Authors: Matthew J. Wolf, Alexander L. Shluger
We present an overview of the structure and properties of clean steps at the surfaces of binary oxides, utilising recent data from scanning probe and spectroscopic experiments, and theoretical calculations. We review and discuss their atomic structure, electronic structure, and interactions with prototypical point defects, using examples from studies on technologically important oxides such as MgO, CeO2, TiO2 and ZrO2. We also review methods of calculating the step formation energy, and discuss reasons for their limited success in explaining the step structures observed in experiments.
Defects at Oxide Surfaces , Springer Series in Surface Sciences Volume 58 , 2015, pp 191-214.
Authors: J. Kullgren, M. J. Wolf, C. W. M. Castleton, P. D. Mitev, W. J. Briels, and K. Hermansson
We propose a resolution to the puzzle presented by the surface defects observed with STM at the (111) surface facet of CeO2 single crystals. In the seminal paper of Esch et al. [Science 309, 752 (2005)] they were identified with oxygen vacancies, but the observed behavior of these defects is inconsistent with the results of density functional theory (DFT) studies of oxygen vacancies in the literature. We resolve these inconsistencies via DFT calculations of the properties of both oxygen vacancies and fluorine impurities at CeO2(111), the latter having recently been shown to exist in high concentrations in single crystals from a widely used commercial source of such samples. We find that the simulated filled-state STM images of surface-layer oxygen vacancies and fluorine impurities are essentially identical, which would render problematic their experimental distinction by such images alone. However, we find that our theoretical results for the most stable location, mobility, and tendency to cluster, of fluorine impurities are consistent with experimental observations, in contrast to those for oxygen vacancies. Based on these results, we propose that the surface defects observed in STM experiments on CeO2 single crystals reported heretofore were not oxygen vacancies, but fluorine impurities. Since the similarity of the simulated STM images of the two defects is due primarily to the relative energies of the 2p states of oxygen and fluorine ions, this confusion might also occur for other oxides which have been either doped or contaminated with fluorine.
Phys. Rev. Lett. 112, 156102
The conference “Oorgandagarna – Inorganic Days” was held in Åhus in Sweden from 17-19 June 2013. Dalton Transactions sponsored two poster prizes, and the two winners were Erik Lewin and Matthew Wolf from Uppsala University. Congratulations to Erik and Matthew, and I hope all the conference attendees had a great time!