Dual Photoluminescence in Low-Temperature Phase of CsSnI₃ Nanocrystals

The expression of metal lone-pair electrons is hypothesized to underpin many of the interesting properties of inorganic halide perovskite semiconductors. Recently, a stable low-temperature monoclinic polar phase was predicted for CsSnBr3 and CsSnI3, opening the possibility of direct investigation of a ferroelectric distorted structure compared to the undistorted structure. To date, there have been no experimental reports of such a structure in CsSnI3, and the low-temperature optical properties of CsSnI3 nanocrystals have remained unexplored. Here we report optical and structural evidence of a phase transition around 240 K in 8.9 nm CsSnI3 nanocrystals. Several changes in optical behavior occur below this transition point, including high-energy photoluminescence (PL) that emits concurrently with the exciton PL. The emergence of this high-energy PL is correlated with X-ray diffraction (XRD) and differential scanning calorimetry (DSC) supporting a phase transition from the orthorhombic structure between 240-200 K. Transient absorption measurements show an increase in the excited state lifetimes, i.e., slowed carrier cooling, at 200 K when photoexciting with photon energies above the high-energy state, consistent with slowed carrier cooling and emergence of high-energy PL. We hypothesize that the slowed carrier cooling is distinctive to this phase transition that modifies both the electronic and phonon structures that dictate excited-state carrier dynamics, and we discuss these changes.