Planar Metal Surface Research Articles

Temperature-Independent Casimir-Polder Forces Despite Large Thermal Photon Numbers

We demonstrate that Casimir-Polder potentials can be entirely independent of temperature even when allowing for the relevant thermal photon numbers to become large. This statement holds for potentials that are due to low-energy transitions of a molecule placed near a plane metal surface whose plasma frequency is much larger than any atomic resonance frequencies. For a molecule in an energy eigenstate, the temperature independence is a consequence of strong cancellations between nonresonant potential components and those due to evanescent waves. For a molecule with a single dominant transition in a thermal state, upward and downward transitions combine to form a temperature-independent potential. The results are contrasted with the case of an atom whose potential exhibits a regime of linear temperature dependence. Contact with the Casimir force between a weakly dielectric and a metallic plate is made.

Evaluating the Role of Pt and Pd Catalyst Morphology on Electrocatalytic Methanol and Ethanol Oxidation

We have investigated the relative efficiency of Pt and Pd catalysts, in the structural formats of nanoporous solids and planar metallic surfaces, for the electro-oxidation of methanol and ethanol in acidic and basic aqueous solutions. There are two issues of primary concern in this comparison. These are the surface area of the two catalyst structures and the crystalline morphology of the metals. We find that the nanoporous solids of Pt and Pd afford greater electro-oxidation efficiency and increased catalytic stability relative to the corresponding planar metallic surface for methanol and ethanol. The increase in current density for the Pt nanoporous solid was greater for basic conditions than for acidic conditions. Although both metals provided increased catalytic activity relative to that of the corresponding planar metal, the reactivity of Pt was greater for methanol, while Pd exhibited greater reactivity for ethanol. Our data point to the utility of nanoporous solids for use as catalytic media.

Transformation of surface plasmon polaritons by surface structures

We present a new, simple, planar structure that effects the all-angle negative refraction of a surface plasmon polariton incident on it. We also predict a step-wise increase in the frequency dependence of the transmissivity of a surface plasmon polariton propagating through a slit of finite length on a planar metal surface at each frequency at which an infinitely long slit can support an additional guided wave polariton.

Dual band terahertz waveguiding on a planar metal surface patterned with annular holes

We report studies of the guiding of terahertz radiation on a copper surface textured with an array of blind annular holes. The structure supports two tightly bound surface plasmon-polaritonlike electromagnetic waves associated with TE11 and TEM coaxial waveguide modes. The TE11-like surface mode has a cutoff determined by the array period while the TEM mode can be arranged to have a lower frequency cut-off by adjusting the hole depth. The ability to guide two modes in independently variable bands could be useful in quantitative chemical sensing.

Wave-packet study of hyperthermal alkali ion neutralization at metal surfaces

Resonant neutralization of 50 eV Na + ions impinging on Cu(100) surfaces is studied, focusing on events where the incident particle undergoes multiple collisions within the first surface layers of the samples. Specific trajectories are generated, via molecular dynamics simulations, in which the projectile comes out interacting, on long time scales, either with a single surface atom or with the valence band of the solid. The resulting three-body problem is modelled by a semi-empirical, one-electron potential that incorporates: (i) the effect of a plane metal surface, with projected band gap, and (ii) the contributions of the projectile, whose charge state will be eventually investigated, as well as of the substrate atom. On this basis, a wave-packet propagation algorithm is constructed to establish the reverse evolution of the affinity orbital of the projectile. The calculated neutralization probability is compared with the results of a model Hamiltonian of the Anderson–Newns type (Nucl Instr Meth 2009;B267:578) and is found in good agreement with the angle resolved neutral fraction measured by Keller et al. (Phys Rev Lett 1995;75:1654).

Photoelectric field emission of electrons: Photon assisted tunneling

This note is a discussion of the enhancement in electric field emission of electrons from a plane metallic surface on irradiation by light. The enhancement in the tunneling coefficient of an electron by absorption of a photon and the consequent increase in the field emission current has been evaluated. A discussion of the results and possible applications concludes the presentation. In particular it is suggested that this phenomenon may be an explanation for the observed increase in photoelectric current with increasing magnitude of negative potential of lunar dust grain simulants.

