Autonomous Artificial Olfactory Sensor Systems with Homeostasis Recovery via a Seamless Neuromorphic Architecture.

Neuromorphic olfactory systems have been actively studied in recent years owing to their considerable potential in electronic noses, robotics, and neuromorphic data processing systems. However, conventional gas sensors typically have the ability to detect hazardous gas levels but lack synaptic functions such as memory and recognition of gas accumulation, which are essential for realizing human-like neuromorphic sensory system. In this study, a seamless architecture for a neuromorphic olfactory system capable of detecting and memorizing the present level and accumulation status of nitrogen dioxide (NO2) during continuous gas exposure, regulating a self-alarm implementation triggered after 147 and 85 s at a continuous gas exposure of 20 and 40 ppm, respectively. Thin-film-transistor type gas sensors utilizing carbon nanotube semiconductors detect NO2 gas molecules through carrier trapping and exhibit long-term retention properties, which are compatible with neuromorphic excitatory applications. Additionally, the neuromorphic inhibitory performance is also characterized via gas desorption with programmable ultraviolet light exposure, demonstrating homeostasis recovery. These results provide a promising strategy for developing a facile artificial olfactory system that demonstrates complicated biological synaptic functions with a seamless and simplified system architecture.

Solution Process-Based Thickness Engineering of InZnO Semiconductors for Oxide Thin-Film Transistors with High Performance and Stability.

To fabricate oxide thin-film transistors (TFTs) with high performance and excellent stability, preparing high-quality semiconductor films in the channel bulk region and minimizing the defect states in the gate dielectric/channel interfaces and back-channel regions is necessary. However, even if an oxide transistor is composed of the same semiconductor film, gate dielectric/channel interface, and back channel, its electrical performance and operational stability are significantly affected by the thickness of the oxide semiconductor. In this study, solution process-based nanometer-scale thickness engineering of InZnO semiconductors was easily performed via repeated solution coating and annealing. The thickness-controlled InZnO films were then applied as channel regions, which were fabricated with almost identical film quality, gate dielectric/channel interface, and back-channel conditions. However, excellent operational stability and electrical performance suitable for oxide TFT backplane was only achieved using an 8 nm thick InZnO film. In contrast, the ultrathin and thicker films exhibited electrical performances that were either very resistive (high positive VTh and low on-current) or excessively conductive (high negative VTh and high off-current). This investigation confirmed that the quality of semiconductor materials, solution process design, and structural parameters, including the dimensions of the channel layer, must be carefully designed to realize high-performance and high-stability oxide TFTs.

Composition Engineering of Indium Zinc Oxide Semiconductors for Damage-Free Back-Channel Wet Etching Metallization of Oxide Thin-Film Transistors.

In contrast to lift-off and shadow mask processes, the back-channel wet etching (BCWE) process is suitable for industrial-scale metallization processes for the large-area and mass production of oxide thin-film transistors (TFTs). However, chemical attacks caused by the corrosive metal etchants used in the BCWE process cause unintended performance degradation of oxide semiconductors, making it difficult to implement oxide TFT circuits through industrial-scale metallization processes. Herein, we propose composition engineering of oxide semiconductors to enhance the chemical durability and electrical stability of oxide semiconductors. The chemical durability of InZnO against Al etchants can be improved by increasing the content of indium oxide, which has a higher chemical resistance than zinc oxide. As a result, A damage-free BCWE-based metallization process was successfully demonstrated for oxide TFTs using In-rich InZnO semiconductors. Furthermore, In-rich InZnO TFTs with wet-etched Al electrodes exhibited electrical performance comparable to that of lift-off Al electrodes, without chemical attack issues.

Intensive harmonized synapses with amorphous Cu2O-based memristors using ultrafine Cu nanoparticle sublayers formed via atomically controlled electrochemical pulse deposition.

