Exploring Doping Mechanisms and Modulating Carrier Concentration in Copper Iodide: Applications in Thermoelectric Materials.

Due to its small hole-effective mass, flexibility, and transparency, copper iodide (CuI) has emerged as a promising p-type alternative to the predominantly used n-type metal oxide semiconductors. However, the lack of effective doping methods hinders the utility of CuI in various applications. Sulfur (S)-doping through liquid iodination is previously reported to significantly enhance electrical conductivity up to 511 S cm-1. In this paper, the underlying doping mechanism with various S-dopants is explored, and suggested a method for controlling electrical conductivity, which is important to various applications, especially thermoelectric (TE) materials. Subsequently, electric and TE properties are systematically controlled by adjusting the carrier concentration from 3.0 × 1019 to 4.5 × 1020 cm-3, and accurately measured thermal conductivity with respect to carrier concentration and film thickness. Sulfur-doped CuI (CuI:S) thin films exhibited a maximum power factor of 5.76 µW cm-1 K-2 at a carrier concentration of 1.3 × 1020 cm-3, and a TE figure of merit (ZT) of 0.25. Furthermore, a transparent and flexible TE power generator is developed, with an impressive output power density of 43 nW cm-2 at a temperature differential of 30 K. Mechanical durability tests validated the potential of CuI:S films in transparent and flexible TE applications.

Effect of Cl2 Radical on Dry Development of Spin-Coated Metal Oxide Resist.

In this study, the effects of Cl2 radicals on dry development of spin-coated metal oxide resist (MOR) and changes in its surface binding states were investigated to verify the mechanism of dry development. Dry development characteristics of tin hydroxide (Tin OH), which is one of the MOR candidates for next generation lithography, were investigated as functions of process time and temperature using a Cl2 radicals source. Non-UV-exposed Tin OH film showed a linear etch rate (1.77 nm/min) from the initial thickness of ∼50 nm, while the UV-exposed film showed slower etch behavior (1.46 nm/min) in addition to the increase of film thickness for up to 3 min during the Cl2 radical dry development. UV-exposed photoresist (PR) contained more oxygen (Sn-O bonding) in the film due to the removal of butyl compounds from the clusters during the UV exposure process. Therefore, due to the lower reaction of chlorine radicals with Sn-O in the UV-exposed Tin OH than the other bindings, the non-UV-exposed PR was preferentially removed compared to the UV-exposed PR. As the temperature decreases, the overall etch rate decreases, but the difference in etch rate between exposed and unexposed Tin OH becomes larger. Finally, at a substrate temperature of -20 °C, the non-UV-exposed Tin OH with a thickness of 50 nm was completely removed, while ∼30 nm thick PR remained for UV-exposed Tin OH. Eventually, a negative tone development was possible with Cl2 radical plasma due to the difference in activation energy between the UV-exposed and non-UV-exposed films. It is believed that dry development using Cl2 radicals will be one of the most important process techniques for next-generation patterning to remove problems such as pattern leaning, line edge roughness, residue, etc., caused by wet development.

Full integration of highly stretchable inorganic transistors and circuits within molecular-tailored elastic substrates on a large scale.

The emergence of high-form-factor electronics has led to a demand for high-density integration of inorganic thin-film devices and circuits with full stretchability. However, the intrinsic stiffness and brittleness of inorganic materials have impeded their utilization in free-form electronics. Here, we demonstrate highly integrated strain-insensitive stretchable metal-oxide transistors and circuitry (442 transistors/cm2) via a photolithography-based bottom-up approach, where transistors with fluidic liquid metal interconnection are embedded in large-area molecular-tailored heterogeneous elastic substrates (5 × 5 cm2). Amorphous indium-gallium-zinc-oxide transistor arrays (7 × 7), various logic gates, and ring-oscillator circuits exhibited strain-resilient properties with performance variation less than 20% when stretched up to 50% and 30% strain (10,000 cycles) for unit transistor and circuits, respectively. The transistors operate with an average mobility of 12.7 ( ± 1.7) cm2 V-1s-1, on/off current ratio of > 107, and the inverter, NAND, NOR circuits operate quite logically. Moreover, a ring oscillator comprising 14 cross-wired transistors validated the cascading of the multiple stages and device uniformity, indicating an oscillation frequency of ~70 kHz.

