Non-reconstructed Surface Research Articles

Ab initio study on the As-stabilized surface structure in AlAs molecular beam epitaxy

The dimer structure of an As-stabilized (001) AlAs surface was analyzed based on the cluster model by using ab initio molecular orbital calculations. The surface As dimer was modeled by Al 4As 4H 12 (cluster 1) and Al 7As 8H 19 (cluster 2) clusters. It was shown that the electron counting model was satisfied for cluster 2, while it was not satisfied for cluster 1. In both clusters, the electronic state of the reconstructed surface was shown to be differ greatly from that of the non-reconstructed surface. The ground state of cluster 2 was 1A 1 , where a σ bond was formed between the two surface As atoms, and each surface As atom had a doubly occupied dangling bond. The bond length of the dimer was calculated to be 2.7 Å. The mechanism whereby the electron counting model was satisfied for cluster 2, but not for cluster 1, was clarified.

Microfacetting of Au(110) electrodes: An optical second-harmonic generation study

The formation and lifting of microfacetting at an Au(110) electrode surface was studied by using in-situ optical Second-Harmonic Generation (SHG). In contrast to the unreconstructed (1 × 1) Au(110) surface which has a C 2v symmetry, the reconstructed surface exhibits a C s symmetry due to essentially (111)-oriented microfacets. Hence, it shows SHG-anisotropy patterns fundamentally different to those of a non-reconstructed surface. Apparently, the nonlinear susceptibility tensor contains an additional threefold symmetry element. Its amplitude was determined using Fourier analysis of SHG-anisotropy curves and, thus, served as a measure of microfacetting of the Au(110) surface. We observed that adsorption of an organic molecule, such as pyridine, has little effect on the electrode-surface crystallography. In contrast, adsorption of bromide ions results in the lifting of microfacetting as indicated by the disappearence of the threefold symmetry term. Potential-step experiments gave time constants for the lifting of microfacetting in the range of 50 < τ 1/2 < 150 ms

Influence of steps on the H 2O adsorption on Ni(s)(111)

To expand previous investigations about the adsorption of H 2O on a stepped Ni(s)(111) surface with nominal (221) orientation, the adsorption of H 2O on a stepped Ni(s)(111) with nominal (11 11 9) orientation was studied in the present paper. The surfaces exhibit the same (111)-terrace and (11 1 )-step orientation but they differ in the terrace width, which is about three times greater for Ni(11 11 9) than for Ni(221). The adsorption was studied with thermal desorption spectroscopy (TDS), low energy electron diffraction (LEED) as well as work function change measurements ( Δφ). The LEED data from the clean Ni(s)(111) surface are consistent with the atomic arrangement expected for the “ideal” non-reconstructed (11 11 9) surface. In TDS experiments five desorption states denoted as A ( T M = 155–160 K), B ( T M = 174–177 K), C ( T M = 225 K), D and E ( T M = 260 K) were found. A and B exist at similar temperatures on flat Ni(111) as well as Ni(221) and are assigned to adsorption of H 2O clusters at terrace sites (B) or adsorption of ice multilayers (A). The step-induced states C, D and E, which were found at higher temperatures, are related to the adsorption of H 2O monomers at steps (C) and the recombination of dissociation products (D and E). As expected, the amount of H 2O molecules adsorbed at step sites as well as the contribution by step molecules to the work function change is about three times smaller than for Ni(221), corresponding to the lower step density of Ni(11 11 9). At higher coverages terrace sites are occupied by H 2O molecules developing a modified bilayer structure. Between 120 and 150 K a (√3 × √3)R30° structure, which has also been found on other flat surfaces with hexagonal symmetry, could be observed. Upon heating to 160 K the LEED pattern changed into a (2 × 2) structure. This behaviour is believed to be due to a temperature- and coverage-induced reordering of state B.

GaAs surface reconstruction obtained using a dry process

We report attaining Ga-terminated (4×2) surface reconstruction on virgin GaAs substrates using a completely dry process at temperatures below the oxide sublimation temperature and without group V overpressure. The native oxides are removed with an electron cyclotron resonance hydrogen plasma treatment, followed by annealing at 500 °C in ultrahigh vacuum, which yields a reconstructed surface suitable for epitaxial overgrowth. Characterization by secondary ion mass spectroscopy and transmission electron microscopy reveals the complete removal of O, reduced C, and high structural order at the epilayer/substrate interface when this preparation method is used before molecular beam epitaxy. Annealing the substrate at a lower temperature yields a nonreconstructed surface possessing significant impurity concentrations, and leads to dislocation defects at the epilayer/substrate interface.

