Raman spectroscopy studies of black phosphorus

Black phosphorus has important applications in many fields such as optics, optoelectronics and thermals. Many of its excellent properties are related to its special anisotropy. In this work, we adopted Raman spectroscopy, which can obtain fast response optical signals without destroying the structure of the sample, to identify its crystal orientation and explore its thermal and SERS properties. We successfully distinguished the armchair and zigzag directions of black phosphorus by angle-resolved polarized Raman spectroscopy of A_g mode in a less studied orthogonal polarization configuration. Then we used temperature dependent Raman spectroscopy to study its thermal properties. It is found that the first order temperature coefficients of its three Raman vibration modes A_g¹, B_2g, and A_g² are -0.0133 cm^-1 K^-1, -0.0232 cm^-1 K^-1 and -0.0229 cm^-1 K^-1, respectively for the 3.2 nm sample. Furthermore, we studied the surface enhancement effect of black phosphorus with different thicknesses as SERS substrates. We found that few-layer black phosphorus has better enhancement effects and its limit of detection for MB and CV are both 10^-6M. The analytical enhancement factor of black phosphorus substrates on CV can achieve 1.2 × 10³ by calculation. These methods can be extended to other similar two-dimensional materials.