Uncovering Hot Hole Dynamics in CdSe Nanocrystals

Single and multiple exciton relaxation dynamics of CdSe/CdZnS nanocrystal quantum dots (QDs) monitored at the two lowest optical transitions, 1S(e)-1S(3/2) and 1S(e)-2S(3/2), have been examined using ultrafast transient absorption (TA) spectroscopy. For the CdSe/CdZnS QDs studied, the 1S(e)-1S(3/2) and 1S(e)-2S(3/2) transitions are widely separated (similar to 180 meV) compared to bare CdSe QDs (similar to 50-100 meV), allowing for dearly distinguishable TA signals attributable to hot hole relaxation. Holes depopulate from the 2S(3/2) state with a lifetime of 7 +/- 2 ps, which is consistent with the predictions for hole relaxation via a phonon coupling pathway to lower-energy hole states, with possible contributions from hole trapping as well. These results suggest that tuning the surface chemistry of semiconductor QDs is a viable route to measure and possibly control their hot hole relaxation dynamics.