271 results on '"Alexander Moewes"'
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2. Bandgap and electronic structure of CaSiN 2 : Experiment and theory
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Tristan de Boer, Teak D. Boyko, Cordula Braun, Wolfgang Schnick, and Alexander Moewes
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Marketing ,Materials Chemistry ,Ceramics and Composites ,Condensed Matter Physics - Published
- 2022
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3. Thermally stable red‐emitting oxide ceramics for laser lighting
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Zhiyu Yang, Tristan de Boer, Patrick M. Braun, Binbin Su, Qinyuan Zhang, Alexander Moewes, and Zhiguo Xia
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Mechanics of Materials ,Mechanical Engineering ,General Materials Science - Published
- 2023
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4. Energy levels of Eu2+ states in the next-generation LED-phosphor SrLi2Al2O2N2:Eu2+
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Muhammad Ruhul Amin, Philipp Strobel, Wolfgang Schnick, Peter J. Schmidt, and Alexander Moewes
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Materials Chemistry ,General Chemistry - Abstract
Our study accesses directly information on electronic and optical properties of phosphors such as intragap states and radiative energy levels. It further describes how the applied methods inform design and understanding of new pc-LED phosphors.
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- 2022
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5. Band gap and electronic structure of defects in the ternary nitride BP3N6: experiment and theory
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Tristan de Boer, Md. Fahim Al Fattah, Muhammad Ruhul Amin, Sebastian J. Ambach, Sebastian Vogel, Wolfgang Schnick, and Alexander Moewes
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Materials Chemistry ,General Chemistry - Abstract
The electronic properties, including the band gap and presence of defects, of the novel ternary nitride BP3N6 are elucidated using synchrotron radiation.
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- 2022
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6. Detecting a Hierarchy of Deep-Level Defects in the Model Semiconductor ZnSiN2
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Tristan de Boer, Jonas Häusler, Philipp Strobel, Teak D. Boyko, Stefan S. Rudel, Wolfgang Schnick, and Alexander Moewes
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General Energy ,0103 physical sciences ,02 engineering and technology ,Physical and Theoretical Chemistry ,021001 nanoscience & nanotechnology ,010306 general physics ,0210 nano-technology ,01 natural sciences ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials - Published
- 2021
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7. Comprehensive Band Gap and Electronic Structure Investigations of the Prominent Phosphors M2Si5N8:Eu2+ (M = Ca, Sr, Ba) Determined Using Soft X-ray Spectroscopy and Density Functional Theory
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Alexander Moewes, Cordula Braun, Thomas M. Tolhurst, and Wolfgang Schnick
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Soft x ray ,Materials science ,Band gap ,Analytical chemistry ,Phosphor ,02 engineering and technology ,Electronic structure ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,0104 chemical sciences ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,General Energy ,Density functional theory ,Physical and Theoretical Chemistry ,0210 nano-technology ,Spectroscopy - Abstract
The suite of LED phosphors M2Si5N8:Eu2+ (M = Ca, Sr, Ba) emerged as indispensable solid-state light sources for the next-generation lighting industry and display systems due to their unique propert...
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- 2021
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8. Tuning the Electronic Band Gap of Oxygen-Bearing Cubic Zirconium Nitride: c-Zr3–x(N1–xOx)4
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Teak D. Boyko, Andreas Zerr, and Alexander Moewes
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Materials science ,Bearing (mechanical) ,Electronic band ,chemistry.chemical_element ,02 engineering and technology ,Zirconium nitride ,021001 nanoscience & nanotechnology ,01 natural sciences ,Oxygen ,Electronic, Optical and Magnetic Materials ,law.invention ,chemistry.chemical_compound ,chemistry ,law ,0103 physical sciences ,Materials Chemistry ,Electrochemistry ,Physical chemistry ,010306 general physics ,0210 nano-technology - Published
- 2021
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9. Structural Influence of Lone Pairs in GeP 2 N 4 , a Germanium(II) Nitridophosphate
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Sebastian J. Ambach, Cody Somers, Tristan de Boer, Lucien Eisenburger, Alexander Moewes, and Wolfgang Schnick
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General Chemistry ,General Medicine ,Catalysis - Published
- 2022
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10. Unraveling the Energy Levels of Eu2+ Ions in MBe20N14:Eu2+ (M = Sr, Ba) Phosphors
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Wolfgang Schnick, M. Ruhul Amin, Alexander Moewes, and Eugenia Elzer
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Materials science ,Analytical chemistry ,Phosphor ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,0104 chemical sciences ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Ion ,General Energy ,Physical and Theoretical Chemistry ,0210 nano-technology ,Energy (signal processing) - Published
- 2021
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11. Electronic Properties of Carbyne Chains: Experiment and Theory
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Dmitry Raikov, Tristan de Boer, A. F. Zatsepin, Alexander Moewes, D. A. Zatsepin, and Ernst Z. Kurmaev
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Carbon chain ,Materials science ,Carbyne ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Allotropes of carbon ,3. Good health ,0104 chemical sciences ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,chemistry.chemical_compound ,General Energy ,chemistry ,Chemical physics ,Physical and Theoretical Chemistry ,0210 nano-technology ,Electronic properties - Abstract
As the known allotropes of carbon have expanded, increasing interest has been shown in one-dimensional carbyne. One approach to studying carbyne has been to synthesize linear carbon chains (LCCs) w...
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- 2021
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12. Direct Evidence of Charge Transfer upon Anion Intercalation in Graphite Cathodes through New Electronic States: An Experimental and Theoretical Study of Hexafluorophosphate
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Mahalingam Balasubramanian, Alexander Moewes, Jeffrey Read, Timothy T. Fister, Jacob G. Lapping, Tristan de Boer, and Jordi Cabana
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Materials science ,Direct evidence ,General Chemical Engineering ,Intercalation (chemistry) ,Inorganic chemistry ,Charge (physics) ,02 engineering and technology ,General Chemistry ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Cathode ,0104 chemical sciences ,Electronic states ,Anode ,law.invention ,chemistry.chemical_compound ,chemistry ,law ,Hexafluorophosphate ,Materials Chemistry ,Graphite ,0210 nano-technology - Abstract
Graphite intercalation compounds continue to be central to technologies for electrochemical energy storage from anodes in established Li-ion batteries to cathodes in beyond Li-ion concepts paired w...
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- 2020
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13. Origin and control of room temperature ferromagnetism in Co,Zn-doped SnO2: oxygen vacancies and their local environment
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Dhamodaran Manikandan, Ramaswamy Murugan, Josha Ho, Tristan de Boer, Brett Leedahl, Patrick M. Braun, and Alexander Moewes
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010302 applied physics ,Materials science ,Annealing (metallurgy) ,02 engineering and technology ,General Chemistry ,Magnetic semiconductor ,021001 nanoscience & nanotechnology ,01 natural sciences ,Condensed Matter::Materials Science ,Transition metal ,Ferromagnetism ,Chemical physics ,Vacancy defect ,0103 physical sciences ,Materials Chemistry ,Density functional theory ,0210 nano-technology ,Luminescence ,Electronic band structure - Abstract
We combine quantitative X-ray spectroscopy and crystal field calculations to study Co and Zn co-doped SnO2 diluted magnetic semiconductor systems, report the location of vacancies, and the new effect of vacancy transfer resulting in optical luminescence only present after annealing. We connect this phenomenon to mid gap states using the band structure of the material obtained via density functional theory. Furthermore, we give a new understanding of the interaction of transition metals within co-doped systems. Finally, we present new evidence that the ferromagnetism is mediated by the oxygen vacancies and the particular local environment of the vacancies can be controlled via the synthesis conditions.
