Anomalous temperature dependence of the the spin-flip thermalization time between the dark and bright exciton states in silicon nanocrystals

Silicon nanocrystals are studied by time-resolved fluorescence spectroscopy. After laser excitation the bright and dark exciton ground state levels are populated at random, but subsequently the decay curves reveal a thermalization between these levels. The characteristic thermalization time is found to be approximately 100 ns for temperatures below 100 K and surprisingly increases for higher temperatures. The decay curves are analyzed using a simple two-state model for the bright and dark exciton ground states.