Quantum Theory of Advanced Materials

Accurate multideterminant ground-state energies of circular quantum dots containing N <= 13 electrons as a function of interaction strength have been evaluated by the diffusion quantum Monte Carlo method. Two unique features are found for these confined two-dimensional systems: (1) as the electron density decreases, the quantum dots favor states with zero orbital angular momentum (L = 0); and (2) for some values of N, the ground state cannot be fully spin-polarized because of a symmetry constraint.

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.

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

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.

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.

We investigate the structural, electronic, and optical properties of hydrogen-passivated silicon nanowires along [110] and [111] directions with diameter d up to 4.2 nm from first principles. The size and orientation dependence of the band gap is investigated and the local-density gap is corrected with the GW approximation. Quantum confinement becomes significant for d<2.2 nm, where the dielectric function exhibits strong anisotropy and new low-energy absorption peaks start to appear in the imaginary part of the dielectric function for polarization along the wire axis.

Various surface passivations for silicon nanowires have previously been investigated to extend their stability and utility. However, the fundamental mechanisms by which such passivations alter the electronic properties of silicon nanowires have not been clearly understood thus far. In this work, we address this issue through first-principles calculations on fluorine, methyl and hydrogen passivated [110] and [111] silicon nanowires. Comparing these results, we explain how passivations may alter the electronic structure through quantum confinement and strain and demonstrate how silicon nanowires may be modelled by an infinite circular quantum well. We also discuss why [110] nanowires are more strongly influenced by their surface passivation than [111] nanowires.

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