| 1. |
- Braud, N., et al.
(författare)
-
Cleaning and tailoring the Pt3Sn(111) surface for surface experiments
- 2023
-
Ingår i: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 732
-
Tidskriftsartikel (refereegranskat)abstract
- The cleaning process of the bimetallic Pt3Sn(111) surface has been studied by means of low-energy electron microscopy (LEEM), microspot low-energy electron diffraction (mu.-LEED), and X-ray photoemission electron microscopy (XPEEM). Different cleaning procedures, performed under ultra-high vacuum conditions (UHV), including sputtering with argon ions and repeated cycles of annealing up to 1500 K were investigated. In this work, we show that a clean Pt3Sn(111) surface of high structural quality with a sharp and brilliant (2 x 2) bulk reconstruction in LEED as well as a perfectly smooth surface with terraces of micron size can be achieved by sputtering, annealing at very high temperatures, followed by a subsequent slow (0.09 K/s) and careful cooling procedure. Additionally, we show the possibility of tailoring the Sn concentration in the topmost layers of Pt3Sn(111) as a function of annealing temperature and subsequent cooling rate. Structural changes of the surface are induced by Sn segregation combined with a surface order-disorder transition at 1340 K. Moreover, two new surface reconstructions depending on the cooling rate are reported.
|
|
| 2. |
- Bruckner, Barbara, et al.
(författare)
-
Neutralization of slow helium ions scattered from single crystalline aluminum and tantalum surfaces and their oxides
- 2020
-
Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 691
-
Tidskriftsartikel (refereegranskat)abstract
- We investigated the impact of surface oxygen on the ion yield for He+ ions scattered from different single crystalline surfaces in low-energy ion scattering. Initially clean Al(111) and Ta(111) were exposed to molecular oxygen and ion spectra for different oxidation stages and different primary energies were recorded. A comparison of ion yields normalized to the differential scattering cross section as well as experimental factors allows obtaining information about the influence of oxygen on charge exchange processes. The decrease in the ion yield of both metals with exposure cannot be explained by different surface coverages exclusively, but requires the neutralization efficiency to be dependent on the chemical structure of the surface. For Ta, additionally, a different energy dependency of the ion yield obtained in the metal and oxide occurs. The ion yield for O shows in both surfaces a significantly weaker energy dependency than the investigated metals.
|
|
| 3. |
- Grespi, A., et al.
(författare)
-
Probing the electrode-liquid interface using operando total-reflection X-ray absorption spectroscopy
- 2024
-
Ingår i: Surface Science. - : Elsevier B.V.. - 0039-6028 .- 1879-2758. ; 748
-
Tidskriftsartikel (refereegranskat)abstract
- Traditional methods to study electrochemical (EC) processes, although successful, are based on current/voltage measurements, providing information about performances rather than offering a direct observation of chemical and structural changes occurring at the electrode surface. These processes are localized at the electrode-electrolyte interface, the structure of which is the main determinant of their behavior, but most surface sensitive experimental techniques are limited to ex situ conditions, owing to the need for an ultra-high vacuum environment. In this contribution, we report operando X-ray absorption spectroscopy in total external reflection geometry (Refle-XAFS) at P64 beamline (DESY, Hamburg), using a simple and versatile EC flow cell designed for multimodal surface sensitive studies with hard X-ray scattering and spectroscopy techniques. We show that the Refle-XAFS method can be used to study chemical surface changes of industrial alloys and model electrodes in harsh electrochemical environments, without being limited to thin film samples. The surface passive film development and breakdown of a corrosion-resistant Ni-Cr-Mo alloy and the electro-oxidation of polycrystalline gold (poly-Au), relevant for fundamental studies on water electrolysis, were investigated. Despite the strong attenuation of the beam by the electrolyte and the PEEK walls of the EC cell, nanoscale surface oxide films were detected using beam energies down to 8 keV. The passivity breakdown region of Ni alloy 59 in 1 M NaCl at pH 7 and pH 12 was identified, showing differences in the composition of the surface oxides during anodic polarization. The electro-oxidation of poly-Au in 0.05 M H2SO4 was observed, showing a progression from two-dimensional Au1+/3+ to three-dimensional thick Au3+ surface oxide/hydroxide during OER.
|
|
| 4. |
- Selegård, Linnéa, et al.
