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Layered hybrid perovskites have attracted much attention in recent years due to their emergent physical properties and exceptional functional performances, but the coexistence of lattice order and structural disorder severely hinders our understanding of these materials. One unsolved problem regards how the lattice dynamics are affected by the dimensional engineering of the inorganic frameworks and the interaction with the molecular moieties. Here, we address this question by using a combination of high-resolution spontaneous Raman scattering, high-field terahertz spectroscopy, and molecular dynamics simulations. This approach enables us to reveal the structural vibrations and disorder in and out of equilibrium and provides surprising observables that differentiate single- and double-layered perovskites. While no distinct vibrational coherence is observed in double-layer perovskites, we discover that an off-resonant terahertz pulse can selectively drive a long-lived coherent phonon mode through a two-photon process in the single-layered system. This difference highlights the dramatic change in the lattice environment as the dimension is reduced. The present findings pave the way for the ultrafast structural engineering of hybrid lattices as well as for developing high-speed optical modulators based on layered perovskites.

We report measurements of the heat capacity and cross-plane thermal conductivity of 2D (CxH2x+1NH3)2[MAPbBr3]n-1PbBr4 (MA = methylammonium) lead bromide perovskites (2D LHPs) at room temperature as a function of both the octahedral layer thickness (n = 1,2,3) and the organic spacer chain length (x=4,5,6,7,8) using differential scanning calorimetry (DSC) and frequency domain thermoreflectance (FDTR) respectively. We observe ultralow thermal conductivities (0.18-0.51 W/m K) for all 2D LHPs studied, but surprisingly minimal suppression of thermal conductivity with respect to bulk MAPbBr3 (0.5 W/m K). Cross-plane thermal conductivity is found to increase monotonically as a function of both the octahedral layer thickness (0.18-0.26 W/m K for n=1-3) and the organic chain length (0.18-0.51 W/m K for x=4-8). Additionally, we measure heat capacities that are well described by composite theory, suggesting bulk-like phonon density-of-states within the separate organic and inorganic subphases of the layered structure. The striking observation of increasing thermal conductivity with increasing organic phase fraction (i.e. increasing organic chain length) indicates efficient thermal transport along the ordered alkyl chain backbone. Our experimental results agree most closely with a predictive model of ballistic phonon transport with diffuse interface scattering - rather than normal thermal conduction within each phase. This study indicates the potential for synthesizing 2D LHPs with thermal conductivity that exceeds the bulk perovskite phase, while also shedding light on relevant phonon transport pathways in 2D LHPs.