Flexible Nanogenerators based on Enhanced Flexoelectricity in Hausmannite 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 naturally occurring 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 7.20 µW cm-2 compared to 1.04 µW cm-2 obtained from NG made up of bulk Mn3O4. We also observe a sensitivity of 108 mV/kPa for applied forces < 10 N in the membrane NGs. We attribute the improved performance to enhanced flexoelectric response in few-layers 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 two-dimensional transition metal dichalcogenides. We also show the presence of a destructive interference between the bending and shear modes in bulk Mn3O4 that almost cancel the overall flexoelectric coefficients. Dimensional reduction in the Mn3O4 suppresses the response to the shear modes and results in large flexoelectric coefficients. As a proof-of-concept, we made flexible triboelectric nanogenerators (TENG) using exfoliated Mn3O4 membranes and used them in self-powered devices such as paper based TENG. This research paves the way for exploration of few-layer membranes of other centrosymmetric oxides for application as energy harvesters.