Surface charge and piezoelectric fields control auger recombination in semiconductor nanocrystals

The dynamics of biexcitons in CdSe nanoparticles are examined as a function of the magnitudes of internal electric fields. We show that the presence of strong internal fields results in rapid Auger recombination. The strengths of the electric fields and hence the Auger recombination rates are controlled in several different ways: specifically, by varying the dielectric constant of the surrounding solvent, by changing the particle surface stoichiometry and hence the magnitude of surface charges, and by inducing a piezoelectric field through the deposition of a lattice-mismatched shell material. Auger recombination is a momentum forbidden process. Fourier transformation of calculated spatial wave functions shows that higher conduction band states have large momentum components that relax the momentum conservation constraints. Relative Auger recombination times depend upon the extent to which the internal electric fields mix conduction band levels, which is easily calculated. Comparison with calculations of valence band states suggests that the excited particle in biexciton Auger recombination is the other electron. The experimental results can therefore be understood in terms of mixing of higher conduction band states with the lowest state from which recombination occurs.