Single molecule observations of the adsorption sites of methyl isocyanide on Pt(111) by low-temperature scanning tunneling microscopy

Scanning tunneling microscopy (STM) has been used to directly investigate the local structure of methyl isocyanide (CNCHa) adsorbed on Pt(111). At low coverages, CNCH₃ is preferentially adsorbed at on-top sites, in agreement with earlier deductions based on vibrational spectroscopy. When dosed at low coverages at 50 K, the molecules tend to adsorb near other CNCH ₃ molecules with preferred distances of √3a and √7a, where a = 2.78 Å is the lattice constant of Pt. Annealing the surface-to 120 K, however, results in a more uniform separation of the molecules. At higher coverages, the CNCHa molecules are observed to occupy both on-top and two-fold bridge sites. On the basis of STM image analysis, CNCH₃ forms an ordered layer of (2 × 3) periodicity at 0.33 ML. Additional details on the structures of CNCH₃ adsorbed at the on-top and two-fold bridge sites are provided by density functional theory (DFT) calculations, At a coverage that saturates the first layer (0.33 ML), the occupation ratio for the on-top and two-fold bridge bonded CNCH₃ is 1:1, which is consistent with the results obtained from the combined use of experimental reflection absorption infrared spectroscopy (RAIRS) data and DFT calculations.