Assessing the Accuracy of Soot Nanoparticle Morphology Measurements Using Three-Dimensional Electron Tomography

Abstract

Morphology plays an important role in determining behaviour and impact of soot nanoparticles, including effect on human health, atmospheric optical properties, contribution to engine wear, and role in marine ecology. However, its nanoscopic size has limited the ability to directly measure useful morphological parameters such as surface area and effective volume. Recently, 3D morphology characterization of soot nanoparticles via electron tomography has been the subject of several introductory studies. So-called '3D-TEM' has been posited as an improvement over traditional 2D-TEM characterization due to the elimination of the error-inducing information gap that exists between 3-dimensional soot structures and 2-dimensional TEM projections. Little follow-up work has been performed due to difficulties with developing methodologies into robust high-throughput techniques. Recent work by the authors has exhibited significant improvements in efficiency, though as yet due consideration has not been given to assessing fidelity of the technique. This is vital to confirm significant and tangible improvements in soot-characterization accuracy that will establish 3D-TEM as a legitimate tool. Synthetic ground-truth data was developed to closely mimic real soot structures and the 3D-TEM volume-reconstruction process. A variety of procedures were tested to assess the magnitude and nuances of deviations from ground-truth values. Results showed average Z-elongation due to the 'missing-wedge' at 3.5% for the previously developed optimized procedure. Mean deviations from ground-truth in volume and surface area were 2.0% and -0.1% respectively. Results indicate highly accurate 3D-reconstruction can be achieved with an optimized procedure that can bridge the gap to permit highthroughput 3D morphology characterization of soot.

Automated particle recognition for engine soot nanoparticles

“…Due to the nanoscopic size of soot (50–500 nm), 13,14 transmission electron microscopy is required for imaging. Transmission electron microscopy (TEM) has been widely used to characterise properties such as maximum length, fractal dimension, radius of gyration and primary particle diameter 13,15–18 . However, TEM produces 2D projections while soot nanoparticles are known to be complex and highly irregular 3D structures, leading to significant errors 17,19–22 .…”

“…Electron tomography (ET) has been used to create 3D reconstructions of soot nanoparticles from a series of 2D TEM images acquired over a wide range of angles (e.g. ±60°) 17,20–25 . ET can produce highly accurate 3D models of soot nanoparticles but is significantly slower due to each particle requiring multiple images, and suitable particles being more difficult to locate on a TEM grid.…”

“…ET can produce highly accurate 3D models of soot nanoparticles but is significantly slower due to each particle requiring multiple images, and suitable particles being more difficult to locate on a TEM grid. Work by our research group has aimed to optimise the ET process for soot by implementing automated image acquisition and processing to identify suitable particles over large areas (∼40 μm 2 ) of the TEM grid 17,19,23 . This procedure also permits rapid measurement of particles in 2D, allowing greatly increased sample sizes and thus increased reliability of TEM‐derived measurements 26 .…”

“…This procedure also permits rapid measurement of particles in 2D, allowing greatly increased sample sizes and thus increased reliability of TEM‐derived measurements 26 . However, due to low contrast of soot against the TEM grid substrate, and high concentrations of non‐soot particles in samples, a manual review process was required to separate real soot structures from false positives 17,23 …”

“…±60 • ). 17,[20][21][22][23][24][25] ET can produce highly accurate 3D models of soot nanoparticles but is significantly slower due to each particle requiring multiple images, and suitable particles being more difficult to locate on a TEM grid.…”