金屬/半導體奈米異質結構之載子動力學與其光能轉換應用

Abstract

本論文主要關注於多種金屬/半導體奈米異質界面材料如Au-CdS、Au-ZnS核殼奈米粒子、ZnS-Au核-衛星奈米複合粒子與Au-Cd1-xZnxS核殼奈米粒子的界面載子傳遞動力學討論，由於Au與半導體端(CdS、ZnS與Cd1-xZnxS)相對能帶結構的關係，光激發產生電子傾向自發性的傳遞至Au端，同時，大量的電洞會存在於半導體殼層，達到載子分離的目的。電子於Au與半導體端界面間之傳遞的行為可利用時間解析螢光光譜技術來量測，亦即量化界面間電子傳遞的現象，載子動力學研究所得之結果與其於光能轉換效率相對呼應，即具有一正比的關係。更進一步，增進Au與半導體端界面間電子傳遞的現象可利用添加Zn元素於Au-CdS核殼奈米粒子的CdS殼層材料來達成，藉由添加Zn元素形成Au-Cd1-xZnxS核殼奈米粒子來提高界面間電子傳遞的驅動力，可觀察到增進一個級數電子傳遞速率常數的結果，此結果展現出利用調控金屬/半導體奈米異質結構的成分組成來改善光能轉換效率之可行性。此研究成果提供一嶄新的金屬/半導體奈米異質結構以提昇其於太陽光能源轉換的效率。 此外，利用dihydolipoic acid作為包覆分子成功製備出具分子軌域吸收特徵與具螢光放光性質之Ag8團簇粒子，樣品乃具有螢光放光波長位於660nm與相當高之量子產率約為4.62%。添加MV2+作為電子傳遞行為之感測分子，藉由觀測MV+.產物的生成，成功地闡明Ag8奈米團簇粒子的界面電子傳遞動力學，結果得到界面間電子傳遞速率常數為2.74 × 1010 s-1，進一步證實金屬奈米團簇粒子參與光催化 還原反應的可行性，這對於了解金屬奈米團簇粒子之激發態程序，與其後續運用於生醫領域如螢光標定、感測與光電轉換系統如光能源轉換、光催化提供具有參考價值之重要資訊。

In this thesis, we investigated and presented the interfacial charge carrier dynamics for a series of metal/semiconductor nanoheterostructures including core-shell Au-CdS nanocrystals, core-shell Au-ZnS nanocrystals, core-satellite ZnS-Au nanoassemblies, and core-shell Au-Cd1-xZnxS nanocrystals. Due to the difference in band structures between Au and semiconductors (CdS, ZnS, and Cd1-xZnxS), the photoexcited electrons of semiconductor would preferentially transfer to Au, simultaneously leaving photogenerated holes at semiconductor domain to achieve charge separation. The electron-charging of Au in the nanoheterostructures can be revealed with time-resolved PL spectroscopy which quantitatively describes the electron transfer event between Au and semiconductors. The result of carrier dynamics measurement for the samples is further correlated with their performance evaluation in photoconversion processes, from which a direct correspondence is observed. To boost the electron transfer from CdS to Au, Zn is incorporated in the CdS shell of Au-CdS to form Au-Cd1-xZnxS nanocrystals. By introducing Zn dopants to enlarge the electron transfer driving force, Au-Cd1-xZnxS nanocrystals show one order of magnitude increase in electron transfer rate constant, demonstrating the possibility of improving the photoconversion efficiency for metal/semiconductor nanoheterostructures by means of composition. The present study gives rise to a new class of highly efficient metal/semiconductor nanoheterostructures which may effectively utilize the solar power. In addition, silver nanoclusters complexed with dihydrolipoic acid (DHLA) exhibit molecular-like excited state properties with well-defined absorption and emission features. The Ag8 clusters exhibit fluorescence maximum at 660 nm with a quantum yield of 4.62 %. By introducing MV2+ as a probe we have succeeded in elucidating the interfacial electro transfer dynamics of Ag nanoclusters. The formation of MV+. as the electron transfer product with a rate constant of 2.74 x 1010 s-1 confirms the ability of these metal clusters to participate in the photocatalytic reduction process. Basic understating of excited state processes in florescent metal clusters paves the way towards the devilment of biological probes, sensors and catalysts in energy conversion devices.