Solution Functionalization of H/HO-Terminated Si(100) Surfaces with Aniline and Pyridine

Adsorption of aniline and pyridine on the H- and HO-terminated Si(100) surfaces has been investigated by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and density functional theory (DFT) calculations. Surface functionalization reactions were conducted under solution-chemistry conditions and an inert atmosphere, and show that adsorption proceeds very differently for aniline and pyridine on H-, and HO-Si(100) surfaces. On hydrogen-terminated silicon surfaces, aniline forms three different types of surface adducts: an expected product defined as covalent Si-NH- bonds and/or Si-N-Si bridge structures, and two products observed on defect surface sites: datively bonded Si←N species on undercoordinated Si, and species physisorbed via hydrogen bonding on surface HO- groups. On HO-terminated surfaces, aniline physisorbs via hydrogen bonding, as the majority product, with a minor contribution from Si-N surface adducts. Pyridine was not found to adsorb on H-Si sites. However, it interacts with the defect sites through hydrogen bonding or dative bonding on surface HO-sites and uncoordinated Si sites, respectively. Surface coverage for pyridine is around 30% of that for aniline within the same range of experimental conditions. AFM analysis revealed a consistent morphological change for both surfaces following functionalization, in which aniline adsorption appears to cause the most noticeable changes in surface morphology. DFT calculations supported the experimental findings, predicting exothermic adsorption and favorable energy barriers for Si-NH- bonding of aniline on both H-Si and HO-Si sites and Si-N-Si formation on hydrogen-terminated surfaces, as well as the enhanced stabilization of physisorbed species due to cooperative effects of coadsorbed water.