ABERRATION-CORRECTED ELECTRON MICROSCOPY

Microscopy allows us to observe objects we cannot see with our 4 eyes alone. With a light microscope, we can distinguish objects at 5 the scale of the wavelengths of visible light just under a microme6 ter. Around 1870 Ernst Abbe, who laid the foundation of modern 7 optics, suggested that the resolution of a microscope would improve 8 by using some yet-unknown radiation with shorter wavelengths than 9 visible light, that is, below 390 nanometers (1 nm= 10−9 m). Elec10 trons can have wavelengths near 1 picometer (1 pm= 10−12 m) and 11 should therefore allow atoms to be distinguished, since they are typ12 ically at least a few hundred pm apart. In this oversimplified view, 13 further decreasing the wavelength of the radiation should allow us to 14 increase the resolution of an electron microscopy even more. However, 15 this can only be done by increasing the energy of the radiation, which 16 would ultimately destroy the samples. Other factors also affect the 17 resolution of electron microscopes, among them non-ideal imaging 18 properties of electromagnetic lenses, which result in false images. It 19 took more than half a century to understand and control these aber20 rations and separate objects less than 1 Ångstrom (1 Å = 10−10 m) 21 apart. 22 At the Technische Universität Berlin in 1931, Max Knoll and his 23 doctoral student Ernst Ruska showed that magnetic coils could be 24 used as lenses for electrons and thereby cleared the path to developing