H. Naramoto

Japan Atomic Energy Agency, Muramatsu, Niigata, Japan

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Publications (229)380.72 Total impact

  • Shiro Entani, Hiroshi Naramoto, Seiji Sakai
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    ABSTRACT: Magnetotransport properties were studied for the vertical spin valve devices with two junctions of permalloy electrodes and a few-layer graphene interlayer. The graphene layer was directly grown on the bottom electrode by chemical vapor deposition. X-ray photoelectron spectroscopy showed that the permalloy surface fully covered with a few-layer graphene is kept free from oxidation and contamination even after dispensing and removing photoresist. This enabled fabrication of the current perpendicular to plane spin valve devices with a well-defined interface between graphene and permalloy. Spin-dependent electron transport measurements revealed a distinct spin valve effect in the devices. The magnetotransport ratio was 0.8% at room temperature and increased to 1.75% at 50 K. Linear current-voltage characteristics and resistance increase with temperature indicated that ohmic contacts are realized at the relevant interfaces.
    Journal of Applied Physics 05/2015; 117(17):17A334. DOI:10.1063/1.4918957 · 2.19 Impact Factor
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    ABSTRACT: Direct growth of graphene on insulators is expected to yield significant improvements in performance of graphene-based electronic and spintronic devices. In this study, we successfully reveal the atomic arrangement and electronic properties of a coherent heterostructure of single-layer graphene and α-Al2O3(0001). The analysis of the atomic arrangement of single-layer graphene on α-Al2O3(0001) revealed an apparentcontradiction. The in-plane analysis shows that single-layer graphene grows not in a single-crystalline epitaxial manner, but rather in polycrystalline form, with two strongly pronounced preferred orientations. This suggests relatively weak interfacial interactions are operative. However, we demonstrate that unusually strong physical interactions between graphene and α-Al2O3(0001) exist, as evidenced by the small separation between the graphene and the α-Al2O3(0001) surface. The interfacial interaction is shown to be dominated by the electrostatic forces involved in the graphene π-system and the unsaturated electrons of the topmost O layer of α-Al2O3(0001), rather than the van der Waals interactions. Such features causes graphene hole doping and enable the graphene to slide on the α-Al2O3(0001) surface with only a small energy barrier despite the strong interfacial interactions.
    Nano Research 05/2015; 8(5). DOI:10.1007/s12274-014-0640-7 · 6.96 Impact Factor
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    ABSTRACT: The interaction between carbon and BN nanotubes (NT) and transition metal Co and Ni supports was studied using electronic structure calculations. Several configurations of interfaces were considered, and the most stable ones were used for electronic structure analysis. All NT/Co interfaces were found to be more energetically favorable than NT/Ni, and conductive carbon nanotubes demonstrate slightly stronger bonding than semiconducting ones. The presence of contact-induced spin polarization was established for all nanocomposites. It was found that the contact-induced polarization of BNNT leads to the appearance of local conductivity in the vicinity of the interface while the rest of the nanotube lattice remains to be insulating.
    Journal of Applied Physics 08/2014; 116(8):084309-084309-4. DOI:10.1063/1.4894157 · 2.19 Impact Factor
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    ABSTRACT: The atomic and electronic structures, mechanical properties and potential barriers of formation of a set of meso–meso b–b fused porphyrin/metalloporphyrin nanopages, nanotapes, nanotubes and 2D nanofabrics were studied by GGA LC-DFT technique using cluster and PBC models. The porphyrin pages of the nanoclusters are connected with each other by graphene fragments formed by meso–meso b–b links. Fusion of all the edges of six porphyrin/metalloporphyrin units produces a novel ~1 nm sized molecule of cubic symmetry with a hollow cage inside. It was found that all studied nanoclusters are metastable with formation energies 0.36–7.57 kcal/mol per atom. Under applied mechanical stress, the nanoclusters exhibit superelastic and ultrastrong properties with binding graphene fragments being the weakest links for mechanical rupture. Depending on the spin-dependent reaction pathways, the hollow caged nanoclusters exhibit almost zero or low potential energy barriers (1–10 kcal/ mol) during the initial stages of self-assembly. All nanoclusters exibit the main features of the electronic structures of the parent porphyrins, in particular the nature of HOMO/LUMO states and the relative energetic positions of the metal d states. The induced curvature of the hollow cage nanoclusters leads to admixture of more than 2% of the dp⊥ states to the ds energy region and formation of vacant superatomic molecular orbitals of d character in cubic ligand field. The Fe-derived hollow-caged nanoclusters reveal extremely high spin states with small energy differences between ferromagnetic and antiferromagnetic configurations, which can be utilized for quantum holonomic computations.
