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Physical Chemistry

Iowa State University

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Articles 1 - 30 of 125

Full-Text Articles in Physics

Characteristics Of Sulfur Atoms Adsorbed On Ag(100), Ag(110), And Ag(111) As Probed With Scanning Tunneling Microscopy: Experiment And Theory, Peter M. Spurgeon, Da-Jiang Liu, Holly Walen, Junepyo Oh, Hyun Jin Yang, Yousoo Kim, Patricia A. Thiel May 2019

Characteristics Of Sulfur Atoms Adsorbed On Ag(100), Ag(110), And Ag(111) As Probed With Scanning Tunneling Microscopy: Experiment And Theory, Peter M. Spurgeon, Da-Jiang Liu, Holly Walen, Junepyo Oh, Hyun Jin Yang, Yousoo Kim, Patricia A. Thiel

Chemistry Publications

In this paper, we report that S atoms on Ag(100) and Ag(110) exhibit a distinctive range of appearances in scanning tunneling microscopy (STM) images, depending on the sample bias voltage, VS. Progressing from negative to positive VS, the atomic shape can be described as a round protrusion surrounded by a dark halo (sombrero) in which the central protrusion shrinks, leaving only a round depression. This progression resembles that reported previously for S atoms on Cu(100). We test whether DFT can reproduce these shapes and the transition between them, using a modified version of the Lang–Tersoff–Hamann ...


Hedgehog Spin-Vortex Crystal Antiferromagnetic Quantum Criticality In Cak (Fe1−Xnix)4 As4 Revealed By Nmr, Qingping Ding, William R. Meier, J. Cui, Mingyu Xu, A. E. Böhmer, Sergey L. Bud’Ko, Paul C. Canfield, Yuji Furukawa Sep 2018

Hedgehog Spin-Vortex Crystal Antiferromagnetic Quantum Criticality In Cak (Fe1−Xnix)4 As4 Revealed By Nmr, Qingping Ding, William R. Meier, J. Cui, Mingyu Xu, A. E. Böhmer, Sergey L. Bud’Ko, Paul C. Canfield, Yuji Furukawa

Ames Laboratory Accepted Manuscripts

Two ordering states, antiferromagnetism and nematicity, have been observed in most iron-based superconductors (SCs). In contrast to those SCs, the newly discovered SC CaK(Fe1−xNix)4As4exhibits an antiferromagnetic (AFM) state, called hedgehog spin-vortex crystal (SVC) structure, without nematic order, providing the opportunity for the investigation into the relationship between spin fluctuations and SC without any effects of nematic fluctuations. Our 75As nuclear magnetic resonance studies on CaK(Fe1−xNix)4As4 (0≤x≤0.049) revealed that CaKFe4As4 is located close to a hidden hedgehog SVC AFM quantum-critical point (QCP). The magnetic QCP without nematicity in CaK(Fe1−xNix)4As4 ...


Reduction Of The Ordered Magnetic Moment And Its Relationship To Kondo Coherence In, Benjamin G. Ueland, Na Hyun Jo, Aashish Sapkota, W. Tian, Morgan W. Masters, Halyna Hodovanets, Savannah S. Downing, Connor Schmidt, Robert Mcqueeney, Sergey L. Bud’Ko, Andreas Kreyssig, Paul C. Canfield, Alan Goldman Apr 2018

Reduction Of The Ordered Magnetic Moment And Its Relationship To Kondo Coherence In, Benjamin G. Ueland, Na Hyun Jo, Aashish Sapkota, W. Tian, Morgan W. Masters, Halyna Hodovanets, Savannah S. Downing, Connor Schmidt, Robert Mcqueeney, Sergey L. Bud’Ko, Andreas Kreyssig, Paul C. Canfield, Alan Goldman

Ames Laboratory Accepted Manuscripts

The microscopic details of the suppression of antiferromagnetic order in the Kondo-lattice series Ce1-&ITx&ITLa&ITx&ITCu2Ge2 due to nonmagnetic dilution by La are revealed through neutron diffraction results for x = 0.20, 0.40, 0.75, and 0.85. Magnetic Bragg peaks are found for 0.20 <= x <= 0.75, and both the Ned temperature T-N and the ordered magnetic moment per Ce mu linearly decrease with increasing x. The reduction in mu points to strong hybridization of the increasingly diluted Ce 4f electrons, and we find a remarkable quadratic dependence of mu on the Kondo-coherence temperature. We discuss our results in terms of local-moment- versus itinerant-type magnetism and mean-field theory and show that Ce1-&ITx&ITLa&ITx&ITCu2Ge2 provides an exceptional opportunity to quantitatively study the multiple magnetic interactions in a Kondo lattice.


