Ehsan Dadrasnia

Terahertz electrical conductivity and optical characterization of composite nonaligned single- and multiwalled carbon nanotubes

Abstract. We employed noncontact terahertz time-domain spectroscopy to investigate and compare the electrical/optical properties of nonaligned single-walled carbon nanotube and nonaligned multiwalled carbon nanotube thin films. Using Drude–Lorentz model together with the Maxwell Garnett effective medium theory, we determined the electrical conductivities from the extracted data of differential complex terahertz analysis in the frequency range of 0.1 to 2 THz. The results demonstrate that the conductivity of isotropic single-walled carbon nanotube thin film is almost two times larger compared to isotropic multiwalled carbon nanotube thin film due to the increased number of surface defects and the availability of mobile carriers. By using Drude–Lorentz model, the broadening optical density and conductivity can be studied at higher frequencies.

Determination of the DC Electrical Conductivity of Multiwalled Carbon Nanotube Films and Graphene Layers from Noncontact Time-Domain Terahertz Measurements

Measuring the DC conductivity of very thin films could be rather difficult because of the electrical contact issue. This DC conductivity can, however, be extracted from noncontact measurements at GHz and THz frequencies using elaborated conductivity models that nicely fit the experimental data. Here we employ this technique to study the DC conductivity of fragile nanometer-thick films of multiwalled carbon nanotubes and monolayer graphene. The THz response of the films is measured by THz time-domain spectroscopy. We show that the THz conductivity of the samples is well fitted by either Drude-Lorentz model or Drude-Smith model, giving information on the physics of electrical conductivity in these materials. This extraction procedure is validated by the good agreement between the so-obtained DC conductivity and the one measured with a classical 4-point probe in-line contact method.

Electrical characterization of silver nanowire-graphene hybrid films from terahertz transmission and reflection measurements

We determined the electrical sheet conductivity of silver nanowire-graphene hybrid films from transmission and reflection terahertz time-domain spectroscopy measurements. The sheet resistance extracted from noncontact terahertz measurement is in good agreement with one measured with a classical 4-point-probe technique. The conductivity is well described by a Drude-Smith model and is calculated to peak around 10 THz.

Sub-THz Characterisation of Monolayer Graphene

We explore the optical and electrical characteristics of monolayer graphene by using pulsed optoelectronic terahertz time-domain spectroscopy in the frequency range of 325–500 GHz based on fast direct measurements of phase and amplitude. We also show that these parameters can, however, be measured with higher resolution using a free space continuous wave measurement technique associated with a vector network analyzer that offers a good dynamic range. All the scattering parameters (both magnitude and phase) are measured simultaneously. The Nicholson-Ross-Weir method is implemented to extract the monolayer graphene parameters at the aforementioned frequency range.

Terahertz conductivity studies in carbon nanotube networks prepared by the vacuum filtration method

The electrical properties of carbon nanostructures have been greatly stimulating to use in the nanotechnology for electronic components. In this paper, we study the AC and DC electrical conductivity responses of multi-walled carbon nanotube films, prepared by the vacuum filtration methods, with noncontact terahertz time-domain spectroscopy (THz- TDS) approach utilizing the extrapolation analysis as well as probe-in line technique.

A Comparison between Different Schemes of Microwave Cancer Hyperthermia Treatment by Means of Left-Handed Metamaterial Lenses

In the hyperthermia therapy, multiple microwave sources can be arranged with appropriate spacing around the tissue containing tumor by using left-handed material (LHM) lenses. We employ some low loss LHM lenses schemes for an effective non-invasive microwave hyperthermia treatment of large tumors up to several centimeters of depth inside the biological tissues. Different configurations of LHM lenses are proposed and compared in order to assess the efficiency of hyperthermia treatment. High-resolution focusing of microwave radiation can be achieved by joint heating of several microwave antennas behind a conformal flat LHM lens. We show that a microwave radiation can be effectively focused in a 1.2 cm diameter tumor located within a lossy breast tissue. The results show that hyperthermia (temperature over 42 ◦ ) is reached and then maintained for one hour without involving the surrounding healthy tissues. Lastly, the heating area is adjusted in both lateral and longitudinal directions changing the position of the microwave sources or selecting LHM lenses with different thickness. This approach confirms that the conformal four-lens system is more efficient to achieve microwave tumor hyperthermia than single- and double-lens schemes.

Terahertz conductivity characterization of nanostructured graphene-like films for optoelectronic applications

Abstract. Difficulty in deposition and integration of fragile graphene-like samples for optoelectronic devices may prevent a multiple contact measurement procedure. We employed noncontact and nondestructive transmission and reflection terahertz (THz)-pulsed spectroscopy to investigate not only the electrical conductivity, but also to study the optical properties of one-dimensional and two-dimensional graphene-like samples. The Drude and non-Drude models were applied to observe and compare the ultrafast carrier transport parameters and high mobility characteristic of such high conductance-nanostructured thin films without requirement for postprocess patterning. The diffusive coefficient and nanoscopic characteristic length from noncontact THz measurement enables us to predict the cut-off frequency of such devices in relevant optoelectronic applications in sub-THz and THz frequencies. The results show that the cut-off frequency of the devices increases with a reduction of the channel length.

THz time-domain spectroscopy in different carbon nanotube thin films

The direct metallic or semiconducting characterization of carbon nanotubes (CNTs) in the high-frequency is one of the key issues to use them in the different state-to-the-art applications. In this work, the terahertz surface conductivity and transmission of carbon nanostructures thin-film utilizing terahertz time-domain spectroscopy (THz-TDS) have been studied. We have also compared the achieved results of single-walled carbon nanotubes thin-film surface conductivity with pervious study as a function of frequency. However, we have improved the obtained conductivity of carbon nanostructures from the microwave to terahertz range by THz-TDS technique with high signal-to-noise ratio.

Optical and electrical characterization of carbon nanotubes by terahertz spectroscopy: comparison between modeling and experimental results

We have studied and analysed in this paper, the modeling of single-walled carbon nanotubes (SWNTs) optical and electrical properties utilizing terahertz (THz) time-domain spectroscopy in the frequency range of 0.1-2 THz. We have compared the measurement data of the THz power absorption coefficient, index of refraction and conductivity of SWNTs film with the experimental results obtained in Ref. [9], being our results based on the combination of Drude- Lorentz and Maxwell-Garnett models. Since we suppose the SWNTs network as an effective medium embedded in air, the comparison shows good agreement.

Free space material characterization of carbon nanotube thin films at sub-terahertz frequencies

Abstract A free space, non-destructive, vectorial continuous wave terahertz analysis method is employed to study the material properties of single-walled and multi-walled carbon nanotube thin films deposited on substrates of fused quartz and silicon. The electrical and optical properties of the thin films have been investigated using a vector network analyzer, in the frequency range of 220–325 GHz (WR 3.4) and 325–500 GHz (WR 2.2). The Nicolson–Ross–Weir method is used to extract the material parameters. The refractive index, absorption coefficient, and the complex conductivity of the single-walled and multi-walled carbon nanotube thin films are extracted and studied.