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Journal Articles

Photo-oxidative doping in $$pi$$-conjugated zig-zag chain of carbon atoms with sulfur-functional group

Ikeura, Hiromi*; Sekiguchi, Tetsuhiro

Applied Physics Letters, 111(23), p.231605_1 - 231605_4, 2017/12

 Times Cited Count:0 Percentile:0(Physics, Applied)

Photo-oxidative doping processes were studied for the trans-polyacetylene (PA) backbone with the -SCH$$_{3}$$ side group as a chemically representative of the S-functionalized zig-zag graphene nanoribbon edge. Sulfur K-edge X-ray absorption near edge structure (XANES) spectroscopy indicates that photochemical reaction of S-CH$$_{3}$$ with atmospheric O$$_{2}$$ forms selectively oxidized products such as -S(O)CH$$_{3}$$ and -SO$$_{3}$$$$^{-}$$ bound to the (PA) backbone. Using the correlation between the oxidation states of sulfur and the XANES peak positions, the partial charge distribution of CH$$_{3}$$S$$^{delta+}$$ -PA$$^{delta-}$$ has been estimated. The results can provide some strategy for area-selective and controllable doping processes of atomic-scale molecular systems with the assistance of UV light.

Journal Articles

Micro-orientation control of silicon polymer thin films on graphite surfaces modified by heteroatom doping

Shimoyama, Iwao; Baba, Yuji; Hirao, Norie*

Applied Surface Science, 405, p.255 - 266, 2017/05

 Times Cited Count:1 Percentile:8.15(Chemistry, Physical)

NEXAFS spectroscopy is applied to study orientation structures of polydimethylsilane (PDMS) films deposited on heteroatom-doped graphite substrates prepared by ion beam doping. The Si ${it K}$-edge NEXAFS spectra of PDMS show opposite trends of polarization dependence for non irradiated and N$$_{2}$$$$^{+}$$-irradiated substrates, and show no polarization dependence for an Ar$$^{+}$$-irradiated substrate. Based on a theoretical interpretation of the NEXAFS spectra via first-principles calculations, we clarify that PDMS films have lying, standing, and random orientations on the non irradiated, N$$_{2}$$$$^{+}$$-irradiated, and Ar$$^{+}$$-irradiated substrates, respectively. Furthermore, photoemission electron microscopy indicates that the orientation of a PDMS film can be controlled with microstructures on the order of $$mu$$m by separating irradiated and non irradiated areas on the graphite surface. These results suggest that surface modification of graphite using ion beam doping is useful for micro-orientation control of organic thin films.

Journal Articles

Electronic structures at phosphorus sites doped in graphite; Spectral analysis of P K-edge NEXAFS with DVX$$alpha$$ calculations

Shimoyama, Iwao; Baba, Yuji

DV-X$$alpha$$ Kenkyu Kyokai Kaiho, 28(1&2), p.62 - 69, 2016/03

We studied electronic structures at phosphorus sites doped in graphite to clarify the dopant effect on catalytic activity of P-doped graphite using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. A sample prepared by high-temperature doping at 700 $$^{circ}$$C showed graphite-like clear polarization dependence of P K-edge NEXAFS spectra indicating planar structure at phosphorus sites. We calculated electronic structures at phosphorus sites in carbon model clusters using the DV-X$$alpha$$ method and compare them with NEXAFS. The electronic structure at planar phosphorus site with three carbon coordination reproduced the NEXAFS spectra. On the other hand, samples prepared by room-temperature doping and post annealing at 800 $$^{circ}$$C showed deterioration of polarization dependence of NEXAFS. We clarified that an electronic structure of a curved carbon model cluster with a pentagon reproduced change of polarization dependence and spectral shape of NEXAFS. This means local structures at phosphorus sites can be controlled by substrate temperatures during ion dopings.

