What Does the Transient Absorption Spectrum of CdSe Quantum Dots Measure?

The roles of conduction and valence band state filling and stimulated emission in the room temperature transient absorption spectroscopy of CdSe and CdSe/CdS quantum dots are discussed. Photoluminescence kinetics and transient absorption (TA) spectra for core-only and core/shell particles with and without an adsorbed hole acceptor p-methylbenzenethiol (MBT) are obtained under identical experimental conditions. The TA spectrum has contributions from ground state depletion, excited state absorption, and stimulated emission. Photoexcitation bleaches the lowest exciton transition, and comparison of the bleach magnitudes for particles with and without MBT permits assessment of the role of valence band state filling and stimulated emission. The present results show that following relaxation into the optically dark fine structure levels, the presence of the hole has only a small effect on the excited state absorption and stimulated emission contributions. This supports the conventional wisdom that the TA bleach in II-VI QDs is dominated by conduction band state filling. We also present effective mass approximation calculations showing that quantitative understanding of the bleach intensity also requires consideration of stimulated emission and the change in excited state absorption due to changes in electron-hole overlap upon hole trapping.