑的量子點之電子轉移速率常數，其值分別為1.42×107、6.80×106以及 1.86×106 s-1，可以與單分子光譜的實驗結果互相吻合。The SMS (Single molecule spectroscopy) which combined confocal microscopy and TCSPC (Time-correlated single photon counting) system was used to detect the fluorescence trajectories and lifetimes of single semiconductor QD (Quantum dot) on a resolution of single molecule level, and the interfacial electron transfer kinetics between single QD and TiO2 film were investigated by this work. Three different diameters CdSe/ZnS QDs of 3.6, 4.6 and 6.4 nm were spin-coated on glass and TiO2 nanoparticles film, and the trajectories and lifetimes were investigated by the SMS with an excitation of 375 nm pulsed laser. The differences of lifetimes showed a size-dependent trend which increased with size decreased, and ET (Electron transfer) rate constants between the interface of QDs and TiO2 film were determined to be (1.5±1.4)×107 and (6.8±8.1)×106 s-1 for the sizes of 3.62 and 4.64 nm, the quantum yields of electron-ejection were also determined to be 22.6 and 13.3 %. However the ET rate constant of the largest size was hard to determine in this measurement due to the tiny difference of lifetimes. A truncated-power law was used to fit the probability density of on-state, and the parameters from the fitting were used to further calculate the ET rate constant by Marcus electron transfer model. The theoretical ET rate constants were determined to be 1.42×107, 6.80×106, and 1.86×106 s-1of three different sizes, and close to our experimental results