  The identification of the crystalline axis of anisotropic black phosphorus
(BP) is key for the study of its physical properties and for its optical and
electronic applications. Herein, we show that by applying an in-plane uniaxial
strain and measuring the changes of Raman shifts, the crystalline axis of BP
can be reliably determined. The strain effects on Raman shifts are
angle-dependent, which can be expressed as a combination of Raman responses
under zigzag and armchair strains. Different from previous polarized optical
spectroscopic means, the proposed method does not rely on the laser
polarization, the excitation wavelength, the sample thickness and the material
of the stretchable substrate. Besides, the effective strain applied on BP from
the stretched substrate is estimated, showing that only 20% to 40% of the
strain can be effectively transferred to BP flakes from polyethylene
terephthalate substrates. Our method provides not only an effective and
universal approach to identify the crystalline orientation of layered BP, but
also a model to extract additional information in strain-related studies, and
can be extended to other 2D anisotropic materials.
