Physical Sciences and Mathematics Commons^™

Stoichiometric Determination Of Hydride Materials At Extreme Conditions, Gregory Alexander Smith

UNLV Theses, Dissertations, Professional Papers, and Capstones

Hydrogen was predicted to be a high-temperature superconductor at near-megabar conditions in 1968,[1] but only recently was been experimentally observed.[2] This is due to the extraneous metrological constraint of requiring 5 megabars of pressure to stabilize. A more practical approach for synthesis of high-temperature superconductors has been pro-posed through the use of hydride compounds. Recently, a surge of rare earth hydrides have achieved critical superconducting transition temperatures (T_C ) close to room temperature.[3, 4, 5, 6] However, due to limitations of the necessary instrumentation to achieve megabar pressures, many techniques traditionally used to measure stoichiometry are unavailable.Three works presented in …

Controllable Co2 Electrocatalytic Reduction Via Ferroelectric Switching On Single Atom Anchored In2se3 Monolayer, Lin Ju, Xin Tan, Xin Mao, Yuantong Gu, Sean Smith, Aijun Du, Zhongfang Chen, Changfeng Chen, Liangzhi Kou

Physics & Astronomy Faculty Research

Efficient and selective CO2 electroreduction into chemical fuels promises to alleviate environmental pollution and energy crisis, but it relies on catalysts with controllable product selectivity and reaction path. Here, by means of first-principles calculations, we identify six ferroelectric catalysts comprising transition-metal atoms anchored on In2Se3 monolayer, whose catalytic performance can be controlled by ferroelectric switching based on adjusted d-band center and occupation of supported metal atoms. The polarization dependent activation allows effective control of the limiting potential of CO2 reduction on TM@In2Se3 (TM = Ni, Pd, Rh, Nb, and Re) as well as the reaction paths and final products on …

Singlet Fission In Acene Blends, Katharina Broch, Clemens Zeiser, Chad Cruz, Luca Moretti, Margherita Maiuri, Eric Chronister, David Reichman, Roel Tempelaar, Giulio Cerullo, Christopher Bardeen

Chemistry and Biochemistry Faculty Research

Singlet fission (SF), the photophysical process converting an excited singlet exciton into two triplet excitons, is a promising approach to boost solar cell efficiencies. It is controlled by various parameters such as intermolecular interactions, energetics, entropy and vibronic coupling and a controlled modification of these parameters is key to a fundamental understanding. Blends of organic semiconductors present an interesting alternative to established methods of chemical functionalization and their potential for the study of SF pathways will be discussed using acene blends as example. Mixed thin films of SF chromophores and weakly interacting, high-bandgap spacer molecules allow one to study the …

Recovery Of High Specific Activity Molybdenum-99 From Accelerator-Induced Fission On Low-Enriched Uranium For Technetium-99m Generators, M. Alex Brown, Nathan Johnson, Artem V. Gelis, Milan Stika, Anna G. Servis, Alex Bakken, Christine Krizmanich, Kristin Shannon, Peter Kozak, Amanda Barnhart, Chad Denbrock, Nicolas Luciani, Terry Grimm, Peter Tkac

Chemistry and Biochemistry Faculty Research

A new process was developed to recover high specific activity (no carrier added) 99Mo from electron-accelerator irradiated U3O8 or uranyl sulfate targets. The process leverages a novel solvent extraction scheme to recover Mo using di(2-ethylhexyl) phosphoric acid following uranium and transuranics removal with tri-n-butyl phosphate. An anion-exchange concentration column step provides a final purification, generating pure 99Mo intended for making 99Mo/99mTc generators. The process was demonstrated with irradiated uranium targets resulting in more than 95% 99Mo recovery and without presence of fission products or actinides in the product.

