Transparent Conducting Oxides-An Up-To-Date Overview

Recent Advances in Transparent Electrodes and Their Multimodal Sensing Applications

This review examines the recent advancements in transparent electrodes and their crucial role in multimodal sensing technologies. Transparent electrodes, notable for their optical transparency and electrical conductivity, are revolutionizing sensors by enabling the simultaneous detection of diverse physical, chemical, and biological signals. Materials like graphene, carbon nanotubes, and conductive polymers, which offer a balance between optical transparency, electrical conductivity, and mechanical flexibility, are at the forefront of this development. These electrodes are integral in various applications, from healthcare to solar cell technologies, enhancing sensor performance in complex environments. The paper addresses challenges in applying these electrodes, such as the need for mechanical flexibility, high optoelectronic performance, and biocompatibility. It explores new materials and innovative techniques to overcome these hurdles, aiming to broaden the capabilities of multimodal sensing devices. The review provides a comparative analysis of different transparent electrode materials, discussing their applications and the ongoing development of novel electrode systems for multimodal sensing. This exploration offers insights into future advancements in transparent electrodes, highlighting their transformative potential in bioelectronics and multimodal sensing technologies.

Structural, elastic, mechanical, electronic, and optical properties of cubic K2Pb2O3 from first-principle study

CONTEXT

Based on first principles, the structure, elasticity, mechanics, electronics, and optical properties of cubic K2Pb2O3 were studied. The structural parameters calculated by this method are close to the previous theoretical results. The elastic constant, bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and mechanical stability are studied, and it is shown that cubic K2Pb2O3 is mechanically stable, isotropic, and brittleness. The electrical conductivity and chemical bonding of cubic K2Pb2O3 were analyzed based on the calculated band structure, density of states (DOS), and bond populations. The dispersion of optical functions, including the dielectric function, refractive index, extinction coefficient, reflectivity, absorption coefficient, and loss function, is displayed and analyzed.

METHODS

All computations have been carried out based on density functional theory (DFT) as implemented in the CASTEP code. The norm conservation pseudopotential method is used to exchange correlation functionals within the generalized gradient approximation (GGA).

Deposition and Optical Characterization of Sputter Deposited p-Type Delafossite CuGaO2 Thin Films Using Cu2O and Ga2O3 Targets

CuGaO2 thin films were deposited using the RF magnetron sputtering technique using Cu2O and Ga2O3 targets. The films were deposited at room temperature onto a quartz slide. The sputtering power of Cu2O remained constant at 50 W, while the sputtering power of Ga2O3 was systematically varied from 150 W to 200 W. The films were subsequently subjected to annealing at temperatures of 850 °C and 900 °C in a nitrogen atmosphere for a duration of 5 h. XRD analysis on films deposited with a Ga2O3 sputtering power of 175 W annealed at 900 °C revealed the development of nearly single-phase delafossite CuGaO2 thin films. SEM images of films annealed at 900 °C showed an increasing trend in grain size with a change in sputtering power level. Optical studies performed on the film revealed a transmission of 84.97% and indicated a band gap of approximately 3.27 eV. The film exhibited a refractive index of 2.5 within the wavelength range of 300 to 450 nm.

Monolithic Use of Inert Gas for Highly Transparent and Conductive Indium Tin Oxide Thin Films

Due to its excellent electrical conductivity, high transparency in the visible spectrum, and exceptional chemical stability, indium tin oxide (ITO) has become a crucial material in the fields of optoelectronics and nanotechnology. This article provides a thorough analysis of growing ITO thin films with various thicknesses to study the impact of thickness on their electrical, optical, and physical properties for solar-cell applications. ITO was prepared through radio frequency (RF) magnetron sputtering using argon gas with no alteration in temperature or changes in substrate heating, followed with annealing in a tube furnace under inert conditions. An investigation of the influence of thickness on the optical, electrical, and physical properties of the films was conducted. We found that the best thickness for ITO thin films was 100 nm in terms of optical, electrical, and physical properties. To gain full comprehension of the impact on electrical properties, the different samples were characterized using a four-point probe and, interestingly, we found a high conductivity in the range of 1.8-2 × 106 S/m, good resistivity that did not exceed 1-2 × 10-6 Ωm, and a sheet resistance lower than 16 Ω sq-1. The transparency values found using a spectrophotometer reached values beyond 85%, which indicates the high purity of the thin films. Atomic force microscopy indicated a smooth morphology with low roughness values for the films, indicating an adequate transitioning of the charges on the surface. Scanning electron microscopy was used to study the actual thicknesses and the morphology, through which we found no cracks or fractures, which implied excellent deposition and annealing. The X-ray diffraction microscopy results showed a high purity of the crystals, as the peaks (222), (400), (440), and (622) of the crystallographic plane reflections were dominant, which confirmed the existence of the faced-center cubic lattice of ITO. This work allowed us to design a method for producing excellent ITO thin films for solar-cell applications.

Intense narrow band blue emission in CuCrO2 delafossite by Ni2+-Mg2+ dual cation doping

For applications involving LED lighting and displays, phosphor materials with narrow emission bands are quite riveting. CuCrO2 is a well-known delafosste for optoelectronic device applications due to its wide bandgap, and high emission at short wavelengths. Here we tried emission band tuning of CuCrO2 by Ni2+ on 0.5 wt% Mg doped CuCrO2 crystallites. The photoluminescence spectra (PL) are observed when excited with 373 nm radiation, indicating that it had a strong blue emission peak at 423 nm with FWHM of 18.26 nm and good color purity of 91.32%. Rietveld refinement of XRD spectra and Raman investigation conveyed a considerable structural variation with Ni2+ doping and band gap decrement by UV-Vis analysis. In Raman studies, shifting of signature peaks and additional defect-induced peaks are observed, manifesting desired microstructural changes, and the UV-Vis absorption investigation revealed marginal decay of the optical band gap from 3.31 eV to 2.63 eV. Refractive index has also been determined from the UV-Vis analysis and their values substantiate the results.

