Memristors Based on Ni(II)-tetraaza[14]annulene Complexes: Toward an Unconventional Resistive Switching Mechanism

Abstract

In this work, a family of Ni-based dibenzotetraaza[14]annulene (dtaa) complexes are investigated for their application in memristors (memory resistors). A series of four Ni(II) complexes with different peripheral substituents of the dtaa ligand are successfully synthesized. Based on these compounds, two-terminal thin-film devices are fabricated in planar architecture. Four metals with different work functions are tested: Mg, Cu, Ni, and Au. It is demonstrated that ITO|[Ni(Me₄dtaa)]|Cu devices show hysteretic behavior and offer stable, robust, and reproducible switching between high- and low-resistive states. An in-depth spectroscopic characterization of the Ni complex is performed, using radiation from infrared, through visible and ultraviolet, to tender X-rays. Operando X-ray fluorescence spectroscopy is used to monitor redox and structural changes upon the polarization of the studied memristor with the external electric field. Density functional theory calculations are used to better understand the electronic structure of the studied material, as well as structural rearrangement after electron injection that may be responsible for the modulation of electric conductivity. Finding a unique case of filamentary-type resistive switching involving redox reactions of stationary molecules within a molecular solid is postulated. Yet, the formation of these filaments is not related to any significant configurational changes at the atomic scale.