Multilevel conductance switching for a monolayer of redox-active metal complexes through various metallic contacts

Abstract

Redox-active metal complexes (e.g., Fe^II, Ru^II, and Co^II biphenylterpyridine) exhibiting multiple electroreduction behaviors show multilevel conductance switching through various metallic contacts, including Au, Pt/Ir, and reduced graphene oxide (rGO), in solid-state molecular junctions (e.g., a metal–molecule–metal junction). At Pt/Ir and Au contacts in the scanning tunneling microscopy (STM)-based junctions or Au/rGO film contacts in monolayer-based devices, current–voltage (I/V) characteristics have different energy distributions depending on the level of injection into the three electron affinity levels of the redox-active metal complexes. These metal complexes can be negatively charged when the energy levels between the Fermi levels of the metal contacts and the molecular reduction states are aligned, which strongly depends upon the molecular conductance states of conjugated ligands coordinated to central metal atoms. Multiple reduction states of redox-active metal complexes measured with solution-phase electrochemistry correspond to the multiple electron affinity levels of the solid-state molecular junctions, which suggests the possibility of multilevel molecular memory components.