Resistive random-access memory (RRAM) devices have significant advantages for neuromorphic computing but have fatal problems of uncontrollability and abrupt resistive switching behaviors degrading their synaptic performance. In this paper, we propose the electrochemical design of an active Cu2O layer containing a strategic sublayer of ultrafine Cu nanoparticles (U-Cu NPs) to form uniformly dispersed conducting filaments, which can effectively improve the reliability for analog switching of RRAM-based neuromorphic computing. The electrochemical pulse deposited (EPD) U-Cu NPs are linked to the bottom electrode through a semi-conductive path within the bottom Cu2O layer, since the EPD is preferentially carried out on the conductive sites. All Cu2O films with U-Cu NPs are developed in situ in the single electrolyte bath without any pause. The proposed U-Cu NPs can concentrate the external electric field and can generate conductive filament paths for analog resistive switching. The applied electric field was uniformly spread to U-Cu NPs at the center of the active layer and displays resistive switching behavior via multiple conductive filaments. This shows a strong harmony between the resistance-switching characteristics and the analog operation of the active layer. This RRAM device shows outstanding gradual analog switching, great linearity, dynamic range, endurance, precision, speed, and retention characteristics simultaneously and adequately for neuromorphic computing by realizing multiple weak filament-type operation.

Progress of Materials and Devices for Neuromorphic Vision Sensors.

The latest developments in bio-inspired neuromorphic vision sensors can be summarized in 3 keywords: smaller, faster, and smarter. (1) Smaller: Devices are becoming more compact by integrating previously separated components such as sensors, memory, and processing units. As a prime example, the transition from traditional sensory vision computing to in-sensor vision computing has shown clear benefits, such as simpler circuitry, lower power consumption, and less data redundancy. (2) Swifter: Owing to the nature of physics, smaller and more integrated devices can detect, process, and react to input more quickly. In addition, the methods for sensing and processing optical information using various materials (such as oxide semiconductors) are evolving. (3) Smarter: Owing to these two main research directions, we can expect advanced applications such as adaptive vision sensors, collision sensors, and nociceptive sensors. This review mainly focuses on the recent progress, working mechanisms, image pre-processing techniques, and advanced features of two types of neuromorphic vision sensors based on near-sensor and in-sensor vision computing methodologies.

Enhanced Light Output Power on Near-Infrared Light-Emitting Diodes with TITO/Ag Multilayer Reflector.

A titanium-indium tin oxide (TITO) multilayer reflector was investigated to improve the light efficiency of high-power, near-infrared, light-emitting diodes (NIR-LEDs). The TITO/Ag was fabricated by combining a patterned TITO and an omnidirectional reflector (ODR). For fabricating a high-power NIR-LED, the wafer bond process required the TITO reflective structure, which has patterns filled by AlAu contact metal, bonded directly to the Ag reflector deposited on the silicon wafer. Among Ag-based single- and multilayer reflectors, the TITO/Ag showed the highest reflectance (R = 96%), which was favorable for wafer-bonded high-power NIR-LEDs. Therefore, the TITO/Ag reflector enabled the production of wafer-bonded NIR-LED chips that exhibit superior output performance (190 mW) compared with conventional cases using a single Ag reflector.

Millimeter-Scale Continuous Film of MoS2 Synthesized Using a Mo, Na, and Seeding Promoter-Based Coating as a Solid Precursor.

While the chemical vapor deposition technique can be used to fabricate 2D materials in a larger area, materials like MoS2 have limited controllability due to their lack of self-controlling nature. This article presents a new technique for synthesizing a void-free millimeter-scale continuous monolayer MoS2 film through the diffusion of a well-controlled Mo, Na, and seeding promoter-based coating under a low-pressure N2 atmosphere. Compared to the conventional method, this technique provides precise control of solid precursors, where MoS2 grows next to the coating. At 800 °C, the synthesized MoS2 showed a uniform single-layer MoS2 film; however, a Na-free coating showed nanoscale voids and poor crystal quality, which are attributed to a higher edge-attachment barrier that slows down the MoS2 lateral growth. The synthesized MoS2 with Na-containing solution showed an intense PL peak with a 1.86 eV band gap. Even at the relatively low temperature of 700 °C, compared to the Na-excluded condition, MoS2 showed almost two times higher area coverage with a comparatively larger crystal size. This finding may assist in the future development of MoS2-based electronic and optoelectronic devices such as transistors and photodetectors.

