Paul McEuen, Physics, finds the miniscule endlessly fascinating. The tagline for his lab sums up the essence of his interest in the nanoworld: Anything, as long as it’s small. There are no barriers to McEuen’s curiosity, and everything is fair game—just as long as it’s no bigger than the size of a single cell and preferably far smaller. These days, he’s turned his attention to nanomachines.
“Back in the 1950s Richard Feynman, a famous physicist, gave an important speech to other scientists where he said, ‘Why don’t you make things really small? Why don’t you make tiny electronics, miniature machines?’” McEuen says. “People went on to make electronics tiny, but nobody really did much with nanomachines. My group, though, has been really interested in them for a while. Recently, we took that interest to the next level and tried to actually build the tiny machines that Feynman envisioned.”
The Cornell Alma Mater, Played on a Nanomachine
In creating their nanomachines, McEuen and his collaborators built on their earlier work with graphene (a resilient material only one or two atoms thick with interesting optical and electrical properties) and carbon nanotubes (graphene rolled into a tiny cylinder with a width of about 10 atoms across). The researchers manipulated graphene in a number of ways, studying its physical properties as they did so.
“We were the first to make a vibrating one-atom-thick graphene sheet that was, in essence, a drum,” McEuen says. “We also made the world’s first nanotube guitar. We used a nanotube as a guitar string and plucked it to make it vibrate.” Since the nanotube guitar string vibrates at megahertz frequencies—much higher than the human ear can hear—the researchers had to figure out how to measure it. They reported their results in a paper forthcoming in the journal Nature. “We not only heard it,” McEuen says with satisfaction, “but we played the Cornell alma mater on it.”
“We had things that know how to bend. We had electronic devices that can carry out tasks. We could shoot light at little voltaic cells to drive the legs. So we combined them.”
For McEuen, playing a tune on a guitar string the width of a strand of DNA is par for the course. He made his reputation manipulating graphene sheets, carbon nanotubes, and other materials, creating some of the smallest electronic or mechanical systems ever invented.
“We had a couple of aims,” he says. “On the technological side, we wanted to help the electronics industry make transistors smaller. That was an attempt to fulfill Moore’s Law, which says electronics will shrink every year. In the meantime, on the physics side, scientists had never studied the behavior of electrons inside tiny materials like these. So we spent a lot of time doing fundamental physics of how electrons behave in one dimension, in the case of the nanotube, and in two dimensions, in the case of the graphene sheet.”
Nanomachines for Electronics, Medicine
Now, the mechanical side of tiny has taken center stage. “On the electronics side, nothing nanoscientists have come up with has displaced silicon transistors,” McEuen says. “They’re still the best at what they do. But with these nanomachines, there isn’t any other competition. We’re doing things that have never been done before.” The researchers use the existing technology of the semiconductor world for their tiny robots. “Little semiconductor devices know how to communicate, how to compute, how to sense certain kinds of things,” McEuen explains. “So we can just hijack that and use it for our machine. Imagine something as small as a piece of dust, but it has a set of solar panels on it to power it; it’s got computation on board to carry out a task, maybe do some sensing; and it’s got a light-emitting diode to blink out information to you.”
The resulting proto machines—everything from voltage and microfluidic sensors to thermometers—are approximately the width of a human hair. McEuen even has a photo of one, resting on the chest of the infinitesimal Abraham Lincoln that sits within one of the windows of the Lincoln Memorial etched on the back of a penny. The nanobots are so small, they could be injected with a needle, which may open up all kinds of medical uses in the future.
“You can put these anywhere,” McEuen says. “One day, one of these little machines might be used to record the voltage when a neuron fires in the brain. So instead of putting a wire in your skull, like they do now, they could implant this and leave it. And it turns out when something is as small as these nanomachines, your body doesn’t realize they’re there. It doesn’t make scar tissue.”
The researchers fabricated the first nanomachines at the Cornell NanoScale Science and Technology Facility, but McEuen envisions a future where the same commercial foundries that make computer chips will also make tiny machines. As a result, the machines will be cheap, perhaps less than a penny to make, even as their computing capacity becomes extremely sophisticated. “One of the reasons to make it so small is because it’s so cheap,” he says. “There’s a lot of up-front costs to developing the technology, but once we have it figure out, they’ll be ubiquitous, even disposable.”
Applying Origami Techniques to Nanomachines—For Movement
Once the researchers had created some working nanomachines, they moved on to the next step—making them mobile. To do that, they turned to techniques used in origami, which they had pioneered in graphene. By punching offset cuts in the two-dimensional graphene, they were able to fold and bend it, creating three-dimensional springs, pyramids, and spirals. They took graphene and other flexible materials and bonded them with a second, more rigid, material into a bimorph. By making the more rigid material contract, McEuen and his collaborators made the flexible material bend and twitch. “We’re building a tool box for our little robots,” he explains. “We are learning how to make things fold.”
Eventually the scientists settled on using voltage to make the bimorph bend. “Now the job was to put these things together,” McEuen says. “We had things that know how to bend. We had electronic devices that can carry out tasks. We could shoot light at little voltaic cells to drive the legs. So we combined them.” In one of McEuen’s videos, multiple nanobots twitch and move in pond water, while a single-celled paramecium—gigantic in comparison—swims past. “There goes a paramecium!” McEuen points out with relish.
In his quest to make these cell-sized smart phones, McEuen has joined with a variety of collaborators at Cornell, including Alyosha C. Molnar, Electrical and Computer Engineering, Chris Xu, Applied and Engineering Physics, and Itai Cohen, Physics.
“I think almost every grant I have is a collaborative grant,” McEuen says. “My group and I enjoy finding someone who’s really good at doing something and working with them rather than trying to figure out everything ourselves. That’s one of the reasons I came to Cornell. I knew it had a very collaborative environment.”