LIPSS formation by picosecond laser irradiation of magnetron sputtered CGO thin films
Résumé
The interaction between ultrashort laser beam pulses (40 ps) with gadolinium-doped ceria (CGO) thin films induces the formation of surface micro/nanostructures. These laser-induced periodic surface structures (LIPSS), or ripples, play a very important role to improve the physico-chemical properties of thin films as specific surface. In the case of electrochemical cells made of thin film assembly, they can further enhance the performance of the electrode by increasing the reactions of the active species at the electrode/electrolyte interface. However, during pulsed laser irradiation, the ablative structure of the thin films of CGO oxides leads us to difficulties with their adhesion to the YSZ electrolyte and therefore prevents us from properly obtaining these periodic ripples.
A Nd: YAG laser beam at the third harmonic (355 nm) is employed to irradiate a 4 X 4 mm2 upon the surface of CGO thin layer, that was deposited by magnetron sputtering.
The morphological characteristics of the thin films and their detachment from YSZ substrate were observed by high resolution scanning electron microscopy (HR-SEM). LIPSS are generally produced in a low fluence laser multi-pulse regime close to the ablation threshold. They were obtained with the period of approximately 220 nm under appropriate values of laser fluence (F from 138 to 417 mJ/cm2) and scanning speed (1 mm/s to 5 mm/s). Even with these rather weak conditions, we can ablate the surface. In agreement with literature, it has been noted that these periodic structures can be distinguished as function of their period, that we can classify as low spatial frequency LIPSS (LSFL) and high spatial frequency LIPSS (HSFL). In this work, we focus on the optimization of laser parameters to generate clear and high resolution LSFL/HSFL without ablating the CGO layer. Using numerical tools for SEM/AFM images, the enhancement of the specific surface will also be discussed.