Nano@Tech Fall 2022 Series | Exciton Polarons in Two-Dimensional Hybrid Metal-Halide Perovskites

Carlos Silva | Professor; Chemistry and Biochemistry at Georgia Tech

Abstract: While polarons --- charges bound to a lattice deformation induced by electron-phonon coupling --- are primary photoexcitations at room temperature in bulk metal-halide hybrid organic-inorganic perovskites (HOIP), excitons --- Coulomb-bound electron-hole pairs --- are the stable quasi-particles in their two-dimensional (2D) analogues. Here we address the fundamental question: are polaronic effects consequential for excitons in 2D-HIOPs? Based on our recent work, we argue that polaronic effects are manifested intrinsically in the exciton spectral structure, which is comprised of multiple non-degenerate resonances with constant inter-peak energy spacing. We highlight measurements of population and dephasing dynamics that point to the apparently deterministic role of polaronic effects in excitonic properties. We contend that an interplay of long-range and short-range exciton-lattice couplings give rise to exciton polarons, a character that fundamentally establishes their effective mass and radius, and consequently, their quantum dynamics.  Given this complexity, a fundamentally far-reaching issue is how Coulomb-mediated many-body interactions---elastic scattering such as excitation-induced dephasing, inelastic exciton bimolecular scattering, and multi-exciton binding---depend upon the specific exciton-lattice coupling within the structured excitation lineshape. We measure the intrinsic and density-dependent exciton dephasing rates of the multiple excitons and their dependence on temperature by means of two-dimensional coherent excitation spectroscopy. We find that diverse excitons display distinct intrinsic dephasing rates mediated by phonon scattering involving different effective phonons, and contrasting rates of exciton-exciton elastic scattering. These findings establish specifically the consequence of distinct lattice dressing on exciton many-body quantum dynamics, which critically define fundamental optical properties that underpin photonics and quantum optoelectronics.

Bio: Carlos Silva earned a Ph.D. in Chemical Physics from the University of Minnesota in 1998 and was then a Postdoctoral Associate in the Cavendish Laboratory, University of Cambridge. In 2001 he became an EPSRC Advanced Research Fellow in the Cavendish Laboratory, and Research Fellow in Darwin College, Cambridge. In 2005, he joined the Université de Montréal as an Assistant Professor, where he held the Canada Research Chair in Organic Semiconductor Materials from 2005 to 2015 and a Université de Montréal Research Chair from 2014 to 2017. He joined Georgia Tech in 2017, where he is currently Professor with joint appointment in the School of Chemistry and Biochemistry and the School of Physics, and Professor by Courtesy Appointment in the School of Materials Science and Engineering. He is also Honorary Professor in the Department of Applied Physics of the Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional (CINVESTAV Unidad Merida). He is a Fellow of the American Physical Society and the Royal Society of Chemistry. His group focuses on optical and electronic properties of organic and hybrid semiconductor materials, mainly probed by nonlinear ultrafast spectroscopies.