Electron Charge Exchange Mechanism Revealed by UCI Scientists
Scientists at the University of California, Irvine, have now developed a new scanning transmission electron microscopy method that enables the visualization of the electric charge density of materials or molecules at sub-angstrom resolution.
With microscopy technique, the UCI researchers were actually able to observe electron distribution between atoms & molecules and also to uncover clues to the origins of ferroelectricity, the capacity of certain crystals to possess the spontaneous electric polarization that can be switched by the application of an electric field. The research study, which is published today in Nature, also revealed the mechanism of charge transfer between two molecules.
Xiaoqing Pan who is the team leader at UCI’s Henry Samueli Endowed Chair in Engineering & a professor of both materials sciences and engineering and physics & astronomy, said that the newly discovered method is an advancement in electron microscopy right from detecting atoms to imaging electrons that could help researchers engineer new materials with desired properties & functionalities for devices used in data storage, energy conversion & quantum computing.
Employing a new aberration-corrected scanning transmission electron microscope with a fine electron probe measuring half an angstrom & a fast-direct electron detection camera, his group was able to acquire two-dimensional raster images of diffraction patterns from a region of interest in the sample. As obtained, the data sets are four-dimensional, since they consist of two-dimensional diffraction patterns from each probe location in two-dimensional scanning areas.
With this new microscope, researchers can routinely form an electron probe as small as 0.6 angstroms, and our high-speed camera with angular resolution can acquire four-dimensional STEM images with 512 x 512 pixels at greater than the 300 frames per sec, Pan said. He added that by using this technique, researchers can see the electron charge distribution between the atoms in 2 different perovskite oxides, non-polar strontium titanate, & ferroelectric bismuth ferrite.
Electron charge density in bulk materials and molecules can be measured by Xray or electron diffraction techniques by assuming the perfectly defect-free structures within the beam illuminated area. Pan said, there remains a challenge in resolving the electron charge density in nanostructured material consisting of the interfaces & defects.
In principle, local electric field & charge density can be determined by electron diffraction imaging using an aberration-corrected scanning transmission electron microscope with sub-angstrom electron probes, he said. While penetrating through the specimens, the electron beam interacts with the internal electric field of materials in its pathway, resulting in changes in its momentum reflected in the diffraction patterns. By measuring these changes, the electric field in a local region of the specimen can actually be delineated, & the charge density can be derived.
Pan said that although this principle has been demonstrated in simulations, no experiments have been successful until now.
Lead author Wenpei Gao, a University of California Irvine postdoctoral researcher in materials science & engineering, said that the electron charge density maps obtained using the four-dimensional STEM method match with theoretical results from the first-principle calculations,” said the research study of the ferroelectric/insulator interface between bismuth ferrite & strontium titanate using the technique directly shows how features of the bismuth compound’s polar atomic structures leak across the interfaces, appearing in the normally non-polar strontium titanate. And as a result, the interface hosts excess electrons confined to a small region which is less than 1 nanometer thick.
Pan added that this research project gives materials researchers, scientists & engineers new tools for evaluating the structures, defects, &interfaces in functional materials and also the nanodevices. He noted that it may soon actually be possible to conduct high-throughput mapping of the charge density of materials &molecules to add to the database of properties aiding in the Materials Genome Initiatives.
As the electron microscopy advances from imaging atoms to probing electrons, it will lead to new understanding & discovery in materials research said co-author Ruqian Wu, UCI professor of physics & astronomy, who led the study’s theoretical work. He further added that the ability to image the charge density distribution around atoms near interfaces,the grain boundaries or other planar defects opens up the new fields for electron microscopy & materials science.
Author: Ria Roy
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