Liquid-Like Thermal Conduction In Intercalated Layered Crystalline Solids, B. Li, H. Wang, Y. Kawakita, Q. Zhang, M. Feygenson, H. L. Yu, D. Wu, K. Ohara, T. Kikuchi, K. Shibata, T. Yamada, X. K. Ning, Y. Chen, J. Q. He, David Vaknin, R. Q. Wu, K. Nakajima, M. G. Kanatzidis Mar 2018

Liquid-Like Thermal Conduction In Intercalated Layered Crystalline Solids, B. Li, H. Wang, Y. Kawakita, Q. Zhang, M. Feygenson, H. L. Yu, D. Wu, K. Ohara, T. Kikuchi, K. Shibata, T. Yamada, X. K. Ning, Y. Chen, J. Q. He, David Vaknin, R. Q. Wu, K. Nakajima, M. G. Kanatzidis

Ames Laboratory Accepted Manuscripts

As a generic property, all substances transfer heat through microscopic collisions of constituent particles1. A solid conducts heat through both transverse and longitudinal acoustic phonons, but a liquid employs only longitudinal vibrations2,3. As a result, a solid is usually thermally more conductive than a liquid. In canonical viewpoints, such a difference also serves as the dynamic signature distinguishing a solid from a liquid. Here, we report liquid-like thermal conduction observed in the crystalline AgCrSe2. The transverse acoustic phonons are completely suppressed by the ultrafast dynamic disorder while the longitudinal acoustic phonons are strongly scattered but survive, and are thus ...


Uncovering The Origin Of Divergence In The Csm(Cro4)2 (M = La, Pr, Nd, Sm, Eu; Am) Family Through Examination Of The Chemical Bonding In A Molecular Cluster And By Band Structure Analysis, Shane S. Galley, Alexandra A. Arico, Tsung-Han Lee, Xiaoyu Deng, Yong-Xin Yao, Joseph M. Sperling, Vanessa Proust, Julia S. Storbeck, Vladimir Dobrosavljevic, Jennifer N. Neu, Theo Siegrist, Ryan E. Baumbach, Thomas E. Albrecht-Schmitt, Nikolas Kaltsoyannis, Nicola Lanata Feb 2018

Uncovering The Origin Of Divergence In The Csm(Cro4)2 (M = La, Pr, Nd, Sm, Eu; Am) Family Through Examination Of The Chemical Bonding In A Molecular Cluster And By Band Structure Analysis, Shane S. Galley, Alexandra A. Arico, Tsung-Han Lee, Xiaoyu Deng, Yong-Xin Yao, Joseph M. Sperling, Vanessa Proust, Julia S. Storbeck, Vladimir Dobrosavljevic, Jennifer N. Neu, Theo Siegrist, Ryan E. Baumbach, Thomas E. Albrecht-Schmitt, Nikolas Kaltsoyannis, Nicola Lanata

Ames Laboratory Accepted Manuscripts

A series of f-block chromates, CsM(CrO4)(2) (M = La, Pr, Nd, Sm, Eu; Am), were prepared revealing notable differences between the Am-III derivatives and their lanthanide analogs. While all compounds form similar layered structures, the americium compound exhibits polymorphism and adopts both a structure isomorphous with the early lanthanides as well as one that possesses lower symmetry. Both polymorphs are dark red and possess band gaps that are smaller than the Ln(III) compounds. In order to probe the origin of these differences, the electronic structure of alpha-CsSm(CrO4)(2), alpha-CsEu(CrO4)(2), and alpha-CsAm(CrO4)(2) were studied ...


Different Topological Quantum States In Ternary Zintl Compounds: Bacax (X = Si, Ge, Sn And Pb), Lin-Lin Wang, Adam Kaminski, Paul C. Canfield, Duane D. Johnson Jan 2018

Different Topological Quantum States In Ternary Zintl Compounds: Bacax (X = Si, Ge, Sn And Pb), Lin-Lin Wang, Adam Kaminski, Paul C. Canfield, Duane D. Johnson

Ames Laboratory Accepted Manuscripts

Topological quantum states require stringent combination of crystal symmetry and spin–orbit coupling (SOC) strength. Here, we report that the ternary Zintl compound series BaCaX (X = Si, Ge, Sn and Pb, Group IV) in the same crystal structure having eight valence electrons per formula unit can host two different topological quantum phases, controlled by atomic size and SOC strength. BaCaSi is a nodal-line semimetal (NLSM) with band inversion protected by mirror symmetry and hosts a strong topological insulator (TI) state when SOC is turned on, thus, a NLSM-TI phase. Moving to larger atomic sizes and heavier atoms, BaCaGe and BaCaSn ...


