Browsing by Author "Ng, A"

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    Comparing the reactivity of alkynes and alkenes on silicon (100) surfaces
    (American Chemical Society, 2009-12-15) Ng, A; Ciampi, S; James, M; Harper, JB; Gooding, JJ
    The relative reactivities of alkynes to alkenes on hydrogen-terminated silicon (100) surfaces, under conditions where a monolayer will be produced via hydrosilylation, were measured using two different approaches. The first approach was to form monolayers from a series of solutions containing different mole fractions of an alkyne, with a trifluorothioacetate distal moiety and an alkene with a terminal carboxylic acid functional. X-ray photoelectron spectroscopic analysis of the resultant surfaces showed that the mole fraction of alkyne on the surface was larger than that in the respective alkyne/alkene mixture. By filling the XPS data, we estimated that the reactivity ratio of alkynec to alkene was approximately 1.7 +/- 0.2 when monolayers were formed at 120 degrees C. The second approach was using a molecule containing both an alkyne at one end and an alkene at the other, non-1-yne-8-ene, as the hydrosilylation reagent such that either end Could attach to the silicon surface. The relative orientation of this molecule, once reacted with it hydrogen-terminated Si(100) surface, was determined by coupling ail additional reagent to the distal end of the monolayer. The reagent used was azidoferrocene, which could attach onto free alkyne moieties on the surface only via the 1,3-Huisgen cycloaddition "click" reaction. Electrochemical analysis was then used to determine how many ferrocene moieties were attached to the SAM surface. In this way, it was shown that the alkyne end reacted preferentially with the silicon surface compared with the alkene end. The reactivity ratio of the alkyne end to the alkene end was increased front 2.0 +/- 0.2 to 9 +/- 1 when the temperature was decreased from 120 to 65 degrees C. © 2009, American Chemical Society
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    Nanoscale condensation of water on self-assembled monolayers
    (Royal Society of Chemistry, 2011-06-07) James, M; Darwish, TA; Ciampi, S; Sylvester, SO; Zhang, Z; Ng, A; Gooding, JJ; Hanley, TL
    We demonstrate that water is almost universally present on apparently dry self-assembled monolayers, even on those considered almost hydrophobic by conventional methods such as water contact goniometry. The structure and kinetics of nanoscale water adsorption onto these surfaces were investigated using X-ray and neutron reflectometry, as well as atomic force microscopy. Condensation of water on hydrophilic surfaces under ambient conditions formed a dense sub-nanometre surface layer; the thickness of which increased with exponentially limiting kinetics. Tapping mode AFM measurements show the presence of nanosized droplets that covered a small percentage ([similar]2%) of the total surface area, and which became fewer in number and larger in size with time. While low vacuum pressures ([similar]10-8 bar) at room temperature did nothing to remove the adsorbed water from these monolayers, heating to temperatures above 65 [degree]C under atmospheric conditions did lead to evaporation from the surface. We demonstrate that water contact angle measurements are not necessarily sensitive to the presence of nanoscale adsorbed water and do not vary with time. For the most part they are a poor indicator of the kinetics and the amount of water condensation onto these surfaces at the molecular level. In summary, this study reveals the need to exclude air containing even trace amounts of water vapor from such surfaces when characterizing using techniques such as X-ray reflectometry.© 2011, Royal Society of Chemistry