For the same reasons that control of electrical currents in silicon transistors is important for electronics, manipulation of the electrical conductivity is also required for molecular electronics. Researchers from Kyoto University, in collaboration with teams from other Japanese universities,1 now demonstrated how the conductance across networks made from gold nanoparticles and small organic molecules can be reversibly switched through irradiation with UV and visible light, respectively.

The photoswitching of these networks originates from the use of diarylethene molecules. Through irradiation with UV light the diarylethene molecules undergo small reconfigurations so that some of the pi-conjugated systems in the molecule alter their bonds. These subtle changes are sufficient to make the molecules highly conductive. The change in bonding structure can be reversed through visible light, which switched the molecules back to their low conductive state.

Fig. 1: Schematic illustration of the network composed of photochromic diarylethenes and gold nanoparticles, embedded within two gold electrodes on both sides.

In order to fabricate devices out of these photochromic molecules, a larger number of molecules need to be stabilized for each device. Therefore, the molecules are attached to gold nanoparticles, so that entire networks form between the two electrodes (Fig. 1).

The current-voltage curves of the devices show a strong dependency on the prior illumination with light. “Conductance switching in a metal nanoparticle network was demonstrated for the first time,” says Kenji Matsuda from the research team. Depending on the precise molecular structure, the conductivity ratio between conducting and insulating states can be significantly different, with a maximum on/off ratio of about 25.

A further advantage of this system in comparison to other materials showing similar conductance switching, is the capability to assemble the network into any desirable structure. Unfortunately, so far the dynamics of the switching remains slow. The sample showing the largest on/off ratio takes about 8 hours, although through an improved design the switching speed could be increased significantly, with switching in less than an hour.

The size of the nanoparticle networks also influences its switching behavior and smaller networks may switch more sharply. “One of our targets is to create a more digital, abrupt switching behavior using a smaller number of molecules,” says Matsuda.