We present a systematic investigation of the electronic properties of bulk and few layer ReS2 van der Waals crystals using low temperature optical spectroscopy. Weak photoluminescence emission is observed from two non-degenerate band edge excitonic transitions separated by ∼ 20 meV. The comparable emission intensity of both excitonic transitions is incompatible with a fully thermalized (Boltzmann) distribution of excitons, indicating the hot nature of the emission. While DFT calculations predict bilayer ReS2 to have a direct fundamental band gap, our optical data suggests that the fundamental gap is indirect in all cases.
@article{arxiv.1811.03461,
title = {Non equilibrium anisotropic excitons in atomically thin ReS$_2$},
author = {J. M. Urban and M. Baranowski and A. Kuc and L. Klopotowski and A. Surrente and Y. Ma and D. Wlodarczyk and A. Suchocki and D. Ovchinnikov and T. Heine and D. K. Maude and A. Kis and P. Plochocka},
journal= {arXiv preprint arXiv:1811.03461},
year = {2018}
}