Spectroscopy of ultrathin epitaxial rutile

Epitaxial ultra-thin titanium dioxide films of 0.3 to ~7nm thickness on a metal single crystal substrate have been investigated by high resolution vibrational and electron spectroscopies. The data complement previous morphological data provided by scanned probe microscopy and low energy electron diffraction to provide very complete characterisation of this system. The thicker films display electronic structure consistent with a stoichiometric TiO2 phase. The thinner films appear nonstoichiometric due to band bending and charge transfer from the metal substrate while workfunction measurements also show a marked thickness dependence. The vibrational spectroscopy shows three clear phonon bands at 368cm, 438cm and 829cm (at 273K) which confirms a rutile structure. The phonon bands intensity scales linearly with film thickness and shift slightly to lower frequencies with increasing temperature, in accord with results for single crystals.