• Mickan, Martin
• Semmar, Nadjib
• Karim, Wael
• Tabbal, Malek
• Petit, Agnès
• Thomann, Anne Lise

Abstract

The interaction between ultrashort laser beam pulses with a material can induce the formation of periodic surface micro/nano structures commonly referred to as LIPSS (Laser Induced Periodic Surface Structures). Controlling such a process can pave the way for the tuning of the physico-chemical properties of the material?s surface. In the case of electrochemical cells made of assembly of thin films incorporating Gadolinium-Doped Ceria (GDC), LIPSS formation can enhance the performance of the electrode by increasing its surface area and thus enhancing the reactions of the active species at the electrode/electrolyte interface. In this work, a Nd: YAG laser beam operating at the third harmonic (355 nm) and emitting 40 ps laser pulses is employed to irradiate a 4x4 mm2 surface of a GDC thin layer, that is deposited by magnetron sputtering on yttria-stabilized zirconia (YSZ) substrate. Using high resolution scanning electron microscopy (HR-SEM), it is found that LIPSS are produced at a lowfluence laser multi-pulse regime close to the ablation threshold. In agreement with the literature, it is found that these periodic structures can be distinguished depending on their spatial period and can be classified as low and high spatial frequency LIPSS, LSFL and HSFL, respectively. However, under the static mode (irradiation of the same area of 500 µm diameter) and under appropriate values of laser fluence (50 to 250 mJ/cm2) with a number of pulses varying from 1 to 70, we have also identified two types of LSFLs that are distinct in their direction and spatial period. LSFL#1 are parallel to the beam polarization, with a typical period of 238 nm and found in the center of the irradiated zone, whereas LSFL#2 are oriented perpendicular to beam polarization with a spatial period of 296 nm and found on the rim of the irradiated zone. Our results suggest that the appearance of the two types of LSFL within the irradiated spot can be attributed to different metallic and dielectric behaviors of the inner and outer zones of the GDC film, respectively. These differences are attributed to increased oxygen losses under the higher beam intensity region. We have also optimized the process parameters to generate well resolved LIPSS under beam scanning conditions. Using numerical tools for SEM/AFM images and thanks to a simple geometric model developed on such structures, the enhancement of the specific surface following laser structuring is estimated to be in the range from 50 to 80%

Topics

• impedance spectroscopy
• surface
• scanning electron microscopy
• thin film
• Oxygen
• atomic force microscopy
• Gadolinium