2013 August

Tailoring properties of surface defects via bulk doping

Monday, 26th August 2013Publication highlights

Importance of long-range charge transfer shown by multiscale modeling at realistic conditions

A unique feature of oxygen vacancies in magnesium oxide is a defect energy level in the band gap. The charge state of the defect is determined by the defect level occupation with electrons. A neutral oxygen vacancy has two electrons on the defect level (left). Doping allows for charge transfer between surface and bulk, and thus influences the defect level occupation, so that the surface defects can get doubly positively charged (right: empty defect level). As a consequence, at the surface the electronic energy bands bend, and surface charge as well as a compensating space charge region build up. Interaction between surface charge and space charge determines the surface defect concentrations.

Point defects such as vacancies, when an atom is missing from the regular crystal lattice, or impurities, when one atom is substituted by a different species, play an important role in many technological applications, and they are fascinating and challenging to study in experiment and theory.

While sometimes oxides of high purity are needed, often defects are indeed desirable. For example, when a metal oxide is used as base material for a catalyst, surface oxygen vacancies can enhance the catalytic selectivity, driving reactions towards the desired products.  Therefore, understanding the defect physics in oxides is very valuable. What makes the task complex is the dependence of defect type, concentration, and charge state on outer conditions such as temperature, pressure, and on concentration of other defects. Experimental determination of point defect stabilities and concentrations is challenging, in particular at realistic conditions, since it is hard to reach thermodynamic equilibrium. In theoretical studies often only the limiting case of small defect concentrations and/or non-interacting defects are considered. However, metal-oxide samples are typically doped, either intentionally or unintentionally. In this work it is shown how charge state and concentration of oxygen vacancies in magnesium oxide are largely determined by electrostatic effects due to charge transfer between surface vacancies and bulk dopants. As a consequence of the charge transfer, a space-charge region is built up that reaches deep into the bulk. To provide a quantitative estimate of the surface defect concentration as a function of outer conditions and dopant concentration, we apply an ab initio theoretical approach that combines high-accuracy calculations of the electronic structure of the material with a thermodynamic model, thus relating microscopic with macroscopic material properties. The gained insights can be used to tune oxygen vacancy concentrations at the surface of metal oxides. Furthermore, the results suggest that experimental information on doping concentrations and profiles may provide new insights into catalytic activity of oxide surfaces.

Original publication:

Norina A. Richter, Sabrina Sicolo, Sergey V. Levchenko, Joachim Sauer, and Matthias Scheffler: Concentration of Vacancies at Metal-Oxide Surfaces: Case Study of MgO(100), Phys. Rev. Lett. 111, 045502 (2013) , doi:110.1103/PhysRevLett.111.045502. http://link.aps.org/doi/10.1103/PhysRevLett.111.045502