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In this study, we propose an array-type gas sensor with high selectivity and response using multiple oxide semiconductors. The sensor array was composed of SnO2 and In2O3, and the detection characteristics were improved by using Pt, Au, and Pd catalysts. All samples were deposited directly on the Pt interdigitated electrode (IDE) through the e-beam evaporator glancing angle deposition (GAD) method. They grew in the form of well-aligned nanorods at off-axis angles. The prepared SnO2 and In2O3 nanorod samples were exposed to CH3COCH3, C7H8, and NO2 gases in a 300℃ dry condition. Au-decorated SnO2, Au-decorated In2O3, and Pd-decorated In2O3 exhibited high selectivity for CH3COCH3, C7H8, and NO2, respectively. They demonstrated a high detection limit of the sub ppb level computationally. In addition, measurements from each sensor were executed in the 40% relative humidity condition. Although there was a slight reduction in detection response, high selectivity and distinguishable detection characteristics were confirmed

Grain enlargement is a crucial requirement to synthesizing two-dimensional (2D) metal chalcogenides because it can minimize the effects of carrier scattering at the grain boundaries. To this end, r...

The outstanding multifunctionality of ferroelectric oxides has opened up new fields in microelectronics. However, the high crystallization temperature of the ferroelectric oxides limits their integ...

Crystal structures and piezoelectric properties of PbZrO3-PbTiO3-Pb(Ni1/3Nb2/3)O3 ceramics near the triple point composition, particularly characteristics of the pseudocubic phase, were investigated. The pseudocubic phase, which formed near the triple point composition, disappeared with increase in the PbZrO3 content. The pseudocubic phase had the Pm3m cubic structure. The tetragonal-pseudocubic morphotropic phase boundary (MPB) structure was developed during the tetragonal-to-cubic phase transformation. However, the rhombohedral phase directly transformed to the cubic phase because the structure of pseudocubic phase was similar to the rhombohedral structure. The specimens with pseudocubic phase and the specimens near pseudocubic phase exhibited nano-sized domains and small coercive electric fields, revealing their low domain wall energies. These specimens exhibited second-order ferroelectric-to-paraelectric phase transition and low Curie temperatures, confirming their low domain wall energies. The enhanced dielectric and piezoelectric properties of these specimens could be attributed to their low domain wall energies.

The synthesis of a continuous and high-quality large-area layer is a key research area in the field of two-dimensional (2D) metal chalcogenides. To date, several techniques, including chemical vapo...

Gas sensors that operate at room temperature have been extensively studied because of sensor stability, lift time, and power consumption. To design effective room-temperature gas sensors, various nanostructures, such as nanoparticles, nanotubes, nanodomes, or nanofibers, are utilized because of their large-surface-to-volume ratio and unique surface properties. In addition, two-dimensional materials, including MoS2, SnS2, WS2, and MoSe, and ultraviolet-activated methods have been studied to develop ideal room-temperature gas sensors. Herein, a brief overview of state-of-the-art research on room-temperature gas sensors and their sensing properties, including nanostructured materials, two-dimensional materials, the ultraviolet-activated method, and ionic-activated gas sensors, is provided.

Gas sensors for Internet of Things applications should meet two requisites – low power consumption and easy mounting universally. To satisfy the conditions, gas sensors need to operate at lower temperature and be flexible. In this study, we demonstrate a flexible gas sensor operating at room temperature using vertically aligned two-dimensional SnS2 nanomaterials. The atomic layer deposition (ALD) technique allows direct growth of SnS2 on a plastic substrate. The morphological structure of SnS2 is engineered by effecting changes in the growth temperature and substrate surface, which leads to the excellent sensing performance with respect to NO2 gas along with superior gas selectivity. The gas response is as high as 309 at 1 ppm of NO2 at room temperature, and a reliably high response is also observed even below 500 ppb of NO2. The fabricated flexible gas sensor exhibits comparable sensing performance and stability upon bending. Furthermore, the ALD achieves excellent uniformity in both the structural and electrical properties of SnS2 over a 4 in. wafer, which is essential for mass production. Therefore, we believe that this work would contribute to realizing the practical application of highly sensitive flexible gas sensors.

