Integrated Logic Circuits Based on Wafer-Scale 2D-MoS2 FETs Using Buried-Gate Structures.

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) materials, such as molybdenum disulfide (MoS2), stand out due to their atomically thin layered structure and exceptional electrical properties. Consequently, they could potentially become one of the main materials for future integrated high-performance logic circuits. However, the local back-gate-based MoS2 transistors on a silicon substrate can lead to the degradation of electrical characteristics. This degradation is caused by the abnormal effect of gate sidewalls, leading to non-uniform field controllability. Therefore, the buried-gate-based MoS2 transistors where the gate electrodes are embedded into the silicon substrate are fabricated. The several device parameters such as field-effect mobility, on/off current ratio, and breakdown voltage of gate dielectric are dramatically enhanced by field-effect mobility (from 0.166 to 1.08 cm2/V·s), on/off current ratio (from 4.90 × 105 to 1.52 × 107), and breakdown voltage (from 15.73 to 27.48 V) compared with a local back-gate-based MoS2 transistor, respectively. Integrated logic circuits, including inverters, NAND, NOR, AND, and OR gates, were successfully fabricated by 2-inch wafer-scale through the integration of a buried-gate MoS2 transistor array.

Monolithic Integration of Semi-Transparent and Flexible Integrated Image Sensor Array with a-IGZO Thin-Film Transistors (TFTs) and p-i-n Hydrogenated Amorphous Silicon Photodiodes.

A novel approach to fabricating a transparent and flexible one-transistor-one-diode (1T-1D) image sensor array on a flexible colorless polyimide (CPI) film substrate is successfully demonstrated with laser lift-off (LLO) techniques. Leveraging transparent indium tin oxide (ITO) electrodes and amorphous indium gallium zinc oxide (a-IGZO) channel-based thin-film transistor (TFT) backplanes, vertically stacked p-i-n hydrogenated amorphous silicon (a-Si:H) photodiodes (PDs) utilizing a low-temperature (<90 °C) deposition process are integrated with a densely packed 14 × 14 pixel array. The low-temperature-processed a-Si:H photodiodes show reasonable performance with responsivity and detectivity for 31.43 mA/W and 3.0 × 1010 Jones (biased at -1 V) at a wavelength of 470 nm, respectively. The good mechanical durability and robustness of the flexible image sensor arrays enable them to be attached to a curved surface with bending radii of 20, 15, 10, and 5 mm and 1000 bending cycles, respectively. These studies show the significant promise of utilizing highly flexible and rollable active-matrix technology for the purpose of dynamically sensing optical signals in spatial applications.

A Bioinspired Ultra Flexible Artificial van der Waals 2D-MoS2 Channel/LiSiOx Solid Electrolyte Synapse Arrays via Laser-Lift Off Process for Wearable Adaptive Neuromorphic Computing.

Wearable electronic devices with next-generation biocompatible, mechanical, ultraflexible, and portable sensors are a fast-growing technology. Hardware systems enabling artificial neural networks while consuming low power and processing massive in situ personal data are essential for adaptive wearable neuromorphic edging computing. Herein, the development of an ultraflexible artificial-synaptic array device with concrete-mechanical cyclic endurance consisting of a novel heterostructure with an all-solid-state 2D MoS2 channel and LiSiOx (lithium silicate) is demonstrated. Enabled by the sequential fabrication process of all layers, by excluding the transfer process, artificial van der Waals devices combined with the 2D-MoS2 channel and LiSiOx solid electrolyte exhibit excellent neuromorphic synaptic characteristics with a nonlinearity of 0.55 and asymmetry ratio of 0.22. Based on the excellent flexibility of colorless polyimide substrates and thin-layered structures, the fabricated flexible neuromorphic synaptic devices exhibit superior long-term potentiation and long-term depression cyclic endurance performance, even when bent over 700 times or on curved surfaces with a diameter of 10 mm. Thus, a high classification accuracy of 95% is achieved without any noticeable performance degradation in the Modified National Institute of Standards and Technology. These results are promising for the development of personalized wearable artificial neural systems in the future.

