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A theoretical study based on density functional theory for H2O dissociation on the metal surface of Pt(111) alloyed simultaneously with Ru and Mo was performed. The determination of the minimum energy path using the climbing image nudged elastic band (CI-NEB) method shows that the dissociation reaction of H2O with this catalyst requires almost no energy cost. This dissociation reaction is not only kinetically favored but also almost thermodynamically neutral and somewhat exothermic. The electronic structure analysis showed that much more charge was released in Mo and was used to bind the adsorbed hydroxyl (OHad). Further analyses of the density of states (DOS) showed that the large number of orbitals that overlap when OH binds to Mo are responsible for the stabilization of the OH-surface bond. The stability of the OHad fragment on the surface is believed to be a descriptor for the dissociation of H2O with an almost spontaneous process.

Quantum dynamics calculation, performed on top of density functional theory (DFT)-based total energy calculations, show dynamical quantum filtering via enhanced scattering of para-H2 on SrTiO3(001). We attribute this to the strongly orientation-dependent (electrostatic) interaction potential between the H2 (induced) quadrupole moment and the surface electric field gradient of ionic SrTiO3(001). These results suggest that ionic surfaces could function as a scattering/filtering media to realize rotationally state-resolved H2. This could find significant applications not only in H2 storage and transport, but also in realizing materials with pre-determined characteristic properties.

A study on the molecular adsorption of a water (H2O) monomer on Pt alloy surfaces with binary Pt-Ru and Pt-Mo and ternary Pt-Ru-Mo surface models was conducted. Some calculations of the interaction between a H2O monomer and the Pt surface were also done for reference. This study is based on density functional theory (DFT) with periodic models and aims to understand the H2O adsorption mechanism. In Pt, Pt-Ru, and Pt-Mo surfaces, H2O preferably adsorbs via an oxygen atom in a flat configuration at the top sites of Pt, Ru, Mo, respectively. However, on the ternary Pt-Ru-Mo surface, the adsorption configuration has the most inclined H2O structure relative to the surface. The results showed that the binding energy of H2O/Pt-Ru-Mo > H2O/Pt-Mo > H2O/Pt-Ru > H2O/Pt. The adsorption mechanism was then clarified by charge transfer and natural bonding. The charge transfer from the surface to the adsorbate is observed in all models, with the greatest charge transfer occurring on the surface of Pt doped with two Mo atoms. This is probably due to the fact that oxygen can attract the most charge on Mo, because the difference in electronegativity is greatest. The calculation results also show that Ru is the most hydrophilic metal for oxygen. However, since the adsorption structure is parallel to the surface, hydrogen (H) is also more sensitive to receive the charge. Subsequently, the most acceptable reason for the most stable adsorption for H2O/Pt-Ru-Mo is that the inclined structure yields the most orbitals hybridization at the H2O's highest occupied molecular orbital (HOMO). This drives the interaction by forming bonding states at the lowest energy and anti-bonding states at the highest energy.

Two types of melanin pigments, brown to black eumelanin and yellow to reddish brown pheomelanin, are biosynthesized through a branched reaction, which is associated with the key intermediate dopaquinone (DQ). In the presence of l-cysteine, DQ immediately binds to the –SH group, resulting in the formation of cysteinyldopa necessary for the pheomelanin production. l-Cysteine prefers to bond with aromatic carbons adjacent to the carbonyl groups, namely C5 and C2. Surprisingly, this Michael addition takes place at 1,6-position of the C5 (and to some extent at C2) rather than usually expected 1,4-position. Such an anomaly on the reactivity necessitates an atomic-scale understanding of the binding mechanism. Using density functional theory-based calculations, we investigated the binding of l-cysteine thiolate (Cys–S−) to DQ. Interestingly, the C2–S bonded intermediate was less energetically stable than the C6–S bonded case. Furthermore, the most preferred Cys–S−-attacked intermediate is at the carbon-carbon bridge between the two carbonyls (C3–C4 bridge site) but not on the C5 site. This structure allows the Cys–S− to migrate onto the adjacent C5 or C2 with small activation energies. Further simulation demonstrated a possible conversion pathway of the C5–S (and C2–S) intermediate into 5-S-cysteinyldopa (and 2-S-cysteinyldopa), which is the experimentally identified major (and minor) product. Based on the results, we propose that the binding of Cys–S− to DQ proceeds via the following path: (i) coordination of Cys–S− to C3–C4 bridge, (ii) migration of Cys–S− to C5 (C2), (iii) proton rearrangement from cysteinyl –NH3+ to O4 (O3), and (iv) proton rearrangement from C5 (C2) to O3 (O4).

