P. Weigand

Surface analysis of PtxNi1−x single crystals

Abstract The surface composition of Pt 10 Ni 90 (100), Pt 25 Ni 75 (111) and Pt 50 Ni 50 (110) single crystal alloys has been determined by ion scattering spectroscopy (ISS) and ultraviolet photoelectron spectroscopy (UPS). Pt enrichment of the topmost layer due to preferential sputtering occurs independently of the surface orientation. Annealing the sputtered alloys results in face specific surface compositions: Pt enrichment for the (111) surface, Ni enrichment for the (110) surface and a composition close to the bulk value for the (100) surface. Exposure of the sputtered surfaces to oxygen induces segregation of Ni. Adsorbate induced segregation is correlated with segregation due to annealing. The determination of workfunctions by UPS results support the ISS results on surface composition. The experimental results on surface segregation are discussed with respect to thermodynamic calculations.

Preferential sputtering and segregation reversal: (100) and (110) surfaces of Pt25Ni75 single crystal alloys

Abstract Low energy ion scattering (LEIS) and Auger electron spectroscopy (AES) are applied to determine the surface composition of Pt25Ni75(100) and Pt25Ni75(110). After annealing at 970 K the topmost atomic layer of Pt25Ni75(100) is observed to be enriched in Pt to a value of 39 at%, whereas the first monolayer of Pt25Ni75(110) is found to consist nearly exclusively of Ni atoms. The second layer of the (110) surface shows strong Pt enrichment. The bombardment with low energy ions at room temperature induces the preferential sputtering of Ni atoms. The LEIS results indicate the superposition of the effects of preferential sputtering and segregation. The dependence of the surface composition on the annealing temperature is rather similar for both Pt25Ni75 crystals under investigation though the segregating component differs. Segregation is found to occur at rather low annealing temperatures, whereas high annealing temperatures are necessary to allow thermodynamic equilibration by bulk diffusion.

The shifted-row reconstruction of PtxNi1−x(100)

Scanning tunneling microscopy on Pt10Ni90(100) and Pt25Ni75(100) single crystals reveals close-packed rows of atoms, which are shifted by 14 〈110〉 along the direction of the rows into a bridge position and slightly outward of the surface. Maximum entropy deconvolution of atomically resolved STM data shows that all atoms between the shifted rows are close to the unreconstructed positions. The density of the shifted rows increases with increasing Pt surface concentration up to a maximum value of each 5th row shifted. The reconstruction shows little dependence on the carbon contamination of the surface, but it is lifted by a full c(2 × 2) coverage of carbon monoxide, which can be imaged simultaneously with the substrate, indicating an on-top position of CO. The driving force of the shifted-row reconstruction is discussed.

Preferential sputtering of Pt-Ni alloy single crystals studied by scanning tunneling microscopy

Abstract Due to its composition, the altered layer of preferentially sputtered alloys has a lattice constant different from that of the bulk. This lattice mismatch can lead to the formation of dislocations or reconstructions, which have been studied on different crystallographic faces. While a subsurface dislocation network exists on the (111) plane, parallel dislocations are found below the (110) surface. The (100) surface exhibits a shifted-row reconstruction, which is tentatively attributed to the stress induced by lattice mismatch between the bulk and the Pt-enriched surface. The annealing process of the sputtered Pt25Ni75(111) surface is studied in detail by evaluation of the mismatch dislocations and low energy ion scattering data.

On the surface composition of Pt10Ni90(110): a low energy ion scattering study

Abstract Low energy ion scattering spectroscopy is applied to determine the surface composition of Pt10Ni90(110). Measurements performed at different scattering geometries allow us to distinguish between scattering from first and second monolayer atoms. Trajectory simulations including neutralization allow the determination of first and second monolayer composition. After annealing at 970 K the topmost atomic layer is found to consist nearly exclusively of Ni atoms, whereas the second monolayer shows a strong Pt enrichment. This oscillating behaviour of the composition depth profile is observed after bombarding the surface with ions as well. After thermal treatment distinct differences in the temperature dependence of the composition between the first and second monolayer are found.

