Direct Observation of Raman Spectra in Black Phosphorus under Uniaxial Strain Conditions

In this paper, we systematically studied the Raman vibration of black phosphorus (BP) transferred onto a germanium (Ge)-coated polydimethylsiloxane (PDMS) substrate, which generates a much higher contrast in BP. This engineered flexible substrate allowed us to directly observe a much thinner BP layer on the flexible substrate at the desired location. Therefore, it enabled us to perform Raman spectroscopy immediately after exfoliation. The Raman spectra obtained from several BP layers with different thicknesses revealed that the clear peak shifting rates for the A_g¹, B_2g, and A_g² modes were 0.15, 0.11, and 0.11 cm^-1/nm, respectively. Using this value to identify a 2⁻3-layered BP, a study on the strain⁻Raman spectrum relationship was conducted, with a maximum uniaxial strain of 0.89%. The peak shifting of A_g¹, B_2g, and A_g² caused by this uniaxial strain were measured to be 0.86, 0.63, and 0.21 cm^-1/Δε, respectively.