Hydrazine (N2H4) and hydroxyl (OH) coadsorption on the Ni(211) surface is investigated using Density Functional Theory (DFT) based calculations. Effects of the vicinal steps and edges on the stability of various N2H4-OH coadsorption configurations are investigated and compared with the case of individual adsorption of the respective molecules. Stabilizing interaction between N2H4 and OH coadsorbate is found to be mediated by hydrogen-bond. The most stable interaction of N2H4-OH is found perpendicular to the step direction, with N2H4 and OH occupying the terrace and edge sites, respectively. Swapping the position of N2H4 and OH reduce the relative stability. However, placing N2H4 on edge sites stabilizes its cis-conformation with noticeable N - H bond elongation. Finally, N2H4 and OH coadsorption with N2H4 and OH coadsorbed on edge-sites is moderate in terms of overall stability but exhibits the weakest N2H4 coadsorption and longest hydrogen bond interaction distance. Nevertheless, these N2H4-OH coadsorption structures are almost equally accessible thermodynamically due to the relatively small energy differences among them (less than 0.2 eV). In an ideal extended periodic system at specific coverage used in this study, the obtained coadsorption reveals the formation of a chain-like structure between N2H4-OH linked by hydrogen bonds. The results demonstrate the importance of steps and edges in effective formations of hydrogen bond stabilizing N2H4 coadsorption with prospects of promoting proton-transfer-reaction from N2H4 to OH.

Elucidating the reaction mechanism of steam methane reforming (SMR) is imperative for the rational design of catalysts for efficient hydrogen production. In this paper, we provide mechanistic insights into SMR on Ru surface using first principles calculations based on dispersion-corrected density functional theory. Methane activation (i.e., C–H bond cleavage) was found to proceed via a thermodynamically exothermic dissociative adsorption process, resulting in (CHy + zH)* species (“*” denotes a surface-bound state, and y + z = 4), with C* and CH* being the most stable adsorbates. The calculation of activation barriers suggests that the conversion of C* into O-containing species via C–O bond formation is kinetically slow, indicating that the surface reaction of carbon intermediates with oxygen is a possible rate-determining step. The results suggest the importance of subsequent elementary reactions following methane activation in determining the formation of stable carbon structures on the surface that deactivates the catalyst or the conversion of carbon into O-containing species.

The phase stability and surface effects on binary transition metal nano-alloy systems were investigated using density functional theory-based first principles calculations. In this study, we evaluated the cohesive and alloying energies of six binary metal alloy bulk systems that sample each type of alloys according to miscibility, i.e., Au-Ag and Pd-Ag for the solid solution-type alloys (SS), Pd-Ir and Pd-Rh for the high-temperature solid solution-type alloys (HTSS), and Au-Ir and Ag-Rh for the phase-separation (PS)-type alloys. Our results and analysis show consistency with experimental observations on the type of materials in the bulk phase. Varying the lattice parameter was also shown to have an effect on the stability of the bulk mixed alloy system. It was observed, particularly for the PS- and HTSS-type materials, that mixing gains energy from the increasing lattice constant. We furthermore evaluated the surface effects, which is an important factor to consider for nanoparticle-sized alloys, through analysis of the (001) and (111) surface facets. We found that the stability of the surface depends on the optimization of atomic positions and segregation of atoms near/at the surface, particularly for the HTSS and the PS types of metal alloys. Furthermore, the increase in energy for mixing atoms at the interface of the atomic boundaries of PS- and HTSS-type materials is low enough to overcome by the gain in energy through entropy. These, therefore, are the main proponents for the possibility of mixing alloys near the surface.