Controlling the structure of a material over a wide range of scales has been extensively pursued because the structure dictates its function. Here, we explore the formation of 3D structures at almost a millimetre scale using single-crystal complex oxides, which has been challenging because of the brittle nature of oxides. Our scheme is to release epitaxial oxide thin film heterostructures from the rigid substrate in order to utilise the elastic epitaxial strain as a driving force for self-shaping the flexible free-standing membrane in a controlled manner. Using an epitaxial free-standing LaAlO3/SrTiO3 membrane as a model system, we were able to create various 3D forms, such as cylindrically-rolled, spherically-bent, and helically-twisted structures, where the inversion-symmetry is broken by the strain gradient via flexoelectric effects. Our results will provide opportunities not only to broaden the application of functional oxides toward flexible electronics, but also to discover new functionalities driven by 3D architectures at various scales.

Triboelectric generators (TEG), based on contact electrification and electro-static induction, has received a significant attention because of their numerous applications. To improve the electrified surface charge density in TEG, increasing the surface area of dielectric materials or forming internal hollow structures are typically employed to increase capacitance. However, the fabrication processes of such structures are complex and time-consuming. Here, we provide a facile and cost-effective synthesis method for the porous PDMS based TEG via a novel vapor encapsulation casting (VEC). The double dielectric layer composed of the porous and dense PDMS layers are formed in-site through VEC. The thickness and the thickness ratio of the double dielectric layer can be precisely controlled by adjusting the uncured PDMS thickness and vapor penetration depth. The double dielectric layer TEG (DTEG) exhibits the improved harvesting performance because the porous dielectric layer increases the capacitance and compressibility, while the dense layer passivates the fully open pores which reduce the charging surface area as completely opening through the dielectric layer without contacting the bottom electrode. We obtain the maximum output voltage of 345 V and short circuit current of 3 μA/cm2 from DTEG having 0.95 porous thickness ratio, resulting 330% enhancement in the power output as compared to the dense PDMS based TEG. We further investigate the performance of DTEG under various operating conditions. We also demonstrate the operation of Bluetooth distance/temperature sensors using capacitors charged by DTEG.

Poly(3,4-ethylenedioxythiophene) (PEDOT) derivatives are synthesized by oxidative polymerization using sodium dodecyl sulfate (SDS) as an anionic surfactant dopant. The resulting polymeric materials featuring nanofiber-type one-dimensional (1D) structures are identified as poly(2‐butyl‐2,3‐dihydrothieno[3,4‐b][1,4]dioxine:dodecyl sulfate (PEDOT-C4:DS) and poly(2‐hexyl‐2,3‐dihydrothieno[3,4‐b][1,4]dioxine:dodecyl sulfate (PEDOT-C6:DS). The ratio of the DS anion doped into PEDOT-C4:DS and PEDOT-C6:DS is 0.16 and 0.23, respectively. The contact angle of water on the PEDOT-C4:DS and PEDOT-C6:DS films is 76.6° and 87.7°, respectively, showing hydrophobic properties similar to that with water on PVDF. It facilitated the fully uniform film formation due to excellent surface matching. Peeling force of PEDOT-C4:DS and PEDOT-C6:DS is stronger than the one of PEDOT:PSS‐CNT composite. GIWAX analysis showed that PEDOT-C4:DS formed the highly ordered edge-on structure and PEDOT-C6:DS formed the bimodal orientation consisting of edge-on structure mainly and face-on structure slightly. The electrical conductivity (σPEDOT‐C4:DS=50.0 S cm−1) of PEDOT-C4:DS is 41.7 times higher than that of PEDOT:PSS (σPEDOT:PSS=1.2 S cm−1). The output signals (maximum voltages/currents) of piezoelectric nanogenerators (PNGs, electrode/PVDF/electrode) using these materials as electrodes are PNG-1 (PEDOT:PSS‐CNT composite) 1.25 V/128.5 nA, PNG-2 (PEDOT-C4:DS) 1.54 V/166.0 nA, and PNG-3 (PEDOT-C6:DS) 1.49 V/159.0 nA. Of these, PNG-2 & PNG-3 show maximum piezoelectric output power of 63.0 nW and 59.9 nW at 9 MΩ compared to PNG-1 (41.0 nW at 10 MΩ). They are enhanced up to 53.7%. The excellent surface matching between a piezoelectric active material and an electrode material leads to high output power.

