Theoretical study of electronic structure, lone pair localization, and electronic transport properties of unconventional bulk and 2D γ-SnSe and γ-SnS

Tin-based monochalcogenides, particularly SnSe and SnS, are of growing interest due to their cost-effectiveness, environmental compatibility, and exceptional thermoelectric properties. Beyond the conventional α-Pnma phase, these materials can adopt alternative bulk and low-dimensional structures with distinct electronic and transport characteristics. The recent experimental discovery of a layered γ-Pnma SnSe phase in 2023 has further stimulated the search for novel structural allotropes within this family and the assessment of their electronic properties. In this study, we employ density functional theory to examine the structural stability, electronic structure, lone-pair characteristics, and thermoelectric performance of γ-SnS and γ-SnSe in both bulk and two-dimensional (2D) monolayer forms. Our results demonstrate that γ-SnSe and γ-SnS monolayers are thermodynamically stable and can be synthesized via mechanical exfoliation. Electronic structure analysis reveals a substantial band gap expansion in the 2D monolayers, increasing by a factor of 4 to 20 compared to the bulk. A detailed investigation of localized lone pairs in the 2D monolayers identifies two distinct p-state contribution schemes for γ- and α-monolayers, with a notable involvement of the Sn 5p state. Additionally, both bulk and monolayer γ-SnSe and γ-SnS exhibit large Seebeck coefficients and power factors, comparable to or exceeding those of the conventional α-Pnma phases.