Quantum Theory of Advanced Materials

Dissociative chemisorption of H-2 on the Al surface is a crucial step in the regeneration of promising hydrogen-storage materials such as alane and alanates. We show from first-principles calculations that transition metals such as V and Nb can act as effective catalysts for H-2 interaction with Al(100). When located at subsurface sites, V and Nb can reduce the activation barrier for H-2 dissociation by significantly larger values than the well-studied catalyst Ti. In addition, the binding energy of a H atom on the surface can be enhanced by as much as 0.4 eV when V or Nb is introduced in the sublayers of Al(100). The diffusion barrier for the adsorbed hydrogen is reduced by similar to 0.1 eV, showing an increased hydrogen mobility. The mechanism of promoting the metal surface reactivity by subsurface alloying with transition metals proposed in this work may serve as a new possible scheme for catalytic reactions on the metal surface.

We have performed calculations of adsorption energetics on the graphene surface using the state-of-the-art diffusion quantum Monte Carlo method. Two types of configurations are considered in this work: the adsorption of a single O, F, or H atom on the graphene surface and the H-saturated graphene system (graphane). The adsorption energies are compared with those obtained from density functional theory with various exchange-correlation functionals. The results indicate that the approximate exchange-correlation functionals significantly overestimate the binding of O and F atoms on graphene, although the preferred adsorption sites are consistent. The energy errors are much less for atomic hydrogen adsorbed on the surface. We also find that a single O or H atom on graphene has a higher energy than in the molecular state, while the adsorption of a single F atom is preferred over the gas phase. In addition, the energetics of graphane is reported. The calculated equilibrium lattice constant turns out to be larger than that of graphene, at variance with a recent experimental suggestion.

The growth of epitaxial graphene on SiC surfaces is accompanied by the evaporation of Si atoms during the growth process. The continuous loss of Si atoms takes place even after the surface graphene layers are formed. Understanding the atomic transport process involved is critical in establishing a growth mechanism to model and control the process. Using density functional theory, we have calculated the potential energy variation and studied the diffusion of Si and C atoms on a single layer of graphene and between graphene sheets. Our results show that Si atoms can move almost freely on graphene and between graphene layers, while C atoms have much larger diffusion barriers. This work provides a detailed description of the energetics of relevant processes in the growth of epitaxial graphene on SiC surfaces.

A p-T phase diagram of graphene nanoribbons (GNRs) terminated by hydrogen atoms is established based on first-principles calculations, where the stable phase at standard conditions (25 degrees C and 1 bar) is found to be a zigzag GNR (zzGNR). The stability of this new GNR is understood based on an electron-counting model, which predicts semiconducting nonmagnetic zzGNRs. Quantum confinement of Dirac fermions in the stable zzGNRs is found to be qualitatively different from that in ordinary semiconductors. Bifurcation of the band gap is predicted to take place, leading to the formation of polymorphs with distinct band gaps but equal thermodynamic stability. A tight-binding model analysis reveals the role of edge symmetry on the band-gap bifurcation.

Graphene is believed to be an excellent candidate material for next-generation electronic devices. However, one needs to take into account the nontrivial effect of metal contacts in order to precisely control the charge injection and extraction processes. We have performed transport calculations for graphene junctions with wetting metal leads (metal leads that bind covalently to graphene) using nonequilibrium Green's functions and density functional theory. Quantitative information is provided on the increased resistance with respect to ideal contacts and on the statistics of current fluctuations. We find that charge transport through the studied two-terminal graphene junction with Ti contacts is pseudo-diffusive up to surprisingly high energies.

The Hofstadter butterfly spectrum for Landau levels in a two-dimensional periodic lattice is a rare example exhibiting fractal properties in a truly quantum system. However, the observation of this physical phenomenon in a conventional material will require a magnetic field strength several orders of magnitude larger than what can be produced in a modern laboratory. It turns out that for a specific range of rotational angles twisted bilayer graphene serves as a special system with a fractal energy spectrum under laboratory accessible magnetic field strengths. This unique feature arises from an intriguing electronic structure induced by the interlayer coupling. Using a recursive tight-binding method, we systematically map out the spectra of these Landau levels as a function of the rotational angle. Our results give a complete description of LLs in twisted bilayer graphene for both commensurate and incommensurate rotational angles and provide quantitative predictions of magnetic field strengths for observing the fractal spectra in these graphene systems.

Electron-phonon coupling (EPC) in Bernal stacked bilayer graphene (BLG) at different doping levels is studied by first-principles calculations. The phonons considered are long-wavelength high-energy symmetric and antisymmetric optical modes. Both are shown to have distinct EPC-induced phonon linewidths and frequency shifts as a function of the Fermi level E-F. We find that the antisymmetric mode has a strong coupling with the lowest two conduction bands when the Fermi level E-F is nearly 0.5 eV above the neutrality point, giving rise to a giant linewidth (more than 100 cm(-1)) and a significant frequency softening (similar to 60 cm(-1)). Our ab initio calculations show that the origin of the dramatic change arises from the unusual band structure in BLG. The results highlight the band structure effects on the EPC in BLG in the high-carrier-density regime.

