Nelson Arboleda

Dissociation and Sticking of H2 on Mg(0001), Ti(0001) and La(0001) Surfaces

We performed quantum dynamics calculations using previously obtained potential energy surfaces (PESs) for the dissociative adsorption of hydrogen molecule incident on a Mg(0001), Ti(0001), and La(0001) surface. Based on the sticking probability plots we obtained as functions of the incidence H 2 beam energy, La is the best material for hydrogen storage, followed by Ti, and then by Mg. This is due to the absence of an activation barrier in the H 2 /La(0001) system. Both H 2 /Ti(0001) and H 2 /Mg(0001) systems have activation barriers, but the H 2 /Ti(0001) system has a very small activation barrier far from the curved region of the reaction path, while the H 2 /Mg(0001) system has a high activation barrier close to the curved region along the reaction path. Our results also indicate that the sticking probability has some dependence on the vibrational state of the impending H 2 molecule for the Mg, Ti and La surfaces. The degree of dependence still varies in each metal. Vibrational effect is most observed w...

Potential Energy of H2 Dissociation and Adsorption on Pt(111) Surface: First-Principles Calculation

We investigate the interaction of molecular hydrogen with the Pt surface. In particular, we calculate the potential energy surfaces (PESs) corresponding to the dissociative adsorption of H2 at different symmetric sites on the Pt(111) surface and for parallel and perpendicular orientations of H2 with respect to the surface, on the basis of the density functional theory. The PES results show the dependence of the activation barriers on the H2 adsorption site and orientation relative to the surface. For parallel orientations, the barrier is lowest (almost zero) when the H2 center of mass (CM) is directly above the top site while the H atoms are directed towards the hcp and fcc hollow sites. The activation barriers for the perpendicular orientation are always much higher than those for the parallel orientation, which indicates that the parallel orientation is favored for H2 adsorption. This result also suggests that an incident H2 initially in a nonparallel (e.g., perpendicular) orientation tends to reorient itself towards a parallel orientation where it is easier for H2 to be adsorbed.

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.

Ab initio Investigation of Hydrogen Atom Adsorption and Absorption on Pd(110) Surface

Ab initio investigation based on density functional theory is performed to determine the behavior of H atom diffusion in Pd(110) surface to the first and second subsurface layers. Potential energy surface is constructed to determine the local minima and activation barriers of H pathways. Contribution of the relaxation of surface atoms in the binding energies of H and activation barriers along the diffusion paths, as well as the zero point energy corrections are also included in this work. The binding energies of H in the second subsurface layer are lower compared to its binding energies in the first subsurface layer and this is attributed to the interaction of H with the surface atoms and the differences in interlayer spacing of the surface layers. Comments on the adsorbate induced Pd(110) (1×2) missing/adding-row reconstruction phenomenon is also given with reference to the observed results in this work as H is absorbed from the surface to the first subsurface layer.

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.

Scattering and dissociative adsorption of H2 on the armchair and zigzag edges of graphite

We performed quantum dynamics calculations on the scattering and dissociative adsorption of hydrogen molecules incident on the armchair and zigzag edges of graphite layers, using relevant potential-energy surfaces (PESs) recently obtained by Dino et al. [e-J. Surf. Sci. Nanotech. 2, 77 (2003), and references therein]. By employing the coupled channel method to determine the reflection and sticking probabilities, we compared the hydrogen scattering and dissociative adsorption dynamics on the two graphite surfaces. Our findings show the different scattering behaviors of H2 for the armchair edge and for the zigzag edge, which enable the identification of an unknown graphite edge from its interaction with H2. The scattering on the zigzag edge is due to the highly curved region of the PES reaction path for H2 interacting with the zigzag edge, whereas the scattering for the armchair edge is caused by a potential barrier. The reflection probability initially decreases with increasing the kinetic energy in both c...

STM-induced switching of the hydrogen molecule in naphthalocyanine

The switching induced by the scanning tunneling microscope (STM) current of an adsorbed hydrogen molecule in the cavity of a naphthalocyanine molecule between two perpendicular orientations is studied. We regard such a system as a good candidate for a molecular size electronic logic gate since it causes a well observed change in the STM tunneling current. To investigate the switching phenomenon theoretically, a dynamical model is proposed in this study. Our suggested model is a three-level system, with the switching as a rotation induced by tunneling electrons considered as a second-order time dependent perturbation, where the electrons tunnel from the STM-tip to the naphthalocyanine molecule then to the metal substrate. The tunneled electrons will excite the hydrogen molecule rotational modes to jump over a potential barrier and then the switching will occur. To verify the model, the probability of the switching is calculated and plotted against the bias voltage at different temperatures using first-principles calculated parameters to fit fairly with experimental observations. In the light of our model and the DFT results, we explain the energy level (highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), and LUMO+1 orbitals) deviations with different substrates (NaCl and RbI) and the charge density distributions of these orbitals in different cases.

Site-Dependent Vibrationally Assisted Sticking Effect on H2–Si(001)2×2 Surface Interactions

We performed quantum dynamics calculations using available potential energy surfaces for the dissociative adsorption of H2 incident on a Si(001)2×2 surface. In this study, we show that the probability of adsorption for H2 and the effect of vibrationally assisted sticking (VAS) vary at different sites along the Si-dimer bond. Based on the sticking probability plots for H2 as functions of its initial translational energy, we show that the H2-silicon surface interactions are all activated, with the lowest potential barrier being found in the case of H2 coming down the Si surface with its center of mass roughly above the down Si-dimer atom. On the other hand, the VAS effect is strongest when H2, while coming down the silicon surface along the surface normal, approaches the point farthest from the up Si-dimer atom along the Si-dimer bond. Here, the potential barrier is closest to the curved region along the reaction path in the corresponding potential energy surface for this system.

DFT Investigation on the Electronic and Water Adsorption Properties of Pristine and N-Doped TiO2 Nanotubes for Photocatalytic Water Splitting Applications

Experimental studies have shown the production of hydrogen through a photocatalytic water splitting process using a titanium dioxide nanotube (TiO2NT) as a photoelectrode. In this study, a theoretical model of pristine and nitrogen-doped TiO2NT based on a TiO2 anatase (101) surface is presented. Spin unrestricted density functional theory calculations were performed to provide a detailed description of the geometries, electronic properties, and adsorption of water (H2O) on pristine and N-doped TiO2NT. The calculations show that doping with N will improve the photocatalytic properties of TiO2NT in two ways: First, the energy barrier of the dissociation reaction of water into hydroxyl radical and hydrogen atom is reduced; and second, the defect-induced states above the valence band lowers the band gap which will result in enhanced visible-light-driven photoactivity. Based on the position of the Fermi level relative to the defect induced energy levels, an optimal doping concentration of around 1.4% is proposed, which is in good agreement with experimental results. This study provides an atomic/molecular level understanding of the photocatalytic water splitting process and may serve as a groundwork for the rational design of more efficient photocatalysts.

H2 adsorption on K decorated germanene surface: an ab-initio investigation

The hydrogen physisorption on K-decorated germanene as potential hydrogen storage was calculated using DFT through VASP. Various approach sites and initial orientations for the H2 were considered for the adsorption. It was found that H2, with a vertical orientation, approaching the valley site was the preferred approach configuration with an associated adsorption energy within the van der Waals interaction energy range. Also, complete site-to-site migration was hindered by some potential barrier inherent in the material suggesting multiple metastable configurations.