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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

Observation of oriented organic semiconductor using Photo-Electron Emission Microscope (PEEM) with polarized synchrotron

Sekiguchi, Tetsuhiro; Baba, Yuji; Hirao, Norie; Honda, Mitsunori; Izumi, Toshinori; Ikeura, Hiromi*

Molecular Crystals and Liquid Crystals, 622(1), p.44 - 49, 2015/12

BB2014-1632.pdf:0.71MB

 Times Cited Count:0 Percentile:0.01(Chemistry, Multidisciplinary)

The molecular orientation is one of the important factors for controlling various properties in organic semiconductor materials. Films are usually heterogeneous. Thus they exist as a mixture of microscopic domains which have a variety of orientation directions. Therefore, it is essential to observe selectively microscopic domains with different orientation direction. In this work, we have developed the photoelectron emission microscopy (PEEM) system combined with the linearly polarized vacuum ultraviolet (VUV) light or synchrotron radiation (SR) X-rays. PEEM images (FOV = ca.50 micro m) for poly(3-hexylthiophene), P3HT thin films were observed under the UV irradiation with various polarization angles, including in-plain and out-of-plain polarization. Morphologies at some bright parts are different each other. The resultant observation suggests that it enables us to distinguish oriented micro-domains with specific directions of polymer chain axis from other amorphous parts.

Journal Articles

Electronic properties of 6,13-Bis(triisopropylsilylethynyl)pentacene probed by inner-shell spectroscopy

Ikeura, Hiromi*; Sekiguchi, Tetsuhiro

Molecular Crystals and Liquid Crystals, 622(1), p.50 - 54, 2015/12

BB2014-1633.pdf:0.39MB

 Times Cited Count:0 Percentile:0.01(Chemistry, Multidisciplinary)

Organic electrically conducting $$pi$$-stacked molecules are widely regarded as promising materials for future application of nanoelectronics. Direct measurements of electronic structures of unoccupied states in organic semiconductors lead to better understanding of mechanism of electron conduction. For probing unoccupied partial density of states (DOS), X-ray absorption spectroscopy (XAS) is commonly used, where selective excitation of the 1s core electron to the unoccupied conduction band is possible. However, XAS cannot distinguish localized and delocalized features without comparing any theoretical approach. In this work, the core-hole-clock method in resonant Auger spectroscopy (RAS) has been applied to probe electron delocalization through the empty conduction band in the attosecond domain for the purpose of exploring electronic materials with high-speed electron transport. We will discuss some organic molecules such as pentacenes.

Journal Articles

Investigation of unoccupied electronic states near the fermi level of polysilane using resonant Auger spectroscopy

Ogawa, Hiroshi*; Ikeura, Hiromi*; Sekiguchi, Tetsuhiro

Molecular Crystals and Liquid Crystals, 622(1), p.164 - 169, 2015/11

 Times Cited Count:0 Percentile:0.01(Chemistry, Multidisciplinary)

Unoccupied electronic states near the Fermi level of poly(dimethylsilane) were probed using Si 1s X-ray absorption spectroscopy (XAS) and Si KL$$_{2,3}$$L$$_{2,3}$$ resonant Auger spectroscopy (RAS). The measured resonance peaks of XAS spectra near Si K-edge have been assigned in comparison with the discrete variational (DV)-X$$alpha$$ molecular orbital calculations. The rapid delocalization of Si 1s core-excited electron through the empty conduction band was observed along the polymer chain with the energy dependent RAS measurement, and the electron delocalization time was estimated based on the core-hole clock method.

Journal Articles

Molecular ordering effect of regioregular poly(3-hexylthiophene) using sulfur K-edge X-ray absorption spectroscopy

Ikeura, Hiromi*; Sekiguchi, Tetsuhiro

Japanese Journal of Applied Physics, 53(2S), p.02BB07_1 - 02BB07_4, 2014/02

 Times Cited Count:11 Percentile:50.47(Physics, Applied)

The electronic structure of the unoccupied conduction band of regioregular poly(3-hexylthiophene) (RR-P3HT) was investigated by X-ray absorption spectroscopy (XAS) near the sulfur K-edge. Angle-dependent XAS studies revealed that polymer chains in films are well aligned and oriented edge-on with respect to the Si substrate. It was clearly observed that the bottom of the conduction band near the Fermi level in a $$pi$$-$$pi$$ stacking film is lower (0.3 eV) than that in a powder. It was demonstrated that $$pi$$-$$pi$$ stacking interactions improve the power conversion efficiency through the reduction of the lowest unoccupied molecular orbital (LUMO) energy level.