Photoelectrochemical Enzyme Biosensor for Malate Using Quantum Dots on Indium Tin Oxide/Plastics as a Sensing Surface

A photoelectrochemical biosensor for malate was developed using an indium tin oxide (ITO) layer deposited on a poly(ethylene terephthalate) plastic sheet as a transparent electrode material for the immobilization of malate dehydrogenase together with CdTe quantum dots. Different approaches were compared for the construction of the bioactive layer; the highest response was achieved by depositing malate dehydrogenase together with CdTe nanoparticles and covering it with a Nafion/water (1:1) mixture. The amperometric signal of this biosensor was recorded during irradiation with a near-UV LED in the flow-through mode. The limit of detection was 0.28 mmol/L, which is adequate for analyzing malic acid levels in drinks such as white wines and fruit juices. The results confirm that the cheap ITO layer deposited on the plastic sheet after cutting into rectangular electrodes allows for the economic production of photoelectrochemical (bio)sensors. The combination of NAD+-dependent malate dehydrogenase with quantum dots was also compatible with such an ITO surface.

Chemical synthesis of complex oxide thin films and freestanding membranes

Oxides offer unique physical and chemical properties that inspire rapid advances in materials chemistry to design and nanoengineer materials compositions and implement them in devices for a myriad of applications. Chemical deposition methods are gaining attention as a versatile approach to develop complex oxide thin films and nanostructures by properly selecting compatible chemical precursors and designing an accurate cost-effective thermal treatment. Here, upon describing the basics of chemical solution deposition (CSD) and atomic layer deposition (ALD), some examples of the growth of chemically-deposited functional complex oxide films that can have applications in energy and electronics are discussed. To go one step further, the suitability of these techniques is presented to prepare freestanding complex oxides which can notably broaden their applications. Finally, perspectives on the use of chemical methods to prepare future materials are given.

Light-enhanced electrical behavior of a Au/Al-doped ZnO/p-Si/Al heterostructure: insights from impedance and current-voltage analysis

In this study, we meticulously deposited an Al-doped ZnO nanoparticle thin film on a p-type silicon substrate using the precise sputtering method. We conducted a comprehensive exploration of the film's structure, morphology, and optical properties. X-ray diffraction (XRD) confirmed its polycrystalline wurtzite configuration with a dominant (002) orientation. High-resolution scanning electron microscopy (SEM) and atomic force microscopy (AFM) revealed a uniformly textured surface adorned with densely packed nanoparticles. Regarding optical properties, the Al-doped ZnO thin film exhibited exceptional transmittance exceeding 80% across visible and near-infrared spectra. Moving on to electrical characteristics, we assessed the Au/Al-doped ZnO/p-Si/Al heterostructure under dark and illuminated conditions. Through current-voltage (I-V) and impedance measurements, we observed significant improvements in conductivity and performance under illumination. Notably, there was an increase in current conduction and a reduction in impedance, highlighting the advantages of illumination. Collectively, these findings emphasize the promising potential of the Au/Al-doped ZnO/p-Si/Al heterostructure, particularly in the realms of optoelectronic devices and photovoltaics. With its ability to efficiently mobilize charges and adeptly assimilate light, this heterostructure stands as a frontrunner for transformative applications in these technologically vital domains.

Fabrication of ITO microelectrodes and electrode arrays using a low-cost CO2 laser plotter

Good electronic (Rs = ∼5 Ω sq-1) and optical properties (transmittance: >83%) make indium tin oxide (ITO) an attractive electrode substrate. Despite the commercial availability of high-quality ITO and some low-cost methods for direct deposition being in use by now, the definition of patterns is still a concern. Putting their popularity and extensive use aside, the manufacturing of ITO electrodes so far lacks a rapid, highly reproducible, flexible, cost-effective, easy patterning process that could surpass difficult, time-consuming techniques such as lithography. Herein, we present a low-cost method based on CO2 laser irradiation for preparing ITO microelectrodes and electrode arrays. Electrodes of different sizes and shapes were examined to identify the performance of the proposed methods. Direct ablation of the ITO layer was optimized for rectangular electrodes of 25, 50, and 100 μm in width, while laser cutting of scotch tape stencils and subsequent wet etching were used to create circular electrodes with a diameter of 1.75 mm. A multielectrode array system consisting 8 of these circular electrodes was fabricated on a (25 × 25) mm2 plate, characterized electrochemically through cyclic voltammetry (CV) and scanning electrochemical microscopy (SECM), and as an example application used for monitoring the anchoring behavior of HeLa and HepG2 cell cultures through cell-based electrochemical impedance spectroscopy. Together, the direct ablation method and preparation of laser cut stencils form a complete toolbox, which allows for low-cost and fast fabrication of ITO electrodes for a wide variety of applications. To demonstrate the general availability of the method, we have also prepared a batch of electrodes using a laser plotter in a local printing shop, achieving high intra-workshop reproducibility.

Transparent Conducting Amorphous IZO Thin Films: An Approach to Improve the Transparent Electrode Quality

It is common knowledge that using different oxygen contents in the working gas during sputtering deposition results in fabrication of indium zinc oxide (IZO) films with a wide range of optoelectronic properties. It is also important that high deposition temperature is not required to achieve excellent transparent electrode quality in the IZO films. Modulation of the oxygen content in the working gas during RF sputtering of IZO ceramic targets was used to deposit IZO-based multilayers in which the ultrathin IZO unit layers with high electron mobility (μ-IZO) alternate with ones characterized by high concentration of free electrons (n-IZO). As a result of optimizing the thicknesses of each type of unit layer, low-temperature 400 nm thick IZO multilayers with excellent transparent electrode quality, indicated by the low sheet resistance (R ≤ 8 Ω/sq.) with high transmittance in the visible range (T¯ > 83%) and a very flat multilayer surface, were obtained.

