Tiny Tech: Nanoscientists and engineers explore the science of the small to develop tomorrow’s technologies
By Andrea Gibson
Illustrations by Christina Ullman
A better picture from your Blu-ray or DVD player. More accurate sensors that can detect hazardous chemicals and materials. A wider range of light emitting diodes (LEDs). More memory for your computer–in less space. Tiny “labs on a chip” that can test out dozens of new drug candidates simultaneously.
In the quest to build the fastest, most efficient, and highest quality technologies for the future, scientists and engineers have been looking for solutions in an entirely new place: the tiny world of atoms and molecules. For decades now, researchers have been exploring the very different laws of physics and behaviors that exist on this extremely small scale and are finding new opportunities to build and power the next generation of devices.
In 2001, Ohio University capitalized on its longtime materials science and engineering research to launch the Nanoscale and Quantum Phenomena Institute (NQPI). More than two dozen faculty members from fields ranging from physics and chemistry to math and chemical engineering now work together on problems that have implications for fields ranging from computer science, telecommunications, and medicine to environmental remediation and energy. Over the last five years, they’ve pulled in almost $20 million in funding from agencies such as the National Science Foundation and U.S. Department of Energy.
“Because we believe nanoscience and nanomedicine will have such a pervasive influence on humanity’s future, we can’t afford not to do this,” says Smith, NQPI director.
Some of the technologies are under development by Ohio University researchers with an eye towards commercialization for the marketplace. Many others are still being explored in the lab, where scientists and engineers are discovering and building intriguing devices at the small scale.
Imagine a flash drive 1,000 times faster than today’s models, or downloading megabyte pictures through your iPhone in the blink of an eye. Scientists have been looking to phase-change memory materials for the next generation of memory devices, but have struggled to overcome a major fault: The devices consume a lot of power as their capacity increases. Physicist Gang Chen is developing a novel solution to the problem. He’s fabricated semiconductor wires and encased them in a silica matrix similar to the material used in fiber optics. When these tiny wires are confined in a small space, they show promise for operating as faster, smaller, less energy-intensive data storage than other technologies under study. Why are these materials so good? It’s partly because they exist in two solid states: ordered and disordered (much in the way that binary code comes in zeroes and ones, the scientist explains) that switch back and forth in a matter of only nanoseconds, says Chen, whose work is funded by the National Science Foundation and U.S. Department of Energy. Now Chen and collaborators are testing out specific types of these materials to determine which ones have the best performance. They’re intrigued by highly porous materials that have been shown to enhance the transition between the two solid states. In the future, the technology could replace conventional information storage devices such as DVD and Blu-ray discs, as well as flash drives.
Conventional electronics are based on the charge of the electron, but nanoscientists say that learning to harness another property of the electron—its spin—can allow researchers to develop smaller, faster, more powerful devices in the future. Theoretical physicist Sergio Ulloa and colleagues explore electron behavior, including its spin, in different materials, from semiconductors and carbon nanotubes to quantum dots, the "artificial atoms" of the nanoscale. Although scientists have controlled spin with magnets, Ulloa has succeeded in using electrical fields to switch spin on and off, a method that's easier and more portable, he says. This spring, doctoral student Ginetom Diniz and Ulloa found that electrical fields generated by strands of DNA could alter the spin of carbon nanotubes, a nanoscale structure made by folding graphene. Another benefit, Ulloa notes, is that scientists may be able to use spin to transfer information with less heat, an attractive feature for new technologies.
Sensors used to detect the presence of chemicals, in applications ranging from environmental restoration to homeland security checks, are largely based on infrared light. But there are many materials that can’t be detected this way, says Ohio University engineer Ralph Whaley. Whaley is exploring the optical properties of amorphous materials, which typically have been used for electronics, LCD displays, and solar cells. These materials have a wider range of interaction with chemicals and could have potential for creating photonic devices based on visible or ultraviolet rays. Such devices could detect anything from anthrax to ammonia to TCE, a solvent used abundantly in cleaning equipment in the manufacturing industry. With funding from the National Science Foundation, Whaley is working with colleagues to fabricate nanosensors made of amorphous zinc oxide films to determine if an integrated device can be manufactured for chemical sensing. The team will use ammonia to test the feasibility of the device, which could be used for either handheld or laboratory applications for homeland security, the energy industry, and environmental cleanup.
