Organic Film Structure Physisorbed Films ESP in SAMS SP in Molecular Films Reforming Catalysts ECH


Structure of Physisorbed Molecular Films on Metal and Organic Substrates

Physisorbed films are deposited under UHV conditions at 25-300 K on Au(111) and Cn/Au(111) substrates by exposure of the surface to low-pressure gases or directed molecular fluxes; systems under investigation include Fe(CO)5/Au, Fe(CO)5/Cn/Au, Ni(CO)4/Au, Ni(CO)4/Cn/Au, and (C5H5)Co(CO)2/Au and (C5H5)Co(CO)2/Cn/Au films.  The IRRAS spectra of these films are obtained in-situ during deposition, during thermal processing, and during desorption to identify film structure, coverage, desorption properties, etc.  The comparison between deposition on the metallic and well-understood organic substrates has proven invaluable in these studies since it allows us to effectively eliminate strong substrate effects, and allows us to systematically control the physical separation between molecular adsorbates and their image dipoles (permanent and/or transition) in ~2.5 Å steps and with sub-angstrom precision; simultaneous IRRAS investigation of the interface between the methyl-terminated organic substrate and the deposited films (via the symmetric and anti-symmetric methyl modes, 2875 cm-1 and 2965 cm-1, respectively) enables the study of sub-surface phenomena.  In the case of Fe(CO)5/Au, we have shown that the as-deposited films are not crystalline solids as previously supposed, but rather are most-likely an amorphous metastable solid; our study of the transformation of these films into the ordered solid using temperature-programmed IRRAS is providing an important 'missing-link' in the spectroscopic interpretation of metal-carbonyl films, and as mentioned below, reveals an extremely high sensitivity of the electron-irradiation cross-sections to the physical phase of the system.

  As part of my sabbatical research project I have studied structural aspects of films physisorbed on Cn/Au(111) substrates using MDS. Some of the most detailed experimental data is for the rare gas (Kr, Xe) overlayers on long-chain substrates; our simulations have quantitatively reproduced all available results for these systems, while providing quantitative predictions for measurements on related systems.  One of the surprising results of these simulations is that the overlayer structures are frequently ‘pinned’ by the corrugation of the methyl terminations of the Cn chains; this effect is most evident in the case of Ar overlayers, where it can cause complete disruption of the overlayer structure.  The calculated structure factors of the Kr and Xe overlayers consistently reveal the presence of the substrate’s periodicities (5.0 Å hexagonal lattice) because of this ‘pinning’ process, and may explain the persistent observation of such periodicities in the diffraction data for these systems (which was previously attributed to diffraction from exposed SAM regions).

Where to from here ?

The next phase of this work will be to extend the MDS simulations to model systems in more complex environments

Work in progress is comparing the tribological properties of the rare gas/SAM interface with that previously reported for rare gas/metal interfaces.  One of the key advantages of these SAM-based friction studies (experimental and theoretical) is that the energy dissipation mechanism is exclusively via phonon creation within the SAM and the organic film, whereas with metal supports there is a controversial (and difficult to isolate) electronic component.  In addition, the interaction potentials of the rare gas/SAM systems are expected to be much less subject to screening effects produced by metal subjects (and thus the key advantages of using rare gas systems is preserved), while maintaining direct experimental accessibility via quartz microbalance techniques.


Relevant Instrumentation (click on image to see apparatus)