| 1. |
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| 2. |
- Allentoft, Morten E., et al.
(författare)
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Population genomics of post-glacial western Eurasia
- 2024
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Ingår i: Nature. - 0028-0836 .- 1476-4687. ; 625:7994, s. 301-311
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Tidskriftsartikel (refereegranskat)abstract
- Western Eurasia witnessed several large-scale human migrations during the Holocene1–5. Here, to investigate the cross-continental effects of these migrations, we shotgun-sequenced 317 genomes—mainly from the Mesolithic and Neolithic periods—from across northern and western Eurasia. These were imputed alongside published data to obtain diploid genotypes from more than 1,600 ancient humans. Our analyses revealed a ‘great divide’ genomic boundary extending from the Black Sea to the Baltic. Mesolithic hunter-gatherers were highly genetically differentiated east and west of this zone, and the effect of the neolithization was equally disparate. Large-scale ancestry shifts occurred in the west as farming was introduced, including near-total replacement of hunter-gatherers in many areas, whereas no substantial ancestry shifts happened east of the zone during the same period. Similarly, relatedness decreased in the west from the Neolithic transition onwards, whereas, east of the Urals, relatedness remained high until around 4,000 bp, consistent with the persistence of localized groups of hunter-gatherers. The boundary dissolved when Yamnaya-related ancestry spread across western Eurasia around 5,000 bp, resulting in a second major turnover that reached most parts of Europe within a 1,000-year span. The genetic origin and fate of the Yamnaya have remained elusive, but we show that hunter-gatherers from the Middle Don region contributed ancestry to them. Yamnaya groups later admixed with individuals associated with the Globular Amphora culture before expanding into Europe. Similar turnovers occurred in western Siberia, where we report new genomic data from a ‘Neolithic steppe’ cline spanning the Siberian forest steppe to Lake Baikal. These prehistoric migrations had profound and lasting effects on the genetic diversity of Eurasian populations.
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| 3. |
- Andersen, Jesper N, et al.
(författare)
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Photoemission Spectroscopy At Max-Lab
- 1991
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Ingår i: Synchrotron Radiation News. - : Informa UK Limited. - 0894-0886 .- 1931-7344. ; 4:4, s. 15-19
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Tidskriftsartikel (refereegranskat)
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| 4. |
- Baraldi, A., et al.
(författare)
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Thermal stability of the Rh(110) missing-row reconstruction: Combination of real-time core-level spectroscopy and ab initio modeling
- 2005
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. ; 72
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Tidskriftsartikel (refereegranskat)abstract
- The thermal stability of the (1×2) missing-row phase of Rh(110), whose first layer is an ordered array of one-dimensional atomic chains, is investigated using high-resolution core-level spectroscopy and density functional calculations. The time evolution of the spectra indicates that this phase is metastable towards deconstruction into a (1×1) phase. Our calculations unveil the mechanism of this process, which is shown to be ignited by surface defects, and provide a value for the activation energy in good agreement with experimental findings.
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| 5. |
- Delheusy, M., et al.
(författare)
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X-ray investigation of subsurface interstitial oxygen at Nb/oxide interfaces
- 2008
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 92:10
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Tidskriftsartikel (refereegranskat)abstract
- We have investigated the dissolution of a natural oxide layer on a Nb(110) surface upon heating, combining x-ray reflectivity, grazing incidence diffuse scattering, and core-level spectroscopy. The natural oxide reduces after heating to 145 degrees C partially from Nb2O5 to NbO2, and an enrichment in subsurface interstitial oxygen by similar to 70% in a depth of 100 A is observed. After heating to 300 degrees C, the oxide reduces to NbO and the surplus subsurface oxygen gets dissolved into the bulk. Our approach can be applied for further investigation of the effect of subsurface interstitial oxygen on the performance of niobium rf cavities. (C) 2008 American Institute of Physics. (C) 2008 American Institute of Physics.
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| 6. |
- Grånäs, Elin, et al.
(författare)
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CO Intercalation of Graphene on Ir(111) in the Millibar Regime
- 2013
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 117:32, s. 16438-16447
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Tidskriftsartikel (refereegranskat)abstract
- Here we show that it is possible to intercalate CO under graphene grown on Ir(111) already at room temperature when CO pressures in the millibar regime are used. From the interplay of X-ray photoelectron spectroscopy and scanning tunneling microscopy we conclude that the intercalated CO adsorption structure is similar to the (3 root 3 X 3 root 3)R30 degrees) adsorption structure that is formed on Ir(111) upon exposure to similar to 1 mbar of CO. Further, density functional theory calculations reveal that the structural and electronic properties of CO-intercalated graphene are similar to p-doped freestanding graphene. Finally we characterize nonintercalated stripes and islands that we always observe in the CO-intercalated graphene. We observe these nonintercalated areas predominately in HCP and FCC areas near step edges and suggest that stress release in graphene is the driving force for their formation, while the weak chemical bonds in HCP and FCC areas are the reason for their area selectivity.
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| 7. |
- Grånäs, Elin, et al.
(författare)
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Water Chemistry beneath Graphene : Condensation of a Dense OH-H2O Phase under Graphene
- 2022
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 126:9, s. 4347-4354
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Tidskriftsartikel (refereegranskat)abstract
- Room temperature oxygen hydrogenation below graphene flakes supported by Ir(111) is investigated through a combination of X-ray photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory calculations using an evolutionary search algorithm. We demonstrate how the graphene cover and its doping level can be used to trap and characterize dense mixed O-OH-H2O phases that otherwise would not exist. Our study of these graphene-stabilized phases and their response to oxygen or hydrogen exposure reveals that additional oxygen can be dissolved into them at room temperature creating mixed O-OH-H2O phases with an increased areal coverage underneath graphene. In contrast, additional hydrogen exposure converts the mixed O-OH-H2O phases back to pure OH-H2O with a reduced areal coverage underneath graphene.
