Observation of elastic heterogeneity and phase evolution in 2D layered perovskites using coherent acoustic phonons

Two-dimensional (2D) organic–inorganic perovskites have shown interesting optical properties due to the natural quantum-well structures. The repetition of soft organic and hard inorganic intercalations also renders 2D perovskites rich phonon dynamics. Here, we investigated the coherent acoustic phonons in (PEA)2PbI4 perovskite films by time-resolved Brillouin spectroscopy. The coherent acoustic phonons were launched indirectly in perovskite films by exciting Au nanoplates which were used as optoacoustic transducers. A longitudinal sound velocity ν = 1937 ± 31 m/s, and an elastic modulus E = 9.84 GPa along the cross-plane direction of perovskites were obtained from analysis of the Brillouin oscillation frequency. Following a bead-spring model, we calculated a spring constant k ≈ 1.709 N m−1 for PEA cations which is comparably small for perovskites. We also demonstrated that coherent acoustic phonons are sensitive to differentiate structural heterogeneity and monitor dynamic phase evolution in perovskite films. Domains of PbI2-rich and PbI2-poor phases were identified. Under light stimulus, PbI2-poor phases were gradually disappearing and PbI2-rich phases became crystallized. The observations of structural and elastic heterogeneity and dynamic phase evolution using coherent acoustic phonons provide a toolbox for submicroscale elastic characterization of perovskites.