The scanning electron microscopy is used not only to exactly measure the surface of material samples but also to determine their chemical composition. Therefore the electron beam excites the atoms of the material sample to fluorescence during the scanning process. The fluorescent radiation contains information regarding location and type of chemical elements within the sample. However especially the light elements in the periodic system emit radiation in an energy range that cannot be resolved exactly enough with energy dispersive spectrometers (EDS). Nevertheless, these materials including lithium, beryllium, boron, carbon and nitrogen, play a significant role in energy materials as well as in functional materials.
The Helmholtz Zentrum Berlin (HZB), a research centre dedicated to materials research, now offers a solution: Professor Alexei Erko who oversees the HZB’s institute of nanometre optics and technology has developed and patented already some time ago novel optics consisting of so-called reflection zone plates. These consist of thousands of concentric and elliptic structures and are used in the analysis of low-energy x-ray analysis at synchrotron sources such as BESSY II. These optics do not cause refraction like conventional glass optics but instead diffraction, generating interferences.
Inspired by these properties, the Institut für Angewandte Photonik (Institute for Applied Photonics) and the Institute for Scientific Instruments GmbH, a commercial company, tested the use of optics based on reflection zone plates in electron microscopes to improve the resolution in the low energy range. As a result of this research project, these partners now presented a functional prototype of a wavelength dispersive spectrometer (WDS). It enables electron microscope users to detect even light elements at very high precision. Besides lithium, boron and beryllium it can be used to verify carbon and oxygen, explained Erko.
The spectrometer consists of an array of 17 reflection zone plates and covers the energy range from 50 eV to 1120 eV. To achieve even higher resolution, the scientists also built e lens with 200 reflection zone plates which delivers almost continuous spectral measurements in the range from 100 eV to 1000 eV.
High-resolution measurement techniques are critical to detect light elements of the periodic system. This is particularly relevant for research on energy materials such as solar cells, batteries, solar fuels and catalysers. "It however could also be useful in the research of magnetic materials and in life sciences", said Erko. "Now we are curious which scientific research questions will be attacked with this innovative tool."