Effect of Cr-doping on the electronic structure and work function of α-Fe 2 O 3 thin films.

We investigate the effect of Cr-doping on the properties of α-Fe ₂ O ₃ (001) thin films with Fe termination using the local density approximation plus a Hubbard U correction. We find that both the doping site and concentration of Cr atoms dramatically affect the electronic structure and work function (W _F ) of α-Fe ₂ O ₃ films. The results demonstrate that it is most energetically favorable for Cr atoms to substitute the Fe atoms in the sub-surface of α-Fe ₂ O ₃ thin films. The doping of Cr atoms in the sub-surface not only lowers the band gap of the film but also greatly enhances the work function by 0.9 eV with respect to the pure α-Fe ₂ O ₃ film. The increase of W _F correlates with the reduction of occupied O p _x /p _y states at the top valence band which leads to a decrease of the Fermi energy. As the Cr concentration changes from 4.2% to 16.7%, the W _F firstly increases, and then drops. The W _F reaches a maximum of 6.61 eV for the Cr concentration of 8.3%. These results suggest that doping Cr atoms in a α-Fe ₂ O ₃ (001) thin film can increase the corrosion potential and benefits the protection of steel from corrosion.