Broadband excitation spectrum of bulk crystals and thin layers of ${\mathrm{PtTe}}_2$

We explore the broadband excitation spectrum of bulk ${\mathrm{PtTe}}_2$ using electron energy-loss spectroscopy and density-functional theory. In addition to infrared modes related to intraband three-dimensional (3D) Dirac plasmon and interband transitions between the 3D Dirac bands, we observe modes at 3.9, 7.5, and 19.0 eV in the ultraviolet region. The comparison of the excitation spectrum with the calculated orbital-resolved density of states allows us to ascribe spectral features to transitions between specific electronic states. Additionally, we study the thickness dependence of the high-energy-loss peak in the ${\mathrm{PtTe}}_2$ thin films. We show that, unlike graphene, the high-energy EELS peak in ${\mathrm{PtTe}}_2$ thin film gets redshifted by $\sim 2.5$ eV with increasing thickness.

Figure 1

Panels (a) and (b) show the top and side views of the atomic crystal structure of ${\mathrm{PtTe}}_2$. The corresponding bulk Brillouin zone of the hexagonal ${\mathrm{PtTe}}_2$ crystal, along with various high-symmetry points is shown in (c). (d) SAED patterns acquired on ${\mathrm{PtTe}}_2$ flakes match with [0001]-oriented single-crystal trigonal ${\mathrm{PtTe}}_2$ (ICSD No. 41373, moncheite). (e) LEED pattern of bulk ${\mathrm{PtTe}}_2$ single crystal oriented along the (0001) direction. (f) XRD pattern of (0001)-oriented planes of ${\mathrm{PtTe}}_2$. Only $\left(00n\right)$ peaks are observed.

Figure 2

(a) Broadband EELS spectrum for bulk ${\mathrm{PtTe}}_2$ measured in reflection mode with a primary electron beam energy of 100 eV. The EELS spectrum shows several distinct peaks at energies 0.5, 1.4, 3.9, 7.5, and 19.0 eV, which are highlighted in (b) and (c). The peak at 0.5 eV is dispersive and it is associated with the intraband density excitations [20] in ${\mathrm{PtTe}}_2$. The experimental broadband EELS spectrum is also reasonably captured by the loss function obtained from ab initio calculations, as shown in (d)–(f) for different momentum values, reported in the legend of panel (e). The 19.0 eV peak in (d) has the highest intensity in experiment as well as the ab initio calculations. The other dominant intraband peaks at 3.9 and 7.5 eV are resolved in panel (f).

Figure 3

Orbital-resolved density of states corresponding to (a) Pt orbitals and (b) Te orbitals. The dominant transitions, corresponding to the observed EELS peaks, are marked by arrows. The corresponding orbital-resolved band-structure plots are shown in Fig. 4.

Figure 4

Orbital-projected band structure of ${\mathrm{PtTe}}_2$. The lowest energy states are dominated by the ${\mathrm{Te}}_{5s}$ orbital. Near the Fermi energy the states have major contributions from the ${\mathrm{Pt}}_{5d}$ and ${\mathrm{Te}}_{5p}$ orbitals. The higher energy states are mostly ${\mathrm{Pt}}_{6p}$ and ${\mathrm{Te}}_{5d}$ states.

Figure 5

High-angle annular dark-field (HAADF)-STEM image of a ${\mathrm{PtTe}}_2$ flake, partially suspended on a hole in the amorphous carbon support film. The portion magnified in (b) was used for STEM-EELS analysis. The points A, B, and C exhibit a lower thickness (lower brightness in HAADF-STEM mode, corresponding to $\sim 0.2\lambda$) than point D (corresponding to $\sim 0.8\lambda$). (c) Only the zero-loss peak appears in the spectrum recorded at point E, i.e., 35 nm far from the ${\text{PtTe}}_2$ flake. (d) The broadband STEM-EELS spectra acquired for points A, B, C, and D. In panel (c) the intensity is normalized to the zero-loss peak maximum, while in panel (d) it is normalized to that of the peak at at $\sim 20$ eV, in order to highlight the dissimilarities in the line shape of the peaks corresponding to different thickness.

Figure 6

Band structure of ${\mathrm{PtTe}}_2$ over a wide energy region. Evidently, there are no energy states in between $-13$ and $-50$ eV.