  Controlling the bandgap through local-strain engineering is an exciting
avenue for tailoring optoelectronic materials. Two-dimensional crystals are
particularly suited for this purpose because they can withstand unprecedented
non-homogeneous deformations before rupture: one can literally bend them and
fold them up almost like a piece of paper. Here, we study multi-layer black
phosphorus sheets subjected to periodic stress to modulate their optoelectronic
properties. We find a remarkable shift of the optical absorption band-edge of
up to ~0.7 eV between the regions under tensile and compressive stress, greatly
exceeding the strain tunability reported for transition metal dichalcogenides.
This observation is supported by theoretical models which also predict that
this periodic stress modulation can yield to quantum confinement of carriers at
low temperatures. The possibility of generating large strain-induced variations
in the local density of charge carriers opens the door for a variety of
applications including photovoltaics, quantum optics and two-dimensional
optoelectronic devices.
