  InSe, a member of the layered materials family, is a superior electronic and
optical material which retains a direct bandgap feature from the bulk to
atomically thin few-layers and high electronic mobility down to a single layer
limit. We, for the first time, exploit strain to drastically modify the bandgap
of two-dimensional (2D) InSe nanoflakes. We demonstrated that we could decrease
the bandgap of a few-layer InSe flake by 160 meV through applying an in-plane
uniaxial tensile strain to 1.06% and increase the bandgap by 79 meV through
applying an in-plane uniaxial compressive strain to 0.62%, as evidenced by
photoluminescence (PL) spectroscopy. The large reversible bandgap change of ~
239 meV arises from a large bandgap change rate (bandgap strain coefficient) of
few-layer InSe in response to strain, ~ 154 meV/% for uniaxial tensile strain
and ~ 140 meV/% for uniaxial compressive strain, representing the most
pronounced uniaxial strain-induced bandgap strain coefficient experimentally
reported in two-dimensional materials.We developed a theoretical understanding
of the strain-induced bandgap change through first-principles DFT and GW
calculations. We also confirmed the bandgap change by photoconductivity
measurements using excitation light with different photon energies. The highly
tunable bandgap of InSe in the infrared regime should enable a wide range of
applications, including electro-mechanical, piezoelectric and optoelectronic
devices.
