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Journal Articles

Electronic structure of Li$$^{+}$$@C$$_{60}$$; Photoelectron spectroscopy of the Li$$^{+}$$@C$$_{60}$$[PF$$_{6}$$$$^{-}$$] salt and STM of the single Li$$^{+}$$@C$$_{60}$$ molecules on Cu(111)

Yamada, Yoichi*; Kuklin, A. V.*; Sato, Sho*; Esaka, Fumitaka; Sumi, Naoya*; Zhang, C.*; Sasaki, Masahiro*; Kwon, E.*; Kasama, Yasuhiko*; Avramov, P. V.*; et al.

Carbon, 133, p.23 - 30, 2018/07

 Times Cited Count:8 Percentile:47.45(Chemistry, Physical)

We report first STM observation of the Li$$^{+}$$ ion endohedral C$$_{60}$$, which is of a new class of endohedral fullerenes, prepared by means of evaporation of high-purity Li$$^{+}$$@C$$_{60}$$[PF$$_{6}$$$$^{-}$$] salt in ultra-high vacuum. Prior to the STM measurements, the electronic structure of Li$$^{+}$$@C$$_{60}$$ in the Li$$^{+}$$@C$$_{60}$$[PF$$_{6}$$$$^{-}$$] salt was also precisely determined. In the salt, it is shown that Li and PF$$_{6}$$ have nearly single positive and negative charge, respectively, and the C$$_{60}$$ cage is nearly neutral, suggesting that Li$$^{+}$$@C$$_{60}$$ in the salt retains its original electronic state.

Journal Articles

Contact-induced spin polarization in BNNT(CNT)/TM (TM=Co, Ni)nanocomposites

Kuzubov, A. A.*; Kovaleva, E. A.*; Avramov, P. V.*; Kuklin, A. V.*; Mikhaleva, N. S.*; Tomilin, F. N.*; Sakai, Seiji; Entani, Shiro; Matsumoto, Yoshihiro*; Naramoto, Hiroshi*

Journal of Applied Physics, 116(8), p.084309_1 - 084309_4, 2014/08

 Times Cited Count:8 Percentile:40.13(Physics, Applied)

Journal Articles

Atomic structure and physical properties of fused porphyrin nanoclusters

Avramov, P. V.*; Kuzubov, A. A.*; Sakai, Seiji; Otomo, Manabu*; Entani, Shiro; Matsumoto, Yoshihiro*; Eleseeva, N. S.*; Pomogaev, V. A.*; Naramoto, Hiroshi*

Journal of Porphyrins and Phthalocyanines, 18(7), p.552 - 568, 2014/07

 Times Cited Count:1 Percentile:6.25(Chemistry, Multidisciplinary)

Journal Articles

Thermo-dynamical contours of electronic-vibrational spectra simulated using the statistical quantum-mechanical methods

Pomogaev, V.*; Pomogaeva, A.*; Avramov, P.; Jalkanen, K. J.*; Kachin, S.*

Theoretical Chemistry Accounts, 130(4-6), p.609 - 632, 2011/12

 Times Cited Count:8 Percentile:21.38(Chemistry, Physical)

Journal Articles

Comparative study of structures and electrical properties in cobalt-fullerene mixtures by systematic change of cobalt content

Sakai, Seiji; Naramoto, Hiroshi*; Avramov, P.; Yaita, Tsuyoshi; Lavrentiev, V.; Narumi, Kazumasa; Baba, Yuji; Maeda, Yoshihito

Thin Solid Films, 515(20-21), p.7758 - 7764, 2007/07

 Times Cited Count:22 Percentile:68.12(Materials Science, Multidisciplinary)

A systematic study is performed for the Co content-dependent structure evolution and electrical properties in the cobalt(Co)-fullerene(C$$_{60}$$) mixtures in the broad content region. Raman analysis confirms that the mixture has a saturation composition of Co$$_{5}$$C$$_{60}$$ for the compoundformation. An important role of the covalent Co-C bonds for the characteristic structure and the properties in the Co-C$$_{60}$$ compounds is concluded from the content-dependent peak shifts and the symmetry lowering of C$$_{60}$$. The Co-dilute mixtures exhibit a semiconductive nature represented bythe activated transport from the shallow mid-gap states. The Co-dense mixtures are characterized by hopping conduction (including variable rangehopping) attributed to the Co clusters/nanoparticles in the compound matrix and by the percolated metallic conduction, depending on the Co content.

