Reaction Mechanism and Regioselectivity of Methyl Oxirane on Si(111)-(7 × 7)

The reaction mechanism and regioselectivity of methyl oxirane (C3H6O) adsorbed on Si(111)-(7 × 7) have been studied using high-resolution electron energy loss spectroscopy (HREELS), in situ scanning tunneling microscopy (STM), and density functional theory (DFT) calculations. The experimental results demonstrate that methyl oxirane chemically binds to Si(111)-(7 × 7) through both dative-bonded addition and ring-opening reaction via cleavage of C–C or one C–O bond within the epoxy group. The STM images also reveal that the adsorption is site-selective with a preference for center adatoms on the faulted half of the (7 × 7) unit cell. The DFT calculations further show that breaking the C(CH3)–O bond is most kinetically favorable with a relatively small barrier of ∼3 kcal/mol. The dative-bonded states are observable on the surface only after three adjacent adatom–rest atom pairs within one-half unit cell are fully reacted with the methyl oxirane molecules during the ring-opening reaction.