Robust 2D MoS2 Artificial Synapse Device Based on a Lithium Silicate Solid Electrolyte for High-Precision Analogue Neuromorphic Computing.

High-precision artificial synaptic devices compatible with existing CMOS technology are essential for realizing robust neuromorphic hardware systems with reliable parallel analogue computation beyond the von Neumann serial digital computing architecture. However, critical issues related to reliability and variability, such as nonlinearity and asymmetric weight updates, have been great challenges in the implementation of artificial synaptic devices in practical neuromorphic hardware systems. Herein, a robust three-terminal two-dimensional (2D) MoS2 artificial synaptic device combined with a lithium silicate (LSO) solid-state electrolyte thin film is proposed. The rationally designed synaptic device exhibits excellent linearity and symmetry upon electrical potentiation and depression, benefiting from the reversible intercalation of Li ions into the MoS2 channel. In particular, extremely low cycle-to-cycle variations (3.01%) during long-term potentiation and depression processes over 500 pulses are achieved, causing statistical analogue discrete states. Thus, a high classification accuracy of 96.77% (close to the software baseline of 98%) is demonstrated in the Modified National Institute of Standards and Technology (MNIST) simulations. These results provide a future perspective for robust synaptic device architecture of lithium solid-state electrolytes stacked with 2D van der Waals layered channels for high-precision analogue neuromorphic computing systems.

Highly Reproducible Heterosynaptic Plasticity Enabled by MoS2/ZrO2-x Heterostructure Memtransistor.

Electrically tunable resistive switching of a polycrystalline MoS2-based memtransistor has attracted a great deal of attention as an essential synaptic component of neuromorphic circuitry because its switching characteristics from the field-induced migration of sulfur defects in the MoS2 grain boundaries can realize multilevel conductance tunability and heterosynaptic functionality. However, reproducible switching properties in the memtransistor are usually disturbed by the considerable difficulty in controlling the concentration and distribution of the intrinsically existing sulfur defects. Herein, we demonstrate reliable heterosynaptic characteristics using a memtransistor device with a MoS2/ZrO2-x heterostructure. Compared to the control device with the MoS2 semiconducting channel, the Schottky barrier height was more effectively modulated by the insertion of the insulating ZrO2-x layer below the MoS2, confirmed by an ultraviolet photoelectron spectroscopy analysis and the corresponding energy-band structures. The MoS2/ZrO2-x memtransistor accomplishes dual-terminal (drain and gate electrode) stimulated multilevel conductance owing to the tunable resistive switching behavior under varying gate voltages. Furthermore, the memtransistor exhibits long-term potentiation/depression endurance cycling over 7000 pulses and stable pulse cycling behavior by the pulse stimulus from different terminal regions. The promising candidate as an essential synaptic component of the MoS2/ZrO2-x memtransistors for neuromorphic systems results from the high recognition accuracy (∼92%) of the deep neural network simulation test, based on the training and inference of handwritten numbers (0-9). The simple memtransistor structure facilitates the implementation of complex neural circuitry.

P-Doped SiOx /Si/SiOx Sandwich Anode for Li-Ion Batteries to Achieve High Initial Coulombic Efficiency and Low Capacity Decay.

Initial reversibility and excellent capacity retention are the key requirements for the success of high-capacity electrode materials in high-performance Li-ion batteries and pose a number of challenges to development. Silicon has been regarded as a promising anode material because of its outstanding theoretical capacity. However, it suffers from colossal volume change and continuous formation of unstable solid electrolyte interphases during lithiation/delithiation processes, which eventually result in low initial Coulombic efficiency (ICE) and severe capacity decay. To circumvent these challenges, a new sandwich Si anode (SiOx /Si/SiOx ) free from prelithiation is designed and fabricated using a combination of P-doping and SiOx layers. This new anode exhibits high conductivity and specific capacity compared to other Si thin-film electrodes. Cells with SiOx /Si/SiOx anodes deliver the highest presently known ICE value among Si thin-film anodes of 90.4% with a charge capacity of 3534 mA h g-1 . In addition, the SiOx layer has sufficient mechanical stability to accommodate the large volume change of the intervening Si layer during charge-discharge cycling, exhibiting high potential for practical applications of Si thin-film anodes.

