Energy dispersive x-ray spectroscopy (EDS) is an important technique in the microscopist’s materials analysis toolbox. Silicon-based detectors are used by most EDS systems to detect characteristic x-rays produced by interactions between the sample and the incident electron beam.
Using elemental mapping in the SEM and EDS spectrum imaging (EDSSI), EDS can detect even trace quantities of elements within a material.
With modern software packages, EDS spectra and maps can now be obtained easily. The tool can be used by both the beginner and advanced EM operator to acquire compositional data of a reaction, with results that can often be interpreted easily.
Recent developments in EDS systems and detectors include multiple detectors in a single system and silicon drift detectors (SDD) with a large detection area (large solid angle). These developments have allowed TEMs with Cs aberration correctors on the probe forming optics to investigate atoms one by one. Atomic resolution EDSSI can now be performed.
It is essential that compatible sample holders are used to fully utilize new and existing EDS systems. In a TEM, an EDS detector is usually located at an angle of ~10 - 20º with regard to the sample. As EDS systems are capable of detecting photons (x-rays), they need direct line of sight from the sample to the detector.
For instance, the design of bulk type heating holders cannot be used with the EDS systems. The sample is surrounded by the furnace without any direct line of sight from the sample to the EDS detector, which limits the TEM’s functionality for in situ analysis.
A flat ceramic membrane is used by the Fusion system that acts as both the sample support and the heating element. By design, the system can be used with EDS because of its direct line of site with the detectors. In fact, Fusion does not restrict EM capabilities and can be used with most tools and techniques, including STEM, EELS, and diffraction. Therefore, compatibility with these tools makes Fusion a powerful and versatile platform for in situ science.
Detectors with an opaque window enable high temperature EDS with Fusion. For systems that have no window or a thin transparent window, EDS spectra can be easily obtained by exploiting the Fusion’s rapid heating and cooling rates (up to 1,000 °C/ms).
Figures 1 and 2 clearly show the EDS spectra in the SEM and TEM. In both experiments, particles were deposited through solvent suspension dispersion onto an E-chip™. As shown in Figure 1, TEM EDSSI spectra of Pd/Rh particles are collected with a JEOL 2010F in STEM mode, followed by collecting Ag and Cu SEM EDS element maps with a JEOL JEM-7600F on Ag/Cu particles (Figure 2).
The sample was subjected to multiple heat cycles and returned to room temperature (RT) for additional SEM imaging and collection of EDS elemental maps.
Figure 1. TEM EDS spectrum images of core/shell Pd/Rh alloy particles
Quantitative TEM EDS spectrum images of Pd/Rh particles are displayed in Figure 1, which shows the spatial distribution of Rh within the particle, and the concentration is indicated by the color bar. By comparing the images before and after heating to 700 °C, the evolution and rearrangement of Rh throughout the particle can be observed.
The particle starts as a core/shell of Pd/Rh, and after the Pd and Rh mix is heated it becomes more uniformly distributed throughout the particle. These particles are being analyzed for hydrogen storage applications, and in this case Rh is used to increase the thermal stability of the particle.
It is important to understand the evolution of Rh as a function of temperature, as Rh exhibits a higher melting point and its distribution is critical to thermal performance. SEM elemental maps are shown In Figure 2, where Cu/Ag particles at RT are shown In Figure 2A and the particles following heating in sequence to 800 °C for 60 seconds and then to 900 °C for 20 seconds are shown in Figure 2B.
Figure 2. Heating Cu/Ag nanoparticles to 900 °C
When the outer shell of Ag is heated, it undergoes a distinct transformation as shown in the figures. Following additional heating, the Ag evaporates and starts to alloy with Cu producing a CuAg intermetallic compound.
This behavior follows the basic bulk CuAg phase diagram, especially in the regime where the Ag percentage is relatively small. The Ag evaporation and alloying process occur rapidly at high temperatures. Bulk heating holders heat and cool too slowly to capture this reaction.
Figure 3 shows an EDS spectrum of the ceramic membrane with a thin carbon overlay collected in the TEM. Oxygen and silicon from the E-chip is detected together with carbon from the thin overlay. Copper is a component in the four contact pins on the holder; although beryllium is also present in the contact pins, it is too light to reliably detect with the EDS systems employed here.
Also, copper or tungsten may appear, which originate from the holder tip materials. The presence of these materials should be considered when analyzing samples. For instance, if a sample contains silicon, it cannot be measured with EDS using E-chips but silicon may be detected with EELS.
Figure 3. EDS spectrum of the MEMS heating membrane
EDS is a versatile and important tool for materials characterization in the EM. Data quality and ease of use are being improved by new types of software and detectors. In the past, many in situ characterization instruments could not be used with such analytical tools.
For example, bulk heating holders are not often compatible with EDS systems, and their rates of cooling and heating are not quick enough to capture many dynamic reactions. The Fusion holder is designed in such a way that full compatibility with all EDS systems is ensured. The materials that make up the E-chips and holder limit analysis of these elements, but despite this fact, many elements can be detected and analyzed.
SEM Experiments: N. Erdman, T. Laudate, S. Mick, Micros. Microanal., vol. 17, pp. 514-515. The TEM EDS measurements were done at Sandia National Laboratories in Livermore, CA.
This information has been sourced, reviewed and adapted from materials provided by Protochips.
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