Β -Technetium: An Allotrope With A Nonstandard Volume-Pressure Relationship, Emily Siska, Dean Smith, Christian Childs, Daniel Koury, Paul M. Forster, Keith V. Lawler, Ashkan Salamat

Chemistry and Biochemistry Faculty Research

We report the synthesis and structure of the second allotrope of technetium, β-Tc. Transformative pathways are accessed at extreme conditions using the laser-heated diamond anvil cell and confirmed with in situ synchrotron x-ray diffraction and Raman spectroscopy. β-Tc is fully recoverable to ambient conditions, although counter to our DFT calculations predicting a face-centered-cubic lattice, we observe a tetragonal structure (I4/mmm) that exhibits further tetragonal distortion with pressure. β-Tc has an expanded volume relative to the hcp ground state phase, that when doped with nitrogen has an unexpected volume lowering. Such anomalous behavior is possibly indicative of a rare electronic phase …

Colossal Density-Driven Resistance Response In The Negative Charge Transfer Insulator Mns2, Dylan Durkee, Nathan Dasenbrock-Gammon, G. Alexander Smith, Elliot Snider, Dean Smith, Christian Childs, Simon A.J. Kimber, Keith V. Lawler, Ranga P. Dias, Ashkan Salamat

Chemistry and Biochemistry Faculty Research

A reversible density driven insulator to metal to insulator transition in high-spin MnS2 is experimentally observed, leading with a colossal electrical resistance drop of 108 ω by 12 GPa. Density functional theory simulations reveal the metallization to be unexpectedly driven by previously unoccupied S22- σ3p∗ antibonding states crossing the Fermi level. This is a unique variant of the charge transfer insulator to metal transition for negative charge transfer insulators having anions with an unsaturated valence. By 36 GPa the emergence of the low-spin insulating arsenopyrite (P21/c) is confirmed, and the bulk metallicity is broken with the system returning to an …

Poly(Pyridinium Salt)S Containing 2,7-Diamino-9,9'-Dioctylfluorene Moieties With Various Organic Counterions Exhibiting Both Lyotropic Liquid-Crystalline And Light-Emitting Properties, Pradip K. Bhowmik, Tae S. Jo, Jung J. Koh, Jongwon Park, Bidyut Biswas, Ronald Carlo G. Principe, Haesook Han, András F. Wacha, Matti Knaapila

Chemistry and Biochemistry Faculty Research

A series of poly(pyridinium salt)s-fluorene main-chain ionic polymers with various organic counterions were synthesized by using ring-transmutation polymerization and metathesis reactions. Their chemical structures were characterized by Fourier Transform Infrared (FTIR), proton (1H), and fluorine 19 (19F) nuclear magnetic resonance (NMR) spectrometers. These polymers showed a number-average molecular weight (Mns) between 96.5 and 107.8 kg/mol and polydispersity index (PDI) in the range of 1.12-1.88. They exhibited fully-grown lyotropic phases in polar protic and aprotic solvents at different critical concentrations. Small-angle X-ray scattering for one polymer example indicates lyotropic structure formation for 60-80% solvent fraction. A lyotropic smectic phase contains 10 …

Coexistence Of Metamagnetism And Slow Relaxation Of Magnetization In Ammonium Hexafluoridorhenate, James Louis-Jean, Samundeeswari M. Balasekaran, Keith V. Lawler, Adrián Sanchis-Perucho, José Martínez-Lillo, Dean Smith, Paul M. Forster, Ashkan Salamat, Frederic Poineau

Chemistry and Biochemistry Faculty Research

© The Royal Society of Chemistry 2021. The (NH4)2[ReF6] (1) salt was studied by X-ray diffraction, Raman spectroscopy, theoretical calculations, and magnetic measurements.1crystallizes in the trigonal space groupP3̄m1 (Re-F = 1.958(5) Å). In the Raman spectrum of1, splitting of the observed peaks was observed and correlated to the valence frequencies of vibration of the [ReF6]2−anion. The study of the magnetic properties of1, through DC and AC magnetic susceptibility measurements, reveals the coexistence of metamagnetism and slow relaxation of magnetization at low temperature, which is unusual in the molecular systems based on the paramagnetic 5d metal ions reported so far.