Interpolymer Complexation As A Strategy For Nanoparticle Assembly And Crystallization, Srikanth Nayak, Nathan Horst, Honghu Zhang, Wenjie Wang, Surya Mallapragada, Alex Travesset, David Vaknin Jan 2018

Interpolymer Complexation As A Strategy For Nanoparticle Assembly And Crystallization, Srikanth Nayak, Nathan Horst, Honghu Zhang, Wenjie Wang, Surya Mallapragada, Alex Travesset, David Vaknin

Chemical and Biological Engineering Publications

Controlled self-assembly of nanoparticles into ordered structures is a major step in fabricating nanotechnology based devices. Here, we report on the self-assembly of high quality superlattices of nanoparticles in aqueous suspensions induced via interpolymer complexation. Using small angle X-ray scattering, we demonstrate that the NPs crystallize into superlattices of FCC symmetry, initially driven by hydrogen bonding and subsequently by van der Waals forces between the complexed coronas of hydrogen-bonded polymers. We show that the lattice constant and crystal quality can be tuned by polymer concentration, suspension pH and the length of polymer chains. Interpolymer complexation to assemble nanoparticles is scalable ...


Chemical Intuition For High Thermoelectric Performance In Monolayer Black Phosphorus, Α-Arsenene And Aw-Antimonene, Bo Peng, Hao Zhang, Hezhu Shao, Ke Xu, Gang Ni, Jing Li, Heyuan Zhu, Costas M. Soukoulis Jan 2018

Chemical Intuition For High Thermoelectric Performance In Monolayer Black Phosphorus, Α-Arsenene And Aw-Antimonene, Bo Peng, Hao Zhang, Hezhu Shao, Ke Xu, Gang Ni, Jing Li, Heyuan Zhu, Costas M. Soukoulis

Ames Laboratory Accepted Manuscripts

Identifying materials with intrinsically high thermoelectric performance remains a challenge even with the aid of a high-throughput search. Here, using a chemically intuitive approach based on the bond-orbital theory, three anisotropic 2D group-V materials (monolayer black phosphorus, alpha-arsenene, and aW-antimonene) are identified as candidates for high thermoelectric energy conversion efficiency. Concepts, such as bond length, bond angle, and bond strength, are used to explain the trends in their electronic properties, such as the band gap and the effective mass. Our first principles calculations confirm that high carrier mobilities and large Seebeck coefficients can be obtained at the same time in ...


A Molecular Debye-Hückel Theory Of Solvation In Polar Fluids: An Extension Of The Born Model, Tiejun Xiao, Xueyu Song Dec 2017

A Molecular Debye-Hückel Theory Of Solvation In Polar Fluids: An Extension Of The Born Model, Tiejun Xiao, Xueyu Song

Ames Laboratory Accepted Manuscripts

A dielectric response theory of solvation beyond the conventional Born model for polar fluids is presented. The dielectric response of a polar fluid is described by a Born response mode and a linear combination of Debye-Hückel-like response modes that capture the nonlocal response of polar fluids. The Born mode is characterized by a bulk dielectric constant, while a Debye-Hückel mode is characterized by its corresponding Debye screening length. Both the bulk dielectric constant and the Debye screening lengths are determined from the bulk dielectric function of the polar fluid. The linear combination coefficients of the response modes are evaluated in ...


Electronic, Magnetic, And Magnetocrystalline Anisotropy Properties Of Light Lanthanides, T. A. Hackett, D. J. Baldwin, Durga Paudyal Nov 2017

Electronic, Magnetic, And Magnetocrystalline Anisotropy Properties Of Light Lanthanides, T. A. Hackett, D. J. Baldwin, Durga Paudyal

Ames Laboratory Accepted Manuscripts

Theoretical understanding of interactions between localized and mobile electrons and the crystal environment in light lanthanides is important because of their key role in much needed magnetic anisotropy in permanent magnet materials that have a great impact in automobile and wind turbine applications. We report electronic, magnetic, and magnetocrystalline properties of these basic light lanthanide elements studied from advanced density functional theory (DFT) calculations. We find that the inclusion of onsite 4f electron correlation and spin orbit coupling within the full-potential band structure is needed to understand the unique magnetocrystalline properties of these light lanthanides. The onsite electron correlation, spin ...


