Search

A combination of experimental methods was used to study the structure of In thin films deposited on the Pd(111) surface and the alloying behavior. X-ray photoelectron spectroscopy (XPS), lowenergy electron diffraction (LEED), and scanning tunneling microscopy results indicate that surface alloying takes place at room temperature. Below 2 monolayer equivalents (MLEs), the LEED patterns show the formation of three rotational domains of InPd(110) of poor structural quality on top of the Pd(111) substrate. Both core-levels and valence band XPS spectra show that the surface alloy does not yet exhibit the electronic structure characteristic of the 1:1 intermetallic compound under these conditions. Annealing the 1 MLE thin film up to 690 K yields to a transition from a multilayer InPd near-surface intermetallic phase to a monolayer-like surface alloy exhibiting a well ordered (√3 × √3) R30° superstructure and an estimated composition close to In2Pd3. Annealing above 690 K leads to further In depletion and a (1 x 1) pattern is recovered. The (√3 × √3) R30° superstructure is not observed for thicker films. Successive annealing of the 2 MLE thin film leads the progressive disappearance of the InPd diffraction spots till a sharp (1 x 1) pattern is recovered above 690 K. In the high coverage regime (from 4 to 35 MLE), the formation of three rotational domains of a bee-In7Pd3 compound with (110) orientation is observed. This In-rich phase probably grows on top of interfacial InPd(110) domains and is metastable. It transforms into a pure InPd(110) near-surface intermetallic phase in a temperature range between 500 and 600 K depending on the initial coverage. At this stage, the surface alloy exhibits core-level chemical shifts and valence band (VB) spectra identical to those of the 1:1 InPd intermetallic compound and resembling Cu-like density of states. Annealing at higher temperatures yields to a decrease of the In concentration in the near-surface region to about 20 at.% and a (1 x 1) LEED pattern is recovered. [ABSTRACT FROM AUTHOR]

We report the adsorption of lead adatoms on the pseudo-10-fold Al 13 Fe 4 (0 1 0) surface using low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). For the submonolayer regime, Pb adatoms remain highly mobile across the surface at 300 K. STM analysis indicates the formation of irregularly shaped islands of monoatomic height. The latter do not coalesce with increasing coverage. At 0.95 MLE coverage, the LEED patterns are consistent with a pseudomorphic growth of the adatoms. This is confirmed by STM measurements which reveal local motifs qualitatively similar to those observed on the clean Al 13 Fe 4 (0 1 0) surface, i.e. prior to dosing. Apart from the absence of plasmons, the XPS measurements of Pb 4 f and Al 2 s core levels are comparable to those observed for the Pb/Al(1 1 1) system. [ABSTRACT FROM AUTHOR]

We report the formation of an ordered C60 monolayer on the Al9Co2(001) surface using scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), x-ray and ultraviolet photoelectron spectroscopy (XPS/UPS), and ab initio calculations. Dosing fullerenes at 300 K results in a disordered overlayer. However, the adsorption of C60 with the sample held between 573-673 K leads to a [4, -2ι 1,3] phase. The growth of C60 proceeds with the formation of two domains which are mirror symmetric with respect to the [100] direction. Within each domain, the superstructure unit cell contains six molecules and this implies an area per fullerene equal to 91 Å2 . The molecules exhibit two types of contrast (bright and dim) which are bias dependent. The adsorption energies and preferred molecular configuration at several possible adsorption sites have been determined theoretically. These calculations lead to a possible scheme describing the configuration of each C60 in the observed superstructure. Several defects (vacancies, protrusions,. . . ) and domain boundaries observed in the film are also discussed. If the sample temperature is higher than 693 K when dosing, impinging C60 molecules dissociate at the surface, hence leading to the formation of a carbide film as observed by STM and LEED measurements. The formation of AI4C3 domains and the molecular dissociation are confirmed by XPS/UPS measurements acquired at different stages of the experiment. The cluster substructure present at the Al9Co2(001) surface dictates the carbide domain orientations. [ABSTRACT FROM AUTHOR]

We report the formation of several complex intermetallics as surface alloys upon the adsorption of Fe on Al 9 Co 2 (001) for different dosing conditions. Up to 4 monolayers equivalent (MLE) of Fe deposited on the substrate held at 593 K, the low energy electron diffraction pattern consists of a ( 2 × 2 )R45° phase with two additional domain types rotated by ± 8 ∘ from it. The scanning tunneling microscopy (STM) measurements show that these three types of domains have the same crystallographic structure. The lattice parameters and structural motifs point towards the formation of the high-temperature Al 8 Fe 5 phase (γ -brass of Zn 8 Cu 5 -type structure). For Fe deposition between 593 K and 873 K, we have identified the formation of two phases, tentatively assigned to a ternary disordered Al 9 (Co,Fe) 2 overlayer and to the monoclinic Al 13 Fe 4 (100). The stoichiometric evolution of the grown structures have been characterized by angle-resolved x-ray photoelectron spectroscopy measurements. Density functional theory based calculations have been performed to model the Al 8 Fe 5 (100)/Al 9 Co 2 (001) interface, its structural stability and to simulate the corresponding STM images. [Display omitted] • Epitaxial growth of simple binary to complex metallic alloy phases. • Growth of the high-temperature Al 8 Fe 5 phase as thin film on Al 9 Co 2 (001). • Identification of the Al 8 Fe 5 (100) surface termination by DFT calculations. • Substrate symmetry operations driving the alternating orientations of domains. • Formation of surface and interface structures of tuneable complexity. [ABSTRACT FROM AUTHOR]

[Display omitted] The (110) and (320) surfaces of the single-phase FeCrMnNiCo solid solution have been studied on two adjacent millimeter size grains using surface science and transmission electron microscopy (TEM) techniques. The structural and chemical evolutions of the high entropy alloy (HEA) surfaces have been determined for various sputtering conditions, annealing temperatures and durations. Up to 873 K, angle-resolved X-ray photoelectron spectroscopy measurements indicate a clear Mn and Ni surface co-segregation. We propose that the surface segregation of Mn is driven by its low surface energy. The attractive interaction between Mn and Ni promotes Ni segregation which accompanied the Mn diffusion to the surface. Regarding the structures investigated by low energy electron diffraction and scanning tunneling microscopy, the (320) surface presents a terraced morphology with an ordered structure consistent with a (1 × 1) termination. On the contrary, the (110) surface reveals an important degree of structural disorder and local reconstructions. Its highly anisotropic morphology resembles rows propagating along the [001] direction. Above 873 K, Mn desorption occurs while the Ni content keeps increasing linearly with the temperature. TEM analysis show no evidence for HEA decomposition into metallic or intermetallic phases even after repeated annealing and sputtering cycles. The above results set the upper temperature limit above which the surface stoichiometry departs from the quinary HEA concept. It also defines the temperature range for the use of FeCrMnNiCo based coating under high vacuum conditions and for aerospace applications. [ABSTRACT FROM AUTHOR]