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- AUGMENTED-WAVE METHOD; SIMPLE METAL-CLUSTERS; BIMETALLIC CLUSTERS; GOLD CLUSTERS; CHEMISTRY; PSEUDOPOTENTIALS; ABUNDANCES; STABILITY; MOLECULES; CATIONS (1)
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Approaching The Singularity In Gowdy Universes, Bartlett Douglas Edmonds Jr.
Approaching The Singularity In Gowdy Universes, Bartlett Douglas Edmonds Jr.
Theses and Dissertations
It has been shown that the cosmic censorship conjecture holds for polarized Gowdy spacetimes. In the more general, unpolarized case, however, the question remains open. It is known that cylindrically symmetric dust can collapse to form a naked singularity. Since Gowdy universes comprise gravitational waves that are locally cylindrically symmetric, perhaps these waves can collapse onto a symmetry axis and create a naked singularity. It is known that in the case of cylindrical symmetry, event horizons will not form under gravitational collapse, so the formation of a singularity on the symmetry axis would be a violation of the cosmic censorship …
Structure And Bonding Of Au5m (M=Na, Mg, Al, Si, P, And S) Clusters, Chiranjib Majumder, Anil K. Kandalam, Puru Jena
Structure And Bonding Of Au5m (M=Na, Mg, Al, Si, P, And S) Clusters, Chiranjib Majumder, Anil K. Kandalam, Puru Jena
Physics Publications
The atomic and electronic structure of Au5M (M=Na, Mg, Al, Si, P, S, and Au) clusters have been investigated using generalized gradient approximation to the density functional theory. Depending on the nature of interaction with different impurity elements a structural transition from planar to nonplanar configuration has been observed in Au5M. With the exception of S, impurities with p electrons (Al, Si, P) yield nonplanar geometries of Au5M clusters, while those with selectrons (Na, Mg) yield planar geometries. The properties of Au5Scluster are anomalous: The cluster not only has a planar geometry, but also is chemically most stable with the …
Dehydrogenation Mechanism In Catalyst-Activated Mgh2, S. Li, Puru Jena, R. Ahuja
Dehydrogenation Mechanism In Catalyst-Activated Mgh2, S. Li, Puru Jena, R. Ahuja
Physics Publications
A small amount of Nb2O5 catalyst is known to substantially improve the desorption thermodynamics and kinetics of MgH2. Using density functional theory in combination with ab initio molecular dynamics simulation, we provide theoretical understanding of the mechanism of dehydrogenation in Nb doped MgH2. We show that the substitution of Nb at the Mg site followed by the clustering of H around Nb is a likely pathway for hydrogen desorption. We also find that dehydrogenation from the vicinity of Mg vacancies is exothermic. However, the vacancies are not likely to play a significant role in hydrogen desorption due to their high …
Electronic Counting Rules For The Stability Of Metal-Silicon Clusters, J. Ulises Reveles, Shiv N. Khanna
Electronic Counting Rules For The Stability Of Metal-Silicon Clusters, J. Ulises Reveles, Shiv N. Khanna
Physics Publications
First principles electronic structure calculations have been carried out to examine the stability of cationic, neutral, and anionic MSi15, MSi16, and MSi17 (M=Sc, Ti, and V) clusters. ScSi16−, TiSi16, and VSi16+ are found to be particularly stable in agreement with recent experiments. It is shown that the enhanced stability can be reconciled within a model where each Si atom coordinated to the metal contributes one electron to the valence pool. Clusters where the total number of valence electrons obtained by summing one electron from each Si site coordinated to metal atom and the valence electrons of the metal attain 20 …
Effect Of Ti And Metal Vacancies On The Electronic Structure, Stability, And Dehydrogenation Of Na3alh6: Supercell Band-Structure Formalism And Gradient-Corrected Density-Functional Theory, S. Li, Puru Jena
Physics Publications
Electronic and structural properties of sodium-aluminum hexahydride (Na3AlH6) formed during the decomposition reaction of sodium alanate (NaAlH4) and the effects of Ti catalyst are studied using supercell approach and density-functional theory. The preferred site of Ti has been determined by substituting it at both the Na and Al sites and comparing the respective formation energies. The least unfavorable site for Ti is found to be the Al site. To examine the role of Ti substitution on the desorption of hydrogen, the energy cost to remove a H atom from the vicinity of Ti was calculated and compared with that from …
Effect Of Au Coating On The Magnetic And Structural Properties Of Fe Nanoclusters For Use In Biomedical Applications: A Density-Functional Theory Study, Q. Sun, A. K. Kandalam, Q. Wang, Puru Jena, Y. Kawazoe, M. Marquez