Estimation of the work of adhesion between ITO and polymer substrates: a surface thermodynamics approach

Indium tin oxide (ITO) is one of the most widely used semiconductor among transparent conducting oxides (TCOs) due to their electrical conductivity and optical transparency properties. Since the development of low temperature deposition methods, coating of ITO on polymer substrates especially for use in flexible electronics has been a popular topic. The existence of adequate adhesion strength between ITO and polymer is critical in producing a successful film. Nowadays, polycarbonate (PC), poly(methyl methacrylate) (PMMA) and polyethyleneterephtalate (PET) are frequently used as substrates for such coatings. However, there may be other polymeric alternatives that have a potential to be used for this purpose in the future. To evaluate these alternatives, work of adhesion (Wa) knowledge between ITO and polymers is necessary, and it has not been handled systematically previously. In this study, the interphase interaction parameters and Wa values between ITO and various polymers were calculated based on the Dupré, Fowkes and Girifalco-Good equations. PC, PMMA, PET, polystyrene (PS), polyphenylene sulfide (PPS), Nylon 66, polypropylene (PP), polyvinylchloride (PVC), styrene-butadiene rubber (SBR), high density polyethylene (HDPE), low density polyethylene (LDPE), polyvinyl acetate (PVAc), polyvinyl fluoride (PVF), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polytrifluoroethylene (PTrFE) and polyperfluoroalkylethyl acrylate (PPFA) were considered as substrate material. Surface free energy (SFE) components calculated by acid-base, geometric mean and harmonic mean approaches for polymeric substrates were used during the calculations. In the present study, the polymers that can be used as substrates were evaluated in terms of adhesion ability to ITO, the significance of calculation methods on Wa values were also investigated simultaneously. It was determined that the Wa between ITO and polymer substrates was directly related with the total SFE value of the polymers.

Numerical Analysis of a CZTS Solar Cell with MoS2 as a Buffer Layer and Graphene as a Transparent Conducting Oxide Layer for Enhanced Cell Performance

Copper zinc tin sulfide (CZTS) can be considered an important absorber layer material for utilization in thin film solar cell devices because of its non-toxic, earth abundance, and cost-effective properties. In this study, the effect of molybdenum disulfide (MoS₂) as a buffer layer on the different parameters of CZTS-based solar cell devices was explored to design a highly efficient solar cell. While graphene is considered a transparent conducting oxide (TCO) layer for the superior quantum efficiency of CZTS thin film solar cells, MoS₂ acts as a hole transport layer to offer electron-hole pair separation and an electron blocking layer to prevent recombination at the graphene/CZTS interface. This study proposed and analyzed a competent and economic CZTS solar cell structure (graphene/MoS₂/CZTS/Ni) with MoS₂ and graphene as the buffer and TCO layers, respectively, using the Solar Cell Capacitance Simulator (SCAPS)-1D. The proposed structure exhibited the following enhanced solar cell performance parameters: open-circuit voltage-0.8521 V, short-circuit current-25.3 mA cm^-2, fill factor-84.76%, and efficiency-18.27%.

Solution Combustion Synthesis of Hafnium-Doped Indium Oxide Thin Films for Transparent Conductors

Indium oxide (In₂O₃)-based transparent conducting oxides (TCOs) have been widely used and studied for a variety of applications, such as optoelectronic devices. However, some of the more promising dopants (zirconium, hafnium, and tantalum) for this oxide have not received much attention, as studies have mainly focused on tin and zinc, and even fewer have been explored by solution processes. This work focuses on developing solution-combustion-processed hafnium (Hf)-doped In₂O₃ thin films and evaluating different annealing parameters on TCO’s properties using a low environmental impact solvent. Optimized TCOs were achieved for 0.5 M% Hf-doped In₂O₃ when produced at 400 °C, showing high transparency in the visible range of the spectrum, a bulk resistivity of 5.73 × 10⁻² Ω.cm, a mobility of 6.65 cm²/V.s, and a carrier concentration of 1.72 × 10¹⁹ cm⁻³. Then, these results were improved by using rapid thermal annealing (RTA) for 10 min at 600 °C, reaching a bulk resistivity of 3.95 × 10 ⁻³ Ω.cm, a mobility of 21 cm²/V.s, and a carrier concentration of 7.98 × 10¹⁹ cm⁻³, in air. The present work brings solution-based TCOs a step closer to low-cost optoelectronic applications.

Unveiling and engineering of third-order optical nonlinearities in NiCo2O4 nanoflowers

In this Letter, we demonstrate for the first time, to the best of our knowledge, NiCo₂O₄ (NCO) as a novel nonlinear optical material with straightforward potential applications in optical limiting. For the 532 nm nanosecond laser, excited state absorption (ESA) and free-carrier absorption give rise to large ESA coefficient (β_ESA) and positive nonlinear n₂. On the other hand, when excited with the 800 nm femtosecond laser, two-photon absorption (TPA) takes place, and bound carriers induce strong negative n₂. The values of β and n₂ obtained for NCO are found to be higher compared to other conventional transition metal oxides and, therefore, are promising for optics and other photonics applications.