Reorientations, Relaxations, Metastabilities, And Multidomains Of Skyrmion Lattices, L. J. Bannenberg, F. Qian, R. M. Dalgliesh, N. Martin, G. Chaboussant, M. Schmidt, Deborah L. Schlagel, Thomas A. Lograsso, Diamond Light Source Ltd., C. Pappas Nov 2017

Reorientations, Relaxations, Metastabilities, And Multidomains Of Skyrmion Lattices, L. J. Bannenberg, F. Qian, R. M. Dalgliesh, N. Martin, G. Chaboussant, M. Schmidt, Deborah L. Schlagel, Thomas A. Lograsso, Diamond Light Source Ltd., C. Pappas

Ames Laboratory Accepted Manuscripts

Magnetic skyrmions are nanosized topologically protected spin textures with particlelike properties. They can form lattices perpendicular to the magnetic field, and the orientation of these skyrmion lattices with respect to the crystallographic lattice is governed by spin-orbit coupling. By performing small-angle neutron scattering measurements, we investigate the coupling between the crystallographic and skyrmion lattices in both Cu2OSeO3 and the archetype chiral magnet MnSi. The results reveal that the orientation of the skyrmion lattice is primarily determined by the magnetic field direction with respect to the crystallographic lattice. In addition, it is also influenced by the magnetic history of the sample ...


Combined Measurement Of Directional Raman Scattering And Surface-Plasmon-Polariton Cone From Adsorbates On Smooth Planar Gold Surfaces, Charles K.A. Nyamekye, Surface Photonics, Inc., Jonathan M. Bobbitt, Emily A. Smith Oct 2017

Combined Measurement Of Directional Raman Scattering And Surface-Plasmon-Polariton Cone From Adsorbates On Smooth Planar Gold Surfaces, Charles K.A. Nyamekye, Surface Photonics, Inc., Jonathan M. Bobbitt, Emily A. Smith

Ames Laboratory Accepted Manuscripts

Directional-surface-plasmon-coupled Raman scattering (directional RS) has the combined benefits of surface plasmon resonance and Raman spectroscopy, and provides the ability to measure adsorption and monolayer-sensitive chemical information. Directional RS is performed by optically coupling a 50-nm gold film to a Weierstrass prism in the Kretschmann configuration and scanning the angle of the incident laser under total internal reflection. The collected parameters on the prism side of the interface include a full surface-plasmon-polariton cone and the full Raman signal radiating from the cone as a function of incident angle. An instrument for performing directional RS and a quantitative study of the ...


Interfacial Self-Assembly Of Polyelectrolyte-Capped Gold Nanoparticles, Honghu Zhang, Srikanth Nayak, Wenjie Wang, Surya K Mallapragada, David Vaknin Oct 2017

Interfacial Self-Assembly Of Polyelectrolyte-Capped Gold Nanoparticles, Honghu Zhang, Srikanth Nayak, Wenjie Wang, Surya K Mallapragada, David Vaknin

Ames Laboratory Accepted Manuscripts

We report on pH- and salt-responsive assembly of nanoparticles capped with polyelectrolytes at vapor–liquid interfaces. Two types of alkylthiol-terminated poly(acrylic acid) (PAAs, varying in length) are synthesized and used to functionalize gold nanoparticles (AuNPs) to mimic similar assembly effects of single-stranded DNA-capped AuNPs using synthetic polyelectrolytes. Using surface-sensitive X-ray scattering techniques, including grazing incidence small-angle X-ray scattering (GISAXS) and X-ray reflectivity (XRR), we demonstrate that PAA-AuNPs spontaneously migrate to the vapor–liquid interfaces and form Gibbs monolayers by decreasing the pH of the suspension. The Gibbs monoalyers show chainlike structures of monoparticle thickness. The pH-induced self-assembly is attributed ...


Nuclear Magnetic Resonance Probe Head Design For Precision Strain Control, T. Kissikov, R. Sarkar, B. T. Bush, Paul C. Canfield, N. J. Curro Oct 2017

Nuclear Magnetic Resonance Probe Head Design For Precision Strain Control, T. Kissikov, R. Sarkar, B. T. Bush, Paul C. Canfield, N. J. Curro

Ames Laboratory Accepted Manuscripts

We present the design and construction of an NMR probe to investigate single crystals under strain at cryogenic temperatures. The probe head incorporates a piezoelectric-based apparatus from Razorbill Instruments that enables both compressive and tensile strain tuning up to strain values on the order of 0.3% with a precision of 0.001%. As NMR in BaFe2As2 reveals large changes to the electric field gradient and indicates that the strain is homogeneous to within 16% over the volume of the NMR coil.


