Tuning the redox potential in molecular monolayers covalently bound to H-Si(100) electrodes via distinct C-C tethering arms

The spatial self-organization of molecular species on an Si oriented surface can be less ideal than that of an SAM on a metal, likely affecting the electronic structure of the resulting hybrids and their electrochemical response as electrodes in solution. In order to investigate such effects, a series of molecular precursors was investigated, consisting of three substituted ferrocenes with a lateral C-C group fully saturated (ethylferrocene) or with a single (vinylferrocene) or double unsaturation (ethynylferrocene). The corresponding functionalized Si(100) wafers were produced following new or literature recipes, starting from hydrogenated Si surfaces. The relationship between the degree of unsaturation in the anchored arm of each adduct and its electronic structure and electrochemical behaviour was investigated by comparing experimental (XPS, electrochemical) and ab initio results of the redox potentials in the series. Density functional theory (DFT) was applied, with inclusion of solute-solvent interactions. Different bond arrangements of the C-C arm with Si surface dimer atoms have been produced theoretically within the series of ferrocenes. Distinct values of redox potentials were displayed by the hybrids, which can be consistently related to the structural differences presented. In fact, measured and computed potentials showed a very satisfactory match only for specific adduct isomers, providing strong indications that the carbon-carbon unsaturation initially present in the anchoring arm is preserved upon addition reaction, an unprecedented result. The demonstrated tunability of a well-defined switching potential in these molecules on silicon can be beneficial to the development of Si-based memory devices.