| 1. |
- Ahola-Tuomi, M., et al.
(författare)
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Properties of self-assembled Bi nanolines on InAs(100) studied by core-level and valence-band photoemission, and first-principles calculations
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 83:24, s. 245401-
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Tidskriftsartikel (refereegranskat)abstract
- We have studied self-assembled bismuth (Bi) nanolines on the Bi-terminated InAs(100) surface by core-level and valence-band photoelectron spectroscopy, and ab initio first-principles calculations. A structural model for this intriguing surface is suggested based on the comparison of the measured and calculated core-level shifts. Also, the atomic origins for the core-level shifts are proposed based on the calculations. A clear peak related to this surface was observed in the valence band 0.34 eV below the Fermi level, which can be used as a "fingerprint" of a well-ordered Bi/InAs(100) nanoline surface.
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| 2. |
- Arman, Alif, et al.
(författare)
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Adsorption of hydrogen on stable and metastable Ir(100) surfaces
- 2017
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 656, s. 66-76
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Using the combination of high resolution core level spectroscopy and density functional theory we present adetailed spectroscopic study for all clean and hydrogen covered phases of Ir(100). The results are complementedby an investigation of the hydrogen desorption process from various phases using temperature programmeddesorption spectroscopy and scanning tunneling microscopy. In total, all experimentally determined core levelshifts match very well with those predicted by density functional theory based on established structural models.In particular, we find for the (bridge site) adsorption on the unreconstructed 1×1 phase that the initial core levelshift of surface Ir atoms is altered by +0.17 eV for each Ir–H bond formed. In the submonolayer regime we findevidence for island formation at low temperatures. For the H-induced deconstructed 5×1-H phase we identifyfour different surface core level shifts with two of them being degenerate. Finally, for the reconstructed 5×1-hexphase also four surface components are identified, which undergo a rather rigid core level shift of +0.15 eV uponhydrogen adsorption suggesting a similarly homogeneous charge transfer to all Ir surface atoms.Thermodesorption experiments for the 5×1-H phase reveal two different binding states for hydrogenindependent of the total coverage. We conclude that the surface always separates into patches of fully covereddeconstructed and uncovered reconstructed phases. We could also show by tunneling microscopy that with thedesorption of the last hydrogen atom from the deconstructed unit cell the surface instantaneously reverts intothe reconstructed state. Eventually, we could determine the saturation coverage upon molecular adsorption forall phases to be θmax = 1.0 ML1×1−H , θ = 0.8 ML max5×1−H , and θ ≥ 1.0 ML max5×1−hex−H .
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| 3. |
- 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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| 4. |
- 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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| 5. |
- 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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| 6. |
- Baran, J. D., et al.
(författare)
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Theoretical and experimental comparison of SnPc, PbPc, and CoPc adsorption on Ag(111)
- 2010
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121 .- 1550-235X. ; 81:7
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Tidskriftsartikel (refereegranskat)abstract
- A combination of normal-incidence x-ray standing-wave (NIXSW) spectroscopy, x-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and density-functional theory (DFT) has been used to investigate the interaction of a number of phthalocyanine molecules (specifically, SnPc, PbPc, and CoPc) with the Ag(111) surface. The metal-surface distances predicted by the DFT calculations for SnPc/Ag(111) (2.48 angstrom) and CoPc/Ag(111) (2.88 angstrom) are in good agreement with our NIXSW experimental results for these systems (2.31 +/- 0.09 and 2.90 +/- 0.05 angstrom, respectively). Good agreement is also found between calculated partial density-of-states plots and STM images of CoPc on Ag(111). Although the DFT and Pb 4f NIXSW results for the Pb-Ag(111) distance are similarly in apparently good agreement, the Pb 4f core-level data suggest that a chemical reaction between PbPc and Ag(111) occurs due to the annealing procedure used in our experiments and that the similarity of the DFT and Pb 4f NIXSW values for the Pb-Ag(111) distance is likely to be fortuitous. We interpret the Pb 4f XPS data as indicating that the Pb atom can detach from the PbPc molecule when it is adsorbed in the "Pb-down" position, leading to the formation of a Pb-Ag alloy and the concomitant reduction in Pb from a Pb2+ state (in bulklike films of PbPc) to Pb-0. In contrast to SnPc, neither PbPc nor CoPc forms a well-ordered monolayer on Ag(111) via the deposition and annealing procedures we have used. Our DFT calculations show that each of the phthalocyanine molecules donate charge to the silver surface, and that back donation from Ag to the metal atom (Co, Sn, or Pb) is only significant for CoPc.
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| 7. |
- Bliem, Roland, et al.
