Phosphonic acid self-assembled monolayers (SAMs) are being considered as versatile surface modification agents due to their ability to attach to surfaces in different configurations, including mono-, bi- or even tri-dentate arrangements. Different chemical strategies for grafting SAMs on oxide-free silicon have been developed. Recently, a novel method for preparing OH-terminated, on otherwise oxide-free silicon has been reported [1] and further expanded for this work. This atomically flat surface contains precisely 1/3 OH and 2/3 H termination groups.

 

Using this model surface, we demonstrate that the phosphonic group of organic molecules can be chemically grafted to the OH group on the surface using a single chemical step, leaving the Si-H termination unaffected, without oxidation of the Si surface. We also show that the nature of solvents is important as they can act as a catalyst. The perfection of the surface (that remains atomically flat throughout the modification) makes it possible to use first principles DFT-based calculations to model the IR and XPS data obtained for this surface. Thus, a detailed structure for the SAMs can be derived on an atomic level. It is found that phosphonic acids are chemically attached to the Si(111) surface as mono-dentate via Si-O-P bond upon reaction with the OH groups. The remaining groups of P=O and P-OH are further oriented by forming a 2D network of hydrogen bonds.

 

[1] D. J. Michalak, S. R. Amy, D. Aureau, M. Dai, A. Esteve and Y. J. Chabal, Nanopatterning Si(111) surfaces as a selective surface-chemistry route, NATURE MATERIALS, Vol. 9, March 2010