Exploring ultrafast electronic processes of quasi-type II nanocrystals by two-dimensional electronic spectroscopy

Colloidal CdTe/CdSe heteronanostructures are model systems for quasi-type II nanocrystals (NCs) and have been examined extensively. However, the complex spectra in these heteronanostructures often make it difficult to reveal details of their optical properties by conventional techniques such as transient absorption spectroscopy. In the present study, two-dimensional electronic spectroscopy (2DES) is used to study colloidal CdTe, CdTe/CdSe, and CdTe/ZnS NCs revealing the nature of absorption bands and ultrafast dynamics in a quasi-type II system. We observe the electronic coupling between the lowest two transitions, oscillations in the population time due to the longitudinal optical (LO) phonon mode, and the high-frequency impulsive Raman modes of the solvent. We observed an excited state absorption near at the band edge only in CdTe/CdSe NCs and established that it is related to the quasi-type II features: the redistribution of excitons among the fine-structured states or the biexciton level shift at the ultrafast time scale.