X - Ray Photoelectron Spectroscopy of Silicate Glasses

X-ray photoelectron spectroscopy (XPS) is a powerful technique capable of providing unique information about the structural chemistry of glasses. However, inappropriate procedures for the preparation of sample surfaces or the lack of reliable methods to overcome electrostatic charging effects during analysis of dielectric materials can cause spurious features to be introduced or intrinsic features to be suppressed in photoelectron spectra which are nominally indicative of bulk phenomena. An adaptation of a biased metal-dot (BMD) energy reference for insulators in XPS to the analysis of fracture surfaces prepared in ultra-high vacuum is described that largely eliminates these difficulties. Based on a study of the charging behavior of SiO glass under conditions of positive and negative bias, a reference of he binding energy scale for an insulator to its surface Fermi level is proposed and shown to be consistent with Schottky barrier formation at metal-insulator interfaces. The utility of this analytical procedure is demonstrated by comparisons of core and valence level spectra acquired from surfaces of fire-polished Si0 2 glass and vacuum-fractured SiO 2 glass and a-quartz. The similarity of core level spectra obtained from fire-polished and vacuum-fractured surfaces of vitreous SiO2 supports a view that extensive surface reconstruction of siloxane bonds occurs at fracture surfaces. Since no significant differences in core level spectra are found between crystalline and vitreous samples of SiO , it is also suggested that the covalent-ionic nature of the Si-O bond goes not change appreciably with the Si-O-Si bridging angle. A structural investigation by XPS of R2 O-SiO glasses and Na 20Al 0 -SiO2 glasses was accomplished by analysis o core level spectra which were acquired from vacuum-fracturedsurfaces and corrected for charging effects by the BMD method. The 0 ls spectrum for Si0 2 glass does not indicate the existence of doubly-bonded oxygens or strained siloxane bonds in detectable concentrations. This result calls into question the assertions of recent crystalline cluster models for glass structure. In accord with earlier XPS work, an analysis of 0 ls spectra for R O-SiO2 glasses agrees with conventional theory for the accommodation of aliali atoms in a vitreous silicon-oxygen network at mole fractions of alkali oxide XR<0.30 but departs negatively from this theory for XR>0.30. The merits of various explanations for this discrepancy are critically appraised. Trends in the binding energies and widths of various core level spectra for Na20-SiO2 glasses support a contention that while singlephase R20-SiO2 glasses do not exhibit any long-range order, chemical details of the short-range atomic structure bear a close resemblance to those in R20-SiO 2 crystals. The proportions of various bridging and nonbridging components of 0 ls spectra for three series of Na20-Al 203-SiO glasses are shown to be ..2 .2 3-SO lse r hw ob directly related to glass composition in a predictable and rather general way within two adjacent compositional regions: 01.0. Thesis Supervisor: Donald R. Uhlmann Title: Cabot Professor of Materials