Statistical Analysis of Uniform Switching Characteristics of Ta2O5-Based Memristors by Embedding In-Situ Grown 2D-MoS2 Buffer Layers.

A memristor based on emerging resistive random-access memory (RRAM) is a promising candidate for use as a next-generation neuromorphic computing device which overcomes the von Neumann bottleneck. Meanwhile, due to their unique properties, including atomically thin layers and surface smoothness, two-dimensional (2D) materials are being widely studied for implementation in the development of new information-processing electronic devices. However, inherent drawbacks concerning operational uniformities, such as device-to-device variability, device yield, and reliability, are huge challenges in the realization of concrete memristor hardware devices. In this study, we fabricated Ta2O5-based memristor devices, where a 2D-MoS2 buffer layer was directly inserted between the Ta2O5 switching layer and the Ag metal electrode to improve uniform switching characteristics in terms of switching voltage, the distribution of resistance states, endurance, and retention. A 2D-MoS2 layered buffer film with a 5 nm thickness was directly grown on the Ta2O5 switching layer by the atomic-pressure plasma-enhanced chemical vapor deposition (AP-PECVD) method, which is highly uniform and provided a superior yield of 2D-MoS2 film. It was observed that the switching operation was dramatically stabilized via the introduction of the 2D-MoS2 buffer layer compared to a pristine device without the buffer layer. It was assumed that the difference in mobility and reduction rates between Ta2O5 and MoS2 caused the narrow localization of ion migration, inducing the formation of more stable conduction filament. In addition, an excellent yield of 98% was confirmed while showing cell-to-cell operation uniformity, and the extrinsic and intrinsic variabilities in operating the device were highly uniform. Thus, the introduction of a MoS2 buffer layer could improve highly reliable memristor device switching operation.

Suppressed Stochastic Switching Behavior and Improved Synaptic Functions in an Atomic Switch Embedded with a 2D NbSe2 Material.

We investigated chemical vapor-deposited (CVD) two-dimensional (2D) niobium diselenide (NbSe2) material for the resistive switching and synaptic characteristics. Three different atomic switch devices with Ag/HfO2/Pt, Ag/Ti/HfO2/Pt, and Ag/NbSe2/HfO2/Pt were studied as both memory and neuromorphic devices. Both the inserted Ti and NbSe2 buffer layers effectively control the stochastic Ag-ion diffusion, leading to suppressed variation of switching characteristics, which is a critical issue in an atomic switch device. Especially, the device with the 2D NbSe2 buffer layer strikingly enhanced the device reliability in both endurance and retention. In conjunction with scanning transmission electron microscopy (STEM) and energy-dispersive spectrometry (EDS) analysis of the control of the Ag-ion migration, it was understood that filament connection is interrelated with the SET and RESET processes. Besides resistive behaviors in the memory device, various synapse functions such as spike-rate-dependent plasticity (SRDP), forgetting curve, potentiation, and depression were demonstrated with an atomic switch with the 2D NbSe2 buffer layer. Furthermore, the emulated long-term synaptic property was simulated using the MNIST 28 × 28 pixel database. Using adopting a CVD 2D NbSe2 blocking layer, the stochastic Ag-ion diffusion behavior is well-controlled and therefore stable switching and synapse functions are attained.

Ultra-flexible and rollable 2D-MoS2/Si heterojunction-based near-infrared photodetector via direct synthesis.

