Advanced Functional Materials and Devices
JIAM scientists are exploring electronic, magnetic, and optical materials and devices that are smaller, more robust, and more energy efficient. These devices, often crafted from new materials, will advance fields as wide ranging as superconductivity, information storage and processing, sensors, energy harvesting, and creation of cheap and clean-burning fuels.
Resistance-Free Electrical Transmission
Superconductors are based on materials that provide resistance-free transmission of electrical current. So efficient are these conducting materials that a current injected into a closed superconducting loop would continue to flow forever, with no loss of energy.
Superconductors could dramatically increase the efficiency of the US power grid or, in the form of powerful magnets, boost the speed and efficiency of transportation systems, including magnetic levitation (Maglev) trains. These trains float on a magnetic field above the rails and are capable of reaching speeds of more than 350 miles per hour. Superconducting magnets are currently used in Magnetic Resonance Imaging (MRI) machines and in particle accelerators, where the magnets are used to bend particle beams.
To achieve zero-resistance conductivity, superconductors must be cooled to a critical temperature. Until the mid-1980s, scientists could achieve superconductivity only at very low temperatures of 10 to 20 degrees Kelvin (-441 to -423 degrees Fahrenheit). Achieving those critical temperatures required liquid helium, which is impractical because of its expense. But in 1986, scientists discovered materials that would superconduct at temperatures above 77 degrees K (-321 degrees F), which are attainable through use of liquid nitrogen, a much more cost-effective cryogenic substance.
While scientists strive to develop superconductors that operate at ever higher temperatures, JIAM’s researchers are at work exploring fundamental questions about the phenomenon of superconductivity, in which, at low temperatures, energy-carrying electrons—particles that, because of their common polarity, should repel each other—actually form into pairs.
JIAM scientists are also developing whole new classes of materials with superconducting properties. These materials are combinations of iron and arsenic substances, including barium potassium iron arsenic, and sodium iron arsenic.
While some JIAM scientists probe the fundamental properties of superconducting materials, others are exploring ways to dampen factors that compromise superconductivity, including magnetic fields. As a large current passes through a wire, it creates a powerful magnetic field, which, as it intensifies, destroys superconductivity. JIAM scientists are researching ways to develop superconductors that are more robust against magnetic fields.
One approach involves using ion beams to introduce defects into a material’s lattice. In the right amounts, these defects can trap—and stop—magnetic field lines and retain superconductivity. Superconducting wires and tapes featuring these defects could be used to increase the efficiency of the US power grid.
JIAM scientists are also wrestling with a very different issue, one of size, working to determine how many—or, more accurately, how few—atoms are necessary to build a superconductor. To that end, scientists are “growing” very thin films, by building them up atom by atom. The quest is to determine how small a device can be and still maintain robust superconductivity. So far, JIAM researchers have created superconducting materials that are only a few atom layers thick.
These minute superconductors could play a role in creating the electronic circuits in nanoscale devices, where heat dissipation remains a key challenge.
In 1988, physicists Albert Fert, a Frenchman, and Peter Grünberg, a German, discovered giant magnetoresistance (GMR), a feature rooted in quantum mechanics that won them the 2007 Nobel Prize and paved the way for the enhanced information storage on modern computers. Based on Fert and Grünberg’s discovery, scientists have devised thin-film materials that comprise magnetic and nonmagnetic layers. Within these devices, scientists are able to control the magnetic orientation of the magnetic layers.
Stacks in which the magnetic orientations of adjacent layers are aligned have low resistance, while stacks in which the magnetic alignment of the layers is anti-parallel have high resistance. The tiny read-write heads on our hard drives rely on this GMR effect in detecting—or reading—the bits (binary digits) of information stored in individual partitions—or domains—on our computers’ hard drives. JIAM researchers have played an active role in investigating the magnetic properties of ultra-small magnetic materials, a research area called ‘nano magnetism’ and the role of magnetism in determining electrical properties at the nanoscale.
Beyond the Silicon Age
While the read-write heads on our hard drives allow us to store and access information, transistors—essentially tiny on-off switches—allow us to process and use the stored information.
While the discovery of GMR provided the means for storing more and more data on smaller and smaller devices, similar advances allow us to pack more and more transistors into an integrated circuit, with parallel increases in processing speed and reduced cost and energy demand. Functions that sixty years ago required entire rooms full of vacuum tubes now are performed by computer chips measuring only a few microns across.
The presence of forbidden zones or ‘band gaps’ in the energy spectrum of a material determines its ability to conduct electricity. Insulating materials have large energy gaps, while conducting materials have very small or nonexistent energy gaps. Semiconductors, like silicon, fall somewhere in between. To make silicon selectively conductive, scientists introduce tiny amounts of foreign atoms (typically at parts-per-million levels) into the semiconductor through a process called doping, which turns the silicon into a semiconductor. Semiconducting properties are prerequisite for building transistors.
Top-down fabrication methods using techniques like optical or electron-beam lithography allow for mass production of silicon-based transistors with features as small as twenty nanometers. As scientists make transistors and other devices smaller and smaller, many of the semiconductor’s properties change, including electrical and thermal conductivity. These changes, which may limit silicon’s usefulness in devices built at the atomic scale, have prompted scientists to look for new classes of semiconducting materials. To that end, JIAM scientists are exploring the use of organic thin films and graphene. Graphene is a one-atom-thick film of carbon that might allow creation of transistors that are much smaller and faster than those crafted from silicon. The discovery of graphene was awarded with the 2010 Nobel Prize for Andre Geim and Konstantin Novoselov.
