Switching from a [2 + 2] Cycloaddition-Like to an Ene-Like Reaction in Covalent Attachment of Halogenated Acetonitrile on Si(100)-2×1: Effect of Halogen Substituent

The adsorption of halogenated acetonitrile (X−CH2C≡N, X = F, Cl, Br) on Si(100)-2×1 has been studied using high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculation. Fluoroacetonitrile chemisorbs on the Si(100)-2×1 surface via a C≡N [2 + 2] cycloaddition-like reaction, while chloroacetonitrile is chemically bound to the Si surface through both C≡N [2 + 2] cycloaddition-like and ene-like reactions. However, bromoacetonitrile was found to attach onto the surface exclusively through the ene-like reaction. The different reaction pathways of these halogenated compounds on the Si(100)-2×1 surface demonstrate that the halogen substitution groups play an important role in manipulating the reaction channels of bifunctional molecules, offering a great flexibility in surface reaction of silicon surfaces.