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

Abstract

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.

Structural Dynamics of a Novel Pseudohalide Perovskite Cs2Pb(SeCN)2Br2 Investigated with Nonlinear Infrared Spectroscopy

“…The PSPP experiments were conducted in the small incidence angle reflection geometry, which preserves the relative polarizations of the pulse when they enter the sample. 20 In this near-normal reflection geometry, the signal is enhanced ∼5 compared to the usual transmission detection. 3.2.…”

“…While it is not as evident in the spectrum of the SeCN-perovskite, it was found for the SCN-perovskite, that the low frequency shoulder is caused by a small side peak. 20 The side peak is shifted from the main peak by ∼5 cm −1 with a slightly larger FWHM. Temperature dependent FT-IR spectra on the SCN-perovskite explicated the nature of the side peak and provided information on its nature.…”

“…Temperature dependent FT-IR spectra on the SCN-perovskite explicated the nature of the side peak and provided information on its nature. 20 At low temperature (120 K), the side peak was too small to detect. As the temperature was increased, the side peak grew in.…”

“…The method of addressing this issue has been developed in previous publications, 20,37,38 and applied to the SCN-perovskite. 20 The observed parallel and perpendicular signals for the ensemble of tilted cones are…”

“…19 We extended the studies of structural dynamics to the purely inorganic perovskites Cs 2 Pb(SCN) 2 Br 2 and Cs 2 Pb(SCN) 2 I 2 , which revealed that the spectral diffusion is slower in these perovskites with time constants of 25 and 20 ps, respectively. 20 The CH 3 NH 3 + cation has orientational degrees of freedom, which Cs + does not have. The loss of these orientational degrees of freedom in the Cs + perovskite may result in less dynamic inhomogeneity.…”