  Atomically thin, two-dimensional (2D) indium selenide (InSe) has attracted
considerable attention due to large tunability in the band gap (from 1.4 to 2.6
eV) and high carrier mobility. The intriguingly high dependence of band gap on
layer thickness may lead to novel device applications, although its origin
remains poorly understood, and generally attributed to quantum confinement
effect. In this work, we demonstrate via first-principles calculations that
strong interlayer coupling may be mainly responsible for this phenomenon,
especially in the fewer-layer region, and it could also be an essential factor
influencing other material properties of {\beta}-InSe and {\gamma}-InSe.
Existence of strong interlayer coupling manifests itself in three aspects: (i)
indirect-to-direct band gap transitions with increasing layer thickness; (ii)
fan-like frequency diagrams of the shear and breathing modes of few-layer
flakes; (iii) strong layer-dependent carrier mobilities. Our results indicate
that multiple-layer InSe may be deserving of attention from FET-based
technologies and also an ideal system to study interlayer coupling, possibly
inherent in other 2D materials.
