Band Structure and Energy Level Alignment of Chiral Graphene Nanoribbons on Silver Surfaces

Abstract

Chiral graphene nanoribbons are extremely interesting structures due to their narrow band gaps and potential development of spin-polarized edge states. Here, we study their band structure on low work function silver surfaces. The use of a curved Ag single crystal provides, within the same sample, regions of disparate step structure and step density. Whereas the former leads to distinct azimuthal growth orientations of the graphene nanoribbons atop, the latter modulates the substrate’s work function and thereby the interface energy level alignment. In turn, we disclose the associated charge transfer from the substrate to the ribbon and assess its effect on the nanoribbon’s properties and the edge state magnetization.

Step-Assisted On-Surface Synthesis of Graphene Nanoribbons Embedded with Periodic Divacancies

“…The length distributions of the polymers (570 K) and GNRs (720 K) at the steps and on the terraces are shown in Figure b and c, respectively. Longer polymers are observed at the steps (Figure b), with an average length of about 9.3 nm, further confirming the template effect of the growth of the linear polymers at low annealing temperatures (480–570 K) (Figures S4 and S7), similar to previous observations. − The average length of the GNRs at the steps is about 7.8 nm, which is much longer than that for those on the terraces (∼3.5 nm), as shown in Figure c. Compared to the results obtained at 623–643 K for forming nanographenes with three divacancies in previous reports , and 570 K for the C66 with a void in Figure c, our results show that the formation of DV8-aGNRs with periodic divacancies has a higher energy barrier.…”

Carbon-based nanostructures as a versatile platform for tunable π-magnetism

“…By way of example, chiral (3, 1)-GNRs with six carbon atoms across their width develop spinpolarized edge states, whereby their band structure widens its gap to avoid the instability associated with large density of states near the Fermi level (figures 20(k) and (l)). However, a charge transfer of around one electron per unit cell, as experimentally observed on silver surfaces, quenches the magnetization and causes the band structure to become like that of nonspin-polarized ribbons (figure 20(m)) [201]. A more detailed analysis of the spin-polarization in this kind of ribbons as a function of charge transfer indeed reveals that an electron transfer of only 0.3 electrons per unit cell is already enough to fully quench any magnetization (figure 20(n)) [201].…”

“…However, a charge transfer of around one electron per unit cell, as experimentally observed on silver surfaces, quenches the magnetization and causes the band structure to become like that of nonspin-polarized ribbons (figure 20(m)) [201]. A more detailed analysis of the spin-polarization in this kind of ribbons as a function of charge transfer indeed reveals that an electron transfer of only 0.3 electrons per unit cell is already enough to fully quench any magnetization (figure 20(n)) [201].…”

“…It should also be mentioned that even for systems in which the magnetism is associated to more electrons, like in chiral or ZGNRs, charge transfer is predicted to similarly affect the magnetism [79,201,205]. By way of example, chiral (3, 1)-GNRs with six carbon atoms across their width develop spinpolarized edge states, whereby their band structure widens its gap to avoid the instability associated with large density of states near the Fermi level (figures 20(k) and (l)).…”

Steering On-Surface Reactions by Kinetic and Thermodynamic Strategies

“…Some vicinal or reconstructed surfaces with grooves may guide molecular adsorption and subsequent diffusion along a certain orientation or site, e.g., along the direction of step edges or a groove template, − promoting the coupling reaction, especially for linear linkages along this direction. For instance, Glaser coupling between linear terminal alkynes turned out to be much more efficient and selective on a vicinal Ag(877) surface with abundant step edges than on a flat Ag(111) surface since the molecules preferentially adsorb near and along the step edges which optimizes reactant geometry readily at the first adsorption step .…”