960
Langmuir 2002, 18, 960-960
Reply to Comment on Highly Doped Silicon Electrodes for the Electrochemical Modification of Self-Assembled Siloxane-Anchored Monolayers: A Feasibility Study
Hoffman et al. contend that the peaks reported in our external reflection FTIR spectra are too intense and that at least some of the peaks are pointed in the wrong direction. As to the peak intensity/film thickness issue, while we did not have ellipsometry data, we did report atomic force microscopy data on the as-deposited methyl 11-(trichlorosilyl)undecanoate (MTU) film and on such films after electrochemistry. These data showed that the MTU film had a roughness almost identical to the silicon oxide surface onto which it was deposited, with a peak-to-trough height of no more than a few angstroms. This observation and the lateral force measurements were consistent with a relatively thin, homogeneous, compact film that follows the substrate topography but, we concede, do not rigorously limit the film to a monolayer. As to the more important issue of peak shape and direction, it is clear that when dealing with combinations of peaks, the intensity in any given direction of external reflection FTIR peaks will be very sensitive to the substrate on which the film is deposited. We are therefore concerned that the spectra reported by Hoffman are not directly comparable to ours. They are dealing with different doping (arsenic versus phosphorus), and the conductivity of the substrates was quite different. The surface preparation of the wafers was different, likely leading to different thicknesses of oxide between the organic film and the substrate. Hoffman et al. acknowledge that FTIR spectra
of adsorbed species in external reflectance mode are highly dependent on the state of the surface, its electrical conductivity, and the nature of the surface film(s) on top of the substrate. Our results relate specifically to highly doped silicon whose conductivity is nearly metallic. The oxide film on our silicon is particularly thin. Hoffman’s experiments with an ODS monolayer on variously doped silicon do not really clarify the picture because these spectra will also depend on the effective conductivity of the substrate and must take into account the thickness and structure of the surface oxide and interactions with the dipoles in the specific surface-bound functional groups. They may also be specific to the nature of the molecules adsorbed and their specific interactions with the conductive substrate. Unless these factors are identical, significant differences (in both directions of peaks and resulting net intensities) among surface-sensitive FTIR spectra can be expected. Our spectra were obtained using a standard commercial (Nicolet) spectrometer and a standard commercial (Spectratech) external reflection accessory, and we are comfortable that they are real. It is likely that differences between them and Hoffman’s expectations are due to differences in the state of the substrate surface. Haviv Grisaru, Yair Cohen, Doron Aurbach,* and Chaim N. Sukenik*
Department of Chemistry, Bar Ilan University, Ramat-Gan, Israel 52900 Received November 13, 2001
10.1021/la015679e CCC: $22.00 © 2002 American Chemical Society Published on Web 01/05/2002
LA015679E
