The gadolinium sesquioxide (Gd2O3) with its bandgap of ∼5.4 eV and high dielectric permittivity and refractive index has been used widely in optics, magnetic resonance imaging, and high k dielectrics. Electron energy loss spectroscopy (EELS) reveals spectral features at 13.5 eV and 15 eV, which can be interpreted as surface and volume plasmons, respectively. The unusual surface exciton polariton, with surface resonances associated with excitonic onsets, was also observed at ∼7.5 eV. Because of the differences in electronic structures between the cubic and the monoclinic phases of Gd2O3, it is straightforward to distinguish the two phases using the low-loss regime and O K-edge as a fingerprint. We further successfully performed EELS and electron diffraction to identify the crystalline phase of a single-crystal Gd2O3 film epitaxially grown on a Si(111) substrate.

The presence of intrinsic defects in topological crystalline insulator materials has been predicted to improve the thermoelectric figure-of-merit values in the literature. Performing atomic-resolved high angle annular dark field imaging, momentum-resolved electron energy loss spectroscopy, and electron spectroscopic diffraction, we observed those intrinsic defects, including interstitial Se atoms and Se vacancies, to cause localized mirror symmetry breaking and further result in the anisotropic π-plasmon dispersion along the ΓX¯ and ΓM¯ directions in single-crystal Pb1−xSnxSe (x = 0 and 0.34). In addition to the anisotropic π plasmon dispersion, the splitting lines along the ΓX¯ direction were revealed with selected π plasmon energy in the electron spectroscopic diffraction pattern.

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The electronic excitations in m-Gd2O3 were systematically studied by low-loss EELS in scanning transmission electron microscopy (STEM) mode and electron diffraction mode to gain both the spatial and momentum (q) resolutions.

In this work, π-plasmon dispersion and plasmon-exciton coupling in single-crystal 1T-TiSe2 along the ΓM¯ direction were investigated by using momentum (q)-resolved electron energy loss spectroscopy (EELS), ω-q map. Both the π plasmon at 7 eV and the π + σ plasmon at ∼19.7 eV were observed in the EELS spectrum. Furthermore, the π plasmon exhibits an unexpected dispersion behavior that transitions from a square root of q dependence (q1/2) to a quadratic dependence (q2) with increasing q values. A low energy excitation at ∼2.3 eV was also observed, which can be attributed to the plasmon-exciton coupling excitation mode (or plexciton) with a linear positive dispersion turning to negative dispersion by following the band structure along the ΓM¯ direction.