Journal Articles

Reconsidering of a polarization rule on atomic arrangements of graphite-like boron carbonitride

Shimoyama, Iwao; Baba, Yuji; Sekiguchi, Tetsuhiro

DV-X$$alpha$$ Kenkyu Kyokai Kaiho, 27(1&2), p.34 - 44, 2015/03

no abstracts in English

Journal Articles

Influence of configuration at dopant sites on catalytic activity of phosphorus-doped graphite

Shimoyama, Iwao; Hakoda, Teruyuki; Shimada, Akihiko; Baba, Yuji

Carbon, 81, p.260 - 271, 2015/01

 Times Cited Count:21 Percentile:64.2(Chemistry, Physical)

The relationship between catalytic activity and local structure at dopant sites of phosphorus-doped graphite prepared by ion implantation was studied by electrochemical measurements and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The P K-edge NEXAFS spectra of samples doped at room temperature and high temperature showed different polarization dependences, with graphite-like clear polarization dependence observed for the sample prepared by high-temperature doping. NEXAFS spectra were analyzed by density functional theory calculations to reveal the chemical-bonding states of P sites. In electrochemical measurements with an acid solution, samples with relatively small polarization dependence of NEXAFS exhibited marked catalytic activity in oxygen reduction reactions while those with larger polarization dependence showed lower catalytic activity. This indicates the influence of the configuration at dopant sites on catalytic activity and suggests that the introduction of curved structures is one way to design carbon catalysts with high catalytic activity.

Journal Articles

Characterization of F$$^{+}$$-irradiated graphite surfaces using photon-stimulated desorption spectroscopy

Sekiguchi, Tetsuhiro; Baba, Yuji; Shimoyama, Iwao; Nath, K. G.

Surface and Interface Analysis, 38(4), p.352 - 356, 2006/04

 Times Cited Count:3 Percentile:8.09(Chemistry, Physical)

We investigated the orientation nature at the top-most layers of F$$^{+}$$-irradiated graphite using polarization dependent near-edge X-ray absorption fine structure (NEXAFS) spectroscopy which incorporates partial electron yield (PEY) detection and photon-stimulated ion desorption (PSID) techniques. The fluorine K-edge NEXAFS spectra conducted in PEY mode show no significant dependence on polarization angles. In contrast, NEXAFS spectra recorded in F$$^{+}$$ ion yield mode show enhanced yields at a feature of $$sim$$689.4 eV assigned as a $$sigma$$*(C-F) state relevant to =C-F sites, which depend on polarization angles. The C-F bonds prefer relatively tilting down the surface at the top-most layer, while the C-F bonds are randomly directed at deeper regions. We conclude that the difference in the orientation structures between the top surface and bulk is reflected in the NEXAFS recorded in the two different detection modes. It was also found that H$$^{+}$$- and F$$^{2+}$$- PSID NEXAFS spectra are helpful in understanding desorption mechanism, thus in analysing NEXAFS data.

Journal Articles

Mechanism of state-specific enhancement in photon-stimulated desorption as studied using a polarization-dependent technique

Sekiguchi, Tetsuhiro; Baba, Yuji; Shimoyama, Iwao; Wu, G.*; Kitajima, Yoshinori*

Surface Science, 593(1-3), p.310 - 317, 2005/11

 Times Cited Count:2 Percentile:12.3(Chemistry, Physical)

Using a newly developed rotatable time-of-flight mass spectrometer(R-TOF-MS) and polarized synchrotron radiation, orientation effect on fragmentation and desorption pathways occurring at the top-most layers of molecular solids have been investigated. Reported will be polarization-angle dependencies of TOF mass spectra, high-resolution electron- and ion-NEXAFS in condensed chlorobenzene.

Journal Articles

Direct and indirect processes in photon-stimulated ion desorption from condensed formamide

Ikeura, Hiromi*; Sekiguchi, Tetsuhiro; Baba, Yuji; Imamura, Motoyasu*; Matsubayashi, Nobuyuki*; Shimada, Hiromichi*

Surface Science, 593(1-3), p.303 - 309, 2005/11

 Times Cited Count:5 Percentile:27.34(Chemistry, Physical)

no abstracts in English

Journal Articles

Substitution effect on orientation of organosilicon compounds (CH$$_{3}$$)$$_{3}$$SiX (X = F, Cl, Br, I, NCO) as studied using NEXAFS spectroscopy

Sekiguchi, Tetsuhiro; Baba, Yuji; Shimoyama, Iwao; Nath, K. G.*; Uddin, M. N.*

Journal of Physics; Condensed Matter, 17(36), p.5453 - 5466, 2005/09

 Times Cited Count:1 Percentile:6.58(Physics, Condensed Matter)

