Angle-resolved polarized Raman study of layered b-AsxPx-1 alloys: Identification of As-P vibrational modes

In this study, the polarization-resolved Raman spectra of b-AsxPx-1 flakes with varying arsenic concentration (x = 0, 0.4, and 0.8) are systematically investigated. A clear polarization dependence is observed in the Raman intensity of all the P-P, As-P, and As-As modes and confirmed by the first-principles calculations. The observed angle-dependence can be related to the structure of the Raman tensors of each peak. In particular, the Raman intensities corresponding to As-P vibration modes are studied in detail. The out of plane As-P mode ($A$$^{1}_{g}$) is clearly identified, but the butterfly-shaped behavior observed in the polar plot around 350 cm-1 is interpreted as the combination of the two in-plane vibrations, one along the armchair and the other along the zigzag direction. This work allows unambiguous assignment of the $A$$^{1}_{g}$, B2g, and $A$$^{2}_{g}$ modes of the As-P vibrational modes and the importance of the consideration of the superposition of the in-plane B2g and $A$$^{2}_{g}$ modes. Such study can be utilized to further characterize the anisotropic structural properties and analyze the local distribution of the As and P in b-AsxPx-1 alloys which will play key roles in electronic and optical properties. Furthermore, this methodology can be further extended to alloyed two-dimensional material systems with in-plane anisotropy in general.