Journal Articles

Magnetic and magnetotransport properties in nanogranular Co/C$$_{60}$$-Co film with high magnetoresistance

Sakai, Seiji; Yakushiji, Kei*; Mitani, Seiji*; Sugai, Isamu; Takanashi, Koki*; Naramoto, Hiroshi*; Avramov, P.; Lavrentiev, V.*; Narumi, Kazumasa; Maeda, Yoshihito

Materials Transactions, 48(4), p.754 - 758, 2007/04

 Times Cited Count:8 Percentile:51.29(Materials Science, Multidisciplinary)

Magnetic properties were investigated for the alternately deposited film of C$$_{60}$$ and Co which has found to exhibit tunnel magnetoresistance (MR) of 10-80 %. Magnetic field and temperature dependences of magnetization showed typical superparamagnetic behaviors with the blocking temperature of 40 K. The magnetization curve at 300 K was well fitted by the Langevin function with the size distribution of Co particles, and the mean diameter and size distribution were evaluated to be 3.1 nm and 1 nm, respectively. Based on the magnetic properties, the detailed structure and magnetotransport properties are discussed.

Journal Articles

Quantum chemical study of atomic structure evolution of Co$$_{x}$$/C$$_{60}$$ (x $$leq$$ 2.8) composites

Avramov, P.; Naramoto, Hiroshi*; Sakai, Seiji; Narumi, Kazumasa; Lavrentiev, V.*; Maeda, Yoshihito

Journal of Physical Chemistry A, 111(12), p.2299 - 2306, 2007/03

 Times Cited Count:13 Percentile:44.97(Chemistry, Physical)

The main features of the local atomic structure of novel Co$$_{x}$$/C$$_{60}$$ (x $$leq$$ 2.8) complex mixtures were studied using the ${it ab initio}$ B3LYP/6-31G$$ast$$ method for a set of low and high energy Co$$_{n}$$(C$$_{60}$$)$$_{m}$$ (n = 1, 2, m = 2, 3) clusters in low and intermediate spin states. For the n = 1 isomers the spin state S = 1/2 is energetically preferable whereas the low energy isomers of n = 2 have an intermediate spin state of S = 1. The $$eta$$$$^{2}$$ (6-6 edge of C$$_{60}$$) type of cobalt ion coordination is preferable for both n = 1 and n = 2 cases. The $$eta$$$$^{2'}$$ (coordination with 6-5 edge) and even the $$eta$$$$^{5}$$ (C$$_{5}$$ fragment) types can serve as low and high energy intermediates for the cobalt ion's migration around the C$$_{60}$$ cage. Formation of cobalt dimers can be the final stage of evolution of Co$$_{x}$$/C$$_{60}$$ atomic structure approaching the equilibrium atomic geometry.

Journal Articles

Tunnel magnetoresistance in Co nanoparticle/Co-C$$_{60}$$ compound hybrid system

Sakai, Seiji; Yakushiji, Kei*; Mitani, Seiji*; Takanashi, Koki*; Naramoto, Hiroshi; Avramov, P.; Narumi, Kazumasa; Lavrentiev, V.; Maeda, Yoshihito

Applied Physics Letters, 89(11), p.113118_1 - 113118_3, 2006/09

 Times Cited Count:58 Percentile:86.98(Physics, Applied)

In this study, the first results are described on the spin-dependent transport in the Co-C$$_{60}$$ hybrid film in which Co nanoparticles are dispersed in a matrix of the Co-C$$_{60}$$ compound. It is found that the hybrid film shows significant tunnel magnetoresistance (MR) of a few 10 %, which is comparable to granular metal-insulator systems, under the small applied bias voltage. Additionally, it is found that the electrical conductivity and MR ratio are strongly influenced by the bias voltage and the MR ratio under the higher bias voltage reaches 80%, which is anomalously high in granular systems. The bias-dependent magnetotransport behaviours are surmised to be related to the electronic structure of the Co-C$$_{60}$$ compound.

Journal Articles

Planar doping of crystalline fullerene with cobalt

Lavrentiev, V.*; Naramoto, Hiroshi*; Narumi, Kazumasa; Sakai, Seiji; Avramov, P.

Chemical Physics Letters, 423(4-6), p.366 - 370, 2006/06

 Times Cited Count:11 Percentile:37.43(Chemistry, Physical)

A study of 100 keV Co$$^{+}$$-implanted C$$_{60}$$ films with Rutherford Backscattering and Raman spectroscopy has revealed the pronounced cobalt translation from the surface layer of amorphous carbon into the deeper crystalline fullerene due to post-implantation annealing at 300 $$^{circ}$$C. Carbon density gradient along the film depth is discussed as a driving force of this effect. Cobalt deficit in the doped fullerene layer, detected by means of ion beam analysis, suggests ionization of the C$$_{60}$$ molecules under the collisions with 2 MeV He$$^{+}$$ ions.

