Interaction of C₆₀ with Isoelectronic Surfaces: Graphite and Hexagonal Boron Nitride

Abstract

In the present study we focus on the interaction of C 60 with sp 2 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 sp 2 surface layer, which covers a cubic-BN film. The interaction and electronic properties of the C 60 -BN system are observed by photoelectron spectroscopy in the xray (XPS) and ultraviolet regime (UPS). The experiment is initiated by sequential deposition of C 60 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 C 60 desorbs from the sp 2 BN surface at 493 K. The remaining C 60 (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 C 60 with either element or a charge transfer are observed. The presence of C 60 induces an upward band bending and related peak shifts in the sp 2 BN surface layer and to a lesser extent in the underlying c-BN bulk. An upward band bending is likewise observed in the C 60 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. INTRODUCTIONThe 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 C 60 with the surface of a wide-bandgap semiconductor, namely sp 2 boron nitride (BN) that is isoelectronic to graphite 6-10 . The bandgap 11 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 process 12-16 . The films consist of an sp 2 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 C 60 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 and the layered structure of the substrate afforded information on the extension of band bending into the film.