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Sökning: L773:0039 6028 OR L773:1879 2758 > (2010-2014)

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1.
  • Bolton, Kim, et al. (författare)
  • DFT study of the adsorption and dissociation of water on Ni(111), Ni(110) and Ni(100) surfaces
  • 2014
  • Ingår i: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 627, s. 1-10
  • Tidskriftsartikel (refereegranskat)abstract
    • Water adsorption and dissociation on catalytic metal surfaces play a key role in a variety of industrial processes, and a detailed understanding of this process and how it is effected by the surface structure will assist in developing improved catalysts. Hence, a comparative study of the adsorption and dissociation of water on Ni(111), Ni(110) and Ni(100) surfaces, which is often used as catalyst, has been performed using density functional theory. The results show that the adsorption energies and dissociation rates depend on the surface structure. The adsorption energies for H2O and OH decrease in the order Ni(110) > Ni(100) > Ni(111), and for the O and H atoms the adsorption energies decrease in the order Ni(100) > Ni(111) > Ni(110). In addition, the splitting of water to OH and H has lower activation energies over less packed Ni(110) and Ni(100) surfaces compared to the highly packed Ni(111) surface. The subsequent splitting of the OH to O and H also has the lowest activation energy on the Ni(110) surface. At 463 K, which is typical for industrial processes that include the water gas shift reaction, the H2O splitting is approximately 6000 and 10 times faster on the Ni(110) surface compared to the Ni(111) and Ni(100) surfaces, respectively, and OH splitting is 200 and 3000 times faster, respectively. The complete water dissociation reaction rate decreases in the order Ni(110) > Ni(100) > Ni(111).
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2.
  • Ericsson, Leif, 1964-, et al. (författare)
  • Photoemission study of ZnO nanocrystals : Thermal annealing in UHV and induced band bending
  • 2013
  • Ingår i: Surface Science. - Amsterdam : Elsevier. - 0039-6028 .- 1879-2758. ; 612, s. 10-15
  • Tidskriftsartikel (refereegranskat)abstract
    • ZnO nanocrystals distributed by spin-coating on SiO2/Si surfaces were annealed in UHV and studied in situ by synchrotron radiation based X-ray Photoelectron Spectroscopy. Changes in chemical composition and electronic structure of ZnO nanocrystal surfaces were found with increasing annealing temperatures. Annealing at 650 °C reduces the surface contaminant levels without any observed de-composition of ZnO. After annealing at 700 °C an initial de-composition of ZnO together with further reduction of contaminants was observed. As a result, 650 °C is found to be the optimal annealing temperature for thermal cleaning of ZnO nanocrystals. Chemical changes and induced point defect formation cause changes in the band structure of the ZnO/SiO2/Si system. An upward band bending of 0.7 eV on the surfaces of the ZnO nanocrystals was found after annealing at 300 °C. The bands on the surfaces of ZnO nanocrystals gradually bend downwards with increasing annealing temperatures. A downward band bending of 1.4 eV is the result after annealing at 750 °C for 1 h. This large downward band bending is explained as due to the change in balance of oxygen vacancies and zinc vacancies on the surfaces of ZnO nanocrystals.
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3.
  • Eriksson, Jan Christer, et al. (författare)
  • Additional remarks related with the discussion inaugurated by the article "Incompatibility of the Shuttleworth equation with Hermann's mathematical structure of thermodynamics" by D. J. Bottomley et al. [Surf. Sci. 603 (2009) 97]
  • 2010
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 604:11-12, s. 1062-1063
  • Tidskriftsartikel (refereegranskat)abstract
    • Jan Christer Eriksson and Anatoly I. Rusanov critically analyze a paper titled 'Incompatibility of the Shuttleworth equation with Hermann's mathematical structure of thermodynamics' by D. J. Bottomley and co-researchers. According to him, the problem of double counting that Bottomley and co-researchers supposed to be due to involving pairs of terms of the kind xdy + ydx, is not a true research issue but rather a pedagogical one. Within the formal scheme adopted by Gibbs, this problem is properly dealt with by means of a Gibbs Duhem condition. The critics underline that the incompatibility with the mathematical structure of thermodynamics erroneously claimed by Bottomley and co-researchers would apply not just to solid but to liquid interfaces as well, thus invalidating even the firmly rooted Gibbs surface tension equation.
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6.
  • Gladh, Jörgen, et al. (författare)
  • Electron- and phonon-coupling in femtosecond laser-induced desorption of CO from Ru(0001)
  • 2013
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 615, s. 65-71
  • Tidskriftsartikel (refereegranskat)abstract
    • We studied femtosecond laser-induced desorption of CO from Ru(0001) using intense near-infrared and visible femtosecond laser pulses. We find a pronounced wavelength dependence with a factor 3-4 higher desorption yield at comparable fluence when desorption is induced via 400 nm light, compared to 800 nm and attribute this difference to the difference in penetration depth of the incident light. All our data can be described using empirical friction-modeling to determine the desorption mechanism with the same mechanism for both wavelengths. We find that both hot electrons and phonons contribute to the desorption process.
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7.
  • Huber, Stefan E., et al. (författare)
  • Large-scale SCC-DFTB calculations of reconstructed polar ZnO surfaces
  • 2014
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 628, s. 50-61
  • Tidskriftsartikel (refereegranskat)abstract
    • We present a theoretical study of a range of surface defects for the most abundant polar ZnO(0001)/(000 (1) over bar) surfaces using a tight binding approach with self-consistent charges (SCC-DFTB). We find that a combination of triangular pits at the Zn-terminated surface and a strongly ordered hexagonal defect pattern at the O-terminated surface constitutes a very stable reconstruction, in excellent agreement with experimental findings. On the whole, the SCC-DFTB method describes the polar surfaces of ZnO very well, and at a low computational cost which allows for the investigation of larger - and more realistic - surface structures compared to previous studies. Such large-scale calculations show that, at the Zn-terminated surface, the reconstruction results in a high density of one-layer deep triangular pit-like defects and surface vacancies which allow for a high configurational freedom and a vast variety of defect motifs. We also present extensive tests of the performance of the SCC-DFTB method in comparison with DFT results.
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8.
