Self-Assembly of a Triphenylene-Based Electron Donor Molecule on Graphene: Structural and Electronic Properties

Abstract

In this study, we report on the self-assembly of the organic electron donor 2,3,6,7,10,11-hexamethoxytriphenylene (HAT) on graphene grown epitaxially on Ir(111). Using scanning tunneling microscopy and low-energy electron diffraction, we find that a monolayer of HAT assembles in a commensurate close-packed hexagonal network on graphene/Ir(111). X-ray and ultraviolet photoelectron spectroscopy measurements indicate that no charge transfer between the HAT molecules and the graphene/Ir(111) substrate takes place, while the work function decreases slightly. This demonstrates that the HAT/graphene interface is weakly interacting. The fact that the molecules nonetheless form a commensurate network deviates from what is established for adsorption of organic molecules on metallic substrates where commensurate overlayers are mainly observed for strongly interacting systems.

Comparing Adsorption of an Electron-Rich Triphenylene Derivative: Metallic vs Graphitic Surfaces

“…The finding of a commensurate structure at a weakly interacting interface was surprising since it goes against the established trend for molecules self-assembled on metal surfaces requiring a substantial molecule–substrate interaction for commensurability. However, similar findings were reported for HAT on Ag(111) and graphene/Ir(111) . Comparison of the HAT ionization potential reveals that it is constant across all three substrates, which points to a weakly interacting interface in each case.…”

“…HAT has recently been studied in the monolayer regime, on Ag(111) 21 and graphene on Ir(111) (Gr/Ir(111)). 22 Herein we report on the self-assembly of HAT on Gr/ Ni(111), studied by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) to obtain knowledge of the molecular ordering and by X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) to gain insight in the electronic properties of the HAT/Gr/Ni(111) interface. We also compare our findings to previous studies of HAT on Ag(111) 21 and Gr/Ir(111).…”

“…Crucial to the functionality of organic semiconductors is the organic–inorganic substrate interface, the structural and electronic properties of which often influence the entire film. ,,− Strong molecule–substrate interactions can induce a molecular ordering for the first molecular layer that is far from the 3D molecular crystal structure which makes the desired layer by layer growth difficult, − but even if the monolayer order is close to that of a certain face of the bulk crystal layer by layer growth is not guaranteed. ,− When it comes to the electronic properties of molecular films, it is advised to study the film growth beginning with the first layer and then continuing gradually to disentangle the contributions of interface effects (e.g., band bending, charge transfer, induced interface states, screening, and interface dipoles) from bulk properties. ,,, Therefore, the study of structural and electronic properties of thin molecular films, starting from the submonolayer regime, remains a topic of considerable interest to date. Recently, particular attention has been given to strategies reducing the interactions at the metal–organic interface to facilitate layer by layer growth, for example by insertion of buffer layers and tailoring of the molecules, respectively. ,− …”

“…Despite this versatility, most studies of surface-supported self-assembled triphenylene derivatives focus on either the strong acceptor 2,3,6,7,10,11-hexacyanohexaazatriphenylene (HATCN) − or the unsubstituted triphenylene. , Here, we focus on the electron donor , molecule 2,3,6,7,10,11-hexamethoxytriphenylene (HAT), which consists of a planar triphenylene backbone functionalized with six methoxy groups (Scheme ). HAT has recently been studied in the monolayer regime, on Ag(111) and graphene on Ir(111) (Gr/Ir(111)) …”

“…We also compare our findings to previous studies of HAT on Ag(111) 21 and Gr/Ir(111). 22 ■ METHODS Synthesis. HAT was synthesized upon oxidative cyclotrimerization of 1,2-dimethoxybenzene with iron(III) chloride in nitromethane and CH 2 Cl 2 at 0 °C for 30 min by following the procedures reported for related alkoxy-substituted triphenylene derivatives previously.…”