The optical properties and functionalities of single-walled carbon nanotubes (SWNTs) are currently attracting considerable attention. New techniques such as alignment of carbon nanotubes have been introduced to elucidate their 1D nature. These techniques include magnetic-field alignment, [1] chemical vapor deposition, [2] and mechanical stretching.[3] Indeed, aligned SWNTs have already provided the opportunity to carry out polarization-dependent spectroscopic studies [4][5][6] and experiments on the directionally dependent Aharanov-Bohm effect in magnetic fields. [7,8] On the other hand, lower frequency regions (gigahertz, terahertz, and far infrared) of the electromagnetic spectrum, which are related to the electrodynamic response of metallic and narrow-gap semiconductors, have been less investigated and are less understood. [9][10][11] This is particularly true for the anisotropic spectral response, though it is crucial for understanding the low-energy electronic structure and carrier dynamics in SWNTs, as well as for exploring polarization-sensitive terahertz applications. In this communication, we report the preparation of aligned SWNTs embedded in polyethylene and their optical properties over a broad frequency range (6-33 000 cm -1 ). The oriented films reveal rich, novel properties involving strong anisotropy as well as morphology and doping dependencies. The discussion based on these results has uncovered intriguing aspects on the infrared response of SWNTs as well as possible functionalities in the terahertz region of the electromagnetic spectrum.We have employed two kinds of SWNTs: laser-ablationgrown SWNTs and commercially available high-pressure CO disproportionation (HiPCO) SWNTs, abbreviated hereafter as L-SWNTs and H-SWNTs. The former was purified by a conventional method, [12] whereas the latter was used as purchased without purification. From a Kataura plot, [13] which relates transitions between van Hove singularities and tube diameter, we have extracted average diameters of 1.4 and 0.95 nm, respectively. Among the three techniques for nanotube alignment, the mechanical stretching of SWNT/polymer composites [14] offers the best flexibility, assuming that the matrix is transparent in the region of interest. Here, we have selected polyethylene as the matrix. All of our SWNTs are bundled; no isolation processes were employed to separate individual tubes. The degree of orientation of our samples has been determined by polarized Raman spectroscopy utilizing a He-Ne laser (633 nm) excitation source. Because resonance Raman scattering is detectable only for polarization parallel to the tube axis, this technique is ideal for determining the degree of alignment in SWNT/polymer composites. [15,16] The maximum entropy analysis [17,18] showed that approximately 90 % of SWNTs are within 25°of the stretching direction, indicating a rather high degree of alignment. A similar degree of alignment was reproducibly obtained both for L-and H-SWNTs. Figure 1 displays the polarized absorption spectra of LSWNTs (top)...

A new method for controlling the hole density in single‐walled carbon nanotube field‐effect transistors (SWCNT‐FETs) by solution‐based chemical doping is presented. The use of organic molecules that adsorb onto SWCNTs from solution is investigated. The transfer characteristics of the SWCNT‐FETs exhibit continuous and precise shifts in threshold voltages (see Figure) upon doping with F4TCNQ molecules, even in air.

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Herein, we have studied a structure of SWNT films doped by organic molecules. In this study, we reacted organic molecules with SWNT films by a vapor phase and a liquid phase, respectively. The structure of doped SWNT films was investigated using synchrotron X-ray powder diffraction measurements, and we found the clear difference between the vapor phase reaction (v-doped) and the liquid phase reaction (I-doped). In v-doped SWNT films, organic molecules are predominantly encapsulated inside SWNTs, although molecules adsorbed on the surface of SWNT bundles in l-doped SWNT films.