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Title: Ab-initio and experimental NEXAFS spectroscopy investigations of graphene : growth and post-processing effects
Author: Rojas Verastegui, Wudmir
ISNI:       0000 0004 7432 2504
Awarding Body: Bangor University
Current Institution: Bangor University
Date of Award: 2018
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Fully commercial exploitation of graphene properties through its integration in the Si-dominated electronic technologies needs to reach challenging growth and processing standards. Indeed, fabrication and subsequent processing of graphene using current methodologies, such as chemical vapor deposition oriented to industrial fabrication, require discriminating and assessing strain and corrugation in order to achieve practical use. For the first time, a combination of ab initio simulations and experimental hyperspectral near-edge X-ray absorption fine structure (NEXAFS) data was successfully employed to monitor mechanical deformation in graphene (i.e., strain and corrugation) at wafer-scale dimensions. Notably, this innovative assessment offers promise towards wafer-scale characterization to inform industrial manufacturing. To this end, the first study presented in this thesis examines the atomic sensitivity to corrugation of the dichroic ratio, orbital vector approximation, and unsupervised machine learning methods. Significantly, the orbital vector approach proved to be the method with the highest sensitivity as reflected by the measured parameter, which also strongly depends on the frequency of the defects. In the second study of this thesis, a new methodology to assess strain with the aid of theoretical samples taken as standards is presented. The most significant results are its accurate strain estimation of chemical vapor deposition-grown graphene on Cu substrates and reasonably accurate estimation of transferred multilayers of graphene onto SiC substrates. In the last study, a method to simulate characteristic angle-resolved NEXAFS spectra (spectral fingerprints) of representative topological defects on graphene is presented and proves to effectively analyze the correlation between the spatial extent of the defect and its spectral fingerprints. This theoretical database of point defects can contribute to the analysis and interpretation of complex experimental spectroscopic data.
Supervisor: Campo, Eva Sponsor: Fujitsu Services Limited ; HPC Wales ; Air Force Research Laboratory
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available