Kinetic oscillations in catalytic CO oxidation on a cylindrical Pt single crystal surface

A Pt single crystal of cylindrical shape (axis II [001]) whose surface exhibits all orientations of the [001] zone has been used to study coupling effects between orientations which exhibit kinetic oscillations in catalytic CO oxidation. On the clean surface, one can distinguish between an orientational range in between (110) and (320) which exhibits a 1×2 reconstruction, a nonreconstructed surface range around (210) and a hex reconstructed surface range around (100). Structural transitions proceed continuously via atomic steps between (110) and (210), while the orientational range extending from (210) to (100) is faceted. With a rotatable Kelvin probe, the orientational dependence of the work function could be followed. The results revealed that the variation of the oxygen sticking coefficient sO2 displays a mirror-like behavior with respect to the work function variation of the clean surface such that the orientation with the lowest work function (210) exhibits the highest sO2. Kinetic oscillations were studied in the 10−5 and 10−4 Torr range. By means of two Kelvin probes, the Δφ oscillations could be followed simultaneously at two different orientations, while the integral behavior of the cylinder surface was monitored via the CO2 production rate. A strong broadening of the oscillatory region in parameter space, as compared to a flat surface, was detected for the oscillatory range around (100). In the vicinity of this orientation, spatial coupling is provided via reaction fronts which propagate from (210) toward (110). Gas-phase coupling is only observed with the (110) orientations. Prolonged oscillation experiments cause faceting resulting in an increase of catalytic activity around (100), while the orientations around (210) lose catalytic activity by faceting.

Self-diffusion on reconstructed and nonreconstructed Ir surfaces

Using the field ion microscope, we have studied random walk diffusion of single Ir atoms and small Ir atomic clusters on Ir surfaces. On the (1×1) Ir {001} surface, diffusion of Ir adatoms occurs by an atomic replacement mechanism along the 〈001〉 directions. On the (1×1) Ir {110} surface, self-diffusion occurs by atomic hopping along the 〈110〉 directions and by atomic replacement along the 〈001〉 directions. On the (1×2) reconstructed Ir {110} surface, self-diffusion occurs only by atomic hopping along the doubly spaced [1̄10] surface channels. Within the experimental accuracy, the pre-exponential factors are all in the 10−3 cm2/s range. Energies needed for these atomic replacements and the barrier heights for these atomic hoppings are also derived. Diffusion of diatomic clusters on the (1×1) Ir {110} surfaces has also been studied.

Formation of Ordered, Nonreconstructed CdTe ( 111 ) Surfaces Electrochemically: Studies by LEED and Auger Electron Spectroscopy

Surface treatments for single crystals have been examined to determine procedures which result in the formation of ordered stoichiometric surfaces, reflecting a truncation of the bulk crystal structure. Nonreconstructed surfaces were obtained in solution using a etching solution followed in sequence by a sulfuric acid rinse and electrochemical reduction. The etching procedure leaves a disordered Te layer and etch products such as . By rinsing in , the can be removed leaving only the Te layer. Subsequent electrochemical reduction converts the Te layer to telluride ions, which are soluble. Low energy electron diffraction (LEED) and Auger electron spectroscopy were used to characterize the surfaces. Ordered (1X1) LEED patterns and stoichiometric surfaces were observed following the procedure just described. Variations in the intensity distributions between spots in the (1X1) LEED patterns for the (111)Cd and (111)Te surfaces were significant enough to provide a clear means of differentiating the two faces. Vacuum studies where ion bombardment has been used to clean surfaces generally result in damaged surfaces, which can be repaired by subsequent annealing. The (111)Cd surface following annealing displays a (2X2) LEED pattern, the result of a surface reconstruction. The (111)Te displays a complex LEED pattern indicative of the formation of pyramid‐shaped facets. Another etching solution consisting of , , and was also investigated but proved too severe; the resulting Te layer was thicker than the one formed in and was difficult to remove. Surface heterogeneity appears to have been introduced by this etching process as well.

On the underlayer and overlayer adsorption of oxygen, nitrogen and carbon on the iron (111) surface

The EHT method has been used to calculate the energy of iron clusters modelling the Fe(111) surface, as well as the energy of iron clusters with oxygen, nitrogen or carbon atoms adsorbed on the reconstructed/non-reconstructed surface. To determine the relative positions of iron atoms and that of the adsorbed atom, a sphere model was employed assuming displacement of one iron atom only. The calculations show that a hollow site in the non-reconstructed Fe(111) surface is the most preferential site for oxygen adsorption. Transition to the underlayer adsorption state is an activated process and in terms of energetics seems to be less likely. For nitrogen or carbon, a hollow site under the iron atom is the preferential site; the transition to such an adsorption state from the adsorption on the non-reconstructed surface requires no activation. On the basis of LEED measurements and the results obtained, sphere models are suggested for two-dimensional structures of iron (111) surfaces with oxygen, nitrogen or carbon adsorbed on them.

