P-type Oxide Research Articles

Switching Enhancement via a Back-Channel Phase-Controlling Layer for p-Type Copper Oxide Thin-Film Transistors.

P-type copper oxide (CuxO) thin-film transistors (TFTs) with enhanced switching characteristics were fabricated by introducing a sputter-processed capping layer capable of controlling the back-channel phase (labeled as phase-controlling layer, PCL). By optimizing the processing conditions (the deposition power and postdeposition annealing parameters), the switching characteristics of the TFTs achieved a subthreshold swing of 0.11 V dec-1, an on/off current ratio (Ion/Ioff) of 2.81 × 108, and a field-effect mobility (μFET) of 0.75 cm2 V-1 s-1, a considerable enhancement in performance compared to that of CuxO TFTs without the PCL. Through optical/electrical analyses and technology computer-aided design simulations, we determined that the performance improvements were because of the CuxO back-channel phase reconstruction through PCL deposition and subsequent annealing. The two factors that occurred during the process, sputtering damage and heat treatment, played key roles in creating the phase reconstruction by inducing a local phase transition that sharply reduced the off-current via controlling back-channel hole conduction. As a sample application, we fabricated a complementary metal oxide semiconductor inverter based on our optimized CuxO TFT and an InGaZnO TFT that demonstrated a large inverter voltage gain of >14. The proposed approach opens up advancements in low-power circuit design by expanding the utilization range of oxide TFTs.

P2-type Fe and Mn-based Na0.67Ni0.15Fe0.35Mn0.3Ti0.2O2 as cathode material with high energy density and structural stability for sodium-ion batteries

P2-type layered cathode materials containing Fe and Mn have attracted much attention due to their elemental abundance, low costs and high reversible capacities, and most related work have focused on attaining cathodes with extremely high reversible capacity. However, the extremely high capacity usually leads to several disadvantages, including extremely wide voltage range, fast capacity fading upon cycling, and inappropriate initial Coulombic efficiency. This work investigates P2-type Na0.67Ni0.15Fe0.35Mn0.5O2 (NFM) and Na0.67Ni0.15Fe0.35Mn0.3Ti0.2O2 (NFMT) as high-energy cathode materials by elevating average operating voltage with Ni2+/Ni4+ and Fe3+/Fe4+ redox couples but limiting (de)sodiation in the voltage of 2.0–4.4 V to obtain an medium reversible capacity. NFMT has high energy density of 471 Wh kg−1 with reversible capacity of 157.2 mAh g−1 and average operating voltage of 3.0 V. NFMT also has an initial Coulombic efficiency of 98.8% and a capacity retention of 85.1% after 50 cycles at 20 mA g−1 (0.1C), better than those of some high-capacity P2-type Fe and Mn-based cathodes, showing great potential for future practical application in the sodium full cells. Ti4+ ion acts as pillar ion in the fully charged NFMT cathode, effectively suppressing thickness reduction of the MO2 slab, volume shrinkage of the cathode, and thus inhibiting P2–O2 phase transition at high voltage, which contribute to the improvement of the structural and cycling stability P-type oxides.