Developments Of Machine Learning Potentials For Atomistic Simulations, Howard Yanxon

UNLV Theses, Dissertations, Professional Papers, and Capstones

Atomistic modeling methods such as molecular dynamics play important roles in investigating time-dependent physical and chemical processes at the microscopic level. In the simulations, energy and forces, sometimes including stress tensor, need to be recalculated iteratively as the atomic configuration evolves. Consequently, atomistic simulations crucially depend on the accuracy of the underlying potential energy surface. Modern quantum mechanical modeling based on density functional theory can consistently generate an accurate description of the potential energy surface. In most cases, molecular dynamics simulations based on density functional theory suffer from highly demanding computational costs. On the other hand, atomistic simulations based on …

Investigations Of Technetium Metal And The Synthesis Of Binary Technetium Nitrides Under Extreme Conditions, Emily Siska

UNLV Theses, Dissertations, Professional Papers, and Capstones

Technetium (Tc) is the lightest of the radioactive elements and has no stable isotopes. Significant quantities of Tc are not naturally occurring on earth. However, technetium is found in high fission yield in nuclear reactors and produced for medical imaging. With its long half life, and high mobility in the environment make it of particular interest. To that end, the fundamental chemistry of Tc and Tc compounds is not as well understood compared to neighboring elements on the periodic table. Therefore, fundamental studies designed to better understand this transition metal, coupled with more targeted investigation at high temperature and pressure …

Chemical And Electronic Surface Structure Of Chalcopyrite-Based Thin Films For Solar Water Splitting, James C. Carter

UNLV Theses, Dissertations, Professional Papers, and Capstones

In recent years, various thin film solar devices have reached markedly high efficiencies on both the laboratory and large area scale. To further evaluate their potential, and help drive device optimization of efficient solar devices, a detailed understanding of the chemical and electronic structure of the surfaces and interfaces is required. It is these interfaces that play a pivotal role in dictating aspects of device performance. Chalcopyrite-based materials, such as Cu(In,Ga)S2 (CIGS) are regarded as one of the most promising absorber materials for use in highly efficient solar devices. In the context of photoelectrochemical (PEC) hydrogen generation, the tunability of …

Nanopatterned Silk Fibroin Films With High Transparency And High Haze For Optical Applications, Corey Malinowski, Fengjie He, Yihong Zhao, Ivan Chang, David W. Hatchett, Shengjie Zhai, Hui Zhao

Chemistry and Biochemistry Faculty Research

Simultaneous high transparency and high haze are necessary for high-efficiency optical, photonic, and optoelectronic applications. However, a typical highly transparent film lacks high optical haze or vice versa. Here, we report a silk fibroin-based optical film that exhibits both ultrahigh optical transparency... (See article for full abstract).

Stereodynamical Control Of A Quantum Scattering Resonance In Cold Molecular Collisions, Pablo G. Jambrina, James F.E. Croft, Hua Guo, Mark Brouard, Balakrishnan Naduvalath, F. Javier Aoiz

Chemistry and Biochemistry Faculty Research

Cold collisions of light molecules are often dominated by a single partial wave resonance. For the rotational quenching of HD (v=1, j=2) by collisions with ground state para-H2, the process is dominated by a single L=2 partial wave resonance centered around 0.1 K. Here, we show that this resonance can be switched on or off simply by appropriate alignment of the HD rotational angular momentum relative to the initial velocity vector, thereby enabling complete control of the collision outcome.

Pressure-Induced Phase Transition In 1,3,5-Triamino-2,4,6-Trinitrobenzene (Tatb), Brad A. Steele, Samantha M. Clarke, Matthew P. Kroonblawd, I-Feng W. Kuo, Philip F. Pagoria, Sergey N. Tkachev, Jesse S. Smith, Sorin Bastea, Laurence E. Fried, Joseph M. Zaug, Elissaios Stavrou, Oliver Tschauner

Geoscience Faculty Publications

Determining the unreacted equation of state of 1,3, 5-triamino-2,4,6-trinitrobenzene (TATB) is challenging because it exhibits low crystal symmetry and low X-ray scattering strength. Here, we present the first high-pressure single-crystal X-ray diffraction (SXD) study of this material. Our SXD results reveal a previously unknown transition to a monoclinic phase above 4 GPa. No abrupt change of the volume occurs but the compressibility changes. Concomitant first principles evolutionary crystal structure prediction USPEX calculations confirm this transition and show that it involves a pressure-induced in-plane shift of the layers of TATB molecules with respect to the ambient-pressure phase.

