Optoelectronic properties of boron monochalcogenide monolayers: Quasiparticle and excitonic effects from first principles

We investigate the linear optical response and excitonic landscape in boron monochalcogenide (BX, X = S, Se, Te) single layers using ab initio many-body perturbation theory. These 2D monochalcogenides are wide band gap semiconductors, with the valence band exhibiting a quasiflat caldera-shaped dispersion in BS and BSe sheets, associated with strong van Hove singularities at the Fermi level in the density of states, an electronic feature that plays a crucial role in the emergence of strong excitonic effects. By solving the Bethe-Salpeter equation on top of G₀W₀ quasiparticle energies, our results reveal that bound excitons arise from direct optical transitions between the highest occupied band and the lowest unoccupied band along the Γ−M and Γ−K paths. Additionally, in BS and BSe monolayers, we identify excitons that are bright for in-plane polarized incident light while becoming dark for out-of-plane polarization, and other excitons with the opposite behavior. The optical selection rules are described using group-theory analysis of wave-function symmetries, determining whether optical transitions are dipole allowed or forbidden. Furthermore, exciton radiative lifetimes are estimated to range from 0.2 ns to 1.6 ns at room temperature, while exciton binding energies are significantly high, ranging from 0.6 eV to 1.2 eV for both indirect ground-state excitons and zero-momentum direct excitons. Finally, the strong electron-hole interactions in these materials lead to the formation of tightly bound excitons with a small radius, paving the way for excitonic Bose-Einstein condensation in BX monolayers. Our study sheds light on the complex excitonic features of single-layer BX, emphasizing its potential for cutting-edge applications in exciton-driven optoelectronics and quantum technologies.