Reconsidering X-ray Photoelectron Spectroscopy Quantification of Substitution Levels of Monolayers on Unoxidized Silicon Surfaces

The covalent functionalization of unoxidized silicon surfaces is of interest for a wide range of applications and for fundamental studies linking surface functionalization and electronic properties. Determination of the level of substitution (yield) of a reaction on a silicon surface is necessary as the number of functional groups bound to the surface is directly linked to properties. X-ray photoelectron spectroscopy, XPS, is the most common analytical method for determining the substitution level of the chemical handle on the silicon surface, typically a Si-H or Si-Cl bond, through which a new stable bond is formed to link the molecule to the surface. Calculations using the atomic ratio of carbon to silicon, as determined by XPS, do not take into account the effect of adventitious hydrocarbons and retained solvent, and the substitution level is typically measured by first assuming 100% substitution of a fictitious hydrocarbon layer with an effective thickness determined by the XPS intensity ratio of C to Si; the actual substitution level is then taken as the ratio of the effective thickness to the theoretical height of the molecule. In this work, we present an alternative and more physically meaningful approach to deriving expressions for the substitution level that is based on the proportionality of the photoelectron attenuation length to the substitution level. For all-hydrocarbon molecules grafted to a silicon surface, this new approach yields the same equations for substitution levels as an earlier effective thickness model. More importantly, unlike the effective thickness models, this method can be extended to include molecules with a heteroatom tag, such as fluorine and chalcogenides, for determining coverage by XPS. The use of the heteroatomic tags is shown to provide a greater degree of certainty with respect to calculating the coverage on silicon. We finish with a simple flowchart to guide the reader to the appropriate equation for both Si(111) and Si(100) surfaces.