Electrical and optical properties of carbon nanotube and silver nanowire layers for low-THz applications

Moving to THz frequencies, modern devices require high efficiency and high functionality but at the same time low cost and small dimensions of the components. Nanomaterials like carbon nanotubes (CNTs) and silver nanowires (AgNWs) are promising candidates as building blocks of such devices. Those nanomaterials possess properties, which are not observed in conventional ones.In this thesis, the millimeter wave and optical properties are studied experimentally and theoretically. The experimental technique used for the nanomaterial layer characterization at low THz frequencies, from 75 to 325 GHz, is based on measurement of dielectric rod waveguide (DRW) loaded with a sample under test. The sample is 20 mm long and 0.5 mm wide thin nanomaterial layer on a low-loss substrate, and the DRW is made of sapphire and has a cross-section of 1.0×0.5 mm2. Such measurements allow to detect the level and behavior of propagation losses, introduced in DRW after deposition of nanomaterial layer. For DRW loaded with a CNT layer, the observed losses are in the range of 5 to 40 dB, depending on the thickness of the layer and the frequency. AgNW layers cause losses in the range of 2-4 dB. Attenuation constant, found from the S-parameter measurements (transmission and reflection), was used as a basic parameter for the developed theoretical model for determination of the complex dielectric constant of nanomaterial layers. In addition, an alternative model for dielectric constant estimation was developed, exploiting a semiclassical model of the surface conductivity of the carbon nanotubes and their length distribution in the layer. The lengths of CNTs in the investigated samples vary from 5 µm to 40 µm. Obtained parameters were used in numerical simulations of a DRW loaded with a nanomaterial layer. Carbon nanotube layers proved to be highly absorbing materials, although this characteristic changes with frequency, while attenuation in AgNW layers is very low and more stable in the wide frequency range. Both models and simulations are in a rather good agreement with measurements. Another part of the thesis deals with optical characterization of transparent conductive coatings based on AgNWs. The resistivity and optical properties of AgNW layer with different lengths and diameters of the nanowires were studied and compared with theoretical predictions. Resistivity measurements show that 3 times longer nanowires give about 10% increase of optical transmittance with the same resistivity. In addition, the aging of the fabricated films was examined.