Table of contents

This special section contains a selection of articles on several different aspects of the physics and chemistry of vicinal surfaces. The aim is not to provide a review of all the aspects of the field, but rather to present a collection of articles concerning problems that have deserved attention during the last few years. Vicinal surfaces have been the subject of theoretical and experimental work for several reasons, such as their catalytic activity or the fact that they are natural templates for the epitaxial growth of a different material. In principle, the selection of the miscut angle allows us to choose the lateral periodicity at the surface, because the miscut angle of a vicinal surface determines the average spacing between steps. Ideally, we should obtain a perfect staircase with the homogenous step spacing determined by the miscut angle. In practice, many different factors conspire to make real vicinal surfaces much more complex than expected, since both step bunching and/or meandering along the steps are favoured from the energetic point of view.

The contribution by Desjonqueres, Spanjaard and co-workers studies the case of transition and noble metal vicinals. Rahman and co-workers consider also the case of vicinal fcc metals, and study the structural relaxations and vibrational dynamics and thermodynamics of these systems. Both kinds of instabilities (step bunching and step meandering) are studied by Ernst and co-workers for Cu homoepitaxy. An extreme case of step bunching is faceting, where crystalline faces different from the nominal orientation are formed. The paper by Sudoh and Iwasaki considers the case of Si(113), and the role of step–step interactions in the surface faceting. Minoda's contribution studies the role of external forces (DC heating in this case) in the structure of vicinal Si(111). Ortega and co-workers summarize how the surface electronic structure is affected by the step periodicity in the case of Cu(111) and Au(111) vicinals, an important property for the further growth of a different material. They also analyse how the surface electronic structure is modified by a superperiodicity in the nanoscale range.

The topic of epitaxial growth on vicinal surfaces is studied in the contribution by Kuhnke andKern, who analyse the role of vicinal metal surfaces as nanotemplates for the growth of low dimensional systems. Several real systems of this kind are also reported in the last papers of the section. Speller and co-workers study the properties of Ag nanostripes deposited on vicinal Cu(111). Rousset and co-workers investigate the role of the step structure in the properties of vicinal Au(111), and also its relevance for the further growth of Co.

To conclude, I would like to thank all the authors of this special section for their contributions.

The energetics of transition and noble metal (Rh, Pd, Cu) vicinal surfaces, i.e., the surface energy, step energy, kink energy and electronic interactions between steps, is studied at 0 K from electronic structure calculations in the tight binding approximation using an s, p and d valence orbital basis set. Then, the surface phonon spectra of copper are investigated in the harmonic approximation with the help of a semi-empirical inter-atomic potential. This allows one to derive the contribution of phonons at finite temperatures to the step free energy and to the interactions between steps. The last part is devoted to the stability of vicinal surfaces relative to faceting with special attention paid to the domain of orientations (100)–(111). Semi-empirical potentials are shown to be not realistic enough to give a reliable answer for this problem. The results derived from electronic structure calculations predict a variety of behaviours and, in particular, a possible faceting into two other vicinal orientations. Finally, temperature effects are discussed. Comparisons are made with other theoretical works and available experiments.

We present here a summary of the calculated structural, dynamical and thermodynamical properties of a number of vicinal surfaces of fcc metals with the aim of identifying trends in their characteristics. In general, multilayer relaxations indicate a contraction in the bonds between surface atoms except for that of the least undercoordinated surface atom ('corner') whose bond length with the nearest neighbour bulk atom displays an impressive expansion. Electronic structure calculations show a rearrangement of charge densities near the steps and the layer resolved density of states highlights the characteristics of the undercoordinated atoms. The frequencies of localized vibrational modes on stepped surfaces point to both softening and stiffening of specific force constants, which lead to enhancement of modes in both the lower and higher ends of the frequency spectrum in the vibrational densities of states. Contributions to the surface vibrational entropy from undercoordinated atoms are found to depend strongly on their atomic coordination and play an important role in determining the step excess free energy for certain step geometries. Comparisons of results are made with available experimental data.

The concept of spontaneous pattern formation in epitaxial growth is currently actively explored as a promising pathway for lateral nanostructuring of surfaces. Often, the origin of self-organization is traced back to the presence of an excess energy barrier for adatom diffusion associated with asymmetric features in the crystalline structure, the Ehrlich–Schwöbel barrier. Upon growth of Cu on vicinal Cu surfaces at moderate substrate temperatures a step-meandering instability develops, resulting in an in-plane patterning of the surfaces at the nanometre scale with a temperature- and flux-dependent characteristic wavelength. This meandering instability is superseded by a step-bunching instability during growth at higher temperatures. Specifically, the meandering instability acts as a precursor to the bunching instability, indicating that a one-dimensional treatment of bunching in step flow growth is not sufficient. These nanostructured surfaces might be used as templates in order to guide the growth of materials which do not show spontaneous self-organization.

