Electronic structure evolution at DBBA/Au(111) interface W/O Bismuth insertion layer

Atomically precise graphene nanoribbons (GNRs) can be on-surface synthesized from halogen containing molecular precursors. Here, we investigated the electronic structure evolution of 10,10'-dibromo-9,9'-bianthracene (DBBA), a famous precursor to 7-AGNRs, on both Au(111) and Bi- 3 x root 3 - Au(111) as a function of film thickness and post-annealing temperature using photoemission spectroscopy, low temperature scanning tunneling microscopy and density functional theories. No obvious changes in electronic structure of DBBA in three STM-observed configurations can be detected, indicating that nonplanar pi-conjugated DBBA is physisorbed on both surfaces. The energy level alignments at the DBBA-substrate interfaces are demonstrated. Bismuth(Bi) insertion layer makes molecular - substrate interaction weaker, and makes the energy levels of DBBA thin film rigidly shift by (similar to)0.70 eV away from Fermi level, which enlarges the hole injection barrier and results in DBBA desorption at (similar to)470 K before dehalogenation occuring. The surface work function reduction can be explained by the push back effect and charge transfer induced interface dipole. Our findings explain why 7-GNRs could not be formed on Bi- 3 x root 3 Au(111).