Localised growth of CuO nanowires by micro-afterglow oxidation at atmospheric pressure: Investigation of the role of stress

• CuO nanostructures are synthesized by atmospheric pressure afterglow.
• Localised growth of nanowires by stress-induced outward diffusion of copper ions
• Evaluation of residual stress by curvature method and by X-ray diffraction

Oxidation of copper thin films was performed by means of a microwave micro-afterglow at atmospheric pressure to grow locally copper oxide nanowires with controlled morphologies. As for thermal oxidation, their growth follows a parabolic rate due to transport limitation by diffusion. TEM analyses indicate that most of the produced nanowires are single crystalline CuO with monoclinic structure and random axial crystallographic orientations. However, some nanowires exhibit a bi-crystalline structure with a twin-boundary along their growth direction. Residual stress was estimated by the curvature method and locally by the XRD sin2ψ method. Compressive (respectively tensile) stress is observed in the Cu2O (respectively CuO) layer. The grain size is larger close to the center than on the edges of the circular treated area, due to strong thermal gradients in the micro-afterglow. This ensures the growth of nanowires with large diameters close to the center and smaller diameters on the edges, although with higher surface density. As for thermal oxidation, the synthesis of CuO nanowires is driven by stress-induced grain boundary diffusion processes due to the development of stress gradients. However, the use of an afterglow increases the oxidation rate at low temperature with respect to thermal oxidation, enhancing the development of stress and thus enabling the synthesis of nanowalls.