Photoemission studies of the surface reactivity of thiophene on Si(1 0 0)-(2×1), Si(1 1 1)-(7×7) and Ge(1 0 0)-(2×1)

The surface reactivity of thiophene with Si(1 0 0)-(2×1), Si(1 1 1)-(7×7) and Ge(1 0 0)-(2×1) has been investigated using valence band photoemission. The data clearly show that for all three surfaces thiophene adsorption leads to the formation of the same surface moiety. Based on comparisons of the photoemission data with gas phase spectra we believe that this moiety is a 2,5-dihydrothiophene-like species. The formation of a 2,5-dihydrothiophene-like species on Si(1 0 0)-(2×1) and Ge(1 0 0)-(2×1) is consistent with a 4+2 cycloaddition (Diels–Alder) mechanism being responsible for the reaction between thiophene and the two surfaces. The relative reactivities of Si(1 0 0)-(2×1) and Ge(1 0 0)-(2×1) towards thiophene are also consistent with a Diels–Alder mechanism. In contrast to the two other surfaces, the formation of a 2,5-dihydrothiophene-like moiety on Si(1 1 1)-(7×7) cannot occur via a Diels–Alder mechanism, because of the absence of the required “π-bonded” silicon dimers. The results presented would imply that although the electronic/physical properties of the three substrates may influence the mechanism of a reaction, they do not appear to significantly affect which species is the most stable product.