Synthesis, Characterization, and Reactivity of Ethynyl- and Propynyl-Terminated Si(111) Surfaces

Abstract

Ethynyl- and propynyl-terminated Si(111) surfaces synthesized using a two-step halogenation/alkylation method have been characterized by transmission infrared spectroscopy (TIRS), high-resolution electron energy-loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), atomic-force microscopy (AFM), electrochemical scanning–tunneling microscopy (EC-STM) and measurements of surface recombination velocities (S). For the ethynyl-terminated Si(111) surface, TIRS revealed signals corresponding to ethynyl ≡C–H and C≡C stretching oriented perpendicular to the surface, HREELS revealed a Si–C stretching signal, and XPS data showed the presence of C bound to Si with a fractional monolayer (ML) coverage (Φ) of ΦSi–CCH = 0.63 ± 0.08 ML. The ethynyl-terminated surfaces were also partially terminated by Si–OH groups (ΦSi–OH = 0.35 ± 0.03 ML) with limited formation of Si3+ and Si4+ oxides. For the propynyl-terminated Si(111) surface, TIRS revealed the presence of a (C–H)CH3 symmetric bending, or “umbrella,” peak oriented perpendicular to the surface, while HREELS revealed signals corresponding to Si–C and C≡C stretching, and XPS showed C bound to Si with ΦSi–CCCH3 = 1.05 ± 0.06 ML. The LEED patterns were consistent with a (1 × 1) surface unit cell for both surfaces, but room-temperature EC-STM indicated that the surfaces did not exhibit long-range ordering. HCC–Si(111) and CH3CC–Si(111) surfaces yielded S values of (3.5 ± 0.1) × 103 and (5 ± 1) × 102 cm s–1, respectively, after 581 h exposure to air. These observations are consistent with the covalent binding of ethynyl and propynyl groups, respectively, to the Si(111) surface.