Faculty Profiles - AKIMOTO Ikuko

Published Papers - AKIMOTO Ikuko

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Fast passage effect in cw-ODMR of an ensemble of NV− centers

Yusuke Maki, Hideto Matsuoka, Ikuko Akimoto

Journal of Chemical Physics 162 054202 - (1-7) 2025.02 [Refereed]

DOI

Cyclotron resonance imaging at cryogenic temperatures enabled by electric field modification in a microwave cavity

Ikuko Akimoto, Takayuki Suzuki, Nobuko Naka, Hideto Matsuoka （Part： Lead author )

Journal of Physics D: Applied Physics 58 025309 2024.10 [Refereed]

DOI

Photogenerate electron- and hole-trap distributions in Al-doped anatase TiO<sub>2</sub> crystals measured by cryogenic EPR imaging

Yamato Kitao, Hideto Matsuoka, Ayane Murano, Takao Sekiya, Ikuko Akimoto （Part： Last author,　Corresponding author )

Physica Scripta ( IOP Publishing ) 99 025514 2024.01 [Refereed]

Abstract

Electron paramagnetic resonance (EPR) imaging was performed on aluminium-doped anatase titanium dioxide (TiO<sub>2</sub>) crystals to demonstrate the distributions of photoinduced trapped electrons and holes at cryogenic temperatures. Complementary distributions of trapped electrons and holes were observed at 25 K; trapped electrons were predominant on the {101} planes, whereas trapped holes were predominant on the (001) and {110} planes in the tetragonal crystal structure. These separate distributions are consistent with the reported facet-dependent photocatalytic efficiency of photoreduction and photooxidation in anatase crystals, resulting from crystal plane-dependent carrier transfer. By this extrinsic trap system, the carrier transport properties were derivatively extracted through electron and hole traps This report provides insight into the fundamental physics governing the facet dependence of photocatalytic effects.

DOI

Intrinsic Mobility of Low-Density Electrons in Photoexcited Diamond

K. Konishi, I. Akimoto, H. Matsuoka, J. Isberg, N. Naka

Physical Review Applied ( American Physical Society (APS) ) 17 ( 3 ) 2022.03 [Refereed]

DOI

Transient coexistence of excitons and charge carriers in high-purity diamond

Sayaka Hamabata, Junichi H. Kaneko, Nobuko Naka, Ikuko Akimoto

Diamond and Related Materials ( Elsevier BV ) 120 108678 - 108678 2021.12 [Refereed]

DOI

Time-Resolved EPR and Theoretical Investigations of Naphthalene Diimide Spin Dynamics in the Excited State

Hideto Matsuoka, Yuki Shibano, Ikuko Akimoto, Yuki Kanzaki

Bulletin of the Chemical Society of Japan ( The Chemical Society of Japan ) 2021.10 [Refereed]

DOI

Diffusion-related lifetime and quantum efficiency of excitons in diamond

K. Konishi, I. Akimoto, J. Isberg, N. Naka

Physical Review B 2020.12 [Refereed]

Low-temperature mobility-lifetime product in synthetic diamond

K. Konishi, I. Akimoto, H. Matsuoka, V. Djurberg, S. Majdi, J. Isberg, N. Naka

Applied Physics Letters 2020.12 [Refereed]

Time-resolved measurement of photocarrier generation in CH3NH3PbI3 single crystals

Y. Tanaka, I. Akimoto

Journal of Physics: Conf. Series 1220 012019 2019.08 [Refereed]

Temperature-dependent carrier injection routes under optical excitation in high-purity diamond crystals

S. Hamabata, I. Akimoto, N. Naka

Journal of Physics: Conf. Series 1220 012023 2019.08 [Refereed]

Photocarrier injection by two-photon excitation in rubrene single crystal

K. Goto, T. Takayama, K. Ohata, K. Matsushita, I. Akimoto

Journal of Physics: Conf. Series 1220 012010 2019.08 [Refereed]

Intrinsic carrier parameters and optical carrier injection method in high-purity diamonds

I. Akimoto, N. Naka

IntechOpen Diamonds in Scientific Research and High Technology 2019.04 [Refereed]

DOI

Evidence of exciton dissociation into carriers induced by electric field in rubrene crystals

