Search

Oxygen vacancies are ubiquitous in oxides, and taking advantage of their mobility is the cornerstone for a variety of future applications. The visualization and quantification of collective defect flow based on electrochromism is a powerful approach to explore oxygen kinetics and electrochemical reaction even in cases that electronic conduction is considerably mixed, but whether or not the measured kinetic properties harmonize with those obtained by the conventional impedance spectroscopy remains veiled. Here, we identify complementary relationships between the two methods by investigating the oxygen vacancy transport in Ca 30%-doped bismuth ferrite thin films epitaxially grown on SrTiO3 (110) substrates. We find that the activation energy of ionic hopping is 0.78 (or 0.92 eV) for the application of an electric bias along [001] (or [1 1 ¯0]) due to the grain elongation along [001]. We anneal the films in an N2 gas environment at high temperatures to suppress the electronic contribution for access to standard impedance spectroscopy. The oxygen kinetic properties obtained from the two methods are consistent with each other, complementarily revealing the collective phase evolution as well as the ionic impedance of the bulk, grain boundary, and interfacial regions. These comparative works provide useful insights into ionic defect conduction in oxides in an intuitive and quantitative manner.

Ionic migration is a key ingredient for applications such as oxide electrolytes and resistive switching memories. We investigate the evolution of ionic conduction pathways based on optical contrast in an epitaxial Bi0.7Ca0.3FeO3−δ thin film where oxygen vacancies are spontaneously produced. We visualize electroforming processes in the hundred-micrometer-scale material channels between coplanar electrodes with a constant electric bias at an elevated temperature, systematically varying the channel orientation with respect to the crystal axis. At the initial stage of electroforming, conducting filaments are created and propagate nearly along the crystal axes ⟨100⟩. The local density of conducting filament regions increases with the elapsed time of bias application and also exhibits a linear dependence on the spatial position at a given time. We also find that the filament-type ionic conduction is abruptly transformed to the bulk conduction when the filament density reaches ∼30%. These results offer useful insight into collective ionic migration in crystalline solids.

Influence of the size of indium clusters on optical properties of green-light-emitting InGaN quantum wells (QWs) was investigated by photoluminescence (PL), cathodoluminescence, PL excitation, and time-resolved PL techniques. Low luminescence efficiency was observed for green-light-emitting InGaN QWs with micron-sized indium clusters, in contrast to the case of InGaN QWs with submicron-sized small indium segregation. Both the thermal activation energy and the carrier lifetime dramatically decreased, while a large Stokes-like shift between absorption edge and PL peak energy was still observed for the InGaN QWs with micron-sized indium clusters. These facts indicate that the effective potential barrier between radiative and nonradiative channels (thus effective carrier localization) rapidly decreases due to the formation of micron-sized large indium clusters possessing a number of nonradiative centers, leading to significant luminescence degradation.

A method for the dry thermal oxidation of a strained SiGe layer is proposed. By oxidation of a graded Si1−xGex layer, the effect of Ge pileup was significantly reduced and the undesirable strain relaxation by defect formation is prohibited. After oxidation, the oxidized SiGe layer was homogenized by postannealing process, and thereby a SiO2/SiGe interface with good structural properties was obtained. During postannealing, the homogenization was significantly enhanced by strain-induced diffusion, and it was clearly proved by the uphill diffusion. This result can propose an alternative oxidation method of strained SiGe/Si heterostructures.

We report on the electronic phase transition in CaMn7O12 quadruple perovskite films synthesized by oxide molecular beam epitaxy on SrLaAlO4 and La0.3Sr0.7Al0.65Ta0.35O3 substrates. We use x-ray diffraction and transmission electron microscopy to confirm that the CaMn7O12 phase has been realized. Temperature dependent resistivity measurements reveal a signature of a charge ordering phase transition at ≈425 K, consistent with bulk CaMn7O12. The transition temperature is found to be relatively invariant to changes in the cation stoichiometry. Density functional theory calculations reveal the changes in atomic and electronic structure induced by the charge ordering transition.

We have investigated the photoexcitation dynamics of poly(1,4-phenylene vinylene) (PPV) and poly(2-fluoro-1,4-phenylene vinylene) (PFPV) by using the femtosecond transient absorption spectroscopic technique. It was demonstrated that photoinduced absorption originates from nonemissive indirect polaron pairs which are more efficiently produced in PFPV than in PPV. The decay processes of polaron pairs are mainly contributed by collisional annihilation and internal conversion by phonon emission in PFPV and PPV, respectively. The differences in photoexcitation dynamics between PPV and PFPV are believed to be caused by the strong electronegativity of substituted fluorine atom.

We report the enhanced p-type conduction properties in BiCuOSe by doping of monovalent ions (K+). As compared with undoped BiCuOSe, simultaneous increase in both the carrier concentration and the Hall mobility was achieved in the K-doped BiCuOSe. The origin of the enhancement was discussed in terms of the two-band structure in the valence band of the BiCuOSe, and the density of state effective masses of the heavy (∼1.1 me) and light hole (∼0.18 me) were estimated by using Pisarenko relation.

We report on the visualization of n-p junctions formed by oxygen vacancy movement under the application of an electric field in a Ca-doped BiFeO3 thin film through spatially resolved scanning photocurrent mapping. The photocurrent mapping, in conjunction with the spectroscopic approach, provides clues to local electronic structures and defect levels associated with oxygen vacancies. These observations provide insights into the spatial redistribution of oxygen vacancies in an electric field.

The effects of Sr/Ba ratio and bias field on the acoustic anomalies of SrxBa1−xNb2O6 (SBN − x) were investigated by Brillouin scattering. The “critical” part in the elastic constant and the absorption coefficient increased linearly with decreasing Sr content. This was attributed to the enhanced coupling between the polarization and the strain due to the formation of quasistatic polar regions in the paraelectric phase. The field-cooling of SBN-0.61 induced discontinuous changes in the elastic properties with some remnant relaxor signature. This was tentatively ascribed to the persisting polar nanoregions that were not aligned due to quenched random fields.

We report on the three-dimensional optical memory utilizing a photoluminescence (PL) change in polymethylmethacrylate. Irradiation with a femtosecond pulsed laser (800 nm, 1 kHz, 100 fs) induced a strong PL spectrum in the visible range, which may result from the photogeneration of emissive radicals. Multilayered patterns were recorded inside the bulk sample by tightly focusing a pulsed laser beam. The pattern images were read out by a reflection-type fluorescent confocal microscope which detected the blue-green emission at 410–510 nm. The stored bits were retrieved with a high signal-to-noise ratio in the absence of any cross-talk.

The 1.5μm emission characteristics of stoichiometric LiNbO3 crystals with different Er3+ doping levels of 0.2, 0.5, 1.0, and 2.0mol% were investigated. All of the emission decays could be fitted by a single exponential curve. The lifetimes of the 1.5μm transition (I13∕24→I15∕24) were measured to be 3.0–3.2ms, which are about 25% longer than those reported for conventional Er-doped congruent LiNbO3. Our analysis showed that the longer lifetime in stoichiometric LiNbO3 is likely to originate from the reduced density of intrinsic defects.