TiNbO5 (TNO) thin films were deposited by electrophoresis at room temperature by using TNO− nanosheets. These TNO films exhibited a large (001) interplanar distance (1.18 nm) owing to the presence of TBA+ between the TNO layers. The TBA+, which were used to synthesize the TNO− nanosheets, were removed from the TNO film after annealing at 600 °C. Two types of structures were developed in the film annealed at 600 °C: type-1 and type-2, which revealed (001) interplanar distances of 0.52 and 0.71 nm, respectively. The TNO film annealed at 600 °C showed a dielectric constant of 48.5, low dielectric loss (0.02), and small leakage current density of 4.16 × 10−7 A/cm2 at 0.6 MV/cm. The dielectric properties were stable with respect to the film thickness and the applied electric field; the dielectric and insulation properties were maintained up to 300 °C. Therefore, TNO films are good candidates for high-temperature capacitors.

Conventional artificial muscles induce bending by aligning large-sized ions within the electrolyte upon bias application. Such design, alike many other actuator types, suffer from volatile actuation where the actuated position gets lost upon switch-off. Here, we develop a non-volatile artificial muscle with ion insertion electrode materials. Upon bias application, the inserted ions pose stress on the electrodes that sustain even after power shut-off. The demonstrated actuator consists of lithium germanide (LixGe) thin films deposited on both sides of a flexible polyimide (PI) substrate. The device exhibits 35.2 mm displacement when operated at 2 V and generates the blocking force of 0.67 mN. The observed stress and volume expansion reach 248 MPa and 8.2% for the 284 nm Li3Ge thin films, respectively. The actuated position is maintained against gravity with 12.1% decay in the actuated distance after 10 min. The novel actuator type proves the potential use of lithium insertion materials as actuation materials and shows that non-volatile actuation can be realized with ion-insertion electrodes.

For a type-1 piezoelectric energy harvester (PEH), in which stress develops in the supporting system of the piezoelectric materials, the electromechanical coupling factor (kij) of the piezoelectric materials is important for the output power at the resonance frequency. Therefore, soft-piezoelectric materials are good candidates for these PEHs. For type-2 PEHs, in which stress develops in the piezoelectric material and supporting system, the figure of merit (FOM) of the output power at the resonance frequency is (kij2 × Qm)/s11E, where Qm and s11E are the mechanical quality factor and the elastic compliance of piezoelectric materials, respectively. In particular, the effect of Qm is very large for these PEHs, indicating that hard-piezoelectric materials are good candidates for type-2 PEHs operating at the resonance frequency. For both type-1 and type-2 PEHs operating at off-resonance frequency, the kij2 × dij × gij is the FOM of the output power of the PEHs, where gij is a piezoelectric voltage constant. Therefore, soft-piezoelectric materials are also good candidates for both type-1 and type-2 PEHs operating at the off-resonance frequency.