K. Goto, T. Takayama, K. Ohata, K. Matsushita, I. Akimoto

Journal of Luminescence 205 46 - 51 2019 [Refereed]

Double Electron–Electron Resonance Between Trapped Electron and Hole in a Semiconductor

Ikuko Akimoto, Takao Sekiya

Applied Magnetic Resonance ( Springer-Verlag Wien ) 49 ( 8 ) 1 - 10 2018.04 [Refereed]

We demonstrate the spin interactions between dispersedly trapped electrons and holes in a semiconductor using the double electron–electron resonance (DEER) method of the pulsed electron paramagnetic resonance (EPR) techniques. An aluminum-doped titanium dioxide crystal is adopted as a spin system, in which optically generated electrons and holes are trapped, to reveal EPR signals that appear close to each other at a selected crystal orientation under an external magnetic field. We used the four-pulse DEER method by applying two microwave frequencies to a microwave cavity for pumping electrons and probing holes at the optimum temperature of 32 K. The dipolar modulation in the probed signal by pumping interacting spins was successfully detected. The observed non-oscillating decay shape indicates that the detected interaction is caused by widely distributed trapped electron and hole spins over long distances. We were able to extract a spin-pair distribution function by the first derivative of a background-corrected curve, referring to a previously reported method.

DOI

Quantitative relevance of substitutional impurities to carrier dynamics in diamond

Takaaki Shimomura, Yoshiki Kubo, Julien Barjon, Norio Tokuda, Ikuko Akimoto, Nobuko Naka

Physical Review Materials 2 094601 2018 [Refereed]

Two optical routes of cold carrier injection in silicon revealed by time-resolved excitation spectroscopy

Ikuko Akimoto, Nobuko Naka

APPLIED PHYSICS EXPRESS ( IOP PUBLISHING LTD ) 10 ( 6 ) 2017.06 [Refereed]

We reveal two routes of optical carrier injection in pure silicon by means of high-resolution excitation spectroscopy on nanosecond cyclotron resonances. Free carriers are generated either by the band-to-band transition assisted by phonon emission, or via two-body collisions of excitons. The first route was previously masked by a strong excitonic response in steady-state optical spectra at low temperatures. Furthermore, valley polarization is achieved for the cold carriers created by the second route with optimized excess energy. These optical carrier injection routes are crucial to initialize the momentum and valley degrees of freedom of carriers in order to enable versatile applications of indirect-bandgap semiconductors. (C) 2017 The Japan Society of Applied Physics

DOI

Excitons and fundamental transport properties of diamond under photo-injection

Nobuko Naka, Hikaru Morimoto, Ikuko Akimoto

PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE ( WILEY-V C H VERLAG GMBH ) 213 ( 10 ) 2551 - 2563 2016.10 [Refereed]

Exciton diffusivity and carrier mobility are key features in determining transport properties of a material. The optimum values required for high-performance devices are expected to be realized in undoped intrinsic materials. Diamond, one of the emergent high-mobility materials, has a wide band gap suited for use in power electronics and optoelectronics. For many years, however, mobility measurement for diamond has been limited by carrier freezing at deep impurity levels. Recently, attempts at utilizing photoexcited carriers in undoped diamond have been made in a wide temperature range and various excitation schemes. A series of investigations demonstrated a wealth of intriguing observations such as dominant role of excitons and emergence of low-temperature anomaly in momentum relaxation. This article highlights recent experimental advances and progresses in understanding transport properties of photoexcited intrinsic diamond. (C) 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

DOI

White-light emission from solid carbon in aqueous solution during hydrogen generation induced by nanosecond laser pulse irradiation

Ikuko Akimoto, Shota Yamamoto, Kosuke Maeda

APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING ( SPRINGER ) 122 ( 7 ) 2016.07 [Refereed]

We previously discovered a novel method of hydrogen generation from high-grade charcoal in an aqueous solution using nanosecond laser pulse irradiation. In this paper, white-light emission during this reaction is reported: A broad spectrum over the visible range is observed above a threshold excitation energy density. The white-light emission is a simultaneous product of the hydrogen generation reaction and is attributed to blackbody radiation in accordance with Planck's Law at a temperature above 3800 K. Consequently, we propose that hydrogen generation induced by laser irradiation proceeds similarly to classical coal gasification, which features reactions at high pressure and high temperature.

