The role of the residual metallic layer in the growth of alpha-Fe2O3 nanowires under thermal oxidation of Fe thin films

This work focuses on the first steps of growth of hematite nanowires by thermal oxidation of thin Fe films. Combining Scanning Electron Microscopy and depth-sensitive Fe K-edge X-ray Absorption Near Edge Structure (XANES) spectroscopy—in fluorescence and total electron yield modes—the oxidation process is monitored at the nanometric scale. The oxidation of the iron film occurs both below and above the original surface, driven by inward oxygen diffusion and Fe out-diffusion, respectively. The results reveal that as oxidation progresses from the surface, a layered structure forms, consisting of α-Fe2O3 (top layer), Fe3O4 (intermediate layer), and metallic Fe in contact with the substrate. The α-Fe2O3 nanowires nucleation starts only once a ∼100 nm thick oxide layer—including a thin hematite surface layer—has developed and continues until the initial metallic layer is fully consumed, reaching an areal density of ≈50μm−2. Their formation occurs through local out-diffusion of iron atoms from the residual metallic sublayer towards the surface via fast diffusion channels in the oxide layers, followed by their oxidation. These conditions can only be achieved for a minimum initial metallic thickness of about 70 nm. This systematic study, considering film thicknesses and annealing durations, provides a comprehensive picture of nanowire formation and growth, following a process that seems independent of the substrate (Si, amorphous SiO2, polyimide). These findings provide crucial constraints, in terms of minimum film thickness and requirement of a residual metallic layer, for the nanofabrication of hematite-based devices requiring a high surface-to-volume ratio.