Polarization-Resolved Raman Study of Bulk-like and Davydov-Induced Vibrational Modes of Exfoliated Black Phosphorus

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 <i>A</i>_<i>g</i>¹, <i>B</i>_2<i>g</i>, and <i>A</i>_<i>g</i>² and the Davydov-induced mode labeled <i>A</i>_<i>g</i>(<i>B</i>_2<i>u</i>). The complex Raman tensor elements reveal that the relative variation in permittivity of all <i>A_g</i> modes is irrespective of the atomic motion involved lowest along the zigzag direction, the basal anisotropy of these variations is most pronounced for <i>A</i>_<i>g</i>² and <i>A</i>_<i>g</i>(<i>B</i>_2<i>u</i>), and interlayer interactions in multilayer samples lead to reduced anisotropy. The bulk-forbidden <i>A</i>_<i>g</i>(<i>B</i>_2<i>u</i>) mode appears for <i>n</i> ≥ 2 and quickly subsides in thicker layers. It is assigned to a Davydov-induced IR to Raman conversion of the bulk IR mode <i>B</i>_2<i>u</i> and exhibits characteristics similar to <i>A</i>_<i>g</i>². 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

Second-Order Raman Scattering in Exfoliated Black Phosphorus

Second-order Raman scattering has been extensively studied in carbon-based nanomaterials, for example, nanotube and graphene, because it activates normally forbidden Raman modes that are sensitive to crystal disorder, such as defects, dopants, strain, and so forth. The sp²-hybridized carbon systems are, however, the exception among nanomaterials, where first-order Raman processes usually dominate. Here we report the identification of four second-order Raman modes, named D₁, D₁^′, D₂ and D₂^′, in exfoliated black phosphorus (P­(black)), an elemental direct-gap semiconductor exhibiting strong mechanical and electronic anisotropies. Located in close proximity to the A_g¹ and A_g² modes, these new modes dominate at an excitation wavelength of 633 nm. Their evolutions as a function of sample thickness, excitation wavelength, and defect density indicate that they are defect-activated and involve high-momentum phonons in a doubly resonant Raman process. Ab initio simulations of a monolayer reveal that the D′ and D modes occur through intravalley scatterings with split contributions in the armchair and zigzag directions, respectively. The high sensitivity of these D modes to disorder helps explaining several discrepancies found in the literature