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| 8. |
- Gustafson, Johan, et al.
(författare)
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Oxygen-induced step bunching and faceting of Rh(553): Experiment and ab initio calculations
- 2006
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121 .- 1550-235X. ; 74:3
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Tidskriftsartikel (refereegranskat)abstract
- Using a combined experimental and theoretical approach, we show that the initial oxidation of a Rh(553) surface, a surface vicinal to (111), undergoes step bunching when exposed to oxygen, forming lower-index facets. At a pressure of about 10(-6) mbar and a temperature of 380 degrees C this leads to (331) facets with one-dimensional oxide chains along the steps, coexisting with (111) facets. Further increase of the pressure and temperature results in (111) facets only, covered by an O-Rh-O surface oxide. Our density functional theory calculations provide an atomistic understanding of the observed behavior.
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| 9. |
- Gustafson, Johan, et al.
(författare)
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Structure of a thin oxide film on Rh(100)
- 2005
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 71:11
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Tidskriftsartikel (refereegranskat)abstract
- The initial oxidation of Rh(100) has been studied using high resolution core level spectroscopy, low energy electron diffraction, surface x-ray diffraction, scanning tunneling microscopy, and density functional theory. We report a structural study of an oxygen induced structure displaying a c(8x2) periodicity at an oxygen pressure above 10(-5) mbar and using a sample temperature of 700 K. Our experimental and theoretical data demonstrate that this structure is due to the formation of a thin surface oxide with a hexagonal trilayer O-Rh-O structure.
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| 10. |
- Hjort, Martin, et al.
(författare)
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Doping profile of InP nanowires directly imaged by photoemission electron microscopy
- 2011
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 99:23
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Tidskriftsartikel (refereegranskat)abstract
- InP nanowires (NWs) with differently doped segments were studied with nanoscale resolution using synchrotron based photoemission electron microscopy. We clearly resolved axially stacked n-type and undoped segments of the NWs without the need of additional processing or contacting. The lengths and relative doping levels of different NW segments as well as space charge regions were determined indicating memory effects of sulfur during growth. The surface chemistry of the nanowires was monitored simultaneously, showing that in the present case, the doping contrast was independent of the presence or absence of a native oxide. (C) 2011 American Institute of Physics. [doi:10.1063/1.3662933]
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| 11. |
- Hjort, Martin, et al.
(författare)
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Surface Chemistry, Structure, and Electronic Properties from Microns to the Atomic Scale of Axially Doped Semiconductor Nanowires.
- 2012
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Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-086X .- 1936-0851.
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Tidskriftsartikel (refereegranskat)abstract
- Using both synchrotron-based photoemission electron microscopy/spectroscopy and scanning tunneling microscopy/spectroscopy, we obtain a complete picture of the surface composition, morphology, and electronic structure of InP nanowires. Characterization is done at all relevant length scales from micrometer to nanometer. We investigate nanowire surfaces with native oxide and molecular adsorbates resulting from exposure to ambient air. Atomic hydrogen exposure at elevated temperatures which leads to the removal of surface oxides while leaving the crystalline part of the wire intact was also studied. We show how surface chemical composition will seriously influence nanowire electronic properties. However, opposite to, for example, Ge nanowires, water or sulfur molecules adsorbed on the exterior oxidized surfaces are of less relevance. Instead, it is the final few atomic layers of the oxide which plays the most significant role by strongly negatively doping the surface. The InP nanowires in air are rather insensitive to their chemical surroundings in contrast to what is often assumed for nanowires. Our measurements allow us to draw a complete energy diagram depicting both band gap and differences in electron affinity across an axial nanowire p-n junction. Our findings thus give a robust set of quantitative values relating surface chemical composition to specific electronic properties highly relevant for simulating the performance of nanoscale devices.
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| 12. |
- Johansson, Niclas, et al.
(författare)
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Ambient pressure phase transitions over Ir(1 1 1): at the onset of CO oxidation
- 2017
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Ingår i: Journal of Physics: Condensed Matter. - 1361-648X. ; 29:44
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Tidskriftsartikel (refereegranskat)abstract
- In this study we report on the adsorbate structures on an Ir(1 1 1) surface during the phase transition from the inactive to the active state during CO oxidation. The CO oxidation over Pt(1 1 1) is used as a reference case. Where Pt(1 1 1) either is inactive and CO covered or active and O covered, Ir(1 1 1) exhibits a transition state with co-existing chemisorbed O and CO. The observed structural differences are explained in terms of DFT-calculated adsorption energies. For Pt(1 1 1) the repulsive CO–O interaction makes co-existing chemisorbed CO and O unfavourable, while for Ir(1 1 1) the stronger O and CO adsorption allows for overcoming the repulsive interaction. At the onset of CO oxidation over Ir(1 1 1), a CO structure containing defects forms, which enables O2 to dissociatively adsorb on the Ir(1 1 1) surface, thus enabling the CO oxidation reaction. At the mass transfer limit, the Ir(1 1 1) surface is covered by a chemisorbed O structure with defects; hence, the active surface is predominately chemisorbed O covered at a total pressure of 0.5 mbar and no oxide formation is observed.
