Alloy formation of Co/Ni/Pt(1 1 1)

The initial growth and alloy formation of ultrathin Co films deposited on 1 ML Ni/Pt(1 1 1) were investigated by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and ultraviolet photoelectron spectroscopy (UPS). A sequence of samples of d_Co Co/1 ML Ni/Pt(1 1 1) (d_Co = 1, 2, and 3 ML) were prepared at room temperature, and then heated up to investigate the diffusion process. The Co and Ni atoms intermix at lower annealing temperature, and Co-Ni intermixing layer diffuses into the Pt substrate to form Ni-Co-Pt alloys at higher annealing temperature. The diffusion temperatures are Co coverage dependent. The evolution of UPS with annealing temperatures also shows the formation of surface alloys. Some interesting LEED patterns of 1 ML Co/1 ML Ni/Pt(1 1 1) show the formation of ordered alloys at different annealing temperature ranges. Further studies in the Curie temperature and concentration analysis, show that the ordered alloys corresponding to different LEED patterns are Ni_xCo_1−xPt and Ni_xCo_1−xPt₃. The relationship between the interface structure and magnetic properties was investigated.     

Adsorption of ethene on Pt(1 1 1) and ordered PtxSn/Pt(1 1 1) surface alloys: A comparative HREELS and DFT investigation

The adsorption of ethene (C₂H₄) on Pt(1 1 1) and the Pt₃Sn/Pt(1 1 1) and Pt₂Sn/Pt(1 1 1) surface alloys has been investigated experimentally by high-resolution electron energy loss spectroscopy and temperature programmed desorption. The experimental results have been compared with density functional theory (DFT) calculations allowing us to perform a complete assignment of all vibration modes and loss features to the species present on the surfaces. On Pt(1 1 1) as well as on the Pt-Sn surface alloys an η² di-σ-bonded conformation of ethene has been found to be the most stable adsorbed form. In addition to this majority species a minor amount of π-bonded ethene has been identified, which is more abundant on the Pt₂Sn surface alloy than on the other surfaces. Additionally the HREELS spectra of ethene on Pt(1 1 1) and the Pt-Sn surface alloys differ only slightly in terms of the energetic positions of the loss peaks.     

A theoretical investigation on Pt3Sn(1 0 2) surface alloy and CO-Pt3Sn(1 0 2) system

The adsorption properties of CO on experimentally verified stepped Pt₃Sn(1 0 2) surface were investigated using quantum mechanical calculations. The two possible terminations of Pt₃Sn(1 0 2) were generated and on these terminations all types of possible adsorption sites were determined. The adsorption energies and geometries of the CO molecule for all those sites were calculated. The most favorable sites for adsorption were determined as the short bridge site on the terrace of pure-Pt row of the mixed-atom-ending termination, atop site at the step-edge of the pure row of pure-Pt-ending termination and atop site at the step-edge of the pure-Pt row of the mixed-atom-ending termination. The results were compared with those for similar sites on the flat Pt₃Sn(1 1 0) surface considering the fact that Pt₃Sn(1 0 2) has terraces with (1 1 0) orientation. The LDOS analysis of bare sites clearly shows that there are significant differences between the electronic properties of Pt atoms at stepped Pt₃Sn(1 0 2) surface and the electronic properties of Pt atoms at flat (1 1 0) surface, which leads to changes in the CO bonding energies of these Pt atoms. Adsorption on Pt₃Sn(1 0 2) surface is in general stronger compared to that on Pt₃Sn(1 1 0) surface. The difference in adsorption strength of similar sites on these two surface terminations is a result of stepped structure of Pt₃Sn(1 0 2). The local density of states (LDOS) of the adsorbent Pt and C of adsorbed CO was utilized. The LDOS of the surface metal atoms with CO-adsorbed atop and of their bare state were compared to see the effect of CO chemisorption on the electron density distribution of the corresponding Pt atom. The downward shift in energy peak in the LDOS curves as well as changes in the electron densities of the corresponding energy levels indicate the orbital mixing between CO molecular orbitals and metal d-states. The present study showed that the adsorption strength of the sites has a direct relation with their LDOS profiles.     

