Sapphire
Electron imaging - Methodology
When the electron beam passes through the specimen in the TEM, it interacts with the electrostatic potential formed by the nucleus and electrons. In particular, the phase shift of the electron wave occurs, and the electrostatic potential inside the specimen can be inferred by measuring phase shift of electron wave. For this, convergent beam electron diffraction (CBED) images are collected from every scan positions. Using the stack of images, information on the specimen is extracted using techniques such as differential phase contrast (DPC) or electron ptychography.
Bonding electron, orbital
Since the electron distribution in the atomic scale is closely related to the chemical, electronic, and optical properties, its measurement has received attention to material science society. Attempts have been made to measure the charge distribution using 4D-STEM based center-of-mass measurement or electron ptychography, but most of the analysis couldn’t reach to provide quantitative analysis but they stopped at the comparison with density functional theory (DFT) calculations. We attempted to detect the distribution of non-symmetrically occupied electrons using electron ptychography. We select the SrTiO3 (STO) as a model system which is a popular material as an oxide substrate. It is widely accepted that electrons are confined at Ti dxy orbital which implies directionality in parallel direction to the surface. We formed an environment in which 2DEG can be formed on the surface was created in TEM using an in-situ heating holder. We applied electron ptychography on the system and measured electrostatic potential for imaging spatial distribution of electrons which is firstly confirmed by using electron energy loss spectroscopy (EELS). The electrons confined in laterally elongated dxy orbital are detected from spatial distribution of potential and charge density in sub-atomic scale.
Electride, Wigner crystal
It is predicted that a group of electrons could crystallize in a solid form under certain conditions to form the phases now known as Wigner crystals. This requires a precise balance between the two forces that affect electrons: electrostatic repulsion and kinetic energy. The latter has a stronger effect, causing the electrons to bounce off randomly, but if the extent can be reduced sufficiently, repulsion can follow and fix the electrons into a uniform lattice, Wigner suggests. It is known that such condition are satisfied when a specific temperature and the number of electrons are controlled. We undertake the imaging of Wigner crystal by using electron ptychography phase contrast imaging.
