Geometric and Electronic Structure of Fullerene Film Growth as a Function of Coverage
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B. REIHL IBM Research Division, Zurich Research Laboratory, 8803 Ruischlikon, Switzerland
ABSTRACT We have employed scanning tunneling microscopy at room and low temperature, i.e. 300, 50, and 5 K, to study the epitaxy and growth of fullerene films on the noble-metal surfaces Ag(110) and Au(110). Initial island growth occurs on terrace sites away from substrate step edges. Particularly at low temperatures where the rotational and vibrational movements of the fullerene molecules are frozen in, different intra-molecular topographic patterns become visible in ordered films, which are characteristic of particular adsorption sites. Complementary tunneling spectroscopy and direct and inverse photoemission measurements reveal distinct differences between the first adsorbed monolayer and additional fullerene layers indicating differences in bonding and charge transfer. Our results are compared to theoretical calculations.
INTRODUCTION The development of a new and inexpensive technique [1] for the mass production of C60 molecules has prompted an intense research activity to explore the novel properties of this new form of solid carbon. The cage-like arrangement of 60 carbon atoms on equivalent sites to form a soccer-ball (named fullerene), which again condenses on fcc lattices site to build up a solid called fullerite, is established by now [2]. An extra impetus came from the discovery of a new kind of high-TC superconductivity in alkali-doped fullerite [3]. Its understanding requires a detailed knowledge of the electronic structure and correlation effects involved. Consequently there exists a great body of experimental and theoretical papers [4-18] dealing with the energy positions of the highest-occupied and lowest-unoccupied molecular orbitals, the resulting HOMO-LUMO gap, and the changes induced by alkali-metal doping. We mention in particular that the electronic properties of KC 60 can be varied from semiconducting (x = 0), to metallic/superconducting (x = 3), to insulating (x = 6) by controlling the doping level of the alkali metal [4,51. Although it seems to be obvious, there exist few detailed investigations on the interplay of the geometric and electronic properties of fullerite formation. In many cases fullerite is formed by growing films of C6o on metal or semiconductor substrate surfaces. Hence, interfacial epitaxy, possible charge transfer, and different adsorption phases play a crucial role and determine the electronic and geometric properties of the first layers of the fullerene film. Here I summarize our findings of C60 adsorption on the clean Ag(l10), Au(ll0)lx2, and Si(100)2xl surfaces employing the techniques of scanning tunneling microscopy (STM), tunneling spectroscopy, photon-emission with the STM, and direct and inverse photoemission. For more details I refer the interested reader to our original publications [19-261. There are many articles and books which describe the various techniques used by us in general terms. Their references may also be found in the original papers [19-26]. 375 Mat. Res. So
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