13 February 2014
We have demonstrated a powerful noncontact, non-destructive tool to study the properties of carbon nanotubes (CNT) thin films by using a Rohde and Schwarz ZVA 24 vector network analyser (VNA) associated with frequency extenders, in the frequency range of 220–500 GHz. The scattering parameters (reflection and transmission of the CNT sample) were measured simultaneously, and the experiment was carried out in a free space environment. We used the Nicholson-Ross-Weir (NRW) approach too, and since the thin films were deposited on a substrate, we have carefully removed the effects of the substrate before proceeding to extract the properties of the films alone. A comprehensive characterisation of the electrical and optical properties of CNTs is necessary, so in order to meet this requirement, we have presented the study of both conductivity and refractive index of multi-walled carbon nanotubes (MWCNTs).
Since the discovery of CNTs, they have been considered as an alternative to indiumtin oxide for different applications because of their unique electrical, optical and mechanical properties. MWCNTs are most attractive as they are simpler to produce than single-walled samples (SWCNT). The noncontact optical and electrical characterisations of MWCNT films are of interest since the probe disturbance can be eliminated, and many materials that are opaque to visible and infrared light are transparent to terahertz (THz) and sub-THz radiations. We chose fused quartz as the substrate material because of its high transparency and low absorption in the THz domain. Lastly, the great interest in the frequency range 220–500 GHz relies on wireless communication applications where new generation of components have to be investigated.
MWCNT samples have been characterised at lower gigahertz (GHz) frequencies before. However, the contacting issue may damage the fragile thin films and thus may prevent a multiple measurement procedure and/or a subsequent use of the tested devices. Nowadays, non-contact THz time domain spectroscopy (TDS) is one of the most common methods to study CNT containing materials in THz frequencies in a wide range. But one of the important issues we face as we come down to the lowest frequencies (<0.3 THz) is the decrease of signal to noise ratio due to a drop in THz power. Thus we investigated the vectorial continuous-wave THz analysis which is very sensitive (here up to 0.5 THz) as complementary to TDS.
The importance of this technique is that the optical and electrical properties of MWCNT thin films deposited on a fused quartz substrate can be studied in free space, which is a non-contact/non-destructive method. This helps to preserve the sample for further studies. Also, VNA offers a great spectral resolution and a good dynamic range in the sub-THz range (60–70 dB, around 500 GHz). Moreover, the technique used is intrinsically vectorial, thus the optical index and losses at THz frequencies can be extracted from phase and amplitude measurements, respectively.
We are now studying the properties of graphene, using the same technique in the sub-THz frequency range. We are also studying CNT thin films deposited on non-transparent substrates such as silicon. Our group is also working on the study of CNT samples and graphene using THz-TDS.
This work has been done within the European project, MIcrowave and TErahertz PHOtonics (MITEPHO) co-ordinated by Grupo de Optoelectrónica y Tecnología Láser (GOTL), Universidad Carlos III de Madrid. The principal research objective of the project was to design and develop efficient, integrated and reliable sources for the generation of microwave and THz waves, providing sufficient power to be useful in communication, spectroscopy, sensing and biomedical applications.
CNTs and graphene films show amazing electrical properties. Therefore, they will certainly be used to design and fabricate new electronic components, especially based on quantum effects thanks to the 2D nanometric scale of the films. Such components are also expected to address the high frequency regime (over 100 GHz), and to affect the telecom market. It is anticipated that such components will be low-cost, compact, and could be included in integrated photonic systems. The manufacture of optimised components requires to both control the thin film fabrication process and to accurately characterise the films.
This article is based on the Letter: ‘Sub-THz characterisation of multiwalled carbon nanotube thin films using a vector network analyser’ (new window).
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