Invited
MAPPING OF CHARGE DISTRIBUTION IN NANOMETER-SCALE SEMICONDUCTOR HETEROSTRUCTURES BY ELECTRON HOLOGRAPHY

Electron holography (EH) is a methodology used in transmission electron microscopy 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 both electrostatic and magnetic fields.

I will present a recent application [1] to characterize charge redistribution in a nanoscale semiconducting hybrid. PbS rock salt core − CdS wurtzite arm multipods were synthesized with epitaxial relations of PbS{111}⟨011⟩∥CdS{0002}⟨112̅0⟩ and a sharp chemical interface. Ensemble fluorescence measurements show quenching of the optical signal from the CdS arms indicating charge separation due to the heterojunction with PbS. A finite-element three-dimensional calculation of the Poisson equation shows a type-I heterojunction, which would prevent recombination of electrons and holes in the CdS arm after optical excitation. A built-in potential of 0.5V is estimated across the junction, though as opposed to the thermal equilibrium calculations, significant accumulation of positive charge at the CdS side of the interface is detected. We find that the multipod geometry prevents efficient removal of generated charge carriers by the high energy electrons of the microscope. Simulations of generated electron−hole pairs in the insulated CdS arm of the NMH indeed show charge accumulation of ~1.7V in agreement with experimental measurements.

Thus, electron holography can be applied as a complementary methodology to ensemble measurements by mapping the charge distribution in single nanoscale heterostructures with complex geometries.

[1] Rajesh Chalasani, Alexander Pekin, Alexander Rabkin, Ran E Abutbul, Oswaldo Diéguez, Yaron Kauffmann, Yuval Golan, Amit Kohn, “Mapping Charge Distribution in Single PbS Core–CdS Arm Nano-Multipod Heterostructures by Off-Axis Electron Holography”, Nano Lett., 17(5), 2778-2787 (2017)