Optical properties of 2D α-In₂Se₃across the low temperature α - α’ phase transition

2D α-In₂Se₃ is an emerging room temperature ferroelectric material with a Mexican hat-shaped valence band and an indirect band gap around 1.35 eV. It undergoes a number of well-studied high temperature phase transitions, from the α – β phase at 473 K, to a γ phase at 650 K, and to a δ phase above 850 K. Electron microscopy studies have also revealed a low temperature transition (α – α’) around 150 K, attributed to a subtle structural distortion. In₂Se₃ retains its ferroelectricity in the low-temperature a’ phase but there are no reports of other properties in this phase. Here, we present low temperature photoluminescence (PL) spectra from bulk single-crystalline α-In₂Se₃ and exfoliated flakes. Upon cooling from room temperature, we observe the expected blueshift of PL emission that changes below 150 K with the appearance of a new peak centered around 1.35 eV. This emission intensity increases significantly below 150 K and persists down to our lowest measured temperature (7 K). Single crystal X-day diffraction measurements reveal a slight change in bond angles and lengths below 150 K that nevertheless causes a dramatic change in the band structure of In₂Se₃. We further explain our findings with the help of temperature-dependent optical transmittance data and density functional theory computations.