STM/STS studies of adsorption and growth of fullerene(C₆₀) on highly oriented pyrolytic graphite (HOPG) surface

Fullerene C60 films from submonolayer to multilayer coverage are obtained on cleaved HOPG (0001) surface and studied by Ultra High Vacuum Scanning Tunneling Microscopy and Spectroscopy (UHV STM/STS). Topographic images reveal that C60 molecules self-assemble and form into ordered, extended overlayers in hesagonal-close-packing (HCP) on the basal plane of HOPG. Monolayer crystalline C60 islands of various sizes on top of the completed overlayer are observed under high flux conditions with well defined boundaries oriented in the symmetry directions of the C60 overlayer underneath. Under low flux conditions, however, more irregular shaped islands and dendritic growth are observed. No intramolecular features of C60 molecules is observed due to rapid, thermally agitated rotation and delocalization of electrons among these molecules, STS study of this system failed due to extremely low. dynamic conductance of the system. Images with different depostition rates from 0.3 monolayer (ML)/sec to 4.17 X 10[to the power of negative one] ML/sec are analyzed. Under the condition of fast deposition or high flux, small number of three dimensional, triple or multi-layer islands are found growing immediately on the first completed C60 overlayer on HOPG. The growth mode resembles the "Stranski-Krastanov" mode. However, further investigations under the slow deposition or low flus shows that the growth mode of the system is more close to the "Frank van der Merwe" mode. The growth mode is therefore dominanted by kinetic factors rather than surface free energies for this particular system This is consistent with theoretical predictions since the intermolecular interactions are only weak van der Waals forces. The island size distribution is also studied as a function of depostion rates. As different from AEK stacking of bulk C60 which has a well known FCC structure, unclear and confusing stacking of C60 islands are observed along different crystalline orientations. The exact mechanism is not knower. Defects like vacancies, dimer molecules, stacking faults, twin-boundaries. etc are observed which provide good evidence for the presence of strain within monolayer islands. Obvious, visible distortions and elongations of unknown nature of C60 molecules at the island boundaries are observed. The observed distortions can be attributed by polarization effects of electron clouds of C60 molecules at the boundaries of islands or due to tip convolution effects. A rudimentary program was written to simulate and demonstrate tip convolution effect. The simulated results are found to be consistent with the observed results and it is believed to be the dominant cause