Adsorption structures and electronic properties of 1,4-phenylene diisocyanide on the Au(111) surface

The adsorption structures and electronic properties of 1,4-phenylene diisocyanide (PDI) on a Au(111) surface have been studied using temperature programmed desorption (TPD), two-photon photoemission (2PPE), and scanning tunneling microscopy (STM). As deposited at 95 K, PDI molecules form disordered islands and short one-dimensional chains on Au(111) terraces. The work function decreases with increasing PDI coverage, and an occupied electronic state appears at 0.88 eV below the Fermi level. Annealing to 300 K causes the PDI molecules to reorganize on the surface and form ordered, one-dimensional molecular chains that extend for many tens of nanometers across the Au(111) terraces. The repeating structure of the molecular chains is consistent with a recently proposed [-Au-PDI-]_n structure in which PDI molecules lie parallel to the surface and are interconnected by Au adatoms released from the Au(111) surface. The formation of the molecular chains is accompanied by a large drop in the work function which we attribute to an increase in the number and magnitude of interfacial dipoles. The electronic structure of the molecular chains is also characterized by occupied and unoccupied states at -0.88 eV below and +3.32 eV above the Fermi level, respectively. The latter are most prominent after annealing a PDI/Au(111) surface to 300 K, indicating that they are associated with interfacial states of the one-dimensional molecular chains.