Optimized Stoichiometry for CuCrO2 Thin Films as Hole Transparent Layer in PBDD4T-2F:PC70BM Organic Solar Cells

The performance and stability in atmospheric conditions of organic photovoltaic devices can be improved by the integration of stable and efficient photoactive materials as substituent of the chemically unstable poly (3,4-ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS), generally used as organic hole transport layer. Promising candidates are p-type transparent conductive oxides, which combine good optoelectronic and a higher mechanical and chemical stability than the organic counterpart. In this work, we synthesize Cu-rich CuCrO2 thin films by aerosol-assisted chemical vapour deposition as an efficient alternative to PEDOT:PSS. The effect of stoichiometry on the structural, electrical, and optical properties was analysed to find a good compromise between transparency, resistivity, and energy bands alignment, to maximize the photovoltaic performances., Average transmittance and bandgap are reduced when increasing the Cu content in these out of stoichiometry CuCrO2 films. The lowest electrical resistivity is found for samples synthesized from a solution composition in the 60-70% range. The optimal starting solution composition was found at 65% of Cu cationic ratio corresponding to a singular point in Hackee's figure of merit of 1 × 10-7 Ω-1. PBDD4T-2F:PC70BM organic solar cells were fabricated by integrating CuCrO2 films grown from a solution composition ranging between 40% to 100% of Cu as hole transport layers. The solar cells integrating a film grown with a Cu solution composition of 65% achieved a power conversion efficiency as high as 3.1%, representing the best trade-off of the optoelectronic properties among the studied candidates. Additionally, despite the efficiencies achieved from CuCrO2-based organic solar cells are still inferior to the PEDOT:PSS counterpart, we demonstrated a significant enhancement of the lifetime in atmospheric conditions of optimal oxides-based organic photovoltaic devices.

ALD Deposited ZnO:Al Films on Mica for Flexible PDLC Devices

In this work, highly conductive Al-doped ZnO (AZO) films are deposited on transparent and flexible muscovite mica substrates by using the atomic layer deposition (ALD) technique. AZO-mica structures possess high optical transmittance at visible and near-infrared spectral range and retain low electric resistivity, even after continuous bending of up to 800 cycles. Structure performances after bending tests have been supported by atomic force microscopy (AFM) analysis. Based on performed optical and electrical characterizations AZO films on mica are implemented as transparent conductive electrodes in flexible polymer dispersed liquid crystal (PDLC) devices. The measured electro-optical characteristics and response time of the proposed devices reveal the higher potential of AZO-mica for future ITO-free flexible optoelectronic applications.

Percolation threshold of curved linear objects

In this study, we investigate the percolation threshold of curved linear objects, describing them as quadratic Bézier curves. Using Monte Carlo simulations, we calculate the critical number densities of the curves with different curviness. We also obtain the excluded area of the curves. When an excluded area is given, we can find the critical number density of the curves with arbitrary curviness. Apparent conductivity exponents are computed for the curves, and these values are found to be analogous to that of sticks in the percolative region for a junction resistance dominant system. These results can be used to analyze the optoelectrical performance of metal nanowire films because the high-aspect-ratio metal nanowires can be easily curved during coating.

Proposal of new spinel oxides semiconductors ZnGaO2, [ZnGaO2]:Mn3+ and Rh3+: ab-initio calculations and prospects for thermophysical and optoelectronic applications

Transparent conducting oxides (TCOs) of semiconductor family gained significant attention due to increasing trends in the optoelectronic and thermo-physical applications. In current work, we reported electronic, optical, transport and thermodynamical properties of spinel oxides ZnGaO₂, [ZnGaO₂]:Mn³⁺ and [ZnGaO₂]:Rh³⁺ compounds. Based on DFT, we employed first-principles calculations implemented in Wien 2k using the modified-Becke-Johnson (mBJ) on parent spinel and generalized-gradient-approximation plus Hubbard potential U (GGA + U) on doped materials, respectively. The calculated band structure shows insulating nature of parent compound, while doped material observed semiconducting nature contains direct band gap for both spin channels with band gaps of [ZnGaO₂]:Mn³⁺ (0.59 up, 2.4 eV dn) and [ZnGaO₂]:Rh³⁺ (2.1 eV up/dn) respectively. The electronic and optical results reveal that hybridization occurred mainly due to O-p/Zn, Mn-d, Rh-d and Ga-s orbitals. It is analyzed that Mn-doped material shows good absorption in the visible region while other are good in UV region. The effective masses of spinel oxides are also computed at high symmetry directions hence varied nonlinearly with the doping. The stability of materials is checked by calculating formation energies which indicate Mn-doped spinel oxide is most stable as that of others. The thermoelectric properties of spinel oxides were carried out by Post-DFT (Boltztrap) calculations. Large values of Seebeck coefficient and power factor of Mn-doped spinel oxide indicate that this material can be used for thermoelectric devices. The thermodynamical properties are calculated by quasi-harmonic Debye model implemented in GIBBS 2 code. Moreover, the pressure and temperature dependence of all (TD) parameters of investigated spinel oxides are analyzed using quasi-harmonic Debye model.

Synthesis, microstructure, multifunctional properties of mayenite Ca12Al14O33 (C12A7) cement and graphene oxide (GO) composites

The Pristine Mayenite Ca12Al14O33 (C12A7) Cement was simply synthesized by using solid-state reaction. The C12A7 and Graphene Oxide (GO) composites (C12A7_GO-x) with various contents of the GO suspension loading (x = 0 wt%, 1 wt%, 2 wt%, 3 wt%, and 4 wt%) were directly prepared by mixing the C12A7 and GO. X-ray diffraction results of pristine C12A7 and all C12A7_GO composites indicated a pure phase corresponding to the standard of C12A7 cement. Raman spectroscopy confirmed the existence of GO in all C12A7_GO samples. Scanning Electron Microscopy (SEM) showed the micrometer grain sizes and the occurrence of grain boundary interfaces for GO incorporation in all C12A7_GO samples. UV-Vis spectroscopy revealed the absorption value of all C12A7_GO samples and red shift near longer wavelengths when increasing the GO concentrations. The dielectric constant of C12A7_GO composites can be explained by the high density of free electron charges for the interfacial polarization on the GO surface. The maximum specific capacitance of C12A7_GO-4 electrode of 21.514 at a current density of 0.2 A g-1 can be attributed to the increase in the electrochemically active surface area for the formation of the electrical double layer capacitors behavior and the effects of high surface area GO connections. Also, the mechanical properties exhibited an increase in Vickers indenter hardness (HV) values with increasing GO contents. The highest HV value was 117.8 HV/2 kg at the C12A7_GO-4 sample. These results showed that the composite materials of the pristine C12A7 cement with GO were highly efficient. All in all, the GO material contained a high potential for enhancing low-cost cement materials in multifunctional properties such as optical, dielectric, electrochemical, and mechanical properties.

