Bistable [2]Catenanes for Data Storage

In these times of big data and artificial intelligence, existing data storage media can become insufficient. The next generation of data storage must meet the demands for high-density storage and energy efficiency. One such technology is resistive random-access memory (RRAM), which stores data through changes in resistance. Yuan Li, Tsinghua University, Beijing, China, and colleagues have developed an approach to making supramolecular memristors, key components in the construction of nano-RRAM.

Molecular Memristors

Memristors could provide data storage that is both fast and reliable. The resistance of a memristor (a portmanteau of memory and resistor) can reversibly change with an applied voltage and then remains stored.

However, constructing a memristor on the molecular scale is a challenge. Although resistance switching can be achieved through redox reactions, and the charged states of molecules can easily be stabilized by counterions in solution, this stabilization is very difficult to achieve in the solid-state junctions required for a memristor.

Supramolecules for Solid-State Devices

The team chose to take a supramolecular approach. It is based on a [2]catenane that is bistable, meaning it is stable in both oxidized and reduced forms. A [2]catenane is a system of two large molecular rings that are interlocked like two links in a chain, but are not chemically bonded.

To build a memristor, the team deposited the catenane (pictured) onto a gold electrode that was coated using S-(CH₂)₃-SO₃^–Na⁺, where it is bound through electrostatic interactions. On top of this, they placed a second electrode made of a gallium-indium alloy coated with gallium oxide. The catenane forms a self-assembled monolayer of flat molecules between the two electrodes. This ensemble, designated as AuTS-S-(CH₂)₃-SO₃–Na+//[2]catenane//Ga₂O₃/EGaIn, forms the memristor.

Promising Performance

These novel supramolecular memristors can be switched between a state of high resistance (off) and a state of low resistance (on), depending on the applied voltage, as is required for RRAMs. The molecular resistance switches achieved at least 1,000 erase-read (ON)-write-read (OFF)–cycles. Switching between on and off occurs in significantly less than one millisecond, which is comparable to commercial inorganic memristors.

The molecular switches “remembered” the set state—ON or OFF—for several minutes. This makes them a promising starting point for effective molecular memristors with non-volatile storage capabilities. In addition, they function as diodes, or rectifiers, which makes them interesting components for the development of molecular nano-RRAMS.