Quantifying nanoscale order in amorphous materials via scattering covariance in fluctuation electron microscopy.
Fluctuation Transmission Electron Microscopy (FTEM) has a unique ability to probe topological order on the 1-3 nm length scale in diffraction amorphous materials. However, extracting a quantitative description of the order has been challenging. We report that the FTEM covariance, computed at two non-degenerate Bragg reflections, is able to distinguish different regimes of size vs. volume fraction of order. The covariance analysis is general and does not require a material-specific atomistic model. We use a Monte-Carlo approach to compute different regimes of covariance, based on the probability of exciting multiple Bragg reflections when a STEM nanobeam interacts with a volume containing ordered regions in an amorphous matrix. We perform experimental analysis on several sputtered amorphous thin films including a-Si, nitrogen-alloyed GeTe and Ge₂Sb₂Te₅. The samples contain a wide variety of ordered states. Comparison of experimental data with the covariance simulation reveals different regimes of nanoscale topological order.
Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.
Tian T
TT
Li
Kristof
K
Darmawikarta
John R
JR
Abelson
D046529:Microscopy, Electron, Transmission / Q000379:methods*; D036103:Nanotechnology / Q000295:instrumentation* / Q000379:methods*
Fluctuation Transmission Electron Microscopy; Nanoscale order; Scattering; Statistics