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| 13. |
- Stierle, A, et al.
(författare)
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Surface x-ray study of the structure and morphology of the oxidized Pd(001) surface
- 2005
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 122:4
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Tidskriftsartikel (refereegranskat)abstract
- The oxidation of Pd(100) and the formation of PdO was studied in situ using surface x-ray diffraction. A bulklike, epitaxial PdO film is formed at oxygen partial pressures beyond 1 mbar and sample temperatures exceeding 650 K. The main orientation is PdO(001)/Pd(001), based upon bulk reflections from the PdO film. By comparing with measurements from the Pd crystal truncation rods, we estimate an rms surface roughness of 6 Angstrom, in good agreement with previous high pressure scanning tunneling microscopy measurements. Finally, we observed the transformation from the (root5 x root5) surface oxide to PdO bulk oxide at 675 K and 50 mbar O-2 pressure.
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| 14. |
- Westerström, Rasmus, et al.
(författare)
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Lack of surface oxide layers and facile bulk oxide formation on Pd(110)
- 2009
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 80:12
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Tidskriftsartikel (refereegranskat)abstract
- The oxidation of the Pd(110) surface has been studied from ultrahigh vacuum up to atmospherical pressures by combining scanning tunneling microscopy, low-energy electron diffraction and high-resolution core-level spectroscopy with in situ surface x-ray diffraction, and density-functional theory calculations. Under in situ conditions, we observe a c(2x4) structure which transforms via the formation of antiphase domain boundaries to a "complex" structure with increasing partial oxygen pressure. Contrary to other closed packed and vicinal Pd surfaces investigated so far, no surface oxide is formed, which allows for the formation of the PdO bulk oxide closer to the thermodynamic limit at temperatures relevant for catalysis.
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| 15. |
- Westerström, Rasmus, et al.
(författare)
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Stressing Pd atoms: Initial oxidation of the Pd(110) surface
- 2008
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 602:14, s. 2440-2447
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Tidskriftsartikel (refereegranskat)abstract
- We have investigated the oxygen induced structures of the Pd(1 1 0) surface in the pressure range of 10(-5)-10(-3) mbar of oxygen, at a sample temperature of around 300 degrees C. These structures, denoted as "(7 x root 3)" and "(9 x root 3)", are studied in detail by the use of a combination of low-energy electron diffraction, scanning tunneling microscopy, high-resolution core level spectroscopy, and ab-initio simulations. Based on our data a model is proposed for these structures containing segments of Pd atoms in the [1 (1) over bar0] direction, in which the Pd rows are decorated by O atoms in a zig-zag pattern. The segments are periodically separated by displaced Pd atoms. Density functional theory calculations show that the displacements reduce the oxygen induced stress significantly, as compared to a structure with no displacements. The calculations also suggest that the new structures are stabilized by domain formation. (C) 2008 Elsevier B.V. All rights reserved.
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| 16. |
- Weststrate, C. J., et al.
(författare)
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CO adsorption on Au(310) and Au(321): 6-Fold coordinated gold atoms
- 2009
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 603:13, s. 2152-2157
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Tidskriftsartikel (refereegranskat)abstract
- The adsorption of CO on Au(310) and Au(321) was studied using a combination of thermal desorption spectroscopy and high resolution core level photoemission spectroscopy. These vicinal Au surfaces both have 6-fold coordinated atoms at the step edges but have a different terrace structure. The CO adsorption behavior was found to be very similar for both surfaces. Three different desorption peaks due to chemisorbed CO were identified, which desorb around 100 K(alpha), 120 K(beta) and 180 K(gamma), respectively. The C1s and O1s spectra of the chemisorbed CO show a complex shake-up structure. Our experimental results indicate that CO only adsorbs on the step atoms. The different desorption peaks are explained by substrate-mediated long-range interactions between the adsorbates. Comparison with literature results shows that the CO adsorption energy is not only dependent on the coordination number of the Au atoms, but that the exact geometrical structure of the surface also plays a role. (C) 2009 Elsevier B.V. All rights reserved.
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| 17. |
- Adams, David L, et al.
(författare)
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Alkali-Aluminum surface alloys
- 2002
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Ingår i: The Elsevier series 'The Chemical Physics of Solid Surfaces'. - 0444511520 ; , s. 225-276
-
Bokkapitel (övrigt vetenskapligt/konstnärligt)
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| 18. |
- Andersen, Jesper N., et al.
(författare)
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Core level binding energy shifts in interfaces between 4d and alkali metals
- 1995
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Ingår i: Journal of Electron Spectroscopy and Related Phenomena. - : Elsevier BV. - 0368-2048. ; 75:C, s. 225-232
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Tidskriftsartikel (refereegranskat)abstract
- High resolution core level photoemission measurements are reported of the binding energy shifts of the 3d core level of Mo, Rh and Pd atoms in interfaces between these 4d metals and alkali metals. The core level binding energies of the interface 4d atoms are found to be almost equal to those of the clean surface atoms for Mo and Rh. For Pd a large alkali induced shift towards higher binding energy is found. The implications of these results for the information derivable from core level binding energy shifts in metallic interfaces are discussed and it is demonstrated that the present results are explained by established total energy based models for core level binding energy shifts in metallic systems.
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| 19. |
- Andersen, Jesper N, et al.
