  Due to the strong quantum confinement effect, few-layer {\gamma}-InSe
exhibits a layer-dependent bandgap, spanning the visible and near infrared
regions, and thus recently draws tremendous attention. As a two-dimensional
material, the mechanical flexibility provides an additional tuning knob for the
electronic structure. Here, for the first time, we engineer the band structures
of few-layer and bulk-like InSe by uniaxial tensile strain, and observe salient
shift of photoluminescence (PL) peaks. The shift rate of the optical gap is
approximately 90-100 meV per 1% strain for 4- to 8-layer samples, which is much
larger than that for the widely studied MoS2 monolayer. Density functional
calculations well reproduce the observed layer-dependent bandgaps and the
strain effect, and reveal that the shift rate decreases with increasing layer
number for few-layer InSe. Our study demonstrates that InSe is a very versatile
2D electronic and optoelectronic material, which is suitable for tunable light
emitters, photo-detectors and other optoelectronic devices.