Faceting is a thermodynamic phase separation where a surface with some arbitrary macroscopic orientation breaks up into a hill and valley structure. Vicinal Si(113) surfaces are typical systems where faceting occurs due to short-range attractive step–step interactions. This paper reviews our recent studies on step dynamics involved in faceting on vicinal Si(113) surfaces, by high temperature scanning tunnelling microscopy and analysis based on the continuum step model.

Study on new types of step instability on a Si(111) vicinal surface induced by the direct current heating effect by reflection electron microscopy and optical microscopy is reviewed. One step instability is in-phase step wandering and the other is a periodic step density wave (PSDW). The former is due to electromigration of Si adatoms and steps are aligned in phase without changing mean step–step distance. This instability occurs at temperatures between 1000 and 1200 °C, under the step-down current heating condition. Steps are in the sinusoidal shape and their wavelength is in the micron order. The wavelength does not depend on the mean step–step distance or off-angle of the vicinal surface nor annealing time. The latter is induced by Au adsorption under step-down current heating at temperature between 830 and 930 °C where extremely straight step bands and Si(111) terraces are aligned periodically. The driving force of the PSDW is the modification of step structure due to electromigration of Au adatoms. These phenomena might be useful for micro-fabrication of surface from the industrial point of view as well as interesting from the fundamental point of view.

Vicinal noble metal surfaces with regular arrays of steps and terraces are very convenient model systems to test the electronic properties of lateral nanostructures. Using angle-resolved photoemission with synchrotron radiation we thoroughly characterize electronic states and wavefunctions in a variety of vicinal Cu(111) and Au(111) surfaces. By tuning the terrace width, we can observe the fundamental transition from arrays of non-interacting nano-objects (terraces), where electron states are confined, to lateral coupling between terraces, which leads to superlattice states.

Vicinal surfaces, which exhibit a regular array of steps, introduce defined arrangements of surface defects, which have the potential to create specific functionalities of the surface. In particular they can be used as templates for the growth of one-dimensional structures using selective step decoration. In this article we discuss the properties of vicinal metal surfaces and how they can be used as nanotemplates. The requirements for the growth of low-dimensional adsorbate structures at step edges of the vicinal (997) and (779) surfaces of platinum will be discussed in detail. Here energetics determined by the different adsorption sites and the kinetics present through the diffusion processes play an essential role. In order to obtain stable arrangements the propensity of the elements for alloy formation must be taken into account. Examples for the properties of the structures obtained and their role in studying one-dimensional systems are discussed, and we give a short outlook on how the principles of step decoration might be extended to a kind of atomic assembly of more complex surface nanostructures.

A new possibility for producing regularly spaced nanostripes with controllable periodicity is the deposition of Ag on vicinal Cu(111) surfaces. This self-structuring originates from two different characteristic length scales of two active physical mechanisms. On the atomic scale, commensurability of Ag(111)-like structures on certain Cu planes leads to spontaneous facet formation. On the mesoscopic scale, a rippled structure is formed in which Ag and Cu nanostripes alternate. A unique property of this hill-and-valley structure is that the periodicity of the stripes can be controlled via the Ag coverage. Predictions on the basis of continuum models with long range elastic interactions can explain quantitatively the periodicities of the nanostripes.

This paper reports on Au(111) vicinal surfaces, either regularly stepped surfaces, reconstructed or not, or periodically faceted surfaces, which are well suited to be used as templates for organized growth of clusters. Angles of misorientation with respect to the (111) plane lie between 1° and 12° and two opposite azimuths are considered: (i) [], that leads to steps with {100} microfacets, and (ii) [], that leads to steps with {111} microfacets. The behaviour of the Au(111) reconstruction in the vicinity of steps depends drastically on the step microstructure, and this is a key point for understanding the various periodic morphologies existing on Au(111) vicinal surfaces. The interaction between the reconstruction and the close-packed steps of the Au(111) surface is interpreted in terms of the relative stability of both types of step. Self-organized morphologies between 10 and 100 nm are interpreted within the framework of elastic theory and by pointing out the crucial role played by the atomic boundary energy term. The microscopic origin of faceting is discussed, proposing two different models depending on each azimuth. Then, we illustrate the use of Au(111) vicinal surfaces as templates for growing long range ordered nanostructures. Examples are given in the case of cobalt growth.