Near-Infrared And Optical Beam Steering And Frequency Splitting In Air-Holes-In-Silicon Inverse Photonic Crystals, Anna C. Tasolamprou, Thomas Koschny, Maria Kafesaki, Costas M. Soukoulis Sep 2017

Near-Infrared And Optical Beam Steering And Frequency Splitting In Air-Holes-In-Silicon Inverse Photonic Crystals, Anna C. Tasolamprou, Thomas Koschny, Maria Kafesaki, Costas M. Soukoulis

Ames Laboratory Accepted Manuscripts

We present the design of a dielectric inverse photonic crystal structure that couples line-defect waveguide propagating modes into highly directional beams of controllable directionality. The structure utilizes a triangular lattice made of air holes drilled in an infinitely thick Si slab, and it is designed for operation in the near-infrared and optical regime. The structure operation is based on the excitation and manipulation of dark dielectric surface states, in particular on the tailoring of the dark states’ coupling to outgoing radiation. This coupling is achieved with the use of properly designed external corrugations. The structure adapts and matches modes that ...


On Magnetic Structure Of Cufe2ge2: Constrains From The 57fe Mössbauer Spectroscopy, Sergey L. Bud’Ko, Na Hyun Jo, Savannah S. Downing, Paul C. Canfield Sep 2017

On Magnetic Structure Of Cufe2ge2: Constrains From The 57fe Mössbauer Spectroscopy, Sergey L. Bud’Ko, Na Hyun Jo, Savannah S. Downing, Paul C. Canfield

Ames Laboratory Accepted Manuscripts

57Fe Mössbauer spectroscopy measurements were performed on a powdered CuFe2Ge2 sample that orders antiferromagnetically at ∼175 K. Whereas a paramagnetic doublet was observed above the Néel temperature, a superposition of paramagnetic doublet and magnetic sextet (in approximately 0.5:0.5 ratio) was observed in the magnetically ordered state, suggesting a magnetic structure similar to a double-Q spin density wave with half of the Fe paramagnetic and another half bearing static moment of ∼0.5-1μB. These results call for a re-evaluation of the recent neutron scattering data and band structure calculations, as well as for deeper examination of details of ...


Thermally Activated Diffusion Of Copper Into Amorphous Carbon, David Appy, Mark Wallingford, Dapeng Jing, Ryan T. Ott, Michael C. Tringides, Gunther Richter, Patricia A. Thiel Jul 2017

Thermally Activated Diffusion Of Copper Into Amorphous Carbon, David Appy, Mark Wallingford, Dapeng Jing, Ryan T. Ott, Michael C. Tringides, Gunther Richter, Patricia A. Thiel

Ames Laboratory Accepted Manuscripts

Using x-ray photoelectron spectroscopy, the authors characterize the thermally activated changes that occur when Cu is deposited on amorphous carbon supported on Si at 300 K, then heated to 800 K. The authors compare data for Cu on the basal plane of graphite with pinning defects, where scanning tunneling microscopy reveals that coarsening is the main process in this temperature range. Coarsening begins at 500–600 K and causes moderate attenuation of the Cu photoelectron signal. For Cu on amorphous carbon, heating to 800 K causes Cu to diffuse into the bulk of the film, based on the strong attenuation ...


A Molecular Debye-Hückel Theory And Its Applications To Electrolyte Solutions: The Size Asymmetric Case, Tiejen Xiao, Xueyu Song Mar 2017

A Molecular Debye-Hückel Theory And Its Applications To Electrolyte Solutions: The Size Asymmetric Case, Tiejen Xiao, Xueyu Song

Chemistry Publications

A molecular Debye-Hückel theory for electrolyte solutions with size asymmetry is developed, where the dielectric response of an electrolyte solution is described by a linear combination of Debye-Hückel-like response modes. As the size asymmetry of an electrolyte solution leads to a charge imbalanced border zone around a solute, the dielectric response to the solute is characterized by two types of charge sources, namely, a bare solute charge and a charge distribution due to size asymmetry. These two kinds of charge sources are screened by the solvent differently, our theory presents a method to calculate the mean electric potential as well ...


