Materials Analysis with Nanoscale Precision Using EDS

Interview conducted by Stuart MilneJun 8 2016

Bert Freitag, Technology Director for the Materials Science business unit at FEI, talks to AZoM about EDS and the important role it plays in material analysis.

Could you provide our readers with an overview of the FEI Company and the sectors that you supply equipment to?

FEI is committed to helping customers find answers that have a positive impact on quality of life. FEI manufactures electron microscopes and designs workflows that incorporate both hardware and software applications.

Researchers in academia, medicine, and industry use FEI instruments to research treatments for diseases such as cancer and to create innovative new materials used for clean energy, transportation, health, and industrial productivity.

Could you briefly explain to our readers how the technique of EDS (Energy-Dispersive X-ray Spectroscopy) works and how it can be integrated with other established techniques?

EDS is a spectroscopy technique providing information on the chemical compostion of material. The electrons pathing through the sample can scatter elastically without energy loss or inelastically with energy loss. Elastic scattering normally provides the image information in the transmission electron microscope, while inelastic scattering provides information about chemical compositions or electronic structure of the material.

In EDS spectroscopy the inelastic scattering of the primary electron excites an electron of the atom shell. The excited atom is responding by a recombination of a higher energy level electron into the empty shell position. During this process an xray is generated carrying the energy difference of the two electron shells involved.

For each atomic species of the periodic table these energy differences are unique and therefor the atom species illuminated by the electron beam can be identified. The intensity of the xray signal is proportional to the concentration of the elements and the ratio between the xray peaks in the spectrum can be used to determine the composition of the material.

In scanning transmission electron microscopy a small electron beam is moved across the sample generating an image by detection of the variation of the elastic scattered electrons in each point(bright field, dark field imaging). In EDS mapping simultaneously the xray emission in each point is acquired, providing a chemical map delivering structural and chemical information in one scan.

3D EDS tomography of Ag-Pt core-shell nanoparticles, where most of the Ag cores shown in the false color of red are covered by green-colored Pt shells with pores to expose partially the cores. Sample courtesy of Professors Yi Ding and Jun Luo at Center for Electron Microscopy, Institute for New Energy Materials & Low-Carbon Technologies, School of Materials Science & Engineering, Tianjin University of Technology, China.

Accurate compositional analyses are essential in the field of materials science. What are the advantages of using EDS over more conventional methods?

In TEM two standard methods are providing chemical information. EDS and EELS (electron energy loss spectroscopy) The energy range in EDS spectroscopy is much larger than in EELS typical 0.1keV to 40keV, while EELS covers efficiently only 0-a few keV in energy range. Therefore, more elements are accessible in EDS than in EELS for spectral analysis.

Moreover EDS has the advantage of providing chemical information on samples, which are thick or vary in thickness over the field of view. EELS suffers from multiple scattering effects which increase with thickness and makes quantification more difficult. Since the background of the EDS spectra are much lower than the background in EELS fitting is simpler compared to EELS.

Nevertheless, the higher energy resolution of EELS is beneficial for bonding state analysis and the detection of light elements compared to EDS. Hence both techniques are complementary.

For users coming from SEM platforms EDS is typically easier to use since EDS analysis is already possible on an SEM. The knowledge about the method is much wider spread in the material science community than EELS, which is exclusive for TEM studies. The limited lateral resolution in chemical analysis of an SEM can be overcome by EDS analysis on a high resolution S/TEM tool.

Please could you explain to our readers what a superalloy is and why you believe EDS is the best analytical technique for their study?

Superalloys are a metal class which is used in high performance applications. The special structure enables high mechanical stability at high temperatures and is therefore used in jet engines of gas turbines to increase their energy efficiency. The structure is not polycrystalline like in normal metals (e.g. steel) but is single crystalline, making its production more complicated in creation of the material and shaping it for engineering.

The single crystal consists of two almost structurally identical phases, gamma and gamma`(prime), which have different chemical composition. The microstructure and this chemical variation on the nanoscale determines their unique performance. EDS is pivotal in providing chemical information on the nanoscale to understand the property function relationship for more powerful superalloys.

EDS is currently being used to characterise catalysts in situ to observe their structural changes upon heating. Why is this research important?

Catalysts enable highly efficient and clean production of various materials.They are key for industries to provide environmentally friendly products. Since the reactions are happening at elevated temperatures, the material has to be examined at these temperatures due to the fact that the structure and the elemental distribution in a nanoparticle can be different under these conditions.

For example, a gold-silver nanoparticle can be a perfect alloy with homogeneous elemental distribution at room temperature, but at high temperature the silver is covering the surface. Obviously the chemical reactivity is strongly influenced by this redistribution, which can only be observed at high temperature.

What do you believe are the advantages of using your EDS technology over other products available on the market?

The windowless and symmetric multi-detector design of the Super-X detector provided the first high sensitivity EDS solution for 3D EDS, in-situ and high efficiency light element detection with the resolution of a S/TEM tool.

The integration in the tool without the need to retract the detectors, which disturbs the experiments, is unique — as is the capability to do EDS up to 800°C with the FEI NanoEx heating holders. The 3D visualisation results from the automatic acquisition of the tomograms and FEI software to provide a complete reproducible workflow for 3D chemical information.

What else does FEI offer that differentiates you from your competitors?  

FEI is committed to helping customers find answers that have a positive impact on quality of life. We pride ourselves in helping our customers get more scientific data and better answers faster.

What steps does FEI take to limit their environmental impact?

The ROHS compliance of our tool and accessories & energy efficient, effective production of our tools. At FEI we keep our promises and use continual improvement. This enables us to satisfy our customers by delivering safe, reliable, valued products and services while complying with applicable quality and regulatory requirements.

How do you see EDS technology progressing in the next decade, and what role do you think FEI will play in this?

Today, for basic research, EDS mapping has replace the energy filtered TEM chemical mapping technique using an imaging filter, because it is easy, allows for faster time to data and even allows atomic chemical resolution or detection low concentration dopants.

For engineers, 3D chemical mapping provide new insight into real structures, not only 2D projections. The in-situ capabilities allow for chemical changes under working conditions of the materials in catalysis in chemical industry.

All these applications need reproducible quantitative chemical information. Evolutionary steps will be provided in the next decade toward stronger signals to push the detection limits, to lower electron dose requirements needed for new material classes, or to increase the range of temperatures for EDS.

The frontier in new applications for chemical mapping are laying in the field of more delicate materials research, which barely survive the imaging methods in modern S/TEM. For example, polymers, 2D materials like graphene, or life science samples need a robust fast spectroscopy technique to reliably provide chemical information with the resolution of a TEM.

Addtionally, in-situ research requires time resolved spectroscopy to understand the kinetics of reactions on the nanoscale and the needs will grow for this information.

These challenges will be addressed with EDS spectroscopy solutions by future FEI solutions.

About Bert Freitag

Bert Freitag obtained his PhD degree in Natural Science on Experimental Physics in the University of Cologne (Ger) in 1995.

He was a postdoctoral researcher in the Institute for Inorganic Chemistry in Bonn (Ger) before joining FEI in 2000 as an application specialist.

During his time at FEI his role has changed from product management to marketing manager for the entire high-end TEM research market served by the Titan platform.

Today he works as Technology Director in the Materials Science BU.