Graphene also shows promise for use in sensors capable of detecting individual molecules of the deadly biological agents that might be used in terrorist attacks. In the presence of these agents, graphene’s electrical properties would change, triggering minute sensors and alerting security teams to the presence of a threat.
While today’s computers require semiconductors to perform logical functions and magnetic materials for information storage and readout, JIAM scientists are currently investigating ways to integrate both functions into one material.
For instance, JIAM researchers are exploring techniques for growing semiconductor films from germanium and doping them with iron, cobalt, nickel, or other magnetic elements in just the right amounts. Magnetic and semiconducting materials generally don’t mix well. The introduction of too much magnetic “dirt” into a semiconducting material creates phase separation—like (de)mixing oil and water. The solution may lie in the bottom up synthesis or artificially-structured semiconductor materials under conditions far away from thermodynamic equilibrium.
As with all electrical functions, the electron will play a starring role. The up-down spin of electrons is contributing to the emergence of a new field of science termed spin electronics, or spintronics.
In a simplified picture, the electron can be viewed as a sphere of electrical charge that spins on an axis like the Earth. As the electron spins, it creates a circulating current that can spin in only two directions, either clockwise or counterclockwise or, in the parlance of scientists, up or down. The spin direction creates the electron’s magnetic moment.
By layering a magnetic semiconductor atop an ordinary semiconductor, scientists can control, or polarize, the spin to force more electrons into the up or down orientation. The imbalance in the population of up and down electrons is essential for creation of a spin transistor.
Abundant, Renewable Energy
Water molecules are composed of two atoms of hydrogen and one atom of oxygen. Once released, hydrogen can be used as a clean-burning fuel. Water, an abundant resource, covers more than 70 percent of the Earth. The task for material scientists is finding an efficient and cost-effective way to create the photochemical reactions necessary to split water into pure hydrogen and oxygen.
In the 1970s, Japanese scientists created an electrochemical cell with one electrode of titanium dioxide, a semiconductor, and a second of platinum in a water solution. In the presence of ultra-violet light, the cell produced hydrogen and oxygen from the water, but the process was slow and inefficient.
Solar light, like water, is an inexhaustible resource, and some of JIAM’s scientists are exploring ways to use titanium oxide and solar light to generate hydrogen from water. The challenge lies in titanium oxide’s energy gap, which is far too wide to readily absorb solar light. Unlike titanium oxide, silicon, with a narrower energy gap, is more efficient at absorbing solar light—which is why it’s used in modern photovoltaic cells. But the environment in an electrochemical cell is corrosive, and silicon would break down in such an environment. Titanium oxide, by contrast, is highly corrosion resistant. It’s also much less expensive than silicon.
JIAM scientists are investigating ways to “dope” titanium oxide with foreign elements to reduce its energy gap and allow it to absorb solar light and act as an efficient catalyst in splitting water into hydrogen and oxygen. Hydrogen gas, when burned, recombines with oxygen and produces water and, thus, closes the loop, making it an idea fuel from both economic and environmental perspectives.
Exploiting the Plasmon
Plasmonic devices combine optical and metallic properties and are based on the behavior and characteristics of the surface plasmon. Plasmons, like water molecules in the ocean, are capable of collectively forming wave-like excitations that move along a metallic surface. This wavelike oscillation is capable of capturing and condensing light, allowing it to travel along the metallic surface and then emerge at a different point. The surface plasmon was discovered in 1957 by UTK’s distinguished alumnus and physicist Rufus Ritchie at ORNL.
While it was once believed that light, with a wavelength that is hundreds of nanometers wide, could not be confined, scientists have shown that light can be confined to very small wavelengths, which effectively intensifies the light. This phenomenon presents a range of potential applications, including new technologies for characterizing materials.
Plasmons might someday carry information on the nanoscale the way fiberoptic cables now convey large amounts of information along our modern telecommunications networks. Integration of plasmonic nanoparticles, which are capable of confining solar light, with semiconductors might lead to highly efficient photovoltaic cells.
Visualization and Characterization Tools
The tools necessary to characterize functional materials and to visualize them at the nanoscale have evolved in parallel with the development of these new materials.
JIAM’s scanning tunneling microscopes (STM), which send electrons through an atomically sharp needle and produce images of a material’s surface at the atomic scale, are based on a design that earned the 1986 Nobel Prize in physics and launched the nanoscience revolution.
The Spallation Neutron Source (SNS), which occupies 80 acres on the ORNL campus, is an accelerator-based neutron source that provides the most intense pulsed neutron beams in the world for scientific research and industrial development. As the neutrons interact with a sample material, some energy is lost. The precise amount of energy loss provides scientists with information about the material’s fundamental structure and properties, including lattice vibration and magnetic excitation.
Along with its sister facility, the High Flux Isotope Reactor, SNS serves as an intellectual hub for neutron scattering research. Neutron research helps scientists improve materials used in a multitude of products, including high-temperature superconductors and powerful lightweight magnets.
JIAM’s high-sensitivity magnetometers make it possible to detect tiny amounts of magnetism in new materials, and pulsed-light lasers provide information on the dynamic chemical processes taking place within a material and allow scientists to follow the time evolution of a vibration moving through a solid. JIAM’s lasers can produce pulses measured in femtoseconds (one quadrillionth of a second).
East Tennessee is also home to two supercomputers that are among the world’s fastest and most powerful. ORNL’s Jaguar-Cray XT5-HE and UT’s Kraken-Cray XT5-HE, both housed on the ORNL campus, allow material scientists to simulate the dynamic molecular relationships within yet-to-be-created materials to determine how they might perform in real-world applications.