Experimental Observation of Strong Exciton Effects in Graphene Nanoribbons

Abstract: Graphene nanoribbons (GNRs) with atomically precise width and edge structures are a promising class of nanomaterials for optoelectronics, thanks to their semiconducting nature and high mobility of charge carriers. Understanding the fundamental static optical properties and ultrafast dynamics of charge carrier generation in GNRs is essential for optoelectronic applications. Combining THz spectroscopy and theoretical calculations, we report a strong exciton effect with binding energy up to 700 meV in liquid-phase-dispersed GNRs with a width of 1.7 nm and an optical bandgap of 1.6 eV, illustrating the intrinsically strong Coulomb interactions between photogenerated electrons and holes. By tracking the exciton dynamics, we reveal an ultrafast formation of excitons in GNRs with a long lifetime over 100 ps. Our results not only reveal fundamental aspects of excitons in GNRs (gigantic binding energy and ultrafast exciton formation etc.), but also highlight promising properties of GNRs for optoelectronic devices.

• The paper demonstrates a strong exciton binding energy of 700 ± 50 meV using optical pump-THz probe spectroscopy to distinguish between exciton and free carrier responses.
• It reveals ultrafast exciton formation within 0.8 ps and long exciton lifetimes exceeding 100 ps, driven by rapid electron-phonon cooling in solution-dispersed graphene nanoribbons.
• Complementary TDDFT calculations corroborate experimental findings by predicting optical transitions and spatial confinement consistent with a Frenkel exciton model.

Experimental Observation of Strong Exciton Effects in Graphene Nanoribbons

Introduction

The study addresses the fundamental photophysics of graphene nanoribbons (GNRs), focusing on exciton dynamics and binding energies in atomically precise structures. GNRs, due to their high carrier mobility and tunable bandgaps, are increasingly scrutinized for optoelectronic implementations, overcoming intrinsic limitations of pristine graphene. The Coulomb interactions in quasi-one-dimensional GNRs are theoretically predicted to be exceptionally strong, yielding large exciton binding energies. Experimental quantification of such effects, specifically in well-dispersed, solution-phase GNRs, remained technically demanding prior to this work. The paper presents a combined terahertz (THz) spectroscopy and first-principles theoretical analysis of these phenomena in model GNRs with robust control over structural parameters and environmental screening.

Experimental Methodology and Observables

The central experimental approach is optical pump-THz probe (OPTP) spectroscopy on GNRs (GNR-AHM) dispersed in a low dielectric medium (toluene), specifically avoiding substrate and aggregation artifacts. Structural uniformity (1.7 nm width, 11 nm average length) is enforced by synthetic design, while optoelectronic properties are investigated under sub-bandgap to high-energy photoexcitation with femtosecond laser pulses.

The time-resolved THz probe unambiguously distinguishes between free carrier (Drude-Smith-like conductivity) and exciton (pure imaginary Lorentzian-like conductivity) responses due to their distinct intraband dynamics in the THz range. Crucially, photoconductivity is mapped as a function of both excitation energy and delay time, enabling direct measurement of exciton generation, dissociation, and lifetime.

Key Results: Exciton Binding Energy and Spectral Features

1. Strong Excitonic Effect

A dramatic exciton binding energy of $700 \pm 50$ meV is observed. This value is deduced from the threshold in excitation energy required to induce transition from bound excitons to unbound, conductive electron-hole plasma, rather than from direct spectral features—necessitated by the unusually large 1S-2P splitting and corresponding THz bandwidth limitations. The interplay of excitation energy, thermal fluctuation, and resultant free carrier probability is modeled quantitatively, yielding the aforementioned binding energy.

2. Ultrafast Formation and Long-Lived Excitons

Exciton formation following high-energy (e.g., 3.1 eV) excitation occurs on sub-picosecond timescales (0.8 ps), with subsequent exciton lifetimes exceeding 100 ps. This separation allows for real-time tracking of transition from hot carrier to exciton-dominated states, facilitated by robust assignment of Drude-Smith and Lorentzian spectral conductivity regimes. Notably, the cooling rate for hot carriers during exciton formation is exceptionally high ($\sim$1.9 meV/fs), attributed to strong electron-phonon coupling, as corroborated by recent non-adiabatic molecular dynamics studies.

3. Theoretical Support

Time-dependent DFT (TDDFT) calculations (range-separated HSE functional, 6-31G* basis, PCM/toluene) on finite GNR segments (tetramer, hexamer) predict main optical transitions in the 1.5-1.8 eV range. The calculated exciton binding energy ranges from 360 to 550 meV, consistent with experiment. Electron-hole density analysis confirms the excitonic transitions are strongly spatially confined (Frenkel-like), with overlap values near unity (0.88–0.91). The effective conjugation length is inferred to be $\sim$6 nm (tetramer), based on energy splitting analysis and comparison with experiment. Theoretical and experimental estimates for exciton size (0.7-2.2 nm) are also in agreement.

Implications for Exciton Physics in Low-Dimensional Carbon Allotropes

The reported exciton binding energies in GNRs approach or even surpass those observed in monolayer transition metal dichalcogenides (TMDCs), but fundamentally differ in exciton nature—strongly localized (Frenkel) in GNRs versus delocalized (Mott-Wannier) in TMDCs. The robust confinement leads to high oscillator strengths and prominent optical signatures in the visible/near-IR range, retained even in solution due to suppressed dielectric screening.

The ultrafast exciton formation and long lifetime highlight GNRs as efficient quantum structures for photoluminescence and photodetection, while the very strong Coulomb binding presents challenges for photovoltaic charge extraction, likely requiring interface engineering for ultrafast charge dissociation.

Outlook and Future Directions

The high intrinsic charge mobility, in conjunction with the demonstrated excitonic performance, positions GNRs as promising candidates for low-dimensional optoelectronic devices, including LEDs and THz photonic components. The direct observation and quantification of strong Frenkel excitons set benchmarks for future GNR design and functionalization strategies aimed at optimizing light–matter interaction.

Several immediate theoretical and experimental directions emerge:

• The impact of edge structure, functional group chemistry, and environmental dielectric engineering on exciton binding and size,
• Detailed characterization of electron-phonon coupling and its influence on hot carrier relaxation and exciton recombination,
• Interface and heterostructure designs to enable effective exciton dissociation for photovoltaic or photodetector applications,
• Extension of these studies to other GNR geometries (zigzag, armchair, chiral) and to on-surface or encapsulated systems for device integration.

Conclusion

The work establishes experimental bounds for exciton binding energy in solution-dispersed GNRs, identifies ultrafast and long-lived exciton dynamics, and supports a Frenkel exciton model via quantum chemical calculations. These findings advance the fundamental understanding of many-body effects in GNRs, with profound implications for their application in future optoelectronic platforms.