Quantum mechanics

Raman spectroscopy

Chemical physics

Polaron

Materials science

Phonon

Perovskite (structure)

Semiconductor

Physics

Exciton

Optics

Molecular physics

Condensed matter physics

Ionic bonding

Crystallography

Chemistry

Electron

Molecular vibration

Optoelectronics

Nature Materials

Hybrid organic-inorganic semiconductors feature complex lattice dynamics due to the ionic character of the crystal and the softness arising from non-covalent bonds between molecular moieties and the inorganic network. Here we establish that such dynamic structural complexity in a prototypical two-dimensional lead iodide perovskite gives rise to the coexistence of diverse excitonic resonances, each with a distinct degree of polaronic character. 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 ($\lesssim 50$\,cm$^{-1}$) optical phonons involving motion in the lead-iodide layers. We thus conclude that different excitons induce specific lattice reorganizations, which are signatures of polaronic binding. This insight on the energetic/configurational landscape involving globally neutral primary photoexcitations may be relevant to a broader class of emerging hybrid semiconductor materials.

Phonon coherences reveal the polaronic character of excitons in two-dimensional lead halide perovskites