Probing Lattice Dynamics in Two-Dimensional Inorganic Pseudohalide Perovskites with Ultrafast Infrared Spectroscopy

Dynamic lattice disorder in two-dimensional (2D) halide perovskites can play an important role in their optical properties through carrier/exciton–lattice interactions. Previously, the 2D halide–pseudohalide perovskite (MA)2Pb­(SCN)2I2 (MA+ = methylammonium) was demonstrated to have a dynamically inhomogeneous lattice with structural evolution occurring on a picosecond time scale. Here, we have investigated two purely inorganic 2D perovskites, Cs2Pb­(SCN)2X2 (X = Br or I). 2D infrared (2D IR) and polarization-selective pump–probe (PSPP) experiments were used to study dynamics and the effects of changing the halide anions. The 12CN stretching mode was used as the vibrational probe. The films were isotopically doped, ∼99% 13CN, to avoid excessive IR absorption, heating, and vibrational excitation transfer. PSPP measurements were performed on thin films using the reflection geometry to improve the signal-to-noise ratio. The results showed that the 12CN vibrational lifetime in Cs2Pb­(SCN)2X2 is significantly longer than that in (MA)2Pb­(SCN)2I2, and the lifetimes were essentially the same for Br and I analogues of Cs2Pb­(SCN)2X2. 2D IR spectroscopy is carried out to measure both spectral diffusion (structural evolution) and homogeneous dephasing of the inhomogeneously broadened CN stretch absorption line. The Cs2Pb­(SCN)2X2 2D perovskites displayed substantially slower structural evolution compared to (MA)2Pb­(SCN)2I2. The spectral diffusion is independent of the halide anion. There is also significant homogeneous dephasing caused by the coupling of the CN stretch to lattice phonons. Our findings provide insights into lattice dynamics of inorganic 2D perovskites and lay a foundation for studies on their complex carrier/exciton–lattice interactions, of importance for applications in optoelectronic devices.