Anisotropic iron-doping patterns in two-dimensional cobalt oxide nanoislands on Au(111)

Abstract

An integrated approach combining density functional theory (DFT) calculations and atomic resolution scanning tunneling microscopy (STM) is used to study well-defined iron-doped cobalt oxide nanoislands supported on Au(111). The focus is on the structure and distribution of Fe dopants within these nanoislands of CoO as a function of Fe to Co ratio. The DFT and STM results agree strongly and complement each other to allow for a more complete understanding of the dopant structure trends on the nanoscale. Using Fe as a marker, we first find that the stacking sequence of the moire structure of the host cobalt oxide nanoislands can be identified unambiguously through a combination of DFT and STM. Using the distinct contrast of the embedded Fe dopant atoms as observed with atom-resolved STM, we find correlations between Fe dopant position and the CoO/Au(111) moire pattern at varying Fe dopant densities. Formation of Fe-dopant clusters within the nanoislands is investigated in detail through DFT and found to agree with the dopant patterns observed in STM. We find that the structural effects of Fe dopants throughout the nanoislands with the basal planes and the two types of edges—the oxygen and metal edges—have different nature. Both DFT calculations and STM images show a strong preference for Fe dopants to be located directly on or near the oxygen edge of the nanoislands as opposed to being directly on or near the metal edge. Taken together, our results illustrate that Fe dopant incorporation and distribution within CoO nanoislands are highly anisotropic and governed by both the moire structure of the basal planes as well as nano-size effects present at the under-coordinated edges of different local geometry and chemistries.