Electronic surface resonances and crystal surface structure

A new method of determination of the lateral structure of crystal surfaces is presented. The method is based on earlier work showing the existence of resonances in the elastic scattering of low-energy electrons at crystal surfaces. The method consists of: (a) Measurement of the surface resonance band structure Ek_∥E, k_∥ respectively denote the electron energy and surface-parallel momentum for which resonances occur) and (b) Interpretation of E(k_∥) to determine the lateral variation of the effective potential acting on electrons at the surface.The surface resonance band structure is measured by a net-current electron reflection method. The measurement method is basically the same as used previously but here its precision is greatly enhanced by the use of digital methods of data handling including a digital filter to remove background due to inelastic and non-resonance elastic scattering. The surface resonance band structure is interpreted by a two-dimensional nearly-free electron scheme. In this scheme the interaction elements are Fourier coefficients of an effective potential which is an average of a pseudopotential with respect to the depth distribution of electron density in a surface resonance — the surface-weighted pseudopotential. Experimental surface resonance band structure for Ni(001), Ni(001) p(2 × 2)O and two different Ni(001) c(2 × 2)O surfaces (one of them with an oxygen-saturated Ni substrate) are presented for E = 1–30 eV and k_∥ running halfway from $\text{}\text{?}$gG towards H? in the surface Brillouin zone for Ni(001). The experimental results are fitted, using the nearly-free electron scheme, to determine the Fourier coefficients of the surface-weighted pseudopotential. Surface potential variations synthesized from the above data are discussed in comparison with the atomic arrangements known from LEED. It is demonstrated that the new method can give a correct picture of the lateral structure of surfaces. It is emphasized that these results are obtained without costly equipment or computations called for by other methods.     

Femtosecond laser-induced surface wettability modification of polystyrene surface

In this paper, we demonstrated a simple method to create either a hydrophilic or hydrophobic surface. With femtosecond laser irradiation at different laser parameters, the water contact angle (WCA) on polystyrene’s surface can be modified to either 12.7° or 156.2° from its original WCA of 88.2°. With properly spaced micro-pits created, the surface became hydrophilic probably due to the spread of the water droplets into the micro-pits. While with properly spaced micro-grooves created, the surface became rough and more hydrophobic. We investigated the effect of laser parameters on WCAs and analyzed the laser-treated surface roughness, profiles and chemical bonds by surface profilometer, scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). For the laser-treated surface with low roughness, the polar (such as C—O, C=O, and O—C=O bonds) and non-polar (such as C—C or C—H bonds) groups were found to be responsible for the wettability changes. While for a rough surface, the surface roughness or the surface topography structure played a more significant role in the changes of the surface WCA. The mechanisms involved in the laser surface wettability modification process were discussed.     

Multipole surface plasmon resonance of an aluminium surface

The surface photoelectric effect and the surface plasmon resonances appear when a p/transverse magnetic polarized laser hits a gas-solid interface. We model this effect in the long wave length (LWL) domain (λ_vac > 10 nm, ?ω < 124 eV) by combining the Ampère-Maxwell equation, written in classical approximation, with the material equation for the susceptibility. The resulting model, called the vector potential from the electron density (VPED), calculates the susceptibility as a product of the bulk susceptibility and the electron density of the actual system. The bulk susceptibility is a sum of the bound electron scalar susceptibility taken from the experiment and of the conduction electron non-local isotropic susceptibility tensor in a jellium metal (Lindhard, 1954 [1]). The electron density is the square of the wave function solution of the Schrödinger equation. The analysis of observables, the reflectance R and the photoelectron yield Y as well as the induced charge density permits to identify and characterize the multipole surface plasmon resonance of Al(111) appearing at ω_m ∼ 0.8ω_p or 11-12 eV.     

Effective surface potential method for calculating surface states

A novel formalism (the effective surface potential method) is developed for calculating surface states. Like the Green function method of Kalkstein and Soven and the transfer matrix method of Falicov and Yndurain, the technique is exact for simple tight binding Hamiltonians. As well as offering an alternative viewpoint, the present method provides a simple analytic expression describing the surface states. At each point k_s in the surface Brillouin zone the semi-infinite solid is viewed as an effective linear chain where each element of the chain is a planar layer. The solution to the linear chain problem can be expressed in terms of an effective potential h(k_s,E) at each energy E. A number of examples are presented in detail; “sp^d” Hamiltonians for a linear chain (d = 1), the honeycomb lattice (d = 2), the 111 surface of silicon (d = 3), and a dissected Bethe lattice. Various exact results are given, e.g. the extremities of surface state bands and the surface density of states of p-like (delta function) bands. The results of Kalkstein and Soven for the 100 surface of a simple cubic solid with a perturbation on the surface layer are rederived.     

