Observation of Magnetic Edge State and Dangling Bond State on Nanographene in Activated Carbon Fibers

The electronic structure of nanographene in pristine and fluorinated activated carbon fibers (ACFs) have been investigated with near-edge x-ray absorption fine structure (NEXAFS) and compared with magnetic properties we reported on previously. In pristine ACFs in which magnetic properties are governed by non-bonding edge states of the \pi-electron, a pre-peak assigned to the edge state was observed below the conduction electron {\pi}* peak close to the Fermi level in NEXAFS. Via the fluorination of the ACFs, an extra peak, which was assigned to the \sigma-dangling bond state, was observed between the pre-peak of the edge state and the {\pi}* peak in the NEXAFS profile. The intensities of the extra peak correlate closely with the spin concentration created upon fluorination. The combination of the NEXAFS and magnetic measurement results confirms the coexistence of the magnetic edge states of \pi-electrons and dangling bond states of \sigma-electrons on fluorinated nanographene sheets.Comment: 4 figures, to appear in Phys. Rev.

Specialization of Generic Array Accesses After Inlining

We have implemented an optimization that specializes type-generic array accesses after inlining of polymorphic functions in the native-code OCaml compiler. Polymorphic array operations (read and write) in OCaml require runtime type dispatch because of ad hoc memory representations of integer and float arrays. It cannot be removed even after being monomorphized by inlining because the intermediate language is mostly untyped. We therefore extended it with explicit type application like System F (while keeping implicit type abstraction by means of unique identifiers for type variables). Our optimization has achieved up to 21% speed-up of numerical programs

Selective chemical bond breaking characteristically induced by resonant core excitation of ester compounds on a surface

Inner-shell excitation possesses an attractive potential to induce selective chemical bond breaking due to its special localization and atomic selectivity. This paper reviews our recent works about site-selective (or site-specific) photon stimulated desorption (PSD) of core-excited ester compounds, poly(methyl methacrylate) (PMMA) thin films, condensed methyl isobutyrate (MIB) multilayers, methyl ester terminated self-assembled monolayers (SAMs), partially deuterided SAMs and ethyl ester terminated SAMs. Exact selective bond breaking was observed for all molecular systems except for MIB multilayers: COO-R (R = CH3, C2H5) breaking in the O 1s(OR) excitations and CO-OR in the excitation. Such resonant excitations take place within the functional group locally and the initial memory of the excited site and its localization character is effectively conserved during the PSD process even after subsequent Auger decays and ionic dissociation/desorption. Variation of selectivity observed here indicates the importance of the molecular environment and the contribution of the indirect dissociation process due to statistical energy relaxation and x-ray-induced electron stimulated desorption. The obvious polarization dependence measured only for selectively desorbing ions also supports the above characteristics. Neutral fragments desorbing from core-excited PMMA follow the profile of the total electron yield spectrum but not the partial ion yield of corresponding ions. This result indicates that ionic dissociation locally promoted by Coulomb repulsion plays an important role to reveal site-selectivity in chemical reactions induced by core-level excitation

Selective Chemical Bond Breaking Characteristically Induced by Resonant Core Excitation of Ester Compounds on a Surface

Inner-shell excitation possesses an attractive potential to induce selective chemical bond breaking due to its special localization and atomic selectivity. This paper reviews our recent works about site-selective (or site-specific) photon stimulated desorption (PSD) of core-excited ester compounds, poly(methyl methacrylate) (PMMA) thin films, condensed methyl isobutyrate (MIB) multilayers, methyl ester terminated self-assembled monolayers (SAMs), partially deuterided SAMs and ethyl ester terminated SAMs. Exact selective bond breaking was observed for all molecular systems except for MIB multilayers: COO-R (R = CH3, C2H5) breaking in the O 1s(OR) excitations and CO-OR in the excitation. Such resonant excitations take place within the functional group locally and the initial memory of the excited site and its localization character is effectively conserved during the PSD process even after subsequent Auger decays and ionic dissociation/desorption. Variation of selectivity observed here indicates the importance of the molecular environment and the contribution of the indirect dissociation process due to statistical energy relaxation and x-ray-induced electron stimulated desorption. The obvious polarization dependence measured only for selectively desorbing ions also supports the above characteristics. Neutral fragments desorbing from core-excited PMMA follow the profile of the total electron yield spectrum but not the partial ion yield of corresponding ions. This result indicates that ionic dissociation locally promoted by Coulomb repulsion plays an important role to reveal site-selectivity in chemical reactions induced by core-level excitation

Observation of magnetic edge state in graphene nanoribbons

The electronic structure and spin magnetism for few-layer-graphene nanoribbons synthesized by chemical vapor deposition have been investigated using near-edge x-ray absorption fine structure (NEXAFS) and electron-spin resonance (ESR). For the pristine sample, a prepeak was observed below the pi* peak close to the Fermi level in NEXAFS, indicating the presence of additional electronic states close to the Fermi level. The intensity of this prepeak decreased with increasing annealing temperature and disappeared after annealing above 1500 degrees C. The ESR spectra, which proved the presence of localized spins, tracked the annealing-temperature-dependent behavior of the prepeak with fidelity. The NEXAFS and ESR results jointly confirm the existence of a magnetic edge state that originates from open nanographene edges. The disappearance of the edge state after annealing at higher temperatures is explained by the decrease in the population of open edges owing to loop formation of adjacent graphene edges