  Two-dimensional (2D) hexagonal boron nitride (hBN) is a wide-bandgap van der
Waals crystal with a unique combination of properties, including exceptional
strength, large oxidation resistance at high temperatures and optical
functionalities. Furthermore, in recent years hBN crystals have become the
material of choice for encapsulating other 2D crystals in a variety of
technological applications, from optoelectronic and tunnelling devices to
composites. Monolayer hBN, which has no center of symmetry, has been predicted
to exhibit piezoelectric properties, yet experimental evidence is lacking.
Here, by using electrostatic force microscopy, we observed this effect as a
strain-induced change in the local electric field around bubbles and creases,
in agreement with theoretical calculations. No piezoelectricity was found in
bilayer and bulk hBN, where the centre of symmetry is restored. These results
add piezoelectricity to the known properties of monolayer hBN, which makes it a
desirable candidate for novel electromechanical and stretchable optoelectronic
devices, and pave a way to control the local electric field and carrier
concentration in van der Waals heterostructures via strain. The experimental
approach used here also shows a way to investigate the piezoelectric properties
of other materials on the nanoscale by using electrostatic scanning probe
techniques.