In order to develop high performance chemoresistive gas sensors for Internet of Everything applications, low power consumption should be achieved due to the limited battery capacity of portable devices. One of the most efficient ways to reduce power consumption is to lower the operating temperature to room temperature. Herein, we report superior gas sensing properties of SnS2 nanograins on SiO2 nanorods toward NO2 at room temperature. The gas response is as high as 701% for 10 ppm of NO2 with excellent recovery characteristics and the theoretical detection limit is evaluated to be 408.9 ppb at room temperature, which has not been reported for SnS2-based gas sensors to the best of our knowledge. The SnS2 nanograins on the template used in this study have excessive sulfur component (Sn:S = 1:2.33) and exhibit p-type conduction behavior. These results will provide a new perspective of nanostructured two-dimensional materials for gas sensor applications on demand.

Sound energy harvesting is one of the promising technologies due to the abundant and clean sound sources. It can be the semi-permanent alternative power supplies for wireless sensor networks (WSNs), which is significant in the Internet of Things (IoT). However, sound waves have the low energy density, so there are many kinds of research in recent years to overcome this problem. This paper provides a comprehensive review of sound energy harvesting, focusing on presenting principles, examples and enhancement methods of sound energy harvesters. In this paper, various approaches are introduced which are classified as sound pressure amplification and transduction mechanism. For sound pressure amplification, two typical types of energy harvesters are presented that one is using a resonator, another one is using an acoustic metamaterial, and these are based on piezoelectric, electromagnetic, and triboelectric mechanisms to convert sound energy to electrical energy.

The breath gas analysis through gas phase chemical analysis draws attention in terms of non-invasive and real time monitoring. The array-type sensors are one of the diagnostic methods with high sensitivity and selectivity towards the target gases. Herein, we presented a 2 × 4 sensor array with a micro-heater and ceramic chip. The device is designed in a small size for portability, including the internal eight-channel sensor array. In2O3 NRs and WO3 NRs manufactured through the E-beam evaporator’s glancing angle method were used as sensing materials. Pt, Pd, and Au metal catalysts were decorated for each channel to enhance functionality. The sensor array was measured for the exhaled gas biomarkers CH3COCH3, NO2, and H2S to confirm the respiratory diagnostic performance. Through this operation, the theoretical detection limit was calculated as 1.48 ppb for CH3COCH3, 1.9 ppt for NO2, and 2.47 ppb for H2S. This excellent detection performance indicates that our sensor array detected the CH3COCH3, NO2, and H2S as biomarkers, applying to the breath gas analysis. Our results showed the high potential of the gas sensor array as a non-invasive diagnostic tool that enables real-time monitoring.

A novel transient electronics technology that is capable of completely dissolving or decomposing in certain conditions after a period of operation offers unprecedented opportunities for medical implants, environmental sensors, and other applications. Here, we describe a biodegradable, flexible silicon-based electronic system that detects NO species with a record-breaking sensitivity of 136 Rs (5 ppm, NO2) and 100-fold selectivity for NO species over other substances with a fast response (~30 s) and recovery (~60 s). The exceptional features primarily depend on not only materials, dimensions, and design layouts but also temperatures and electrical operations. Large-scale sensor arrays in a mechanically pliable configuration exhibit negligible deterioration in performance under various modes of applied loads, consistent with mechanics modeling. In vitro evaluations demonstrate the capability and stability of integrated NOx devices in severe wet environments for biomedical applications. Degradable devices that monitor health by measuring gases emitted by the body have been developed by researchers in South Korea, the UK and the USA. Biodegradable electronics, devices that decay naturally over time, are potentially useful for medical devices. They can be used internally as implants that don’t need to be surgically removed or externally as wearable health monitors. Chong-Yun Kang and Suk-Won Hwang from Korea University in Seoul and their colleagues have developed a biodegradable, flexible silicon nanomembrane to detect nitric oxide with a high sensitivity andselectivity and a fast response time. Nitric oxide sensors are important for health monitoring as the gas is associated with many physiological processes including regulation of the vascular system and blood flow. In vitro experiments in aqueous solutions demonstrated the feasibility of their devices for disposable medical implants. Soft, transient silicon-based gas sensing system capable of detecting nitrogen oxides with remarkable sensitivity and selectivity is presented in this report. The results provide materials, device layouts, manufacturing process, and theorectical modeling illlustrating the capabilities and operational aspects. In vitro experiments demonstrate the possibilities for disposable environmental monitors and temporary biomedical implants.

