Thomson, Mark D. and Zouaghi, Wissem and Meng, Fanqi and Wiecha, Matthias M. and Rabia, Kaneez and Heinlein, Thorsten and Hussein, Laith and Babu, Deepu and Yadav, Sandeep and Engstler, Jörg and Schneider, Jörg J. and Nicoloso, Norbert and Rychetský, Ivan and Kužel, Petr and Roskos, Hartmut G. (2018):
Dielectric properties of vertically aligned multi-walled carbon nanotubes in the terahertz and mid-infrared range.
In: Journal of Physics D: Applied Physics, 51 (3), pp. 034004. ISSN 0022-3727,
DOI: 10.1088/1361-6463/aa9e42,
[Article]

Abstract

We investigate the broadband dielectric properties of vertically aligned, multi-wall carbon nanotubes (VACNT), over both the terahertz (THz) and mid-infrared spectral ranges. The nominally undoped, metallic VACNT samples are probed at normal incidence, i.e. the response is predominantly due to polarisation perpendicular to the CNT axis. A detailed comparison of various conductivity models and previously reported results is presented for the non-Drude behaviour we observe in the conventional THz range (up to 2.5 THz). Extension to the mid-infrared range reveals an absorption peak at $ \newcommand{\thz}{~{\rm THz}} {\sim}24\thz$ , reminiscent of that observed in single-wall CNT, only there it arises for polarisation parallel to the CNT axis. To account for the observed resonance here, we apply a Bergman-type effective-medium theory, based on first-principles' electromagnetic simulations for the perpendicular polarisation including both the intra- and inter-tube response, which can reproduce the observed spectrum if one assumes a much higher plasma frequency and scattering rate than that reflected in the low-frequency spectra, and proposes an explanation for the non-Drude behaviour at low-frequencies.

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