Measuring Charge State at the Single-Atomic-Column-Base with Four-Dimensional Scanning Transmission Electron Microscopy

Abstract

Charge state influences many chemical and physical properties of an atom, such as the state of oxidation or reduction, the availability in forming bonds with other elements, or the ability to promote charge transfer. Engineering the charge state of atoms makes it possible to control the electronic, magnetic and chemical properties. For example, in catalysts, the surface charge state defines the chemical activity of an atom. For small gold clusters, the modification of electron orbits transforms the metallic bond into a covalent bond, converting gold into a non-metallic state.[1] Different approaches can be used to measure the charge states of atoms, including X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, electron energy loss spectroscopy (EELS),[2] or scanning probe microscopy. The spectroscopic methods rely on the electron-orbital interactions, where the absorption of photon or electron energy reflects the status of orbital electron distribution. The X-ray based spectroscopic probes usually offer a spatial resolution of about micrometers, with location related properties mostly missing. While EELS in scanning transmission electron microscopy (STEM) can offer better spatial resolution, the detection sensitivity is limited by noise, making it even more challenging to measure the charge states of heavy elements such as Bi, Pb and Sr, where the major edges are around or more than 2000 eV. Recently, by coupling a high-speed pixelated electron detector with an aberration corrected STEM, electric field mapping can be obtained using scanning diffraction or four-dimensional (4D) STEM. Projection of charge density in two-dimension (2D) can be derived by using the divergence of the electric field map, here we show the possibility to detect the charge state of atoms using the charge density image.