氧化鋅量子點中不受激發功率影響之激子態研究

Abstract

利用溶膠-凝膠法藉由改變溶液濃度來合成不同晶粒大小的氧化鋅量子點，並量測其光學特性。從低溫螢光光譜隨氧化鋅量子點尺寸變小而藍移的現象可觀察到在氧化鋅量子點中存在量子侷限效應。然而，量測變功率螢光光譜發現，當激發光強度改變了1000倍，除了光譜強度會隨激發強度增加而變大外，NBE的光譜形狀幾乎都不變，且觀察不到雙激子的形成與激子與激子的散射現象，此情形與氧化鋅塊材的特性相當不一樣。另外，自由激子與表面束縛激子的發光能量幾乎不隨激發光強度改變及線性的發光強度與激發能量關係，表示當激發功率增加，仍沒有激子間的交互作用，因此在氧化鋅量子點中，存在穩定的激子態。

ZnO QDs are synthesized by a simple sol-gel method and the average size of QDs can be tailored under solution concentration. Size-dependent blue shifts of photoluminescence give the evidence of the quantum confinement effect. Furthermore, the unchanged spectral profiles of near-bandedge emission (NBE) for a fixed size of ZnO quantum dots (QDs) reveal no signature of biexciton formation and exciton-exciton scattering under excitation density covering three orders of magnitude. These results quite differ from that of the micrometer-sized powder. The intensities of NBE peaks attributed to free exciton and surface bound exciton states exhibit linear dependence on the excitation power density that confirms the invariable exciton states with no effect of two-exciton interaction in ZnO QDs upon increasing the pump intensity.