Phenyl Isocyanide on Cu(111): Bonding and Interfacial Energy Level Alignment

Abstract

The bonding and interfacial energy level alignment of 2,5-dimethylphenyl isocyanide (dmPNC, (CH3)2C6H3N⋮C) on Cu(111) have been studied using temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS). In TPD, there is no evidence for dissociation after dosing at 160 K. A tiny amount of residual carbon left after TPD to 850 K is attributed tentatively to surface oligomerization. Evidence is presented for two coverage- and temperature-dependent dmPNC bonding geometriesσ-bonded (narrow 300 K TPD peak) and σ/π-synergetic bonded (broad 400 K TPD peak). The σ/π-form is dominant at low coverages and shifts to the σ-bonded form as the coverage increases toward 1.0 monolayer of dmPNC. Upon annealing the latter to 340 K, the coverage drops to 0.63 monolayer and the σ/π-form returns and dominates. The sharpness of the 300 K peak is attributed to desorption of some σ-bonded dmPNC accompanied by the transition of the remainder to σ/π-bonded dmPNC. As-dosed at 160 K, dmPNC lowers the work function by as much as 2.55 eV, a result consistent with a large, vertically aligned dipole associated with the −N⋮C group of adsorbed dmPNC. A relatively strong UPS signal at 3.8 eV binding energy for low coverages is attributed to electron density in a weak chemisorption bonding orbital between dmPNC and Cu that involves mixing of the lowest lying π* orbital of dmPNC with valence orbitals of Cu.