M. V. Shuba

Theory of multiwall carbon nanotubes as waveguides and antennas in the infrared and the visible regimes

The propagation of azimuthally symmetric guided waves in multiwalled carbon nanotubes (MWCNTs) was analyzed theoretically in the mid-infrared and the visible regimes. The MWCNTs were modeled as ensembles of concentric, cylindrical, conducting shells. Slightly attenuated guided waves and antenna resonances due to the edge effect exist for not-too-thick MWCNTs in the far- and mid-infrared regimes. Interband transitions hinder the propagation of guided waves and have a deleterious effect on the performance of a finite-length MWCNT as an antenna. Propagation of surface-plasmon waves along an MWCNT with a gold core was also analyzed. In the near-infrared and the visible regimes, the shells behave effectively as lossy dielectrics suppressing surface-plasmon-wave propagation along the gold core.

Electromagnetic wave propagation in an almost circular bundle of closely packed metallic carbon nanotubes

An equivalent-multishell approach for the approximate calculation of the characteristics of electromagnetic waves propagating in almost circular (azimuthally symmetric), closely packed bundles of parallel, identical, and metallic carbon nanotubes (CNTs) yields results in reasonably good agreement with a many-body technique, for infinitely long bundles when the number of CNTs is moderately high. The slow-wave coefficients for azimunthally symmetric guided waves increase with the number of metallic CNTs in the bundle, tending for thick bundles to unity, which is characteristic of macroscopic metallic wires. The existence of an azimuthally nonsymmetric guided wave at low frequencies in a bundle of a large number of finite-length CNTs stands in contrast to the characteristics of guided-wave propagation in a single CNT. The equivalent-multishell approach yields the polarizability scalar and the antenna efficiency of a bundle of finite-length CNTs in the long-wavelength regime over a wide frequency range spanning the terahertz and the near-infrared regimes. Edge effects give rise to geometric resonances in such bundles. The antenna efficiency of a CNT bundle at the first resonance can exceed that of a single CNT by four orders of magnitude, which is promising for the design and development of CNT-bundle antennas and composite materials containing CNT-bundles as inclusions.

Electromagnetic shielding efficiency in Ka-band: carbon foam versus epoxy/carbon nanotube composites

Abstract. The wide application of microwaves stimulates searching for new materials with high electrical conductivity and electromagnetic (EM) interference shielding effectiveness (SE). We conducted a comparative study of EM SE in Ka-band demonstrated by ultra-light micro-structural porous carbon solids (carbon foams) of different bulk densities, 0.042 to 0.150  g/cm3, and conventional flexible epoxy resin filled with carbon nanotubes (CNTs) in small concentrations, 1.5 wt.%. Microwave probing of carbon foams showed that the transmission through a 2 mm-thick layer strongly decreases with decreasing the pore size up to the level of 0.6%, due to a rise of reflectance ability. At the same time, 1 mm thick epoxy/CNT composites showed EM attenuation on the level of only 66% to 37%. Calculating the high-frequency axial CNTs’ polarizability on the basis of the idea of using CNT as transmission lines, we devised a strategy to improve the EM SE of CNT-based composites: because of the high EM screening of inner shells of multi-walled CNTs in the GHz range, it is effective to use either single-walled CNT or multi-walled CNTs with a relatively small number of walls (up to 15, i.e., those taking part in the EM interaction, if the CNT length is 20 μm).

Anisotropic electromagnetic properties of polymer composites containing oriented multiwall carbon nanotubes in respect to terahertz polarizer applications

Polystyrene composites with 0.5 wt. % loading of oriented multiwall carbon nanotubes (MWCNTs) have been produced by forge rolling method. The composites showed anisotropy of transmission and reflection of terahertz radiation depending on sample orientation relative to the polarization of electromagnetic wave. The structural characteristics of composites (nanotube ordering, length, defectiveness) were estimated by fitting the theoretical dependencies calculated within the Clausius-Mossotti formalism for cylindrical particles to the experimental data. The presented model was used for prediction of electromagnetic response of composites containing oriented MWCNTs with various structural parameters in THz region.