(författare)
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High resolution core level photoemission of clean and adsorbate covered metal surfaces
- 2001
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Ingår i: Journal of Physics C: Solid State Physics. - 0022-3719. ; 13:49, s. 11267-11291
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Forskningsöversikt (refereegranskat)abstract
- A brief review is given of some fundamental aspects of high-resolution core-level photoemission and of its application to studies of pure and adsorbate covered metal surfaces. We first give a short description of the basic theory and then turn to a number of examples. The first group of examples is aimed at demonstrating the use of core-level binding energy shifts for deriving the surface and interface geometrical structure whereas the second group deals with the core-level lineshape and, in particular, the influence of vibrational and other excitations.
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| 20. |
- Andersen, Jesper N, et al.
(författare)
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Strong Phonon Replicas in Be 1 s Photoemission Spectra
- 2001
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Ingår i: Physical Review Letters. - 1079-7114. ; 86:19, s. 4398-4401
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Tidskriftsartikel (refereegranskat)abstract
- The Be 1s core level photoemission line from metallic Be is shown to contain unexpected internal fine structure. We argue that this fine structure is caused by intrinsic excitation of a narrow band of optical phonons in the 1s photoemission process. The general importance of the present results for high resolution core level photoemission investigations of metals is pointed out.
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| 21. |
- Ataman, Evren, et al.
(författare)
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Adsorption of L-cysteine on rutile TiO2(110)
- 2011
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 605:1-2, s. 179-186
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Tidskriftsartikel (refereegranskat)abstract
- We have used X-ray photoelectron spectroscopy to study the adsorption of L-cysteine on a rutile TiO2(110) surface at room temperature and -65 degrees C. For the molecules in direct contact with the surface our results suggest that the molecules bind dissociatively to the fivefold-coordinated Ti atoms of the surface through their deprotonated carboxylic groups. A second, dissociative interaction occurs between the molecular thiol groups and the surface. It is attributed to a dissociative bond to the bridging oxygen vacancies. Most likely, the thiol groups are deprotonated and a bond is formed between the thiolates and defects. In an alternative scenario, the C-S bond is cleaved and atomic sulfur binds to the defects. With regard to the molecular amino groups, they remain neutral at the lowest investigated coverages (0.3-0.5 ML), but already starting from around 0.7 ML nominal coverage protons are being transferred to them. The fraction of protonated amino groups increases with coverage and becomes dominating in multilayers prepared at room temperature and -65 degrees C. In these multilayers the carboxylic groups are deprotonated. (C) 2010 Elsevier B.V. All tights reserved.
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| 22. |
- Ataman, Evren, et al.
(författare)
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Modification of the Size of Supported Clusters by Coadsorption of an Organic Compound: Gold and l-Cysteine on Rutile TiO(2)(110).
- 2011
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 27:18, s. 11466-11474
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Tidskriftsartikel (refereegranskat)abstract
- Using X-ray photoelectron spectroscopy we studied the coadsorption of the amino acid l-cysteine and gold on a rutile TiO(2)(110) surface under ultrahigh vacuum conditions. Irrespective of the deposition order, i.e., irrespective of whether l-cysteine or gold is deposited first, the primary interaction between l-cysteine and the gold clusters formed at the surface takes place through the deprotonated thiol group of the molecule. The deposition order, however, has a profound influence on the size of the gold clusters as well as their location on the surface. If l-cysteine is deposited first the clusters are smaller by a factor two to three compared to gold deposited onto the pristine TiO(2)(110) surface and then covered by l-cysteine. Further, in the former case the clusters cover the molecules and thus form the outermost layer of the sample. We also find that above a minimum gold cluster size the gold cluster/l-cysteine bond is stronger than the l-cysteine/surface bridging oxygen vacancy bond, which, in turn, is stronger than the gold cluster/vacancy bond.
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| 23. |
- Ataman, Evren, et al.
(författare)
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Unconventional Zwitterionic State of L-Cysteine
- 2011
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 2:14, s. 1677-1681
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Tidskriftsartikel (refereegranskat)abstract
- We report the signature of an unconventional zwitterion in thick (>40 angstrom) L-cysteine films grown at low temperature (100 K) by means of sublimation in ultrahigh vacuum. Using X-ray photoelectron spectroscopy (XPS) we find that protonated amino groups (57%) are less prevalent than in films grown at higher temperatures. A large majority of the protonated amino groups receive the hydrogen through deprotonation of the thiol group (45%) and, in contrast, only a small fraction of the carboxylic groups (13%) is deprotonated. This shows, for the first time, the unconventional zwitterion to be the dominant species in L-cysteine. We believe the unusual growth parameters are responsible for this novel observation.
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| 24. |
- Balmes, Olivier, et al.
(författare)
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Reversible formation of a PdCx phase in Pd nanoparticles upon CO and O-2 exposure
- 2012
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Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 14:14, s. 4796-4801
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Tidskriftsartikel (refereegranskat)abstract
- The structure and chemical composition of Pd nanoparticles exposed to pure CO and mixtures of CO and O-2 at elevated temperatures have been studied in situ by a combination of X-ray Diffraction and X-ray Photoelectron Spectroscopy in pressures ranging from ultra high vacuum to 10 mbar and from room temperature to a few hundred degrees celsius. Our investigation shows that under CO exposure, above a certain temperature, carbon dissolves into the Pd particles forming a carbide phase. Upon exposure to CO and O-2 mixtures, the carbide phase forms and disappears reversibly, switching at the stoichiometric ratio for CO oxidation. This finding opens new scenarios for the understanding of catalytic oxidation of C-based molecules.
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| 25. |
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| 26. |
- Baraldi, Alessandro, et al.
