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Physicists Discover Why Thin Carbon Nanotubes Have Metallic Conductivity

21 November 2017

An international team of researchers from MIPT, Levedev Physical Institute, RAS, Prokhorov General Physics Institute, RAS, Soltech and Alato University has examined the optical and dielectric properties of thin, macroscopic films. These films are based on single-walled carbon nanotubes. The researchers explain the metallic nature of their conductivity using infrared and terahertz spectroscopy.

A single-walled carbon nanotube, or SWNT, can be pictured as a graphene sheet rolled into a cylinder. SWNTs are light, strong and resistant to high temperatures. They can be used as additives to make composite materials more durable, or as building blocks to fabricate aerosol filters and electrochemical sensors. Transparent and flexible carbon nanotube films have a wide variety of potential applications. They can be used as supercapacitors or transparent electrodes in flexible electronics, for example. The study of the charge transfer mechanisms in these films is important for basic research and practical applications.

Atomic force microscopy image of the surface of a carbon nanotube film. The fragment seen on the image is 2.5 by 2.5 micrometers. The false color bar indicates the penetration depth of the microscope tip. Source: MIPTAtomic force microscopy image of the surface of a carbon nanotube film. The fragment seen on the image is 2.5 by 2.5 micrometers. The false color bar indicates the penetration depth of the microscope tip. Source: MIPT

The research team measured the optical and electrical properties of the films by terahertz-infrared spectroscopy at a variety of temperatures, from minus 268 degrees Celsius to room temperature, and in a variety of incident radiation wavelengths, from ultraviolet to terahertz. The study of the interaction between films and radiation yielded fundamental data on the electrodynamics of the films.

The SWNT films were synthesized by using aerosol chemical vapor deposition (CVD). A vapor of the catalyst precursor, ferrocene, is briefly supplied into the CVD reactor, where it decomposes the atmosphere of carbon monoxide, which forms nanometer-sized catalyst particles. On the surface, carbon monoxide (CO) disproportion occurs when the SWNTs grow. They are then at the outlet of the reactor and SWNTs are collected onto the nitrocellulose filter. Different thicknesses can be achieved through varying the duration of the collection time. The SWNT films can be easily transferred to different substrates by dry deposition or used in their free-standing form, without a substrate. This enables the production of high-quality nanotubes with no 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,” comments Professor Albert Nasibulin of Skoltech.

During their studies, the scientists coated nanotubes filled with iodine and copper chloride by placing them in the atmosphere of the appropriate vapors. This treatment increases charge carrier density in the filled tubes and reduces contact resistance between them, which enables flexible transparent electrodes and materials with selective charge transfer for use in optoelectronics and spintronics.

In order to be used in electronics, films need to be efficient charge carriers so the physicists examined the broadband spectrum of their dielectric permittivity. But flexible electronics will require the films to be transparent, so their optical conductivity was measured as well. Both analyses were conducted in a wide temperature range. The data obtained in terahertz and far infrared regions of the spectrum were particularly interesting to the researchers. While prior research findings pointed to a peak in the terahertz conductivity spectrum, the research from this study reports no clear indications of the phenomenon. The authors say this is because of the high quality of their films.

Because the analysis of optical and dielectric properties of the films at frequencies of below 1,000 cm⁻¹ revealed spectral features that were typical for conducting materials, like metals, the team decided to employ the corresponding conductivity models that were developed by Paul Drude. According to this model, the charge in the conductors is transferred by free carriers. Like the ideal gas molecules, they move between the ions in the lattice and scatter upon collision with its vibrations, defects or impurities.

In this study, the charge carriers are also scattered by the energy barriers at the intersections of individual nanotubes. But, as the analysis suggests, these barriers are insignificant and allow the electrons to move almost freely across the film. Using the Drude model, the researchers were able to quantitatively analyze the temperature dependencies of the carriers’ effective parameters, which are responsible for the electrodynamic properties 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 non-contact manner. Our findings show that such films may be successfully used as components or assemblies in various micro- and nanoelectronic devices,” says Elena Zhukova, deputy head of the Laboratory of Terahertz Spectroscopy at MIPT.

A paper on this research was published in the journals, Carbon and Nanotechnology.

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