Interaction of C₆₀ with Tungsten: Modulation of Morphology and Electronic Structure on the Molecular Length Scale

The evolution of morphology and electronic structure in sequential depositions of W and C₆₀ on graphite has been studied by scanning tunneling microscopy/spectroscopy. The deposition sequence decisively controls morphology expression. W deposited on a graphite surface forms small clusters whose morphology is consistent with the predictions of a liquid droplet model in the size regime below 5 nm in diameter; these small clusters then agglomerate without sintering. These agglomerates are immobilized by the subsequent C₆₀ deposition. C₆₀ shows very little interaction with the W-cluster agglomerates, and the formation of typical close packed fullerene islands is observed. The inverse deposition sequence, W deposition on the surface of C₆₀ multilayer islands, leads simultaneously to the formation of ultrasmall W clusters (d < 2 nm) due to limited mobility on the highly corrugated surface, and the intercalation of W in the C₆₀ matrix. The signature of intercalation is cessation of molecule rotation, which is recognized by imaging of molecular orbitals. The electronic structure of C₆₀ is not significantly modified by the presence of W agglomerates, clusters, and intercalation of W. However, if W is deposited on a single layer of C₆₀ its impact on the electronic structure is considerable and expressed in a compression of the band gap, which might be attributable to charge screening due to image charges, or the onset of molecule breakdown. The morphology as well as the electronic structure of this layer is highly inhomogeneous and can be described as a composite of W and C₆₀ due to accumulation of W at the graphite substrate–C₆₀ interface.