Fermi surface, surface states, and surface reconstruction in Sr2RuO4

The electronic structure of Sr2RuO4 is investigated by high angular resolution ARPES at several incident photon energies. We address the controversial issues of the Fermi surface (FS) topology and the van Hove singularity at the M point, showing that a surface state and the replica of the primary FS due to sqrt[2]xsqrt[2] surface reconstruction are responsible for previous conflicting interpretations. The FS thus determined by ARPES is consistent with the de Haas-van Alphen results, and it provides additional information on the detailed shape of the alpha, beta, and gamma sheets.     

Non-equilibrium surface state properties at clean cleaved silicon surface as measured by surface photovoltage

Surface photovoltage transients were measured at clean cleaved silicon (111) faces in ultrahigh vacuum. The temperature and doping of the samples, the intensity of the stimulating light pulses (energy less than band gap), and the surface coverage (clean and adsorbed water vapor) were varied systematically. The results yield information on the charge transfer at the surface and on surface recombination. The calculation of the surface photovoltage (using only the generation rates into and out of the surface states and data of thermal equilibrium) shows, that only one bulk band (conduction band for n-doped samples and valence band for p-doped samples) controls completely signal height and its relaxation via charge transfer to the surface states. The determined surface state parameters are: relaxation time constants, capture cross-sections for photons and transition probabilities. On the basis of the model all decay curves can be reproduced quantitatively.     

Unoccupied surface states and surface barrier in metals

Recent progress in the spectroscopy of empty electronic states at metal surfaces allows for measuring the energy vs. momentum dispersion of both crystal-induced and image-potential surface states with high precision. This allows for deriving the effective barrier potential for an electron near a metal surface with considerable accuracy by comparing the experimental data with corresponding calculations based on the one-step model of inverse photoemission. The method is demonstrated for Cu(100) where four empty surface states are known experimentally.     

Optimum surface roughness for surface enhanced Raman scattering

Surface enhanced Raman scattering from copper phthalocyanine thin films deposited onto Ag films roughened by underlayers of gas-evaporated Si particles was investigated. The surface roughness was systematically varied by varying the average size of Si particles. Results of quantitative intensity measurements indicate that there exists an optimum surface roughness depth for SERS. The maximum enhancement factor obtained is ≈1.5 x 10⁴ and a crude estimate of the optimum roughness depth is of the order of 100 Å. The origin of presently observed enhancement is thought to be purely electromagnetic, involving the excitation of the surface plasmon modes via the surface roughness.     

Modification of rubber surface by UV surface grafting

Rubber surface is subjected to ultraviolet radiation (UV) in the presence of allylamine and radiation sensitizer benzophenone (BP). Fourier transform infrared spectral studies reveal the presence of allylamine on the surface. The presence of irregular needle shapes on the surface as observed in scanning electron micrographs also confirms the polymerized allylamine on the surface. Allylamine coatings have been further confirmed from atomic force microscopy (AFM) analysis. Thermogravimetric analysis (TGA) reveals that allylamine coating on the rubber surface lowers the thermal degradation rate. The contact angle between the water and rubber surface decreases for the modified rubber surface confirming the surface modification due to UV surface grafting.     

Quasi-steady-state surface acoustic waves at a rough solid surface

Sound propagation along an inhomogeneous solid-vacuum interface is considered. The frequency-wavenumber relation has been obtained for the transverse-polarized surface wave and the range of existence of this wave has been analyzed. The surface roughness is shown to produce additional damping.     

The attenuation of rayleigh surface waves by surface roughness

This paper presents a calculation of the attenuation length of Rayleigh surface waves in the presence of surface roughness. We consider Rayleigh waves on the surface of a semi-infinite isotropic elastic continuum, and the method we use produces the contribution to the attenuation rate proportional to the square of the rms amplitude of the roughness. We obtain explicit expressions for the contribution to the attenuation rate from roughness-induced scattering into bulk transverse and longitudinal acoustic waves, and into Rayleigh waves. Our derivation makes use of a Green's function method. When the wavelength λ of the Rayleigh wave is long compared to the transverse correlation length a that characterizes the surface roughness, all contributions to the attenuation rate are proportional to the fifth power of the frequency. When λ is comparable to or smaller than a, the attenuation constant varies more slowly with frequency. For a model of the surface roughness, we present numerical calculations of the relative magnitude and frequency dependence of the various contributions to the attenuation rate. The Green's functions used here may be applied to a number of calculations. A derivation of their form is provided in an Appendix.     

