Quantum-Disk Excitons and Their Interaction with Phonons in BiI3

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  • Kim DaeGwi
    Department of Physics, Graduate School of Science, Osaka City University
  • Karasawa Tsutomu
    Department of Physics, Graduate School of Science, Osaka City University
  • Iida Takeshi
    Department of Physics, Graduate School of Science, Osaka City University
  • Komatsu Teruo
    Department of Physics, Graduate School of Science, Osaka City University

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  • Quantum-Disk Excitons and Their Interaction with Phonons in BiI<sub>3</sub>

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We investigate excitonic transitions and exciton–phonon interactions in disk-shaped microcrystallites of layered BiI3 in CdI2 matrices. We have observed the optical spectra and analyze the results theoretically introducing a quantum disk model of the disk-shaped microcrystallites. The absorption band of the excitons confined in the microcrystallite has been observed in the energy region higher than the bulk exciton transition energy. For the resonant excitations within the absorption band, luminescence bands have been observed with Stokes shifts whose magnitudes are different depending on the excitation energies; the Stokes shift decreases with increasing the excitation energy. This is the first observation of the luminescence from the relaxed state of exciton, that is, the self-trapped exciton (STE) state in the microcrystallite. We show that the observed absorption and luminescence spectra can be reproduced by the model calculations with taking into account the disk-size dependent exciton–phonon interaction and the disk-size distribution observed in our previous work. From the temporal behavior of the luminescence, we find that the decay-time constant becomes shorter with the decrease in the detecting energy, while it becomes longer with decreasing temperature especially below 4.2 K. It is shown that such anomalous behavior of the size and temperature dependence of the decay time constant are well described in terms of the spin-multiplet splitting of the STE states and the size-dependent acoustic phonon interaction.

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