Chemical\nStability toward O₂ and H₂O of Si(111) Grafted\nwith CH₃, CH₂CH₂CH₃, CHCHCH₃,\nand CCCH₃

Abstract

The\nchemical stability of compact monolayers on silicon toward\noxidizing agents is a key issue for the use of such monolayers in\ndevices such as solar cells or in the electronics industry. In this\nwork, we investigated the reactivity toward H₂O, O₂, and OH species of monolayers terminated with a methyl group\nto unveil the mechanisms that prevent the oxidation of the underlying\nsilicon. Density functional theory calculations were performed to\ninvestigate the reaction pathways for the two competing processes\ninvolved: diffusion through the monolayer and reaction with the terminal\nmethyl group. Activation energy barriers for the diffusion of H₂O and O₂ are very sensitive to the monolayer structure,\nand they increase in the order CH₂CH₂CH₃ < CCCH₃ < CHCHCH₃ with energy\nbarriers of 0.0 kcal/mol (0.0 kcal/mol), 35.0 kcal/mol (42.5 kcal/mol),\nand 57.0 kcal/mol (64.1 kcal/mol), respectively, for H₂O (O₂). This agrees with ordering of stabilities reported\nexperimentally for these monolayers. The oxidation of the terminal\nmethyl group by O₂ is less affected by steric constraints.\nThe formation of the CH₂OOH species has an energy\nbarrier of 56.5 kcal/mol on the rigid CH₃ monolayer,\nwhereas this barrier decreases to 40.7 kcal/mol on the CCCH₃ monolayer. In the case of the methyl monolayer, the abstraction\nof a H atom of the CH₃ group has smaller energy\nbarriers with singlet O₂ and OH reactants, with values\nof 38.4 and 3.5 kcal/mol, respectively. The high energy barriers of\nall of the processes investigated indicate that compact monolayers\nhinder the oxidation of the underlying substrate. The passivating\ncapability of the monolayers correlates with the steric constraints\nfor H₂O and O₂ diffusion.