Atomic two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted significant attention for application in various optoelectronic devices such as image sensors, biomedical imaging systems, and consumer electronics and in diverse spectroscopic analyses. However, a complicated fabrication process, involving transfer and alignment of as-synthesized 2D layers onto flexible target substrates, hinders the development of flexible high-performance heterojunction-based photodetectors. Herein, an ultra-flexible 2D-MoS2/Si heterojunction-based photodetector is successfully fabricated through atmospheric-pressure plasma enhanced chemical vapor deposition, which enables the direct deposition of multi-layered MoS2 onto a flexible Si substrate at low temperature (<200 °C). The photodetector is responsive to near infrared light (λ = 850 nm), showing responsivity of 10.07 mA W-1 and specific detectivity (D*) of 4.53 × 1010 Jones. The measured photocurrent as a function of light intensity exhibits good linearity with a power law exponent of 0.84, indicating negligible trapping/de-trapping of photo-generated carriers at the heterojunction interface, which facilitates photocarrier collection. Furthermore, the photodetectors can be bent with a small bending radius (5 mm) and wrapped around a glass rod, showing excellent photoresponsivity under various bending radii. Hence, the device exhibits excellent flexibility, rollability, and durability under harsh bending conditions. This photodetector has significant potential for use in next-generation flexible and patchable optoelectronic devices.

Electrodeposition of High-Surface-Area IrO2 Films on Ti Felt as an Efficient Catalyst for the Oxygen Evolution Reaction.

Under acidic conditions, IrO2 exhibits high catalytic activity with respect to the oxygen evolution reaction (OER). However, the practical application of Ir-based catalysts is significantly limited owing to their high cost in addition to the scarcity of the metal. Therefore, it is necessary to improve the efficiency of the utilization of such catalysts. In this study, IrO2-coated Ti felt (IrO2/Ti) electrodes were prepared as high-efficiency catalysts for the OER under acidic conditions. By controlling the surface roughness of the Ti substrate via wet etching, the optimum Ti substrate surface area for application in the IrO2/Ti electrode was determined. Additionally, the IrO2 film that was electrodeposited on the 30 min etched Ti felt had a large surface area and a uniform morphology. Furthermore, there were no micro-cracks and the electrode obtained (IrO2/Ti-30) exhibited superior catalytic performance with respect to the OER, with a mass activity of 362.3 A g Ir - 1 at a potential of 2.0 V (vs. RHE) despite the low Ir loading (0.2 mg cm-2). Therefore, this proposed strategy for the development of IrO2/Ti electrodes with substrate surface control via wet etching has potential for application in the improvement of the efficiency of catalyst utilization with respect to the OER.

Surface Engineering of Low-Temperature Processed Mesoporous TiO2 via Oxygen Plasma for Flexible Perovskite Solar Cells.

A major problem in the application of mesoporous TiO2 as an electron transport layer for flexible perovskite solar cells is that a high-temperature sintering process is required to remove organic additives from the TiO2 layer. A facile oxygen plasma process is herein demonstrated to fabricate mesoporous-structured perovskite solar cells with significant photovoltaic performance at low temperatures. When the low-temperature processed TiO2 layer is modified via oxygen plasma, the organic additives in the TiO2 layer that hinder the charge transport process are successfully decomposed. The oxygen plasma treatment improves the wettability and infiltration of the perovskite layer and also passivates the oxygen vacancy related traps in TiO2. Hence, the oxygen plasma treatment evidently enhances charge extraction and transport, thereby improving photovoltaic performance and decreasing hysteresis.

Artificial 2D van der Waals Synapse Devices via Interfacial Engineering for Neuromorphic Systems.

Despite extensive investigations of a wide variety of artificial synapse devices aimed at realizing a neuromorphic hardware system, the identification of a physical parameter that modulates synaptic plasticity is still required. In this context, a novel two-dimensional architecture consisting of a NbSe2/WSe2/Nb2O5 heterostructure placed on an SiO2/p+ Si substrate was designed to overcome the limitations of the conventional silicon-based complementary metal-oxide semiconductor technology. NbSe2, WSe2, and Nb2O5 were used as the metal electrode, active channel, and conductance-modulating layer, respectively. Interestingly, it was found that the post-synaptic current was successfully modulated by the thickness of the interlayer Nb2O5, with a thicker interlayer inducing a higher synapse spike current and a stronger interaction in the sequential pulse mode. Introduction of the Nb2O5 interlayer can facilitate the realization of reliable and controllable synaptic devices for brain-inspired integrated neuromorphic systems.

Facile Interfacial Engineering of Mesoporous TiO2 for Low-Temperature Processed Perovskite Solar Cells.

