Polarization-Resolved Raman Study of Bulk-like and Davydov-Induced Vibrational Modes of Exfoliated Black Phosphorus.
basic_science · Level V
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Abstract
Owing to its crystallographic structure, black phosphorus is one of the few 2D materials expressing strongly anisotropic optical, transport, and mechanical properties. We report on the anisotropy of electron-phonon interactions through a polarization-resolved Raman study of the four vibrational modes of atomically thin black phosphorus (2D phosphane): the three bulk-like modes A<sub>g</sub><sup>1</sup>, B<sub>2g</sub>, and A<sub>g</sub><sup>2</sup> and the Davydov-induced mode labeled A<sub>g</sub>(B<sub>2u</sub>). The complex Raman tensor elements reveal that the relative variation in permittivity of all A<sub>g</sub> modes is irrespective of the atomic motion involved lowest along the zigzag direction, the basal anisotropy of these variations is most pronounced for A<sub>g</sub><sup>2</sup> and A<sub>g</sub>(B<sub>2u</sub>), and interlayer interactions in multilayer samples lead to reduced anisotropy. The bulk-forbidden A<sub>g</sub>(B<sub>2u</sub>) mode appears for n ≥ 2 and quickly subsides in thicker layers. It is assigned to a Davydov-induced IR to Raman conversion of the bulk IR mode B<sub>2u</sub> and exhibits characteristics similar to A<sub>g</sub><sup>2</sup>. Although this mode is expected to be weak, an electronic resonance significantly enhances its Raman efficiency such that it becomes a dominant mode in the spectrum of bilayer 2D phosphane.