Fluctuation Electron Microscopy (FEM) has become an effective materials structure characterization technique, capable of probing any medium-range order (MRO) that may be present in amorphous materials. Although its sensitivity to MRO has been exercised in numerous studies, FEM is not yet a quantitative technique. The holdup has been the discrepancy between the computed kinematical variance and the experimental variance, which previously was attributed to source incoherence. Although high-brightness, high coherence, electron guns are now routinely available in modern electron microscopes, they have not eliminated this discrepancy between theory and experiment. FEM of amorphous carbon, amorphous silicon and ultra nanocrystalline diamond samples, as well as modeling, was carried out in an attempt to explore and reveal the reasons behind this conundrum. Decoherence, caused mainly by beam-induced atomic motion in the sample, was identified as the source of this discrepancy. It is likely that decoherence is a strong contributor to the so-called Stobbs factor – a previously-reported discrepancy between calculated and experimental intensity in high-resolution TEM lattice images.
For further information please contact Renu Sharma, 301-975-2418, renu.sharma [at] nist.gov (renu[dot]sharma[at]nist[dot]gov)