DEVELOPMENT OF SURFACE SENSITIVITY IN SCANNING X-RAY MICROSCOPY AND NEXAFS SPECTROSCOPY OF ORGANOSULPHUR COMPOUNDS

Abstract

In this thesis, two objectives related to Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy have been studied. The first objective was to develop surface sensitivity in Scanning Transmission X-ray Microscopy (STXM) at the Canadian Light Source (CLS) and the Advanced Light Source (ALS). The second objective was to study sulphur 1s NEXAFS spectra of organosulphur compounds relevant to petroleum by experimental and computational methods. Towards the first objective, Total Electron Yield (TEY) detection has been implemented in a STXM microscope, by conventional sample current and single electron counting detection modes. This provides improved surface-sensitive detection, simultaneous with existing bulk-sensitive transmission detection in the STXM microscopes. Both approaches provide improved surface sensitive imaging and spectroscopy, although channeltron-based detection is superior. TEY-STXM provides surface sensitive imaging of ultrathin films such as phase-separated Langmuir-Blodgett monolayer films, phase separated polymer thin films, as well as differentiation of surface and bulk oxides of patterned metal thin films. The challenge for TEY-STXM measurements is the poor vacuum environment in the STXM chamber at the CLS, which greatly impacts the function of the channeltron as well as the rate of the photodeposition. Although the effect of photodeposition can be minimal in bulk spectroscopy of organic samples, it is a challenge in spectroscopy of organic thin films, where the photodeposits can dominate the weak signal originating from the surface. Chapter 4 of this thesis discusses the details of this study along with the challenges encountered in the development of this new TEY-STXM technique. The second goal of my research was the detailed study of sulphur 1s NEXAFS spectra of organosulphur compounds by experimental and computational methods to obtain a complete database of sulphur 1s NEXAFS spectra. The speciation and quantification of sulphur compounds is of great interest in different areas such as fossil fuel studies, biology, geology, and archaeology. Sulphur 1s NEXAFS spectroscopy can be used for speciation and quantification of these compounds. For this purpose a firm understanding of NEXAFS spectra of sulphur compounds is required. Therefore, the sulphur 1s NEXAFS spectra of different sulphur functionalities have been studied including thiols, thioethers, disulphides, sulfoxides, sulfones, and thiophenic compounds in gas and condensed phases. These highly resolved spectra have been further analyzed with the aid of ab initio calculations. The highly resolved experimental spectra showed fine features predicted by calculations. The combination of experiment and calculation has been used to improve assignment of spectroscopic features relevant for the speciation and quantification of sulphur compounds.