| 1. |
- 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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| 2. |
- 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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| 3. |
- 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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| 4. |
- Faraggi, M. N., et al.
(författare)
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Role of Deprotonation and Cu Adatom Migration in Determining the Reaction Pathways of Oxalic Acid Adsorption on Cu(111)
- 2011
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 115:43, s. 21177-21182
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Tidskriftsartikel (refereegranskat)abstract
- Scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and first principles theoretical calculations have been used to gain insight into the fundamental processes involved in the adsorption and self-assembly of oxalic acid on Cu(111). The experimental data demonstrate that several reaction pathways are involved in the chemisorption of oxalic acid on Cu(111), one of which leads to deprotonation of the acid into oxalate molecules that form ordered structures on the surface. Theoretical calculations indicate that the adsorption of oxalate molecules is not stable on the surface unless copper adatoms are taken into consideration. Coordination with copper adatoms prevents oxalate molecules from getting closer to the substrate, precluding the expected decomposition of oxalate into carbon dioxide. Our results, thus, suggest that the 2D gas of diffusing copper adatoms might play a very important role in the self-assembly of the molecules not only by catalyzing the deprotonation of oxalic acid but also by decreasing the surface reactivity.
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| 5. |
- Isvoranu, Cristina, et al.
(författare)
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Adsorption of ammonia on multilayer iron phthalocyanine.
- 2011
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 134:11
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Tidskriftsartikel (refereegranskat)abstract
- The adsorption of ammonia on multilayers of well-ordered, flat-lying iron phthalocyanine (FePc) molecules on a Au(111) support was investigated by x-ray photoelectron spectroscopy. We find that the electron-donating ammonia molecules coordinate to the metal centers of iron phthlalocyanine. The coordination of ammonia induces changes of the electronic structure of the iron phthalocyanine layer, which, in particular, lead to a modification of the FePc valence electron spin.
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| 6. |
- Isvoranu, Cristina, et al.
(författare)
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Ammonia adsorption on iron phthalocyanine on Au(111): Influence on adsorbate-substrate coupling and molecular spin.
- 2011
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 134:11
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Tidskriftsartikel (refereegranskat)abstract
- The adsorption of ammonia on Au(111)-supported monolayers of iron phthalocyanine has been investigated by x-ray photoelectron spectroscopy, x-ray absorption spectroscopy, and density functional theory calculations. The ammonia-induced changes of the x-ray photoemission lines show that a dative bond is formed between ammonia and the iron center of the phthalocyanine molecules, and that the local spin on the iron atom is quenched. This is confirmed by density functional theory, which also shows that the bond between the iron center of the metalorganic complex and the Au(111) substrate is weakened upon adsorption of ammonia. The experimental results further show that additional adsorption sites exist for ammonia on the iron phthalocyanine monolayer.
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| 7. |
- Isvoranu, Cristina, et al.
(författare)
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Comparison of the Carbonyl and Nitrosyl Complexes Formed by Adsorption of CO and NO on Mono layers of Iron Phthalocyanine on Au(111)
- 2011
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 115:50, s. 24718-24727
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Tidskriftsartikel (refereegranskat)abstract
- The interaction between monolayers of iron phthalocyanine on a Au(111) support and carbon monoxide and nitric oxide is studied by X-ray photoelectron spectroscopy and density functional theory calculations. We find several carbon monoxide and nitric oxide adsorbate species, and in particular species that bind to the iron ions of the phthalocyanine compound. The formation of phthalocyanine carbonyl and nitrosyl complexes leads to a redistribution of the electrons in the iron 3d levels resulting in a change of the spin state. Further, the adsorption results in an electronic decoupling of the iron phthalocyanine adsorbates from the substrate. The extent of the spin change and adsorbate substrate decoupling depends on which ligand is used. The X-ray photoelectron spectroscopy results suggest that a covalent bond is formed between the NO and CO adsorbates and the FePc iron ion, and that the NO and CO valence states hybridize with metal ion d states. The density functional theory calculations show that CO adsorbs in a linear configuration, while NO adsorption assumes a tilted geometry.
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| 8. |
- Isvoranu, Cristina, et al.
