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The shape of individual building blocks is an important parameter in bottom-up self-assembly of nanostructured materials. A simple shape change from sphere to spheroid can significantly affect the assembly process due to the modification to the orientational degrees of freedom. When a layer of spheres is placed upon a layer of spheroids, the strain at the interface can be minimized by the spheroid taking a special orientation. C fullerenes represent the smallest spheroids, and their interaction with a sphere-like C is investigated. We find that the orientation of the C within a close-packed C layer can be steered by contacting a layer of C. This orientational steering phenomenon is potentially useful for epitaxial growth of multilayer van der Waals molecular heterostructures.

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Self-assembly of binary and multi-component colloidal crystals is an effective approach for the fabrication of structured new materials. The formation of colloidal crystals in most cases is controlled by optimized space occupation by the particles. C 60 and C 70 molecules, when treated as the smallest "colloidal particles", can be used as model systems to study the formation of multicomponent molecular crystals in which the intermolecular forces are mainly of the van der Waals type. By deposition of C 60 and C 70 molecules onto Au(111), the mixing of the two molecules has been investigated. Both C 60 and C 70 form close-packed layers on Au(111), but with different lattice constants. At room temperature, the molecules within the molecular layer have complete freedom to rotate. The boundary between the C 60 and the C 70 domains consists of row dislocations. Adding C 60 onto a C 70 layer has an interesting effect in that C 60 molecules can push their way into the C 70 layer and force the C 70 molecules to take a fixed upright configuration where the long axis of the molecule is perpendicular to the interface. Significant interlayer diffusion takes place at elevated temperatures. When C 70 is added to a C 60 layer, there is little evidence of C 70 getting into the C 60 layer. The second layer C 70 forms a lattice-matched structure by taking the upright orientation.

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