Growth And Characterization Of Baznga, Na Hyun Jo, Qisheng Lin, Udhara S. Kaluarachchi, William R. Meier, Soham Manni, Savannah S. Downing, Anna E. Böhmer, Tai Kong, Yang Sun, Valentin Taufour, Cai-Zhuang Wang, Kai-Ming Ho, Sergey L. Bud’Ko, Paul C. Canfield Jan 2017

Growth And Characterization Of Baznga, Na Hyun Jo, Qisheng Lin, Udhara S. Kaluarachchi, William R. Meier, Soham Manni, Savannah S. Downing, Anna E. Böhmer, Tai Kong, Yang Sun, Valentin Taufour, Cai-Zhuang Wang, Kai-Ming Ho, Sergey L. Bud’Ko, Paul C. Canfield

Ames Laboratory Accepted Manuscripts

We report the growth, structure and characterization of BaZnGa, identifying it as the sole known ternary compound in the Ba–Zn–Ga system. Single crystals of BaZnGa can be grown out of excess Ba–Zn and adopt a tI36 structure type. There are three unique Ba sites and three M = Zn/Ga sites. Using DFT calculations we can argue that whereas one of these three M sites is probably solely occupied by Ga, the other two M sites, most likely, have mixed Zn/Ga occupancy. Temperature-dependent resistivity and magnetization measurements suggest that BaZnGa is a poor metal with no electronic ...


125te Nmr And Seebeck Effect In Bi2te3 Synthesized From Stoichiometric And Te-Rich Melts, E. M. Levin, Trevor M. Riedemann, A. Howard, Na Hyun Jo, Sergey L. Bud’Ko, Paul C. Canfield, Thomas A. Lograsso Nov 2016

125te Nmr And Seebeck Effect In Bi2te3 Synthesized From Stoichiometric And Te-Rich Melts, E. M. Levin, Trevor M. Riedemann, A. Howard, Na Hyun Jo, Sergey L. Bud’Ko, Paul C. Canfield, Thomas A. Lograsso

Materials Science and Engineering Publications

Bi2Te3 is a well-known thermoelectric material and, as a new form of quantum matter, a topological insulator. Variation of local chemical composition in Bi2Te3 results in formation of several types of atomic defects, including Bi and Te vacancies and Bi and Te antisite defects; these defects can strongly affect material functionality via generation of free electrons and/or holes. Nonuniform distribution of atomic defects produces electronic inhomogeneity, which can be detected by 125Te nuclear magnetic resonance (NMR). Here we report on 125Te NMR and Seebeck effect (heat to electrical energy conversion) for two single crystalline samples: (#1) grown from stoichiometric ...


Comparison Of S-Adsorption On (111) And (100) Facets Of Cu Nanoclusters, Jeffrey S. Boschen, Jiyoung Lee, Theresa L. Windus, James W. Evans, Patricia A. Thiel, Da-Jiang Liu Oct 2016

Comparison Of S-Adsorption On (111) And (100) Facets Of Cu Nanoclusters, Jeffrey S. Boschen, Jiyoung Lee, Theresa L. Windus, James W. Evans, Patricia A. Thiel, Da-Jiang Liu

Physics and Astronomy Publications

In order to gain insight into the nature of chemical bonding of sulfur atoms on coinage metal surfaces, we compare the adsorption energy and structural parameters for sulfur at four-fold hollow (4fh) sites on (100) facets and at three-fold hollow (3fh) sites on (111) facets of Cu nanoclusters. Consistent results are obtained from localized atomic orbital and plane-wave based density functional theory using the same functionals. PBE and its hybrid counterpart (PBE0 or HSE06) also give similar results. 4fh sites are preferred over 3fh sites with stronger bonding by ∼0.6 eV for nanocluster sizes above ∼280 atoms. However, for ...


What Is The Best Method To Fit Time-Resolved Data? A Comparison Of The Residual Minimization And The Maximum Likelihood Techniques As Applied To Experimental Time-Correlated, Single-Photon Counting Data, Kalyan Santra, Jinchun Zhan, Xueyu Song, Emily A. Smith, Namrata Vaswani, Jacob W. Petrich Feb 2016

What Is The Best Method To Fit Time-Resolved Data? A Comparison Of The Residual Minimization And The Maximum Likelihood Techniques As Applied To Experimental Time-Correlated, Single-Photon Counting Data, Kalyan Santra, Jinchun Zhan, Xueyu Song, Emily A. Smith, Namrata Vaswani, Jacob W. Petrich

Chemistry Publications

The need for measuring fluorescence lifetimes of species in subdiffraction-limited volumes in, for example, stimulated emission depletion (STED) microscopy, entails the dual challenge of probing a small number of fluorophores and fitting the concomitant sparse data set to the appropriate excited-state decay function. This need has stimulated a further investigation into the relative merits of two fitting techniques commonly referred to as “residual minimization” (RM) and “maximum likelihood” (ML). Fluorescence decays of the well-characterized standard, rose bengal in methanol at room temperature (530 ± 10 ps), were acquired in a set of five experiments in which the total number of “photon ...


