Exploring 2D LaOBr and LaOI monolayers: Stability, electronic, phononic, thermal, and optical properties using DFT and AIMD approaches

This work aims to investigate the stability, electronic, thermal, and optical properties of two-dimensional LaOBr and LaOI monolayers using density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations. The energetic, dynamic, and thermal stability of both monolayers are confirmed via the calculations of formation energy, phonon dispersion, and AIMD, respectively. LaOBr has a larger binding energy, suggesting stronger interlayer interactions and thus higher structural stability. The $d$-orbitals of La atoms are more localized in LaOBr. The La atoms thus do not overlap significantly with orbitals of neighboring atoms reducing the hybridization and broadening of the conduction band. A narrower conduction band typically corresponds to a higher energy gap between the valence and conduction bands, potentially increasing the band gap of LaOBr to 3.89 eV (GGA) and 6.27 eV (HSE06) compared to LaOI with band gap of 3.41 eV (GGA) and 5.27 eV (HSE06). Consequently, the optical conductivity of LaOBr is blue shifted to a higher photon energy with a lower static dielectric function and refractive index. LaOBr and LaOI monolayers exhibit distinct thermal conductivity due to varying group velocities, suggesting their complementary applications from low to high-frequency thermal management. The lattice thermal conductivity of LaOBr is much higher than that of LaOI due to it’s higher group velocity. We confirm that both monolayers could have powerful applications in thermoelectric and optoelectronic devices.