Department of Electrical and Computer Engineering, Rice University, 6100 Main St., MS-378, Houston, Texas 77005, USA.
Nanoscale. 2013 Feb 21;5(4):1411-39. doi: 10.1039/c2nr32769d.
Single-wall carbon nanotubes provide ideal model one-dimensional (1-D) condensed matter systems in which to address fundamental questions in many-body physics, while, at the same time, they are leading candidates for building blocks in nanoscale optoelectronic circuits. Much attention has been recently paid to their optical properties, arising from 1-D excitons and phonons, which have been revealed via photoluminescence, Raman scattering, and ultrafast optical spectroscopy of semiconducting carbon nanotubes. On the other hand, dynamical properties of metallic nanotubes have been poorly explored, although they are expected to provide a novel setting for the study of electron-hole pairs in the presence of degenerate 1-D electrons. In particular, (n,n)-chirality, or armchair, metallic nanotubes are truly gapless with massless carriers, ideally suited for dynamical studies of Tomonaga-Luttinger liquids. Unfortunately, progress towards such studies has been slowed by the inherent problem of nanotube synthesis whereby both semiconducting and metallic nanotubes are produced. Here, we use post-synthesis separation methods based on density gradient ultracentrifugation and DNA-based ion-exchange chromatography to produce aqueous suspensions strongly enriched in armchair nanotubes. Through resonant Raman spectroscopy of the radial breathing mode phonons, we provide macroscopic and unambiguous evidence that density gradient ultracentrifugation can enrich ensemble samples in armchair nanotubes. Furthermore, using conventional, optical absorption spectroscopy in the near-infrared and visible range, we show that interband absorption in armchair nanotubes is strongly excitonic. Lastly, by examining the G-band mode in Raman spectra, we determine that observation of the broad, lower frequency (G(-)) feature is a result of resonance with non-armchair "metallic" nanotubes. These findings regarding the fundamental optical absorption and scattering processes in metallic carbon nanotubes lay the foundation for further spectroscopic studies to probe many-body physical phenomena in one dimension.
单壁碳纳米管为解决多体物理中的基本问题提供了理想的一维（1-D）凝聚态物质模型，同时也是构建纳米尺度光电电路的首选材料。由于一维激子和声子的存在，单壁碳纳米管的光学性质引起了广泛关注，这些性质已经通过半导体碳纳米管的光致发光、拉曼散射和超快光学光谱学得到了揭示。另一方面，金属纳米管的动力学性质研究较少，尽管预计它们为在简并 1-D 电子存在下研究电子-空穴对提供了新的环境。特别是，（n，n）-手性或扶手椅金属纳米管是真正无带隙的，载流子质量为零，非常适合用于汤姆松-拉廷格液体的动力学研究。不幸的是，由于纳米管合成中固有的问题，即同时生成半导体和金属纳米管，阻碍了此类研究的进展。在这里，我们使用基于密度梯度超速离心和基于 DNA 的离子交换色谱的后合成分离方法，来制备强烈富集扶手椅纳米管的水溶液悬浮液。通过对径向呼吸模声子的共振拉曼光谱研究，我们提供了宏观且明确的证据，表明密度梯度超速离心可以使扶手椅纳米管在样品中富集。此外，我们使用近红外和可见光范围内的常规光吸收光谱，表明扶手椅纳米管的带间吸收是强烈的激子吸收。最后，通过检查拉曼光谱中的 G 带模式，我们确定观察到的宽的低频（G(-)）特征是与非扶手椅“金属”纳米管共振的结果。这些关于金属碳纳米管基本光吸收和散射过程的发现，为进一步的光谱研究奠定了基础，以探索一维中的多体物理现象。