Investigation of Pt25Ni75(111): preferential sputtering and surface segregation

The surface composition of Pt25Ni75(111) has been determined by ion scattering spectroscopy and Auger electron spectroscopy. The surface layers are found to be enriched in Pt due to preferential sputtering. Thermal treatment induces further segregation of Pt at the topmost atomic layer. From the different sampling depths of ISS and AES composition profiles are deduced, which show a strong dependence on the temperature during thermal treatment. At an annealing temperature of 770 K the thermally induced Pt segregation is determined by the composition of the sputter induced altered layer being enriched in Pt. However, at an annealing temperature of 970 K the sputter induced altered layer has disappeared and the Pt segregation at the topmost atomic layer is determined by the bulk composition.

Surface composition of PtxNi1−x single crystal alloys

Abstract We present results of the surface composition of the topmost layer of three low index planes of Pt x Ni 1−x single crystal alloys: Pt 10 Ni 90 (100), Pt 25 Ni 75 (111) and Pt 50 N 50 (110). The alloys were investigated by ion scattering spectroscopy (ISS). We observed Pt enrichment of the surface layers caused by preferential sputtering. After exposing the surfaces to oxygen, adsorbate induced segregation of Ni was measured. Annealing the sputtered alloys results in a site specific change in surface composition. We found Pt enrichment for the (111) surface and Ni enrichment for the (110) surface. The surface of Pt 10 Ni 90 (100) showed a Pt concentration close to the bulk composition. Work functions were determined by ultraviolet photoelectron spectroscopy (UPS). The UPS results support the ISS results of the surface composition. The results of thermodynamic calculations are discussed with respect to the experimental results on surface segregation.

Pt25Ni75(100) and (110) single crystals: preferential sputtering and segregation reversal

Abstract After annealing at 970 K low energy ion scattering (LEIS) reveals the topmost atomic layer of Pt 25 Ni 75 (100) to be enriched in Pt to a value of 39 at%, whereas for Pt 25 Ni 75 (110) the first monolayer is found to consist nearly exclusively of Ni atoms and the second layer shows strong Pt enrichment. LEIS measurements performed after ion bombardment at room temperature indicate the superposition of the effects of preferential sputtering of Ni atoms and segregation. Segregation is found to start at rather low annealing temperatures, whereas high annealing temperatures are necessary to allow thermodynamic equilibration by bulk diffusion. Most of the theoretical models fail to reproduce the orientation dependent change in the segregating component of the PtNi system. One of the few successful approaches is a thorough thermodynamic description which is used here to calculate the composition profile and to discuss the segregation behaviour. Good agreement between measurement and calculation is found.

Surface composition of Pt25Ni75(111) investigated by ISS and STM

SummaryIon scattering spectroscopy results show Pt enrichment in the topmost atomic layers of a Pt25Ni75(111) single crystal due to preferential sputtering. The increased lattice constant caused thereby leads to subsurface lattice mismatch dislocations, which have been studied by scanning tunneling microscopy. Agreement is found for the Pt concentration estimated from the density of dislocations and the results obtained by ISS. Thermal annealing induces further segregation of Pt in the topmost atomic layer. The composition of the subsurface layers has been studied and a strong dependence on the annealing temperature has been found. The observed Pt enrichment in the first monolayer for the thermodynamic equilibrium state agrees well with a thermodynamic theory.

Surface composition of Pt10Ni90(110)

Abstract Low energy ion scattering spectroscopy results are presented which allow to derive the first and second layers composition of the Pt 10 Ni 90 (110) single crystal. After annealing at 970 K the topmost layer is found to be nearly pure Ni, whereas the second layer shows a strong enrichment in Pt. After bombarding the surface with ions a similar depth profile is preserved with a slight Ni enrichment in the first monolayer. Thermal treatment of the sputtered surface induces site changes at low temperatures between the first and second layer and at high temperatures equilibration between surface and bulk. Only very few theoretical models successfully describe the oscillating segregation profile and the orientation dependent segregating component of the PtNi system in the equilibrium state. A thorough thermodynamic description (previously applied in a monolayer approximation) will be used here in multilayer calculations to study the influence of the different effects on the composition profile. Pt and Ni differ negligibly in the surface free energies giving way to a competition between an ordering effect and a size effect. Good agreement between measurement and calculation is found.