Faculty Profiles - OSADA Minoru

Papers - OSADA Minoru

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Controlled doping of semiconducting titania nanosheets for tailored spinelectronic materials Reviewed

Minoru Osada, Satoshi Yoguchi, Masayuki Itose, Bao-Wen Li, Yasuo Ebina, Katsutoshi Fukuda, Yoshinori Kotani, Kanta Ono, Shigenori Ueda, Takayoshi Sasaki

NANOSCALE Vol. 6 ( 23 ) page： 14227 - 14236 2014

Language：English Publishing type：Research paper (scientific journal) Publisher：ROYAL SOC CHEMISTRY

Ti1-x-yFexCoyO2 nanosheets are synthesized in which the (Fe/Co) content is systematically controlled in the range of 0 <= x <= 0.4 and 0 <= y <= 0.2. A key feature of this new preparation is the use of (Li/Fe)-, (Fe/Co)- and (Li/Co)-co-substituted layered titanates as starting materials. In exfoliated nanosheets, the composition can be intentionally modified by controlled Fe/Co substitution into Ti sites during the solid-state synthesis of the starting layered compounds. The composition of the host layers is maintained in the subsequent exfoliation process, which is very helpful in the rational design of nanosheets through the use of controlled doping. Through this controlled doping, we achieve exquisite control of the electronic properties of Ti-1-dO(2) nanosheets, including the position of impurity bands, the Fermi energy and ferromagnetic properties. From photoelectron spectroscopy and first-principles studies, we have observed that the use of Fe/Co co-doping with higher Fe and Co oxidation states is necessary to bring the highest occupied Fe/Co impurity states to the Fermi level. This band engineering transforms the Ti1-x-yFexCoyO2 nanosheet into a room-temperature half-metallic ferromagnet, thus accomplishing the main requirements of future spinelectronics.

DOI： 10.1039/c4nr04465g

BaTiO3 nanocube and assembly to ferroelectric supracrystals Reviewed

Kazumi Kato, Ken-Ichi Mimura, Feng Dang, Hiroaki Imai, Satoshi Wada, Minoru Osada, Hajime Haneda, Makoto Kuwabara

Journal of Materials Research Vol. 28 ( 21 ) page： 2932 - 2945 2013.11

Language：English Publishing type：Research paper (scientific journal)

New strategies for materials fabrication are of fundamental importance in the advancement of science and technology. Nanocrystals, especially with an anisotropic shape such as cubic, are candidates for building blocks for new bottom-up approaches to materials assembly, yielding a functional architecture. Such materials also receive attention because of their intrinsic size-dependent properties and resulting applications. Here, we report synthesis and characteristics of BaTiO3 and SrTiO3 nanocubes and the ordered assemblies as ferroelectric supracrystals. BaTiO3 and SrTiO3 nanocubes with narrow size distributions were obtained in an aqueous process. BaTiO3 films made up of ordered nanocube assemblies were fabricated on various substrates by evaporation-induced self-assembly method. Regardless of the substrate, the nanocubes exhibited {100} orientations and a high degree of face-to-face ordering, which remained even after heat treatment at 850 °C. Piezoresponse force microscopy was carried out on the supracrsytal films to obtain plots of the d 33 piezoelectric coefficient against the poling field, and ferroelectric hysteresis curves were shown. © 2013, Materials Research Society. All rights reserved.

DOI： 10.1557/jmr.2013.299

Tunable and highly reproducible surface-enhanced Raman scattering substrates made from large-scale nanoparticle arrays based on periodically poled LiNbO3 templates Reviewed

Xiaoyan Liu, Kenji Kitamura, Qiuming Yu, Jiajie Xu, Minoru Osada, Nagata Takahiro, Jiangyu Li, Guozhong Cao

SCIENCE AND TECHNOLOGY OF ADVANCED MATERIALS Vol. 14 ( 5 ) 2013.10

Language：English Publishing type：Research paper (scientific journal) Publisher：NATL INST MATERIALS SCIENCE

This work describes novel surface-enhanced Raman scattering (SERS) substrates based on ferroelectric periodically poled LiNbO3 templates. The templates comprise silver nanoparticles (AgNPs), the size and position of which are tailored by ferroelectric lithography. The substrate has uniform and large sampling areas that show SERS effective with excellent signal reproducibility, for which the fabrication protocol is advantageous in its simplicity. We demonstrate ferroelectric-based SERS substrates with particle sizes ranging from 30 to 70 nm and present tunable SERS effect from Raman active 4-mercaptopyridine molecules attached to AgNPs when excited by a laser source at 514 nm.