(författare)
-
Step by step rare-earth catalyzed SiOx annealing and simultaneous formation of Europium- silicide by low coverage of Eu doped Gd2O3 nanoparticles
- 2021
-
Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 704
-
Tidskriftsartikel (refereegranskat)abstract
- We report the formation of silicide by annealing of a SiOx surface, with low coverage of Eu doped Gd2O3 nanoparticles. The annealing temperature required for removal of native oxide from the Si substrate decreases with close to 200 °C in presence of the nanoparticles. X-ray photoemission electron microscopy, low-energy electron microscopy and mirror electron microscopy are used to monitor the silicide formation and SiOx removal. Fragmentation of the nanoparticles is observed, and the SiOx layer is gradually removed. Eu migrates to clean Si areas during the annealing process, while Gd is found in areas where oxide is still present. This annealing process is clearly facilitated in the presence of rare-earth based nanoparticles, where nanoparticles are suggested to function as reaction sites to catalyze the oxygen removal and simultaneously form Eu based silicide. Reduction of the annealing temperature of SiOx substrates is also observed in presence of pure Eu3+ and Gd3+ ions. Simultaneous oxygen removal and EuSi formation enable this new rare-earth catalyzed annealing and silicide formation to find applications both within optoelectronics and processing microelectronic industry.
|
|
| 5. |
- Soldemo, Markus, et al.
(författare)
-
Sulfur dioxide interaction with thin iron oxide films on low-index surfaces of iron
- 2021
-
Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 714
-
Tidskriftsartikel (refereegranskat)abstract
- The adsorption of sulfur dioxide (SO2) at room temperature on iron oxide surfaces has been studied using corelevel photoelectron spectroscopy. A variety of iron oxides, from adsorbate structure to thin film, with different stoichiometries and terminations were grown on common low-index single crystal iron surfaces to model a range of structures in the initial stages of atmospheric oxidation. This permits well-controlled comparisons of differences and similarities in SO2 interaction. Both non-dissociative and dissociative adsorption of SO2 were observed, to different relative and absolute coverages, on all surfaces. The only identified non-dissociated species is SO4. For all surfaces, at least some amounts of atomic sulfur are observed while only for the submonolayer adsorbate structure, also tentative dimerization of sulfur into S2 or formation of S-Osurface is observed. For the two oxides terminated by a complete well-ordered oxygen layer, the absolute sulfur coverage is low. The complete oxygen layer terminated oxide film exhibiting a moire acute accent -pattern had a saturation coverage of one SO4 species per surface super cell, which indicates a tentative preferred adsorption site. The surface showing the highest SO4 formation is the Fe3O4(100) thin film surface with a corrugated row structure. The SO4 formation is suggested between surface oxygen atoms within the same row.
|
|
| 6. |
- Starfelt, Samuel, et al.
(författare)
-
Initial quantum well states in Ag thin films on the In/Si(111)- √3 × √3 surface
- 2020
-
Ingår i: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 692, s. 1-7
-
Tidskriftsartikel (refereegranskat)abstract
- Silver thin films have been formed by room temperature deposition of Ag on In/Si(111)-√3 × √3. The Ag films have been investigated using both angle-resolved photoelectron spectroscopy (ARPES) and scanning tunnelingmicroscopy and spectroscopy (STM/STS). This creates a powerful link between the electronic structures and the film morphology. The valence band spectra show a clear evidence of quantum well state (QWS) formation already for a 2 monolayer (ML) film. This QWS moves towards the Fermi level for the 3 ML film, which also reveals a second QWS. The QWSs’ dispersions have been plotted along the ΓM and ΓK symmetry lines of the 1×1 surface Brillouin zone (SBZ), where the ΓM direction shows the umklapp-mediated QWSs. The valence band spectra for the 3 ML Ag film also show a strong Ag sp band close to the edge of the Ag(111) 1×1 SBZ. In the STS spectrum from 2 ML, two peaks are visible below the Shockley surface state. These peaks are compared with the ARPES data and attributed to different features of the QWS
|
|
| 7. |
- Starfelt, Samuel, et al.
(författare)
-
Layer-by-layer control of Ag film growth on Sn/Si(111)-(√3 ×√3)-R30°
- 2020
-
Ingår i: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 701, s. 1-9
-
Tidskriftsartikel (refereegranskat)abstract
- Silver thin films have been formed on the Sn/Si(111)-(√3 ×√3)-R30 degrees surface at room temperature. The film morphologies, growth and electronic structures have been studied by scanning tunneling microscopy/spectroscopy (STM/STS), low-energy electron diffraction (LEED) and angle-resolved photoelectron spectroscopy (ARPES). The first layer of Ag forms an interface which consists of atomic rows with three-fold symmetry oriented along the Si(111)-1 x 1 directions. On top of the interface, Ag grows as an uniform thin film, following a layer-by-layer mode. The electronic structures of the films have been studied by STS for coverages between 1-5 monolayers (MLs). The STS spectra show peaks in the occupied electronic states which move towards the Fermi level with increased film thicknesses. These peaks have been attributed to quantum well states. ARPES measurements have been performed for 1 and 2 ML Ag coverage on Sn/Si(111)-(√3 ×√3)-R30 degrees, where the resulting thicknesses were confirmed by STM. The spectra reveal that quantum well states appear first for the 2 ML film.
|
|
| 8. |
- Starfelt, Samuel, et al.
