Abstract
Thin films of multi-walled carbon nanotubes forming random networks were produced by vacuum filtration method, and their broadband electromagnetic radiation transmittance spectra are presented. Thickness of the nanotube films was between 100 nm and 1 μm, and the transmission properties are demonstrated for the wavelength range from 300 nm to 400 μm. It is observed that transmittance is an increasing function of a radiation wavelength, and for the thickest films it almost saturates above 1 μm wavelength. To explain the experimental results in the ultraviolet–near infrared range, we employed effective medium theory (in the form of symmetric Bruggeman model) correlating properties of multi-walled carbon nanotubes with the effective dielectric function of a nanotube network. The optical properties of a single multi-walled carbon nanotube that were used for calculations were based on ordinary and extraordinary dielectric functions of bulk graphite. The proposed theoretical model has been successfully fitted to the experimental results. It has been also found that despite the fact that radiation undergoes multiple internal reflections at the film interfaces, the transmittance–thickness relation can be still described by exponential decay.
Highlights
The interaction between electromagnetic radiation and carbon nanotubes has been continually the subject of scientific interest, both from the theoretical and experimental point of view
For multi-walled carbon nanotubes (MWCNTs) with diameters larger than few nanometers, the curvature effects can be neglected and optical properties of a single MWCNT can be derived from the dielectric functions of bulk graphite [3]
The relative transmittance spectra Trel(k) of MWCNT thin films collected for six different samples on glass substrates are presented in Fig. 2, and they are calculated with respect to the substrate spectrum Tsub(k) so that they better demonstrate the properties of data demonstrated by Kamaras et al [14] for free standing layers of MWCNTs
Summary
The interaction between electromagnetic radiation and carbon nanotubes has been continually the subject of scientific interest, both from the theoretical and experimental point of view. For multi-walled carbon nanotubes (MWCNTs) with diameters larger than few nanometers, the curvature effects can be neglected and optical properties of a single MWCNT can be derived from the dielectric functions of bulk graphite [3]. For a theoretical description of electromagnetic properties of such materials, the literature delivers different models being classified under effective medium theory (EMT) and so far they were mainly applied to systems of vertically aligned nanotubes [17,18,19]. The internal structure of the obtained thin films is similar to a network of nanotubes tending to arrange in a layered structure rather than chaotic bundles This can be deduced from the scanning electron microscope images of the MWCNT networks shown, b.
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