Electronic and optical properties of two-dimensional flat band triphosphides

Abstract

In this work we use first-principles density-functional theory (DFT) calculations combined with the maximally localized Wannier function tight binding Hamiltonian (MLWF-TB) and Bethe-Salpeter equation (BSE) formalism to investigate quasi-particle effects in 2D electronic and optical properties of triphosphide based two-dimensional materials XP$_3$ (X = Ga, Ge, As; In, Sn, Sb; Tl, Pb and Bi). We find that with exception of InP$_3$, all structures have indirect band gap. A noticeable feature is the appearance of flat valence bands associated to phosphorous atoms, mainly in InP$_3$ and GaP$_3$ structures. Furthermore, AIMD calculations show that 2D-XP$_3$ is stable at room temperature, with exception of TlP$_3$ monolayer, which shows a strong distortion yielding to a phase separation of the P and Tl layers. Finally, we show that monolayered XP$_3$ exhibits optical absorption with strong excitonic effects, thus revealing exciting features of these monolayered materials.