(författare)
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Electron spectroscopy study of the initial stages of iron phthalocyanine growth on highly oriented pyrolitic graphite.
- 2009
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 131:21
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Tidskriftsartikel (refereegranskat)abstract
- The nature of the intermolecular and substrate bonds of iron phthalocyanine adsorbed on highly oriented pyrolitic graphite has been investigated by x-ray photoelectron spectroscopy and x-ray absorption spectroscopy. We find that the molecules grow in a highly ordered fashion with the molecules essentially plane-parallel to the surface in both the mono- and multilayers. The spectra obtained on both types of film are virtually identical, which shows that the bonds both between the adsorbate and substrate and between the molecular layers have a pure van der Waals nature. Supporting density functional theory results indicate that the layers are stabilized by weak hydrogen bonds within the molecular layers.
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| 9. |
- Isvoranu, Cristina
(författare)
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Metal-Organic Complexes at Surfaces
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The adsorption of different metal-organic complexes at surfaces was investigated by spectroscopic techniques (x-ray photoemission spectroscopy, x-ray absorption spectroscopy, resonant photoemission spectroscopy) and by scanning tunnelling microscopy. The investigation methods are able to provide atomic scale information on the studied systems, which moreover originates from the top-most atomic layers in a sample, meaning that they are surface sensitive techniques. Since the interest is always directed towards adsorbate-substrate interfaces, surface sensitivity is essential. The spectroscopic techniques also ensure atomic and chemical specificity. The information obtained relates to the electronic and geometric structure of the adsorbates on surfaces. The systems studied are iron phthalocyanine, oxalic acid and bi-isonicotinic acid molecules on different supports (Au(111), Cu(111) and highly oriented pyrolitic graphite). A significant part of the research deals with the study of iron phthalocyanine at surfaces and its interaction with different small gas molecules such as ammonia, pyridine, carbon monoxide and nitric oxide. It is interesting to note that the iron phthalocyanine shows different coupling strength on a Au(111) substrate compared to highly oriented pyrolitic graphite. On Au(111), the adsorbate-substrate coupling is relatively strong resulting in covalent-type interactions, while on graphite the coupling is weak and of van der Waals type. The adsorption of different gases on iron phthalocyanine structures on Au(111) results in the formation of bonds between the iron atom and the gas molecule which acts as a molecular ligand. This leads to significant changes in the electronic structure of the phthalocyanine molecular network. First, a decoupling of the phthalocyanine adsorbates from the Au(111) substrate is observed and second, the spin state of the phthalocyanine molecule is significantly changed. The spin changes range from a lowering of the spin to a complete spin quench, depending on the gas molecule that is bound to the iron site. By comparing the adsorption of different molecules it is possible to show that the spin state can be tuned to different values by using different gas molecules. On the other hand the adsorption of oxalic acid on Cu(111) shows the possibility of forming an iron –oxalate coordination network on a Cu(111) surface by incorporation of iron atoms inside a previously formed oxalate network. The study of of bi-isonicotinic acid on Au(111) by resonant photoemission spectroscopy shows the possibility of femtosecond charge transfer from the substrate to the bi-isonicotinic acid molecules, while charge transfer in the opposite direction does not seem to take place.
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| 10. |
- Isvoranu, Cristina, et al.
(författare)
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Pyridine Adsorption on Single-Layer Iron Phthalocyanine on Au(111)
- 2011
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 115:41, s. 20201-20208
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Tidskriftsartikel (refereegranskat)abstract
- The adsorption of pyridine on monolayers of well-ordered, flat-lying iron phthalocyanine molecules on Au(111) is investigated by X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and density functional theory. It is found that pyridine both coordinates to the iron site of iron phthalocyanine and binds weakly to other sites. The iron coordination causes significant changes in the electronic structure of the iron phthalocyanine compound, with the implication of a change of the spin properties of the iron atoms due to the strong ligand field created by the pyridine axial ligand. Both low coverages and multilayer coverages of pyridine are considered. At low doses, the pyridine molecules are ordered, whereas in multilayers, no preferred orientation is observed. The orientation of the FePc molecules with respect to the Au(111) surface is not affected by the adsorption of pyridine.
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