Atomic lattice disorder in charge-density-wave phases of exfoliated dichalcogenides (1T-TaS ₂ )

Abstract

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...

Possible lattice and charge order in CuxBi2Te2Se

“…The electronic charge density, a scalar quantity and a natural order parameter of a CDW, is normally assumed to be linearly coupled to the longitudinal PLD, with the ordered CDW period an integral or fractional multiple of the period of the atomic lattice or the PLD. Such a situation could also arise due to planar defects such as stacking faults [19,20], as is likely to be the case in our samples, given the large changes in lattice constants and phonon frequencies in the 0.2x0.3 region.…”

Direct observation of mono-layer, bi-layer, and tri-layer charge density waves in 1T-TaS2 by transmission electron microscopy without a substrate

“…There is a possibility that the properties differ from those in higher layers (bulks). [13][14][15] There has been a recent debate on whether the commensurate CDW phase exists in ultrathin films of 1T-TaS 2 . [16][17][18][19][20] However, the samples considered in these studies either contain a substrate or interacts with the surrounding environment, which inevitably alter the properties of the material.…”

Total-Ionizing-Dose Effects on Threshold Switching in $1{T}$ -TaS2 Charge Density Wave Devices

“…Prior work has shown that exfoliated 1T-TaS 2 may contain stacking disorder but not vacancies or other point defects that can act as charge trapping sites [18]. Thus, electronhole pairs created during irradiation can readily recombine, greatly reducing the charge yield within the 1T-TaS 2 .…”