Electronic and vibrational properties of thiophene on Si(100)

The surface reactions of thiophene on Si(100)-2x1 have been investigated as part of a larger study on the interaction between five-membered heterocyclic aromatic molecules and silicon substrates. Using XPS, UPS, and HREELS, two adsorption states are identified at 120 K, corresponding to physisorbed and chemisorbed thiophene. The former desorbs below 200 K, whereas the latter strongly binds to the surface, showing lower C(1s) and S(2s) binding energies. HREELS reveals that chemisorbed thiophene is a 2,5-dihydrothiophene-like species which exhibits coverage-dependent reorientation from nearly parallel to tilted relative to the surface plane. Based on the frontier molecular orbital theory and work function measurements, an inverse Diels-Alder cycloaddition mechanism for thiophene with Si(100)-2x1 is proposed. Above 400 K, chemisorbed species either desorbs as molecular thiophene or decomposes possibly via alpha -thiophenyl and Si-H, and metallocycle-like intermediate and atomic S through a sulfur abstraction mechanism. By 1000 K, the S(2s) peak has disappeared, leaving silicon carbide on the substrate.