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- 2020
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14. Inverse‐Tunable Red Luminescence and Electronic Properties of Nitridoberylloaluminates Sr 2− x Ba x [BeAl 3 N 5 ]:Eu 2+ ( x =0–2)
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Eugenia Elzer, Philipp Strobel, Volker Weiler, Muhammad R. Amin, Peter J. Schmidt, Alexander Moewes, and Wolfgang Schnick
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Organic Chemistry ,General Chemistry ,Catalysis - Published
- 2022
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15. Inverse-Tunable Red Luminescence and Electronic Properties of Nitridoberylloaluminates Sr
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Eugenia, Elzer, Philipp, Strobel, Volker, Weiler, Muhammad R, Amin, Peter J, Schmidt, Alexander, Moewes, and Wolfgang, Schnick
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The nitridoberylloaluminate Ba
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- 2021
16. Electronic structure and structural defects in 3d-metal doped In2O3
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N. V. Gavrilov, Seif O. Cholakh, Alexander Moewes, Josha Ho, Ivan S. Zhidkov, V. I. Brinzari, Brett Leedahl, Ernst Z. Kurmaev, J. Becker, Andrey I. Kukharenko, and Danil W. Boukhvalov
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010302 applied physics ,Materials science ,Dopant ,business.industry ,Electronic structure ,Magnetic semiconductor ,Condensed Matter Physics ,01 natural sciences ,Atomic and Molecular Physics, and Optics ,Electronic, Optical and Magnetic Materials ,Ion ,Metal ,Semiconductor ,Chemical physics ,Lattice (order) ,visual_art ,0103 physical sciences ,visual_art.visual_art_medium ,Electrical and Electronic Engineering ,Thin film ,business - Abstract
Dilute magnetic semiconductors (DMSs) are a highly attractive field of research due to their potential to open new technological functionality. Here, we perform a systematic study of In2O3 thin films with dopant ions of Mn, Co, Ni, and Fe to investigate the unique interaction of each of these ions and their incorporation into the semiconductor lattice. We report substitutional positioning of Fe atoms into the In3+ site and a mixture of interstitial, metallic clustering, and substitutional positioning for Co, Mn, and Ni, discriminating between oxidation states for all dopant atoms.
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- 2019
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17. Bandgap and Electronic Structure Determination of Oxygen-Containing Ammonothermal InN: Experiment and Theory
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T. de Boer, M. Ruhul Amin, Alexander Moewes, Jan Hertrampf, Peter Becker, and Rainer Niewa
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X-ray absorption spectroscopy ,Materials science ,Absorption spectroscopy ,Band gap ,Analytical chemistry ,chemistry.chemical_element ,02 engineering and technology ,Electronic structure ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Oxygen ,Synchrotron ,0104 chemical sciences ,3. Good health ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,law.invention ,General Energy ,chemistry ,law ,Emission spectrum ,Physical and Theoretical Chemistry ,0210 nano-technology - Abstract
The electronic structure and band gap of InN synthesized by the ammonothermal method are studied by synchrotron-based soft X-ray absorption spectroscopy (XAS), emission spectroscopy (XES), X-ray ex...
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- 2019
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18. Paving the way towards green catalytic materials for green fuels: impact of chemical species on Mo-based catalysts for hydrodeoxygenation
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L. Díaz-García, Jorge Aburto, Alexander Moewes, Diego Valencia, Luis Felipe Ramírez-Verduzco, and Amir Qamar
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Chemistry ,General Chemical Engineering ,Sulfidation ,Biomass ,02 engineering and technology ,General Chemistry ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,0104 chemical sciences ,Catalysis ,Palmitic acid ,Chemical species ,chemistry.chemical_compound ,Chemical engineering ,Biomass feedstock ,0210 nano-technology ,Hydrodeoxygenation - Abstract
A series of Mo-based catalysts were synthesized by tuning the sulfidation temperature to produce mixtures of MoO3 and MoS2 as active phases for the hydrodeoxygenation (HDO) of palmitic acid. Differences in the oxidation states of Mo, and the chemical species present in the catalytic materials were determined by spectroscopic techniques. Palmitic acid was used as a fatty-acid model compound to test the performance of these catalysts. The catalytic performance was related to different chemical species formed within the materials. Sulfidation of these otherwise inactive catalysts significantly increased their performance. The catalytic activity remains optimal between the sulfidation temperatures of 100 °C and 200 °C, whereas the most active catalyst was obtained at 200 °C. The catalytic performance decreased significantly at 400 °C due to a higher proportion of sulfides formed in the materials. Furthermore, the relative proportion of MoO3 to MoS2 is essential to form highly active materials to produce O-free hydrocarbons from biomass feedstock. The transition from MoS2 to MoO3 reveals the importance of Mo–S and Mo–O catalytically active species needed for the HDO process and hence for biomass transformation. We conclude that transitioning from MoS2 to MoO3 catalysts is a step in the right direction to produce green fuels.
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- 2019
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19. Electronic structure investigation of wide band gap semiconductors-Mg
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Md Fahim, Al Fattah, Muhammad Ruhul, Amin, Mathias, Mallmann, Safa, Kasap, Wolfgang, Schnick, and Alexander, Moewes
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The research on nitridophosphate materials has gained significant attention in recent years due to the abundance of elements like Mg, Zn, P, and N. We present a detailed study of band gap and electronic structure of M
- Published
- 2020
20. Luminescence of an Oxonitridoberyllate: A Study of Narrow-Band Cyan-Emitting Sr[Be6ON4]:Eu2+
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Volker Weiler, Peter J. Schmidt, Tristan de Boer, Philipp Strobel, Alexander Moewes, and Wolfgang Schnick
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Materials science ,General Chemical Engineering ,Cyan ,Doping ,Analytical chemistry ,Phosphor ,02 engineering and technology ,General Chemistry ,Crystal structure ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,0104 chemical sciences ,law.invention ,Full width at half maximum ,law ,Materials Chemistry ,Atomic ratio ,0210 nano-technology ,Luminescence ,Light-emitting diode - Abstract
Oxo- and (oxo)nitridoberyllates show exceptional potential as host lattices for application in illumination grade phosphor converted (pc)LEDs due to their remarkable electronic and structural characteristics, allowing highly efficient narrow-band emission upon doping with Eu2+. Sr[Be6ON4]:Eu2+, the first example of an oxonitridoberyllate phosphor, exhibits narrow-band cyan emission (λem = 495 nm; full width at half-maximum, fwhm = 35 nm; ≈1400 cm–1), comparable to the emission of the oxonitridosilicate BaSi2O2N2:Eu2+ (fwhm = 35 nm) or a cyan-emitting primary LED (fwhm = 27 nm). Sr[Be6ON4]:Eu2+ reveals a highly condensed rigid 3D network with a remarkably large degree of condensation [i.e., atomic ratio Be:(O,N)] of κ = 1.2 that is achieved by interconnection of highly condensed layers of BeN4 tetrahedra by Be2ON6 units via common edges. The crystal structure of Sr[Be6ON4]:Eu2+ was solved on the basis of single-crystal and powder XRD data (C2/c, no. 15, a = 13.9283(14), b = 5.7582(6), c = 4.9908(5) A, β = ...
