Current polarized Raman-based techniques for identifying the crystalline orientation of black phosphorus suffer significant uncertainty and unreliability because of the complex interference involving excitation laser wavelength, scattering light wavelength, and sample thickness. Herein, for the first time, we present a new method, optothermal Raman spectroscopy (OT-Raman), for identifying crystalline orientation. With a physical mechanism based on the anisotropic optical absorption of the polarized laser and the resulting heating, the OT-Raman can identify the crystalline orientation explicitly, regardless of excitation wavelength and sample thickness, by Raman frequency-power differential Φ (=∂ω/∂P). The parameter Φ has the largest (smallest) value when the laser polarization is along the armchair (zigzag) direction. The OT-Raman technique is robust and is able to identify the crystalline orientation of BP samples with thicknesses up to 300 nm at a minimum and potentially as high as 1200 nm.
Keywords: Raman spectroscopy; black phosphorus; crystalline orientation; nanostructures; polarization dependence.
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