H. Naramoto

Nuclear Physics Institute of the AS CR, Řež, Central Bohemia, Czech Republic

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Publications (193)281.83 Total impact

  • [show abstract] [hide abstract]
    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 01/2014; 104(5):051604-051604-4. · 3.79 Impact Factor
  • Vacuum. 12/2013;
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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). · 2.21 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. · 1.15 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. · 3.77 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. · 6.59 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. · 1.83 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). · 2.21 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 - FULLER NANOTUB CARBON NANOSTR. 01/2012; 20:328-335.
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    ABSTRACT: The tunneling magnetoresistance (TMR) effect of granular C60-Co films at low temperatures and also at small applied voltages is studied in the current-perpendicular-to-plane (CPP) geometry to elucidate the spin-dependent tunneling process, bringing about a remarkably high magnetoresistance (MR) as compared to the expectations from the conventional theory on sequential tunneling. The current-voltage characteristics showed ohmic and power-law dependences in the Coulomb blockade regime, which are interpreted as the occurrence of cooperative tunneling (so-called cotunneling) through a few to several Co nanoparticles. The zero-bias MR ratios are in the range of 50%–90% at a few degrees Kelvin and show strong and unconventional temperature dependence depending on the temperature range. Furthermore, the spin polarization of tunneling electrons evaluated based on the cotunneling model is in the range of 50%–80%, suggesting that the enhanced spin polarization of tunneling electrons at the interface between Co nanoparticles and a C60-based matrix (C60-Co compound) is crucial for large TMR effects.
    Physical review. B, Condensed matter 05/2011; 83(17).
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    ABSTRACT: We have studied the influence of the interface formation of graphene with noble metals (Ag and Au) on its vibrational properties by using confocal micro-Raman spectroscopy. The interactions at the metal/graphene interface are investigated by comparing the results from two different regions, the heterostructure and pristine graphene regions, with and without noble metals on the same graphene sheet. In Ag/graphene, the Raman signal intensity was increased by the surface enhanced Raman scattering process, and the enhanced signals are found to be composed of the broadened D and G peak components emitted from the Ag/graphene interface. The precise evaluation of graphene-layer-number-dependence of the D and G bands revealed that the disordered graphitic carbons were adhered on the glass substrate during the sample preparation by the micromechanical cleavage method. In the 2D band, no obvious peak shift induced by the heterostructure formation was observed in Ag/graphene, whereas a large shift (more than ˜15 cm-1) was observed at Au/single layer graphene. This is considered to be due to the difference in the amount of the doped carriers in graphene between two heterostructures.
    Japanese Journal of Applied Physics 01/2011; 50. · 1.07 Impact Factor
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    ABSTRACT: X-ray magnetic circular dichroism (XMCD) spectroscopy was employed to investigate the electronic and magnetic structures of the bilayers of a C60–Co compound and Ni. A few–several nm thick C60–Co compound layers on the Ni(111) surface are found to show intense XMCD signals attributed to the localized Co d-spins only with the remanent magnetization of Ni layer. It is suggested that the region of the C60–Co compound within 3nm from the interface is ferromagnetically coupled with Ni due to the indirect exchange interaction mediated by C60, probably relevant to the interlayer charge transfer.
    Chemical Physics Letters 01/2011; 511(1):68-72. · 2.15 Impact Factor
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    ABSTRACT: Atomic structure of graphene, bi-, tri-, tetralayer graphenes and graphite as well was studied using ab initio HSE, LDA and PBE DFT approaches in periodic boundary conditions. Based on comparison of theoretical results with experimental data the performance of the methods was estimated. It was found that long-range corrected HSE potential is the most reliable DFT approximation to reproduce the atomic structure of weakly bound multilayer graphenes and graphite as well.
    Chemical Physics Letters 01/2011; 508(1):86-89. · 2.15 Impact Factor
