The method of the ELNES theoretical calculation with van der Waals interaction

Abstract

In recent year, the investigation of the local electronic and atomic structure of materials has been attracting attention due to their importance on the macroscopic properties. The electron energy‐loss near edge structure (ELNES) in the electron energy‐loss spectrum (EELS) reflects partial electronic state of the material. Therefore, ELNES becomes one of the most powerful method to analyze the local electronic and atomic structure because it is observed with a transmission electron microscopy (TEM) and scanning TEM (STEM), which accomplishes an atomic resolution [1]. The TEM/STEM‐ELNES method can determine detailed electronic and atomic structures and detect their small but important changes in the interpretation of the ELNES spectra. In order to interpret the experimental ELNES and acquire the information of electronic and atomic structures, such as the bonding state, coordination environment and valence state, a theoretical calculation of the ELNES is essential. The ELNES simulation with the first‐principles calculation is almost established and can reproduce the experimental spectrum with high accuracy. Although, there is still room for improvement, for example, the effect of the van der Waals (vdW) interaction. In this study, we focused on that of vdW interaction on the ELNES calculation. The power of the vdW interaction was much smaller interaction than that of the ionic and covalent bonding. However, this interaction is acted on the all materials, and the ELNES provides a great benefit to investigate the vdW interaction at a local region. Therefore, in this study, we investigated the effects of vdW interaction in the theoretical calculation of ELNES. The vdW effect differs depending on the state, the solid, liquid and gaseous states were systematically investigated to influence of the vdW interaction on the ELNES calculation. We composed the solid, liquid and gaseous models, and calculated their ELNES spectra. These structural models are shown in FIG.1 (1–6). The ELNES spectra of these structures were calculated using the first‐principles plane‐wave basis pseudopotential method. To introduce the vdW interaction into the ELNES calculation, we applied the semi‐empirical vdW‐TS approach of Tkatchenko and Scheffler [2]. This method is reported to be able to calculate vdW interaction accurately. In the case of solid and liquid, due to the influence of the vdW interaction, the transition energies decreased by approximately 0.1 eV in FIG.2 (1–2). The energy shift implies that the vdW interaction exerts more influence on the excited state than the ground state. It is for this reason that the excited state has more electrons at the delocalized unoccupied bands. Furthermore, the relationships between the vdW interaction and local electronic structure and molecular configuration were discussed in Fig. 3. In contrast to the case of the solid and liquid structures, the gaseous models are little dependent of the vdW interaction owing to the large intermolecular distance in FIG.2 (3) [3]. We are going to talk about detailed results in my presentation.