Metal surfaces in interaction with molecular adsorbates

For a finite-step barrier model of a metal surface, it is shown that structure in the local density of states, around the bulk density of states, is induced deep into the metal. In a non-free-electron metal, it is stressed that the range over which such structure persists into the bulk metal depends on the shape of the Fermi surface. A summary is then given of the way in which chemical bonds are affected by a metal surface in physisorbed and chemisorbed regimes. A bond parallel to a planar metal surface is weakened and lengthened. Polyatomic molecules are discussed, with particular reference to H2O on the (0001) surface of Ru metal. The principle of maximum overlap predicts the center geometry for the O atom, not the overhead site. The spillout of electron density affects the angle of H2O and near the united atom it opens up the angle a little compared with the free space value for a given O—H length. Finally, the linear response function of the infinite-step barrier model, plus a model potential designed to simulate a flat molecule like benzene adsorbed parallel to the surface, is used to calculate the charge density displaced in the metal. Near the surface, this displaced electron density, along an axis through the center of the benzene ring and perpendicular to it, is shown to be sensitive to the dimensions of the adsorbed molecule. It would therefore be of interest if field gradients due to the displaced charge could be measured.

Manipulating microwaves with ‘spoof’ surface plasmons

The newly discovered ability to confine, guide, and manipulate microwaves in open structures on scales that are much smaller than their wavelengths appears to be heralding a new era in microwave technology. Since the days of Marconi, metal surfaces have been known to support surface electromagnetic waves. In many ways, such waves are identical to simple grazing radiation because the electromagnetic fields in the air above the planar metal surface extend over distances of order many tens of wavelengths and are almost completely excluded from the metal. Since these waves are only weakly localized at the surface, they are readily perturbed by objects placed some distance away. They are consequently not particularly useful for forming surface-guided wave structures. However, if the distance over which the electromagnetic field extends above the metal is considerably reduced, so that the mode is much more strongly bound to the surface, ideally over distances of less than the wavelength of the incident radiation, then surface-wave manipulation of microwaves is more readily achieved. By patterning a metal surface with grooves or holes with a characteristic dimension less than the wavelength of the incident radiation, one can alter the electromagnetic boundary conditions to strongly localize microwave radiation to that surface (see Figure 1). This highly localized wave is—using language borrowed from optical technology—a ‘spoof’ or ‘designer’ surface plasmon. In the visible regime, a surface plasmon is the trapped surface wave associated with density oscillations of the conduction electrons. (Observations of surface-plasmon excitation in the visible domain were first recorded, unknowingly, by Wood in 1902, who found that—for light impinging on a metal grating—a strong absorption of power occurred for a particular angle of incidence.) For the visible region, the degree of trapping of the mode at the metal surface and the usefulness of the surface plasmon depend strongly on the metal used. Figure 1. Schematic representation of electric fields associated with a mode propagating along the surface of a metal. (a) At microwave frequencies, the metal is almost perfectly conducting and the field (Ez) extends far beyond the metal. (b) By perforating the substrate with an array of subwavelength holes, the field is localized near the interface.

Double resonant neutralization in hyperthermal energy alkali ion scattering at clean metal surfaces

Resonant neutralization of hyperthermal Na+ ions impinging on clean Cu(100) surfaces is studied, focussing on long lived electronic interactions involving the projectile and a target atom. Specific trajectories are considered where the incident particle undergoes multiple collisions within the first surface layers, interacting simultaneously with several target atoms, which leads to single emission of a surface atom that can resonantly exchange charge with both the solid and the projectile. The system is described via a semi-empirical, one-electron potential that includes the effect of a plane metal surface, with projected band gap, the projectile, whose charge state will be eventually investigated, and the substrate atom. On this basis, a model Hamiltonian of the Anderson–Newns type is constructed and the calculated neutralization probability is compared with the angle resolved neutral fraction measured by Keller et al. [C.A. Keller, C.A. DiRubio, G.A. Kimmel, B.H. Cooper, Phys. Rev. Lett. 75 (1995) 1654].