A small amount of catalyst, such as Ti, was found to greatly improve the kinetics of hydrogen reactions in the prototypical hydrogen storage compound sodium alanate (NaAlH(4)). We propose a near-surface alloying mechanism for the rehydrogenation cycle based on a detailed analysis of available experimental data as well as first-principles calculations. The calculated results indicate that the catalyst remains at subsurface sites near the Al surface, reducing the dissociation energy barrier of H(2). The binding between Ti and Al modifies the surface charge distribution, which facilitates hydrogen adsorption and enhances hydrogen mobility on the surface.

The surface states of ABC-stacked few-layer graphene ( FLG) are studied based on density-functional theory. These states form flat bands near the Fermi level, with the k-space range increasing with the layer number. Based on a tight-binding model, the characteristics of these surface states and their evolution with respect to the number of layers are examined. The infrared optical conductivity is then calculated within the single-particle excitation picture. We show that the surface states introduce unique peaks at around 0.3 eV in the optical conductivity spectra of ABC-stacked FLG when the polarization is parallel to the sheets, in good agreement with recent experimental measurement. Furthermore, as the layer number increases, the absorption amplitude is greatly enhanced and the peak position redshifts, which provides a feasible way to identify the number of layers for ABC-stacked FLG using optical conductivity measurements.

We have performed first-principles calculations to study the lattice vibrational modes and their Raman activities in silicon nanowires (SiNWs). Two types of characteristic vibrational modes are examined: high-frequency optical modes and low-frequency confined modes. Their frequencies have opposite size dependence with a red shift for the optical modes and a blue shift for the confined modes as the diameter of SiNWs decreases. In addition, our calculations show that these vibrational modes can be detected by Raman scattering measurements, providing an efficient way to estimate the size of SiNWs.

It has been demonstrated that replacing Li(2)NH with the mixed imide Li(2)Mg(NH)(2) improves the reaction conditions for the hydrogen-storage system Li(2)NH + H(2) <-> LiNH(2) + LiH, at the expense of reducing the gravitational hydrogen capacity from 6.5% to 5.6%. In this article, we report from first-principles calculations a possible mixed imide Li(6)Mg(NH)(4) that has less Mg concentration and higher gravimetric capacity for hydrogen storage than Li(2)Mg(NH)(2). We find that Li(6)Mg(NH)(4) is thermodynamically more stable than the phase-separated mixture of Li(2)Mg(NH)(2) and Li(2)NH over a large temperature range. The reaction LiH + 1/4Mg(NH(2))(2) + 1/2LiNH(2) <-> 1/4Li(6)Mg(NH)(4) + H(2) can be completed via two steps and releases 6.0 wt % hydrogen in total, at a temperature about 40 degrees C lower than that for the cycling between LiNH(2) and Li(2)NH.

A twisted graphene bilayer consists of two graphene monolayers rotated by an angle. with respect to each other. Theory predicts that charge-neutral twisted graphene bilayers display a drastic reduction of their Fermi velocity v(F) for 0 less than or similar to 0 less than or similar to 20 degrees and 40 less than or similar to 0 less than or similar to 60 degrees. In this paper we present evidence for an additional anisotropic reduction of v(F) in the presence of external electrostatic fields. We also discuss in quantitative detail velocity renormalization for other relevant bands in the vicinity of the K point. Except for a rigid energy shift, electrostatic fields and doping by metal atoms give rise to similar renormalization of the band structure of twisted graphene bilayers.

It has been suggested that the diffusion of AlH(3) vacancies plays an essential role in the decomposition of NaAlH(4), a prototypical material for hydrogen storage. We find from first-principles calculations that the AlH(3) vacancy induces several isolated vibrational modes that are highly localized in the vacancy region with frequencies within the phonon gaps of pure NaAlH(4) in both the a and g phases. Thus, the proposed existence of AlH(3) vacancies in the dehydrogenation reaction of NaAlH(4) can be possibly confirmed with the experimental detection of these unique vibrational modes associated with the AlH(3) vacancy.

In this paper, we study the low-lying energy spectra of a two-dimensional (2D) graphene-based magnetic dot in a perpendicular and radially inhomogeneous magnetic field with the use of the massless Dirac-Weyl equation. Numerical calculations are performed using 2D harmonic basis states for direct diagonalization. Effects of both the dot size and the magnetic field on the low-lying energy spectra are discussed.