(författare)
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The (1x1)-> hexagonal structural transition on Pt(100) studied by high-energy resolution core level photoemission
- 2007
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 127:16
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Tidskriftsartikel (refereegranskat)abstract
- The (1 x 1)-> quasihexagonal (HEX) phase transition on a clean Pt(100) surface was investigated by monitoring the time evolution of the Pt4f(7/2) core level photoemission spectra. The spectral component originating from the atoms forming the (1 x .1) metastable unreconstructed surface was found at -570 +/- 20 meV with respect to the bulk peak. Ab initio calculations based on density functional theory confirmed the experimental assignment. At temperatures above 370 K, the (1 x 1) phase irreversibly reverts to the more stable HEX phase, characterized by a surface core level shifted component at -185 +/- 40 meV. By analyzing the intensity evolution of the core level components, measured at different temperatures in the range of 393-475 K, we determined the activation energy of the phase transformation, E=0.76 +/- 0.04 eV. This value is considerably lower than the one previously determined by means of low energy electron diffraction. Possible reasons for this discrepancy are discussed.
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| 27. |
- Birgersson, Martin, et al.
(författare)
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Density-functional theory applied to Rh(111) and CO/Rh(111) systems: Geometries, energies, and chemical shifts
- 2003
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 67:4, s. 1-04502
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Tidskriftsartikel (refereegranskat)abstract
- We present extensive density-functional theory (DFT) based calculations of the clean Rh(111) surface and of CO/Rh(111) overlayer systems. We study both ground-state structural properties and core-level shifts from differences in total energies at different coverages and adsorption sites. Most results are obtained using using norm-conserving or ultrasoft pseudopotentials. The overall reliability of the pseudopotential method is analyzed theoretically, and computationally by way of all-electron calculations. In general, core corrections are required in order to correctly simulate all-electron total energies, although the corrections are rather small for the systems considered here. Overall there is a very good agreement both between the pseudopotential and all-electron results as well as with high-resolution experimental spectra. The obtained agreement between theoretical and experimental core-level energies, however, requires that the correct geometrical parameters are used. For instance, inclusion of bucklings of the first Rh layer in the (2x2)-1CO and (root3x3)R30degrees-1CO overlayers is essential. For the overlayers studied here, different competing adsorption sites give almost the same frozen-lattice adsorption energies. However, the C 1s binding energy shows large differences between CO adsorbed in different sites. Thus calculations of the C 1s shifts allow us to predict the adsorption sites despite the small differences in ground-state energies. We also analyze sources of the shifts in terms of differences in Hartree potential and relaxation at different sites. As the DFT core eigenvalue lies above rather than below the core excitation energy some care is required in order to properly identify a relaxation energy in a DFT framework. In order to clarify the question we relate the DFT approach for core energies to approaches based on self-energies or the Hartree-Fock approximation.
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| 28. |
- Blomberg, Sara, et al.
(författare)
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A high pressure X-ray photoelectron spectroscopy study of oxidation and reduction of Rh(100) and Rh nanoparticles
- 2014
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 628, s. 153-158
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Tidskriftsartikel (refereegranskat)abstract
- We have studied the oxidation and reduction of Rh(100) and SiO2 supported Rh particles using high pressure X-ray photoelectron spectroscopy. We show that the formation and reduction of Rh bulk oxide can be followed in situ in O-2 and CO pressures in the range of 0.1 Torr. In general, the oxidation/reduction processes are similar on Rh(100) and the nanoparticles, but there are significant differences in temperature dependence. Already at a sample temperature of 140 degrees C, the particles show clear signs of a thin bulk oxide, while an ultra-thin so-called surface oxide covers the single crystal at the same temperature. Both of these oxide films, however, hinder further oxidation, and a thick oxide is only found at a temperature of at least 300 degrees C, for both samples. The reduction, in contrast, starts at a higher temperature on the particles as compared to the single crystal, but once started the particles are completely reduced at lower temperatures. (C) 2014 Elsevier B.V. All rights reserved.
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| 29. |
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| 30. |
- Blomberg, Sara, et al.
(författare)
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Structure of the Rh2O3(0001) surface
- 2012
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 606:17-18, s. 1416-1421
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Tidskriftsartikel (refereegranskat)abstract
- We have studied the (0001) surface termination of Rh2O3 on a Rh(111) single crystal using a combination of high resolution core level spectroscopy, low energy electron diffraction, scanning tunneling microscopy and density functional theory. By exposing the Rh(111) to atomic oxygen we are able to grow Rh2O3 layers exposing the (0001) surface. The experiments support the theoretical predictions stating that the surface is terminated with an O-Rh-O trilayer yielding a RhO2 termination instead of a bulk Rh2O3 termination. The structural details as found by the DFT calculations are presented and reasons for the previously observed strong differences in catalytic activity between the structurally similar RhO2 surface oxide, and the Rh2O3(0001) surface are discussed. (C) 2012 Elsevier B.V. All rights reserved.
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| 31. |
- Borg, Mikael, et al.
(författare)
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Density of configurational states from first-principles calculations: The phase diagram of Al-Na surface alloys
- 2005
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Ingår i: ChemPhysChem. - : Wiley. - 1439-7641 .- 1439-4235. ; 6:9, s. 1923-1928
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Tidskriftsartikel (refereegranskat)abstract
- The structural phases of AlxNa1-x surface alloys have been investigated theoretically and experimentally. We describe the system using a lattice gas Hamiltonian, determined from density functional theory together with Monte Carlo (MC) calculations. The obtained phase diagram reproduces the experiment on a quantitative level. From calculation of the (configurational) density of states by recently introduced Wang-Landau MC algorithm, we derive thermodynamic quantities such as free energy and entropy which are not directly accessible from conventional MC simulations. We accurately reproduce the stoichiometry, as well as the temperature at which an order-disorder phase transition occurs, and demonstrate the crucial role, and magnitude of the configurational entropy.