- Published
- 2018
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21. Ultrasmall Au nanocatalysts supported on nitrided carbon for electrocatalytic CO2 reduction: the role of the carbon support in high selectivity
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Pu Wang, Josha Ho, Jie He, Peter Kerns, Huiqin Yao, Ben Liu, Yue Yang, Laura A. Achola, Aaron Lopes, Lei Jin, Yong Pei, and Alexander Moewes
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Materials science ,Inorganic chemistry ,chemistry.chemical_element ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Electrochemistry ,01 natural sciences ,7. Clean energy ,Nanomaterial-based catalyst ,0104 chemical sciences ,Catalysis ,chemistry ,Reversible hydrogen electrode ,General Materials Science ,Lewis acids and bases ,0210 nano-technology ,Selectivity ,Carbon ,Faraday efficiency - Abstract
Au is one of the most promising electrocatalysts to convert CO2 into CO in an aqueous-phase electrochemical reduction. However, ultrasmall Au nanocatalysts (AuNCs
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- 2018
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22. How functional groups change the electronic structure of graphdiyne: Theory and experiment
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Thomas M. Tolhurst, Yuliang Li, Niloofar Ketabi, Brett Leedahl, Alexander Moewes, and Huibiao Liu
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Chemistry ,Band gap ,Dirac (software) ,02 engineering and technology ,General Chemistry ,Electronic structure ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Molecular physics ,Spectral line ,0104 chemical sciences ,General Materials Science ,Density functional theory ,Emission spectrum ,Atomic physics ,0210 nano-technology ,Electronic band structure ,Absorption (electromagnetic radiation) - Abstract
Graphdiyne's electrons have been verified to display near massless behavior, as was predicted by the Dirac cone-like shape of its band structure, and has thus resulted in an exceptionally promising semiconducting material. We present a study of three graphdiyne samples with different thicknesses grown using a cross coupling reaction. Their electronic structures were examined using synchrotron soft X-ray absorption and emission spectroscopy, together with complementary full-potential, all-electron density functional theory calculations. Excellent agreement between the measured and calculated spectra was achieved, indicating strong evidence that the correct structural model was found. We show the existence of oxygen molecules and hydroxyl functional groups, as well as pyridinic nitrogen sites in each graphdiyne sample studied. Our study shows that the defect type varies with sample thickness, which in turn strongly depends on the synthesis conditions. The band gaps of three graphdiyne samples were measured to be 0.6 eV, 0.8 eV, and 0.9 eV in agreement with our calculated values. We propose that controlling the thickness of graphdiyne films may provide a novel method for tuning their band gaps.
- Published
- 2017
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23. Direct Measurements of Energy Levels and Correlation with Thermal Quenching Behavior in Nitride Phosphors
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Wolfgang Schnick, Philipp Strobel, Alexander Moewes, Peter J. Schmidt, and Thomas M. Tolhurst
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Materials science ,business.industry ,Scattering ,General Chemical Engineering ,Phosphor ,02 engineering and technology ,General Chemistry ,Nitride ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,0104 chemical sciences ,law.invention ,law ,Excited state ,Materials Chemistry ,Optoelectronics ,Quantum efficiency ,Atomic physics ,0210 nano-technology ,business ,Thermal quenching ,Energy (signal processing) ,Light-emitting diode - Abstract
Highly efficient narrow-band red emitting (RE) phosphors are the most desired and requested materials for developing illumination grade phosphor-converted light emitting diodes (pcLEDs). This study presents direct measurements of RE energy levels, critical to the color and efficiency of LED phosphors. For the first time, we experimentally determine the energetic separation of the Eu 5d state and the conduction band, which is the key indicator of quantum efficiency. This was achieved for the next-generation pcLED phosphors Li2Ca2[Mg2Si2N6]:Eu2+, Ba[Li2(Al2Si2)N6]:Eu2+, and Sr[LiAl3N4]:Eu2+ using resonant inelastic X-ray scattering. Band to band and 4f to valence band transitions are directly observed in X-ray excited optical luminescence spectra of Sr[LiAl3N4]:Eu2+ and Sr[Mg3SiN4]:Eu2+. These techniques are widely applicable and create a comprehensive, experimental picture of the Eu2+ energy levels in these compounds, leading to a complete understanding of all pertinent electronic processes. This study for...
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- 2017
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24. Bulk vs. Surface Structure of 3d Metal Impurities in Topological Insulator Bi2Te3
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P. Huu Le, Andrey I. Kukharenko, Danil W. Boukhvalov, Ivan S. Zhidkov, Ernst Z. Kurmaev, Seif O. Cholakh, C. Wei Luo, Brett Leedahl, N. V. Gavrilov, and Alexander Moewes
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Multidisciplinary ,Materials science ,Dopant ,Science ,02 engineering and technology ,021001 nanoscience & nanotechnology ,01 natural sciences ,Metal ,Crystal ,Oxidation state ,Impurity ,Chemical physics ,visual_art ,Topological insulator ,0103 physical sciences ,Atom ,visual_art.visual_art_medium ,Medicine ,010306 general physics ,0210 nano-technology ,Surface states - Abstract
Topological insulators have become one of the most prominent research topics in materials science in recent years. Specifically, Bi2Te3 is one of the most promising for technological applications due to its conductive surface states and insulating bulk properties. Herein, we contrast the bulk and surface structural environments of dopant ions Cr, Mn, Fe, Co, Ni, and Cu in Bi2Te3 thin films in order to further elucidate this compound. Our measurements show the preferred oxidation state and surrounding crystal environment of each 3d-metal atomic species, and how they are incorporated into Bi2Te3. We show that in each case there is a unique interplay between structural environments, and that it is highly dependant on the dopant atom. Mn impurities in Bi2Te3 purely substitute into Bi sites in a 2+ oxidation state. Cr atoms seem only to reside on the surface and are effectively not able to be absorbed into the bulk. Whereas for Co and Ni, an array of substitutional, interstitial, and metallic configurations occur. Considering the relatively heavy Cu atoms, metallic clusters are highly favourable. The situation with Fe is even more complex, displaying a mix of oxidation states that differ greatly between the surface and bulk environments.
