Tanglaw Roman

Potential energy of hydrogen atom motion on Pd(111) surface and in subsurface: A first principles calculation

We calculate the adiabatic potential energy for hydrogen atom motion on a Pd(111) surface and in a subsurface within the framework of the density functional theory in order to understand the diffusion mechanism of a hydrogen atom from the Pd(111) surface to the subsurface. According to the calculated adiabatic potential energy surface for the hydrogen atom motion up to the third atom layer, an effective diffusion path of the hydrogen atom into the Pd bulk starts from the fcc hollow site on the Pd(111) surface. Moreover, the diffusion path passes through the octahedral site between the first and the second Pd atom layers, the tetrahedral site beneath a Pd atom of the first layer or above the Pd atom of the third layer, and the octahedral site between the second and third layer.

Change of the work function of platinum electrodes induced by halide adsorption.

Summary The properties of a halogen-covered platinum(111) surface have been studied by using density functional theory (DFT), because halides are often present at electrochemical electrode/electrolyte interfaces. We focused in particular on the halogen-induced work function change as a function of the coverage of fluorine, chlorine, bromine and iodine. For electronegative adsorbates, an adsorption-induced increase of the work function is usually expected, yet we find a decrease of the work function for Cl, Br and I, which is most prominent at a coverage of approximately 0.25 ML. This coverage-dependent behavior can be explained by assuming a combination of charge transfer and polarization effects on the adsorbate layer. The results are contrasted to the adsorption of fluorine on calcium, a system in which a decrease in the work function is also observed despite a large charge transfer to the halogen adatom.

Realizing a carbon-based hydrogen storage material

In response to the current need for an efficient, safe, and compact system for storing hydrogen in mobile applications, a scheme for maximizing and controlling hydrogen storage in graphite is proposed by modifying substrate reactivity through the exploitation of intrinsic vibrational modes in pristine and fully-hydrogenated graphite systems. Calculations within density functional theory suggest that infrared radiation of distinct frequencies can be used to independently induce graphite lattice restructuring and recrystallization for promoting hydrogen uptake and discharge, respectively. Effects of the initial attachment of hydrogen on graphite sheets are discussed, with computational results showing that additional hydrogen adsorption can proceed through easier reaction routes.

Quantum states of hydrogen atom motion on the Pd(111) surface and in the subsurface

We investigate the quantum states of hydrogen atom motion on Pd(111) surface and in its subsurface by calculating the wavefunctions and the eigenenergies for hydrogen atom motion within the framework of the variation method on an adiabatic potential energy surface (PES), obtained through first-principles calculations, for the hydrogen atom motion. The calculated results show that the ground-state wavefunction for the hydrogen atom motion localizes on the face-centered cubic (fcc) hollow site of the surface. The higher excited state wavefunctions are distributed between the first and second layers, and subsequently delocalized under the second atom layer. These suggest that an effective diffusion path of the hydrogen atom into the subsurface area passes through the fcc hollow site to the octahedral sites in the subsurface. Moreover, activation energies for diffusion of H and D atoms over the saddle point of the PES between the fcc hollow site and the first (second) octahedral site are estimated as 598 (882) meV and 646 (939) meV, respectively. Furthermore, the activation energies for diffusion of H and D atoms over the saddle point of the PES between the first (second) octahedral site and the fcc hollow site are estimated as 285 (483) meV and 323 (532) meV, respectively.

Diameter Dependent Magnetic and Electronic Properties of Single-Walled Carbon Nanotubes with Fe Nanowires

We investigate the electronic and magnetic properties of single-walled carbon nanotubes (SWNTs) filled with Fe nanowires, based on the spin-polarized density functional theory. We find that in the stable structure, the Fe-filled (3,3) and (5,0) SWNTs exhibit semiconducting properties, and the magnetic moment of Fe nanowires inside disappears. On the other hand, the Fe-filled (4,4), (5,5), (6,6) and (6,0) SWNTs, having larger radii, are metallic and exhibit ferromagnetic properties. The corresponding magnetic moment increases with increasing nanotube diameter.

Reactive Ion Etching Process of Transition-Metal Oxide for Resistance Random Access Memory Device

The reactive ion etching (RIE) of the binary transition-metal oxides (TMOs) NiO, CuO and CoO, which are expected to be key materials of resistance random access memory (RRAMTM), was investigated. We found that inductively coupled plasma using CHF3-based discharge, which is highly compatible with conventional semiconductor RIE, is effective for the TMOs studied here. Furthermore, device fabrication using Pt/CoO/Pt trilayers is carried out, and a large change in resistance, which is an essential functionality of RRAM, was successfully observed. This should be definite evidence of a successful RIE realized in the present device fabrication.

