Thickness And Stacking Sequence Determination Of 1T-Tantalum Disulfide Using Scanning Transmission Electron Microscopy

The layered transition metal dichalcogenide (TMD), 1T-tantalum disulfide (1T-TaS2) exhibits a range of electronic properties when cooled down, including phase transitions associated with charge-density-waves (CDWs), metal-insulator transitions as well as superconductivity upon doping or high pressure. The CDW phase transitions can be modulated by the thickness of 1T-TaS2. To understand the material's potential for electronic applications it is therefore important to identify the dimension and atomic layer stacking of 1T-TaS2 samples with both accuracy and precision. In this work, our goal is to develop a reliable method to extract the thickness and stacking sequence of 1T-TaS2 directly from experimentally recorded images. High-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) is a powerful method to image materials with atomic resolution. However, considering the two-dimensional nature of projected HAADF-STEM images, we choose the technique of convergent-beam electron diffraction (CBED) performed with atomically small electron beams from STEM. We show that combining experimental and simulated CBED patterns with rich information in the third-direction (i.e. thickness and stacking sequence) is an effective approach to study the structures of layered dichalcogenides. I will discuss the development of a method to extract the thickness and stacking sequence of exfoliated 1T-TaS2 layered structures directly from experimental CBED patterns and STEM images by comparison with simulation results. Starting with multislice simulations of both HAADF-STEM images and CBED patterns for 1T-TaS2 thickness and stacking sequence, I will demonstrate the influence on the image contrast. By comparison of these results with experimental data I show that the thickness of 1TTaS2 with simple stacking (all-A stacking) can be determined with a ± 1 unit cells uncertainty. Variations in the symmetry and the intensity distribution in CBED patterns can be used to distinguish simple all-A (AA….A) stacking from other stacking. Finally, I will briefly discuss the limitations of this method and future work. i