New relaxation phenomena in the outer atomic layers of Fe{211}

A low-energy electron diffraction analysis of a {211} surface of body-centered cubic iron reveals relaxations in the directions perpendicular and parallel to the surface plane. Both relaxations alternate in successive layers. The perpendicular relaxation goes from contraction of 10.5% to expansion of 5% to contraction of 1%. The parallel relaxation goes from a shift of the first layer of 0.24Å (10% of the nearest neighbor distance) toward more symmetrical registration with the second layer, to an opposite shift of 0.035 Å of the second layer with respect to the third.     

Theoretical studies of fast He atom scattering from a W{100} surface

Theoretical calculations of the scattering of fast neutral He atoms from W{100} are presented which are directly compared to the results from recent experiments of Nielsen and Delchar. The experiments which were performed for He atom energies between 150 and 1000 eV, and for incident polar angles between 0° and 65° as measured from the surface normal, displayed peaks in the polar angle distributions at 72°, 56° and ±10°. The results from classical dynamics calculations are employed here to explain the scattering phenomena that give rise to these peaks in the polar distributions. The calculations indicate that the peak at 72° is primarily due to scattering from the first layer W atoms. The peaks at ± 10° and 56° are unusual in that there are a multitude of different collision paths that result in the He atom being scattered into the same final angle. The peaks at ± 10° result from He atoms scattering mainly from the second, third and fourth layers of W atoms. The He atoms are focused on the outward path into the near normal direction by two first and two second layer W atoms. Subsurface scattering is also responsible for the peak at 56°. In this case the channel of first and second layer W atoms that focuses the outgoing He atoms is oriented at 54.7° with respect to the surface normal. It is proposed that slight variations of the experimental data from the calculated values are due to surface reconstruction of W{100} and that a more thorough analysis could reveal the microscopic nature of this structure.     

Point-defect properties of and sputtering events in the {001} surfaces of Ni3Al I. Surface and point-defect properties

Constant-area and fully relaxed molecular dynamics methods are employed to study the properties of the surface and point defects at and near {001} surfaces of bulk and thin-film Ni, Al and Ni₃Al respectively. The surface tension is larger than the surface energy for all {001} surfaces considered in the sequence: Al (1005?mJ?m^?2)<?Ni₃Al (mixed Ni–Al plane outermost, 1725?mJ?m^?2)<?Ni₃Al (all-Ni-atoms plane outermost, 1969?mJ?m^?2)<?Ni (1993?mJ?m^?2). For a surface of bulk Ni₃Al crystal with a Ni–Al mixed plane outermost, Al atoms stand out by 0.0679?Å compared with the surface Ni atoms and, for the all-Ni-atoms surface, Al atoms in the second layer stand out by 0.0205?Å compared with Ni atoms in the same layer. Vacancy formation energies are about half the bulk values in the first layer and reach a maximum in the second layer where the atomic energy is close to the bulk value but the change in embedding energy of neighbouring atoms before and after vacancy formation is greater than that in the bulk. Both the vacancy formation energy and the surface tension suggest that the fourth layer is in a bulk state for all the surfaces. The formation energy of adatoms, antisite defects and point-defect pairs at and near {001} surfaces of Ni₃Al are also given.     

3D atom probe study of gas adsorption and reaction on alloy catalyst surfaces II: Results on Pt and Pt-Rh

