Fig. 2: First principles studies of BiOI.
From: Layered BiOI single crystals capable of detecting low dose rates of X-rays
a, Phonon dispersion curve of bulk BiOI. Red circles indicate the two A1g phonon modes QA and QB. b Fully relativistic single-shot GW band structure of BiOI. The highest occupied state is set to 0 eV c Top: spatial distribution of the electron constituting the lowest-lying direct exciton when the hole (red square) is fixed on an iodine atom. Bottom: spatial distribution of the hole constituting the lowest-lying direct exciton when the electron (blue square) is localised on an oxygen atom. d Two-dimensional cut of the potential energy surface corresponding to the lowest-lying direct exciton. The axes correspond to the positive displacements along the Raman active modes illustrated on the right-hand side, with the minimum labelled by a grey cross. QA = 10 corresponds to the collective displacement of all I atoms out of the layer by c = 0.63 Å, while QB = 10 corresponds to the displacement of Bi atoms into the layer by c = 0.27 Å. e Relaxation coordinate diagram illustrating the lowering of the exciton energy upon distortion along Raman modes QA and QB. It is evident that the renormalisation (stabilisation) of the excited state (upon distortion) is much smaller than the destabilisation of the ground state. The energy minimum of the excited state is at QA = 9, QB = 6. Inset: Electron density of the excited state after structural displacement along the relaxation coordinate highlights that the wavefunction remains fully delocalised. The corresponding hole wavefunction is depicted in the Supplementary Fig. 31.