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| 32. |
- Borg, Mikael, et al.
(författare)
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Experimental and theoretical surface core-level shifts of aluminum (100) and (111)
- 2004
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Ingår i: Physical Review B. - 1550-235X. ; 69
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Tidskriftsartikel (refereegranskat)abstract
- The surface core-level shifts of Al(111) and Al(100) have been measured using high-resolution core-level photoemission spectroscopy and calculated using density functional theory (DFT). For Al(100), the 2p core-level shift of the first (second) layer was determined to be –75 meV (+20 meV) from experiment and –71 meV (+20 meV) from the DFT calculations. For Al(111), the corresponding values are –27 meV (0 meV) from experiment and –14 meV (–) from the DFT calculations. Core-level splittings caused by the low-symmetry crystal fields at the (111) and (100) surfaces have also been studied. These splittings turn out to be much smaller than previously reported provided proper care is taken of the influence of the core hole screening and of core–valence exchange beyond the DFT level. Finally, the experimental Al 2p line shape was found to contain structure caused by a sharp no-phonon line and a broad and weak phonon replica.
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| 33. |
- Borg, Mikael, et al.
(författare)
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Structure and formation of the Al(100)-(root 5 x root 5) R27 degrees-Na phase: a LEED, DFT and HRCLS study
- 2002
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Ingår i: Surface Science. - 0039-6028. ; 515:2-3, s. 267-280
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Tidskriftsartikel (refereegranskat)abstract
- Adsorption of 0.2 ML Na on Al(1 0 0) at room temperature yields a disordered Al(1 0 0)-(1 x 1)-Na phase, which transforms reversibly to a well-ordered Al(1 0 0)-(root5 x root5)R27degrees-Na phase on cooling below 250 K. Based on low energy electron diffraction (LEED) and high resolution core-level spectroscopy (HRCLS) measurements, and on ab initio calculations, it is concluded that the structure of the Al(1 0 0)-(root5 x root5-)R27degrees-Na phase consists of Na atoms occupying substitutional sites. The structural parameters obtained from LEED and density functional theory analyses are in quantitative agreement. Adsorption of 0.2 ML Na at 100 K yields an Al(1 0 0)-c(2 x 2)-Na island structure, which transforms irreversibly into the (root5 x root5)R27degrees-Na structure by annealing above 190 K. The nature of the reversible and irreversible phase transformations to the (root5 x root5)R27degrees structure is investigated by HRCLS and LEED. (C) 2002 Elsevier Science B.V. All rights reserved.
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| 34. |
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| 35. |
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| 36. |
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| 37. |
- Dick Thelander, Kimberly, et al.
(författare)
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Improving InAs nanotree growth with composition-controlled Au-In nanoparticles
- 2006
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 17:5, s. 1344-1350
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Tidskriftsartikel (refereegranskat)abstract
- Au nanoparticles are commonly used as seeds for epitaxial growth of III-V semiconductor nanowires. However, the interaction between Au and In-containing III-V materials makes it difficult to control the growth of more complex nanowire structures in materials such as InAs. Here we report the growth of InAs nanowires and branched nanotrees using Au and Au-In nanoparticles. We show that the initial composition of the particle does not affect the morphology of the first-generation nanowires, nor does it affect the final composition of the particle after growth. However, when the Au-In particles were used to seed a second generation of nanowires, producing nanotrees, the branches exhibited a 2-3 times higher growth rate and more regular shape than those seeded by pure Au particles. This result is attributed to the decreased interaction between the seed particle and the trunk nanowires when Au-In particles are used. Thus the incorporation of In into the seed particle during particle production allows for modification of the particle-wire interaction.
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| 38. |
- Gerber, Timm, et al.
(författare)
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CO-Induced Smoluchowski Ripening of Pt Cluster Arrays on the Graphene/Ir(111) Moire
- 2013
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Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-086X .- 1936-0851. ; 7:3, s. 2020-2031
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Tidskriftsartikel (refereegranskat)abstract
- Regular Pt cluster arrays grown on the moire template formed by graphene on Ir(111) were tested for their stability with respect to CO gas exposure. Cluster stability and adsorption-Induced processes were analyzed as a function of cluster size, with In situ scanning tunneling microscopy and X-ray photoelectron spectroscopy. Small clusters containing fewer than 10 atoms were unstable upon CO adsorption. They sintered through Smoluchowski ripening-cluster diffusion and coalescence rather than the frequently reported Ostwald ripening mediated by metal-adsorbate complexes. Larger dusters remained immobile upon CO adsorption but became more three-dimensional. Careful analysis of the experimental data complemented by ab initio density functional theory calculations provides insight Into the origin of the CO-induced Pt cluster ripening and shape transformations.
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| 39. |
- Gladys, M. J., et al.
