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Smith Breaks New Ground with Domain Wall Research

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Kevin Smith, recent Ph.D. graduate from the department of chemistry, and Professor of Chemistry Janice Musfeldt have published the results of a collaborative investigation into the properties of ferroelectric domain walls. This research has generated a greater understanding of both a specific material, and domain walls in general, expanding the foundational knowledge critical to effectively using domain walls in future technologies.

Smith joined the chemistry department as a graduate student in 2015 and very quickly began investigating domain walls. Domain walls act as the boundaries between regions, or domains, of materials and have the potential to impact the properties and uses of that material.

Smith’s work specifically investigates the domain walls of ferroelectric materials, which have been a source of interest in the development of electronics. Efforts have been made to use domain walls as functional parts of devices as they could offer high speed memory reading and writing while requiring less energy to function.

Before ferroelectric domain walls can be successfully leveraged, researchers must develop a fundamental understanding of them and how they behave. It has long been hypothesized that these domain walls are atomically thin and conductive, but this had never been confirmed with a direct measurement at the wall. Smith and Musfeldt began investigating ferroelectric domain walls not with the intention of addressing this long-held belief, but with the goal of uncovering foundational information that could contribute to a greater understanding of these materials.

A collaboration with a group of physicists at Rutgers university, led by Henry Rutgers Professor Sang-Wook Cheong, provided Smith the material with which to begin his exploration.

“Our synthetic collaborators at Rutgers grew the material for us and provided some basic mapping on where to look for the domain walls,” said Smith. “We performed a line scan of the material with the near-field infrared microscope at Beamline 2.4 of the Advanced Light Source, or ALS, at Lawrence Berkeley National Lab. That’s when we started seeing these differences that we weren’t expecting to see.”

When thinking of a solid object, the expectation is often that the object is fairly uniform and that the components creating it are evenly distributed throughout that object. However, with the material Smith was investigating, the scan’s results were pointing toward different organizations of the material’s component parts in different regions of the material.

Smith and Musfeldt knew if they were going to uncover the source of these differences, they were going to need to investigate the material further, using the high-resolution infrared technique at the ALS to scan the material more thoroughly.

Beamline 2.4 of the ALS couples an atomic force microscope with synchrotron-generated infrared light to perform nanospectroscopy to examine materials on a much smaller scale than traditional microscopes. The microscope uses extremely sharply focused light delivered to an object at a very close distance. The response of the light as it interacts with the object is then collected and used to determine what is happening in that object.

“Using the ALS allowed us to examine these differences we were seeing in much greater detail. The material that we were studying was grown in such a way that it had two different types of metals in its A-site, scandium and lutetium. The ALS let us tease out three compositional arrangements for these materials that explained the differences. We found regions that were fairly evenly distributed, as well as both scandium-rich and lutetium-rich regions,” said Smith.

In addition to explaining the differences in domains with slightly different local composition, Smith and Musfeldt were able to determine the domain walls themselves were, in fact, much wider than traditionally believed. They also concluded that while they may have different conductivity than the surrounding regions, the domain walls were not metallic.

By successfully imaging ferroelectric domain walls, Smith and Musfeldt have accomplished something that has never been done before. As a result, they not only created a deeper understanding of these domain walls in a specific material, but also upended long-held beliefs about domain walls in general, paving the way for future innovation. Their work further highlights the importance of foundational and exploratory research in the development of future breakthroughs.

“This project really highlights the importance of curiosity in research,” said Musfeldt. “Kevin took an exploratory project and turned it into the most exciting thing in our lab with far-reaching implications.”

New materials are one potential path to improving existing technologies and generating new means of meeting the modern needs of people and society. Materials, however, are only useful insofar as they can be understood. Smith and Musfeldt’s work digs into the fundamental science behind a material’s properties, simultaneously creating a better understanding of that material and creating a roadmap for more effective uses for it in the future.

The full publication describing this research can be read here.

Alum Spotlight – Neal

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Sabine Neal, born and raised in western Montana, graduated with her PhD working with the Musfeldt Lab in May 2021.