Sulfur-Induced Structural Motifs On Copper And Gold Surfaces, Holly Walen Jan 2016

Sulfur-Induced Structural Motifs On Copper And Gold Surfaces, Holly Walen

Graduate Theses and Dissertations

The interaction of sulfur with copper and gold surfaces plays a fundamental role in important phenomena that include coarsening of surface nanostructures, and self-assembly of alkanethiols. Here, we identify and analyze unique sulfur-induced structural motifs observed on the low-index surfaces of these two metals. We seek out these structures in an effort to better understand the fundamental interactions between these metals and sulfur that lends to the stability and favorability of metal-sulfur complexes vs. chemisorbed atomic sulfur.

We choose very specific conditions: very low temperature (5 K), and very low sulfur coverage (≤ 0.1 monolayer). In this region of temperature-coverage ...


Discontinuous Non-Equilibrium Phase Transition In A Threshold Schloegl Model For Autocatalysis: Generic Two-Phase Coexistence And Metastability, Chi-Jen Wang, Da-Jiang Liu, James W. Evans Apr 2015

Discontinuous Non-Equilibrium Phase Transition In A Threshold Schloegl Model For Autocatalysis: Generic Two-Phase Coexistence And Metastability, Chi-Jen Wang, Da-Jiang Liu, James W. Evans

Physics and Astronomy Publications

Threshold versions of Schloegl's model on a lattice, which involve autocatalytic creation and spontaneous annihilation of particles, can provide a simple prototype for discontinuous non-equilibrium phase transitions. These models are equivalent to so-called threshold contact processes. A discontinuous transition between populated and vacuum states can occur selecting a threshold of N ≥ 2 for the minimum number, N, of neighboring particles enabling autocatalytic creation at an empty site. Fundamental open questions remain given the lack of a thermodynamic framework for analysis. For a square lattice with N = 2, we show that phase coexistence occurs not at a unique value but ...


Platinum Nanoparticle During Electrochemical Hydrogen Evolution: Adsorbate Distribution, Active Reaction Species, And Size Effect, Teck L. Tan, Lin-Lin Wang, Jia Zhang, Duane D. Johnson, Kewu Bai Mar 2015

Platinum Nanoparticle During Electrochemical Hydrogen Evolution: Adsorbate Distribution, Active Reaction Species, And Size Effect, Teck L. Tan, Lin-Lin Wang, Jia Zhang, Duane D. Johnson, Kewu Bai

Ames Laboratory Publications

For small Pt nanoparticles (NPs), catalytic activity is, as observed, adversely affected by size in the 1–3 nm range. We elucidate, via first-principles-based thermodynamics, the operation H* distribution and cyclic voltammetry (CV) during the hydrogen evolution reaction (HER) across the electrochemical potential, including the underpotential region (U ≤ 0) that is difficult to assess in experiment. We consider multiple adsorption sites on a 1 nm Pt NP model and show that the characteristic CV peaks from different H* species correspond well to experiment. We next quantify the activity contribution from each H* species to explain the adverse effect of size ...


Crystal Structure Of The Alcanivorax Borkumensis Ydah Transporter Reveals An Unusual Topology, Jani Reddy Bolla, Chih-Chia Su, Jared A. Delmar, Pattathil Radhakrishnan, Nitin Kumar, Tsung-Han Chou, Feng Long, Kanagalaghatta R. Rajashankar, Edward Yu Jan 2015

Crystal Structure Of The Alcanivorax Borkumensis Ydah Transporter Reveals An Unusual Topology, Jani Reddy Bolla, Chih-Chia Su, Jared A. Delmar, Pattathil Radhakrishnan, Nitin Kumar, Tsung-Han Chou, Feng Long, Kanagalaghatta R. Rajashankar, Edward Yu

Physics and Astronomy Publications

The potential of the folic acid biosynthesis pathway as a target for the development of antibiotics has been clinically validated. However, many pathogens have developed resistance to these antibiotics, prompting a re-evaluation of potential drug targets within the pathway. The ydaH gene of Alcanivorax borkumensis encodes an integral membrane protein of the AbgT family of transporters for which no structural information was available. Here we report the crystal structure of A. borkumensis YdaH, revealing a dimeric molecule with an architecture distinct from other families of transporters. YdaH is a bowl-shaped dimer with a solvent-filled basin extending from the cytoplasm to ...