  • Karlsson, P. G., et al. (författare)
  • TiO2 chemical vapor deposition on Si(111) in ultrahigh vacuum: Transition from interfacial phase to crystalline phase in the reaction limited regime
  • 2011
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 605:13-14, s. 1147-1156
  • Tidskriftsartikel (refereegranskat)abstract
    • The interaction between the metal organic precursor molecule titanium(IV) isopropoxide (TTIP) and three different surfaces has been studied: Si(111)-(7 x 7), SiOx/Si(111) and TiO2. These surfaces represent the different surface compositions encountered during TTIP mediated TiO2 chemical vapor deposition on Si(111). The surface chemistry of the titanium(IV) isopropoxide precursor and the film growth have been explored by core level photoelectron spectroscopy and x-ray absorption spectroscopy using synchrotron radiation. The resulting film morphology has been imaged with scanning tunneling microscopy. The growth rate depends on both surface temperature and surface composition. The behavior can be rationalized in terms of the surface stability of isopropoxy and isopropyl groups, confirming that growth at 573 K is a reaction limited process. (C) 2011 Elsevier B.V. All rights reserved.
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9.
  • Kuernsteiner, Philipp, et al. (författare)
  • Matrix effects in the neutralization of He ions at a metal surface containing oxygen
  • 2013
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 609, s. 167-171
  • Tidskriftsartikel (refereegranskat)abstract
    • Charge exchange between He ions and a Ni(111) surface containing oxygen was studied by Low-Energy Ion Scattering, using 1.25 key He+ as primary ions. The energy resolved yield of positive ions was detected after backscattering from Ni or O for different exposures of Ni(111) to molecular oxygen. Pronounced changes in the neutralization efficiency due to the presence of oxygen are observed for both, the adsorbate phase at low oxygen dose, and the NiO phase at high dose. The presence of O in the surface makes resonant charge transfer in a close collision possible. Evidence for a strong matrix effect is found: O in NiO neutralizes much more efficiently than O in the adsorbate phase. Independently, the different interaction stages of Ni-O and the surface structure were monitored by Photoelectron-Emission-Microscopy and Low-Energy Electron Diffraction.
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10.
  • Kuzmin, M., et al. (författare)
  • Photoemission and density functional theory study of Ge(100) : Clean surface and Yb-induced (2x4) reconstruction
  • 2013
  • Ingår i: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 615, s. 88-96
  • Tidskriftsartikel (refereegranskat)abstract
    • Clean and metal-adsorbed (100) surfaces of group-IV semiconductors, such as Si and Ge, often exhibit electronically and structurally similar reconstructions. However, the fundamental bulk properties of group-IV materials can have an impact on particular features of such systems, which are related, e.g., to final-state relaxation in photoemission and thus determine their spectral line shape. Here we have studied Yb/Ge(100)(2 x 4) reconstruction as well as clean Ge(100) surface by high-resolution photoelectron spectroscopy and ab initio calculations. An atomic geometry of both surfaces is thoroughly investigated. A detailed analysis of Ge 3d core-level photoemission, atomic origins of surface-shifted components, and final-state screening effects is presented. In particular, it is demonstrated that the core-hole screening plays an essential role in Ge 3d measurements, and that its amount in the complete screening model correlates well with the core-level binding energy of respective Ge atoms in the initial state. The results are discussed in the proper context of related reconstructions on Si(100).
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11.
  • Lang, J. J. K., et al. (författare)
  • Tin-stabilized (1 x 2) and (1 x 4) reconstructions on GaAs(100) and InAs(100) studied by scanning tunneling microscopy, photoelectron spectroscopy, and ab initio calculations
  • 2011
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 605:9-10, s. 883-888
  • Tidskriftsartikel (refereegranskat)abstract
    • Tin (Sn) induced (1 x 2) reconstructions on GaAs(100) and InAs(100) substrates have been studied by low energy electron diffraction (LEED), photoelectron spectroscopy, scanning tunneling microscopy/spectroscopy (STM/STS) and ab initio calculations. The comparison of measured and calculated STM images and surface core-level shifts shows that these surfaces can be well described with the energetically stable building blocks that consist of Sn-III dimers. Furthermore, a new Sn-induced (1 x 4) reconstruction was found. In this reconstruction the occupied dangling bonds are closer to each other than in the more symmetric (1 x 2) reconstruction, and it is shown that the (1 x 4) reconstruction is stabilized as the adatom size increases.
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12.
  • Nisar, J., et al. (författare)
  • Structural, electronic and energetic properties of water adsorbed on beta-Si3N4 (0001) surface : First-principles calculations
  • 2010
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 604:5-6, s. 617-622
  • Tidskriftsartikel (refereegranskat)abstract
    • Structural, energetic and electronic properties of water molecules adsorbed on beta-Si3N4 (0 00 1) surface, at various coverages, are investigated using density functional theory. At low coverages (0 <= 0.5), it is found that all H2O molecules undergo spontaneous dissociation forming hydroxyl (OH) and imino (NH) groups where the reactive sites are identified, a result shown for the first time using ab initio theory. For higher coverages (0 > 0.5), only partial dissociation takes place where some of the molecules stay intact being bound via H-bond in good agreement with experimental findings. The driving force for the water dissociation has been identified to be dangling bonds on lower coordinated N and Si surface atoms showing that not all surface atoms are reactive corroborating with previous experimental findings.
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13.
  • Näslund, Lars-Åke (författare)
  • Reaction kinetics for the oxygen hydrogenation process on Pt(111) derived from temperature-programmed XPS
  • 2013
  • Ingår i: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 618, s. 42-48
  • Tidskriftsartikel (refereegranskat)abstract
    • Oxygen hydrogenation under ultra high vacuum conditions at the platinum surface was explored using temperature-programmed X-ray photoelectron spectroscopy. Through modeling of the oxygen consumption, information on the reaction kinetics was obtained indicating that the reaction rate of the oxygen hydrogenation process depends on the hydrogen diffusion and on the lifetime of hydroxyl intermediates. The reaction rate is, however, enhanced when an autocatalytic process stabilizes the hydroxyl intermediates through hydrogen bonding to neighboring water molecules. The overall activation energy for the hydrogenation of atomic oxygen to form water was determined to be 0.20 eV with a frequency factor of only 10(3) s(-1).
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14.
  • Pal, Prabir, et al. (författare)
  • Pseudogap behavior in Pr0.5Sr0.5MnO3 : A photoemission study
  • 2011
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 605:9-10, s. 875-877
  • Tidskriftsartikel (refereegranskat)abstract
    • The valence band electronic structure of Pr0.5Sr0.5MnO3 has been investigated across its paramagnetic metallic (PMM)-ferromagnetic metallic (FMM)-antiferromagnetic insulator (AFMI) transition. Using surface sensitive high resolution photoemission we have conclusively demonstrated the presence of a pseudogap of magnitude 80 meV in the near Fermi level electronic spectrum in the PMM and FMM phases and finite intensity at the Fermi level in the charge ordering (CO)-AFMI phase. The pseudogap behavior is explained in terms of the strong electron-phonon interaction and the formation of Jahn Teller UT) polarons, indicating the charge localizations. The finite intensity at the Fermi level in the insulating phase showed a lack of charge ordering in the surface of the Pr0.5Sr0.5MnO3 samples.
