Molecular diodes which perform 50 times better than all prior designs have been developed by Columbia University engineering researchers.
The new techology, created under the direction of Latha Venkataraman, associate professor of applied physics at Columbia, is the first to develop a single-molecule diode that may have real-world technological applications for nanoscale devices.
“Our new approach created a single-molecule diode that has a high (>250) rectification and a high “on” current (~ 0.1 micro Amps). 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.”
Electronic Devices Getting Smaller Every Day
The field of molecular electronics is becoming ever more important for solving the problem of further miniaturization. Single molecule devices represent the limit of miniaturization.
The idea of creating a uni-molecular diode was suggested in 1974 by Arieh Aviram and Mark Ratner. They theorized that a molecule could act as a rectifier, a one-way conductor of electric current.
Ever since, scientists have been exploring the charge-transport properties of molecules.
They have shown that single-molecules attached to metal electrodes, single-molecule junctions, can be made to act as a variety of circuit elements, including resistors, switches, transistors, and, indeed, diodes. They have found that it is possible to see quantum mechanical effects, such as interference, manifest in the conductance properties of molecular junctions.
Because diodes act as an electrical valve, their structure needs to be asymmetric so that electricity flowing in one direction experiences a different environment than electricity flowing in the other direction. In order to develop a single-molecule diode, researchers have simply designed molecules that have asymmetric structures.
Lead author Brian Capozzi, a PhD student working with Venkataraman, explained:
“While such asymmetric molecules do indeed display some diode-like properties, they are not effective. 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 surmount the issues associated with asymmetric molecular design, Venkataraman and her colleagues focused instead on developing an asymmetry in the environment around the molecular junction. They created an environmental asymmetry through a rather simple method.
They surrounded the active molecule with an ionic solution and used gold metal electrodes of different sizes to contact the molecule.
“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.”
Their results achieved rectification ratios as high as 250: 50 times higher than earlier designs.
Brian Capozzi, Jianlong Xia, Olgun Adak, Emma J. Dell, Zhen-Fei Liu, Jeffrey C. Taylor, Jeffrey B. Neaton, Luis M. Campos & Latha Venkataraman
Single-molecule diodes with high rectification ratios through environmental control
Nature Nanotechnology (2015) doi:10.1038/nnano.2015.97
Illustration: molecule used by Columbia Engineering professor Latha Venkataraman to create the first single-molecule diode with a non-trivial rectification ratio overlaid on the raw current versus voltage data. Diodes are fundamental building blocks of integrated circuits; they allow current to flow in only one direction. Credit: Latha Venkataraman, Columbia Engineering