Transient Absorption Spectroscopy of CdSe Nanoplatelets

Transient absorption (TA) spectra of 4.5 monolayer thick CdSe nanoplatelets are analyzed in terms of the heavy hole electronic structure. The nanoplatelets are treated with 4-methylbenzenethiol (MBT), a hole acceptor, and the spectra and kinetics with and without MBT are compared in order to assess the role of valence band state filling in the TA spectra. Analogous TA spectra are obtained for spherical CdSe quantum dots, showing that the presence of a valence band hole produces qualitatively different behavior in the spherical vs two-dimensional (2D) nanoparticle cases. In quantum dots, the exciton to biexciton transition is fully allowed and has close to the same energy and oscillator strength as the negatively charged particle to negative trion transition, which leads to the absence of any hole dynamics being seen in TA measurements. In the case of the platelets, the large shape anisotropy splitting lifts the light/heavy hole degeneracy, causing the exciton to biexciton transition from the lowest hole level to be forbidden. The larger spatial extent of the excitonic wave functions also reduces the exchange interaction, leading to significant population of the bright exciton state. As a result, stimulated emission significantly contributes to the amplitude of the heavy hole bleach band, and the amplitude of the photoinduced absorption of the heavy hole transition is sensitive to both conduction band and valence band state filling.