(författare)
-
Spin coupling in Ag film growth on Sn/Ge(111)-√3×√3
- 2022
-
Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 719, s. 122043-122043
-
Tidskriftsartikel (refereegranskat)abstract
- Silver has been deposited on the Sn/Ge(111)-(√3×√3)-R30° surface at room temperature. The Ag growth and resulting surface morphology have been investigated using scanning tunneling microscopy. The first layer of silver forms an interface with domains of two different phases. One structure consists of short atomic rows with three-fold symmetry, oriented in the directions of the √3×√3 surface. These rows are separated by a distance equal to and are found to fit a 2√3×√3 unit cell. The other phase is a 3×3 honeycomb structure, oriented in the Ge(111) 1 × 1 directions. Atomic structural models for the two interface phases are proposed, based on two different spin arrangements of the Sn/Ge(111)-√3×√3 surface. The results highlight the topological coupling of the two interface faces. Both interface structures are preserved with additional silver deposition. The second layer of Ag grows with a bulk-like lattice thickness on top of both interfaces. Low-energy electron diffraction on a mostly two layer Ag film reveals that it consists of domains where Ag grows in different orientations. These domains are rotated 30° with respect to each other, and thus mirror the symmetry directions of the two interfacial phases.
|
|
| 9. |
- Tolias, Panagiotis, 1984-
(författare)
-
Non-retarded room temperature Hamaker constants between elemental metals
- 2020
-
Ingår i: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 700
-
Tidskriftsartikel (refereegranskat)abstract
- The Lifshitz theory of van der Waals forces is utilized for the systematic calculation of the non-retarded room temperature Hamaker constants between 26 identical isotropic elemental metals that are embedded in vacuum or in pure water. The full spectral method, complemented with a Drude-like low frequency extrapolation, is employed for the elemental metals benefitting from the availability of extended-in-frequency reliable dielectric data. The simple spectral method is employed for pure water and three dielectric representations are explored. Numerical truncation and low frequency extrapolation effects are shown to be negligible. The accuracy of common Lifshitz approximations is quantified. The Hamaker constants for 100 metal combinations are reported; the geometric mixing rule is revealed to be highly accurate in vacuum & water.
|
|
| 10. |
- Tzomos, E., et al.
(författare)
-
Ag-oxide signature in Ag 3d photoelectron spectra : A study on free nanoparticles
- 2023
-
Ingår i: Surface Science. - : ELSEVIER. - 0039-6028 .- 1879-2758. ; 733
-
Tidskriftsartikel (refereegranskat)abstract
- Over decades the Ag 3d-level binding energy has been puzzling researchers with its unusual sign and value in silver oxides. For the absolute majority of metals, the metal-to-oxide binding energy shifts are positive and depend significantly on the oxidation state, while in Ag-oxides the oxide shift was time after time reported negative, small, and close for the two very different Ag(I) and Ag(III) oxidation states. In the current work, a photoelectron spectroscopy (PES) investigation on the in -situ created free nanoparticles simultaneously containing both metallic silver and silver-oxide parts provided the grounds to reconsider the old consensus on the Ag-oxide shifts. The Ag 3d energies for the metallic and the oxide parts established in the current experimental work allowed estimating a approximate to 1.2 eV positive shift for the realized oxidation state. This estimate was made possible by using a beam of free nanoparticles with finely controlled composition. The PES experiments on such a beam allowed for a continuous and fast renewal of the poorly conducting sample and for a reliable and accurate calibration relative to vacuum. The constant oxide shift observed at several different oxidation conditions, as well as the relatively narrow and symmetric oxide peaks, point to one dominating oxidation state being present in the particles.
|
|
| 11. |
- Zhang, Chu, et al.
