Polarized Raman Scattering of In‐Plane Anisotropic Phonon Modes in α‑MoO₃

Abstract

The recent discovery of in‐plane hyperbolic phonon polaritons in biaxial polar crystal—α‐MoO3, has aroused a huge research upsurge on anisotropic photonic quasiparticles in van der Waals materials. The physical origin of these highly anisotropic phonon polaritons is attributed to the strong anisotropy of crystal phonons. However, the orientation‐dependent phonon modes have not been fully understood from crystallographic point of view. This work systematically performs in situ angle‐resolved polarized Raman spectroscopy on α‐MoO3 to investigate its anisotropic photo‐phonon interactions. The sample orientation and Raman tensors are unambiguously determined under different polarization configurations. Specifically, the in‐plane anisotropic phonon modes of α‐MoO3 can be well characterized from the highly polarization‐dependent Raman‐active modes with periodic intensity variation. It can be concluded that these findings cannot only offer a better understanding of anisotropic phonon modes in α‐MoO3, but also provide a valuable reference for other anisotropic van der Waals crystals.

Optical Effect Modulation in Polarized Raman Spectroscopy of Transparent Layered α‐MoO3

“…In the parallel polarization configuration (Figure 3b), we observed four anomalous ARPRS responses of A g modes on bulk-like α-MoO 3 as shown in Figure 3c-e and Figure S2, Supporting Information. Specifically, in the responses of modes A g 3 (217.0 cm −1 ), A g 4 (337.5 cm −1 ), A g 6 (818.2 cm −1 ), and A g 7 (994.2 cm −1 , with a response similar to that of A g 4 ), there is a secondary maximum (which of A g 4 and A g 7 are too large to show an isotropic-like response) that cannot be reproduced using the real Raman tensor commonly used in previous studies on α-MoO 3 [32,33,35,38] as shown by the blue lines in Figure 3c-e. However, using the birefringence-induced Raman selection rule shown below, the ARPRS responses of A g modes can be well-fitted, as shown by the red lines.…”

“…Previous polarized Raman studies on α-MoO 3 have tended to focus on selected Raman modes or sample thicknesses. [32][33][34][35] Several additional physical questions remain unanswered such as the exploration and reproduction of anomalous ARPRS responses in α-MoO 3 with rich Raman fingerprints that cannot be fitted with real Raman tensors widely used for non-absorbent Optical anisotropy, which is quantified by birefringence (Δn) and linear dichroism (Δk), can significantly modulate the angle-resolved polarized Raman spectroscopy (ARPRS) response of anisotropic layered materials (ALMs) by external interference. This work studies the separate modulation of birefringence on the ARPRS response and the intrinsic response by selecting transparent birefringent crystal α-MoO 3 as an excellent platform.…”

Spent Catalyst‐Derived Mo‐MOF: Triboelectric Nanogenerators and Energy Harvesting

“…where 8A g , 8B 1g , 4B 2g , 4B 3g are Raman active, and others are infrared-active modes. [42,43] Peaks at 992 and 814 cm À1 correspond to symmetric stretching of Mo═O and Mo─O─Mo in MoO 3 , [32] whereas the peaks between 1600 and 1300 cm À1 correspond to the stretching vibrations of the imidazole ring and 800-990 cm À1 corresponds to Mo-O stretching and bending vibrations in case of Mo-MOF. [44,45] SEM analysis is also a valuable method for studying the samples' shape, size, and microstructure information.…”

Emerging 2D Metal Oxides: From Synthesis to Device Integration

“…[58] Recently, α-MoO 3 has attracted numerous research interests on anisotropic photonic quasi-particles in vdW materials due to the discovery of in-plane hyperbolic phonon polaritons in such a biaxial polar crystal. [59][60][61][62] The twisting stacked α-MoO 3 slabs have achieved a configurable light-matter waves-phonon polaritons over a broad range of twist angles, giving the potential for tunable nanophotonic devices. [61,62] Figure 3c exhibits the electrical conductivity anisotropy of α-V 2 O 5 .…”