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Quantitative surface structure determination

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Title: Quantitative surface structure determination
Authors: Ryan, Paul
Item Type: Thesis or dissertation
Abstract: In this thesis work, normal incidence X-ray standing waves (NIXSW) and energy scanned photoelectron diffraction (PhD) were utilised to determine the surface structures of a number of adsorbate/ surface systems. The systems under study can be separated into two subject areas: those concerning the study of adsorbates on the magnetite (001) facet (Fe3O4(001)) and those that comprise the adsorption of 2,3,5,6-tetra uro-7,7,8,8- tetracyanoquinodimethane (F4-TCNQ) on coinage metals which is proposed to form charge transfer salts. Formic acid dissociatively adsorbs on the (001) facet of magnetite (Fe3O4(001)) producing an adsorbed formate and a surface hydroxyl. The NIXSW results of this thesis corroborate with prior studies which place the formate bidentate bonding on the surface to the surface Fe cations. An additional structural study, using PhD, was uniquely able to determine the location of the surface hydroxyl as being at a surface interstitial site. It has been proposed that deposited Ni metal sits at the adatom site on the Fe3O4(001) surface as well in the subsurface cation vacancies of the subsurface cation vacancy termination. NIXSW results presented in this thesis unequivocally determine this to be the case. The vertical height of the subsurface Ni, as determined by NIXSW, likely shows that it sits in subsurface sites and not at the surface. Finally, the NIXSW technique was used to determine the effect CO adsorption has on the structure of Ag adatoms on the Fe3O4(001) surface. The NIXSW results find that the CO pulls the Ag adatom away from the surface and these results have been used to benchmark computationally affordable approaches to calculating the adatom adsorption heights and other properties of the system. Prior NIXSW studies of 7,7,8,8-tetracyanoquinodimethane (H4-TCNQ) on the coinage metals show that it does not adsorb on coinage metals in a planar geometry. It has been determined that the molecule twists and bends on the surface and incorporates substrate adatoms into a charge transfer salt. The NIXSW results of this thesis show that F4-TCNQ also adsorbs on many of the coinage metals, not in a planar geometry, but twisted at either end. Such twisting of the molecule is analogous to that of H4- TCNQ and could potentially indicate that F4-TCNQ also forms charge transfer salts on coinage metals. This would corroborate with ARPES, STM and LEED results from prior investigations. These results provide a benchmark for future theoretical calculations investigating the various electronic properties of these adsorbate/ metal surface systems.
Content Version: Open Access
Issue Date: Oct-2020
Date Awarded: Apr-2021
URI: http://hdl.handle.net/10044/1/89258
DOI: https://doi.org/10.25560/89258
Copyright Statement: Creative Commons Attribution Non-Commercial Licence
Supervisor: Payne, David
Sponsor/Funder: Diamond Light Source (Firm)
Department: Materials
Publisher: Imperial College London
Qualification Level: Doctoral
Qualification Name: Doctor of Philosophy (PhD)
Appears in Collections:Materials PhD theses

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