  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 (DFT) 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.
