Polymer adhesion: first-principles calculations of the adsorption of organic molecules onto Si surfaces

The structures and energetics of organic molecules adsorbed onto clean and H-passivated Si(001)-(2$\times$1) surfaces have been calculated using density functional theory. For benzene adsorbed on the clean Si surface the tight-bridge structure was found to be stable and the butterfly structure metastable. Both carbonic acid H$_2$CO$_3$ and propane C$_3$H$_8$ dissociate on contact with the surface. Passivation of the Si surface with H-atoms has a dramatic effect on the surface properties. The passivated surface is very inert and the binding energy of all the molecules is very weak.Comment: 8 pages, 13 figure

A hierarchical dualscale study of bisphenol-A-polycarbonate on a silicon surface : structure, dynamics and impurity diffusion

A previously developed and studied coarse-grained model is used to investigate the properties of bisphenol-A-polycarbonate (BPA-PC) in contact with the Si(001)-(2 x 1) surface. The surface interaction potentials are based on density functional calculations. Both a smooth wall potential and a site-dependent wall potential were used to represent the surface. For both types of surface potential it was found that only the chain ends adsorb and the density profiles and conformations in each case are similar. The site-dependent surface slows the dynamics of the polymer at the interface by an order of magnitude compared to the bulk dynamics for the chain lengths considered. The diffusion of non-adsorbing impurity particles for both surface potentials was investigated and the concentration and dynamics of the impurity particles were analysed

Comment on "Binding of hydrogen molecules by a transition-metal ion"

A Comment on the Letter by Niu, Rao, and Jena Phys. Rev. Lett. 68, 2277 (1992).Peer reviewe

Atomic-scale modeling of the ion-beam-induced growth of amorphous carbon

The results of a detailed molecular-dynamics study of the growth of amorphous carbon (a−C) are reported. Carbon atoms with kinetic energies between 10 and 150 eV are deposited on a−C surface originating from bulk a−C. Earlier simulation results of an optimal energy window at 40–70 eV are confirmed. Additionally, it is found that the growth rate is at maximum at around 40 eV. At low implantation energies (Ebeam≈10 eV), the growth of amorphous carbon takes place on the surface. At higher energies, the growth proceeds increasingly in the subsurface region by global film expansion and single atom diffusion towards the surface. Scattering events (e.g., the deposited atom does not adsorb to the surface) at intermediate energies Ebeam≈100 eV result in a densification of the growing film. Moreover, at Ebeam≈150 eV, nonpermanent diamond formation is observed.Peer reviewe

Valence-band offsets at the AlxGa0.5-xIn0.5P-ZnSe(001) lattice-matched interface

The difficulty in making good Ohmic contact at the interfaces with p-doped ZnSe is an important problem hindering the realization of blue-light-emitting diode lasers based on the II-VI semiconductor technology. So far no metal or semiconductor material has been found to have a low enough barrier at the (001) interface with ZnSe. A possible solution to this problem is the insertion of a so-called barrier-reduction layer at the interface with ZnSe. We have investigated the interface formation energies and valence-band offsets at the (001) interface between AlxGa0.5−xIn0.5P and ZnSe. The results of our calculations show the existence of a strong interdependence between the valence-band offset and the interface geometric structure. The interface is found to have structural and electronic similarities to the GaAs-ZnSe(001) system. The very low values obtained for the valence-band offset confirm the possibility of using this material as a major constituent of the barrier-reduction layer.Peer reviewe

Energetics of Sr adatom interactions on the Mo(112) surface

First-principles methods are used to investigate the formation and structure of the ordered phases of Sr atoms adsorbed on the furrowed Mo(112) surface. The energetics of various commensurate and incommensurate adatom structures providing information on lateral interactions between adatoms is determined for coverages 0.11⩽Θ⩽1 monolayer. It is found that the binding energy of Sr atoms decreases with increasing coverage. The experimentally observed p(8×1) and p(5×1) adatom chains are found to belong to the most favored structures for Θ<0.5. The energetic difference between these two structures amounts to 20meV. The experimental work function variation with Sr adatom coverage is very well reproduced. The energy barriers for Sr diffusion along the atomic troughs are calculated and discussed.Peer reviewe

First-principles calculation of Li adatom structures on the Mo(112) surface

The formation and structure of the ordered phases of Li atoms adsorbed on the Mo(112) surface are investigated by performing first-principles density-functional calculations. Large inward relaxation for the first atomic interlayer distance of the clean Mo(112) surface is found. The vertical relaxations of the substrate are only little influenced by the adsorbed Li atoms. The magnitude of lateral shifts of atoms along the atomic rows is found to be small and in line with measured values. The energetics of chain structures of Li adatoms for coverages 0.125<~Θ<~1 monolayer is determined and the p(4×1) and p(2×1) adatom structures are found to be the most favored ones in agreement with experiment. The binding energy of Li atoms decreases with increasing coverage. The calculated work function changes with Li adatom coverage are in good agreement with experiment. The energy barriers for adatom diffusion are determined and the energetics of lateral interactions between adatoms is discussed.Peer reviewe

Resistivity of Ca-Al metallic glasses

The resistivity of Ca-Al metallic glass is calculated as a function of temperature and composition. The diffraction model is used, including the partial dynamic structure factors. To account for the blurring of the Fermi surface we propose a new model which preserves the wave-packet character of the scattering electron. The results compare favorably with the experimental measurements available, indicating that the diffraction model is applicable also for high-resistivity systems. The use of t-matrix formulation of scattering is seen essential and the value of the Fermi wave vector to be used is shown to need careful consideration. The Fermi-surface blurring is found to affect the resistivity but the changes are not drastic. The origins of the large resistivity are seen to be in the strong scattering from Ca atoms, as expected, but also in the changes in the atomic and electronic structure.Peer reviewe

Modeling charge-imbalanced NaNbO3/SrTiO3 superlattices: Lattice relaxation and metallicity

The electronic and structural properties of different charge-imbalanced perovskite oxide NaNbO3/SrTiO3 superlattices are investigated with density-functional theory (local density approximation and local spin density approximation+U) methods. Metallic or insulating behavior of such a superlattice depends on the types of interfaces present: nonstoichiometric composition of a superlattice introduces holes to O p orbitals or extra electrons to Nb/Ti d orbitals. Lattice parameters, superlattice volume, and the extent of conduction electron or hole states are found to depend on interface type. The extent of the metallic state may also depend on the NaNbO3/SrTiO3 ratio. Octahedral rotations and other low-symmetry phases increase the gap between p and d orbitals but do not affect metallicity. Adding a Hubbard U to account for possible electronic correlations does not affect electron localization. Within LSDA+U, the delocalized holes align ferromagnetically.Peer reviewe