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Journal Articles

Controllable fabrication of amorphous Si layer by energetic cluster ion bombardment

Lavrentiev, V.*; Vorli$v{c}$ek, V.*; Dejneke, A.*; Chvostova, D.*; J$"a$ger, A.*; Vacik, J.*; Jastrabik, L.*; Naramoto, Hiroshi*; Narumi, Kazumasa

Vacuum, 98, p.49 - 55, 2013/12

https://doi.org/10.1016/j.vacuum.2013.05.017
 Times Cited Count：1 Percentile：4.63(Materials Science, Multidisciplinary)

Journal Articles

Polymerization of solid C$$_{60}$$ under C$$_{60}$$$$^{+}$$ cluster ion bombardment

Lavrentiev, V.*; Vacik, J.*; Naramoto, Hiroshi*; Narumi, Kazumasa

Applied Physics A, 95(3), p.867 - 873, 2009/06

https://doi.org/10.1007/s00339-009-5088-3
 Times Cited Count：3 Percentile：14.44(Materials Science, Multidisciplinary)

The structure transformation occurring in fullerene films under bombardment with 50-keV C$$_{60}$$$$^{+}$$ cluster ions is reported. The Raman spectra of the irradiated C$$_{60}$$ films reveal a new peak rising at 1458 cm$$^{-1}$$ with increasing ion fluence. This feature of the Raman spectra suggests linear polymerization of solid C$$_{60}$$ induced by the cluster ion impacts. The aligned C$$_{60}$$ polymeric chains composing about 5-10 fullerene molecules have been distinguished on the film surface after the high-fluence irradiation using atomic force microscopy (AFM). The surface profiling analysis of the irradiated films has revealed pronounced sputtering during the treatment. The obtained results indicate that the C$$_{60}$$ polymerization occurs in a deep layer situated more than 40 nm below the film surface. The deep location of the C$$_{60}$$ polymeric phase indirectly confirms the dominant role of shock waves induced by C$$_{60}$$-ion bombardment in the detected C$$_{60}$$ phase transformation.

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

https://doi.org/10.1016/j.tsf.2007.03.141
 Times Cited Count：24 Percentile：68.43(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

https://doi.org/10.2320/matertrans.48.754
 Times Cited Count：9 Percentile：51.33(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

https://doi.org/10.1021/jp0655874
 Times Cited Count：14 Percentile：45.25(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

https://doi.org/10.1063/1.2354035
 Times Cited Count：59 Percentile：86.21(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

Polymeric chains in C$$_{60}$$ and Co mixture

Lavrentiev, V.*; Abe, Hiroaki*; Naramoto, Hiroshi*; Sakai, Seiji; Narumi, Kazumasa

Chemical Physics Letters, 424(1-3), p.101 - 104, 2006/06

https://doi.org/10.1016/j.cplett.2006.04.046
 Times Cited Count：14 Percentile：43.41(Chemistry, Physical)

We present transmission electron beam identification of C$$_{60}$$-based polymeric chains in the mixture between Co and C$$_{60}$$ simultaneously deposited on (001) NaCl at room temperature. Estimation of intrachain distances taken from the direct image suggests bridging position of Co atoms between buckyballs. The chains tend to be coupled due to Coulomb interaction between the charged units of the polymers. We have confirmed the charge transfer between Co and C$$_{60}$$ detecting 0.6 eV upward shift of C$$_{60}$$ LUMO-derived peak in the electron-energy-loss spectrum of mixture. The systematic shifts of energy-loss $$sigma$$$$^{*}$$ peaks imply distortion of C$$_{60}$$ cages belonging to the polymeric chains.

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

https://doi.org/10.1016/j.cplett.2006.03.087
 Times Cited Count：11 Percentile：35.52(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.

Journal Articles

Polymeric Co-C$$_{60}$$ compound phase evolved in atomistically mixed thin films

Sakai, Seiji; Naramoto, Hiroshi*; Lavrentiev, V.*; Narumi, Kazumasa; Maekawa, Masaki; Kawasuso, Atsuo; Yaita, Tsuyoshi; Baba, Yuji

Materials Transactions, 46(4), p.765 - 768, 2005/04

https://doi.org/10.2320/matertrans.46.765
 Times Cited Count：17 Percentile：70.79(Materials Science, Multidisciplinary)

A systematic study of the atom-level mixtures between Co and C$$_{60}$$ has made possible to find a new C$$_{60}$$-based compound phase, Co$$_{x}$$C$$_{60}$$ (x$$leq$$5). It is confirmed with the EXAFS analysis and the positron annihilation study that the polymeric structure is developed three-dimensionally by bridging two C$$_{60}$$ molecules with the covalently bonded Co atom.

