In-Plane Anisotropic Raman Spectroscopy of van der Waals α-MoO₃

Abstract

Van der Waals (vdW) α-MoO3 has a strong anisotropic phonon polariton effect in the middle- and far-infrared up to terahertz band. This can be of a great importance for developing new nanophotonics and optoelectronic devices. The anisotropic phonon polaritons originate from the anisotropic lattice vibrational properties. Therefore, the phonon modes of α-MoO3, especially the in-plane anisotropic phonon modes, should be systematically studied. To the best of our knowledge, there are no correlative theoretical and experimental studies on the phonon behaviors in vdW α-MoO3 thin films (∼100 nm). Here, the lattice vibrational properties of α-MoO3 were investigated using density functional theory and polarization-resolved Raman spectroscopy. Several Ag modes, whose polarization-resolved Raman spectra could be used for identifying the orientation of the α-MoO3 crystal, were observed. Furthermore, it was demonstrated that modes with higher frequency have longer lifetimes, and the phonon modes in bulk α-MoO3 have longer lifetimes than those in monolayer α-MoO3. The group velocity of each mode was also obtained. These results can help understand the physical origin of the highly anisotropic phonon polaritons in α-MoO3 and other types of vdW crystals.

Ultrabroadband Nanocavity of Hyperbolic Phonon–Polaritons in 1D-Like α-MoO3

“…Such a complex structure leads to a great variety of phonon modes in the mid- to far-IR, as depicted by the nearly coincident Raman spectra of the bulk crystal and nanobelts in Figure e. The peaks at 994, 814, 663, 473, 373, 332, and 286 cm –1 are assigned to vibrational modes of the orthorhombic α-MoO 3 phase. ,− The 286 cm –1 peak is attributed to OMoO wagging vibrations. The peaks at 332 and 373 cm –1 are assigned to O 3 –Mo–O 3 bending mode and O–Mo–O scissoring, respectively.…”

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

“…[8,9] The natural polaritonic anisotropy of α-MoO 3 is aroused by its highly anisotropic crystalline structure, which gives rise to diverse phonon modes that are infraredactive along different crystallographic directions. [10][11][12] The optical response of α-MoO 3 is governed by its phonon absorption and the different directional optical phonons yield multiple reststrahlen bands (RBs) within the infrared range: RB 1 in 11.8-18.4 µm arises from the phonon mode along the [001] crystal direction, RB 2 in 10.4-12.2 µm from the phonon mode along the [100] crystal direction, and RB 3 in 9.9-10.5 µm from the phonon mode along the [010] crystal direction. [7,13,14] These bands originate from the degeneracy breaking of transverse optical (TO) and longitudinal optical (LO) phonons, in which the permittivity's real part Re(ε) becomes negative.…”

Tunable Hyperbolic Phonon Polaritons in a Suspended van der Waals α‐MoO3 with Gradient Gaps

“…Typical Raman fingerprint of α‐MoO 3 can be observed at 818 cm −1 (A g mode) (Supporting information, Figure S2a), corresponding to O–Mo–O stretching vibration along the [100] direction. [ 40 ] Two‐dimensional Raman spectrum intensity distribution was then recorded around 818 cm −1 . The image shows that the variations of phonon frequency are within 0.5 cm −1 across the suspended flake (Supporting information, Figure S2b).…”