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- 2017
25. Designing Luminescent Materials and Band Gaps: A Soft X-ray Spectroscopy and Density Functional Theory Study of Li2Ca2[Mg2Si2N6]:Eu2+ and Ba[Li2(Al2Si2)N6]:Eu2+
- Author
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Wolfgang Schnick, Thomas M. Tolhurst, Philipp Strobel, Peter J. Schmidt, and Alexander Moewes
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Materials science ,Band gap ,Doping ,Analytical chemistry ,Phosphor ,Context (language use) ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,7. Clean energy ,Molecular physics ,0104 chemical sciences ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Characterization (materials science) ,General Energy ,Density of states ,Density functional theory ,Physical and Theoretical Chemistry ,0210 nano-technology ,Spectroscopy - Abstract
A large band gap is a prerequisite for efficient emissions from a rare earth doped phosphor and is consequently a prerequisite for its application in high-quality lighting. We present a detailed characterization of luminescent materials Li2Ca2[Mg2Si2N6]:Eu2+ and Ba[Li2(Al2Si2)N6]:Eu2+ using soft X-ray spectroscopy and density functional theory calculations, including a rigorous experimental determination, and theory-based elucidation, of their band gaps. The band gap of Li2Ca2[Mg2Si2N6]:Eu2+ is determined to be 4.84 ± 0.20 eV, while that of Ba[Li2(Al2Si2)N6]:Eu2+ is 4.82 ± 0.20 eV. The origin of the band gaps is discussed in the context of the calculated DOS of each material and compared to benchmark luminescent materials Sr[LiAl3N4]:Eu2+ and Sr[Mg3SiN4]:Eu2+. Critically, the elements determining the band gaps are identified using the calculated density of states, as well as experimental resonant X-ray emission measurements. This allows for predictive power when searching for new nitridosilicates and rela...
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- 2017
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26. Intercalation-Induced Exfoliation and Thickness-Modulated Electronic Structure of a Layered Ternary Vanadium Oxide
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Thomas M. Tolhurst, Justin L. Andrews, Sarbajit Banerjee, Luis R. De Jesus, Peter M. Marley, and Alexander Moewes
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Materials science ,Electronic correlation ,Band gap ,General Chemical Engineering ,Intercalation (chemistry) ,Nanotechnology ,02 engineering and technology ,General Chemistry ,Electronic structure ,Electron ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Vanadium oxide ,0104 chemical sciences ,Chemical physics ,Lattice (order) ,Materials Chemistry ,0210 nano-technology ,Ternary operation - Abstract
Solid-state compounds wherein electrons cannot be described as noninteracting particles and instead show strongly correlated behavior are of interest both as systems manifesting novel quantum chemical phenomena as well as for electronic device applications. In the absence of predictive theoretical descriptors, modulation of the properties of these compounds tends to be challenging, and generalizable strategies for modulating closely coupled lattice, orbital, and spin degrees of freedom are exceedingly sparse. Here, it is shown that exfoliation mediated by cation intercalation can serve as a powerful means of modulating the electronic structure of layered correlated materials. Using a strongly correlated and charge-ordered layered compound, δ-Sr0.50V2O5, as a model system, it is shown that the band gap can be drastically altered from ca. 1.07 to 2.32 eV and the electron correlation strength can be greatly modified by intercalation-driven exfoliation to 2D nanosheets upon elimination of structural coherence...
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- 2017
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27. The electronic structure of ε′-V2O5: an expanded band gap in a double-layered polymorph with increased interlayer separation
- Author
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Justin L. Andrews, Brett Leedahl, Thomas M. Tolhurst, Sarbajit Banerjee, and Alexander Moewes
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Materials science ,Renewable Energy, Sustainability and the Environment ,Band gap ,Intercalation (chemistry) ,02 engineering and technology ,General Chemistry ,Crystal structure ,Electronic structure ,021001 nanoscience & nanotechnology ,01 natural sciences ,Crystallography ,Metastability ,0103 physical sciences ,General Materials Science ,Density functional theory ,010306 general physics ,0210 nano-technology ,Spectroscopy ,Ternary operation - Abstract
Selective elimination of network connectivity has emerged as an effective means of modifying the electronic structure of materials. Given its unique properties and diversity of polymorphs, V2O5 is an outstanding candidate. Recent studies have highlighted the benefit of utilizing metastable materials as cathode materials for multivalent ion batteries. In particular, novel polymorphs accessible from topochemical modification of ternary vanadium oxide bronzes have been identified as particularly interesting intercalation hosts. This is a study of the electronic structure of one such polymorph, e′-V2O5, using soft X-ray spectroscopy measurements and density functional theory calculations. This new double-layered polymorph of V2O5 has an increased interlayer separation that is found to lead to a dramatic increase in the band gap. Furthermore, the distortions brought on by the exfoliation process lead to a complex RIXS spectrum, showing d–d excitations, as well as low-energy charge transfer excitations. The comparison of the measurements and calculations is used to refine the crystal structure of e′-V2O5, which cannot be directly determined from X-ray diffraction data. In addition, distinct aspects of the electronic structure that make such polymorphs useful for correlated electron devices and electrode materials for intercalation batteries are discussed and linked to the crystal structure.
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- 2017
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28. A Probe of Valence and Conduction Band Electronic Structure of Lead Oxide Films for Photodetectors
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Alla Reznik, Oleksandr Grynko, Alexander Moewes, Muhammad Ruhul Amin, Amir Qamar, and O. Semeniuk
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Materials science ,Valence (chemistry) ,Condensed matter physics ,Annealing (metallurgy) ,business.industry ,Band gap ,02 engineering and technology ,Electronic structure ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Atomic and Molecular Physics, and Optics ,0104 chemical sciences ,Amorphous solid ,Semiconductor ,Density of states ,Density functional theory ,Physical and Theoretical Chemistry ,0210 nano-technology ,business - Abstract
We investigate how the electronic structure of amorphous lead oxide (a-PbO) films deposited on ITO substrate is changed after annealing at various temperatures. Both experimental soft X-ray spectroscopic and density functional theory (DFT) based computational techniques are used to explore the electronic structure of this material. X-ray emission, resonant X-ray inelastic scattering, and X-ray absorption spectroscopic techniques are employed to directly probe the valence and conduction bands. We discover that the films are very stable and remain amorphous when exposed to temperatures below 300 °C. An amorphous-to-polycrystalline (α-PbO phase) transformation occurs during annealing at 400 °C. At 500 °C, an alpha to beta phase change is observed. These structural modifications are accompanied by the band gap value changing from 1.4±0.2 eV to 2.0±0.2 eV upon annealing at 400 °C and to 2.6±0.2 eV upon annealing at 500 °C. A difference between surface and bulk structural properties is found for all samples annealed at 500 °C and above; these samples also exhibit an unexpected suppression of O : 2p density of states (DOS) near the bottom of the conduction band, whereas additional electronic states appear well within the valence band. This study provides a significant step forward to understanding the electronic properties of two polymorphic forms of PbO needed for optimization of this material for use in X-ray sensors.
