Flexible Nanogenerators based on Enhanced Flexoelectricity in Mn3O4 Membranes

Atomically thin, few-layered membranes of oxides show unique physical and chemical properties compared to their bulk forms. We have exfoliated manganese oxide (Mn3O4) membranes from the naturally occurring mineral Hausmannite and used them to make flexible, high-performance nanogenerators. We observe an enhanced power density in the membrane nanogenerators (NG) with the best-performing device showing a power density of 7.99 mW m-2 compared to 1.04 µW m-2 obtained from NG made up of bulk Mn3O4. We also observe a sensitivity of 108 mV kPa-1 for applied forces < 10 N in the membrane NGs. We attribute the improved performance of these NGs to enhanced flexoelectric response in few-layers of Mn3O4. Using first-principles calculations, we calculate the flexoelectric coefficients of monolayer and bilayer Mn3O4, and find them to be 50 – 100 times larger than other two-dimensional transition metal dichalcogenides (TMDCs). Using a model based on classical beam theory, we find increasing activation of the bending mode with decreasing thickness of the oxide membranes, which in turn leads to a large flexoelectric response. As a proof-of-concept, we made flexible nanogenerators using exfoliated Mn3O4 membranes and used them in self-powered paper-based devices. This research paves the way for the exploration of few-layered membranes of other centrosymmetric oxides for application as energy harvesters.