DOI： 10.1088/1468-6996/14/5/055011

Investigation of PbTiO3 thin films with reduced and re-oxidized treatment using Raman spectroscopy Reviewed

Ken Nishida, Minoru Osada, Hiroki Moriwake, Joe Sakai, Nobuaki Ito, Masamichi Nishide, Hirotaka Oka, Masashi Matsuoka, Rikyu Ikariyama, Hiroshi Funakubo, Takashi Katoda, Takashi Yamamoto

JOURNAL OF THE CERAMIC SOCIETY OF JAPAN Vol. 121 ( 1417 ) page： 859 - 862 2013.9

Language：English Publishing type：Research paper (scientific journal) Publisher：CERAMIC SOC JAPAN-NIPPON SERAMIKKUSU KYOKAI

Investigations of the lattice dynamics of defects, such as oxygen vacancies, are important to understand the properties of PbTiO3 and related materials. Herein polarized Raman spectra of epitaxial PbTiO3 thin films with reduced and re-oxidized treatment were investigated. The oxygen-reduced spectrum displayed an additional mode related to oxygen vacancies, whereas the re-oxidized spectrum was the same as the initial one. The oxygen vacancy content increases as the intensity of additional mode increases, while the intensity of the B-1 mode related to oxygen ions decreases. First-principles calculations of the phonon mode related to oxygen vacancies support the assignment of the additional mode. (C)2013 The Ceramic Society of Japan. All rights reserved.

DOI： 10.2109/jcersj2.121.859

Enhanced dielectric response induced by controlled morphology in rutile TiO2 nanocrystals Reviewed

Minoru Osada, Makoto Kobayashi, Masato Kakihana

JOURNAL OF THE CERAMIC SOCIETY OF JAPAN Vol. 121 ( 1416 ) page： 593 - 597 2013.8

Language：English Publishing type：Research paper (scientific journal) Publisher：CERAMIC SOC JAPAN-NIPPON SERAMIKKUSU KYOKAI

High-quality rutile TiO2 nanocrystals have been synthesized by a simple hydrothermal treatment of a watersoluble titanium-glycolate complex at 200 degrees C. The obtained samples were composed of rod- or whisker-like nanocrystals. The aspect ratios of these nanocrystals could be controlled by adding glycolic acid. Combined characterizations with X-ray diffraction, Raman scattering and high-resolution transmission electron microscopy revealed the formation of highly phase-pure and well-crystallized rutile nanorods/nanowhiskers with the c-axis orientation. Room-temperature dielectric measurements indicated that the permittivity for the rutile nanowhiskers is similar to 150 at 100 kHz, which is higher than that for the bulk rutile powder. These observations pave a way for morphological controlled rutile nanocrytals to find a broad class of technological uses. (C) 2013 The Ceramic Society of Japan. All rights reserved.

DOI： 10.2109/jcersj2.121.593

Oxygen vacancies in PbTiO3 thin films probed by resonant Raman spectroscopy Reviewed

Ken Nishida, Minoru Osada, Joe Sakai, Nobuaki Ito, Takashi Katoda, Rikyu Ikariyama, Hiroshi Funakubo, Hiroki Moriwake, Takashi Yamamoto

JOURNAL OF THE CERAMIC SOCIETY OF JAPAN Vol. 121 ( 1416 ) page： 598 - 601 2013.8

Language：English Publishing type：Research paper (scientific journal) Publisher：CERAMIC SOC JAPAN-NIPPON SERAMIKKUSU KYOKAI

Resonant Raman spectroscopy was applied to evaluate oxygen vacancies in PbTiO3-x thin films that were heat treated in a hydrogen atmosphere at various temperatures. Additional mode related to oxygen vacancies occurred in the resonant Raman measurement condition, and its intensity was in proportion to the oxygen vacancy concentration. This correlation offers a simple and useful probe for oxygen vacancies in oxide-based devices. (C) 2013 The Ceramic Society of Japan. All rights reserved.

