The cycloaddition of benzonitrile with Si(111)-7×7 has been investigated as a model system for understanding the interaction of conjugated π-electron systems with Si(111)-7×7 using high-resolution electron energy loss spectroscopy, x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy, scanning-tunneling microscopy (STM), and density-functional-theory calculation (perturbative Beck-Perdew functional in conjugation with a basis set of ${\mathrm{DN}}^{\mathrm{*}\mathrm{*}}$). Vibrational features of chemisorbed benzonitrile unambiguously demonstrate that the cyano group directly interacts with Si surface dangling bonds, evidenced in the disappearance of C≡N stretching mode around 2256 ${\mathrm{cm}}^{\mathrm{-}1}$ coupled with the appearance of C=N stretching mode at 1623 ${\mathrm{cm}}^{\mathrm{-}1}$ and the retention of all vibrational signatures of phenyl ring. XPS shows that both C $1s$ and N $1s$ core levels of the cyano groups display large down-shifts by 2.5 and 1.5 eV, respectively, after chemisorption. A smaller down-shift of ∼0.8 eV is observed for the C $1s$ core level of phenyl group due to the weaker inductive effect of the formed C=N groups in chemisorbed benzonitrile than that of C≡N groups in physisorbed molecules. Compared with physisorbed molecules, the photoemission from ${\pi }_{\mathrm{CN}}$ orbitals of chemisorbed benzonitrile is significantly reduced, suggesting the direct involvement of ${\pi }_{\mathrm{CN}}$ in the surface binding. These experimental results show that the covalent attachment of benzonitrile on Si(111)-7×7 occurs in a selective manner through the (2+2) cycloaddition between the cyano group and the adjacent adatom-rest atom pair. The cycloadduct formed contains an intact phenyl ring protruding into vacuum, retaining aromaticity. This functionalized Si surface may serve as a substrate for further modification or act as an intermediate for fabrication of multilayer organic thin films or materials syntheses in vacuum.

• Received 18 September 2001

DOI:https://doi.org/10.1103/PhysRevB.65.115311