Vibronic and Excitonic Structure of a Template-Engineered Molecular-Graphene Heterostructure

Area of Science:

• Condensed Matter Physics
• Materials Science
• Organic Electronics

Background:

• Strong electron-phonon coupling, described by the Holstein Hamiltonian, is crucial for the optical properties of molecular quantum materials.
• Understanding excitonic behavior and vibronic coupling is key to designing advanced organic electronic devices.

Purpose of the Study:

• Investigate the optical response and electronic structure of 2,3,6,7,10,11-hexamethoxytriphenylene (HMT) grown on graphene/SiC.
• Characterize the vibronic manifold and excitonic splitting in this novel molecular heterostructure.
• Establish a scalable platform for studying dark excitons and electron-phonon interactions in organic quantum materials.

Main Methods:

• Epitaxial growth of HMT overlayers on graphene/SiC.
• Fourier transform photocurrent spectroscopy, photoluminescence, and Raman spectroscopy.
• Angle-resolved photoemission spectroscopy and surface-sensitive microscopy.
• Tight-binding model parametrization using experimental data.

Main Results:

• Resolved a vibronic manifold exhibiting Davydov splitting due to P63/m crystal symmetry.
• Identified the lifting of the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) degeneracy into bright and dark excitonic branches.
• Determined key parameters including intermolecular coupling, polarization energy, Huang-Rhys factor, and Herzberg-Teller corrections.
• Observed polaron-mediated relaxation into the lower-energy exciton branch, consistent with Kasha's rule.

Conclusions:

• The graphene-supported HMT heterostructure enables detailed study of excitonic phenomena and vibronic coupling.
• The observed Davydov splitting and polaron dynamics provide insights into the optical response governed by electron-phonon interactions.
• This scalable platform opens avenues for future research into dark excitons and organic quantum materials.