Classification of Urine Odour Using Artificial Neural Networks

Introduction An odour create a distinctive smell that is caused by specific volatile molecules; it can be either a single chemical compound or a mixture of them. Volatile organic compounds are currently of great interest for non-invasive disease diagnoses such as diabetes [2] and prostate cancers [3]. In this work, the detection of human urine is studied in order to aid incontinent individuals and prevent social embarrassment. Urine odour is known to comprise of more than 200 different types of chemical compounds, including ketones, aldehydes, and esters [1]. Gas Sensing Device The proposed sensing device is compact with three custom-made CMOS non-specific metal oxide semiconductors coated with n-type tungsten trioxide, Pd/Pt doped tin oxide, and p-type copper oxide, respectively. The sensors are interfaced to an integrated Teensy 3.6 microcontroller; and the response data are collected at a 10 Hz sampling rate. This device has been tested via a custom gas testing station with synthetic urine as well as its three individual interference compounds, ammonia, ethyl acetate and acetone. The maximum concentration levels tested in air were 25 ppm, 20 ppm and 200 ppm, respectively, and 5% for synthetic urine. Five repetitions were performed with five concentration steps and three different humidity levels for each gas of interest. The three sensors responded to the target compounds differently with a specific pattern that can be classified through pre-processing and supervised artificial neural networks. Artificial Neural Networks The collected time-series data were first pre-processed to extract the change of sensor response, and auto-ranged to spread the sample variance evenly across all sensors. This step allows the input sensor data to contribute equally to the neural network models and increase the learning speed and prediction accuracy. Two methods were trialled, a shallow neural network with multi-layer perceptrons, and a deep convolutional neural network (CNN). The shallow neural network topology consists of 4 input neurons (no concentration information), 2 hidden layers of 8 neurons each and 4 output neurons for each odour class. This supervised feed-forward backpropagation algorithm is based on the Levernberg-Marquardt approach with sigmoid function for hidden layers. The convolutional neural network has two convolution layers and two pooling layers before the fully connected output layer, which works in the same manner as a shallow neural network. Unlike a typical 2D CNN used in image processing, this 1D network has the kernel size of 1 × n and a single row filtering window tiling down the dataset. In both models, four experiment repeats were used for training and one for testing, until all had been independently tested. Results and Conclusions Confusion matrices are used to demonstrate the odour prediction result of both models, as presented in Table 1 for the shallow neural network and Table 2 for the convolutional neural network. These tables are the two model results for the same dataset. Two performance evaluation methods are studied here, the cross entropy and the precision. The cross entropy loss is used to measure the model performance by comparing the predicted outputs with the target outputs. It is calculated through a log equation. For a classification model, cross entropy of 0 indicates perfect results. In the shallow neural network model, the cross entropy is 0.0269 for Table 1, and 0.0183 for Table 2 with the CNN model. Precision is defined as the accuracy of positive predictions and calculated on a per-class basis. The precision of urine odour prediction is 0.93 for the shallow neural network and 1.0 for the CNN, which is ideal. Therefore, while both have high overall accuracies, 92.56% for the shallow neural network and 97.22% for the CNN, the CNN provides better results not just in the urine odour detection, but also the predictions of other similar compounds. The shallow neural network has its merits in terms of easy implementation of a real-time classification device for incontinence individuals, and low memory requirement with relatively high accuracy. Overall, this work proves the feasibility of using these low-cost metal oxide sensors for urine prediction in an open environment that could contain interfering VOCs such as solvents.

