Ligand effects on the angular momentum fine structure of CdTe quantum dots

Ligand effects on the angular momentum fine structure of 3.2 nm diameter CdTe quantum dots have been studied using room temperature static spectroscopy and lifetime measurements. This study focuses on the comparison of particles having charged (octadecylphosphonic acid) and uncharged (octanethiol) ligands. In both cases, polarized photoluminescence excitation spectroscopy provides a measure of the extent to which fine structure states with orthogonal polarizations can be selectively excited, and the photoluminescence Stokes shift provides a measure of the amount of luminescence coming from fine structure states with different energies. Radiative lifetime measurements provide a measure of the relative oscillator strengths of fine structure states with different energies. We demonstrate that the changes in the nature of the surface ligands (charged or uncharged) measurably change the polarized photoluminescence excitation spectrum, Stokes shift, and radiative lifetimes. We suggest that this is due to large internal electric fields associated with the charged ligands that alter the angular momentum fine structure mixings and energetics. The results are analyzed in terms of a simple two-parameter effective mass approximation model for quasi-spherical II-VI quantum dots [Efros et al., Phys. Rev. B 54, 4843-4856 (1996)], which is the most commonly used model of angular momentum fine structure states. We find that this model qualitatively explains the results. However, quantitative analysis of these results gives a significant deviation from the model predictions; it requires that the fine structure splittings are smaller and/or more complicated and that the lower energy states have more oscillator strength than predicted by the Efros model.