A typical resistive memory device is composed of one layer of active materials sandwiched between two conductive electrodes and the conductivity of a binary memory can be switched between a low- ("OFF") and high-conductance ("ON") states under an external electric stimulus (figure 1). Due to the high capacity, high flexibility, good scalability, low cost, and low power consumption, resistive memory is promising for the next-generation high-density data storage. Transition metal complexes have recently attracted considerable scientific interest for resistive memory switching. The interplay between metal ions and ligand endows transition metal complexes with rich and tunable electrochemical and photophysical properties, making them beneficial for the formation of electrical bi- or multistates in resistive memory devices.
Figure 1. Basic configuration of a resistive memory device
Gold, cobalt, and rhodium complexes are used here for modulating the switching behavior of resistive memories, and the specific applications are shown below.
Researchers' interest in gold (III) complexes stems from their relatively high stability, environmental benignancy, and abundant photoluminescent behavior, which together make them ideal candidates for potential applications in optoelectronics. In this regard, a class of phosphole oxide-containing gold (III) complex has been synthesized, structurally characterized, and applied in solution-processable resistive memory devices . High ternary memory performance has been demonstrated with distinct and low switching threshold voltages, high OFF/ON1/ON2 current ratio of 1/103 /107, and good stability of the three conductive states. The multilevel memory behavior has been attributed to the two independent charge-trapping sites provided by the cyclometalated ligand and the benzophosphole oxide moiety.
Figure 2. (a) Perspective view of one enantiomer of phosphole oxide-containing gold (III) complex (b) Schematic diagram of the device structure
Cobalt (II) polypyridine complexes have drawn a lot of interest in many fields because of their enriched optoelectronic properties. In particular, the presence of well-defined multiple redox processes of Cobalt (II) complexes may make them beneficial for resistive switching. Zhong and co-workers  used triphenylamine-appended bisterpyridine cobalt (II) complex as the active layer to fabricate resistive switching memory devices. The devices exhibit a flash-type rewritable memory switching behaviour with a large ON/OFF ratio (> 103) and low operation voltages (< ±3 V).
Figure 3. Schematic representation of the sandwiched memory device structure
Similar to the common transition metal complexes, rhodium complexes possess clear and reversible redox properties, which are quite important for resistive memory. Goswami and co-workers  studied the memory device properties of the rhodium (III) complex with three aromatic azopyridyl ligands. These complexes possessed multistep reversible redox couples and showed excellent memory switching properties with a large ON/OFF ratio.
Figure 4. (a)Triplet azo-anion di-radical complex of rhodium (III) (b) Cross-sectional view of the device layout for the memory device
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- Vivian, W. W. Y.; et al. A phosphole oxide-containing organogold (III) complex for solution-processable resistive memory devices with ternary memory performances. J. Am. Chem. Soc. 2016, 138: 6368-6371.
- Zhong, Y. W.; et al. Resistive memory based on a triphenylamine-decorated non-precious cobalt (II) bis-terpyridine complex. Chem. Commun. 2017,53: 11925-11928.
- Paul, N.; et al. Azo anion radical complex of rhodium as a molecular memory switching device: isolation, characterization, and evaluation of current-voltage characteristics J. Am. Chem. Soc. 2012, 134: 6520-6523.