Journal Articles

Chemically driven microstructure evolution in cobalt-fullerene mixed system

Lavrentiev, V.; Naramoto, Hiroshi; Abe, Hiroaki*; Yamamoto, Shunya; Narumi, Kazumasa; Sakai, Seiji

Fullerenes, Nanotubes, and Carbon Nanostructures, 12(1-2), p.519 - 528, 2004/00

https://doi.org/10.1081/FST-120027216
 Times Cited Count：3 Percentile：16.41(Chemistry, Physical)

no abstracts in English

Journal Articles

Structure evolution and corresponding electrical properties in weakly bound Co-C60 mixture

Sakai, Seiji; Naramoto, Hiroshi; Xu, Y.; Priyanto, T. H.; Lavrentiev, V.; Narumi, Kazumasa

Materials Research Society Symposium Proceedings, Vol.788, p.L11.49.1 - L11.49.6, 2004/00

https://doi.org/10.1557/PROC-788-L11.49

Mixture films between cobalt and C$$_{60}$$ (CoxC$$_{60}$$, x = 0.5-700) were prepared by co-evaporation technique under UHV conditions. The composition-dependent features of dilatation and downshift of Raman peak suggest that the mixtures are composed of cobalt particles and a C$$_{60}$$-based phase in which certain number of cobalt atoms are coordinated with C$$_{60}$$ molecules. It is deduced that the equilibrated number of cobalt atoms in the C$$_{60}$$-based phase is 4 atoms per C$$_{60}$$ molecule and in which an electron transfer occurs from a cobalt atom to a C$$_{60}$$ molecule. The evaluation of their temperature dependences reveals that: (1) The mixtures with x $$leq$$ 4 are thermally activated-type, and their conductive nature can be attributed to the C$$_{60}$$-based phase. Further (2) the mixtures between 4 $$<$$ x $$<$$ 60 are also thermally activated, however, the electron hopping process between the isolated cobalt particles is supposed to be operative (variable-range-hopping). And (3) in the mixtures with x $$geq$$ 60 which corresponds to a percolation threshold for cobalt particles, the system changes into metallic conduction.

Journal Articles

Isolation of Co nanoparticles by C$$_{60}$$ molecules in co-deposited film

Lavrentiev, V.; Abe, Hiroaki*; Yamamoto, Shunya; Naramoto, Hiroshi; Narumi, Kazumasa

Materials Letters, 57(24-25), p.4093 - 4097, 2003/08

https://doi.org/10.1016/S0167-577X(03)00272-6
 Times Cited Count：9 Percentile：38(Materials Science, Multidisciplinary)

no abstracts in English

Journal Articles

Formation of promising Co-C nanocompositions

Lavrentiev, V.; Abe, Hiroaki; Yamamoto, Shunya; Naramoto, Hiroshi; Narumi, Kazumasa

Surface and Interface Analysis, 35(1), p.36 - 39, 2003/01

https://doi.org/10.1002/sia.1489
 Times Cited Count：8 Percentile：22.7(Chemistry, Physical)

no abstracts in English

Journal Articles

Formation of carbon nanotubes under conditions of Co+C$$_{60}$$ film

Lavrentiev, V.; Abe, Hiroaki; Yamamoto, Shunya; Naramoto, Hiroshi; Narumi, Kazumasa

Physica B; Condensed Matter, 323(1-4), p.303 - 305, 2002/10

https://doi.org/10.1016/S0921-4526(02)01031-1
 Times Cited Count：15 Percentile：59.47(Physics, Condensed Matter)

no abstracts in English

Journal Articles

Co and C$$_{60}$$ interaction under conditions of mixture

Lavrentiev, V.; Abe, Hiroaki; Yamamoto, Shunya; Naramoto, Hiroshi; Narumi, Kazumasa

Molecular Crystals and Liquid Crystals, 386, p.139 - 143, 2002/00

https://doi.org/10.1080/713738824
 Times Cited Count：5 Percentile：37.03(Chemistry, Multidisciplinary)

no abstracts in English

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 targets

Narumi, Kazumasa; Naramoto, Hiroshi*; Lavrentiev, V.*

no journal, , 

no abstracts in English

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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