Correlated Visible-Light Absorption and Intrinsic Magnetism of SrTiO₃ Due to Oxygen Deficiency: Bulk or Surface Effect?

The visible-light absorption and luminescence of wide band gap (3.25 eV) strontium titanate (SrTiO₃) are well-known, in many cases, to originate from the existence of natural oxygen deficiency in the material. In this study based on density functional theory (DFT) calculations, we provide, to the best of our knowledge, the first report indicating that oxygen vacancies in the bulk and on the surfaces of SrTiO₃ (STO) play different roles in the optical and magnetic properties. We found that the doubly charged state of oxygen vacancy (V_O²⁺) is dominant in bulk SrTiO₃ and does not contribute to the sub-band gap photoexcitation or intrinsic magnetism of STO. Neutral oxygen vacancies (V_O⁰) on (001) surfaces terminated with both TiO₂ and SrO layers induce magnetic moments, which are dependent on the charged state of V_O. The calculated absorption spectra for the (001) surfaces exhibit mid-infrared absorption (<0.5 eV) and sub-band gap absorption (2.5–3.1 eV) due to oxygen vacancies. In particular, V_O⁰ on the TiO₂-terminated surface has a relatively low formation energy and magnetic moments, which can explain the recently observed spin-dependent photon absorptions of STO in a magnetic circular dichroism measurement [Rice, W. D.; et al. Nat. Mater. 13, 481, 2014]

Tuning the Electronic and Magnetic Properties of Phosphorene by Vacancies and Adatoms

We report a density functional theory (DFT) study regarding the effects of atomic defects, such as vacancies and adatom adsorption, on the electronic and magnetic properties of phosphorene (a two-dimensional monolayer of black phosphorus). A monovacancy in the phosphorene creates an in-gap state in the band gap of pristine phosphorene and induces a magnetic moment, even though pristine phosphorene is nonmagnetic. In contrast, both planar and staggered divacancies do not change the magnetic properties of phosphorene, although a staggered divacancy creates states in the gap. Our DFT calculations also show that adsorption of nonmetallic elements (C, N, and O) and transition metal elements (Fe, Co, and Ni) can change the magnetic properties of phosphorene with or without vacancies. For example, the nonmagnetic pristine phosphorene becomes magnetic after the adsorption of N, Fe, or Co adatoms, and the magnetic phosphorene with a monovacancy becomes nonmagnetic after the adsorption of C, N, or Co atoms. We also demonstrate that for O- or Fe-adsorbed monovacancy structure the electronic and magnetic properties are tunable via the control of charge on the phosphorene system. These results provide insight for achieving metal-free magnetism and a tunable band gap for various electronic and spintronic devices based on phosphorene

Unexpected Roles of Interstitially Doped Lithium in Blue and Green Light Emitting Y₂O₃:Bi³⁺: A Combined Experimental and Computational Study

To enhance the photoluminescence of lanthanide oxide, a clear understanding of its defect chemistry is necessary. In particular, when yttrium oxide, a widely used phosphor, undergoes doping, several of its atomic structures may be coupled with point defects that are difficult to understand through experimental results alone. Here, we report the strong enhancement of the photoluminescence (PL) of Y₂O₃:Bi³⁺ via codoping with Li⁺ ions and suggest a plausible mechanism for that enhancement using both experimental and computational studies. The codoping of Li⁺ ions into the Y₂O₃:Bi³⁺ phosphor was found to cause significant changes in its structural and optical properties. Interestingly, unlike previous reports on Li⁺ codoping with several other phosphors, we found that Li⁺ ions preferentially occupy interstitial sites of the Y₂O₃:Bi³⁺ phosphor. Computational insights based on density functional theory calculations also indicate that Li⁺ is energetically more stable in the interstitial sites than in the substitutional sites. In addition, interstitially doped Li⁺ was found to favor the vicinity of Bi³⁺ by an energy difference of 0.40 eV in comparison to isolated sites. The calculated DOS showed the formation of a shallow level directly above the unoccupied 6p orbital of Bi³⁺ as the result of interstitial Li⁺ doping, which may be responsible for the enhanced PL. Although the crystallinity of the host materials increased with the addition of Li salts, the degree of increase was minimal when the Li⁺ content was low (<1 mol %) where major PL enhancement was observed. Therefore, we reason that the enhanced PL mainly results from the shallow levels created by the interstitial Li⁺

Synergistic Oxygen Evolving Activity of a TiO₂‑Rich Reconstructed SrTiO₃(001) Surface

In addition to composition, the structure of a catalyst is another fundamental determinant of its catalytic reactivity. Recently, anomalous Ti oxide-rich surface phases of ternary oxides have been stabilized as nonstoichiometric epitaxial overlayers. These structures give rise to different modes of oxygen binding, which may lead to different oxidative chemistry. Through density functional theory investigations and electrochemical measurements, we predict and subsequently show that such a TiO₂ double-layer surface reconstruction enhances the oxygen evolving activity of the perovskite-type oxide SrTiO₃. Our theoretical work suggests that the improved activity of the restructured TiO₂(001) surface toward oxygen formation stems from (i) having two Ti sites with distinct oxidation activity and (ii) being able to form a strong O–O moiety (which reduces overbonding at Ti sites), which is a direct consequence of (iii) having a labile lattice O that is able to directly participate in the reaction. Here, we demonstrate the improvement of the catalytic performance of a well-known and well-studied oxide catalyst through more modern methods of materials processing, predicted through first-principles theoretical modeling