Impurity effects on adhesive energies

Some simple arguments are made about the effect of impurities on adhesive interactions at solid junctions. A universal adhesive force relation is derived for brittle adhesion. The adhesive binding energy, ΔE, is an important parameter in brittle adhesion forces. ΔE has also been shown by others to be important when there is plastic flow. We found that impurity effects on ΔE are determined by the segregation energies to the junction and to the free surfaces. At low temperatures, if it is energetically more favorable for impurities to segregate to the surfaces than to the junction, then the impurities will decrease ΔE. The converse is also true. For example, for self junctions which are in registry, ΔE is decreased if surface segregation is exothermic and increased if it is endothermic. These segregation energy relationships are consistent with the results of a number of experiments on the effects of impurities on adhesion forces and grain boundary embrittlement.     

Impurity effects on ionic-liquid-based supercapacitors

Small amounts of an impurity may affect the key properties of an ionic liquid and such effects can be dramatically amplified when the electrolyte is under confinement. Here the classical density functional theory is employed to investigate the impurity effects on the microscopic structure and the performance of ionic-liquid-based electrical double-layer capacitors, also known as supercapacitors. Using a primitive model for ionic species, we study the effects of an impurity on the double layer structure and the integral capacitance of a room temperature ionic liquid in model electrode pores and find that an impurity strongly binding to the surface of a porous electrode can significantly alter the electric double layer structure and dampen the oscillatory dependence of the capacitance with the pore size of the electrode. Meanwhile, a strong affinity of the impurity with the ionic species affects the dependence of the integral capacitance on the pore size. Up to 30% increase in the integral capacitance can be achieved even at a very low impurity bulk concentration. By comparing with an ionic liquid mixture containing modified ionic species, we find that the cooperative effect of the bounded impurities is mainly responsible for the significant enhancement of the supercapacitor performance.     

Impurity effects in graphene

Polarization-independent and omnidirectional tunable bandpass filters have been proposed and demonstrated. It is seen that a bilayer period structure composed of single-negative materials can act as a tunable filter, which can be changed from a filter with two symmetrical passband to one with a single pass bandpass, or from one with a wide to one with a narrow pass band. Moreover, the proposed filter is insensitive to the incident angle and polarization of light, which is an omnidirectional, polarization-independent filter for incidence angles between 0° to 90°.     

Impurity effects in electron hole droplets

We use a simple model to describe the effects of impurities on the electron hole droplet emission spectrum. In this model, an attractive interaction between the droplet and impurities causes a decrease in droplet density and an increase in the droplet work function. The predictions of the model are in good agreement with experiment.     

Impurity effects on lowest-energy Cooper pairing in an antiferromagnet

Isotropic scattering of electrons from nonmagnetic impurities does not suppress lowest-energy Cooper pairing in an antiferromagnet at all, and effects of non-isotropic scattering are expected to be small in magnitude. For this state, impurities substituted for magnetic ions affect the superconductivity mainly through their effects on the antiferromagnetism. Effects of nonmagnetic impurities on lowest-energy Cooper pairing in an antiferromagnet are just as though the pairing were s-wave in a nonmagnetic superconductor: in this state anisotropy of the pairing is purely a spin-density anisotropy and not a charge-density anisotropy. The Cooper pairing scheme which has lowest free energy in a perfect-crystal antiferromagnet also has lowest energy in a dirty antiferromagnet.     

Impurity effects in amorphous germanium and silicon

A systematic study of the hoping conductivity of amorphous germanium-transition metal (Cr, Co, Fe) films reveals an exponential decrease of the hopping parameter, T_o, as a function of the transition metal concentration. A similar, but often unnoticed, behavior is found in the literature. A linear decrease in the energy gap as a function of concentration is postulated as an explanation.     

A model for hydrogen chemisorption including image charge effects

The Green function method is applied to an Anderson hamiltonian including the interaction of the chemisorbed hydrogen atom with the surface plasmon of the metal substrate. The resulting adatom density of states presents a periodic structure with a period equal to the surface plasmon energy. Numerical results are obtained for the case of hydrogen adsorbed on tungsten explaining the peaks observed in photo-emission experiments.     

