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.

References:
[1] H. Raether, Excitation of plasmons and interband transitions by electrons, Springer Tracts in Modern Physics, v.88 (1980), Springer, Berlin
[2] P. A. Midgley, Ultramicroscopy 76 (1999), p.91
[3] E. A. Tapt and H. R. Philipp, Phys. Rev. 138 (1965), A197
[4] K. Zeppenfeld, Optic Commun. 1 (1969), p119
[5] The research leading to these results has received funding from the European Union Seventh Framework Program [FP7/2007-2013] under grant agreement no. 312483 (ESTEEM2).

Keywords: transition metal dichalcogenides, dispersion, momentum-resolved EELS