Volume 2, Issue 11
Nanoscience's Master Mechanic
This fall, UC Berkeley is launching a new $11.9 million center for researchers to design and test novel motors, sensors, batteries, and other mechanical components. What will set this center apart from most engineering endeavors is that none of the devices built there will be visible without an electron microscope. The Center of Integrated Nanomechanical Systems (COINS) aims to develop a storehouse of mechanical components hundreds of times smaller than the diameter of a human hair. Physics professor Alex Zettl is leading the charge, having recently demonstrated a conveyor belt for ferrying atoms, the world's smallest nanomotor, and an oscillator based on drops of liquid metal that weight just one-quadrillionth of a gram.
Alexander Zettl holds a model of a carbon nanotube, the molecular basis for his nanoscale devices. (courtesy Berkeley Lab)
"We're designing nanoscale building blocks to make them accessible to the point where you could order them like you order lumber at a lumberyard and assemble them," says Zettl, director of COINS and a staff scientist at Lawrence Berkeley National Laboratory.
COINS is one of six Nanoscale Science and Engineering Centers across the country funded by the National Science Foundation. Consisting of more than two dozen researchers from UC Berkeley, UC Merced, Stanford University, and the California Institute of Technology, the Berkeley-based center is unique in that its specific focus is on mechanics at the nanoscale. The COINS Center will directly provide new specialized nanoscience and nanoengineering facilities as well as take advantage of several state-of-the-art nanoscale "machine shops" now under construction, including Lawrence Berkeley National Laboratory's Molecular Foundry, the Nanofabrication Facility in the new headquarters of the Center for Information Technology Research in the Interest of Society (CITRIS), and the Biomolecular Nanotechnology Center at Stanley Hall, future home to the Department of Bioengineering and the California Institute of Quantitative Biomedical Research (QB3).
Zettl's laboratory pioneered what has become quintessential COINS research. Over the last few years, he and his colleagues fabricated pistons, electromechanical switches, and transistors from nanotubes, which are rolled-up crystalline sheets of carbon atoms. The devices, Zettl explains, demonstrate how nanoscience can break down the physical walls faced by engineers trying not only to shrink integrated circuits but also tiny machines. Two decades ago, UC Berkeley led the development of micro-electromechanical system (MEMS) — gears, pumps, and sensors no bigger than the period at the end of this sentence. Now, MEMS are found in everything from automobile airbags to desktop video projectors.
A time series of four transmission electron microscopy (TEM) video images showing one period of oscillator action. The second frame from the left shows the participating liquid metal droplets, labeled I and II. The suspended nanotube substrate, which is visible crossing the images diagonally, is carrying electrical current from the lower left to the upper right corners. In the first frame, the droplet at Position II is barely visible, but by the third frame it has grown to the point where it is nearly touching the droplet at Position I. In the fourth frame, the oscillator is seen directly after the subsequent relaxation event, when the mass distribution has been reset to a condition like that of the first frame. (courtesy Zettl Research Group, Lawrence Berkeley National Laboratory and University of California at Berkeley)
"The machines are quite sophisticated but the materials aren't really well-suited for scaling them down any further," Zettl says. "At the same time, electronic devices have hit their physical limits. But nanotechnology is a nice meeting ground for these fields that have been so well developed at the microscale."
Zettl imagines that MEMS and microelectronics could be integrated with nanostructures to create an entire new breed of devices. For example, a nanoscale cantilever could flex like a diving board when a particular molecule binds to it, enabling the development of a highly sensitive chemical or biological wireless sensor. Or, in the very far future, a nanoscale engine might power a microscale robot traveling through the body to attack a tumor.
"Sometimes it's not just about making a device smaller because you can," Zettl says. "Often, physics on the nanoscale enable devices to work better."
Computer-generated animation of the "conveyor belt," depicting metal transport from left to right along a single carbon nanotube. (courtesy Zettl Research Group, Lawrence Berkeley National Laboratory and University of California at Berkeley)
The smaller a device, the less power it consumes, Zettl says. Nanodevices could potentially operate for very long periods on very little energy. Meanwhile, small forces that are hardly noticeable at the macroscale become very useful at the nanoscale. Think of the way some insects are capable of walking on water while humans are not. That's because the small insects can take advantage of surface tension, an attractive force exerted on the surface of a liquid by the molecules underneath. Zettl and his research group published a design for a nanoscale oscillator that takes advantage of this same force.
"If I built a regular engine, the surface tension is miniscule," Zettl explains. "But at the nanoscale, that force wins out over everything else."
The oscillator mechanism consists of two separate droplets of liquid metal – one small and one large – on a nanotube. Applying an electrical current to the nanotube causes metal from the larger droplet (Droplet 1) to move to the smaller droplet (Droplet 2). Eventually, Droplet 2 becomes large enough to contact Droplet 1.
A series of scanning electron microscope pictures of the spinning rotor of a nanomotor. The entire electric motor is about 500 nanometers across, 300 times smaller than the diameter of a human hair. (courtesy Zettl Research Group, Lawrence Berkeley National Laboratory and University of California at Berkeley)
"When they touch and the surface tension kicks in, they merge into one droplet and the cycle starts again," Zettl says.
To use the mechanism as an oscillator, a microscale lever could be positioned near Droplet 2 so that when it reaches a certain size it pushes the lever arm. Increasing the electrical voltage across the nanotube moves the metal from droplet to droplet faster, thereby increasing the frequency by which the lever is thrown.
"We tune a nanoscale device just by turning a big knob that controls the electrical current," Zettl says. "It's a real electromechanical coupling between the macro and nanoworld."
The oscillator is based on a nanoscale conveyor belt that Zettl and his colleagues demonstrated last year. In that experiment, they applied a small electrical current to a carbon nanotube laden with indium particles. The voltage caused the metal atoms to scoot along the surface of the nanotube "track" like parts moving along an assembly line. Eventually, the mass conveyor belt could become a key component in the COINS machine shop.
"Right now, we make most nanodevices one at a time, sometimes as laboriously as atom by atom," Zettl says. "That approach can prove a concept, but if you can't scale up then it becomes a curiosity instead of a viable technology. We need automated assembly at the nanoscale."