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    ABSTRACT: Studies are conducted for the influence of the interface formation of graphene with various transition metals on its vibrational properties by using confocal micro-Raman spectroscopy. Micrometer-scale heterostructures consisting of patterned regions of the single layer and multilayer graphene (SLG and MLG, hereafter) covered with and without metals on the same graphene sheet were fabricated by thin-film deposition on the graphene surface through a shadow mask. Comparative analysis for these two different regions (SLG and MLG) fabricated within an identical graphene sheet enables us to investigate the interactions at and the doping effect from the metal/graphene interface as a function of the layers number of graphene without the influence of the unintentional doping. Confirmed dependences of the peaks shifts of the Raman bands (D, G, and 2D bands) on the graphene layers number and metal species (Co, Ni, and Au) reveal that the interfacial interactions are dramatically different between single layer and multilayer graphenes. In the metal/MLG heterostructures, the Raman band shifts are reasonably attributed to carrier doping from metals. It is found that the type of the doped carriers (electrons or holes) is different between Co/MLG and Au/MLG, irrespective of almost the same work functions of Co and Au. These analyses also provide the effective thickness of carrier doping (2−3 graphene layers) from the interfaces. In the metal/SLG heterostructures, significant differences from the metal/MLG heterostructures were observed for the Raman parameters of the G and 2D bands. It is suggested that there exist strong interactions at the metal/SLG interfaces different from those at the metal/MLG interfaces.
    11/2010;
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    ABSTRACT: Composition dependence of magnetic and magnetotransport properties in C60Cox thin films exhibiting large magnetoresistance (MR) effect was investigated in the Co composition range of x = 8–20, where x denotes the number of Co atoms per C60 molecule. From the superparamagnetic magnetization curves observed, the average diameter (dave) of Co nanoparticles dispersed in the matrix phase was evaluated to be approximately 1 nm for the sample of x = 8, and increased with the Co composition, x. By analyzing the temperature (T) dependence of resistivity based on the model by Abeles et al. [Adv. Phys. 24, 407 (1975)] , the average charging energies (〈C〉) of Co nanoparticles were evaluated to be 2–9 meV for the samples of x = 8–17 while the considerably weak temperature dependence suggested much smaller values of 〈C〉 for the samples of x>17. The composition dependence of dave and 〈C〉 revealed a structural transition from well-defined granular structures in the range of x = 8–17 to magnetically and electronically coupled states of Co nanoparticles over x ∼ 17. As a result of the structural change, the MR behavior became different between the two composition regions separated at x ∼ 17. In particular, for the samples of x = 8–17, the bias-voltage (V) dependence of the MR ratio in the low-V region fits well with an unusual exponential form of MR = MR0 exp(−V/Vc) at T<20 K, and it is most remarkable that the zero-bias MR ratio (MR0) is in proportion to 〈C〉 and also that the fitting parameter Vc is closely correlated with 〈C〉. These results indicate that the charging effect of Co nanoparticles plays an important role in the anomalously large MR effect of C60–Co granular films. In addition, the power-law dependence of MR on T (MR∝T−α,α ∼ 2) was also observed at relatively high temperature range T ≥ 10 K in the wide range of the composition.
    Journal of Applied Physics 09/2010; 108(6):063920-063920-7. · 2.21 Impact Factor
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    ABSTRACT: Effect of deposition temperature (Ts) on structure of Co-C60 nanocomposite (NC) prepared by simultaneous deposition of cobalt and fullerene on sapphire is presented. The NC structure variations with Ts increasing from room temperature (RT) to 400 degrees C have been analyzed using scanning electron microscopy (SEM), atomic force microscopy (AFM) and Raman spectroscopy. AFM and SEM show granule-like structure of the Co-C60 film. The mixture film deposited at RT includes the hills on the surface suggesting accumulation of internal stress during phase separation. Raman spectra show 25 cm(-1) downshift of Ag(2) C60 peak suggesting -Co-C60- polymerization in C60-based matrix of the NC film. Analysis of Raman spectra has revealed existence of amorphous carbon (a-C) in the NC matrix that argues C60 decomposition. The Ts increase to 200 degrees C causes the surface hills smoothing. In parallel, downshift of the Ag(2) peak decreases to 16 cm(-1) that implies more pronounced phase separation and lower -Co-C60- polymerization efficiency. Also, amount of a-C content slightly increases. Further Ts increasing to 400 degrees C changes the NC structure dramatically. AFM shows evident enlargement of the granules. According to Raman spectra the high Ts deposition yields pronounced C60 decomposition increasing the a-C content. Features of a-C Raman peak imply nucleation of graphitic islands at the NC interfaces. Abundant decomposition of C60 in the mixture film deposited at 400 degrees C is referred to cobalt catalytic effect.