We presented a detailed study of the oxidation functional groups (epoxide and hydroxyl) on graphene based on density-functional calculations. Effects of single functional groups and their various combinations on the electronic and structural properties are investigated. It is found that single functional groups can induce interesting electronic bound states in graphene. Detailed energetics analysis shows that epoxy and hydroxyl groups tend to aggregate on the graphene plane. Investigations of possible ordered structures with different compositions of epoxy and hydroxyl groups show that the hydroxyl groups could form chainlike structures stabilized by the hydrogen bonding between these groups, in close proximity of the epoxy groups. Our calculations indicate that the energy gap of graphene oxide can be tuned in a large range of 0-4.0 eV, suggesting that functionalization of graphene by oxidation will significantly alter the electronic properties of graphene.

Lithium imide (Li2NH) has been considered as a promising medium for hydrogen storage with the following reaction: LiNH(2)+LiH <-> Li(2)NH+H(2). All possible phases involved in the reaction need to be fully characterized in order to understand the right pathway connecting the two end compounds LiNH(2) and Li(2)NH and to further improve its reaction condition to meet the requirements of practical applications. We study from first-principles calculations the possible intermediate compounds Li(2-x)NH(1+x) between Li(2)NH and LiNH(2). Based on the energetics results, possible intermediate phases are identified for 0

Both the size and the magnetic-field dependences of low-lying spectra of two-dimensional (2D) graphene based magnetic dot and ring in perpendicular inhomogeneous magnetic fields, where the magnetic field is zero inside the dot and ring, and constant elsewhere, are studied by the massless Dirac-Weyl equation. Numerical results obtained from direct diagonalization with 2D harmonic basis show that, under nonuniform magnetic fields, the higher Landau levels (N >= 1) for such massless Dirac electron interacting system in general become nondegenerate and split into discrete angular momentum states with level crossings with the lowest one (N=0) being an exception. (C) 2010 American Institute of Physics. [doi:10.1063/1.3435478]

In this paper, we present the direct observation of quantum size effects (QSE) on the work function in ultrathin Pb films. By using scanning tunneling microscopy and spectroscopy, we show that the very existence of quantum well states (QWS) in these ultrathin films profoundly affects the measured tunneling decay constant kappa, resulting in a very rich phenomenon of "quantum oscillations" in kappa as a function of thickness, L, and bias voltage, V(s). More specifically, we find that the phase of the quantum oscillations in kappa vs. L depends sensitively upon the bias voltage, which often results in a total phase reversal at different biases. On the other hand, at very low sample bias (vertical bar V(s)vertical bar < 0.03 V) the measurement of kappa vs. L accurately reflects the quantum size effect on the work function. In particular, the minima in the quantum oscillations of kappa vs. L occur at the locations where QWS cross the Fermi energy, thus directly unraveling the QSE on the work function in ultrathin films, which was predicted more than three decades ago. This further clarifies several contradictions regarding the relationship between the QWS locations and the work function.

We report on a first-principles study of the conductance through graphene suspended between Al contacts as a function of junction length, width, and orientation. The charge transfer at the leads and into the freestanding section gives rise to an electron-hole asymmetry in the conductance and in sufficiently long junctions induces two conductance minima at the energies of the Dirac points for suspended and clamped regions, respectively. We obtain the potential profile along a junction caused by doping and provide parameters for effective model calculations of the junction conductance with weakly interacting metallic leads.

We present a first-principles study of the electron-phonon (e-ph) interactions and their contributions to the linewidths for the optical-phonon modes at Gamma and K in one-layer to three-layer graphene. It is found that, due to the interlayer coupling and the stacking geometry, the high-frequency optical-phonon modes in few-layer graphene couple with different valence and conduction bands, giving rise to different e-ph interaction strengths for these modes. Some of the multilayer optical modes derived from the Gamma-E(2g) mode of monolayer graphene exhibit slightly higher frequencies and much reduced linewidths. In addition, the linewidths of K-A(1)(') related modes in multilayers depend on the stacking pattern and decrease with increasing layer numbers.

We present a first-principles investigation to study the possible alloy phases of sodium and lithium alanates. Structural and energetics properties of alloy systems Na(1-x)Li(x)AlH(4) and Na(3(1-x))Li(3x)AlH(6) are studied via phase interpolation. Alloy system Na(1-x)Li(x)AlH(4) is found to have a small mixing energy (<5 kj/mol). The equilibrium structure undergoes a transition from a tetragonal structure to a monoclinic structure between x = 0.25 and 0.5. Within each structure the cell volume decreases with increasing x, which can be explained by Li having a smaller ion size than Na. Alloy system Na(3(1-x))Li(3x)AlH(6) is also studied, and one intermediate composition Na(2)LiAlH(6) is found to be stable in agreement with experimental findings. (C) 2009 Elsevier B.V. All rights reserved.