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15.
  • Pitkänen, H., et al. (författare)
  • Ab initio study of the surface properties of austenitic stainless steel alloys
  • 2013
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 609, s. 190-194
  • Tidskriftsartikel (refereegranskat)abstract
    • Using ab initio calculations we investigated the surface energies of paramagnetic Fe1-c-nCrcNin random alloys within the concentration range of 0.12 <= c <= 032 and 0.04 <= n <= 0.32. These alloys crystallize mainly in the face centred cubic (fcc) structure and constitute the main building blocks of austenitic stainless steels. It is shown that all alloys have the lowest surface energies along the most close packed crystal orientation, namely the fcc (111) surfaces. The amount of Ni seems to have little effect on the surface energy, while almost all composition-driven change may be attributed to the changes in the Cr content. Within the studied compositional range, the change of the surface energy with the composition is of the order of 10%. Trends of the surface energy can be related to the magnetic structure of surfaces. Using the total energy as a function of the concentration, we determine the effective chemical potentials in bulk and at the surface, which can be used to estimate the surface segregation energies.
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16.
  • Punkkinen, Marko P. J., et al. (författare)
  • Structure of ordered oxide on InAs(100) surface
  • 2012
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 606:23-24, s. 1837-1841
  • Tidskriftsartikel (refereegranskat)abstract
    • It was recently found that oxygen induces ordered reconstructions on several III-V surfaces. The most oxygen-rich reconstruction shows (3x1) periodicity. Based on first-principles investigations, a detailed atomic model is presented for this reconstruction. The uncommon periodicity is attributed to the highly stable In - O - In trilayer below surface which also leads to stabilizing additional bonds within the surface layer. The strain induced by the trilayer is more effectively accommodated within the (3 x 1) reconstruction than within the competing (2 x 1) reconstruction due to smaller number of dimers. It is proposed that the experimentally found semiconductivity is reached by substitutional atoms within the surface layer. Suitable substitution preserves the magnitude of the bulk band gap. (C) 2012 Elsevier B.V. All rights reserved.
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17.
  • Ragazzon, Davide, et al. (författare)
  • Chemical vapor deposition of ordered TiOx nanostructures on Au(111)
  • 2013
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 617, s. 211-217
  • Tidskriftsartikel (refereegranskat)abstract
    • The deposition of TiOx (x <= 2) structures on Au(111) by chemical vapor deposition (CVD) in ultrahigh vacuum (UHV) has been investigated with high-resolution core level photoelectron spectroscopy (PES), low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Using titanium tetra-isopropoxide as single source precursor it is possible to form different TiOx phases on the surface after deposition: at low coverages, we observe large two-dimensional (2D) honeycomb-lattice Ti2O3 islands with a (2 x 2) registry with the substrate. Higher coverages are dominated by the formation of three-dimensional (3D) TiO2 structures. The TiO2 structures are atomically well ordered provided that the deposition temperature is high enough (500 degrees C). The ordered structure exhibits a LEED pattern characteristic for a rectangular surface unit cell. By performing the deposition at different temperatures it is possible to tune the balance between the 2D and 3D phases: Growth at 500 degrees C significantly favors the formation of 3D TiO2 islands as compared to growth at 200 degrees C and 300 degrees C. (C) 2013 Elsevier B.V. All rights reserved.
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18.
  • Raymand, David, 1981-, et al. (författare)
  • Water adsorption on stepped ZnO surfaces from MD simulation
  • 2010
  • Ingår i: Surface Science. - : Elsevier B.V.. - 0039-6028 .- 1879-2758. ; 604:9-10, s. 741-752
  • Tidskriftsartikel (refereegranskat)abstract
    • This work presents a ReaxFF reactive force-field for use in molecular dynamics simulations of the ZnO–water system. The force-field parameters were fitted to a data-set of energies, geometries and charges derived from quantum-mechanical B3LYP calculations. The presented ReaxFF model provides a good fit to the QM reference data for the ZnO–water system that was present in the data-set. The force-field has been used to study how water is adsorbed, molecularly or dissociatively, at monolayer coverage on flat and stepped ZnO surfaces, at three different temperatures (10 K, 300 K, and 600 K). The stepped surfaces were created by introducing steps along the (0 0 0 1)-direction on the -surface. Equilibrium between molecular and dissociated water was observed on the terraces, resulting in a half dissociated, half molecular water monolayer. The equilibrium between dissociated and molecular water on the surface was found to be reached quickly (<10 ps). When water molecules desorb and the coverage falls, the 1:1 water–hydroxyl ratio is maintained on terraces, while steps remain largely hydroxylated. The results show that structures that promote hydrogen bonding are favored and that the presence of steps promotes an increased level of hydroxylation in the water monolayers.
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19.
  • Sangiovanni, Davide, et al. (författare)
  • Ab-initio and classical molecular dynamics simulations of N2 desorption from TiN(001) surfaces
  • 2014
  • Ingår i: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 624, s. 25-31
  • Tidskriftsartikel (refereegranskat)abstract
    • Ab initio molecular dynamics simulations based on density functional theory show that N adatoms are chemisorbed in threefold sites close to a N surface atom and between the two diagonally opposed neighboring Ti surface atoms on TiN(001). The most probable N adatom reaction pathway, even in the presence of nearby N adatoms, is for the N adatom and N surface atom pair to first undergo several exchange reactions and then desorb as a N2 molecule, resulting in a surface anion vacancy, with an activation barrier Edes of 1.37 eV and an attempt frequency Ades = 3.4 × 1013 s− 1. Edes is essentially equal to the N adatom surface diffusion barrier, Es = 1.39 eV, while As is only three to four times larger than Ades, indicating that isolated N adatoms migrate for only short distances prior to N2 desorption. The probability of N2 desorption via recombination of N adatoms on TiN(001) is much lower due to repulsive adatom/adatom interactions at separations less than ~ 3 Å which rapidly increase to ~ 2 eV at a separation of 1.5 Å. We obtain good qualitative and quantitative agreement with the above results using the modified embedded atom method potential to perform classical molecular dynamics simulations.
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20.
