Exciton Localization and Radiative Lifetimes in CdSe Nanoplatelets

The spatial extents of CdSe nanoplatelet (NPL) excitons are investigated using transient absorption (TA) spectroscopy. The bleach magnitudes of a series of NPLs with varying lateral dimensions are compared with a quantum dot (QD) standard, allowing the relative magnitude of the heavy hole (HH) bleach to be determined as a function of size. The relative bleach of the HH absorbance decreases with increasing NPL area, while the excitonic sizes calculated from the bleach magnitudes are found to be independent of the lateral dimensions. This result is consistent with a model that considers the relative intensities of photoinduced absorption (PA) and stimulated emission (SE) contributed by distinct regions of the platelet occupied by the electron only, both the electron and hole, or neither. Using this model gives an average excitonic area of 21.2 +/- 2.5 nm(2). Considering the small spatial extent of the relaxed NPL exciton compared with the number of oscillators participating in ground state absorption partially accounts for the discrepancy between the HH extinction coefficients and the slow radiative rates, which deviate from the values calculated from the Einstein relations by a factor between similar to 30 and 80. The electron-hole overlap integral is estimated from the ratio of HH bleach magnitudes before and after trapping the hole by 4-methylbenzenethiol (MBT), a hole acceptor, and is then factored into calculations of the radiative lifetimes. Together with the measured singlet-triplet splitting and e-h overlap, these considerations allow the NPL radiative kinetics to be semiquantitatively reproduced.