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

Abstract

Ethynyl- and propynyl-terminated\nSi(111) surfaces synthesized using\na two-step halogenation/alkylation method have been characterized\nby transmission infrared spectroscopy (TIRS), high-resolution electron\nenergy-loss spectroscopy (HREELS), X-ray photoelectron spectroscopy\n(XPS), low-energy electron diffraction (LEED), atomic-force microscopy\n(AFM), electrochemical scanning–tunneling microscopy (EC-STM)\nand measurements of surface recombination velocities (<i>S</i>). For the ethynyl-terminated Si(111) surface, TIRS revealed signals\ncorresponding to ethynyl C–H and CC\nstretching oriented perpendicular to the surface,\nHREELS revealed a Si–C stretching signal, and XPS data showed\nthe presence of C bound to Si with a fractional monolayer (ML) coverage\n(Φ) of Φ_Si–CCH = 0.63 ± 0.08 ML.\nThe ethynyl-terminated surfaces were also partially terminated by\nSi–OH groups (Φ_Si–OH = 0.35 ±\n0.03 ML) with limited formation of Si³⁺ and Si⁴⁺ oxides. For the propynyl-terminated Si(111) surface, TIRS revealed\nthe presence of a (C–H)_CH₃ symmetric\nbending, or “umbrella,” peak oriented perpendicular\nto the surface, while HREELS revealed signals corresponding to Si–C\nand CC stretching, and XPS showed C bound to Si with Φ_Si–CCCH₃ = 1.05 ± 0.06 ML. The LEED\npatterns were consistent with a (1 × 1) surface unit cell for\nboth surfaces, but room-temperature EC-STM indicated that the surfaces\ndid not exhibit long-range ordering. HCC–Si(111) and CH₃CC–Si­(111) surfaces yielded <i>S</i> values\nof (3.5 ± 0.1) × 10³ and (5 ± 1) ×\n10² cm s^–1, respectively, after 581 h\nexposure to air. These observations are consistent with the covalent\nbinding of ethynyl and propynyl groups, respectively, to the Si(111)\nsurface.