  • Sangiovanni, Davide, et al. (författare)
  • Ti adatom diffusion on TiN(001) : Ab initio and classical molecular dynamics simulations
  • 2014
  • Ingår i: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 627, s. 34-41
  • Tidskriftsartikel (refereegranskat)abstract
    • Ab initio and classical molecular dynamics (AIMD and CMD) simulations reveal that Ti adatoms on TiN(001) surfaces migrate between neighboring fourfold hollow sites primarily along in-plane less than100greater than channels. less than100greater than and less than110greater than single jumps, as well as less than100greater than double jump rates, obtained directly from MD runs as a function of temperature, are used to determine diffusion activation energies Ea, and attempt frequencies A, for the three preferred Ti adatom migration pathways on TiN(001). From transition rates Aexp[-Ea / (k(B)T)], we determine adatom surface distribution probabilities as a function of time, which are used to calculate adatom diffusion coefficients D(T). AIMD and CMD predictions are consistent and complementary. Using CMD, we investigate the effect on the adatom jump rate of varying the phonon wavelength degrees of freedom by progressively increasing the supercell size. We find that long-wavelength phonons significantly contribute to increasing adatom mobilities at temperatures less than= 600 K, but not at higher temperatures. Finally, by directly tracking the Ti adatom mean-square displacement during CMD runs, we find that Ti adatom jumps are highly correlated on TiN(001), an effect that yields lower D-s values (D-s(corr)) than those estimated from uncorrelated transition probabilities. The temperature-dependent diffusion coefficient is D-s(corr) (T) = (4.5 x 10(-4) Cm-2 s(-1)) exp[-0.55 eV / (k(B)T)].
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21.
  • Schaefer, A., et al. (författare)
  • Chemistry of thin film formation and stability during praseodymium oxide deposition on Si(111) under oxygen-deficient conditions
  • 2010
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 604:15-16, s. 1287-1293
  • Tidskriftsartikel (refereegranskat)abstract
    • The growth of thin praseodymium oxide films on silicon (111) using small deposition rates under oxygen-deficient conditions was investigated in the range from submonolayer up to six monolayers coverage by transmission electron microscopy (TEM) and photoemission spectroscopy (PES). A detailed analysis of the silicon 2p and oxygen 1 s core level and valence band spectra reveals chemical reactions between deposited species, substrate, and the growing film. Silicate, silicide and oxide species are coexisting over the entire range of coverages investigated. Cross sectional TEM shows silicide inclusions extending from the surface several nanometers into the substrate and affecting the substrate band bending at the interface. The reactivity of the praseodymia overlayer leads to reactions in the as-deposited film even at room temperature and render it unstable. The article aims at providing a coherent picture of the chemistry proceeding during interface formation and film growth at low rates of deposition (0.06 nm/min). The results will be discussed in comparison to studies using higher rates, emphasizing the possibility of growth rate dependent reactions between substrate and deposited material and, consequently, distinctly different film compositions and structures for different rates of deposition. (C) 2010 Elsevier B.V. All rights reserved.
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22.
  • Shimizu, Kenichi, et al. (författare)
  • X-ray photoelectron spectroscopy of fast-frozen hematite colloids in aqueous solutions. 4. Coexistence of alkali metal (Na+, K+, Rb+, Cs+) and chloride ions
  • 2012
  • Ingår i: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 606:13-14, s. 1005-1009
  • Tidskriftsartikel (refereegranskat)abstract
    • Colloidal suspensions of hematite in contact with aqueous solutions of 50 mM alkali metal chloride electrolytes (NaCl, KCl, RbCl, CsCl) were investigated by cryogenic X-ray photoelectron spectroscopy (XPS) and electrophoretic mobility. Suspension pH values were varied from 2 to 11 in order to evaluate effects of positively- and negatively-charged hematite surfaces. XPS revealed coexisting cations and chloride ions both below and above the point of zero charge. Concentration profiles of adsorbed cations point to a Hofmeister series in the order of Na+ > K+ > Rb+ ≈ Cs+. Binding energies of photoelectrons emitted from electrolyte ions increased with pH at roughly 0.04 eV per pH unit. This shift was attributed to variations in the surface electric potential of hematite. This effect, compounded by rises in aliphatic carbon signals with pH, called for referencing of all spectra to the 530.0 eV oxide component of the hematite O1s spectrum. This departure from the traditional use of the external C 1s 285.0 eV peak is hereby proposed for cryogenic XPS studies of interfacial reactions involving hematite.
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23.
  • Simonov, K. A., et al. (författare)
  • Controllable oxidation of h-BN monolayer on Ir(111) studied by core-level spectroscopies
  • 2012
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 606:3-4, s. 564-570
  • Tidskriftsartikel (refereegranskat)abstract
    • The effect of atomic oxygen adsorption on the structure and electronic properties of monolayer hexagonal boron nitride (h-BN) grown on Ir(111) has been studied using near edge X-ray absorption fine structure spectroscopy (NEXAFS), photoelectron spectroscopy (PES), and low-energy electron diffraction (LEED). It has been shown that the oxidation of the h-BN monolayer occurs through a gradual substitution of N by O in the h-BN lattice. This process leads to the formation of defect sites corresponding to three different types of the B atom environment (BN3-xOx with x=1,2,3). The oxidation of the h-BN monolayer is very different from the case of graphene on Ir(111), where adsorption of atomic oxygen results mainly in the formation of epoxy groups [J. Phys. Chem. C. 115, 9568 (2011)]. A post-annealing of the h-BN monolayer after oxygen exposure results in further destruction of the B N bonds and formation of a B2O3-like structure. (C) 2011 Elsevier B.V. All rights reserved.
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24.
  • Sohail, Hafiz Muhammad, et al. (författare)
  • Electronic and atomic structures of the Ag induced √3x√3 superstructure on Ge(111)
  • 2014
  • Ingår i: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 625, s. 23-29
  • Tidskriftsartikel (refereegranskat)abstract
    • The Ag/Ge(111) surface together with Ag/Si(111) constitutes a set of surfaces that is ideally suited for fundamental studies related to low dimensional physics. We here focus on the atomic and electronic structures of the two-dimensional superstructure induced by Ag on Ge(111), a surface that is significantly less studied than the Si counterpart. Extensive information on the surface band structure obtained by angle resolved photoelectron spectroscopy (ARPES) is presented, complemented by atomic information from scanning tunneling microscopy (STM). The results reveal new findings that are important for the understanding of the Ag induced structure, acting as a prototype for semiconductor/metal interfaces. i) We have identified a new occupied surface band near the -point of the surface Brillouin zone. ii) The Ag/Ge(111) surface exhibits a partially occupied surface band, S1, with a parabolic-like shape at Γ¯. At low temperature (≈ 100 K) this band splits into two bands, S1U and S1D. The identification of two bands is significantly different from the case of Ag/Si(111) for which just one band has been reported. Besides these specific results, our extensive ARPES study reveals four surface bands at room temperature (RT), while five surface bands were identified at ≈ 100 K (LT). Room temperature empty state STM images show, depending on the tunneling bias, both honeycomb and hexagonal periodicities which are consistent with the honeycomb chained trimer and the in-equivalent trimer models, respectively.
