Scientia: Research at the University of Tennessee
She Sees the Colors Electric
By LAURA BUENNING
For this UT chemist, a spectrometer reveals much more than the properties of light and provides clues to the complexity of novel materials
Jan Musfeldt is fascinated by color. But her interest extends beyond the hues an artist might apply to canvas to include colors that range over the full electromagnetic spectrum. “I’m interested in what color can tell us about the properties of complex materials,” Musfeldt says, pulling out a page of schematic diagrams depicting simple zero-, one-, quasi-one-, and two-dimensional bulk and nanoscale materials. Think of a zero-dimensional solid as a dot, a single molecule magnet, or nanoparticle. One-dimensional solids include a line of atoms or molecules or a nanotube. Ladder-like solids are quasi-one dimensional, and two-dimensional materials might include molecules on a surface, a thin film, or solids like graphite or superconducting copper oxides.
Musfeldt, a member of the University of Tennessee chemistry faculty and co-director of the UT Chemical Physics Program, brings a unique combination of chemical, physical, and engineering insights to her research. In her lab she introduces her graduate and postdoctoral team, just back from a week of data collection at Florida’s National High Magnetic Field Laboratory (NHMFL). At NHMFL the team works with the world’s strongest steady magnetic fields and a series of spectrometers to investigate the magneto-optical properties of various compounds. But the team also relies on UT’s complement of sophisticated tools. “At UT, our spectrometers cover a wide energy range, giving us the flexibility to investigate all kinds of excitations in electronic and magnetic materials,” Musfeldt says, pointing out several large-photocopier–sized instruments (their electronic innards visible through clear windows) for measuring color properties under variable temperature and magnetic-field conditions.
Musfeldt’s far-infrared spectrometer, for instance, measures responses—electromagnetic “colors”—in the range of wavenumbers from 10 to 600 cm-1. (In comparison, the colors visible to humans have wavenumbers from 13,000 to 18,000 cm-1.) Other spectrometers in the lab measure middle-infrared and short-ultraviolet frequencies. The team also has an infrared microscope for working with very tiny crystals.
Excitations, Musfeldt explains, derive from the magnetic structure of a material, the vibrational modes, and the different types of low- and high-energy electronic structure. As she describes the process, one of her associates releases a cloud from a tank of liquid helium that the team uses to achieve the extremely low temperatures necessary for some experiments. Musfeldt’s team uses variable temperature spectroscopy to observe the microscopic aspects of phase transitions, but cryogenic temperatures (4 kelvins or below) are also useful to “freeze in” electronic and vibrational motion and probe the ground (lowest energy) state of a material.
Tuning Color
Musfeldt recently discovered a whole class of materials that change color when exposed to magnetic fields. Called magneto-dielectric compounds, the materials have tunable optical properties. “Our current challenge is to understand the mechanisms that cause the magneto-dielectric effect,” she says. She also plans to explore how to increase the color contrast and whether the magneto-dielectric effect can be achieved at room temperature.
“You can imagine that a magneto-dielectric material would be useful for camouflage or data storage,” she says, describing how information linked to color might be read by sensors tuned to their minuscule differences.
Reducing Friction
Musfeldt envisions new types of nanomaterials with both fundamentally interesting science and novel engineering properties. The dream, she says, is to roll any of the well-known layered transition metal oxides into a tube, much like the better-known carbon-based nanotubes, and investigate the consequences of this new shape and structure. For instance, tungsten disulfide (WS2) nanoparticles show great promise as solid-state lubricants, with substantially improved performance over that of graphite powders and sprays now used to reduce friction between moving machine parts.
“WS2 nanoparticles have a fifty percent lower friction coefficient than the best corresponding bulk commercial lubricant. Less friction means less fuel or energy loss, so you can imagine environmental applications,” says Musfeldt. The team’s ongoing work suggests that charge and covalency differences may be behind the improved engineering properties. “This sort of application is a special bonus, above and beyond the fundamental science,” says Musfeldt.
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For more information, contact Jan Musfeldt, (865) 974-3392, or e-mail musfeldt@utk.edu.