C 60 bonding to graphite and boron nitride surfaces

Abstract

The present study focuses on the interaction of C 60 with the surfaces of highly oriented pyrolitic graphite (HOPG) and sp 2 -bonded boron nitride (BN). The nanocrystallineBNfilm was deposited by mass selected ion beams and features an sp 2 -bonded surface layer, which covers a cubic phase BNfilm. The first part of the experiment is the sequential deposition of C 60 , which is monitored by photoelectron spectroscopy in the x-ray(XPS) and ultraviolet (UPS) regime. The growth of the C 60 layer on HOPG is close to a layer-by-layer growth mode, but on the BN surface island growth is favored. No charge transfer or chemical reaction (e.g., carbide formation) between the fullerene layer, and the underlying substrate is observed in either case. In the second part of the experiment the samples are heated at a rate of 10 K/min while simultaneously recording the UPS VB spectra. The complete desorption of C 60 from the HOPG surface occurs in a small temperature interval between 510–530 K. For the sp 2 BN surface the majority of C 60 desorbs around 493 K, about half a monolayer (ML) remains, and the C 60 concentration decreases gradually with increasing temperature; less than a tenth of a ML can be detected even at 1000 K. The first desorption event at 493 K is attributable to the multilayer desorption from islands. The remaining C 60 directly in contact with the BN surface is then removed in a large temperature interval between 500 and 1000 K which indicates the presence of a multitude of adsorption sites. The presence of C 60 on the BNfilm surface also induces a band bending and related B 1s and N 1s core level shifts. An upward band bending is present in the C 60 overlayer, which indicates that defects are responsible for the pinning of the Fermi level at the interface.