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Retour du Jeudi - 09/05/19 - Momentum resolved spectroscopy of the dielectric response by TEM

Momentum resolved spectroscopy of the dielectric response by TEM

by Frédéric Fossard, LEM – UMR CNRS/ONERA

Considering dielectric properties of 2D materials, specific questions arise among the screening effects of charge carriers or effects induced by the substrate and highlight the necessity of studying these properties on free standing pristine layers. This can be done in a Transmission Electron Microscope using electron energy loss spectroscopy. Furthermore, in contrast to standard optical spectroscopies, the transfer momentum dependence of the excitations is accessible from the measured loss function provided to operate an angular resolved EELS. Measuring the q dependence of the loss function needs to build a data cube in the diffraction space relying energy losses and q vectors in the diffraction plane.

I will present two acquisition modes that provide complementary pieces of information and offer a global view of excitations in reciprocal space. I will show the capabilities of this setup through the study of various 2D layered materials. I will show that we can access to the loss function of 2D materials and that the results are comparable to that obtained from non resonant x-ray inelastic scattering but with advantageous specificities (enhanced sensitivity at low q, local measurement) that make it well adapted to the study of two-dimensional materials and related heterostructures. As a final part of the presentation, I will shortly introduce arising techniques that give access to more physical data in the sample and discuss the main applications of our new TEM project, in particular in the field of semiconductors and their application to solar cells

(left) Energy filtered scattering patterns of hBN in the basal plane measured at different transferred energies. Comparison between measured and calculated loss function at 8 eV (middle) and 12 eV (right). Dashed lines represent the in-plane Brillouin zone boundaries

Date : Jeudi 9 Mai, 13h30, amphi Becquerel