The control and utilisation of individual molecules adsorbed on insulating substrates could have profound technological applications, particularly in the area of molecular electronics. To understand mechanisms of adsorption and diffusion we have performed calculations of organic molecules on the MgO (001) and and TiO2 (011) surfaces - both technologically important oxides that have been imaged on the atomic scale with scanning probe microscopes.
Using high quality quantum chemical calculations we studied the adsorption of a variety of small prototype organic molecules on the perfect and defective MgO (001) surface. It was found that hydrocarbon molecules (i.e. benzene and methane) are very weakly bound to the surface and will not be de-protonated. Polar molecules, such as nitromethane, methanal and pyridene bind more strongly, and could form the basis of 'anchoring' groups to limit the diffusion of a larger molecule. Benzene and larger aromatic hydrocarbons will bind more strongly to charged oxygen vacancies in the surface (F+ and F2+ centres) and it was found that these defects can significantly lower the un-occupied energy levels of these molecules. This property could be useful in controlling the conductivity of a molecule or modifying the interaction with metallic electrodes.
The adsorption of larger organic molecules on the TiO2 (011) surface has been simulated using inter atomic potentials that we have previously developed from quantum mechanical calculations. The mechanism of diffusion of these molecules on the surface has been investigated using both transition path calculations and molecular dynamics simulations. We have found that subtle differences in the degree of freedom and flexibility of anchoring groups can significantly affect rates of diffusion on the surface. |