- Published
- 2019
29. Energy band gaps and excited states in Si QD/SiO
- Author
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Anatoly F, Zatsepin, Evgeny A, Buntov, Dmitry A, Zatsepin, Ernst Z, Kurmaev, Vladimir A, Pustovarov, Alexey V, Ershov, Neil W, Johnson, and Alexander, Moewes
- Abstract
X-ray and optical spectroscopies were applied in order to study the band structure and electronic excitations of the SiO
- Published
- 2019
30. Frontispiece: Oxygen Vacancy Induced Structural Distortions in Black Titania: A Unique Approach using Soft X-ray EXAFS at the O-K Edge
- Author
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Brett Leedahl, Tristan de Boer, Xiaotao Yuan, and Alexander Moewes
- Subjects
Organic Chemistry ,General Chemistry ,Catalysis - Published
- 2019
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31. Tunability of Room Temperature Ferromagnetism in Spintronic Semiconductors through Non-magnetic Atoms
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Kyle LeBlanc, Alexander Moewes, Zahra Abooalizadeh, and Brett Leedahl
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Spin pumping ,Condensed Matter - Materials Science ,Materials science ,Spintronics ,Condensed matter physics ,Strongly Correlated Electrons (cond-mat.str-el) ,business.industry ,Materials Science (cond-mat.mtrl-sci) ,FOS: Physical sciences ,02 engineering and technology ,Electronic structure ,Magnetic semiconductor ,021001 nanoscience & nanotechnology ,7. Clean energy ,01 natural sciences ,Condensed Matter - Strongly Correlated Electrons ,Semiconductor ,Octahedron ,Ferromagnetism ,Lattice (order) ,0103 physical sciences ,010306 general physics ,0210 nano-technology ,business - Abstract
The implementation and control of room temperature ferromagnetism (RTFM) by adding magnetic atoms to a semiconductor's lattice has been one of the most important problems in solid state state physics in the last decade. Herein we report for the first time, to our knowledge, on the mechanism that allows RTFM to be tuned by the inclusion of \emph{non-magnetic} aluminum in nickel ferrite. This material, NiFe$_{2-x}$Al$_x$O$_4$ (x=0, 0.5, 1.5), has already shown much promise for magnetic semiconductor technologies, and we are able to add to its versatility technological viability with our results. The site occupancies and valencies of Fe atoms (Fe$^{3+}$ T$_d$, Fe$^{2+}$ O$_h$, and Fe$^{3+}$ O$_h$) can be methodically controlled by including aluminum. Using the fact that aluminum strongly prefers a 3+ octahedral environment, we can selectively fill iron sites with aluminum atoms, and hence specifically tune the magnetic contributions for each of the iron sites, and therefore the bulk material as well. Interestingly, the influence of the aluminum is weak on the electronic structure (supplemental material), allowing one to retain the desirable electronic properties while achieving desirable magnetic properties.
- Published
- 2019
- Full Text
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32. Fundamental crystal field excitations in magnetic semiconductor SnO2: Mn, Fe, Co, Ni
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Danil W. Boukhvalov, Alexander Moewes, Seif O. Cholakh, Brett Leedahl, Andrey I. Kukharenko, D. J. McCloskey, Ivan S. Zhidkov, N. V. Gavrilov, E. Z. Kurmaev, and V. I. Brinzari
- Subjects
X RAY SCATTERING ,Materials science ,Field (physics) ,CRYSTAL FIELD SPLITTINGS ,General Physics and Astronomy ,FOS: Physical sciences ,02 engineering and technology ,Electron ,010402 general chemistry ,01 natural sciences ,Crystal ,Metal ,VACANCY FORMATION ,Condensed Matter - Strongly Correlated Electrons ,Transition metal ,Physical and Theoretical Chemistry ,CRYSTAL FIELD EXCITATIONS ,RESONANT INELASTIC X-RAY SCATTERING ,COORDINATION SPHERE ,Condensed Matter - Materials Science ,Dopant ,Condensed matter physics ,Strongly Correlated Electrons (cond-mat.str-el) ,Scattering ,ELECTRONIC EXCITATION ,Materials Science (cond-mat.mtrl-sci) ,TRANSITION METALS ,SEMICONDUCTING MATERIALS ,Magnetic semiconductor ,021001 nanoscience & nanotechnology ,0104 chemical sciences ,CRYSTALS ,visual_art ,ELECTRONIC AND MAGNETIC PROPERTIES ,visual_art.visual_art_medium ,0210 nano-technology ,MAGNETIC SEMICONDUCTORS - Abstract
Directly measuring elementary electronic excitations in dopant 3d metals is essential to understanding how they function as part of their host material. Through calculated crystal field splittings of the 3d electron band it is shown how transition metals Mn, Fe, Co, and Ni are incorporated into SnO2. The crystal field splittings are compared to resonant inelastic X-ray scattering (RIXS) experiments, which measure precisely these elementary dd excitations. The origin of spectral features can be determined and identified via this comparison, leading to an increased understanding of how such dopant metals situate themselves in, and modify the host's electronic and magnetic properties; and also how each element differs when incorporated into other semiconducting materials. We found that oxygen vacancy formation must not occur at nearest neighbour sites to metal atoms, but instead must reside at least two coordination spheres beyond. The coordination of the dopants within the host can then be explicitly related to the d-electron configurations and energies. This approach facilitates an understanding of the essential link between local crystal coordination and electronic/magnetic properties. © 2019 the Owner Societies.
- Published
- 2019
33. Oxygen Vacancy Induced Structural Distortions in Black Titania -- A Unique Approach using Soft X-Ray EXAFS at the O K-Edge
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Brett Leedahl, Xiaotao Yuan, Tristan de Boer, and Alexander Moewes
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Condensed Matter - Materials Science ,Materials science ,Extended X-ray absorption fine structure ,Silicon drift detector ,Strongly Correlated Electrons (cond-mat.str-el) ,010405 organic chemistry ,Organic Chemistry ,Detector ,Materials Science (cond-mat.mtrl-sci) ,FOS: Physical sciences ,General Chemistry ,010402 general chemistry ,01 natural sciences ,Molecular physics ,Measure (mathematics) ,Signal ,Catalysis ,0104 chemical sciences ,Condensed Matter - Strongly Correlated Electrons ,Condensed Matter::Materials Science ,K-edge ,Photocatalysis ,Absorption (electromagnetic radiation) - Abstract
Unknown changes in the crystalline order of regular TiO$_2$ result in the formation of black titania, which has garnered significant interest as a photocatalytic material due to the accompanying electronic changes. Herein, we determine the nature of the lattice distortion caused by an oxygen vacancy that in turn results in the formation of mid-band gap states found in previous studies of black titania. We introduce an innovative technique using a state-of-the-art silicon drift detector, which can be used in conjunction with extended x-ray absorption fine structure (EXAFS) to measure bulk interatomic distances. We illustrate how the energy dispersive nature of such a detector can allow us an unimpeded signal, indefinitely in energy space, thereby sidestepping the hurdles of more conventional EXAFS, which is often impeded by other absorption edges.