The orientation nature of multilayer organosilicon compounds has been investigated by measuring the dependence of the Si K-shell near-edge X-ray absorption fine structures (NEXAFS) on the polarization angle. Two approaches helped to elucidate the orientation mechanism: the substitution effect and the deposition-rate dependence. The orientation angles of Si-X bond axes were obtained for trimethylsilyl halides, (CH$$_{3}$$)$$_{3}$$SiX (X =F Cl Br I NCO, condensed on Cu(111) at a low (82 K) temperature: the angles are 60, 73, 61, 55, and 55 degrees with respect to the surface normal, for X =F, Cl, Br, I, and NCO, respectively. Chloride (X =Cl) produces the most parallel tilt angle. The specific orientation nature of chloride is attributed to its strong dipole moment as well as the regular tetrahedron shape of the molecule. The molecular volumes calculated verify this view. Furthermore, deposition rates are found to greatly influence the growth manner: namely, high deposition rates led to a slightly perpendicular orientation of Si-X bond axis.

Journal Articles

Structure of sub-monolayered silicon carbide films

Baba, Yuji; Sekiguchi, Tetsuhiro; Shimoyama, Iwao; Nath, K. G.

Applied Surface Science, 237(1-4), p.176 - 180, 2004/10

 Times Cited Count:7 Percentile:38.81(Chemistry, Physical)

no abstracts in English

Journal Articles

Electronic structure analysis of a $$h$$-BN thin film on Ni(111) using NEXAFS spectroscopy

Shimoyama, Iwao; Baba, Yuji; Sekiguchi, Tetsuhiro; Nath, K. G.

Hyomen Kagaku, 25(9), p.555 - 561, 2004/09

no abstracts in English

Journal Articles

NEXAFS spectra of an epitaxial boron nitride film on Ni(111)

Shimoyama, Iwao; Baba, Yuji; Sekiguchi, Tetsuhiro; Nath, K. G.

Journal of Electron Spectroscopy and Related Phenomena, 137-140, p.573 - 578, 2004/07

 Times Cited Count:25 Percentile:70.72(Spectroscopy)

Hexagonal boron nitride (h-BN) thin film has been epitaxially formed on Ni(111) by chemical vapor deposition using borazine gas. The electronic structure of this system is studied by near edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS). The thickness of the h-BN is estimated to be about two-layers from XPS. B K-edge NEXAFS spectra show new $$pi$$* peak which is not observed in the spectrum for bulk h-BN. From a polarization dependence analysis of NEXAFS, we propose this new $$pi$$* peak originates from the interaction between the h-BN and Ni(111). This new $$pi$$* peak clearly proves that conduction band of h-BN/Ni(111) is different from that of bulk h-BN.

Journal Articles

Near-edge X-ray absorption fine structure of DNA nucleobases thin film in the nitrogen and oxygen K-edge region

Fujii, Kentaro; Akamatsu, Ken; Yokoya, Akinari

Journal of Physical Chemistry B, 108(23), p.8031 - 8035, 2004/06

 Times Cited Count:49 Percentile:77.34(Chemistry, Physical)

Near-edge X-ray absorption fine structure (NEXAFS) of evaporated DNA nucleobase thin films on to an Au-coated Si surface at room temperature has been investigated around the nitrogen and oxygen K-edge region using highly monochromatized synchrotron soft X-rays. Each nucleobase shows a specific absorption spectrum originating from resonant excitation of a 1s electron of nitrogen or oxygen to antibonding states, some of which strongly depend on the polarization angle of incident soft X-rays. From the analysis of polarization angle dependences for $$pi$$* resonant absorption intensity, purines are orientated to the multilayer surface with the angle 15$$pm$$6$$^{circ}$$ for adenine and 38$$pm$$1$$^{circ}$$ for guanine, respectively. Uracil has an orientation of 16$$pm$$4$$^{circ}$$, although other pyrimidines such as thymine and cytosine are randomly orientated to the surface.

