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Surface X-ray diffraction studies of graphene on ruthenium and hexagonal boron nitride on PtRh-alloy surfaces


Kalichava, Irakli. Surface X-ray diffraction studies of graphene on ruthenium and hexagonal boron nitride on PtRh-alloy surfaces. 2014, University of Zurich.

Abstract

sp2 single layers of graphene and hexagonal boron nitride (h-BN) on transition metal substrates show many interesting properties not observed in the three-dimensional crystalline form. When these layers are grown on substrates with different lattice parameters, the lattice mismatch induces strain which with the surface-area effect impose the formation of superstructures. In general, the interaction of the sp2 layer with the metal substrate involves both weak van der Waals forces and strong chemical bonds that can have a considerable effect, depending on the lattice mismatch and the type of metal. In some cases the lattice mismatch strain causes the buckling of the sp2 layer and results in a periodically varying strength of the interaction with the substrate across the surface. Such structural changes can have a decisive influence on the electronic properties. Depending on the substrates, significant charge transfer and the site-specific orbital hybridization between the metal and the sp2 layer may occur. These corrugated superstructures are both interesting from a fundamental point of view as well as for their potential in technological applications. Surface X-ray diffraction (SXRD) offers powerful and non-destructive possibilities for the investigation for two-dimensional systems such as surface structures, interfaces and thin films. It is a straightforward process to determine the reconstruction and the superstructure size with picometer resolution. In SXRD, the full structure studies represent the main challenge, as one is confronted with the phase problem. Different phase-retrieval and structure-refinement approaches allow one to overcome this problem. This thesis describes SXRD structural studies of graphene and h-BN grown on ruthenium, rhodium, and platinum-rhodium alloy. Graphene on ruthenium forms a corrugated 25-on-23 superstructure which was extensively studied by means of different techniques. In this work, in conjunction with density functional theory calculations (DFT), this system has been further elucidated and important structural parameters have been refined. SXRD simulations based on the present DFT structure were performed and compared to the experimental result. The simulated superstructure rods exhibit all essential structural characteristics and are fairly consistent with experiment. The model has a goodness of fit better than that of previous models. Furthermore the DFT optmized model was refined using the parameterization procedure and its physical validity was successfully evaluated. The h-BN grown on PtRh 50:50 alloy surface forms a typical commensurate nanomesh structure. In this work, SXRD was applied to this system and an 11-on-10 reconstruction was determined. The recorded crystal truncation rods for the bare PtRh were compared to those from h-BN/PtRh. This allowed the elucidation of the influence of the h-BN nanomesh on the near-surface structure of PtRh. It was found the h-BN nanomesh imposes compositional changes in the PtRh substrate. In addition to qualitative findings, semi-quantitative structure fitting was performed where the Rh-composition profile was obtained for the bare PtRh and h-BN/PtRh samples. h-BN nanomeshes may exhibit modulation of the adsorption potential which allows template functionality to produce well-ordered arrays of various adatoms, molecules and atomic clusters. h-BN on Rh forms a well-ordered nanomesh structure and shows a strong modulation of the binding energy. In order to demonstrate a possible template functionality of this system, xenon crystals were grown on h-BN/Rh and several in-situ SXRD experiments were performed in the framework of this thesis

Abstract

sp2 single layers of graphene and hexagonal boron nitride (h-BN) on transition metal substrates show many interesting properties not observed in the three-dimensional crystalline form. When these layers are grown on substrates with different lattice parameters, the lattice mismatch induces strain which with the surface-area effect impose the formation of superstructures. In general, the interaction of the sp2 layer with the metal substrate involves both weak van der Waals forces and strong chemical bonds that can have a considerable effect, depending on the lattice mismatch and the type of metal. In some cases the lattice mismatch strain causes the buckling of the sp2 layer and results in a periodically varying strength of the interaction with the substrate across the surface. Such structural changes can have a decisive influence on the electronic properties. Depending on the substrates, significant charge transfer and the site-specific orbital hybridization between the metal and the sp2 layer may occur. These corrugated superstructures are both interesting from a fundamental point of view as well as for their potential in technological applications. Surface X-ray diffraction (SXRD) offers powerful and non-destructive possibilities for the investigation for two-dimensional systems such as surface structures, interfaces and thin films. It is a straightforward process to determine the reconstruction and the superstructure size with picometer resolution. In SXRD, the full structure studies represent the main challenge, as one is confronted with the phase problem. Different phase-retrieval and structure-refinement approaches allow one to overcome this problem. This thesis describes SXRD structural studies of graphene and h-BN grown on ruthenium, rhodium, and platinum-rhodium alloy. Graphene on ruthenium forms a corrugated 25-on-23 superstructure which was extensively studied by means of different techniques. In this work, in conjunction with density functional theory calculations (DFT), this system has been further elucidated and important structural parameters have been refined. SXRD simulations based on the present DFT structure were performed and compared to the experimental result. The simulated superstructure rods exhibit all essential structural characteristics and are fairly consistent with experiment. The model has a goodness of fit better than that of previous models. Furthermore the DFT optmized model was refined using the parameterization procedure and its physical validity was successfully evaluated. The h-BN grown on PtRh 50:50 alloy surface forms a typical commensurate nanomesh structure. In this work, SXRD was applied to this system and an 11-on-10 reconstruction was determined. The recorded crystal truncation rods for the bare PtRh were compared to those from h-BN/PtRh. This allowed the elucidation of the influence of the h-BN nanomesh on the near-surface structure of PtRh. It was found the h-BN nanomesh imposes compositional changes in the PtRh substrate. In addition to qualitative findings, semi-quantitative structure fitting was performed where the Rh-composition profile was obtained for the bare PtRh and h-BN/PtRh samples. h-BN nanomeshes may exhibit modulation of the adsorption potential which allows template functionality to produce well-ordered arrays of various adatoms, molecules and atomic clusters. h-BN on Rh forms a well-ordered nanomesh structure and shows a strong modulation of the binding energy. In order to demonstrate a possible template functionality of this system, xenon crystals were grown on h-BN/Rh and several in-situ SXRD experiments were performed in the framework of this thesis

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Additional indexing

Item Type:Dissertation (monographical)
Referees:Willmott Philip R
Communities & Collections:UZH Dissertations
Dewey Decimal Classification:Unspecified
Language:English
Place of Publication:Zürich
Date:2014
Deposited On:04 Apr 2019 06:58
Last Modified:04 Apr 2019 06:58
Number of Pages:93
OA Status:Green
Related URLs:https://www.recherche-portal.ch/primo-explore/fulldisplay?docid=ebi01_prod010407442&context=L&vid=ZAD&search_scope=default_scope&tab=default_tab&lang=de_DE (Library Catalogue)

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