X-ray Photoelectron Spectroscopy (XPS) is a widely used surface analytical tool that provides information about the near surface regions of materials. And while indispensable for XPS data analysis, peak fitting of narrow scans is often a fairly subjective exercise. Herein we introduce the equivalent width (EW) as an additional and less subjective figure of merit for XPS narrow scans. We believe that this parameter will prove particularly useful for analyzing series of similar or nominally identical spectra, perhaps as a component of an expert software system for the machine interpretation of spectra. It also appears to be useful, shedding light on the chemical state of materials, when additional information about a sample is known. The EWXPS is simply defined as the area of a narrow scan divided by the height of the maximum of its peak envelope. To limit any ambiguity in EWXPS for a series of spectra, we may also list the peak position of the maximum of the envelope (PEmax). The potential usefulness and limitations of the EWXPS and PEmax parameters are demonstrated by their application to the narrow scans of: (i) four sets of ozone-treated carbon nanotubes (EWXPS ~ 2.11-2.16 eV for a Shirley background, and up to 2.88 eV for no background, PEmax ~ 284.4-284.5 eV), (ii) a series of silicon wafers with different oxide thicknesses (EWXPS ~ 1.5-2.8 eV, PEmax ~ 99-103 eV), (iii) hydrogen-terminated silicon before and after derivatization with pentyl groups, and after annealing of the pentyl-modified material (EWXPS ~ 0.7-1.0 eV, PEmax ~ 25.9-26.1 eV), and (iv) five nanodiamond samples, where three of the spectra showed charging (EWXPS ~ 2.6-4.9 eV, PEmax ~ 272.7-293.9 eV). In this final example, EWXPS was plotted against PEmax to identify the region corresponding to the materials that showed the least charging. EWXPS and PEmax appear to correlate with the expected chemistries of all the systems studied. We calculate EWXPS using a Shirley baseline and with no baseline at all. In setting the baseline limits for EWXPS, we consider the derivative of C 1s narrow scans. We also show the application of EWXPS to single, fitted components within a narrow scan.
Singh, Bhupinder, Daniel Velazquez, Jeff Terry, Matthew R. Linford
Journal of Electron Spectroscopy and Related Phenomena