Electrons and holes get closer

molybdenum and tungsten dichalcogenides, some of which have been predicted to have larger piezoelectric coefficients than MoS2 (ref. 3). Piezoelectricity has been reported in few-layer C3N4 nanosheets7. Future work may involve the study of higher-order electromechanical effects in few-layer materials that have potential to generate curvature in membranes5. The atomically thin nature of these materials also provides avenues to endowing nonpiezoelectric materials with piezoelectric properties through the adsorption of adatoms or molecules that change the local symmetry of the material. Graphene has an inversion centre and is not piezoelectric, but the single-sided adsorption of adatoms has been predicted to provide it with some non-zero elements of the piezoelectric tensor8. Two-dimensional piezoelectric materials could improve the performance of piezoelectrics in existing applications, such as surface acoustic wave devices. These materials may also enable new applications due to their nanoscale nature. For example, it might be possible to achieve integration of electronic and acoustic elements in a single layer. Furthermore, very large tensile strains of more than 20% have been reported in two-dimensional materials, pointing to the potential for applications in flexible electronic devices. In fact, Wang and colleagues suggest that twodimensional piezoelectrics could be used for energy harvesting1. The piezoelectric effect may also play an important role in the function of devices whose operation is not based on piezoelectricity. For example, in GaN/AlN superlattices, which are used to make optoelectronic devices, the interface between the layers can host two-dimensional electron or hole gases that result from piezoelectric effects. Similarly, such effects could also play a role in interfaces between two-dimensional materials. Practical applications for single-layer and few-layer piezoelectrics will be facilitated by the fabrication of larger-area single crystals, which are currently at micrometre length scales. Another challenge may be the precise control of the required layer thickness, which is needed because of the sensitivity of the piezoelectric effect to the number of layers in the material. ❐