2 research outputs found
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 electronâphonon 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><sub><i>g</i></sub><sup>1</sup>, <i>B</i><sub>2<i>g</i></sub>, and <i>A</i><sub><i>g</i></sub><sup>2</sup> and the Davydov-induced
mode labeled <i>A</i><sub><i>g</i></sub>(<i>B</i><sub>2<i>u</i></sub>). The complex Raman tensor
elements reveal that the relative variation in permittivity of all <i>A<sub>g</sub></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><sub><i>g</i></sub><sup>2</sup> and <i>A</i><sub><i>g</i></sub>(<i>B</i><sub>2<i>u</i></sub>), and interlayer interactions in multilayer
samples lead to reduced anisotropy. The bulk-forbidden <i>A</i><sub><i>g</i></sub>(<i>B</i><sub>2<i>u</i></sub>) mode appears for <i>n</i> â„ 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><sub>2<i>u</i></sub> and exhibits characteristics similar to <i>A</i><sub><i>g</i></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
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 sp<sup>2</sup>-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 D<sub>1</sub>, D<sub>1</sub><sup>âČ</sup>, D<sub>2</sub> and D<sub>2</sub><sup>âČ</sup>, in exfoliated black phosphorus (PÂ(black)), an elemental
direct-gap semiconductor exhibiting strong mechanical and electronic
anisotropies. Located in close proximity to the A<sub>g</sub><sup>1</sup> and A<sub>g</sub><sup>2</sup> 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 DâČ 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