A group of scientists led by Dr Latha Venkataraman of Columbia University has developed a single-molecule diode that may have real-world technological applications for nanodevices. The study is funded by the National Science Foundation, the Department of Energy, and the Packard Foundation.
The idea of creating a single-molecule diode – a circuit element that directs current flow – was first suggested more than 40 years ago, in 1974, by researchers Arieh Aviram of IBM Thomas J. Watson Research Center and Mark Ratner of New York University. Scientists have since been exploring the charge-transport properties of molecules. They have shown that single-molecules attached to metal electrodes can be made to act as a variety of circuit elements, including resistors, switches, transistors, and, indeed, diodes. They have learned that it is possible to see quantum mechanical effects, such as interference, manifest in the conductance properties of molecular junctions. A single-molecule diode with the highest on–off current (or rectification) ratio to date has been unveiled by a team of physicists and chemists in the US. While single-molecule diodes have been made in the past, they suffered from low conductance and very low rectification ratios. The new diode could be used to study the fundamental electronic properties of materials on the molecular scale, and might lead to the development of better nanoscale electronic devices. Electronic devices made from single molecules, including single-electron transistors, memory elements and optical switches, have been around since the 1990s. However, making single-molecule diodes – the most basic of all electronic elements – has proved to be a difficult task. A single-molecule diode is a two-terminal electronic component that allows current to flow in only one direction; the idea of such a device was first proposed more than 40 years ago in a theory paper. The concept involved an asymmetric “donor-bridge-acceptor” molecule, and was expected to work like the semiconductor p–n junction in a conventional diode. Since then, researchers have made several single-molecule diodes featuring asymmetric molecules. However, despite improvements in the properties of these devices over the decades, they still suffer from low conductance and low rectification ratios (of less than 11). They also require high operating voltages of around 1 V. Since a diode acts as an electricity valve, its 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. “While such asymmetric molecules do indeed display some diode-like properties, they are not effective,” said team member Brian Capozzi, a PhD student at Columbia University. In order to overcome the issues associated with asymmetric molecular design, the scientists focused on developing an asymmetry in the environment around the molecular junction. They surrounded the active molecule (oligomer of thiophene-1,1-dioxide) with an ionic solution and used gold metal electrodes of different sizes to contact the molecule. The ‘on’ current flow in their devices can be more than 0.1 microamps, which is a lot of current to be passing through a single-molecule. And, because this new technique is so easily implemented, it can be applied to all nanodevices of all types, including those that are made with graphene electrodes.
Symmetric molecule works
A molecular diode normally needs to have an asymmetric structure so that the current flow is also asymmetric in terms of direction. This is usually achieved by using an inherently asymmetric molecule or by using electrodes made from different materials. Now, a team of researchers led by Latha Venkataraman of Columbia University in New York has succeeded in building asymmetry into a molecular diode using a symmetric molecule and electrodes made from the same metal (gold). This was done by adjusting the electrostatic environments where the molecule is attached to each electrode, which involved having one end of the molecule in contact with a planar electrode with a large surface area. The other end of the molecule is in contact with a sharp-tipped electrode coated with wax, so it offers a much smaller surface area (see figure). The researchers also operated the device in a polar solvent and exposed different areas of the electrodes to this ionic medium.
Asymmetric charge distribution
The result of this interface asymmetry is that double layers of differing charge densities develop at the two electrodes–molecule interfaces. These double layers originate from ions in the solvent that propagate towards the interfaces to screen out the electric field generated by electrical charges in the gold. “This asymmetric charge distribution is responsible for the enhanced current rectification we observed,” explains Venkataraman. “Our technique to enhance current rectification in these single-molecule structures is simple and robust. It also alleviates the need for complex synthesis strategies required to design asymmetric molecules,” says team member Brian Capozzi. The researchers say they achieved rectification ratios of more than 200 at voltages as low as 370 mV using molecules comprising symmetric oligomers of thiophene-1,1-dioxide. The same junctions immersed in non-polar solvents do not show any rectification, which the team says proves that the environment around the electrodes plays a key role in the operation of the devices.
Fundamental electronic structure
“Combined with the high rectification and currents that we have measured, our technique might also be used to make real-world devices, and could be applied to other nanoscale device components, not just single-molecule junctions,” says Venkataraman. And that is not all: the method provides a way to experimentally probe how energy levels are aligned in single-molecule junctions – something that could be useful for studying the fundamental electronic structure of a variety of other device components. The team, which includes groups lead by Luis Campos of Columbia University and Jeffrey Neaton of the University of California, Berkeley, says that it is now busy optimizing and developing even better single-molecule diodes.
The Team of Researchers
Columbia Engineering researchers are the first to create a single-molecule diode the ultimate in miniaturization for electronic devices with potential for real-world applications. Under the direction of Latha Venkataraman, associate professor of applied physics at Columbia Engineering, researchers have designed a new technique to create a single-molecule diode, and, in doing so, they have developed molecular diodes that perform 50 times better than all prior designs. Venkataraman’s group is the first to develop a single-molecule diode that may have real-world technological applications for nanoscale devices.
With electronic devices becoming smaller every day, the field of molecular electronics has become ever more critical in solving the problem of further miniaturization, and single molecules represent the limit of miniaturization. The idea of creating a single-molecule diode was suggested by Arieh Aviram and Mark Ratner who theorized in 1974 that a molecule could act as a rectifier, a one-way conductor of electric current. Researchers have since 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 learned that it is possible to see quantum mechanical effects, such as interference, manifest in the conductance properties of molecular junctions. Since a diode acts as an electricity valve, its 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. “While such asymmetric molecules do indeed display some diode-like properties, they are not effective, explains 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. In order to overcome the issues associated with asymmetric molecular design, Venkataraman and her colleagues—Chemistry Assistant Professor Luis Campos’ group at Columbia and Jeffrey Neaton’s group at the Molecular Foundry at UC Berkeley—focused 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. Their results achieved rectification ratios as high as 250: 50 times higher than earlier designs. The “on” current flow in their devices can be more than 0.1 microamps, which, Venkataraman notes, is a lot of current to be passing through a single-molecule. And, because this new technique is so easily implemented, it can be applied to all nanoscale devices of all types, including those that are made with graphene electrodes.
“Our new approach created a single-molecule diode that has a high (>250) rectification and a high “on” current (~ 0.1 micro Amps),” says Venkataraman. “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’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.”
Conclusion
Research teams from California University have designed a single molecular diode a practice gift to nano-devices & applications. Till not the innovation is still under labs and studies but hopefully may found its significant roles in many curtail applications.