Algal biofuels serve as a promising alternative energy source for liquid fuels. However, one of the bottlenecks in the conversion of microalgae to biofuels is the drying process. A moisture content of at most 10 % is desired for algal biomass prior to oil extraction to maximise biofuel yield. Conventional means of drying results to longer drying time and uneven drying of algal biomass. This study investigated the drying characteristics of microwave for microalgae (Chlorella vulgaris). Three microwave intensity levels (300, 600, and 900 W) were considered to dry 10, 20, and 30 of algal mass. Page model gave a better fit on the moisture ratio with time of microwave drying than the exponential model. Furthermore, the specific energy requirement was computed, and a relationship was found between moisture ratio with power and mass. Fourier transform infrared spectroscopy results showed significant reduction of infrared signal intensities of the functional groups present in the algae after drying at higher microwave power level. It was concluded that the 20 W/g microwave drying setting gave a lower specific energy requirement with good quality of remaining high lipid content qualitatively. Furthermore, it was recommended to use gas chromatography mass spectroscopy to further quantify the algal lipids and other functional groups.

A density functional theory (DFT)-based study of ethylene (C2H4) adsorption on a reduced anatase titanium dioxide (TiO2) (001) surface, i.e., with a surface oxygen vacancy (Ovac), is presented. It was found that C2H4 preferably adsorbs on the Ovac-site. The excess electrons originating from the removed oxygen weaken the C2H4 CC double bond by filling the lowest unoccupied molecular orbital (LUMO) or of C2H4, and simultaneously enhance the binding of C2H4 to the reduced anatase TiO2 (001). The bonding between the two C2H4 C atoms and the two Ti atoms nearest to the Ovac-site produces two σ-type bonds, leading to the emergence of a new localized mid-gap state. This hybrid 3d defect state can account for the ∼0.7 eV decrease in the band-gap observed in previous optical measurements of carbon-coated TiO2, formed after TiO2 exposure to C2H4 gas (Mater. Lett. 108, 2013, 134). Subsequent calculations on two initial decomposition pathways of C2H4 adsorbed on the Ovac-site show that the C–H bond and C–C bond cleaving require high activation barriers where the C–H bond cleaving is slightly easier compared to the C–C bond cleaving, 2.94 eV and 3.01 eV, respectively, (as calculated using DFT-D2+Ud = 3 eV) and the final states of the two initial decomposition pathways show similar endothermic characteristics. This finding indicates that the surface Ovac-sites tend to favor the formation of molecularly adsorbed Ti-bound sp3-C2H4 (–Ti–CH2CH2–Ti–) as compared to the dissociative adsorption case.

We investigated the effect of local curvature on the adsorption of oxygen on single-walled carbon nanotubes based on density functional theory calculations with van der Waals corrections. The results showed that as the curvature increases, the interaction of the nanotubes with oxygen increases as well. An oxygen atom was strongly chemisorbed on the bridge site of the nanotubes accompanied by a significant transfer of charge from the surface to the oxygen atom. Larger curvature enhanced both the adsorption energy and charge transfer due to the greater strain on the carbon atoms that led to a better interaction with oxygen. The oxygen molecule was physisorbed on the nanotubes with the interaction arising mostly from the long-range van der Waals interactions. The adsorption energy was also enhanced by greater curvature. The results were compared with the flat graphene sheet to confirm the effects of surface curvature.

Theoretical calculations based on dispersion-corrected density functional theory (DFT-D2) has been performed to investigate hydrazine adsorption and N–N bond cleaving on closed packed surfaces of 3d metals: Fe(110), Co(0001), Ni(111), Cu(111), and Zn(0001). The activation energies of N–N bond cleaving of hydrazine on each surface are estimated using climbing-image nudged elastic band (CINEB) method. The results showed that the activation energies for this process have increasing trend from Fe(110) to Zn(0001). By examining the electronic structure of the adsorbed hydrazine, it is found that this trend is related to occupation of derived 6σ* orbitals. It is also found that approximate linear relationship between reaction energy and activation energy (the Bronsted–Evans–Polanyi (BEP) relationship) holds for those surfaces.

The underlying mechanism of H atom absorption in the Pd(1 1 0) (1 × 2) missing-row reconstructed surface is investigated by performing density functional theory based calculations. The stronger binding energy of H on ridge than on trough site of the missing-row surface is due to the more pronounced creation of derived bonding state as had been depicted from the electronic structure of the system. Hydrogen absorption takes place with the involvement of other incoming H atoms through an assisted absorption process that is facilitated by the repulsion between the incoming H and the absorbing H. The geometry of the missing-row surface enables the Pd atoms to accommodate the H atoms efficiently leading to H absorption as well as H 2 dissociation.

