Tadashi C. Ozawa

Oriented Monolayer Film of Gd2O3:0.05 Eu Crystallites: Quasi‐Topotactic Transformation of the Hydroxide Film and Drastic Enhancement of Photoluminescence Properties

Caught on film: A semitransparent and intensely luminescent monolayer film of oriented Gd(2)O(3):0.05 Eu platelet crystallites is fabricated by annealing the precursor hydroxide film (see scheme). The photoluminescence properties of the as-transformed film are greatly improved over those of the hydroxide film, and are much more pronounced than those of the corresponding Gd(2)O(3):0.05 Eu powder.

Engineered interfaces of artificial perovskite oxide superlattices via nanosheet deposition process.

Combining different materials into desired superlattice structures can produce new electronic states at the interface and the opportunity to create artificial materials with novel properties. Here we introduce a new, rather unexpected, and yet simple way to such a superlattice assembly of perovskite oxides: in the Dion-Jacobson phase, a model system of layered perovskites, high-quality bicolor perovskite superlattices (LaNb(2)O(7))(nL)(Ca(2)Nb(3)O(10))(nC) are successfully fabricated by a layer-by-layer assembly using two different perovskite nanosheets (LaNb(2)O(7) and Ca(2)Nb(3)O(10)) as a building block. The artificially fabricated (LaNb(2)O(7)/Ca(2)Nb(3)O(10)) superlattices are structurally unique, which is not feasible to create in the bulk form. By such an artificial structuring, we found that (LaNb(2)O(7)/Ca(2)Nb(3)O(10)) superlattices possess a new form of interface coupling, which gives rise to ferroelectricity.

Chemistry of layered d-metal pnictide oxides and their potential as candidates for new superconductors

Abstract Layered d-metal pnictide oxides are a unique class of compounds which consist of characteristic d-metal pnictide layers and metal oxide layers. More than 100 of these layered compounds, including the recently discovered Fe-based superconducting pnictide oxides, can be classified into nine structure types. These structure types and the chemical and physical properties of the characteristic d-metal pnictide layers and metal oxide layers of the layered d-metal pnictide oxides are reviewed and discussed. Furthermore, possible approaches to design new superconductors based on these layered d-metal pnictide oxides are proposed.

Synthesis of a Solid Solution Series of Layered EuxGd1−x(OH)2.5Cl0.5·0.9H2O and Its Transformation into (EuxGd1−x)2O3 with Enhanced Photoluminescence Properties

The synthesis of a series of new layered rare-earth hydroxide solid solutions and their transformation into (Eu(x)Gd(1-x))(2)O(3) crystallites are described. Highly crystalline platelets of Eu(x)Gd(1-x)(OH)(2.5)Cl(0.5) x 0.9 H(2)O solid solutions with various Eu(3+)/Gd(3+) ratios were prepared through a homogeneous precipitation method. The hydroxide solid-solution samples exhibited characteristic Eu(3+) photoluminescence properties through the energy transfer from Gd(3+) to Eu(3+) and the self-excitation of Eu(3+). Cubic (Eu(x)Gd(1-x))(2)O(3) crystallites were obtained via quasi-topotactic transformation of Eu(x)Gd(1-x)(OH)(2.5)Cl(0.5) x 0.9 H(2)O solid solutions above 800 degrees C. The as-transformed cubic (Eu(x)Gd(1-x))(2)O(3) crystallites well retained the original platelet morphology and single crystalline nature, and exhibited greatly enhanced photoluminescence properties with respect to the precursor hydroxides. The Eu(3+) content of 0.05 in the cubic (Eu(x)Gd(1-x))(2)O(3) gave a maximum luminescence intensity, which is comparable with that of a commercial Y(2)O(3):Eu phosphor.

Oriented films of layered rare-earth hydroxide crystallites self-assembled at the hexane/water interface.

Layered rare-earth hydroxide crystallites self-assembled at the hexane/water interface were transferred to various substrates to form a monolayer film, which exhibited photoluminescence properties and ion-exchange ability.

Versatile van der Waals epitaxy-like growth of crystal films using two-dimensional nanosheets as a seed layer: orientation tuning of SrTiO3 films along three important axes on glass substrates

One of the basic requirements for attaining a good epitaxy is a close structural matching between a substrate and a growing crystal epilayer. This restrictive requirement causes a major obstacle for its wide application to a range of functional crystal films in electronic, magnetic or optical devices. One approach for overcoming this problem is the so-called van der Waals epitaxy (VDWE) method, which can effectively implement the epitaxy of various crystals on cleaved faces of layered materials having no dangling bonds. The weak adatom–substrate interaction without directional covalent bonding plays a crucial role in the initial stage of VDWE, which drastically relaxes the lattice matching limitation. However, the method requires special materials for use as a substrate, thereby meaning that its applicability is limited. In this study, the concept is extended to the two-dimensional (2D) lattice of inorganic nanosheets, which are molecularly thin 2D crystals produced via artificial exfoliation of layered metal oxides. The nanosheets can neatly cover the surface of conventional substrates such as glass via a facile solution-based process. Similar to the above-mentioned cleaved faces of layered materials, such substrates can promote VDWE-like crystal growth because of their dangling bond-free nature. Based on this principle, we have demonstrated a selective deposition of highly textured (100), (110) and (111) SrTiO3 films, a fundamentally important archetype of functional crystals, on glass substrates covered with single-layer nanosheets with suitable 2D periodicities as a trigger for VDWE-like film growth. The rich varieties of nanosheet structures and their facile deposition onto almost any kinds of substrates provide a significant advantage, expanding potential applications for a range of devices based on functional crystal films.

