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Langmuir 2002, 18, 958-959
Comments Comment on Highly Doped Silicon Electrodes for the Electrochemical Modification of Self-Assembled Siloxane-Anchored Monolayers: A Feasibility Study
In a recent paper by H. Grisaru et al.,1 the authors explore the suitability of highly doped silicon substrates as electrodes for the electrochemical modification of adsorbed alkylsiloxane monolayers. They use the reduction of a terminal carboxylic ester group in the monolayer film to the corresponding alcohol (-COOCH3 f -CH2OH) as a model reaction to test whether sufficient charge transfer rates from the bulk silicon through the native oxide layer and through a long-chain hydrocarbon adsorbate film can be achieved for in situ electrochemical surface transformations. On the basis of cyclic voltametry, atomic force microscopy, contact angle measurements, and IR reflection spectra of the monolayer films, the authors find some evidence for a partial conversion of the terminal ester groups but conclude that the electrical resistance of the silicon oxide layer necessary for monolayer anchoring via siloxane bonds is too high for practically useful electrochemical transformations of these systems. In Figure 6 and Figure 9 of their paper, external reflection IR spectra are presented of a self-assembled monolayer of methyl 11-(trichlorosilyl) undecanoate (MTU) before (Figure 6) and after (Figure 9) a potential scan from 0 to -1.5 V (versus a standard calomel electrode), the range in which the electrochemical reduction of the ester to the alcohol groups should take place. Our concern is related to the shape and intensities of the absorption bands in these IR spectra, which in our opinion are incompatible with a monolayer film on a dielectric substrate such as silicon. In Figure 1, the external reflection spectrum of an MTU monolayer film prepared and measured in our lab2 under the same conditions as in ref 1 (4 cm-1 resolution, 80° incidence angle) is shown together with the spectrum of the same sample after chemical reduction with LiAlH4. A film thickness of 1.5 ( 0.15 nm was measured by ellipsometry for both the MTU and the reduced MTU film. Whereas the peak frequencies of the major MTU absorptions in Figure 1 agree reasonably well with the peak positions measured in ref 1, there are two fundamental differences: All absorption bands in our spectra (Figure 1) are inverted, i.e., the peaks are downward-looking in a conventional absorbance-scale spectrum, and the band intensities are more than 1 order of magnitude smaller compared to the spectra in ref 1. We have shown previously3 that the band intensities and band directions of monolayer films on nonmetal, dielectric substrates depend strongly on the particular transition dipole moment orientation. This can result in very complex, (1) Grisaru, H.; Cohen, Y.; Aurbach, D.; Sukenik, C. N. Langmuir 2001, 17, 1608. (2) Experimental details on film preparation and IR spectra measurements are described in: Brunner, H.; Vallant, T.; Mayer, U.; Hoffmann, H. Langmuir 1996, 12, 4614. p-Doped, (100)-oriented silicon wafers (14-30 Ω‚cm resistivity) with a native oxide layer of 1.3 ( 0.1 nm were used. (3) Hoffmann, H.; Mayer, U.; Krischanitz, A. Langmuir 1995, 11, 1304. (b) Brunner, H.; Mayer, U.; Hoffmann, H. Appl. Spectrosc. 1997, 51, 209.
Figure 1. External reflection infrared spectra of monolayer films of MTU (OxSi-(CH2)10COOCH3) and chemically reduced MTU (OxSi-(CH2)10CH2OH) on native silicon (Si/SiO2) substrates.
Figure 2. CH-stretching vibrations in external reflection infrared spectra of ODS (OxSi-(CH2)17CH3) monolayer films on native silicon (Si/SiO2) substrates with different dopant concentrations: (A) p-dotation (boron), (100) orientation, 1430 Ω‚cm resistivity; (B) n-dotation (arsenic), (100) orientation,
