Ab initio investigation of spin–orbit coupling on structural, electronic, and optical properties for quaternary chalcogenide 2D-layered ACu2BS3 (A = K, Na; B = Bi, Sb) compounds

Abstract

Two-dimensional (2D) materials have triggered broad interest owing to their unique physical and chemical properties that have pioneering applications in electronic and optical devices. In this study, a group of quaternary chalcogenide 2D-layered ACu₂BS₃ (A = K, Na; B = Bi, Sb) compounds are investigated using ab initio density functional theory calculations. The first-principles calculations are computed by generalised gradient approximation (GGA), such as Perdew–Burke–Ernzerhof (PBE) exchange–correlation functions, with the incorporation of spin–orbit coupling (SOC). The structural, electronic, and optical properties are investigated systematically, revealing that all these compounds exhibit direct band gap nature. However, the inclusion of SOC in KCu₂BiS₃ (KCBS) and NaCu₂BiS₃ (NCBS) compounds results in an indirect band gap. The change in the band gap from direct to indirect arises with the effective splitting of Bi-p and S-p states in the conduction band (CB) offset. The novel alkali-metal Na⁺ compounds, NaCu₂BiS₃ (NCBS) and NaCu₂SbS₃ (NCSS), exhibit better structural, electronic, and optical properties than the existing K⁺, KCu₂BiS₃ (KCBS) and KCu₂SbS₃ (KCSS) compounds. Thus, gaining theoretical insights into novel 2D materials paves the way for the synthesis and fabrication of photovoltaic cells (PVC) in the future.