Ralph Scheicher - Academia.edu (original) (raw)

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Papers by Ralph Scheicher

The adsorption energies of hydrogen molecules at all possible adsorption sites of covalent organi... more The adsorption energies of hydrogen molecules at all possible adsorption sites of covalent organic framework-1 (COF-1) are studied by density functional theory (DFT) and second-order Møller-Plesset perturbation theory (MP2). The most favorable adsorption sites from our DFT results are on the top of an oxygen atom for the B3O3 ring and on the top of the center of the C-C bond for the benzene ring when a single H2 is interacting with the COF-1. The adsorption energy trend obtained from the DFT calculations is found to be in good agreement with the MP2 binding energy trend. The binding preferences are slightly changed when high hydrogen loading is considered. H2 molecules prefer to be trapped on the top of the carbon atoms of the benzene ring and also on the top of the oxygen atoms of the B3O3 ring. These findings are confirmed by hydrogen center-of-mass distribution results obtained using molecular dynamics simulations. Moreover, our DFT results reveal that the hydrogen adsorption energies are boosted when we increase the number of hydrogen loadings due to attractive H2-H2 interactions. In addition, the nonavailability of the remaining adsorption sites in the COF-1 leads to a reduction of the H2 mobility.

Density functional theory has been used to study the physisorption energies of hydrogen at all po... more Density functional theory has been used to study the physisorption energies of hydrogen at all possible adsorption sites near the metal oxide cluster in both Cd- and Zn-based Metal Organic Framework-5 (MOF-5). Three types of exchange- correlation functionals (LDA, GGA-PW91, and GGA-PBE) were compared. The binding for all adsorption sites in Cd-based MOF- 5 was found to be generally stronger

Scientific reports, 2015

We investigate the influence of uniaxial strain on the site occupancy of hydrogen in vanadium, us... more We investigate the influence of uniaxial strain on the site occupancy of hydrogen in vanadium, using density functional theory. The site occupancy is found to be strongly influenced by the strain state of the lattice. The results provide the conceptual framework for the atomistic description of the observed hysteresis in the to phase transition in bulk, as well as the preferred octahedral occupancy of hydrogen in strained V layers.

The results presented by us allow for an understanding of pressure-induced superconductivity of t... more The results presented by us allow for an understanding of pressure-induced superconductivity of tri-hydrides with a particular focus on YH3. We show that a structural phase transition from hexagonal to cubic structure occurs at 20 GPa, which is in good agreement with recent experiments. This structural phase transition is seen to be accompanied by an insulator-to-metal transition in our quasi-particle calculations. Furthermore, we present an analysis of the superconducting behavior in cubic YH3. At the lowest possible pressure (17.7 GPa), cubic YH3 is superconducting with a Tc of 40 K and turns into the normal metallic phase at 25 GPa due to a change of s-d hybridization between hydrogen and yttrium. This hitherto unprecedented low pressure should make superconducting YH3 a very attractive system to study experimentally among the hydrogen-rich superconductors. Finally, we also predict that the superconducting phase reemerges at 45 GPa. J. S. de Almeida, D. Y. Kim, C. Ortiz, M. Klint...

Proceedings of the Japan Academy, Series B, 2007

A marked difference in spin relaxation behavior due to hemoglobin magnetism was found for positiv... more A marked difference in spin relaxation behavior due to hemoglobin magnetism was found for positive muons (µ + ) in deoxyhemoglobin in comparison with that observed in oxyhemoglobin in aqueous solution at room temperature under zero and external longitudinal magnetic fields upto 0.4 Tesla. At the same time, small but significant unique relaxation pattern was observed in nonmagnetic oxyhemoglobin. Combined with our previous measurements on hemoglobin in human blood, application of this type of measurement to the studies of the level of oxygenation in various regions of the human brain is suggested.

Physical Review Letters, 2009

Metallization in pure hydrogen has been proposed to give rise to high-temperature superconductivi... more Metallization in pure hydrogen has been proposed to give rise to high-temperature superconductivity at pressures which still lie beyond the reach of contemporary experimental techniques. Hydrogen-dense materials offer an opportunity to study related phenomena at experimentally achievable pressures. Here we report the prediction of high-temperature superconductivity in yttrium hydride (YH3), with a T(c) of 40 K at 17.7 GPa, the lowest reported pressure for hydrogen-dense materials to date. Specifically, we find that the face-centered cubic structure of YH3 exhibits superconductivity of different origins in two pressure regions. The evolution of T(c) with pressure follows the corresponding change of s-d hybridization between H and Y, due to an enhancement of the electron-phonon coupling by a matching of the energy level from Y-H vibrations with the peak of the s electrons from the octahedrally coordinated hydrogen atoms.

