Connecting the Dots: The Kinetics and Thermodynamics of Hot, Cold, and Surface-Trapped Excitons in Semiconductor Nanocrystals

The excitonics of semiconductor nanocrystals (NC) depend upon temperature in a complex manner due to the interplay between the kinetics of hot exciton relaxation/trapping and the thermodynamics leading to cold exciton recombination. We apply a semidassical electron transfer model of surface trapping to temperature-dependent absorption and emission data to elucidate a microscopic picture of the factors which govern the fate of hot and cold excitons. The linear absorption spectra reveal a unique temperature-dependence to the energies of higher excitonic states, while oscillator strength is shown to be temperature invariant. We identify the phonon based origin to the anomalous low temperature peak energy trend in photoluminescence (PL) spectra. PL intensities, PL lifetimes, and absorption spectra are used to demonstrate that variation of quantum yield with temperatures arises from the thermally controlled fraction of NC which emit, rather than from an activated nonradiative pathway common to all NCs. Experimental quantum yield spectra are shown for several NCs and we perform a much-needed analysis of the role of surface PL in quantum yield. Finally, we show that a semiclassical electron transfer model including hot excitonic effects can explain experimental quantum yield spectra and suggests how to probe kinetic trapping processes via simple steady-state spectroscopy.