Diffusiophoresis of a spherical particle normal to an air–water interface

The diffusiophoresis of a spherical colloidal particle normal to an air–water interface subject to a uniform electrolyte concentration gradient is investigated theoretically. The governing electrokinetic equations are solved numerically with a pseudo-spectral method based on Chebyshev polynomials. Key parameters such as the distance between the particle and the interface, the double-layer thickness, and the surface potential of the particle are examined to analyze their respective effect on the diffusiophoretic velocity of the particle. Distinctive features pertinent to an air–water interface are investigated in particular as compared with a solid metal surface. It is found in this study, among other things, that the diffusiophoretic velocity of a particle moving toward an air–water interface is always less than that toward a planar metal surface when the diffusivities of cations and anions are identical. This can be explained nicely by the classic image-charge analogue in electrostatics, where the former (air–water interface) stands for an image-charge of the same sign, while the latter (planar metal surface) stands for an image-charge of the opposite sign. In the case of distinct diffusivities of cations and anions, the situation is much more complicated. No such simple and convenient analogue is observed.

Sheath analysis in collisional electronegative plasmas with finite temperature of positive ions

The sheath which occurs in front of a plane metallic surface immersed in an electronegative plasma is analysed taking into account the positive ion collisions and thermal motion. Collisions do not affect the equations of the sheath but have a significant influence on the initial conditions, which are obtained from the plasma solution. The combined influence of collisions and positive ion thermal motion on the sheath, the sheath thickness, the positive ion saturation current and the floating potential is analysed.

Enhanced optical reflection and multiple-peak transmission from a subwavelength slit with surface corrugations

We have observed a new phenomenon of the enhanced optical reflection and transmission from a subwavelength metallic slit with surface corrugations. The reflected energy is focused by the periodic structure and our experiment shows that it is more effective than a planar metal surface in real situations. The transmission spectrum performs the feature of a multipeak narrow-band transmission with a wavelength spacing of 1.4 nm resulting from the FP cavity resonance in the substrate film. Such optical response properties of our sample will be very useful for applications in many fields including wavelength-division-multiplexing (WDM) optical communications systems.

Nanoscale surface chemistry over faceted substrates: structure, reactivity and nanotemplates

Faceting is a form of self-assembly at the nanometre-scale on adsorbate-covered single-crystal surfaces, occurring when an initially planar surface converts to a "hill and valley" structure, exposing new crystal faces of nanometre-scale dimensions. Planar metal surfaces that are rough on the atomic scale, such as bcc W(111), fcc Ir(210) and hcp Re(1231), are morphologically unstable when covered by monolayer films of oxygen, or by certain other gases or metals, becoming "nanotextured" when heated to temperatures above approximately 700 K. Faceting is driven by surface thermodynamics (anisotropy of surface free energy) but controlled by kinetics (diffusion, nucleation). Surfaces can spontaneously rearrange to minimize their total surface energy (by developing facets), even if this involves an increase in surface area. In this critical review, we discuss the structural and electronic properties of such surfaces, and first principles calculations are compared with experimental observations. The utility of faceted surfaces in studies of structure sensitive reactions (e.g., CO oxidation, ammonia decomposition) and as templates for growth of metallic nanostructures is explored (122 references).

Electromagnetic model and calculations of the surface-enhanced Raman-shifted emission from Langmuir-Blodgett films on metal nanostructures

We present a theoretical study of the electromagnetic contribution to surface-enhanced Raman scattering (SERS) from a Langmuir-Blodgett film close to a metal surface. This macroscopic dipolar model fully accounts for the Raman-shifted emission so that meaningful SERS (electromagnetic) enhancement factors that do not depend only on the local electromagnetic field enhancement at the pump frequency are defined. For a plane metal surface, analytical SERS enhancement factors that are consistent for all pump beam polarization and molecular orientation are obtained. In order to investigate SERS on complex nanostructured metal surfaces, we introduce this model into the formally exact, Green's theorem surface integral equation formulation of the scattered electromagnetic field. This formulation is thus employed to calculate numerically the near-field and far-field emissions at the Raman-shifted frequency for very rough, random nanostructured surfaces, with emphasis on the impact of collective processes for varying pump frequency and Raman shift. Our results reveal that the widely used |E|4 approximation tends to overestimate average SERS enhancement factors.

