Second-Order Raman Scattering in Exfoliated Black Phosphorus.

Area of Science:

• Condensed Matter Physics
• Materials Science
• Nanotechnology

Background:

• Second-order Raman scattering is well-studied in carbon nanomaterials, activating normally forbidden modes sensitive to crystal disorder.
• First-order Raman processes typically dominate in sp²-hybridized carbon systems, making them an exception.

Purpose of the Study:

• To identify and characterize second-order Raman scattering modes in exfoliated black phosphorus (P(black)).
• To investigate the nature and origin of these newly observed modes in an anisotropic semiconductor.

Main Methods:

• Experimental Raman spectroscopy on exfoliated black phosphorus with varying thickness, excitation wavelength, and defect density.
• Ab initio simulations of monolayer black phosphorus to understand scattering mechanisms.

Main Results:

• Identification of four new second-order Raman modes (D₁, D₁^', D₂, D₂^') in black phosphorus, dominating at 633 nm excitation.
• These modes are defect-activated and involve high-momentum phonons via a doubly resonant Raman process.
• Ab initio simulations attribute D' and D modes to intravalley scattering along armchair and zigzag directions, respectively.

Conclusions:

• Discovery of defect-activated second-order Raman modes in black phosphorus, challenging previous assumptions for elemental semiconductors.
• The high sensitivity of these modes to disorder provides a tool for characterizing defects in black phosphorus.
• These findings help resolve discrepancies in existing literature regarding Raman scattering in black phosphorus.