Optimum excitation photon energy for CdSe–ZnS core–shell quantum dot based luminescence imaging

Abstract

The authors present a comparative study of photoluminescence excitation (PLE) and absorption spectra of CdSe–ZnS core–shell quantum dot ensembles, with mean core diameters ranging from 2.6 to 7.2 nm. The larger dots show a threshold phenomenon wherein for excitation photon energy greater than a value Eth, the PLE and absorption spectra diverge abruptly. Beyond Eth the PLE signal tends to decrease while absorption continues to increase. Results are compared with electronic structure calculations based on a spherical core–shell potential model. One finds that Eth matches with the minimum photon energy required to create holes with energy equal to the core–shell potential energy barrier for holes. These holes, more exposed to shell surface defects, are likely to be lost through non-radiative pathways thereby reducing luminescence. Part of the increase in absorption at higher energies can be attributed to normally forbidden transitions, whose oscillator strength is shown to increase with increase in the transition energy.