Surface Review and Letters

The influence of surface roughness on surface melting phase transition is discussed within the molecular field theory. The roughness is characterized by the surface order parameter averaged over all the density fluctuations whose description corresponds to the discrete Gaussian solid-on-solid model. The potential governing the transition between the rough surface and the surface melting is considered in terms of the modified van der Waals equation of state. Its effective shape represents two intersecting parabolas with nonequal curvatures for the solid and liquid phases. The phase diagram shows the coexistence of two phases with rough and wet surfaces.

With near-UV Ar⁺-laser-induced photolysis of gaseous CH₂I₂, efficient, large-area or localized deposition of iodine-free amorphous carbon is possible at room temperature. The method allows a separate substrate heating and thereby a tuning of the deposits properties from soft and polymer-like to diamond-like. This article reports on an observed photon–surface interaction resulting in an enhanced carbon film formation. The shape of the deposit indicates that the deposition process is supported by an increased absorption cross section of surface-adsorbed intermediate molecular fragments, with respect to flat-angle surface-impinging photons.

The adsorption and desorption processes accompanying the deposition of silane on Cu(111) and Ni(111) have been investigated using work function change (Δϕ), Auger electron spectroscopy (AES) and thermal desorption spectroscopy (TDS). In the early stages of deposition (0–2 L), at 130 K, on the Cu(111) surface, a predominantly hydrogenic phase forms. Heating the saturated surface to T_s>400 K produces a two-phase system: Cu₂Si alloy and clean Cu(111). The regenerated Cu(111) phase extends across (16 ± 1)% of the total surface area and this process of clean surface generation can be repeated by redosing onto the mixed phase surface. The temperature coefficient of the work function of the Cu₂Si alloy surface is -(7 ± 2) × 10^-5 eV/K. Dosing silane onto the 110 K Ni(111) surface to saturation deposits a relatively small amount (0.05 ML) of Si. However, the Δϕ and TDS spectra are significantly distorted from their pure H/Ni(111) counterparts.

Initially clean, (2 × 1)-reconstructed Si(100) surfaces were exposed to NH₃, until saturation, at sequentially increased temperatures of up to 600°C. The resulting surfaces have been studied by low energy electron diffraction (LEED) and by photoemission yield and Auger electron spectrometries (PYS and AES). The room temperature saturation — for which NH₃ is adsorbed in a dissociated form as NH₂ and H, either of them bonded to a single Si dimer — remains essentially unchanged up to 250°C. Beyond 250°C, NH radicals form and replace the Si dimers by flat Si–NH–Si bridges along the surface, which ends up being stabilized by a full monolayer of such bridges at 400–450°C. Beyond that starts the actual nitridation process which concerns deeper Si layers.

In this investigation, several Mo/ZSM-5 catalysts with various Mo loadings and various reaction times were prepared, and methane conversion to aromatics without the presence of oxygen was studied by microreactor evaluation, FTIR and XPS. It was found, for the first time, that oxygen treatment before the reaction could shorten the induction period of this reaction. In situ FTIR study indicated the formation of species (888 cm^-1) and O–Al (670 cm^-1) on the surface of the catalyst as a result of the oxygen treatment. The two IR bands shifted to 854 and 659 cm^-1 respectively when ¹⁸O₂ was used. These oxygen species may take part in the initial reaction and shorten the induction time. The XPS study revealed the coexistence of Mo₂C and MoO₃ species on working catalyst surfaces, and a proper Mo₂C/MoO₃ ratio (~0.38) was identified for the best aromatization Mo/ZSM-5 catalyst. The active sites for methane conversion to aromatics should include Mo oxide as well as Mo carbide.

This study involves the adsorption of Cs on Si(111)-(7 × 7) surfaces at room and elevated temperatures, with increasing coverage of Cs to saturation, in ultrahigh vacuum (UHV) conditions. The techniques of low energy electron diffraction (LEED), Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS) and work function (WF) measurements were utilized. A WF change at the minimum, found to be Δϕ=-3.2 eV at an approximate 0.23 ML Cs coverage, corresponds to half the saturation coverage. We propose that, at that coverage, the adatom dangling bonds of the Si(111)-(7 × 7) substrate are completely filled. At saturation coverage (0.47 ML), Cs forms a single saturation layer in a near-metallic surface state, where the atomic radius of the Cs adatoms is likely to be 2.2× 10^-8 cm. The surface exhibits a high degree of disorder with 0.47 ML of adsorbed Cs, and it is likely that the adsorbate remains in cluster-like domains within the center of the disordered 7×7 unit cells, instead of evenly covering the Si surface. Saturation coverage of Cs on Si(111)-(7 × 7) surfaces exhibits a WF value 0.5 eV less than that of pure metallic Cs. Cesium adatoms form a stronger bond to the Si(111)-(7 × 7) surface than to Ni(100), with a calculated energy of 1.64 eV/atom. The induced surface disorder and strong binding energy are evidence of the strong Cs–Si interaction, which most likely prevents the formation of a purely metallic Cs overlayer.

The reactive behavior of NO on Pd(111) has been studied using a semiempirical theoretical method. The adsorption sites and the related electronic structure have been considered. In particular, the dissociation process has been studied and compared with CO. Different dissociation mechanisms have been proposed and the formation of NCO species has been considered. The results follow the trends reported in the experimental literature.

