Electron nanodiffraction methods for measuring medium-range order.

Electron nanodiffraction in a scanning transmission electron microscopy (STEM) instrument with a beam diameter of the order of 1 nm can be used to assess the medium-range ordering, or the correlation of atom positions over distances of 1-3 nm, in thin films of disordered materials. Proposals are made for measurements of medium-range order by use of a thin annular detector in STEM, to give the equivalent of the variable-coherence microscopy of Treacy and Gibson (Acta Cryst. A 52 (1996) 212) and by measuring the correlation of diffraction intensities from neighboring nanometer-diameter regions. Two simpler methods for measuring the average dimensions of regions of correlated structure, by observing the persistence of diffraction spots as the beam is translated over the specimen, and by observing the dimensions of spots in greatly defocused diffraction patterns, have been proposed and applied to the study of thin films of amorphous carbon, silica and silicon nitride.