We describe a novel solid state crystallisation method for optimising a thin film transparent conductive oxide when deposited on flexible polymer substrates. The method is based on ultra-short non-thermal laser sintering of indium tin oxide (ITO) thin films. In this study, we used commercial ITO thin films deposited on a flexible polyethylene terephthalate substrate with a relatively low melting temperature compared with ITO on glass. We demonstrate the use of laser scanning with high pulse overlapping at fluences seven times less than the threshold required for melting/damage of ITO. The results confirm greater than four times enhancement in the mobility of charge carriers of ITO thin films after laser scanning and sheet resistance can be reduced up to 25%. There is no reduction in optical transparency observed in laser treated samples. Surface morphology and x-ray diffraction analyses confirm the improvement in crystallite sizes by laser sintering, resulting in a greater than 37% increase in grain size due to enhanced crystallization. Comparison of experimental and simulation based on a delayed two temperature model confirms that ITO thin film crystallization occurred at about one-third of the melting temperature of ITO.

Silver thin films are well known as the most sensitive material for surface plasmon resonance (SPR) based analysis. However, the use of silver for this purpose is limited by three main issues, namely poor adhesion to plastic substrates, chemical instability in both air and aqueous environments and hence the difficulty in functionalizing the silver coated substrate for immobilizing biomolecular ligands by conventional liquid phase methods. In this work, we have successfully addressed these problems using gas-phase coating processes. We demonstrate highly adherent sputter-deposited silver coatings on low cost polymer substrates using a sputter-deposited thin gold adhesion layer. The problems of chemical instability and functionalisation have been addressed by using the gas phase process of plasma enhanced chemical vapour deposition (PECVD) to deposit thin films with a base SiO_xC_yH_z layer (using tetraethyl orthosilicate precursor) functionalised with carboxylic acid (from sequential deposition with acrylic acid precursor). The resultant coating serves as a protective layer against degradation of the optical properties of silver under long term storage and use in ambient conditions. The reactive carboxyl functionality is used for the covalent immobilization of biomolecules. The successful stabilisation and functionalization of silver films on plastic sensor chips is demonstrated by mouse IgG immunoassays. The expected superior performance of the silver thin films over gold thin films for SPR analysis is demonstrated.

The fabrication of low-cost and mechanically robust flexible electronic patterns has increasingly gained attention due to their growing applications in flexible displays, touch screen panels, medical devices, and solar cells. Such applications require cost-effective deposition of metals in a well-controlled manner potentially using nanoparticles (NPs). The presence of solvent and precursors in NP based inks impacts the electrical conductivity of the printed pattern and a post-processing heating step is typically performed to restore the electrical properties and structure of the material. We report printing with picolitre droplet volumes of silver (Ag) NPs on flexible substrates using an acoustic microdroplet dispenser. The low-cost, controlled deposition of Ag ink is performed at room temperature on photopaper, polyimide and clear polyimide substrates. A localized, ultrashort pulsed laser with minimal heat affected zone is employed to sinter printed Ag patterns. For comparison, oven sintering is performed, and the results are analysed with scanning electron microscopy, four-point probe and Hall measurements. The femtosecond laser sintering revealed highly organized, connected nanostructure that is not achievable with oven heating. A significant decrease in sheet resistance, up to 93% in Ag NPs on clear polyimide confirms the laser sintering improves the connectivity of the printed film and as a result, the electrical properties are enhanced. The surface morphology attained by the laser sintering process is interpreted to be due to a joining of NPs as a result of a solid-state diffusion process in the near surface region of NPs.