Coadsorption of Na and K on Ag(001) at room temperature has been studied by low energy electron diffraction (LEED). A 3 × 3 LEED pattern was observed irrespective of the order of adsorption. For this formation, it is necessary to deposit Na and K atoms with appropriate coverage. We have determined the 3 × 3 structure by a tensor LEED analysis. It is a restructured surface and is very similar to the previously determined 3 × 3 structure formed on Ag(001) by pure Na adsorption. In the coadsorption, Na and K atoms occupy preferable sites selectively, and construct an ordered mixed structure on Ag(001). That is, small Na atoms are located in the missing row sites, while large K atoms sit on the hollow sites of four-Ag-atom islands. The reason for the site selectivity of Na and K atoms in the mixed 3 × 3 structures is discussed.

We present a dynamical tensor low energy electron diffraction (LEED) study of α-Fe₂O₃(0001) surface structures that form in an oxygen pressure range from 10^-5 to 1 mbar. Epitaxial α-Fe₂O₃(0001) films were prepared on Pt(111) in defined oxygen partial pressures at temperatures of around 1100 K. In 1 mbar O₂ strongly relaxed oxygen-terminated surface structures are formed, while in 10^-5 mbar O₂ three different surface structures yield rather good Pendry R factors. Further experimental evidence from scanning tunneling spectroscopy (STM) and ion scattering spectroscopy (ISS), in combination with a critical review of the literature, is only consistent with a hydroxyl termination forming in 10^-5 mbar O₂. The stabilization of both structures is discussed on the basis of electrostatic arguments considering the boundary conditions at the oxide–gas as well as the oxide–substrate interface (autocompensation). For oxygen pressures between 10^-4 and 10^-1 mbar O₂, the two domains coexist as analyzed using a new, modified version of the symmetrized automated tensor LEED program package.

The system investigated in this study turns out to be very complex and the LEED analysis alone is not capable of identifying the involved surface structures unambiguously. Only in combination with results from other surface-sensitive methods was it possible to deduce models for the most likely surface structures.

Yb:YAl₃(BO₃)₄ (YbYAB) is a self-doubling frequency laser crystal. Based on atomic force microscopy (AFM) observations, a series of growth morphologies formed after different growing times on the smooth face are reported in this paper. No two-dimensional nuclei or growth steps have been found on this face after growth of several seconds, or even several minutes. This reflects the fact that at the beginning stage, the growth rate of the face is much slower than that of the cleavage face of YbYAB crystal (reported in our previous work). The reason is the different properties of nonsaturated bonds on the surfaces between a cleavage face and a natural face. After growth of 50 min, checklike morphologies of a special kind develop on the face. These morphologies result from twin structure. The growth steps with small heights (~ 0.6 nm) cannot cross the two twinned crystal regions, but those with large heights (6–8 nm) can cross by changing the directions at the twin boundaries. We have also observed the rough morphology of the face obtained by spontaneous nucleation growth at a high rate. The rough morphology is composed of many small faces with the same orientation but very different heights. This reflects the fact that the rough face does not grow by accumulating the growth units irregularly.

Nb thin films have been prepared with e-beam evaporation under UHV conditions, and by RF magnetron sputtering. Al thin films were deposited by resistive heating in the UHV chamber. The preparation of these films and the trilayers of Nb/AlO_x/Nb are intended for their use in Josephson junctions. Surface studies of these films are undertaken by using an atomic force microscope in the noncontact mode. These studies have revealed that the sputter-deposited Nb film surface is smoother than that of the UHV e-beam evaporated with R_rms values of 3.5 and 4.0 nm respectively. Al thin films have a very smooth surface, with an R_rms value of only 0.9 nm. Consequently, UHV-evaporated Nb thin films deposited on top of Al thin films are smoother, with a surface roughness of 1.8 nm.

The growth of gold and copper nanoparticles on a γ-Al₂O₃ single crystal surface has been investigated using LEED and STM. The clusters were grown on the sample by exposing the substrate to a metal vapour. The particle size distributions are determined as a function of the metal vapour dosage and, furthermore, the Volmer–Weber growth mechanism and a semi-log scaling law are deduced from these data.

There are three primary explanations for the degradation mechanism of field-emitter arrays (FEA's): oxidation, sputtering damage and ion implantation. Theoretical analysis and calculation are carried out in this paper to investigate the issue deeply. Adsorption and oxidation of FEA's fit best the experimental phenomenon and are the most reasonable causes for it. The process of adsorption and oxidation is described and directions for further experiments are suggested.

Further to the previous part [Surf. Rev. Lett.8(3/4), 367 (2001)], this part deals with VLEED numerical quantification of the Cu₃O₂ bond-forming kinetics on the Cu(001) surface. Besides the solution-number certainty that has been ensured, the modeling exercises have enabled the full capacity of VLEED to be explored, revealing that the bond geometry and the elastic potential define the shapes of the VLEED fine-structure features while the inelastic potential contributes to the absolute intensity. VLEED, covering the valence band in energy, is found to be able to collect nondestructive information from the top atomic layer, as the inelastic damping dominates in this region. Except for the bond geometry, dislocation of a single atom in the unit cell produces no feature that can match the VLEED spectral change, indicating the essentiality of the parametrization techniques developed. It is the right premise that it has enabled the four-stage O–Cu(001) reaction kinetics, revealed by VLEED, to be quantified as the evolution of the Cu₂ to the Cu₃O₂. In addition, it is revealed that annealing provides disturbance rather than driving force strengthening the bond energy. Therefore, formulation in terms of bond making and its consequence on the behavior of atoms and valence electrons, and the corresponding parametrization, are demonstrated to be able to correctly reflect the real process of reaction and the correlation of the parameters as well as their natural link to the observations using LEED, STM and PES, or equivalent approaches.