Origin and enhancement of the piezoelectricity in monolayer group IV monochalcogenides under strain and in the presence of vacancies

Piezoelectric materials are a critical component in many electronic devices from the nanoscale to the macroscale. Monolayer group IV monochalcogenides can provide particularly large piezoelectric coefficients. To investigate the origin of this strong piezoelectricity, we conduct an atomic-level analysis of the charge redistribution under mechanical strain. Our results show that it arises from charge transfer between strong and weak chemical bonds. We demonstrate that the piezoelectric coefficients can be substantially enhanced by mechanical strain and the presence of vacancies, for instance in the case of monolayer SnSe by up to 112% by 2% compression and by up to 433% by an Sn-Se vacancy density of 5.5%.