(författare)
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Chemical composition and reactivity of water on clean and oxygen-covered Pd{111}
- 2008
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 602:22, s. 3540-3549
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Tidskriftsartikel (refereegranskat)abstract
- The chemical composition and dissociation behaviour of water adsorbed on clean and oxygen pre-covered Pd{111} was studied using high-resolution time-resolved and temperature-programmed X-ray photoelectron spectroscopy. We find that water remains intact at all temperatures up to desorption on the clean surface and at high oxygen coverage(0.69 ML) when a surface oxide is formed. The highest desorption peaks occur at 163 K from the clean surface and at 172 K from the surface oxide. At the intermediate coverage of 0.20 ML oxygen reacts with coadsorbed water at 155 K, to generate a mixed H2O/OH layer exhibiting a (root 3- x root 3)R30 degrees diffraction pattern, which is stable up to 177 K. The measured ratio between intact water and the hydroxyl species in this layer varies between 1.5 and 2 depending on temperature. (C) 2008 Elsevier B.V. All rights reserved.
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| 40. |
- Gladys, M. J., et al.
(författare)
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Modifying the adsorption characteristics of water on Ru{0001} with preadsorbed oxygen
- 2008
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 78:3
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Tidskriftsartikel (refereegranskat)abstract
- The coadsorption of water and preadsorbed oxygen on Ru{0001) was studied by synchrotron-based high-resolution x-ray photoelectron spectroscopy. A dramatic change was observed in the interaction of water with oxygen between low and high oxygen precoverages. Low oxygen coverages below 0.18 ML induce partial dissociation, which leads to an adsorbed layer of H2O and OH. Around half the oxygen atoms take part in this reaction. All OH recombines upon heating to 200 K and desorbs together with H2O. Oxygen coverages between 0.20 and 0.50 ML inhibit dissociation, instead a highly stable intact water species is observed, which desorbs at 220 K. This species is significantly more stable than intact water on the clean surface. The stabilization is most likely due to the formation of hydrogen bonds with neighboring oxygen atoms. For intermediate oxygen coverages around 0.18 ML, the dissociation behavior depends on the preparation conditions, which points toward possible mechanisms and pathways for partial dissociation of water on Ru{0001}.
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| 41. |
- Gladys, MJ, et al.
(författare)
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Water adsorption on O-covered Ru {0001}: Coverage-dependent change from dissociation to molecular adsorption
- 2005
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614. ; 414:4-6, s. 311-315
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Tidskriftsartikel (refereegranskat)abstract
- When water is coadsorbed with oxygen at coverages above 0.25ML an intact water species is observed in high resolution X-ray photoelectron spectroscopy up to 220 K, which is significantly more stable than intact water on the clean surface. The presence of this species causes a shift in the O 1s binding energy of the pre-adsorbed oxygen, which indicates the formation of hydrogen bonds between the two adsorbates. Low coverages of oxygen induce partial dissociation and recombinative desorption in the same temperature range, which illustrates that desorption temperatures alone cannot be used to determine whether water is molecularly intact or not.
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| 42. |
- Grånäs, Elin, et al.
(författare)
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Hydrogen intercalation under graphene on Ir(111)
- 2016
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 651, s. 57-61
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Tidskriftsartikel (refereegranskat)abstract
- Using high resolution X-ray photoelectron spectroscopy and scanning tunneling microscopy we study the intercalation of hydrogen under graphene/Ir(111). The hydrogen intercalated graphene is characterized by a component in C 1s that is shifted −0.10 to −0.18 eV with respect to pristine graphene and a component in Ir 4f at 60.54 eV. The position of this Ir 4f component is identical to that of the Ir(111) surface layer with hydrogen atoms adsorbed, indicating that the atomic hydrogen adsorption site on bare Ir(111) and beneath graphene is the same. Based on co-existence of fully- and non-intercalated graphene, and the inability to intercalate a closed graphene film covering the entire Ir(111) surface, we conclude that hydrogen dissociatively adsorbs at bare Ir(111) patches, and subsequently diffuses rapidly under graphene. A likely entry point for the intercalating hydrogen atoms is identified to be where graphene crosses an underlying Ir(111) step.
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| 43. |
- Grånäs, Elin, et al.
(författare)
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Oxygen Intercalation under Graphene on Ir(111): Energetics, Kinetics, and the Role of Graphene Edges.
- 2012
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Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-086X .- 1936-0851.
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Tidskriftsartikel (refereegranskat)abstract
- Using X-ray photoemission spectroscopy (XPS) and scanning tunneling microscopy (STM) we resolve the temperature-, time-, and flake size-dependent intercalation phases of oxygen underneath graphene on Ir(111) formed upon exposure to molecular oxygen. Through the applied pressure of molecular oxygen the atomic oxygen created on the bare Ir terraces is driven underneath graphene flakes. The importance of substrate steps and of the unbinding of graphene flake edges from the substrate for the intercalation is identified. With the use of CO titration to selectively remove oxygen from the bare Ir terraces the energetics of intercalation is uncovered. Cluster decoration techniques are used as an efficient tool to visualize intercalation processes in real space.
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| 44. |
- Grönbeck, Henrik, 1966, et al.
(författare)
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Mechanism for reversed photoemission core-level shifts of oxidized Ag
- 2012
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969 .- 1098-0121. ; 85:11
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Tidskriftsartikel (refereegranskat)abstract
- Density functional theory calculations and high-resolution core-level spectroscopy are used to explore the remarkable observation of decreased Ag 3d binding energy upon silver oxidation. The shift in Ag 3d binding energy is investigated at different degrees of oxidation and compared to results for Pd 3d, which exhibits a normal shift. Analysis of initial-state effects and valence electronic structure shows that the onsite Ag core potential is insensitive to oxidation despite a clear metal-to-oxygen charge transfer. The substantial negative shift for oxidized Ag is instead attributed to final-state effects as screening of the core-hole occurs in metal s states of bonding character.
