The New Detection Method For Hole Surface Trapping Dynamics In Quantum Dots:Electron Spin Manipulation

The surface state of nanocrystal has important effect on the photoelectron properties of materials, one of which is charge carrier trapping at the surface. Carrier trapping dynamics is usually probed in an indirect way using time-resolved photoluminescence and transient absorption spectroscopies, which are based on monitoring the depopulation process of band-edge excitonic states.In addition to carrier trapping, radiative and nonradiative recombination processes of excitons also contribute to the depopulation dynamics of band-edge excitonic states. Therefore weak excitation conditions combined with many measurement techniques only then can analyze hole trapping process.A new detection technique, pump-spinorientation-probe ultrafast spectroscopy, is developed to directly monitor the hole trapping dynamics. The hole surface trapping process spatially separates the electron-hole pairs excited by the pump pulse, leaves the core negatively charged, and thus enhances the electron spin signal generated by the orientation pulse. The spin enhancement transients signal as a function of the pump-orientation delay reveal a fast and a slow hole trapping process with respective time constants of ～10 ps and ～100 ps. This new technique can also be applied to high excitation conditions. The power dependence of hole trapping dynamics elucidates the saturation process and relative number of traps, and suggests that there are three subpopulations of nanoparticles related to hole surface trapping, one with the fast trapping pathway only, another with the slow trapping pathway only, and the third with both pathways together.