R. Baudoing-Savois

Monotonous concentration profile and reconstruction at Pt80Fe20(111): LEED study of a catalyst

Low energy electron diffraction (LEED) is used to determine the structure, the composition of the surface region and the substrate order of Pt80Fe20(111). The concentration profile is monotonously decreasing in contrast with disordered PtxNi1−x alloys for the concentration oscillates around the bulk value: the top layer, which is buckled (dz1 = 0.09 ± 0.02 A), is almost pure Pt (C1 = 96 ± 4at% Pt); the second and third layers are also enriched with Pt (C2 = 84 ± 7%, C3, = 85 ± 15%). The interlayer very close to the substrate spacing Δd12 = 0.3 ± 1% and Δd23 = −0.6 ± 2%. In addition, we show that the concentration profile the geometry can be solved independently: for surface alloys of this type, the composition profile can be extracted in the subsitution disorder approximation with a large gain in computing time.

A NEW UHV DIFFRACTOMETER FOR SURFACE STRUCTURE AND REAL TIME MOLECULAR BEAM DEPOSITION STUDIES WITH SYNCHROTRON RADIATIONS AT ESRF

Abstract We describe a new surface X-ray diffractometer, which is optimized to combine surface X-ray diffraction (SXRD), absorption spectroscopy (SEXAFS) and grazing incidence small angle scattering (GISAXS). This instrument is particularly well suited for real time studies of material elaborated in-situ with molecular beam deposition (MBD) techniques. The goniometer allows for large in-plane and out-of-plane momentum transfer with high accuracy. Owing to the flipping mechanism of the sample holder, it can perform absorption experiments with polarisation directions normal and parallel to the sample surface, while keeping a grazing incidence. Several MBD sources as well as complementary surface sensitive electron techniques (RHEED and Auger spectroscopy) can be used simultaneously with X-rays. Finally, the samples can easily be inserted in the X-ray chamber via an UHV transport system and a fast entry load-lock module.

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.

LEED study of Pt25Co75(111)

A quantitative LEED study of the Pt 25 Co 75 (111) surface (in the substitutionally disordered state) shows weak relaxations of interlayer spacings (Δd 12 /d bulk , Δd 23 /d bulk , Δd 34 /d bulk )=(-1.9, -1.4, 3.6)%, no stacking fault with respect to the ABC stacking but a significant oscillatory composition profile (C 1 , C 2 , C 3 )=(46, 3, 37) at% Pt. This behaviour is very close to the general trend observed for PtNi(111) surfaces as a function of the bulk concentration C and in good agreement with the predictions of the tight binding Ising model of Treglia, Legrand and Ducastelle and of the embedded atom method results of Lundberg

Thickness effect on alloying of ultrathin Co films on Pt(111): a real time and in situ UHV study with synchrotron X-ray diffraction

Abstract We study the alloying of ultrathin Co deposits (3 and 10 monolayer) on a Pt(111) substrate. The surface film evolution is followed in real time during annealing by X ray diffraction, which provides the depth profile with atomic resolution. We fully characterize the formation of Pt–Co surface alloys before dissolution; the kinetics is shown to depend on the Co thickness. For the 10 monolayer film, the dominant hcp phase exhibits a strong resistance to interdiffusion while for the 3 monolayer deposit the surface film is progressively enriched in Pt. Finally, whatever the annealing treatment and the film thickness, the stabilized alloy is close to Pt60Co40 bulk-like, after heating around 450°C.

LEED analysis of ultra-thin cobalt layers grown on Cr(100)

Abstract Cobalt overlayers grown at room temperature on a chromium (100) surface have been studied with low energy electron diffraction (LEED) up to a thickness of four atomic layers. A quantitative analysis via dynamical LEED calculations is given for zero and two cobalt layers grown on Cr(100). A metastable bcc phase of cobalt is evidenced. The influence of nitrogen contamination of the films is discussed.

Reconstruction of the Pt50Ni50(100) surface: a LEED and STM study

The structure of the Pt50Ni50(100) surface was investigated by low energy electron diffraction (LEED) and scanning tunnelling spectroscopy (STM). Superstructures corresponding to 12 A— 1 and 19 A— ...

Segregation profile and surface structure of Pt35Co65(110) : Pt25Co75(110) revisited

Abstract We address the general question of segregation in deep layers of bimetallic alloys and crystal preparation by heat treatment. A new analysis of LEED I(V) spectra collected on an originally Pt25Co75(110) surface reveals that: (1) LEED is very sensitive to the bulk concentration and lattice parameter; (2) the bulk was enriched with Pt to form a Pt35Co65(110) alloy. The results obtained for the segregated surface layers differ very little from those assuming a 25% Pt substrate: the concentration in the five top layers are respectively 1, 99, 0, 55, 0% Pt. The first interlayer spacing is strongly reduced (−16%) with respect to the bulk value, the second one is expanded by about 10% and the third one is almost bulklike (−1.9%). For this bulk composition, no metastable state corresponding to a Pt-rich top layer is observed although the predictions from TBIM and EAM left this possibility as an open question.

Structural investigation of the PtCo(0001) interface by GIXS

The crystallographic structure of the Pt/Co(0001) interface has been investigated by grazing incidence X-ray scattering. The occurrence of strained epitaxial layers in a twinned-fcc structure are reported.

LONG RANGE ORDER IN ULTRATHIN Pt–Co(111) ALLOYS

Long range order (LRO) in ultrathin Pt–Co films was studied by surface X-ray diffraction (XRD). Several fcc alloys of nanometric thickness were grown in situ by annealing at 460°C Co layers deposited onto a Pt(111) single crystal. Superstructure reflections were observed, which agreed with the extinction rules of either the L12 or the L10 chemically ordered bulk phases. The relationship between their structure factor and the atomic scattering factors was found with anomalous scattering performed near both Pt LIII and Co K edges. This method is very promising for studies of surface alloying. The films were also studied in real time during annealing. They evolved in a quite different way, depending on the initial Co thickness, but LRO always occurred by heating above 400°C. At this temperature the films became Pt-rich, with a stoichiometry close to Pt60Co40. We did not succeed in obtaining long range chemical order in a Co-rich alloy by annealing a Co/Pt(111) deposit, contrary to what happens in films grown by codeposition.