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 photogen- erated 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 have 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., hole-less) CdSe nanocrystals leads to short p electron lifetimes comparable to those calculated in the presence of a photogenerated hole delocalised in the dot core. Furthermore, it is shown that efficient energy transfer can also be achieved between a delocalised electron and a surface-trapped hole leading to short p electron lifetimes in the (sub-) ps range. These results are in quantitative agreement with experiment and consistent with the Auger interpretation of the electron relaxation. The fast sub-ps electron relaxation times calculated in the presence of a hole localised in a shallow surface trap raise the intriguing question of whether in earlier measurements in TOPO-capped nanocrystals the hole was indeed delocalised within the dot core, as it was believed at the time, or whether it could have been in a trap state.