Charge Separation And Electron Spin Coherence Measurements In ?-? Colloidal Quantum Dots

Due to the existence of unsaturated bonds on the surfaces and large surface-to-volume ratio,charge separation is a commom phenomenon in colloidal quantum dots?QDs?.The spin lifetime can be prolonged by charge separation,which is benefit for the spin-based applications.II-VI colloidal QDs are the most investigated system to study photophysical processes among various colloidal QDs.In this dissertation,charge separation and relevant spin phenomena of II-VI colloidal QDs are investigated by transient spectroscopy.The main research contents and innovative results are summarized as follows:1.The electron spin dynamics of as-grown CdSe,core/shell CdSe/ZnS and CdSe colloidal QDs with hole or electron acceptors are investigated by time-resolved Faraday rotation and ellipticity spectra at room temperature.The experimental results show that there are two Larmor precession frequencies in spin signals of CdSe QDs,and the spin amplitudes depend on the type and quantity of the charge separation.We have demonstrated that both Larmor precession frequencies belong to electrons in photocharged QDs,which are either confined by the QD potential or additionally localized in the vicinity of the surface.Thereby,we solve the long-standing problem of the origin of the two Larmor precession frequencies in the coherent spin dynamics of colloidal CdSe QDs.2.Photo-induced charging in CdSe colloidal QDs with a diameter of 2.4 nm is investigated by time-resolved ellipticity spectroscopy at room temperature.By addition of 1-octanethiol to the CdSe QD solution in toluene,an extremely long-lived negative photocharging is detected which lives exceeding one month in N₂ atmosphere and hours in air atmosphere at room temperature.1-Octanethiol not only acts as a hole acceptor,but also results in a reduction of the oxygen-induced photo-oxidation in CdSe QDs,allowing an air-stable negative photocharging.The observed extremely long-lived charge separated states,are potentially useful for applications in low-threshold lasing,photocatalysis and spintronics.3.The electron spin dynamics of CdS colloidal QDs dissolved in toluene and water-soluble CdSe colloidal QDs are investigated by time-resolved ellipticity spectroscopy.Compared with as-grown CdS QDs,the spin amplitude of CdS QDs with hole acceptor phenothiazine increases effectively,while the Larmor spin frequency keeps the same?12.30 GHz?.Via addition of methylviologen electron acceptor,two different Larmor frequencies show up in spin dymanics of CdS QDs?v₁=11.17 GHz,v₂=12.30 GHz?.Therefore,we can conclude that the lower frequency is relevant to electron trapping and the higher frequency is related to hole trapping.With addition of methylviologen electron acceptor,the amplitude of electron spin dynamics in water-soluble CdSe QDs decreases,while Larmor precession frequency keeps the same.The choice of suitable electron or hole acceptors is of great significance to charge separation and spin control in colloidal QDs.