Faculty Profiles - AKIMOTO Ikuko

Published Papers - AKIMOTO Ikuko

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20pBP-7 Dynamics of optically generated carriers in high-purity silicon crystals

Akimoto I., Ohata K., Naka N.

Meeting Abstracts of the Physical Society of Japan ( The Physical Society of Japan (JPS) ) 71 1507 - 1507 2016

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23pPSA-34 Relaxation of excitons and holes in diamond measured by the time-resolved cyclotron resonance

Akimoto I., Naka N.

Meeting Abstracts of the Physical Society of Japan ( The Physical Society of Japan (JPS) ) 70 1580 - 1580 2015

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Fluorescent silica nanoparticles modified chemically with terbium complexes as potential bioimaging probes: their fluorescence and colloidal properties in water

Yoshio Nakahara, Yoichi Tatsumi, Ikuko Akimoto, Shusuke Osaki, Motomichi Doi, Keiichi Kimura

NEW JOURNAL OF CHEMISTRY ( ROYAL SOC CHEMISTRY ) 39 ( 2 ) 1452 - 1458 2015 [Refereed]

It generally requires a complicated reaction protocol for the synthesis of silylated fluorescent lanthanide complexes. In this study, a silylated terbium complex was prepared by a very simple procedure through formation of a Schiff base between a terbium complex bearing a formyl group and 3-aminopropyltriethoxysilane. Using the silylated terbium complex, highly fluorescent silica nanoparticles modified chemically with terbium complexes (Tb-SNPs) were efficiently synthesized by a reverse micelle method with Triton X-100 as a surfactant in cyclohexane. The fluorescence properties of Tb-SNPs were remarkably improved by photostability, pH dependence and fluorescence lifetime, compared to free terbium complexes. Also, Tb-SNPs hardly aggregated under aqueous conditions with different salt concentrations and pHs. From these results, it was found that Tb-SNPs are applicable under physiological aqueous conditions. Furthermore, as an application model, Tb-SNPs were used as the fluorescent label for the imaging of African green monkey kidney cells.

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18pPSA-22 Comparison of steady- and time-resolved- cyclotron resonance methods for photocarriers in a high-resistivity silicon crystal

Ohata K., Aiba K., Handa Y., Akimoto I., Naka N.

Meeting Abstracts of the Physical Society of Japan ( The Physical Society of Japan (JPS) ) 70 1274 - 1274 2015

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17aCE-1 Excitation wavelength dependence of photocarrier generation processes in high-resistivity silicon crystal

Akimoto I., Ohata K., Handa Y., Naka N.

Meeting Abstracts of the Physical Society of Japan ( The Physical Society of Japan (JPS) ) 70 1191 - 1191 2015

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Nanosecond cyclotron resonance in ultrapure diamond

Nobuko Naka, Katsuyuki Fukai, Yushi Handa, Ikuko Akimoto

JOURNAL OF LUMINESCENCE ( ELSEVIER SCIENCE BV ) 152 93 - 97 2014.08 [Refereed]

We have investigated temporal properties of photo-excited carriers in ultrapure diamond by the nanosecond cyclotron resonance (CR) method. The ramp time and the amplitude of the CR signals are found to be dependent on the incident laser pulse energy as well as on the photon energy for excitation. The dependences are discussed based on the photoabsorption process due to the indirect excitons in diamond, which is assisted by absorption of the transverse-acoustic phonons. Similarities to the free-carrier generation via two-body excitonic collisions in a dipole-forbidden direct gap semiconductor, Cu2O, are pointed out. (C) 2014 Elsevier B.V. All rights reserved.

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High carrier mobility in ultrapure diamond measured by time-resolved cyclotron resonance

Ikuko Akimoto, Yushi Handa, Katsuyuki Fukai, Nobuko Naka

APPLIED PHYSICS LETTERS ( AMER INST PHYSICS ) 105 ( 3 ) 2014.07 [Refereed]

We have performed time-resolved cyclotron resonance measurements in ultrapure diamond crystals for the temperature range of T = 7.3-40 K and obtained the temperature-dependent momentum relaxation times based on the cyclotron resonance widths for optically generated electrons and holes. The relaxation time follows a T-3/2 law down to 12 K, which is expected for acoustic-phonon scattering without impurity effect because of the high purity of our samples. The deviation from the law at lower temperatures is explained by the impurity scattering and the breakdown of the high-temperature approximation for the phonon scattering. We extract the carrier drift mobility by using the directly measured effective masses and the relaxation times. The mobility at 10 K for 600 ns delay time after optical injection is found to be mu(e) = 1.5 x 10(6) cm(2)/V s for the electrons, and mu(lh) = 2.3 x 10(6) cm(2)/V s and mu(hh) = 2.4 x 10(5) cm(2)/V s for the light and heavy holes, respectively. These high values are achieved by our high-sensitivity detection for low-density carriers (at < 10(11) cm(-3)) free from the carrier-carrier scattering as well as by the suppression of the impurity scattering in the high-purity samples. (C) 2014 AIP Publishing LLC.