I use the method of classical density-functional theory in the weighted-density approximation of Tarazona to investigate the phase diagram and the interface structure of a two-dimensional lattice-gas model with three phases—vapour, liquid and triangular solid. While a straightforward mean-field treatment of the interparticle attraction is unable to give a stable liquid phase, the correct phase diagram is obtained when including a suitably chosen square-gradient term in the system grand potential. Taking this theory for granted, I further examine the structure of the solid–vapour interface as the triple point is approached from low temperature. Surprisingly, a novel phase (rather than the liquid) is found to grow at the interface, exhibiting an unusually long modulation along the interface normal. The conventional surface-melting behaviour is recovered only by artificially restricting the symmetries being available to the density field.

Amorphous carbon (a-C) and carbon nitride (a-CN_x) thin films prepared by sputter deposition on thin Corning glass substrates were studied using isothermal thermogravimetry and the differential method. Two typical reaction kinetics, the sigmoid and deceleratory reactions, were observed, which correspond to the films sputtered at higher (0.9–3 mTorr) and lower vacuums (7–16 mTorr), respectively. The structure bonding ratios and activation energies for thermal decompositions, which increased with the kinetic energy of the carbon species during deposition, were used to interpret the observations of reactions. The free bonded and weakly bonded compositions, the amounts of which increased with sputtering pressure, could be active nucleus sites in early reactions determining the reaction kinetics. The films with higher structure bonding ratios showed an induction period for generation of infrequently formed nuclei and had typically sigmoid-type reaction kinetics, while those with large amounts of free and weakly bonded compositions produced large amounts of quickly formed nuclei in early reactions and showed deceleratory reaction kinetics.

The dynamics of the ferroelectric phase transition in a SrBi₂Ta₂O₉ film of 5.5 µm thickness deposited on sapphire was studied by means of time-domain terahertz transmission spectroscopy (30–300 K) and Fourier transform far-infrared transmission spectroscopy (300–950 K). The optical soft mode near 28 cm⁻¹ (at 300 K) exhibits a weak monotonic softening down to 20 cm⁻¹ at 950 K with no significant anomaly near either the ferroelectric or ferroelastic phase transition. An additional relaxation process below the phonon frequencies was resolved in the terahertz transmission spectra. The critical slowing down of its relaxation frequency could be responsible for the dielectric anomaly near the ferroelectric transition temperature. This indicates that the ferroelectric transition is predominantly of order–disorder type.

We employ density functional theory to investigate and compare Al/TiC and Al/TiN interfaces by electronic structures, relaxed atomic geometries and adhesions. The results show that the preferred bonding site is the interfacial Al atoms above the ceramic's metalloid atoms for both systems. The calculated adhesion energies are quantitatively in agreement with other calculated and experimental results of Al on the carbide and nitride. A detailed comparison of the adhesion energies and relaxed structures shows weaker bonding and less relaxation in the Al/nitride case, which is correlated with the lower surface energy of the ceramic. We have thoroughly characterized the electronic structure and determined that the polar covalent Al3sp–C(N)2s bonds constitute the primary interfacial bonding interaction. The larger overlapping bonding states at the Al/TiC interface reveal the reason why it exhibits relatively larger adhesion energy. Cleavage may take place preferentially at the interface, especially for the Al/TiN, which is in agreement with experimental results.

We present first-principles calculations of the magnetic hyperfine fields H_hf of 5sp impurities on the (001), (111), and (110) surfaces of Ni. We examine the dependence of H_hf on the coordination number by placing the impurity in the surfaces, on top of them at the adatom positions, and in the bulk. We find a strong coordination dependence of H_hf, different and characteristic for each impurity. The behaviour is explained in terms of the on-site s–p hybridization as the symmetry is reduced at the surface. Our results are in agreement with recent experimental findings.

Thin films of a-SiO_x with values of x ranging from 1.13 to 1.89 were prepared by reactive evaporation of SiO in a controlled oxygen environment. The oxygen pressure in the deposition chamber was varied so as to obtain films with different values of x. The films were studied by x-ray photoelectron spectroscopy and optical spectrophotometry. An attempt was made to analyse the Si 2p core-level spectra in terms of five chemically shifted components corresponding to basic Si bonding units Si–(Si_4−nO_n) with n = 0,1,...,4. The concentration of these bonding units as a function of oxygen concentration was in reasonable agreement with the random-bonding model, with the exception that the Si–(Si₃O) component was almost completely suppressed for all stoichiometries. Films with x<1.65 consisted of elemental Si and oxides of silicon, while those with were almost free of Si. Films containing Si have higher refractive indices and degrees of absorption in the visible region compared with those which were free of Si.