“The Musfeldt lab provided me with a lot of opportunity. Musfeldt knew I was a single parent and looked past that, believing in me, and giving me so many invaluable experiences,” Neal said. “I had a chance to work at two national labs, collaborate with scientists all over the world, travel to conferences, and employ cutting edge technology to study two-dimensional systems.”

Neal began working at Brookhaven National Lab in January 2021 as a Research Associate in Materials Science as a part of the Interface Science and Catalysis group at the Center for Functional Nanomaterials.

Neal’s expertise primarily lies in infrared and Raman spectroscopy and currently works on a broad array of instrumentation including both NanoIR, Photothermal, and nanoprobe systems to study high energy materials. She also uses LEEM and LEED to grow and characterize thin films. 

“UT’s chemistry graduate program helped me prepare in several ways. First, the hands-on training in the lab was crucial. I know how to trouble shoot, maintain lab equipment, and work independently.  Second, the many conferences, visitor presentations, and group talks helped to cultivate my communication skills,” Neal said. “Being a TA also helped me learn how to communicate effectively to different groups/skill levels of people. I really enjoyed working with the students and general chemistry staff.  Finally, I had three stand-out professors that aided in my personal journey to obtain my degree: Musfeldt, Sharma, and Kilbey.”

“As a single parent, most people have told me what I couldn’t do. I couldn’t get a bachelor’s degree. I did. I couldn’t get a master’s. I did. And most wouldn’t have believed I could earn a PhD. But I did,” Neal said. “You can do anything that you want if you put your mind to it and work hard. Stand up for yourself. Do what makes you happy. There is no limit!”

Musfeldt Group Published in 2D Materials

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The Musfeldt group published their work “Exploring few and single layer CrPS4 with near-field infrared spectroscopy” in 2D Materials. First author is Sabine Neal, UT chemistry alum.

“We combine synchrotron-based near-field infrared spectroscopy and first principles lattice dynamics calculations to explore the vibrational response of CrPS4 in bulk, few-, and single-layer form,” Neal said. “Analysis of the mode pattern reveals a C2 polar + chiral space group, no symmetry crossover as a function of layer number, and a series of non-monotonic frequency shifts in which modes with significant intralayer character harden on approach to the ultra-thin limit whereas those containing interlayer motion or more complicated displacement patterns soften and show inflection points or steps.”

This is different from MnPS3 where phonons shift as 1/size2 and are sensitive to the three-fold rotation about the metal center that drives the symmetry crossover. “We discuss these differences as well as implications for properties such as electric polarization in terms of presence or absence of the P–P dimer and other aspects of local structure, sheet density, and size of the van der Waals gap,” Neal said.

Figure 1. (a) Crystal structure of CrPS4 at 300 K [23]. The sheet thickness and size of the van der Waals gap are indicated. (b) Schematic of the near-field infrared technique in which an atomic force microscope (AFM) cantilever tip directs light to the sample surface. (c) High resolution AFM image of exfoliated CrPS4. (d) Schematic diagram of the symmetry subgroup relationships as they pertain to the presence or absence of a phosphorus dimer.

Musfeldt Group Published in npj 2D Materials and Applications

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The Musfeldt Group published their work “Chemical bonding and Born charge in 1T-HfS2” in npj 2D Materials and Applications. This is a collaborative research with the Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN. 

Their research combines infrared absorption and Raman scattering spectroscopies to explore the properties of the heavy transition metal dichalcogenide 1T-HfS2. They employ the LO–TO splitting of the Eu vibrational mode along with a reevaluation of mode mass, unit cell volume, and dielectric constant to reveal the Born effective charge.

 In addition to resolving the controversy over the nature of chemical bonding in this system, we decompose Born charge into polarizability and local charge. Polar displacement-induced charge transfer from sulfur p to hafnium d is responsible for the enhanced Born charge compared to the nominal 4+ in hafnium. 1T-HfS2 is thus an ionic crystal with strong and dynamic covalent effects. 

This work places the vibrational properties of 1T-HfS2 on a firm foundation and opens the door to understanding the properties of tubes and sheets.

