Fluctuation transmission electron microscopy: detecting nanoscale order in disordered structures.

The structures of many disordered materials are not ideally random, but contain structural order on the scale of 1-3 nm. However, such nanoscale order, called medium-range order, cannot be detected by conventional diffraction methods in most cases. Fluctuation transmission electron microscopy (FTEM) has the capability to detect medium-range order in disordered materials based on statistical analysis of nanodiffraction patterns or dark-field images from TEM.

Observation of the role of subcritical nuclei in crystallization of a glassy solid.

Phase transformation generally begins with nucleation, in which a small aggregate of atoms organizes into a different structural symmetry. The thermodynamic driving forces and kinetic rates have been predicted by classical nucleation theory, but observation of nanometer-scale nuclei has not been possible, except on exposed surfaces. We used a statistical technique called fluctuation transmission electron microscopy to detect nuclei embedded in a glassy solid, and we used a laser pump-probe technique to determine the role of these nuclei in crystallization.