Electron-induced chemistry of pyridine on Si(111)7 × 7: a LEED and TDS study

Abstract

The adsorption of pyridine on Si(111)7 × 7 at room temperature (RT) is investigated by thermal desorption spectrometry (TDS). The molecular (mass 79) desorption profile, with desorption maxima at ∼ 380 and ∼ 470 K, is found to resemble that of five-membered heterocyclic aromatic compounds including thiophene and furan. The corresponding molecular TDS profile for pyridine exposed to a sputtered Si surface consists of a single broad peak at ∼ 420 K with similar intensity as that found for the 7 × 7 surface, thus suggesting that a specific type of active sites is not essential for the pyridine adsorption. On the other hand, the large reduction in mass 79 desorption observed for pyridine exposure to an oxidized Si surface shows that Si dangling bonds are required for the initial adsorption of pyridine. In addition, a 50 L RT exposure of pyridine on Si(111)7 × 7 results in a “modified” 7 × 7 LEED pattern in which the fractional-order spots become less intense except for the off-axis (± 37, ± 37), fractional-order spots that remain relatively intense. Furthermore, the rapid conversion of the modified 7 × 7 pattern to a 1 × 1 pattern during the LEED experiment suggests that the adsorbed pyridine molecules undergo an electron-induced dissociation reaction on the Si surface. The corresponding mass 79 TDS profile is found to become broaden and less intense with increasing electron irradiation current and incident electron energy. The electron irradiation also appears to significantly increase the mass 4 (D2) desorption for RT exposure of d5-pyridine to Si(111)7 × 7, which further indicates that electron-induced dissociation of pyridine results in the deposition of C, N, and D atoms on the Si surface. It is, however, unclear if the electron-induced dissociation is a direct process or a stepwise process involving smaller intermediate fragments that further decompose during thermal annealing. Comparison of the respective TDS profiles of pyridine and benzene shows that the heteroatom appears to have the effect of reducing the adsorption of pyridine on the Si surface (in comparison to that of benzene) while inducing a greater surface reactivity. The present work further illustrates that the similarities observed in the TDS profiles for a series of homologous five-membered and six-membered aromatic compounds may not necessarily entail similar adsorption configurations and that the complex surface chemistry involved can indeed be quite different for these molecules on the Si(111) surfaces.