An atomic switch consists of two electrodes, one of which is a solid electrolyte. When a bias in a certain direction is applied across the electrodes, electrochemical precipitation results in a protrusion on the solid electrolyte. This protrusion acts as a bridge to span the gap between the electrodes, turning the switch ON. When a bias in the opposite direction is applied, the protrusion becomes smaller, turning the device OFF. It is not surprising that this process would require very narrow gaps between the electrodes. Up to now, atomic switches have had gaps of approximately 1 nm. Achieving such nanogaps can be challenging, but there is much interest in these devices because they represent the smallest of switches and would be quite useful for nanodevices.

In recent work from Japan’s Osaka University and National Institute for Materials Science, atomic switches with gaps ranging from 10 to 80 nm are shown to be possible. The atomic switches of Tanaka, Hasegawa, and their co-workers includes a photoconductive organic layer covering the electrodes and the gap. This design not only allows for simper fabrication techniques than that required for previous atomic switches, but also for additional functionality. Initially, light is needed in combination with a bias to complete the formation of the atomic-scale bridge. Without the appropriate light, the organic layer does not have the required photocurrent to allow for bridge formation. After initiation however, the device acts as a normal atomic switch. Consequently, it has two levels of switching: initiation and the standard on/off switching. Such a switch is therefore programmable and could be useful for photosensing memory devices.

The studies also demonstrate that the initiation times of the devices vary, depending on the gap size; larger gaps need longer illumination times for devices to be fully initiated. These differences could lead to additional ways to control the switch. Although the recent report includes only one type of organic layer, it should be possible to similarly incorporate other films. An array of such devices with organic layers that respond to different wavelengths could act as sophisticated photosensors allowing image detection.