Chemical Stability toward O2 and H2O of Si(111) Grafted with CH3, CH2CH2CH3, CHCHCH3, and CCCH3

The chemical stability of compact monolayers on silicon toward oxidizing agents is a key issue for the use of such monolayers in devices such as solar cells or in the electronics industry. In this work, we investigated the reactivity toward H2O, O2, and OH species of monolayers terminated with a methyl group to unveil the mechanisms that prevent the oxidation of the underlying silicon. Density functional theory calculations were performed to investigate the reaction pathways for the two competing processes involved: diffusion through the monolayer and reaction with the terminal methyl group. Activation energy barriers for the diffusion of H2O and O2 are very sensitive to the monolayer structure, and they increase in the order CH2CH2CH3 < CCCH3 < CHCHCH3 with energy barriers of 0.0 kcal/mol (0.0 kcal/mol), 35.0 kcal/mol (42.5 kcal/mol), and 57.0 kcal/mol (64.1 kcal/mol), respectively, for H2O (O2). This agrees with ordering of stabilities reported experimentally for these monolayers. The oxidation of the terminal methyl group by O2 is less affected by steric constraints. The formation of the CH2OOH species has an energy barrier of 56.5 kcal/mol on the rigid CH3 monolayer, whereas this barrier decreases to 40.7 kcal/mol on the CCCH3 monolayer. In the case of the methyl monolayer, the abstraction of a H atom of the CH3 group has smaller energy barriers with singlet O2 and OH reactants, with values of 38.4 and 3.5 kcal/mol, respectively. The high energy barriers of all of the processes investigated indicate that compact monolayers hinder the oxidation of the underlying substrate. The passivating capability of the monolayers correlates with the steric constraints for H2O and O2 diffusion.