Dopant Control of Solution-Processed CuI:S for Highly Conductive p-Type Transparent Electrode.

Copper iodide (CuI) has garnered considerable attention as a promising alternative to p-type transparent conducting oxides owing to its low cation vacancy formation energy, shallow acceptor level, and readily modifiable conductivity via doping. Although sulfur (S) doping through liquid iodination has exhibited high efficacy in enhancing the conductivity with record high figure of merit (FOM) of 630 00 MΩ-1, solution-processed S-doped CuI (CuI:S) for low-cost large area fabrication has yet to be explored. Here, a highly conducting CuI:S thin-film for p-type transparent conducting electrode (TCE) is reported using low temperature solution-processing with thiourea derivatives. The optimization of thiourea dopant is determined through a comprehensive acid-base study, considering the effects of steric hindrance. The modification of active groups of thioureas facilitated a varying carrier concentration range of 9 × 1018-2.52 × 1020 cm-3 and conductivities of 4.4-390.7 S cm-1. Consequently, N-ethylthiourea-doped CuI:S exhibited a FOM value of 7 600 MΩ-1, which is the highest value among solution-processed p-type TCEs to date. Moreover, the formulation of CuI:S solution for highly conductive p-type TCEs can be extended to CuI:S inks, facilitating high-throughput solution-processes such as inkjet printing and spray coating.

In Situ Radiation Hardness Study of Amorphous Zn-In-Sn-O Thin-Film Transistors with Structural Plasticity and Defect Tolerance.

Solution-processed metal-oxide thin-film transistors (TFTs) with different metal compositions are investigated for ex situ and in situ radiation hardness experiments against ionizing radiation exposure. The synergetic combination of structural plasticity of Zn, defect tolerance of Sn, and high electron mobility of In identifies amorphous zinc-indium-tin oxide (Zn-In-Sn-O or ZITO) as an optimal radiation-resistant channel layer of TFTs. The ZITO with an elemental blending ratio of 4:1:1 for Zn/In/Sn exhibits superior ex situ radiation resistance compared to In-Ga-Zn-O, Ga-Sn-O, Ga-In-Sn-O, and Ga-Sn-Zn-O. Based on the in situ irradiation results, where a negative threshold voltage shifts and a mobility increase as well as both off current and leakage current increase are observed, three factors are proposed for the degradation mechanisms: (i) increase of channel conductivity, (ii) interface-trapped and dielectric-trapped charge buildup, and (iii) trap-assisted tunneling in the dielectric. Finally, in situ radiation-hard oxide-based TFTs are demonstrated by employing a radiation-resistant ZITO channel, a thin dielectric (50 nm SiO2), and a passivation layer (PCBM for ambient exposure), which exhibit excellent stability with an electron mobility of ∼10 cm2/V s and aΔVth of <3 V under real-time (15 kGy/h) gamma-ray irradiation in an ambient atmosphere.

Transformation of K2Sb8Q13 and KSb5Q8 Bulk Crystals to Sb2Q3 (Q = S, Se) Nanofibers by Acid-Base Solution Chemistry.

The ability to manipulate crystal structures using kinetic control is of broad interest because it enables the design of materials with structures, compositions, and morphologies that may otherwise be unattainable. Herein, we report the low-temperature structural transformation of bulk inorganic crystals driven by hard-soft acid-base (HSAB) chemistry. We show that the three-dimensional framework K2Sb8Q13 and layered KSb5Q8 (Q = S, Se, and Se/S solid solutions) compounds transform to one-dimensional Sb2Q3 nano/microfibers in N2H4·H2O solution by releasing Q2- and K+ ions. At 100 °C and ambient pressure, a transformation process takes place that leads to significant structural changes in the materials, including the formation and breakage of covalent bonds between Sb and Q. Despite the insolubility of the starting crystals in N2H4·H2O under the given conditions, the mechanism of this transformation can be rationalized by applying the HSAB principle. By adjusting factors such as the reactants' acid/base properties, temperature, and pressure, the process can be controlled, allowing for the achievement of a wide range of optical band gaps (ranging from 1.14 to 1.59 eV) while maintaining the solid solution nature of the anion sublattice in the Sb2Q3 nanofibers.