CO adsorption on epitaxially grown Pt layers on Ru(0 0 0 1)

The adsorption of CO on epitaxially grown Pt films of variable thickness has been studied using infrared-absorption spectroscopy, scanning tunnelling microscopy and thermal desorption spectroscopy. Depending on the number of pseudomorphic Pt layers (N_Pt = 1-4) the internal and external CO stretching modes (ν_C-O and ν_Pt-CO, respectively) display characteristic frequency shifts due to the vanishing influence of the underlying Ru(0 0 0 1) substrate and Pt/Ru interface. For thicker layers (N_Pt ? 5) when this influence has become negligible, the compressive stress within the Pt film is gradually relieved, leading to a dislocation network. The structural heterogeneity during the ongoing relaxation process of the Pt film is reflected in the ν_C-O line shape; no line broadening is observed for either pseudomorphic or very thick films (N_Pt ? 15). For N_Pt ? 3 the adsorption of CO on Pt/Ru(0 0 0 1) films closely resembles CO on Pt(1 1 1), with residual deviations in line position and desorption temperatures gradually converging to zero.     

Interaction of CO with atomically well-defined PtxRuy/Ru(0 0 0 1) surface alloys

The interaction of CO with structurally well-defined Pt_xRu_y surface alloys supported on Ru(0 0 0 1) was investigated by thermal desorption spectroscopy and infrared reflection-absorption spectroscopy. The surface composition and the distribution of the surface atoms were controlled by high resolution scanning tunneling microscopy. On these surfaces, which have a nearly random distribution of the two surface species, the adsorption (and desorption) of CO is strongly modified compared to the pure elemental surfaces, by strain effects and electronic ligand effects. CO adsorbs exclusively in a linear configuration on Pt and Ru atoms for all surfaces investigated. The adsorption energy of CO is lowered on the alloy surfaces with respect to both Pt(1 1 1) and Ru(0 0 0 1), similar as for pseudomorphic monolayer Pt films. For both Pt and Ru sites the adsorption strength decreases with increasing Pt concentration.     

Growth and alloy formation of ultrathin Ni films on Co/Pt(1 1 1)

Low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES) were used to study the growth and the structural evolution of Ni/Co/Pt(1 1 1) following high-temperature annealing. From the oscillation of the specular beam of the LEED and Auger uptake curve, we concluded that the growth mode of thin Ni films on 1 ML Co/Pt(1 1 1) is at least 2 ML layer-by-layer growth before three-dimensional island growth begins. The alloy formation of Ni/1 ML Co/Pt(1 1 1) was analyzed by AES. The temperature for the intermixing of Ni and Co layers in the upper interface without diffusing into the bulk of Pt is independent of the thickness of Ni when a Co buffer is one atomic monolayer. After the temperature was increased, formations of Ni-Co-Pt alloy, Ni-Pt alloy and Co-Pt alloy were observed. The temperature required for the Ni-Co intermixing layer to diffuse into Pt bulk increases with the thickness of Ni. The interlayer distance as a function of annealing temperature for 1 ML Ni/1 ML Co/Pt(1 1 1) was calculated from the I-V LEED. The evolution of LEED patterns was also observed at different annealing temperatures.     

Seedlayer interface enhanced magnetic anisotropy in CoPt (0 0 0 2)-textured films

The effects of interface roughness of Ta seedlayer on the structural and magnetic properties of Co₇₂Pt₂₈(20 nm)/Ru(30 nm)/Pt(2 nm)/Ta(5 nm)/glass were investigated. Uniaxial perpendicular magnetic anisotropy (8.6×10⁶ ergs/cc), coercivity (5.5 kOe) and nucleation field (−2.8 kOe) in the Co₇₂Pt₂₈ thin film sputter-deposited on relatively smooth surface of Ta seedlayer were achieved. The results showed that relatively smoother interface roughness of Ta seedlayer improved the CoPt/Ru (0 0 0 2) texture and magnetic properties.     

