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

Abstract

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

Intralayer Negative Poisson's Ratio in 2D Black Arsenic by Strain Engineering

“…[51] In contrast, Raman intensity of B 2g mode remains unchanged and shows the repeated polarization dependence. [36,52] As shown in Figure 1e, in the parallel configuration, when the two main lattice axes are aligned with the laser polarization, the B 2g mode cannot be detected due to matrix cancellation. Therefore, it allows us to conveniently identify the ZZ or AC directions of b-As flake.…”

Suspended Black Arsenic Nanoribbons with Anisotropic Elastic Properties for Nanomechanical Devices

“…Figure d demonstrates the typical Raman spectrum of the exfoliated b-As nanoflake before the FIB process, which displayed three characteristic phonon modes: the out-of-plane A g 1 and in-plane B 2g and A g 2 . To accurately identify the lattice orientation of b-As, ARPRS and TDPRS were jointly explored. − Based on the identical lattice dynamics and group theory with b-P, the ARPRS results showed that the Raman intensity I (B 2g ) of the B 2g mode of the b-As nanoflake varied with the polarization angle θ of the excitation source, leading to the periodic polar plot with 4-fold symmetry. As exhibited in Figure e, the obtained data about its Raman intensity versus polarization angle can be well fitted with I (B 2g ) ∝ cos 2 2θ, where the minimum values imply the AC or ZZ direction of b-As.…”

“…Importantly, the Raman shift of the A g 2 mode presents more significant temperature dependence when the laser polarization is aligned with ZZ (−0.0230 cm −1 /K) than the counterpart AC (−0.0199 cm −1 /K) direction in Figure 1f (more data shown in Figure S1), mostly due to its anisotropic phonon velocity and thus superior thermal conductivity, which is well consistent with literature data. 34 Next, we turn to measure the anisotropic mechanical properties of the prepared b-As nanoribbons along different lattice orientations by using the AFM nanoindentation method and further attempt to extract the elastic modulus through the force−indentation depth curves. As shown in Figure 2a, it is obvious that b-As nanoribbons along the ZZ direction was suspended on the trench of the SiO 2 /Si substrate.…”

Investigation of the Electron–Phonon Coupling in Dirac Semimetal PdTe2 via Temperature‐Dependent Raman Spectroscopy

“…Raman Spectroscopy Measurements: We used an inVia Qontor system (Renishaw, UK) to perform the Raman scattering measurements on the PdTe 2 flakes using 532 nm laser excitation with a spot size smaller than 1 μm and 1800 lines mm À1 grating. [46] The laser power were maintained low enough to avoid a heating impact on the flakes. For TDRS measurements, the samples were put in a controlled stage (Linkam THMS600, UK), where the temperature was taken from 80 to 580 K with a step of 10 K. Initially, the temperature was held at 100 °C with the aid of N 2 gas for 20-30 min to ensure a clean sample and to eliminate water.…”