Observation of piezoelectricity in free-standing monolayer MoS2

Piezoelectricity allows precise and robust conversion between electricity and mechanical force, and arises from the broken inversion symmetry in the atomic structure1–3. Reducing the dimensionality of bulk materials has been suggested to enhance piezoelectricity4. However, when the thickness of a material approaches a single molecular layer, the large surface energy can cause piezoelectric structures to be thermodynamically unstable5. Transition-metal dichalco-genides can retain their atomic structures down to the single-layer limit without lattice reconstruction, even under ambient conditions6. Recent calculations have predicted the existence of piezoelectricity in these two-dimensional crystals due to their broken inversion symmetry7. Here, we report experimental evidence of piezoelectricity in a free-standing single layer of molybdenum disulphide (MoS2) and