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25.
  • Szamota-Leandersson, Karolina, et al. (författare)
  • Correlated development of a (2x2) reconstruction and acharge accumulation layer on the InAs(111)-Bi surface
  • 2011
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 605:1-2, s. 12-17
  • Tidskriftsartikel (refereegranskat)abstract
    • We have studied the formation of a Bi induced (2x2) reconstruction on the InAs(111)Bsurface. In connection to the development of the (2x2) reconstruction, a two dimensionalcharge accumulation layer located in the bottom of the InAs conduction band appears as seenthrough a photoemission structure at the Fermi level. Not well ordered Bi layers do not inducea charge accumulation. The Bi induced reconstruction reduces the polarisation of the pristinesurface and changes the initial charge distribution. InAsBi alloying occurs below the surfacewhere Bi act as charge donor leading to the charge accumulation layer.
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26.
  • Tholander, Christopher, 1986-, et al. (författare)
  • Effect of Al substitution on Ti, Al, and N adatom dynamics on TiN(001), (011), and (111) surfaces
  • 2014
  • Ingår i: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 630, s. 28-40
  • Tidskriftsartikel (refereegranskat)abstract
    • Substituting Al for Ti in TiN(001), TiN(011), and N- and Ti-terminated TiN(111) surfaces has significant effects on adatom surface energetics which vary strongly with the adatom species and surface orientation. Here, we investigate Ti, Al, and N adatom surface dynamics using density functional methods. We calculate adatom binding and diffusion energies with both a nudged elastic band and grid-probing techniques. The adatom diffusivities are analyzed within a transition-state theory approximation. We determine the stable and metastable Ti, Al, and N binding sites on all three surfaces as well as the lowest energy migration paths. In general, adatom mobilities are fastest on TiN(001), slower on TiN(111), and slowest on TiN(011). The introduction of Al has two major effects on the surface diffusivity of Ti and Al adatoms. First, Ti adatom diffusivity on TiN(001) is significantly reduced near substituted Al surface atoms; we observe a 200% increase in Ti adatom diffusion barriers out of fourfold hollow sites adjacent to Al surface atoms, while Al adatom diffusivity between bulk sites is largely unaffected. Secondly, on TiN(111), the effect is opposite; Al adatoms are slowed near the substituted Al surface atom, while Ti adatom diffusivity is largely unaffected. In addition, we note the importance of magnetic spin polarization on Ti adatom binding energies and diffusion path. These results are of relevance for the atomistic understanding of Ti1-xAlxN alloy and Ti1-xAlxN/TiN multilayer thin-film growth processes.
  •  
27.
  • Walle, L. E., et al. (författare)
  • Competing water dissociation channels on rutile TiO2(110)
  • 2014
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 621, s. 77-81
  • Tidskriftsartikel (refereegranskat)abstract
    • The interplay between two different water dissociation channels on rutile TiO2(110) was studied with the use of synchrotron radiation photoelectron spectroscopy. It was found that water dissociation at oxygen vacancies competes with water dissociation on defect-free regions such that one vacancy assisted dissociation event cancels one dissociation event on defect-free regions. The quenching affects the thermally most stable dissociated species that form at low coverage on the defect free surface but does not affect the stability of molecular water. As a result, molecular adsorption becomes favored at low coverage on a surface where all vacancies have been hydroxylated. The presence of competitive dissociation channels rationalizes the difficulties in identifying dissociated species on defect-free regions in previous studies. (C) 2013 Elsevier B.V. All rights reserved.
  •  
28.
  • Watcharinyanon, Somsakul, et al. (författare)
  • Hydrogen intercalation of graphene grown on 6H-SiC(0001)
  • 2011
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 605:17-18, s. 1662-1668
  • Tidskriftsartikel (refereegranskat)abstract
    • Atomic hydrogen exposures on a monolayer graphene grown on the SiC(0001) surface are shown to result in hydrogen intercalation. The hydrogen intercalation induces a transformation of the monolayer graphene and the carbon buffer layer to bi-layer graphene without a buffer layer. The STM, LEED, and core-level photoelectron spectroscopy measurements reveal that hydrogen atoms can go underneath the graphene and the carbon buffer layer and bond to Si atoms at the substrate interface. This transforms the buffer layer into a second graphene layer. Hydrogen exposure results initially in the formation of bi-layer graphene islands on the surface. With larger atomic hydrogen exposures, the islands grow in size and merge until the surface is fully covered with bi-layer graphene. A (root 3 x root 3)R30 degrees periodicity is observed on the bi-layer areas. ARPES and energy filtered XPEEM investigations of the electron band structure confirm that after hydrogenation the single pi-band characteristic of monolayer graphene is replaced by two pi-bands that represent bi-layer graphene. Annealing an intercalated sample, representing bi-layer graphene, to a temperature of 850 degrees C. or higher, re-establishes the monolayer graphene with a buffer layer on SiC(0001). (C) 2011 Elsevier B.V. All rights reserved.
  •  
29.
  • Watcharinyanon, Somsakul, et al. (författare)
  • Rb and Cs deposition on epitaxial graphene grown on 6H-SiC(0001)
  • 2011
  • Ingår i: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 605:21-22, s. 1918-1922
  • Tidskriftsartikel (refereegranskat)abstract
    • Epitaxial graphene grown on the silicon-terminated SiC(0001) is doped by alkali metals deposited on the surface. The synchrotron radiation based photoelectron spectroscopy results presented reveal that Rb and Cs deposited on monolayer graphene samples, grown on the silicon-terminated SiC(0001), gives rise to n-type doping, i.e. electron transfer from the metal to the graphene layer. The Dirac point of the single pi-cone is found to shift downwards away from the Fermi level by ca. 1.0 eV after alkali metal depositions. The adsorbed Rb and Cs atoms do initially disrupt the bonds in the graphene layer but after heating the bonds appear to be recreated when the alkali metal coverage start to decrease due to thermal desorption. None of these two alkali metals do intercalate into the graphene and buffer layer after depositing at room temperature or after heating. This is contrary to the case of Li where intercalation occurred directly after deposition.
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30.
