Metal oxide heterojunctions using a printable nickel oxide ink
Wide band gap metal oxides are ideally suited for inorganic optoelectronic devices. While zinc oxide is a commonly used n-type material, there is still a lot of ongoing work for finding suitable p-type oxides. In this work, we describe a two-step route to formulate a stable and conducting p-type nickel oxide (NiO) nanofluid. NiO nanoparticles were synthesised using a bottom-up wet chemical approach and dispersed in ethylene glycol to form a nanofluid. The viscosity and surface tension of the nanofluid were optimised for printing. The printing was done using an extrusion-based direct writer. The NiO nanofluid was printed onto an aluminum-doped zinc oxide layer and annealed at different temperatures. Electrical characterisation of the junction was used to extract the junction barrier for carriers across the interface. The resulting heterojunction was found to exhibit rectifying behaviour, with the highest rectification ratio occurring at an annealing temperature of 250 °C. This annealing temperature also resulted in the lowest junction barrier height, and was in excellent agreement with theoretically predicted values. The development of a printed p-type ink will help in the realisation of oxide-based printed electronic devices.
Solution processed Li-Al2O3/LiNbO3/Li-Al2O3 stacked gate dielectric for a non-volatile ferroelectric thin film transistor
Lithium niobate (LiNbO3) gate dielectric based SnO2 ferroelectric thin film transistor (FETFT) is fabricated by a simple solution processed technique. However, LiNbO3 alone is not a suitable candidate for a gate insulator of a TFT because of its low band gap. Therefore, Li-Al2O3/LiNbO3/Li-Al2O3 stacked gate dielectric has been used that reduces the gate leakage current by an order of magnitude compared to the LiNbO3 only device. Moreover, ionic polarization of Li-Al2O3 thin films that originated from mobile Li+ of Li-Al2O3, compensate for the ferroelectric charge polarization of LiNbO3 film. By reducing gate leakage current and compensating ferroelectric charge polarization, it becomes possible to achieve ferroelectric memory retention up to 7.2× 103 s of time with a difference of ON/OFF state by 3 times whereas the reference LiNbO3 device almost merges to each other very quickly. Besides, these ferroelectric TFTs (FETFT) can operate within 2 V operating voltage due to the strong ionic polarization of the gate dielectric. The carrier mobility of 1.9 cm2. V−1.s−1, current ON/OFF ratio of 1.6⨭104 and subthreshold swing (SS) of 167 mV.decade−1 has been achieved under 2 V operation of this FETFT, whereas memory retention time has been studied at 0 V gate and 1 V drain bias.
Precursor-based bismuth ferrite ink for direct writing
We report the synthesis of a bismuth ferrite (BiFeO3) (BFO) precursor-based ink via wet chemical route followed by extrusion-based direct writing on fluorine-doped tin oxide (FTO), indium-doped tin oxide (ITO), and quartz substrates. A stable aggregation-free ink is developed, based on sol–gel route that yields phase-pure BFO characterised by X-ray diffraction, scanning electron microscopy, Raman, UV–vis spectroscopy, contact angle and viscosity. Optical bandgap varies from 2.5 to 2.8 eV, as a function of particle size going from 47 to 116 nm. The viscosity of the synthesized ink is 60 mPa.s with contact angles below 90°, indicating good substrate wettability, compatibility, and suitability for printing. Printed BFO films, due to their porous nature, indicate photocatalysis and gas sensing to be promising avenues for future work.
Focal plane characterization of spot focusing horn antennas for free space microwave dielectric NDE
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.
Direct writing of silver nanowire-based ink for flexible transparent capacitive touch pad
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.
Annealing-induced changes in optoelectronic properties of sputtered copper oxide films
Copper (I) oxide thin films are deposited on quartz substrates by DC magnetron reactive sputtering. This study examines the effect of post-annealing on their optoelectronic properties in detail. The films are grown by sputtering from copper in an atmosphere of argon and oxygen. The substrate temperature is held at 200 °C, while annealing in ambient atmosphere has been carried out between 100 and 600 °C. X-ray diffraction analysis, Raman and UV–Vis spectroscopy, and four-probe measurements were used to characterise the films. XRD indicates that deposited Cu2O has a preferred orientation of (110). Post-annealing did not show any measurable conversion to copper (II) oxide until about 500 °C, and the process was incomplete even at 600 °C. The highest conductivity is observed in the sample post-annealed at 100 °C. These results are of substantial technological importance for using Cu2O for a variety of applications, including transparent solar cell fabrication.
Photoresponse of a printed transparent silver nanowire-zinc oxide nanocomposite
Zinc oxide (ZnO) is widely used as an absorber layer in photodetectors (PDs). Here, we demonstrate a printed transparent PD with an absorber layer based on a nanocomposite of ZnO with silver nanowires (Ag NWs). The nanocomposite shows good optoelectronic properties; a single pass film exhibits 93% transparency at 550 nm and a low sheet resistance of 28.7 Ω sq-1, which allows for the device to operate at low voltages. Its formulation as printable ink with a low annealing temperature of 100 C makes it suitable for a single step patterned deposition and compatible with flexible substrates that cannot withstand higher temperature. Unlike conventional PDs, the photoconductivity decreases under illumination, i.e. the resistance of the nanocomposite is higher than in dark condition. This anomalous behavior can be explained based on the band alignment between ZnO and Ag in the nanocomposite. The Schottky barrier between ZnO and Ag prevents photo-generated electrons in ZnO from moving to the NWs, while the photo-generated holes recombine with the electrons flowing in the NWs leading to a resistance increase. The PD exhibits an improved photoresponsivity of 35 mA W-1, at a relatively low biasing voltage of 1 V, compared to a pure ZnO absorber layer with a responsivity of 14 mA W-1 at 5 V bias for an illumination at 365 nm. The properties of the nanocomposite make it suitable for single layer, low cost, and large area transparent PDs. The anomalous resistance change can also be extended to fabricating other kinds of sensors, such as gas or humidity sensors, with this nanocomposite.
Top-down synthesis of zinc oxide based inks for inkjet printing
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.
Printable Silver Nanowire and PEDOT:PSS Nanocomposite Ink for Flexible Transparent Conducting Applications
Patterned deposition of highly flexible transparent conducting materials is essential to realize stretchable optoelectronic devices. Silver nanowires (NWs) are suitable for these applications because they possess high electrical conductivity and good optical transparency. However, NWs have poor surface adhesion and large roughness. Embedding them in a conducting polymer, such as poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS), is one way to overcome these disadvantages without affecting the optoelectronic properties. However, this is normally a two-step deposition process and difficult to pattern directly. In this work, we have formulated a stable and printable nanocomposite ink consisting of Ag NWs and PEDOT:PSS. This ink can be directly used for patterned deposition in a single-step process. The printed film shows 86% transparency and 23 ω/sq sheet resistance, which is suitable for flexible transparent electrode applications. The printed film shows good adhesion and excellent stability to mechanical deformation, with less than 20% resistance variation after 10,000 bending cycles. The nanocomposite also exhibits improved thermal stability, planarity, reduced contact resistance, and good optical transparency when compared to pure Ag NWs. We demonstrate suitability of this nanocomposite using two applications -a printed transparent flexible antenna radiating at Wi-Fi frequencies and a printed transparent flexible heater suitable for antifogging applications. The nanocomposite properties make it suitable as a transparent electrode in flexible optoelectronic devices such as photovoltaics and light-emitting diodes.
Templated electroless nickel deposition for patterning applications
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.