Found a new non-contact method for measuring the thickness of films of carbon nanotubes

Skoltech scientists, in collaboration with Russian and Finnish colleagues, have found a new, non-contact method for measuring the thickness of films of single-walled carbon nanotubes, which has prospects for use in a wide variety of fields – from solar energy to smart tissue.

The results of the study are published in the journal Applied Physics Letters. A single-walled carbon nanotube (SWCNT) is a single atom thick sheet of graphite rolled into a tube. SWCNTs are one of the allotropic forms of carbon along with fullerenes, graphene, diamond, and graphite. Single-walled carbon nanotubes have wide prospects for industrial applications in a wide range of fields, from solar panels and LEDs to ultrafast lasers, transparent electrodes, and smart tissue.

However, many applications require fairly accurate measurements of film thickness and its optical properties. “For a number of applications, the thickness of the film is of great importance, which determines the amount of light that it can transmit in the visible spectral range, that is, the higher the transparency of the film, the smaller its thickness.

In addition, the accuracy of controlling the thickness of the film and its optical constants is extremely important in creating effective transparent electrodes. For example, in the case of solar panels, it is necessary to know the film thickness in order to improve the antireflection properties of the surface of a transparent layer of SWCNTs. To evaluate and then use the mechanical properties of SWCNT films, you must first know their geometric dimensions, ”says Professor Albert Nasibulin, Head of the Laboratory of Nanomaterials of the Center for Photonics and Quantum Materials of Skoltech.

Carbon nanotubes

Now, methods of absorption spectroscopy and spectroscopy of characteristic energy losses by electrons are used to measure optical properties, and transmission electron microscopy, scanning electron microscopy, or atomic force microscopy are used to determine geometric parameters. All these methods are very laborious and require expensive equipment and special sample preparation, which can affect the measured properties of SWCNT films.

A group of researchers led by Professor Skoltech and Aalto University Albert Nasibulin managed to develop a fast, universal, non-contact method for accurately assessing the thickness of SWCNT films and their dielectric functions. Scientists have chosen a non-standard path, deciding to use the fast and highly sensitive non-destructive method of spectroscopic ellipsometry.

“Ellipsometry is an indirect non-contact method that can be used to determine film parameters, but, unfortunately, far from always standard data processing algorithms give a satisfactory result. At first glance, it might seem that a film of carbon nanotubes is a very inconvenient object for ellipsometry since it consists of millions of individual and connected nanometer-sized tubes randomly oriented.

Such films are characterized by strong absorption in the entire spectral range, weak reflection, and anisotropy of optical properties. Nevertheless, the first author of the article, a student of the joint master program of Skoltech and MIPT, Georgy Ermolaev developed an elegant algorithm that allows obtaining data on the thickness and optical constant films of nanotubes from one set of optical measurements, ”notes one of the authors of the work, Skoltech senior researcher Yuri Gladush.

The researchers fabricated SWCNT films of various thicknesses with an absorption in the range from 90% to 45% in the spectral region of about 550 nanometers and determined the refractive index in a wide spectrum (250–3300 nm) and the corresponding film thickness.

“We expected that the optical properties of the films would depend on the packing density of carbon nanotubes, but did not expect this dependence to be so significant. When one drop of ethanol is added, the film is densified, and the refractive index increases from 1.07 to 1.7, which allows controlling the optical properties of SWCNT films, ”adds Nasibulin.

Researchers believe that the results can be used by other scientists as applied not only to carbon nanotubes but also to other similar structures. The work was carried out with the participation of specialists from the Center for Photonics and Two-Dimensional Materials, MIPT, as well as GrapheneTek and Canatu Ltd.