Characterization of self-assembled monolayers (SAMs) using x-ray photoelectron spectroscopy (XPS) measurements of the gold (4f) attenuation from increasing molecular length alkane thiols were pioneered by Bain and Whitesides[1]. Since then, the gold attenuation has been used by many others as a method for determining the length/thickness of molecular films formed as SAMs on gold. We have done similarly with carboxylic acid (COOH) terminated alkane thiols SAMs deposited on gold with the aid of additives[2], and we obtained similar results. We discovered, however, that the attenuation of the sulfur (2p) signal did not correspond with the gold attenuation. Additionally, neither the gold or sulfur attenuation could correctly account for the observed carbon (1s) signal in the XPS measurements. This fact was originally noted, but not addressed, by Bain and Whitesides.

 

In conjunction with experiments, we will present our successful solution to the modeling of XPS measurements of molecular films prepared by self-assembly and by the Langmuir-Blodgett (LB) technique. In a classic alkanethiol SAM, one third of the surface gold atoms, typically, are bound to a thiol-terminated molecule, due to the steric effect of a radial shell created by the molecule. Thus, the gold signal is only partially attenuated by the molecule. Use of the attenuation of the sulfur signal associated with the S-Au bond (obtained by fitting the XPS signal), we find that we are able to correctly determine the electron escape depth (λ) for sulfur and carbon through the molecular film. Using a poly(thiomethyl methacrylate) thin film as a carbon and sulfur XPS standard combined with the correct λ, we developed a model for the packing density and molecular orientation of COOH terminated alkane thiols that is consistent with gold, sulfur, and carbon XPS measurements as a function of molecule length. We have expanded our model to include SAMs formed of sterically crowded tertiary thiols, where fewer molecules per gold atom can attach to the surface, and to LB films formed from carboxylic acid terminated alkanes on gold surfaces, where no sulfur linkage is made. The consistent interpretation provided by our model will be presented at the talk.

 

[1] C.D. Bain and G.M. Whitesides, J. Phys.Chem. 93, 1670 (1989).

[2] A.W. Snow, G.G. Jernigan, and M.G. Ancona, Analyst 136, 4935 (2011).