We report density-functional-theory-based calculations of hydrazine adsorption and its N N bond cleaving on clean Fe(110) surface. It is found that hydrazine may adopt several adsorption configurations among which the most energetically stable is the bridging configuration. Adsorption on short bridge site generally has larger adsorption energies than those on long bridge site. N N bond cleaving is an exothermic process with reaction energies of 1.90 and 1.67 eV on long and short bridge site, respectively. Nudged elastic band method is used to estimate the activation energies of N N bond cleaving. Our results indicate that N N bond cleaving on long bridge site has lower activation energy (0.27 eV) compared to that of short bridge site (0.36 eV). By examining the molecular orbitals of the initial state it is found that this difference stems from stronger bond between the two NH 2 fragments adsorbed on short bridge site as compared to long bridge.

We performed density functional theory (DFT) based calculations to investigate the effects of O-vacancies on the adsorption of O 2 on anatase TiO 2 (001). Our calculation results show that we can promote O 2 adsorption on an initially inert stoichiometric TiO 2 (001) by introducing O-vacancy. The resulting excess electrons from the introduction of the O-vacancy redistribute around the neighboring Ti ions. An incoming O 2 can then adsorb at the O-vacancy site, either in the superoxide state (O 2 − ) or the peroxide state (O 2 2 − ). In the O 2 − state, the O O bond is oriented parallel to the surface, along [010]. In the O 2 2 − state, the O O bond is oriented perpendicular to the surface, along [001]. Healing of the surface occurs when one of the O atoms of the perpendicularly adsorbed O 2 fills the vacant site and the other atom diffuses, recovering the stoichiometric surface.

The role of the presence of Ag atom in Pd 3 Ag(111) surface on the adsorption of N and H atoms and NH species was studied by using first principles calculations based on density functional theory (DFT). The adsorption energies of N and NH species are weakened by at least 0.50 eV when Ag atom is one of the nearest neighbors, in contrast to the case of H, in which the adsorption energies are weakened by at most 0.15 eV. Local density of states (LDOS) profiles show that for N and NH adsorption near the silver alloy atom, the derived anti-bonding states are shifted below the Fermi level and hence the adsorption energy is weakened on the alloyed surface. In the presence of adsorbed N, the adsorption energies of H on the nearby sites are also reduced. Nonetheless, this reductions in H adsorption energies on the most stable sites are lower in Pd 3 Ag(111) surface. NH formations with H moving across the Pd atom on both surfaces show comparable activation barriers but the barrier is increased by 0.2–0.3 eV when the formation happens across Ag atom.

We have investigated the physisorption states of H-2 on Ag(111) surfaces. To clarify the accurate adsorption properties of H-2 on Ag(111), we performed first-principles calculations based on spin-polarized density functional theory (DFT) with the semiempirical DFT-D2 method and the newly-developed exchange functional with the non-local correlation functional vdW-DF2 (rev-vdW-DF2). We constructed exhaustive potential energy surfaces, and revealed that non-negligible out-of-plane potential anisotropy with a perpendicular orientation preference exists even for H-2 physisorption on planar Ag(111), as predicted by previous results of resonance-enhanced multiphoton ionization spectroscopy and temperature-programmed desorption experiments. Therefore, the molecular rotational ground states of ortho-H-2 split into two energy levels in the anisotropic potential. The obtained adsorption energy and the number of bound states, including the zero-point energies and the rotational energy shift, agree with diffractive and rotationally mediated selective adsorption scattering resonance measurements. The origin of the potential anisotropy on Ag(111) is a combination of the London dispersion interaction and the virtual transition of the metal electron to the unoccupied molecular state.

The decomposition of methane (CH4) is a catalytically important reaction in the production of syngas that is used to make a wide spectrum of hydrocarbons and alcohols, and a principal carbon deposition pathway in methane reforming. Literatures suggest that stepped Ni surface is uniquely selective toward methane decomposition to atomic C, contrary to other catalysts that favor the CH fragment. In this paper, we used dispersion-corrected density functional theory-based first principles calculations to identify the electronic factors that govern this interesting property of stepped Ni surface. We found that the adsorption of atomic C on this surface is uniquely characterized by a 5–coordinated bonding of C with Ni atoms from both the surface and subsurface layers. Comparison with Ru surface indicates the importance of the subsurface atoms of stepped Ni surface on its selectivity toward methane decomposition to atomic C. Interestingly, we found that substituting these subsurface atoms with other elements can dramatically change the reaction mechanism of methane decomposition, suggesting a new approach to catalyst design for hydrocarbon reforming applications.

