Nanotube films open up new perspectives for electronics

Nanotube films open up new perspectives for electronics

Physicists at MIPT and Skoltech have found a way to purposely modify and fine-tune the electronic properties of carbon nanotubes to meet the requirements of new electronic devices. The article appeared in the carbon diary.

Carbon nanomaterials form a broad class of compounds that includes graphene, fullerenes, nanotubes, nanofibers, and more. Although the physical properties of many of these materials already appear in textbooks, scientists continue to create new structures and find ways to use them in real applications. Macro structures designed as randomly oriented films made of carbon nanotubes look like very thin cobwebs with an area reaching several dozen square centimeters and a thickness of a few nanometers.

Figure 1. Carbon nanotube film under a scanning electron microscope.
Figure 2. Oxygen plasma treatment creates defects that change the electrical characteristics of the carbon nanotubes (left). The top box shows the surface resistance versus frequency for treated (red curve) and uncontaminated (blue curve) (right) films. The lower inset shows the coefficients of resistance to temperature (TCR) versus temperature for the same films.

Carbon nanotube films exhibit an extraordinary combination of physical and chemical properties, such as mechanical stability, flexibility, extensibility, excellent adhesion to various substrates, chemical inertness, and outstanding electrical and optical properties.

Unlike metal films, these highly conductive films are lightweight and flexible, and hence, can be used in various electrical devices, such as electromagnetic screens, modulators, antennas, bolometers, and so on.

Knowledge of the underlying physical principles is essential for effective use of the electrical and electrodynamic properties of films in real life. Of particular interest are the terahertz and far infrared spectral bands with wavelengths from 2 mm to 500 nm where the films show typical properties of metallic conductors.

Scientists from MIPT and Skoltech studied the conductivity of films in the terahertz bands and in the infrared using films synthesized with the gas phase deposition method. Some of the films consisted of nanotubes with lengths ranging from 0.3 to 13 µm, while others were treated with oxygen plasma for 100-400 seconds and changed their electrodynamic properties in the process.

In a previous study, the authors demonstrated that the conductivity of high quality pristine films can be accurately described using the conductivity model valid for metals. In these films, the free electrons have enough energy to overcome potential barriers at the intersections of individual nanotubes and can move fairly easily over the entire film, which results in high conductivity.

However, shortening the length of the tubes (down to 0.3 μm) or exposing films to plasma (for more than 100 s) leads to a drop in conductivity at low terahertz frequencies (<0.3 THz). The team found that in both cases the conductivity changes in much the same way and produces similar results. Exposure to plasma causes a greater amount of defects and, therefore, a greater amount of potential barriers for traveling electrons. For shorter nanotubes, the number of barriers per unit area also increases. Barriers strongly affect the conductivity of both nanotubes and direct current (DC) films and low enough frequencies, because at low temperatures electrons lack kinetic energy to overcome potential barriers. The authors demonstrated that at sufficiently high frequencies the electrons move freely as if the barriers were not present. At low frequencies and in the DC case, films made from short or plasma-treated tubes exhibit a higher temperature coefficient of resistance (TCR) which shows how resistance changes with temperature.

For plasma exposure greater than 100 seconds or nanotube lengths less than 0.3 μm, the TCR reaches saturation. The effect can be considered a precursor of TCR reduction in films that are exposed to plasma for a very long time when the separate tubes are severely damaged and lose their unique electrical properties.

Researchers from MIPT and Skoltech intend to continue studying modified films, including those stretched in one or more directions. Boris Gorshunov, co-author of the article and head of the Terahertz Spectroscopy Laboratory at MIPT, comments: “Unlike nanotubes which have long been studied in great detail, research on macroobjects, such as nanotube films, has only recently begun. . Nanotube films are much lighter and chemically and mechanically stable than metal films and, therefore, are more attractive for electronic applications. Because we know the fundamental physics behind the electrical properties of films, we can fine-tune them for specific real-life applications. Research on the terahertz band that will soon become ubiquitous in telecommunications is of particular relevance ”.

“It was found that the controlled destruction of this extraordinary material by the microwave plasma results in a number of surprising properties, such as a noticeable increase in TCR in films made of single-walled carbon nanotubes. This happens because the competing contributions of metal and semiconductor tubes to conductivity no longer play an important role and the conductivity of the film is mainly determined by the amount of defects. This feature is very interesting for the design of new generation devices, such as high-speed bolometers at room temperature, “notes Professor Albert Nasibulin, head of Skoltech’s Laboratory of Nanomaterials.

Find out more about the original article here.

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