From 2-D to 3-D to 4-D electron microscopy: an outline of the essence and promise of unconventional crystallography and spectroscopy

After outlining his debt to, and indirect contact with, W.L. Bragg, who was his predecessor-but-one as Director of the Royal Institution, the author first outlines how the simplest of 2-D electron microscopic techniques - the gold decoration method of Betghe - was used to determine point-defect (vacancies) and line defect (screw dislocations) concentrations in layered solids such as graphite. There follows an illustrative summary of the various modes of utilizing 3-D electron microscopy that encompasses electron crystallography, electron energy-loss spectroscopy and electron tomography. These shed light on the structural and morphological features of solids, and they reveal in exceptional detail, often at sub-unit cell level, both regular and irregular intergrowths. In contrast to X-ray or neutron-based methods of structural elucidation, which produce structural features averaged over as many as 1014 unit cells, 3-D electron microscopy probes as little as 103 or so unit cells. It also enables the visualization of numerous features through direct imaging. Finally, the essence of 4-D electron microscopy is adumbrated. Here, the time-domain (down to the femtosecond level) is added to the 3-D spatial ones, thereby promising, and already providing, unprecedented insights into the dynamics of change within, and at, the surfaces of solids.