Effect Of Au Coating On The Magnetic And Structural Properties Of Fe Nanoclusters For Use In Biomedical Applications: A Density-Functional Theory Study, Q. Sun, A. K. Kandalam, Q. Wang, Puru Jena, Y. Kawazoe, M. Marquez
Physics Publications
In this paper, we report the first systematic theoretical study of gold-coated iron nanoclusters, aiming at understanding the magnetic properties of this core-shell structure used in biomedical applications. The calculations based on density-functional theory focus on the effect of gold coating on the magnetic and structural properties of iron clusters of various sizes, and the reaction of the bare and coated iron clusters with oxygen. Our results show that the magnetic moment of iron nanocore with gold coating is still significantly higher than that in bulk Fe; the coupling between Fe atoms remained ferromagnetic and is insensitive to the thickness …
Structure And Stability Of Co-N(Pyridine)(M)(-) Clusters: Absence Of Metal Inserted Structures, B. Douglas Edmonds, A. K. Kandalam, S. N. Khanna, X. Li, A. Grubisic, I. Khanna, K. H. Bowen
Structure And Stability Of Co-N(Pyridine)(M)(-) Clusters: Absence Of Metal Inserted Structures, B. Douglas Edmonds, A. K. Kandalam, S. N. Khanna, X. Li, A. Grubisic, I. Khanna, K. H. Bowen
Physics Publications
A synergistic approach combining the experimental photoelectron spectroscopy and theoretical electronic structure studies is used to probe the geometrical structure and the spin magnetic moment of Con(pyridine)−m clusters. It is predicted that the ground state of Co(pyridine)− is a structure where the Co atom is inserted in a CH bond. However, the insertion is marked by a barrier of 0.33eV that is not overcome under the existing experimental conditions resulting in the formation of a structure where Co occupies a site above the pyridine plane. For Co2(pyridine)−, a ground-state structure is predicted in which the Co2 diametric moiety is inserted …
Hydrogen Storage And The 18-Electron Rule, Boggavarapu Kiran, Anil K. Kandalam, Puru Jena
Hydrogen Storage And The 18-Electron Rule, Boggavarapu Kiran, Anil K. Kandalam, Puru Jena
Physics Publications
We show that the 18-electron rule can be used to design new organometallic systems that can store hydrogen with large gravimetric density. In particular, Ti containing organic molecules such as C4H4, C5H5, and C8H8 can store up to 9wt% hydrogen, which meets the Department of Energy target for the year 2015. More importantly, hydrogen in these materials is stored in molecular form with an average binding energy of about 0.55eV∕H2 molecule, which is ideal for fast kinetics. Using molecular orbitals we have analyzed the maximum number of H2 molecules that can be adsorbed as well as the nature of their …
Gold As Hydrogen: Structural And Electronic Properties And Chemical Bonding In Si3au3+/0/- And Comparisons To Si3h3+/0/-, Boggavarapu Kiran, Xi Li, Hua-Jin Zhai, Lai-Sheng Wang
Gold As Hydrogen: Structural And Electronic Properties And Chemical Bonding In Si3au3+/0/- And Comparisons To Si3h3+/0/-, Boggavarapu Kiran, Xi Li, Hua-Jin Zhai, Lai-Sheng Wang
Physics Publications
A single Au atom has been shown to behave like H in its bonding to Si in several mono- and disilicon gold clusters. In the current work, we investigate the Au∕H analogy in trisilicon gold clusters, Si3Au+∕0∕−3. Photoelectron spectroscopy and density functional calculations are combined to examine the geometric and electronic structure of Si3Au−3. We find that there are three isomers competing for the ground state of Si3Au−3 as is the case for Si3H−3. Extensive structural searches show that the potential energy surfaces of the trisilicon gold clusters (Si3Au−3, Si3Au3, and Si3Au+3) are similar to those of the corresponding silicon …
Strong Suppression Of The Yellow Luminescence In C-Doped Gan In Air Ambient, Michael A. Reshchikov
Strong Suppression Of The Yellow Luminescence In C-Doped Gan In Air Ambient, Michael A. Reshchikov
Physics Publications
The authors observed a drastic reduction of the yellow luminescence (YL) intensity in carbon-doped semi-insulating GaN in air or oxygen ambient as compared to the intensity in vacuum. The YL intensity dropped about 300 times while the exciton emission remained almost unchanged. The authors assume that the donor-acceptor-pair transitions involving a gallium vacancy complex in a thin near-surface region cause the strong YL. Oxygen molecules or ions induce the surface states acting as a very efficient channel of nonradiative recombination. The results indicate that carbon may not be involved in the acceptor responsible for the YL band in GaN:C.