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Alcohol additive effect in hydrogen generation from water with carbon by photochemical reaction

Kosuke Maeda, Nobuhiko Ozaki, Ikuko Akimoto

JAPANESE JOURNAL OF APPLIED PHYSICS ( IOP PUBLISHING LTD ) 53 ( 5 ) 2014.05 [Refereed]

We have found a novel photochemical activity of carbon powder to generate hydrogen-included gas by irradiating nanosecond laser pulse on carbon aqueous suspension at room temperature. In this study, we investigate alcohol additive effect to improve the gas generation efficiency in Bincho-tan charcoal aqueous suspension. We find that the generated gas volume with 50 wt% ethanol addition increases twice as much as that without ethanol. Components of the generated gas are analyzed to be 50% of hydrogen, 20% of carbon monoxide, and others, being similar to that generated with pure water suspension. Possible roles of ethanol are discussed as an agent in photochemical reactions and/or as a hole scavenger reagent in a photocatalytic reaction. (C) 2014 The Japan Society of Applied Physics

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Hydrogen generation by laser irradiation of carbon powder in water

Ikuko Akimoto, Kousuke Maeda, Nobuhiko Ozaki

Journal of Physical Chemistry C 117 ( 36 ) 18281 - 18285 2013.09 [Refereed]

We report the photochemical activity of carbon powder in the generation of hydrogen from water by laser irradiation, without any electrodes and photocatalysts. The gas was obtained by laser irradiation in the VIS-NIR range and was dependent nonlinearly on the laser power density. From a gas component analysis and a repeated irradiation experiment, it was found that the carbon powder was oxidized and acted as a sacrificial reagent in the photochemical hydrogen generation. In addition, a highly carbonized charcoal, known as Bincho-tan, was found to effectively work for the hydrogen generation. © 2013 American Chemical Society.

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Free-carrier generation by two-photon resonant excitation to the excitonic states in cuprous oxide

Nobuko Naka, Ikuko Akimoto, Masanobu Shirai

PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS ( WILEY-V C H VERLAG GMBH ) 250 ( 9 ) 1773 - 1776 2013.09 [Refereed]

We have investigated nanosecond temporal responses of the cyclotron resonance signal of optically generated carriers in Cu-2 O. The rise time is found to increase significantly as the excitation photon energy approaches to the phonon-assisted absorption edge of the 1s exciton. We model the generation of free carriers by two-body collisions of excitons, which leads to the reasonable estimation of the Auger coefficient. We also demonstrate the excitation spectroscopy of cyclotron resonance by using a two-photon transition to the even-parity excitonic states. Their dynamics is discussed based on the quantum-number dependence of the resonance linewidth. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Direct measurement via cyclotron resonance of the carrier effective masses in pristine diamond

Nobuko Naka, Katsuyuki Fukai, Yushi Handa, Ikuko Akimoto

PHYSICAL REVIEW B ( AMER PHYSICAL SOC ) 88 ( 3 ) 2013.07 [Refereed]

We have determined effective mass parameters for ultrapure diamond by time-resolved cyclotron resonance of optically excited carriers. The transverse and longitudinal electron masses are found to be m(t)/m(0) = 0.280(2) and m(t)/m(0) = 1.56( 2), where m(0) is the free-electron mass. The Luttinger parameters are extracted as gamma(1) = 2.67(2), gamma(2) = -0.40(2), and gamma(3) = 0.68(2), which give the light- and heavy-hole bands with considerable anisotropy and the split-off hole mass. The resonance widths indicate scattering times longer than 100 ps and carrier mobilities as high as 10(6) cm(2) V-1 s(-1) at 10 K.

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Temporal shift from magnetoplasma resonance to cyclotron resonance of photo-carriers generated from 1s-exciton in cuprous oxide crystal

I. Akimoto, S. Torai, N. Naka, M. Shirai

EUROPEAN PHYSICAL JOURNAL B ( SPRINGER ) 85 ( 11 ) 2012.11 [Refereed]

We have established a method for time-resolved cyclotron resonance measurements using an electron spin resonance cavity with a temporal resolution 15 ns under photo-excitation in an undoped semiconductor, cuprous oxide. The observed photo-carriers are generated via long-lived 1s-excitons excited at the phonon assisted absorption edge below the yellow gap. This work focuses on the fast response that is within 80 ns after photo-excitation of high-density excitons. A shift of the photo-carriers from a magnetoplasma resonance to a cyclotron resonance, previously known as a steady resonance in highly doped semiconductors, is observed for the first time in the temporal domain. We successfully explain the real and imaginary parts of the transient microwave absorption using spectral simulations, taking account of the depolarization factors in two different sample shapes and configurations in the cavity.

