Energy-filtered TEM and low-loss EELS of 2D materials

Abstract

In this work, we perform aberration-corrected energy-filtered TEM (EFTEM) and low-loss electron energy-loss spectroscopy (EELS) with two-dimensional (2D) materials. In particular, the carbon K-edge EFTEM signal is analyzed for a monolayer of graphene, and the low-loss EELS spectra of graphene and molybdenum disulfide (MoS₂) heterostructures are investigated by the technique of momentum-resolved EELS. With EFTEM in the Cc/Cs-corrected “SALVE III” low-voltage transmission electron microscope, we demonstrate lattice contrast in zero-loss, plasmon-loss, and C-K-edge images. Both bright-atom contrast and dark-atom contrast are observed in all three cases, and focus series show that even for the C-K edge, contrast inversions can be caused by a change in the defocus. For an analysis of the image contrast in direct space, all raw EFTEM data are averaged over a large number of unit cells to improve the signal-to-noise-ratio. In the low-loss regime of EELS (0–50 eV), our momentum-resolved experimental data for 2D heterostructures of graphene and MoS₂ can be understood by a combination of ab initio simulations and dielectric model calculations. Therefore, the low-loss EELS signals of 2D multilayers and heterostructures are described on the basis of time-dependent density functional theory calculations for the constituent monolayers. Expanding on a layered-electron gas model for graphene, multilayers and heterostructures with MoS₂ are eventually modeled by microscopic dielectric calculations that are in good agreement with our experimental EELS data. The necessity of models beyond the layered electron-gas model is demonstrated by calculations for multilayer and bulk MoS₂.