47155-AC5
Molecular Adsorption on Metallic Nanostructures: Tailoring Chemisorption by Quantum Confinement of Electrons
The goal of this research is to identify how quantum size effects in nanoscale metal structures impact their interaction with small molecules. The research employs ultrathin metal films that exhibit so-called metallic quantum well (MQW) states whose energy changes as a function of film thickness. Earlier work focused on how adsorbate-surface interactions are modified as changing film thickness causes MQW states to cross the Fermi level. In this funding period concentrated on the fccCo/Cu(100) system where the MQW states are > 1 eV above the Fermi level, but are nearly degenerate with adsorbate molecular orbitals. Moreover, the Co film is in a metastable fcc structure, so adsorption properties are expected to differ from those of hcp-Co single crystal surfaces. We have found that adsorption of CO on the fccCo/Cu(100) surface indeed differs from that on single crystal Co, and that the adsorption properties depend on film thickness. In addition, we have explored the adsorption of dimethyl-disulfide on the Cu/fccCo/Cu(100) MQW system, and see pronounced differences from adsorption on Cu(100). The CO/fccCo/Cu(100) system was studied with an array of surface sensitive techniques including inverse photoemission spectroscopy (IPS), low energy electron diffraction (LEED), temperature programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS). RAIRS and TPD indicate that, when adsorbed at low temperature and at low coverages, CO binds at an atop site, but with a binding energy that is lower than that for CO on single crystal Co surfaces. As the coverage approaches saturation, CO bonds more weakly at a variety of additional sites giving rise to new structure in the RAIRS spectrum that are correlated with the appearance of low temperature features in TPD. While the precise nature of these bonding configurations is difficult to identify, comparison with gas phase data from cobalt carbonyls suggest the presence of a compressed phase with more than one CO molecule bound to a single Co surface atom.[1] The adsorption of dimethyl-disulfide on the Cu/fccCo/Cu(100) system also exhibits features that differ significantly from adsorption on the single crystal Cu(100) surface. In both cases, the molecule dissociates into methyl-sulfide which appears to bond through its S-end. In RAIRS measurements, the C—H stretch mode of the methyl group show a significant shift as a function of coverage, indicting increasing molecule-molecule interactions. Pyrolysis causes scission of the C-S bond and methyl radicals are observed in the TPD spectra. The desorption peak temperature of these radicals from Cu/fccCo/Cu(100) system was significantly lower than from single crystal Cu(100) surface. Scanning tunneling microscopy studies of the surface after annealing to 300 C show an ordered superstructure that is very similar to what is found for the HS/Cu(100) system, which is interpreted as atomic S on the Cu surface. In the upcoming months we will investigate how the properties of these two adsorption systems, CO/fccCo/Cu(100) and (CH3S)2/Cu/fccCo/Cu(100) behave as a function of metal overlayer thickness. Such studies will reveal the influence of quantum size effects on the nature of bonding and intermolecular interaction in these systems. [1] “Carbon monoxide adsorption on the fccCo/Cu(100) metallic quantum well system,” L. Tskipuri and R.A. Bartynski, Surface Science, to be published (2009)