A computational study of H2 dissociation and CO adsorption on the PtML/WC(0 0 0 1) surface

We studied computationally the relative stability of Pt_ML/WC(0 0 0 1) [pseudomorphic monolayer of Pt(1 1 1) on WC(0 0 0 1)] interfacial structures using a density functional slab model approach. The work of adhesion was calculated for six different interfacial structures, taking into account both W- and C-terminations of the carbide. The results show that the optimal interfacial structure of Pt_ML/WC(0 0 0 1) is the W-terminated WC(0 0 0 1) with Pt atoms adhesion on the hcp site (W-hcp). The nature of metal/carbide bonding for the W-hcp interfacial geometry was determined on the basis of the partial density of states (PDOS). Adsorption of atomic hydrogen and dissociation of the hydrogen molecule on the W-hcp Pt_ML/WC(0 0 0 1) was investigated and compared to that on Pt(1 1 1). It is found that the most favorable H₂ dissociation channels need similar activation energies of 5.28 and 4.93 kJ/mol on Pt_ML/WC(0 0 0 1) and Pt(1 1 1), respectively, with the release of considerable reaction energies. Furthermore, adsorption of CO on the W-hcp Pt_ML/WC(0 0 0 1) and Pt(1 1 1) was also investigated. The results indicate that Pt_ML/WC(0 0 0 1) is much less susceptible to CO poisoning than Pt(1 1 1), especially at the low coverage of CO.     

DFT study of Pt adsorption on low index SrTiO3 surfaces: SrTiO3(1 0 0), SrTiO3(1 1 1) and SrTiO3(1 1 0)

Density Functional Theory has been used to determine the energetically preferred structures of submonolayer, monolayer, and multilayer Pt films on both ideal terminations of SrTiO₃(1 0 0), SrTiO₃(1 1 1), and SrTiO₃(1 1 0). The strength of the resulting metal/metal oxide interfaces was characterized by the adsorption energy of the film and the film’s work of separation. The two polar surfaces, SrTiO₃(1 1 1) and SrTiO₃(1 1 0), form significantly stronger interfaces than the non-polar SrTiO₃(1 0 0) surface. Approximate criteria were applied to predict the growth mode of Pt on each surface.     

Carbon monoxide adsorption on Co deposited Pt(1 0 0)-hex: IRRAS and LEED investigations

Infrared reflection absorption spectroscopy (IRRAS) was used to investigate carbon monoxide (CO) adsorption on Pt(1 0 0) surfaces deposited with Co layers with different thicknesses. Pt(1 0 0) surfaces cleaned in ultrahigh vacuum showed surface reconstruction, i.e., Pt(1 0 0)-hex: two absorption bands ascribable to adsorbed CO on the 1 × 1 surface and hex domains emerge at 2086 and 2074 cm⁻¹, respectively, after 1.0 L CO exposure. Deposition of a 0.3-nm-thick-Co layer on Pt(1 0 0)-hex at 333 K changes the low-energy electron diffraction (LEED) pattern from hex to p(1 × 1), indicating that the deposited Co lifts the reconstruction. The IRRAS spectrum for 1.0-L-CO-exposed Co_0.3 nm/Pt(1 0 0)-hex fabricated at 333 K yields a single absorption band at 2059 cm⁻¹. For Co_0.3 nm/Pt(1 0 0)-hex fabricated at 693 K, the LEED pattern shows a less-contrasted hex and the pattern remains nearly unchanged even after CO exposure of 11 L, although only 1.0 L CO exposure to Pt(1 0 0)-hex lifts the surface reconstruction. A Co_0.3 nm/Pt(1 0 0)-hex surface fabricated at 753 K exhibits an absorption band at 2077 cm⁻¹, which is considered to originate from CO adsorbed on the Pt-enriched surface alloy. Co_0.3 nm/Pt(1 0 0)-hex surfaces fabricated above 773 K show a clear hex-reconstructed LEED pattern, and the frequencies of the adsorbed CO bands are comparable to those of Pt(1 0 0)-hex, indicating that the deposited Co atoms are diffused near the surface region. The outermost surface of the 3.0-nm-thick-Co-deposited Pt(1 0 0)-hex is composed of Pt-Co alloy domains even at a deposition temperature of 873 K. Based on the LEED and IRRAS results, the outermost surface structures of Co_x/Pt(1 0 0)-hex are discussed.     