- Published
- 2019
- Full Text
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34. Experiment-Driven Modeling of Crystalline Phosphorus Nitride P3 N5 : Wide-Ranging Implications from a Unique Structure
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Cordula Braun, Thomas M. Tolhurst, Teak D. Boyko, Alexander Moewes, and Wolfgang Schnick
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Absorption spectroscopy ,010405 organic chemistry ,Chemistry ,Band gap ,Organic Chemistry ,Nanotechnology ,General Chemistry ,Electronic structure ,010402 general chemistry ,01 natural sciences ,Catalysis ,Semimetal ,0104 chemical sciences ,Chemical physics ,Density of states ,Direct and indirect band gaps ,Density functional theory ,Spectroscopy - Abstract
Nitridophosphates have emerged as advanced materials due to their structural variability and broad technical applicability. Their binary parent compound P3N5, a polymeric network of corner- and edge-sharing PN4 tetrahedra with N[2] and N[3] sites, is a particularly interesting example. We present a study of the band gap and electronic structure of α-P3N5 by using soft X-ray spectroscopy measurements and DFT calculations. The band gap, which is crucial for all applications, is measured to be 5.87±0.20 eV. This agrees well with the calculated, indirect band gap of 5.21 eV. The density of states are found to show dramatic variation between the nonequivalent N sites and a high degree of covalency. Coupled to these results is what is, to our knowledge, the largest core hole shift reported to date for a soft X-ray absorption spectrum. We propose an intuitive bonding scheme for α-P3N5 that explains the observed band gap and unique density of states, while providing a framework for predicting these properties in known and yet to be discovered PN compounds. We briefly consider the implications of these results for new low-dimensional P and PN materials, which alongside graphene, could become important materials for nanoelectronics.
- Published
- 2016
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35. The hardness of group 14 spinel nitrides revisited
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Teak D. Boyko and Alexander Moewes
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Materials science ,Band gap ,Spinel ,Metallurgy ,02 engineering and technology ,General Chemistry ,Nitride ,engineering.material ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Group (periodic table) ,0103 physical sciences ,Materials Chemistry ,Ceramics and Composites ,engineering ,010306 general physics ,0210 nano-technology ,Solid solution - Published
- 2016
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36. Contrasting 1D tunnel-structured and 2D layered polymorphs of V2O5: relating crystal structure and bonding to band gaps and electronic structure
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Brett Leedahl, Alexander Moewes, Peter M. Marley, Justin L. Andrews, Sarbajit Banerjee, and Thomas M. Tolhurst
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Materials science ,Condensed matter physics ,Band gap ,General Physics and Astronomy ,02 engineering and technology ,Electronic structure ,Crystal structure ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Semimetal ,0104 chemical sciences ,Crystallography ,Local symmetry ,Density functional theory ,Orthorhombic crystal system ,Physical and Theoretical Chemistry ,0210 nano-technology ,Spectroscopy - Abstract
New V2O5 polymorphs have risen to prominence as a result of their open framework structures, cation intercalation properties, tunable electronic structures, and wide range of applications. The application of these materials and the design of new, useful polymorphs requires understanding their defining structure-property relationships. We present a characterization of the band gap and electronic structure of nanowires of the novel ζ-phase and the orthorhombic α-phase of V2O5 using X-ray spectroscopy and density functional theory calculations. The band gap is found to decrease from 1.90 ± 0.20 eV in the α-phase to 1.50 ± 0.20 eV in the ζ-phase, accompanied by the loss of the α-phase's characteristic split-off dxy band in the ζ-phase. States of dxy origin continue to dominate the conduction band edge in the new polymorph but the inequivalence of the vanadium atoms and the increased local symmetry of [VO6] octahedra results in these states overlapping with the rest of the V 3d conduction band. ζ-V2O5 exhibits anisotropic conductivity along the b direction, defining a 1D tunnel, in contrast to α-V2O5 where the anisotropic conductivity is along the ab layers. We explain the structural origins of the differences in electronic properties that exist between the α- and ζ-phase.
- Published
- 2016
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37. Searching for pure iron in nature: the Chelyabinsk meteorite
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Alexander Moewes, A. S. Belozerov, Victor I. Grokhovskii, Brett Leedahl, Ernst Z. Kurmaev, Ivan S. Zhidkov, V. I. Anisimov, Andrey I. Kukharenko, Aleksander V. Korolev, Seif O. Cholakh, and Sergey L. Skornyakov
- Subjects
Materials science ,010504 meteorology & atmospheric sciences ,Magnetism ,General Chemical Engineering ,Metallurgy ,Alloy ,Analytical chemistry ,chemistry.chemical_element ,General Chemistry ,engineering.material ,010502 geochemistry & geophysics ,01 natural sciences ,Nickel ,Ferromagnetism ,chemistry ,Meteorite ,Phase (matter) ,engineering ,Curie temperature ,0105 earth and related environmental sciences ,Phase diagram - Abstract
Herein we aimed to use thermomagnetic analysis (TMA) to determine the nature of iron and nickel in the Chelyabinsk meteorite, and their effect on the meteorite's magnetism. Our magnetic measurements show that 3% of the meteorite is metallic and consists of two ferromagnetic phases with Curie temperatures of TC1 = 1049 K and TC2 = 800 K. Using an Fe–Ni phase diagram, we show that the lower of the two temperatures is due to an Fe–Ni alloy with 51% Ni, while the higher Curie temperature phase is due to a pure or nearly pure (Ni-free) iron phase, for which we can be certain the Ni content is less than 1%. X-ray absorption (XAS) measurements show there are two clearly distinct iron oxidation environments: metallic and 2+, with the 2+ regions differing significantly from the standard FeO phase. We also demonstrate that beneath the immediate surface, iron exists virtually entirely in a metallic state. We are then able to estimate the surface composition using XPS, for which we found that 10% of iron on the surface is still surprisingly unoxidized. Finally, our theoretical calculations show how the density of states for both Fe and Ni atoms is affected for different nickel concentrations.
- Published
- 2016
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38. Electronic structure investigation of wide band gap semiconductors—Mg2PN3 and Zn2PN3: experiment and theory
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Muhammad Ruhul Amin, Wolfgang Schnick, Alexander Moewes, Fahim Al Fattah, Mathias Mallmann, and Safa Kasap
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Materials science ,Absorption spectroscopy ,Band gap ,Wide-bandgap semiconductor ,02 engineering and technology ,Electronic structure ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,0103 physical sciences ,Density of states ,General Materials Science ,Direct and indirect band gaps ,Density functional theory ,Atomic physics ,010306 general physics ,0210 nano-technology ,Spectroscopy - Abstract
The research on nitridophosphate materials has gained significant attention in recent years due to the abundance of elements like Mg, Zn, P, and N. We present a detailed study of band gap and electronic structure of M2PN3 (M = Mg, Zn), using synchrotron-based soft x-ray spectroscopy measurements as well as density functional theory (DFT) calculations. The experimental N K-edge x-ray emission spectroscopy (XES) and x-ray absorption spectroscopy (XAS) spectra are used to estimate the band gaps, which are compared with our calculations along with the values available in literature. The band gap, which is essential for electronic device applications, is experimentally determined for the first time to be 5.3 ± 0.2 eV and 4.2 ± 0.2 eV for Mg2PN3 and Zn2PN3, respectively. The experimental band gaps agree well with our calculated band gaps of 5.4 eV for Mg2PN3 and 3.9 eV for Zn2PN3, using the modified Becke-Johnson (mBJ) exchange potential. The states that contribute to the band gap are investigated with the calculated density of states especially with respect to two non-equivalent N sites in the structure. The calculations and the measurements predict that both materials have an indirect band gap. The wide band gap of M2PN3 (M = Mg, Zn) could make it promising for the application in photovoltaic cells, high power RF applications, as well as power electronic devices.
