Positron annihilation in carbon nanotubes studied by coincidence Doppler broadening spectroscopy
Positron annihilation in carbon nanotubes studied by coincidence Doppler broadening spectroscopy
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DOI：
10.12693/aphyspola.113.1479
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发表时间：
2008-05-01
影响因子：
0.7
通讯作者：
Sano, M.
中科院分区：
文献类型：
--
作者：
Murakami, H.;Sato, K.;Sano, M.
为了确定正电子湮没的位置，测量了高取向热解石墨晶体、石墨粉、多壁碳纳米管（MNTs）和杯状碳纳米管（CNTs）的符合多普勒展宽谱。石墨粉末的光谱相对于高取向热解石墨（HOPG）的光谱标准化，在7 × 10（-3）~ 13 × 10（-3）m（c）c的动量区域内几乎是平坦的，具有接近1的比率。平坦的光谱表明，正电子注入到石墨粉中湮灭在堆叠的石墨片的层间空间中，以相同的方式在高取向的热解石墨湮灭在散装。碳纳米管和碳纳米管的符合多普勒增宽谱与高取向热解石墨的符合多普勒增宽谱有很大的不同，表明注入碳纳米管和碳纳米管的正电子不是在体相湮灭，而是在表面湮灭。多壁碳纳米管的正电子寿命谱被分析为一个单一的组件，由于表面捕获的正电子，而碳纳米管的正电子寿命谱被分解为三个组件归因于对位正电子素表面捕获的正电子和邻正电子素。碳纳米管的符合多普勒增宽谱与碳纳米管的符合多普勒增宽谱之间的差异被解释为碳纳米管的锯齿形表面上的正电子湮没，这是由石墨片和石墨边缘平面。
In order to assign the sites of positron annihilation, coincidence Doppler broadening spectra were measured for a highly oriented pyrolytic graphite crystal, graphite powder, multi-walled carbon nanotubes (MNTs) and cup-stacked carbon nanotubes (CNTs). The spectrum for graphite powder normalized to that for highly oriented pyrolytic graphite (HOPG) is almost flat in the momentum region from 7 x 10(-3) to 13 x 10(-3) m(c)c, having a ratio close to unity. The flat spectrum demonstrates that positrons injected into graphite powder annihilate in the interlayer spaces of piled graphite sheets, in the same manner as positrons in highly oriented pyrolytic graphite annihilate in the bulk. The coincidence Doppler broadening spectra for MNTs and CNTs are quite different from that for highly oriented pyrolytic graphite, which indicates that positrons injected into MNTs and CYTs annihilate not in the bulk, but on surface. The positron lifetime spectrum for multi-walled carbon nanotubes is analyzed in terms of a single component due to surface-trapped positrons, while that for CNTs is decomposed into three components attributable to para-positronium surface-trapped positrons and ortho-positronium. The difference between the coincidence Doppler broadening spectrum for CNTs and that for MNTs is explained in terms of positron annihilation on zigzag surfaces of CNTs which are composed of both graphite-sheet and graphite-edge planes.