Abstract
Abstract Core holes are an important contributing factor to the core-loss fine structure in electron energy-loss spectroscopy (EELS). While there has been much work on bulk materials, less is known about core hole screening in more complex dielectric environments, such as free surfaces or interfaces between two different materials, even though the latter is frequently encountered in high spatial resolution EELS analysis. In this work, experimental O K- and Mg L3,2-EELS edges from the free surfaces of a MgO cube are measured using scanning transmission electron microscopy (STEM). The free surface O K-spectrum shows extra intensity at the edge onset compared with the “bulk” spectrum. Core hole screening near a free surface is modeled using electrodynamic theory. It is shown that much of the extra intensity is due to reduced core hole screening, while genuine surface states make a smaller contribution to the fine structure. The low energy Mg L3,2-edge, however, does not show any significant change at a free surface. This is because the measurement is less surface sensitive due to strong delocalization.
Highlights
In many materials, accurate modeling of the energy loss near-edge structure (ELNES) in electron energy-loss spectroscopy (EELS) is impossible without considering core hole interactions
The results demonstrate the importance of the dielectric environment on the observed ELNES
It is assumed that the energy loss ΔE is small enough to have a negligible effect on the high energy electron trajectory, and the work done is equal to the change in electrostatic potential energy
Summary
Accurate modeling of the energy loss near-edge structure (ELNES) in electron energy-loss spectroscopy (EELS) is impossible without considering core hole interactions. Core hole screening by valence electrons for (say) a free surface will be different from the bulk material, since the electrostatic potential must satisfy Maxwell’s boundary conditions at the free surface. Surface core hole effects can be simulated either using electronic structure methods or by modifying the electrodynamic model (Mendis & Ramasse, 2021) for the given boundary conditions. The former method is, computationally expensive, since large supercell sizes are required due to the material geometry being intrinsically non-periodic, and because the interaction between neighboring core holes must be minimized (Seabourne et al, 2010). The results demonstrate the importance of the dielectric environment (in this case a free surface, and includes interfaces between two different materials) on the observed ELNES
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