- Published
- 2020
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39. Understanding of Luminescence Properties Using Direct Measurements on Eu 2+ ‐Doped Wide Bandgap Phosphors
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Muhammad Ruhul Amin, Alexander Moewes, Philipp Strobel, Wolfgang Schnick, Tobias Giftthaler, and Amir Qamar
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Materials science ,business.industry ,Band gap ,Doping ,Phosphor ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Atomic and Molecular Physics, and Optics ,0104 chemical sciences ,Electronic, Optical and Magnetic Materials ,Optoelectronics ,0210 nano-technology ,Luminescence ,business - Published
- 2020
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40. Ultrasmall Au nanocatalysts supported on nitrided carbon for electrocatalytic CO
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Lei, Jin, Ben, Liu, Pu, Wang, Huiqin, Yao, Laura A, Achola, Peter, Kerns, Aaron, Lopes, Yue, Yang, Josha, Ho, Alexander, Moewes, Yong, Pei, and Jie, He
- Abstract
Au is one of the most promising electrocatalysts to convert CO2 into CO in an aqueous-phase electrochemical reduction. However, ultrasmall Au nanocatalysts (AuNCs,2 nm) have proven to be favorable for water reduction over CO2, although they possess a large surface-to-volume ratio and potentially are ideal for CO2 reduction. We herein report that ultrasmall AuNCs (1.9 ± 0.3 nm) supported on nitrided carbon are remarkably active and selective for CO2 reduction. The mass activity for CO of AuNCs reaches 967 A g-1 with a faradaic efficiency for CO of ∼83% at -0.73 V (vs. reversible hydrogen electrode) that is an order of magnitude more active than the state-of-the-art results. The high activity is endowed by the large surface area per unit weight and the high selectivity of ultrasmall AuNCs for CO2 reduction originates from the cooperative effect of Au and the nitrided carbon support where the surface N sites act as Lewis bases to increase the surface charge density of AuNCs and enhance the localized concentration of CO2 nearby catalytically active Au sites. We show that our results can be applied to other pre-synthesized Au catalysts to largely improve their selectivity for CO2 reduction by 50%. Our method is expected to illustrate a general guideline to effectively lower the cost of Au catalysts per unit weight of the product while maintaining its high selectivity for CO2 reduction.
- Published
- 2018
41. Electronic Structure, Bandgap, and Thermal Quenching of Sr[Mg3SiN4]:Eu2+in Comparison to Sr[LiAl3N4]:Eu2+
- Author
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Sebastian Schmiechen, Thomas M. Tolhurst, Alexander Moewes, Wolfgang Schnick, Philipp Pust, and Peter J. Schmidt
- Subjects
X-ray spectroscopy ,Materials science ,Band gap ,Phosphor ,02 engineering and technology ,Electronic structure ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Atomic and Molecular Physics, and Optics ,0104 chemical sciences ,Electronic, Optical and Magnetic Materials ,Density of states ,Density functional theory ,Direct and indirect band gaps ,Atomic physics ,0210 nano-technology ,Spectroscopy - Abstract
In this study the band gap and electronic structure of the next-generation, red phosphor Sr[Mg3SiN4]:Eu2+ (SMS) are explored through a combination of soft X-ray spectroscopy, density functional theory calculations, and thermal quenching data for the Eu2+ 5d -> 4f emissions. The results will be compared to those for the high-efficiency phosphor Sr[LiAl3N4]:Eu2+ (SLA), which shows different, yet exceptional emission characteristics in the red-spectral region. It is found that SMS has an indirect band gap of 3.28 +/- 0.20 eV, strong uniformity in the density of states of its nonequivalent nitrogen sites, and an estimated energetic separation between the lowest Eu2+ 5d state and the conduction band of approximate to 0.13 eV. The Eu2+ 5d-conduction band separation in SLA is found to be approximate to 0.28 eV, which points to why the visible emissions of SLA, and not SMS, show outstanding thermal stability. A bonding scheme explaining the band gap difference of SMS and SLA is proposed based on the density of states of SMS. Modifications to each lattice are put forward for achieving optimized phosphor characteristics for use in pc-LEDs.
- Published
- 2015
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42. Stability and Electronic Characteristics of Epitaxial Silicene Multilayers on Ag(111)
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Neil W. Johnson, Alexander Moewes, David Muir, and Ernst Z. Kurmaev
- Subjects
Biomaterials ,Materials science ,Condensed matter physics ,Silicene ,Electrochemistry ,Nanotechnology ,Condensed Matter Physics ,Epitaxy ,Electronic, Optical and Magnetic Materials - Published
- 2015
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43. Band gap and electronic structure of MgSiN2determined using soft X-ray spectroscopy and density functional theory
- Author
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Teak D. Boyko, Wolfgang Schnick, Tristan de Boer, Cordula Braun, and Alexander Moewes
- Subjects
Soft x ray ,Materials science ,Band gap ,General Materials Science ,Density functional theory ,Electronic structure ,Soft X-ray emission spectroscopy ,Condensed Matter Physics ,Spectroscopy ,Molecular physics - Published
- 2015
- Full Text
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44. The electronic structure of zirconium in hydrided and oxidized states
- Author
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Adrian Hunt, Xiaoyu Cui, Alexander Moewes, Hamed Akhiani, and Jerzy A. Szpunar
- Subjects
Zirconium ,Materials science ,Mechanical Engineering ,Zirconium alloy ,Inorganic chemistry ,Metals and Alloys ,chemistry.chemical_element ,Electronic structure ,Zirconium hydride ,XANES ,Crystallography ,X-ray photoelectron spectroscopy ,chemistry ,Mechanics of Materials ,Materials Chemistry ,Molecule ,Density functional theory - Abstract
Valence band energy shifts for pure zirconium and a model zirconium alloy (Zircaloy-4) in oxidized and hydrided states have been investigated with X-ray photoelectron spectroscopy (XPS) and X-ray Absorption Near-Edge Structure (XANES) technique. With XANES, we show that O/H interactions in oxidized Zr can be detected in the near-edge region of O K. Using density functional theory (DFT) simulations, we have determined where H atoms bond in the monoclinic ZrO2 lattice. The preferred stoichiometry is ZrO2:H, but the O–H bond is weak; increasing H causes the H atoms to form H2 molecules rather than O–H bonds. These interactions cause energy shifts in the Zr 3d XPS spectra. The results illustrate the complex processes of hydrogen and oxygen interactions at the Zr surface.