DOI

Time-resolved cyclotron resonance on dislocation-free HPHT diamond

I. Akimoto, N. Naka, N. Tokuda

Diamond and Related Materials ( Elsevier Ltd ) 63 38 - 42 2016.03 [Refereed]

Intrinsic transport properties in diamond are remained frontiers in material research of semiconductors. Here, we apply the time-resolved cyclotron resonance (TRCR) method to dislocation-free HPHT diamonds in order to elucidate the momentum relaxation mechanisms of photoexcited carriers. Two specimens of single sector crystals oriented along [001] and [111] directions are examined. The both specimens show clear TRCR signals below 40 K, where the signals in the (111)-oriented diamond decay faster than those in the (001)-oriented one. From the temperature dependence of the CR spectrum in the (111)-oriented diamond, carrier momentum relaxation rates at the low temperature limit are extracted. The rates are two/four times larger for heavy/light holes in the HPHT diamond than the previous results obtained in a high-purity CVD diamond, and include temperature-independent scattering by neutral impurities. We also deduce that the TRCR detection is rather insensitive to crystalline dislocations by considering the cyclotron radii of carriers.

DOI

21pPSA-19 Infrared spectroscopy on high-purity CVD and HPHT diamond crystals

Shimomura T., Naka N., Tokuda N., Akimoto I.

Meeting Abstracts of the Physical Society of Japan ( The Physical Society of Japan (JPS) ) 71 1579 - 1579 2016

DOI

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

DOI

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

DOI

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.

DOI

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

DOI

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

DOI

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.

DOI

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.

DOI

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

DOI

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.

DOI

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

DOI

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.

DOI

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.

DOI

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.

DOI

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.

DOI

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]

DOI

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.

DOI

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.

DOI

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.

DOI

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

DOI

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.

DOI

Vibrational, electronic, and vibronic excitations of polar C60F18 molecules: Experimental and theoretical study

A. A. Popov, V. M. Senyavin, V. I. Korepanov, I. V. Goldt, A. M. Lebedev, V. G. Stankevich, K. A. Menshikov, N. Yu. Svechnikov, O. V. Boltalina, I. E. Kareev, S. Kimura, O. Sidorova, K. Kanno, I. Akimoto

PHYSICAL REVIEW B ( AMER PHYSICAL SOC ) 79 ( 4 ) 2009.01 [Refereed]

Polarized optical conductivity spectra in the IR and visible range and variable-temperature photoluminescence spectra of the C60F18 single crystals in the 4-300 K temperature range are reported. Density-functional theory (DFT) calculations of vibrational spectra and time-dependent DFT calculations of the excitation energies of the C60F18 molecule are performed to interpret experimentally observed phenomena. Orientation of the C-3v-symmetric C60F18 molecules in the single crystal is revealed by comparison of the experimental polarized IR spectra and vibrational DFT calculations. A(2) symmetry is assigned to the lowest energy singlet excited state of C60F18, and hence pure electronic S-0 -> S-1 excitation is found to be dipole forbidden. Fine vibronic structure found at low temperature is interpreted in terms of Herzberg-Teller and Franck-Condon mechanisms. Considerable similarity with vibronic spectra of C-60 is found and explained by presumable localization of the lowest energy electronic excitation of C60F18 on the fullerenelike part of the molecule.

DOI

Up to 30 times enhancement of deep UV emission at room temperature by prolonged excitation of localized exciton in NaCl: I crystal

Ikuko Akimoto, Masahiro Shimozato, Ken-Ichi Kan'No

Physica Status Solidi (C) Current Topics in Solid State Physics 6 ( 1 ) 342 - 345 2009 [Refereed]

A material emitting DUV light at room temperature is promising for an application to a new laser action. We report the remarkable phenomena that deep ultra-violet (DUV) emission at 220 nm, so-called NE emission, in NaCl:I crystal is enhanced up to 30 times at room temperature by prolonged excitation of the localized exciton. Such an accumulated effect is kept even if the excitation is interuped for a few minutes but is completely reset by white light irradiation at room temperature. The phenomena are discussed considering thermal ionization of iodine anion by the VUV light excitation, trapping of dissociated electrons in hole centers and recombination of electrons and iodine hole centers. © 2009 WILEY-VCH Verlag GmbH &amp
Co. KGaA.