The 3-dimensional atom probe (3DAP) is a unique instrument providing chemical analysis at the atomic scale for a wide range of materials. A dedicated 3DAP has been built specifically for analysing reactions at metal surfaces, called the catalytic atom probe (CAP). This paper presents an overview of results from the CAP on structural and chemical transformations to surface layers of Pt and Pt-17.4 at.%Rh catalysts following exposure to a number of gases typically emitted by vehicle engine exhausts, normally for 15 min at pressures of 10 mbar. Following exposure to the oxidising gases NO on Pt, and NO, O₂ or N₂O on Pt-Rh, both surfaces appear disrupted, while for Pt-Rh, Rh enrichment of the surface atomic layer is noted over the entire specimen apex for exposure temperatures up to 523 K. However, for oxidising exposures at 573-773 K relatively clean, Rh-depleted surfaces are observed on {0 0 1}, {0 1 1} and {0 1 2} crystallographic regions of Pt-Rh. It is suggested that this result is due to surface diffusion of oxide species over the specimen apex, towards the {1 1 1}-orientated areas where the oxides appear to be stabilised. In contrast, CO exposure appears to have little effect on the either the surface structure or composition of the Pt-Rh alloy. Finally, combinations of two gases (NO + CO, O₂ + NO) were also dosed onto Pt-Rh alloys in the same exposure. These revealed that while NO and CO can co-adsorb without interference, CO prevents the build up of oxide layers and reduces the extent of Rh segregation seen under NO exposure alone. On exposing Pt-Rh to NO after an oxygen exposure, heavily oxidised surfaces, Rh segregation and no intact NO molecules were seen, confirming the ability of oxidised Pt-Rh to dissociate nitric oxide.     

Research of the behaviour of O chemisorption on the (110) surface of Rhx--Pt1－x alloy

An atomic group model of the disordered binary alloy Rhx-Pt1-x has been constructed to investigate surface segregation. According to the model, we have calculated the electronic structure of the Rhx-Pt1-x alloy surface by using the recursion method when O atoms are adsorbed on the Rhx-Pt1-x （110） surface under the condition of coverage 0.5. The calculation results indicate that the chemical adsorption of O changes greatly the density of states near the Fermi level, and the surface segregation exhibits a reversal behaviour. In addition, when x 〈 0.3, the surface on which O is adsorbed displays the property of Pt; whereas when x 〉 0.3 it displays the property of Rh.     

Orientation of chemisorbed species from orthogonal-plane arups: Tilted CO on Pt{110} and upright CO on Pt{111}.

Angle-resolved photoemission spectra obtained with electron collection in the plane orthogonal to the incidence plane, using a He II photon source, are used to provide an accurate measure of the orientation of chemisorbed diatomic molecules. As previously established, CO in the Pt{111}c(4 × 2) structure is found to be chemisorbed in an upright configuration, whereas in the Pt{110}(2 × 1) p1g1 structure the CO molecular axis is tilted 26 ± 2° away from the surface normal, in a direction between [211] and [433].     

Radiation damage in single-crystal ice studied by 100 keV proton channeling

A detailed study has been undertaken of the Ni{100} (2 × 2)C structure formed by cracking ethylene on a clean Ni{100} surface. The LEED pattern shows characteristic missing spots which can be attributed to the presence of glide lines and indicate a space group symmetry of p4g. We show that this can be readily interpreted in terms of a distortion of the top nickel layer both parallel and perpendicular to the surface, which accompanies the carbon adsorption. Detailed comparisons of LEED intensity data with dynamical calculations indicate that the top layer nickel atoms are displaced 0.35 ± 0.05 Å parallel to the surface, 0.20 ± 0.05 Å outwards from the surface, and that the carbon atoms are in 4-fold hollows (now distorted) at a spacing of 0.1 ± 0.1 Å from the surface. These conclusions lead to a nickel-carbon nearest neighbour spacing of 1.803 ± 0.015 Å.     

A combined XPS/AES study of Cu segregation to the high and low index surfaces of a Cu-Ni alloy

