Epitaxial growth of C 60 thin films on mica
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Single crystal films of C6o of different thickness values have been deposited on mica substrates by resistance evaporation. Electron diffraction and high resolution microscopy have been used to assess the orientational ordering and the nature of the defects present in these face-centered cubic films which exhibit a (111) direction normal to the film surface.
The ability to be able to fabricate buckminsterfullerenes, C60 molecules,1 in reasonably large quantities2'3 has made possible studies of the structural properties of this form of carbon when it is in a crystalline state. Well-formed three-dimensional bulk crystals up to 0.4 mm diameter have been reported free of solvents using an extraction process, where the Cso material was heated in a quartz tube and allowed to condense in a cooler region.4 At room temperature, single crystal x-ray diffraction shows that the molecules are centered on sites of a face-centered cubic (fee) Bravais lattice, with a high degree of rotational disorder, aQ = 14.2 A. More recently it was also found that a low temperature phase exists below 249 K with diffraction peaks that can be indexed as simple-cubic reflections with mixed odd and even indices (i.e., forbidden reflections5). Although somewhat earlier in time, a study of C6O/C7o bulk material using electron diffraction and high resolution electron microscopy has revealed a hexagonal closepacked phase which rapidly degraded under the electron beam.6 Although it was not noted, the reflections can be classified as being of the mixed type. However, because of the appreciable solvent concentration, these forbidden reflections would be caused by localized defects producing void-like regions and hence local regions of simple cubic structure rather than a hexagonal structure. It is reasonable to expect the highly ordered C6o thin films will have superior properties to bulk material for certain applications such as superconductivity. To this end, several film studies have been undertaken to understand the early stages of C60 growth. Monolayer and multilayer growths of C60 on (110) GaAs at 300 K and elevated temperatures have been studied by scanning tunneling microscopy.7 Both equilibrium and nonequilibrium structures were found in these van der Waals solids, and there is lack of preference of binding sites on the substrate, except where large corrugations or substrate steps exist. A similar type of study of the monolayer growth regime of sublimed C6o on mica substrates has also been performed by helium atom scattering.8 The main conclusion of this work was the demonstration of a single monolayer C60 epitaxial growth at room tem784
http://journals.cambridge.org
J. Mater. Res., Vol. 7, No. 4, Apr 1992
Downloaded: 15 Mar 2015
perature conditions with a periodicity of 10.4 A, which is double that of (001) mica (5.2 A). Very recently a preliminary investigation of thick film fabrication of C60 on MoS 2 substrates was reported producing 0.1 fim thick films measuring 1.5 cm x 1.5 cm. Electron diffraction was used and the films were said to
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