The optical properties of the films approach those of fused silica (SiO₂) as the values of x increase. For the films with the largest value of x (= 1.89), the refractive index is smaller than that of fused silica. The density of these films was estimated to be smaller than that of fused silica by about 13%.

We report a Monte Carlo investigation on the excitonic diffusion process in semiconductor quantum wells (QW). The rates of interface-roughness scattering and phonon scattering are calculated. By comparing the scattering rates, we find that, in a realistic QW sample with typical well-width and interface quality, the interface-roughness scattering is the dominant mechanism in limiting the diffusivity. Spatiotemporal dynamics of the exciton distribution function are obtained by the simulations and the excitonic diffusivity under different conditions is deduced. We find that the diffusivity increases rapidly with increasing the correlation length of the interface fluctuation and with decreasing the amplitude of the fluctuation. In a typical QW sample, the diffusivity increases exponentially with increasing the width of the well.

The ultraordinary transmission of light through an array of holes in both perfect and real metallic films is simulated with the three-dimensional finite-difference time-domain method. We demonstrate that two distinguishing physical processes dominate the transmission behaviour: transverse surface current excited on the entrance surface of the metallic film transports light energy to the holes, while the evanescent coupling of incident and reflected waves inside the holes transmits light from the entrance to the exit of the holes. Surface plasmons excited on the flat parts of metallic hole arrays make a positive impact in the energy transportation of the first process, but are not necessary for the enhanced transmission. Our results are contrary to some recent theoretical results but agree with the experimental observations available.

The magnetism of Cr/Ir vicinal systems is investigated using the self-consistent tight-binding method in the unrestricted Hartree–Fock approximation with the Hubbard Hamiltonian. These structures exhibit a ferromagnetic structure of unequal local magnetic moments with the lowest values on the kink atoms and the highest on the edge atoms. The net magnetization decreases as a function of the step length.

A single-crystal film of α-Al₂O₃ was grown on the Ru(0001) surface with thicknesses of 6.5, 12 and 30 Å and a overlayer plasmon was found for 6.5 and 12 Å thick films. The dispersion relation as a function of wavevector parallel to the surface was observed to have a negative gradient in the direction using angle-resolved electron-energy-loss spectroscopy (AREELS). This relation is as expected for the overlayer plasmon due to the collective motion of electrons in the oxide layer along the direction perpendicular to the surface. The energy of the overlayer plasmon is closely related to the interband transition energy: the latter plus depolarization shift equals the former. From the energy loss at the point and the dispersion relation we arrived at the conclusion that intrinsic bandgap narrowing occurred. AREELS measurements with τ = 12 and 30 Å support that the overlayer plasmon leading to the bandgap narrowing is caused by a strong interaction of the excited electron and hole inside the thin film with the image dipole induced in the metal substrate.

Novel nanowires patterned with holes and nanotubes of silicon oxide have been fabricated on silicon substrates by a catalyst-assisted route. The diameters of these nanowires and nanotubes vary from 20 to 100 nm. The patterned nanowires have encapsulated holes along the longitudinal direction. High-resolution transmission electron microscopy reveals that the patterned nanowires and nanotubes are amorphous. The growth of the nanostructures is probably controlled by a modified vapour–liquid–solid process.

Single-phase Ga₂Se₃ films have been deposited with a growth rate of about 30 nm min⁻¹ on clean and Mo-coated soda-lime glass substrates by chemical close-spaced vapour transport. The use of HCl/H₂ as a transport agent results in a stoichiometric volatilization of the binary Ga₂Se₃ powder source material and the growth of Ga₂Se₃ films with reproducible composition. The films have been characterized using x-ray diffraction measurements, scanning electron microscopy observations, energy dispersive x-ray analysis, x-ray fluorescence spectrometry and elastic recoil detection analysis. A p-type conductivity was determined by means of the thermoelectric probe method. A Ga₂Se₃ band gap energy E_g = 2.56 eV has been found by optical measurements.

The adsorption of oxygen on a Ba-covered Ni(110) surface has been investigated mainly by Auger electron spectroscopy (AES) and work function (WF) measurements. Low energy AES lines indicate that O interacts with Ba and Ni as well. Both Ba–O and Ni–O interactions take place simultaneously on the surface, progressively leading to BaO and NiO formation. Ba enhances the oxidation of the substrate due to the higher sticking coefficient of O on the Ba/Ni surface. The oxygen interacts with the nickel substrate even at high adsorbate coverage, incorporating under the Ba layer. A part of the Ba adatoms remains non-oxidized even at a high O exposure.