Single-molecule diode in new current record
A single-molecule diode with the highest ON-OFF current (or rectification) ratio to date has been unveiled by researchers in the US. While single-molecule diodes have been made before now, they suffered from low conductances and very low rectification ratios. The new diode might be used to study the fundamental electronic properties of materials on the molecular scale and so make better nanoscale device components.
Electronic devices made from single molecules that mimic standard microelectronics components, such as single-electron transistors, memory elements or photoswitches, have been around since the 1990s. However, making single-molecule diodes, which are one of the most basic of al electronic elements, has proved to be more difficult.
A single-molecule diode is a two-terminal electronic component that allows current to flow in only one direction and the idea of such a device was first put forward over 40 years ago in a theoretical paper. It consisted of an asymmetric donor-bridge-acceptor molecule and was expected to work like a semiconductor p-n junction. Since then, researchers have constructed several single-molecule diodes featuring asymmetric molecular backbones and molecule-electrode linkers. However, despite improvements in the properties of these devices over the decades, they still suffer from low conductances and low rectification ratios (of less than 11), and require high operating voltages of around 1 V.
Symmetric molecule works
A molecular diode normally needs to have an asymmetric structure so that current flow can be controlled in the device. This is usually done 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) by changing the electrostatic environment around the molecular junction in the device.
The researchers say they did this by contacting the active molecular elements making up the diode to electrodes that have considerably different surface areas. They also operated the device in a polar solvent and exposed different areas of the electrodes to this ionic medium.
Asymmetric charge distribution enhances current rectification
Thanks to the asymmetry in the areas around the electrodes, double layers of differing charge densities develop at the interfaces of the two electrodes. These double layers originate from ions in the solvent that propagate towards the interfaces to screen out the electric field generated by 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,” team member Brian Capozzi tells nanotechweb.org. "It also alleviates the need for complex synthesis strategies required to design asymmetric molecules.”
The researchers say they achieved rectification ratios of over 200 at voltages as low as 370 mV using novel molecular systems comprising symmetric oligomers of thiophene-1,1-dioxide. The same junctions immersed in nonpolar solvents do not show any rectification, which proves that the environment around the electrodes plays a key role here.
Useful for studying fundamental electronic structure
Combined with the high rectification and ON 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 us with 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 too.
The team, which includes groups lead by Luis Campos at Columbia and Jeffrey Neaton at the University of California, Berkeley, says that it is now busy optimizing and developing even better single-molecule diodes.
http://nanotechweb.org/cws/article/tech/61434
.......funny, was having a convo today about this: Some are already on it. : )
Looking Forward
A single-molecule diode with the highest ON-OFF current (or rectification) ratio to date has been unveiled by researchers in the US. While single-molecule diodes have been made before now, they suffered from low conductances and very low rectification ratios. The new diode might be used to study the fundamental electronic properties of materials on the molecular scale and so make better nanoscale device components.
Electronic devices made from single molecules that mimic standard microelectronics components, such as single-electron transistors, memory elements or photoswitches, have been around since the 1990s. However, making single-molecule diodes, which are one of the most basic of al electronic elements, has proved to be more difficult.
A single-molecule diode is a two-terminal electronic component that allows current to flow in only one direction and the idea of such a device was first put forward over 40 years ago in a theoretical paper. It consisted of an asymmetric donor-bridge-acceptor molecule and was expected to work like a semiconductor p-n junction. Since then, researchers have constructed several single-molecule diodes featuring asymmetric molecular backbones and molecule-electrode linkers. However, despite improvements in the properties of these devices over the decades, they still suffer from low conductances and low rectification ratios (of less than 11), and require high operating voltages of around 1 V.
Symmetric molecule works
A molecular diode normally needs to have an asymmetric structure so that current flow can be controlled in the device. This is usually done 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) by changing the electrostatic environment around the molecular junction in the device.
The researchers say they did this by contacting the active molecular elements making up the diode to electrodes that have considerably different surface areas. They also operated the device in a polar solvent and exposed different areas of the electrodes to this ionic medium.
Asymmetric charge distribution enhances current rectification
Thanks to the asymmetry in the areas around the electrodes, double layers of differing charge densities develop at the interfaces of the two electrodes. These double layers originate from ions in the solvent that propagate towards the interfaces to screen out the electric field generated by 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,” team member Brian Capozzi tells nanotechweb.org. "It also alleviates the need for complex synthesis strategies required to design asymmetric molecules.”
The researchers say they achieved rectification ratios of over 200 at voltages as low as 370 mV using novel molecular systems comprising symmetric oligomers of thiophene-1,1-dioxide. The same junctions immersed in nonpolar solvents do not show any rectification, which proves that the environment around the electrodes plays a key role here.
Useful for studying fundamental electronic structure
Combined with the high rectification and ON 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 us with 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 too.
The team, which includes groups lead by Luis Campos at Columbia and Jeffrey Neaton at the University of California, Berkeley, says that it is now busy optimizing and developing even better single-molecule diodes.
http://nanotechweb.org/cws/article/tech/61434
.......funny, was having a convo today about this: Some are already on it. : )
Looking Forward