Journal Articles

Orientation effect of organic semiconducting polymer revealed using Photo-Electron Emission Microscope (PEEM)

Sekiguchi, Tetsuhiro; Baba, Yuji; Shimoyama, Iwao; Hirao, Norie; Honda, Mitsunori; Izumi, Toshinori; Ikeura, Hiromi*

Photon Factory Activity Report 2013, Part B, P. 546, 2014/00

The molecular orientation is one of the important factors for controlling various properties in organic semiconductor materials. Films are usually heterogeneous. Thus they exist as a mixture of microscopic domains which have a variety of orientation directions. Therefore, it is essential to observe selectively microscopic domains with different orientation direction. In this work, we have developed the photoelectron emission microscope (PEEM) system combined with the linearly polarized vacuum ultraviolet (VUV) light or synchrotron radiation (SR) X-rays. PEEM images for poly(3-hexylthiophene), P3HT thin films were observed under synchrotron X-ray irradiation with linearly polarization. In conclusion, it was found that PEEM with polarized synchrotron can be a powerful tool that gives information of molecular orientation in nano-meter scale.

Journal Articles

Molecular orientation of pentacene derivative

Ikeura, Hiromi*; Sekiguchi, Tetsuhiro

Photon Factory Activity Report 2013, Part B, P. 518, 2014/00

Organic electrically conducting $$pi$$-stacked small molecules are widely regarded as promising materials for future application of low-cost and flexible nanoelectronics. Pentacene is one of the most promising organic semiconductors because of its excellent device performance. Direct measurements of electronic structures of unoccupied states of organic semiconductors lead to better understanding of mechanism of electron conduction. For probing unoccupied partial density of states (DOS), X-ray absorption spectroscopy (XAS) is commonly used, where selective excitation of the 1s core electron to the unoccupied conduction band is possible. The molecular orientation of pentacene derivative has been investigated by angle dependent XAS measurements. Electronic states were calculated by DVX$$alpha$$ method.

Journal Articles

Electronic structure of regioregular poly(3-hexylthiophene)

Ikeura, Hiromi*; Sekiguchi, Tetsuhiro

Photon Factory Activity Report 2012, Part B, P. 118, 2013/00

We have investigated the electronic property of RR-P3HT (regioregular-poly(3-hexylthiophene)) films with $$pi$$-$$pi$$ stacking using sulfur K-edge X-ray absorption fine structure (XAFS) spectroscopy. It was observed that the LUMO of $$pi$$-$$pi$$ stacking film is lower in energy by 0.3 eV than that of the polycrystalline powder, although these compounds are the same origin. This shift was explained that the interaction between the two molecules in co-facial leads to splitting of the LUMO level to LUMO and LUMO+1, resulting in reduced LUMO energy.

Journal Articles

Molecular orientation of regioregular poly(3-hexylthiophene) film

Ikeura, Hiromi*; Sekiguchi, Tetsuhiro

Photon Factory Activity Report 2011, Part B, P. 96, 2012/00

The regioregularity (2head-to-5'tail regularity) for organic semiconductor polymer backbone plays a dominant role in controlling molecular orientation, thus strongly influences the performance of organic solar cells. In the present work, we have investigated the orientation structure of donor regioregular poly(3-hexylthiophene) (P3HT) films using angle-resolved near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy. The films on silicon substrates were prepared by solution spin cast method. The lowest unoccupied molecular orbital (LUMO) levels were obtained from the edge energies of S 1s inner-shell excitation, from which energy conversion efficiencies for solar cells were estimated. The results of them were compared with those which were measured for amorphous P3HT films.

