With advances in instrument stability and electron-optical hardware, especially aberration correction, atomic resolution imaging has become routine in the scanning transmission electron microscope (STEM) over the past decade. The more recent addition of high-speed, high-dynamic range electron detectors, [1-3] has opened up new experimental capabilities in the microscope. Every pixel in every image is its own scattering experiment, providing a rich source of information for determining materials’ structures including strain, atomic ordering, the presence of dopants, etc. Here we investigate how the additional information captured by these detectors can be used to improve “atom counting” methods. [4-9]
Atomic resolution high-angle annular dark field (HAADF) STEM imaging can produce images with contrast sensitive to the atomic number of the constituent atoms. Under certain conditions, this can be used to overcome the ‘projection issue’, where three-dimensional shape is not inherently obvious as a specimen is imaged in transmission. If carefully calibrated HAADF-STEM imaging is combined with simulation, the experimental transmitted probe intensity can be converted to specimen thickness to gain information about a specimen’s 3D shape. However, to achieve this with atomic level precision and accuracy is challenging and can require detailed knowledge of the incident electron wavefield, crystal structure of the specimen, and detector response. [4-9]
By making use of the new generation of STEM detectors to measure the full angular distribution of scattered electrons, rather than just integrating high-angle scattering, far more information about the electron-specimen interaction at each probe position, or pixel, is available. This paper will discuss how this information might be used to determine the number of atoms in each atomic column with greater accuracy and precision, while also requiring less a priori knowledge about the specimen and experimental conditions. The method will be illustrated for determining the shape of metallic nanoparticles developed for plasmonic applications.