    Journal of Porphyrins and Phthalocyanines 05/2014; 18:1-17. DOI:10.1142/S1088424614500291 · 1.36 Impact Factor
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    ABSTRACT: Hexagonal boron nitride (h-BN) is a promising barrier material for graphene spintronics. In this Letter, spin-polarized metastable de-excitation spectroscopy (SPMDS) is employed to study the spin-dependent electronic structure of monolayer h-BN/Ni(111). The extreme surface sensitivity of SPMDS enables us to elucidate a partial filling of the in-gap states of h-BN without any superposition of Ni 3d signals. The in-gap states are shown to have a considerable spin polarization parallel to the majority spin of Ni. The positive spin polarization is attributed to the π-d hybridization and the effective spin transfer to the nitrogen atoms at the h-BN/Ni(111) interface.
    Applied Physics Letters 02/2014; 104(5):051604-051604-4. DOI:10.1063/1.4863324 · 3.52 Impact Factor
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    ABSTRACT: We report on the fabrication of an amorphous Si (a-Si) thin layer by means of bombardment of a Si(100) surface using monoenergetic C-60 cluster ions with energies from 50 keV to 400 keV. The C-60 cluster implantation produces nanogranules on the surface of a-Si layer detected by atomic force microscopy. The structural disorder and thickness of the modified layer were identified using Raman spectrometry, ion channelling, spectroscopic ellipsometry (SE) and transmission electron microscopy (TEM). According to SE and TEM data the thickness of a-Si layer gradually increases with cluster ion energy reaching to about 30 nm in the 200 keV C-60-bombarded Si sample. There is also thin layer of nanocrystalline Si found between the a-Si layer and pristine Si crystal. The obtained results represent an attractive method for creation of the a-Si layer as a functional material for opto- and nano-electronics. The study describes nanostructure created by cluster ion implantation as well as demonstrates the structural consequences of fast cluster energy dissipation in solids such as local heating and shock waves.
    Vacuum 12/2013; 98:49-55. DOI:10.1016/j.vacuum.2013.05.017 · 1.43 Impact Factor
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    ABSTRACT: A process of tunneling conduction and the spin-dependent resistivity change (so-called tunneling magnetoresistance effect) in the Fe-doped C60 film with a granular structure is investigated for the current-into-plane device. Cooperative tunneling (cotunneling) through several Fe nanoparticles is suggested to be operative at temperatures lower than 20 K. By considering the effect of cotunneling on the magnetoresistance ratio, it is successfully shown that the spin polarization of tunneling electrons generated at the Fe/C60 interface is much higher than that in Fe crystal at low temperature in a similar fashion to that at the Co/C60 interface in the Co-doped C60 films. A strong temperature dependence of spin polarization is observed, suggesting a possible influence by the thermally induced disorders ascribed to the Fe atoms bonded with C60 in the C60Fe compound.
    Synthetic Metals 06/2013; 173:22–25. DOI:10.1016/j.synthmet.2012.10.027 · 2.22 Impact Factor
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    ABSTRACT: The spin-polarized electronic structures across the interface between single-layer graphene and a Ni(111) thin film are explored by employing depth-resolved X-ray absorption and magnetic circular dichroism spectroscopy with atomic layer resolution. The depth-resolved Ni L2,3-edge analysis clarifies that the Ni atomic layers adjacent to the interface show a transition of the spin orientation to the perpendicular one in contrast to the in-plane one in the bulk region. The C K-edge analysis reveals the intensification of the spin–orbit interactions induced by the π–d hybridization at the interface as well as out-of-plane spin polarization in the π band region of graphene. The present study indicates the importance of the interface design at the atomic layer level for graphene-based spintronics.
