Concentration Dependence of Dopant Electronic Structure in Bottom-up Graphene Nanoribbons

Bottom-up fabrication techniques enable atomically precise integration of dopant atoms into the structure of graphene nanoribbons (GNRs). Such dopants exhibit perfect alignment within GNRs and behave differently from bulk semiconductor dopants. The effect of dopant concentration on the electronic structure of GNRs, however, remains unclear despite its importance in future electronics applications. Here we use scanning tunneling microscopy and first-principles calculations to investigate the electronic structure of bottom-up synthesized <i>N</i> = 7 armchair GNRs featuring varying concentrations of boron dopants. First-principles calculations of freestanding GNRs predict that the inclusion of boron atoms into a GNR backbone should induce two sharp dopant states whose energy splitting varies with dopant concentration. Scanning tunneling spectroscopy experiments, however, reveal two broad dopant states with an energy splitting greater than expected. This anomalous behavior results from an unusual hybridization between the dopant states and the Au(111) surface, with the dopant–surface interaction strength dictated by the dopant orbital symmetry

Molecular Self-Assembly in a Poorly Screened Environment: F₄TCNQ on Graphene/BN

We report a scanning tunneling microscopy and noncontact atomic force microscopy study of close-packed 2D islands of tetrafluoro­tetracyanoquinodimethane (F₄TCNQ) molecules at the surface of a graphene layer supported by boron nitride. While F₄TCNQ molecules are known to form cohesive 3D solids, the intermolecular interactions that are attractive for F₄TCNQ in 3D are repulsive in 2D. Our experimental observation of cohesive molecular behavior for F₄TCNQ on graphene is thus unexpected. This self-assembly behavior can be explained by a novel solid formation mechanism that occurs when charged molecules are placed in a poorly screened environment. As negatively charged molecules coalesce, the local work function increases, causing electrons to flow into the coalescing molecular island and increase its cohesive binding energy