Tight-binding Ising-model study of chemisorption effects on surface segregation in transition-metal alloys

The tight-binding Ising model is generalized for the system consisting of a transition-metal alloy with chemisorbed adatoms. The effect of chemisorption on the surface segregation is calculated within the scope of a semi-infinite linear chain model for hydrogen chemisorbed on a Cu-Ni alloy. The charge-neutrality condition is incorporated into the model and its effect on the calculated surface segregation is demonstrated.     

Coverage effects under atomic chemisorption: Morse-potential modeling based on bond-order conservation

Our recent analytical Morse-potential model based on bond-order conservation has been extended to treat coverage (θ) effects on the heat of atomic chemisorption Q. For highly symmetric surfaces such as fcc(111), fcc(100), and bcc(100), explicit expressions for Q versus θ have been obtained projecting regularities of Q(θ) and of the overlayer structures, in encouraging agreement with experiment. In particular, the model predicts that Q should typically decrease with θ (though at very low θ, Q can sometimes increase) and that there may be some critical coverage θ_c<1 beyond which the second-order phase transition (hollow→bridge or on-top) will occur.     

Atomic chemisorption on simple metals: Chemical trends and core-hole relaxation effects

We have obtained essentially exact numerical solutions for a simple model of atomic chemisorption on simple metals. The approximations constituting the model are the semi-infinite jellium simulation of the metal substrate and the self-consistent local density theory of exchange and correlation. The solutions provide a detailed picture of the electronic charge making up the chemisorption bond. The variation of this picture with the valence of the adatom exhibits in a direct and microscopic way the roles of electronegativity, charge transfer, and covalency. Predicted bond energies, bond lengths, and dipole moments are consistent with measurements and independent theoretical considerations.     

Hydrogen chemisorption on ferromagnetic metals

The problem of hydrogen chemisorption on ferromagnetic substrates is studied using the Anderson-Newns Hamiltonian with the explicit inclusion of the substrate magnetization. A model density of states is used to approximate the particular case of nickel. A polarized solution is obtained for the adatom, which can also explain the rather large width of the photoemission peak observed for hydrogen chemisorbed on nickel.     

Hydrogen chemisorption on metal surfaces

The adsorption of H atoms on metal (jellium) surfaces has been investigated using linear response theory within the density functional formalism. The adsorbate is represented initially by a 1S orbital on the H atom, which perturbs the jellium surface and indirectly the adsorbate itself. The interaction energy curves, atomic binding energies, induced dipole moments, chemical shifts associated with the adsorbate, and vibrational excitation energies at the equilibrium internuclear separation have been calculated for a single H atom chemisorbed on metal surfaces. The sum of the atomic binding energy and the ionization potential of the H 1S level may be regarded as the initial state energy in the case of photoemission from the chemisorbed H. The rather satisfactory overall agreement between the theory and the experimental results for binding energies, vibrational excitation energies, and dipole moments suggests that this simple formalism could also have useful applications in more complicated chemisorbed systems.     

Alkali-metal chemisorption

The linear response function in the density functional formalism developed by Ying, Smith and Kohn is applied to alkali chemisorption on metallic surfaces, the adatom being represented by a pseudopotential perturbing the surface.Binding energies, induced dipoles and vibrational frequencies at the equilibrium position are calculated for a series of alkalis. The theoretical values for these quantities are in good agreement with experimental results where available.     

Impurity effects in the optical absorption of quantum rings

We study the interplay between impurity scattering and Coulomb interaction effects in the absorption spectrum of neutral bound magnetoexcitons confined in quantum-ring structures. Impurity scattering breaks the rotational symmetry of the ring system, introducing characteristic features in the optical emission. Signatures of the optical Aharonov–Bohm effect are still present for weak scattering and strong Coulomb screening. Furthermore, an impurity-induced modulation of the absorption strength is present even for a strong impurity potential and low screening. This behavior is likely responsible of recent experimental observations in quantum-ring structures.