Investigating fluorination of thin metal oxides through spin coating and vapor transport treatments

We have studied the effects of two ‘fluorination’ treatments, spin coating and vapor transport, on the structural, magnetic, and electrical properties of La0.67 Ca0.33MnO 3-y (LCMO) thin films. Spin coating fluorination treatment involves coating the films with a fluorine containing polymer Poly-Vinyl Difluoride (PVDF) followed by an ex-situ heat treatment. Spin coating fluorination treatment of oxygen deficient films decreases the resistivity by several orders of magnitude, compared to as grown. and induces the insulator-metal transition. X-ray diffraction shows an additional phase which corresponds to a shortened c-lattice parameter. Identical control samples subject to the same thermal treatment without the PVDF coating do not show any significant changes.The drastic decrease in resistivity and the occurrence of the insulator-to-metal transition indicate that incorporation of fluorine at oxygen vacancy sites may lead to increase in the hole doping thus promoting a higher Mn4+ to Mn3+ ratio. Vapor transport fluorination (VTF) involves heating PVDF pellets and in a tube furnace under flowing argon gas. Similar experiments on another perovskite material (SrFeO3) reported in literature shows that VTF process lengthens the c-lattice parameter of thin film samples as compared to as grown films, the opposite result of the spin coating treatment, suggesting that VTF is reductive. We will present our results comparing the effects of fluorination on LCMO thin films by spin coating and vapor transport.