Development of an Accurate Force Field for Self-assembled Alkanethiol Monolayers on Au(111)


1. Introduction:

Self-assembled monolayers (SAMs) are formed by a spontaneous chemisorption of organic molecules on a substrate surface. The chemical nature of precursor molecules (e.g., head group, terminal group, and molecular chain length) determines and controls chemical and structural properties of SAMs. The novel properties of SAMs make them ideal for applications to biomaterials, biosensors, and corrosion and wear inhibition. A number of classes of SAM systems have been developed and characterized, including organosulfur/coinage metal, organosilicon/oxide, and carboxylic acid/metal oxide systems. Of these monolayers, alkanethiol SAMs on Au(111) surface are most commonly used as a model system for both experimental and theoretical studies

For alkanethiol SAMs on gold, the interaction between S and Au is one of the important terms in the force field. Ab initio quantum calculations were carried out and several force fields were developed.  Unfortunately, none of the above models are able to reproduce the c(42) superlattice structures at ambient temperature as observed by low-energy atom diffraction (LEAD), grazing-incidence X-ray diffraction (GIXD), scanning tunneling microcopy (STM), and atomic force microscopy (AFM). In this work, we performed ab initio QC calculations of alkanethiols on Au(111). Both clusters and periodic systems were used and no disulfide was considered. Based on the ab initio results, an accurate force field was developed. Molecular mechanics (MM) and MD simulations with the developed force field were performed on alkanethiol SAMs on Au(111) with different chain lengths to predict c(42) superlattice structures.

 

2. Clusters for Ab initio QM calculations:

 

3. All-atom model with the Morse potential for force field developments:

 

4. Molecular structure predicted by the developed force field.

 

5. C(4x2) predicted by the developed force field and obseved by STM.

(scale: 8nm x 8nm)

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January 21, 2002.