Electron Spin Dynamics In Negatively Photocharged Colloidal Quantum Dots

The electron spin state of semiconductor quantum dots(QDs)is expected to be used in the storage and processing of quantum information.Due to the electron spatial localization effect and the weak electron-nuclear spin hyperfine interaction,?-? colloidal QDs have a long electron spin dephasing/relaxation time.In this paper,timeresolved ellipticity spectroscopy is used to study the photocharging process and spin phenomenons of ?-? colloidal QDs.The main research contents and innovative results are summarized as follows:1.The effects of hole acceptors on the charging phenomena and spin signals of CdSe and CdS colloidal QDs have been studied at room temperature.The experimental results show that stable negatively charged QDs can be formed in a series of CdSe and CdS QD solutions with different QD sizes under nitrogen atmosphere by adding hole acceptor lithium triethylborohydride(Li[Et₃BH]).In the as-grown QDs without Li[Et₃BH],spin coherence dynamics have two precession frequencies(a smaller frequency v₁ component and a larger frequency v₂ component).After addition of the hole acceptor Li[Et₃BH],the v₁ component signal was significantly enhanced,while the v₂ component signal disappeared.Furthermore,the addition of another hole acceptor 1-octanethiol can also produce negatively photocharged QDs,but the effect will be different for different sizes of QDs and in air or nitrogen atmosphere.In certain samples,the addition of 1-octanethiol in nitrogen atmosphere enhanced the v₁ component signal,while the v₂ component signal disappeared.In contrast,the addition of 1-octanethiol in air atmosphere enhanced the v₂ component signal,while the v₁ component signal disappeared.The effect of Li[Et₃BH] and 1-octanethiol on the charging phenomena and spin signals of CdSe and CdS QDs of different sizes was investigated,which pave the way for the in-depth study of spin dynamics in chapter 3 and chapter 4.2.A series of negatively charged CdSe and CdS QDs with different sizes were obtained by adding hole acceptor Li[Et₃BH].The size dependence and transverse and longitudinal magnetic field dependence of these negatively charged QDs were systematically studied by time-resolved ellipticity spectroscopy at room temperature.It is shown that the spin dephasing of all negatively photocharged QDs is dominated by electron-nuclear spin hyperfine interaction in zero magnetic field.The electron spin dephasing time of these samples is ¹-2 ns at room temperature and decreases with decreasing of the size,which contains a time component proportional to size to the power of 3/2(D^3/2)and a time component independent of size D.With the addition of a transverse magnetic field,the spin dephasing time of all CdSe QDs becomes shorter as the magnetic field increases.However,the spin dephasing time of all CdS QDs has little change within the measured magnetic field range(0-700 mT).With the addition of a weak longitudinal magnetic field(50 mT),the hyperfineinduced spin dephasing is strongly suppressed in all the CdSe and CdS QD samples.3.The electron spin dynamics of CdS colloidal QDs with a diameter of 5.3 nm were studied by time-resolved ellipticity spectroscopy at room temperature with the addition of hole acceptor 1-octanethiol in either air or nitrogen atmosphere.The results show that the addition of hole acceptor 1-octanethiol in air and nitrogen atmosphere will lead to different negatively charged states in QDs,and the corresponding electron spin precession frequency,spin g factor and spin dephasing time are obviously different.Although electron spin dephasing processes are dominated by hyperfine interactions in both zero and weak magnetic fields,the hyperfine-induced spin dephasing time is 1.74 ns in nitrogen atmosphere and 3.42 ns in air atmosphere,which is about twice as long as the spin dephasing time in nitrogen atmosphere.When the transverse magnetic field increases from 50 mT to 700 mT,the spin dephasing time of the sample decreases from 1.72 ns to 1.57 ns under nitrogen atmosphere,which is weakly dependent on the transverse magnetic field,indicating that the spread of the g factor is very small(?g =0.0010).However,the spin dephasing time of the sample in air atmosphere decreases from 2.73 ns to 1.45 ns with the increase of transverse magnetic field(from 50 mT to 700 mT),which is relatively dependent on the transverse magnetic field,indicating that the spread of the g factor is large(?g =0.0056).