Diameter-dependent shielding effectiveness and terahertz conductivity of multiwalled carbon nanotubes

In the present contribution we have experimentally demonstrated the diameter dependence of terahertz (THz) shielding and THz conductivity of multiwalled carbon nanotubes (MWNTs) and performed detailed theoretical analysis to extract the mechanism of shielding at different MWNT diameters. Self-standing films of three different types of MWNT having the same average length but different outer tube diameter (namely, MWNT_7, MWNT_25, and MWNT_40 nm) are prepared by the vacuum filtration technique. The shielding effectiveness (SE) of these films in the frequency range of 0.4–2.2 THz is measured at room temperature, and the results are analyzed using a theoretical model. Shielding due to absorption (SEA) turns out to be the dominant shielding mechanism for the MWNT_7 and MWNT_25 nm films, while the contribution of shielding due to reflection (SER) dominates for the MWNT_40 nm films in the smaller frequency region (<0.8  THz). Considering the films as a composite of MWNTs and air gaps, we have modeled the dielectric properties of the films using a combination of Maxwell–Garnett effective medium theory and the Drude–Lorentz model. THz conductivity is found to be increasing with increasing MWNT diameter due to the increasing number of Drude-like free electrons. No systematic dependence of the THz conductivity peak (TCP) frequency has been observed on the diameter of the tubes, which negates the idea of a curvature-induced bandgap as the sole origin of the much debated TCP in carbon nanotubes. Our results reveal intriguing aspects on THz response of MWNT films as a function of MWNT diameter.