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| 45. |
- Gustafson, Johan, et al.
(författare)
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Identification of Step Atoms by High Resolution Core Level Spectroscopy
- 2003
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Ingår i: Physical Review Letters. - 1079-7114. ; 91:5
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Tidskriftsartikel (refereegranskat)abstract
- Vicinal Rh(111) surfaces are studied with high resolution core level photoemission. We demonstrate the possibility to distinguish between the different kinds of surface atoms on these surfaces by virtue of their 3d core level binding energies. In particular, the low coordinated step atoms are found to exhibit a clear fingerprint in Rh 3d spectra. We demonstrate how this may be used to show that initial oxygen adsorption occurs on the steps and not on the terraces of the vicinal surfaces.
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| 46. |
- Gustafson, Johan, et al.
(författare)
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One-dimensional PtO(2) at Pt steps : Formation and reaction with CO
- 2005
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 95:25, s. 1-4
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Tidskriftsartikel (refereegranskat)abstract
- Using core-level spectroscopy and density functional theory we show that a one-dimensional (1D) PtO(2) oxide structure forms at the steps of the Pt(332) surface after O(2) exposure. The 1D oxide is found to be stable in an oxygen pressure range, where bulk oxides are only metastable, and is therefore argued to be a precursor to the Pt oxidation. As an example of the consequences of such a precursor exclusively present at the steps, we investigate the reaction of CO with oxygen covered Pt(332). Albeit more strongly bound, the oxidic oxygen is found to react more easily with CO than oxygen chemisorbed on the Pt terraces.
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| 47. |
- Gustafson, Johan, et al.
(författare)
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Self-limited growth of a thin oxide layer on Rh(111)
- 2004
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Ingår i: Physical Review Letters. - 1079-7114. ; 92:12
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Tidskriftsartikel (refereegranskat)abstract
- The oxidation of the Rh(111) surface at oxygen pressures from 10(-10) mbar to 0.5 bar and temperatures between 300 and 900 K has been studied on the atomic scale using a multimethod approach of experimental and theoretical techniques. Oxidation starts at the steps, resulting in a trilayer O-Rh-O surface oxide which, although not thermodynamically stable, prevents further oxidation at intermediate pressures. A thick corundum like Rh2O3 bulk oxide is formed only at significantly higher pressures and temperatures.
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| 48. |
- Gustafson, J., et al.
(författare)
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Sensitivity of catalysis to surface structure: The example of CO oxidation on Rh under realistic conditions
- 2008
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 78:4
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Tidskriftsartikel (refereegranskat)abstract
- Using a combination of surface x-ray diffraction and mass spectrometry at realistic pressures, the CO oxidation reactivity of Rh(111) and Rh(100) model catalysts has been studied in conjunction with the surface structure. The measurements show that the presence of a specific thin surface oxide is crucial for the high activity of the Rh based CO oxidation. As this oxide is readily formed on all Rh facets, we conclude that the specific Rh crystal planes exposed during catalysis will not directly influence the reactivity. This is fortified by the very close similarity between the Rh(111) and the Rh(100) results.
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| 49. |
- Gustafson, Johan, et al.
(författare)
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Structure and catalytic reactivity of Rh oxides
- 2009
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Ingår i: Catalysis Today. - : Elsevier BV. - 0920-5861. ; 145:3-4, s. 227-235
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Konferensbidrag (refereegranskat)abstract
- Using a combination of experimental and theoretical techniques, we show that a thin RhO2 surface oxide film forms prior to the bulk Rh2O3 corundum oxide on all close-packed single crystal Rh surfaces. Based on previous reports, we argue that the RhO2 surface oxide also forms on vicinal Rh surfaces as well as on Rh nanoparticles. The detailed structure of this film was previously determined using UHV based techniques and density functional theory. In the present paper, we also examine the structure of the bulk Rh2O3 corundum oxide using surface X-ray diffraction. Being armed with this structural information, we have explored the CO oxidation reaction over Rh(1 1 1), Rh(1 0 0) and Pt25Rh75(1 0 0) at realistic pressures using in situ surface X-ray diffraction and online mass spectrometry. In all three cases we find that an increase of the CO2 production coincides with the formation of the thin RhO2 surface oxide film. In the case of Pt25Rh75(1 0 0), our measurements demonstrate that the formation of bulk Rh2O3 corundum oxide poisons the reaction, and argue that this is also valid for all other Rh surfaces. Our study implies that the CO oxidation reaction over Rh surfaces at realistic conditions is insensitive to the exact Rh substrate orientation, but is rather governed by the formation of a specific surface oxide phase. (C) 2008 Elsevier B.V. All rights reserved.
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| 50. |
- Gustafson, Johan, et al.
(författare)
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The identification of step-atoms by high resolution core level spectroscopy
- 2002
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Ingår i: 7th International Conference on Nanometer-Scale Science and Technology and 21st European Conference on Surface Science.
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Konferensbidrag (refereegranskat)abstract
- We have studied the vicinal Rh(553) surface by the use of High Resolution Core Level Spectroscopy (HRCLS). We demonstrate that we are able to resolve the photo-emitted electrons from the Rh3d5/2 level originating from the step atoms with respect to those emitted from the atoms in the (111) terrace. Our study therefore shows that it is possible to obtain a core level "fingerprint" from the step atoms, opening up a new area of possible experiments for the technique of HRCLS
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