Unveiling Davydov-Split Excitons in a Template-Engineered Molecular-Graphene Heterostructure

Abstract

The realization of high-fidelity organic-inorganic quantum emulators is frequently hindered by the interfacial imperfections introduced during device fabrication. Here, we demonstrate a robust nanofabrication protocol that restores the atomic-scale purity of epitaxial graphene on SiC to UHV-equivalent levels, as confirmed by Low-Energy Electron Diffraction, and Microscopy. This pristine interface enables the emergence of macroscopic excitonic coherence in epitaxial overlayers of 2,3,6,7,10,11-hexamethoxytriphenylene (HMTP), a model molecular system characterized by intense electron-phonon coupling. Through a combination of high-sensitivity Fourier Transform Photo-current Spectroscopy, photoluminescence, and dynamic Raman mapping, we resolve a complex vibronic manifold governed by Davydov splitting. We show that the $P6_3/m$ crystalline symmetry of the HMTP overlayer lifts the degeneracy of the HOMO-LUMO transition, creating discrete bright and dark excitonic branches. Using an analytical tight-binding model parameterized by ARPES-derived intermolecular coupling and Raman vibrational modes validated by molecular dynamics simulations, we quantify the polarization energy, the Huang-Rhys factor, and Herzberg-Teller corrections to the Franck-Condon model. Our results reveal that the dark-state branch dominates the radiative channel, following a polaron-mediated relaxation pathway consistent with Kasha's rule. By reconciling macroscopic device architecture with UHV-level surface science, this work establishes a scalable platform for the study of dark-exciton dynamics and the development of solid-state molecular quantum memories.