Thomson, Mark D. ; Zouaghi, Wissem ; Meng, Fanqi ; Wiecha, Matthias M. ; Rabia, Kaneez ; Heinlein, Thorsten ; Hussein, Laith ; Babu, Deepu ; Yadav, Sandeep ; Engstler, Jörg ; Schneider, Jörg J. ; Nicoloso, Norbert ; Rychetský, Ivan ; Kužel, Petr ; 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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