Ryota Takahashi | The University of Tokyo (original) (raw)

Papers by Ryota Takahashi

MRS Bulletin

Spontaneous noble metal dopant segregation in an oxide lattice can lead to the formation of metal... more Spontaneous noble metal dopant segregation in an oxide lattice can lead to the formation of metallic clusters and extended acicular inclusions. In a thin-film process, the shape and orientation of such noble metal inclusions are governed by the crystal growth direction, giving rise to a composite material with lattice-matched metal nanopillars embedded vertically in an insulating or semiconducting oxide matrix. An interesting application of such composites is in photoelectrochemical cell electrodes, where the metallic nanopillars take on three distinct roles: forming a Schottky junction with the host matrix, providing a low-loss current path from bulk to surface, and creating an efficient electrocatalytic active site on the electrode surface. In particular, we discuss the application of vertically aligned metal–oxide nanopillar composites in photoelectrochemical water-splitting cells used for direct solar-powered hydrogen generation.

Nature communications, Jun 3, 2016

Production of chemical fuels by direct solar energy conversion in a photoelectrochemical cell is ... more Production of chemical fuels by direct solar energy conversion in a photoelectrochemical cell is of great practical interest for developing a sustainable energy system. Various nanoscale designs such as nanowires, nanotubes, heterostructures and nanocomposites have been explored to increase the energy conversion efficiency of photoelectrochemical water splitting. Here we demonstrate a self-organized nanocomposite material concept for enhancing the efficiency of photocarrier separation and electrochemical energy conversion. Mechanically robust photoelectrodes are formed by embedding self-assembled metal nanopillars in a semiconductor thin film, forming tubular Schottky junctions around each pillar. The photocarrier transport efficiency is strongly enhanced in the Schottky space charge regions while the pillars provide an efficient charge extraction path. Ir-doped SrTiO3 with embedded iridium metal nanopillars shows good operational stability in a water oxidation reaction and achieves...

Applied Physics Letters, 2015

We study magnetic coupling between hole-doped manganite layers separated by either a perovskite o... more We study magnetic coupling between hole-doped manganite layers separated by either a perovskite or a rock-salt barrier of variable thickness. Both the type and the quality of the interface have a strong impact on the minimum critical barrier thickness where the manganite layers become magnetically decoupled. A rock-salt barrier layer only 1 unit cell (0.5 nm) thick remains insulating and is able to magnetically de-couple the electrode layers. The technique can therefore be used for developing high-performance planar oxide electronic devices such as magnetic tunnel junctions and quantum well structures that depend on magnetically and electronically sharp heterointerfaces.

Physical Review B, 2010

The dynamics of a strain-induced resistive phase transition in a VO 2 thin film was studied by us... more The dynamics of a strain-induced resistive phase transition in a VO 2 thin film was studied by using a low-frequency crystal bending stage. It was found that the resistance response of the film to strain excitation is determined by two time constants, interpreted as a nucleation lifetime, in the 10 −6-10 −4 s range, and a domain growth lifetime of ϳ10 −3 s. The variation of the domain nucleation lifetime is discussed in terms of the barrier height separating the coexisting insulating and metallic phases in a VO 2 film. Below the transition temperature, the phase transition is nucleation limited. Above the transition temperature, the resistance change is strongly affected by the growth of metallic domains. Direct imaging of metallic domains by local-probe current mapping showed that the phase transition in a thin film is limited by the presence of grain boundaries.

Physical Review B, 2012

We have investigated the spontaneous polarization in Fe3O4 thin films by using dynamic and static... more We have investigated the spontaneous polarization in Fe3O4 thin films by using dynamic and static pyroelectric measurements. The magnetic and dielectric behavior of Fe3O4 thin films grown on Nb:SrTiO3(001) substrates was consistent with bulk crystals. The well-known metal-insulator (Verwey) transition was observed at 120 K. The appearance of a pyroelectric response in the Fe3O4 thin films just below the Verwey temperature shows that spontaneous polarization appeared in Fe3O4 at the charge ordering transition temperature. The polar state characteristics are consistent with bond-and site-centered charge ordering of Fe 2+ and Fe 3+ ions sharing the octahedral B sites. The pyroelectric response in Fe3O4 thin films was dependent on the dielectric constant. Quasi-static pyroelectric measurement of Pd/Fe3O4/Nb:SrTiO3 junctions showed that magnetite has a very large pyroelectric coefficient of 735 nC/cm 2 K at 60 K.

