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Optical and vibrational properties of toroidal carbon nanotubes

机译：环形碳纳米管的光学和振动特性

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Toroidal carbon nanotubes (TCNTs), which have been evaluated for their potential applications in terahertz communication systems, provide a challenge of some magnitude from a purely scientific perspective. A design approach to TCNTs, as well as a classification scheme, is presented based on the definition of the six hollow sections that comprise the TCNT, slicing each of them to produce a (possibly creased) planar entity, and projecting that entity onto a graphene lattice. As a consequence of this folding approach, it is necessary to introduce five- and seven-membered rings as defect sites to allow the fusing together of the six segments into final symmetric TCNTs. This analysis permits the definition of a number of TCNT geometry families containing from 108 carbons up to much larger entities. Based on density functional theory (DFT) calculations, the energies of these structural candidates have been investigated and compared with [60]fullerene. The structures with the larger tube diameters are computed to be more stable than C_(60), whereas the smaller diameter ones are less stable, but may still be within synthetic reach. Computational studies reveal that, on account of the stiffness of the structures, the vibrational frequencies of characteristic low-frequency modes decrease more slowly with increasing ring diameter than do the lowest optical excitation energies. It was found that this particular trend is true for the "breathing mode" vibrations when the diameter of the tubes is small, but not for more flexible toroidal nanotubes with larger diameters.
机译：已评估了其在太赫兹通信系统中的潜在应用的环形碳纳米管（TCNT），从纯粹的科学角度来看，提出了一定程度的挑战。基于组成TCNT的六个中空部分的定义，提出了TCNT的设计方法以及分类方案，将每个中空部分切成一个（可能是折痕的）平面实体，并将该实体投影到石墨烯上格子。作为这种折叠方法的结果，有必要引入五元和七元环作为缺陷位点，以允许将六个片段融合在一起成为最终的对称TCNT。该分析允许定义多个TCNT几何族，这些族包含从108个碳到更大的实体。基于密度泛函理论（DFT）的计算，已经研究了这些结构候选物的能量并将其与[60]富勒烯进行了比较。计算得出，具有较大管直径的结构比C_（60）更稳定，而具有较小管直径的结构则较不稳定，但可能仍处于合成范围之内。计算研究表明，由于结构的刚度，特征低频模式的振动频率随环直径的增加而下降的速度比最低的光学激发能下降得更慢。已经发现，当管的直径较小时，这种特殊趋势对于“呼吸模式”振动是正确的，但是对于较大直径的更柔性的环形纳米管却不是这样。

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