DOI： 10.2109/jcersj2.121.598

Realization of graphene field-effect transistor with high-kappa HCa2Nb3O10 nanoflake as top-gate dielectric Reviewed

Wenwu Li, Song-Lin Li, Katsuyoshi Komatsu, Alex Aparecido-Ferreira, Yen-Fu Lin, Yong Xu, Minoru Osada, Takayoshi Sasaki, Kazuhito Tsukagoshi

APPLIED PHYSICS LETTERS Vol. 103 ( 2 ) 2013.7

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER INST PHYSICS

A high-quality HCa2Nb3O10 (HCNO) nanoflake (epsilon(r) = 9.1) consisting of high-kappa perovskite nanosheets is adopted as a gate dielectric for graphene-based electronics. A dual-gated device was physically constructed by directly dry-transferring a 22-nm-thick HCNO nanoflake as a top gate dielectric onto graphene. The fabricated graphene field-effect transistor could be operated at biases <1.5V with a gate leakage below 1 pA. The top-gate capacitance and mobility of the dual-gated graphene device at room temperature were estimated to be 367 nF/cm(2) and 2500 cm(2)/V.s, respectively. These results show that HCNO can be employed as an alternative dielectric for graphene-based devices. (C) 2013 AIP Publishing LLC.

DOI： 10.1063/1.4813537

Successive phase transformation in stoichiometric glassy Li2Ge4O9: Isothermal and nonisothermal study Reviewed

Yoshihiro Takahashi, Jun Kunitomo, Takamichi Miyazaki, Minoru Osada, Takumi Fujiwara

JOURNAL OF APPLIED PHYSICS Vol. 114 ( 3 ) 2013.7

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER INST PHYSICS

Stoichiometric glassy germanate is an important material in geophysics and materials science. Despite the binary Li2O-GeO2 system being primitive, its crystallization mechanism remains undetermined. In this study, we investigated crystallization and structural ordering in glassy ferroelectric Li2Ge4O9. Using isothermally and nonisothermally prepared samples, we demonstrated that the glassy Li2Ge4O9 shows no direct structural change to the stoichiometric phase; it rather forms alpha-GeO2 nanocrystals, i.e., it undergoes successive phase transformation. The results suggest that the metastable immiscibility is related to the ordering that occurs prior to crystallization of the stoichiometric phase. (C) 2013 AIP Publishing LLC.

DOI： 10.1063/1.4813783

Oriented Film Growth of Ba1-xSrxTiO3 Dielectrics on Glass Substrates Using 2D Nanosheet Seed Layer Reviewed

Chulho Jung, Tsuyoshi Ohnishi, Minoru Osada, Kazunori Takada, Takayoshi Sasaki

ACS APPLIED MATERIALS & INTERFACES Vol. 5 ( 11 ) page： 4592 - 4596 2013.6

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER CHEMICAL SOC

An approach to fabricate Ba0.5Sr0.5TiO3 (BST) films with a preferred orientation on a glass substrate by pulsed laser deposition was developed. To ensure a preferred crystallographic orientation, we utilized a molecularly thin Ca2Nb3O10 perovskite nanosheet as a seed layer and successfully fabricated BST films with a nearly perfect (100)axis orientation. The 100 nm films after annealing at 450 degrees C in air showed a good dielectric performance (epsilon(r) > 400), which was comparable to the epsilon(r) value of epitaxially grown films with the same thickness. These results indicate that the nanosheet seed layer plays a crucial role in controlled film growth, realizing a nearly intrinsic performance of BST.

