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Atomic-Scale Visualizations of Low-Temperature Phase Transitions

Probing the structure of layered materials offers a gateway to better understand and potentially manipulate their electronic properties. While interactions within the individual layers are dominant, the proximity of neighboring layers significantly impacts the properties of such quasi-2D systems. Transition-metal dichalcogenides containing tellurium are especially noteworthy for their modulated structures and prominent interlayer contributions.

Layered materials

Figure: STEM imaging of 1T'-TaTe2 at room temperature and cryogenic temperature (93K). (bottom) Structure and corresponding contrast modulation of the low-temperature phase in comparison to contrast modulation at high temperature, based on theoretical modeling.

Here, members of PARADIM’s in-house research team combine cryogenic scanning transmission electron microscopy (cryo-STEM) with theoretical modeling to visualize and understand complex patterns of displacements of atoms in the layered material TaTe2. At room temperature, Ta atoms form small clusters in a staggered three-layer stacking. At low temperatures, the Ta clusters become even more distorted, creating a superstructure that includes subtle displacement of the Te atoms. Snapshots of the structure showed faint contrast modulations which could only be quantified by pushing the resolution and stability of a cryogenic sample holder available to all users of PARADIM. Theoretical calculations revealed a connection between the contrast modulations and the pattern of atomic displacements, which opens the door for visualizing a range of new materials with complex structural arrangements.

What Has Been Achieved: Transition-metal dichalcogenides containing tellurium anions show remarkable charge-lattice modulated structures and prominent interlayer character. Using cryogenic scanning transmission electron microscopy (STEM), we map the atomic-scale structures of the high temperature (HT) and low temperature (LT) modulated phases in 1T’-TaTe2. At HT, we directly show in-plane metal distortions which form trimerized clusters and staggered, three-layer stacking. In the LT phase at 93 K, we visualize an additional trimerization of Ta sites and subtle distortions of Te sites by extracting structural information from contrast modulations in plan-view STEM data. Coupled with density functional theory calculations and image simulations, this approach opens the door for atomic-scale visualizations of low temperature phase transitions and complex displacements in a variety of layered systems.

Importance of the Achievement: Layered transition-metal dichalcogenides often host novel electronic and structural transitions at low temperatures. Despite their two-dimensional nature, the structural arrangement between the layers can profoundly alter the overall macroscopic electronic properties. These interlayer arrangements are, however, difficult to probe at the atomic scale from a single orientation, as in the case of 1T’-TaTe2. By improving the resolution and signal-to-noise ratio under challenging cryogenic conditions, subtle contrast modifications appeared in cryogenic STEM images of the low temperature phase of 1T’-TaTe2. These tiny changes were rationalized and quantified by comparing to theoretical calculations and simulations of crystal structures. This synergistic approach can be applied to other 2D materials with complex layer stackings to map hitherto uncharted atomic-scale arrangements underlying electronic transitions.

Unique Feature(s) of the MIP that Enabled this Achievement: A cryogenic sample holder offering unprecedented position stability enables low temperature scanning transmission electron microscopy (STEM) of an exotic phase transition in TaTe2. The close collaboration between theory and electron microscopy enabled the development of new approaches to visualize low-temperature phase transitions and making it available to the PARADIM community.

Publication: I. El Baggari, N. Sivadas, G.M. Stiehl, J. Waelder, D.C. Ralph, C.J. Fennie, and L.F. Kourkoutis, "Direct Visualization of Trimerized States in 1T'-TaTe2," Phys. Rev. Lett. 125, 165302 (2020), https://www.doi.org/10.1103/PhysRevLett.125.165302.

Data Availability: Raw data will be provided through the PARADIM Data Collective at data.paradim.org/89001.

Acknowledgement: This work was supported by the National Sciences Foundation (NSF) through the Platform for the Accelerated Realization, Analysis, and Discovery of Interface Materials (DMR-1539918) and made use of the shared facilities of the Cornell Center for Materials Research with funding from the NSF MRSEC program (DMR-1719875). Assistance provided by G. M. S. in the preparation of samples was funded by the US Department of Energy (DE-SC0017671). The FEI Titan Themis 300 was acquired through NSF-MRI-1429155, with additional support from Cornell University, the Weill Institute, and the Kavli Institute at Cornell.

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