| There are usually a large number of unsaturated bonds on the surfaces and interfaces of semiconductor quantum dots grown by colloidal chemistry method.Photoexcitation generates electron-hole pairs within the quantum dots,which are easily trapped by those unsaturated bonds,leaving the cores of quantum dots with net negative(or positive)charges.This process is called negative photocharging or positive photocharging,which is a common phenomenon in colloidal quantum dot materials.Due to electron-hole spatial separation,electron-hole exchange interactions and recombination processes that lead to spin relaxation are largely reduced after the photocharging.Therefore,the spin lifetime can be extended,which is beneficial to the application of quantum information processing.In this thesis,the photocharging of CdSe colloidal quantum dots and its effect on the signals of electron spin are studied by using the pump-orientation-probe technique.The main results include:(1)CdSe colloidal quantum dots have a fast hole surface trapping process,leading to the formation of negative photocharging which enhances the electron spin signals.In turn,the dynamics of the hole surface trap dynamics(i.e.,the formation dynamics of negative photocharging)is revealed from the enhancement transients of spin signals.At room temperature,our experiments show that such dynamics contains two processes,with time constants of about 6 and 73 ps,respectively.(2)Previous studies have found that there are usually two spin-precession frequencies in CdSe colloidal quantum dots,but the physical mechanism for this is still not clear.By adding electron acceptor or hole acceptor to change the surface state of the quantum dot,and by using the pump-orientation-probe technique,we show that the two spin-precession frequencies of CdSe colloidal quantum dot arise from negative photocharging and positive photocharging,respectively.In addition,we find that the temperature has an important effect on the photocharging phenomena. |
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