Angular Resolved low loss EELS for Materials Characterization

Electron Energy Loss Spectrometry (EELS) in Transmission Electron Microscopy (TEM) is a powerful tool for the investigation of the electronic structure of materials. In the low loss regime, one can access the optical properties which are governed by the dielectric function ε(ω), which may be retrieved thanks to the so-called "loss function". The energy range covered by EELS goes from the infrared (less than 1 eV) to the hard X-rays (about 3 keV), each region having its own interest. The low loss region (0–100 eV) serves to investigate the valence electron structure and the inter-band transition behavior. Using angular resolved EELS gives further information about the dispersion behavior of the investigated material. This work is a part of a large project aiming a closer agreement between theory and experiment in angular resolved EELS by improving the state of the art in both fields. The purpose of this work is to set up a reliable experimental technique to retrieve the momentum transfer resolved single scattering distribution at very low energy with good accuracy and resolutions. One of the main challenges was to use the newly installed JEOL 2200 FS transmission electron microscope that is equipped with an in-column Ω filter for retrieval of the loss function in diffraction energy filtered TEM. Issues such as the angular resolution, contamination, image-coupling in diffraction mode and plural scattering have been solved. Several optical alignments and operational modes of the JEOL 2200 FS have been tested, and finally nano-beam diffraction was found to be the optimal running mode. This work provides for the first time a full 3D energy loss data cube in diffraction mode with energy and angular resolutions of 1 eV and 0.5 nm−1 respectively. After processing, the energy and angular relevant data span an energy loss range from 2.75 eV to 40.25 eV and the angular momentum transfer up to 12.5 nm−1. Although theoretically, without CCD camera binning and the maximum available camera length of the JEOL (250 cm), a momentum transfer resolution of 0.04 nm−1 would have been possible, the effective angular resolution was limited by the smallest obtainable beam convergence as well as by the remaining aberrations of the microscope. The presented technique can be reproduced on any transmission electron microscope equipped with an in-column energy filter. Furthermore, it is demonstrated that the proposed method provides working ranges in energy, spatial and angular resolutions limited only by the perfor- mance of the microscope. Previous techniques did not provide experimental results showing an energy resolution independent of the spatial and angular resolutions.