Electrochemical Biosensors Employing Natural and Artificial Heme Peroxidases on Semiconductors

Heme peroxidases are widely used as biological recognition elements in electrochemical biosensors for hydrogen peroxide and phenolic compounds. Various nature-derived and fully synthetic heme peroxidase mimics have been designed and their potential for replacing the natural enzymes in biosensors has been investigated. The use of semiconducting materials as transducers can thereby offer new opportunities with respect to catalyst immobilization, reaction stimulation, or read-out. This review focuses on approaches for the construction of electrochemical biosensors employing natural heme peroxidases as well as various mimics immobilized on semiconducting electrode surfaces. It will outline important advances made so far as well as the novel applications resulting thereof.

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.

Highly Conductive Sb-SnO2 Nanocrystals Synthesized by Dual Nonthermal Plasmas

Nonthermal plasma synthesis of transparent conducting oxide nanocrystals can offer advantages, for example, ligand-free surfaces, over traditionally used colloidal synthesis methods. When it comes to multicomponent (doped) metal oxide nanocrystal synthesis, uniform distribution of different metal elements and suppressing surface segregation of secondary resistive phases have been concerns. Specifically, surface segregation of resistive secondary phases reduces the electrical conductivity of nanocrystal assemblies. In this work, we demonstrate a nonthermal dual-plasma synthesis method capable of forming Sb-SnO₂ (ATO) nanocrystals with a uniform composition distribution and apparently insignificant surface segregation of the dopant. A drastic increase in conductivity was observed in ATO thin films comprised of nanocrystals formed using a dual-plasma configuration compared to nanocrystals formed using a single-plasma configuration. The conductivity values of as-deposited porous films comprised of ATO nanocrystals, prepared using the dual-plasma approach, were on the order of 0.1 S cm^-1, which to our knowledge is the highest conductivity reported to-date for that type of high surface area material. Annealing the films comprised of ATO nanocrystals at 500 °C for 2 h in air increased the conductivity and improved ambient stability, without significantly affecting the crystallite size.

Resonant Ta Doping for Enhanced Mobility in Transparent Conducting SnO2

Transparent conducting oxides (TCOs) are ubiquitous in modern consumer electronics. SnO₂ is an earth abundant, cheaper alternative to In₂O₃ as a TCO. However, its performance in terms of mobilities and conductivities lags behind that of In₂O₃. On the basis of the recent discovery of mobility and conductivity enhancements in In₂O₃ from resonant dopants, we use a combination of state-of-the-art hybrid density functional theory calculations, high resolution photoelectron spectroscopy, and semiconductor statistics modeling to understand what is the optimal dopant to maximize performance of SnO₂-based TCOs. We demonstrate that Ta is the optimal dopant for high performance SnO₂, as it is a resonant dopant which is readily incorporated into SnO₂ with the Ta 5d states sitting ∼1.4 eV above the conduction band minimum. Experimentally, the band edge electron effective mass of Ta doped SnO₂ was shown to be 0.23m ₀, compared to 0.29m ₀ seen with conventional Sb doping, explaining its ability to yield higher mobilities and conductivities.

Active IrO2 and NiO Thin Films Prepared by Atomic Layer Deposition for Oxygen Evolution Reaction

Atomic layer deposition (ALD) is a special type of chemical vapor deposition (CVD) technique that can grow uniformed thin films on a substrate through alternate self-limiting surface reactions. Recently, the application of these thin film materials to catalytic systems has begun to attract much attention, and the capacity to deposit these catalytic films in a highly controlled manner continues to gain importance. In this study, IrO2 and NiO thin films (approximately 25 to 60 nm) were deposited on industrial Ni expanded mesh as an anode for alkaline water electrolysis. Different ALD operating parameters such as the total number of deposition cycles, sublimation and deposition temperatures, and precursors pulse and purge lengths were varied to determine their effects on the structure and the electrochemical performance of the thin film materials. Results from the electrochemical tests (6 M KOH, 80 °C, up to 10 kA/m2) showed the catalytic activity of the samples. Oxygen overpotential values (ηO2) were 20 to 60 mV lower than the bare Ni expanded mesh. In summary, the study has demonstrated the feasibility of using the ALD technique to deposit uniformed and electroactive thin films on industrial metallic substrates as anodes for alkaline water electrolysis.

Water-Oxidation Electrocatalysis by Manganese Oxides: Syntheses, Electrode Preparations, Electrolytes and Two Fundamental Questions