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Size dependent optical properties of quinacridonequinone nanoparticles prepared by liquid laser ablation in water

Ikuko Akimoto, Masahiro Ohata, Nobuhiko Ozaki, Ping Gu

CHEMICAL PHYSICS LETTERS ( ELSEVIER SCIENCE BV ) 552 102 - 107 2012.11 [Refereed]

We investigated optical properties of colloidal solutions of nanoparticles prepared by laser ablation of quinacridonequinone powder in a poor solvent, water. Blue shift of the absorption peak energy was observed corresponding to the increase of laser fluence or to the prolonged irradiation time. For a series of colloidal solutions, we found a linear correlation between the blue shift of the absorption peak energy and the decrease in the diameter of the nanoparticles. The blue shift is considered to be due to the surface state, such as surface excitons or bond modified surface states. (C) 2012 Elsevier B.V. All rights reserved.

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Time-resolved cyclotron resonance in cuprous oxide

Nobuko Naka, Ikuko Akimoto, Masanobu Shirai, Ken-Ichi Kan'No

Physical Review B - Condensed Matter and Materials Physics 85 ( 3 ) 2012.01 [Refereed]

We have demonstrated cyclotron resonance with a temporal resolution of 15 ns by using an electron-spin-resonance cavity. In an undoped direct-gap semiconductor, cuprous oxide, we observe clear low-field shifts of the cyclotron resonance peaks shortly after generation of photoexcited carriers. Based on the plasma shift of the cyclotron resonance, we evaluate the carrier density and quantitatively discuss the interaction between free carriers and excitons. With increasing time delay, the hole resonance asymptotically reaches the constant value corresponding to an effective mass of 0.575 times the free electron mass, providing a definitive answer to the controversy on the effective mass of holes in cuprous oxide. © 2012 American Physical Society.

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Time-resolved cyclotron resonance in cuprous oxide

Nobuko Naka, Ikuko Akimoto, Masanobu Shirai, Ken-ichi Kan'no

PHYSICAL REVIEW B ( AMER PHYSICAL SOC ) 85 ( 3 ) 2012.01 [Refereed]

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Formation of nanoparticles of organic molecules by liquid laser ablation

Ikuko Akimoto, Masahiro Ohata, Nobuhiko Ozaki, Gu Ping

Materials Research Society Symposium Proceedings 1455 55 - 60 2012 [Refereed]

We carried out laser ablation of three organic molecules, rubrene (Rb), Oralith Brilliant Pink R (BP) and quinacridonequinone (QQ) in a poor solvent, water. As a result, nanoparticles of BP and QQ were formed, but those of Rb were not formed because of photodissociation. For a rigid molecule, QQ, optical properties of colloidal solutions were investigated in relation to the size of the included nanoparticles. A linear correlation between the blue shift of the absorption peak energy and the decrease in the diameter of the nanoparticles was found, indicating that the nanoparticle diameter can be easily estimated from the absorption spectrum of a colloidal solution. From the solution, a nanoparticle film was fabricated on an electrode by the electrophoretic deposition method. © 2012 Materials Research Society.

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Investigation of yellow emission from a co-polymer utilizable as an emitting material for white OLED

I. Akimoto, S. Tsuzuki, H. Uzawa, M. Hinatsu, Y. Nishide, H. Osuga, H. Sakamoto

PROCEEDINGS OF THE 16TH INTERNATIONAL CONFERENCE ON LUMINESCENCE AND OPTICAL SPECTROSCOPY OF CONDENSED MATTER: POSTERS ( ELSEVIER SCIENCE BV ) 29 46 - 49 2012 [Refereed]

A novel co-polymer, P-PBTx, is a promising emitting material for white OLED. Appearance of unexpected yellow emission in EL is a key point, but its origin has not been clarified yet. Photo-luminescence properties of the copolymer were investigated in comparison with those of its monomer as well as two component molecules. An PL yellow band was also observed even in a film of a chromophore molecule at low temperature, depending on the position of an inhomogeneous film. Then, modified molecule center such as a dimer or an excimer of neighboring chromophores is a possible origin of the PL yellow emission.

