Size Dependence of Excitation-Energy-Related Surface Trapping Dynamics in PbS Quantum Dots

Using ultrafast transient absorption spectroscopy, we investigated the surface carrier trapping dynamics in various sized PbS quantum dots (QDs) when either a hot or cold exciton is photogenerated by different pump-energy. We observed that hot carriers exhibit distinctly different surface trapping dynamics from the cold exciton, in which their corresponding transient absorption (TA) spectral evolutions show clear differences in the long wavelength region (less than a band gap energy, E-g). We observed a rapid growth in the degree of surface trapping with an increase in the pump-energy. On the other hand, the degree of surface trapping in terms of the number of created excitons (< N-x >) shows negligible variation upon photoexcitation at any given wavelength. The photoinduced electron-hole separation followed by carrier trapping was characterized by ultrafast trapping rate. The surface trapping rate was solely dependent on the PbS QD size; the surface trapping rate becomes faster as the QD size increases. Furthermore, we explain the dependence of QD size on the surface trapping rate in terms of the size-dependent exciton binding energy (E-eb).