DOI： 10.1021/am400849z

Gigantic plasmon resonance effects on magneto-optical activity of molecularly thin ferromagnets near gold surfaces Reviewed

Minoru Osada, Natália Hajduková-Šmídová, Kosho Akatsuka, Satoshi Yoguchi, Takayoshi Sasaki

Journal of Materials Chemistry C Vol. 1 ( 14 ) page： 2520 - 2524 2013.4

Language：English Publishing type：Research paper (scientific journal)

We report on a new magneto-plasmonic material consisting of 1 nm thick ferromagnetic nanosheets and Au nanoparticles. The magneto-optical (MO) Kerr spectra of Ti0.8Co0.2O2 nanosheets near Au surfaces showed a gigantic MO response (∼106 deg cm-1) in the visible wavelength region (380-600 nm), not present in Ti 0.8Co0.2O2 nanosheets on a bare glass substrate. The observed peaks correspond to intrinsic d-d* electronic transitions in Ti0.8Co0.2O2 and are consistent with the near-field enhancement of the MO response resulting from the spectral overlap of the surface-plasmon-resonance (SPR) in the Au surface with the electronic transitions in Ti0.8Co0.2O2. Similar SPR effects were also achieved in ferromagnetic nanosheet/Au with different compositions and with different separation distances (&lt
5 nm). This demonstration of SPR-enhanced magneto-optics in the ferromagnetic/plasmonic nanosystem may enable designing of nanoarchitectures for miniaturized high-performance MO devices and for remote sensing and imaging of magnetic fields. © The Royal Society of Chemistry 2013.

DOI： 10.1039/c3tc00952a

Synthesis of Highly Strained Mesostructured SrTiO3/BaTiO3 Composite Films with Robust Ferroelectricity Reviewed

Norihiro Suzuki, Mohamed B. Zakaria, Nagy L. Torad, Kevin C-W Wu, Yoshihiro Nemoto, Masataka Imura, Minoru Osada, Yusuke Yamauchi

CHEMISTRY-A EUROPEAN JOURNAL Vol. 19 ( 14 ) page： 4446 - 4450 2013.4

Fabrication of Ruthenium Metal Nanosheets via Topotactic Metallization of Exfoliated Ruthenate Nanosheets Reviewed

Katsutoshi Fukuda, Jun Sato, Takahiro Saida, Wataru Sugimoto, Yasuo Ebina, Tatsuo Shibata, Minoru Osada, Takayoshi Sasaki

INORGANIC CHEMISTRY Vol. 52 ( 5 ) page： 2280 - 2282 2013.3

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER CHEMICAL SOC

The metallization behavior of molecularly thin RuO2 nanosheets obtained from complete delamination of layered ruthenates was studied. Interestingly, the RuO2 nanosheets in a monolayer state topotactically transformed into a single layer of Ru atoms, i.e., ruthenium metal nanosheets, which can be regarded as a new family of nanosized metals.

DOI： 10.1021/ic302720d

Low-frequency Raman scattering in binary silicate glass: Boson peak frequency and its general expression Reviewed

Kensaku Nakamura, Yoshihiro Takahashi, Minoru Osada, Takumi Fujiwara

Journal of the Ceramic Society of Japan Vol. 121 ( 1420 ) page： 1012 - 1014 2013

Language：English Publishing type：Research paper (scientific journal) Publisher：Ceramic Society of Japan

Low-frequency Raman measurement was performed in alkali and alkaline-earth silicate glasses to elucidate the effect of introduction of network modifiers on boson peak nature. Our study illustrated that introduction of small/medium ions tended to increase in the boson peak frequency (ωBP), whereas the introduction of large ions tended to decrease the frequency. Although ωBP has been suggested to be proportional to the shear modulus, no linear relationship was obtained in these binary systems. Instead, we experimentally demonstrated that mean atomic volume can explain variations in ωBP regardless of the trend of boson peaks. © 2013 The Ceramic Society of Japan. All rights reserved.

DOI： 10.2109/jcersj2.121.1012

Room temperature multiferroic heterostructure: Nd: BiFeO3/YMnO3 Reviewed

Hongyang Zhao, Hideo Kimura, Zhenxiang Cheng, Xiaolin Wang, Qiwen Yao, Minoru Osada, Baowen Li

Journal of Crystal Growth Vol. 365 page： 19 - 23 2013

Language：English Publishing type：Research paper (scientific journal) Publisher：Elsevier B.V.

