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 μm 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.
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 A g 1 and A g 2 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 A g 1 to A g 2 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.L ayered crystalline materials such as graphene, transition metal dichalcogenides (TMDCs), and black phosphorus (BP) have attracted tremendous interest owing to their high carrier mobility, large specific surface area, and thicknessdependent physical properties, 1−3 etc. Monoelemental group-VA (P, As, Sb, Bi) layered materials in a puckered honeycomb structure exhibit strong in-plane anisotropy and have emerged in recent years as new platforms for novel electrical and optical devices. 4−10 Among them, BP has been extensively investigated to show the layer-dependent optical direct band gap, 11 carrier mobility over 10 4 cm 2 V −1 s −1 , 12 anisotropies, 6,13 and abnormal polarized Raman scattering. 13 However, BP flakes are easily degraded in ambient conditions.Arsenic sits just below phosphorus in the periodic table. There mainly exist three arsenic allotropes: gray, orthorhombic, and yellow arsenic. The orthorhombic arsenic has been found in natural arsenolamprite (Copiapo area, northern Chile) 14 and is black and has a metallic luster, as shown in the optical image in Figure 1a, suggesting its high air stability. It is also named black arsenic (b-As). The high quality and atomic structure of such natural b-As crystals have been proven in previous reports. 15−18 Similar to BP, each arsenic atom in b-As is covalently bonded with three other adjacent ones to form a puckered honeycomb structure (as shown in the top and side views in Figure 1b). In-plane band a-axes correspond to the armchair (AC) and zigzag (ZZ) directions, respectively. The out-of-plane van der Waals (vdW) stacking direction is the caxis. In contrast to BP, monolayer b-As is an indirect band gap semiconductor. The carrier mobility and band gap of b-As are
Layered Palladium ditelluride (PdTe2) is an interesting noble‐transition‐metal dichalcogenide with high electrical and thermal conductivity, exhibiting superconductivity and a type‐II Dirac semimetallic phase due to the increased electron–phonon (e–ph) coupling. Herein, the e–ph coupling constant λ of E g (in‐plane) and A 1g (out‐of‐plane) modes in the exfoliated PdTe2 nanoflakes via temperature‐dependent Raman spectroscopy within the temperature range from 80 to 580 K is determined. Both E g mode with a Gaussian line shape and A 1g mode with a Breit–Wigner–Fano line shape show nonlinear frequency redshift and linewidth broadening with temperature. The extracted e–ph coupling constants are λ E g = 1.54 and λ A 1 g = 0.55, respectively. The Raman results confirm that PdTe2 is a phonon‐mediated Bardeen–Cooper–Schrieffer superconductor.
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