Nano Motor

"It's the smallest synthetic motor that's ever been made," said Alex Zettl, professor of physics at UC Berkeley and faculty scientist at Lawrence Berkeley National Laboratory. "Nature is still a little bit ahead of us - there are biological motors that are equal or slightly smaller in size - but we are catching up." Zettl and his research group report their feat in the current issue of Nature.

The electrostatic motors represent a significant step foward in nanotechnology, and prove that nanotubes and other nanostructures several hundred times smaller than the diameter of a human hair can be manipulated and assembled into true devices.

The motor is about 500 nanometers across, 300 times smaller than the diameter of a human hair. While the part that rotates, the rotor, is between 100 and 300 nanometers long, the carbon nanotube shaft to which it is attached is only a few atoms across, perhaps 5-10 nanometers thick.

The motor's shaft is a multiwalled nanotube, that is, it consists of nested nanotubes much like the layers of a leek. Annealed both to the rotor and fixed anchors, the rigid nanotube allows the rotor to move only about 20 degrees. However, the team was able to break the outer wall of the nested nanotubes to allow the outer tube and attached rotor to freely spin around the inner tubes as a nearly frictionless bearing.

To build the motor, Zettl and his team made a slew of multiwalled nanotubes in an electric arc and deposited them on the flat silicon oxide surface of a silicon wafer. They then identified the best from the pile with an atomic force microscope, a device capable of picking up single atoms.
A gold rotor, nanotube anchors and opposing stators were then simultaneously patterned around the chosen nanotubes using electron beam lithography. A third stator was already buried under the silicon oxide surface. The rotor was annealed to the nanotubes and then the surface selectively etched to provide sufficient clearance for the rotor.
When the stators were charged with up to 50 volts of direct current, the gold rotor deflected up to 20 degrees, which was visible in the SEM. With alternating voltage, the rotor rocked back and forth, acting as a torsional oscillator. Such an oscillator, probably capable of microwave frequency oscillations from hundreds of megahertz to gigahertz, could be useful in many types of devices - in particular, communications devices such as cell phones or computers.
With a strong electrical jolt to the stators, the team was able to jerk the rotor and break the outer wall of the nested nanotubes, allowing the rotor to spin freely on the nested nanotube bearings. Zettl had made similar bearings several years ago, but this was the first time he had put them to use.

Nanotube arrays

Despite the attractive electrical properties and physical features of single-walled carbon nanotubes, incorporating them into scalable integrated circuits has proven to be a challenge because of difficulties in manipulating and positioning these molecular scale objects and in achieving sufficient current outputs.

Now, researchers at the University of Illinois, Lehigh University and Purdue University have developed an approach that uses dense arrays of aligned and linear nanotubes as a thin-film semiconductor material suitable for integration into electronic devices.
The nanotube arrays can be transferred to plastic and other unusual substrates for applications such as flexible displays, structural health monitors and heads-up displays. The arrays also can be used to enhance the performance of devices built with conventional silicon-based chip technology.

"The aligned arrays represent an important step toward large-scale integrated nanotube electronics," said John A. Rogers, a Founder Professor of Materials Science and Engineering at Illinois, and corresponding author of a paper accepted for publication in the journal Nature Nanotechnology, and posted on its Web site.

To create nanotube arrays, the researchers begin with a wafer of single-crystal quartz, on which they deposit thin strips of iron nanoparticles. The iron acts as a catalyst for the growth of carbon nanotubes by chemical vapor deposition. As the nanotubes grow past the iron strips, they lock onto the quartz crystal, which then aligns their growth.

The resulting linear arrays consist of hundreds of thousands of nanotubes, each approximately 1 nanometer in diameter (a nanometer is 1 billionth of a meter), and up to 300 microns in length (a micron is 1 millionth of a meter). The nanotubes are spaced approximately 100 nanometers apart.

The arrays function as an effective thin-film semiconductor material in which charge moves independently through each of the nanotubes. In this configuration, the nanotubes can be integrated into electronic devices in a straightforward fashion by conventional chip-processing techniques.

A typical device incorporates approximately 1,000 nanotubes, and can produce current outputs 1,000 times higher than those of previously reported devices that incorporate just a single nanotube. Many devices can be built from each array, with good device-to-device uniformity. Detailed theoretical analysis of these unusual devices reveals many aspects of their operation.
Using the arrays, the researchers built and tested a number of transistors and logic gates, and compared the properties of nanotube arrays with those of individual nanotubes.

"This is the first study that shows properties in scalable device configurations that approach the intrinsic properties of the tubes themselves, as inferred from single-tube studies," said Rogers, who also is a researcher at the university's Beckman Institute.

Nanotube arrays aren't likely to replace silicon, Rogers said, but could be added to a silicon chip and exploited for particular purposes, such as higher speed operation, higher power capacity and linear behavior for enhanced functionality. They can also be used in applications such as flexible devices, for which silicon is not well suited.

"Nanotubes have shown potential in the past, but there hasn't been a clear path from science to technology," said Moonsub Shim, a professor of materials science and engineering at Illinois, and a co-author of the paper. "Our work seeks to bridge this gap."