Physics at Virginia

"Exciton polarons in two-dimensional organic-inorganic hybrid perovskites"

Professor Carlos Silva , Georgia Tech
[Host: Seunghun Lee]

Owing to both electronic and dielectric confinement effects, two-dimensional organic-inorganic hybrid perovskites sustain strongly bound excitons at room temperature. In this seminar, we demonstrate that there are non-negligible contributions to the excitonic correlations that are specific to the lattice structure and its polar fluctuations, both of which are controlled via the chemical nature of the organic counter-cation. In these systems, organic cations not only serve as spacers between slabs consisting of corner-sharing metal-halide octahedra, but also determine lattice structure by inducing varying degree of distortion of the octahedra via the organic-inorganic interactions. We present a phenomenological yet quantitative framework to simulate excitonic absorption line shapes in single-layer organic-inorganic hybrid perovskites, based on the two-dimensional Wannier formalism. We include four distinct excitonic states separated by 35±5 meV, and additional vibronic progressions. Intriguingly, the associated Huang-Rhys factors and the relevant phonon energies show substantial variation with temperature and the nature of the organic cation. This points to the hybrid nature of the line shape, with a form well described by a Wannier formalism, but with signatures of strong coupling to localized vibrations, and polaronic effects perceived through excitonic correlations. Furthermore, by means of high-resolution resonant impulsive stimulated Raman spectroscopy, we identify vibrational wavepacket dynamics that evolve along different configurational coordinates for distinct excitons and photocarriers. Employing density functional theory calculations, we assign the observed coherent vibrational modes to various low-frequency (≲50 cm−1) optical phonons involving motion in the lead iodide layers. This supports our conclusion that different excitons induce specific lattice reorganizations, which are signatures of polaronic binding. Excitonic correlations (exciton and biexciton binding energies) and exciton dynamics (e.g. uni- and bimolecular population decay mechanisms, pure dephasing processes, excitation-induced dephasing, etc.) reflect the polar solvation-like processes induced by organic cation components of the hybrid lattice in a broad structural space. I will address how ultrafast nonlinear spectroscopies yield deep insight on the multiparticle properties in compelx semiconductor materials.


Condensed Matter Seminar
Thursday, May 9, 2019
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

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