Adsorption and diffusion of organic molecules at surfaces

We have studied the adsorption and diffusion of large functionalized organic molecules on an insulating ionic surface at room temperature using a noncontact atomic force microscope (NCAFM) and theoretical modeling. Custom designed syn-5,10,15-tris(4-cyanophenylmethyl)truxene molecules are adsorbed onto the nanoscale structured KBr(001) surface at low coverages and imaged with atomic and molecular resolution with the NC-AFM. The molecules are observed rapidly diffusing along the perfect monolayer step edges and immobilized at monolayer kink sites. Extensive atomistic simulations elucidate the mechanisms of adsorption and diffusion of the molecule on the different surface features. The results of this study suggest methods of controlling the diffusion of adsorbates on insulating and nanostructured surfaces.

Individual molecules of Co-Salen, a small chiral paramagnetic metal organic Schiff base complex, were deposited on NaCl(001) and subsequently imaged with noncontact atomic force microscopy employing Cr coated tips in a cryogenic ultrahigh vacuum environment. Images were obtained in which both the position and orientation of the adsorbed molecules and the atomic structure of the surface are resolved simultaneously, enabling the determination of the exact adsorption site. Density functional theory calculations were used to identify the ionic sublattice resolved with the Cr tip and also to confirm the adsorption site and orientation of the molecule on the surface. These calculations show that the central Co atom of the molecule physisorbs on top of a Cl on and is aligned along < 110 >-directions in its lowest energy configuration. In addition, a local energy minimum exists along (100)-directions. Due to the chirality of the molecule, two mirror symmetric configurations rotated by approximately +/- 5 degrees away from these directions are energetically equivalent. The resulting 16 low energy configurations are observed in the experimental images.

We show that the overall structure and flexibility of an organic molecule has a profound effect on the mechanism of diffusion and the effective diffusion rate on a surface. Calculations were performed to model the diffusion of a set of large organic molecules with polar binding groups on the perfect TiO(2) (110) surface. These simulations involved determining the accessible states of the molecule surface system and the energy barriers that separate them using realistic atomistic simulations employing a set of specifically developed potentials. With the complete set of accessible states and energy barriers for each molecule, we then performed kinetic Monte Carlo simulations to determine the mechanisms of diffusion and the effective diffusion rates of the molecules. These calculations suggest ways in which the mobility of large molecules on surfaces can be controlled through careful design of the molecular structure.