Insight into Trimeric Formation of Nitric Oxide on Cu(111): A Density Functional Theory Study

We have studied the adsorption of small nitric oxide (NO) clusters [(NO)(n) (n = 1-3)] on Cu(1 1 1), by means of the van der Waals density functional. We have found that a single NO molecule preferably adsorbs in an upright N-down configuration at the fcc-hollow site, whereas all constituent NO molecules of the NO dimer and trimer adsorb at the fcc-hollow sites, in inclined N-down geometries. Among three NO clusters, the NO trimer is the most stable regardless of lateral periodicities, in good agreement with the scanning tunneling microscopy experiment. van der Waals interaction dominates the stabilization of the NO dimer, while the hybridization among 2 pi* orbitals of NO plays an important role in addition to the van der Waals interaction in the formation of the NO trimer. The relatively weak interaction between the NO and Cu substrate is crucial for the stabilization of the NO trimer. Our vibrational analysis has revealed that the N-O stretching mode is blue-shifted from monomer to trimer, and the latter agrees well with the electron energy loss spectroscopy data.