Kosho Akatsuka

Construction of highly ordered lamellar nanostructures through Langmuir-Blodgett deposition of molecularly thin titania nanosheets tens of micrometers wide and their excellent dielectric properties.

Exfoliated unilamellar titania nanosheets of Ti(0.87)O(2) with a lateral size of 10-30 microm were deposited layer-by-layer onto various substrates by Langmuir-Blodgett procedure to produce a highly ordered lamellar nanofilms. The nanosheets dispersed in an aqueous suspension containing quaternary ammonium ions as a supporting electrolyte floated spontaneously at the air/liquid interface, and they were successfully transferred onto the substrate after surface compression. Neat tiling of the nanosheets could be realized at an optimized surface pressure. The film thus obtained was exposed to UV light to turn the substrate surface hydrophilic, which was helpful for stable repetition of monolayer deposition. Layer-by-layer growth was confirmed by UV-visible absorption spectra, which showed progressive enhancement of an absorption band due to the nanosheet. Cross-sectional transmission electron microscopy images visualized the ultrathin film homogeneously deposited on the substrate surface and a lamellar fringe of the layer-by-layer assembled nanosheets was clearly resolved at a higher magnification. X-ray diffraction data on the films showed sharp basal reflections up to the seventh order, and Williamson-Hall analysis of the pattern indicated that the film was coherent across the total thickness with respect to X-ray and that the lattice strain was extremely small. In addition, the first basal reflection was accompanied by small satellite peaks, which are accounted for by the Laue interference function. All these features clearly indicate the formation of a highly ordered lamellar nanostructure of the titania nanosheets comparable to artificial lattice films produced via modern vapor-phase deposition processes. The obtained films showed superior dielectric and insulating properties as a reflection of the highly organized film nanoarchitecture.

Exfoliated Nanosheet Crystallite of Cesium Tungstate with 2D Pyrochlore Structure: Synthesis, Characterization, and Photochromic Properties

Layered cesium tungstate, Cs(6+x)W(11)O(36), with two-dimensional (2D) pyrochlore structure was exfoliated into colloidal unilamellar sheets through a soft-chemical process. Interlayer Cs ions were replaced with protons by acid exchange, and quaternary ammonium ions were subsequently intercalated under optimized conditions. X-ray diffraction (XRD) measurements on gluelike sediment recovered from the colloidal suspension by centrifugation showed a broad pattern of a pronounced wavy profile, which closely matched the square of calculated structure factor for the single host layer. This indicates the total delamination of the layered tungstate into nanosheets of Cs(4)W(11)O(36)(2-). Microscopic observations by transmission electron microscopy and atomic force microscopy clearly revealed the formation of unilamellar crystallites with a very high 2D anisotropy, a thickness of only approximately 2 nm versus lateral size up to several micrometers. In-plane XRD analysis confirmed that the 2D pyrochlore structure was retained. The colloidal cesium tungstate nanosheet showed strong absorption of UV light with sharp onset, suggesting a semiconducting nature. Analysis of the absorption profile provided 3.6 eV as indirect band gap energy, which is 0.8 eV larger than that of the bulk layered precursor, probably due to size quantization. The nanosheet exhibited highly efficient photochromic properties, showing reversible color change upon UV irradiation.

Robust High-κ Response in Molecularly Thin Perovskite Nanosheets

Size-induced suppression of permittivity in perovskite thin films is a fundamental problem that has remained unresolved for decades. This size-effect issue becomes increasingly important due to the integration of perovskite nanofilms into high-κ capacitors, as well as concerns that intrinsic size effects may limit their device performance. Here, we report a new approach to produce robust high-κ nanodielectrics using perovskite nanosheet (Ca2Nb3O10), a new class of nanomaterials that is derived from layered compounds by exfoliation. By a solution-based bottom-up approach using perovskite nanosheets, we have successfully fabricated multilayer nanofilms directly on SrRuO3 or Pt substrates without any interfacial dead layers. These nanofilms exhibit high dielectric constant (>200), the largest value seen so far in perovskite films with a thickness down to 10 nm. Furthermore, the superior high-κ properties are a size-effect-free characteristic with low leakage current density (<10(-7) A cm(-2)). Our work provides a key for understanding the size effect and also represents a step toward a bottom-up paradigm for future high-κ devices.

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.

Unusually stable ~100-fold reversible and instantaneous swelling of inorganic layered materials

Cells can swell or shrink in certain solutions; however, no equivalent activity has been observed in inorganic materials. Although lamellar materials exhibit increased volume with increase in the lamellar period, the interlamellar expansion is usually limited to a few nanometres, with a simultaneous partial or complete exfoliation into individual atomic layers. Here we demonstrate a large monolithic crystalline swelling of layered materials. The gallery spacing can be instantly increased ~100-fold in one direction to ~90 nm, with the neighbouring layers separated primarily by H2O. The layers remain strongly held without peeling or translational shifts, maintaining a nearly perfect three-dimensional lattice structure of >3,000 layers. First-principle calculations yield a long-range directional structuring of the H2O molecules that may help to stabilize the highly swollen structure. The crystals can also instantaneously shrink back to their original sizes. These findings provide a benchmark for understanding the exfoliating layered materials.

