Electronic coherence has attracted considerable attention for its possible role in energy and charge transfer processes. The development of multi-dimensional coherent visible spectroscopies has allowed the discovery of oscillating signals in light harvesting systems and conjugated polymers at room-temperature. These quantum beats were interpreted as the signature of coherent superposition of electronic states, i.e. states will well-defined relative phase. Importantly, the lifetime of these coherences can be sometimes comparable to the time scale of energy or charge transfer, leading thus to the question if they are relevant in these processes (initially thought as classical).
The detection of electronic coherence is however challenged by system interaction with the bath (homogenous broadening) and ensemble effects (inhomogenous broadening), that result in fast dephasing. Numerous reported electronic coherences remain controversial or turned out to be vibrational in nature. Long-lived coherences, i.e. lasting for hundreds of fs, have been rationalized by invoking a strong correlation in the mechanism of decoherence or by involving resonant vibrational modes capable of sustaining these coherences.
Colloidal semiconductor nanocrystals exhibit unique optical properties that depend on their size, shape and composition. It is relatively recently that a new class of colloidal nanocrystals have been developed, with a two-dimensional structure. These colloidal quantum wells, or nanoplatelets, have garnered particular attention due to their huge absorption cross section and narrow lineshapes. Colloidal semiconductor nanoplatelets are an ideal system to study electronic coherence and their possible role in the mechanism of charge transfer in the case of heterostructures.
After describing the femtosecond two-dimensional spectroscopy technique we use in our group to study ultrafast processes, I will present my postdoctoral research on the examination of electronic coherence in semiconductor nanoplatelets. In CdSe and CdSe/CdS core/shell nanostructures, quantum beats were observed up to about 100 fs in solution at room-temperature and were unambiguously attributed to coherent superposition among the “heavy hole” and “light hole” excitons.
Exciton coherence dephasing times were examined with the optical coherence times (coherence between the ground and excited states) related to optical transition linewidth. Comparing these exciton and optical coherence times evidences a partial correlation in the energy fluctuation of the two excited states and allows us to elucidate the decoherence mechanisms occurring in these samples.
Finally, I will present interesting results on the study of charge transfer in in-plane heteronanostructures with a type-II band alignement, CdSe-CdTe nanoplatelets, notably a sub-25 fs delocalization of the electron wavefunction over the two materials.
Frick Chemistry Department, Princeton University