Surface Engineering of Graphite and Graphene by Viologen Self-Assembling: From Global to Local Architectures

Despite holding unique electronic, optical, mechanical, and thermal properties, some inherent characteristics of graphene restrict its widespread use in technological applications such as digital electronics. The introduction of ordered organic adlayers on top of graphene has been identified as a potential approach to tune its electronic band structure via quantum confinement effects caused by molecular-scale ordering within the adsorbate layers. To this end, the redox-dependent self-assembled molecular architectures of dibenzyl viologen (DBV) on graphitic surfaces including graphene are investigated under electrochemical control. Molecular-resolution electrochemical scanning tunneling microscopy results reveal three architectures, namely, the mobile, dimer, and stacking phases, corresponding to different DBV redox states, that is, DBV2+, DBV•+, and DBV0, respectively, formed and stabilized on both highly oriented pyrolytic graphite and chemical vapor deposition graphene on a copper foil. The phase transition is fully controlled by the applied electrode potential. The patterned functionalization of graphitic surfaces via laterally confined self-assembly of DBV molecules was demonstrated. These findings may help pave the way to functionalize graphene and other two-dimensional materials in both global and patterned manners by means of viologen-based self-assemblies.