Density-functional calculations were performed to study Pt–O bonding interaction on core–shell Ni@Pt and pure Pt clusters composed of 55 atoms each. Based on oxygen adsorption energy and ν(Pt–O) stretching frequency, the global trend of Pt–O bonding strength in the clusters accommodates the successive order of bridge (and/or hcp-like) > fcc-like > vertex site. Further analysis on oxygen-induced reconstruction effect using first nearest neighbor (1NN) analysis shows that 1NN Pt–Pt distribution pattern of surface atoms changes abruptly after oxygen adsorbed. The profiles of distorted 1NN distribution pattern are greatly determined by the position and the number of adsorbed oxygen on the clusters including whether the cluster type is core–shell or not. The broadening distribution pattern clearly indicates deviation of Pt–Pt bond length due to Pt–O bonds formation and the general trend is similar with the one observed in platinum-related spectroscopy studies.

The absorption of H atom in H-covered Pd(110) (1 × 2) missing-row surface is investigated using density functional theory based calculations. For 1.5 monolayer H coverage, interaction among the adsorbed H atoms is found to be negligible and was verified by analyzing the electronic structure of the system and by comparing the calculated binding energies with low H coverage case. Neither the monoatomic absorption of H nor the assistance from the initially adsorbed H atom explains the experimentally observed presence of subsurface H in Pd(110) at high H coverage. Instead, H absorption is realized through the assistance of incoming H atoms from vacuum. The absorption of H in the H-covered surface is non-activated while the dissociation of H 2 is the rate-limiting process. Dissociation on ridge site is energetically more preferred than on trough site and the difference in the activation barrier is elucidated by investigating the electronic structures of the system.

Some effects of cationic charge density distribution on single water molecule dissociation of [M(H2O)6]3+ clusters with M=Cr, Mn, Fe, Co, and Ni have been investigated using first-principles study. The molecular structures, molecular binding energies, hydration enthalpies, cation and water molecules orbitals, infrared vibrational frequencies, and potential energy surfaces/curves had been computed to fulfill the purpose of research. Our investigation results have revealed that the hollow shape of charge density of central Cr(III) makes the Cr-O bonds rigid as indicated by a higher activation energy for releasing one water molecule from [Cr(H2O)6]3+. The isotropic charge density of Fe(III) affects in lowering water rotational rigidities which is responsible for augmenting the activation energy, while the cones-shape charge density of Ni(III) weakens the ion-dipole interactions by increasing interactions between nearest neighbour water molecules.

The stability of Pt-skin on Pt 3 Co alloy against Co and Pt dissolution with respect to Pt-skin thickness, surface structure and composition is investigated using density functional theory calculations. It is found that, even under oxygen environment, Co migration to the surface is suppressed by the thicker Pt-skin (2–3 Pt atomic layers), confirming experiments. However, the instability of single Pt layer is attributed to the electronic effect of oxygen. The adsorbed oxygen redistributes charge from Pt–Co region to Pt–O region, weakening the Pt–Co bond and facilitating Co migration to surface via Co–O bond formation. We further note that in this system, the Co migrates easier in (111) surface than in (100), attributed to the difference in the oxygen adsorption sites (fcc vs bridge). A minimal negative electrode potential shift of 0.06–0.09 V for Pt dissolution is noted for thicker Pt-skin systems, indicating stability close to pure Pt. The Pt-skin composition is varied by introducing different Migration Barrier Layers (MBLs) = Ru, Rh, Pd, Os, Ir, in the mid-Pt-skin region and found that when MBL is Os or Pd, a novel low-cost composition and more stable MBL-substituted Pt-skin/Pt 3 Co ORR catalyst emerges.

We investigate the spin polarization of a non magnetic material, e.g., a carbon material made from ten C atoms forming a hexagonal structure with total spin S = 0, induced by a ferromagnetic material, e.g., two Fe atoms with a total spin S = 4. Based on the density functional theory, we calculate the total spin density of the system. Our preliminary results show that the total spin for the ten C atoms changes from S = 0 to S = 4, while the total spin of the two Fe atoms changes from S = 4 to S =0. These results seem to indicate that there is a promising possibility to induce spin polarization on a carbon material by Fe atoms.

