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


Structural Studies of Organic Self-Assembly at Metal Interfaces

We are using ex-situ and in-situ scanning tunneling microscopy (STM), infrared reflection absorption spectroscopy (IRRAS) and molecular dynamics simulations (MDS) to investigate the balance of interactions that lead to the spontaneous adsorption and ordering of organic species on Au surfaces. Our studies are concentrating on the poorly characterized short-chain alkanethiol (containing fewer than 6 methylene groups) monolayers in order to elucidate the nature of the S-Au interface, which is particularly difficult to examine directly by spectroscopic techniques. Our STM measurements have provided the first evidence of a stable ground state structure for butanethiol/Au films (the c(4x2) superlattice); the direct in-situ observation of its nucleation and growth from organic solvents (and the pre-requisite conditions for its formation) provides a reliable target structure for molecular simulations. We have also characterized the long-term evolution of this structure and its degradation into lower-symmetry structures of pinstripes and amorphous liquid-like domains. The molecular simulations of Goddard et al., who have also provided energetic arguments that show how short-chain systems are more easily disrupted, have recently confirmed our identification of the ground state structure.

IRRAS provides complementary structural information that can be directly interpreted in terms of molecular orientation and average film properties. We have shown that the kinetics of chain re-organization during self-assembly can be followed using this technique, and that the spectral characteristics of short-chain systems are just as amenable to quantitative analysis as the more commonly explored long-chain systems. We have determined that the molecular tilts of butanethiol adsorbates are less inclined with respect to the surface normal than the long-chain systems (~14o and ~25o, respectively), and this in turn imposes severe constraints on the angular dependencies of the S-Au binding. Our MDS of these systems shows that the most commonly adopted S-Au interaction potential (Sellers et al.) cannot duplicate this chain-length dependence of the tilt angle, while MDS performed with the more recent Beardmore potential are in close agreement with our experimental results.


Where to from here ?

The next phase of this work will be the study of self-assembled systems bound to Hg and Ga surfaces, at temperatures below, at and above the substrate melting points.

Our interest is in understanding how the structure and the internal degrees of freedom of the substrate can influence the adsorbate structures. By considering a mobile subphase, we can elucidate the role of this interface in annealing out defects in the organic structure, and the role of the surface rigidity in thermalizing excess energy.


Relevant Instrumentation (click on image to see apparatus)