Atomic structure of amorphous solids from high resolution electron microscopy - a technique for the new millenium?

A high performance electron lens can distinguish scattering from different regions of a sample, i.e., atomic density ?atm(r)=?i?(r?Ri), with a resolution approaching not, vert, similar1 Å – information which is not available from any other technique. Early high resolution electron microscopy (HREM) studies of amorphous solids faced scepticism due to the association of ‘amorphous' with ‘random'. This paper demonstrates two ways in which HREM gives information about atomic structure, which is not available from diffraction. Firstly, while diffraction depends on pair correlations, HREM depends on ??atm(r) dz, and hence provides a probe for triplet correlations (for example). Secondly, while diffraction depends on A(k)2 (where A(k)=FT{?atm(r)} is the scattering amplitude), HREM depends on A(k). This provides a probe for locally anisotropic medium range order (MRO), such as in the quasi-Bragg plane model for MRO in a-SiO2 by Gaskell et al. These relationships are demonstrated using computer simulations on models of ta-C.