Yutaka Kaburagi

Observation of zigzag and armchair edges of graphite using scanning tunneling microscopy and spectroscopy

The presence of structure-dependent edge states of graphite is revealed by both ambient and ultrahigh-vacuum (UHV) scanning tunneling microscopy and scanning tunneling spectroscopy observations. On a hydrogenated zigzag (armchair) edge, bright spots are (are not) observed together with a

Experimental evidence of a single nano-graphene

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Graphite films prepared from carbonized polyimide films

superlattice near the Fermi level (

Carbonization and graphitization of BPDA/PDA polyimide films: effect of structure of polyimide precursor

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Microtexture and magnetoresistance of glass-like carbons

for a peak of the local density of states) under UHV, demonstrating that a zigzag edge is responsible for the edge states, although there is no appreciable difference between as-prepared zigzag and armchair edges in air. Even in the hydrogenated armchair edge, however, bright spots are observed at defect points, at which partial zigzag edges are created in the armchair edge.

Growth of iron clusters and change of magnetic property with carbonization of aromatic polyimide film containing iron complex

A single nano-sized graphene sheet is prepared by a combination of electrophoretic deposition (EPD) and heat-treatment of diamond nano-particles on a highly oriented pyrolytic graphite (HOPG) substrate. Heat-treatment at 1600 °C converts diamond nano-particles to single nano-graphenes, where the mean in-plane size and the inter-layer distance from the substrate are estimated at 10–15 and 0.35–0.37 nm, respectively. The considerably large inter-layer distance compared to bulk graphite suggests a large reduction of inter-layer interaction, although a nano-graphene is placed epitaxially on the substrate. The isolated single nano-graphene provides an important model of nano-sized π-electron system, for which recent theory predicts unconventional electronic structure of edge-inherited non-bonding state.

Highly crystallized graphite films prepared by high-temperature heat treatment from carbonized aromatic polyimide films

Abstract Graphitizability of various carbonized polyimide films heat treated at 2950°C was studied by X-ray and magnetoresistance measurements and SEM observation. The films used were commercially available polyimide films, Kapton and Upilex, and two kinds of laboratory-prepared polyimide films. Results obtained show experimental evidence that there are three conditions involved in obtaining highly graphitizable carbon films; 1. (1) flatness of original organic molecules, 2. (2) high degree of their orientation and 3. (3) simple release of non-carbon atoms during carbonization.

Carbonization and graphitization of Kapton-type polyimide films prepared from polyamide alkyl ester

Poly(amide acid) was prepared from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and p-phenylenediamine (PDA) and was then reacted with NaH and various kinds of alkyl iodides to transform it into its alkyl esters. The cast films were imidized as fixed on glass substrates to give polyimide films. The tensile modulus of polyimide films prepared from the methyl ester precursor was lower than that from poly(amide acid), but it increased with increasing size of the alkyl group. The polyimide films were carbonized by heating to 900°C, and then further heated to 2800°C for graphitization. Their degrees of graphitization and orientation of graphite crystallites as a function of weight loss at imidization were studied by measurements of X-ray diffraction at room temperature and magnetoresistance at liquid nitrogen temperature. Both measurements clearly indicate that the graphitized films prepared from poly(amide ester) have high degrees of graphitization. It was also made clear that the orientation of the graphitized films prepared from poly(amide ester) with small alkyl groups is lower than that from poly(amide acid), but the orientation increased with increase in the size of the alkyl group.

Carbonization and graphitization of Kapton-type polyimide film having boron-bearing functional groups

Abstract Microtextures of glass-like carbons commercially available were investigated through scanning electron microscopy and measurement of their magnetoresistance. A field-emission electron gun type SEM, which is characterized by a specimen chamber equipped in its objective lens, was operated with the acceleration voltage of 2 kV and high fidelity observations were thereby achieved with a resolving power of 2 nm. All the specimens exhibit granular microtextures with the mean grain diameter spanning 6–13 nm. The mean grain diameter correlates to the mean crystallite size La(110) determined by the X-ray powder diffractometry. The density of each sample calculated by using the mean diameter and the X-ray parameters reproduces well its catalog value. Negative magnetoresistance was observed just for the specimens whose average grain sizes are not smaller than 7.3 nm. The crystallite size La(110) of the specimen whose grain size shows the minimum value for appearance of negative magnetoresistance is estimated to be 2.7 nm. The absolute value of negative magnetoresistance under a constant magnetic field increases with increasing the average grain size.

Carbonization and graphitization of polyimide films: Effect of size of leaving group at imidization

Abstract Carbonization process of a polyimide film containing iron of 0.6 at% (Fe-PIF) was investigated in detail by the measurements of XRD and mass magnetization. Because of the existence of fine iron particles as a catalyst, the Fe-PIF films heated in a temperature range from 700 to 1200°C showed turbostratic structure having interlayer spacing d 002 of about 0.343–0.344 nm and mean crystallite size L a ranging from 3.6 to 6.0 nm. The formation of α-Fe particles occurred mainly during carbonization process after pyrolysis of Fe-PIF. Mean crystallite size L 110 of the α-Fe particles was evaluated to be 16 nm for the film heated at 800°C and 25 nm at 1200°C. Magnitudes of the magnetization for Fe-PIF and the heat-treated films were independent of magnetic field direction applied, indicating random orientation of the crystallographic directions of iron particles. Films heated at 800°C and below were superparamagnetic (SP), while those at 900°C and above ferromagnetic (FM). However, all the films were found to have both magnetic components, SP and FM ones; 2.55 wt% (0.58 at%) and 0.0007 wt% (0.00016 at%) for Fe-PIF and 6.56 wt% (1.48 at%) and very small, but still present, for the 1200°C treated Fe-PIF, respectively. Irreversible behavior was observed between zero field cooled magnetization and field cooled for the heat-treated films, but not for original Fe-PIF because of fine clusters of iron. The irreversible effect are attributed to freezing of spin directions of iron particles at low temperatures.