Multiple exciton systems in two-dimensional CdSe nanoplatelets
Laurens Siebbels, Optoelectronic Materials Section, Delft University of Technology, The Netherlands
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]
Photoexcitation leads predominantly to formation of excitons rather than free charge carriers. Interestingly, excitons in CdSe nanosheets 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 is 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.[2] This is a particular result for 2D materials, which does not hold for 3D semiconductors.
We used our theoretical model to calculate the yield of biexcitons as a function of temperature and photoexcitation density. We predict that biexcitons are formed even at room temperature and mutual distance between excitons exceeding their lateral size.[3]
References
[1] R. Tomar et al., J. Phys. Chem. C 123, 9640 (2019)
[2] F. García Flórez et al., Phys. Rev. B 100, 245302 (2019)
[3] F.García Flórezet al., Phys. Rev. B102, 115302(2020)