title: Real-Time Observation of Organic Cation Reorientation in Methylammonium Lead Iodide Perovskites.
abstract: The introduction of a mobile and polarized organic moiety as a cation in 3D lead-iodide perovskites brings fascinating optoelectronic properties to these materials. The extent and the time scales of the orientational mobility of the organic cation and the molecular mechanism behind its motion remain unclear, with different experimental and computational approaches providing very different qualitative and quantitative description of the molecular dynamics. Here we use ultrafast 2D vibrational spectroscopy of methylammonium (MA) lead iodide to directly resolve the rotation of the organic cations within the MAPbI3 lattice. Our results reveal two characteristic time constants of motion. Using ab initio molecular dynamics simulations, we identify these as a fast (∼300 fs) "wobbling-in-a-cone" motion around the crystal axis and a relatively slow (∼3 ps) jump-like reorientation of the molecular dipole with respect to the iodide lattice. The observed dynamics are essential for understanding the electronic properties of perovskite materials.
authors.affiliation: FOM Institute AMOLF , Science Park 104, Amsterdam, 1098 XG, The Netherlands.|Cavendish Laboratory, University of Cambridge , JJ Thomson Avenue, Cambridge CB30HE, United Kingdom.
authors.forename: Artem A
authors.initials: AA
authors.lastname: Bakulin
authors.affiliation: FOM Institute AMOLF , Science Park 104, Amsterdam, 1098 XG, The Netherlands.
authors.forename: Oleg
authors.initials: O
authors.lastname: Selig
authors.affiliation: FOM Institute AMOLF , Science Park 104, Amsterdam, 1098 XG, The Netherlands.
authors.forename: Huib J
authors.initials: HJ
authors.lastname: Bakker
authors.affiliation: FOM Institute AMOLF , Science Park 104, Amsterdam, 1098 XG, The Netherlands.
authors.forename: Yves L A
authors.initials: YL
authors.lastname: Rezus
authors.affiliation: Institute for High-Frequency Technology, TU Braunschweig , Schleinitzstr. 22, 38106 Braunschweig, Germany.|InnovationLab GmbH , Speyerer Str. 4, 69115 Heidelberg, Germany.|Kirchhoff-Institute for Physics, Heidelberg University , Im Neuenheimer Feld 227, D-69120 Heidelberg, Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Müller
authors.affiliation: InnovationLab GmbH , Speyerer Str. 4, 69115 Heidelberg, Germany.|Kirchhoff-Institute for Physics, Heidelberg University , Im Neuenheimer Feld 227, D-69120 Heidelberg, Germany.
authors.forename: Tobias
authors.initials: T
authors.lastname: Glaser
authors.affiliation: Institute for High-Frequency Technology, TU Braunschweig , Schleinitzstr. 22, 38106 Braunschweig, Germany.|InnovationLab GmbH , Speyerer Str. 4, 69115 Heidelberg, Germany.
authors.forename: Robert
authors.initials: R
authors.lastname: Lovrincic
authors.affiliation: LPEM-UMR 8213, ESPCI-ParisTech/CNRS/Université Pierre et Marie Curie , 10 Rue Vauquelin, 75005 Paris, France.
authors.forename: Zhenhua
authors.initials: Z
authors.lastname: Sun
authors.affiliation: LPEM-UMR 8213, ESPCI-ParisTech/CNRS/Université Pierre et Marie Curie , 10 Rue Vauquelin, 75005 Paris, France.
authors.forename: Zhuoying
authors.initials: Z
authors.lastname: Chen
authors.affiliation: Department of Chemistry, University of Bath , Claverton Down, Bath BA2 7AY, United Kingdom.
authors.forename: Aron
authors.initials: A
authors.lastname: Walsh
authors.affiliation: Department of Chemistry, University of Bath , Claverton Down, Bath BA2 7AY, United Kingdom.
authors.forename: Jarvist M
authors.initials: JM
authors.lastname: Frost
authors.affiliation: Zernike Institute for Advanced Materials, University of Groningen , Nijenbough 4, 9747 AG, Groningen, The Netherlands.
authors.forename: Thomas L C
authors.initials: TL
authors.lastname: Jansen
keywords: methylammonium lead iodide; molecular dynamics; transient anisotropy; ultrafast spectroscopy; vibrational spectroscopy