Second-order Raman scattering in exfoliated black phosphorus

"Second-order Raman scattering in exfoliated black phosphorus", Alexandre Favron, Félix Antoine Goudreault, Vincent Gosselin, Julien Groulx, Michel Côté, Richard Leonelli, Jean-Francis Germain, Anne-Laurence Phaneuf-L'Heureux, Sébastien Francoeur et Richard Martel. Nano Lett. 2018, 18, 2, 1018-1027. (2018) 

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.

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