Piezoelectric materials, which convert energy between electricity and mechanical force, are the core of clocks, ultrasonics and microbalances. We have experimentally demonstrated that a single-layer crystal of less than 1-nm thick can be piezoelectric, and are tough enough for constructing Nano-ElectroMechanical Systems (Zhu et al, Nature Nanotechnology 10, 151).
The freestanding single-layer MoS2 NEMS device. A force is generated by electric field and picked up by Atomic Force Microscope.
The ratio between the in-plane stress and the electric field, i.e. piezoelectric coefficient, for this 2D crystal is comparable with conventional 3D crystals like quartz and AlN. The crystal can be chemically engineered to show out-of-plane piezoelectricity as well (Lu et al, Nature Nanotechnology 12, 744).