Solely σ-Atop Site Bonding of Phenyl Isocyanide on Au(111)? Comparison with on Cu(111)

The interfacial bonding and electronic structure of 4-methylphenyl isocyanide (CH 3 C 6 H 4 NC, mPNC) on Au(111) dosed at 160 K have been studied using temperature-programmed desorption (TPD) time-of-flight (TOF) mass spectrometry and X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS), in comparison with that on Cu(111). The TPD profiles for Au(111) show a multilayer peak at 200 K and two comparable sub-monolayer peaks at 305 and 375 K, direct evidence of two different chemisorbed binding structures on Au. No evidence of dissociation was observed from TPD, XPS, and UPS. For Cu(111), a multilayer and a broad major sub-monolayer peaks were observed at 210 and 360 K, respectively. A weak minor peak was also observed at 290 K. On the basis of the N(1s) XPS binding energies, 400 eV for Au(111) and 398 eV for Cu(111), we conclude that the σ-bonded linear N⋮C form is dominant for Au(111), and the σ/π-bonded bent N C form is dominant for Cu(111). For 1.0 monolayer mPNC on Au(111) and on Cu(111), the work functions were measured to be 4.0 and 2.85 eV, respectively. For multilayer coverages, the work function for Au(111) is 0.8 eV lower while that for Cu(111) is 0.08 eV higher. For the 1.0 monolayer mPNC on Au(111), the HOMO level is located at 2.8 eV below the Fermi level. Upon annealing this to 325 K, the coverage is reduced, and the UPS peak at 3.4 eV becomes stronger while the peak at 2.8 eV becomes weaker, due to a change in binding geometry.