Self-diffusion on the reconstructed and nonreconstructed Ir(110) surfaces.

On the (1\ifmmode\times\else\texttimes\fi{}1) Ir(110) surface the diffusion of single Ir adatoms is two dimensional; i.e., an adatom can diffuse along the [11\ifmmode\bar\else\textasciimacron\fi{}0] surface channel by atomic hopping as well as across the channel by atomic replacement. The energy needed for an Ir adatom to hop along the channel is 0.80\ifmmode\pm\else\textpm\fi{}0.04 eV, or 0.09 eV larger than needed to replace a lattice atom in the cross-channel jump. In contrast, Ir adatom diffusion on the (1\ifmmode\times\else\texttimes\fi{}2)-reconstructed surface, with double-space [11\ifmmode\bar\else\textasciimacron\fi{}0] channels, is one dimensional; i.e., an adatom can only hop along these channels. The activation energy, 0.86\ifmmode\pm\else\textpm\fi{}0.03 eV, is higher than that for either cross-channel or along-the-channel diffusion on the nonreconstructed surface.

CO adsorption on stepped Ni(111) surfaces

The CO adsorption on Ni(331) and Ni(221) [referred to as Ni(s)(111)] was studied with low-energy electron diffraction (LEED), thermal desorption spectroscopy (TDS), and work function change measurements. LEED from the clean Ni(s)(111) crystals give evidence for nonreconstructed surfaces with atomic arrangements as expected from the ideal (331) and (221) orientations; these surfaces can be interpreted as stepped Ni(111) surfaces with (111) terraces and (111) steps. CO desorption shows on both surfaces several desorption states, which are sequentially filled with increasing CO exposure. The desorption state with the highest peak temperature [Tm=455 K for Θ(CO)≊0] is identified on both surfaces and related to CO desorption from (111) terraces. For higher coverages, due to the presence of steps, new desorption states fill in with lower desorption temperature in the range of 200–350 K. These desorption states are not present on flat Ni(111). With LEED in a definite regime of CO coverages half and one fourth-order streaks in [1̄10] directions of Ni(s)(111) are observed and related to a one-dimensional ordering of CO in directions parallel to the step directions. Ordered LEED overlayers with c(4×2) [Ni(331) and Ni(221)] and (2×1) [Ni(221)] are established on both surfaces near the CO saturation coverage at 120 K. These overlayers demonstrate that the barrier for lateral interaction introduced by the steps is overcome at higher coverages.

Alkali ion impact collision scattering at Pt(111)

The features of alkali low energy ion scattering (ISS) in the impact collision mode (ICISS) have been investigated by scattering of 2000 eV Na + at a clean, non-reconstructed surface, Pt(111). The measured ICISS patterns show sharp intensity maxima at the positions expected from the surface geometry on the basis of strong focussing of the scattered ions via shadowing and blocking effects. The self calibration procedure based on the experimental determination of an effective shadow cone is an important advantage and results in a simple calculation scheme for the locations of intensity slopes in the ICISS patterns. The alkali ion ICISS shows a way to overcome the dilemma of neutralization effects known in noble gas ISS, as well as the complexity of alkali ISS in forward direction due to multiple scattering.

Zero-slope points and electronic energy bandmapping

Angle resolved photoemission spectra of Au (111) have been measured as a function of polar angle. The wavevector components of the electrons have been determined by symmetry arguments only. The method applied is based on zero-slope points and extrema in the bindung energy caused by symmetry. Results obtained from a predominantly nonreconstructed Au surface are presented for the ∑ direction.

He-beam scattering study of the dynamics of oxygen induced reconstruction of the Pt(997) surface: II. Shape changes of the He diffraction patterns induced by reconstruction and oxygen adsorption

The changes induced in the shape of the He diffraction patterns by the reconstruction of the Pt(997) surface reflect the presence of terraces with increased width. The locations of the fractional order peaks in the He patterns indicate that the distribution of the terrace widths on the reconstructed surface is more complicated than it was inferred from LEED patterns. The changes of the He diffraction patterns of both, reconstructed and non-reconstructed surfaces, induced by oxygen adsorption are 2-fold: a dramatic shape deformation and an overall reduction of the coherent intensity. The first is shown to be due to oxygen chemisorbed at step sites, while the second mainly to oxygen chemisorbed at terrace sites. Evidence for the important role played by the attractive interaction potential around step edges in He scattering is presented.