Surface Functionalization of Reconstructed Si(111) with Methionine

Understanding the fundamental interactions between biological and inorganic systems enables better control of the molecular adsorption and nanofilm growth processes, allowing the manipulation of interfacial phenomena for potential applications in nanobiotechnology and biomedical sciences. Here, we provide the first study of coverage-dependent adsorption behavior of L-methionine molecules on the Si(111)7x7 surface by using X-ray photoelectron spectroscopy (XPS) and large-scale ab initio density functional theory (DFT)-D2 calculations (that include van der Waals corrections), particularly to follow the evolution of the adsorption structures and growth mechanisms as a function of coverage. Our DFT-D2 calculations show that the most stable adsorption configuration involves a dehydrogenated methionine adspecies N-bonded on Si with its S atom undertaking long-range interaction with Si at the early growth stage. XPS analysis further reveals a three-stage growth process, from a chemisorbed interfacial layer (first stage) to a transitional layer (second stage) and finally to a zwitterionic multilayer film (third stage), upon increasing methionine exposure. In the interfacial layer, a single prominent N is feature corresponding to the formation of a Si-N covalent-bonded structure involving a dehydrogenated amino group is observed. At a higher coverage, the presence of two N is peaks corresponding to N center dot center dot center dot H-O hydrogen bonding and protonated amino (-NH3+) group supports the formation of a transitional layer and a zwitterionic layer. In contrast to cysteine, which has been observed to unidentately attach through either the dehydrogenated amino or thiol group, methionine [obtained by replacing a thiol (-SH) group in cysteine with a terminal methylthio methylene (-CH2SCH3) group] is found to exhibit remarkably different bonding interaction involving S dative bond. The discernibly longer molecular backbone in methionine also leads to unique bonding features that significantly affect the details of the nanofilm growth.