Quantitative information was obtained regarding the equilibrium segregation of Cu to the high index surfaces (cut 5° off the (100) plane in the [001] and [011] directions) and low index surfaces ((100), (111) and (110)) of a Cu: Ni, 5: 95% alloy crystal. Data regarding segregation to the (111) surface of a Cu: Ni, 50: 50% alloy crystal was also obtained. Equilibrium surfaces were obtained by careful annealing in the temperature range 850–920 K to avoid Cu loss by evaporation. The surface composition profile was calculated using data from the combination of the X-ray Photo-electron Spectroscopy and Auger Electron Spectroscopy techniques. Using these techniques, a wide range of electron kinetic energies and associated electron escape depths can be probed, yielding information about the topmost layer concentration and about the concentration profile into the bulk. Extensive Cu segregation was observed for the 5% and 50% Cu alloys. Topmost layer compositions of 85–100% Cu were found for both the high and low index surfaces of the 5% Cu alloy. In the next two or three atomic layers the composition was found to drop swiftly to near bulk Cu levels. For atomic layers deeper than this, some experimental evidence suggested a rise in Cu composition over three or four layers to 8–17% Cu before bulk levels were finally regained. For the 50% Cu alloy sample, a topmost layer composition was found of 95–100% Cu. Bulk levels of Cu were regained in about four atomic layers. These results are discussed in relation to other theoretical and experimental studies of segregation in these Cu-Ni alloys. The significance of the measured composition in relation to an equilibrium surface is also considered.     

The effect of Pd doped in Ru layer on theinterlayer coupling in Co／RuPd multilayers

2.0nmCo/tnmRu层间反磁铁耦合 磁性录音 振动交换 添加效应2.0nmCo/tnmRu1-xPdx multilayers with x=0, 0.05, 0.08, 0.24, 0.39 and 0.48 were prepared by magnetron sput-tering. The spacer layer thickness of both Ru (before doping Pd) and RuPd (after doping Pd) varies from 0.2nm to 1.6nm. Two effects have been investigated: (1) the dependence of the interlayer coupling on the thickness of Ru1-xPdx as a function of x and (2) the dependence of the interlayer coupling on Pd doping density, x, as a function of thickness of Ru1-xPdx. Our results indicate that the interlayer coupling is strongly dependent on the doping density and the spacer layer thickness. The saturation field Hs increases when very low concentration of Pd doped in the Ru layers and a suitable spacer thickness are adopted.     

Thermal roughening of FEM-clean Pt{110} and its vicinal areas

After heat treatment at ∼1600 K and rapid quenching, thermal roughening through kink formation could be observed on “FEM-clean” Pt{110} and on all its vicinal areas (which are made up of {110} terraces). Noticeable exceptions were the areas between {771} and {331} (including {771}), which are located on the [1&#x0304;10] zone. These areas may remain topographically unchanged, or, more likely, may form {111}-microfacets during heat treatment. Upon annealing, the high-temperature roughness on {110} started to decrease at 830 K (formation of an 1 × 2 structure) and reached a minimum at 960 ± 25 K (≡ transition temperature, T_c). Thereafter, it increased precipitously until 1030 K was reached (surface roughening/deconstruction). Surface roughening could be suppressed readily by gas phase and/or bulk impurities (surface segregation). Vapour deposited Si, P, SiO, TiO₂, Al₂O₃ and C (graphite) prevented surface roughening on Pt{110}. All investigations were carried out by FEM.     

Near-degenerate stereomorphs of the doubly-chiral hcp-{213̅1} surface

The surfaces of hcp crystals can show a variety of structural features and classes of symmetry that differ markedly from those of simpler fcc or bcc crystals. The hcp-{213?1} surface, for example, can occur in four distinct stereomorphs, interconverted by a combination of mirror operations (linking degenerate enantiomorphically related surfaces) and/or the removal of the outermost atomic layer (linking non-degenerate diamorphically related surfaces). The strict pattern of degeneracy amongst these stereomorphs is analogous to that found for molecules with two chiral centres, and hence it is possible to view this system as doubly-chiral. Simple nearest-neighbour bond-counting arguments, however, suggest that for {213?1} even the diamorphically related cases should be near-degenerate, despite the fact that they differ in having either a notably short or notably long interlayer spacing between the outermost layers (ideal spacing ratio 1:5). In the present work, this counterintuitive result is confirmed at the level of density functional theory, both for the ideal and relaxed {213?1} surfaces of Co, Ru and Re.     