DOI

Up to 30 times enhancement of deep UV emission at room temperature by prolonged excitation of localized exciton in NaCl:I crystal

Ikuko Akimoto, Masahiro Shimozato, Ken-ichi Kan'no

PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 6, NO 1 ( WILEY-V C H VERLAG GMBH ) 6 ( 1 ) 342 - 345 2009 [Refereed]

A material emitting DUV light at room temperature is promising for an application to a new laser action. We report the remarkable phenomena that deep ultra-violet (DUV) emission at 220 urn, so-called NE emission, in NaCl:I crystal is enhanced up to 30 times at room temperature by prolonged excitation of the localized exciton. Such an accumulated effect is kept even if the excitation is interuped for a few minutes but is completely reset by white light irradiation at room temperature. The phenomena are discussed considering thermal ionization of iodine anion by the VUV light excitation, trapping of dissociated electrons in hole centers and recombination of electrons and iodine hole centers. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

DOI

Multifrequency EPR analysis of the positive polaron in I-2-doped poly(3-hexylthiophene) and in poly[2-methoxy-5-(3,7-dimethyloctyloxy)]-1,4-phenylenevinylene

Aranzazu Aguirre, Peter Gast, Sergey Orlinskii, Ikuko Akimoto, Edgar J. J. Groenen, Hassane El Mkami, Etienne Goovaerts, Sabine Van Doorslaer

PHYSICAL CHEMISTRY CHEMICAL PHYSICS ( ROYAL SOC CHEMISTRY ) 10 ( 47 ) 7129 - 7138 2008 [Refereed]

The W-band continuous-wave electron paramagnetic resonance (EPR) analysis of chemically induced polarons in drop-cast and spin-coated polyphenylenevinylene-type and polythiophene-type polymer films reveals rhombic g tensors in both cases. The dependence of the W-band EPR signals on the orientation of the spin-coated films with respect to the magnetic field indicates a high degree of backbone alignment with the substrate and allows a partial assignment of the g tensor orientation. The derived molecular orientations of the polymer chains in the spin-coated films show clear differences between the two types of polymers. The proton hyperfine interactions obtained from X-band HYSCORE (hyperfine sublevel correlation) and Q- and W-band pulsed ENDOR (electron-nuclear double resonance) experiments are interpreted in terms of earlier theoretical studies on the extension of the polarons.

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Investigation of optical properties of fluorofullerene C60F18 single crystals

A. M. Lebedev, K. A. Menshikov, N. Yu. Svechnikov, V. G. Stankevich, O. V. Boltalina, I. V. Goldt, S. Kimura, T. Nishi, I. Akimoto, K. Kan'no

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT ( ELSEVIER SCIENCE BV ) 575 ( 1-2 ) 96 - 98 2007.05

For the first time optical investigations of C60F18 single crystals were carried out in the temperature region (300-4) K. Intensity of the obtained photoluminescence (PL) spectra has a strong brake near 100 K. At low temperature the clear fine structure of luminescence spectra is observed. The reflectance spectra have shown main changes for the energy about 4 eV (typical for pi-pi* transitions), and for E > 8 eV for deeper electronic levels. (C) 2007 Elsevier B.V. All rights reserved.

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Decay dynamics of photo-luminescence in tris(2-phenylpyridine)iridium phosphor

I. Akimoto, K. Nagao, T. Tashiro, H. Osuga, K. Tanaka, K. Kan'no

JOURNAL OF LUMINESCENCE ( ELSEVIER SCIENCE BV ) 122 ( 1-4 ) 440 - 443 2007.01 [Refereed]

The phosphor material, tris(2-phenylpyridine)iridium (Ir(ppy)(3)), has attracted much attention as an emitter of highly efficient electrophosphorescent devices. In the present work, decay dynamics of photo-luminescence and its temperature dependence have been investigated in the Ir(ppy)(3) emitting center. From the analyses of luminescent decay profiles and time-resolved luminescent spectra, thermally activated population transfer from every triplet sub-level to a singlet state is suggested. (c) 2006 Elsevier B.V. All rights reserved.