Controlling Rotational Quenching Rates In Cold Molecular Collisions, James F.E. Croft, Balakrishnan Naduvalath

Chemistry and Biochemistry Faculty Research

The relative orientation and alignment of colliding molecules plays a key role in determining the rates of chemical processes. Here, we examine in detail a prototypical example: rotational quenching of HD in cold collisions with H2. We show that the rotational quenching rate from j = 2 → 0, in the v = 1 vibrational level, can be maximized by aligning the HD along the collision axis and can be minimized by aligning the HD at the so called magic angle. This follows from quite general helicity considerations and suggests that quenching rates for other similar systems can also be …

Optimization Of Useful Hard X-Ray Photochemistry, David Lewis Goldberger

UNLV Theses, Dissertations, Professional Papers, and Capstones

X-ray induced damage is generally considered a nuisance, but in the field of Useful Hard X-ray Photochemistry we harness the highly ionizing and penetrating properties of hard X-rays (> 7 keV) to initiate novel photochemical decomposition and synthesis at ambient and extreme conditions. Preliminary experiments suggest that the energy of irradiating photons and the sample pressure play roles in determining the nature of X-ray induced damage. Here, we present the X-ray energy dependence of damage induced in strontium oxalate, strontium nitrate, and barium nitrate, as well as the pressure dependence of X-ray induced damage of strontium oxalate. Our results indicate …

The Electrochemical Deposition Of Samarium And Europium Dissolved In Ionic Liquid Solvent, Bea Martinez

UNLV Theses, Dissertations, Professional Papers, and Capstones

Rare-earth elements include the lanthanide series in the periodic table with the addition of scandium and yttrium. China produces approximately 95% of the world’s rare-earths supply and is the largest consumer of the world’s rare earth supply. Domestic production of rare-earth metals is a priority in the US. The domestic demand for rare-earth elements is largely based on their use in electronic devices, catalytic converters, and more importantly defense applications. Therefore, China’s monopoly of rare-earth elements is viewed as a threat to national security. Although capital investments have resulted in an increase in domestic mining and refining of rare-earth materials, …

Surface And Interface Characterization Of Solution-Processed Metal Oxides And Pedot:Pss Using Photoelectron Spectroscopy, Lynette M. Kogler

UNLV Theses, Dissertations, Professional Papers, and Capstones

Solution-processed materials are appealing for use in printable electronics as a means to lower production costs, but precise control of the process is crucial for achieving the desired properties in the final materials and their interfaces. Electronic interface properties depend on both the involved materials and their fabrication processes, impacting the development and commercialization of these materials. Analyzing the chemical and electronic structure of these materials, particularly at the surfaces and interfaces, is important not only for insuring that the materials have the desired properties, but also for understanding the effects of the fabrication process and how to modify properties …

Structural Behavior Of Nbsexte2-X Superconductors Under High Pressure, Vahe Mkrtchyan

UNLV Theses, Dissertations, Professional Papers, and Capstones

Niobium chalcogenide compounds have recently gained a great deal of interest due to the fact that a superconducting phase coexists with the charge density wave state (CDW), as well as their potential for numerous applications. Two superconducting compositions, NbSexTe2-x (x=2, 1.5) were prepared by solid state route using high purity Nb, Se, and Te powders. Powder X-ray diffraction patterns collected at ambient conditions for NbSe2 and NbSe1.5Te0.5 showed a single phase with hexagonal crystal structure, with space group P63mmc. High-pressure X-ray diffraction measurements were performed at the Advanced Photon Source at Argonne National Laboratory to investigate structural stability up to …

Spectroscopic Investigation Of The Chemical And Electronic Properties Of Chalcogenide Materials For Thin-Film Optoelectronic Devices, Kimberly Horsley

UNLV Theses, Dissertations, Professional Papers, and Capstones

Chalcogen-based materials are at the forefront of technologies for sustainable energy production. This progress has come only from decades of research, and further investigation is needed to continue improvement of these materials.