Growth and stability of carbon islands on platinum surfaces

Details of the growth of C(graphite) islands and their stability on Pt surfaces were studied by FEM, UHV-SEM and very high resolution scanning AES. Initial nucleation of the C occurs on dislocations in the curved high index surface areas. Above 1150 K these randomly distributed islands dissolve and face specific layers are formed on {110} which can extend along the 〈100〉 zones all the way to the {100} planes. The sequence of stability of graphite layers on Pt is: {110} > all other {hk0} on the 〈100〉 zone except {100} > {100} followed closely by {111}. Concerning this layer stability, epitaxial mismatch plays a subservient role to the dipole interaction between metal substrate and graphite layer.     

Ru thickness-dependent interlayer coupling and ultrahigh FMR frequency in FeCoB/Ru/FeCoB sandwich trilayers

The antiferromagnetic (AFM) interlayer coupling effective field in a ferromagnetic/non-magnetic/ferromagnetic (FM/NM/FM) sandwich structure, as a driving force, can dramatically enhance the ferromagnetic resonance (FMR) frequency. Changing the non-magnetic spacer thickness is an effective way to control the interlayer coupling type and intensity, as well as the FMR frequency. In this study, FeCoB/Ru/FeCoB sandwich trilayers with Ru thickness ($t_{\rm Ru}$) ranging from 1 Å to 16 Å are prepared by a compositional gradient sputtering (CGS) method. It is revealed that a stress-induced anisotropy is present in the FeCoB films due to the B composition gradient in the samples. A $t_{\mathrm{Ru}}$-dependent oscillation of interlayer coupling from FM to AFM with two periods is observed. An AFM coupling occurs in a range of $2 {\rm Å} \le t_{\rm Ru} \le 8 {\rm Å}$ and over 16 $\mathrm{Å}$, while an FM coupling is present in a range of $t_{\rm Ru}< 2$ Å and $9 {\rm Å} \le t_{\rm Ru} \le 14.5 Å$. It is interesting that an ultrahigh optical mode (OM) FMR frequency in excess of 20 GHz is obtained in the sample with ${t}_{\mathrm{Ru}}= 2.5 \mathrm{Å}$ under an AFM coupling. The dynamic coupling mechanism in trilayers is simulated, and the corresponding coupling types at different values of $t_{\mathrm{Ru}}$ are verified by Layadi's rigid model. This study provides a controllable way to prepare and investigate the ultrahigh FMR films.     

Ion-beam crystallography of clean and sulfur covered Ni(110)

Medium energy ion scattering has been used to determine the atomic structure of a Ni(110) surface covered with 0.5 monolayer of sulfur. After having confirmed that the sulfur atom resides in a fourfold-coordinated hollow site, it was found that its distance above the plane of the first Ni layer is 0.87 ± 0.03 Å. We measured a 6 ± 3% outward relaxation effect for the sulfur covered Ni(110) surface layer and an inward relaxation of 4 ± 1% when this surface is clean.     

Interface dislocation structures at the onset of coherency loss in nanoscale Ni–Cu bilayer films

We have performed a transmission electron microscopy study, using weak beam imaging, of the interface dislocation arrays that form initially at the (001) Ni–Cu interface during coherency loss. Interface dislocations were absent in the 2.5?nm Ni/100?nm Cu bilayers, but were present in the 3.0?nm Ni samples, indicating that the critical Ni film thickness for coherency loss is between 2.5 and 3?nm. The key features of the interface dislocation structure at the onset of coherency loss are: (i) the majority of interface dislocations are 60° dislocations, presumably formed by glide of threading dislocations in the coherently stressed Ni layer, and have Burgers vector in the {111} glide plane; (ii) the interface contained approximately 5% Lomer edge dislocations, with Burgers vector in the {001} interface plane, and an occasional Shockley partial dislocation and (iii) isolated segments of interface dislocations terminating at the surface are regularly observed. Possible mechanisms that lead to these dislocation configurations at the interface are discussed. This experimental study shows that near the critical thickness, accumulation of interface dislocations occurs in a somewhat stochastic fashion with favourable regions where coherency is first lost.     

