Publication date: 10th April 2024
We present a universally applicable measurement technique for studying diffusion in thin films. By covering parts of the measured sample with a protective layer, the electrochemical insertion of ions into the thin films can be locally confined to a small stripe-like gap, allowing us to explore lateral diffusion of inserted species away from the gap into the thin film under the protective layer. By using this method, it is possible to perform in situ Raman measurements at various distances from the gap. This allows us to record temporally and spatially resolved information about the crystal structure and optical properties of the sample. Raman measurements at a fixed position allow us to probe changes in crystal structure as the concentration of the inserted species varies. As the distance from the gap increases, the diffusion profile becomes more extended and the concentration gradients become smaller. Therefore, the temporal resolution of the measurements improves significantly. When combined with spatially resolved transmission measurements, this approach can provide detailed information about the crystal structure and diffusion profile of the thin films under investigation.
We used this method to investigate tungsten trioxide thin films and their electrochemical behavior with respect to hydrogen-induced structural phase transitions and their impact on the diffusion process. Spatially resolved transmission measurements in the wavelength range of 633±55 nm provide the diffusion profile perpendicular to the stripe like gap and are used to characterize the diffusion. The time-resolved Raman measurements at different distances from the electrolyte contact area are conducted with a 633 nm laser. Our results reveal an unambiguous correlation between the structural phase transition of the thin film (from the orthorhombic to the tetragonal phase) and the variation of the diffusion coefficient with hydrogen content. A larger diffusion coefficient of the tetragonal phase compared to the orthorhombic phase is due to the higher symmetry of the latter.
Furthermore, a numerical simulation was conducted whose results support the findings regarding the concentration-dependent diffusion.
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