Stable atomic structure and magnetism of Pt–Cr binary surface alloys on Pt(0 0 1): First-principle calculations

The possibility of Pt–Cr surface alloys formation on Pt(0 0 1) was investigated and their magnetism was calculated by the full-potential linearized augmented plane wave (FLAPW) method with eight different atomic configurations. The most stable structure was calculated to be the Pt-segregated L1₂ ferromagnetic surface alloy. A₃B types (L1₂ or D0₂₂) were more stable compared to AB types (L1₀). It implies that the A₃B type surface alloys may be formed when depositing a monolayer of Cr on Pt(0 0 1). It was found from the total energy calculations that there exists a strong tendency of the Pt segregation. The segregation further stabilizes the surface alloy significantly. The work function of the most stable surface alloy was calculated to be 6.02 eV and the magnetic moment of the surface Cr was much enhanced to 3.3 μ_B. It is a quite interesting finding that the coupling between Cr and Pt atoms on the surface plane is ferromagnetic in the Pt-segregated L1₂ ferromagnetic surface alloy, while the coupling is antiferromagnetic in the bulk.     

Growth of Pt thin films on WSe2

The morphology and structure of Pt deposited on a WSe₂(0 0 0 1) van der Waals surface have been investigated by reflection high energy electron diffraction and scanning tunneling microscopy. At room temperature, the initial growth is characterized by the formation of three-dimensional fcc Pt islands with (1 1 1) orientation. In contrast, at higher temperatures of about 450 °C the formation of a novel chemically ordered Pt-Se alloy is observed. Based on the diffraction patterns, a tetragonal DO₂₂-type structure of a Pt₃Se compound is suggested. With increasing Pt thickness, this chemically ordered alloy disappears and an additional superstructure occurs, which is accompanied by the coalescence of the islands. The observed superstructure is attributed to a strong Se diffusion towards the growth surface, forming most likely a PtSe₂ alloy with the CdI₂-type layered structure on the top surface. Due to the lateral lattice mismatch between the Pt(1 1 1) layers and the PtSe₂(1 1 1) top layer, a Moiré pattern with a period of 1.1 nm is created, which might be used as a long-range atomic pattern for further nanostructure growth.     

Diffusions of small clusters on Pt(1 1 1) and Cu(1 1 1) surfaces

Diffusions of small cluster Pt₆ on Pt(1 1 1) surface and Cu₆ on Cu(1 1 1) are studied by molecular dynamics simulation, respectively. The atomic interaction is modeled by the semiempirical potential. The results show that the diffusion processes in the two systems are far different. For example, on Pt(1 1 1) surface, the hopping of single atom and the shearing of two atoms of hexamer only occur on the adatom(s) adsorbed at B-step, while on Cu(1 1 1) surface they can appear on the adatom(s) either at A-step or B-step. To the concerted translation of the parallelogram hexamer, the anisotropy in the diffusion path is observed in the two systems, the mechanisms and then the preferential paths, however, are completely different. The reasons for these diffusion characteristics and differences are discussed.     

Surface core-level shift of Pd at the AgcPd1−c(1 1 1) surface: Nonlinear subsurface effects

The surface core-level binding-energy shift (SCLS) of Pd at the Ag_cPd_1−c(1 1 1) surface is calculated as a function of bulk concentration of the alloy. The equilibrium volume and the surface concentration profile used in the calculations refer to the 0 K case. The SCLSs are evaluated within the Z + 1 approximation. The results are analysed using the mixing enthalpy of the alloy and the bulk and surface chemical potentials. A relation of the SCLS to the bulk concentration is considered. This relation is shown to be mediated by the surface concentration profile which induces the observed nonlinear behaviour. The results are interpreted using a simple model for the alloy electronic structure.     

