2, Rensselaer Polytechnic Institute, Troy, New York, United States
As a derivative of organic-inorganic halide perovskites (e.g. CH3NH3PbI3), van der Waals Ruddlesdon-Popper perovskites (C4H9NH3)2PbX4 became popular for their self-assembled electronic quantum well structure and huge exciton binding energy. However, electron phonon coupling, a fundamental process that limits various optoelectronic performances, remains obscure, partly due to the lack of systematic studies on high-quality single crystalline samples free from exfoliation/transfer. In this work, we performed temperature-dependent photoluminescence (PL) studies on single crystalline (C4H9NH3)2PbI4 flakes grown by CVD on Si, SiO2/Si or muscovite mica with thickness of about 15-100 nm. Epitaxial relation were observed between perovskite flakes and mica/Si substrates. Due to the substrate effect therein, the structural phase transition at around 240 K were hindered and room-temperature phase were stabilized till liquid nitrogen temperature. By analyzing the temperature-dependent PL line width (FWHM), we showed that the Pb-I longitudinal optical (LO) phonon couples strongly to excitons via Fröhlich mechanism and the coupling strength decreased significantly as flake thickness was reduced. This trend was explained by the LO phonon confinement in out-of-plane direction of perovskite flakes. In contrast, the PL line width of three-dimensional hybrid perovskites CH3NH3PbBr3 increased significantly as flake thickness was reduced to several unit cells. This discrepancy was attributed to the lack of highly polar interface in layered van der Waals perovskites, which in turn reduces interface electron-phonon scattering. Our work demonstrates the controllability in electron-phonon scattering of ultrathin layered van der Waals perovskites and their potential in various optoelectronic applications.