A Monte Carlo study of surface segregation in alloys

The Monte Carlo method has been applied to the study of surface segregation in a multi-layer, regular solution model of alloy surfaces. Three different alloy configurations have been investigated: semi-infinite slabs, thin films and small particles. The results show that the alloy component with the lowest surface energy tends to segregate to the first three or four surface atom layers and that segregation is greater in clustering alloys than in ordering alloys. Furthermore, segregation is more pronounced in low coordination surfaces, as evidenced by a comparison of {110} and {100}-oriented surfaces of fcc alloys. The degree of surface segregation in thin films and small particles (in the particle size range studied) tends to be smaller than in semi-infinite slabs, because of mass conservation constraints, and decreases with decreasing film thickness and particle size. The results obtained are contrasted with previous calculations and possible avenues for improving surface segregation models are discussed.     

Surface Structure of Large Synthetic Diamonds by High Temperature and High Pressure

With NiMnCo and FeCoNi alloys as solvent metals, large single-crystal diamonds of about 3mm across are grown by temperature gradient method （TGM） under high temperature and high pressure （HPHT）. Although both {100} and {111} surfaces are developed by a layer growth mechanism, some different characteristic patterns are seen clearly on the different surfaces, no matter whether NiMnCo or FeCoNi alloys are taken as the solvent metals. For {100} surface, it seems to have been melted or etched greatly, no dendritic patterns to be found, and only a large number of growth hillocks are dispersed net-likely; while for {111} surface, it often seems to be more smooth-faced, no etched or melted traces are present even when a lot of depressed trigonal growth layers. This distinct difference between {111} and {100} surfaces is considered to be related to the difference of surface-atom distribution of different surfaces, and {111} surfaces should be more difficult to be etched and more steady than {100} surfaces.     

Morphology and composition of nanoscale surface oxides on Fe–20Cr–18Ni{1 1 1} austenitic stainless steel

Surface oxidation ranging from initial stages to the onset of passive oxide layer formation have been investigated on Fe–20Cr–18Ni{1 1 1} single crystal surface by X-ray photoelectron spectroscopy (XPS). Surface segregation of the alloying elements and the morphology of the surface oxide nanostructure were characterized quantitatively by inelastic electron background analysis. Our results demonstrate that by increasing the oxidation temperature the relative concentrations of Fe²⁺ and Fe³⁺ cations increase due to their enhanced mobility. Higher temperature also improves the mobility of chromium, thus enhancing its segregation to the oxygen-rich surface and thereby reinforcing the passive layer on the alloy. This is in agreement with the results showing the sudden decrease in oxide film thickness at the oxidation temperatures exceeding 600 K. Additionally, a pronounced segregation of metallic nickel is found in the interface between the surface oxide layer and the bulk alloy.     

Atomic structure of Fe {111}

A clean Fe {111} surface was prepared and studied with LEED (low-energy electron diffraction) and AES (Auger electron spectroscopy). A LEED intensity analysis was carried out with a new computational scheme (THIN) specially designed for short interlayer spacings. The results are, for the fust interlayer spacing, d₁₂ = 0.70 ± 0.03 Å and for the inner potential V₀ = 11.1 ± 1.1 eV, the confidence intervals referring to 95% confidence level. Thus, the Fe {111} surface is contracted 15.4% with respect to the bulk (0.827 Å).     

Growth and desorption of indium epitaxial layers investigated by high field microscopy

Growth of indium single crystals on tungsten field emission tips was carried out by deposition of indium from vapour in ultra high vacuum, using substrate temperatures in the range of 293–420 K. Two different tungsten tips were used as the substrate: a perfect W single crystal in one case and a bi-crystal with a distinct grain boundary in the other. No influence of the grain boundary on the epitaxial growth was found. Two orientation relationships were observed mostly: {111}In ∥ {110}W with 〈110〉In ∥ 〈111〉W and {111}In ∥ {100}W with 〈110〉In ∥ 〈110〉W. In the first case the growth was initiated by the indium nucleus created on the ledges of the {110}W plane. A field strength of 0.9 V/Å was found for the evaporation field of indium. The field strength of the desorption of In-W interfacial layer atoms was found to be 4.4–5.2 V/Å. A mechanism of the growth of indium crystals has been proposed.