Scanning tunnelling microscopy studies of growth morphology in highly epitaxial c-axis oriented Pt thin film on (0 0 1) SrTiO3

A noble metal Pt thin film was successfully grown on (0 0 1) SrTiO₃ substrate by using a DC-sputtering technique. The surface morphology and growth features of the as-grown Pt films were investigated by scanning tunnelling microscopy. Growth conditions, such as pre-sputtering, deposition ambience, and oxygen ratio are found to greatly affect the orientation, the crystallinity, and the epitaxial behavior of Pt films on (0 0 1) SrTiO₃. Single-crystalline Pt films have been achieved by introducing a few percentage oxygen into the sputtering ambient. The in-plane-relationship of the c-axis oriented Pt thin films on (0 0 1) SrTiO₃ was determined to be (0 0 1)_Pt∥(0 0 1)_SrTiO3 and [0 0 1]_Pt∥[0 0 1]_SrTiO3. Oxygen in the sputtering ambient was found to be a key factor to achieve the epitaxial Pt films.     

Ab-initio investigation of Ni(Fe)/ZrO2(0 0 1) and Ni-Fe/ZrO2(0 0 1) interfaces

The atomic and electronic structures of Me/ZrO₂(0 0 1) interfaces, where Me is Ni, Fe or a Ni-Fe alloy, are investigated by the plane wave pseudopotential method within density-functional theory. The work of separation of metal films from oxide substrate for the O- and Zr-terminated Me/ZrO₂(0 0 1) interfaces is calculated. High adhesion at both Me/(ZrO₂)_O and Me/(ZrO₂)_Zr interfaces is found. The effect of oxygen vacancies on the adhesion at the metal-ceramic interfaces is also investigated. It is shown that Ni(Fe)-O interaction at the O-terminated interface weakens in the presence of interfacial oxygen vacancies. At interfaces with Ni-Fe alloys the adhesion depends strongly on the composition of the interfacial layers and their magnetic properties.     

Adsorption and reaction of bicyclic hydrocarbons at Pt(1 1 1) and Sn/Pt(1 1 1) surface alloys: trans-decahydronaphthalene (C10H18) and bicyclohexane (C12H22)

Adsorption and desorption of trans-decahydronaphthalene (C₁₀H₁₈) and bicyclohexane (C₁₂H₂₂) can be used to probe important aspects of non-specific dehydrogenation leading to surface carbon accumulation and establish better estimates of activation energies for C-H bond cleavage at Pt-Sn alloys. This chemistry was studied on Pt(1 1 1) and the (2 × 2)-Sn/Pt(1 1 1) and (√3 × √3)R30°-Sn/Pt(1 1 1) surface alloys by using temperature programmed desorption (TPD) mass spectroscopy and Auger electron spectroscopy (AES). These hydrocarbons are reactive on Pt(1 1 1) surfaces and fully dehydrogenate at low coverages to produce H₂ and surface carbon during TPD. At monolayer coverage, 87% of adsorbed C₁₀H₁₈ and 75% C₁₂H₂₂ on Pt(1 1 1) desorb with activation energies of 70 and 75 kJ/mol, respectively. Decomposition of C₁₀H₁₈ is totally inhibited during TPD on these Sn/Pt(1 1 1) alloys and decomposition of C₁₂H₂₂ is reduced to 10% of the monolayer coverage on the (2 × 2)-Sn/Pt(1 1 1) alloy and totally inhibited on the (√3 × √3)R30°-Sn/Pt(1 1 1) alloy. C₁₀H₁₈ and C₁₂H₂₂ are more weakly chemsorbed on these two alloys compared to Pt(1 1 1) and these molecules desorb in narrow peaks characteristic of each surface with activation energies of 65 and 73 kJ/mol on the (2 × 2) alloy and 60 and 70 kJ/mol on the (√3 × √3)R30°-Sn/Pt(1 1 1) alloy, respectively. Alloyed Sn has little influence on the monolayer saturation coverage of these two molecules, and this is decreased only slightly on these two Sn/Pt(1 1 1) alloys. The use of these two probe molecules enables an improved estimate of the activation energy barriers E* to break aliphatic C-H bonds in alkanes on Sn/Pt alloys; E* = 65-73 kJ/mol on the (2 × 2)-Sn/Pt(1 1 1) alloy and E* ? 70 kJ/mol on the (√3 × √3)R30°-Sn/Pt(1 1 1) alloy.     

