INKJET PRINTING OF HIGH REFRACTIVE STRUCTURES BASED ON TiO₂ SOL

Subject of Research. The paper deals with description of novel inkjet printing method for optical elements formed by structures based on TiO₂ sol. The proposed approach presents effective way to obtain such optical nano objects as diffraction structures and transparent interference patterns. Methods. TiO₂ nanoparticles were synthesized by hydrolysis of titanium isopropoxide (0.36 mol)  in deionized water (33.3 mol) under vigorous stirring and using nitric acid (0.04 mol) as a protonating agent. Viscosity was determined by Brookfield HA/HB viscometer, and surface tension - by Kyowa DY-700 tensiometer. Titanium dioxide inks were deposited on commercially available microembossed PET film with a thickness of 20 µm. To print titania ink Canon Pixma IP 2840 desktop office printer was used with a drop volume of 2 pL. The thickness of an inkjet TiO₂ layer after drying in the air and removal of the solvents did not exceed 500 nm with a refractive index not less than 2.08 in the entire visible range.Main Results. The synthesis of aqueous TiO₂ sol was used to obtain the ink with desirable rheological characteristics: viscosity and surface tension. The required rheology was regulated by controlling parameters of sol-gel transition in the system of aqueous titanium dioxide sol and by adding ethanol that affects the charge of double electrical layer of disperse phase. The reviled ratio of titanium dioxide sol and ethanol in the system gives such values of viscosity and surface tension that make this material convenient for inkjet printing. The coatings created by sol have a high refractive index in the entire visible range (not less than 2.08). We have shown that the deposition of optical transparent microstructures with diffraction effect has an ability to be applied on the transparent surfaces. The morphology of particles and the topology of printed structures were analyzed by optic and atomic-force microscopes.Practical Relevance. We have proposed the approach to obtain colorful interference patterns using one type of high refractive inks. The method opens new opportunities for “roll-to-roll” production technology of protected optical structures on the flexible polymer substrates.

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PICTURES

 Fig. 1. Dependence of surface tension (a) and viscosity (b) from the degree of titanium dioxide sol dilution with ethanol in the composition of the ink

 Fig.2. The dependence of reflection index on the wavelength of normally incident optical radiation onto the homogeneous surface: 1 – SiO₂ glass, 2 –TiO₂ film on glass surface SiO₂ (fused silica)

 Fig. 3. The dependence of refractive index of nanocrystalline TiO₂ film, deposited on the substrate surface, on the wavelength

 ig. 4. A picture of deposited and dried titanium-oxide ink drop performed on the Biolam M-1 LOMO microscope

 Fig. 5. The mechanism of drop transformation from liquid to solid state at the deposition by inkjet printing. The model of ink drop drying and its turning into a solid structure (a); the relief model of obtained structure (b); the AFM image of obtained structure (c); cross-section profilogram of obtained image(d); the topography of plateau surface for dried-up drop obtained on AFM (d)

 Fig. 6. Model (a, b, c) and photos of experiment (d, e, f) for laser beam passing through a diffraction grating: (a) and (d) – a laser beam that has not passed through a diffraction grating; (b) and (e) – the model gratings obtained by depositing the structures by means of the inkjet printing; (c)and (e) – diffraction patterns of the model and real experiment

 Fig. 7. Photograph of the encoded holographic image reconstruction

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