Journal Articles

Orientation effect on phot-fragmentation and ion-desorption from the topmost layers of molecular solids

Sekiguchi, Tetsuhiro; Ikeura, Hiromi*; Baba, Yuji

Surface Science, 532-535(1-3), p.1079 - 1084, 2003/06

Using a newly developed rotatable time-of-flight mass spectrometer(R-TOF-MS) and polarized synchrotron radiation, we have investigated orientation effect on fragmentation and desorption pathways occurring at the top-most layers of molecular solids. Reported will be polarization-angle dependencies of TOF mass spectra, high-resolution electron- and ion-NEXAFS in condensed formic acid, formamide and benzene. For condensed formamide(HCOND$$_{2}$$), marked orientation effect was observed for the enhanced H$$^{+}$$-yields following C1s $$rightarrow$$ $$sigma$$*$$_{C-H}$$ resonance. Direct photodissociation and charge-neutralization play an important role in the effect. For some fragment species, however, the bond scission showed no polarization dependence when dissociation sites were far from core-excited atoms. This is the case for N-D scission and D$$^{+}$$-desorption following C1s excitation, suggesting that indirect process governs, where secondary electrons would induce the fragmentation.

Journal Articles

X-ray-induced ion desorption from solid surfaces

Baba, Yuji

Trends in Vacuum Science & Technology, Vol.5, p.45 - 74, 2002/10

This article reviews recent works on the ion desorption from solid surfaces induced by the irradiation of soft X-rays ranging from 100 eV to 3 keV. The data presented here are focused on the positive ion desorption from adsorbed, condensed, and solid molecules following the core-level excitations. The element-specific and site-specific fragment-ion desorptions are clearly realized, when we tune the photon energy at the core-to-valence resonance regions. The specificity of the ion desorption more clearly shows up following the deep-core excitations, i.e., 1s$$rightarrow$$4p resonant excitations in third-row elements, rather than the shallow-core excitations. Since the main decay channels after the core-level excitation are the Auger transitions, the Auger decay spectra excited by the photons around the core-level thresholds are also presented for some of the adsorbed systems. The mechanism of the observed highly specific ion desorption is discussed on the basis of the photon-energy dependencies of the electron and ion yields and the analysis of the Auger decay spectra.

Journal Articles

Local structure of the silicon implanted in a graphite single crystal

Baba, Yuji; Shimoyama, Iwao; Sekiguchi, Tetsuhiro

Hyomen Kagaku, 23(7), p.417 - 422, 2002/07

no abstracts in English

Journal Articles

Local structures analysis of carbon nitride films by utilizing NEXAFS spectroscopy

Shimoyama, Iwao

Hoshako, 15(1), p.12 - 19, 2002/01

Since the prediction of the possibility that carbon nitride would have super hard property which exceeds that of diamond, considerable efforts to synthesize this material have been employed. However, local structures in carbon nitride films synthesized have not been revealed enough. We study the local structures utilizing synchrotron radiation and revealed the existence of three kinds of local structures in carbon nitride films, i.e., cyanic structure which has C≡N triple bond, pyridine-like structure which has C=N double bond, and graphite-like structure in which carbon atom is substituted by nitrogen atom in graphite network. In this article, I explain the method of near edge X-ray absorption fine structure (NEXAFS) that we used for this study, and from the comparison with X-ray photoelectron spectroscopy (XPS), I comment the availability of NEXAFS to study the local structures in new materials.

Journal Articles

Study of electronic structure of graphite-like carbon nitride

Shimoyama, Iwao; Wu, G.; Sekiguchi, Tetsuhiro; Baba, Yuji

Journal of Electron Spectroscopy and Related Phenomena, 114-116, p.841 - 848, 2001/03

 Times Cited Count:45 Percentile:86.22(Spectroscopy)

no abstracts in English

Journal Articles

Adsorption structure of formic acid on Si(100) studied by surface NEXAFS

Sekiguchi, Hiromi*; Sekiguchi, Tetsuhiro

Surface Science, 433-435, p.549 - 553, 1999/00

 Times Cited Count:24 Percentile:75.92(Chemistry, Physical)

no abstracts in English

Journal Articles

The Orientation of chemisorbed formate using ${it K}$-edge photoabsorption spectroscopy

Sekiguchi, Hiromi*; Sekiguchi, Tetsuhiro

Atomic Collision Research in Japan, No.24, p.116 - 117, 1998/11

no abstracts in English

28 (Records 1-20 displayed on this page)