Charge-density waves (CDWs) and their concomitant periodic lattice distortions (PLDs) govern the electronic properties in several layered transition-metal dichalcogenides. In particular, 1T-TaS 2 undergoes a metal-to-insulator phase transition as the PLD becomes commensurate with the crystal lattice. Here we directly image PLDs of the nearly commensurate (NC) and commensurate (C) phases in thin, exfoliated 1T-TaS 2 using atomic resolution scanning transmission electron microscopy at room and cryogenic temperature. At low temperatures, we observe commensurate PLD superstructures, suggesting ordering of the CDWs both in-and out-of-plane. In addition, we discover stacking transitions in the atomic lattice that occur via one-bond-length shifts. Interestingly, the NC PLDs exist inside both the stacking domains and their boundaries. Transitions in stacking order are expected to create fractional shifts in the CDW between layers and may be another route to manipulate electronic phases in layered dichalcogenides. or TaSe 2 , are prototypical charge-density-wave (CDW) systems that spontaneously break lattice symmetry through periodic lattice distortions (PLDs). PLDs are associated with dramatic electronic changes such as metal-to-insulator transitions (1). Upon cooling from the normal metal phase at >543 K, the 1T polymorph of TaS 2 undergoes several CDW transitions until it finally enters a strongly insulating phase at low temperature where the PLD is commensurate with the crystal lattice (2, 3). In addition to thermal and pressure-induced transitions (4), recent work on thin, exfoliated 1T-TaS 2 flakes has demonstrated thicknesstuned conductivity and external electronic control (5). Whereas CDWs at the surface of bulk crystals have been carefully mapped using scanning tunneling microscopy (STM), less is known about the CDW/PLD structure and stacking order in thin, exfoliated TMDs.Recent theoretical calculations (6-8) and surface measurements (9-11) suggest that the electronic structure of 1T-TaS 2 is critically dependent on the CDW stacking order along the c axis. However, previous work has focused on phase changes of the CDWs alone and variations in atomic lattice stacking were not discussed. Such changes in local topology can have a large influence on the implementation of actual devices based on 2D materials (12). This became apparent in bilayer graphene, where stacking boundaries dominate the bulk transport behavior (13,14). The layered TMDs have additional complexities-CDW/PLD structure, sensitivity to oxidation (5), and lattice stacking order-that solicit real-space characterization with atomic resolution.Here, we use aberration-corrected and cryogenic scanning transmission electron microscopy (STEM) paired with modern exfoliation techniques to interrogate the PLD structure of thin 1T-TaS 2 in both plan-view and cross-section, revealing local variations in PLD coherence across layers and the presence of stacking faults in the atomic lattice. We demonstrate that STEM provides a direct measurement of PLD structures...

Layered transition metal dichalcogenides (TMDs) have attracted interest due to their promise for future electronic and optoelectronic technologies. As one approaches the two-dimensional (2D) limit, thickness and local topology can greatly influence the macroscopic properties of a material. To understand the unique behavior of TMDs it is therefore important to identify the number of atomic layers and their stacking in a sample. The goal of this work is to extract the thickness and stacking sequence of TMDs directly by matching experimentally recorded high-angle annular dark-field scanning transmission electron microscope images and convergent-beam electron diffraction (CBED) patterns to quantum mechanical, multislice scattering simulations. Advantageously, CBED approaches do not require a resolved lattice in real space and are capable of neglecting the thickness contribution of amorphous surface layers. Here we demonstrate the crystal thickness can be determined from CBED in exfoliated 1T-TaS2 and 2H-MoS2 to within a single layer for ultrathin ≲9 layers and ±1 atomic layer (or better) in thicker specimens while also revealing information about stacking order-even when the crystal structure is unresolved in real space.

Layered transition metal dichalcogenides (TMD) have attracted growing interest due to their promise for future technologies. As one approaches the 2D limit, the thickness and local topology can greatly influence the materials macroscopic properties [1]. To understand their potential for electronic applications it is therefore important to identify the dimension and atomic layer stacking of TMDs. The goal of this work is to extract the thickness and stacking sequence of TMDs directly by matching experimentally recorded HAADF images and convergent beam electron diffraction (CBED) patterns to multislice simulations. We demonstrate the accuracy to which thickness and stacking order can be determined in exfoliated TaS2.

No abstract