Charge density wave order and commensuration in TaS₂ heterostructures

Two-dimensional charge density wave (CDW) materials are promising platforms for energy-efficient electronics. However, principles governing the formation and evolution of CDW quantum phases remain elusive. In this work, we examine novel TaS₂ polymorphs and shed light on the interplay between atomic structure, CDW order and electronic properties. We thermally treat 2D 1T-TaS₂flakes to make metastable polymorphs of TaS₂ comprising 1H layers embedded in a 1T matrix. Different ratios of 1H to 1T layers can be realized with our thermal process, as revealed by differential-phase-contrast scanning transmission electron (DPC STEM) imaging. All synthesized heterostructures exhibit two types of CDW domains, characterized by different rotational tiling of CDW clusters. These so-called mirror domains, which are not commonly coeval, were characterized with room-temperature 4-dimensional (4D) STEM. 4D-STEM revealed that nanoscale mirror domains form in the out-of-plane direction, resulting in a CDW superlattice. The observed CDW structure is accompanied by increased commensuration of the CDW order parameter at room temperature. Further, temperature evolution of the CDW order was probed using a combination of cryogenic Raman spectroscopy and cryogenic TEM diffraction. Findings of these studies were correlated with variable-temperature quantum transport to establish how CDW structure shapes the electronic signatures of engineered TaS₂ polytypes.