The research team of Tomsk Polytechnic University analyzed the effect of the surface structure of the substrate on optical signals in nano-spectroscopy. The team managed to identify several so-called artifacts, which affect the quality of the obtained nano-images. The research article was published in the Ultramicroscopy journal.

The international research team headed by Prof. Rodriguez Raul David from the Research School of Chemistry & Applied Biomedical Sciences and Prof. Evgenia Sheremet from the Research School of High-Energy Physics, develops a new field in optical nano-spectroscopy and plasmon and sensor materials for biomedicine and electronics at TPU.

Recently Eugenia Sheremet has won the prestigious Women in Science fellowship, a joint project of UNESCO and L’Oreal. The researchers implement the nano-optics techniques using nano-antennas, when studying individual nanoparticles, e.g., a single nanotube or nanocluster and internal inhomogeneities. This is the probe enhanced Raman spectroscopy, when the nano-antenna is placed on a probe of an atomic force microscope to scan the studied surface. The nano-antenna is irradiated with a laser, and the spectrum of the scattered light is detected by the Raman spectrometer.

The following method is necessary to optimize nanostructure in the development of solar cells, organic or solid-state transistors, or organic light-emitting diodes. It provides the detection of chemical composition, chemical reactions, and mechanical stress in very small areas.

Moreover, the team implements a highly reflective substrate of gold or silver to achieve a resolution of less than 10 nanometers. The surface of this substrate may be uneven, which will cause the artifact, a certain image distortion. In the published paper they specified the ways these artifacts affect image quality. “Nanospectroscopy artifacts were considered only in several articles by researchers from Romania, the USA, and other countries. However, these studies considered other aspects of this effect.

In our case we consider both convex and concave surfaces as well as the impact of the probe shape,” Evgenia Sheremet says. The researchers studied three systems. The first one is a gold substrate with a layer of organic dye molecules with a thickness of 2 to 40 nanometers. The second one is a silicon substrate with gold structures and dye molecules. The third system is a concave surface in the form of a substrate of polystyrene spheres coated with a metal layer (gold or silver) and dye molecules.

As a result, the team succeeded to describe previously unstudied types of artifacts as well as to present a more complete picture of probe-surface interaction.

“The probe is a very sharp needle. When we scan a surface, we figuratively “touch” it with a probe. Moreover, if there is an obstacle on the substrate surface, then the electromagnetic interaction of the nanoantennas with the surface can distort the image obtained. How do these artifacts impact the results?

Firstly, when we analyze the signal intensity, then in classical microspectroscopy it depends on the amount of substance. In nanospectroscopy, the surface structure impacts the intensity much more than the amount of matter. Secondly, artifacts can cause distortion in the size of structures. Thirdly, the image may shift in relation to the actual position of structure,”

– the professor says.

The study is supported by the Russian Foundation for Basic Research (No. 18-42-700014 r_a). The project was carried out jointly with colleagues from the Chemnitz Technical University (Germany) and the Rzhanov Institute of Semiconductor Physics (SB RAS).

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