  • Watcharinyanon, Somsakul, et al. (författare)
  • Studies of Li intercalation of hydrogenated graphene on SiC(0001)
  • 2012
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 606:3-4, s. 401-406
  • Tidskriftsartikel (refereegranskat)abstract
    • The effects of Li deposition on hydrogenated bilayer graphene on SiC(0001) samples, i.e. on quasi-freestanding bilayer graphene samples, are studied using low energy electron microscopy, micro-low-energy electron diffraction and photoelectron spectroscopy. After deposition, some Li atoms form islands on the surface creating defects that are observed to disappear after annealing. Some other Li atoms are found to penetrate through the bilayer graphene sample and into the interface where H already resides. This is revealed by the existence of shifted components, related to H–SiC and Li–SiC bonding, in recorded core level spectra. The Dirac point is found to exhibit a rigid shift to about 1.25 eV below the Fermi level, indicating strong electron doping of the graphene by the deposited Li. After annealing the sample at 300–400 °C formation of LiH at the interface is suggested from the observed change of the dipole layer at the interface. Annealing at 600 °C or higher removes both Li and H from the sample and a monolayer graphene sample is re-established. The Li thus promotes the removal of H from the interface at a considerably lower temperature than after pure H intercalation.
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31.
  • Xia, Chao, et al. (författare)
  • Detailed studies of Na intercalation on furnace-grown graphene on 6H-SiC(0001)
  • 2013
  • Ingår i: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 613, s. 88-94
  • Tidskriftsartikel (refereegranskat)abstract
    • The effects induced by Na deposited on furnace grown graphene on SiC(0001) and after subsequent annealing are investigated using LEEM, mu-LEED, mu-PES and XPEEM. Intercalation in between carbon layers and at the interface is observed to occur both on the 1 ML and 2 ML areas directly after Na deposition. Annealing at a temperature around 100 degrees C is found to strongly promote Na intercalation. Exposure to the electron beam or the focused synchrotron radiation in the LEEM/XPEEM is also found to promote the intercalation, which is confirmed to begin at domain boundaries between the 1 ML and 2 ML areas, and also as stripe/streak-like features on the 1 ML areas. The XPEEM data show Na adsorption on the surface and intercalation at the interface to be quite non-uniform. When annealing at higher temperatures Na starts to de-intercalate and leave the sample, but Na is still detectable on the sample after annealing at 240 degrees C.
  •  
32.
  • Önsten, Anneli, et al. (författare)
  • SO2 interaction with Zn(0001) and ZnO(0001) and the influence of water
  • 2013
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 608, s. 31-43
  • Tidskriftsartikel (refereegranskat)abstract
    • Photoelectron spectroscopy has been used to study room temperature adsorption of sulfur dioxide on clean and water exposed (0001) surfaces of zinc and zinc oxide. Water has no significant effect either on clean or on SO2 exposed Zn(0001) at the low water pressures used (p < 10(-7) mbar). In the Case of the zinc-terminated ZnO(0001) surface, however, water adsorbs dissociatively and OH groups are shown to have a considerable effect on SO2 surface reactions. A strong oxidation reaction occurs between Zn(0001) and SO2 giving various sulfur containing species. On ZnO(0001), SO2 interacts mainly with oxygen sites giving SO3 or SO4 species. It is shown that the ZnO(0001) sample preparation procedure can have large effects on surface chemical and physical properties. Samples cleaned by four different preparation procedures are investigated, namely sputtering only and sputtering followed by annealing at 450 degrees C, 530 degrees C and 600-650 degrees C. Annealing at 600 degrees C leads to a transition from a partly OH-terminated surface to a triangularly structured surface free from OH groups. Adsorption of SO2 on the latter surface leads to a decreased surface conductivity, which hampers photoemission measurements. Water is shown to block SO2 adsorption sites on both 450 degrees C and 530 degrees C annealed samples. On the latter sample. SO2 reduction has been observed to a small extent on the clean surface and to a larger extent when the surface is prehydroxylated. Here, we speculate that water, similar to hydrogen, generates surface zinc clusters on ZnO(0001). Zinc clusters could enable charge transfer to the antibonding LUMO of the SO2 molecule and subsequent dissociation.
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33.
  • Ataman, Evren, et al. (författare)
  • Adsorption of L-cysteine on rutile TiO2(110)
  • 2011
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 605:1-2, s. 179-186
  • 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.
  •  
34.
  • Blomberg, Sara, et al. (författare)
  • A high pressure X-ray photoelectron spectroscopy study of oxidation and reduction of Rh(100) and Rh nanoparticles
  • 2014
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 628, s. 153-158
  • 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.
  •  
35.
  • Blomberg, Sara, et al. (författare)
  • Generation and oxidation of aerosol deposited PdAg nanoparticles
  • 2013
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 616, s. 186-191
  • Tidskriftsartikel (refereegranskat)abstract
    • PdAg nanoparticles with a diameter of 10 nm have been generated by an aerosol particle method, and supported on a silica substrate. By using a combination of X-ray Energy Dispersive Spectroscopy and X-ray Photoelectron Spectroscopy it is shown that the size distribution of the particles is narrow and that the two metals form an alloy with a mixture of 75% Pd and 25% Ag. Under oxidizing conditions, Pd is found to segregate to the surface and a thin PdO like oxide is formed similar to the surface oxide previously reported on extended PdAg and pure Pd surfaces. (C) 2013 Elsevier B.V. All rights reserved.
  •  
36.
  • Blomberg, Sara, et al. (författare)
  • Structure of the Rh2O3(0001) surface
  • 2012
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 606:17-18, s. 1416-1421
  • 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.
  •  
37.
  • Fernandes, V. R., et al. (författare)
  • Reduction behavior of oxidized Pd(100) and Pd75Ag25(100) surfaces using CO
  • 2014
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 621, s. 31-39
  • Tidskriftsartikel (refereegranskat)abstract
    • The reduction of the (A root 5x root 5)R27 degrees surface oxide on Pd(100) and Pd75Ag25(100) surfaces by CO has been investigated by high-resolution X-ray photoelectron spectroscopy in combination with kinetic analysis using the Avrami-Erofeev theory. For both surfaces we observed faster reduction with increasing temperature. Kinetic analysis indicates that the reduction process is phase boundary controlled for Pd(100) in the temperature range investigated, from 30 degrees C to 120 degrees C On Pd75Ag25(100) the surface oxide reduction is significantly slower compared to Pd(100). Also in this case, a phase boundary controlled reduction of the surface oxide is observed at temperatures of 120 C and above, while at T <= 70 degrees C the reduction is found to be diffusion limited. Density functional theory calculations show that the presence of silver in the outermost surface layer significantly increases the CO diffusion barriers on the reduced areas, supporting a diffusion limited reduction process for Pd75Ag25(100) at lower temperatures. (C) 2013 Elsevier B.V. All rights reserved.
  •  
38.
