Enabling enelike reactions on Si(111)-7x7 through tuning organic molecular structures.

We previously demonstrated that acetonitrile (N[triple bond]C-CH(3)) binds to the adjacent adatom-rest atom pair of Si(111)-7x7 through a [2+2]-like cycloaddition reaction, forming a (Si)N=C(Si)-CH(3)-like surface species [Tao et al., J. Phys. Chem. B 106, 3890 (2002)]. Current investigation clearly showed that chloroacetonitrile (N[triple bond]C-CH(2)Cl), propargyl chloride (HC[triple bond]C-CH(2)Cl), and 3-chloropropionitrile (N[triple bond]C-CH(2)-CH(2)Cl) react with the surface via enelike reactions, concurrently involving N[triple bond]C/C[triple bond]C as well as the breakage of the C-Cl/C-H bond. Further separation of the unsaturated bond (C[triple bond]C) from the C-Cl bond using CH(2) spacers in 5-chloro-1-pentyne (CH[triple bond]C-CH(2)CH(2)CH(2)Cl) would direct the reaction to a [2+2]-like cycloaddition. These experimental results clearly suggest the possibility of controlling the surface reaction pathways by tuning the organic molecular structures. This strategy can be useful in designing and fabricating functional molecular templates on Si(111)-7x7.

Binding of glycine and L-cysteine on Si(111)-7 x 7.

The adsorption of glycine and l-cysteine on Si(111)-7 x 7 was investigated using high-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS). The observation of the characteristic vibrational modes and electronic structures of NH3+ and COO- groups for physisorbed glycine (l-cysteine) demonstrates the formation of zwitterionic species in multilayers. For chemisorbed molecules, the appearance of nu(Si-H), nu(Si-O), and nu(C=Omicron) and the absence of nu(O-H) clearly indicate that glycine and l-cysteine dissociate to produce monodentate carboxylate adducts on Si(111)-7 x 7. XPS results further verified the coexistence of two chemisorption states for each amino acid, corresponding to a Si-NH-CH2-COO-Si [Si-NHCH(CH2SH)COO-Si] species with new sigma-linkages of Si-N and Si-O, and a NH2-CH2-COO-Si [NH2CH(CH2SH)COO-Si] product through the cleavage of the O-H bond, respectively. Glycine/Si(111)-7 x 7 and l-cysteine/Si(111)-7 x 7 can be viewed as model systems for further modification of Si surfaces with biological molecules.

Binding mechanisms of methacrylic acid and methyl methacrylate on si(111)-7 x 7 -- effect of substitution groups.

The interaction of methacrylic acid and methyl methacrylate with Si(111)-7 x 7 has been investigated using high-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS). While methacrylic acid chemisorbs dissociatively through O-H bond cleavage, methyl methacrylate is covalently attached to the silicon surface via a [4+2] cycloaddition. The different reaction pathways of these two compounds on Si(111)-7 x 7 demonstrate that the substitution groups play an important role in determining the reaction channels for multifunctional molecules, leading to the desired flexibility in the organic modification of silicon surfaces.