Nothing moves a surface: vacancy mediated surface diffusion

We report scanning tunneling microscopy observations, which imply that all atoms in a Cu(001) surface move frequently, even at room temperature. Using a low density of embedded indium "tracer" atoms, we visualize the diffusive motion of surface atoms. Surprisingly, the indium atoms seem to make concerted, long jumps. Responsible for this motion is an ultralow density of surface vacancies, diffusing rapidly within the surface. This interpretation is supported by a detailed analysis of the displacement distribution of the indium atoms, which reveals a shape characteristic for the vacancy mediated diffusion mechanism that we propose.     

Determination of surface state parameters from surface photovoltage transients

Photo-stimulated electron transitions from the surface states into the conduction band which are involved in surface photovoltage spectroscopy are analyzed in terms of phenomenological surface state parameters. The surface state parameters are determined frolm photovoltage transients on the basis of relationships derived for a general case where the mechanism of electron transients is not specified, for the case where the surface states are in equilibrium with the bulk and the case where the surface states are not in equilibrium with tpe bulk. The procedure is illustrated utilizing experimental data obtained on CdS surfaces.     

Influence of surface relaxation on the surface d-band width

We show that the surface d-band width strongly depends on surface atomic relaxation. A compression of the first interplanar spacing equal to a few per cent is sufficient to cancel the surface effect on the d-band width.     

Effect of the surface roughness on Bean-Livingston surface barrier

Bean-Livingston barrier for a single Abrikosov vortex interacting with the rough surface is calculated as a function of the random characteristics of the surface roughness. It has been shown that even a respectively smooth surface roughness results in an essential decrease of the Bean-Livingston surface barrier.     

Effect of impurities on surface self-diffusion and surface structure

Measurements using field emission techniques of the activation energy for surface selfdiffusion of several of the refractory transition metals when carbon or silicon is present on the surface show large increases which are dependent on the degree of surface coverage. Maximum values obtained were: 8.5 eV for carbon on tungsten, 7.0 eV for silicon on tungsten, 4.9 eV for carbon on tantalum, 4.5 eV for carbon on molybdenum and 2.8 eV for silicon on molybdenum. In addition, two anomalous effects have been observed in which surface changes occur at critical temperatures, (a) Sharp discontinuities occur in the plots of activation energy versus temperature for carbon on tungsten at about 2300 °K and for silicon on tungsten at about 2000 °K. In both cases the activation energy drops from the respective high value to that for the clean substrate material of 3.0 eV. Concomitant with this transition the emission patterns change in appearance from those typical of a contaminated surface to those typical of a clean surface, (b) For carbon on tungsten and silicon on tungsten, (433) planes are observed which decrease in size with temperature and suddenly disappear at a very sharp critical temperature. It is suggested that the presence of these impurities causes a restructuring of the surface layers even when present in much less than stoichiometric amounts and that surface phase changes occur independent of bulk changes.     

Elastic interactions between surface adatoms and between surface clusters

One major term is omitted in most conventional treatments of interactions between surface adatoms or groups of adatoms. This is the elastic interaction, in which adatoms interact through mutual distortion of the substrate. The indirect elastic interaction explains a range of observed surface phenomena in a consistent quantitative way. These phenomena include static and dynamic effects ranging from ordered structures to correlated motions of adatom clusters. It is likely also that substrate distortion is important in clean surface reconstruction. The elastic interaction is typically comparable with or larger than the indirect electronic interactions usually presumed. It follows that detailed calculations which ignore the substrate distortions produced will be of limited value only.     

Effect of surface roughness on surface plasmon resonance absorption

The absorption of light by surface plasmons has been studied using the method of attenuated total reflection. The reflectance from a quartz-Ag interface has been measured as a function of angle and surface structure for the wavelength region from 3600 to 6000 . It is shown that the reflectance minimum for a smooth Ag film is changed in both angular position and spectral half-width by roughening the Ag surface with CaF₂ underlayers. Dispersion curves are presented which show that the wave vector of a surface plasmon propagating on an irregular surface is greater than that of an equally energetic surface plasmon propagating on a planar surface.