Picosecond laser beam nanostructuring of GDC thin films: exchange surface enhancement by LIPSS

In ceramic electrochemical cells, gadolinium-doped ceria (GDC) thin film is deposited between the electrolyte (yttria-stabilized zirconia, YSZ) and the upper electrode to improve the aging resistance. Increasing the interface surface area is expected to be beneficial to the cell performance. This work investigates the formation of LIPSS (Laser induced periodic surface structures) on the surface of a 600 nm GDC thin films grown by magnetron sputtering on screen-printed YSZ. A picosecond Nd:YAG laser operating at its third harmonic (lambda = 355 nm, 40 ps, 10 Hz) was used to structure the surface of the GDC/YSZ layer after deposition, thanks to static and scanning irradiation modes. The different structures formed on the film surface following laser irradiation, mainly quasi-periodic LIPSS, were investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Parallel and perpendicular LIPSS were simultaneously formed with periods varying from 220 to 300 nm, as a function of the beam fluence ranging from 30 to 160 mJ/cm(2), and within a spot size of similar to 500 mu m in the static mode. The coexistence of the two orientations disappeared at a higher number of shots (N> 20), where only parallel structures were formed. Increasing the number of shots up to N> 50 led to a progressive ablation of the film followed by the start of a process of YSZ substrate nano-structuring. The formation of parallel LIPSS is mainly attributed to the thermochemical process that occurs on the GDC/YSZ film in the center of the irradiated zone corresponding to the highest local fluence. The same LIPSS orientations were also found in the scanning mode by varying the laser fluence and the scanning stage parameters. For the latter mode, the effect of laser irradiation on the elemental composition of the films was investigated by energy dispersive X-ray spectroscopy (EDX) and Rutherford back-scattering (RBS) to check the film chemical stability under the process of soft ablation by accumulation effect. Finally, the surface area enhancement by LIPSS formation was estimated thanks to atomic force microscopy (AFM) and using a geometrical enhancement coefficient that returned 'theoretical' maximum values of 57% and 78% for 1D (regular LIPSS) and 2D periodic structures, respectively, and 41% for the experimental case. [GRAPHICS] .