These technologies don’t exist yet, but the ultra-small pieces needed to make them real do.
Carbon nanotubes (CNTs) are the potential superstar structures of molecular engineering because of their remarkable electronic and mechanical properties. Already they are used in making flat panel display screens and sensing devices that can detect substances in very low concentrations.
Junhong Chen, UWM assistant professor of mechanical engineering, believes CNTs – invisibly thin sheets of graphite that are rolled into a cylindrical shape – are only the beginning. His laboratory focuses on new uses for CNTs, and for nanoparticles, bits of matter that are nanoscale in all three dimensions. (A few nanometers is roughly 50,000 times smaller than the width of a human hair.)
He sees uniting the structures into a hybrid material as vital to pushing nanotechnology forward, and his lab is pioneering ways to do it.
“These interesting multi-component structures will open up new opportunities in several interdisciplinary fields,” Chen says, “including medical diagnostics, green energy technology, and sensors for everything from food flavor to invisible toxic gas.”
When big and small worlds collide
While manipulating atoms in order to produce molecular-scale devices is still in its infancy, CNTs are proving to be the building blocks of choice.
They conduct electricity like either copper or silicon, are stronger than steel, pliable like polymers (kinds of plastics) and can be made from a range of raw materials. Combined with nanoparticles, CNTs can do even more, says Chen.
Since their properties change with size, the challenge is to coax nanostructures to behave in predictable ways.
Junhong Chen, UWM assistant professor of mechanical engineering, and his lab have discovered an easy way to merge two kinds of nanostructures that can do amazing things.
Manipulating CNTs and nanoparticles is tricky business because so many conditions affect their behavior. Just below nanoscale, at the level of individual atoms, matter acts quite differently than it does lumped together in bulk.
With the help of graduate student Ganhua Lu, Chen has pioneered a method for creating hybrid structures by coating CNTs with aerosol nanoparticles. His lab also has produced a low-cost way to make “custom” nanoparticles that gives them full control over the structure’s final properties.
Already they have devised a gas sensor using only nanoparticles of tin oxide.
Their process for producing hybrid structures is far superior to the method currently available, and their work advances understanding of how materials in the quantum world interact with those in the “seen” world.
Nanotech achievement at UWM
“My goal is to make something real, that people can see and use and that has tangible results,” says Chen, who came to UWM in 2003 after a year as a post-doctoral scholar at the California Institute of Technology.
With two patents pending and funding from sources such as the National Science Foundation and the Xerox Corporation, he is well on his way. His and Lu’s work was recently featured in the journal Nanotechnology and the online news source Nanowerk.
Chen is one of a cluster of UWM scientists – in engineering, chemistry and physics – who conduct research into nano- and surface science. In fact, UWM’s Laboratory for Surface Studies, a UW Center of Excellence, brings together the work of 13 faculty who explore the structure and properties of solid surfaces and the interaction of surfaces with atoms and molecules.
The lab’s research encompasses topics such as thin films and laminates, spintronics, molecular wires, optical fiber sensing, and properties such as catalysis, corrosion and friction.
Molecular building with control
When it comes to size, there’s tiny and then there’s tiny all over. Nanostructures can be at nanoscale in either one, two or three dimensions, or any combination. The number of dimensions at nanoscale determines the structure’s properties. So do the structure’s shape and the material it’s made of.
CNTs can behave like an electrical conductor or a semiconductor, depending on the diameter and the twist of the tube. Nanoparticles have their own unique characteristics. Once attached to CNTs, they can transfer their abilities to the tube.
For example, nanoparticles could potentially give an insulator like silicon the ability to conduct electricity.
A happy union of the two structures offers a chance to brainstorm new applications, says Chen. For example, the hybrid can be altered to absorb and emit various wavelengths of light, giving it optical properties.
“You get an opportunity to make a material that could potentially display not only the properties of the CNT and the nanoparticles, but also some additional properties because of the interaction between the two.
“So one plus one may be greater than two,” he says. “That’s the whole idea.”
Before Chen’s technique for fusing the two nanostructures, the process took hours.
CNTs are produced in a gas phase from carbon precursors, but nanoparticles are made in a solution, he says. “You had to combine the dry with the wet before you can make something out of it,” he says. “It’s not very compatible.”
Also the surface of each structure had to be modified, and the chemistry would be different for each new material involved. “What you would get at the end could be very different from what you were trying to get,” he says.
So Chen and his lab developed a way to make nanoparticles in the gas (dry) phase. Then they applied an electrostatic force to attract any kind of nanoparticle to the CNT. The one-step process takes only minutes.
“It works like an indoor air cleaner, using the electric field to capture dust particles,” he says.
Another advantage of the process is Chen can control the size of the nanoparticle that is fastened to the tube, and that determines the final properties of the hybrid structure. The higher the electric field, the larger the particles it will attract.
Chen calls his lab’s work in developing hybrid materials an “important contribution,” but believes that the link with industry that students experience is also essential.
“Our research creates a wonderful vehicle for educating students,” he says. “Many of our projects lie at the intersection of fundamental science and industrial applications with ample opportunities for new discoveries.”
For more on the Laboratory for Surface Studies:
To access the article on Chen and Lu’s research in Nanowerk: