Silicone engineered anisotropic lithography for ultrahigh-density OLEDs.

Ultrahigh-resolution patterning with high-throughput and high-fidelity is highly in demand for expanding the potential of organic light-emitting diodes (OLEDs) from mobile and TV displays into near-to-eye microdisplays. However, current patterning techniques so far suffer from low resolution, consecutive pattern for RGB pixelation, low pattern fidelity, and throughput issue. Here, we present a silicone engineered anisotropic lithography of the organic light-emitting semiconductor (OLES) that in-situ forms a non-volatile etch-blocking layer during reactive ion etching. This unique feature not only slows the etch rate but also enhances the anisotropy of etch direction, leading to gain delicate control in forming ultrahigh-density multicolor OLES patterns (up to 4500 pixels per inch) through photolithography. This patterning strategy inspired by silicon etching chemistry is expected to provide new insights into ultrahigh-density OLED microdisplays.

Circularly polarized light-sensitive, hot electron transistor with chiral plasmonic nanoparticles.

The quantitative detection of circularly polarized light (CPL) is necessary in next-generation optical communication carrying high-density information and in phase-controlled displays exhibiting volumetric imaging. In the current technology, multiple pixels of different wavelengths and polarizers are required, inevitably resulting in high loss and low detection efficiency. Here, we demonstrate a highly efficient CPL-detecting transistor composed of chiral plasmonic nanoparticles with a high Khun's dissymmetry (g-factor) of 0.2 and a high mobility conducting oxide of InGaZnO. The device successfully distinguished the circular polarization state and displayed an unprecedented photoresponsivity of over 1 A/W under visible CPL excitation. This observation is mainly attributed to the hot electron generation in chiral plasmonic nanoparticles and to the effective collection of hot electrons in the oxide semiconducting transistor. Such characteristics further contribute to opto-neuromorphic operation and the artificial nervous system based on the device successfully performs image classification work. We anticipate that our strategy will aid in the rational design and fabrication of a high-performance CPL detector and opto-neuromorphic operation with a chiral plasmonic structure depending on the wavelength and circular polarization state.

Vacuum-free solution-based metallization (VSM) of a-IGZO using trimethylaluminium solution.

This research demonstrates a method to reduce the resistance of amorphous indium-gallium-zinc-oxide (a-IGZO) using a "vacuum-free solution-based metallization" (VSM) process, which revolutionizes the metallization process thanks to its simplicity, by simply dipping the a-IGZO into trimethyl aluminium (TMA, (CH3)3Al) solution. From the XPS results, it was found that oxygen vacancies were generated after the VSM process, resulting in the enhanced conductivity. Various metallization time and solution temperature conditions were investigated, and the measured conductivity of the a-IGZO could be enhanced up to 20.32 S cm-1, which is over 105 times larger compared to that of the untreated a-IGZO. By utilizing the VSM process, self-aligned top-gate (SATG) a-IGZO thin-film-transistors (TFTs) were successfully fabricated, and to provide an explanation for the mechanism, X-ray photoelectron spectroscopy (XPS) was employed.

Synaptic transistors based on a tyrosine-rich peptide for neuromorphic computing.

In this article, we propose an artificial synaptic device based on a proton-conducting peptide material. By using the redox-active property of tyrosine, the Tyr-Tyr-Ala-Cys-Ala-Tyr-Tyr peptide film was utilized as a gate insulator that shows synaptic plasticity owing to the formation of proton electric double layers. The ion gating effects on the transfer characteristics and temporal current responses are shown. Further, timing-dependent responses, including paired-pulse facilitation, synaptic potentiation, and transition from short-term plasticity to long-term plasticity, have been demonstrated for the electrical emulation of biological synapses in the human brain. Herein, we provide a novel material platform that is bio-inspired and biocompatible for use in brain-mimetic electronic devices.

Proton-enabled activation of peptide materials for biological bimodal memory.

The process of memory and learning in biological systems is multimodal, as several kinds of input signals cooperatively determine the weight of information transfer and storage. This study describes a peptide-based platform of materials and devices that can control the coupled conduction of protons and electrons and thus create distinct regions of synapse-like performance depending on the proton activity. We utilized tyrosine-rich peptide-based films and generalized our principles by demonstrating both memristor and synaptic devices. Interestingly, even memristive behavior can be controlled by both voltage and humidity inputs, learning and forgetting process in the device can be initiated and terminated by protons alone in peptide films. We believe that this work can help to understand the mechanism of biological memory and lay a foundation to realize a brain-like device based on ions and electrons.

Quantitative analysis of the coupling between proton and electron transport in peptide/manganese oxide hybrid films.

Understanding how electrons and protons move in a coupled manner and affect one another is important to the design of proton-electron conductors and achieving biological transport in synthetic materials. In this study, a new methodology is proposed that allows for the quantification of the degree of coupling between electrons and protons in tyrosine-rich peptides and metal oxide hybrid films at room temperature under a voltage bias. This approach is developed according to the Onsager principle, which has been thoroughly established for the investigation of mixed ion-electron conductors with electron and oxide ion vacancies as carriers at high temperatures. Herein, a new device platform using electron-blocking electrodes provides a new strategy to investigate the coupling of protons and electrons in bulk materials beyond the molecular level investigation of coupled proton and electron transfer. Two Onsager transport parameters, αi* and σe', are obtained from the device, and the results of these transport parameters demonstrate that the coupled transport of electrons and protons inside the hybrid film plays an important role in the macroscopic-scale conduction. The results suggest that an average of one electron is dragged by one proton in the absence of a direct driving force for electron movement ∇ηe.

Hexagonal Boron Nitride for Surface Passivation of Two-Dimensional van der Waals Heterojunction Solar Cells.

Two-dimensional (2D) semiconductors can be promising active materials for solar cells due to their advantageous electrical and optical properties, in addition to their ability to form high-quality van der Waals (vdW) heterojunctions using a simple process. Furthermore, the atomically thin nature of these 2D materials allows them to form lightweight and transparent thin-film solar cells. However, strategies appropriate for optimizing their properties have not been extensively studied yet. In this paper, we propose a method for reducing the electrical loss of 2D vdW solar cells by introducing hexagonal boron nitride (h-BN) as a surface passivation layer. This method allowed us to enhance the photovoltaic performance of a MoS2/WSe2 solar cell. In particular, we observed ∼74% improvement of the power conversion efficiency owing to a large increase in both short-circuit current and open-circuit voltage. Such a remarkable performance enhancement was due to the reduction of the recombination rate at the junction and surface of nonoverlapped semiconductor regions, which was confirmed via a time-resolved photoluminescence analysis. Furthermore, the h-BN top layer was found to improve the long-term stability of the tested 2D solar cell under ambient conditions. We observed the evolution of our MoS2/WSe2 solar cell for a month and found that h-BN passivation effectively suppressed its degradation speed. In particular, the degradation speed of the passivated cell was twice as low as that of a nonpassivated cell. This work reveals that h-BN can successfully suppress the electrical loss and degradation of 2D vdW heterojunction solar cells under ambient conditions.

Thermo and flex multi-functional array ionic sensor for a human adaptive device.

Recently, electronic skin that mimics human skin in measuring tactile stimuli, temperature, and humidity and having a self-healing function was developed. Furthermore, with the advances in the field of artificial intelligence and health monitoring, various materials and methods have been studied for e-skin. The limitations to work on actual human skin include device flexibility and large-area applications through array structures, and many studies are underway to overcome these problems. Polymeric materials containing ionic liquids can be used to easily fabricate devices in the solid state. They are highly sensitive to both pressure and temperature, making them suitable for multi-sensing devices. Resistive and capacitive sensors have the advantage of having a simple structure, which makes them easy to fabricate. In a single device, both types work well. For resistive sensors, the temperature sensitivity (1.1/°C) is relatively high. Conversely, capacitive sensors have a low temperature sensitivity (0.3/°C). However, they have the advantage of being uniformly variable under each condition and having a smaller error range. In the array structure, independent flex and thermo sensors are arranged repeatedly. The resistive type shows changes in temperature and bending, but in the capacitive type, it is difficult to obtain results from the pixels due to parasitic capacitance.

Optical properties of the crumpled pattern of selectively layered MoS2.

Crumple-structured two-dimensional MoS2 was evaluated as an essential element for future optoelectronic and stretchable devices owing to its interesting optical properties. This Letter reports the characteristics of the crumpled structure of MoS2 directly layered on a MoS2 sheet by chemical vapor deposition. The crumpling structure is presented as a method for selectively layering MoS2 with crumpled layered patterning and tunable optical properties as a crumpled structure on a single substrate. Optical analysis by the fast Fourier transform revealed the distribution characteristics of the crumple structure, and a Raman, photoluminescence, and optical absorption analysis confirmed the change in peak shift and intensity according to the degree of the crumpled structure. This material has potential future optoelectronic applications.

Two-Dimensional WSe2/MoS2 p-n Heterojunction-Based Transparent Photovoltaic Cell and Its Performance Enhancement by Fluoropolymer Passivation.

As a means to overcome the limitation of installation space and to promote the utilization of the solar cell in various applications, a transparent thin-film solar cell has been studied by many researchers. To achieve a transparent solar cell, the choice of materials which are transparent enough and showing the photovoltaic property at the same time is the key. Here, we suggest a two-dimensional (2D) p-n heterojunction of WSe2/MoS2 and an indium tin oxide electrode to fabricate a transparent thin-film photovoltaic cell. Because of advantages that 2D materials possess, a highly transparent (∼80%) solar cell with considerable efficiency was achieved. Furthermore, by introducing a transparent passivation layer composed of a fluoropolymer, the photovoltaic performance was much improved. With the passivation layer, our WSe2/MoS2 transparent photovoltaic cell reached an efficiency of ∼10%. A comparison of photovoltaic parameters before and after applying passivation and analysis on the origin of such differences are also discussed. To the best of our knowledge, this is the first report to fabricate a 2D material-based fully transparent photovoltaic device. Our result exhibits a great potential of the van der Waals p-n heterojunction of 2D semiconductors to be utilized for an active layer of a highly transparent and lightweight thin-film solar cell.

Physically Transient Field-Effect Transistors Based on Black Phosphorus.

Black phosphorus (BP) has shown great potential as a semiconductor material beyond graphene and MoS2 because of its intrinsic band gap and high mobility. Moreover, the biocompatibility of the final biodegradation products of BP has led to extensive research on biomedical applications. Herein, physically transient field-effect transistors (FETs) based on black phosphorus have been demonstrated using peptide insulator as a gate dielectric layer. The fabricated devices show high hole mobility up to 468 cm2 V-1 s-1 and on-off current ratio over 103. The combined use of black phosphorus, peptide, and molybdenum provides rapid disappearance of the devices within 36 h. Dissolution kinetics and cytotoxicity of black phosphorus are assessed to clarify its availability to be applied in transient electronics. This work provides transient FETs with high degradability and high performance based on biocompatible black phosphorus.

Ultrasensitive, Low-Power Oxide Transistor-Based Mechanotransducer with Microstructured, Deformable Ionic Dielectrics.

The development of a highly sensitive artificial mechanotransducer that mimics the tactile sensing features of human skin has been a big challenge in electronic skin research. Here, we demonstrate an ultrasensitive, low-power oxide transistor-based mechanotransducer modulated by microstructured, deformable ionic dielectrics, which is consistently sensitive to a wide range of pressures from 1 to 50 kPa. To this end, we designed a viscoporoelastic and ionic thermoplastic polyurethane (i-TPU) with micropyramidal feature as a pressure-sensitive gate dielectric for the indium-gallium-zinc-oxide (IGZO) transistor-based mechanotransducer, which leads to an unprecedented sensitivity of 43.6 kPa-1, which is 23 times higher than that of a capacitive mechanotransducer. This is because the pressure-induced ion accumulation at the interface of the i-TPU dielectric and IGZO semiconductor effectively modulates the conducting channel, which contributed to the enhanced current level under pressure. We believe that the ionic transistor-type mechanotransducer suggested by us will be an effective way to perceive external tactile stimuli over a wide pressure range even under low power (<4 V), which might be one of the candidates to directly emulate the tactile sensing capability of human skin.

Effects of proton conduction on dielectric properties of peptides.