    Journal of Materials Chemistry A 01/2013; 1:5533-5537. DOI:10.1039/c3tc30872c · 7.44 Impact Factor
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    ABSTRACT: Atomic and electronic structure of graphene/Ni(111), h-BN/Ni(111) and graphene/h-BN/Ni(111) nanocomposites with different numbers of graphene and h-BN layers and in different mutual arrangements of graphene/Ni and h-BN/Ni at the interfaces was studied using LDA/PBC/PW technique. Using the same technique corresponding graphene, h-BN and graphene/h-BN structures without the Ni plate were calculated for the sake of comparison. It was suggested that C-top:C-fcc and N-top:B-fcc configurations are energetically favorable for the graphene/Ni and h-BN/Ni interfaces, respectively. The Ni plate was found to induce a significant degree of spin polarization in graphene and h-BN through exchange interactions of the electronic states located on different fragments.
    Journal of Applied Physics 12/2012; 112(11). DOI:10.1063/1.4767134 · 2.19 Impact Factor
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    ABSTRACT: We report the creation of a functional nanostructure on a Si crystal surface by 200 keV C60(++) cluster ion bombardment (CIB). We found that the modified layer produced by CIB includes two sublayers with different nanostructures. The top 24-nm-thick sublayer is an agglomeration of 5-nm-sized amorphous Si nanodots (a-Si NDs). The deeper 10-nm-thick sublayer is a transient layer of disordered Si as an interface between the a-Si top sublayer and the bulk Si(100). The top a-Si sublayer and the nc-Si transient layer are formed by the local heating effect and shock wave effect, respectively, induced by the cluster ion impacts. The photoluminescence (PL) spectra of the CIB-modified Si samples revealed an emission line centered at a photon energy of 1.92 eV. The absorption spectra of the modified samples exhibit enhanced light absorption at this photon energy. The parameters of the PL line require ascribing the emission origin to the quantum-confinement-induced optical transitions in the a-Si nanodots. The core-shell structure of a-Si NDs is confirmed by detection of an additional PL line centered at 2.5 eV. Analysis of the Rutherford backscattering (RBS) and the PL spectra implies the existence of -Si--O- bonds in the nanodot outer shells, which are responsible for the additional PL line. The obtained results demonstrate the valuable potential of CIB for the controllable fabrication of Si surface nanostructures, which is attractive for optoelectronics and nanoelectronics. The obtained results elucidate the evolution of structure modification occurring in silicon due to the injection of energetic C60 cluster ions with an energy of hundreds of keV.
    Journal of Nanoscience and Nanotechnology 12/2012; 12(12):9136-41. DOI:10.1166/jnn.2012.6782 · 1.34 Impact Factor
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    ABSTRACT: The graphane with chemically bonded alkali metals (Li, Na, K) was considered as potential material for hydrogen storage. The ab initio calculations show that such material can adsorb as many as four hydrogen molecules per Li, Na, and K metal atom. These values correspond to 12.20, 10.33, and 8.56 wt% of hydrogen, respectively, and exceed the DOE requirements. The thermodynamic analysis shows that Li-graphane complex is the most promising for hydrogen storage with ability to adsorb three hydrogen molecules per metal atom at 300 K and pressure in the range of 5–250 atm.
    Physical Review B 08/2012; 86(8):085435. DOI:10.1103/PhysRevB.86.085435 · 3.66 Impact Factor
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    ABSTRACT: The atomic and electronic structure of narrow zigzag nanoribbons with finite length, consisting of graphene terminated by fluorine on one side, hexagonal (h) h-BN, and h-SiC were studied with density functional theory. It is found that the asymmetry of nanoribbon edges causes a uniform curvature of the ribbons due to structural stress in the aromatic ring plane. Narrow graphene nanoribbons terminated with fluorine on one side demonstrate a considerable out-of-plane bend, suggesting that the nanoribbon is a fraction of a conical surface. It is shown that the intrinsic curvature of the narrow nanoribbons destroys the periodicity and results in a systematic cancellation of the dipole moment. The in- and out-of-plane curvature of thin arcs allows their closure in nanorings or cone fragments of giant diameter. Using the fragment molecular orbital method, we optimized the structure of a planar giant arc and a closed ring of h-BN with a diameter of 105 nm.
    Journal of Physical Chemistry Letters 07/2012; 3(15):2003–2008. DOI:10.1021/jz300625t · 6.69 Impact Factor