Journal Articles

Femto- and attosecond electron dynamics in 5'-Guanosine monophosphate interface as probed by resonant Auger spectroscopy

Ikeura, Hiromi*; Sekiguchi, Tetsuhiro

Surface and Interface Analysis, 42(6-7), p.1085 - 1088, 2010/06

AA2012-0699.pdf:0.38MB

 Times Cited Count:5 Percentile:12.46(Chemistry, Physical)

To help in our understanding of the electron transport properties of deoxyribonucleic acid (DNA), it is useful to study the interfacial interaction between each nucleotide building block of DNA. Electron transfer properties between phosphate and groups in microcrystalline 5'-Guanosine monophosphate (GMP) were probed using energy-dependent resonant Auger spectroscopy. Results show that the phosphate group of GMP forms an extended state along the phosphate directions similar to the case of DNA phosphate backbones. Electron delocalization time of the phosphate groups in GMP is faster than those of DNA, as estimated using the core-hole clock method. This suggests that interface between GMP phosphate groups would have the lowest tunneling barrier among DNA related systems. Although the $$pi$$-$$pi$$ coupling of nucleobasis is only recognized as charge-transfer pathway in DNA and related systems, another conduction pathway through phosphate groups would be possible.

Journal Articles

Enhancement effect in photon-stimulated ion desorption for benzene adsorbed on silicon surfaces observed using angle-dependent technique

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

Applied Surface Science, 254(23), p.7812 - 7816, 2008/09

BB2008-4109.pdf:0.79MB

 Times Cited Count:0 Percentile:0.01(Chemistry, Physical)

We have constructed a novel rotatable time-of-flight (TOF) mass detector for desorbed fragment-ions produced by irradiation of synchrotron radiation (SR). This makes the best use of advantages in synchrotron, including variable X-ray energy, linear polarization, and pulsed capability. The detector allows us to measure excitation-energy dependences of desorbing ion yields from solid surfaces in a variety of incident angles of X-ray beams. We propose that this angle-dependent technique provides insights into orientation of chemical bonds lying at the top surface. The present paper reports our recent findings that the mass-patterns and yields of desorbed ions drastically depend on the incident angles less than 10 degrees. The results suggest that direct desorption mechanism prevails in extremely small angles and indirect desorption mechanism induced through collision excitation by secondary electrons is suppressed.

Journal Articles

Unoccupied electronic states in polythiophene as probed by XAS and RAS

Ikeura, Hiromi*; Sekiguchi, Tetsuhiro

Surface and Interface Analysis, 40(3-4), p.673 - 675, 2008/03

AA2008-0169.pdf:0.26MB

 Times Cited Count:14 Percentile:37.21(Chemistry, Physical)

Polythiophene is one of the typical conducting polymers. Unoccupied electronic states near the Fermi level of polythiophene polycrystalline powder were probed by S 1s X-ray absorption spectroscopy (XAS) and S$$KL_{2,3}$$, $$L_{2,3}$$ resonant Auger spectroscopy (RAS). Its overall XAS spectral shape is similar to those of 2,-2'-bithiophene and alpha-terthiophene polycrystalline powder. Any hybridization of the energy levels of thiophene unit was not observed in the $$pi^{*}$$-LUMO (the lowest unoccupied molecular orbital). It is found that the position of $$sigma^{*}$$(C-C) resonance shifts to lower energy as increasing thiophene repeating units. This finding reflects the different bond lengths of C-C. Although $$pi^{*}$$ and $$sigma^{*}$$(S-C) states in XAS are not resolved due to their overlap of electronic transitions, those were clearly separated by plotting spectator Auger yields as a function of excitation energy. If the Auger spectator shifts of the corresponding states are different each other, such unresolved states in XAS can be distinguished by RAS technique. RAS also indicates that the $$pi^{*}$$-LUMO of polythiophene has localized character. The present study demonstrated that excitation energy-dependent RAS can overcome the limitation of conventional XAS. The obtained results suggest that polythiophene is poorly conjugated, where the $$pi^{*}$$ delocalization is interrupted by the conformational disorder.

Journal Articles

Attosecond electron delocalization in the conduction band through the phosphate backbone of Genomic DNA

Ikeura, Hiromi*; Sekiguchi, Tetsuhiro

Physical Review Letters, 99(22), p.228102_1 - 228102_4, 2007/11

 Times Cited Count:34 Percentile:80.28(Physics, Multidisciplinary)

Partial density of states in the empty conduction band of the phosphate backbone sites in DNA was probed using energy-dependent resonant Auger spectroscopy. Results show that genomic DNA with periodic backbones exhibits an extended state despite separation of each phosphate group by an insulating sugar group. In antisense DNA with an aperiodic backbone, the equivalent state is localized. Remarkably rapid electron delocalization occurs at ca. 740 attoseconds for wet DNA, as estimated using the core-hole clock method. Such delocalization is comparable to the Fermi velocity of carbon nanotubes.

