  When a two-dimensional material, adhered onto a compliant substrate, is
subjected to compression it can undertake a buckling instability yielding to a
periodic rippling. Interestingly, when black phosphorus flakes are compressed
along the zig-zag crystal direction the flake buckles forming ripples with a
40% longer period than that obtained when the compression is applied along the
armchair direction. This anisotropic buckling stems from the puckered honeycomb
crystal structure of black phosphorus and a quantitative analysis of the ripple
period allows us to determine the Youngs's modulus of few-layer black
phosphorus along the armchair direction (EbP_AC = 35.1 +- 6.3 GPa) and the
zig-zag direction (EbP_ZZ = 93.3 +- 21.8 GPa).