- Published
- 2015
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45. Investigations of the Electronic Structure and Bandgap of the Next-Generation LED-Phosphor Sr[LiAl3N4]:Eu2+-Experiment and Calculations
- Author
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Teak D. Boyko, Neil W. Johnson, Alexander Moewes, Wolfgang Schnick, Thomas M. Tolhurst, and Philipp Pust
- Subjects
Materials science ,Band gap ,business.industry ,Phosphor ,02 engineering and technology ,Electronic structure ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Atomic and Molecular Physics, and Optics ,0104 chemical sciences ,Electronic, Optical and Magnetic Materials ,Optoelectronics ,Density functional theory ,0210 nano-technology ,business - Published
- 2015
- Full Text
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46. X-ray spectroscopic study of amorphous and polycrystalline PbO films, α-PbO, and β-PbO for direct conversion imaging
- Author
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Alexander Moewes, Yuanming Pan, Alla Reznik, J. Lin, Amir Qamar, K. LeBlanc, and O. Semeniuk
- Subjects
0301 basic medicine ,Multidisciplinary ,Materials science ,Absorption spectroscopy ,Band gap ,lcsh:R ,Analytical chemistry ,lcsh:Medicine ,02 engineering and technology ,Electronic structure ,021001 nanoscience & nanotechnology ,Article ,Amorphous solid ,03 medical and health sciences ,030104 developmental biology ,Density functional theory ,lcsh:Q ,Emission spectrum ,Crystallite ,0210 nano-technology ,lcsh:Science ,Lead oxide - Abstract
We investigated the electronic structure of Lead Oxide (PbO) – one of the most promising photoconductor materials for direct conversion x-ray imaging detectors, using soft x-ray emission and absorption spectroscopy. Two structural configurations of thin PbO layers, namely the polycrystalline and the amorphous phase, were studied, and compared to the properties of powdered α-PbO and β-PbO samples. In addition, we performed calculations within the framework of density functional theory and found an excellent agreement between the calculated and the measured absorption and emission spectra, which indicates high accuracy of our structural models. Our work provides strong evidence that the electronic structure of PbO layers, specifically the width of the band gap and the presence of additional interband and intraband states in both conduction and valence band, depend on the deposition conditions. We tested several model structures using DFT simulations to understand what the origin of these states is. The presence of O vacancies is the most plausible explanation for these additional electronic states. Several other plausible models were ruled out including interstitial O, dislocated O and the presence of significant lattice stress in PbO.
- Published
- 2017
47. Recent Developments in Soft X-Ray Absorption Spectroscopy
- Author
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Alexander Moewes
- Subjects
Soft x ray ,Materials science ,Absorption spectroscopy ,Analytical chemistry ,XANES - Published
- 2017
- Full Text
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48. Structure-Induced Switching of the Band Gap, Charge Order, and Correlation Strength in Ternary Vanadium Oxide Bronzes
- Author
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Alexander Moewes, Sarbajit Banerjee, Justin L. Andrews, Peter M. Marley, Thomas M. Tolhurst, and Brett Leedahl
- Subjects
Condensed matter physics ,Electronic correlation ,Chemistry ,Band gap ,Organic Chemistry ,Nanowire ,02 engineering and technology ,General Chemistry ,Electron ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Catalysis ,0104 chemical sciences ,Crystallography ,Charge ordering ,Density functional theory ,0210 nano-technology ,Spectroscopy ,Ternary operation - Abstract
Recently, V2O5 nanowires have been synthesized as several different polymorphs, and as correlated bronzes with cations intercalated between the layers of edge- and corner-sharing VO6 octahedra. Unlike extended crystals, which tend to be plagued by substantial local variations in stoichiometry, nanowires of correlated bronzes exhibit precise charge ordering, thereby giving rise to pronounced electron correlation effects. These developments have greatly broadened the scope of research, and promise applications in several frontier electronic devices that make use of novel computing vectors. Here we present a study of δ-SrxV2O5, expanded δ-SrxV2O5, exfoliated δ-SrxV2O5 and δ-KxV2O5 using a combination of synchrotron soft x-ray spectroscopy and density functional theory calculations. We experimentally determine the band gaps of each system, and discuss their calculated electronic structures from the perspective of the measured spectra. Band gaps ranging from 0.66 ± 0.20 eV to 2.32 ± 0.20 eV are found, and linked to the underlying structure of each material. This demonstrates that the band gap of V2O5 can be tuned across a large portion of the range of greatest interest for device applications. The potential for metal-insulator transitions, tuneable electron correlations and charge ordering in these systems is discussed within the framework of our measurements and calculations, while highlighting the structure-property relationships that underpin them.
- Published
- 2017
49. Study of the Structural Characteristics of 3d Metals Cr, Mn, Fe, Co, Ni, and Cu Implanted in ZnO and TiO2—Experiment and Theory
- Author
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Alexander Moewes, L. Cui, D. A. Zatsepin, Ivan S. Zhidkov, Brett Leedahl, N. V. Gavrilov, Danil W. Boukhvalov, Robert J. Green, Seif O. Cholakh, Sang Sub Kim, and Ernst Z. Kurmaev
- Subjects
Materials science ,Dopant ,Spintronics ,Alloy ,Inorganic chemistry ,chemistry.chemical_element ,Zinc ,Crystal structure ,engineering.material ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Metal ,General Energy ,chemistry ,visual_art ,engineering ,visual_art.visual_art_medium ,Physical chemistry ,Density functional theory ,Atomic number ,Physical and Theoretical Chemistry - Abstract
Herein we systematically study a range of dopants (Cr, Fe, Ni, Cu, and an MnCo alloy) in ZnO and TiO2 using several X-ray spectroscopic techniques. We identify the dopant’s local environment and interaction with the host lattice by employing crystal field multiplet calculations and hence clarify their potential applicability for spintronic technologies. Our density functional theory (DFT) calculations predict a decreasing probability of direct cation (Zn/Ti) substitution by dopant atoms as atomic number increases, as well as a much greater likelihood of metallic clustering in TiO2. Our spectroscopic measurements confirm that in all cases, except Mn, metallic clusters of dopant atoms form in the TiO2 crystal lattice, thus making it unfit for spintronic capabilities. On the other hand, in ZnO, the dopants substitute directly into zinc sites, which is promising for spintronic technologies.
- Published
- 2014
- Full Text
- View/download PDF
50. Electronic Structure of FeSe1–xTex Studied by X-ray Spectroscopy and Density Functional Theory
- Author
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Alexander Moewes, V. Ortiz, John A. McLeod, I. Perez, Raul Escamilla, and Robert J. Green
- Subjects
Physics ,Superconductivity ,X-ray spectroscopy ,Spin states ,Condensed matter physics ,Scattering ,02 engineering and technology ,Electronic structure ,021001 nanoscience & nanotechnology ,01 natural sciences ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,General Energy ,0103 physical sciences ,Density of states ,Density functional theory ,Physical and Theoretical Chemistry ,010306 general physics ,0210 nano-technology ,Spectroscopy - Abstract
We study the electronic properties of the $\textrm{Fe}\textrm{Se}_{1-x}\textrm{Te}_x$ system ($x=0$, 0.25, 0.5, 0.75, and 1) from the perspective of X-ray spectroscopy and density functional theory (DFT). The analysis performed on the density of states reveals marked differences in the distribution of the $5p$ states of Te for $x>0$. We think that this finding can be associated with the fact that superconductivity is suppressed in FeTe. Moreover, using resonant inelastic X-ray scattering, we estimate the spin state of our system which can be correlated to the magnetic order. We find that the spin state of the $\textrm{Fe}\textrm{Se}_{1-x}\textrm{Te}_x$ system fluctuates, as a function of $x$, between $S=0$ and $S=2$ with Fe in FeSe in the highest spin state. Finally, our DFT calculations nicely reproduce the X-ray emission spectra performed at the Fe $L$-edge (which probe the occupied states) and suggest that the $\textrm{Fe}\textrm{Se}_{1-x}\textrm{Te}_x$ system can be considered at most as a moderately correlated system.
- Published
- 2014
- Full Text
- View/download PDF
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