Cylinder microstructure on non-planar and functional surface based on photo-patternable TiO2 sol-gel and colloidal photolithography
Résumé
Subwavelength micro-structured surfaces are often used as antireflection and light-trapping structures whose main optical based applications concern solar energy and efficiency improvements of existing products (PhotoVoltaic cells and modules, Concentrated Solar Power systems…). The geometry of the overwhelming majority of diffractive optical microstructures is planar. The microstructuring of warped surfaces in the 3D-space is much less developed in particular cylindrical grating. The authors demonstrate here a unique low cost process to print directly TiO2 gratings on both planar surfaces and cylindrical components without etching and/or lift-off process. Furthermore, TiO2 offers interesting optical properties, good mechanical and chemical stability and thus can be directly used as a functional photocatalytic microstructure for outdoor applications.A specific sol–gel formulation has been adapted from titanium isopropoxyde orthotitanate (TIPT) complexed by benzoyl acetone (BzAc) in alcoholic solvent. Easily deposited by spin coating or dip coating to form a thin xerogel layer, it creates under UVA exposition, a contrast of solubility in alcohol between illuminated and non-illuminated areas to achieve direct microstructuring like a negative photoresist. This protocol [1,2] was implemented as part of a new technique of micro-structuration compatible with cylindrical substrates. An hexagonal pattern of silica microspheres of 1 µm diameter deposited by Langmuir Blodgett process on the photosensitive xerogel layer has been illuminated by a collimated source emitting at 365 nm wavelength. Each silica microsphere acts as a convergent super-lens, focusing the light to create a so-called photonic nano-jet beam propagating in the TiO2 underlying photoresist layer. After washing in solvent, nanopillars are revealed, according to the photonic nanojet distribution. The nanopillars adopt the previous hexagonal pattern of the microsphere and their periodicity, line-space ratio and aspect ratio are easily controllable. To print 2D nanopillars all around a cylinder, a specific optical set-up has been developed. It consists to illuminate the cylinder with a UV extended mercury lamp while the cylinder is turning to achieve a complete insolation all around the cylinder. Since the submicron hexagonal pattern is imposed by the spheres arrangement and not by the optical beam, mechanical and optical tolerances are very good. The authors will present modeling of the electric field distribution underneath the spheres during the illumination process with the RCWA method (Rigorous Coupled Wave Analysis) and preliminary results of TiO2 nano-pillars on planar substrate. Then, authors will present the adaptation of this photolithography method on cylindrical substrates