DIFFER Seminar: Exciting Electrons in Optoelectronic Nanomaterials

Abstract: Semiconductor nanomaterials such as quantum dots, nanowires and two-dimensional materials are of interest for optoelectronic applications, including solar cells, photodetectors, LEDs, lasers and photocatalysis. We study electronic excited states (excitons), charge carriers and many-body complexes thereof. Excitons and charge carriers are generated by photoexcitation with ultrashort laser pulses and detected by time-resolved optical absorption and terahertz conductivity measurements. We analyze the measured data on the basis of quantum mechanical calculations of electronic band structures and electron dynamics.

The first part of the talk involves photoexcited electronic states in CdSe nanoplatelets with thickness of a few atomic layers and lateral sizes of tens of nanometers [1]. The motion of excitons in these platelets can be described by the quantum mechanical particle in-a-box model [2]. Surprisingly, excitons appear to be 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 counterintuitive 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 [3,4,5]. The stability of excitons is beneficial for application of CdSe nanoplatelets as laser gain medium or in LEDs.

In a conventional solar cell, the energy of a photon in excess of the band gap is lost as heat. This energy loss can be reduced if photoexcited electrons with sufficient energy excite other electrons across the band gap via impact ionization. We theoretically explain why the impact ionization efficiency measured in the 2D MoTe₂ van der Waals material is much higher than for e.g. PbSe and PbS [6]. Our recent experimental results for MoSe₂ will also be discussed.

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. F. García Flórez et al., Phys. Rev. B 100, 245302(2019).
4. F. García Flórez et al., Phys. Rev. B 102, 115302(2020).
5. M. Failla et al., J. Phys. Chem. C, 127, 1899 (2023).
6. S. Weerdenburg et al., J. Phys.Chem. C, 128, 3693 (2024).