Role of impact ionization in multiple exciton generation in PbSe nanocrystals

In the impact ionization process, a hot carrier relaxes by generating an exciton. We present tight binding calculations showing that the rate of this process in PbSe nanocrystals has a strong energy dependence and that the relaxation energy increases linearly with the carrier excess energy. The impact ionization can be extremely fast $(\sim \mathrm{fs})$ but it is not enhanced by the confinement in contrast to the usual belief. It explains the multiple exciton generation observed experimentally in these dots except at high photon energy where more complex many-particle phenomena are involved.