Hybrid-DFT Modeling of Lattice and Surface Vacancies in MnO

We have investigated the properties of defects in MnO bulk and at (100) surfaces, as used in catalytic applications, using hybrid-level density functional theory (i.e., inclusion of exact exchange within the exchange-correlation evaluation) in a hybrid QM/MM embedded-cluster approach. Initially, we calculate the formation energy for bulk Mn and O vacancies, comparing charged-defect compensation with charge carriers at the Fermi Level (ϵf) and through Schottky defect formation. Oxygen vacancies were also investigated at the (100) surface, where the vacancy formation energy is very similar to the bulk. Defect levels associated with the most stable vacancies are calculated using the ΔSCF method: all are positioned mid band gap, with surface environments failing to alter strongly the overall nature of the defect relative to bulk. Chemical activity of the (100) MnO surface was considered through the adsorption of a probe CO2 molecule, which is considered the initial step in the transformation of CO2 into hydrocarbons. CO2 adsorption was strongest over a neutral oxygen vacancy, where the associated trapped electrons of the defect transfer to the adsorbate and thus activate it. However, we have shown with our embedded-cluster approach that the neutral oxygen vacancy is not necessarily the dominant species, which has implications when interpreting results for future catalytic applications

Limits to doping of wide band gap semiconductors

The role of defects in materials is one of the long-standing issues in solid-state chemistry and physics. On one hand, intrinsic ionic disorder involving stoichiometric amounts of lattice vacancies and interstitials is known to form in highly ionic crystals. There is a substantial literature on defect formation and the phenomenological limits of doping in this class of materials; in particular, involving the application of predictive quantum mechanical electronic structure computations. Most wide band gap materials conduct only electrons and few conduct holes, but rarely are both modes of conduction accessible in a single chemical system. The energies of electrons and holes are taken from the vertical ionization potentials and electron affinities; polaronic trapping of carriers is excluded. While the focus here is defect energetics, the atomic and electronic structures have been carefully examined in all cases to ensure physical results were obtained.</p

Limits to doping of wide band gap semiconductors

The role of defects in materials is one of the long-standing issues in solid-state chemistry and physics. On one hand, intrinsic ionic disorder involving stoichiometric amounts of lattice vacancies and interstitials is known to form in highly ionic crystals. There is a substantial literature on defect formation and the phenomenological limits of doping in this class of materials; in particular, involving the application of predictive quantum mechanical electronic structure computations. Most wide band gap materials conduct only electrons and few conduct holes, but rarely are both modes of conduction accessible in a single chemical system. The energies of electrons and holes are taken from the vertical ionization potentials and electron affinities; polaronic trapping of carriers is excluded. While the focus here is defect energetics, the atomic and electronic structures have been carefully examined in all cases to ensure physical results were obtained.</p

Limits to Doping of Wide Band Gap Semiconductors

Limits to Doping of Wide Band Gap Semiconductor