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).
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.