Strong Anisotropy in the Far‐Infrared Absorption Spectra of Stretch‐Aligned Single‐Walled Carbon Nanotubes

Abstract

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)...

Mechanism of the Far-Infrared Absorption of Carbon-Nanotube Films

“…It exhibits a broad peak centered at a resonance frequency ! res =2 of roughly 4 THz in agreement with previous reports [8][9][10][11][12][13][14]21]. As seen in Fig.…”

“…Second, @! res increases when the mean tube diameter of a NT ensemble decreases and, thus, the mean G increases [2,10,14]. However, Fig.…”

“…2(c) implies that the temperature dependence of electronic transitions with such low energy would be puzzlingly weak. Consequently, alternative mechanisms such as infrared-active phonons [9] and particle plasmons [10] were suggested to cause the broad FIR absorption feature. We exclude the phonon scenario since theory neither predicts broad phonon bands nor do the vibrating carbon atoms produce large electric dipole moments required for an effective absorption of THz radiation [21].…”

“…As a consequence, phonons [9] and, as sketched in Fig. 1(b), particle plasmons [10] were suggested as alternative mechanisms. It is crucial to clarify this point, since the FIR absorption has already been extensively used as a sensitive probe of side chains covalently bound to the NT wall [11] and of the doping level of the tubes [12][13][14].…”

Fundamental optical processes in armchair carbon nanotubes

“…125 In the case of semiconducting nanomaterials, their observed color is due to their size-dependent band gaps, which for armchair SWCNTs, is zero. Additionally, as shown by Hároz et al 57 for armchair-enriched SWCNT material and by Green and Hersam 71 for bulk ν = 0-enriched SWCNT material, optical absorption spectra and hence color is not affected by aggregation size.…”