A Hemispherical Image Sensor Array Fabricated with Organic Photomemory Transistors.

Hemispherical image sensors simplify lens designs, reduce optical aberrations, and improve image resolution for compact wide-field-of-view cameras. To achieve hemispherical image sensors, organic materials are promising candidates due to the following advantages: tunability of optoelectronic/spectral response and low-temperature low-cost processes. Here, a photolithographic process is developed to prepare a hemispherical image sensor array using organic thin film photomemory transistors with a density of 308 pixels per square centimeter. This design includes only one photomemory transistor as a single active pixel, in contrast to the conventional pixel architecture, consisting of select/readout/reset transistors and a photodiode. The organic photomemory transistor, comprising light-sensitive organic semiconductor and charge-trapping dielectric, is able to achieve a linear photoresponse (light intensity range, from 1 to 50 W m-2 ), along with a responsivity as high as 1.6 A W-1 (wavelength = 465 nm) for a dark current of 0.24 A m-2 (drain voltage = -1.5 V). These observed values represent the best responsivity for similar dark currents among all the reported hemispherical image sensor arrays to date. A transfer method was further developed that does not damage organic materials for hemispherical organic photomemory transistor arrays. These developed techniques are scalable and are amenable for other high-resolution 3D organic semiconductor devices.

Multiscale structural control of thiostannate chalcogels with two-dimensional crystalline constituents.

Chalcogenide aerogels (chalcogels) are amorphous structures widely known for their lack of localized structural control. This study, however, demonstrates a precise multiscale structural control through a thiostannate motif ([Sn2S6]4-)-transformation-induced self-assembly, yielding Na-Mn-Sn-S, Na-Mg-Sn-S, and Na-Sn(II)-Sn(IV)-S aerogels. The aerogels exhibited [Sn2S6]4-:Mn2+ stoichiometric-variation-induced-control of average specific surface areas (95-226 m2 g-1), thiostannate coordination networks (octahedral to tetrahedral), phase crystallinity (crystalline to amorphous), and hierarchical porous structures (micropore-intensive to mixed-pore state). In addition, these chalcogels successfully adopted the structural motifs and ion-exchange principles of two-dimensional layered metal sulfides (K2xMnxSn3-xS6, KMS-1), featuring a layer-by-layer stacking structure and effective radionuclide (Cs+, Sr2+)-control functionality. The thiostannate cluster-based gelation principle can be extended to afford Na-Mg-Sn-S and Na-Sn(II)-Sn(IV)-S chalcogels with the same structural features as the Na-Mn-Sn-S chalcogels (NMSCs). The study of NMSCs and their chalcogel family proves that the self-assembly principle of two-dimensional chalcogenide clusters can be used to design unique chalcogels with unprecedented structural hierarchy.

Photo-Regeneration of Zeolite-Based Volatile Organic Compound Filters Enabled by TiO2 Photocatalyst.

Indoor air filtration received significant attention owing to the growing threat to the environment and human health caused by air pollutants such as volatile organic compound (VOC) gases. However, owing to the limited adsorption capacity of VOC adsorbents, such as activated carbon, a rapid breakthrough can occur, reducing the service life of the filter. Therefore, TiO2-coated zeolite (TiO2/zeolite) was utilized as a photo-regenerative VOC adsorbent to increase the service life of VOC filters. In particular, with photoactive TiO2 forms on zeolite, efficient and repetitive photo-regeneration is attainable through the dissociation of VOC molecules by the photocatalytic reaction. We optimized the TiO2 coating amount to obtain TiO2/zeolite particles with a high surface area (BET surface area > 500 m2/g) and high adsorption capacity. A VOC filter with an adsorption efficiency of 72.1% for formaldehyde was realized using TiO2/zeolite as the adsorbent. Furthermore, the TiO2/zeolite filter exhibits a photo-regeneration efficiency of >90% for the initial two regeneration cycles using ultraviolet illumination, and >60% up to five cycles. Based on these observations, we consider that TiO2/zeolite is a potential adsorbent candidate for photo-regenerative VOC filters.