(författare)
-
Steps and catalytic reactions : CO oxidation with preadsorbed O on Rh(553)
- 2022
-
Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 715
-
Tidskriftsartikel (refereegranskat)abstract
- Industrial catalysts are often comprised of nanoparticles supported on high-surface-area oxides, in order to maximise the catalytically active surface area and thereby utilise the active material better. These nanoparticles expose steps and corners that, due to low coordination to neighboring atoms, are more reactive and, as a consequence, are often assumed to have higher catalytic activity. We have investigated the reaction between CO and preadsorbed O on a stepped Rh(553) surface, and show that CO oxidation indeed occurs faster than on the flat Rh(111) surface at the same temperature. However, we do find that this is not a result of reactions at the step sites but rather at the terrace sites close to the steps, due to in-plane relaxation enabled by the step. This insight can provide ways to optimize the shape of the nanoparticles to further improve the activity of certain reactions.
|
|
| 12. |
- Hagman, Benjamin, et al.
(författare)
-
Oxygen induced faceting of Cu(911)
- 2022
-
Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 715
-
Tidskriftsartikel (refereegranskat)abstract
- The oxidation of copper is essential for several fields, such as corrosion, catalytic methanol synthesis, and CO2 electroreduction. However, the understanding of the oxidation of copper under various conditions is not complete. Here, we study the oxidation of the vicinal Cu(911) surface by O2 with in-situ Surface X-ray Diffraction. It was found that the surface facets to (410), (401), and (100) which are stable in the parameter range of T = 25–400 °C and p = 10−10–10−5 mbar O2. The (410) and (401) facets are present until the surface is further oxidized to Cu2O, at 10−5 mbar and above. These results further the knowledge on the oxidation of copper and its surfaces, which may be of importance for a wide range of applications.
|
|
| 13. |
- Polley, C. M., et al.
(författare)
-
Electronic structure of thin epitaxial Be films on Si(111)
- 2024
-
Ingår i: Surface Science. - 0039-6028. ; 741
-
Tidskriftsartikel (refereegranskat)abstract
- We show that single crystal films of Be can be grown on Si(111) substrates, exhibiting nearly identical surface electronic bandstructure to that found on the pristine Be(0001) surface of bulk Be. Segregation of silicon atoms into the film causes strong doping of the surface states. For sufficiently thin films, the quality and uniformity are sufficient to observe well-defined quantum well states. In addition to making single-crystal beryllium more accessible for future studies, thin film growth of beryllium on high-resistivity silicon substrates may open up new possibilities to investigate earlier suggestions of surface superconductivity.
|
|
| 14. |
- Posada Borbon, Alvaro, 1990, et al.
(författare)
-
On the signatures of oxygen vacancies in O1s core level shifts
- 2021
-
Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 705
-
Tidskriftsartikel (refereegranskat)abstract
- Density functional theory calculations are used to investigate O1s surface core level shifts for MgO(100), ZnO(101¯0), In2O3(111) and CeO2(111). Shifts are calculated for the pristine surfaces together with surfaces containing oxygen vacancies and dissociated H2. Pristine surfaces show small negative shifts with respect to the bulk components and vacancies are found to have a minor effect on the O1s binding energies of neighboring oxygen atoms. OH-groups formed by H2 dissociation yield binding energies shifted to higher energies as compared to the oxygen atoms in the bulk. The results stress the difficulties in assigning core-level shifts and suggest that assignments of shifts in O1s binding energies to neighboring oxygen vacancies for the explored oxides should be reconsidered.
|
|
| 15. |
- Zhdanov, Vladimir, 1952
(författare)
-
Manifestation of Onsager's off-diagonal fluxes in diffusion of coadsorbed particles
- 2021
-
Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 704
-
Tidskriftsartikel (refereegranskat)abstract
- According to Onsager's phenomenological theory, the diffusion fluxes of coadsorbed particles are expressed via the gradients of their chemical potentials and can contain the off-diagonal terms. In the conventional two-dimensional lattice-gas models with jumps of monomers to nearest-neighbour vacant sites, such terms are absent at the mean-field (MF) level or in the framework including the statistical correlations related to lateral interaction between particles but neglecting the dynamical correlations. The role of the off-diagonal terms in these models with dynamical correlations and in other unconventional models is still not clear. Herein, the situation in this area is clarified from two perspectives by using Monte Carlo simulations and analytically, respectively. First, the scale of the related effects is shown for conventional diffusion with emphasis on the concentration profiles under steady-state conditions and diffusion-limited associative desorption under transient conditions. In these cases, the role of the off-diagonal terms is well manifested at the level above the MF approximation provided the coverages are appreciable and the difference between the jump rates of different particles is appreciable as well. Second, the off-diagonal terms are demonstrated to be available and important already at the MF level in unconventional diffusion occurring via concerted exchange and pair jumps.
|
|
