Bucky Balls - Buckyballs Explored

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.

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.