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. FTEM has been successful in demonstrating the theoretically predicted development of nanoscale nuclei in amorphous chalcogenides, as well as in revealing the subtle effect of different preparation routes on the medium-range order in amorphous semiconductors and metals. The fluctuation principle can also be applied to study structural order on longer length scales in polymers and other disordered materials using X-rays or visible light. Further advances in theory and practice of FTEM will greatly increase our understanding of amorphous structures and nucleation phenomena.

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. This study provides a convincing proof of the time- and temperature-dependent development of nuclei, information that will play a critical role in the development of advanced materials for phase-change memories.