Catalytic Metal-Accelerated Crystallization of High-Performance Solution-Processed Earth-Abundant Metal Oxide Semiconductors.

As an alternative strategy for conventional high-temperature crystallization of metal oxide (MO) channel layers, the catalytic metal-accelerated crystallization (CMAC) process using a metal seed layer is demonstrated for low-temperature crystallization of solution-processed MO semiconductors. In the CMAC process, the catalytic metal layer plays the role of seed sites for initiating and accelerating the crystallization of amorphous MO films. Generally, the solution-processed crystalline-TiO2 (c-TiO2) films required high-temperature crystallization conditions (≥500-600 °C), showing low electrical performance with a high defect density. In contrast, the suggested CMAC process could effectively lower crystallization temperature of the a-TiO2 films, enabling high-quality c-TiO2 films with well-aligned anatase grains and low-defect density. The various crystalline catalytic layers were deposited over the earth-abundant n-type amorphous titanium oxide (a-TiO2) films. Also, then, the CMAC process was performed for facile low-temperature translation of solution-processed a-TiO2 to a highly crystallized state. In particular, the Al-CMAC process using the crystalline thin-aluminum (Al) catalytic metal seed layer facilitates low-temperature (≥300 °C) crystallization of the solution-processed a-TiO2 films and the fabrication of high-performance solution-processed c-TiO2 thin-film transistors with superior field-effect mobility, good on/off switching behavior, and improved operational stability.

Smart Bifunctional Sb2 Se3 Nanorods for Integrated Water Purification: Insoluble Liquid Separation and Photoelectrochemical Degradation.

Antimony selenide (Sb2 Se3 ) nanostructures enable bifunctional water purification by a single membrane through i) physical separation of water-insoluble oil and ii) photoelectrocatalytic degradation of water-soluble organic compounds. Sb2 Se3 nanorods with exposed surfaces of {h 0 0} and {h 0 l} planes exhibit superhydrophobicity (water contact angle of ≈159°) owing to extremely low surface energy of those dangling-bond-free van der Waals planes. Based on crystallographic understanding, superhydrophobic Sb2 Se3 nanorods were produced on a mesh-type substrate for utilization as a membrane for physical water/oil separation. Sb2 Se3 exhibited an optimal photocathodic response with p-type electrical conductivity under visible light along the longitudinal crystal direction. This indicated that the nanorods could be used as photoelectrocatalytic material for chemical water purification. A smart membrane with Sb2 Se3 nanostructures was proposed as a candidate for integrated water purification that can simultaneously accomplish water/oil separation and photoelectrocatalytic degradation of organic compounds in wastewater. Linear sweep voltammetry measurements of the Sb2 Se3 -membrane showed cathodic photocurrent generation (up to approximately 10 mA cm-2 at 0 V vs. reversible hydrogen electrode), which was enough to reduce O2 to an oxygen radical (O2 .- ) for degradation of methyl orange. Consequently, solar-driven integrated water purification was demonstrated for the first time by using a single material with a dual function of superhydrophobicity and photoactivity.

Environment-Adaptable Artificial Visual Perception Behaviors Using a Light-Adjustable Optoelectronic Neuromorphic Device Array.

Emulating the biological visual perception system typically requires a complex architecture including the integration of an artificial retina and optic nerves with various synaptic behaviors. However, self-adaptive synaptic behaviors, which are frequently translated into visual nerves to adjust environmental light intensities, have been one of the serious challenges for the artificial visual perception system. Here, an artificial optoelectronic neuromorphic device array to emulate the light-adaptable synaptic functions (photopic and scotopic adaptation) of the biological visual perception system is presented. By employing an artificial visual perception circuit including a metal chalcogenide photoreceptor transistor and a metal oxide synaptic transistor, the optoelectronic neuromorphic device successfully demonstrates diverse visual synaptic functions such as phototriggered short-term plasticity, long-term potentiation, and neural facilitation. More importantly, the environment-adaptable perception behaviors at various levels of the light illumination are well reproduced by adjusting load transistor in the circuit, exhibiting the acts of variable dynamic ranges of biological system. This development paves a new way to fabricate an environmental-adaptable artificial visual perception system with profound implications for the field of future neuromorphic electronics.

