Characterization of Langmuir-Blodgett film using differential charging in X-ray photoelectron spectroscopy

Application of differential charging in XPS for structural study of Langmuir-Blodgett films

Differential charging in XPS was recently shown to be a novel technique for studying in-depth structural information of discrete cadmium layers in Langmuir-Blodgett (LB) multilayer films. Here we report structural modification in multilayer LB films after sulfidation using differential charging in angle-dependent XPS that are not observable by the x-ray reflectivity technique. An AFM study suggests less modification in compact LB films in comparison to the non-compact ones. The differential charging in the LB multilayers changes after sulfidation due to the formation of cadmium sulfide nanostructures in the cadmium arachidate LB matrix, which was reflected prominently in the differential charging of Cd 3d(5/2) XPS peaks. It was found that, even after sulfidation, the compact multilayer LB films were differentially charged in the out-of-plane (in-depth) direction, whereas this kind of differential charging was not apparent in rough and non-compact films. Our results clearly indicate that for LB films with compact structure a partial layered structure survives the impact of particle formation, whereas for non-compact films the modification is large and no specific conclusion could be drawn. While x-ray reflectivity cannot provide specific information about the internal structure of the post-sulfidation films it shows that the total thickness of the films reduces in all cases.

Response of polyelectrolyte layers to the SiO2 substrate charging as probed by XPS

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 -SO3+ counterions of the second layer.