A first principles study on the stability and electronic and optical properties of 2D SbXY (X = Se/Te and Y = I/Br) Janus layers†

Motivated by the exceptional optoelectronic properties of 2D Janus layers (JLs), we explore the properties of group Va antimony-based JLs SbXY (X = Se/Te and Y = I/Br). Using Bader charges, the electric dipole moment in the out-of-plane direction of all the JLs is studied and the largest dipole moment is found to be in the SbSeI JL. Our results on the formation energy, phonon spectra, elastic constants, and ab initio molecular dynamics (AIMD) simulation provide insights into the energetic, vibrational, mechanical, and thermal stability of JLs. After confirming the stability, the three-dimensional phase diagram is investigated to propose the experimental conditions required to fabricate the predicted JLs. Then, the electronic band structure is calculated using different levels of theory, namely, the generalized gradient approximation (GGA), GGA + spin–orbit coupling (GGA + SOC), hybrid Heyd–Scuseria–Ernzerhof (HSE) functional, and many-body perturbation theory-based Green's function method (GW). According to the HSE results, JLs show band gaps between 1.653 and 1.852 eV. The GGA + SOC calculations reveal Rashba spin splitting in these JLs. The calculated carrier mobility using deformation potential theory shows that the electrons have exceptionally high mobility compared to holes, which assists the spatial separation of both charge carriers. The optical spectra are determined using GGA, HSE, and GW methods. With respect to GGA results, HSE and GW optical spectra show a blue shift. More accurate calculations using the GW–Bethe Salpeter equation (BSE) yield optical absorption spectra that are dominated by strong excitonic effects with the excitonic binding energy (BE_ex) in the range of 550–800 meV. Compared to the GW–BSE method, the Mott–Wannier (MW) model predicts a lower BE_ex. A strong e–h coupling is observed for dispersions along K−M in the Brillouin zone from the fat band analysis. Our study suggests that the SbSeI JL is a potential candidate for photocatalytic and photovoltaic applications due to its largest dipole moment and low excitonic binding energy.