Toward Robust Photoelectrochemical Operation of Cuprous Oxide Nanowire Photocathodes Using a Strategically Designed Solution-Processed Titanium Oxide Passivation Coating.

To date, TiO2 films prepared by atomic layer deposition are widely used to prepare Cu2O nanowire (NW)-based photocathodes with photoelectrochemical (PEC) durability as this approach enables conformal coating and furnishes chemical robustness. However, this common approach requires complicated interlayers and makes the fabrication of photocathodes with reproducible performance and long-term stability difficult. Although sol-gel-based approaches have been well established for coating surfaces with oxide thin films, these techniques have rarely been studied for oxide passivation in PEC applications, because the sol-gel coating methods are strongly influenced by surface chemical bonding and have been mainly demonstrated on flat substrates. As a unique strategy based on solution processing, herein, we suggest a creative solution for two problems encountered in the conformal coating of surfaces with oxide layers: (i) how to effectively prevent corrosion of materials with hydrophilic surfaces by simply using a single TiO2 surface protection layer instead of a complex multilayer structure and (ii) guaranteeing perfect chemical durability. A Cu(OH)2 NW can be easily prepared as an intermediate phase by anodization of a Cu metal, where the former inherently possesses a hydrophilic hydroxylated surface and thus, enables thorough coating with TiO2 precursor solutions. Chemically robust nanowires are then generated as the final product via the phase transformation of Cu(OH)2 to Cu2O via sintering at 600 °C. The coated NWs exhibit excellent PEC properties and a stable performance. Consequently, the perfect chemical isolation of the Cu2O NWs from the electrolyte allows a remarkable PEC operation with the maintenance of the initial photocurrent for more than one day.

Toward Adequate Operation of Amorphous Oxide Thin-Film Transistors for Low-Concentration Gas Detection.

We suggest the use of a thin-film transistor (TFT) composed of amorphous InGaZnO (a-IGZO) as a channel and a sensing layer for low-concentration NO2 gas detection. Although amorphous oxide layers have a restricted surface area when reacting with NO2 gas, such TFT sensors have incomparable advantages in the aspects of electrical stability, large-scale uniformity, and the possibility of miniaturization. The a-IGZO thin films do not possess typical reactive sites and grain boundaries, so that the variation in drain current of the TFTs strictly originates from oxidation reaction between channel surface and NO2 gas. Especially, the sensing data obtained from the variation rate of drain current makes it possible to monitor efficiently and quickly the variation of the NO2 concentration. Interestingly, we found that enhancement-mode TFT (EM-TFT) allows discrimination of the drain current variation rate at NO2 concentrations ≤10 ppm, whereas a depletion-mode TFT is adequate for discriminating NO2 concentrations ≥10 ppm. This discrepancy is attributed to the ratio of charge carriers contributing to gas capture with respect to total carriers. This capacity for the excellent detection of low-concentration NO2 gas can be realized through (i) three-terminal TFT gas sensors using amorphous oxide, (ii) measurement of the drain current variation rate for high selectivity, and (iii) an EM mode driven by tuning the electrical conductivity of channel layers.

An All Oxide-Based Imperceptible Thin-Film Transistor with Humidity Sensing Properties.

We have examined the effects of oxygen content and thickness in sputtered InSnO (ITO) electrodes, especially for the application of imperceptible amorphous-InGaZnO (a-IGZO) thin-film transistors (TFTs) in humidity sensors. The imperceptible a-IGZO TFT with 50-nm ITO electrodes deposited at Ar:O2 = 29:0.3 exhibited good electrical performances with Vth of -0.23 V, SS of 0.34 V/dec, µFE of 7.86 cm²/V∙s, on/off ratio of 8.8 × 107, and has no degradation for bending stress up to a 3.5-mm curvature. The imperceptible oxide TFT sensors showed the highest sensitivity for the low and wide gate bias of -1~2 V under a wide range of relative humidity (40-90%) at drain voltage 1 V, resulting in low power consumption by the sensors. Exposure to water vapor led to a negative shift in the threshold voltage (or current enhancement), and an increase in relative humidity induced continuous threshold voltage shift. In particular, compared to conventional resistor-type sensors, the imperceptible oxide TFT sensors exhibited extremely high sensitivity from a current amplification of >10³.