(författare)
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Adsorption and incorporation of transition metals at the magnetite Fe3O4(001) surface
- 2015
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 92:7
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Tidskriftsartikel (refereegranskat)abstract
- The adsorption of Ni, Co, Mn, Ti, and Zr at the (root 2 x root 2)R45 degrees-reconstructed Fe3O4(001) surface was studied by scanning tunneling microscopy, x-ray and ultraviolet photoelectron spectroscopy, low-energy electron diffraction (LEED), and density functional theory (DFT). Following deposition at room temperature, metals are either adsorbed as isolated adatoms or fill the subsurface cation vacancy sites responsible for the (root 2 x root 2)R45 degrees reconstruction. Both configurations coexist, but the ratio of adatoms to incorporated atoms depends on the metal; Ni prefers the adatom configuration, Co and Mn form adatoms and incorporated atoms in similar numbers, and Ti and Zr are almost fully incorporated. With mild annealing, all adatoms transition to the incorporated cation configuration. At high coverage, the (root 2 x root 2)R45 degrees reconstruction is lifted because all subsurface cation vacancies become occupied with metal atoms, and a (1 x 1) LEED pattern is observed. DFT+U calculations for the extreme cases, Ni and Ti, confirm the energetic preference for incorporation, with calculated oxidation states in good agreement with photoemission experiments. Because the site preference is analogous to bulk ferrite (XFe2O4) compounds, similar behavior is likely to be typical for elements forming a solid solution with Fe3O4.
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| 8. |
- Borck, O., et al.
(författare)
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Adsorption of methylamine on Ni3Al(111) and NiAl(110)-a high resolution photoelectron spectroscopy and density functional theory study
- 2010
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Ingår i: Journal of Physics: Condensed Matter. - : IOP Publishing. - 1361-648X .- 0953-8984. ; 22:39
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Tidskriftsartikel (refereegranskat)abstract
- Methylamine adsorption on the ordered Ni3Al(111) and NiAl(110) surfaces has been investigated by high resolution photoelectron spectroscopy and density functional theory calculations. Methylamine adsorbs molecularly at both surfaces at low temperature (90 K). The experiments show that methylamine interacts with the surface aluminium atoms on both surfaces, resulting in a positive binding energy shift relative to the Al 2p bulk contributions. A shift towards lower binding energy is also observed on NiAl(110) attributed to first and second layer surface Al atoms not bonded to methylamine. According to total energy calculations methylamine binds through its N atom to Al on-top sites on NiAl(110) while the Ni on-top site is found to be slightly preferred over the Al on-top site on Ni3Al(111). Calculated adsorbate induced shifts are, however, in good agreement with the experimental values only when methylamine is situated in the Al on-top site on both surfaces. In both cases, a lone pair bonding mechanism is found.
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| 9. |
- Britton, Andrew J., et al.
(författare)
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Charge transfer between the Au(111) surface and adsorbed C-60: Resonant photoemission and new core-hole decay channels
- 2010
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 133:9
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Tidskriftsartikel (refereegranskat)abstract
- The interaction of C-60 with the Au(111) surface has been investigated using synchrotron radiation-based electron spectroscopy. Resonant photoelectron spectroscopy and autoionization spectroscopy have been used to probe the coupling between the molecule and the substrate. Three distinct high energy spectator Auger features were observed that are only evident for a monolayer of C-60 chemisorbed to the Au (111) surface and not a multilayer or the clean surface itself. Combined with C 1s x-ray absorption and valence band spectra, the data suggest a decay process not previously reported for this system. This is a spectator decay channel involving electrons transferred from the gold substrate to the adsorbed molecule, either in the ground state or during the timescale of the core-hole lifetime. Both possibilities are considered in the interpretation of the results, which support, on balance, a ground state charge transfer. (C) 2010 American Institute of Physics. [doi:10.1063/1.3488299]
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| 10. |
- Cant, David J. H., et al.
(författare)
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Surface Properties of Nanocrystalline PbS Films Deposited at the Water-Oil Interface: A Study of Atmospheric Aging
- 2015
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 31:4, s. 1445-1453
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Tidskriftsartikel (refereegranskat)abstract
- Nanocrystalline thin films of PbS are obtained in a straightforward reaction by precipitation at the interface between toluene (containing a Pb precursor) and water (containing Na2S). Lead thiobiuret [Pb(SON(CN(i)Pr-2)(2))(2)] and lead diethyldithiocarbamate [Pb(S2CNEt2)(2)] precursors are used. The films are characterized by X-ray diffraction and electron microscopy, revealing typical particle sizes of 10-40 nm and preferred (200) orientation. Synchrotron-excited depth-profiling X-ray photoelectron spectroscopy (XPS) is used to determine the depth-dependent chemical composition as a function of surface aging in air for periods of up to 9 months. The as-synthesized films show a 1:1 Pb/S composition. Initial degradation occurs to form lead hydroxide and small quantities of surface-adsorbed -SH species. A lead-deficient Pb1-xS phase is produced as the aging proceeds. Oxidation of the sulfur occurs later to form sulfite and sulfate products that are highly localized at the surface layers of the nanocrystals. These species show logarithmic growth kinetics, demonstrating that the sulfite/sulfate layer acts to passivate the nanocrystals. Our results demonstrate that the initial reaction of the PbS nanocrystals (forming lead hydroxide) is incongruent. The results are discussed in the context of the use of PbS nanocrystals as light-harvesting elements in next-generation solar technology.
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