  • Foehlisch, A., et al. (författare)
  • Charge transfer dynamics in molecular solids and adsorbates driven by local and non-local excitations
  • 2012
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 606:11-12, s. 881-885
  • Tidskriftsartikel (refereegranskat)abstract
    • Charge transfer pathways and charge transfer times in molecular films and in adsorbate layers depend both on the details of the electronic structure as well as on the degree of the initial localization of the propagating excited electronic state. For C6F6 molecular adsorbate films on the Cu(111) surface we determined the interplay between excited state localization and charge transfer pathways. In particular we selectively prepared a free-particle-like LUMO band excitation and compared it to a molecularly localized core-excited C1s -> pi* C6F6 LUMO state using time-resolved two-photon photoemission (tr-2PPE) and core-sole-clock (CHC) spectroscopy, respectively. For the molecularly localized core-excited C1s -> pi* C6F6 LUMO state, we separate the intramolecular dynamics from the charge transfer dynamics to the metal substrate by taking the intramolecular dynamics of the free C6F6 molecule into account Our analysis yields a generally applicable description of charge transfer within molecular adsorbates and to the substrate. (C) 2011 Published by Elsevier B.V.
  •  
39.
  • Gustafson, Johan, et al. (författare)
  • Depth-profiling the composition of bimetallic nanoparticles using medium energy ion scattering
  • 2011
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 605:1-2, s. 220-224
  • Tidskriftsartikel (refereegranskat)abstract
    • The near monolayer depth resolution of medium energy ion scattering is utilized to develop a probe of the depth dependent composition of bimetallic nanoparticles supported on planar oxide supports. The approach fits spectra of scattered ion intensity versus ion energy at well-defined scattering angles taking into account the asymmetric line shape in such spectra and also the depth dependent loss processes encountered by incident ions as they pass through the bimetallic particles. (C) 2010 Elsevier B.V. All rights reserved.
  •  
40.
  • Hilner, Emelie, et al. (författare)
  • Surface structure and morphology of InAs(111)B with/without gold nanoparticles annealed under arsenic or atomic hydrogen flux
  • 2010
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 604:3-4, s. 354-360
  • Tidskriftsartikel (refereegranskat)abstract
    • We have investigated the structure and morphology of the InAs(1 1 1)B surface using Low Energy Electron Diffraction (LEED), Scanning Tunneling Microscopy (STM) and Scanning Electron Microscopy (SEM). The surface was prepared by annealing in the presence of an arsenic or atomic hydrogen pressure. A (2 x 2) reconstruction that changes into a (1 x 1) unreconstructed surface after prolonged annealing was observed irrespective of preparation method, while the surface morphology was distinctly different in the two cases. Detailed atomic scale models are proposed to explain the behavior. Deposition of An aerosol nanoparticles on the sample prior to annealing was found to have no effect on the surface reconstruction. The Au particles were found to sink into the surface. (C) 2009 Elsevier B.V. All rights reserved.
  •  
41.
  • Iwasaki, Takayuki, et al. (författare)
  • Formation and structural analysis of twisted bilayer graphene on Ni(111) thin flims
  • 2014
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 625, s. 44-49
  • Tidskriftsartikel (refereegranskat)abstract
    • We synthesized twisted bilayer graphene on single crystalline Ni(111) thin films to analyze the statistical twist angle distribution on a large scale. The twisted bilayer graphene was formed by combining two growth methods, namely the catalytic surface reaction of hydrocarbons and carbon segregation from Ni. Low energy electron diffraction (LEED) investigations directly revealed dominant twist angles of 13 degrees, 22 degrees, 38 degrees, and 47 degrees. We show that the angle distribution is closely related to the sizes of Moire superlattices which form at commensurate rotation angles. In addition to the commensurate angles, quasi-periodic Moire structures were also formed in the vicinity of the dominant angles, confirmed by microscopic observations with low energy electron microscopy and scanning tunneling microscopy (STM). The quasi-periodic Moire patterns are presumably caused by insufficient mobility of carbon atoms during the segregation growth while cooling. Micro-LEED studies reveal that the size of single twisted domains is below 400 nm. Atomic-scale characterization by STM indicates that the twisted layer grown by segregation is located underneath the layer grown by surface reaction, i.e. between the Ni surface and the top single-crystal graphene layer. (C) 2014 Elsevier B.V. All rights reserved.
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42.
  • Klacar, Simon, 1984, et al. (författare)
  • Facile NOx interconversion over preoxidized Ag(111)
  • 2013
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 617, s. 167-174
  • Tidskriftsartikel (refereegranskat)abstract
    • X-ray photoelectron spectroscopy and density functional theory calculations are used to investigate NO adsorption at low (100 K) and room temperature (RT) over preoxidized Ag(111). At 100 K, the data indicates presence of NO and N2O2, with little or no nitrite/nitrate formation. This is consistent with the calculated surface core level shifts and the pronounced barrier for nitrite formation. At RT, the recorded spectra indicate a complex interconversion between adsorbed species with an initial formation of a p(4 x 4) nitrate overlayer. With increasing NO pressure, the experimental results are best rationalized by partial nitrate decomposition into nitrites and subsequent NO physisorption, which leads to the formation of N2O3-like species.
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43.
  • Lundgren, Edvin, et al. (författare)
  • Surface composition of clean and oxidized Pd75Ag25(100) from photoelectron spectroscopy and density functional theory calculations
  • 2012
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 606:23-24, s. 1777-1782
  • Tidskriftsartikel (refereegranskat)abstract
    • High resolution photoelectron spectroscopy and density functional theory calculations have been used to study the composition of clean and oxidized Pd75Ag25( 100). The results for the clean surface confirm earlier reports of surface segregation by Wouda et al. (1998), where the top most layers are rich in Ag. The Pd 3d core level component from the surface region is observed at higher binding energies than the contribution from the bulk which is found to be a signature of Pd embedded in Ag. Low energy electron diffraction and scanning tunneling microscopy measurements reveal that oxidation of the Pd75Ag25(100) surface results in a (root 5 x root 5)R27 degrees-O structure similar to the one reported for Pd(100). The calculations suggest that the stable structure is a PdO(101) monolayer supported on a (100) surface rich in Ag at the interface to the stoichiometric alloy. The calculated core level shifts for the oxidized surface are in good agreement with the experimental observations.
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44.
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45.