Engineer Savas Kaya is growing gold nanowires that he says can be developed into miniscule optical and chemical sensors, as well as novel electronic and medical devices. Kaya has created an aluminum scaffolding that he calls a “nanocomb” with tiny pores. By applying electrochemistry techniques, the engineer has successfully prompted gold and silver to sprout through the pores into long strands. Kaya envisions attaching these nanowires to biological membranes to create addressable sensors in microfluidic devices. These “lab-on-a-chip” systems hold promise for drug development, as hundreds of interactions can be tested at once. In addition to growing materials, the engineer also is using electron-beam lithography to fabricate new devices at the nanoscale. In a recent experiment, Kaya drew geometric shapes on a thin layer of gold with electron beams, a process that is easier and less expensive than other strategies to build molecular devices from the ground up, he says.
Gallium nitride is a semiconductor used for bright blue light-emitting optical devices. By adding manganese to the surface of this material, Arthur Smith and colleagues have found interesting properties that might make it a good candidate for the development of a “nanospintronic” device, which would operate on the basis of the property of electron spin instead of just electron charge. Such a nanospintronic material should, in principle, lead to much more energy-efficient devices compared to conventional devices, he says. To study this new material, Smith uses a special tool called a spin-polarized scanning tunneling microscope, which has the ability to extract both spin and electronic information at the atomic scale from the materials under study. The combination of manganese/gallium nitride might be an attractive new nanospintronic material, he says, because devices made from it may be able to operate at room temperature, whereas many conventional materials have to be cooled to low temperatures to operate.
How do you construct a strand of artificial DNA? Theoretical physicist Sasha Govorov, doctoral student Z. Fan, and colleagues used gold nanoparticles—not molecules— as building blocks to construct an artificial helix that resembles a protein. They’re especially excited about the fact that their structure has chirality, a property important for fostering strong molecular interactions. Chiral objects can’t be superimposed on top of each other—two human hands are a common example. But chiral objects can tightly link, as two right hands do when people shake in greeting. It’s been challenging for scientists to recreate this phenomenon in manmade nanostructures, which so far are less sophisticated than their biological counterparts, explains Govorov, whose study was published in the journal Nature and funded by the Volkswagen Foundation and the National Science Foundation. But the quest continues, as chiral structures can become the basis for more sophisticated optical materials and might be more sensitive detectors for biomolecules, a key factor in the search for new drugs. Such nanostructures also could be used as building blocks in computers, integrated transistors, and one day efficient solar energy conversion.
Scientists have developed a new class of semiconductors that can emit the red, green, and blue lights for the fully integrated color displays needed for technologies such as high-definition television screens and Blu-ray applications. But engineer Wojciech Jadwisienczak and others continue to search for better materials for the red light-emitting material that can be more effectively integrated with their blue and green III-nitride semiconductor counterparts, which will improve the performance and energy efficiency of these electronics. The answer could lie with rare earth ions such as Europium, which has been shown to emit a pure red light, he says. These materials can contain dopants that boast strong magnetic properties attractive for spintronics. Jadwisienczak is exploring how to harness specific nanoscale properties, such as spin, of these materials, for possible use in spin-polarized optoelectronic technologies.
Graphene, a single sheet of carbon atoms that forms graphite (think pencil “lead”) in its layered form, is a hot material in nanoscience. Researchers covet its strength, stiffness, thinness, and conductivity. Depending on how these graphene sheets are terminated, the combination of two of the material’s electron properties—charge velocity and spin orientation—can produce remarkable results. In particular, the material can acquire a chiral structure, which can increase the material’s ability to conduct electricity. By analyzing different models, theoretical physicist Nancy Sandler is examining the effects of graphene terminations to determine the most optimal, reliable structure for conductivity at room temperature. The material has some limitations at the nanoscale, she’s discovered. In a previous study, she found that thin graphene ribbons—proposed as a possible “nanowire”—were poor conductors of electricity due to the repulsion between equal charges that is enhanced in a confined environment. But graphene has many other attributes. Sandler is exploring properties that allow the material to exist in a topological insulator phase. Topological insulators, materials that only conduct electricity at certain points on their surfaces and/or edges, may be important for future spintronic devices. If scientists can control the chirality and spin of the material by choosing appropriate terminations and substrates, she says, graphene could be used reliably in better, faster encryption devices and computers.