DOI

Magnetic field dependence of triplet state in iridium complex phosphor

Ikuko Akimoto, Kazuhiro Nagao, Ken-ichi Kan'no

PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 4, NO 3 ( WILEY-V C H VERLAG GMBH ) 4 ( 3 ) 813 - + 2007 [Refereed]

The magnetic field dependence of photo-luminescent spectrum in tris (2-phenylpyridine) iridium complex is reported. Upon applying magnetic field from 0 to 6 T, remarkable change of spectral band shape was observed at 4.2 K. Comparing the spectral variation under magnetic field with the time resolved spectra, it is clarified that the luminescence involves distinctive contributions from, at least, two spectral sets of zero-field-split triplet sub-levels, and an additional fast decay component which is presumably associated with the nearly singlet manifold. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

DOI

Photoluminescence and relaxation processes in the optically excited states of fluorinated fullerenes

I. Akimoto, M. Mori, K. Kan'No

AIP Conference Proceedings 786 33 - 36 2005.09

Origin of the photoluminescence and relaxation processes in the optically excited states of fluorinated fullerenes has been investigated by site selective excitation and time resolved spectroscopy. The intrinsic luminescence consists of at least two components with the different kind of decay kinetics. One of which is of single exponential with τd ∼ 6 ns and the other is of non-exponential but proportional to t-1. The latter component seems to be recombination luminescence due to an intermolecular polaron, in which an electron and a hole are separately localized with accompanying some structural relaxation. © 2005 American Institute of Physics.

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Photo-luminescence properties and exciton-carrier interaction in 1,2-diarylethylene derivatives films

Akimoto, I, KI Kan'no, H Osuga, K Tanaka

JOURNAL OF LUMINESCENCE ( ELSEVIER SCIENCE BV ) 112 ( 1-4 ) 341 - 344 2005.04

New organic materials, 1,2-diarylethylene derivatives consisting of ortho-fused heterocyclic system like benzodithiophene (BDT) have been reported as candidates of thermally stable fluorescent materials, promisingly used in organic electro-lummescence devices. Optical properties of the compound possessing methyl or phenyl substituents in ethylene moiety are investigated. Photo-luminescence (PL) is observed at blue-green region accompanied by molecular vibrational structures. The quantum efficiency of the fluorescence in H-BDT toluene solution is estimated as 0.32 at room temperature. On the basis of quantum chemical calculation, the molecular structure and electronic structure are discussed. We also investigated the electronic and optical properties of H-BDT thin films prepared by sublimation method. For a preliminary single-layered device (fTO/H-BDT/Al:Mg), the I-V property and the effect of applied electric field on PL intensity were investigated and a possibility of the exciton-carrier interaction in the organic layer was pointed out. (c) 2004 Elsevier B.V. All rights reserved.

DOI

Luminescent decay kinetics in a quasi-one-dimensional crystal, C5H10NH2PbI3

Akimoto, I, M Sakai, K Kan'no

JOURNAL OF LUMINESCENCE ( ELSEVIER SCIENCE BV ) 108 ( 1-4 ) 31 - 35 2004.06 [Refereed]

Photoluminescence and its decay kinetics for a single crystal of piperidinium triiodoplumbate C5H10NHPbI3 have been investigated. The luminescent spectrum exhibits a single emission band at 2.1 eV with FWHM 0.3 eV, which has been attributed to a self-trapped exciton. The luminescent kinetics is discussed for the temperature range below 70K, where the integrated luminescent intensity is almost constant. The exponential decay time tau(d) apparently decreases from 44 mus at 5 K to similar to2 mus at 70 K while the initial intensity I-0 increases by one order of magnitude, so that the product I-0 x tau(d) is almost constant. Such a phenomenon is explained by assuming that the initial state of the luminescence consists of at least two sub-levels, between which population transfer is thermally achieved with an activation energy of 6 meV. (C) 2004 Elsevier B.V. All rights reserved.

