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Structural studies of surfaces and interfaces by scanned-angle x-ray photo-electron diffraction

Gleeson, Michael A (1996) Structural studies of surfaces and interfaces by scanned-angle x-ray photo-electron diffraction. PhD thesis, Dublin City University.

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Abstract

Scanned-angle X-ray photo-electron diffraction (XPD) was examined as a candidate for use in the qualitative determination of the surface crystallography of a variety of surface systems of chemical and physical relevance, including the growth of strained ultra-thin metal films, the co-adsorption of CO with alkali metals and the geometry/compositional profile of bi-metallic alloy surfaces. Where appropriate, ancillary techniques such as Near Edge X-ray Absorption Fine Structure (NEXAFS), Low Energy Ion Scattering Spectroscopy (LEISS) and Angle-Resolved Ultra-violet Photo-emission Spectroscopy (ARUPS) were used to further characterise these surface systems. The effect of strain on the structure of ultra-thin metal films grown on fee substrates was examined by XPD. Cobalt films grown on Pd {100} and {111} substrates were shown to have a Volmer-Weber growth mode, and to be initially strained in-plane by the substrate. In both cases the Co overlayers were shown to grow in an fee structure, rather than the normal hep phase adopted by Co at room temperature. In the case o fP d {100}, the Co overlayers grew in a novel highly tetragonally distorted fee layer structure ( c /a = l .l l - 1.15; fee c/a=1.41, bcc c/a=1.0), which persisted to high coverages (>20 eML). On the {111} plane, the Co adlayers relaxed more rapidly toward their bulk fee lattice parameters with complete relaxation occuring in the 1-5 eML coverage range. There was again evidence of a distorted lattice structure induced by the Pd substrate for very thin Co films. In contrast, Cu was shown to grow on Pd {110} at 300 K in the Stranski-Krastanov growth mechanism with flat clusters growing above a 1 ML pseudomorphic Cu slab. Again XPD indicated evidence of growth of a distorted fee structure. The fee structure of the Cu films was studied by angle-resolved photo-emission using synchrotron radiation. The strain imposed by the Pd substrate manifested itself by changes in the film electronic structure relative to bulk Cu {110}. A range of stoichiometric K/CO co-adsorbed monolayers on Co {1010} were studied by NEXAFS, XPS and XPD, including a c(2x4)-(K+CO), a c(2x2)-<K+CO) and a p(3xl)/c(2x2)-(K+CO) mixed phase. The orientation of the CO molecule and changes in the CO bondlength were determined via NEXAFS. The CO was found to be upright on the c(2x4) and c(2x2) overlayers. However, evidence for a tilted CO species on the p(3xl)/c(2x2) phase was found. Co-adsorption with K was shown to induce a lengthening of the CO molecular bond (monotonically increasing with K:CO stoichiometry) of up to -0.13 A relative to CO on clean Co. Various co-adsorption induced NEXAFS features, such as the appearance of an additional resonance with nsymmetry -11.7 eV above the Fermi level and a splitting o f the characteristic CO nresonance, were identified as being consistent with a direct chemical bonding interaction between the K 4s-resonance and the CO 2k orbitals. Low-energy scanned-angle XPD was utilised to determine the structure o f the c(2x2)-(K+CO) overlayer. The overlayer was found to consist o f an intimately mixed K/CO phase, with the potassium and oxygen species nearly co-planar and at a lateral separation o f -2 .9 A, which is consistent with a K-0 chemical bonding distance. The results indicate that low-energy scanned-angle XPD has definite potential as a quantitative structural analysis tool for complex systems containing two or more co-adsorbates. The surface structure of two CuPd alloys, a Cuo.85Pdo.15 {110} bulk crystal and a Cu{100}-c(2x2)-Pd surface alloy was examined by polar XPD. In the case o f the surface alloy (formed by deposition o f -0.5 ML of Pd on Cu {100}, XPD clearly demonstrated that the alloy was not a homogeneous, single layer structure, but was composed o f domains o f c(2x2) CuPd alloy and areas of pure Cu {100}. Approximately 50% of the Pd atoms were found to reside in the second layer of the selvedge. In the case of the bulk alloy, polar XPD clearly demonstrated that the alloy surface did not adopt a simple fee lattice structure. Various models consistent with the observed XPD measurements have been proposed, which should be tested by independent structural determination techniques. Scanned angle XPD was shown to be a rapid and reliable method of obtaining the major aspects of the surface geometry for a wide variety of surface systems. The use of a single scattering approximation in the analysis of scanned-angle measurements was found to be sufficient. The results of the co-adsorption study indicate that even at relatively low kinetic energies (-120 eV), single scattering modelling can still yield reliable results.

Item Type:Thesis (PhD)
Date of Award:1996
Refereed:No
Supervisor(s):Barnes, Colin
Uncontrolled Keywords:Diffraction; Photochemistry; Surface crystallography
Subjects:Physical Sciences > Chemistry
DCU Faculties and Centres:DCU Faculties and Schools > Faculty of Science and Health > School of Chemical Sciences
Use License:This item is licensed under a Creative Commons Attribution-NonCommercial-No Derivative Works 3.0 License. View License
ID Code:18753
Deposited On:30 Jul 2013 14:30 by Celine Campbell. Last Modified 30 Jul 2013 14:30

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