X-ray Reflectivity (XR) measurements allow an atomic scale determination of the morphological characteristics of thin films, such as their thickness and surface roughness. In the present work, the Energy Dispersive (ED) variant of the conventional XR technique is used in-situ to obtain, for the first time, the (minimal) morphological changes of gas sensing thin films of Ruthenium Phtalocyanine (RuPc)2 upon working. The real-time collection of Reflectivity spectra during the exposure of the films to a gas flux of nitrogen oxides (NOX) molecules enabled to sample the evolution of the two morphological parameters (thickness and roughness) with an unprecedented density, providing new information on the gas-film interaction.
To perform this Reflectometry experiment, the advantages of the ED technique on the Angular Dispersive (AD) counterpart, connected to the immobility of the experimental apparatus during data collection, were essential. Indeed, in the grazing geometry required for this kind of measurements, even minimal misalignments of the sample may induce relevant relative errors during the angular scan. Moreover, if many scans have to be carried out consecutively, as in the present case, reproducibility concens may arise because of the mechanical movements of the diffractometer arms.
This experiment enables a discrimination to be made among the proposed models that account for the different processes that occur during the absorption of the film and diffusion of the gas. Indeed, the evolution of the morphological parameters described by any realistic model is to be consistent with the x-ray reflectometry results.
The present work studies the kinetics of interaction between the NOX gas and two (RuPc)2 films. The experiment consisted of collecting some fifty X-ray reflection spectra, each acquired for fifteen minutes, over a total observation time of about twelve hours. The analysis of the data gave an insight in the kinetic evolution of the gas-film interaction process and allowed to discriminate two distinct reaction stages. The interaction with the gas involves both the surface and the bulk of the film: the surface interaction between the gas and (RuPc)2, inducing the increase in roughness, is more rapid and not influenced by the underlying bulk i.e. independent on the film thickness. However, the bulk process, involving the intercalation of the gas molecules inside the film, needs a longer time and is related to the film's initial thickness. Therefore, the hypothesis of a two step process of the interaction mechanisms could be confirmed by a direct measurement of the film morphologic parameters.
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