(Plenary) Design Strategies for High-Performance Oxide Semiconductor Gas Sensors

Oxide semiconductor gas sensors have been widely used to detect explosive and harmful gases because of their irreplaceable advantages such as high sensitivity, excellent reversibility, simple structure, facile integration, and cost effectiveness, and their applications are being rapidly expanded to environmental monitoring, disease diagnosis from exhaled breath, food quality control, smart farming, and artificial olfaction. In particular, oxide semiconductor chemiresistors and their array can be integrated within smart phones and portable devices, which will open a wide range of new applications through the sensor connection using Internet of Things, multivariate pattern recognition, and machine learning of big gas sensing data in the near future. To date, various nanostructures with high surface area to volume ratio, such as nanoparticles, nanowires, nanosheets have been explored to enhance the gas response and sensing materials have been loaded with noble metal or oxide catalysts for tailoring gas selectivity. However, for demand-based design of high performance gas sensors and artificial olfaction, many issues still remain unsolved, which include the detection of ultralow concentration of analyte gas, highly selective detection of a specific gas, moisture-independent gas sensing, and the establishment of distinctive gas sensing library toward numerous analyte gases as well as complex odors. How can we overcome above challenges? The diversity of gas sensing materials provides a rational solution, which can be accomplished by introducing new sensing materials, designing new nano-archiectures, making new methods to tune the reforming/oxidation of analyte/interference gases, establishing new strategies to maximize/control catalytic activity, and using different ways to control electronic/chemical sensitization. In this presentation, various new, promising and general strategies to design sensitive, selective, and reliable chemiresistors for next-generation gas sensors and artificial olfaction will be suggested, which are as follows: Hollow and hierarchical nanostructures with high gas accessibility for effective and raid gas detection [1]P-type oxide semiconductors with abundant oxygen adsorption, facile redox reaction, and high catalytic activity for detecting new gas species and for expanding the gas sensing materials [2]Multi-valence additives for humidity-independent oxide semiconductor gas sensors [3,4]Catalyst-loaded or catalytic micro-reactors such as yolk-shell spheres and multiroom-structured spheres for highly selective detection of aromatic compounds via the reforming of less-reactive gases into more reactive species [5]Bilayer sensors consisting of sensing layer and nanoscale catalytic overlayer for separating catalytic and sensing reactions as well as for tailoring gas selectivity and response without limiting the gas transport [6]Hetero-nanostructures consisting of two different materials with dissimilar catalytic activity and work function for enhancing catalytic activity and for promoting electronic sensitization via the charge transfer across hetero-interface [7]Semiconducting multinary oxides consisting of two or more catalytic components [8].Monolayer oxide nanofiber sensor with tailored exposure of catalytic electrode for controlling gas selectivity [9] References [1] J.-H. Lee, Gas sensors using hierarchical and hollow oxide nanostructures: Overview, Sens. Actuators B 140 (2009) 319-336. doi:10.1016/j.snb.2009.04.026.[2] H.-J. Lee, J.-H. Lee, Highly sensitive and selective gas sensors using p-type oxide semiconductors: Overview, Sens. Actuators B 192 (2014) 607-627. doi:10.1016/j.snb.2013.11.005.[3] H.-R. Kim, A. Haensch, I.-D. Kim, U. Weimar, J.-H. Lee, Role of NiO doping in reducing the humidity impact on the performance of SnO2-based gas sensors: synthesis strategies, phenomenological and spectroscopic studies, Adv. Funct. Mater. 21 (2011) 4229-4240. doi:10.1002/adfm.201101154.[4] J.‐W.Yoon, J.‐S.Kim, T.‐H. Kim, Y. J. Hong, Y. C. Kang, J.‐H. Lee, A new strategy for humidity independent oxide chemiresistors: Dynamic self‐refreshing of In2O3 sensing surface assisted by layer‐by‐layer coated CeO2 nanoclusters, Small 12 (2016) 4456-4463. doi:10.1002/smll.201601507.[5] J.-W.Yoon, Y.J.Hong, G.D.Park, S.-J.Hwang, F.Abdel-Hady, A.A.Wazzan, Y.C.Kang, J.-H.Lee, Kilogram-scale synthesis of Pd-loaded quintuple-shelled Co3O4 microreactors and their application to ultrasensitive and ultraselective detection of methylbenzenes, ACS Appl. Mater. Interfaces 7 (2015) 7717-7723. doi: 10.1021/acsami.5b00706.[6] S.-Y.Jeong, J.-W.Yoon, T.-H.Kim, H.-M.Jeong, C.-S.Lee, Y.C.Kang, J.-H.Lee, Ultra-selective detection of sub-ppm-level benzene using Pd-SnO2 yolk-shell micro-reactors with a catalytic Co3O4 overlayer for monitoring air quality, J. Mater. Chem. A 5 (2017) 1446-1454. doi:10.1039/C6TA09397C.[7] T.-H. Kim, C.-H. Kwak, J.-H. Lee, NiO/NiWO4 composite yolk−shell spheres with nanoscale NiO outer layer for ultrasensitive and selective detection of sub ppm-level p-xylene, ACS Appl. Mater. Interfaces 9 (2017) 32034-32043. doi:10.1021/acsami.7b10294.[8] K. H. Lee, B.-Y. Kim, J.-W. Yoon, J.-H. Lee, Extremely selective detection of ppb levels of indoor xylene using CoCr2O4 hollow spheres activated by Pt doping. Chem. Commun. 55 (2019) 751-754. doi:10.1039/C8CC08186G.[9] K. Lim, Y.-M. Jo, J.-W. Yoon, J.-H. Lee, Metal oxide patterns of one-dimensional nanofibers: on-demand, direct-write fabrication, and application as a novel platform for gas detection, J. Mater. Chem. A 7 (2019) 24919-24928. doi:10.1039/C9TA09708B.