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Twisted molecular geometry and localized electronic structure of the triplet excited gem-diphenyltrimethylenemethane biradical: substituent effects on thermoluminescence and related theoretical calculations

Yasunori Matsui, Hayato Namai, Ikuko Akimoto, Ken-ichi Kan'no, Kazuhiko Mizuno, Hiroshi Ikeda

TETRAHEDRON ( PERGAMON-ELSEVIER SCIENCE LTD ) 67 ( 38 ) 7431 - 7439 2011.09 [Refereed]

Substituent effects on the energies of electronic transitions (ETs) between the triplet excited and ground states of gem-diphenyltrimethylenemethane biradicals ((3)2a center dot center dot) were explored by using thermoluminescence (TL) spectroscopy and density functional theory (DFT) including time-dependent (TD) DFT. Linear free energy (Hammett) analyses of TL energies of a variety of para-substituted aryl derivatives of (3)2 center dot center dot* gave reasonable correlations with the substituent constant, sigma(center dot). The slope of Hammett plots of the data are nearly identical to one obtained from a similar analysis of the photoluminescence (PL) energies of the structurally-related 1,1-diarylethyl radicals (3 center dot*). The results suggest that TL of (3)2 center dot center dot* and PL of 3 center dot* derive from a common diarylmethyl radical fluorophore. This interpretation is also supported by the DFT and TDDFT calculated electronic structures and ET energies of (3)2a center dot center dot and 3 center dot. Thermodynamic and kinetic analyses of the charge recombination (CR) process between 2 center dot+ and 1, which generates (3)2 center dot center dot*, revealed that substituents not only alter the TL energies but also the TL intensities of (3)2 center dot center dot*. The observations made in this effort demonstrate that (3)2 center dot center dot* as well as (3)2a center dot center dot and 2 center dot+ have greatly twisted molecular geometries and highly localized electronic structures. (C) 2011 Elsevier Ltd. All rights reserved.

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Origin of yellow emission in white OLED with single emitting material

Ikuko Akimoto, Sayaka Tsuzuki, Hidetoshi Uzawa, Masako Hinatsu, Yosuke Nishide, Hideji Osuga, Hidefumi Sakamoto

PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 8, NO 1 ( WILEY-V C H VERLAG GMBH ) 8 ( 1 ) 2011 [Refereed]

The photo-luminescence (PL) and electro-luminescence (EL) of a novel organic material, benzodithiophene/triphenylamine copolymer (P-PBTx, n=5-6) were investigated. The PL exhibited wide blue emission at room temperature, which is attributed to fluorescence from a chromophore, benzodithiophene moiety, from results of spectral analysis and decay profile. The EL, however, exhibited white emission consisting of wide blue (B) and yellow (Y) bands, although it was prepared with P-PBTx as a single emitting material. The Y-band was slightly observed at 7 K and found to possess long lived component of 19 ms lifetime. The origin of the Y-band efficiently stimulated by carrier injection is supposed to be a charge-transfer state between benzodithiophene and triphenylamine moieties. The long lived component could be explained by lowered transition probability caused by charge separation. (c) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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X-ray-Triggered Thermoluminescence and Density Functional Theory Characterization of a gem-Diphenyltrimethylenemethane Biradical

Hiroshi Ikeda, Yasunori Matsui, Ikuko Akimoto, Ken-ichi Kan'no, Kazuhiko Mizuno

AUSTRALIAN JOURNAL OF CHEMISTRY ( CSIRO PUBLISHING ) 63 ( 9 ) 1342 - 1347 2010 [Refereed]

Thermoluminescence (TL) from the excited triplet state of a gem-diphenyltrimethylenemethane biradical ((3)4(center dot center dot)*) is triggered by X-irradiation at 77 K followed by annealing to similar to 140 K. The new X-irradiation method reported here is simpler and more convenient than the previously employed gamma-irradiation method. The TL spectrum of (3)4(center dot center dot)* is similar to the photoluminescence spectrum of the 1,1-diphenylethyl radical (5(center dot)). The results of density functional theory (DFT) and time-dependent-DFT calculations of the ground state biradical (3)4(center dot center dot) suggest that no significant electronic interaction takes place between the diphenylmethyl and allyl radical moieties owing to its twisted geometry. Accordingly, the results also suggest that the excited state biradical (3)4(center dot center dot)* has a similar molecular geometry and electronic structure as the triplet ground state. Both the experimental and computational results obtained for (3)4(center dot center dot) and 5(center dot) confirm that the main fluorophore of (3)4(center dot center dot)* is the diphenylmethyl radical moiety.

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Published Papers - AKIMOTO Ikuko