Momentum‐resolved energy loss spectra from ultra‐thin metal oxide layers obtained at high spatial resolution

Abstract

Momentum‐resolved electron energy loss spectroscopy (MREELS) probes the momentum (q) dependence (dispersion) of energy losses from characteristic excitations such as excitons, plasmon, and interband excitations thus probing bandstructures, indirect excitations, and dipole‐forbidden excitations. 1 However, the ability to obtain momentum‐resolved spectra both at high spatial and momentum resolutions has remained difficult. As such, layer‐resolved momentum spectra from ultra‐thin films, heterostructures as well as from nanostructures has not been widely reported. We present an experimental approach that enables the acquisition of momentum resolved spectra at high spatial resolution down to 2 nm using a nano‐beam electron diffraction approach. 2 Through this approach we have obtained momentum resolved spectra from individual, differently‐oriented nano‐domains in an ultra thin (12 nm) PrNiO 3 layer as well as spectra from different positions in a LaNiO 3 thin film (70 nm). Figure 1(a) displays a Z‐contrast image of the LaNiO 3 thin layer grown on a LaSrAlO 4 substrate. A nano‐beam electron diffraction pattern from the LaNiO 3 layer is shown in Figure 1(b). A slit is used to select the 200, ‐200 set of diffraction spots (shown by the red rectangle) allowing electrons that have been scattered to certain scattering angles into the spectrometer. This corresponds to the Г‐X direction of the Brillouin zone in the pseudo‐cubic symmetry of LaNiO 3 . The resulting image (ω‐q map) shown in figure 1(c) displays energy losses as function of momentum transfer along the Г‐X (q// [100] ) direction in the reciprocal space. The presented approach will enable the acquisition of momentum resolved spectra from nano‐structured materials, thin films, interfaces, surfaces, and heterostructures at high spatial, energy, and momentum resolutions.