Most of what we know about the structure of amorphous materials is limited to the pair correlation (or radial distribution) function. The existence of ordering on a medium length scale (~10 bond lengths) is poorly understood, yet likely of major importance to properties of amorphous materials. Direct imaging of atomic structure in amorphous materials is possible but very challenging, and it is difficult to examine a statistically significant number of atoms by direct imaging. Instead, the technique of fluctuation electron microscopy offers a powerful complementary solution, which also promises application to radiation sensitive materials.

Dr. Gibson will review the method, which relies on statistical analysis of coherent nanobeam electron diffraction patterns (4DSTEM), and demonstrate its sensitivity to higher order atomic correlations and medium range order. In his work he has primarily studied the structure of amorphous Si, but he will also discuss the work of some other groups on other materials, and some recent preliminary work on carbon.

Finally, he will discuss developments in the fluctuation microscopy technique and the potential for broader application to amorphous and disordered materials.