Biaxial strain engineering of the band structure in atomically thin InSe

InSe emerges as an attractive two-dimensional semiconductor with potential applications in the near-infrared spectral range, featuring strongly layer-dependent direct bandgaps, prominent P-band emission, and high carrier mobilities. Owing to the superior mechanical flexibility of InSe, strain has been proposed as an effective way to modulate its electronic and optical properties. Here, we engineer the band structure of few-layer and bulk InSe with the thickness down to trilayers by biaxial compressive strain, enabled through thermal contraction of the substrate. We observe a significant bandgap tunability, with the blueshift rate of the bandgap transition (A transition) in the photoluminescence spectra as large as ∼200 meV per percent strain, almost twice of that obtained in the case of uniaxial straining. The biaxial strain effects show no signs of obvious layer dependence for InSe from trilayers to bulk. Moreover, we are surprised to find that the B transition with a deeper valence band involved is more sensitive to temperature than the A transition. Our results pave the way for atomically thin InSe in the applications of strain tunable optoelectronic devices and sensitive strain sensors.