    Journal of Nanoscience and Nanotechnology 04/2010; 10(4):2624-9. · 1.15 Impact Factor
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    ABSTRACT: The effect of the strong centrifugal force, mega-gravity (MG) on inter-diffusion between Au and Cu thin films was studied by using Au(60nm)/Cu(500nm)/α-Al2O3 (0001) films. The Rutherford backscattering analysis of the Au and Cu depth profiles shows that Cu atoms diffuse through the Au layer, resulting in the formation of the Cu layer on the surface under both the thermal annealing at 220°C and the application of 0.61×106G at 220°C. The results indicate that the MG application enhances the layer thickness of the Cu layer on the surface.
    Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 01/2010; 268(11):1867-1870. · 1.27 Impact Factor
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    ABSTRACT: The Co-C60 nano-composite film prepared by simultaneous deposition of Co and C60 at room temperature (RT) on sapphire with high content of cobalt (50 at.% Co) is recognized as ensemble of the fcc-Co crystals (5-6 nm in size) separated by the C60-based matrix. It is shown formation of internal stress in the composition arising due to the phase separation. The internal stress causes the phenomena sufficiently influencing structure of the nano-composite. One of them is locking the Co atoms within the C60-based matrix (retained Co atoms) occurring during the separation process. Analysis of the Raman spectrum argues that the retained Co atoms are included in the Co-C60 polymer dominating in the matrix of nano-composite. It is suggested importance of the internal stress for the polymer formation. Another phenomenon is the structural relaxation releasing the internal stress. These phenomena are tested through applying different thermal treatments. Raman spectrum of the mixture film deposited at 200 degrees C shows the lower polymerization efficiency in the C60-based matrix due to the more complete phase separation decreasing number of the retained Co atoms. Post-deposition annealing of the RT-deposited Co-C60 mixture film done at 300 degrees C for 1 hour induces the structural relaxation as conversion of fullerene into the regular carbon structure. According to the Raman analysis the regular carbon structure corresponds to the single-wall carbon nanotubes (SWNT) doped by cobalt. Similar analysis of the 200 degrees C-deposited mixture film treated by the following annealing reveals formation of SWNT only after much longer annealing. These experiments designate the Co diffusion as a main process driven by the carbon nanotube formation. The results demonstrate remarkable opportunity to control structure of the Co-C60 nano-composite using proper thermal treatments.
    Journal of Nanoscience and Nanotechnology 07/2009; 9(7):4305-10. · 1.15 Impact Factor
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    Chemical Physics Letters 05/2009; 474(s 1–3):238. · 2.15 Impact Factor
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    ABSTRACT: Hybridization and thermal evolution of the Ni+C60 composites, deposited on Si(001) at room temperature, were studied using Scanning Electron Microscopy, mu-Raman spectroscopy and Rutherford Backscattering. As-deposited, the hybrid films exhibited a granular nano-structure with Ni nano-particles encapsulated in C60 polymerized rinds. The Ni and C (C60) distributions in a top layer were found homogeneous with a stable Ni/C (C60) ratio; in the larger depth the distributions were inhomogeneous and their ratio dramatically varied. At elevated temperatures, all structural parameters were changed. In the subsurface layer Ni-and C (C60)-rich zones were formed (due to the induced phase separation), C60-molecules decayed and their fragments were transformed into amorphous carbon (a-C). The free volume distribution of the stressed hybrid matter was analyzed by the Hg marker that (in a form of vapors) in-diffused in to the samples. The RBS analysis showed that the Hg distribution of the as-deposited films followed the character of distribution of the C (C60) component. At elevated temperatures, however, the Hg profiles decreased dramatically (especially in the subsurface layer) suggesting a significant drop-off of the free volume concentration due to the formation of the spatially compact a-C clusters.
    AIP Conference Proceedings. 03/2009; 1099(1).

Publication Stats

172 Citations
281.83 Total Impact Points

Institutions

  • 2012
    • Nuclear Physics Institute of the AS CR
      Řež, Central Bohemia, Czech Republic
  • 1994–2010
    • Japan Atomic Energy Agency
      • • Advanced Science Research Center
      • • Quantum Beam Science Directorate
      Muramatsu, Niigata-ken, Japan
  • 2006
    • Kyoto Institute of Technology
      • Department of Chemistry and Materials Technology
      Kyoto, Kyoto-fu, Japan
  • 1998
    • Kyoto University
      Kioto, Kyōto, Japan
    • Houston Advanced Research Center
      Houston, Texas, United States
  • 1997
    • Himeji Institute of Technology
      Himezi, Hyōgo, Japan