Olga Shavdina

Elaboration and surface modification of structured poly(l-lactic acid) thin film on various substrates

Biosourced or biodegradable polymers like poly(lactic acid) (PLA) are often base-material for tissue-engineered scaffolds. However, in most of the cases, their bioadhesion properties are not satisfactory. Since the adhesion is controlled both by roughness and surface chemistry, PLA films were textured by applying the breath figure procedure and, then, plasma-treated. Depending on physicochemical characteristics of the breath figure technique, nice hexagonal structures were obtained. Their surface properties, i.e. hydrophobic-hydrophilic balance were controlled by plasma modification. However, their surface decoration could be only preserved with some specific plasma parameters depending on the applied energy and also on the induced surface chemistry.

Control Methods in Microspheres Precision Assembly for Colloidal Lithography

Colloidal lithography based on the assembly of microspheres is a powerful tool for the creation of a large variety of two dimensional micro or nanostructures patterned. However very few studies examine the control, qualification and quantification of the ordering of the particles once deposited on the substrate. We have developed two unique methods working at microscopic and macroscopic scales, respectively called Microfixe® and Macrofixe®, for the analysis of grain morphology in the case of hexagonal closed packed (HCP) monolayers of spherical microparticles. The processing of the images taken at microscopic scale uses Delaunay triangulation and histograms of lengths and orientations of Delaunay triangles sides. At the macroscopic scale, six camera images are required of the sample illuminated under six different incidence angles. Image treatment consists in the comparison of the six images and eventually subdivision of these images to sharpen the analysis. At the end, the two softwares constitute artificial images of particle deposit, giving at microscopic and macroscopic scales significant information about grain size, grain morphology, orientation distributions, defaults (voids, stacking)…With these two new control methods, colloidal lithography is emerging as an industrial process.

Periodic TiO2 Nanostructures with Improved Aspect and Line/Space Ratio Realized by Colloidal Photolithography Technique

This paper presents substantial improvements of the colloidal photolithography technique (also called microsphere lithography) with the goal of better controlling the geometry of the fabricated nano-scale structures—in this case, hexagonally arranged nanopillars—printed in a layer of directly photopatternable sol-gel TiO2. Firstly, to increase the achievable structure height the photosensitive layer underneath the microspheres is deposited on a reflective layer instead of the usual transparent substrate. Secondly, an increased width of the pillars is achieved by tilting the incident wave and using multiple exposures or substrate rotation, additionally allowing to better control the shape of the pillar’s cross section. The theoretical analysis is carried out by rigorous modelling of the photonics nanojet underneath the microspheres and by optimizing the experimental conditions. Aspect ratios (structure height/lateral structure size) greater than 2 are predicted and demonstrated experimentally for structure dimensions in the sub micrometer range, as well as line/space ratios (lateral pillar size/distance between pillars) greater than 1. These nanostructures could lead for example to materials exhibiting efficient light trapping in the visible and near-infrared range, as well as improved hydrophobic or photocatalytic properties for numerous applications in environmental and photovoltaic systems.

Cost-effective and large-area process for the functionalization and surface microstructuring using photo-patternable TiO2 sol-gel process and colloidal photolithography

The authors demonstrate a low cost direct process to directly print TiO 2 grating on both planar and non-conventional substrate such as cylinder components. A well collimated i-line source emitting at a 365nm wavelength illuminates a mono layer of 1μm diameter silica microspheres deposited on a photosensitive TiO 2 -based sol-gel layer. No etching process is required since this layer is directly UV photo patternable like a negative photo-resist. Furthermore this thin layer offers interesting optical properties (high refractive index) and good mechanical and chemical stability. The paper describes the photochemistry of the TiO 2 sol-gel layer process, the modeling of the electric field distribution below the spheres during the illumination process and preliminary results of TiO 2 nanopillars of 200 nm diameter.

Nano-composite magnetic material embedded on TiO2 pillars to realize magneto-optical resonant guided mode gratings

Periodic structuration of magnetic material is a way to enhance the magneto-optical behavior of optical devices like isolators. It is useful to reduce the footprint of such integrated devices or to improve their features. However, the structuration and/or integration of efficient magnetic materials on photonic platforms is still a difficult problem, because classical magneto-optical materials require an annealing temperature as high as 700°C. A novel wafer-scale approach is to incorporate that material into an already structured template through a single step deposition at low temperature. Using the dip-coating method, a magneto-optical thin film (~300nm) of CoFe2O4 nanoparticles in silica matrix prepared by sol-gel technique was coated on a 1D and 2D TiO2 subwavelength gratings. Such gratings were realized by the patterning of TiO2 films obtained by a sol-gel process. It was confirmed by Scanning Electron Microscope images that the magneto-optical composite completely occupies the voids of the 2D structuration showing a good compatibility between both materials. This composite shows a specific Faraday rotation of about 200°cm-1 at 1,5μm for 1% of volume fraction of nanoparticles. Spectral studies of the transmission and the reflection of a 1D TiO2 grating filled with the MO composite have evidenced the presence of a guided-mode optical resonance at 1,55μm. The position of this resonance was confirmed by numerical simulations, as well as its quite low efficiency. Based on simulations results, one can conclude that an increase of the grating depth is required to improve the efficiency of the resonance.