Characterizing heterogeneous particles with SEM/SDD-EDS mapping and NIST Lispix

Abstract

Compositional measurements of microscopic particles by scanning electron microscopy/energy dispersive x-ray spectrometry (SEM/EDS) typically assume particle homogeneity so that a representative EDS spectrum can be collected by continuously bracket scanning the particle (overscanning). Particles are often found to be complex structures comprised of smaller entities that are elementally different, knowledge of which is inevitably lost with particle overscanning. Heterogeneity can be directly visualized with x-ray spectrum image mapping performed in a high resolution thermal field emission gun SEM combined with the silicon drift detector (SDD)-EDS. SDD-EDS is capable of x-ray collection with output count rates in excess of 1 MHz, enabling spectrum image mapping with useful pixel density (128x128 or more), intensity range (8 - 16 bits), and compositional sensitivity (detection to approximately 1 weight percent) with a total time of 3 - 30 s when a high beam current (e.g., 50 nA) is utilized. Spectrum image datacubes can range from 100 Mbyte to several gigabytes. NIST Lispix contains extensive image processing tools to extract spectral and image information from such large datacubes. In addition to particle chemical heterogeneity, spectrum image mapping can directly reveal the effects of geometric factors (size, shape, curvature) that modify x-ray generation and emission from particles and which must be considered in particle quantification calculations.