  • Mikkelsen, Anders, et al. (författare)
  • Surface science of free standing semiconductor nanowires
  • 2013
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 607, s. 97-105
  • Tidskriftsartikel (refereegranskat)abstract
    • Due to the wide range of possible applications, there is a strong current interest in semiconductor nanowires, that began around the start of the millennia. As a result, a number of important new surface science challenges of both fundamental and practical nature have emerged. Surfaces govern important processes for nanowire growth, physical properties and the ability of nanowires to interact with their surroundings. However, experimental studies of nanowire surfaces are difficult as many important surface science tools are not well suited to access these highly one dimensional objects. Still, recent studies has shown that, by designing experiments in an appropriate fashion, it is possible to both uniquely contribute to the understanding of the seed particle driven growth of nanowires and to explore the surfaces of nanowires with crystal structures and materials combinations not found in the bulk. In this prospective, recent results obtained using surface sensitive electron microscopy/spectroscopy and scanning tunneling microscopy will be highlighted and future directions will be discussed. (c) 2012 Elsevier B.V. All rights reserved.
  •  
46.
  • Monsen, A. F., et al. (författare)
  • Surface stoichiometry of La0.7Sr0.3MnO3 during in vacuo preparation; A synchrotron photoemission study
  • 2012
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 606:17-18, s. 1360-1366
  • Tidskriftsartikel (refereegranskat)abstract
    • We present a study of the surface stoichiometry and contamination of La0.7Sr0.3MnO3 thin films following exposure to air and subsequent in vacuo preparation. Samples were studied using both soft X-ray synchrotron photoemission (hv = 150 to 350 eV) and traditional Mg-K alpha XPS (hv = 1253.6 eV) whilst annealing incrementally to approximate to 510 degrees C in low pressures of O-2. In all cases, a Mn depleted and Sr rich surface oxide layer is observed, it is of reduced crystalline quality and is charge depleted. This surface layer is weakly affected by subsequent annealing, and is partially reversed by annealing in higher O-2 pressure. Surface carbon contamination is incrementally removed by annealing at increased temperatures, and at 270 degrees C, it is reduced to approximate to 0.4% of the topmost unit cell. The modification of the surface stoichiometry and electronic properties is consistent with the reported loss of magnetic properties in thin LSMO films. (C) 2012 Elsevier B.V. All rights reserved.
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47.
  • Shavorskiy, Andrey, et al. (författare)
  • Surface chemistry of glycine on Pt{111} in different aqueous environments
  • 2013
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 607, s. 10-19
  • Tidskriftsartikel (refereegranskat)abstract
    • Adsorption of glycine on Pt{111} under UHV conditions and in different aqueous environments was studied by XPS (UHV and ambient pressure) and NEXAFS. Under UHV conditions, glycine adsorbs in its neutral molecular state up to about 0.15 ML Further deposition leads to the formation of an additional zwitterionic species, which is in direct contact with the substrate surface, followed by the growth of multilayers, which also consist of zwitterions. The neutral surface species is most stable and decomposes at 360 K through a multi-step process which includes the formation of methylamine and carbon monoxide. When glycine and water are co-adsorbed in UHV at low temperatures (<170 K) inter-layer diffusion is inhibited and the surface composition depends on the adsorption sequence. Water adsorbed on top of a glycine layer does not lead to significant changes in its chemical state. When glycine is adsorbed on top of a pre-adsorbed chemisorbed water layer or thick ice layer, however, it is found in its zwitterionic state, even at low coverage. No difference is seen in the chemical state of glycine when the layers are exposed to ambient water vapor pressure up to 0.2 Torr at temperatures above 300 K. Also the decomposition temperature stays the same, 360 K, irrespective of the water vapor pressure. Only the reaction path of the decomposition products is affected by ambient water vapor. (C) 2012 Elsevier B.V. All rights reserved.
  •  
48.
  • Shipilin, Mikhail, et al. (författare)
  • Quantitative surface structure determination using in situ high-energy SXRD: Surface oxide formation on Pd(100) during catalytic CO oxidation
  • 2014
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 630, s. 229-235
  • Tidskriftsartikel (refereegranskat)abstract
    • We have performed a quantitative structure determination of the SQRT(5×5)R27° surface oxide, formed on Pd(100) under semi-realistic conditions for catalytic CO oxidation, using in situ high-energy surface X-ray diffraction. We describe the experiment and the extraction of quantitative data in detail. The structural results are in agreement with previous reports of a system consisting of a single layer of PdO(101) formed in pure O2 on top of Pd(100) and studied under ultra high vacuum conditions.
  •  
49.
  • Song, F., et al. (författare)
  • Extracting the near surface stoichiometry of BiFe0.5Mn0.5O3 thin films; a finite element maximum entropy approach
  • 2012
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 606:23-24, s. 1771-1776
  • Tidskriftsartikel (refereegranskat)abstract
    • The surface and near-surface chemical composition of BiFe0.5Mn0.5O3 has been studied using a combination of low photon energy synchrotron photoemission spectroscopy, and a newly developed maximum entropy finite element model from which it is possible to extract the depth dependent chemical composition. In the uppermost few unit cells, an overabundance of Bi, and a deficiency of Fe and Mn are observed. In deeper layers, the measurements are consistent with bulk-like stoichiometry. Additionally, a definitive identification of all the observed species together with their abundance and depth dependence is given, and the mixed Fe and Mn valencies are estimated. In addition to the expected bulk valencies Mn3+ and Fe3+, some Fe2+ and a small amount of Mn4+ are also observed. The maximum entropy finite element model demonstrated here is also discussed in more general terms and its potential application to the broader field of perovskite thin films is made apparent. (C) 2012 Elsevier B.V. All rights reserved.
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50.
  • Syres, K. L., et al. (författare)
  • Pyrocatechol as a surface capping molecule on raffle TiO2 (110)
  • 2012
  • Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 606:3-4, s. 273-277
  • Tidskriftsartikel (refereegranskat)abstract
    • A 'cap and dip' method of adsorbing ruthenium di-2,2'-bipyridy1-4,4'-dicarboxylic acid diisocyanate (N3 dye) on a rutile TiO2 (110) surface was investigated using pyrocatechol as a capping molecule. This method involves cleaning the rutile surface in ultra-high vacuum (UHV), depositing pyrocatechol onto the surface to 'cap' the adsorption sites, removing from vacuum, 'dipping' in an N3 dye solution and returning to vacuum. Photoemission measurements following the return of the crystal to vacuum suggest that the pyrocatechol keeps the surface free from contamination on exposure to atmosphere. Photoemission spectra also indicate that the pyrocatechol capping molecules are replaced by the N3 dye in solution and that the N3 dye is adsorbed intact on the nitile TiO2 (110) surface. This technique may allow other large molecules, which are thermally unstable to evaporation in UHV, to be easily deposited onto TiO2 surfaces. (C) 2011 Elsevier B.V. All rights reserved.
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