HfO2-doped Silica Thin Films by XPS

The development of molecularly homogeneous multicomponent oxide based glasses is a key concern in many fields of materials chemistry. Oxide nanoparticles covalently embedded in dielectric matrices represent a class of nanocomposite systems endowed with outstanding optical, mechanical, electronic, and thermal properties. In this study, silica thin films embedding HfO2 were prepared by dip-coating on silica glass via a modified sol-gel processing. This powerful synthetic method, mainly based on the hydrolysis and condensation of metal alkoxides, provides a reliable route to oxide materials which can be processed into a variety of different forms ~thin films, powders, monoliths, etc.!. The novel synthetic route here presented is based on the copolymerization of the organically modified oxohafnium clusters ~Hf4O2~OMc!12 with OMc 5 OC~O!-C~CH3!5CH2!! with ~methacryloxymethyl!triethoxysilane ~MAMTES!. The crystalline clusters, which are the precursors for the corresponding metal oxide ~MO2! were prepared via the sol-gel route by reaction of hafnium butoxide with methacrylic acid. The copolymerization of the cluster with previously prehydrolysed methacrylate-functionalized siloxane, allows the anchoring of the oxocluster to the forming silica network. Thin films were prepared starting from a THF ~tetrahydrofurane! solution with molar ratios Hf4O2~OMc!12:MAMTES of 1:44 and a MAMTES:THF molar ratio of 1:5, which was stirred at room temperature for 8 h. After deposition, the film was annealed 3 h at 800 °C in air to promote the decomposition of the hafnium oxocluster to give the corresponding HfO2 . The obtained HfO2-SiO2 film resulted transparent and homogeneous. The composition of the film was investigated by secondary ionization mass spectrometry ~SIMS! and x-ray photoelectron spectroscopy ~XPS!, which were used also to investigate the in-depth distribution. The depth profiles evidenced a very homogenous distribution of hafnium within the whole silica film and a sharp film-substrate interface. TEM micrographs revealed the formation of isolated nanocrystalline particles, thus ruling out the formation of a HfO2-SiO2 mixture. Concerning the herewith reported XPS analyses, the main XPS core-levels were analyzed for the sample annealed 3 h at 800 °C. The formation of hafnium oxide was evidenced. Furthermore, a very homogeneous distribution of the guest oxide in the host matrix was evidenced, in agreement with SIMS data.