publications

UConn-Technion Collaboration Develops Model for Affordable Fuel Cell Catalysts

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UConn researchers and collaborators at Technion – Israel Institute of Technology developed a theoretical model that will expediate the development of affordable fuel cells, a key technology for sustainable energy.

Radenka Maric, UConn’s vice president for research, innovation and entrepreneurship; Dario Dekel from Technion’s Chemical Engineering Department; S. Pamir Alpay, UConn’s associate dean for research and industrial partnerships; and Sanjubala Sahoo, a research scientist in Alpay’s group published their findings in ACS Catalysis in February.

Thomas and Sanjeev Published in Physical Review B!

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This research is a uniquely rigorous and systematic study of the effect of exchange-correlation functionals (XCFs) (in the framework of density functional theory) on the outcome of geometric and electronic structure calculations in layered binary chalcogenide (A2B3) systems.

Abstract

Topological insulators (TIs) are materials that are insulating in the bulk but have zero band-gap surface states with linear dispersion and are protected by time-reversal symmetry. These unique characteristics could pave the way for many promising applications that include spintronic devices and quantum computations. It is important to understand and theoretically describe TIs as accurately as possible to predict properties. Quantum mechanical approaches, specifically first-principles density-functional-theory (DFT)-based methods, have been used extensively to model electronic properties of TIs. Here we provide a comprehensive assessment of a variety of DFT formalisms and how these capture the electronic structure of TIs. We concentrate on Bi2Se3 and Bi2Te3 as examples of prototypical TI materials. We find that the generalized gradient approximation (GGA) and kinetic density functional (metaGGA) increase the thickness of the TI slab, whereas we see the opposite behavior in DFT computations using LDA. Accounting for van der Waals (vdW) interactions overcomes the apparent over-relaxations and retraces the atomic positions toward the bulk. Based on a systematic computational study, we show that GGA with vdW treatment is an appropriate method for structural optimization. However, the vdW corrections recover the experimental bulk parameters, and do not influence the charge density implicitly. Thus, electronic structures derived from the base GGA functional, employing experimental lattice parameters, is sufficient.
DOI: https://doi.org/10.1103/PhysRevB.101.085140

Congrats! Ayana and Dennis Earn Top Distinction in Scientific Reports

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Ayana Ghosh’s and Dennis Trujillo’s article,  Electronic and Magnetic Properties of Lanthanum and Strontium Doped Bismuth Ferrite: A First-Principles Study, received 2,603 article downloads in 2019, placing it as one of the top 100 downloaded physics papers for Scientific Reports in 2019.

Scientific Reports published more than 1,072 physics papers in 2019, and so a position in the top 100 most downloaded articles is an extraordinary achievement.

Kevin, Sanjubala and Colleagues Published in Clays and Clay Minerals

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DFT was used to determine counter-hydrogen ion retention strength in Montmorillonite clays, a common aluminosilicate mineral found in soil, as a measure of cation adsorption strength. Results show that impurity Mg-Fe separation distance is responsible for three possible adsorption strengths regimes and are consistent with experimental data.

Abstract

Although multiple types of adsorption sites have long been observed in montmorillonite, a consistent explanation about the chemical structure of these adsorption sites has not yet been established. Identifying the cation interlayer adsorption sites based on the octahedral cation distribution on montmorillonite was investigated in this study by using a Density Functional Theory (DFT) simulation. A clay structural model (H[Al6MgFe]Si16O40(OH)8) with a similar composition to Wyoming SWy-1 montmorillonite was built, where two octahedral Al were respectively substituted by Fe and Mg, and H+ was the charge compensating cation. This model had twenty-one different possible configurations as a function of the distribution of octahedral Al, Fe, and Mg cations. The DFT simulations of 15 of these different configurations showed no preference for the formation of any configuration with a specific octahedral Fe-Mg distance. However, the H+ adsorption energy was separated into three distinct groups based on the number of octahedral jumps from Fe to Mg atoms. The H+ adsorption energy significantly decreased with increasing number of octahedral jumps from Fe to Mg. Assuming an even probability of occurrence of 21 octahedral structures in montmorillonite, the percentages of these three groups are 43, 43, and 14%, respectively, which are very close to the three major sites on montmorillonite from published cation adsorption data. These DFT simulations offer an entirely new explanation for the location and chemical structure of the three major adsorption sites on montmorillonite, namely, all three sites are in the interlayer, and their adsorption strengths are a function of the number of octahedral jumps from Fe to Mg atoms.

Tulsi & Kevin Publish in the Journal of Applied Physics

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Congratulations to Tulsi & Kevin on their publication, “Ferroelectric Films on Metal Substrates: The Role of Thermal Expansion Mismatch on Dielectric, Piezoelectric, and Pyroelectric properties”, in the Journal of Applied Physics for their findings on the role of metallic substrates on the functional properties of PZT solid solutions.

