Concurrence of negative in-plane piezoelectricity and photocatalytic properties in 2D ScAgP 2 S 6 monolayers

Abstract Apart from poly(vinylidene fluoride) and its copolymers, 2D multilayered CuInP 2 S 6 (CIPS) is experimentally known to show negative out-of-plane piezoelectric coefficient ( d 33 ) [2019 Sci. Adv. 5 1–10], which is attributable to weak interlayer van der Waals binding. However, CIPS monolayers exhibit positive in-plane piezoelectricity ( d 11 ) [ Phys. Status Solidi RRL , 14 2000321], as per DFT calculations. We ascribe it to strong in-plane interatomic bonding. Weakening of the in-plane interatomic bonding may even induce a negative in-plane piezoelectric coefficient, which is exceptionally rare, as observed in this work based on DFT calculations. Much smaller in-plane elastic constants found in ScAgP 2 S 6 monolayers relative to that in CIPS monolayers indicate much weaker in-plane interatomic bonding, which induces negative in-plane piezoelectric coefficient, d 22 = −2.81 pm V −1 , in ScAgP 2 S 6 monolayers. In addition to its all-around stability, ScAgP 2 S 6 monolayer is a semiconducting showing a bandgap of 2.04 eV, thereby making it suitable for electronics and solar energy harvesting. Upon the application of 4% biaxial strain on this flexible and stretchable 2D material, d 22 has been found to be drastically increased to −591 pm V −1 . The factors causing such an enhancement in d 22 have been ascertained through the maximum rise in the Born effective charges and maximum drop in the elastic constants. With the further drop in elastic constants relative to ScAgP 2 S 6 monolayer, d 22 reaches −3.45 pm V −1 and −13.48 pm V −1 in ScAgP 2 Se 6 and ScAgP 2 Te 6 monolayers respectively. 2D ScAgP 2 S 6 monolayer is found to be a promising photocatalyst as well, since its band edges straddle the water redox potential over the entire range in pH (0–14), in addition to having a high absorption coefficient (≈10 5 cm −1 ) in the visible region. It is understood that 2D ScAgP 2 S 6 will substantially expand the scope for its applications in piezoelectricity and visible light harvesting at the nanoscale.