Thesis: Imen BEN KHALED

Reporters:

• Gabriel VELU - University of Artois - France
• Brice GAUTIER - INSA Lyon - France

Examiners:

• Michel VOUE - University of Mons - Belgium
• Sophie BARRAU - University of Lille - France
• Damien THIRY - University of Mons - Belgium
• Rabah BOUKHERROUB - University of Lille - France

Thesis supervisors:

• El Hadj DOGHECHE - Université Polytechnique Hauts-de-France - France
• Philippe LECLERE - University of Mons - Belgium

Zinc oxide (ZnO) has for several decades been one of the most promising piezoelectric materials known for its piezoelectric properties for energy recovery. This green material can be a substitute for materials containing toxic elements such as lead. In this research activity, the ultimate goal is to fabricate Yttrium (Y)-doped and undoped materials at several scales, including thin films (developed by RF magnetron sputtering) and nanowires (synthesized using a low-carbon, environmentally friendly hydrothermal technique), featuring a robust crystalline structure and vertically oriented microstructure. We investigated the influence of the ZnO seed layer and hydrothermal synthesis time on nanowire growth, in particular on density and orientation. In order to optimize the germination layer
necessary for the vertical growth of nanowires, two parameters were analyzed: the silver/oxygen ratio and the layer thickness. The germination layers and nanowires obtained were characterized by scanning electron microscopy coupled to X-ray energy dispersive analysis (eSEM/EDS), X-ray diffraction (XRD) and atomic force microscopy (AFM). eSEM and AFM analyses highlighted the granular organization on the surface of the germination layer, as well as the characteristic hexagonal morphology of the nanowires. At the same time, their elemental composition was stoichiometric. The electrical properties of the resulting structures were also assessed by conductive atomic force microscopy (CAFM): the I-V curves show non-linear characteristics, confirming the formation of Schottky contacts. Finally, the piezoelectric behavior of ZnO NFs was studied both by forcepiezoelectric microscopy (PFM). A piezoelectric coefficient of around 30 pm/V was obtained for nondoped NFs. In contrast, for Yttrium-doped NFs, a stronger coefficient of the order of 57pm/V is observed.