The (001) surface of silicon contains pairs of atoms that are held together with a strong σ bond and a weak π bond. The interaction of styrene with the Si(001) surface has been investigated as a model system for understanding the interaction of conjugated π-electron systems to π-bonded semiconductor surfaces. Scanning tunneling microscopy images show one primary bonding configuration, slightly off-center from the middle of a dimer row. Infrared spectra using isotopically labeled styrene establish that attachment occurs in a highly selective way, bonding through the external vinyl group and leaving the aromatic ring almost completely unperturbed. Ab initio calculations reveal that the interaction between the π electrons of the vinyl group of styrene and the electron-deficient end of a SiSi dimer is strongly attractive. It is proposed that this attraction facilitates a low-symmetry interaction between the surface dimers and the vinyl group, leading to a highly selective reaction pathway for which Woodward−Hoffmann rules do not apply. The implications for selective attachment of other conjugated π-electron systems to other π-bonded semiconductor surfaces are discussed.

The adsorption of pyrrole, aniline, 3-pyrroline, and pyrrolidine on the Si(001)-(2 × 1) surface has been studied using Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). Both pyrrole and aniline retain their aromatic character after bonding to the surface. Spectroscopic evidence indicates that each of these aromatic molecules can attach to the Si(001) surface via cleavage of one N-H bond, linking the molecule to the surface through a Si-N tether. Isotopic studies of pyrrole show evidence for additional cleavage of C-H bonds. While strong selectivity favoring bonding through the nitrogen atom is observed for the aromatic molecules, the unsaturated molecule 3-pyrroline shows evidence for at least two bonding configurations. XPS and FTIR data show that 3-pyrroline can bond either through the nitrogen atom with cleavage of an N-H bond, or through the CdC bond via the surface equivalent of a [2 + 2] cycloaddition reaction. Pyrrolidine appears to bond only through the nitrogen atom. Potential factors controlling the selectivity in bonding and the role of aromaticity in controlling reaction pathways on silicon surfaces are discussed. † Part of the special issue "John T. Yates, Jr. Festschrift".

Benzenethiol (C6H5SH) and diphenyl disulfide (C6H5S−SC6H5) were used as model systems to compare the interaction of chemically similar π-conjugated molecules with the Si(001)-2×1 surface. The bonding behavior of these substituted aromatic hydrocarbons on the Si(001) surface was investigated using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM). Both FTIR and XPS indicate that benzenethiol molecules chemisorb on the Si(001) surface predominantly through the sulfur atom via deprotonation of the thiol substituent group. There also is evidence that a small minority of benzenethiol molecules may adsorb on the surface through the phenyl ring or undergo further fragmentation. Diphenyl disulfide appears to bond to the Si(001) surface in one primary configuration in which the S−S bond of diphenyl disulfide is cleaved and the two sulfur−phenyl moieties are bonded to the silicon surface through the sulfur atoms. Thermal studies indicate that the sulfur-tethered aromatic rings of benzenethiol and diphenyl disulfide are stable to temperatures above 520 K. Furthermore, STM studies show that these molecules chemisorb to the silicon surface within a single dimer row and, in the case of diphenyl disulfide, appear to form ordered rows of sulfur-tethered aromatic rings. This new chemistry demonstrates remarkable potential as a means of selectively attaching π-conjugated systems to technologically useful semiconductor surfaces.

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The interactions of toluene, para-xylene, meta-xylene and ortho-xylene with the (001) surface of silicon have been investigated using Fourier-transform infrared spectroscopy. Infrared spectra show that these methyl-substituted aromatic hydrocarbons are chemisorbed and oriented on the Si(001) surface at both 110 and 300 K. Peaks in the Si–H stretching region indicate that some dissociation occurs upon adsorption. Comparisons of infrared spectra of these molecules with deuterated and nondeuterated methyl groups reveal that the major source of decomposition is likely from C–H cleavage of the substituent groups, leaving the ring intact. Additionally, the striking similarity of the infrared spectra of benzene, toluene and the xylene isomers suggests that the methyl-substituted aromatic rings interact with the Si(001) surface in much the same way as benzene. Differences in relative peak intensity point to the possibility that the methyl substituent groups may steer the ring into different ratios of specific bonding geometries.

We discuss two inequivalent ways for describing magnetized D-branes wrapped N times on a torus T 2 . The first one is based on a non-abelian gauge bundle U(N ), while the second one is obtained by means of a Narain T-duality transformation acting on a theory with non-magnetized branes. We construct in both descriptions the boundary state and the open string vertices and show that they give rise to different string amplitudes. In particular, the description based on the gauge bundle has open string vertex operators with momentum dependent Chan-Paton factors.

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