Investigating the structural, electrochemical, and optical properties of p-type spherical nickel oxide (NiO) nanoparticles

A facile and efficient co-precipitation method has been used to synthesize pure nickel oxide (NiO) nanoparticles. The synthesis of NiO has been conducted at two different pH values (11.00, 12.33) and in alkaline media of LiOH and NaOH, and the variations in structural, electrochemical, and optical properties have been analyzed. The particle sizes of all of the synthesized samples were in the range 8.000–20.00 nm, and their energy band gaps were in the range 3.500–4.000 eV. Williamson–Hall plots indicate that strain was introduced in the samples during the synthesis process. NiO nanoparticles synthesized in the LiOH medium at pH 12.33 showed the smallest crystallite size of 8.830 nm, with a lower strain value of 9.620E-6. The charge-transport and charge-transfer properties of the NiO samples have been investigated by means of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements. CV study revealed that the redox process is diffusion-controlled, with a diffusion constant of the order of 10−10 cm2 s−1. Nyquist plots obtained by the EIS technique indicate how charge-transfer resistance and Warburg diffusion vary with pH. The NiO nanoparticles synthesized in LiOH medium at pH 12.33 showed the lowest charge-transfer resistance, indicating high electrical conductivity. In the present study, NiO nanoparticles with small crystallite size and lower strain value has been obtained, which form a better thin film suitable for use as a hole-transporting layer in solar cells. Lower charge-transfer resistance facilitates better charge transport in the obtained NiO nanomaterial, facilitating use in electrochromic devices.

Nickel vacancy acceptor in nickel oxide: Doping beyond thermodynamic equilibrium

This work reports on temperature-induced out-diffusion and concentration decay of the prominent intrinsic point defect VNi (nickel vacancy) in the wide-gap p-type semiconductor nickel oxide (NiO). VNi can easily be introduced into NiO thin films by offering high oxygen partial pressures during film growth, rendering nonstoichiometric semiconducting structures. However, exposure to lower oxygen supply after growth, e.g. in a standard atmosphere, usually leads to a gradual decrease of film conductivity, because the vacancy concentration equilibrates. In this study, we observe this process in situ by performing temperature-dependent measurements of the electrical conductivity on a room temperature-grown NiO film. At a temperature of 420K under exclusion of oxygen, the doping level decreases by a factor of 8 while the associated room temperature dc conductivity drops by six orders of magnitude. At the same time, out-diffusion of the mobile VNi species can be indirectly observed through the occurrence of electrode polarization characteristics.

Synthesis and Characterization of Spinel Cobaltite (Co3O4) Thin Films for Function as Hole Transport Materials in Organometallic Halide Perovskite Solar Cells

Conventional inorganic p-type conductive oxides, for example, NiO, CuOX, and CuCrOX, can serve as low-cost and efficient hole transport materials for wide-bandgap organolead halide perovskites [for...

Design of PMOS and NMOS Input Folded Cascode Amplifier using 180nm SCL Technology Node for Low Power Application

This paper presents the details design and simulation of the Folded Cascode amplifier using Source-Coupled-Logic (SCL) technology node for both the P-Type Metal Oxide Semiconductor (PMOS) and N-Type Metal Oxide Semiconductor (NMOS) input. The different way to implement the circuit design for a given specification has clearly described including all the design equation has been presented. All the parameter like open loop gain, Unity Gain Bandwidth (UGB) and Phase Margin (PM) are compared for both the NMOS and PMOS input fully differential folded cascode op-amp circuit are discussed and finally we have got after performance analysis that NMOS input fully differential folded cascode op-amp is the best choice for low power high speed application like in pipeline Analog to Digital (ADC). The circuit has been simulated using cadence virtuoso tool in 0.18μm SCL technology node.

