Abstract
Two-dimensional Ruddlesden–Popper organic–inorganic hybrid layered perovskites (2D RPs) are solution-grown semiconductors with prospective applications in next-generation optoelectronics. The heat-carrying, low-energy acoustic phonons, which are important for heat management of 2D RP-based devices, have remained unexplored. Here we report on the generation and propagation of coherent longitudinal acoustic phonons along the cross-plane direction of 2D RPs, following separate characterizations of below-bandgap refractive indices. Through experiments on single crystals of systematically varied perovskite layer thickness, we demonstrate significant reduction in both group velocity and propagation length of acoustic phonons in 2D RPs as compared to the three-dimensional methylammonium lead iodide counterpart. As borne out by a minimal coarse-grained model, these vibrational properties arise from a large acoustic impedance mismatch between the alternating layers of perovskite sheets and bulky organic cations. Our results inform on thermal transport in highly impedance-mismatched crystal sub-lattices and provide insights towards design of materials that exhibit highly anisotropic thermal dissipation properties.
Highlights
Two-dimensional Ruddlesden–Popper organic–inorganic hybrid layered perovskites (2D RPs) are solution-grown semiconductors with prospective applications in next-generation optoelectronics
We investigate coherent longitudinal acoustic phonons (CLAPs) in a series of 2D RPs, (CH3(CH2)3NH3)2(CH3NH3)N−1 PbNI3N+1, or (BA)2(MA)N-1PbNI3N+1 (N = 1–6), which are denoted as N = 1–6 for short
We find 2D RPs exhibit significantly lower group velocities compared to the 3D counterpart, methylammonium lead iodide (MAPbI3), which corresponds to N = ∞
Summary
Two-dimensional Ruddlesden–Popper organic–inorganic hybrid layered perovskites (2D RPs) are solution-grown semiconductors with prospective applications in next-generation optoelectronics. Through experiments on single crystals of systematically varied perovskite layer thickness, we demonstrate significant reduction in both group velocity and propagation length of acoustic phonons in 2D RPs as compared to the three-dimensional methylammonium lead iodide counterpart. As borne out by a minimal coarse-grained model, these vibrational properties arise from a large acoustic impedance mismatch between the alternating layers of perovskite sheets and bulky organic cations. The interlocked soft organic and hard inorganic sublattices raise intriguing questions regarding the existence and properties of low-energy vibrational excitations, and their ability to carry heat along the cross-plane direction of these highly anisotropic materials with natural quantum-well-like electronic structures. We expect our findings based on such model material class to be applicable to a broader class of systems, such as nanocrystal arrays[26], biological organic–inorganic interfaces[27], and self-assembled monolayers on inorganic solids[28,29]
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