All-Nanosheet Ultrathin Capacitors Assembled Layer-by-Layer via Solution-Based Processes

All-nanosheet ultrathin capacitors of Ru0.95O20.2-/Ca2Nb3O10-/Ru0.95O20.2- were successfully assembled through facile room-temperature solution-based processes. As a bottom electrode, conductive Ru0.95O20.2- nanosheets were first assembled on a quartz glass substrate through a sequential adsorption process with polycations. On top of the Ru0.95O20.2- nanosheet film, Ca2Nb3O10- nanosheets were deposited by the Langmuir-Blodgett technique to serve as a dielectric layer. Deposition parameters were optimized for each process to construct a densely packed multilayer structure. The multilayer buildup process was monitored by various characterizations such as atomic force microscopy (AFM), ultraviolet-visible absorption spectra, and X-ray diffraction data, which provided compelling evidence for regular growth of Ru0.95O20.2- and Ca2Nb3O10- nanosheet films with the designed multilayer structures. Finally, an array of circular films (50 μm ϕ) of Ru0.95O20.2- nanosheets was fabricated as top electrodes on the as-deposited nanosheet films by combining the standard photolithography and sequential adsorption processes. Microscopic observations by AFM and cross-sectional transmission electron microscopy, as well as nanoscopic elemental analysis, visualized the sandwich metal-insulator-metal structure of Ru0.95O20.2-/Ca2Nb3O10-/Ru0.95O20.2- with a total thickness less than 30 nm. Electrical measurements indicate that the system really works as an ultrathin capacitor, achieving a capacitance density of ∼27.5 μF cm(-2), which is far superior to currently available commercial capacitor devices. This work demonstrates the great potential of functional oxide nanosheets as components for nanoelectronics, thus contributing to the development of next-generation high-performance electronic devices.

Tantalum oxide nanomesh as self-standing one nanometre thick electrolyte

Tantalum oxide (TaO3) nanosheets coated on the surface of a LiCoO2 cathode decrease its interfacial resistance in a solid-state battery by two orders of magnitude. Since the interfacial resistance is rate-determining in the solid-state system, the interfacial structure of the nanosheet is anticipated to pave the way for realising high-performance solid-state lithium batteries. The reduction in the interfacial resistance also strongly suggests that the TaO3 nanosheet is a self-standing solid electrolyte layer with an ultimate thinness of 1 nm. It has a wide band gap and a mesh structure with openings that are almost the same in size as the lithium ion, which prevents electronic conduction and allows the penetration of lithium ions, respectively.

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.

Photochromogenic Nanosheet Crystallites of Tungstate with a 2D Bronze Structure

Layered rubidium tungstate, Rb(4)W(11)O(35), with a two-dimensional (2D) bronze-type tunnel structure was successfully delaminated into colloidal nanosheets via a soft-chemical process involving acid exchange and subsequent intercalation of tetrabutylammonium ions. Characterizations by transmission electron microscopy and atomic force microscopy confirmed the formation of unilamellar 2D nanosheet crystallites with a unique thickness of ∼3 nm and an average lateral size of 400 nm. The obtained nanosheets exhibited reversible color change upon UV-light excitation via an optical band gap of 3.5 eV. The ultimate 2D aspect ratio favorable for an adsorption of charge-compensating cations to trapped electrons working as a color center is presumably responsible for highly efficient photochromic behavior. Its coloration mainly consists of a broad band at a wavelength of 1800 nm and longer, which is much different from that of the common tungstate nanomaterials. Thus, the chromogenic nanosheet obtained in this study features the intense UV absorption and optically switchable visible-to-IR absorption, which may be useful for window applications such as cutoff filters and heat-absorbing films.

High Thermal Robustness of Molecularly Thin Perovskite Nanosheets and Implications for Superior Dielectric Properties

A systematic study has been conducted to examine the thermal stability of layer-by-layer assembled films of perovskite-type nanosheets, (Ca2Nb3O10(-))n (n = 1-10), which exhibit superior dielectric and insulating properties. In-plane and out-of-plane X-ray diffraction data as well as observations by atomic force microscopy and transmission electron microscopy indicated the high thermal robustness of the nanosheet films. In a monolayer film with an extremely small thickness of ∼2 nm, the nanosheet was stable up to 800 °C, the temperature above which segregation into CaNb2O6 and Ca2Nb2O7 began. The critical temperature moderately decreased as the film thickness, or the number of nanosheet layers, increased, and reached 700 °C for seven- and 10-layer films, which is comparable to the phase transformation temperature for a bulk phase of the protonic layered oxide of HCa2Nb3O10·1.5H2O as a precursor of the nanosheet. This thermal stabilization of perovskite-type nanosheets should be associated with restricted nucleation and crystal growth peculiar to such ultrathin 2D bound systems. The stable high-k dielectric response (εr = 210) and highly insulating nature (J < 10(-7) A cm(-2)) remained substantially unchanged even after the nanosheet film was annealed up to 600 °C. This study demonstrates the high thermal stability of 2D perovskite-type niobate nanosheets in terms of structure and dielectric properties, which suggests promising potential for future high-k devices operable over a wide temperature range.