| AVS 63rd International Symposium & Exhibition | |
| Surface Science | Tuesday Sessions |
| Session SS2+AS+HC+NS-TuM |
| Session: | Nanostructures: Growth, Reactivity, and Catalysis |
| Presenter: | Ehsan Monazami, University of Virginia |
| Authors: | E. Monazami, University of Virginia J.B. McClimon, University of Pennsylvania J.M. Rondinelli, Northwestern University P. Reinke, University of Virginia |
| Correspondent: | Click to Email |
The current literature on annealing of fullerene molecules on tungsten surfaces indicates a complete dissociation of the fullerene cage and the formation of a carbide phase. However, our measurements with high resolution STM and STS illustrate a complex intermediate reaction sequence. Upon annealing of C60 adsorbed on a tungsten thin film grown on MgO (001) in UHV, C60 does not dissociate and the spherical C60 shape is retained up to a temperature of at least 973 K. During the annealing, the band gap of the molecular layer decreases gradually from the wide bandgap of fullerene to a fully metallic electronic state. This transition occurs in a narrow temperature range between 600 K and 700 K. After this transition, the near-spherical particles are termed “nanospheres.” This progression was observed with a series of high resolution scanning tunneling spectra and detailed spectral mapping. The bandgap variation presents an approach to achieve the formation of densely packed nanoclusters (nanospheres) with variable bandgap, which are stable at elevated temperatures. Experimental results for sub-ML fullerene coverage on tungsten show that the fullerene molecules are mobile at room temperature, but they become stationary after annealing above 500 K. This immobilization of molecules indicates a strong interaction and likely a covalent bond between the molecule and substrate that is triggered by annealing.
The progression to metallic nanospheres is hypothesized to occur either by gradual substitution of W-atoms or by wetting the molecule with W-atoms and thus formation of W-C bonds in a solid state reaction. These models were tested using density functional theory (DFT) calculations. Two simulation strategies were used. In the first, C60 carbon atoms were substituted by W in the molecule and the resulting electronic properties and bandgap were calculated. In the second approach, different adsorption geometries of a C60 molecule on the tungsten (110) surface were considered. The variation of the band gap due to different C60 orientations relative to the tungsten substrate and various types of hetero-fullerenes will be discussed.