A GO/CoMo3S13 chalcogel heterostructure with rich catalytic Mo-S-Co bridge sites for the hydrogen evolution reaction.

Molybdenum disulfide (MoS2)-based materials are extensively studied as promising hydrogen evolution reaction (HER) catalysts. In order to bring out the full potential of chalcogenide chemistry, precise control over the active sulfur sites and enhancement of electronic conductivity need to be achieved. This study develops a highly active HER catalyst with an optimized active site-controlled cobalt molybdenum sulfide (CoMo3S13) chalcogel/graphene oxide aerogel heterostructure. The highly active CoMo3S13 chalcogel catalyst was achieved by the synergetic catalytic sites of [Mo3S13]2- and the Mo-S-Co bridge. The optimized GO/CoMo3S13 chalcogel heterostructure catalyst exhibited high catalytic HER performance with an overvoltage of 130 mV, a current density of 10 mA cm-2, a small Tafel slope of 40.1 mV dec-1, and remarkable stability after 12 h of testing. This study presents a successful example of a synergistic heterostructure exploiting both the appealing electrical functionality of GO and catalytically active [Mo3S13]2- sites.

Ferroelectric PVDF nanofiber membrane for high-efficiency PM0.3 air filtration with low air flow resistance.

The significant public health concerns related to particulate matter (PM) air pollutants and the airborne transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) have led to considerable interest in high-performance air filtration membranes. Highly ferroelectric polyvinylidene fluoride (PVDF) nanofiber (NF) filter membranes are successfully fabricated via electrospinning for high-performance low-cost air filtration. Spectroscopic and ferro-/piezoelectric analyses of PVDF NF show that a thinner PVDF NF typically forms a ferroelectric ß phase with a confinement effect. A 70-nm PVDF NF membrane exhibits the highest fraction of ß phase (87%) and the largest polarization behavior from piezoresponse force microscopy. An ultrathin 70-nm PVDF NF membrane exhibits a high PM0.3 filtration efficiency of 97.40% with a low pressure drop of 51 Pa at an air flow of 5.3 cm/s owing to the synergetic combination of the slip effect and ferroelectric dipole interaction. Additionally, the 70-nm PVDF NF membrane shows excellent thermal and chemical stabilities with negligible filtration performance degradation (air filtration efficiency of 95.99% and 87.90% and pressure drop of 55 and 65 Pa, respectively) after 24 h of heating at 120 °C and 1 h immersion in isopropanol.

Retina-Inspired Color-Cognitive Learning via Chromatically Controllable Mixed Quantum Dot Synaptic Transistor Arrays.

Artificial photonic synapses are emerging as a promising implementation to emulate the human visual cognitive system by consolidating a series of processes for sensing and memorizing visual information into one system. In particular, mimicking retinal functions such as multispectral color perception and controllable nonvolatility is important for realizing artificial visual systems. However, many studies to date have focused on monochromatic-light-based photonic synapses, and thus, the emulation of color discrimination capability remains an important challenge for visual intelligence. Here, an artificial multispectral color recognition system by employing heterojunction photosynaptic transistors consisting of ratio-controllable mixed quantum dot (M-QD) photoabsorbers and metal-oxide semiconducting channels is proposed. The biological photoreceptor inspires M-QD photoabsorbers with a precisely designed red (R), green (G), and blue (B)-QD ratio, enabling full-range visible color recognition with high photo-to-electric conversion efficiency. In addition, adjustable synaptic plasticity by modulating gate bias allows multiple nonvolatile-to-volatile memory conversion, leading to chromatic control in the artificial photonic synapse. To ensure the viability of the developed proof of concept, a 7 × 7 pixelated photonic synapse array capable of performing outstanding color image recognition based on adjustable wavelength-dependent volatility conversion is demonstrated.

Vertically Stacked Full Color Quantum Dots Phototransistor Arrays for High-Resolution and Enhanced Color-Selective Imaging.

