Spin Dynamics Of Localized Electrons In Diverse Dimensional Semiconductors

Electron spins in semiconductors are expected to be used in spin electronic devices and quantum computing.Localized electrons mean that the wavefunction of the electron is spatially localized in the small scale(usually nanometer scale or below),such as the electrons in quantum dots whose three dimensions are of nanoscale sizes and the localized states caused by defects or impurities in the bulk or other dimensional materials.The spin-orbit interaction of localized electrons is greatly suppressed,resulting in longer spin lifetime and dephasing time,compared with those of free electrons.In this dissertation,the transient spectroscopies are used to study the spin dynamics of localized electrons in various materials such as zero-dimensional CdSe colloidal quantum dots,two-dimensional monolayer MoS₂ semiconductor and ZnO bulk materials.The main research contents and innovative achievements are summarized as follows:1.The electron spin dynamics in zero-dimensional colloidal quantum dots is studied by time-resolved ellipticity at room temperature.It is found that the surface conditions have a very important effect on the electron spin dynamics in zero-dimensional colloidal quantum dots.The hole acceptors Li[Et₃BH]and 1-octanethiol are added in CdSe colloidal quantum dots,showing significant differences in the electron spin precessing frequency,spin g factor,spin relaxation/dephasing time and mechanisms.In CdSe quantum dots with Li[Et₃BH],the electron spin dephasing/relaxation are dominated by electron-nuclear hyperfine interactions,and the spin dephasing time can reach up to 2ns in zero and weak magnetic fields.In contrast,hyperfine interactions,electron carrier lifetimes,and exchange interactions between electrons and holes or surface dangling bond spins control the electron spin dynamics in CdSe quantum dots with 1-octanethiol with the spin dephasing time being up to 0.66ns.Inhomogeneous dephasing limits the spin dephasing time in strong transverse magnetic fields for both hole acceptor cases,but with distinct different g-factor inhomogeneity.2.The electron spin dynamics in two-dimensional monolayer MoS₂ is studied by time-resolved Faraday rotation spectroscopy at T=5 K.It is found that there are both free and localized electrons in monolayer MoS₂.The free electron spin relaxes rapidly in an external transverse magnetic field without spin precession signals.In contrast,three spin precession frequencies exist in the spin signals of localized electrons,which are attributed to three kinds of localized electrons.Each of them has two spin relaxation/dephasing processes,and the spin dephasing time of the slow process can reach up to 6.9ns.Their slow processes are dominated by electron-nuclear hyperfine interactions in zero and weak magnetic fields,and dominated by the inhomogeneous dephasing mechanism.However,the fast process is independent of the transverse and longitudinal magnetic fields,which may be dominated by the anisotropic exchange Dzyaloshinskii-Moriya interaction between adjacent localized electrons.3.The electron spin dynamics are studied in Ga-doped ZnO single crystals by time-resolved Faraday and Kerr rotation spectroscopies.It is found that there are both free and localized electrons in Ga-doped ZnO single crystals.At low temperature, the spin signals of both free and localized electrons can be observed.With the increase of temperature,the relaxation rate of free electrons increases rapidly,and the spin signal cannot be observed when the temperature is higher than 100 K.However,the spin dephasing time of localized electrons is still very long at room temperature,which can reach up to 5.2ns.Through the laser wavelength dependence and laser power dependence measurements,it is found that there are two kinds of localized electrons.The spin dephasing(relaxation)processes of the two localized electrons are independent of the transverse(longitudinal)magnetic fields,indicating that the spin dephasing is not induced by the g-factor inhomogeneity,electron-nuclear hyperfine interactions or spin-phonon interactions.It is shown that the spin dephasing processes of the two kinds of localized electrons are dominated by the anisotropic exchange Dzyaloshinskii-Moriya interaction between adjacent localized electrons.