E. Platzgummer

Submonolayer growth of Pb on Cu( 111): surface alloying and de-alloying

Abstract In spite of the immiscibility of Pb in bulk Cu, atomically resolved scanning tunneling microscopy reveals surface alloy formation of Pb deposited on Cu ( 111 ), even at 300 K. Due to kinetic limitations at room temperature, the incorporation of Pb is restricted to advance from step edges, while after annealing to 470 K or higher, embedded Pb atoms are found to be randomly distributed over terraces. At low tunneling voltages, standing waves of surface-state electrons scattered by embedded Pb atoms could be observed. The maximum packing density of the surface alloy is about 40% ( = 0.4 ML) of a close-packed Pb overlayer. Thus, deposition above 0.4 ML and subsequent annealing results in hexagonal close-packed Pb regions, whereas on the non-annealed surface hexagonal close-packed Pb islands are already found at 0.2 ML. Eventually, at 1 ML the surface alloy is entirely replaced by a Pb overlayer.

Surface alloying and superstructures of Pb on Cu(100)

Abstract The growth of submonolayer and monolayer Pb films on Cu(100) has been investigated by STM. In the submonolayer region a disordered surface alloy is found in spite of the immiscibility of bulk Cu and Pb. At a coverage of θ = 3 8 a c(4 × 4) superstructure is observed. The atomic arrangement of the c(4 × 4) superstructure unit cell could be revealed; it is formed by linear chains of Pb atoms in two rotational domains. Increasing the coverage to θ = 0.5, a c(2 × 2) structure can be observed. The transition to the c(5√2 × √2)R45° superstructure of the dense overlayer proceeds via the insertion of antiphase domain boundary. The model for the c(5√2 × √2)R45° superstructure presented in literature is confirmed.

Temperature-dependent segregation on Pt25Rh75(111) and (100)

Abstract Surface segregation is studied on Pt25Rh75(111) and Pt25Rh75(100) by LEED intensity analysis and LEIS. Although both equilibrated surfaces are strongly Pt-enriched (up to 80xa0at.%), we find an interesting difference in the segregation behavior when annealing the sputtered surfaces. The Pt concentration grows continuously on Pt25Rh75(111) until 1000°C, whereas it reaches a maximum enrichment around 500°C on Pt25Rh75(100) and decreases thereafter. This contrasting behavior results solely from the kinetic limitation in the low-temperature regime, and is not due to energetic reasons. From temperature-dependent composition profiles we determine the segregation kinetics as well as the annealing temperature necessary for thermodynamic equilibration. We find that an equilibrium is acquired on the Pt25Rh75(100) surface by the interchange of Pt and Rh atoms within the near-surface layers, and on the Pt25Rh75(111) surface by a diffusion of Pt atoms from bulk to the near-surface region. The latter leads to an overall Pt enrichment of several layers, and is only observed after annealing at 1100°C. The presence of carbon contamination on the Pt25Rh75(100) surface causes a significant reduction of the Pt segregation. There is excellent agreement between the top-layer concentrations derived by LEIS and quantitative LEED.

Chemical ordering and reconstruction of Pt25Co75(100): an LEED/STM study

The surface of a disordered Pt25Co75(100) alloy has been investigated using quantitative LEED, AES and UHV-STM at room temperature. Atomic-resolution images reveal that it reconstructs with close-packed rows shifted by half the interatomic distance, from hollow to bridge sites. The density of shifted rows increases with the surface Pt concentration, leading to (1 × 5), (1 × 6) and (1 × 7) patterns. Segregation and chemical ordering lead to the formation of c(2 × 2) domains between the shifted rows. Chemical resolution was achieved with STM: the apparent height of the Pt atoms in the STM topographs is about 0.1–0.4 A above that of Co, whereas LEED shows that Pt atoms are geometrically ∼0.04 A higher. The composition was determined down to the fourth layer. An oscillatory segregation profile is observed, with Pt-rich layers (〈C1〉 = 62.6% Pt, 〈C3〉 = 53.5%) and Pt-depleted layers (〈C2〉 = 6.9%, 〈C4〉 = 2.7%). Chemical ordering is present in the third layer and the four-layer surface slab stabilises with a structure and a composition quite similar to that of the L12 PtCo3 phase. As regards the composition and ordering of the top layer, there is a remarkable agreement between chemically resolved STM analysis and LEED analysis.

