Abstract
Hydrophobic to hydrophilic transition of HF-treated Si surface strongly depends upon the metal ions, which are present in the headgroups of the deposited Langmuir–Blodgett (LB) film. Structure of LB films studied by X-ray reflectivity technique and chemical analysis of LB film–substrate interfaces studied by X-ray photoelectron spectroscopy combinedly suggest that the partial transition or partial oxidation of the HF-treated Si surface takes place under the subphase water but further transition or oxidation is possible only in the presence of metal ions. Electrovalent and covalent natures of the metal ions tune this transition or oxidation. Ni ions, for which bonding with headgroups are electrovalent in nature, are favorable for such transition/oxidation and as a result, complete transition/oxidation takes place when nickel arachidate LB film is deposited. On the other hand, Cd ions, for which bonding with headgroups show covalent nature, is not favorable for such transition and can not oxidize the underlying H-passivated Si substrate totally when cadmium arachidate LB film is deposited on such HF-treated Si surface. This ion-specific hydrophobic to hydrophilic transition is visualized by X-ray reflectivity, contact angle and X-ray photoelectron spectroscopy measurements.
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