Analytical and computational modeling of fluctuation electron microscopy from a nanocrystal/amorphous composite.

A nanocrystal/amorphous composite is an idealized model of the medium-range order structure in a variety of amorphous materials. We have investigated the fluctuation electron microscopy (FEM) signal from such a model analytically and with computer simulations. In the analytical modeling, we improved the previous model by Stratton and Voyles (Ultramicroscopy 108, 727 (2008)) by introducing the partial occupancy of nanocrystals in a column, the effect of the deviation parameter on the diffracted intensity, and a distribution of nanocrystal sizes. The improved model no longer has a maximum in the FEM signal as a function of the volume fraction of nanocrystals. In the computer modeling, we compared the variance calculated using kinematic scattering and dynamical scattering and investigated the effects of disorder and strain inside the nanocrystals on the variance. The variance based on dynamical scattering is about 15% smaller than that based on kinematic scattering. Disorder introduced in the nanocrystal reduces the variance peaks at all scattering vectors, with a much larger reduction at longer scattering vector.