Low voltage-driven oxide phototransistors with fast recovery, high signal-to-noise ratio, and high responsivity fabricated via a simple defect-generating process.

We have demonstrated that photo-thin film transistors (photo-TFTs) fabricated via a simple defect-generating process could achieve fast recovery, a high signal to noise (S/N) ratio, and high sensitivity. The photo-TFTs are inverted-staggered bottom-gate type indium-gallium-zinc-oxide (IGZO) TFTs fabricated using atomic layer deposition (ALD)-derived Al2O3 gate insulators. The surfaces of the Al2O3 gate insulators are damaged by ion bombardment during the deposition of the IGZO channel layers by sputtering and the damage results in the hysteresis behavior of the photo-TFTs. The hysteresis loops broaden as the deposition power density increases. This implies that we can easily control the amount of the interface trap sites and/or trap sites in the gate insulator near the interface. The photo-TFTs with large hysteresis-related defects have high S/N ratio and fast recovery in spite of the low operation voltages including a drain voltage of 1 V, positive gate bias pulse voltage of 3 V, and gate voltage pulse width of 3 V (0 to 3 V). In addition, through the hysteresis-related defect-generating process, we have achieved a high responsivity since the bulk defects that can be photo-excited and eject electrons also increase with increasing deposition power density.

Periodically pulsed wet annealing approach for low-temperature processable amorphous InGaZnO thin film transistors with high electrical performance and ultrathin thickness.

In this paper, a simple and controllable "wet pulse annealing" technique for the fabrication of flexible amorphous InGaZnO thin film transistors (a-IGZO TFTs) processed at low temperature (150 °C) by using scalable vacuum deposition is proposed. This method entailed the quick injection of water vapor for 0.1 s and purge treatment in dry ambient in one cycle; the supply content of water vapor was simply controlled by the number of pulse repetitions. The electrical transport characteristics revealed a remarkable performance of the a-IGZO TFTs prepared at the maximum process temperature of 150 °C (field-effect mobility of 13.3 cm(2) V(-1) s(-1); Ion/Ioff ratio ≈ 10(8); reduced I-V hysteresis), comparable to that of a-IGZO TFTs annealed at 350 °C in dry ambient. Upon analysis of the angle-resolved x-ray photoelectron spectroscopy, the good performance was attributed to the effective suppression of the formation of hydroxide and oxygen-related defects. Finally, by using the wet pulse annealing process, we fabricated, on a plastic substrate, an ultrathin flexible a-IGZO TFT with good electrical and bending performances.

Dual electrical behavior of multivalent metal cation-based oxide and its application to thin-film transistors with high mobility and excellent photobias stability.

The effect of multivalent metal cations, including vanadium(V) and tin (Sn), on the electrical properties of vanadium-doped zinc tin oxide (VZTO) was investigated in the context of the fabrication of thin-film transistors (TFTs) using a single VZTO film and VZTO/ZTO bilayer as channel layers. The single VZTO TFT did not show any response to the gate voltage (insulator-like behavior). On the other hand, the VZTO/ZTO bilayer TFT exhibited a typical TFT transfer characteristic (semiconducting behavior). X-ray photoelectron spectroscopy revealed that, in contrast to what is commonly true in many oxides, oxygen vacancies (V(O)) in VZTO did not provide a dominant contribution to the total carrier concentration, because the V(O) peak area in the single VZTO film was 5.4% and reduced to 4.5% in VZTO/ZTO bilayer. Instead, Sn 3d5/2 and V 2p3/2 spectra suggest that the significant reduction in Sn and V ions is strongly related to the insulator-like behavior of the VZTO film. In negative-bias illumination tests and illumination tests with various photon energies, the VZTO/ZTO bilayer TFT had much better stability than the ZTO TFT. This result is attributed to the reduction of donor-like states ([Formula: see text]O) that can be positively ionized by blue and green illumination.