Interaction of C 60 with Tungsten: Modulation of Morphology and Electronic Structure on the Molecular Length Scale J. Brandon McClimon, † Ehsan Monazami, and Petra Reinke* University of Virginia, 395 McCormick Road, P.O. Box 400745, Charlottesville, Virginia 22904-4745, United States * S Supporting Information ABSTRACT: The evolution of morphology and electronic structure in sequential depositions of W and C 60 on graphite has been studied by scanning tunneling microscopy/spectros- copy. 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 C 60 deposition. C 60 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 C 60 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 C 60 matrix. The signature of intercalation is cessation of molecule rotation, which is recognized by imaging of molecular orbitals. The electronic structure of C 60 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 C 60 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 C 60 due to accumulation of W at the graphite substrate-C 60 interface. ■ INTRODUCTION The study of interfaces between metals and organic molecules has been a frequent topic of study over the last several decades due to the attendant technologies, 1-3 such as organic light emitting diodes and organic solar cells where these interfaces are critical to functionality. 4 In particular, C 60 and its derivatives are of interest since C 60 has a direct band gap and electron acceptor properties that make it and its derivatives particularly suitable for photovoltaic applications. 5-7 The interaction of C 60 with metal and semiconductor surfaces has been studied extensively, and in contrast, the deposition of metals on the C 60 layers, where the metal is the highly mobile reactant, has only been observed in a few systems despite the frequent use of this process in the assembly of device structures. 8-10 The metal-fullerene interaction can have a strong impact on the electronic structure of the molecular layer, and as such is critical to device design. Several studies of submonolayer (ML) films of C 60 on single crystal metal surfaces have found strongly reduced band gaps compared to those present in the bulk phase. 11-15 In all cases, the effect has been attributed to image charge effects arising due to electron donation from the metal to the lowest unoccupied molecular orbital (LUMO) of the C 60 molecule. This explanation has been corroborated by ab initio calculations. 13,14 If the metal is deposited on a C 60 surface, a much larger variety of behavior is observed. For example, Au atoms have very high mobility on top of the C 60 layer and will travel to the edge of the layer before nucleating clusters. 8 Cr nucleates into clusters on the C 60 surface, while Ti and La form a conformal layer due to a chemisorption interaction. 16 Numerous attempts have been undertaken to form intercalation compounds where metal atoms diffuse into a C 60 matrix thereby forming exohedrally doped fullerene complexes. Such studies have been generally successful for alkali 17 and alkaline earth metals, 18 but not for transition metals. 16 This difference has been attributed to the higher cohesive energies of the transition metals which make metal cluster formation thermodynamically preferred over intercalation. 11 Nevertheless, this problem can be sidestepped by the use of nonequilibrium conditions which discourage metal clustering, as has been demonstrated for both Ti 19 and Ag 20 with C 60 . Two different reaction sequences are distinguished in the present study of C 60 -W composite material: the first type of experiment began by depositing the W on HOPG followed by the deposition of C 60 . In the second type of experiment the sequence was inverted and C 60 was deposited first followed by W deposition. The inversion of the deposition sequence leads to significant differences in the final morphology of the thin Received: July 3, 2014 Revised: September 18, 2014 Published: September 25, 2014 Article pubs.acs.org/JPCC © 2014 American Chemical Society 24479 dx.doi.org/10.1021/jp506618b | J. Phys. Chem. C 2014, 118, 24479-24489