Physical Review B, 2007

We report the results of our first-principles investigation on the interaction of the nucleobases... more We report the results of our first-principles investigation on the interaction of the nucleobases adenine (A), cytosine (C), guanine (G), thymine (T), and uracil (U) with graphene, carried out within the density functional theory framework, with additional calculations utilizing Hartree-Fock plus second-order Møller-Plesset perturbation theory. The calculated binding energy of the nucleobases shows the following hierarchy: G > T ≈ C ≈ A > U, with the equilibrium configuration being very similar for all five of them. Our results clearly demonstrate that the nucleobases exhibit significantly different interaction strengths when physisorbed on graphene. The stabilizing factor in the interaction between the base molecule and graphene sheet is dominated by the molecular polarizability that allows a weakly attractive dispersion force to be induced between them. The present study represents a significant step towards a first-principles understanding of how the base sequence of DNA can affect its interaction with carbon nanotubes, as observed experimentally. PACS numbers: 68.43.-h, 81.07.De, 82.37.Rs

Physical Review B, 2009

We have employed ab initio molecular dynamics simulations in an attempt to study a temperature-in... more We have employed ab initio molecular dynamics simulations in an attempt to study a temperature-induced order-disorder structural phase transformation that occurs in Li2NH at about 385 K. A structural phase transition was observed by us in the temperature range 300-400 K, in good agreement with experiment. This transition is associated with a melting of the cation sublattice (Li+) , giving

Nano Letters, 2009

A synergistic approach involving experiment and first-principles theory not only shows that carbo... more A synergistic approach involving experiment and first-principles theory not only shows that carbon nanostructures can be used as catalysts for hydrogen uptake and release in complex metal hydrides such as sodium alanate, NaAlH(4), but also provides an unambiguous understanding of how the catalysts work. Here we show that the stability of NaAlH(4) originates with the charge transfer from Na to the AlH(4) moiety, resulting in an ionic bond between Na(+) and AlH(4)(-) and a covalent bond between Al and H. Interaction of NaAlH(4) with an electronegative substrate such as carbon fullerene or nanotube affects the ability of Na to donate its charge to AlH(4), consequently weakening the Al-H bond and causing hydrogen to desorb at lower temperatures as well as facilitating the absorption of H(2) to reverse the dehydrogenation reaction. In addition, based on our experimental observations and theoretical calculations it appears the curvature of the carbon nanostructure plays a role in the catalytic process. Ab initio molecular dynamics simulation further reveals the time evolution of the charge transfer process.

Bulletin of the American Physical Society, Mar 17, 2006

Standing at the intersection of the biological regime and the nanomaterials world, DNA-coated car... more Standing at the intersection of the biological regime and the nanomaterials world, DNA-coated carbon nanotubes (CNT) possess features which can make them attractive for a range of applications, eg, as highly specific nanosensors [1], or as a method to efficiently separate CNTs according to their structure [2]. It is therefore certainly worthwhile to obtain a detailed understanding of the binding between the bases in DNA and the surface of CNTs, preferably from first principles. To this end, we have studied the interaction between single ...

Bulletin of the American Physical Society, Mar 15, 2010

We investigate the adsorption of the nucleic acid bases, adenine (A), guanine (G), cytosine (C), ... more We investigate the adsorption of the nucleic acid bases, adenine (A), guanine (G), cytosine (C), thymine (T) and uracil (U) on the outer wall of a high curvature semiconducting single-walled boron nitride nanotube (BNNT) by first principles density functional theory calculations. The calculated binding energy shows the order: G$>$A$\approx CCC\approx TTT\approx $U implying that the interaction strength of the (high-curvature) BNNT with the nucleobases, G being an exception, is nearly the same. A higher binding energy for the G-BNNT conjugate ...

One of the most technically challenging barriers to the widespread commercialization of hydrogen-... more One of the most technically challenging barriers to the widespread commercialization of hydrogen-fueled devices and vehicles remains hydrogen storage. More environmentally
friendly and effective nonmetal catalysts are required to improve hydrogen sorption. In this paper, through a
combination of experiment and theory, we evaluate and
explore the catalytic effects of layered graphene nanofibers
toward hydrogen release of light metal hydrides such as
sodium alanate. Graphene nanofibers, especially the helical
kind, are found to considerably improve hydrogen release from
NaAlH4, which is of significance for the further enhancement
of this practical material for environmentally friendly and effective hydrogen storage applications. Using density functional theory, we find that carbon sheet edges, regardless of whether they are of zigzag or armchair type, can weaken Al−H bonds in sodium
alanate, which is believed to be due to a combination of NaAlH4 destabilization and dissociation product stabilization. The helical
form of graphene nanofibers, with larger surface area and curved configuration, appears to benefit the functionalization of carbon sheet edges. We believe that our combined experimental and theoretical study will stimulate more explorations of other microporous or mesoporous nanomaterials with an abundance of exposed carbon edges in the application of practical complex light metal hydride systems.