The spatial resolution of energy dispersive spectroscopy (EDS) analysis is fundamentally limited by the interaction volume of the characteristic X-ray emission.
Typical EDS analysis in an SEM is performed at relatively high energy (> 10 kV), leading to a huge interaction volume on the order of micrometers. Lowering the energy of the primary electron beam reduces the interaction volume significantly, but brings many practical challenges, including low count rates during analysis.
An alternative method to reduce the interaction volume is to use a thin sample, on the order of tens of nanometers. The sample is now thin enough to be transparent to the electron beam of a transmission scanning electron microscopy, making STEM-EDS possible.
High resolution EDS mapping of Co nanoparticles embedded in mesoporous silica measured at 30 kV. Individual Co nanoparticles approximately 10 nanometers in size are resolved.The STEM-EDS technique is especially suitable for nanomaterials. Due to their nanometer-scaled size in at least one of their dimensions, they are naturally transparent to a high energy electron beam, and thus can be prepared by simply dispersing them on a typical copper grid with a carbon film.
In the example shown above, the Co nanoparticles dispersed in mesoporous silica are imaged at 30 kV in high angular dark field mode. The Co nanoparticles have an average size around 10 nanometers and are deposited inside the nanometer sized channels of the mesoporous silica. The elemental mapping shown demonstrates the capability of STEM-EDS to resolve nanometer-scales objects even below a size of 10 nanometers.
However, there are practical challenges to STEM-EDS resulting from stray X-ray signals. This technology note reviews how to overcome these challenges for successful STEM-EDS with nanometer resolution.
Read the technology note for the full details and caveats of nanometer scaled resolution STEM-EDS analysis.
Learn about ZEISS scanning electron microscopes.
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