Using density functional theory and model Hamiltonian analysis, we investigate the localized states induced by an oxygen vacancy in rutile TiO 2 . We identify two classes of localized states-the hybrid and the polaron. The hybrid state is caused by the orbital overlap between three Ti atoms next to a vacancy and is mainly derived from the Ti e g orbitals. The polaron state is caused by the local lattice distortion and is mainly composed of one particular t 2g orbital from a single Ti atom. The first principles calculation shows that the polaron state is energetically favored, and the tight-binding analysis reveals the underlying connection between the bulk band structure and the orbital character of the polaron. The magnetic coupling between two nearby polaron states is found to be ferromagnetic. Using this picture, we analyze the results of recent theoretical calculations and experiments and discuss the connection to vacancies in SrTiO 3 . V C 2015 AIP Publishing LLC. [http://dx.

The electronic structure of Eu sesquioxide (Eu2O3) presents a significant challenge to the electronic structure theory due to the presence of correlated Eu semicore 4f electrons. The bandgap values do not agree between computational methods, and even experimentally, there are discrepancies between reports. Eu2O3 was grown epitaxially in a thin film form on n-type GaN (0001) by molecular beam epitaxy. The film was analyzed using UV and x-ray photoemission spectroscopies as well as inverse photoelectron spectroscopy in order to characterize both occupied and unoccupied states. Signatures of Eu2+ are detected after annealing in UHV or after exposure to air, which can be removed by subsequent O2 annealing. The sample reduction is shown to strongly affect the electronic structure. The bandgap of 4.3 eV, electron affinity of 2.2 eV, and band alignment to the substrate with a valence band offset of 0.2 eV for a stoichiometric Eu2O3 film were extracted from the measurements of the occupied and unoccupied electronic states. The electronic structure is interpreted in view of recent theoretical models, and the energy band alignment across the Eu2O3/GaN interface is discussed.

Heterostructures of complex transition metal oxides are known to induce extraordinary emergent quantum states that arise from broken symmetry and other discontinuities at interfaces. Here we report the emergence of unusual, thickness-dependent properties in ultrathin CaRuO3 films by insertion of a single isovalent SrO layer (referred to as δ-doping). While bulk CaRuO3 is metallic and nonmagnetic, films thinner than or equal to ~15-unit cells (u.c.) are insulating though still nonmagnetic. However, δ-doping to middle of such CaRuO3 films induces an insulator-to-metal transition and unusual ferromagnetism with strong magnetoresistive behavior. Atomically resolved imaging and density-functional-theory calculations reveal that the whole δ-doped film preserves the bulk-CaRuO3 orthorhombic structure, while appreciable structural and electronic changes are highly localized near the SrO layer. The results highlight delicate nature of magnetic instability in CaRuO3 and subtle effects that can alter it, especially the role of A-site cation in electronic and magnetic structure additional to lattice distortion in ruthenates. It also provides a practical approach to engineer material systems via highly localized modifications in their structure and composition that may offer new routes to the design of oxide electronics.

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The high-pressure hexagonal phase of Eu2O3 has been grown epitaxially on C-plane GaN (0001) by molecular beam epitaxy. A structural phase transition from the hexagonal to the monoclinic phase is observed with increasing film thickness by ex-situ X-ray diffraction. The critical thickness for the structural transition is between 2 and 6 nm. The observed epitaxial relationships between the substrate and the film are GaN (0001) ǁ Eu2O3 (0001), GaN ⟨112¯0⟩ ǁ Eu2O3 ⟨112¯0⟩ for the hexagonal phase, and GaN (0001) ǁ Eu2O3 (201¯), GaN ⟨112¯0⟩ ǁ Eu2O3 [020] with six rotational domains for the monoclinic phase. The (0.8 ± 0.2) eV conduction band offset and bulk dielectric constant of ∼14 makes Eu2O3 a possible gate dielectric for a GaN-based field effect transistor.

In the quest for materials sustainability for grid‐scale applications, graphene quantum dot (GQD), prepared via eco‐efficient processes, is one of the promising graphitic‐organic matters that have the potential to provide greener solutions for replacing metal‐based battery electrodes. However, the utilization of GQDs as electroactive materials has been limited; their redox behaviors associated with the electronic bandgap property from the sp2 carbon subdomains, surrounded by functional groups, are yet to be understood. Here, the experimental realization of a subdomained GQD‐based anode with stable cyclability over 1000 cycles, combined with theoretical calculations, enables a better understanding of the decisive impact of controlled redox site distributions on battery performance. The GQDs are further employed in cathode as a platform for full utilization of inherent electrochemical activity of bio‐inspired redox‐active organic motifs, phenoxazine. Using the GQD‐derived anode and cathode, an all‐GQD battery achieves a high energy density of 290 Wh kgcathode−1 (160 Wh kgcathode+anode−1), demonstrating an effective way to improve reaction reversibility and energy density of sustainable, metal‐free batteries.