How Metallic Fe Controls the Composition of its Native Oxide

We have studied in situ the oxidation of ultrathin iron layers and monitored the chemical changes induced by subsequent deposition of Fe metal using hard x-ray absorption spectroscopy. The site sensitivity of the technique allows us to quantify the composition of the layer throughout the oxidation or deposition process. It is found that the thin native oxide incorporates a significant fraction of Fe atoms remaining in a metallic configuration even in the saturated state. Subsequent deposition of Fe leads to a complete reduction of the oxide that adopts an FeO-like structure containing ${\mathrm{Fe}}^{2+}$ sites only.

Figure 1

(a) Fe $K$-edge XANES spectra of reference samples for Fe, FeO, ${\mathrm{Fe}}_3{\mathrm{O}}_4$, and $\gamma \mathrm{\text{−}}{\mathrm{Fe}}_2{\mathrm{O}}_3$. (b) Evolution of the absorption edge position and shape for a 0.8 nm Fe layer exposed to an increasing amount of oxygen. (c) Evolution of the spectrum upon coverage of the saturated oxide by Fe. (d) Least square fitting of the saturated oxide spectra using a linear combination of reference spectra. (e) Evolution of the relative concentration of the different chemical sites as a function of oxygen exposure (left) and as a function of the Fe coverage on top of the saturated oxide (right).

Figure 2

Evolution of the ${\mathrm{Fe}}^0$ concentration in the saturated (6500 L) oxide as a function of the initial Fe layer thickness.

Figure 3

Electron energy diagram (left-hand panels) and net charge distribution (right-hand panels) at different stages of the oxidation or reduction. $E_F$ is the Fermi level. ${\varphi }_m$ and ${\varphi }_o$ are the metal and ${\mathrm{O}}^{-}$ level work functions, respectively. $E_C$ and $E_V$ are the energy levels of the conduction and valence band of the oxide, respectively. (a) Oxidation. The Mott potential $V_M={\varphi }_o-{\varphi }_m$ appears across the oxide layer, promoting the cation diffusion. (b) When the oxygen supply is shut off, the system is in equilibrium and only a small residual electric field is present at the metal-oxide interface due to the induced space charge in the oxide. (c) After deposition of Fe. The space charge in the oxide induces a net charge on the ultrathin Fe adsorbate which promote the Fe diffusion in the oxide. The right-hand inset shows the progressive growth of the ${\mathrm{Fe}}^{2+}$ layer.

Figure 4

(a) Fourier transform (FT) modules in the $r$ space of the EXAFS signal of unoxidized Fe layers of different thickness. The inset shows the evolution of the mean square displacement ${\sigma }^2$ of the first coordination shell. (b) FT of the EXAFS signal for the saturated native oxide (6500 L) and subsequent coverage with 0.2 nm Fe ($+0.2\mathrm{nm}$ Fe). The fitted radial distribution of the latter was obtained by fitting the filtered EXAFS signal constrained to the first two subshells ($r<0.3\mathrm{nm}$). For comparison, a FeO reference for bulk FeO is given (dashed line).