The efficient catalysis of the four-electron oxidation of water to molecular oxygen is a central challenge for the development of devices for the production of solar fuels. This is equally true for artificial leaf-type structures and electrolyzer systems. Inspired by the oxygen evolving complex of Photosystem II, the biological catalyst for this reaction, scientists around the globe have investigated the possibility to use manganese oxides (“MnOx”) for this task. This perspective article will look at selected examples from the last about 10 years of research in this field. At first, three aspects are addressed in detail which have emerged as crucial for the development of efficient electrocatalysts for the anodic oxygen evolution reaction (OER): (1) the structure and composition of the “MnOx” is of central importance for catalytic performance and it seems that amorphous, MnIII/IV oxides with layered or tunnelled structures are especially good choices; (2) the type of support material (e.g. conducting oxides or nanostructured carbon) as well as the methods used to immobilize the MnOx catalysts on them greatly influence OER overpotentials, current densities and long-term stabilities of the electrodes and (3) when operating MnOx-based water-oxidizing anodes in electrolyzers, it has often been observed that the electrocatalytic performance is also largely dependent on the electrolyte’s composition and pH and that a number of equilibria accompany the catalytic process, resulting in “adaptive changes” of the MnOx material over time. Overall, it thus has become clear over the last years that efficient and stable water-oxidation electrolysis by manganese oxides can only be achieved if at least four parameters are optimized in combination: the oxide catalyst itself, the immobilization method, the catalyst support and last but not least the composition of the electrolyte. Furthermore, these parameters are not only important for the electrode optimization process alone but must also be considered if different electrode types are to be compared with each other or with literature values from literature. Because, as without their consideration it is almost impossible to draw the right scientific conclusions. On the other hand, it currently seems unlikely that even carefully optimized MnOx anodes will ever reach the superb OER rates observed for iridium, ruthenium or nickel-iron oxide anodes in acidic or alkaline solutions, respectively. So at the end of the article, two fundamental questions will be addressed: (1) are there technical applications where MnOx materials could actually be the first choice as OER electrocatalysts? and (2) do the results from the last decade of intensive research in this field help to solve a puzzle already formulated in 2008: “Why did nature choose manganese to make oxygen?”.

Low-cost uncooled MWIR PbSe quantum dots photodiodes

A mid-wave infrared (MWIR) uncooled PbSe-QDs/CdS p–n heterojunction photodiode has been fabricated using a wet-chemical synthesis route. This offers a low-cost alternative to traditional monocrystalline photodiodes relying on molecular beam epitaxy (MBE) technology. It was demonstrated that the post-annealing is critical to tailor the photoresponse wavelength and to improve the performance of photodiodes. After annealing at 673 K in air for 0.5 h, the ligand-free PbSe-QDs/CdS photodiode exhibits a MWIR spectral photoresponse with a cutoff wavelength of 4.2 μm at room temperature. Under zero-bias photovoltaic mode, the peak responsivity and specific detectivity at room temperature are 0.36 ± 0.04 A W⁻¹ and (8.5 ± 1) ×10⁸ cm Hz^1/2 W⁻¹, respectively. Temperature-dependent spectral response shows an abnormal intensity variation at temperatures lower than 200 K. This phenomenon is attributed to the band alignment transition from type II to type I, resulting from the positive temperature coefficient of PbSe. In addition, it was proved that In doped CdSe (CdSe:In) films could be used as a promising new candidate of infrared transparent conductive electrodes, paving the way for monolithic integration of uncooled low-cost MWIR photodiodes on Si readout circuitry.

A mid-wave infrared (MWIR) uncooled PbSe-QDs/CdS p–n heterojunction photodiode has been fabricated using a wet-chemical synthesis route.

Importance of Local Bond Order to Conduction in Amorphous, Transparent, Conducting Oxides: The Case of Amorphous ZnSnOy

In this report, reactive and nonreactive sputtering of amorphous ZnSnO_y (a-ZnSnO_y) was investigated, and extensive composition maps have been measured by X-ray photoelectron spectroscopy. The comprehensive analysis of the ((ZnO)_x(SnO₂)_1-x) composition reveals that the best Zn/Sn ratio for high conductivity of the material can vary depending on the deposition technique utilized. Best conductivities of 225 S/cm were found to occur at x = 0.32 for reactive sputtering of a Sn target and x = 0.27 for nonreactive sputtering of a SnO₂ target. These values correspond to unstable polymorphs of a-ZnSnO_y, ZnSn₂O₅, and ZnSn₃O₇. Distinct local bonding arrangements have been confirmed by Raman spectroscopy.

Transparent and Flexible Mayan-Pyramid-based Pressure Sensor using Facile-Transferred Indium tin Oxide for Bimodal Sensor Applications

Transparent and conducting flexible electrodes have been successfully developed over the last few decades due to their potential applications in optoelectronics. However, recent developments in smart electronics, such as a direct human-machine interface, health-monitoring devices, motion-tracking sensors, and artificially electronic skin also require materials with multifunctional properties such as transparency, flexibility and good portability. In such devices, there remains room to develop transparent and flexible devices such as pressure sensors or temperature sensors. Herein, we demonstrate a fully transparent and flexible bimodal sensor using indium tin oxide (ITO), which is embedded in a plastic substrate. For the proposed pressure sensor, the embedded ITO is detached from its Mayan-pyramid-structured silicon mold by an environmentally friendly method which utilizes water-soluble sacrificial layers. The Mayan-pyramid-based pressure sensor is capable of six different pressure sensations with excellent sensitivity in the range of 100 Pa-10 kPa, high endurance of 105 cycles, and good pulse detection and tactile sensing data processing capabilities through machine learning (ML) algorithms for different surface textures. A 5 × 5-pixel pressure-temperature-based bimodal sensor array with a zigzag-shaped ITO temperature sensor on top of it is also demonstrated without a noticeable interface effect. This work demonstrates the potential to develop transparent bimodal sensors that can be employed for electronic skin (E-skin) applications.