This work investigated the adsorption of C 2 H 4 (ethylene) on anatase TiO 2 (001) surface by means of first-principles calculation. Four adsorption configurations that correspond to two main types of interaction of ethylene–solid surface, namely π-bonded and di-σ-bonded interactions, are studied. We found that ethylene adsorbed on top of unsaturated Ti 5c (five-fold coordinated Titanium) is the most stable configuration and this configuration corresponds to the π-bonded interaction. For the most stable configuration, the adsorption energy is calculated to be ~ − 0.2 eV (0.49 eV with vdW correction) and ethylene retains its initial sp 2 planar configuration upon adsorption. Vibrational frequency analysis shows that some small modifications are observed for each reported vibrational modes of ethylene. These findings suggest that ethylene is weakly adsorbed on clean TiO 2 surface and vdW interaction is found to have a significant contribution to the total binding energy of ethylene.

We investigated the reactivity of CeO2-supported Pt4 cluster (denoted as Pt4/CeO2(1 1 1)) toward O2 adsorption and dissociation as well as the geometry/electronic properties associated with such metal oxide supported cluster system using density functional theory and on-site Coulomb interaction correction via the Hubbard-like term, U (DFT+U). It was found that Pt4 binds strongly to CeO2(1 1 1) via Pt O Ce bonds which act as “anchors” between the surface and the cluster, confirming its non-sintering as found in experiments. The adsorption of the cluster involves net electron transfer to CeO2, however, charge redistribution also happens within the cluster (from Pt atom bonded to the surface to the Pt on top of the cluster). This charge couples to the top Pt leading to reduce its spin moment as compared to that of unsupported cluster. When O2 adsorbs on Pt4/CeO2(1 1 1), while it prefers Pt vertex site near the CeO2 surface, the O O bond elongation is more profound at the Pt Pt edges. The energy barrier for dissociating O2 from this edge site precursor state is smallest. A correlation between the O O bond length at the precursor state and the stability at the transition state is revealed. Finally, the barrier for dissociation in unsupported Pt4 is lower, indicating suppression of the cluster's reactivity due to the support. We attribute this to the hybridization of Pt-5d orbitals with O-2p orbitals in CeO2(1 1 1) leading to the broadening of Pt-5d states near the Fermi level.

Periodic density functional theory calculations are performed to investigate the nature of the BH 4ad and its interaction with H 2 O ad in the presence of homogenous electric field. We observed a significant charge polarity of BH 4ad on Os(1 1 1) and such property could explain the electrostatic interaction with water monomer (H ad ) with its HOH plane parallel to the surface. This interaction changes the BH ad molecular structure to BH 3ad + H ad . In the presence of homogenous electric field, the water co-adsorption effect is reduced due to the stabilization of H 2 O ad on the surface and the deviation of the O–H bond from the plane, decreasing the electrostatic interaction between BH 4ad and H 2 O ad . These fundamental findings imply accessible control of borohydride structures on an electrode surface, which could be relevant for direct borohydride fuel cell (DBFC) and reversible hydrogen storage/release applications.

Recent studies of the ballistic transport in the mesoscopic systems performed in our laboratory are reviewed in the present paper. In the first half of the present paper, we investigate the characteristics of ballistic transport in quantum wires of which the effective confined length (the effective width) varies along the direction parallel to the current. It is shown that the step-wise variation of the conductance as a function of the Fermi energy, corresponding to the ideal quantization of the conductance, is smeared when the effective confined length of the wire varies along the direction parallel to the current and when the temperature increases. In the second half of the present paper, we show the numerical results of wave functions of the stationary states for the geometry of the semi-infinite two­ dimensional region with a narrow channel as an injector in the external magnetic field. In the presence of the magnetic field, we obtain a periodic peak structure in the modulus squared of the wave function along the boundary in the semi-infinite two-dimensional region, the period of which is nearly equal to the classical cyclotron diameter. Moreover, it is shown that small peaks exist between the periodic main peaks, which is considered to be one of the characteristic aspects of the quantum interference effects. The quantization of the conductance is one of the most remarkable findings in the ballistic transport phenomena, which has recently been discovered in the two­ dimensional electron system of GaAs-AlGaAs heterostructures_l),z) An explanation of the observed quantization of the conductance has been given on the basis of the assumption of the quantization of the motion along the direction perpendicular to the current. For simplicity, let us consider a quasi-one-dimensional straight wire. Although the electronic states along the wire are described in terms of plane wave functions, in the direction perpendicular to the current the electronic states are described by localized orbitals, and the energy of the electronic states in this direction is quantized. That is, the electronic states have subband structures. If the number of subbands intersecting the Fermi level is Nc (the number of open channels), the conductance of the system can be given by G=(2e 2 /h)Nc on the basis of the Landauer