In order to search candidate multiferroics, a heterostructure of Nd:BiFeO3/YMnO3 (BFO/YMO) was designed. It was fabricated using a pulsed laser deposition (PLD) method on (111) Nb: SrTiO3 substrate through layer-by-layer growth. The particularly interesting phases for Nd:BiFeO3 (P4mm) and YMnO3 (hexagonal and orthorhombic) were observed. This film has a high ferromagnetic Curie temperature at 420 K. The ferroelectric domain switching was observed and it exhibited large electromechanical coupling with maximum d33∼73 pm/V at 10 V. © 2012 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jcrysgro.2012.12.023

Structural relaxation and quasi-elastic light scattering in glass: Approach by ferroelectric and ion-conducting phases Reviewed

Yoshihiro Takahashi, Kensaku Nakamura, Minoru Osada, Takumi Fujiwara

SCIENTIFIC REPORTS Vol. 2 2012.10

Language：English Publishing type：Research paper (scientific journal) Publisher：NATURE PUBLISHING GROUP

Inelastic light scattering has been utilized for examining the structure of glass and its relaxation. However, the quasi-elastic-light-scattering (QLS) phenomenon has not been addressed in much detail. In this study, we observed pronounced QLS-intensity variations in two temperature domains-supercooled liquid (SCL) state (alpha-relaxation regime) and below the glass-transition temperature (beta-relaxation regime)-in niobium-oxide (Nb2O5)-rich glass. These variations may be interpreted on the basis of the concept of ferroelectric and ion-conducting phases. It was suggested that the observed QLS originates as a result of the polarization fluctuation of NbO6 units, which is due to the dynamics of the nanometric phase separation in the SCL phase (alpha-regime), and the fluctuation due to the migration/hopping of conductible ions that are localized in the vicinity of the NbO6 units (beta-regime).

DOI： 10.1038/srep00714

The formation of graphene-titania hybrid films and their resistance change under ultraviolet irradiation Reviewed

Pengzhan Sun, Renzhi Ma, Minoru Osada, Takayoshi Sasaki, Jinquan Wei, Kunlin Wang, Dehai Wu, Yao Cheng, Hongwei Zhu

CARBON Vol. 50 ( 12 ) page： 4518 - 4523 2012.10

Language：English Publishing type：Research paper (scientific journal) Publisher：PERGAMON-ELSEVIER SCIENCE LTD

We report the fabrication of hybrid films of graphene and monolayer titania using a simple electrostatic self-assembly method. Ultraviolet (UV) responses of the hybrid films based on the graphene-titania structure were investigated. We observed that the resistance of the graphene-titania hybrid increased exponentially with UV irradiation time and decreased exponentially when UV was turned off. Time constants of the order of hundreds of seconds were identified and found to be sensitive to the gas environment of graphene. The UV response as well as the time constant is tunable by varying the number of titania layers. Our results confirmed that UV irradiation played a significant role in the resistance modulation of graphene as well as graphene-titania hybrid films. (C) 2012 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.carbon.2012.05.035

Interfacial phenomena in solid-state lithium battery with sulfide solid electrolyte Reviewed

Kazunori Takada, Narumi Ohta, Lianqi Zhang, Xiaoxiong Xu, Bui Thi Hang, Tsuyoshi Ohnishi, Minoru Osada, Takayoshi Sasaki

SOLID STATE IONICS Vol. 225 page： 594 - 597 2012.10

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

The effects of surface coating on electrode properties of LiMn2O4 in a sulfide solid electrolyte were investigated. The surface coating with LiNbO3 reduced the electrode resistance by two orders of magnitude. Changes in the electrode properties were very similar to those observed for the corresponding LiCoO2 electrodes, which strongly suggest that the space-charge layer formed at the high-voltage cathode/sulfide electrolyte interface is rate-determining and must be controlled to improve the rate capability. (c) 2012 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.ssi.2012.01.009

Nano-Materials Design for High-TC Ferromagnets of Ti1-xCoxO2 Nanosheets Reviewed

Minoru Osada, Takayoshi Sasaki, Kanta Ono

INTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY Vol. 9 ( 5 ) page： 936 - 941 2012.9

