Anomalous stokes shift in CdSe nanocrystals

We investigate the temperature dependence of exciton fluorescence, absorption, and Stokes shift for five sizes of high quality CdSe/ZnS nanocrystals from the perspective of the fine structure model and exciton-acoustic phonon coupling. The Stokes shift is found to have a weak temperature dependence for all sizes. Within the fine structure model, we use the temperature dependence of the Stokes shift to infer the upper level energy and oscillator strength. We also interpret our ensemble measurements using an exciton-acoustic phonon scattering model. We find that neither the fine structure nor exciton-acoustic phonon scattering is able to adequately explain our data in isolation and conclude that a comprehensive theory which includes the physics of both models on an equal footing is required.