(Invited) Rich Electron-Phonon Interactions in Atomically-Thin Black Phosphorus
Authors: Francoeur Sebastien 
Published: May 2020

Black phosphorus is the most stable allotrope of phosphorus and exhibits a layered structure that can be exfoliated down to a single monolayer. This elemental 2D semiconductor offers several distinctive features unmatched by none of the other 2D materials. For instance, it offers high carrier mobilities, strongly anisotropic optical, electrical and mechanical properties along all three principal crystal axes, a thickness-tunable gap spanning the near-IR and the short wavelength IR spectral region, a direct gap that efficiently interacts with light irrespective of sample thickness, and, most usually, a gap that can be dynamically tuned by an external electric field to an extent rarely seen from other semiconductor materials. For these reasons, black phosphorus opens many interesting prospects for the design of optoelectronic devices with novel functionalities.
I will introduce this 2D material, discuss its most intriguing characteristics and then present results from an elaborate Raman study. In addition to bulk modes, atomically-thin black phosphorus exhibits unexpected bulk-forbidden Raman-allowed modes. The most obvious ones result from the lack of translational symmetry in few-layer samples, where the combination of infrared-allowed modes in adjacent layers combine to generate new Raman-allowed modes. Normally very weak, this phenomenon is prominent in the bilayer due to resonant effects in the electronic band structure. Polarization-resolved measurements on first order modes reveal powerful insights on the complex Raman tensor elements, their relative strengths, and their evolution as a function of sample thickness. For instance, anisotropy dominates the monolayer response and interlayer interactions in multilayers reduce all observed anisotropies.
In addition to first-order Raman modes, four new second-order modes are identified in the close vicinity of the well-established symmetrical Ag modes. 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 calculations of the monolayer band structure and Raman intensity simulations reveal that phonon scattering predominantly involves intravalley contributions either in the zigzag or armchair cristal directions. Second order modes have played an important role in the development of carbon-based materials, including graphene, and their existence in black phosphorus offers new ways to probe carrier mobility, defect density, doping levels as well as chemical reactivity and mechanical stress.
Instrumental in the scientific and technological development of 2D materials, Raman spectroscopy appears to be an even more powerful technique for the study of orthorhombic materials: the Raman-allowed modes express changes in permittivity as a function of atomic motion, couple in a unique way with the anisotropic electronic band structure through very restrictive selection rules. These connections will be illustrated by experimental results discussed using arguments relating to the symmetry of black phosphorus.