Swaminathan Parasuraman

Printed electronics is an emerging field involving the fabrication of electronic devices by the patterned deposition of material inks. For many systems, producing stable printable inks is the key challenge. In this work, the formulation of a silver nanowire-based ink for printed transparent electrode applications is described. The nanowire length and rheology of the ink are adjusted for printing, with a single layer printed film having a sheet resistance of approximately 30 Ω/□ and a transmittance of 94% at 550 nm. The number of printed layers and volume per layer are optimized to get maximum transparency with good electrical conduction. A transparent capacitive touch pad, in the form of a 2 2 matrix is implemented, using this ink and with PDMS as the dielectric. The touch pad has a high degree of flexibility with a resistance variation less than 2% after 10 000 bending cycles. The formulated nanowire ink can be extended for other flexible and stretchable transparent sensing applications.

Developing printed optoelectronic devices based on metal oxide inks requires synthesizing stable suspensions of the desired materials. In this work, pure and manganese doped zinc oxide inks were synthesized by a top-down wet milling route and the role of solvent in ink stabilization and printing was analyzed. Fluid properties of the as-prepared inks were measured and used for studying jettability criteria for inkjet printing. Among the various solvents evaluated, ethylene glycol produced stable oxide inks and satisfied the jetting conditions. The inks were evaluated using a commercial drop-on-demand piezoelectric inkjet printer. The morphology of the patterns printed on glass substrates, for various ink volumes, was investigated and was found to be continuous and showed good optical transmittance. We also investigated the particle segregation in these inks using a custom built direct writing system. This top-down approach, by separating the material development and ink synthesis, can be extended to a variety of metal oxide based inks.

Electroless nickel deposition is a chemical reduction technique commonly used for coating on metals. Deposition on non-metallic substrates is challenging owing to their poor ability to catalyse the reaction. The kinetics data for electroless nickel is limited, especially for different activation techniques. In this work, we show that gold‑palladium sputter activation can be used for electroless nickel growth on glass substrates. Deposition kinetics is measured and compared with standard metallic substrates to elucidate the growth mechanism. By tracking film thickness as a function of temperature and time, we extracted the activation energies for different substrates and surface activation techniques. Activation energy, at a pH of 5.6, for bare SS316 is 3.4 kJ/mol, while for sputter activated substrate it is 73.9 kJ/mol. We also demonstrate that sputter activation can be used to obtain patterned electroless films, by performing lithography assisted area selective deposition and by fabricating a patterned transmission line, with close dimensional tolerance. The process might be extended to other electroless systems as well.

The focal plane characterization of spot focusing horn antennas used for microwave NDE is presented using three experimental methods. Reflections from a metal sphere were analyzed in time domain to quantify the focal spot and spot size in the first method. In the second method, a rectangular waveguide padded with absorber was used as the sensor. Finally, graphene based miniaturized electric (E)-field sensor printed on photo paper using inkjet printing was used to measure antenna focal plane characteristics. The focal plane measured using the metal ball and waveguide techniques was within 5% of the simulated value. Higher error was recorded in focal spot measurement (≥25%) for the first two methods. The focal plane and spot size measured by the miniaturized E-field sensor was within 10% of the simulated antenna characteristics. The results indicate that the flexible printed sensor has better measurement accuracy and simple setup for field measurement.

Aluminium doped zinc oxide is prepared using spark plasma sintering. The process leads to a tailored optical band gap extending into the visible region with increasing aluminium concentration. Microstructural, optical, and electrical characterization were carried out to investigate the effect of the sintering process and aluminium concentration on the grain size, band gap, and electrical conductivity. The delocalization of impurity energy states created during sintering, causes band gap narrowing and leads to an improved conductivity. The impurity states are primarily oxygen vacancies and zinc interstitials. Spark plasma sintering offers a promising approach to synthesize doped conductive metal oxides with control over the band gap and can be extended to a variety of other systems.

The focal plane characterization of spot focusing horn antennas used for microwave NDE is presented using three experimental methods. Reflections from a metal sphere were analyzed in time domain to quantify the focal spot and spot size in the first method. In the second method, a rectangular waveguide padded with absorber was used as the sensor. Finally, graphene based miniaturized electric (E)-field sensor printed on photo paper using inkjet printing was used to measure antenna focal plane characteristics. The focal plane measured using the metal ball and waveguide techniques was within 5% of the simulated value. Higher error was recorded in focal spot measurement (≥25%) for the first two methods. The focal plane and spot size measured by the miniaturized E-field sensor was within 10% of the simulated antenna characteristics. The results indicate that the flexible printed sensor has better measurement accuracy and simple setup for field measurement.

In this work, we describe the preparation of a zinc oxide-ethylene glycol nanoparticle ink and the parameters that control the printing using a custom-built direct writer system. The ink (nanoparticle dispersion) was prepared using a two-step wet synthesis method, without using any surfactant. Its viscosity was found to be in the suitable range for printing and straight lines were printed on cleaned glass substrates. The influence of various printing parameters, such as total dispersed volume, number of printed layers, substrate temperature, drying temperature and time, and particle loading, on the morphology of the printed patterns was investigated. In-situ and post-printing drying of the printed pattern, at the same temperature, produced different morphologies, which can be attributed to the direction of heat transfer and solvent removal. Optimization of these printing parameters enabled us to obtain a continuous printed pattern with uniform morphology using a direct writing system, which can be extended to a variety of nanoparticle based inks.

We report on the anti-reflection characteristics of silica nanopillars fabricated using bimetallic copper-silver nanoparticles as an etch mask. The particles were formed by dewetting and their size and shape were controlled by the film thickness and annealing conditions. Our results showed that annealing of a copper-silver film results in formation of segregated bimetallic nanoparticles. Using these bimetallic nanoparticles as a mask for reactive ion etching produced dual diameter nanopillars, compared to single diameter nanopillars for monometallic nanoparticles. The height and shape of the nanopillars were controlled by the initial film thickness, annealing temperature, and etching parameters. The fabricated silica nanopillars showed a reduction in reflectance as compared to the bare substrate in the UV–Vis-NIR range, and dual diameter nanopillars were better as compared to single diameter nanopillars. Increasing etch duration resulted in increase in pillar height and a further reduction in reflectance. These silica nanopillar structures can be used as anti-reflection coatings.

We report on the effect of added silver on the solid state dewetting behavior of copper thin films, grown by thermal evaporation. The films were annealed under different conditions, conventional furnace annealing in air and argon and rapid thermal annealing in nitrogen. Our results show that for all annealing conditions, addition of silver to copper films suppresses the dewetting when compared to pure copper films. Fully dewet silver–copper films produce roughly spherical nanoparticles, with the amount of silver controlling the particle areal density and size distribution. This provides an additional parameter for controlled synthesis of nanoparticles by a bottom–up approach.

Manganese-doped zinc oxide powders were synthesized by solid state reaction of the respective oxides. The high-temperature conditions were chosen such that multiple valence states of manganese were doped in the host zinc oxide lattice. Structural characterization was carried out to confirm the doping and to find the maximum amount of manganese that can be incorporated. Diffuse reflectance spectroscopy was used to measure the optical band gap of the doped sample and the lowering with respect to pure ZnO was attributed to the presence of higher oxidation states of manganese. The presence of these oxidation states was confirmed using x-ray photoelectron spectroscopy. The study shows that a solid state reaction is a viable route for synthesizing doped metal oxides with desired optical properties.