Language：English Publishing type：Research paper (scientific journal) Publisher：WILEY-BLACKWELL

We present materials design of high-TC ferromagnetic nanomaterials based on first-principles calculations. Two-dimensional (2D) titania nanosheet, a recently discovered room-temperature (RT) ferromagnetic nanomaterial, is an important target for 2D ferromagnets. In Ti1-xCoxO2 nanosheets with Co2+ oxidation state, the ferromagnetic states are highly stabilized up to x=0.5 with a modest doping dependence of magnetic moment (1 mu B/Co). Moreover, we find that tailoring properties could be achieved by tuning Co oxidation states, and the optimally doped Ti1-xCoxO2 nanosheets (Co3+; x=0.125, 0.25) are found to be an excellent RT half-metallic ferromagnet with a large magnetic moment (similar to 4 mu B/Co). 2D nanosheet is therefore expected to provide a very useful platform for nanoscale spinelectronics devices.

DOI： 10.1111/j.1744-7402.2012.02783.x

SE(R)RS excitation profile of free-base 5,10,15,20-tetrakis(1-methyl-4-pyridyl) porphyrin on immobilized gold nanoparticles Reviewed

Natalia Hajdukova-Smidova, Marek Prochazka, Minoru Osada

VIBRATIONAL SPECTROSCOPY Vol. 62 page： 115 - 120 2012.9

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE BV

Surface-enhanced (resonance) Raman scattering - SE(R)RS - excitation profile (dependence of SE(R)RS intensity on excitation wavelength) is a key point for optimizing of electromagnetic enhancement of particular SE(R)RS system. In this paper, we measured SE(R)RS excitation profile of free-base 5,10,15,20-tetrakis(1-methyl-4-pyridyl) porphyrin (TMPyP) on immobilized Au nanoparticles using confocal Raman microspectrometer with six excitation wavelengths (457.9, 488.0, 514.5, 530.9, 568.2 and 647.1 nm). Au colloidal nanoparticles were immobilized to a glass slide via a self-assembled monolayer of 3-aminopropyltrimethoxysilane. SE(R)RS excitation profile is correlated with the corresponding surface plasmon extinction (SPE) spectrum of immobilized Au nanoparticles although the maximum of SE(R)RS intensity is slightly shifted from maximum of SPE. In our case of resonant molecule, SE(R)RS enhancement is coupled with molecular resonance enhancement and SE(R)RS excitation profile for particular vibrational mode depends on its TMPyP molecular resonance contribution. SE(R)RS excitation profile shows that maximal SE(R)RS intensity is obtained for 568.2 nm excitation that provides limit of detection (LOD) of TMPyP 2 x 10(-8) M in soaking solution. (C) 2012 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.vibspec.2012.04.007

Percolative BaTiO3/Carbon-Nanotube Composite Films Employing Aerosol Deposition Reviewed

Yoon-Hyun Kim, Minoru Osada, Hong-Ki Kim, Song-Min Nam

JAPANESE JOURNAL OF APPLIED PHYSICS Vol. 51 ( 9 ) 2012.9

Language：English Publishing type：Research paper (scientific journal) Publisher：IOP PUBLISHING LTD

Percolative BaTiO3/multiwalled carbon nanotube (MWNT) composite thick films were developed using an aerosol deposition (AD) process at room temperature. Well-dispersed mixed powders with various volume ratios were utilized as the starting powders for AD, and dense BaTiO3/MWNT composite thick films were successfully fabricated with the starting mixed powders with 0.01 vol% MWNTs. The existence of MWNTs in the composite films was confirmed using Raman spectroscopy. The composite films show a high relative dielectric constant (657 at 1 kHz), which is approximately 7 times larger than that (91 at 1 kHz) of the BaTiO3 films. In addition, the strong frequency dispersion of the dielectric constant in the low-frequency range was confirmed for the composite films. The dielectric properties of the composite films were investigated using the dielectric mechanism used in percolative composites. (C) 2012 The Japan Society of Applied Physics

DOI： 10.1143/JJAP.51.09LC07

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