Radiofrequency field absorption by carbon nanotubes embedded in a conductive host

Understanding the electromagnetic response of carbon nanotubes (CNTs) in the radio frequency range is very important for experimental development of therapeutic and diagnostic CNT applications, including selective thermolysis of cancer cells and thermoacoustic imaging. In this study, we present the theory of electromagnetic wave scattering by several finite length CNT configurations, including singlewall CNT’s having a surfactant coating, CNT bundles, and multiwall CNTs. Absorption cross-sections of these structures in a conductive host region are theoretically studied in the radio frequency range. Strong local field enhancement due to edge effects is predicted to be inherent to metallic singlewall CNTs in the near-field zone, providing an additional mechanism of energy dissipation in a conductive host. Due to the screening effect the application of singlewall CNTs for the enhancement of energy dissipation is more effective than the application of multiwall CNTs or CNT bundles at the same mass fraction of...

Soft cutting of single-wall carbon nanotubes by low temperature ultrasonication in a mixture of sulfuric and nitric acids.

To decrease single-wall carbon nanotube (SWCNT) lengths to a value of 100-200 nm, aggressive cutting methods, accompanied by a high loss of starting material, are frequently used. We propose a cutting approach based on low temperature intensive ultrasonication in a mixture of sulfuric and nitric acids. The method is nondestructive with a yield close to 100%. It was applied to cut nanotubes produced in three different ways: gas-phase catalysis, chemical vapor deposition, and electric-arc-discharge methods. Raman and Fourier transform infrared spectroscopy were used to demonstrate that the cut carbon nanotubes have a low extent of sidewall degradation and their electronic properties are close to those of the untreated tubes. It was proposed to use the spectral position of the far-infrared absorption peak as a simple criterion for the estimation of SWCNT length distribution in the samples.

The Effect of Sample Holder Geometry on Electromagnetic Heating of Nanoparticle and NaCl Solutions at 13.56 MHz

Electromagnetic absorption and subsequent heating of nanoparticle solutions and simple NaCl ionic solutions is examined for biomedical applications in the radiofrequency range at 13.56 MHz. It is shown via both theory and experiment that for in vitro measurements the shape of the solution container plays a major role in absorption and heating.

Effects of inclusion dimensions and p-type doping in the terahertz spectra of composite materials containing bundles of single-wall carbon nanotubes

Experiments recently showed that the finite lengths of single-wall carbon nanotubes (SWNTs) randomly dispersed and randomly aligned in a composite material are responsible for the appearance of a broad peak in its terahertz conductivity. We investigated, both theoretically and experimentally, the influences of the cross-sectional diameter and the acid-induced p-type doping of SWNT bundles in composite materials on their terahertz conductivity peaks (TCPs). We found that the TCP blue-shifts if the inclusion diameter is larger, and that doping enhances the effective conductivity of the composite material. But, a theoretical prediction of the blue-shifting of the TCP by p-type doping was only weakly supported by experimental evidence. All experimental observations turned out to be in good qualitative agreement with the concept of localized plasmon resonance in SWNTs.

Terahertz sensing with carbon nanotube layers coated on silica fibers: Carrier transport versus nanoantenna effects

Carbon nanotube layers prepared as coatings on silica fibers are found to be suitable for terahertz detection in 0.5–7.3 THz range within temperatures of 4.2–70 K. In time-domain of terahertz excitation, two following constituents in the photoresponse are discriminated: the first one is attributed to the bolometric effect while the other one is related to the photoconductivity caused by the terahertz-induced hopping effect. In frequency domain, nonmonotonic behavior of the photoconductivity is associated with prevailing carbon nanotube-induced antenna effects in the electronic transport. The experimental observations are supported by theoretical estimates.

Absorption Cross-Section and Near-Field Enhancement in Finite-Length Carbon Nanotubes in the Terahertz-to-Optical Range

Electromagnetic characteristics of single-walled finite-length carbon nanotubes - absorption cross-section and field enhancement in the near zone - are theoretically studied in a wide frequency range from terahertz to visible. The analysis is based on the impedance-type effective boundary conditions and the integral equation technique. Comparison with experimental results is carried out allowing qualitative physical interpretation of low-frequency (far-IR and terahertz) absorption band observed in experiments. Potentiality of CNTs for the IR photothermolysis of living cells is discussed. Strong local field enhancement is predicted to be inherent to metallic CNTs in the near-field zone providing necessary mechanism for far-IR and terahertz near-field optics.