Peptides have been overlooked for their use in the field of electronics, even though they are one of the most commonly found bio-induced materials, and are not only easy to mass-produce but also exhibit a high dielectric constant. Additionally, unlike proteins, which are gaining considerable interest with materials researchers, peptides are much simpler, rendering their original characteristics easier to maintain without significant alteration of their structure. On the other hand, proteins tend to deform due to their susceptibility to environmental changes. Combining such superb dielectric properties with their relatively stable nature, peptides could be utilized as a component of electronic devices ranging from basic capacitors to more complex thin-film transistors. In this paper, a peptide chain (YYACAYY) composed of tyrosine, alanine, and cysteine was extensively studied using an impedance analyzer to determine its innate charge movement mechanism in order to extend our understanding of the electric properties of peptides. The movement of mobile protons inside the peptide insulator was found to be the source of the high relative permittivity of the peptide insulator, and the dielectric constant of the peptide insulator was found to be over 17 in humid conditions. By widening the understanding of the dielectric properties of the peptide insulator, it is expected that the peptide can be further utilized as an insulator in various electronic devices.

Grains in Selectively Grown MoS2 Thin Films.

Transition metal dichalcogenides (TMDCs) have recently been studied using various synthesis methods, such as chemical vapor deposition for large-scale production. Despite the realization of large-scale production with high material quality, a range of approaches have been made to solve the patterning issue of TMDCs focusing on the application of integrated devices; however, patterning is still under study to accurately represent nanoscale-sized patterns, as well as the desired positions and shapes. Here, an insulating substrate is treated selectively with O2 plasma, and MoS2 growth is induced in the superhydrophilic area. Selectively well-grown MoS2 patterns are confirmed by atomic force microscopy and Raman and photoluminescence spectroscopy. In addition, the grain size, according to the growth size, and grain boundary are analyzed by annual dark field transmission electron microscopy (TEM) and spherical aberration-corrected scanning TEM to confirm the selective growth. An analysis of the device performance and the optical properties reveals an enhancement with increasing grain size. This method presents the path of the growth technique for patterning, as well as the direction that can be applied to devices and integrated circuits.

Enhancement of the Device Performance and the Stability with a Homojunction-structured Tungsten Indium Zinc Oxide Thin Film Transistor.

Tungsten-indium-zinc-oxide thin-film transistors (WIZO-TFTs) were fabricated using a radio frequency (RF) co-sputtering system with two types of source/drain (S/D)-electrode material of conducting WIZO (homojunction structure) and the indium-tin oxide (ITO) (heterojunction structure) on the same WIZO active-channel layer. The electrical properties of the WIZO layers used in the S/D electrode and the active-channel layer were adjusted through oxygen partial pressure during the deposition process. To explain enhancements of the device performance and stability of the homojunction-structured WIZO-TFT, a systematic investigation of correlation between device performance and physical properties at the interface between the active layer and the S/D electrodes such as the contact resistance, surface/interfacial roughness, interfacial-trap density, and interfacial energy-level alignments was conducted. The homojunction-structured WIZO-TFT exhibited a lower contact resistance, smaller interfacial-trap density, and flatter interfacial roughness than the WIZO-TFT with the heterojunction structure. The 0.09 eV electron barrier of the homojunction-structured WIZO-TFT is lower than the 0.21 eV value that was obtained for the heterojunction-structured WIZO-TFT. This reduced electron barrier may be attributed to enhancements of device performance and stability, that are related to the carrier transport.

Honeycomb-Like Interconnected Network of Nickel Phosphide Heteronanoparticles with Superior Electrochemical Performance for Supercapacitors.

Transition-metal-based heteronanoparticles are attracting extensive attention in electrode material design for supercapacitors owing to their large surface-to-volume ratios and inherent synergies of individual components; however, they still suffer from limited interior capacity and cycling stability due to simple geometric configurations, low electrochemical activity of the surface, and poor structural integrity. Developing an elaborate architecture that endows a larger surface area, high conductivity, and mechanically robust structure is a pressing need to tackle the existing challenges of electrode materials. This work presents a supercapacitor electrode consisting of honeycomb-like biphasic Ni5P4-Ni2P (NixPy) nanosheets, which are interleaved by large quantities of nanoparticles. The optimized NixPy delivers an ultrahigh specific capacity of 1272 C g-1 at a current density of 2 A g-1, high rate capability, and stability. An asymmetric supercapacitor employing as-synthesized NixPy as the positive electrode and activated carbon as the negative electrode exhibits significantly high power and energy densities (67.2 W h kg-1 at 0.75 kW kg-1; 20.4 W h kg-1 at 15 kW kg-1). These results demonstrate that the novel nanostructured NixPy can be potentially applied in high-performance supercapacitors.

