Tribo-Piezoelectricity in Group III Nitride Bilayers: A Density Functional Theory Investigation.

The notable out-of-plane piezoelectric effect caused by the large electronegativity of the constituent elements makes two-dimensional (2D) group III nitrides appealing for nanoscale energy-harvesting applications. Here, we demonstrate by extensive density functional theory investigations that the vertical piezoelectricity is enhanced significantly in 2D XN (X = B, Al, Ga) bilayers due to in-plane interlayer sliding. The sliding operation generates tribological energy from the vertical resistance force between the monolayers. A maximum shear strength between the monolayers of 1-25 GPa is recorded during vertical sliding. We elucidate the tribo-piezoelectricity generation mechanism of XN bilayers using the tribological energy conversion to overcome the interfacial sliding barrier. The strongest out-of-plane piezoelectricity is found when the bilayers are in the A-A stacking arrangement. Any reduction in the interlayer distance between group III nitride bilayers enhances out-of-plane polarization due to the increase in sliding energy resistances, leading to an increased inductive voltage output. An induced voltage of ∼3.5 V is achieved during vertical compressive sliding of the upper layer. Using these phenomena, we present a compression-slide XN bilayer nanogenerator strategy capable of tuning the produced tribo-piezoelectric energy through sliding and compression.