The mesoporous TiO2 nanoparticle-based scaffold structure is the best electron transport layer (ETL) for perovskite solar cells (PSCs) and is still used in most PSCs with optimal photovoltaic characteristics. However, the high sintering temperature of TiO2 nanoparticles required to remove binders from the TiO2 paste limits PSC application to flexible electronics. In this study, a simple interface modification process involving ethanol rinsing is developed to enhance the photovoltaic characteristics of low-temperature processed PSCs. This easy and fast technique could enable remarkable performance by PSCs by significantly increasing the fill factor and current density, leading to a power conversion efficiency more than four times that of untreated solar cells.

Highly Sensitive and Selective Nanogap-Enhanced SERS Sensing Platform.

This paper reports a highly sensitive and selective surface-enhanced Raman spectroscopy (SERS) sensing platform. We used a simple fabrication method to generate plasmonic hotspots through a direct maskless plasma etching of a polymer surface and the surface tension-driven assembly of high aspect ratio Ag/polymer nanopillars. These collapsed plasmonic nanopillars produced an enhanced near-field interaction via coupled localized surface plasmon resonance. The high density of the small nanogaps yielded a high plasmonic detection performance, with an average SERS enhancement factor of 1.5 × 107. More importantly, we demonstrated that the encapsulation of plasmonic nanostructures within nanofiltration membranes allowed the selective filtration of small molecules based on the degree of membrane swelling in organic solvents and molecular size. Nanofiltration membrane-encapsulated SERS substrates do not require pretreatments. Therefore, they provide a simple and fast detection of toxic molecules using portable Raman spectroscopy.

Hierarchical multi-level block copolymer patterns by multiple self-assembly.

Uniform, well-ordered sub-20 nm patterns can be generated by the templated self-assembly of block copolymers (BCPs) with a high Flory-Huggins interaction parameter (χ). However, the self-assembled BCP monolayers remain limited in the possible structural geometries. Here, we introduce a multiple self-assembly method which uses di-BCPs to produce diverse morphologies, such as dot, dot-in-honeycomb, line-on-dot, double-dot, pondering, dot-in-pondering, and line-on-pondering patterns. To improve the diversity of BCP morphological structures, we employed sphere-forming and cylinder-forming poly(styrene-block-dimethylsiloxane) (PS-b-PDMS) BCPs with a high χ. The self-assembled mono-layer and double-layer SiOx dot patterns were modified at a high temperature (∼800 °C), showing hexagonally arranged (dot) and double-hexagonally arranged (pondering) SiOx patterns, respectively. We successfully obtained additional new nanostructures (big-dot, dot-in-honeycomb, line-on-dot, pondering, dot-in-pondering, and line-on-pondering types) through a second self-assembly of cylinder-forming BCPs using the dot and pondering patterns as guiding templates. This simple approach can likely be extended to the multiple self-assembly of many other BCPs with good functionality, significantly contributing to the development of various nanodevices.

Facile Fabrication of a Two-Dimensional TMD/Si Heterojunction Photodiode by Atmospheric-Pressure Plasma-Enhanced Chemical Vapor Deposition.

A growth technique to directly prepare two-dimensional (2D) materials onto conventional semiconductor substrates, enabling low-temperature, high-throughput, and large-area capability, is needed to realize competitive 2D transition-metal dichalcogenide (TMD)/three-dimensional (3D) semiconductor heterojunction devices. Therefore, we herein successfully developed an atmospheric-pressure plasma-enhanced chemical vapor deposition (AP-PECVD) technique, which could grow MoS2 and WS2 multilayers directly onto PET flexible substrate as well as 4-in. Si substrates at temperatures of <200 °C. The as-fabricated MoS2/Si and WS2/Si heterojunctions exhibited large and fast photocurrent responses under illumination of a green light. The measured photocurrent was linearly proportional to the laser power, indicating that trapping and detrapping of the photogenerated carriers at defect states could not significantly suppress the collection of photocarriers. All the results demonstrated that our AP-PECVD method could produce high-quality TMD/Si 2D-3D heterojunctions for optoelectronic applications.

A Hybrid Gate Dielectrics of Ion Gel with Ultra-Thin Passivation Layer for High-Performance Transistors Based on Two-Dimensional Semiconductor Channels.

