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The presence and potential ordering of oxygen vacancies play an important role in determining the electronic, ionic, and thermal transport properties of many transition metal oxide materials. Controlling the concentration of oxygen vacancies, as well as the structures of ordered oxygen vacancy domains, has been the subject of many experimental and theoretical studies. In epitaxial thin films, the concentration of oxygen vacancies and the type of ordering depend on the structure of the substrate as well as the lattice mismatch between the thin films and the substrate. However, the role of temperature or structural phase transitions in either the substrate or the epitaxial thin films in the oxygen vacancy ordering has remained largely unexplored. In particular, atomic-resolution imaging and spectroscopy analysis of oxygen vacancy ordering in thin films at temperatures below 300 K have not yet been reported. Here, we use aberration-corrected scanning transmission electron microscopy combined with in-situ cooling experiments to characterize the atomic/electronic structures of oxygen-deficient La0.5Sr0.5CoO3−δ thin films grown on SrTiO3 across its antiferrodistortive phase transition at 105 K. We demonstrate that atomic-resolution imaging and electron energy-loss spectroscopy can be used to examine variations in the local density of states as a function of sample temperature. [ABSTRACT FROM AUTHOR]

The Li-V2O5 system has been well studied electrochemically, but there is a lack of systematic in-situ studies involving direct investigations of the structural changes that accompany the lithiation process. The open-cell battery setup inside a transmission electron microscope is ideal for studying the reaction pathway of intercalation of Li+ into nanowire cathodes. In this work, we utilize in-situ transmission electron microscopy to study the Li-V2O5 system. More specifically, we employ electron beam diffraction and electron energy-loss spectroscopy (EELS) in an open-cell battery setup to examine the phase changes within α-V2O5 nanowire cathodes upon in-situ lithiation. Our results suggest that the pristine α-V2O5 nanowire forms a Li oxide shell which then acts as a solid state electrolyte to conduct Li+ ions, and the bulk of the V2O5 nanowire undergoes transformation to the γ - Li2V2O5 phase. [ABSTRACT FROM AUTHOR]

Identifying different phases of VO2 during the metal-insulator phase transition is critical for device application due to the difference of electrical, mechanical and magnetic properties of phases. However, most studies so far were carried out using microprobe analyses, which lack the spatial resolution needed to identify nanoscale phases and changes. Taking advantage of in situ low temperature aberration-corrected scanning transmission electron microscopy, we observed the existence of M2 phase alongside M1 and R phase in the W-doped nanowires close to transition temperature. The localized stress caused by adding W in the structure results in the stabilization of nanosize grains of M2 phase in structure along with M1 and R phases. The observation of the metastable M2 phase even for unclamped nanowires suggests the possibility of finely modulating the phase diagram of VO2 through a combination of finite size and doping. [ABSTRACT FROM AUTHOR]

The integration of functional oxide thin-films on compound semiconductors can lead to a class of reconfigurable spin-based optoelectronic devices if defect-free, fully reversible active layers are stabilized. However, previous first-principles calculations predicted that SrTiO3 thin films grown on Si exhibit pinned ferroelectric behavior that is not switchable, due to the presence of interfacial vacancies. Meanwhile, piezoresponse force microscopy measurements have demonstrated ferroelectricity in BaTiO3 grown on semiconductor substrates. The presence of interfacial oxygen vacancies in such complex-oxide/semiconductor systems remains unexplored, and their effect on ferroelectricity is controversial. Here, we use a combination of aberration-corrected scanning transmission electron microscopy and first-principles density functional theory modeling to examine the role of interfacial oxygen vacancies on the ferroelectric polarization of a BaTiO3 thin film grown on GaAs. We demonstrate that interfacial oxygen vacancies enhance the polar discontinuity (and thus the single domain, out-of-plane polarization pinning in BaTiO3), and propose that the presence of surface charge screening allows the formation of switchable domains. [ABSTRACT FROM AUTHOR]

This study focuses on the effects of electron beam induced irradiation to the layered oxide Li2MnO3. Aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy are used to characterize structural and electronic transitions in the material during irradiation, with a focus on changes in Mn valence and O content. This truly in situ irradiation allows for specific particle tracking, dose quantification, and real-time observation, while demonstrating many parallels to the oxide's structure evolution observed during electrochemical cycling. Furthermore, it is demonstrated that typical imaging conditions are not severe enough to induce damage to the pristine oxide. [ABSTRACT FROM AUTHOR]

The full phonon spectra of two rational approximants to the incommensurate thermoelectric oxide material Ca3Co4O9 (CCO) are calculated from first principles within density functional theory (DFT) and its DFT+U extension. The computed phonon dispersion curves are then used to evaluate the constant volume heat capacity, as well as the full and partial (subsystem-projected) lattice thermal conductivity of CCO. The results are discussed in terms of the effects of the Hubbard U on the computed properties, the size of the rational approximants used to model CCO, the relative contributions of the two subsystems that comprise the misfit-layered material, and the anisotropic nature of the computed thermal conductivity. We obtain good agreement with available experimental data. [ABSTRACT FROM AUTHOR]

Previous studies of Ca3Co4O9 suggested that the abrupt changes in the magnetic susceptibility and electrical resistivity at ∼420 K can be attributed to Co spin state transitions. In this letter, we study the possible transitions above 420 K by variable-temperature Z-contrast imaging and electron energy-loss spectroscopy. We find that there is no observable change in the structure and Co valence state upon in situ heating to 500 K, compared to room temperature. However, an intensity decrease of the prepeak in the O K-edge near edge fine structure indicates a Co3+-ion spin state transition has occurred from a low to an intermediate spin state. [ABSTRACT FROM AUTHOR]