Temperature effects of adsorption ofC60molecules on Si(111)-(7×7) surfaces

The adsorption of submonolayers ${\mathrm{C}}_{60}$ molecules on Si(111)-(7×7) surfaces, deposited at room temperature and then annealed to various elevated temperatures, is studied using scanning tunneling microscopy. For room temperature adsorption, the ${\mathrm{C}}_{60}$ molecules were observed to favor bonding to adatom bridge sites within a triangular half cell. After annealing to 600 °C, however, they were found to bond dominantly to adatom or restatom sites. The adsorbate-substrate interaction at room temperature is characterized by charge transfer from the substrate to the molecules. After the annealing process, however, the adsorbates bond covalently to the substrate. It is found that the desorption of the submonolayer adsorbates due to the annealing process is minimal. The surface diffusion of the adsorbates begins when annealing the sample to 700 °C, at which point the initial clustering of the adsorbates takes place. When annealing the sample to 850 °C, the ${\mathrm{C}}_{60}$ molecules decomposed on the sample surface and reacted with the Si atoms to form SiC islands.