Color-selective multifunctional and multiplexed photodetectors have attracted considerable interest with the increasing demand for color filter-free optoelectronics which can simultaneously process multispectral signal via minimized system complexity. The low efficiency of color-filter technology and conventional laterally pixelated photodetector array structures often limit opportunities for widespread realization of high-density photodetectors. Here, low-temperature solution-processed vertically stacked full color quantum dot (QD) phototransistor arrays are developed on plastic substrates for high-resolution color-selective photosensor applications. Particularly, the three different-sized/color (RGB) QDs are vertically stacked and pixelated via direct photopatterning using a unique chelating chalcometallate ligand functioning both as solubilizing component and, after photoexposure, a semiconducting cement creating robust, insoluble, and charge-efficient QD layers localized in the a-IGZO transistor region, resulting in efficient wavelength-dependent photo-induced charge transfer. Thus, high-resolution vertically stacked full color QD photodetector arrays are successfully implemented with the density of 5500 devices cm-2 on ultrathin flexible polymeric substrates with highly photosensitive characteristics such as photoresponsivity (1.1 × 104 AW-1 ) and photodetectivity (1.1 × 1018 Jones) as well as wide dynamic ranges (>150 dB).

Highly Reliable Implementation of Optimized Multicomponent Oxide Systems Enabled by Machine Learning-Based Synthetic Protocol.

Multicomponent oxide systems are one of the essential building blocks in a broad range of electronic devices. However, due to the complex physical correlation between the cation components and their relations with the system, finding an optimal combination for desired physical and/or chemical properties requires an exhaustive experimental procedure. Here, a machine learning (ML)-based synthetic approach is proposed to explore the optimal combination conditions in a ternary cationic compound indium-zinc-tin oxide (IZTO) semiconductor exhibiting high carrier mobility. In particular, by using support vector regression algorithm with radial basis function kernel, highly accurate mobility prediction can be achieved for multicomponent IZTO semiconductor with a sufficiently small number of train datasets (15-20 data points). With a synergetic combination of solution-based synthetic route for IZTO fabrication enabling a facile control of the composition ratio and tailored ML process for multicomponent system, the prediction of high-performance IZTO thin-film transistors is possible with expected field-effect mobility as high as 13.06 cm2 V-1 s-1 at the In:Zn:Sn ratio of 63:27:10. The ML prediction is successfully translated into the empirical analysis with high accuracy, validating the protocol is reliable and a promising approach to accelerate the optimization process for multicomponent oxide systems.

Engineering Copper Iodide (CuI) for Multifunctional p-Type Transparent Semiconductors and Conductors.

Developing transparent p-type semiconductors and conductors has attracted significant interest in both academia and industry because metal oxides only show efficient n-type characteristics at room temperature. Among the different candidates, copper iodide (CuI) is one of the most promising p-type materials because of its widely adjustable conductivity from transparent electrodes to semiconducting layers in transistors. CuI can form thin films with high transparency in the visible light region using various low-temperature deposition techniques. This progress report aims to provide a basic understanding of CuI-based materials and recent progress in the development of various devices. The first section provides a brief introduction to CuI with respect to electronic structure, defect states, charge transport physics, and overviews the CuI film deposition methods. The material design concepts through doping/alloying approaches to adjust the optoelectrical properties are also discussed in the first section. The following section presents recent advances in state-of-the-art CuI-based devices, including transparent electrodes, thermoelectric devices, p-n diodes, p-channel transistors, light emitting diodes, and solar cells. In conclusion, current challenges and perspective opportunities are highlighted.

A highly efficient nanofibrous air filter membrane fabricated using electrospun amphiphilic PVDF-g-POEM double comb copolymer.

Current global emergencies, such as the COVID-19 pandemic and particulate matter (PM) pollution, require urgent protective measures. Nanofibrous air filter membranes that can capture PM0.3 and simultaneously help in preventing the spread of COVID-19 are essential. Therefore, a highly efficient nanofibrous air filter membrane based on amphiphilic poly(vinylidene fluoride)-graft-poly(oxyethylene methacrylate) (PVDF-g-POEM) double comb copolymer was fabricated using atomic transfer radical polymerization (ATRP) and electrospinning. Fourier transform infrared spectroscopy, X-ray diffraction, proton nuclear magnetic resonance, transmission electron microscopy, differential scanning calorimetry, and thermogravimetric analysis were employed to successfully characterize the molecular structure of the fabricated amphiphilic PVDF-g-POEM double comb copolymer. The nanofibrous air filter membrane based on amphiphilic PVDF-g-POEM double comb copolymer achieved a low air resistance of 4.69 mm H2O and a high filtration efficiency of 93.56 % due to enhanced chemical and physical adsorption properties.

