Monolayers of terminal alkynes with long hydrocarbon chains CnH2n+1C[CH (n = 10, 13, 16) were prepared on Si(100) substrates via thermally induced hydrosilylation and the surface orientation of the hydrocarbon chains was investigated by external reflection infrared spectroscopy. It was found that under rigorous exclusion of oxygen in the monolayer preparation process, all three compounds yield highly characteristic IR reflection spectra, consisting of upward-pointing v(CH2) absorptions and downward-pointing v(CH3) absorptions, indicative of a highly ordered anisotropic film structure. Via spectral simulations it was found that the methylene backbones (CH2)n in these films adopt a uniform, all-trans conformation with a tilt angle of about 30° toward the surface normal, whereas the chain termini are disordered and give an isotropic film surface composed of randomly oriented CH3 groups. Lower quality films, which are hardly distinguishable from highly ordered films by other methods, but have been shown to exhibit inferior electrical properties, are clearly identified in their infrared (IR) spectra as partly disordered structures. External reflection IR therefore proves to be an exceptionally sensitive tool to detect structural defects in these monolayers.
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