Low-loss EELS of 2D materials

Electron energy-loss spectroscopy (EELS) is a very useful method for studying the interactions of fast electrons with the specimen by measuring the energy losses of the incident electrons. EELS performed in a TEM also enables high spatial resolution, unmatched by any other method. Due to the high resolution of the TEM, TEM-EELS signals can be obtained even from single nanoscale objects. Moreover, in diffraction mode, energy-loss spectra can be acquired with momentum resolution.

In low-loss EELS experiments, the energy distribution of the primary electrons that lost a few electronvolts (eV) while interacting with the sample are analysed. This energy distribution gives information about the sample’s dielectric properties, and it is closely related to the sample’s dielectric function. In terms of light optics, the energy losses of the low loss spectra correspond to a large spectral range from the infrared (IR) to far UV. The excitations in this energy range comprise phonons, intra- and interband transitions, excitons, and plasmons.

For 2D materials, these excitations can differ strongly from those in the well-known bulk counterparts. This implies novel dielectric properties which can be interesting for future applications of 2D materials such as graphene, 2D transition metal dichalcogenides (TMDs) and hexagonal boron nitride (hBN). The assembly of vertical and lateral 2D heterostructures allows to combine materials with different dielectric response and leads to a vast number of structures with “tailored” properties.

We combine TEM-EELS experiments and ab-initio calculations to study the interesting dielectric properties of low-dimensional materials and their heterostructures. In particular, we focus on the acquisition and interpretation of momentum-resolved and spatially resolved EELS. In the last years we have investigated the plasmon dispersion in free-standing graphene [1,2], as well as plasmon modes in few-layer graphene [3] and the interactions between plasmons in graphene/MoS2 heterostructures [4].

Acquisition of momentum-resolved EELS: (a) Electron diffraction pattern of 1H-MoS2. Momentum transfers q along the ΓΜ direction can be selected with a slit aperture (red rectangle). (b) ”E-q map” with simultaneously acquired EELS signals for all selected momentum transfers q. The extracted spectrum for q = 0.3 Å-1 is shown exemplarily on the right.

References

[1] M. K. Kinyanjui et al., EPL 97 (2012), 57005

[2] P. Wachsmuth et al., Phys. Rev. B 88 (2013), 075433

[3] P. Wachsmuth et al., Phys. Rev. B 90 (2014), 235434

[4] M. J. Mohn et al., Phys. Rev. B 97 (2018), 235410

Collaborations

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Projects / Funding

DFG and MWK Baden-Württemberg in the frame of the SALVE project