Chemical Physics Seminar: Mapping of Charge Distribution in Nanometer-scale materials by Electron Holography

Abstract:

Electron holography (EH) is a methodology used in transmission electron microscopy (TEM) to reconstruct the phase of an electron wave after it traversed through the sample. When applied at nanometer scale spatial resolution, the reconstructed phase enables to map electrostatic and magnetic fields.

I will present recent applications of EH combined with TEM imaging and spectroscopy to characterize charge redistribution in nanoscale semiconductor and dielectric materials.

The first case is a semiconducting hybrid of a PbS core and CdS arms that exhibit epitaxial relations. In these structures, fluorescence emission from CdS arms is quenched, attributed to energy band alignment with the PbS core. EH measurements of single structures compared to 3-dimensional Poisson calculations show a Type I interface in which the electric field is mostly redistributed on the CdS arm, causing charge carriers to separate and prevent recombination.

The second case is non-stoichiometric granular spinel, MgO∙Al2O3, for which EH measurements show that the space charge potential and grain boundary segregation are controlled by excess cation species and also by the grain dimensions with respect to the Debye length. Moreover, by applying an electric field during sintering, the magnitude of the space charge potential is decreased.