Modelling of fullerenes on silicon surfaces

An extension to the capabilities of an ab-initio density functional theory package, PLATO, has been undertaken. This concerned the calculation of Slater-Koster integrals and their derivatives, via the recursive methods initially proposed by Podolskiy and Vogl, and developed by Elena and Meister. This extension provides the ability to include the previously unavailable f -orbitals (and beyond) within PLATO calculations. Calculations have been performed, including f - orbitals, on silver, silicon and nitrogen systems. The results show a modest improvement, in terms of the convergence of the total energies calculated, when comparing the calculations including f -orbitals to those without. The impact on computational time is mixed, with both decreases and increases in compuational time demonstrated, dependent on the system in question and the type of calculation performed. The interactions between C60 molecules and the Si (100) surface, as well as the interactions between the endohedrally doped N@C60 molecules and the Si (100) surface have been explored via ab-initio total energy calculations. Configurations which have the cage located upon the dimer row bonded to two dimers (r2) and within the dimer trench bonded to four dimers (t4) have been investigated, as these have previously been found to be the most stable for the C60 molecule. We show that our results for the adsorption of the C60 molecule upon the Si (100) surface are comparable with previous studies. We have investigated the differences between the adsorption of the C60 and N@C60 molecules upon the Si (100) surface and found that there are only minimal differences. It is shown that the effects on the endohedral nitrogen atom, due to its placement within the fullerene cage, are small. Bader analysis has been used to explore differences between the C60 and N@C60 molecules. The interactions between pairs of C60 molecules adsorbed upon the Si (100) surface have also been studied. The same selection of t4 configurations used for the isolated fullerenes is explored in all possible pairs of fullerene configuration combinations. A previous study by Frangou explored pairs of fullerenes in adjacent bonding sites on the silicon surface, this study, however, investigates bonding sites separated by one silicon dimer. Comparisons between the two studies confirm the trend of the combinations becoming more favourable at a greater fullerene separation. There are several cases where the combined pair of fullerenes are less favourable than the two isolated cases, so these are studied indepth. The separation chosen in our study reflects the experimental separation observed by Moriarty et al..