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. Manganese oxide (Mn₃O₄) membranes are exfoliated from the naturally occurring mineral Hausmannite and used to make flexible, high-performance nanogenerators (NGs). An enhanced power density in the membrane NG is observed with the best-performing device showing a power density of 7.99 mW m^-2 compared to 1.04 µW m^-2 in bulk Mn₃O₄. A sensitivity of 108 mV kPa^-1 for applied forces <10 N in the membrane NG is observed. The improved performance of these NGs is attributed to enhanced flexoelectric response in a few layers of Mn₃O₄. Using first-principles calculations, the flexoelectric coefficients of monolayer and bilayer Mn₃O₄ are found to be 50-100 times larger than other 2D transition metal dichalcogenides (TMDCs). Using a model based on classical beam theory, an increasing activation of the bending mode with decreasing thickness of the oxide membranes is observed, which in turn leads to a large flexoelectric response. As a proof-of-concept, flexible NGs using exfoliated Mn₃O₄ membranes are made and used 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.