Imagine a single molecule that could act as an electrical diode, holding the power to control the direction of current flowing within a circuit. If scientists are successful in forging such a molecule, it could spell the beginning of a new era of tiny technology.
This idea was first suggested 40 years ago, and there have been some prototypes in the past. But now, scientists from Columbia University have created a diode that blows all previous attempts out the water. It can transform current from alternating current (AC) to direct current (DC) at a rate 50 times higher than previous designs. This process is called "rectification."
"Our new approach created a single-molecule diode that has a high (>250) rectification and a high 'on' current (~ 0.1 micro Amps)," says Latha Venkataraman, associate professor of applied physics at Columbia University. "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."
It may sound like a tough challenge to create this sort of selective device at the molecular level, but the way that scientists have solved this problem is to make the molecule asymmetric. This means that if electricity tries to flow one way, it has a different experience than if it tries to flow through the other way. This effectively creates a tiny diode.
So how did the team create an asymmetric molecule that is better than any of the previous designs? As described in Nature Nanotechnology, the researchers submerged half of the molecule in an ionic solution—the different environments around the two halves of the molecule emphasized the asymmetry. To further increase the asymmetry, the gold contacts that connected the diode to the rest of the circuit were different sizes.
Screenshot of video of molecular diode being used to bridge the gap between two contacts (red) via The National Science Foundation
It's a fantastic technique that marries the concepts of chemistry and physics.
“It’s amazing to be able to design a molecular circuit, using concepts from chemistry and physics, and have it do something functional,” Venkataraman says. “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.”
But making the best so far isn't enough for Venkataraman and her team: They want to continue to test different molecular systems to make the diode even more efficient.