Crumpled Graphene-MoS₂-Graphene Heterostructure for Flexoelectric Energy Harvester

Flexoelectric effect induces an electric polarization from strain gradient. Theoretically, since the size of the strain gradient grows quadratically as the material’s length scale decreases, the flexoelectricity-induced polarization grows quadratically as the material’s length scale decreases as well. Consequently, the flexoelectric-induced electric polarization can be maximized in atomically thin two-dimensional (2D) materials. In this study, we demonstrate flexoelectric energy harvesters by utilizing crumpled 2D molybdenum disulfide (MoS₂) as an active material to generate power output from strain gradient. We assembled graphene-MoS₂-graphene vertical stack on a pre-stretched very high bond (VHB) substrate followed by a release of the stretch to create a crumpled heterostructure. The fabricated flexoelectric energy devices showed 1 V of open circuit voltage (OCV) and 10 nA of short circuit current (SCC) under repetitive bending motions. However, the devices showed limited capability on harvesting energy from the translational motion. To overcome this challenge, we employed an asymmetric device configuration. The asymmetric device configuration was achieved by two-step stamping-crumpling methods and resulted in a periodic contact between one graphene electrode and the MoS₂ active layer while maintaining a full contact between the other graphene electrode and the MoS₂ active layer. As a result, the device with asymmetric device configuration showed 3 mV of OCV and 0.01 nA of SCC under repetitive translational motions. Furthermore, the crumpled structure allowed high deformation endurance, and the devices demonstrated robust energy harvesting ability up to 100 % and 50 % stretching strain for symmetric and asymmetric configurations, respectively.