QuMat seminar

Abstract:

We studied excitons and charge carriers in CdSe nanoplatelets with thickness of a few atomic layers and lateral sizes of tens of nanometers. Excitons and charge carriers were generated by photoexcitation with ultrashort laser pulses and detected by time-resolved optical absorption and terahertz conductivity measurements.[1] The shape of photoluminescence and absorption spectra varies with the lateral sizes of the nanoplatelets. We attribute this to quantum-confinement effects on the center-of-mass motion of excitons in the plane of the nanoplatelets. The spectra can be reproduced very accurately by a theoretical description of excitons based on the quantum mechanical particle-in-a-box model.[2] The initial photogeneration quantum yields of free charge carriers versus excitons were found to increase with photon energy. Biexcitons and trions were observed due to formation of an exciton by a probe photon near an already present exciton or charge carrier left after the pump laser pulse. After 1 picosecond excitons and charges have relaxed to the same energy distribution, irrespective of the pump photon energy. We found that excitons are stable even at high densities where they start to exhibit spatial overlap. A crossover to an electron-hole plasma of uncorrelated free electrons and holes was not observed. This counter intuitive result can be understood theoretically from the fact that the Coulomb screening length, and thus the exciton binding energy, remain non-zero even at high density.[4,5]

References
[1] R. Tomar et al., J. Phys. Chem. C 123, 9640 (2019)
[2] M. Failla et al., Phys. Rev. B., 102, 195405 (2020)
[3] M. Failla et al., J. Phys. Chem. C, doi.org/10.1021/acs.jpcc.2c07292
[4] F. García Flórez et al., Phys. Rev. B 100, 245302 (2019)
[5] F. García Flórez et al., Phys. Rev. B 102, 115302 (2020)

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