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Photoluminescence and Near-Edge Optical Absorption in the Low-Temperature Phase of Pristine C60 Single Crystals

Ikuko Akimoto, Ken-Ich Kan'no

Journal of the Physical Society of Japan 71 ( 2 ) 630 - 643 2002.02 [Refereed]

Photoluminescence and near-band-edge electronic structure of pristine C60 single crystals have been investigated in the low temperature phase below 90 K. By applying two-photon excitation spectroscopy, a monomorphic spectrum is extracted from the several crystals which exhibit apparent diversity for ordinary one-photon excitation. Also, a localized absorption band with discrete vibronic sub-peaks is clearly confirmed at 1.685 eV below the fundamental absorption edge (1.815 eV). Overall features of the luminescence-line-narrowing spectrum obtained by site-selective excitation into the localized absorption band are assigned to the vibronic progressions coupled with the Raman-active gerade-symmetry modes of individual C60 molecules (i.e., eight h g, two ag, and their combinations), accompanied by phonon-sidebands due to cooperative librations and intermolecular translations. The origin of photoluminescence and its relaxation kinetics are fully discussed in connection with the inhomogeneously broadened localized states which are presumably due to perturbed Frenkel excitons stabilized with symmetry-lowering by adjacent intrinsic defect or some lattice disorder.

DOI

Dynamics of the photoinduced ionic-to-neutral phase transition in tetrathiafulvalene-p-chloranil studied by femtosecond time-resolved reflection spectroscopy

I. Akimoto, K. Tanimura, S. Koshihara

Phase Transitions 75 ( 7-8 ) 815 - 822 2002.01 [Refereed]

DOI

Femtosecond time-resolved spectroscopy of photoinduced ionic-to-neutral phase transition in tetrathiafulvalen-p-chloranil crystals

K Tanimura, Akimoto, I

JOURNAL OF LUMINESCENCE ( ELSEVIER SCIENCE BV ) 94 483 - 488 2001.12 [Refereed]

Recent experimental results are reviewed of the photoinduced phase transformation between ionic and neutral phases in tetrathiafulvalen-p-chloranil crystals. Spectroscopic studies have revealed different features of Frenkel-type and charge transfer (CT)-type excited states in inducing photo-induced structural phase transition: the transition can be induced only above threshold-excitation intensities in the case of CT excitation. The thresholds imply that non-linear processes of CT excited states are crucial in the transition. On the other hand, time-resolved studies have demonstrated a dynamical aspect of the transition. optical reflectance signals that probe the evolution of the transition show dumped-oscillatory changes before establishing N-phase domains at around 100 ps after excitation. Mechanism of the transition has been discussed based on these results. (C) 2001 Elsevier Science B.V. All rights reserved.

Origin of photoluminescence and spectral analysis of vibronic structure resolved in C-60 single crystals

Akimoto, I, K Kan'no

JOURNAL OF LUMINESCENCE ( ELSEVIER SCIENCE BV ) 87-9 788 - 790 2000.05 [Refereed]

Origin of the photoluminescence and band-edge electronic structure of pristine C-60 single crystals have been studied in the temperature range from 4 to 300 K by applying several spectroscopic techniques such as absorption, photoluminescence under one-photon and two-photon excitation, and site-selective luminescence line narrowing spectroscopy. Photo-absorption characterized by the prominent vibrational structures is found around 1.688 eV below the fundamental absorption edge (1.814 eV), which apparently exhibits mirror images of the well-known spectrum of photoluminescence called type A. Upon selective excitation into the band, luminescence line narrowing is observed: The spectra consists of sharp vibrational lines attributed to the progressions of eight H-g and two A(g) modes and libron sidebands. These results indicate that type A luminescence originates from the inhomogeneously broadened localized state which may be stabilized by symmetry lowering and local rotational disorder spread over more than one molecule. (C) 2000 Elsevier Science B.V. All rights reserved.

Luminescence from C-60 single crystals in glassy phase under site-selective excitation

Akimoto, I, M Ashida, K Kan'no

CHEMICAL PHYSICS LETTERS ( ELSEVIER SCIENCE BV ) 292 ( 4-6 ) 561 - 566 1998.08 [Refereed]

Luminescence from C-60 single crystals has been investigated at liquid He temperature under selective excitation into the tail part below the fundamental absorption edge. Luminescence line narrowing is observed for resonant excitation, in which a characteristic vibrational quantum is found similar to 420 cm(-1) in the excited state. It is revealed that the spectrum which has been known as photo-luminescence from C-60 single crystals is an superposition of site-selective luminescence from several localized states. Such inhomogeneity is most likely to arise from fluctuation of the intermolecular orientational correlation in a pair of C-60 molecules over which the excitation is delocalized. (C) 1998 Elsevier Science B.V. All rights reserved.

