Low Voltage Scanning Transmission Electron Microscopy of Oxide Interfaces
For over 40 years, electron microscopy, in particular scanning transmission electron microscopy has been used to visualize condensed matter structures with atomic resolution . Beginning in 1970, Crewe et al. showed for the first time that using a highly convergent 30 kV electron beam (convergence angle ∼20 mrad), single uranium and thorium atoms on 20 nm thick amorphous carbon films can be directly imaged using an annular dark field detector . At the time, the authors reported the smallest possible probe-size of 5 Å, which is sufficient to directly image individual heavy atoms on carbon support, but well in excess of the interatomic spacings in many crystals structures. Atomic-resolution lattice imaging of crystalline materials using annular dark field imaging was not possible using such an electron probe. Following Crewe's initial discovery, over the next few decades the trend in scanning transmission electron microscopy (STEM) moved towards increasing the electron energy from 30 keV up to the 1 MeV regime to decrease the electron wavelength, and therefore also increase the spatial resolution up to the ambitious goal of 0.5 Å in the STEM images . It is interesting to point out here that even at 30 kV, the wavelength of the electron beam is only about 7 pm, that is about 70 ...