Molecular simulation and experimental studies of thin films on solid surfaces

Our understanding of static interfaces has advanced considerably, however, very little is known about dynamic interfaces. The research work focused on the dynamic interfaces in the presence of thin films. Structural, rheological and tribological behaviors of the dynamic interfaces were investigated by ab initio quantum chemical (QC) calculations, molecular simulations, and scanning probe microscopy (SPM) experiments. This work consists of four major components: (1) development of an accurate force field for self-assembled monolayers (SAMs) of alkanethiols on Au(111), (2) molecular tribology of microelectromechanical systems (MEMS) by molecular simulations, (3) friction and adhesion of alkyl monolayers on Si(111) by SPM, and (4) rheology of confined fluids in equilibrium with the bulk by non-equilibrium molecular dynamics (NEMD) simulations. The first component concentrated on QC studies of interactions between sulfur and gold for alkanethiol SAMs/Au(111). An accurate force field was developed based on ab initio calculations to predict c(4 x 2) structures observed by experiments. In the second component, MD simulations were performed on the optimal molecular packing of alkyl monolayers on Si(111) found by molecular mechanics to investigate the friction of the monolayers with varying hydrophobicities and the effect of relative humidity on friction. The third component focused on alkyl monolayers with different terminal groups ranging from hydrophobic (-CH3) to hydrophilic (-COOH) on atomically flat Si(111) surface. The monolayers were prepared and characterized by various techniques such as contact angle and X-ray photoelectron spectroscopy (XPS). Friction and adhesion were quantitatively measured by SPM. In the fourth component, NEMD simulations were performed on confined n-decane in contact with the bulk. Constant-load and constant-height MD simulations were carried out to calculate the structural and rheological properties of confined fluids. This work will advance our fundamental understanding of the dynamic interfacial behaviors at the molecular level and guide us to rationally design various thin films for applications such as thiol/gold as platforms for biosensors, alkyl coatings for microelectromechanical systems (MEMS) devices, and lubricants for engines. It will also provide an integrated technique approach (quantum chemistry, force field development, molecular simulations, and SPM experiments) applicable to other problems, such as protein adsorption on man-made surfaces.