Space Charge Drives Electromechanical Conversion in Ion-Implanted Polymers via an Apparent Piezoelectric Effect

Ion implantation is a powerful tool to modify material chemistry and structure. The implantation process was considered to result in a net-neutral material, due to implanted ionic charge being compensated by the host materials lattice. Here, we show ion implantation into polytetrafluoroethylene (PTFE) results in an uncompensated "space charge" region-requiring a reconsideration of ion implantation into polymers. This is demonstrated via electromechanical testing of Cu implanted PTFE as a triboelectric nanogenerator (TENG). Previously, ion implantation into polymers has been shown to increase TENG performance, attributed to increasing the prevalence of electron transfer during contact-separation testing. This attribution to electron transfer is incorrect, with significant electromechanical conversion being observed in 1×10^{16} at.cm^{-2} Cu^{+} implanted polytetrafluoroethylene (PTFE) in both piezoelectric mode testing and in noncontact induction measurements-where electron transfer cannot occur. These results indicate that the implantation of Cu ions creates a space charge effect in the PTFE matrix, and the subsequent charge asymmetry creates an electric field enhancing TENG performance, analogous to hybrid piezoelectric TENGs. These results demonstrate that ion implanted polymers possess space charge and can be used directly for sensing, creating a new pathway for electromechanical conversion materials.