 

Ferroelectric Films on Metal Substrates: The Role of Thermal Expansion Mismatch on Dielectric, Piezoelectric, and Pyroelectric properties
T. A. Patel, K. Co, R. J. Hebert, and S. P. Alpay, J. Appl. Phys. 126, 134103 (2019)
DOI: DOI: https://doi.org/10.1063/1.5116134

ABSTRACT
We present here a comprehensive analysis of the effect of thermal stresses on the functional properties of ferroelectric oxides on metal substrates. We use a Landau-Devonshire formalism to quantitatively assess the role of in-plane thermal strains that arise from the coefficient of thermal expansion (CTE) mismatch between lead zirconate titanate [PbZrxTi1–xO3, PZT x/(1 – x)] films and Al, Cu, Fe, Ni, and Ti-based substrates. Specifically, we compute Curie transition temperatures, spontaneous polarizations, dielectric permittivities, piezoelectric coefficients, and pyroelectric responses of tetragonal PZT compositions as a function of the growth/processing temperature. To provide a rapid evaluation, we also develop Ashby diagrams of property coefficients as a function of PZT composition, processing temperature, and CTE of the substrate. Our results show that thermal strains in PZT may significantly alter the ferroelectric transition temperature, dielectric, piezoelectric, and pyroelectric properties. For example, for PZT 50/50 films on Ni-based superalloys processed/annealed at 700 °C, we predict monodomain intrinsic dielectric, piezoelectric, and pyroelectric responses to be 234, 152 pC/N, and 0.021 μC cm−2 °C−1, respectively, compared to bulk PZT 50/50 values of 381, 326 pC/N, and 0.045 μC cm−2 °C−1. These are substantial variations which show that thermal strains must be considered in the design and development of built-in functionality obtained through ferroelectric films in structural, aerospace components.

Co and Nayak published in Phys. Rev. B

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Great work Kevin and Dr. Nayak! They published in Physical Review B, Polarization rotation in Bi4Ti3O12 by isovalent doping at the fluorite sublattice.

Abstract

Bismuth titanate, Bi4Ti3O12 (BiT), is a complex layered ferroelectric material that is composed of three perovskitelike units and one fluoritelike unit stacked alternatively along the transverse direction. The ground-state crystal structure is monoclinic with the spontaneous polarization (∼50μC/cm2) along the plane. BiT typically grows along the c direction in thin-film form, and having the polarization vector aligned with the growth orientation can be beneficial for several potential device applications. It is well known that judicious doping of ferroelectrics is an effective method in adjusting the magnitude and the orientation of the spontaneous polarization. Here, we show using first-principles density-functional theory and a detailed phonon analysis that Bi atoms in the fluoritelike layers have significantly more impact on the magnitude and orientation of the spontaneous polarization vector as compared to the perovskitelike layer. The low-energy hard-phonon modes are characterized by fluoritelike layers experiencing transverse displacements and large changes in Born effective charges on Bi atoms. Thus, the breaking of symmetry caused by doping of Bi sites within the fluoritelike layer leads to the formation of uncancelled permanent dipole moments along the transverse direction. This provides an opportunity for doping the Bi site in the fluoritelike layer. Isovalent dopants P, As, and Sb were studied. P is found to be most effective in the reorientation of the spontaneous polarization. It leads to a threefold enhancement of the out-of-plane component of polarization and to a commensurate rotation of the spontaneous polarization vector by 36.2° towards the transverse direction. DOI: https://doi.org/10.1103/PhysRevB.99.014101

Dr. Sanjeev Nayak publishes in Scripta Mater. on GP Zone Formation

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Dr. Sanjeev Nayak in collaboration with Dr. Rainer Heber and MSE PhD student Cain Hung develop methods to describe GP zone formation in Al-Cu and Al-Ag alloys. Results published in Scripta Mater.

S. K. Nayak, C. Hung, R. J. Hebert, and S. P. Alpay, “Atomistic Origins of Guinier-Preston Zone Formation and Morphology in Al-Cu and Al-Ag Alloys from First Principles,” Scripta Mater. 162, 235 –240 (2019)

Graphene Supported Single Atom Transition Metal Catalysts

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Dr. Sanjubala Sahoo and colleagues have provided insight for the origin of high catalytic activity for graphene-supported single transition metal atoms for activation of methane molecules. The paper entitled “Graphene Supported Single Atom Transition Metal Catalysts for Methane Activation” published in ChemCatChem, Volume 10, page 3229 (2018) co-authored by S. Sahoo, S. L. Suib and S. P. Alpay was a cover feature for the journal.