Efficient Auger electron cooling in seemingly unfavourable configurations: hole traps and electrochemical charging

The absence of a phonon bottleneck in the intraband relaxation between p-like and s-like electron states in CdSe nanocrystals is generally ascribed to efficient inelastic scattering with the photogenerated hole (Auger cooling). However, the fast relaxation of electrons observed in the absence of a hole or in the presence of a hole trapped in a surface state has raised serious questions about the suitability of this model. The semiempirical pseudopotential calculations reported here show that electron−electron scattering in chemically reduced or electrochemically charged (i.e., holeless) CdSe nanocrystals leads to short p electron lifetimes comparable to those calculated in the presence of a photogenerated hole delocalized in the dot core. Furthermore, it is shown that efficient energy transfer can also be achieved between a delocalized electron and a surface-trapped hole, leading to short p electron lifetimes in the (sub-) picosecond range. These results are in quantitative agreement with experiment and are consistent with the Auger interpretation of the electron relaxation. The fast sub-picosecond electron relaxation times calculated in the presence of a hole localized in a shallow surface trap raise the intriguing question of whether in earlier measurements in TOPO-capped nanocrystals the hole was indeed delocalized within the dot core, as it was believed at the time, or whether it could have been in a trap state.