Abstract
Electronic structure modifications due to strain are an effective method for tailoring nano-scale functional materials. Demonstrated on nickel oxide (NiO) thin films, Resonant Inelastic X-ray Scattering (RIXS) at the transition-metal M2,3-edge is shown to be a powerful tool for measuring the electronic structure modification due to strain in the near-surface region. Analyses from the M2,3-edge RIXS in comparison with dedicated crystal field multiplet calculations show distortions in 40 nm NiO grown on a magnesium oxide (MgO) substrate (NiO/MgO) similar to those caused by surface relaxation of bulk NiO. The films of 20 and 10 nm NiO/MgO show slightly larger differences from bulk NiO. Quantitatively, the NiO/MgO samples all are distorted from perfect octahedral (Oh) symmetry with a tetragonal parameter Ds of about -0.1 eV, very close to the Ds distortion from octahedral (Oh) symmetry parameter of -0.11 eV obtained for the surface-near region from a bulk NiO crystal. Comparing the spectra of a 20 nm film of NiO grown on a 20 nm magnetite (Fe3O4) film on a MgO substrate (NiO/Fe3O4/MgO) with the calculated multiplet analyses, the distortion parameter Ds appears to be closer to zero, showing that the surface-near region of this templated film is less distorted from Oh symmetry than the surface-near region in bulk NiO. Finally, the potential of M2,3-edge RIXS for other investigations of strain on electronic structure is discussed.
Highlights
The electronic structure of nano-sized layers on surfaces of functional solid materials may be different to that of bulk samples
The SolidFlexRIXS setup that we used was not set to the optimal resolution in order to improve count rates, we want to stress here that for M-edge RIXS nowadays sub-100 meV resolutions are achieved with rather compact instruments, very much in contrast to L2,3-edge RIXS of 3d-transition metal materials, where such energy resolution can only be achieved with huge experimental effort
Current HHG sources do not provide sufficient flux to perform the here presented RIXS experiments, we expect future developments increasing photon flux in combination with increased efficiency of RIXS optics,[33,34] such that one could foresee that this type of strain engineering analyses with M-edge RIXS may be performed in the lab in the future
Summary
The electronic structure of nano-sized layers on surfaces of functional solid materials may be different to that of bulk samples. The SolidFlexRIXS setup that we used was not set to the optimal resolution in order to improve count rates, we want to stress here that for M-edge RIXS nowadays sub-100 meV resolutions are achieved with rather compact instruments, very much in contrast to L2,3-edge RIXS of 3d-transition metal materials, where such energy resolution can only be achieved with huge experimental effort.
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