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Musfeldt Group Published in Nano Letters

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The Musfeldt Group published their work “Excitations of Intercalated Metal Monolayers in Transition Metal Dichalcogenides” in Nano Letters.

They combine Raman scattering spectroscopy and lattice dynamics calculations to reveal the fundamental excitations of the intercalated metal monolayers in the FexTaS2 (x = 1/4, 1/3) family of materials. Both in- and out-of-plane modes are identified, each of which has trends that depend upon the metal–metal distance, the size of the van der Waals gap, and the metal-to-chalcogenide slab mass ratio.

They test these trends against the response of similar systems, including Cr-intercalated NbS2 and RbFe(SO4)2, and demonstrate that the metal monolayer excitations are both coherent and tunable.

They discuss the consequences of intercalated metal monolayer excitations for material properties and developing applications.

Musfeldt Group Published in Nature Communications

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The Musfeldt group published their work “Site-specific spectroscopic measurement of spin and charge in (LuFeO3)m/(LuFe2O4)1 multiferroic superlattices” in a collaborative piece in Nature Communications.

Interface materials offer a means to achieve electrical control of ferrimagnetism at room temperature as was recently demonstrated in (LuFeO3)m/(LuFe2O4)1 superlattices. A challenge to understanding the inner workings of these complex magnetoelectric multiferroics is the multitude of distinct Fe centres and their associated environments. This is because macroscopic techniques characterize average responses rather than the role of individual iron centres.

In this article, researchers combine optical absorption, magnetic circular dichroism and first-principles calculations to uncover the origin of high-temperature magnetism in these superlattices and the charge-ordering pattern in the m = 3 member. In a significant conceptual advance, interface spectra establish how Lu-layer distortion selectively enhances the Fe2+ →  Fe3+ charge-transfer contribution in the spin-up channel, strengthens the exchange interactions and increases the Curie temperature.

Comparison of predicted and measured spectra also identifies a non-polar charge ordering arrangement in the LuFe2O4 layer. This site-specific spectroscopic approach opens the door to understanding engineered materials with multiple metal centres and strong entanglement.

Musfeldt Group’s Recent Achievements

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The Musfeldt Group’s research area focuses on experimental materials chemistry and physics. They employ a variety of spectroscopic methods to reveal and control the properties of quantum materials. External stimuli are used to tune these properties in order to explore new physical phenomena and uncover properties of technological relevance.

The Musfeldt Group provides a very unique opportunity for students with the DMREF team (Designing Materials to Revolutionize and Engineer our Future). This year, the team received “The Creativity Extension which is the highest honor in DMR,” Musfeldt said. “The team received an extra $450,000 for it this year.”

The group has also been busy publishing papers such as “Piezochromism in the magnetic chalcogenide MnPS3” in npj Quantum Materials. Nathan Harms, graduate student in the Musfeldt Group, is the lead author. This research explores combining high-pressure optical spectroscopies and first-principles calculations to reveal piezochromism in MnPS3. Photographs are of piezochromic MnPS3 inside the diamond anvil cell at several characteristic pressures and also after release at room temperature. These images show a gasket hole diameter of 325 μm. The diamond culets are 500 μm.

Musfeldt was also published a cover article in Physics Today titled “Nanotubes from layered transition metal dichalcogenides.”

 

Musfeldt Group Published in Inorganic Chemistry

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The Musfeldt Group recently published their work “Spin-Lattice Coupling Across the Magnetic Quantum-Phase Transition in Copper-Containing Coordination Polymers” in Inorganic Chemistry.

The group employs a series of copper-containing coordination polymers as a platform for exploring spin−lattice coupling across the magnetic quantum-phase transition. This interaction, which they quantify for the out-of-plane pyrazine bending mode as a function of the magnetic and structural dimensionality, reaches a maximum in ladderlike [Cu(pyz)1.5(4-HOpy)2](ClO4)2 because of the intermediate dimensionality.

They also sought to reveal spin−phonon coupling under compression but instead discovered a pressure-induced transition in the ladder to a state that is likely ferroelectric.