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

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.