Modeling of adsorption and atomic force microscopy imaging of molecules on insulating surfaces

• Main Author: Gao, D.
• Published: University College London (University of London) 2015
• Subjects: 621.381
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.666800

The study of atoms and molecules on bulk insulating surfaces is of critical importance to many fields of surface science including lubrication, and molecular electronics. I studied these systems by using a variety of theoretical methods to predict adsorption geometry, diffusion pathways, and film structure, and to interpret noncontact atomic force microscopy (NCAFM) data. I began by using molecular dynamics (MD) simulations to predict that Pd atoms deposited onto MgO(100) exhibit some transient mobility. They were initially able to move across the surface, but were eventually captured at an adsorption site when enough energy had been dissipated. Similarly, deposited molecules may also be able move around and find nucleation sites such as step edges or kinks before becoming stabilized on surface terraces at low temperature. I then moved on to study the properties of single molecules on oxide surfaces. I combined my theoretical calculations with experimental data to compare adsorption sites and geometries of Co-Salen molecules on NaCl(100) and NiO(100). I used density functional theory calculations (DFT) to show that minor differences in commensurability between the molecule and the surface can qualitatively change adsorption. Both surfaces are bulk insulators with simple cubic crystal structures, however, a much higher adsorption energy and distortion of the molecule on NiO(100) produced a significant vertical dipole moment. Single molecules adsorbed onto insulators can be directly imaged with chemical resolution using metal coated NCAFM tips. However, accurate interpretation of the results is needed. I studied metallic tips using DFT calculations and developed a point dipole model to represent the Cr coated tips used experimentally. I then fit the position and magnitude of the point dipole in this model directly to experimental scan-lines and was able to produce virtual AFM (VAFM) images and scan-lines that were in quantitative agreement with experiment. This method simultaneously reduced the complexity of interpretation of experimental data and the computational cost of producing VAFM images. Finally, I studied larger systems using a hybrid quantum mechanics/molecular mechanics (QM/MM) and parametrized classical force fields using genetic algorithm (GA) methods. This allowed me to study CDB, a large organic molecule, on KCl(100). Static DFT calculations and classical MD simulations using these force fields showed that adsorbed CDB molecules are mobile at room temperature and stabilized at step edges due to increased adsorption energy. These results provide insight into the processes and mechanisms that govern deposition, adsorption, and diffusion of atoms and molecules on insulating surfaces and can help guide the design of functional molecules and films.