Olefin Additions on H−Si(111): Evidence for a Surface Chain Reaction Initiated at Isolated Dangling Bonds

We report the surface chain reaction of styrene with hydrogen-terminated Si(111), H−Si(111) initiated at isolated surface dangling bonds. Electrons from a scanning tunneling microscope in ultrahigh vacuum are used to create surface isolated dangling bonds on H−Si(111) prepared by aqueous ammonium fluoride etch. Exposure of dangling bonds on an otherwise hydrogen terminated surface to as little as 5 langmuirs of styrene leads to the formation of compact islands containing multiple styrene adsorbates bonded to the surface through individual C−Si bonds. From this observation we conclude that these adsorbate islands are the result of a surface chain reaction of styrene with H−Si(111). This result suggests that the key radical-chain propagation step of hydrogen atom abstraction from the silicon surface for the proposed mechanism of reaction of liquid phase terminally unsaturated organic molecules with hydrogen-terminated silicon under free-radical conditions readily occurs.