An optic solution for the measurement of gas diffusion in thin films

Thin films are increasingly used both for the development of innovative gas sensors and for surface protection purposes due to availability of new surface treatments, such as those based on low pressure and atmospheric plasma processes. In many cases sequential treatments are required, where, after the initial deposition, the film is exposed to different chemical compounds that both react and diffuse inside the film, usually to enhance its properties. These reaction/diffusion processes depend on many parameters connected to the layer morphology and are, therefore, complex and difficult to preview. Several authors have studied the problem and have developed powerful mathematical models, whose validation, however, is difficult since making measurements during the transient process, especially for films with nanometric thickness, is not easy. In this paper, the authors investigate two optical solutions that can be applied to many practical cases to follow the diffusion process of gases in real time and thus tune and validate the models. The first solution is based on the well-known Surface Plasmon Resonance (SPR) phenomenon and allows precise evaluations to be obtained, but requires a lab-type setup. The other solution employs a Plastic Optical Fiber (POF) and exploits the evanescent field interaction to arrange a simple and cheap setup, that can be used also for in situ tests. Both approaches are applied to the study of the diffusion of sulfide vapors in silver with subsequent surface tarnishing, a topic that represents a practical case of relevant importance in the field of cultural heritage preservation.