Molecularly Resolved Protein Electromechanical Properties

Previous work has shown that protein molecules can be trapped between the conductive surfaces presented by a metal-coated AFM probe and an underlying planar substrate where their molecule-specific conductance characteristics can be assayed. Herein, we demonstrate that transport across such a derived metal-protein-electrode junction falls within three, pressure-dependent, regimes and, further, that pressure-dependent conductance can be utilized in analyzing temporal variations of protein fold. Specifically, the electronic and mechanical properties of the metalloprotein azurin have been characterized under conditions of anisotropic vertical compression through the use of a conducting atomic force microscope (CP-AFM). By utilizing the ability of azurin to chemically self-assemble on the gold surface presented either by the apex of a suitably coated AFM probe or a planar metallic surface, molecular-level transport characteristics are assayable. Under conditions of low force, typically less than 2 nN, the weak physical and electronic coupling between the protein and the conducting contacts impedes tunneling and leads to charge buildup followed by dielectric breakdown. At slightly increased force, 3-5 nN, the copper protein exhibits temporal electron occupation with observable negative differential resistance, while the redox-inactive zinc mutant does not. At imposed loads greater than 5 nN, appreciable electron tunneling can be detected even at low bias for both the redox-active and -inactive species. Dynamic current-voltage characteristics have been recorded and are well-described by a modified Simmons tunneling model. Subsequent analyses enable the electron tunneling barrier height and barrier length to be determined under conditions of quantified vertical stress. The variance observed describes, in essence, the protein's mechanical properties within the confines of the tunnel junction.

Fermi edge singularities in ion-induced electron emission from plane metal surfaces

Auger electron emission – following low energy monoatomic ion impact on metal surfaces – is studied with a many body theory that includes both the band structure and the Fermi singular response of metal electrons (to the sudden neutralization of the projectiles). Application is made to the experimental kinetic energy distributions of electrons ejected from polycrystalline Al by Ar+-projectiles at varying incident energies and angles. Excellent agreement is found between the theory and the measurements.

Wave packet study of the secondary emission of negatively charged, monoatomic ions from sputtered metals

Secondary emission of Ag− and Au− particles, following the sputtering of clean Ag(100) and Au(100) targets, respectively, is studied with a Crank–Nicholson wave-packet propagation method. A one-electron pseudo-potential is used to describe the plane metal surface, with a projected band gap, the ejected ion, whose charge state is investigated, and its nearest-neighbor substrate ion, put in motion by the collision cascade generated by the primary ion beam. Time-dependent Schrödinger equation is solved backwards in time to determine the evolution of the affinity orbital of the negative particles from an instant when they are unperturbed, at distances of the order of ∼102a.u. from the surface, to the instant of ejection. The probability that a band electron will be eventually detected in affinity state of the ejected particle is, thus, calculated and compared with the result of another method based on the spectral decomposition of the one-electron Hamiltonian.

Auger electron emission from metals induced by low energy ion bombardment: Effect of the band structure and Fermi edge singularity

Calculations of the kinetic energy distributions of electrons ejected from plane metal surfaces by Auger neutralization of slow monoatomic ions are reported. A many body theory is used that includes both the band structure of the target material and the Fermi singular response of metal electrons (to the sudden neutralization of the projectile). Application is made to experiments of electron emission from polycrystalline Al by Ar +-ions, at varying incident energies and angles. Adjustment of the broadening parameters of the distribution of shake-up electrons leads to excellent agreement between the theory and the measurements.

Negative ionization of the secondary ions of silver and gold sputtered from their elemental surfaces

Calculations of the ionization probabilities of Ag− and Au− particles, ejected during sputtering of clean Ag(100) and Au(100) surfaces, respectively, are reported. An effective one-electron theory is used to describe: the plane metal surface, with a projected band gap, the secondary emitted atom, whose charge state is investigated, and its nearest-neighbor substrate atom, put in motion by the collision cascade generated by the primary ion beam. Suitable rectilinear trajectories are selected to describe the motion of these two atoms outside the solid. A good agreement is found with van Der Heide’s experiments (P.A.W. van Der Heide, Nucl. Instr. and Meth. B 157 (1999) 126).

Levitation of atoms over a metal surface

The potential of the interaction of an alkali metal with a planar metal surface was calculated by solving a two-dimensional Schrodinger equation. The potential energy of the valence electron of the atom was derived from a model pseudopotential based on the image potential. It was demonstrated that the potential energy curve for the interaction of an alkali metal atom with the surface of a heavier alkali metal has a minimum at a certain distance from the surface, a feature that gives rise to equilibrium states of levitating atoms.