Nature Communications, 2011

Chemical and structural heterogeneity and the resulting interaction of coexisting phases can lead... more Chemical and structural heterogeneity and the resulting interaction of coexisting phases can lead to extraordinary behaviours in oxides, as observed in piezoelectric materials at morphotropic phase boundaries and relaxor ferroelectrics. However, such phenomena are rare in metallic alloys. Here we show that, by tuning the presence of structural heterogeneity in textured Co 1 − x Fe x thin films, effective magnetostriction λ eff as large as 260 p.p.m. can be achieved at lowsaturation field of ~10 mT. Assuming λ 100 is the dominant component, this number translates to an upper limit of magnetostriction of λ 100 ≈ 5λ eff > 1,000 p.p.m. microstructural analyses of Co 1 − x Fe x films indicate that maximal magnetostriction occurs at compositions near the (fcc + bcc)/bcc phase boundary and originates from precipitation of an equilibrium Co-rich fcc phase embedded in a Fe-rich bcc matrix. The results indicate that the recently proposed heterogeneous magnetostriction mechanism can be used to guide exploration of compounds with unusual magnetoelastic properties.

Advanced Functional Materials, 2010

Within the class of materials showing a reversible martensitic phase transformation and a shape m... more Within the class of materials showing a reversible martensitic phase transformation and a shape memory effect (SME), NiTi enjoys widespread and growing use in a variety of applications. Owing to its unique combination of properties: ductility, strength, corrosion resistance, biocompatibility, robust pseudoelasticity, and one-and two-way SME, it is commercially used in medical and actuator applications. Despite such attributes, there remain undesirable functional and structural fatigue properties of NiTi. Functional fatigue of shape memory alloys (SMAs) resulting in changes of physical, mechanical, and shape memory (SM) properties [1] during cyclic thermal or mechanical loading is related to irreversible processes, that is, generation of dislocations, [2,3] which take place during the martensitic phase transformation. Only recently, the mechanism for the multiplication of dislocations during martensitic transformations has been uncovered, based on transmission electron microscopy (TEM) investigations on singlecrystal NiTi. [4] Accumulation of these defects eventually causes internal crack formation and growth, leading to catastrophic failure for many applications. [1,5] The number of dislocations generated during the phase transformation is believed to be closely related to the geometric compatibility at the developing interface between austenite (hightemperature phase) and martensite (low-temperature phase). Due

CrystEngComm, 2015

Silver oxide clathrate Ag7O8NO3epitaxial films were synthesized photo-electrochemically on rutile... more Silver oxide clathrate Ag7O8NO3epitaxial films were synthesized photo-electrochemically on rutile-type Nb-doped TiO2(110) single-crystal substrates.

Review of Scientific Instruments, 2019

A pulsed laser deposition system with rapid beam deflection (RBD-PLD) by a galvanometer mirror sc... more A pulsed laser deposition system with rapid beam deflection (RBD-PLD) by a galvanometer mirror scanner has been developed for alternating ablation of multiple targets with a single laser instrument. In this system, the alternating deposition of different target materials is carried out by scanning the laser beam between the positionally fixed targets with a galvanometer mirror instead of mechanically switching the target positions on a fixed optical path of the laser beam as is done in conventional pulsed laser deposition (PLD) systems. Thus, the "wait" time required for switching target materials to be deposited, which typically takes several seconds in a conventional system, can be made as short as a few milliseconds. We demonstrate some of the advantages of this PLD system in several technologically important aspects of thin film synthesis: (1) fast fabrication of binary alloy films, (2) preparation of natural composition spread libraries, (3) effect of the target switching time on the deposition of volatile compounds, (4) control of the degree of mixing of two different materials in a film, and (5) efficient growth of compositionally graded thin films.