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    ABSTRACT: Voltage-dependence of the tunneling magnetoresistance effect in the granular C60–Co films has been investigated for the samples with the current-perpendicular-to-plane geometry. The transport measurements under this geometry demonstrate that the granular C60–Co films show an unusual exponential bias voltage dependence of the magnetoresistance ratio down to zero voltage. Small characteristic energies of less than 10's meV are derived from the temperature dependences of the characteristic voltage in the exponential relationship. Considering the magnitudes of the voltage drop between Co nanoparticles and also the effect of cotunneling on the energy values, the characteristic energies for the voltage-induced degradation of the spin polarization are found to show a satisfactory agreement with that for the thermally-induced one. It can be reasonably expected that the onset of magnetic disorder to the localized d-electron spins at the interface region of the C60-based matrix (C60–Co compound) with Co nanoparticles leading to the unusual voltage and temperature dependence of the magnetoresistance ratio and the spin polarization at low temperatures.
    Journal of Magnetism and Magnetic Materials 06/2012; 324(12):1970–1974. DOI:10.1016/j.jmmm.2012.01.033 · 2.00 Impact Factor
  • V. Lavrentiev, J. Vacik, H. Naramoto
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    ABSTRACT: The factors driving the structure organization in the Co-C60 films fabricated by simultaneous deposition were analyzed. It is shown that phase separation is the major factor yielding the nanocomposite (NC) structure in the co-deposited film (fcc-Co nanocrystals immersed into C60-based matrix). The phase separation is accompanied by accumulation of compressive stress in the film mixture. The metal–fullerene chemical bonding is the next important factor responsible for creation of the –Co-C60- polymer in the NC matrix. Thermal treatment can significantly influence the structure organization through enhancement of the main factors (phase separation or chemical bonding) that yields the remarkable structural effects. In particular, annealing at the elevated temperatures (Ta = 300–500°C) induces the regular carbon structure transformations in the NC matrix followed by creation of carbon nanotubes or graphitic-like shells (depending on Ta). The C-structure transformation is controlled by catalytic effect of Co atoms retaining in the NC matrix.
    Fullerenes Nanotubes and Carbon Nanostructures 05/2012; 20(4-7):328-335. DOI:10.1080/1536383X.2012.655118 · 0.64 Impact Factor
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    ABSTRACT: In situ analysis was performed on the graphene growth in ultrahigh vacuum chemical vapor deposition by exposing the epitaxial Ni(111) thin film to benzene vapor at 873 K. It is shown that the highly uniform single- and bi-layer graphenes can be synthesized by the control of benzene exposure in the range of 10–105 langmuirs, reflecting a change in the graphene growth-rate by three orders of magnitude in between the first and second layer. Electron energy loss spectroscopy measurements of single- and bi-layer graphenes indicates that the interface interaction between bi-layer graphene and Ni(111) is weakened in comparison with that between single-layer graphene and Ni(111). It is also clarified from the micro-Raman analysis that the structural and electrical uniformities of the graphene film transformed on a SiO2 substrate are improved remarkably under the specific exposure conditions at which the growths of single- and bi-layer graphenes are completed.
    Journal of Applied Physics 03/2012; 111(6). DOI:10.1063/1.3694662 · 2.19 Impact Factor

Publication Stats

1k Citations
380.72 Total Impact Points

Institutions

  • 1988–2014
    • Japan Atomic Energy Agency
      • Advanced Science Research Center
      Muramatsu, Niigata, Japan
  • 2009–2013
    • Nuclear Physics Institute of the AS CR
      Řež, Central Bohemia, Czech Republic
  • 2007
    • Kyoto University
      Kioto, Kyōto, Japan
  • 2006
    • Kyoto Institute of Technology
      • Department of Chemistry and Materials Technology
      Kioto, Kyōto, Japan
  • 2004
    • Academy of Sciences of the Czech Republic
      • Nuclear Physics Institute
      Praha, Praha, Czech Republic
  • 2000
    • Tsinghua University
      • School of Materials Science and Engineering
      Peping, Beijing, China
  • 1998
    • Houston Advanced Research Center
      The Woodlands, Texas, United States
    • Tohoku University
      • Institute for Materials Research
      Miyagi, Japan
  • 1997
    • Department of Atomic Energy
      Mumbai, Maharashtra, India