Unreconstructed Au(100) monolayers on a Au3Pd(100) single-crystal surface

Abstract The Au 3 Pd(100) single-crystal surface was studied with ion scattering methods, low-energy electron diffraction (LEED) and scanning tunneling microscopy. The crystal is covered at room temperature with a pure, (100)-ordered gold layer. Palladium is found in the second layer only. The lattice constant of the gold surface as evaluated by ion scattering and a tensor low-energy electron diffraction (TLEED) analysis is equal to the bulk lattice constant of 4.017xa0A as evaluated by X-ray analysis. The surface lattice constant of the gold layer on the alloy surface is 0.08xa0A smaller than that of bulk gold.

Temperature-dependent segregation and (1×2) missing-row reconstruction of Pt25Rh75(110)

Abstract The surface structure and composition of the clean Pt25Rh75(110) surface is investigated by low energy electron diffraction (LEED) and low energy ion scattering (LEIS). For the equilibrated Pt25Rh75(110) surface we observe a (1×2) missing-row reconstruction in analogy to the pure Pt(110) surface, and a significant Pt enrichment of the topmost atomic layer (up to 80xa0at.% Pt). As the same strong surface enrichment in Pt was found in a previous study on the (100) and (111) surface of the same bulk composition, this means that in contrast to Pt–Ni and Pt–Co alloys, for Pt25Rh75 alloys the segregation behavior is not influenced extensively by the surface orientation. In addition to the structure analysis by LEED we performed LEIS experiments to determine the temperature-induced changes of the surface composition and structure. Since the Pt segregation is less pronounced at elevated temperature, the surface reveals a temperature-induced deconstruction of the (1×2) structure around 750°C, resulting in an fcc(110) (1×1) surface at high temperature. Temperature-dependent measurements further show a hysteresis-like behavior of the top-layer composition, which is attributed to an enhanced Pt segregation on the (1×2) reconstructed surface.

The accuracy of quantitative LEED in determining chemical composition profiles of substitutionally disordered alloys : a case study

We explore the accuracy of chemical composition profiles of substitutionally disordered alloys determined experimentally by LEED (low-energy electron diffraction) I(E) analysis. We analyse experimental I(E) spectra of pure Rh(111) for its known chemical composition by comparing them to calculations assuming a substitutionally disordered PtxRh1−x alloy surface. The layer concentrations known to be 100% Rh are reproduced with a maximum error of 8% when the Pendry R-factor (RP) is employed. This error is considerably smaller than estimated by error bars derived from the variance of RP. We argue that the same accuracy can be expected for compositional depth profiles to be determined for alloys exhibiting weak chemical order and negligible lattice distortions such as PtxRh1−x.

A quantitative LEED analysis of the oxygen-induced p(3×1) reconstruction of Pt25Rh75(100)

