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

A high performance electron lens can distinguish scattering from different regions of a sample, i.e., atomic density ρatm(r)=∑iδ(r−Ri), with a resolution approaching ∼1 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 `amorphousʹ with `randomʹ. 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 ∫ρatm(r) dz, and hence provides a probe for triplet correlations (for example). Secondly, while diffraction depends on |A(k)|2 (where A(k)=FT{ρatm(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.