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.

Figures (12)

• Figure 1: Structural characterization of HMTP submonolayers. LEED pattern of the graphene interdigitated device following the post-nanofabrication cleaning procedure (a) before, and (b) after HMTP submonolayer growth; the sharp diffraction spots in (a) confirm the removal of polymer residues. (c) Numerical Fourier Transform (FFT) of the STM image in (f), showing reciprocal space points consistent with the LEED symmetry in (b). (d) LEEM image of a 3 $\mu$m wide graphene electrode before and (e) after HMTP deposition; scale bars represent 0.5 $\mathrm{\mu m}$. (f) High-resolution STM image of the HMTP layer on an unpatterned graphene substrate, revealing the molecular self-assembly.
• Figure 2: Morphological characterization of HMTP-based devices. (a) Optical micrograph of a Fourier transform photocurrent device featuring the prefabricated metal interdigitated electrode array. (b) SEM image of interdigitated contacts patterned from epitaxial graphene on SiC. (c) AFM topography of a 50 nm thick HMTP film deposited onto the metal-electrode platform shown in (a). (d) AFM topography of the 50 nm HMTP film at the interface between the graphene contact (upper region) and the bare SiC substrate (lower region).
• Figure 3: Vibrational characterization and Molecular Dynamics (MD) validation. (a) Raman spectra of HMTP compared to the bare substrate ($\lambda_\text{exc}=532$ nm). Spectra from the SiC substrate (blue) and the HMTP film at two distinct spatial locations (red and orange) are shown. (b) Magnified view of the low-intensity region, highlighting the subtle molecular vibrational features. Vertical solid lines denote characteristic HMTP modes; peaks overlapping with the blue SiC reference represent substrate phonon modes. (c) Calculated Vibrational Density of States (VDOS) of HMTP obtained from MD simulations. Simulated transitions are convoluted with a Gaussian broadening function to match experimental resolution. Vertical dashed and dotted lines correlate the primary and secondary experimental modes with the simulated VDOS peaks.
• Figure 4: Crystallographic unit cell of 2,3,6,7,10,11-hexamethoxytriphenylene (HMTP) is depicted by a blue rhombic prism. The spheres represent carbon (black), oxygen (red), and hydrogen (white) atoms. The arrangement corresponds to the $P6_3/m$ space group symmetry.
• Figure 5: (Top) Experimental photoluminescence (red shaded area) and absorption (blue shaded area) spectra. Solid vertical lines indicate the vibrational manifold of the primary Davydov branch, while dashed vertical lines denote the minor (secondary) branch. (Bottom) Theoretical simulation based on a Franck-Condon model incorporating Herzberg-Teller corrections. Solid and dashed curves represent the calculated transitions for the main and minor manifolds, respectively. The model is benchmarked against experimentally extracted peak positions and intensities (black open circles with error bars). The unperturbed monomer HOMO-LUMO transition ($E_{mon} = 3.4$ eV) is indicated for reference.