The azobenzene moiety—two nitrogen atoms double-bonded to each other with benzene rings attached—is useful for memory applications because its conductance can be switched optically or electrically.

En-Tang Kang and colleagues at the National University of Singapore1 show that they can change memory-type using changes in molecular structure with polymers containing azobenzene side chains. By simply modifying the terminal groups attached to the benzene rings they are able to convert the memory from write-once, read-many-times (WORM) to 'FLASH' memory that can be rewritten many times.

The conductance switching of azobenzene molecules arises from electrically induced charge trapping. The trap-free and trap-filled states of the molecules have different conductivities, which can be read to make electrical memory devices.

Kang and colleagues created five different types of polymers with similar structures. They varied the terminating groups on the azobenzene rings from electron-donating groups such as a methoxy configiration, to electron-withdrawing groups such as a nitro structures.

Fig. 1: Schematic diagram of the device structure.

The researchers created memory devices by sandwiching the polymers between conducting electrodes (Fig.1). The as-made devices were in the low conductivity, OFF state. However, when the device voltage was swept to about –1.5V, the conductivity suddenly increased by five orders of magnitude, indicating charge trapping in the azobenzene.

For the nitro-functionalized polymers, this conductivity change could not be reversed when a positive voltage was applied—the characteristics of a WORM device. However, when the nitro group was replaced with a methoxy group, the conductivity could be reversed to OFF, where a positive voltage resulted in the conductivity returning to its low value. This corresponds to a rewritable FLASH memory. The researchers believe that the ON state is stabilized by the electron-withdrawing groups, making the nitro-containing polymer difficult to turn OFF when the applied voltage is reversed.

In the future, the authors hope to take advantage of the polymers’ unique attributes. “Azobenzene molecules can be manipulated both optically and electronically,”says group member Siew Lay Lim.“We hope to explore the possibility of combining both optical and electronic manipulation to fabricate azobenzene-based memories with additional functionalities.”