March 28, 1994 - March 28, 2027

  • Date:11SundayJune 2023

    Probing nanocrystal photophysics with spectator excitons

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    Time
    10:00 - 11:00
    Location
    Gerhard M.J. Schmidt Lecture Hall
    Lecturer
    Prof. Sanford Ruhman
    Institute of Chemistry, HUJI
    Organizer
    Department of Molecular Chemistry and Materials Science
    Contact
    AbstractShow full text abstract about Femtosecond pump-probe experiments on nanocrystals are inter...»
    Femtosecond pump-probe experiments on nanocrystals are interpreted primarily in terms of state filling of the states involved in the intense band edge absorption features, and bi-exciton shifting which changes the resonance energy of the probe pulse due to presence of pump induced excitations. Results have been interpreted to show 1) that “hot” excitons will relax to the lowest available levels in the conduction band in ~1 ps, and 2) that said intense band edge exciton transition will be bleached linearly with excitons until the underlying states are completely filled. In the talk we describe a new approach involving “spectator excitons” to test these accepted views. It consists of comparing pump-probe experiments on pristine samples, with equivalent scans conducted on the same sample after it has been saturated in cold mono-excitons. We show how this method has uncovered previously unrecognized spin blockades in the relaxation of hot multi-exciton states in CdSe NCs, and simply detects stimulated emission signals even in presence of overlapping absorption. We report specific difficulties of applying this approach on perovskite crystals leading to controversial determination that in quantum confined CsPbBr3 bi-exciton interactions are positive (repulsive) and describe recent time resolved emission data which challenges this result.
    Lecture