Tuning the surface charge of 2D oxide nanosheets and the bulk-scale production of superlatticelike composites.

The surface charge of various anionic unilamellar nanosheets, such as graphene oxide (GO), Ti0.87O2(0.52-), and Ca2Nb3O10(-) nanosheets, has been successfully modified to be positive by interaction with polycations while maintaining a monodispersed state. A dilute anionic nanosheet suspension was slowly added dropwise into an aqueous solution of high molecular weight polycations, which attach on the surface of the anionic nanosheets via electrostatic interaction. Surface modification and transformation to positively charged nanosheets were confirmed by various characterizations including atomic force microscopy and zeta potential measurements. Because the sizes of the polycations used are much larger than the nanosheets, the polymer chains may run off the nanosheet edges and fold to the fronts of the nanosheets, which could be a reason for the continued dispersion of the modified nanosheets in the suspension. By slowly adding a suspension of polycation-modified nanosheets and pristine anionic nanosheet dropwise into water under suitable conditions, a superlatticelike heteroassembly can be readily produced. Characterizations including transmission electron microscopy and X-ray diffraction measurements provide evidence for the formation of the alternately stacked structures. This approach enables the combination of various pairs of anionic nanosheets with different functionalities, providing a new opportunity for the creation of unique bulk-scale functional materials and their applications.

Impact of perovskite layer stacking on dielectric responses in KCa2Nan−3NbnO3n+1 (n=3–6) Dion–Jacobson homologous series

The dielectric properties of KCa2Nan−3NbnO3n+1 (n=3–6) Dion–Jacobson homologous series have been investigated. The dielectric constants (e) increase with the number of octahedral units (n), and the n=6 compound (KCa2Na3Nb6O19) exhibits a stable dielectric response with e=∼500 between 1 kHz and 1 MHz. This n-dependent behavior is similar to those observed in other layered perovskites such as Ruddlesden–Popper and Aurivillius phases. Raman scattering studies reveal that increase in n in KCa2Nan−3NbnO3n+1 leads to higher polarizability of the lattice and softening of the lowest-frequency phonon mode, which is responsible for the observed enhancement in e with n.

A-Site-Modified Perovskite Nanosheets and Their Integration into High-κ Dielectric Thin Films with a Clean Interface

We investigated dielectric properties of La1-xEuxNb2O7 perovskite nanosheets in order to study the effect of A-site modification on dielectric properties. Langmuir–Blodgett deposition was employed to fabricate multilayer nanofilms of perovskite nanosheets. In these nanosheets, A-site modification with Eu3+ ions improves the leakage current characteristics and, at the same time, reduces permittivity. The slight modification with Eu3+ ions in La0.95Nb2O7 nanosheets causes a 50% reduction in er value. We also discuss the high-κ properties of La0.95Nb2O7 nanosheets by performing detailed investigations based on first-principles calculations and interfacial structures.

An alkali-metal ion extracted layered compound as a template for a metastable phase synthesis in a low-temperature solid-state reaction: preparation of brookite from K0.8Ti1.73Li0.27O4.

We have designed a new approach to synthesize brookite, i.e., to extract alkali-metal ions from K(0.8)Ti(1.73)Li(0.27)O(4) (KTLO) and to apply simultaneous heat treatment to the remaining lepidocrocite-type layers of TiO(6) octahedra. For the alkali-metal ion extraction and the simultaneous heat treatment, KTLO was heated at 400 degrees C with polytetrafluoroethylene (PTFE) in flowing Ar. PTFE has been found to be an effective agent to extract strongly electropositive alkali-metal ions from KTLO because of the strong electronegativity of F as its component. The product of this reaction consists of a mixture of brookite, K(2)CO(3), LiF, and PTFE derivatives, indicating the complete extraction of K(+) and Li(+) from KTLO and formation of brookite from the lepidocrocite-type layer of TiO(6) octahedra as a template. This brookite has a partial replacement of O(2-) with F(-) and/or slight oxygen deficiency; thus, its color is light-bluish gray. Fully oxidized brookite formation and complete decomposition of PTFE derivatives have been achieved by further heating in flowing air, and coproduced alkali-metal salts have been removed by washing in water. Powder X-ray diffraction, Raman spectroscopy, and chemical analysis results have confirmed that the final brookite product treated at 600 degrees C is single phase, and it is white. The method to extract alkali-metal ions from a crystalline material using PTFE is drastically different from the common methods such as soft-chemical and electrochemical reactions. It is likely that this new synthetic approach is applicable to other layered systems to prepare a diverse family of compounds, including novel metastable ones.