A number of studies have been devoted to the organization and local properties of C60 films deposited on metallic or semi-conducting surfaces. Much less attention has been paid to C70 overlayers, which present a greater variety of morphologies and properties than C60 because the molecule symmetry is lowered from Ih to Dh and has an ellipsoidal shape.
A few STM studies of C70 overlayers on Au, Ag, Cu, Si, and GaAs evidence the existence of a local order. In most cases, the C70 overlayer is produced and observed in UHV. STM observation of C70 in air has also been reported but leads to a poor resolution. Surprisingly no attempt has been made to observe C70 by STM at a liquid-solid interface, a convenient technique which allows to prepare molecular self-assemblies and observe them in situ, in ambient conditions and without contamination.
We report here on the STM study of C70 (sub)-monolayers deposited at the tetradecane/Au(111) interface. We compare adsorption of C70 with previous STM results that we obtained with C60 in the light of their geometric and electronic differences.
Similarly to C60, at room temperature C70 molecules keep rotate at the liquid/gold interface thus preventing STM intramolecular resolution. Our study shows that the C70 molecules adopt two types of arrangements: 1/ a 2Ö3´2Ö3 R30° structure with long molecular axis perpendicular to the Au (111) surface (“a-domains”), and 2/ a quasi-hexagonal in-phase packing with long axis parallel to the surface (“b-domains”).
In the a-domains, C70 molecules form a commensurate structure against the <112> direction. C70 adopt an upright position, with the short axis parallel to the surface. In the b-domains, the molecular rows are parallel to the <110> direction of gold. Their structure is quasi-hexagonal with a unit cell elongated in the <110> direction. This type of packing is reminiscent of the in-phase structure observed for C60 monolayers on gold in UHV.
Finally, we checked the possibility to distinguish C70 from C60 in a binary C60/C70 mixture (80:20). The deposition of this mixture on Au(111) generates an overlayer presenting close-packed areas among which a number of brighter spots are observed (see Figure). Based on their larger apparent height (by ~1-2 Å) we attribute these bright spots to C70.
Further work is presently underway to investigate the conduction properties of both structures of C70 on Au(111) by means of STS.
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