Electronic structure changes during the on-surface synthesis of nitrogen-doped chevron-shaped graphene nanoribbons

Utilizing suitable precursor molecules, a thermally activated and surface-assisted synthesis results in the formation of defect-free graphene nanoribbons (GNRs), which exhibit electronic properties that are not present in extended graphene. Most importantly, they have a band gap in the order of a few electron volts, depending on the nanoribbon width. In this study, we investigate the electronic structure changes during the formation of GNRs, nitrogen-doped (singly and doubly N-doped) as well as non-N-doped chevron-shaped CGNRs on Au(111). Thus we determine the optical gaps of the precursor molecules, the intermediate nonaromatic polymers, and finally the aromatic GNRs, using high-resolution electron energy loss spectroscopy and density functional theory calculations. As expected, we find no influence of N-doping on the size of the optical gaps. The gap of the precursor molecules is around 4.5 eV. Polymerization leads to a reduction of the gap to a value of 3.2 eV due to elongation and thus enhanced delocalization. The CGNRs exhibit a band gap of 2.8 eV, thus the gap is further reduced in the nanoribbons, since they exhibit an extended delocalized $\pi$-electron system.

Figure 1

Precursor molecules (1–3) for the generation of chevron-shaped graphene nanoribbons (CGNRs) with defined N-doping via on-surface polymerization followed by cyclodehydrogenation. Shown are monomers with either zero (pristine), one (+N), or two (+2N) N atoms, while the synthesis is exemplified for the case of doubly N-doped CGNR.

Figure 2

Shown are the optimized structures of the monomer (a), the unit cells of the polymer (b), and the CGNR (c) in the case of doping with two N atoms (+2N). The calculations for the monomer were done at the PBE0/6-311G** level of theory, while the periodic calculations were done at the PBE0/6-31G* level of theory (for details, see the text). Carbon atoms are colored gray, hydrogen atoms white, nitrogen atoms blue, and bromine atoms red.

Figure 3

Changes in the vibrational HREELS data observed during the cyclodehydrogenation reaction step from the singly doped (+N) polymer to the corresponding nanoribbon (+1N-CGNR) recorded with a primary electron energy of 4.5 eV in specular and $7^{\circ }$ off-specular scattering geometry.

Figure 4

Electronic structure changes during the on-surface synthesis of chevron-shaped graphene nanoribbons on Au(111). (a)–(c) Electronic HREEL data of the nondoped (pristine) compounds, (d)–(f) the corresponding spectra for the singly doped, and (g)–(i) for the doubly N-doped derivatives.

Figure 5

Different geometries of the pristine polymer with different planarity (for details, see the text). Part (a) shows the optimized polymer, in (b) all dihedral angles determining the orientation of the terminal phenyl rings are set to ${90}^{\circ }$, in (c) these dihedral angels are set to ${60}^{\circ }$, and in (d) they are set to ${30}^{\circ }$ (the same color code as in Fig. 2 is used).