Response of Polyelectrolyte Layers to the SiO 2 Substrate Charging As Probed by XPS Can Pinar Conger and Sefik Suzer* Department of Chemistry and Institute of Materials and Nanotechnology, Bilkent UniVersity, 06800 Ankara, Turkey ReceiVed October 7, 2008. ReVised Manuscript ReceiVed NoVember 26, 2008 A single layer of the cationic polyelectrolyte poly(allyamine) hydrochloride (PAH) deposited, using the layer- by-layer technique, on a silicon substrate containing 5 nm oxide layer is investigated by XPS while applying an external potential bias to the sample to control and manipulate the charge built-up on the oxide layer. Under application of a -10 V bias, the oxide layer is positively charged due to photoemission process, evidenced by the measured Si2p binding energy of 104.4 eV. Application of a +10 V bias attracts the low energy neutralizing electrons, stemming from a hot filament, and leads to a negatively charged oxide layer, also evidenced by the measured Si2p binding energy of 102.9 eV. The single polyelectrolyte overlayer also responds to this polarity change of the oxide layer underneath by displaying a somewhat larger shifts both in the C1s and N1s peaks. In addition to the shifts in the positions, the N1s peaks undergo a significant intensity depletion, mostly on the positively charged -N + component. We interpret this intensity depletion to be the result of reorientation of some of the dangling positively charged groups by moving toward the negatively charged oxide underlayer. To our knowledge this is the first time that a chemically specific response to an electrical stimuli is reported using XPS. A bilayer LbL film consisting of PAH and PSS, exhibits even a larger charging shift, but this time no intensity alteration is observed, most probably due to locking of the -N + groups by the -SO 3 + counterions of the second layer. Introduction Control of interfacial properties of materials such as wetting, by means of external chemical/physical stimuli, is still a grand challenge to chemists, physicists, biologists, and material scientists. Permanent and/or reversible alteration of wettability upon application of chemical, electrochemical, thermal, and optical stimuli has been reported and the underlying chemical and physical forces at the molecular level have been determined and extensively discussed. 1-6 A recent study by Lahann et al. demonstrated the feasibility of reversibly switching the surface between hydrophobic and hydrophilic states, in response to an applied electrical potential to the underlying substrate. 7 Wetting property of a surface is generally assessed by the traditional water-contact-angle method, which gives limited information at the molecular level. For obtaining detailed molecular information various types of spectroscopic and/or imaging methods must be utilized. The recently developed and highly sensitive imaging techniques like STM, AFM, etc. unfortunately do not have any chemical specificity, and common spectroscopic methods like IR are not sensitive to interfaces, unless used in very special and difficult modifications like SFG techniques. 1-7 Electron spectroscopic techniques, like XPS, on the other hand offer both chemical specificity and interface sensitivity. 8-10 In addition, it is also possible to detect the electrical potentials externally applied and/or internally developed as a consequence of charge built-up. In XPS analysis, the photoelectrons emitted from the sample may lead to positive charging if the sample is not conducting. These charges are usually eliminated by directed flow of low energy electrons or ions, which, under certain conditions, may lead to build-up of excess electrons, i.e. negative charging. 11,12 The sign and the extent of charging can easily be controlled by application of an external voltage bias to the sample rod, which, in turn, can be utilized for extracting additional analytical and/or electrical information about the sample under investigation, as was recently shown by us and others. 13-22 Using the same strategy, in the present contribution, we show that, during our XPS analysis, by switching the sign of the external bias, we force a thin dielectric layer of SiO 2 substrate to switch from a positively charged state to a negatively charged state, which in turn causes a depletion only in the N1s signal of the quaternary ammonium ions (-N + ) belonging to the polyelectrolyte overlayer. 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