Nano Tech Views, em 03/02/2011
Combining two different techniques, researchers at the National Synchrotron Light Laboratory (LNLS), Campinas (SP), developed a new methodology capable of mapping in three dimensions the chemical composition of nanostructures.
According to Antonio Ramirez, the methodology determines the chemical composition of three-dimensional structures with dimensions of few nanometers from images obtained by transmission electron microscopy, high resolution (HRTEM).
“These images were obtained with a technique known as reconstruction of focal series (FSR), a procedure used to obtain images free of aberrations of the optical system of the microscope. These images, with high quality and resolution picometres were combined with the analysis technique geometric phase (GPA), which allows a two-dimensional mapping of structural distortions, “Ramirez said.
Structural deformations
According to the researcher, the approach adopted by the Brazilian group consisted of using the results obtained with the combination of two techniques to calculate the chemical composition of nanostructures from measurements of its structural weaknesses.
“Such distortions in different crystallographic directions, provided information on three-dimensional arrangement of chemical nanostructures with nanoscale spatial resolution. By combining the techniques, we can reconstruct the nanostructural chemical composition of these structures,” he said.
The Electron Microscopy Laboratory (LME) LNLS is a multi-user laboratory, which receives about 150 researchers per year. In his 11 years, became a reference in electron microscopy, especially in the area of chemistry.
Germanium-silicon islands
To conduct the survey, the group used the islands epitaxial silicon-germanium. “This is a classic system of semiconductors. It was one of the authors, Gilberto Medeiros-Ribeiro, who developed this system,” said Ramirez.
In the system, germanium is deposited on a surface of silicon nanocrystals – which is characterized by fairly flat – preheated to a temperature of about 500 ° C. As the layers are being deposited, the germanium pyramidal shape – or “islands” – on the silicon.
“Germanium and silicon have the same crystalline structure, that is, their atoms are arranged in similar order. But their sizes are slightly different and therefore the accumulation of several layers of germanium distorts so pronounced that form the islands. Part silicon, however, is drawn into these structures, “he said.
High precision measurement
Ramirez explains that the characteristics of this system of crystal structures had already been measured in other studies using X-ray diffraction But this is a very general measure, which requires certain distributions.
“In our case, it is as if we had produced pictures of the composition of these structures from various angles. Thus, we have obtained information from several directions, with a mathematical treatment, we started a three-dimensional mapping of the chemistry of nanostructures” he said.
According to the researcher, an important aspect of this work is that instead of just using the data obtained by transmission electron microscopy, the study has added knowledge to contribute to the development of new techniques.
“Being in a national laboratory that offers these advanced techniques, is part of our obligation not only to use them, but help develop them. This methodology could open new doors for the study and characterization of technologically important materials in areas such as electronics, “he said.
