Electronic structure of a subnanometer wide bottom-up fabricated graphene nanoribbon: End states, band gap, and dispersion

Angle-resolved two-photon photoemission and high-resolution electron energy loss spectroscopy are employed to derive the electronic structure of a subnanometer atomically precise quasi-one-dimensional graphene nanoribbon (GNR) on Au(111). We resolved occupied and unoccupied electronic bands including their dispersion and determined the band gap, which possesses an unexpectedly large value of 5.1 eV. Supported by density functional theory calculations for the idealized infinite polymer and finite size oligomers, an unoccupied nondispersive electronic state with an energetic position in the middle of the band gap of the GNR could be identified. This state resides at both ends of the ribbon (end state) and is only found in the finite sized systems, i.e., the oligomers.

Figure 1

(a) Scheme of the surface-supported fabrication of graphene nanoribbons (GNRs) via a two-step process: (i) dehalogenation of precursor molecule $10,{10}^{\prime }$-dibromo-$9,9^{\prime }$-bianthryl followed by a C–C coupling and (ii) cyclodehydrogenation. (b) Changes in the vibrational HREEL spectrum observed during the cyclodehydrogenation step from the linear polymer to the nanoribbon measured with a primary electron energy of 3.5 eV.

Figure 2

(a) Electronic HREEL spectrum of the GNR-covered gold surface recorded at a primary electron energy of 15 eV. Two electronic transitions are observed which are fitted using two Gaussian peaks. (b) Two-color 2PPE spectra recorded at photon energies of 2.26 eV and 4.50 eV of GNR/Au(111). (c) Energetic position of GNR-derived electronic states. The Fermi level of Au(111) serves as reference.

Figure 3

(a) Angle-resolved one-color 2PPE spectra. (Inset: Measuring geometry in angle-resolved 2PPE experiments. Among the randomly oriented GNRs on the surface, mainly those lying in the plane perpendicular to the rotation axis of the sample are detected for off-normal emission.) (b) Dispersion of the spectral feature labeled as A, B, and C. While the states A and C are localized, state B shows a strong dispersion, viz., it is delocalized.

Figure 4

(a) Density of states obtained by Lorentzian broadening for the periodic case (black line) and for the occupied (blue lines) and virtual orbitals (red lines) of the heptamer and the hexamer together with the experimental energies, which have been aligned to the middle of the band gap of the periodic GNR. (b) HOMO and LUMO for the heptamer, which are localized at the ends of the oligomer (all calculations at the CAM-B3LYP/6-311G** level of theory).