Abstract Pt25Rh75(100) forms a p(3×1) reconstruction at saturation coverage of oxygen (23xa0L O2, 600°C). A previous STM study on O/Pt50Rh50(100) suggests that every third row of the first substrate layer is shifted by half a lattice constant (“shifted rows”). We present a LEED I(E) analysis of Pt25Rh75(100) confirming the shifted-row model and find that oxygen resides in threefold-coordinated sites on both sides of the shifted rows. The adsorbate occupies those of the threefold-coordinated sites that are directly separated by the metal atom in the shifted row. Further I(E) calculations exclude the alternative threefold-coordinated adsorption site beside the shifted row as well as the fourfold-coordinated site symmetrically in between the shifted rows. We achieve a Pendry R-factor of 0.14 for the best-fit structure. Oxygen has equal bond lengths to its three metal neighbours, amounting to 1.95xa0A. The first substrate layer relaxes outward by 8.8% of the bulk value to 2.08xa0A, but we do not observe any significant relaxations of deeper layer spacings. The shifted rows pop out of the surface by 0.38xa0A. After determination of the oxygen adsorption site with LEED, we examine local adsorption structures on Pt25Rh75(100) at low oxygen coverage with STM. We resolve the shifted rows in real-space, and for special tip conditions, we find maxima of apparent height at in-plane positions that coincide with the oxygen position as established by quantitative LEED. We determine chemical-composition depth-profiles by quantitative LEED for three surface preparations occurring during sample preparation. While the first substrate layer of clean and annealed Pt25Rh75(100) is enriched in Pt (76%) as compared to the bulk value (25%), that in p(3×1)-O/Pt25Rh75(100) is enriched in Rh (90%). Oxygen adsorption at a moderate temperature (600°C) and formation of the p(3×1) structure reverse segregation on Pt25Rh75(100). Finally, oxygen can be removed at room temperature by exposure of the surface to hydrogen. This lifts the reconstruction but keeps the Rh enrichment of the first substrate layer.

Temperature-dependent segregation reversal and (1×3) missing-row structure of Pt90Co10(110)

The surface structure and composition of the clean Pt90Co10(110) surface are investigated by low-energy electron diffraction (LEED) and low-energy ion scattering (LEIS). Through LEED I–V analysis, we find a (1×3) missing-row reconstruction on the equilibrated Pt90Co10(110) surface — comparable with the pure Pt(110) (1×3) surface — in which all atomic positions in the topmost layer and in the (111) oriented microfacets are Pt-enriched. Due to the fact that the unreconstructed Pt25Co75(110) surface is known to exhibit an almost pure Co top layer, the Pt segregation reported in this study is undoubtedly connected to the existence of the missing-row reconstruction. The proposed structural influence on the composition is confirmed by LEIS experiments performed on the hot Pt90Co10(110) surface, in which simultaneously temperature-induced changes of the surface composition and qualitative changes in the surface structure are monitored. The measured low-energy ion spectra not only reproduce the calculated first-layer composition of the LEED analysis but also show a less pronounced Pt segregation at temperatures around 750°C, and eventually a reversed Pt segregation above 750°C, i.e. Co enrichment of the Pt90Co10(110) surface with respect to the bulk concentration. We find a clear correlation between the thermal deconstruction and the surface composition. The striking segregation reversal during temperature variation is attributed to the high excess value of the mixing enthalpy, which implies a structure-dominated segregation behavior.

Anti-corrugation and nitrogen c(2 × 2) on Cr(100): STM on atomic scale and quantitative LEED

We present a LEED I-V analysis of c(2 × 2)-NCr(100). We found nitrogen residing in fourfold hollow sites and exclude adsorption models in which nitrogne adsorbs on a metal site (on-top, substitutional or second-layer interstitial). We achieved a Pendry R-factor of 0.16 for the best-fit structure. Nitrogen resides at a vertical distance of 0.36 A above the first chromium layer. The interlayer spacing between the first and the second chromium layer is expanded to 1.55 A (7.5% with respect to the bulk value of 1.44 A). The interlayer spacing between the second and the third layer is contracted to 1.41 A. The second chromium layer is buckled (0.13 A). The second-layer chromium atom beneath a nitrogen atom resides deeper in the bulk. The nitrogen bond length to the four first-layer chromium atoms amounts to 2.07 A, the bond length to the second-layer chromium atom amounts to 1.97 A. The nitrogen position in c(2 × 2)-N determined by LEED is used to identify hollow sites in scanning tunnelling microscopy images. We found that hollow sites in p(1 × 1)-Cr(100) are imaged as hillocks and chromium atoms as depressions. This is anti-corrugation of clean Cr(100). Anti-corrugation seems to be related to a surface state of clean Cr(100) and is lifted in p(1 × 1)-NCr(100) at a (local) nitrogen coverage of 1 monolayer.