Tunable indirect-direct transition of few-layer SnSe via interface engineering.

Tin selenide (SnSe) is one of the best thermoelectric materials reported to date. The possibility of growing few-layer SnSe helped boost the interest in this long-known, earth abundant material. Pristine SnSe in bulk, mono- and few-layer forms are reported to have indirect electronic bandgaps. Possible indirect-direct transition in SnSe is attractive for its optoelectronic-related applications. Based on the results from first principles density functional theory calculations, we carefully analyzed electronic band structures of bulk, and bilayer SnSe with various interlayer stackings. We report the possible stacking-dependent indirect-direct transition of bilayer SnSe. By further analysis, our results reveal that it is the directionality of interlayer interactions that determine the critical features of their electronic band structures. In fact, by engineering the interface stacking between layers, it is possible to achieve few-layer SnSe with direct electronic band gap. This study provides fundamental insights to design few-layer SnSe and SnSe heterostructures for electronic/optoelectronic applications, where the interface geometry plays a fundamental role in device performance.