For this dissertation, a number of chalcogenide systems were studied, which have applications in optoelectronic devices, such as LEDs and Photovoltaics. The systems studied include Cu(In,Ga)Se2 (CIGSe) and CuInSe2 (CISe) thin-film absorbers, CdTe-based photovoltaic structures, and CdTe-ZnO nanocomposite materials. For each project, a sample set was prepared through collaboration with outside institutions, and a suite of spectroscopy techniques was employed to answer specific questions about the system. These …

K-Shell X-Ray Spectroscopy Of Atomic Nitrogen, M. M. Sant'anna, Gunnar Ohrwall, Wayne C. Stolte, Alfred S. Schlachter, Dennis W. Lindle, B. M. Mclaughlin

Chemistry and Biochemistry Faculty Research

Absolute K-shell photoionization cross sections for atomic nitrogen have been obtained from both experiment and state-of-the-art theoretical techniques. Because of the difficulty of creating a target of neutral atomic nitrogen, no high-resolution K-edge spectroscopy measurements have been reported for this important atom. Interplay between theory and experiment enabled identification and characterization of the strong 1s → np resonance features throughout the threshold region. An experimental value of 409.64 ± 0.02 eV was determined for the K-shell binding energy.

Neutron Diffraction Of Nabd4: Phase Transition, Rietveld Structure Refinements, And Equation Of State, Guillermo Esparza, Esparza, Patricia Kalita, Andrew Cornelius

Undergraduate Research Opportunities Program (UROP)

NaBH4 is a hydride with possible applications as a hydrogen storage material for future renewable energy technologies. It’s dehydrogenation properties are enhanced with the mixture of particular catalysts through ball-milling techniques during which local pressures may exceed several GPa’s. It is for this reason that understanding the behavior of pressure induced phase changes of its crystalline unit cell is an area of interest.

Equilibrium Speciation Of Select Lanthanides In The Presence Of Acidic Ligands In Homo- And Heterogeneous Solutions, Troy Robinson

UNLV Theses, Dissertations, Professional Papers, and Capstones

This dissertation explores lanthanide speciation in liquid solution systems related to separation schemes involving the acidic ligands: bis(2-ethylhexyl) phosphoric acid (HDEHP), lactate, and 8-hydroxyquinoline. Equilibrium speciation of neodymium (Nd³⁺ ), sodium (Na⁺ ), HDEHP, water, and lactate in the TALSPEAK liquid-liquid extraction system was explored under varied Nd³⁺ loading of HDEHP in the organic phase and through extraction from aqueous HCl and lactate media. System speciation was probed through vapor pressure osmometry, visible and Fourier Transform Infrared (FTIR) spectroscopy, ²² Na and ¹³ C labeled lactate radiotracer distribution measurements, Karl Fischer titrations, and equilibrium pH measurements. Distribution …

Synthesis, Extraction, Dissolution, And Voltammetry Of F-Species In A Room Temperature Ionic Liquid Solvent, Wendy Jacqueline Pemberton

UNLV Theses, Dissertations, Professional Papers, and Capstones

Room temperature ionic liquids (RTILs) are salts that are liquids from as low as -96 °C to up to 100 °C. RTILs are of high interest in many fields of study because of their negligible vapor pressure, high electrochemical stability, high conductivity, and wide electrochemical windows. These diverse solvents have recently been used in organic synthesis, as extraction solvents, and as electrochemical solvents. It is as a direct result of the multifunctional capabilities of these RTIL solvents that they are examined in this work. The ability to probe the chemistry of lanthanides and actinides is based on the unique properties …

Electrochemistry Of Technetium Analogues Rhenium And Molybdenum In Room Temperature Ionic Liquid, Pauline Nancy Serrano

UNLV Theses, Dissertations, Professional Papers, and Capstones

Rhenium was used as an analog for Technetium to study the electrochemical redox properties because the two elements share the same stable oxidation states in aqueous solutions. However, Tc-99 is radioactive and is not readily available for experimentation purposes. Molybdenum is also of interest because when Mo-99 is irradiated, the decay products are Tc-99m and Tc-99. Approximately 30% Tc-99m is eluted from the Mo columns for radiopharmaceutical use with the remaining Mo-99 source decaying to Tc-99 which is discarded as radioactive hospital waste. Currently there are no viable procedures for the reclamation of the radioactive Tc-99 from either fission streams …