Midinfrared Intersubband Absorption In Znxcd1−Xse/Znx′Cdy′Mg1−X′−Y′Se Multiple Quantum Well Structures, H. Lu, A. Shen, M. C. Tamargo, C. Y. Song, H. C. Liu, S. K. Zhang, R. R. Alfano, M. Muñoz
Midinfrared Intersubband Absorption In Znxcd1−Xse/Znx′Cdy′Mg1−X′−Y′Se Multiple Quantum Well Structures, H. Lu, A. Shen, M. C. Tamargo, C. Y. Song, H. C. Liu, S. K. Zhang, R. R. Alfano, M. Muñoz
Physics Publications
The authors report the observation of intersubband absorption in ZnxCd(1−x)Se/Znx′Cdy′Mg(1−x′−y′)Semultiple quantum wells. Lattice-matched samples were grown by molecular beam epitaxy on InP (001) substrates. Photoluminescence measurements indicate that the samples have excellent material quality. The peak absorption wavelengths measured by Fourier transform infrared spectroscopy are 3.99 and 5.35μm for two samples with ZnxCd(1−x)Se well widths of 28 and 42Å, respectively. These values fall within the 3–5μm wavelength range, which is of interest for midinfrared intersubband devices, such as quantum cascade lasers and quantum well infrared photodetectors. Their experimental results fit well with theoretical predictions based on the envelope function approximation. …
Determination Of Acceptor Concentration In Gan From Photoluminescence, Michael A. Reshchikov
Determination Of Acceptor Concentration In Gan From Photoluminescence, Michael A. Reshchikov
Physics Publications
The concentration of the acceptor responsible for the yellow luminescence (YL) band at about 2.2eV in GaN is determined from photoluminescence. The YL band intensity increases linearly with excitation power density and partially saturates above some critical value. The dependence is quantitatively described within a phenomenological model accounting for recombination statistics in GaN layer and saturation of acceptors with photogenerated holes. The incomplete saturation of the YL intensity at high excitation intensities is explained by gradual saturation of acceptors at different distances from the sample surface. The identity of deep and shallow acceptors in GaN is discussed.
Silicon Oxide Nanoparticles Reveal The Origin Of Silicate Grains In Circumstellar Environments, Peneé Armaize Clayborne
Silicon Oxide Nanoparticles Reveal The Origin Of Silicate Grains In Circumstellar Environments, Peneé Armaize Clayborne
Theses and Dissertations
The processes leading to the widespread presence of crystalline silicates throughout the galaxy and the origin of silicon nanoparticles thought to be responsible for the observed extended red emission in diffuse galactic background are still far from being understood. One of the most abundant oxygen bearing species in molecular astronomical regions is SiO. It has been conjectured that silicate formation probably proceeds via the agglomeration of these molecular species; however there are no studies to reveal the microscopic mechanism. We have used a synergistic approach combining experiments in molecular beams and first principles theoretical calculation to demonstrate that the passage …
Determination Of The Band Gap Bowing Parameter Of A1xga1-Xn With Contactless Electroreflectance, Laura C. Mcglinchey
Determination Of The Band Gap Bowing Parameter Of A1xga1-Xn With Contactless Electroreflectance, Laura C. Mcglinchey
Theses and Dissertations
Contactless electroreflectance (CER), a modulation spectroscopy (MS) technique, has been used to study the A and C exciton transitions in A1xGa1-xN layers for a composition range of 0 ≤ x ≤ 0.48 at room temperature. Taking the entire composition range (0 ≤ x ≤ 1) into account by incorporating a previously reported band gap energy for AlN, the dependence of the A-exciton transition on composition showed a downward bowing from linearity. A bowing parameter of b = 1.7 eV was found. Analysis of the lower composition range 0 ≤ x ≤ 0.48 resulted in a linear fit, as did the …
