Enhanced periodic modulation of electronic states in a hexagonal iron-nitride monolayer on Cu(001) via interfacial interaction

Metal nitride has robust bonding between metal and nitrogen atoms. We have synthesized a monoatomic layer of a hexagonal-type iron nitride with strong Fe-N bond on the Cu(001) substrate with a fourfold lattice symmetry. From atomic-scale observation using scanning tunneling microscopy, we have found that the Fe-N film exhibits a strip structure consisting of regular and deformed hexagonal lattices. The lattices are deformed along a hypothetical Moir\'e pattern made by a perfect hexagonal lattice on the square Cu(001) lattice. With increasing the lattice deformation, the strain energy increases, whereas the periodic deformation of the Fe-N lattice can minimize the total lattice energy including the strain energy of the Fe-N in-plane bonding and interfacial bonding energy between the Fe-N layer and the Cu substrate. In $\mathrm{d}I/\mathrm{d}V$ spectra, periodic spatial change of the unoccupied density of states due to the lattice strain was observed. This is attributed to an energy shift of the antibonding states due to compressive strain.