  We report on the mechanical properties of few-layer black phosphorus (BP)
nanosheets, in high vacuum and as a function of time of exposure to atmospheric
conditions [1]. BP flakes with thicknesses ranging from 4 to 30 nm suspended
over circular holes are characterized by nanoindentations using an atomic force
microscope tip. From measurements in high vacuum an elastic modulus of 46+/-10
GPa and breaking strength of 2.4+/-1 GPa are estimated. Both magnitudes are
independent of the thickness of the flakes. Our results show that the exposure
to air has substantial influence in the mechanical response of flakes thinner
than 6 nm but small effects on thicker flakes.