Oral presentation

Quantum chemical study of atomic structure evolution of the Co$$_{x}$$/C$$_{60}$$(x$$leq$$2) composites

Avramov, P.; Naramoto, Hiroshi; Sakai, Seiji; Narumi, Kazumasa; Lavrentiev, V.

no journal, , 

Atomic and electronic structure of a new class of Co$$_{x}$$/C$$_{60}$$ (x$$leq$$2) composites synthesized using energy beams has been studied using B3LYP/6-31G$$^{*}$$ approach. For the x$$leq$$1 the $$eta$$$$^{2}$$ type of coordination is the lowest energy structure. Other isomers like $$eta$$$$^{2'}$$ or mixed structures $$eta$$$$^{2}$$/$$eta$$$$^{5}$$, $$eta$$$$^{2'}$$/$$eta$$$$^{5}$$ and $$eta$$$$^{2'}$$/$$eta$$$$^{2}$$ have energies between the $$eta$$$$^{2}$$ and $$eta$$$$^{5}$$ clusters ($$sim$$30kcal/mol). The theoretical data confirm the indirect qualitative structural data. A comparison of the theoretical Co-C bond lengths with XAFS analysis shows that B3LYP/6-31G$$^{*}$$ data (2.06 ${AA}$ ) match well the experimental value (2.01 ${AA}$). For the clusters with two cobalt ions a formation of metallic dimmers is preferable even in case of formation of additional C-C bonds between the fullerene molecules. To study the atomic structure evolution of Co$$_{x}$$/C$$_{60}$$ composites we have calculated the structure of all possible linear and bent (C$$_{60}$$-Co-C$$_{60}$$-Co-C$$_{60}$$) isomers of Co$$_{2}$$(C$$_{60}$$)$$_{3}$$ clusters. Like in the case of Co(C$$_{60}$$)$$_{2}$$ clusters, the $$eta$$$$^{2}$$:$$eta$$$$^{2}$$/$$eta$$$$^{2}$$:$$eta$$$$^{2}$$ and $$eta$$$$^{2}$$:$$eta$$$$^{2}$$/$$eta$$$$^{2}$$:$$eta$$$$^{2'}$$ types of coordination have lowest energies whereas the $$eta$$$$^{5}$$:$$eta$$$$^{5}$$/$$eta$$$$^{5}$$:$$eta$$$$^{2'}$$ and $$eta$$$$^{5}$$:$$eta$$$$^{5}$$/$$eta$$$$^{5}$$:$$eta$$$$^{5}$$ are the high energy isomers. It was shown that the closer the cobalt ions are, the lower the relative energy of the structures is. The formation of the cobalt dimmers on the final stage of the transition metal migration around the C$$_{60}$$ cage sufficiently ($$sim$$20-50kcal/mol) lowers the relative energies of the structures.

Oral presentation

Radiation effect of low energy C$$_{60}$$ ions on crystalline C$$_{60}$$ and Si targets

Narumi, Kazumasa; Naramoto, Hiroshi; Lavrentiev, V.*; Sakai, Seiji; Avramov, P.; Maeda, Yoshihito

no journal, , 

no abstracts in English

Oral presentation

The Theoretical DFT study of the local atomic and electronic structure of the Co$$_{x}$$/C$$_{60}$$ (x $$leq$$ 2.8) composites

Avramov, P.; Naramoto, Hiroshi*; Sakai, Seiji; Narumi, Kazumasa; Lavrentiev, V.*; Maeda, Yoshihito

no journal, , 

The main features of the local atomic structure of novel Co$$_{x}$$/C$$_{60}$$ (x $$leq$$ 2.8) complex mixtures were studied using the ab initio B3LYP/6-31G$$^{*}$$ method for a set of low and high energy Co$$_{n}$$(C$$_{60}$$)$$_{m}$$ (n = 1, 2, m = 2, 3) clusters in low and intermediate spin states. For the n = 1 isomers the spin state S = 1/2 is energetically preferable whereas the low energy isomers of n = 2 have an intermediate spin state of S = 1. The $$eta$$$$^{2}$$ (6-6 edge of C$$_{60}$$) type of cobalt ion coordination is preferable for both n = 1 and n = 2 cases. The $$eta$$$$^{2'}$$ (coordination with 6-5 edge) and even the $$eta$$$$^{5}$$ (C$$_{5}$$ fragment) types can serve as low and high energy intermediates for the cobalt ion's migration around the C$$_{60}$$ cage. Formation of cobalt dimers can be the final stage of evolution of Co$$_{x}$$/C$$_{60}$$ atomic structure approaching the equilibrium atomic geometry.

Oral presentation

Radiation effect of low energy C$$_{60}$$ ions on crystalline C$$_{60}$$ and Si targets

Narumi, Kazumasa; Naramoto, Hiroshi*; Lavrentiev, V.*; Sakai, Seiji; Avramov, P.; Maeda, Yoshihito

no journal, , 

no abstracts in English

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