The Purdue team has shown that single electrons can tunnel through a layer of ultrasmall gold clusters -- one to two nanometers in diameter -- at room temperature, passing electrical current in a stair-step fashion that largely eliminates the problem of heat buildup found in current electrical devices.
"This provides a new way of thinking about how such a small electrical device would work. Instead of having current go through it continuously, it operates by a series of discrete single electron transfers," says Clifford Kubiak, professor of chemistry and part of the interdisciplinary team at Purdue. "You would never have to pass a whole lot of current through this structure, because the basic response you're trying to create is effected by just a single electron jumping."
The achievement marks the first time a miniature unit based on single electron tunneling has been able to run at room temperature, and it provides a prototype for constructing molecule-sized electronic components. Details of the study were presented in July at the International Symposium on Small Particles and Inorganic Clusters, and were published in the May 31 issue of the journal Science .
The idea of using very small components, or "nanotechnology," to make computers and electrical devices, including biomedical devices that could be inserted into the body, has been the subject of much scientific interest and research. Nanotechnology refers to components only a few nanometers in size. A nanometer is one-billionth of a meter.
Efforts to build such small electronic components have produced active elements on computer memory chips as small as 500 nanometers. But attempts to build smaller units have been hindered because current processes such as photolithography cannot create structures that small, and because of the heat buildup that occurs when electrical current passes through such small structures.
"Small structures just can't tolerate large electrical loads," Kubiak says.
The structure developed at Purdue -- made of gold clusters attached to a gold substrate by organic molecules -- sidesteps this problem with its unique structure, which act as a sort of "turnstile" to limit the amount of current that passes through the module.
"It's like a large group of people all heading toward a single turnstile at a busy subway station," Kubiak says. "If all the gates are open, there is no resistance, and the people all go rushing through. But with only one gate open, there is much more control, and only a single person passes through at a time."
The Purdue structure was designed using an approach called self-assembly, a method that allows scientists to produce a structure atom-by-atom.
"Self-assembly follows the principles used in nature, where, given the right starting ingredients, the molecules will interact to form a specific structure with a defined shape and function, similar to a snowflake or crystal," Kubiak says.
The key to using self-assembly in the laboratory is to find the right complementary relationship between the starting ingredients so that they come together in a very deliberate way. Given the right mixture, scientists can use this technique to develop molecule-sized structures with interconnections that are both highly organized and complex, Kubiak says.
To keep the structure simple and small, the Purdue group used sulfur atoms to connect the gold clusters to the gold substrate, because sulfur bonds readily to gold.
The scientists first produced a set of molecules shaped like a barbell with a sulfur atom on each end. When exposed to a flat gold surface, the barbells stood on end, with one sulfur atom firmly adhering to the surface and the other sulfur atom exposed. The scientists then attached preformed crystallites, containing 100 to 200 gold atoms, to the exposed ends.
"We had to do some careful spectroscopy to show that these double-ended sulfur molecules were in fact standing up, not laying down," Kubiak says. "With one sulfur bonded to the surface, and the other presented up, we were able use the outermost sulfur on each of these molecules to "catch" one of the gold clusters and immobilize it in a fixed position."
The shape and size of the gold clusters were carefully designed to allow for a staircase effect in the voltage vs. current characteristics of the module, says Ronald Andres, professor of chemical engineering and part of the research team.
Using a scanning tunneling microscope -- a device designed to probe nanoscale structures -- the group was able to image the attached clusters and measure the relationship between current and voltage as electrons passed through the structure.
At room temperature, the current-voltage data showed the desired "staircase" behavior, showing that electrons advanced through the structure one step at a time, rather than in a steady stream as would be expected.
"Usually Ohm's law predicts that the relationship between current and voltage is a simple straight line, so this staircase effect is a surprising finding, but one that can be explained from the fact that each step represents a single electron tunneling through the nanostructure," Kubiak says.
This staircase effect previously has been seen in small structures only at temperatures near absolute zero. This is the first time the effect has been documented at room temperature.
Andres attributes this feat to the component's unique structure and size, which provide a model for designing components tens to hundreds of times smaller than those currently in use today. Because the molecule-sized unit built at Purdue operates at room temperature and doesn't heat up, it has many potential applications.
"The number of practical applications for many other microstructures is limited, because the units produce large amounts of heat when excited by electrical current," Kubiak says. "In order for these tiny structures to work, they must be cooled to temperatures near absolute zero, which is around -450 degrees Fahrenheit."
The molecule-sized components built at Purdue also may allow researchers to produce products with more precision and flexibility than current methods allow.
"One of the most exciting things about this research is that we've shown we can use molecular assembly to manipulate molecules and clusters of atoms to position them where we want them, and to make them do what we want them to do," Kubiak says.
The interdisciplinary research team at Purdue that developed the new device includes Kubiak and Andres, Thomas Bein, professor of chemistry, Supriyo Datta, professor of electrical and computer engineering, David Janes, senior research scientist of electrical and computer engineering, and Ronald Reifenberger, professor of physics.
The research was funded by the U.S. Army Research Office and Purdue University.
Sources: Clifford Kubiak, (765) 494-5323; Internet, firstname.lastname@example.org
Ronald Andres, (765) 494-4047; Internet, email@example.com
Writer: Susan Gaidos, (765) 494-2081; Internet, firstname.lastname@example.org
Purdue News Service: (765) 494-2096; e-mail, email@example.com
"Coulomb Staircase" at Room Temperature in a Self-Assembled Molecular Nanostructure
Double-ended aryl dithiols [ , '-xylyldithiol (XYL) and 4,4'-biphenyldithiol] formed self-assembled monolayers (SAMs) on gold (111) substrates and were used to tether nanometer-sized gold clusters deposited from a cluster beam. An ultrahigh-vacuum scanning tunneling microscope was used to image these nanostructures and to measure their current-voltage characteristics as a function of the separation between the probe tip and the metal cluster. At room temperature, when the tip was positioned over a cluster bonded to the XYL SAM, the current-voltage data showed "Coulomb staircase" behavior. These data are in good agreement with semiclassical predictions for correlated single-electron tunneling and permit estimation of the electrical resistance of a single XYL molecule.
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