Interaction of C 60 with Isoelectronic Surfaces: Graphite and Hexagonal Boron Nitride
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INTERACTION OF C60 WITH ISOELECTRONIC SURFACES: GRAPHITE AND HEXAGONAL BORON NITRIDE P. Reinke, H. Feldermann, 2. Physikalisches Institut, Universität Göttingen, Bunsenstr. 7-9, 37073 Göttingen, Germany, P. Oelhafen, Institut für Physik, Universität Basel, Klingelbergstr. 82, 4056 Basel, Switzerland ABSTRACT In the present study we focus on the interaction of C60 with sp2 boron nitride (BN), a surface that is isoelectronic to graphite. The nanocrystalline BN substrate was deposited by mass selected ion beam and consists of an sp2 surface layer, which covers a cubic-BN film. The interaction and electronic properties of the C60-BN system are observed by photoelectron spectroscopy in the xray (XPS) and ultraviolet regime (UPS). The experiment is initiated by sequential deposition of C60 and the overlayer growth proceeds via island formation. In a second step the sample is annealed at a rate of 5-10 K/minute while simultaneously recording the UPS spectra. The majority of C60 desorbs from the sp2 BN surface at 493 K. The remaining C60 (initially about 0.6 ML) is gradually removed with increasing temperature (up to 813 K) but never completely desorbed and presumably attached at surface imperfections or grain boundaries of the nanocrystalline material. No carbide formation, preferential interaction of C60 with either element or a charge transfer are observed. The presence of C60 induces an upward band bending and related peak shifts in the sp2 BN surface layer and to a lesser extent in the underlying c-BN bulk. An upward band bending is likewise observed in the C60 overlayer, and the Fermi level has therefore to be pinned by defects in the interface region. The layered structure of the BN film allows to probe the extension of the space charge layer in the BN film. INTRODUCTION The application of fullerene layers as functional surface layers and as part of multilayer structures has been reported a few times 1-5, but information on the chemical and electronic structure of the interfaces and surface is necessary to further develop this type of materials. In the present study we chose to concentrate on the interaction of C60 with the surface of a wide-bandgap semiconductor, namely sp2 boron nitride (BN) that is isoelectronic to graphite6-10. The bandgap11 exists because of the ionicity of the BN bond in contrast to the purely covalent bonds present in graphite, which is a semimetal. BN thin films, which are grown by ion-assisted methods, feature a layered structure introduced through the ion driven growth process12-16. The films consist of an sp2 bonded surface layer and a cubic BN bulk, if grown in the appropriate deposition regime defined here by the substrate temperature and ion energy. The experiments described on the following pages deal with the growth of the C60 layer and its subsequent desorption as a function of temperature. Photoelectron spectroscopy was used to monitor the changes in surface composition and study the adsorbate-surface interaction. The electronic properties of the interface can be deduced an
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