Using the strange properties of quantum mechanics may revolutionize the way computers operate. Unlike a classical computer, where information exists in a definite state, a quantum computer could exist in multiple states at once. For example, with four quantum bits of information one could simultaneously hold all 16 possible combinations of those bits, explains physicist Eric Stinaff. One of the major building blocks in the nanoscale world, which may form the basis of a future quantum computer, is quantum dots, which behave like artificial atoms. When these dots absorb light, the charges inside form an exciton. Using a pulsed laser system in his lab, Stinaff uses electric fields to prompt the excitons to emit photons very quickly—in a matter of nano or picoseconds. The scientist has successfully lengthened the lifetime of these particles, an important step in using them consistently to manipulate data at the quantum scale.
Call it the world’s tiniest construction project. Saw-Wai Hla is building superconductors, switches, and motors with atoms and molecules. Using a technique called scanning tunneling microscopy, the scientist has found that he can manipulate atoms on a surface at very cold temperatures. To demonstrate this technique, Hla created and captured images of a “smiley face” and the letters “OU” for Ohio University made entirely out of individual atoms. There are potentially big applications for this small work. Hla’s biological switch, made from a chlorophyll molecule extracted from spinach, could be used for nanoscale logic circuits or mechanical switches for future medical, computer technology, or green energy applications. His discovery of the world’s smallest superconductor, a sheet of four pairs of molecules less than one nanometer wide, provides the first evidence that nanoscale molecular superconducting wires can be fabricated, which could be used for nanoscale electronic devices and energy technologies. Up next: Hla and colleagues are designing molecular machines with nanoscale rotors.
Cucurbit[n]urils are curious molecules that are creating a lot of buzz in the nanoscience field. Named for the plant order that includes gourds and squashes, cucurbit[n]urils are shaped like a pumpkin. These molecules’ strong “skin,” hollow interior, and nontoxic properties make them good candidates for drug delivery, energy applications, and use as nanovessels in which scientists can perform miniscule lab experiments, says organic chemist Eric Masson. There’s been an explosion of research on cucurbit[n]urils over the past decade, including Masson’s own studies that have shown the molecule’s ability to work as a nanoscale light switch, mimicking an electronic circuit. His team also has found that cucurbit[n]urils could aid in the manufacture of silver nanoparticles, which hold promise for their ability to speed wound healing of damaged skin tissue. Masson’s experiments show that cucurbit[n]urils keep the nanoparticles from aggregating, a common problem that would prevent their use as biomedical treatments. A full year after the experiment started, the pumpkin-shaped molecules have continued to keep the nanoparticles stable in aqueous solution, he reports.
Distinguished Professor Tadeusz Malinski has developed nanosensors that can probe the smallest cell or neuron to detect biological activity. One of his primary focuses is tracking nitric oxide, a substance produced by the endothelial cells that supports healthy cardiovascular function by relaxing blood vessels and maintaining good blood flow. Nitric oxide dysfunction can cause major health problems, from cardiovascular disease, high blood pressure, and arteriosclerosis to strokes and Alzheimer’s disease. In addition to studies aimed at improving the diagnosis and treatment of these conditions, Malinski and colleagues also are using the nanosensors to take a critical look at the impact of some nanoparticles on human health. Two of his recent studies suggested that nanoparticles of amorphous silica—commonly used in industry and produced by activities such as the operation of coal-fired power plants—are small enough that they can pass through the lungs and skin and enter the blood vessels. The particles prompted the release of a high level of beneficial nitric oxide, but also triggered high levels of toxic nitroxidative stress, which, in turn, caused inflammation and cell death in the human tissue under study, Malinski reports. The research also found that the high levels of nitroxidative stress produced in response to the presence of nanoparticles caused blood platelets to aggregate, which can form blood clots that can lead to strokes. These studies are the first to provide an early warning to those in the medical community considering nanoparticles as a drug delivery vehicle or as a medium for imaging and early diagnosis in nanomedicine. Researchers should weigh the possible serious adverse health impacts, including strokes and cancer, of these nanoparticles on the human body, Malinksi says.
For more examples of Ohio University nanotechnology research, visit www.ounqpi.org.
This article appears in the Spring/Summer 2012 issue of Ohio University's Perspectives research magazine.