Second-order Raman scattering has been extensively studied in carbon-based nanomaterials, \emph{e.g.} nanotube and graphene, because it activates normally forbidden Raman modes that are sensitive to crystal disorder, such as defects, dopants, strain, etc. The sp2-hybridized carbon systems are, however, the exception among most 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 Ag1 and Ag2 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. \emph{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.
@article{arxiv.2408.09010,
title = {Second-Order Raman Scattering in Exfoliated Black Phosphorus},
author = {Alexandre Favron and Félix Antoine Goudreault and Vincent Gosselin and Julien Groulx and Michel Côté and Richard Leonelli and Jean-Francis Germain and Anne-Laurence Phaneuf-L'Heureux and Sébastien Francoeur and Richard Martel},
journal= {arXiv preprint arXiv:2408.09010},
year = {2024}
}