Interfacial chalcogen pair-mediated, biaxial strain- and interlayer distance-tuneable type-2 band alignment in an SnSSe homogeneous bilayer: a density functional theory-based analysis

This study presents a comprehensive density functional theory-based investigation of the electronic properties of homogeneous bilayer Janus tin sulphide selenide (SnSSe) with different interfacial chalcogen pair configurations. This study focuses on interlayer band alignment, wherein the effects of interlayer distance modulation and applied biaxial strain are methodically studied. Next, the spatial distribution and lifetime of excitonic states are systematically analysed from the in-plane and inter-layer atomic orbital interactions, spatial separation of electron-hole wave functions, and spread of the conduction band in energy. Asymmetric interfacial chalcogen pairing exhibits a finite interlayer dipole moment and interlayer electric field. This reduces the energy bandgap and induces prominent type-2 band alignment between individual SnSSe layers, where band edge Bloch states are strongly localized in alternating layers. An increasing interlayer distance and applied biaxial tensile strain were found to retain the type-2 interlayer band alignment. In contrast, reducing the interlayer distance and biaxial compressive strain significantly altered interlayer atomic orbital interactions and annihilated the type-2 band alignment with a sharp reduction in the energy bandgap owing to the strong delocalization of band edge Bloch states. Thus, this study demonstrates the strain tuneable type-2 band alignment and interlayer excitonic properties of homogeneous bilayer SnSSe with broken interlayer mirror symmetry.