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

Characterization and degradation of ZEP520 resist film by TOF-PSID and NEXAFS

Ikeura, Hiromi*; Sekiguchi, Tetsuhiro; Koike, Masaki*

Journal of Electron Spectroscopy and Related Phenomena, 144-147, p.453 - 455, 2005/06

 Times Cited Count:15 Percentile:59.09(Spectroscopy)

no abstracts in English

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

Inner shell excitation and dissociation of condensed formamide

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

Applied Surface Science, 169-170, p.282 - 286, 2001/01

 Times Cited Count:8 Percentile:45.98(Chemistry, Physical)

Carbon and nitrogen K-shell excitation and dissociation of condensed formamide at 96 K were studied by near edge X-ray absorption fine structure (NEXAFS) recorded by total electron yield (TEY), total ion yield (TIY) and photon-stimulated desorption (PSD) yield of H+ measurements. It was found that electronic transitions from the C1s or the N1s to the $$sigma$$*(C-H) and/or (N-H) enhanced ion yield of H+ from the C-H and/or N-H functional group. This selective dissociation indicates that the corresponding unoccupied molecular orbital has an antibonding character of the C-H and/or N-H. To investigate the molecular orientation of condensed formamide, incidence-angle-dependent TEY-NEXAFS spectra were measured. The CNO plane of the adsorbed formamide molecule is determined to be tilted away from the surface by an averaged angle of about 42 degrees.

Journal Articles

Kinetic energy released distributions of H$$^{+}$$ ions desorbed from H$$_{2}$$O/Si(100) adsorption system by O 1s excitation

Sekiguchi, Tetsuhiro; Ikeura, Hiromi*; Obi, Kinichi*; Tanaka, Kenichiro*

Photon Factory Activity Report, P. 128, 1994/00

no abstracts in English

Oral presentation

Localized or delocalized unoccupied electronic states in polymers

Ikeura, Hiromi*; Sekiguchi, Tetsuhiro

no journal, , 

X-ray absorption spectroscopy (XAS) is commonly used as probing unoccupied electronic states. However, XAS can not distinguish localized and delocalized electronic states. In metal system, plotting the normal Auger yields (in Resonant Auger spectroscopy (RAS)) around the XAS region exhibit unoccupied conduction band state near the Fermi level (EF). In polymer system, RAS spectra can be interpreted based on two competing decay channels: that is, core-hole decay and core-excited resonant electron delocalization. If the resonantly excited electron remains long enough to be localized in the vicinity of a core-hole site during the core-hole decay, the decay process results in final states of two holes with one electron (2h1e), known as "spectator Auger". In contrast, if it delocalizes to the conduction band prior to the core-hole decay, the decay process results in final states of two holes (2h), known as "normal Auger". Thus, If polymer has a delocalized conduction band, an excited electron can delocalize along the polymer chain before core-hole decay. Consequently, the normal Auger yields exhibit the conduction band structure, as similar to the metal system. We show that the localized or delocalized band structure can be probed by the normal Auger yields.

Oral presentation

Adsorption, photo-fragmentation, and ion-desorption pathways of organic molecules at semiconductor surfaces using synchrotron radiation techniques

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

no journal, , 

The chemistry of organic molecules on semiconductor surfaces has attracted much attention due to the perspective of the molecular wires, self-assembled monomolecular layers (SAM), and semiconductor-organic hybrid formation such as SiC. We have investigated the adsorption interaction, photo-dissociation, and ion-desorption of multifunctional organic molecules such as acetone [(CH3)2C=O] adsorbed on Si substrates. We used synchrotron radiation techniques including polarization-angular dependences of near-edge X-ray absorption fine structure (NEXAFS) and photon-stimulated ion-desorption (PSID). We conclude PSID and NEXAFS techniques are useful tools to investigate chemical states such as configuration, orientation, and charge-transfer interaction of adsorbed molecules at surfaces.

30 (Records 1-20 displayed on this page)