Due to the discovery of the exceptional electronic and physical properties of graphene, and thanks to hard-ware aberration correction in TEM, a new research area on the atomic structure of other two-dimensional (2D) layered materials has emerged such as transition metal dichalcogenides (TMDs) whose properties differ strongly from those of the semimetallic character of graphene. The diverse properties of TMDs depend on their composition. These materials can be semiconductors, semimetals or true metals, and superconductors. Metallic TMDs like 1T/2H-TaSe₂, 1T/2H-TaS₂ or 2H-NbSe₂ can produce ―depending on temperature, doping and pressure ― so-called charge density waves (CDWs) [1]. These waves are periodic modulations of the charge density in a material accompanied by periodic lattice distortions (PLDs), forming a superstructure in the material. They can occur during a metal-insulator transition due to electron-phonon coupling. CDWs/PLDs in TMDs are of great interest, because they are a model system to understand phenomena like superconductivity, spin density waves, and metal-insulator transitions. Already since the 1970s it is known that bulk 1T-TaSe₂ shows a commensurate CDW (CCDW)/PLD at temperatures below 473 K [1] and the CCDW/PLD is characterized by a  superstructure with a lattice parameter of a₀ = 3.48 Å. Owing to the layered structure of the TMD materials, nowadays single layers of these materials can be mechanical exfoliated. However, so far neither much is known about the stability and the characteristics of CDWs/PLDs in single- layer TMDs, nor whether they do exist at all in single-layers and/or few-layers due to confinement effects.

Here we present the experimental characterization of PLDs in single-layer and few-layer 1T-TaSe₂ as well as in 1T-TaSe₂-graphene heterostructures using aberration-corrected high-resolution transmission electron microscopy (AC-HRTEM) and selected area electron diffraction (SAED). Experimental observations of PLDs in single-layer 1T-TaSe₂ are a challenge due to difficulties in sample preparation and due to electron beam damage occurring during the TEM experiment. In 1T-TaSe2 / graphene heterostructures, graphene is used as a support material for imaging a few micrometer large exfoliated 1T-TaSe₂ monolayer. Moreover, graphene sandwiching is used to reduce radiation damage effects [2].  We perform our TEM experiments at a low accelerating voltage of 80 kV, however, the material is not stable under the electron beam and low-dose operation is required. Figure 1 -shows that PLDs can be found in few-layer-thick 1T-TaSe₂ graphene heterostructures, Figure 1(a) shows an AC-HRTEM image of this heterostructure and Figure 1(b) the corresponding fast Fourier transformation (FFT). We will also present a method to analyze PLDs in single-layer 1T-TaSe₂ using atomic scale mapping. In addition we shall show our first experiments obtained with the new spherical and chromatic aberration-corrected SALVE instrument operating in the range between 20kV and 80kV at exceptionally high-resolution. Results proving better understanding of electron-sample interaction will be also discussed. [3]

References

[1] J. A. Wilson, F. J. Di Salvo, and S. Mahajan, Advances in Physics 24.2 (1975): 117-201.

[2] G. Algara-Siller, S. Kurasch, M. Sedighi, O. Lehtinen, & U. Kaiser, Applied Physics Letters 103.20 (2013): 203107.

[3] The authors acknowledge funding from the German Research Foundation (DFG) and the Ministry of Science, Research and the Arts (MWK) of the federal state Baden-Württemberg, Germany in the frame of the SALVE (Sub-Angstroem Low-Voltage) project.

Figures:

Figure 1: 80kV AC-HRTEM image of few-layer 1T-TaSe2 on graphene, (a) shows small contrast variations caused by the superstructure, (b) FFT of (a) showing the reflexes of 1T-TaSe2 , the reflexes of the superstructure, and the reflexes of graphene (a Wiener filter and a Hann window have been used on the HRTEM image). Scale bar is 5 nm.

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