Interaction of C60 with transition metals for thin film applications

Abstract: The fullerene molecule C60 is known to undergo many different chemical reactions, including intercalation of alkali- and alkaline earth metals. Relatively little is still known about the reaction between transition metals and C60, and the objective of this thesis was therefore-to investigate the chemical interaction between C60 and some group IV-VI transition metals. The method chosen for this purpose was thin film studies employing either co-evaporation of the metal together with C60 in a ultrahigh vacuum (UHV) system or by the reaction of gaseous C60 with a transition metal halide in a chemical vapor deposition (CVD) process. In addition, adsorption studies of some transition metal halides were also performed in UHV.In situ x-ray photoelectron spectroscopy (XPS) of films obtained by co-evaporating Ti and C60 at approximately 100°C under UHV conditions showed two different reaction pathways depending on the Ti/C60 ratio in the film: (i) Deposition of films with Ti/C60>~3.5 resulted in disruption of the C60 molecule and titanium carbide (TiC) formation. Based on this reaction, it was shown that epitaxial TiC films could be deposited on substrates like MgO or SiC at substrate temperatures as low as 250°C. X-ray diffraction, low energy electron diffraction, and transmission electron microscopy were used to confirm the epitaxial growth behaviour of TiC. Moreover, the feasibility of using this process in the deposition of refractory carbide multilayers was demonstrated by growing a TiC/VC superlattice. (ii) For Ti/C60<~3.5, a new compound (titanium fulleride, TixC60) was formed. Soft x-ray spectroscopy and Raman spectroscopy also supported compound formation. This TixC60 phase showed enhanced electrical conductivity and optical reflectivity compared to pure C60. Detailed information of the physical properties was impeded by the rapid oxidation upon air exposure. Finally, C60 surfaces showed no adsorption of transition metal halides such as Ticl4 and WF6 and it was shown that this property can be used for selective CVD.