Science and Technology of Advanced Materials, 2017

As an extension of combinatorial molecular layer epitaxy via ablation of perovskite oxides by a p... more As an extension of combinatorial molecular layer epitaxy via ablation of perovskite oxides by a pulsed excimer laser, we have developed a laser molecular beam epitaxy (MBE) system for parallel integration of nano-scaled thin films of organic-inorganic hybrid materials. A pulsed infrared (IR) semiconductor laser was adopted for thermal evaporation of organic halide (A-site: CH 3 NH 3 I) and inorganic halide (B-site: PbI 2) powder targets to deposit repeated A/B bilayer films where the thickness of each layer was controlled on molecular layer scale by programming the evaporation IR laser pulse number, length, or power. The layer thickness was monitored with an in situ quartz crystal microbalance and calibrated against ex situ stylus profilometer measurements. A computer-controlled movable mask system enabled the deposition of combinatorial thin film libraries, where each library contains a vertically homogeneous film with spatially programmable A-and B-layer thicknesses. On the composition gradient film, a hole transport Spiro-OMeTAD layer was spin-coated and dried followed by the vacuum evaporation of Ag electrodes to form the solar cell. The preliminary cell performance was evaluated by measuring I-V characteristics at seven different positions on the 12.5 mm × 12.5 mm combinatorial library sample with seven 2 mm × 4 mm slits under a solar simulator irradiation. The combinatorial solar cell library clearly demonstrated that the energy conversion efficiency sharply changes from nearly zero to 10.2% as a function of the illumination area in the library. The exploration of deposition parameters for obtaining optimum performance could thus be greatly accelerated. Since the thickness ratio of PbI 2 and CH 3 NH 3 I can be freely chosen along the shadow mask movement, these experiments show the potential of this system for high-throughput screening of optimum chemical composition in the binary film library and application to halide perovskite solar cell.

EPL (Europhysics Letters), 2016

Ferromagnetic insulator Josephson junctions consisting of Nb/Au/Pr0.8Ca0.2MnO3/La1.85Sr0.15CuO4 l... more Ferromagnetic insulator Josephson junctions consisting of Nb/Au/Pr0.8Ca0.2MnO3/La1.85Sr0.15CuO4 layers were fabricated. Non-linear current-voltage characteristics suggest the presence of superconducting coupling between the Nb/Au and La1.85Sr0.15CuO4 layers across a ferromagnetic Pr0.8Ca0.2MnO3 tunnel barrier. Tunneling spectra showed clear conductance peaks due to quasi-particle tunneling. Superconducting gap energies determined from the tunneling spectra were consistent with the temperature dependence of the critical current of the junctions. We argue that magnetic inhomogeneity in the ultrathin tunnel barrier plays a role in linking the superconducting states in Nb/Au and La1.85Sr0.15CuO4.

Nanotechnology, 2009

We report the fabrication of PbTi0 3 nanomesas down to 30 nm lateral size and 4 nm high on nanost... more We report the fabrication of PbTi0 3 nanomesas down to 30 nm lateral size and 4 nm high on nanostructured SrRu0 3 /SrTi0 3 templates by off-axis radio frequency magnetron sputtering. The templates were prepared using a top-down lithography approach based on scanning tunneling microscopy. The growth rate of the PbTi0 3 nanomesas was found to decrease with increasing growth temperature as well as with shrinking template size. Piezoresponse force microscopy measurements for the PbTi0 3 nanomesas showed a strong increase in response with decreasing lateral size. A decrease of the coercive voltage was also observed for the same lateral size range. This laterally size-dependent behavior is attributed to reduction of in-plane strain, when shrinking the nanomesa lateral dimensions.

Journal of the Physical Society of Japan, 2013

The effect of nonmagnetic substitution in a perovskite ferromagnetic insulator Pr 0:8 Ca 0:2 MnO ... more The effect of nonmagnetic substitution in a perovskite ferromagnetic insulator Pr 0:8 Ca 0:2 MnO 3 was studied in epitaxial Pr 0:8 Ca 0:2 Mn 1Ày Sc y O 3 thin films grown on SrTiO 3 substrates. An increasing Sc substitution level weakened the ferromagnetic order, resulting in a decrease of saturation magnetization and the ferromagnetic transition temperature. Analysis of transport properties and Raman spectra indicates that besides simple spin dilution, weakening of both the double-exchange interaction and orbital order contributes to the reduction of ferromagnetic order.

