Synthesis of WTe2 Nanowires with Increased Electron Scattering.

Tungsten ditelluride (WTe2) has many interesting properties such as its extremely large nonsaturating magnetoresistance and quantum spin Hall state in the monolayer limit. The anisotropic crystal structure of WTe2 can allow for isolation of particular crystal directions to study the predicted Weyl states or crystal-symmetry-dependent magnetoresistance when studied at limited dimensions. In particular, the recent demonstration of superconductivity in WTe2 monolayer suggests that realizing nanowire geometry for WTe2 may be important to investigate potential Majorana zero modes predicted in one-dimensional topological superconductors. In this work, we demonstrate a large-yield, low-temperature synthesis of WTe2 nanowires, an approximate one-dimensional system, by converting WO3 nanowires via tellurization. The nanowires are single crystalline and have a higher resistivity than WTe2 exfoliated flakes with similar thickness. The increased resistivity is attributed to increased scattering from imperfect surfaces and higher surface-to-volume ratios of the WTe2 nanowires. We demonstrate that the synthesis method is generalizable to other transition-metal dichalcogenides, laying the foundation for further study of this class of materials in the one-dimensional limit.

Thickness and Stacking Sequence Determination of Exfoliated Dichalcogenides (1T-TaS2, 2H-MoS2) Using Scanning Transmission Electron Microscopy.

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.

Atomic lattice disorder in charge-density-wave phases of exfoliated dichalcogenides (1T-TaS2).

Charge-density waves (CDWs) and their concomitant periodic lattice distortions (PLDs) govern the electronic properties in several layered transition-metal dichalcogenides. In particular, 1T-TaS2 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-TaS2 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.

Mode manipulation and near-THz absorptions in binary grating-graphene layer structures.

The excitation and absorption properties of grating coupled graphene surface plasmons were studied. It was found that whether a mode can be excited is mainly determined by the frequency of incident light and the duty ratio of gratings. In the structure consisting graphene bilayer, a blueshift of the excitation frequency existed when the distance between neighbor graphene layer were decreased gradually. In graphene-grating multilayer structures, a strong absorption (approximately 90% at maximum) was found in near-THz range.