Investigations of transition metal dichalcogenides with momentum-resolved electron energy-loss spectroscopy
Michael R. S. Huang, Wilfried Sigle, and Peter A. van Aken
Max Planck Institute for Solid State Research, Stuttgart, Germany
When fast electrons pass through a thin film, various solid-state excitations occur through mutual Coulomb interactions, which convey useful information relevant to the fundamental materials properties. Measurements by momentum-dependent electron energy-loss spectroscopy (q-dependent EELS or ω – q map) directly access the dispersion relation, enabling the physical origins of the intrinsic electronic excitations to be explored [1]. This research is concentrated on anisotropic titanium diselenide (TiSe2), a two-dimensional material in the transition metal dichalcogenide group. The ω – q maps were acquired in the Zeiss sub-electron-volt-sub-angstrom microscope (SESAM), which is equipped with a monochromator and the advanced in-column Mandoline energy filter. To enhance the angular resolution, the specimen was purposely raised above the eucentric height, which significantly extends the effective camera length (ECL) beyond the originally achievable specification [2]. Figure 1 shows the characteristic dispersion of TiSe2 recorded along the momentum transfer parallel to the Γ-K direction. The specific selection of scattering vector in reciprocal space is achieved through a narrow slit well positioned in the filter entrance pupil plane. With further increase of the ECL as well as in the spectral magnification, the details of the dispersion become clearly resolved (Figure 2). Similar to other layer- structured crystals such as graphite or molybdenite (MoS2) [3,4], the two dominant features at approximately 6.5 and 19.7 eV can be interpreted as the π and π + σ plasmons, which stem from the collective oscillations of the π and π + σ valence electrons, respectively. Moreover, another weak spectral feature at a lower energy of about 1.9 eV without significant dispersive behavior was also noticed. This excitation could probably be attributed either to the interband transition or to another plasmon resonance as a result of the negative real part of the dielectric function within this regime. However, more comprehensive investigations are required for clarity.
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Figures:
Figure 1: The ω - q map of TiSe2 acquired along the momentum transfer parallel to the Γ-K direction. The scheme on the right side shows the geometric relationship between the selection slit and the diffraction pattern along the c-axis (red dashed line: the first Brillouin zone).
Figure 2: The ω - q map of TiSe2 recorded at a larger ECL and a higher spectral magnification (top: raw image; bottom: log-scaled and color-coded intensity profile). The arrows at about 1.9, 6.5, and 19.7 eV indicate the major dispersive features.
To cite this abstract:
Huang Michael R. S., Sigle Wilfried , van Aken Peter A.; Investigations of transition metal dichalcogenides with momentum-resolved electron energy-loss spectroscopy. The 16th European Microscopy Congress, Lyon, France. https://emc-proceedings.com/abstract/investigations-of-transition-metal-dichalcogenides-with-momentum-resolved-electron-energy-loss-spectroscopy/. Accessed: December 4, 2023« Back to The 16th European Microscopy Congress 2016
EMC Abstracts - https://emc-proceedings.com/abstract/investigations-of-transition-metal-dichalcogenides-with-momentum-resolved-electron-energy-loss-spectroscopy/