Journal of Crystal Growth, 2005

We propose a general term flux-mediated epitaxy (FME) to single crystal quality of complex oxide ... more We propose a general term flux-mediated epitaxy (FME) to single crystal quality of complex oxide thin films in vapor-phase epitaxy. The key is a flux, which is frequently used in a bulk process for lowering a process temperature and suppressing incongruent melt. The successful application of the flux to the bulk single crystal growth allows us to expect a similar benefit even in the case of vapor-phase epitaxy. In this paper, we discuss on the capability of this general concept 'FME' for controlling phases and crystallinity of the complex oxide films, showing some examples such as optical, ferromagnetic oxide and high-T c superconductor.

Japanese Journal of Applied Physics, 2001

We fabricated the FePt film on thermal oxidized Si substrate by rapid thermal annealing (RTA), an... more We fabricated the FePt film on thermal oxidized Si substrate by rapid thermal annealing (RTA), and investigated the order parameter and the orientation parameter as a function of annealing temperature. We also observed grains by a transmission electron microscope (TEM). The joining of the grains begins at between 300 • C and 400 • C, and this is the threshold for obtaining highly (001) oriented FePt films. We also clarify the difference in the magnetization and the structure between the films annealed from Pt/Fe (Fe-first) and from Fe/Pt (Pt-first) multilayers as the starting materials. The film annealed from the Fefirst multilayers has the perpendicular magnetization showing higher (001) orientation and magnetic anisotropy than the film annealed from the Pt-first multilayers.

$Research paper thumbnail of Recognition of the Atomic Terminating Layer in Perovskite Oxide Substrates by Reflection High Energy Electron Diffraction$

Japanese Journal of Applied Physics, 2003

The chemical surface structure in perovskite oxides has been identified by monitoring the oscilla... more The chemical surface structure in perovskite oxides has been identified by monitoring the oscillation intensity of reflection high energy electron diffraction patterns during initial growth of MO (M=Sr, Ba) films on the perovskite substrate. This successful analysis technique is demonstrated for the film growth on A and B-site oxides terminated ABO 3 substrates. Epitaxial growth of MO thin films is dominated by chemical interaction between the growing lattice and the underlying atomic layer to follow the crystal habit of forming a more stable layer unit cell. This simple technique can be used as a substitute to relevant physical techniques such as coaxial impact-collision ion scattering spectroscopy and friction force microscopy that are currently used techniques to determine the termination layer.

Crystal Growth & Design, 2014

Crystal Growth & Design, 2012

ABSTRACT We have investigated self-template control of Fe3O4 thin film orientation on SrTiO3(001)... more ABSTRACT We have investigated self-template control of Fe3O4 thin film orientation on SrTiO3(001) substrates. The growth orientation of Fe3O4 films on SrTiO3(001) is dependent on the preparation temperature, with a crossover from the (001) to (111) grain orientation occurring at around 600 °C. In order to grow high-quality (001)-oriented Fe3O4 thin films at high temperature, a self-template technique was used, where an 8-nm-thick nucleation layer was deposited on a SrTiO3(001) substrate at 400 °C, followed by main film growth at 700 °C. This method achieved films that showed pure (001) grain orientation with bulk-like magnetic and transport behavior.

Applied Surface Science, 2006

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Applied Physics Letters, 2008

ABSTRACT The ferroelectric domain ground state of PbTiO3 thin films on Nb:SrTiO3 substrates has b... more ABSTRACT The ferroelectric domain ground state of PbTiO3 thin films on Nb:SrTiO3 substrates has been studied as a function of film thickness. High resolution linear Qx x-ray diffraction profiles were recorded in order to probe the presence of stripe domains, which were invariably observed for as-grown samples. After photochemical treatment, films thinner than 50 unit cells were switched to a monodomain state that could be reversed upon subsequent annealing. The stripe-domain state is linked to a Qx modulation of the Qz parameter in the reciprocal space, which disappears upon switching.