PVD growth of spiral pyramid-shaped WS2 on SiO2/Si driven by screw dislocations.

Atomically thin layered transition metal dichalcogenides (TMDs), such as MoS2 and WS2, have been getting much attention recently due to their interesting electronic and optoelectronic properties. Especially, spiral TMDs provide a variety of candidates for examining the light-matter interaction resulting from the broken inversion symmetry, as well as the potential new utilization in functional optoelectronic, electromagnetic and nanoelectronics devices. To realize their potential device applications, it is desirable to achieve controlled growth of these layered nanomaterials with a tunable stacking. Here, we demonstrate the Physical Vapor Deposition (PVD) growth of spiral pyramid-shaped WS2 with ∼200  µ m in size and the interesting optical properties via AFM and Raman spectroscopy. By controlling the precursors concentration and changing the initial nucleation rates in PVD growth, WS2 in different nanoarchitectures can be obtained. We discuss the growth mechanism for these spiral-patterned WS2 nanostructures based on the screw dislocations. This study provides a simple, scalable approach of screw dislocation-driven (SDD) growth of distinct TMD nanostructures with varying morphologies, and stacking.

In-Plane Phonon Anisotropy and Anharmonicity in Exfoliated Natural Black Arsenic.

Group-VA two-dimensional layered materials in a puckered honeycomb structure exhibit strong in-plane anisotropy and have emerged as new platforms for novel devices. Here, we report on systematic Raman investigations on exfoliated b-As flakes on the Ag1 and Ag2 polarization dependence on their symmetry, excitation wavelength, and flake thickness. The intensity maximums of both phonons are corrected in the b-As armchair direction under 633 nm excitation regardless of the flake thickness upon considering optical birefringence effects and interference effects. The intensity ratio of Ag1 to Ag2 modes under 532 nm excitation is useful for b-As crystalline orientation identification. Temperature-dependent Raman investigations reveal the linearly anharmonic behaviors of both phonons in the range from 173 to 293 K and a slightly greater first-order temperature coefficient in the zigzag direction. Our findings give deep insight into the in-plane phonon anisotropy and anharmonicity of b-As and provide a step toward future device applications.