Effect of thermal oxidation on the optical, electrical, and chemical properties of zinc nitride films grown by reactive magnetron sputtering

We have studied the effect of post growth annealing in oxygen ambient on the optical, chemical and electronic properties of zinc nitride thin films. The films were prepared by reactive radio frequency magnetron sputtering of zinc target in the presence of argon and nitrogen gasses at a substrate temperature of 300 °C. Post growth annealing was performed at annealing temperatures of 300, 400, 500, 600, and 700 °C in the oxygen environment to transform the zinc nitride films into p-type zinc oxide. Atomic force microscopy of the films reveal that surface roughness and grain size of the films are directly proportional to the post-growth annealing temperature. X-ray photoelectron spectroscopy of the films revealed that as-grown film has the highest intensity of nitrogen in the film whereas little nitrogen remains in the films that were annealed at 500 °C and higher temperatures. Films annealed at higher temperatures have lower carrier concentration but higher Hall mobility compared to the as-grown film and the films annealed at lower temperature. Moreover, as-grown film and the films annealed at lower temperature have n-type conductivity whereas films annealed at temperatures above 500 °C exhibit p-type conductivity. Films annealed at lower temperature have bandgap values closer to the zinc oxynitride but bandgap of the films annealed at higher temperatures have values close to the bandgap of zinc oxide. This indicates that post growth annealing of zinc nitride films at 500 °C in oxygen ambient transforms it into p-type zinc oxide.

Wide-Gap Zn1−xNixO Alloy: A Transparent p -Type Oxide

The development of transparent bipolar oxide devices is largely hampered by the lack of oxides with reliable p-type conductivity. Recent calculations show that rock salt (RS) structured $\mathrm{Zn}\mathrm{O}$ alloys are promising candidates for transparent p-type oxides. Here, we synthesize wide-gap ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}\mathrm{O}$ alloy thin films over the entire composition range on glass substrates by means of radiofrequency magnetron sputtering. We find that the ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}\mathrm{O}$ alloy thin films undergo a phase transition from wurtzite (WZ) to RS structure as the $\mathrm{Ni}$ content x increases to x \ensuremath{\sim} 0.27. Interestingly, the band gap of RS-${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}\mathrm{O}$ is about 4.6 eV at the WZ to RS transition composition (x \ensuremath{\sim} 0.27) and decreases with x to the value of RS-$\mathrm{Ni}\mathrm{O}$ (\ensuremath{\sim}3.8 eV). Nominally undoped alloy thin films sputtered in pure $\mathrm{Ar}$ are semi-insulating, while $\mathrm{O}$-rich RS-${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}\mathrm{O}$ thin films with relatively high x (e.g., x \ensuremath{\ge} 0.5) sputtered in ${\mathrm{Ar}}^{+}$ 1.4% ${\mathrm{O}}_{2}$ and/or with $\mathrm{Cu}$ doping exhibit good p-type conductivity at room temperature. The hole transport in these RS-${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}\mathrm{O}$ alloy thin films follows a small polaron hopping process, with activation energy ranges from 0.19 to 0.32 eV. Drastic differences in the electronic band structure of the WZ and RS ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}\mathrm{O}$ alloys are also observed with a type II band offset for alloys at the WZ to RS transition composition. The valence band maximum (VBM) of the RS phase is g1 eV above that of the WZ phase, making them energetically more favorable for the formation of native acceptor defects. The much higher VBM position of RS alloys also favors their extrinsic p-type doping efficiency.

High Responsivity and External Quantum Efficiency Photodetectors Based on Solution-Processed Ni-Doped CuO Films.