Electronic structure and topography of annealed SrTiO3(1 1 1) surfaces studied with MIES and STM

Perovskites of ABO₃ type like strontium titanate (SrTiO₃) are of great practical concern as materials for oxygen sensors operating at high temperatures. It is well known that the surface layer shows different properties compared to the bulk. Numerous studies exist for the SrTiO₃(1 0 0) and (1 1 0) surfaces which have investigated the changes in the electronic structure and topography as a function of the preparation conditions. They have indicated a rather complex behaviour of the surface and the near surface region of SrTiO₃ at elevated temperatures. Up to now, the behaviour of the SrTiO₃(1 1 1) surfaces under thermal treatment is not sufficiently known. This contribution is intended to work out the relation between alteration of the surface topography with respect to the preparation conditions and the simultaneous changes of the electronic structure. We applied scanning tunneling microscopy (STM) to investigate the surface topography and, additionally, metastable impact electron spectroscopy (MIES) to study the surface electronic structure of reconstructed SrTiO₃(1 1 1) surfaces. The crystals were heated up to 1000 °C under reducing and oxidizing conditions. Both preparation conditions cause strong changes of the surface topography and electronic structure. A microfaceting of the topmost layers is found.     

Epitaxial growth of Au and Pt on Fe3O4(1 1 1) surface

We have studied the epitaxial growth of Au and Pt layers on Fe₃O₄(1 1 1) as a function of the deposition temperature and thickness. The layers were deposited by UHV sputtering and the structural properties were investigated by reflection high energy electron diffraction (RHEED), X-ray experiments and transmission electron microscopy (TEM). The epitaxial growth of both metals was obtained whatever the deposition conditions but the wetting is however different for the two metals. Comparison between the coverage ratios of Au and Pt is correlated with their surface and interfaces energies. The optimum conditions to achieve a 2D flat epitaxial metallic layer are determined.     

Surface morphology of epitaxial lattice-matched Ba0.7Sr0.3O on Si(0 0 1) and vicinal Si(0 0 1)-4°[1 1 0] substrates

Exploring ways for self-organized structuring of insulating thin films, we investigated the possibility to produce replicas of step trains, given by a vicinal Si(0 0 1)-4°[1 1 0] surface, in layers of crystalline and perfectly lattice-matched Ba_0.7Sr_0.3O. For this purpose, we carried out high-resolution spot profile analyses in low-energy electron diffraction (SPA-LEED) both on flat Si(0 0 1) and on Si(0 0 1)-4°[1 1 0]. Oxide layers were generated by evaporating the metals in oxygen ambient pressure with the sample at room temperature. Our G(S) analysis of these mixed oxide layers reveals a strong influence of local compositional fluctuations of Sr and Ba ions and their respective scattering phases, which appears as an unphysically large variation of layer distances. Nevertheless, we are able to show that quite smooth and closed oxide films are obtained with an rms roughness of about 1 ML. These Ba_0.7Sr_0.3O films directly follow the step train of Sr-modified vicinal Si surfaces that form (1 1 3) oriented facets after adsorption of a monolayer of Sr. This proves that self-organized structuring of insulating films can indeed be an effective method.