The first transistors to be fashioned from a single "buckyball" - a molecule of carbon-60 - have been reported by scientists with the Lawrence Berkeley National Laboratory (Berkeley Lab).

by Lynn Yarris.

Taking advantage of a phenomenon that is largely viewed as a problem by the electronics industry, the team of Berkeley Lab and UC Berkeley researchers created a separation between two gold electrodes that was about one nanometer (one billionth of a meter) across. This tiny gap could accommodate the insertion of a single buckyball in order to create a molecular-sized electronic device.

"Nature long ago solved the problem of making electronic devices on a molecular-scale and we're now beginning to learn how to do things the way Nature does," says Paul McEuen, a physicist who holds joint appointments with Berkeley Lab's Materials Sciences Division, and with UC Berkeley's Physics Department.

McEuen was one of the co-authors of a paper in the journal Nature (September 7, 2000) that described this research. The other authors were Hongkun Park, Jiwoong Park, Andrew Lim, Erik Anderson, and Paul Alivisatos.

The ability to use individual molecules as functional electronic devices is a much coveted prize in the computer industry because of the potential for dramatically shrinking the silicon-based microelectronic systems of today. As electronic devices are reduced in size to a nanometer scale, the atoms with which silicon must be doped will eventually begin to move about, resulting in poor or uneven performances. Nanoscale sizes should not pose a problem for devices based on single large molecules of carbon as the atoms in these molecules are covalently bonded and therefore firmly locked in place.

Within the past few years, a number of research groups, including McEuen's, have made transistors from carbon nanotubes - tiny sheets of graphite that have been curled and connected along the seam. Although considered a single molecule of carbon, these elongated tubes were several times larger than the soccer-ball shaped carbon-60 molecules used by McEuen and his colleagues to make their newest transistors. Buckyballs are so tiny that, as transistors, they only permit one electron at a time to move through them. This opens the door to the study of single-electron transport effects.

"Transport measurements of these single carbon-60 transistors provide evidence for coupling between the center-of-mass motion of the carbon-60 and single-electron hopping, a novel conduction mechanism that has not been observed in previous quantum-dot studies," the authors stated in their Nature paper. "The transport measurements demonstrate that single-electron tunneling events can be used both to excite and probe the motion of a molecule."

McEuen likens the carbon-60 molecule to a ball tethered to a spring that rests on the surface of a gold electrode. When an electron hops onto the carbon-60, the "spring" is compressed as the charge of the additional electron draws the molecule closer to the gold surface. When the electron hops off the carbon-60, the spring is released. In this manner, electron-hopping causes the molecule to oscillate, like a ball on a spring bouncing up and down. McEuen says this quantized nano-mechanical movement of the carbon-60 might serve as a logic gate, a means of storing information in the position of the molecule that would be more stable and much faster than the current technology. 

To make their transistors, McEuen and his colleagues capitalized on a phenomenon known as "electromigration." If two electrodes are physically connected to one another and a large current is sent through them, the movement of the electrons can create nanometer-sized fissures between the electrodes. Opening up cracks between the electrodes is not usually desirable when making electronic devices, but this was a case, McEuen says, of using lemons to make lemonade, as the cracks in the gold electrodes were a good fit for buckyballs. Transport measurements showed that the conductance across the cracks was substantially enhanced when a solution of carbon-60 was deposited onto the connected electrodes, indicating that individual buckyballs had filled those cracks. Measurements were also found to be in excellent agreement with theoretical predictions.

Fullerene Structure Library Buckminsterfullerene, C60, the molecule that started it all.

The gold electrodes used in this study were fabricated on Berkeley Lab's "Nanowriter," an ultra-high resolution lithography machine that can generate an electron beam at energies up to 100,000 volts with a diameter of only five nanometers.

Says Erik Anderson of the Center for X-ray Optics, a collaborator on this study who helped design the Nanowriter's pattern generator and control system, "The Nanowriter's high-resolution, excellent placement accuracy, and modest throughput capabilities enabled us to make a large number of high quality gold electrode structures which we could then break apart with good reliability."

The devices created with the buckyballs are analogous MOSFETs (metal-oxide semiconductor field effect transistors). Though McEuen says they probably hold no commercial use at this time (the carbon-60 molecules can be readily blown out of the junction between the electrodes with too much voltage), they do represent one of the first actual experiments with a device for the upcoming age of nanoelectromechanical systems or NEMS.