Near Band Edge excitation in 2D materials by Transmission Electron Microscopy

Abstract

In this work, we report on the characterization of near band edge excitation by electron energy loss spectroscopy (EELS). This technique is operated in a Transmission Electron Microscope and allows to rely the structure of a material obtained by HR‐TEM with its chemical and physical properties deduced from EELS. Indeed, when the energy transfered by a transmitted electron remains below 50 eV, it is possible to have access to the electronic structure of the material and more precisely to its dielectric function. [1] In other words, we are able to obtain informations such as, plasmon resonances, interband transitions and band gap measurements. We used a Libra 200 equipped with an electrostatic monochromator operating at 80 kV. Thanks to the in‐column filter, energy loss signal is recorded on a CCD camera with a spectral resolution of 150 meV. The sharp cut‐off of the omega filter allows to probe the dielectric properties of semiconducting materials down to 1eV losses. We are able to determine bandgaps in several 2D materials and rely them to the number of layers. For instance, we can see the blue shift of the “optical absorption” from several MoS 2 layers (1.4 eV) down to a single layer (1.8 eV). Recently, thanks to dedicated operating modes [2,3], we have been able to obtain additional informations on the plasmons and interband transitions over the Brillouin Zone in hexagonal Boron Nitride (hBN). Energy Filtered scattering patterns have been recorded in the TEM to have access to the symmetries of the dipole matrix elements involved in the observed transitions. Moreover, by dispersing the energy along specific crystallographic directions, we accessed to the related dispersion of plasmons and interband transitions with the so‐called ω‐q maps [2] as representated on fig 1. We show that, due to a strong electron‐hole interaction, the observed dispersion is related to the one of the exciton [4]. The experimental results are in good agreements with inelastic X‐ray scattering experiments [5] and calculations [6] as shown on fig 3.