Configurational Thermodynamics Of Alloyed Nanoparticles With Adsorbates, Lin-Lin Wang, Teck L. Tan, Duane D. Johnson Nov 2014

Configurational Thermodynamics Of Alloyed Nanoparticles With Adsorbates, Lin-Lin Wang, Teck L. Tan, Duane D. Johnson

Ames Laboratory Publications

Changes in the chemical configuration of alloyed nanoparticle (NP) catalysts induced by adsorbates under working conditions, such as reversal in core–shell preference, are crucial to understand and design NP functionality. We extend the cluster expansion method to predict the configurational thermodynamics of alloyed NPs with adsorbates based on density functional theory data. Exemplified with PdRh NPs having O-coverage up to a monolayer, we fully detail the core–shell behavior across the entire range of NP composition and O-coverage with quantitative agreement to in situ experimental data. Optimally fitted cluster interactions in the heterogeneous system are the key to enable ...


Statistical Mechanical Models For Dissociative Adsorption Of O2 On Metal(100) Surfaces With Blocking, Steering, And Funneling, Da-Jiang Liu, James W. Evans May 2014

Statistical Mechanical Models For Dissociative Adsorption Of O2 On Metal(100) Surfaces With Blocking, Steering, And Funneling, Da-Jiang Liu, James W. Evans

Physics and Astronomy Publications

We develop statistical mechanical models amenable to analytic treatment for the dissociative adsorption of O2 at hollow sites on fcc(100) metal surfaces. The models incorporate exclusion of nearest-neighbor pairs of adsorbed O. However, corresponding simple site-blocking models, where adsorption requires a large ensemble of available sites, exhibit an anomalously fast initial decrease in sticking. Thus, in addition to blocking, our models also incorporate more facile adsorption via orientational steering and funneling dynamics (features supported by ab initio Molecular Dynamics studies). Behavior for equilibrated adlayers is distinct from those with finite adspecies mobility. We focus on the low-temperature limited-mobility regime ...


Taking Advantage Of Gold’S Electronegativity In R4mn3–Xau10+X (R = Gd Or Y; 0.2 ≤ X ≤ 1), Saroj L. Samal, Abhishek Pandey, D. C. Johnston, John D. Corbett, Gordon J. Miller Jan 2014

Taking Advantage Of Gold’S Electronegativity In R4mn3–Xau10+X (R = Gd Or Y; 0.2 ≤ X ≤ 1), Saroj L. Samal, Abhishek Pandey, D. C. Johnston, John D. Corbett, Gordon J. Miller

Chemistry Publications

Ternary R4Mn3–xAu10+x (R = Gd or Y; 0.2 ≤ x ≤ 1) compounds have been synthesized and characterized using single-crystal X-ray diffraction. The structure is a ternary variant of orthorhombic Zr7Ni10 (oC68, space group Cmca) and is isostructural with Ca4In3Au10. The structure contains layers of Mn-centered rectangular prisms of gold (Mn@Au8), interbonded via Au atoms in the b-c plane, and stacked in a hexagonal close packed arrangement along the a direction. These layers are bonded via additional Mn atoms along the a direction. The ...


Controlling Reactivity Of Nanoporous Catalyst Materials By Tuning Reaction Product-Pore Interior Interactions: Statistical Mechanical Modeling, Jing Wang, David M. Ackerman, Victor S.-Y. Lin, Marek Pruski, James W. Evans Jan 2013

Controlling Reactivity Of Nanoporous Catalyst Materials By Tuning Reaction Product-Pore Interior Interactions: Statistical Mechanical Modeling, Jing Wang, David M. Ackerman, Victor S.-Y. Lin, Marek Pruski, James W. Evans

Physics and Astronomy Publications

Statistical mechanical modeling is performed of a catalytic conversion reaction within a functionalized nanoporous material to assess the effect of varying the reaction product-pore interior interaction from attractive to repulsive. A strong enhancement in reactivity is observed not just due to the shift in reaction equilibrium towards completion but also due to enhanced transport within the pore resulting from reduced loading. The latter effect is strongest for highly restricted transport (single-file diffusion), and applies even for irreversible reactions. The analysis is performed utilizing a generalized hydrodynamic formulation of the reaction-diffusion equations which can reliably capture the complex interplay between reaction ...