Photodetectors based on p-type metal oxides are still a challenge for optoelectronic device applications. Many effects have been paid to improve their performance and expand their detection range. Here, high-quality Cu1-xNixO (x = 0, 0.2, and 0.4) film photodetectors were prepared by a solution process. The crystal quality, morphology, and grain size of Cu1-xNixO films can be modulated by Ni doping. Among the photodetectors, the Cu0.8Ni0.2O photodetector shows the maximum photocurrent value (6 × 10-7 A) under a 635 nm laser illumination. High responsivity (26.46 A/W) and external quantum efficiency (5176%) are also achieved for the Cu0.8Ni0.2O photodetector. This is because the Cu0.8Ni0.2O photosensitive layer exhibits high photoconductivity, low surface states, and high crystallization after 20% Ni doping. Compared to the other photodetectors, the Cu0.8Ni0.2O photodetector exhibits the optimal response in the near-infrared region, owing to the high absorption coefficient. These findings provide a route to fabricate high-performance and wide-detection range p-type metal oxide photodetectors.

Optical and electronic properties of chemical bath deposited p-CuO and n-ZnO nanowires on silicon substrates: p-CuO/n-ZnO nanowires solar cells with high open-circuit voltage and short-circuit current

P-type cupric oxide (p-CuO) and p-type cupric-oxide/n-type zinc oxide nanowires (p-CuO/n-ZnO NWs) heterojunction diodes were fabricated by employing chemical bath deposition technique on n-silicon (n-Si) substrate. The work demonstrates the potential of zinc oxide nanowires as window layer owing to its wide and direct band gap for metal oxide based solar cells. Optical and electronic properties of the CuO film and ZnO nanowires including refractive index, absorption coefficient and band gaps were investigated by using spectroscopic ellipsometric analysis. Impact of nanowire morphology, chemical composition and crystallite structures on optical and photovoltaic performance of p-CuO/n-Si and p-CuO/n-ZnO NWs were investigated. Absorption coefficient in the near UV–visible range was observed to improve significantly by incorporating n-ZnO nanowires which was desired for the solar cell applications. The comparative photovoltaic performance showed improvements of both the short-circuit current density (Jsc) and open circuit voltage (Voc), due to reduced interface recombination and enhanced absorption for the increased surface-to-volume ratio of ZnO nanowires in the p-CuO/n-ZnO NWs heterojunction. Photovoltaic performance with Jsc, Voc and power conversion efficiency of 11.1 mA/cm2, 1.1 V and 4.19% was obtained. Therefore, the work emphasizes importance of using the combination of n-ZnO nanowires and p-CuO film as the wide- and low- band gap materials for improving performance of the future low-cost solar cell with superior efficiency.

Visible Light-Driven p-Type Semiconductor Gas Sensors Based on CaFe2O4 Nanoparticles.

In this work, we present conductometric gas sensors based on p-type calcium iron oxide (CaFe2O4) nanoparticles. CaFe2O4 is a metal oxide (MOx) with a bandgap around 1.9 eV making it a suitable candidate for visible light-activated gas sensors. Our gas sensors were tested under a reducing gas (i.e., ethanol) by illuminating them with different light-emitting diode (LED) wavelengths (i.e., 465–640 nm). Regardless of their inferior response compared to the thermally activated counterparts, the developed sensors have shown their ability to detect ethanol down to 100 ppm in a reversible way and solely with the energy provided by an LED. The highest response was reached using a blue LED (465 nm) activation. Despite some responses found even in dark conditions, it was demonstrated that upon illumination the recovery after the ethanol exposure was improved, showing that the energy provided by the LEDs is sufficient to activate the desorption process between the ethanol and the CaFe2O4 surface.

Reactive Templated Grain Growth and Thermoelectric Power Factor Enhancement of Textured CuFeO2 Ceramics

The delafossite-type CuFeO2 (CFO) is a promising p-type thermoelectric oxide reported to show a large Seebeck coefficient, but the lack of a texturing method for CFO ceramics limits their thermoele...

