We have grown fee crystalline islands of C6o on a clean Si(l 11)7x7 surface. Scanning tunneling microscopy reveals that the islands are oriented primarily in tv^o distinct directions independent of the island height and size. We show that this is a result of the pinning of C6o molecules at the corner holes associated with a disorder-to-order transition at the interface. Interlayer molecular interactions play a vital role in the process. The presence of two distinct island orientations is a manifestation of the formation of double domain solid Ceo on the Si(111 )7 x 7 surface.PACS numbers: 68.35.Bs, 61.16.Ch, 68.35.Fx, 68.35.Rh The epitaxial growth of C6o on semiconductor and other substrates has attracted much attention following the advance in synthesizing abundant quantities of pure fullerenes [!]. While it is well established that the interaction in crystals of C6o is governed by the van der Waals forces [2], fundamental questions about the nature of the interaction at the semiconductor-Ceo interfaces remain unanswered. Do substrate atoms interact in concert with C60 molecules as a whole or do they form covalent bonds with just a few of the nearest carbon atoms within each C60 molecule? In either case, how can we best describe the interaction force and its strength?On GaAsd 10), Li et al. [3] observed that at submonolayer coverage, Ceo molecules form monolayer islands that are well ordered and commensurate with the substrate. Their results indicate that Ceo has fairly high mobility on the surface due to its weak interaction with the substrate. On Si(lOO), on the other hand, Hashizume et al. [4] showed that the sticking probability for Ceo is much higher and that although island formation begins after the first disordered layer is completed, ordered fee islands are only found above the third layer. Stronger still is the interaction between Ceo molecules and the Sid 11)7x7 surface. Neither of these two groups observed any ordered structures of Ceo on Si(l 11)7x7 [5,6]. These examples indicate that the strength of the interfacial interaction varies appreciably for different semiconductor surfaces.In this Letter we show that crystalline islands of Ceo can, in fact, be grown on the Si(l 11)7x7 substrate. More surprisingly, these Ceo islands orient themselves primarily in two distinct directions independent of their height or their size. We shall demonstrate that these two preferred orientations are clear manifestations of the formation of a double domain Ceo solid on the Si(l 11)7x7 surface. The process leading to such a unique structure involves a disorder-to-order interface transition in which interplanar molecular interactions play an indispensable role. Our observations suggest that the force binding the Ceo molecules to the semiconductor substrates is comparable to the van der Waals forces between the molecules of the Ceo solid in general, but it is extremely sensitive to the detailed local atomic and molecular configurations.The experiments were performed in a UHV system equipped with a homemade scanning t...

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