Swaminathan Parasuraman

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.

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.

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.

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.

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.

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.

In a DC magnetron sputtering system, the deposition rate is significantly affected by the sputtering power and working pressure, and the precise nature of this dependence varies from machine to machine. An understanding of the effect of sputtering parameters on the deposition rate is required before undertaking further studies. To do so, we have carried out a design of experiments study on the optimization of deposition process parameters using a Taguchi design methodology. Taking an example of silver (Ag) deposited on silicon (100) substrates using the DC magnetron sputtering method, we use a Taguchi L9 design to reduce the experimental design space from 27 to 9 combinations. The attributes of the Ag thin films were quantified with respect to surface roughness (Ra), thickness (t), and sheet resistance (R s). The objectives of this study are 3-fold: establishing the effect of sputtering process parameters, namely the power and working pressure, on the deposition rate for the system under consideration; optimizing the sputtering process parameters within the chosen design space to ensure the formation of smooth conductive films using Taguchi analysis and response surface models; and analyzing the effect of annealing on the thin film characteristics. We found that the deposition rate increases with increasing the sputtering power. The highest deposition rate for the maximum power considered (25 W) was achieved at an intermediate working pressure of 6.1 × 10−3 mbar. The lowest deposition rate was obtained for the minimum power (5 W) and the highest working pressure (8.5 × 10−3 mbar) considered. For the given substrate–material system, we also found that the critical thickness below which the deposited films are non-conductive was 16 nm, which agrees with the existing literature.

Copper nanowires (Cu NWs) are a promising alternative to silver NWs to develop transparent conducting films (TCFs) due to their comparable electrical conductivity and relative abundance. Postsynthetic modifications of the ink and high-temperature postannealing processes for obtaining conducting films are significant challenges that need to be addressed before commercial deployment of these materials. In this work, we have developed an annealing-free (room temperature curable) TCF with Cu NW ink that requires minimal postsynthetic modifications. Organic acid pretreated Cu NW ink is used for spin-coating to obtain a TCF with a sheet resistance of 9.4 Ω/sq. and optical transparency of 67.4% at 550 nm. For oxidation protection, the Cu NW TCF is encapsulated with polydimethylsiloxane (PDMS). The encapsulated film is tested as a transparent heater at various voltages and shows good repeatability. These results demonstrate the potential of Cu NW-based TCFs as a replacement for Ag-NW based TCFs for a variety of optoelectronic applications, such as transparent heaters, touch screens, and photovoltaics.

Understanding the kinetics of metal nanoparticle self-assembly on functionalized surfaces is key for a variety of applications. In this work, we present a combined experimental and Monte Carlo simulation analysis of the monolayer formation of Au nanoparticles (Au NPs) on a glass surface functionalized with 3-aminopropyltrimethoxysilane (APTMS). The effect of particle size on the deposition process is analyzed by a wet chemical synthesis of Au NPs with sizes ranging from 14.65 ± 1.85 to 101.81 ± 10.3 nm. The adsorption kinetics is studied by measuring the peak optical absorbance, which increases with the surface concentration of Au NPs on the glass. Also, with the increase in nanoparticle size the surface concentration is found to decrease. To understand the adsorption process, the Frumkin isotherm is used to analyse the adsorption isotherms by choosing Au NPs of three different sizes, 38.31 ± 3.55, 77.32 ± 7.14, and 101.81 ± 10.3 nm. The fitted parameters of this isotherm indicate that as the size increases the decrease in the affinity between the particles and the modified surface leads to reduced surface saturation. Correspondingly, the increased attractive interaction between the Au NPs causes agglomeration. To further interpret the experimental results, a Monte Carlo simulation was carried out to relate the adsorption kinetics with the surface coverage of the substrate. The simulations confirmed the linear relationship between the probability of NPs being adsorbed on the substrate and solution bulk concentration of the NPs. The immobilization of nanoparticles is governed by both the electrostatic interaction between the substrate and the nanoparticles and the bulk diffusion of the particles. As the diffusivity of the nanoparticles is inversely proportional to the size, the deposition decreases with particle size. The overall insight of this study helps to develop a systematic framework for the fabrication of a monolayer of Au NPs with different particle sizes for various applications.

Carbon dioxide (CO2) being a greenhouse gas whilst generally present in the environment, excess concentration leads to health discomfort and even death. So monitoring and detecting CO2 is critical to ensure both personal and environmental safety. Traditional CO2 sensors operate at high temperatures (100–400 °C) because thermal energy is needed for better sensitivity. But high temperatures also lead to added complexities and costs and hence there is a preference for developing sensors capable of operating at room temperature. This study reports on the room temperature sensing of carbon dioxide (CO2), based on a composite of n-type zinc oxide (ZnO) and p-type nickel oxide (NiO). The composite is prepared by co-precipitation route and different compositions are investigated along with pure ZnO and NiO for reference. The prepared composites are characterized by a variety of techniques including X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence (PL), and UV–Visible spectroscopy. A custom-built setup is used to measure the gas sensing performance and the results show that an optimum composition of 80 % ZnO with 20 % NiO exhibits an excellent response to CO2, with a sensitivity of 61.1% (measured in terms of the resistance change during exposure) for 40 mg/L CO2 concentration. The performance of the sensor is explained by the formation and random distribution of n−p junctions in the composite. The results open up the possibility of further improving the sensitivity by using other heterojunction-based composites. The low processing temperature also makes this approach suitable for flexible substrates.