Hierarchical NiCo2O4 microspheres assembled by nanorods with p-type response for detection of triethylamine

The morphological and structural design provides an efficient protocol to optimize the performance of gas sensing materials. In this work, a gas sensor with high sensitivity for triethylamine (TEA) detection is developed based on p-type NiCo2O4 hierarchical microspheres. The NiCo2O4 microspheres, synthesized by a hydrothermal route, have a three-dimensional (3D) urchin-like structure assembled by nanorod building blocks. The structure-property correlation has been investigated by powder X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscope, scanning electron microscope, N2 adsorption-desorption tests and comprehensive gas sensing experiments. The influence of calcination temperature on the morphological structure and sensing performances has been investigated. Results reveal that the material annealed at 300 °C has a very large specific surface area of 125.27 m2/g, thereby demonstrating the best TEA sensing properties including high response and low limit of detection (145 ppb), good selectivity and stability. The further increase of the calcination temperature leads to the collapse of the 3D hierarchical structure with significantly decreased surface area, which is found to decline the sensing performances. This work indicates the promise of ternary p-type metal oxide nanostructures for application in highly sensitive gas sensors.

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.

Design principle for a p-type oxide gate layer on AlGaN/GaN toward normally-off HEMTs: Li-doped NiO as a model

The band alignment matching between p-type oxide and AlGaN/GaN is revealed to be key factor for p-type oxide gated normally-off HEMTs.

The effect of post-deposition annealing conditions on structural and thermoelectric properties of sputtered copper oxide films.

The development of thin-film thermoelectric applications in sensing and energy harvesting can benefit largely from suitable deposition methods for earth-abundant materials. In this study, p-type copper oxide thin films have been prepared on soda lime silicate glass by direct current (DC) magnetron sputtering at room temperature from a pure copper metallic target in an argon atmosphere, followed by subsequent annealing steps at 300 °C under various atmospheres, namely air (CuO:air), nitrogen (CuO:N) and oxygen (CuO:O). The resultant films have been studied to understand the influence of various annealing atmospheres on the structural, spectroscopic and thermoelectric properties. X-ray diffraction (XRD) patterns of the films showed reflexes that could be assigned to those of crystalline CuO with a thin mixed Cu(I)Cu(II) oxide, which was also observed by near edge X-ray absorption fine structure spectroscopy (NEXAFS). The positive Seebeck coefficient (S) reached values of up to 204 μV K−1, confirming the p-type behavior of the films. Annealing under oxygen provided a significant improvement in the electrical conductivity up to 50 S m−1, resulting in a power factor of 2 μW m−1 K−2. The results reveal the interplay between the intrinsic composition and the thermoelectric performance of mixed copper oxide thin films, which can be finely adjusted by simply varying the annealing atmosphere.

Synthesis of Co3O4 nanoparticles for sensing toxic gas at room temperature

Over the past few decades, there has been an increasing demand for inexpensive, accurate, portable and reliable gas sensors which can be used to detect combustible, flammable and toxic gases, and oxygen depletion. Typically, gases of interest include CO, NO, NO2, NH3, SO2, CO2, CH4 and other hydrocarbons. These gases can be harmful to human health if present beyond a certain concentration. Among various metal oxide semiconductors, p-type Cobalt oxide semiconductors are excellent materials for fabricating highly sensitive and selective gas sensors of high-performance. In this study, a novel and low cost chemical route has been developed to synthesize Co3O4 nanostructures. The efficiency of Co3O4 nanomaterials is improved by means of introducing n-type dopants. The synthesized nanomaterials were characterized by different characterization techniques like UV double beam spectrophotometry, X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray analysis (EDAX). The Cobalt oxide nanoparticles are observed to have a good response and sensitivity to ammonia gas at room temperature.

Towards sustainable and efficient p-type metal oxide semiconductor materials in dye-sensitised photocathodes for solar energy conversion.

In order to meet the ever-growing global energy demand for affordable and clean energy, it is essential to provide this energy by renewable resources and consider the eco-efficiency of the production and abundance of the utilised materials. While this is seldom discussed in the case of technologies still in the research stage, addressing the issue of sustainability is key to push research in the right direction. Here we provide an overview of the current p-type metal oxide semiconductor materials in dye-sensitised photocathodes, considering element abundance, synthetic methods and large scale fabrication as well as the underlying physical properties that are necessary for efficient solar harvesting devices.