A Microelectrochemical Diode with Submicron Contact Spacing Based on the Connection of Two Microelectrodes Using Dissimilar Redox Polymers

Closely spaced, 0.2-1 μm, Au microelectrodes (50 μm long, 1–2 μm wide, and 0.1 μm thick) on Si₃N₄ can be functionalized with poly(vinylferrocene), PVFc^+/0, or with an N, N-dibenzyl-4, 4-bipyridinium-based polymer, (BPQ^2+/+)n, derived from hydrolysis of N, N'-bis[(p-trimethoxysilyl)benzyl]-4, 4^'-bipyridinium (I). Two-or eight-microelectrode arrays have been functionalized with PVFc^+/o or (BPQ²⁺/⁺)n. Adjacent microelectrodes can be connected with either polymer in the sense that net current can pass from one microelectrode to another, through the polymer, when one electrode is held at a potential where the polymer is oxidized and the other electrode is held at a potential where the polymer is reduced. From such steady-state current an estimate of the diffusion coefficient for charge transport, D_CT, in the polymer can be made; values in the range l0^-9-10^-10 cm²/s are found and accord well with earlier measurements of D_CT for the polymers studied. A two-terminal diode can be fabricated by coating one electrode with (BPQ^2+/+)n and an adjacent electrode with PVFc^+/0 such that there is a connection between the microelectrodes via the (BPQ²⁺/⁺)_n/PVFc^+/° contact. Current passes when the applied potential is such that the negative lead is attached to the (BPQ^2+/+)_n-coated electrode and the positive lead is attached to the PVFc^+/0-coated electrode. When the applied potential approaches the difference in the E^os of the two polymers, current flows with the crucial feature being a downhill (by ~0.9 V) cross redox reaction at the (BPQ^2+/+)_n/PVFc^+/0 interface, BPQ⁺ + Fc⁺ —> BPQ²⁺ + Fc°. Current does not flow between the microelectrodes when the applied potential is in the opposite sense, because the reaction BPQ²⁺ + Fc° —> BPQ⁺ + Fc⁺ is uphill by ~0.9 V. The switching time of a microelectrochemical diode is controlled by the time required to oxidize and reduce the polymers.