TPU researchers develop a unique method for measuring optical properties of nanostructures

Raul Rodriguez, the Professor of the Research School of Chemistry & Applied Biomedical Sciences is developing a unique method for measuring optical absorption of material in different ranges of visible light with the resolution up to four nanometers.  Currently, it is the world most detailed resolution in the measurement of optical absorption. The research results were published in the journal ACS Photonics (IF: 6,756; Q1).

Photo: TPU researchers Raul Rodriguez and Evgeniya Sheremet

Even using the world most powerful microscope for studying the optical properties of nanostructures, it is possible to obtain a resolution only up to hundreds of nanometers. However, Raul Rodriguez and his team proved that in the optical range it is possible to achieve resolution of up to four nanometers. Today it is a record resolution limit in the measurement of optical absorption.

“Due to organic solar cells, one can receive electricity simply by painting the house roof with a special sensitive paint. However, this type of cells has problems with efficiency and stability. To find out what is happening there at the molecular level, at the level of polymers, it is necessary to measure the absorption distribution for different wavelengths. In case of nanoresolution, it can be done by our method”,

 — comments Raul Rodriguez.  

The method is based on the use of cantilevers for atomic force microscopy. A probe is a needle at the end of the cantilever with a radius of five to eight nanometers (for comparison, the size of one DNA base is two nanometers). It is taken close to a sample; as a result there should be either attraction or repulsion between them.  Consequently, one can observe changes in the cantilever oscillations amplitude. In these conditions, the microscope can scan and display a 3D picture of the surface.

However, along with the shape of this surface, it is also necessary to measure its optical properties. To do this, Raul Rodriguez team uses laser radiation. If a material absorbs this radiation, then the former begins to heat up and expand thermally.  This expansion is very small; therefore, one should use a sensitive method of atomic force microscopy to measure it. This method will allow one to see a change in altitude of one atom.

"To implement it technically, we use several additional "tricks". For example, the laser is modulated at the probe resonant frequency. It means, that the laser light turns on and off with the probe oscillation frequency.  Then our material begins to pulsate at the laser frequency, the probe amplifies an object pulsation due to resonant effects. Consequently, it becomes more than enough to measure it", - says a member of the research team.

It is worth noting, that the probes in this study are made of gold. It is an important moment, due to the optical properties of the metal. At the tip of the probe, the laser light intensifies more and the needle heats quickly and efficiently.

The method illustration: the gold cantilever with a needle-probe is illuminated by modulated laser radiation. As a result, carbon nanotubes (black) on glass (SiO2) expand thermally, changing amplitude of cantilever oscillations. On the right, below: the result of measurements at the nanotube-glass interface.

The research is conducted by Raul Rodriguez together with a team of scientists, including Evgeniya Sheremet, Professor of the Research School of High-Energy Physics, invited international experts Teresa I. Madeira, Evgeny Borshchagovsky, Ashutosh Mukherjee, Michael Hitchhold, Dietrich R.T. Zan, and international students.

We first thought about this project in 2012, when Anasys co. developed a method for measuring absorption in the infrared range with nanoscale. It is based on a similar method as was proposed by the university research team but works in a different spectral range.

"The Anasys method is a huge step in science, especially in organic and biology, since now it is possible to visualize the chemical composition of substances on the surface in the nanoscale. Neither Anasys nor any other researchers have worked in the optical range. We decided that could be the first",

highlights Evgeniya Sheremet.

The team began work in the scope of the SMINT FOR 1713 grant (Sensoric Micro- and Nanosystems). It is a large program funded by the German research community (Deutsche Forschungsgemeinschaft, DFG). The project researchers needed methods for studying nanoelectronics based on carbon nanotubes. The Rodriguez team studied transistors and contacts using TERS method (tip-enhanced Raman spectroscopy) and then became part of COST Action MP1302 - a European network of scientists who work in the field of nanospectroscopy, interact with the world best researchers and invite them to their laboratories.

"Due to COST Action, we managed to assemble a team of experts in the theoretical and engineering fields of methodology development and created such an installation in Chemnitz, Germany. The results presented in the article were received there. Having arrived at TPU, we are gathering resources to recreate and develop this methodology at the university. This will be a unique method in the world", 

— says Evgeniya Sheremet.  - Currently, we have shown the possibilities of resolution, and now we need to investigate the effects of optical absorption and thermal conductivity. We hope that experts of the School of Advanced Manufacturing Technologies will help us with this project since they have the equipment and expertise necessary for these studies. 

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