Effects of mechanical deformation on energy conversion efficiency of piezoelectric nanogenerators.

Piezoelectric nanogenerators (PNGs) are capable of converting energy from various mechanical sources into electric energy and have many attractive features such as continuous operation, replenishment and low cost. However, many researchers still have studied novel material synthesis and interfacial controls to improve the power production from PNGs. In this study, we report the energy conversion efficiency (ECE) of PNGs dependent on mechanical deformations such as bending and twisting. Since the output power of PNGs is caused by the mechanical strain of the piezoelectric material, the power production and their ECE is critically dependent on the types of external mechanical deformations. Thus, we examine the output power from PNGs according to bending and twisting. In order to clearly understand the ECE of PNGs in the presence of those external mechanical deformations, we determine the ECE of PNGs by the ratio of output electrical energy and input mechanical energy, where we suggest that the input energy is based only on the strain energy of the piezoelectric layer. We calculate the strain energy of the piezoelectric layer using numerical simulation of bending and twisting of the PNG. Finally, we demonstrate that the ECE of the PNG caused by twisting is much higher than that caused by bending due to the multiple effects of normal and lateral piezoelectric coefficients. Our results thus provide a design direction for PNG systems as high-performance power generators.

Influence of post-annealing on the off current of MoS2 field-effect transistors.

Two-dimensional materials have recently been spotlighted, due to their unique properties in comparison with conventional bulk and thin-film materials. Among those materials, MoS2 is one of the promising candidates for the active layer of electronic devices because it shows high electron mobility and pristine band gap. In this paper, we focus on the evolution of the electrical property of the MoS2 field-effect transistor (FET) as a function of post-annealing temperature. The results indicate that the off current drastically decreased at 200°C and increased at 400°C while other factors, such as the mobility and threshold voltage, show little variation. We consider that the decreasing off current comes from the rearrangement of the MoS2 film and the elimination of the surface residue. Then, the increasing off current was caused by the change of the material's composition and adsorption of H2O and O2.

A high-performance complementary inverter based on transition metal dichalcogenide field-effect transistors.

For several years, graphene has been the focus of much attention due to its peculiar characteristics, and it is now considered to be a representative 2-dimensional (2D) material. Even though many research groups have studied on the graphene, its intrinsic nature of a zero band-gap, limits its use in practical applications, particularly in logic circuits. Recently, transition metal dichalcogenides (TMDs), which are another type of 2D material, have drawn attention due to the advantage of having a sizable band-gap and a high mobility. Here, we report on the design of a complementary inverter, one of the most basic logic elements, which is based on a MoS2 n-type transistor and a WSe2 p-type transistor. The advantages provided by the complementary metal-oxide-semiconductor (CMOS) configuration and the high-performance TMD channels allow us to fabricate a TMD complementary inverter that has a high-gain of 13.7. This work demonstrates the operation of the MoS2 n-FET and WSe2 p-FET on the same substrate, and the electrical performance of the CMOS inverter, which is based on a different driving current, is also measured.

Bendability optimization of flexible optical nanoelectronics via neutral axis engineering.

The enhancement of bendability of flexible nanoelectronics is critically important to realize future portable and wearable nanoelectronics for personal and military purposes. Because there is an enormous variety of materials and structures that are used for flexible nanoelectronic devices, a governing design rule for optimizing the bendability of these nanodevices is required. In this article, we suggest a design rule to optimize the bendability of flexible nanoelectronics through neutral axis (NA) engineering. In flexible optical nanoelectronics, transparent electrodes such as indium tin oxide (ITO) are usually the most fragile under an external load because of their brittleness. Therefore, we representatively focus on the bendability of ITO which has been widely used as transparent electrodes, and the NA is controlled by employing a buffer layer on the ITO layer. First, we independently investigate the effect of the thickness and elastic modulus of a buffer layer on the bendability of an ITO film. Then, we develop a design rule for the bendability optimization of flexible optical nanoelectronics. Because NA is determined by considering both the thickness and elastic modulus of a buffer layer, the design rule is conceived to be applicable regardless of the material and thickness that are used for the buffer layer. Finally, our design rule is applied to optimize the bendability of an organic solar cell, which allows the bending radius to reach about 1 mm. Our design rule is thus expected to provide a great strategy to enhance the bending performance of a variety of flexible nanoelectronics.