Semiempirical Peak Fitting Guided by ab Initio Calculations of X-ray Photoelectron Spectroscopy Narrow Scans of Chemisorbed, Fluorinated Silanes.

Abstract

Here, we address the issue of finding correct CF2/CF3 area ratios from X-ray photoelectron spectroscopy (XPS) C 1s narrow scans of materials containing -CH2CH2(CF2)nCF3 (n = 0, 1, 2, ...) moieties. For this work, we modified silicon wafers with four different fluorosilanes. The smallest had a trifluoropropyl (n = 0) moiety, followed by nonafluorohexyl (n = 3), tridecafluoro (n = 5), and finally, heptadecafluoro (n = 7) moieties. Monolayer deposition of the fluorosilanes was confirmed by spectroscopic ellipsometry, wetting, and XPS. Analysis of the trifluoropropyl (n = 0) surface and a sample of polytetrafluoroethylene provided pure-component XPS spectra for -CF3 and -(CF2)n- moieties, respectively. Initial XPS C 1s peak fitting, which follows the literature precedent, was not entirely adequate. To address this issue, six different fitting approaches with increasing complexity and/or input from the Hartree-Fock theory (HF) were considered. Ultimately, we show that by combining HF results with empirical analyses, we obtain more accurate CF2/CF3 area ratios while maintaining high-quality fits.