Archives for August 2019

Department Welcomes New Faculty Member

August 29, 2019 by Kayla Benson

Fred Heberle Aug. 2019
Photo by Steven Bridges

Fred Heberle joined the Department of Chemistry as an assistant professor. Heberle comes from a small town in eastern Montana, and earned his PhD at Cornell University in the lab of Gerald Feigenson. Heberle spent five years in the Neutron Sciences Division at Oak Ridge National Laboratory for his postdoctoral research and is a PI on an NSF grant for the study of asymmetric membranes.

“Our lab uses biophysical, biochemical, and computational methods to answer fundamental questions about biomembrane structure and organization,” Heberle said. “We use model systems spanning a vast range of complexity, from simple liposomes made from a single type of lipid, to multicomponent vesicles with engineered lipid asymmetry, to the plasma membrane of a living cell itself. We use techniques ranging from calorimetry, to fluorescence, to neutron and X-ray scattering.”

Feigerle Gives Talk About Diamond Foils for ORNL’s SNS

August 9, 2019 by Kayla Benson

Chuck Feigerle, head of the Department of Chemistry at the University of Tennessee, Knoxville, spent a summer learning how to make diamond films with Jim Butler from the Naval Research Laboratory in Washington, D.C. Feigerle and his associates grew the first diamond films for Buehler Hall’s Spallation Neutron Source (SNS). Then, with Department of Energy funds, a microwave plasma reactor was purchased for Bob Shaw’s lab at ORNL, where diamond foils were grown and tested for the SNS over the next few years. Shaw and Feigerle led the research group. He credits Leslie Wilson with carrying out much of the research.

Each foil is grown by chemical vapor deposition on a silicon wafer formed to have a desired corrugated surface pattern that makes the film rigid. The silicon substrate is dissolved away using acid, leaving just enough to serve as a handle for mounting the foil.

Feigerle spoke about ORNL and UT research in developing and testing this key component of the SNS in a recent talk.

The SNS employs nanocrystalline diamond foils made at ORNL to strip the electrons from negatively charged hydrogen atoms in a beam accelerated to almost 90 percent the speed of light.

Feigerle presented results on the development and use of nanocrystalline diamond stripper foils at the SNS. He described studies at ORNL of the transformations that occur in crystalline structure, emissivity and the carbon state of the foils from deposition of beam energy into the foil.

One finding is that after long exposure to high beam currents that increase the temperature to above 1,500 degrees Celsius, the diamond foils become more like a graphite, gray, crystalline, form of carbon used as a solid lubricant, in pencils and as a moderator in nuclear reactors (e.g., the Graphite Reactor, the world’s first continuously operated reactor that originally made ORNL famous).

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Calhoun Lab Illuminates ‘Dark’ States in Nano Letters Paper

August 7, 2019 by Kayla Benson

Researchers in the Department of Chemistry at the University of Tennessee, Knoxville, are shooting lasers at quantum dots to illuminate ‘dark’ states and provide new insights that can steer the design of future materials.

Tiny crystals known as quantum dots have become so ubiquitous that you can find them in modern commercial televisions. Yet there are still key questions that have remained unanswered despite decades of research about how these crystals work. In particular, it is not clear exactly how the surfaces of quantum dots affect how they interact with light.

When light hits a quantum dot, the energy is stored in energy levels or states. When this energy moves to states on the surface of the quantum dot, it becomes “trapped” and lost for potential use. These surface states, however, are invisible to basic optical experiments because they cannot directly absorb the light, and this leaves the energies of these surface states unknown.

That was until a collaborative UT/Oak Ridge National Laboratory research team, led by Tessa Calhoun in the University of Tennessee’s Department of Chemistry, developed a new way to shed light on them. Their electronic sum frequency generation microspectroscopy technique simultaneously mixes different colors of ultrashort laser pulses to generate new colors of light that describe these elusive ‘dark stats’ on the quantum dots and is detailed in their recent paper published in Nano Letters.

“I was excited by just how many energy levels we could detect with a single measurement at ambient conditions,” Calhoun said.

In addition to being able to match dozens of known energy levels from literature, their studies were able to detect multiple elusive surface states.

“While many other experiments had suggested the presence of one or more of these states, we are the first to be able to directly show that there is more than one surface state in these quantum dots.” Calhoun said.

In addition to Calhoun, other team members were Brianna Watson, the lead author, and Benjamin Doughty, a staff scientist in the Chemical Sciences Division at ORNL. Brianna was a graduate student in the UT Department of Chemistry and is now conducting postdoctoral research using microscopy at Boston Children’s Hospital.

This new information about the energies of dark states promises to provide an avenue to control these generally undesirable defects to improve performance in devices.

Knowing more about these surface states will allow scientists to design better nanoparticles and Calhoun’s group is excited for the future systems they can explore with their new microspectroscopy.

Read more about this research in the paper titled “Energetics at the Surface: Direct Optical Mapping of Core and Surface Electronic Structure in CdSe Quantum Dots using Broadband Electronic Sum Frequency Generation Microspectroscopy.”