2019/12/24 by Yuri D. Glinka, Glinka, Yuri D., Rui Cai +9 · 1 citation
Engineering · Materials Science · #Perovskite Materials and Applications #Solid-state spectroscopy and crystallography #Crystal Structures and Properties
paper · pdf · doi:10.48550/arxiv.1912.11295
Although the structural phase transitions in single-crystal hybrid\nmethyl-ammonium (MA) lead halide perovskites (MAPbX3, X = Cl, Br, I) are common\nphenomena, they have never been observed in the corresponding nanocrystals.\nHere we demonstrate that two-photon-excited photoluminescence (PL) spectroscopy\nis capable of monitoring the structural phase transitions in MAPbX3\nnanocrystals because nonlinear susceptibilities govern the light absorption\nrates. We provide experimental evidence that the orthorhombic-to-tetragonal\nstructural phase transition in a single layer of 20-nm-sized 3D MAPbBr3\nnanocrystals is spread out within the 70 - 140 K range. This structural phase\ninstability range arises because, unlike in single-crystal MAPbX3, free\nrotations of MA ions in the corresponding nanocrystals are no longer restricted\nby a long-range MA dipole order. The resulting configurational entropy loss can\nbe even enhanced by the interfacial electric field arising due to charge\nseparation at the MAPbBr3/ZnO heterointerface, extending the\northorhombic-to-tetragonal structural phase instability range from 70 to 230 K.\nWe conclude that the weak sensitivity of conventional one-photon-excited PL\nspectroscopy to the structural phase transitions in 3D MAPbX3 nanocrystals\nresults from the structural phase instability providing negligible distortions\nof PbX6 octahedra. In contrast, the intensity of two-photon-excited PL and\nelectric-field-induced one-photon-excited PL still remains sensitive enough to\nweak structural distortions due to the higher rank tensor nature of nonlinear\nsusceptibilities involved. We also show that room-temperature PL originates\nfrom the radiative recombination of the optical-phonon vibrationally excited\npolaronic quasiparticles with energies might exceed the ground-state Frohlich\npolaron and Rashba energies due to optical-phonon bottleneck.\n