C60 and its derivatives at Si surfaces : experimental and theoretical studies of electronic structure

• Main Author: Phillips, Michael Andrew
• Published: University of Nottingham 2004
• Subjects: 530.417; QD146 Inorganic chemistry
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.514851

There has been a considerable amount of research into the effects of the modifcation of C60 on its properties in the bulk and at metal surfaces. Strong covalent interactions between C60, its derivatives, and a silicon substrate, however, are not well understood, nor is their role in determining the electronic properties of a modified fullerene film. In conjunction with existing knowledge concerning the behaviour of unmodified C60 at silicon surfaces, the study of modified C60 at these surfaces may provide some insight into the nature of the adsorbate{substrate interactions. Furthermore, if the tunability o®ered by fullerene modification is to be exploited in the production of novel nanoscale devices, an understanding of the role of these interactions is of key importance. From a theoretical perspective, modelling a fullerene{substrate system can prove to be computationally costly, and an approach to the reduction of this expense has been tested. Relative stabilities of various binding geometries of C60 to the Si(100)-(2 x 1) surface have been determined. Additionally, an agreement between experimental results and a rather simplistic model of the C60/Si interaction suggests that the electronic structure of C60 adsorbed at Si surfaces is not strongly dependent upon adsorption geometry --- much of the structure in the valence region is inherent to the formation of localised covalent bonds. Photoemission spectroscopy has been used to study alkali- and noble-metal doping of C60 at Si surfaces, and the effects of functional groups on electronic structure and adsorption. An alternative method for the determination of charge transfer from a dopant to the fullerene cage has been proposed and tested. The results show that a metallic phase of KxC60 can exist at the Si(111)-(7x7) surface, but not at the Si(100)-(2x1) surface, and that the LUMO-derived band is split, in both cases. In contrast to the considerable charge transfer seen upon adsorption of C60 at noble-metal surfaces, a Si substrate severely limits the interaction of both Ag and Au with thin films of C60. Furthermore, Au demonstrates an interaction with the encapsulated Si surface that is su±ciently strong to displace adsorbed C60. A mechanism for this interaction is proposed. Finally, the addition of phenyl groups to C60 is observed to have a profound effect on the chemistry of the fullerene cage itself.