N.J. Tao and his colleagues at Arizona State University’s Biodesign Institute have found a way to make a key electrical component on a phenomenally tiny scale. We are talking very, very tiny. Single molecule tiny. The ASU scientists describe their single-molecule diode the October 2009 online edition of the journal Nature Chemistry.
In the electronics world, diodes are a versatile component. They exist everywhere and appear in many shapes and sizes. Diodes are used in an endless array of devices. They are essential ingredients for the semiconductor industry. Making components including diodes smaller, cheaper, faster and more efficient has been the never ending goal of an exploding electronics field, that now is probing the nanoscale realm.
Smaller size means cheaper cost and better performance for electronic devices. Tao says that the first-generation computer CPU used a few thousand transistors. “Today, even simple, inexpensive computers use millions of transistors on a single chip.”However, the task of miniaturization has become much more difficult in recent years. Consider the famous dictum known as Moore’s law. It states that the number of silicon-based transistors on a chip doubles every 18 to 24 months. Eventually, we will reach the physical limits of the material.
“Transistor size is now reaching a few tens of nanometers. That’s only about 20 times larger than a molecule,” Tao says. “It’s one of the reasons people are excited about this idea of molecular electronics.”
Diodes are critical components for a broad array of applications. They are used in power conversion equipment and radios, logic gates, photodetectors, and light-emitting devices. Diodes are essential to each device. They are the components that allow current to flow in one direction around an electrical circuit but not the other. For a molecule to perform this feat, Tao says it must be physically asymmetric. One end must be capable of forming a covalent bond with the negatively charged anode. The other end must bond with the positive cathode terminal.
In their study, the ASU scientists compared a symmetric molecule with an asymmetric one. They then detailed the performance of each in terms of electron transport.
“If you have a symmetric molecule, the current goes both ways, much like an ordinary resistor,” Tao says. This is potentially useful. But the diode is a more important (and difficult) component to replicate. The idea of surpassing silicon limits with a molecule-based electronic component is not new. “Theoretical chemists Mark Ratner and Ari Aviram proposed the use of molecules for electronics such as diodes back in 1974,” Tao says. “People have been trying to accomplish this for more than 30 years.” Most efforts to date have involved many molecules, Tao says, referring to molecular thin films. Only very recently have serious attempts been made to surmount the obstacles to single-molecule designs. One of the challenges is to bridge a single molecule to at least two electrodes that are supplying current to that molecule. Another challenge involves the proper orientation of the molecule in the device. “We are now able to do this. We can build a single molecule device with a well-defined orientation,” Tao says. The technique developed by Tao’s group relies on a property known as AC modulation. “Basically, we apply a little, periodically varying mechanical perturbation to the molecule,” Tao says. “If there’s a molecule bridged across two electrodes, it responds in one way. If there’s no molecule, we can tell.” The interdisciplinary project involved Luping Yu, a professor at the University of Chicago. Yu supplied the molecules for study. Providing theoretical insight was Ivan Oleynik, a professor from the University of South Florida. The team used conjugated molecules for the study. These are molecules in which atoms are stuck together with alternating single and multiple bonds. Such molecules display large electrical conductivity. They also have asymmetrical ends capable of spontaneously forming covalent bonds with metal electrodes to create a closed circuit. The project’s results raise the prospect of building single molecule diodes – the smallest devices one can ever build. “I think it’s exciting. We are now able to look at a single molecule and play with it,” Tao says. “We can apply a voltage, a mechanical force, or optical field, measure current and see the response. As quantum physics controls the behaviors of single molecules, this capability allows us to study properties distinct from those of conventional devices.” Chemists, physicists, materials researchers, computational experts and engineers all play a central role in the emerging field of nanoelectronics. A veritable zoo of available molecules exists. Each has a different function and provides the raw material for innovation. Tao also is examining the mechanical properties of molecules. Specifically, he studies their ability to oscillate. Binding properties between molecules make them attractive candidates for a new generation of chemical sensors. “Personally, I am interested in molecular electronics,” the ASU scientist explains. “And not just because of their potential to duplicate today’s silicon applications.” Instead, molecular electronics will benefit from unique electronic, mechanical, optical and molecular binding properties that set them apart from conventional semiconductors. This work may lead to applications that complement rather than replace silicon devices.