We propose a hybrid gate structure for ion gel dielectrics using an ultra-thin Al2O3 passivation layer for realizing high-performance devices based on electric-double-layer capacitors. Electric-double-layer transistors can be applied to practical devices with flexibility and transparency as well as research on the fundamental physical properties of channel materials; however, they suffer from inherent unwanted leakage currents between electrodes, especially for channel materials with low off-currents. Therefore, the Al2O3 passivation layer was introduced between the metal electrodes and ion gel film as a leakage current barrier; this simple approach effectively reduced the leakage current without capacitance degradation. In addition, we confirmed that a monolayer MoS2 transistor fabricated with the proposed hybrid gate dielectric exhibited remarkably enhanced device properties compared to a transistor using a normal ion gel gate dielectric. Our findings on a simple method to improve the leakage current properties of ion gels could be applied extensively to realize high-performance electric-double-layer transistors utilizing various channel materials.

Effect of Nb Doping on Chemical Sensing Performance of Two-Dimensional Layered MoSe2.

Here, we report that Nb doping of two-dimensional (2D) MoSe2 layered nanomaterials is a promising approach to improve their gas sensing performance. In this study, Nb atoms were incorporated into a 2D MoSe2 host matrix, and the Nb doping concentration could be precisely controlled by varying the number of Nb2O5 deposition cycles in the plasma enhanced atomic layer deposition process. At relatively low Nb dopant concentrations, MoSe2 showed enhanced device durability as well as NO2 gas response, attributed to its small grains and stabilized grain boundaries. Meanwhile, an increase in the Nb doping concentration deteriorated the NO2 gas response. This might be attributed to a considerable increase in the number of metallic NbSe2 regions, which do not respond to gas molecules. This novel method of doping 2D transition metal dichalcogenide-based nanomaterials with metal atoms is a promising approach to improve the performance such as stability and gas response of 2D gas sensors.

Alloyed 2D Metal-Semiconductor Heterojunctions: Origin of Interface States Reduction and Schottky Barrier Lowering.

The long-term stability and superior device reliability through the use of delicately designed metal contacts with two-dimensional (2D) atomic-scale semiconductors are considered one of the critical issues related to practical 2D-based electronic components. Here, we investigate the origin of the improved contact properties of alloyed 2D metal-semiconductor heterojunctions. 2D WSe2-based transistors with mixed transition layers containing van der Waals (M-vdW, NbSe2/WxNb1-xSe2/WSe2) junctions realize atomically sharp interfaces, exhibiting long hot-carrier lifetimes of approximately 75,296 s (78 times longer than that of metal-semiconductor, Pd/WSe2 junctions). Such dramatic lifetime enhancement in M-vdW-junctioned devices is attributed to the synergistic effects arising from the significant reduction in the number of defects and the Schottky barrier lowering at the interface. Formation of a controllable mixed-composition alloyed layer on the 2D active channel would be a breakthrough approach to maximize the electrical reliability of 2D nanomaterial-based electronic applications.

Two-Dimensional Atomic-Layered Alloy Junctions for High-Performance Wearable Chemical Sensor.

We first report that two-dimensional (2D) metal (NbSe2)-semiconductor (WSe2)-based flexible, wearable, and launderable gas sensors can be prepared through simple one-step chemical vapor deposition of prepatterned WO3 and Nb2O5. Compared to a control device with a Au/WSe2 junction, gas-sensing performance of the 2D NbSe2/WSe2 device was significantly enhanced, which might have resulted from the formation of a NbxW1-xSe2 transition alloy junction lowering the Schottky barrier height. This would make it easier to collect charges of channels induced by molecule adsorption, improving gas response characteristics toward chemical species including NO2 and NH3. 2D NbSe2/WSe2 devices on a flexible substrate provide gas-sensing properties with excellent durability under harsh bending. Furthermore, the device stitched on a T-shirt still performed well even after conventional cleaning with a laundry machine, enabling wearable and launderable chemical sensors. These results could pave a road toward futuristic gas-sensing platforms based on only 2D materials.