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 opticalproperties through carrier/exciton-lattice interactions. Previously,the 2D halide-pseudohalide perovskite (MA)2Pb(SCN)2I2(MA+= methylammonium) was demonstrated to have a dynamicallyinhomogeneous lattice with structural evolution occurring on apicosecond time scale. Here, we have investigated two purelyinorganic 2D perovskites, Cs2Pb(SCN)2X2(X = Br or I). 2Dinfrared (2D IR) and polarization-selective pump-probe (PSPP)experiments were used to study dynamics and the effects ofchanging the halide anions. The12CN stretching mode was used asthe vibrational probe. Thefilms were isotopically doped,similar to 99%13CN, to avoid excessive IR absorption, heating, and vibrationalexcitation transfer. PSPP measurements were performed on thinfilms using the reflection geometry to improve the signal-to-noiseratio. The results showed that the12CN vibrational lifetime in Cs2Pb(SCN)2X2is 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 spectroscopyis carried out to measure both spectral diffusion (structural evolution) and homogeneous dephasing of the inhomogeneouslybroadened CN stretch absorption line. The Cs2Pb(SCN)2X22D perovskites displayed substantially slower structural evolutioncompared to (MA)2Pb(SCN)2I2. The spectral diffusion is independent of the halide anion. There is also significant homogeneousdephasing caused by the coupling of the CN stretch to lattice phonons. Ourfindings provide insights into lattice dynamics ofinorganic 2D perovskites and lay a foundation for studies on their complex carrier/exciton-lattice interactions, of importance forapplications in optoelectronic devices