Relaxation dynamics into two types of luminescence states in C-60 single crystals below 80 K

Akimoto, I, J Azuma, M Ashida, K Kan'no

JOURNAL OF LUMINESCENCE ( ELSEVIER SCIENCE BV ) 76-7 206 - 210 1998.02 [Refereed]

Luminescence spectra from pure C-60 Single crystals and their time behavior have been studied in the temperature range below 80 K under the excitation into two distinctive energy ranges. In case of the excitation into the higher-lying band slates, two kinds of fluorescence bands, called types A and B, were observed with their onsets at 733 and 684 nm around LHeT. These bands exhibit the anti-correlated temperature dependence, indicating thermally activated population transfer from the initial state of type B to that of type A. On the other hands, the selective excitation into the tail part (similar to 733 nm) below the fundamental absorption edge stimulates not only a series of Raman lines but also the resonant luminescence which evolves into the type A luminescence with inhomogeneous behavior. It is noteworthy that for just resonance a novel vibronic structure with Delta similar to 120 cm(-1) is clearly resolved. The present study strongly suggests that fluctuation in the local intermolecular orientational correlation of molecules is essential to the relaxation processes of photo-excited states in C-60 Single crystal. (C) 1998 Elsevier Science B.V. All rights reserved.

Pulse-annealing study of photo-stimulated luminescence and trapped-hole states in BaFBr:Eu2+

A Ohnishi, Akimoto, I, K Kanno, Y Iwabuchi, N Mori

JOURNAL OF LUMINESCENCE ( ELSEVIER SCIENCE BV ) 72-4 775 - 777 1997.06 [Refereed]

Optically detected magnetic resonance and zero-field splitting energy of the self-trapped exciton in NaBr

K. Kan'no, M. Shirai, T. Matsumoto, I. Akimoto

Materials Science Forum ( Trans Tech Publications Ltd ) 239-241 569 - 572 1997

The triplet states of the type I self-trapped exciton (STE) in NaBr have been studied using a pulsed X-band (9.4 GHz) ODMR technique for the magnetic field H//[110] under the two-photon excitation with KrF laser pulses: An EPR signal from the Mz=0 sublevel in the [110]-oriented STE was time-resolved at 5.4 K, by monitoring a transient change induced in the decay curve of triplet emission ("π band" at 4.6 eV) when a microwave pulse (10 μs duration) was applied at 20 μs after the laser excitation. The low-field transition of (Mz=0 ⇒ Mz=-1) was confirmed to appear around 1.44T as a broad EPR band with partially resolved seven lines. The Zero-field splitting was unambiguously determined to be D=0.2 meV, in good agreement with our previous study of magnetic field effect on luminescence lifetimes. The D value thus determined is one order of magnitude larger than that of the type III STE in KBr (0.03 meV). Such a large D value comes most likely from a geometry of on-center type, i.e. [VK + e] configuration, though it is still open problem whether the type I STE in NaBr strictly keeps the D2h symmetry.

DOI

Emission spectra and decay characteristics in photo-stimulated (CnH2n+1NH3)2CdCl4 : n=1, 2, 3

A. Ohnishi, T. Yamada, T. Yoshinari, I. Akimoto, K. Kan'no, T. Kamikawa

Journal of Electron Spectroscopy and Related Phenomena ( Elsevier ) 79 163 - 166 1996 [Refereed]

Recombination luminescence from (CnH2n+1NH3)2CdCl4 : n=1, 2, 3 has been studied using synchrotron radiation pulses. A largely stokes-shifted emission band was found to appear at 2.50eV for the n=2 crystal when excited into the excitonic absorption range, while at 2.25eV when excited into the band-to-band transition range. Decay curves of both bands are composed of fast and slow components. Non-exponential decay of slow components suggests that the 2.25eV band is induced by tunneling recombination between self-trapped electrons and self-trapped holes. As for the 2.50eV band, recombination of correlated electron-hole pairs with strong lattice relaxation is supposed.

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