AN STM STUDY OF C-60 ADSORPTION ON SI(100)-(2X1) SURFACES - FROM PHYSISORPTION TO CHEMISORPTION

The adsorption of C-60 molecules on Si(100)-(2 X 1) surfaces before and after annealing has been studied using scanning tunneling microscopy (STM). For room temperature deposition, the STM images reveal that the C-60 molecules adsorb predominantly at the four-dimer sites in the troughs between dimer rows. C-60 molecules bonded to two-dimer sites are also observed in small molecular clusters forming even at a low coverage. The nature of the interaction between the adsorbates and the Si(100)-(2 X 1) surface is explained in terms of a dipole-induced dipole interaction. Subsequent annealing of the samples to 600 degrees C changes the adsorption characteristics of the C-60 molecules. First, the annealing causes a strong covalent bonding between the carbon atoms of the C-60 molecules and the silicon atoms of the substrate, and modifies the bonding sites of the adsorbates from locations in the troughs to locations on the dimer rows. Second, the annealing causes some initial surface diffusion and clustering of the C-60 molecules, which now tend to stick to the ends, rather than the sides, of the dimer rows. Furthermore, after the annealing process, it is observed that small silicon islands form on the substrate terraces along with isolated and clustered adsorbates.