Subpicosecond Time-Resolved Raman Studies Of Field-Induced Transient Transport In An Inxga1−Xas-Based P-I-N Semiconductor Nanostructure, K. T. Tsen, Juliann G. Kiang, D. K. Ferry, Hadis Morkoç
Subpicosecond Time-Resolved Raman Studies Of Field-Induced Transient Transport In An Inxga1−Xas-Based P-I-N Semiconductor Nanostructure, K. T. Tsen, Juliann G. Kiang, D. K. Ferry, Hadis Morkoç
Physics Publications
Electron transient transport in an InxGa1−xAs-based (x=0.53) p-i-nnanostructure under the application of an electric field has been studied by time-resolvedRaman spectroscopy on a subpicosecond time scale and at T=300K. The experimental results reveal the time evolution of the electron distribution function and electron drift velocity with subpicosecond time resolution. These experimental results are compared with those of both InP-based and GaAs-based p-i-nnanostructures and provide a consistent understanding and better insight of electron transient transport phenomena in semiconductors.
Clustering Of Cr In Gan Nanotubes And The Onset Of Ferrimagnetic Order, Q. Wang, Q. Sun, Puru Jena, Y. Kawazoe
Clustering Of Cr In Gan Nanotubes And The Onset Of Ferrimagnetic Order, Q. Wang, Q. Sun, Puru Jena, Y. Kawazoe
Physics Publications
A comprehensive theoretical study of the structure and magnetic properties of Cr-doped GaN nanotubes yields some interesting and unexpected results: (1) A single wall GaN nanotube constructed from the GaN wurtzite crystal relaxes to a carbon-like zigzag SWNT structure and remains stable at 300K, while a multiwall GaN nanotube retains its original wurtzite form. (2) Cr atoms prefer to form clusters and the underlying magnetism depends on the degree of clustering. (3) The coupling between two Cr atoms mediated by the neighboring N is ferromagnetic, but changes to ferrimagnetic as the cluster grows. These results are based on spin polarized …
Ferromagnetism In Al1−Xcrxn Thin Films By Density Functional Calculations, Q. Wang, A. K. Kandalam, Q. Sun, Puru Jena
Ferromagnetism In Al1−Xcrxn Thin Films By Density Functional Calculations, Q. Wang, A. K. Kandalam, Q. Sun, Puru Jena
Physics Publications
We report the results of a theoretical study of magnetic coupling between Cr atoms doped in bulk AlN as well as AlN (112¯0) thin films having wurtzite structure. The calculations are based on density fuctional theory with the generalized gradient approximation to the exchange and correlation potential. In the thin film, modeled by a slab of finite thickness, Cr atoms are found to cluster around N on the surface layer and couple ferromagnetically. The results for the Cr-doped AlN crystal are similar, namely, Cr atoms cluster around N and couple ferromagnetically. In the thin film, the preference of Cr to …
Structural, Electronic, And Chemical Properties Of Multiply Iodized Aluminum Clusters, N. O. Jones, J. Ulises Reveles, S. N. Khanna, D. E. Bergeron, P. J. Roach, A. W. Castleman Jr.
Structural, Electronic, And Chemical Properties Of Multiply Iodized Aluminum Clusters, N. O. Jones, J. Ulises Reveles, S. N. Khanna, D. E. Bergeron, P. J. Roach, A. W. Castleman Jr.
Physics Publications
The electronic structure, stability, and reactivity of iodized aluminum clusters, which have been investigated via reactivity studies, are examined by first-principles gradient corrected density functional calculations. The observed behavior of Al13I−x and Al14I−x clusters is shown to indicate that for x⩽8, they consist of compact Al−13 and Al++14 cores, respectively, demonstrating that they behave as halogen- or alkaline earthlike superatoms. For x>8, the Al cores assume a cagelike structure associated with the charging of the cores. The observed mass spectra of the reacted clusters reveal that Al13I−x species are more stable for even x while Al14I−x exhibit enhanced stability …