Abstract
We present a theoretically oriented analysis of the appearance and properties of plausible candidates for the anionic defects observed in scanning tunneling microscopy (STM) experiments on CeO2(111). The simulations are based on density functional theory (DFT) and cover oxygen vacancies, fluorine impurities and hydroxyl groups in the surface and sub-surface layers. In the surface layer, all three appear as missing spots in the oxygen sublattice in filled state simulated STM images, but they are distinguishable in empty state images, where surface oxygen vacancies and hydroxyls appear as, respectively, diffuse and sharp bright features at oxygen sites, while fluorine defects appear as triangles of darkened Ce ions. In the sub-surface layer, all three defects present more complex patterns, with different combinations of brightened oxygen ion triangles and/or darkened Ce ion triangles, so we provide image maps to support experimental identification. We also discuss other properties that could be used to distinguish the defects, namely their diffusion rates and distributions.
Highlights
The functional properties of technologically relevant materials can depend critically on the properties of point defects in the lattice structure, which may be present unavoidably as contaminants or introduced deliberately as dopants
With the above discussion in mind, the principal purpose of the present paper is to offer a consistent set of simulated scanning tunneling microscopy (STM) images of three plausible candidate defects at anionic sites on CeO2(111), drawing attention to their similarities and differences, which we believe will aid in the interpretation of STM experiments on this system
STM images of the ceria (111) surface exhibit a number of point defects in an otherwise regular hexagonal pattern
Summary
The functional properties of technologically relevant materials can depend critically on the properties of point defects in the lattice structure, which may be present unavoidably as contaminants or introduced deliberately as dopants. Typically one must rely on spectroscopic probes, which provide spatially averaged information, but at the surface one can make use of spatially resolving techniques such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM). Of these latter two methods, STM remains more widely used, despite the fact that its applicability is limited to materials with sufficient conductivity, and atomic resolution STM imaging of surfaces is fairly routine. Substantial success has been achieved by combining experimental data with the results of electronic structure calculations, most commonly based on density functional theory (DFT) (see e.g., Setvín et al, 2017 for a relevant review of such studies on oxide surfaces)
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