Stability of Three-Hydrogen Clusters on Graphene

Realizing high hydrogen uptakes on surfaces is one of the essential aspects of practical hydrogen storage in solid-state materials. To achieve this, it is always beneficial to know how the road to adsorption saturation on the surface looks like in terms of the physical mechanisms involved, and how we can control required reactions given this knowledge. On this topic, work has been carried out on how the simplest groups—pairs—of hydrogen behave on graphite/graphene. In that study it was shown that hydrogen pair interaction cannot be described by a simple function of interadsorbate separation, and that only certain pairing geometries on the surface are energetically favored (we note here that full relaxation of the substrate atoms was not found to change these conclusions). As detecting and discriminating singly adsorbed and small groups of adsorbates is an essential step to knowing how saturation can be reached, we have subsequently shown how probing surface electronic states can be used to identify an atomic hydrogen adsorbate, and distinguish it from the closely-spaced hydrogen pairs on the surface, affirming previously published experimental work on this subject, particularly that in ref. 3. In this paper we comment on the next step towards saturation: the formation of hydrogen clusters of three, i.e. hydrogen trios, on graphene. Results are discussed with respect to results obtained from hydrogen pairs adsorbed on graphene. Stable hydrogen adsorption configurations were determined through geometry optimization calculations using the VASP code, which implements the projector augmentedwave method for density functional theory-based electronic structure calculations. All calculations were spin-polarized, and utilized the exchange–correlation functional based on the PBE version of the generalized gradient approximation. We applied a 400 eV cutoff to limit the plane-wave basis set without compromising computational accuracy, and a 4 4 1 Monkhorst–Pack special k point grid for Brillouin zone sampling. Three H atoms on a 48-C atom single sheet comprise the unit cell, with C–C nearest-neighbor distances of 1.42 A before relaxation. All atoms were completely unrestricted in the geometry optimization. A 15.0 A vacuum separating adjacent sheets was used. Figure 1 shows the different clusters of three hydrogen atoms systems included in the computations of this study. We specifically choose the fourteen most closely-packed combinations of three atoms adsorbed on C atom ‘‘top’’ sites. Upon reconstruction hydrogen atom lateral positions generally don’t deviate much from the positions on receiving C atoms shown in Fig. 1. A trio is named based on its smallest H pairing component (o = ortho, m = meta, p = para) and distance of the third member of the trio from the pair center. This means, for example, that the trio labeled to1 is the three-hydrogen cluster comprised of a pair of adjacently adsorbed (ortho) hydrogen, and a third H atom adsorbed in the closest possible distance from the aforementioned pair. Table I shows the trios arranged by adsorption energy, starting with the most stable geometry. Table values were computed using the following expressions: Eads 1⁄4 Egr+3H(ads) ðEgr þ 3EH(g)Þ; Es 1⁄4 Eads=3; Eo 1⁄4 Egr+3H(ads) ðEgr+2H(ortho) þ EH(g)Þ; Em 1⁄4 Egr+3H(ads) ðEgr+2H(meta) þ EH(g)Þ; and Ep 1⁄4 Egr+3H(ads) ðEgr+2H(para) þ EH(g)Þ, where the terms Egr, EH(g), Egr+3H(ads), and Egr+2H are the total energies for the graphene sheet, a gas phase H atom, the system comprised of an adsorbed H trio and graphene, and the system comprised of an adsorbed H pair and graphene, respectively. The adsorption energy of an isolated H atom on graphene Eiso 1⁄4 Egr+H(ads) ðEgr þ EH(g)Þ is 0:77 eV, a value which differs slightly from previous calculations due to the different adsorbate coverage used in these studies. For the same reason pair and trio adsorption energies reported here also differ slightly from corresponding values reported in refs. 2 and 8. Adsorbed trios are generally stable: the Eads (and Es) values are all negative, meaning all adsorbed three-hydrogen groups are stable with respect to hydrogen atoms located far from the graphene surface. If Es for a given trio is less than Eiso, the trio is more stable compared with a system comprised of three isolated adsorbed H on graphene. In other words it would be more energetically favorable for the hydrogen atoms to clump together than to separate from each other on the surface, i.e., the net interaction shows an ‘attractive’ character. Only one trio— tm1— is found to1 to4

High-uptake graphene hydrogenation: a computational perspective

We review the physical mechanisms that lead toward the conversion of graphene into its fully hydrogenated counterpart, which is a material that possesses properties closer to those of diamond than graphene. These are discussed from a theoretical perspective, i.e., from calculations based on density functional theory. We first discuss stability trends in small clusters of adsorbed hydrogen, as well as surface structure and concurrent reactivity changes for graphene one-face and two-face hydrogenation. Effects of adsorbed hydrogen on graphene electronic states, which are essential to adsorbed hydrogen structure discrimination, are also discussed.

Electric and Magnetic Properties of Co-filled Carbon Nanotube

We investigate the electric and magnetic properties of a (3,3) single-walled carbon nanotube filled with a linear Co nanowire. We carry out first-principle calculations based on the spin-polarized density functional theory, and find that in the stable structure, it shows half metallic ferromagnetic behavior, i.e., the majority-spin electrons show metallic behavior while the minority-spin electrons have a semiconducting bandgap.

Identifying Hydrogen Atoms on Graphite

We comment on the identification of a hydrogen atom adsorbed on graphite and distinguishing it from closely-spaced pairs under the scanning tunneling microscope (STM) through electronic state calculations based on density functional theory. The presence of the H atom should be very well observable through a distinct feature most directly associated with the adsorbate itself, a threefold symmetry most apparent through the third-nearest neighbor C atoms, and through sublattice visibility differences. A comparison with effects on the electronic states brought about by closely-spaced pairs shows visible differences in the aforementioned three factors, which should enable us to discriminate among adsorbed structures. Results compare well with STM measurements of adsorbed deuterium on graphite.