Infrared and Terahertz Spectroscopy Used to Investigate Metallic Conductivity of SWNT Films

The dielectric and optical characteristics of thin macroscopic films have been investigated by a team of international researchers — from MIPT; Lebedev Physical Institute, RAS; Prokhorov General Physics Institute, RAS; Skoltech and Aalto University (Finland) — depending upon single-walled carbon nanotubes.

The team has also derived an interpretation of the metallic conductivity of the films by adopting infrared and terahertz spectroscopy. The outcomes of the study have been reported in two journals — Carbon and Nanotechnology.

Credit: Moscow Institute of Physics and Technology

A single-walled carbon nanotube (i.e. SWNT) can be imagined to be a graphene sheet enclosed inside a cylinder. SWNT are strong, lightweight and resistant to higher temperatures, hence they can be applied as elementary units for making electrochemical sensors and aerosol filters, or as additives to composite materials to render them highly durable. Flexible and transparent carbon nanotube films, which are 2-D structures made of intersecting nanotubes, find a broad range of prospective applications, for instance, as transparent electrodes or supercapacitors in flexible electronics, or electronic devices with the propensity to be folded, bent and twisted without being broken. Hence the analysis of the charge transfer mechanisms in these films is highly significant not only for fundamental research but also practical uses.

The physicists adopted terahertz-infrared spectroscopy at temperatures ranging from -268°C to ambient temperature, and under a broad array of incident radiation wavelengths (from ultraviolet to terahertz, that is, wavelengths of about 1 mm), to measure the electrical and optical characteristics of the films. The investigation of the interaction of the films with the incident radiation produced basic data related to the electrodynamics of the films.

The SWNT films were produced by means of aerosol chemical vapor deposition (CVD) method. In short, vapor of ferrocene (a catalyst precursor) is administered into the CVD reactor, where it gets disintegrated under the atmosphere of carbon monoxide (CO) and forms nanometer-sized catalyst particles. CO disproportionation (or simultaneous oxidation and reduction) occurs on their surface, and eventually SWNT grow. The flow from the reactor’s outlet is filtered, and SWNT are collected over the nitrocellulose filter.

Films with varying thicknesses can be produced by altering the duration of the collection time. Specifically, the SWNT films can be simply conveyed onto various substrates through dry deposition or can be used in their independent form, or with no substrate. The technique allows high-quality nanotubes to be produced without any amorphous carbon impurities.

Since all carbon atoms in SWNTs are located on their surface, it is relatively easy to alter the electrical properties of this unique material. We can improve the conductivity of the films either by incorporating dopants into the nanotubes or by coating them with electron-acceptor or -donor molecules.

Professor Albert Nasibulin, Skoltech

As part of their investigations, the researchers coated gold chloride on the samples, which functioned as the doping agent, and acquired films from nanotubes including with iodine and copper chloride by positioning them in an ambience of the suitable vapors. Apart from increasing the charge carrier density in the filled tubes, the process minimizes contact resistance between them, thereby allowing the production of materials with selective charge transfer and flexible transparent electrodes that can be applied in spintronics and optoelectronics.

If films have to be used in electronics, they must be efficient charge carriers. Therefore, the team investigated the broadband spectrum of the dielectric permittivity of the films. Yet, flexible electronics also mandate the transparent nature of the films; hence the team also measured the optical conductivity. The two investigations were performed in a broad temperature range, that is, from several degrees above absolute zero to room temperature.

The data acquired in the terahertz as well as far infrared areas of the spectrum are of specific interest. In contrast to earlier research outcomes that indicated a peak in the terahertz conductivity spectrum at frequencies ranging from 0.4 to 30 THz, based on the study, this study has not reported clear signs of the phenomenon. The team accredits such outcomes to the higher quality of the films.

As the investigation of the dielectric and optical features of the films at frequencies lesser than 1,000 per cm exhibited spectral features characteristic of conducting materials (such as metals), the researchers made up their mind to adopt the related conductivity model developed by Paul Drude. As stated by the model, the charge in the conductors is conveyed by free carriers: similar to the ideal gas molecules, they progress between the ions in the lattice and get scattered as a result of collision with its defects, vibrations or impurities.

In this research, the charge carriers were also observed to be scattered by the energy barriers at the junctions of individual nanotubes. Yet, as indicated by the investigation, the barriers are trivial and enable the electrons to move nearly free inside the film. By applying the Drude model, the authors could quantitatively investigate the reliance of the effective parameters of the carriers (such as mobility, concentration, time between collisions and mean free path) on temperature. It is these parameters that are responsible for the electrodynamic characteristics of the films.

Our research has clearly demonstrated that terahertz spectroscopy provides an efficient tool for studying the conductivity mechanisms in macro-scale carbon nanotube films and determining the effective parameters of charge carriers in a noncontact manner. Our findings show that such films may be successfully used as components or assemblies in various micro- and nanoelectronic devices.

Elena Zhukova, Deputy Head, The Laboratory of Terahertz Spectroscopy, MIPT

The Ministry of Education and Science of the Russian Federation (Project 5-100, Federal Target Program Grant No. RFMEFI59417X0014) and the Russian Foundation for Basic Research (Grant No. 15-12-30041) supported the research.

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