Scientists have designed a new way to create a single-molecule diode that performs 50 times better than past models.
These single-molecule diodes are the first that could be used for real-world applications in nanoscale devices, Columbia University School of Engineering and Applied Science reported. The idea of creating a single-molecule diode was first proposed in the 1970s by Arieh Aviram and Mark Ratner, who theorized that a molecule could act as a "rectifier" to conduct one-way currents.
"Constructing a device where the active elements are only a single molecule has long been a tantalizing dream in nanoscience. This goal, which has been the 'holy grail' of molecular electronics ever since its inception with Aviram and Ratner's 1974 seminal paper, represents the ultimate in functional miniaturization that can be achieved for an electronic device," said Latha Venkataraman, associate professor of applied physics at Columbia Engineering.
Since the 1974 paper, scientists have determined single-molecules attached themselves to metal electrodes, and act as a number of circuit elements such as switches, resistors, and diodes. A diode works as an "electricity valve," and requires an asymmetrical structure in order to create different environments for electricity flowing in each direction.
"While such asymmetric molecules do indeed display some diode-like properties, they are not effective," said Brian Capozzi, a PhD student working with Venkataraman and lead author of the paper. "A well-designed diode should only allow current to flow in one direction-the 'on' direction-and it should allow a lot of current to flow in that direction. Asymmetric molecular designs have typically suffered from very low current flow in both 'on' and 'off' directions, and the ratio of current flow in the two has typically been low. Ideally, the ratio of 'on' current to 'off' current, the rectification ratio, should be very high."
To remedy this, the researchers worked to develop asymmetry in the environment around the molecular junction. They accomplished this by surrounding the active molecule with an ionic solution and employed the use of gold metal electrodes that differed in size to contact the molecule. The method led to rectification ratios as high as 250, which is 50 times higher than earlier designs.
"It's amazing to be able to design a molecular circuit, using concepts from chemistry and physics, and have it do something functional," Venkataraman said. "The length scale is so small that quantum mechanical effects are absolutely a crucial aspect of the device. So it is truly a triumph to be able to create something that you will never be able to physically see and that behaves as intended."
The findings were published in a recent edition of the journal Nature Nanotechnology.