We report results of our experimental and theoretical studies on the oxidation of Cu-Au alloy surfaces, viz., Cu3Au(111), CuAu(111) and Au3Cu(111), using hyperthermal O2 molecular beam (HOMB). We observed strong Au segregation to the top layer of the corresponding clean (111) surfaces. This forms a protective layer that hinders further oxidation into the bulk. The higher the concentration of Au in the protective layer formed, the higher the protective efficacy. As a result, of the three Cu-Au surfaces studied, Au3Cu(111) is the most stable against dissociative adsorption of O2, even with HOMB. We also found that this protective property breaks down for oxidations occurring at temperatures above 300 K.

The adsorption of borohydride on 3d transition metals (Cr, Mn, Fe, Co, Ni and Cu) was studied using first principles calculations within spin-polarized density functional theory. Magnetic effect on the stability of borohydride is noted. Molecular adsorption is favorable on Co, Ni and Cu, which is characterized by the strong s–dzz hybridization of the adsorbate-substrate states. Dissociated adsorption structure yielding one or two H adatom fragments on the surface is observed for Cr, Mn and Fe.

The resistance random access memory (RRAM™) device, with its electrically induced nanoscale resistive switching capacity, has attracted considerable attention as a future nonvolatile memory device. Here, we propose a mechanism of switching based on an oxygen vacancy migration-driven change in the electronic properties of the transition-metal oxide film stimulated by set pulse voltages. We used density functional theory-based calculations to account for the effect of oxygen vacancies and their migration on the electronic properties of HfO2 and Ta/HfO2 systems, thereby providing a complete explanation of the RRAM™ switching mechanism. Furthermore, computational results on the activation energy barrier for oxygen vacancy migration were found to be consistent with the set and reset pulse voltage obtained from experiments. Understanding this mechanism will be beneficial to effectively realizing the materials design in these devices.

We focused on the basic adsorption properties of two types of cations, viz., 1-methoxymethyl-1-methylpyrrolidinium (MMMP) and 1-ethyl-3-methylimldazolium (EMI), on graphene, as a first step in investigating the properties of carbon electrodes in an electric double layer capacitor. To clarify the basic adsorption properties, we used first principles calculation based on the density functional theory with the generalized gradient approximation for the exchange-correlation energy. We investigated the electrical dipole moment formed between the cations and graphene and found that electric dipole moment per cation formed by MMMP is larger than by EMI. In addition, we found that intramolecular bindings of MMMP are stronger than those of EMI. Based on these results, we conclude that MMMP is more suitable as the material for electric double layer capacitor than EMI.

We report a spin-polarized density functional theory (spin-polarized DFT) study of the electronic properties of defected GaAs bulk modeled by a supercell of 216 atoms. MetaGGA (mBJ-LDA) is used to describe the exchange-correlation and the PAW method is used to integrate spin–orbit (SO) interaction to suitably account for the broken inversion-symmetry of GaAs. We note that the band-gap and the split-off band at Γ of the clean (no defects) GaAs bulk are in agreement with that of the experiment and with other sophisticated yet very expensive methods, GW and HSE. For the GaAs bulk with defects, we found that at ~1% concentration of As-antisite (AsGa), mid-gap state appears. The structural origin of this state is a AsGa-centered tetrahedral cluster and not the AsGa point defect alone. The energy level of the mid-gap state, obtained with respect to the conduction band minimum, is 0.791 eV, in excellent agreement with the below band-gap excitation at 0.790 eV. This energy level reduces to 0.645 eV when SO interaction is neglected, indicating the significance of SO effects in the stabilizing mid-gap states. The defected GaAs also exhibits split-off band at Γ with a value similar to that of clean GaAs.