Self-Assembling Organic Semiconductors With Tunable Electronic Properties Based On Novel Asymmetric Phenazine And Bisphenazine, Kyoungmi Jang

UNLV Theses, Dissertations, Professional Papers, and Capstones

Current demands in the area of organic semiconductors focus on both electronic and self-assembling properties. Particularly, one-dimensionally grown nanostructures of small organic semiconductors have drawn much attention for nanodevice fabrication. Self-assembly through various intermolecular interactions has been widely used to produce one-dimensionally grown nanostructures which can be induced by various methods such as rapid solution dispersion, a phase transfer method, vapor annealing, crystallization, and organogelation in conjunction with proper molecular design. Controlling the morphology of the nanostructures plays an important role in achieving desirable properties in optoelectronic device applications. While significant advancements have been made in developing molecular architectures for …

Chemical And Electronic Structure Of Surfaces And Interfaces In Compound Semiconductors, Sujitra Pookpanratana

UNLV Theses, Dissertations, Professional Papers, and Capstones

The interface formation between two different materials is important in applications for optoelectronic devices. Often, the success or performance of these devices is dependent on the formation of these heterojunctions. In this work, the surface and interfaces in such materials for optoelectronic devices are investigated by a suite of X-ray analytical techniques including X-ray photoelectron (XPS), X-ray excited Auger electron (XAES), and X-ray emission (XES) spectroscopies to provide novel insight.

For the group III-nitrides (e.g., AlxGa1-xN) used in many light emitting devices, a significant challenge exists to form an Ohmic contact. The electron affinities and band gaps of GaN and …

Jahn-Teller Coupling And Fragmentation After Core-Shell Excitation Ini Cf4 Investigated By Partial-Ion-Yield Spectroscopy, Renaud Guillemin, Wayne C. Stolte, Maria Novella Piancastelli, Dennis W. Lindle

Chemistry and Biochemistry Faculty Research

We investigate fragmentation processes induced by core-level photoexcitation in CF4 at both the carbon and fluorine K edges by means of partial-ion-yield spectroscopy. The molecule CF₄ is a textbook example of systems in which Jahn-Teller coupling strongly manifests itself in the photoabsorption spectrum. Spectral features related to Jahn-Teller and quasi-Jahn-Teller splitting are observed, and important differences in the fragmentation pathways are revealed depending on the symmetries of the core-excited states. We interpret these experimental observations on the grounds of symmetry lowering from the Td to the C3v point group as well as preferential orientation with respect …

Full Multiple Scattering Analysis Of Xanes At The Cd L 3- And O K- Edges In Cdo Films Combined With A Soft-X-Ray Emission Investigation, I. N. Demchenko, J. D. Denlinger, M. Chernyshova, K. M. Yu, D. T. Speaks, P. Olalde-Velasco, Oliver Hemmers, W. Walukiewicz, A. Derkachova, K. Lawniczak-Jablonska

Environmental Studies Faculty Publications

X-ray absorption near edge structure (XANES) at the cadmium L3 and oxygen K edges for CdO thin films grown by pulsed laser deposition method, is interpreted within the real-space multiple scattering formalism, FEFF code. The features in the experimental spectra are well reproduced by calculations for a cluster of about six and ten coordination shells around the absorber for L3 edge of Cd and K edge of O, respectively. The calculated projected electronic density of states is found to be in good agreement with unoccupied electronic states in experimental data and allows to conclude that the orbital character of the …

Sulfur K-Edge Photo-Fragmentation Of Ethylene Sulfide, Wayne C. Stolte, Gunnar Ohrwall

Chemistry and Biochemistry Faculty Research

We have investigated the photofragmentation properties of the three-membered ring heterocyclic molecule ethylene sulfide or thiirane, C₂H₄S, by time-of-flight mass spectroscopy. Positive ions have been collected as a function of photon energy around the S K ionization threshold. Branching ratios were derived for all detected ions, which are informative of the decay dynamics and photofragmentation patterns of the core-excited species. We present a new assignment of the spectral features around the S K-edge.