Enhanced Sol-Gel Route to Obtain a Highly Transparent and Conductive Aluminum-Doped Zinc Oxide Thin Film

The electrical and optical properties of sol–gel derived aluminum-doped zinc oxide thin films containing 2 at.% Al were investigated considering the modifying effects of (1) increasing the sol H₂O content and (2) a thermal treatment procedure with a high-temperature approach followed by an additional heat-treatment step under a reducing atmosphere. According to the results obtained via the TG-DTA analysis, FT-IR spectroscopy, X-ray diffraction technique, and four-point probe resistivity measurements, it is argued that in the modified sample, the sol hydrolysis, decomposition of the deposited gel, and crystallization of grains result in grains of larger crystallite size in the range of 20 to 30 nm and a stronger c-axis preferred orientation with slightly less microstrain. The obtained morphology and grain-boundary characteristics result in improved conductivity considering the resistivity value below 6 mΩ·cm. A detailed investigation of the samples’ optical properties, in terms of analyzing their absorption and dispersion behaviors through UV-Vis-NIR spectroscopy, support our reasoning for the increase of the mobility, and to a lesser extent the concentration of charge carriers, while causing only a slight degradation of optical transmittance down to nearly 80%. Hence, an enhanced performance as a transparent conducting film is claimed for the modified sample by comparing the figure-of-merit values.

Beyond the display: phase-only liquid crystal on Silicon devices and their applications in photonics [Invited]

Existing for almost four decades, liquid crystal on Silicon (LCOS) technology is rapidly growing into photonic applications. We review the basics of the technology, from the wafer to the driving solutions, the progress over the last decade and the future outlook. Furthermore we review the most exciting industrial and scientific applications of the LCOS technology.

Using Oxygen Plasma Pretreatment to Enhance the Properties of F-Doped ZnO Films Prepared on Polyimide Substrates

In this study, a radio frequency magnetron sputtering process was used to deposit F-doped ZnO (FZO) films on polyimide (PI) substrates. The thermal expansion effect of PI substrates induces distortion and bending, causing FZO films to peel and their electrical properties and crystallinity to deteriorate. To address these shortcomings, oxygen (O₂) plasma was used to pretreat the surface of PI substrates using a plasma-enhanced chemical vapor deposition system before the FZO films were deposited. The effects of O₂ plasma pretreatment time on the surface water contact angle, surface morphologies, and optical properties of the PI substrates were investigated. As the pretreatment time increased, so did the roughness of the PI substrates. After the FZO films had been deposited on the PI substrates, variations in the surface morphologies, crystalline structure, composition, electrical properties, and optical properties were investigated as a function of the O₂ plasma pretreatment time. When this was 30 s, the FZO films had optimal optical and electrical properties. The resistivity was 3.153 × 10^-3 Ω-cm, and the transmittance ratios of all films were greater than 90%. The X-ray photoelectron spectroscopy spectra of the FZO films, particularly the peaks for O_1s, Zn 2p_1/2, and Zn 2p_3/2, were determined for films with O₂ plasma pretreatment times of 0 and 30 s. Finally, a HCl solution was used to etch the surfaces of the deposited FZO films, and silicon-based thin-film solar cells were fabricated on the FZO/PI substrates. The effect of O₂-plasma pretreatment time on the properties of the fabricated solar cells is thoroughly discussed.

Atomic layer deposition of nickel-cobalt spinel thin films

We report the atomic layer deposition (ALD) of high-quality crystalline thin films of the spinel-oxide system (Co_1-xNi_x)₃O₄. These spinel oxides are ferrimagnetic p-type semiconductors, and promising material candidates for several applications ranging from photovoltaics and spintronics to thermoelectrics. The spinel phase is obtained for Ni contents exceeding the x = 0.33 limit for bulk samples. It is observed that the electrical resistivity decreases continuously with x while the magnetic moment increases up to x = 0.5. This is in contrast to bulk samples where a decrease of resistivity is not observed for x > 0.33 due to the formation of a rock-salt phase. From UV-VIS-NIR absorption measurements, a change from distinct absorption edges for the parent oxide Co₃O₄ to a continuous absorption band ranging deep into the near infrared for 0 < x ≤ 0.5 was observed. The conformal deposition of dense films on high-aspect-ratio patterns is demonstrated.

Quantifying the Performance of P-Type Transparent Conducting Oxides by Experimental Methods

Screening for potential new materials with experimental and theoretical methods has led to the discovery of many promising candidate materials for p-type transparent conducting oxides. It is difficult to reliably assess a good p-type transparent conducting oxide (TCO) from limited information available at an early experimental stage. In this paper we discuss the influence of sample thickness on simple transmission measurements and how the sample thickness can skew the commonly used figure of merit of TCOs and their estimated band gap. We discuss this using copper-deficient CuCrO2 as an example, as it was already shown to be a good p-type TCO grown at low temperatures. We outline a modified figure of merit reducing thickness-dependent errors, as well as how modern ab initio screening methods can be used to augment experimental methods to assess new materials for potential applications as p-type TCOs, p-channel transparent thin film transistors, and selective contacts in solar cells.

Thermoelectric and Transport Properties of Delafossite CuCrO₂:Mg Thin Films Prepared by RF Magnetron Sputtering

P-type Mg doped CuCrO₂ thin films have been deposited on fused silica substrates by Radio-Frequency (RF) magnetron sputtering. The as-deposited CuCrO₂:Mg thin films have been annealed at different temperatures (from 450 to 650 °C) under primary vacuum to obtain the delafossite phase. The annealed samples exhibit 3R delafossite structure. Electrical conductivity σ and Seebeck coefficient S of all annealed films have been measured from 40 to 220 °C. The optimized properties have been obtained for CuCrO₂:Mg thin film annealed at 550 °C. At a measurement temperature of 40 °C, this sample exhibited the highest electrical conductivity of 0.60 S·cm⁻¹ with a Seebeck coefficient of +329 µV·K⁻¹. The calculated power factor (PF = σS²) was 6 µW·m⁻¹·K⁻² at 40 °C and due to the constant Seebeck coefficient and the increasing electrical conductivity with measurement temperature, it reached 38 µW·m⁻¹·K⁻² at 220 °C. Moreover, according to measurement of the Seebeck coefficient and electrical conductivity in temperature, we confirmed that CuCrO₂:Mg exhibits hopping conduction and degenerates semiconductor behavior. Carrier concentration, Fermi level, and hole effective mass have been discussed.

