Synthesis and characterization of chemically pure nanometer-thin zero-valent iron films and their surfaces

The synthesis and characterization of 15, 25, 50, and 70 nm thin iron films having chemical impurities below the detection limit of various analytical techniques is reported. As established herein, the films are chemically pure and formed by electron beam deposition from inexpensive and readily available iron sources of 3N5 purity. Chemical purity of the thin films was achieved using mean deposition rates of 0.3 nm/s or higher, at which point the melting point of iron is reached at the iron source surface and a shutter is opened, from which point on the rate of transfer of impurities present in the source to the target is low enough that they are not observed in the film as confirmed via X-ray photoelectron spectroscopy (XPS), reported here for energies between 0 and 1200 eV. Nanoindentation measurements indicate the iron films to be 14 times harder than bulk iron. The iron films are shown by XPS to be coated with a 3 nm thin overlayer of Fe³⁺, which is possibly present in the form of Fe₃O₄, even though other forms of iron oxide are likely to be present as well, as indicated by Raman and XPS spectroscopy. Grazing incidence angle X-ray diffraction experiments indicate the presence of crystalline Fe⁰ with low index faces exposed but no crystallinity of the iron oxide overlayer. Atomic force microscopy of the iron film surfaces indicates narrowing and shifts to lower heights in the height distribution of nanoscale features formed during the film deposition process as the film thickness decreases. Second harmonic generation is then used to determine that the interfacial charge density of the thinnest iron film is -0.007(3) C/m² at pH 7.