Highly-Sensitive Textile Pressure Sensors Enabled by Suspended-Type All Carbon Nanotube Fiber Transistor Architecture.

Among various wearable health-monitoring electronics, electronic textiles (e-textiles) have been considered as an appropriate alternative for a convenient self-diagnosis approach. However, for the realization of the wearable e-textiles capable of detecting subtle human physiological signals, the low-sensing performances still remain as a challenge. In this study, a fiber transistor-type ultra-sensitive pressure sensor (FTPS) with a new architecture that is thread-like suspended dry-spun carbon nanotube (CNT) fiber source (S)/drain (D) electrodes is proposed as the first proof of concept for the detection of very low-pressure stimuli. As a result, the pressure sensor shows an ultra-high sensitivity of ~3050 Pa-1 and a response/recovery time of 258/114 ms in the very low-pressure range of <300 Pa as the fiber transistor was operated in the linear region (VDS = -0.1 V). Also, it was observed that the pressure-sensing characteristics are highly dependent on the contact pressure between the top CNT fiber S/D electrodes and the single-walled carbon nanotubes (SWCNTs) channel layer due to the air-gap made by the suspended S/D electrode fibers on the channel layers of fiber transistors. Furthermore, due to their remarkable sensitivity in the low-pressure range, an acoustic wave that has a very tiny pressure could be detected using the FTPS.

High-performance p-channel transistors with transparent Zn doped-CuI.

'Ideal' transparent p-type semiconductors are required for the integration of high-performance thin-film transistors (TFTs) and circuits. Although CuI has recently attracted attention owing to its excellent opto-electrical properties, solution processability, and low-temperature synthesis, the uncontrolled copper vacancy generation and subsequent excessive hole doping hinder its use as a semiconductor material in TFT devices. In this study, we propose a doping approach through soft chemical solution process and transparent p-type Zn-doped CuI semiconductor for high-performance TFTs and circuits. The optimised TFTs annealed at 80 °C exhibit a high hole mobility of over 5 cm2 V-1 s-1 and high on/off current ratio of ~107 with good operational stability and reproducibility. The CuI:Zn semiconductors show intrinsic advantages for next-generation TFT applications and wider applications in optoelectronics and energy conversion/storage devices. This study paves the way for the realisation of transparent, flexible, and large-area integrated circuits combined with n-type metal-oxide semiconductor.

Highly Efficient Photo-Induced Charge Separation Enabled by Metal-Chalcogenide Interfaces in Quantum-Dot/Metal-Oxide Hybrid Phototransistors.

Quantum dot (QD)-based optoelectronics have received great interest for versatile applications because of their excellent photosensitivity, facile solution processability, and the wide range of band gap tunability. In addition, QD-based hybrid devices, which are combined with various high-mobility semiconductors, have been actively researched to enhance the optoelectronic characteristics and maximize the zero-dimensional structural advantages, such as tunable band gap and high light absorption. However, the difficulty of highly efficient charge transfer between QDs and the semiconductors and the lack of systematic analysis for the interfaces have impeded the fidelity of this platform, resulting in complex device architectures and unsatisfactory device performance. Here, we report ultrahigh detective phototransistors with highly efficient photo-induced charge separation using a Sn2S64--capped CdSe QD/amorphous oxide semiconductor (AOS) hybrid structure. The photo-induced electron transfer characteristics at the interface of the two materials were comprehensively investigated with an array of electrochemical and spectroscopic analyses. In particular, photocurrent imaging microscopy revealed that interface engineering in QD/AOS with chelating chalcometallate ligands causes efficient charge transfer, resulting in photovoltaic-dominated responses over the whole channel area. On the other hand, monodentate ligand-incorporated QD/AOS-based devices typically exhibit limited charge transfer with atomic vibration, showing photo-thermoelectric-dominated responses in the drain electrode area.