Electrochemical behaviour of low temperature grown iron fluoride thin films

By coupling a homemade substrate holder with a refrigerated ethanol cryogenic system, we succeeded in growing thin films by Pulsed Laser Deposition at substrate temperatures as low as −50 °C. The benefit of enlarging substrate temperatures to negative values is illustrated through the example of iron fluoride thin films, for which the substrate temperature is a key factor governing the FeF 2 or/and FeF 3 phase deposition. Using a FeF 3 target, the X-ray diffraction study shows that the “FeF x ” thin films grown at 600 °C correspond to a single well-crystallized FeF 2 phase (S.G.: P4 2/ mnm) as opposed to a mixture of FeF 3 and FeF 2 phases for room temperature substrate, and a single FeF 3 phase (S.G.: R 3 ¯ c ) having quite an intense (0 1 2) Bragg peak at low temperature substrate (−50 °C). Such assignments were confirmed by complementary HRTEM and Mössbauer measurements with the exception of the −50 °C grown film that was shown to contain amorphous FeF 2 together with crystallized FeF 3 phases. The electrochemical behaviour of the FeF x thin films, namely their voltage profiles, was found to be dependent on the substrate grown temperature.