High-Throughput Continuous Hydrothermal Synthesis of Transparent Conducting Aluminum and Gallium Co-doped Zinc Oxides

High-throughput continuous hydrothermal flow synthesis was used to generate a library of aluminum and gallium-codoped zinc oxide nanoparticles of specific atomic ratios. Resistivities of the materials were determined by Hall Effect measurements on heat-treated pressed discs and the results collated into a conductivity-composition map. Optimal resistivities of ∼9 × 10^-3 Ω cm were reproducibly achieved for several samples, for example, codoped ZnO with 2 at% Ga and 1 at% Al. The optimum sample on balance of performance and cost was deemed to be ZnO codoped with 3 at% Al and 1 at% Ga.

Codoping and Interstitial Deactivation in the Control of Amphoteric Li Dopant in ZnO for the Realization of p-Type TCOs

We report on first principle investigations about the electrical character of Li-X codoped ZnO transparent conductive oxides (TCOs). We studied a set of possible X codopants including either unintentional dopants typically present in the system (e.g., H, O) or monovalent acceptor groups, based on nitrogen and halogens (F, Cl, I). The interplay between dopants and structural point defects in the host (such as vacancies) is also taken explicitly into account, demonstrating the crucial effect that zinc and oxygen vacancies have on the final properties of TCOs. Our results show that Li-ZnO has a p-type character, when Li is included as Zn substitutional dopant, but it turns into an n-type when Li is in interstitial sites. The inclusion of X-codopants is considered to deactivate the n-type character of interstitial Li atoms: the total Li-X compensation effect and the corresponding electrical character of the doped compounds selectively depend on the presence of vacancies in the host. We prove that LiF-doped ZnO is the only codoped system that exhibits a p-type character in the presence of Zn vacancies.

Colloidal Synthesis of Bipolar Off-Stoichiometric Gallium Iron Oxide Spinel-Type Nanocrystals with Near-IR Plasmon Resonance

We report the colloidal synthesis of ∼5.5 nm inverse spinel-type oxide Ga₂FeO₄ (GFO) nanocrystals (NCs) with control over the gallium and iron content. As recently theoretically predicted, some classes of spinel-type oxide materials can be intrinsically doped by means of structural disorder and/or change in stoichiometry. Here we show that, indeed, while stoichiometric Ga₂FeO₄ NCs are intrinsic small bandgap semiconductors, off-stoichiometric GFO NCs, produced under either Fe-rich or Ga-rich conditions, behave as degenerately doped semiconductors. As a consequence of the generation of free carriers, both Fe-rich and Ga-rich GFO NCs exhibit a localized surface plasmon resonance in the near-infrared at ∼1000 nm, as confirmed by our pump–probe absorption measurements. Noteworthy, the photoelectrochemical characterization of our GFO NCs reveal that the majority carriers are holes in Fe-rich samples, and electrons in Ga-rich ones, highlighting the bipolar nature of this material. The behavior of such off-stoichiometric NCs was explained by our density functional theory calculations as follows: the substitution of Ga³⁺ by Fe²⁺ ions, occurring in Fe-rich conditions, can generate free holes (p-type doping), while the replacement of Fe²⁺ by Ga³⁺ cations, taking place in Ga-rich samples, produces free electrons (n-type doping). These findings underscore the potential relevance of spinel-type oxides as p-type transparent conductive oxides and as plasmonic semiconductors.

Microscopic Examination of Cold Spray Cermet Sn+In2O3 Coatings for Sputtering Target Materials

Low-pressure cold spraying is a newly developed technology with high application potential. The aim of this study was to investigate potential application of this technique for producing a new type of transparent conductive oxide films target. Cold spraying technique allows the manufacture of target directly on the backing plate; therefore the proposed sputtering target has a form of Sn+In₂O₃ coating sprayed onto copper substrate. The microstructure and properties of the feedstock powder prepared using three various methods as well as the deposited ones by low-pressure cold spraying coatings were evaluated, compared, and analysed. Produced cermet Sn+In₂O₃ targets were employed in first magnetron sputtering process to deposit preliminary, thin, transparent conducting oxide films onto the glass substrates. The resistivity of obtained preliminary films was measured and allows believing that fabrication of TCO (transparent conducting oxide) films using targets produced by cold spraying is possible in the future, after optimization of the deposition conditions.

Spectroscopic identification of the chemical interplay between defects and dopants in Al-doped ZnO

Contributions to the spectroscopic response of defects and dopants in Al-doped ZnO films are determined combining X-ray spectroscopies and DFT.

Secondary Electron Emission Materials for Transmission Dynodes in Novel Photomultipliers: A Review

Secondary electron emission materials are reviewed with the aim of providing guidelines for the future development of novel transmission dynodes. Materials with reflection secondary electron yield higher than three and transmission secondary electron yield higher than one are tabulated for easy reference. Generations of transmission dynodes are listed in the order of the invention time with a special focus on the most recent atomic-layer-deposition synthesized transmission dynodes. Based on the knowledge gained from the survey of secondary election emission materials with high secondary electron yield, an outlook of possible improvements upon the state-of-the-art transmission dynodes is provided.

Recent Progress in Materials and Devices toward Printable and Flexible Sensors

Printable electronics present a new era of wearable electronic technologies. Detailed technologies consisting of novel ink semiconductor materials, flexible substrates, and unique processing methods can be integrated to create flexible sensors. To detect various stimuli of the human body, as well as specific environments, unique electronic devices formed by "ink-based semiconductors" onto flexible and/or stretchable substrates have become a major research trend in recent years. Materials such as inorganic, organic, and hybrid semiconductors with various structures (i.e., 1D, 2D and 3D) with printing capabilities have been considered for bio and medical applications. In this review, we report recent progress in materials and devices for future wearable sensor technologies.