Sub-Bandgap Photoinduced Transient Absorption Features in CdSe Nanostructures: The Role of Trapped Holes

Transient absorption (TA) is widely used to study the dynamics of various processes, such as trapping, nonradiative decay, or transferring of photoexcited carriers in semiconductor nanocrystals. TA spectra of these systems show photoinduced absorption (PA) features that appear lower in energy than those of the band edge, which have been attributed to sub-bandgap absorptions of photoexcited electrons and holes. Here, we perform atomistic, semiempirical pseudopotential calculations in CdSe nanostructures to compute oscillator strengths for sub-bandgap transitions of conduction band electrons, valence band holes, and surface-trapped holes. We find that sharper peaks in the infrared (IR) range and broader features in the near-IR range (0.5-1.0 eV) are due to near-band-edge transitions of electrons and holes, respectively. Additionally, we focus on the region from 1.45 to 1.9 eV (850-650 nm), in which broad features have been observed and assigned to the PA of holes populating surface traps of nanocrystals. While there has been experimental justification of this assignment, there has been little theoretical investigation. We find that, in this region of interest from 1.45 to 1.9 eV, oscillator strengths for transitions of trapped holes are significantly larger than those of electrons or valence band holes. We conclude that the low symmetry of localized surface trap states and optimal spatial overlap with highly oscillatory states deep in the valence band lead to large electric dipole matrix elements and increased oscillator strengths. Our results are consistent for CdSe and CdS cores, CdSe-CdS core-shell quantum dots, and CdSe nanorods.