Chemical state analysis of carbon alloys using soft x-ray synchrotron radiation

为了开发先进的功能碳材料，从电子/分子结构的角度了解其功能的起源是很重要的。因此，我们用软x射线同步辐射研究了它们的电子结构和化学态。在本研究中，我们实现了(1)各种基本碳化合物的X射线光谱数据库的建立作为基础研究，(2)各种工业碳材料的化学状态分析作为应用研究。(1)软X射线数据库：利用同步辐射测量了各种多环芳烃(PAH)的软X射线发射光谱。从它们的光谱特征来看，峰特征与分子大小之间存在新的关系。这表明，从软X射线发射光谱可以很容易地估计纳米石墨的纳米尺寸或加氢比例。这为评价纳米石墨炭材料…的纳米尺寸提供了一种新的方法工业碳材料的化学状态分析：利用同步辐射的高分辨率软X射线光谱对日本烟熏屋顶上的碳膜进行了表征。通过与参考碳化合物的软X射线发射光谱和吸收光谱的比较，确定了川原表面的碳膜主要由类碳黑的sp2碳原子组成，表面也含有类聚乙烯的sp3碳原子。出射/入射角与X射线发射/吸收光谱的关系表明，类碳黑sp2碳原子部分形成了平行于粘土基面的层状结构，碳膜的取向度约为高取向热解石墨的50%。Kawa上碳膜的微结构是指在X射线驻波条件下，Ru/B_4C多层膜的BK区和CK区，一半的类碳黑的sp2碳原子形成平行于粘土基面的层状结构的碳原子，其余的碳原子形成的无规层状团簇刚性地连接在一起，从而达到无损化学键状态分析的目的。Ru/B_4C多层膜样品被单色波荡器辐射选择性地激发，在360 eV附近的Bragg反射条件和能量足够高到BK和CK阈值的关闭条件下被激发。布拉格反射条件下和非布拉格反射条件下激发的光谱特征差随布拉格反射条件附近激发能量的变化而对称变化。这种对称性的光谱特征表明了Ru/B4C多层膜中过渡层和块体层之间的化学键合态的差异。用软X射线发射和同步辐射吸收光谱对掺硼金刚石中半导体态和金属态之间的电子结构转变进行了元素选择性的观察。对于轻掺B的金刚石，价带的B2P态密度(DOS)增强，费米能级附近呈现陡峭的边缘特征，在导带的B2P和C2P-DOS中都明显观察到了半导体特有的局域受主能级。对于重掺B的金刚石，局域受主能级发展成扩展的能级，产生的新的能级形成与价带重叠的扩展导带结构。因此，金属能带结构实际上是由重掺硼形成的。软X射线发射和吸收光谱观察到的这些价带和导带结构解释了掺B金刚石的电学性质。较少

For the development of advanced functional carbon materials, it is important to understand the origin of their functions from the view point of electronic/molecular structures. We have therefore investigated their electronic structure and chemical-state using soft x-ray synchrotron radiation. In this study, we have achieved (1) the data base formation of x-ray spectra of various fundamental carbon compounds as basic research, and (2) chemical-state analysis of various industrial carbon materials as applied research.(1) Soft x-ray database :Soft x-ray emission spectra of various polycyclic aromatic hydrocarbons (PAH) were measured using synchrotron radiation. From their spectral features, the new relation between the peak features and the molecular size. This indicates that nano-size or hydrogenation ratio of nano-graphite can be easily estimated from the soft x-ray emission spectra. This can therefore be proposed as a new method to evaluate the nano-size of nano-graphitic carbon materi … More als.(2) Chemical state analysis of industrial carbon materials :Carbon films on the Japanese smoked roof the "Ibushi-Kawara" were characterized by high-resolution soft X-ray spectroscopy using synchrotron radiation. By comparing the soft x-ray emission and absorption spectra of Kawara with the reference carbon compounds, it was determined that the carbon films on Kawara consist of mostly carbon-black-like sp_2 carbon atoms and that the surface also contains polyethylene-like sp_3 carbon atoms. The take-off/incident-angle dependence on the x-ray emission/absorption spectra of Kawara shows that the carbon-black-like sp_2 carbon atoms partially form a layer structure oriented parallel to the basal clay plane, and that the degree of orientation of the carbon films is estimated to be 50% that of highly oriented pyrolytic graphite (HOPG). The microstructure of the carbon films on Kawara is one in which half of the carbon-black-like sp_2 carbon atoms form layer-structured clusters parallel to the basal clay plane and the rest of the carbon atoms form random-structured clusters which rigidly connect the layer-structured clustersSoft x-ray emission spectra in the BK and CK regions of Ru/B_4C multilayers under the x-ray standing-wave conditions, aiming at nondestructive chemical-bonding-state analysis of the interlayers. The Ru/B_4C multilayer samples were selectively excited by the monochromatized undulator radiation with on the Bragg reflection condition around 360 eV and off- conditions which were sufficiently higher energy to BK and CK thresholds. Spectral feature differences excited between on-Bragg-reflection-condition and on the off-conditions were symmetrically changed against the excitation energy around the Bragg reflection condition. This symmetrical spectral feature should indicate the chemical-bonding-state difference between the interlayers and bulk layers in the Ru/B_4C multilayers.The electronic structure transition between the semiconducting and metallic states in boron (B)-doped diamonds was element-selectively observed by soft x-ray emission and absorption spectroscopy using synchrotron radiation. For lightly B-doped diamonds, the B2p-density of states (DOS) in the valence band were enhanced with a steep-edge-feature near the Fermi level, and localized acceptor levels, which is characteristic of semiconductors, were clearly observed both in B2p- and C2p-DOS in the conduction bands. For heavily B-doped diamonds, the localized acceptor levels developed into extended energy levels and the new energy levels generated formed an extended conduction band structure that overlapped with the valence band. Thus, the metallic energy band structure is actually formed by heavy boron doping. These valence and conduction band structures observed by soft x-ray emission and absorption spectroscopy accounted for the electrical properties of B-doped diamonds. Less