Reactions of substituted aromatic hydrocarbons with the Si(001) surface

Abstract

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.

Chemical manipulation of multifunctional hydrocarbons on silicon surfaces

“…A study by Li et al employed a combination of low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), thermal desorption, and DFT calculations to specifically examine the adsorption of para-xylene (1, 4-dimethylbenzene) and noted that this hydrocarbon adsorbs very similarly to benzene on the Si(100)-2×1 substrate [114]. In a contrast with the studies of Coulter et al [113], no vibrational spectroscopy investigation was undertaken by this group; however, a deuterium-substitution approach confirmed that while some of para-xylene molecules desorb intact at 400 and 470 K (from two different geometries of surface adducts), the majority of these molecules are strongly chemisorbed after the loss of hydrogen from methyl groups [114].…”

“…By investigating isotopicallysubstituted compounds it was concluded that the source of surface hydrogen was indeed the methyl groups and not the aromatic ring. Temperature-dependent studies suggested that the effect of methyl groups can be both thermodynamic (as stabilizing factor in the initial surface adducts) and kinetic (in reducing the reaction barriers) [113]. The chemical reactivity of the methyl group in toluene was supported by the first principles calculation studies by Costanzo et al [12,13].…”

“…Even a substituent as innocuous and seemingly stable as a methyl group in the studies of toluene and xylenes exhibited noticeable amount of C-H dissociation [113]. By investigating isotopicallysubstituted compounds it was concluded that the source of surface hydrogen was indeed the methyl groups and not the aromatic ring.…”

Formation of a Benzoimine-like Conjugated Structure through the Adsorption of Benzonitrile on Si(100)

“…Previous work in this area mainly involves unsaturated hydrocarbons containing unpolarized covalent bonds of CC or C⋮C, including typical ethylene, , acetylene, , five-membered aromatics, , benzene and its derivatives, − and other unsaturated hydrocarbons. − Specially, the covalent binding of benzene and its derivatives on Si surfaces have been well demonstrated. Benzene adsorbs readily on Si(100) , and Si(111)-7 × 7 11 through [4 + 2]-like cycloaddition mechanism involving 1,4-C atoms of benzene and silicon surface dangling bonds.…”

“…Some benzene derivatives bind on silicon surfaces through a dissociative reaction pathway or/and molecular chemisorption via the direct participation of phenyl ring, indicating the loss of the phenyl ring skeleton and aromaticity. For example, for toluene and xylene/Si(100), IR studies revealed the dissociative adsorption together with molecular chemisorption involving the cycloaddition of their 2,5-carbon atoms to the silicon surface dimer. In the case of chlorobenzene/Si(111)-7 × 7, HREELS results demonstrated the molecular chemisorption mechanism of a typical [4 + 2] cycloaddition involving 2,5-C atoms of chlorobenzene and adatom-rest atom pair of Si(111)-7 × 7.…”