Piezoelectric energy harvesters (PEHs) aim to generate sufficient power to operate targeting device from the limited ambient energy. PEH includes mechanical-to-mechanical, mechanical-to-electrical, and electrical-to-electrical energy conversions, which are related to PEH structures, materials, and circuits, respectively; these should be efficient for increasing the total power. This critical review focuses on PEH structures and materials associated with the two major energy conversions to improve PEH performance. First, the resonance tuning mechanisms for PEH structures maintaining continuous resonance, regardless of a change in the vibration frequency, are presented. Based on the manual tuning technique, the electrically- and mechanically-driven self-resonance tuning (SRT) techniques are introduced in detail. The representative SRT harvesters are summarized in terms of tunability, power consumption, and net power. Second, the figure-of-merits of the piezoelectric materials for output power are summarized based on the operating conditions, and optimal piezoelectric materials are suggested. Piezoelectric materials with large kij , dij , and gij values are suitable for most PEHs, whereas those with large kij and Qm values should be used for on-resonance conditions, wherein the mechanical energy is directly supplied to the piezoelectric material. This comprehensive review provides insights for designing efficient structures and selection of proper piezoelectric materials for PEHs.

The fluoride-based electrostrictive terpolymers are attractive in electromechanical applications. To obtain high electromechanical performance, the terpolymers are hybridized with various fillers such as carbon materials. However, the previous hybrid films have been fabricated with thickness of 20–100 μm due to poor dispersion of the fillers, indicating that these electrostrictive films require high driving voltages of more than 200 V. Herein, we have demonstrated the electrostrictive P(VDF-TrFE-CTFE) thin film hybridized with highly dispersed carbon blacks (CB). The CBs were chemically oxidized to improve the dispersion in the polymer matrix, thus leading to a successful fabrication of the oxidized CB/P(VDF-TrFE-CTFE) hybrid films with 8 μm thickness using solution casting method. The P(VDF-TrFE-CTFE) thin film with 2.75 wt% oxidized CB shows 1.6 fold increased dielectric constant and maximum polarization with low loss factor compared to the pure terpolymer. These enhancements of the 8 μm thick hybrid film enable to yield useful mechanical output at low driving voltages below 100 V. To evaluate the electromechanical performance of hybrid thin films, a unimorph cantilever was fabricated. With a low applied voltage of 90 V, the cantilever based on P(VDF-TrFE-CTFE) thin film with 2.75 wt% oxidized CB produces a displacement twice as high as that of the pure terpolymer. These results provide the first feasibility study of electrostrictive composites for practical applications, particularly human-related applications requiring a low driving voltage.

Vertically aligned and dense InGaAs nanowires were grown on Si (111) substrates by Au-assisted molecular beam epitaxy, and their antireflection characteristics were studied. The bandgap of InGaAs nanowires was tuned to be about 1.0 eV by adjusting the In to Ga ratio. The grown nanowires were vertically aligned with a diameter of ~ 20 nm near the top and ~ 44 nm at the bottom, with a slightly tapered structure. This tapered nanostructure was formed due to the different surface diffusivities and affinities of In and Ga to the Au catalyst. The grown InGaAs nanowires have no significant stacking, kinking, and bending defects. High-resolution transmission electron microscopy study showed that the grown InGaAs nanowires have a pure wurtzite single crystalline structure with the maximum length of ~ 18 µm. Photo-reflectometry measurement showed a significant reduction in the reflectance less than ~ 5% at normal incidence in the wavelength range of 200–1700 nm. In addition, spectroscopic ellipsometry study showed a reduced reflectance at various incident angles of 30–70° in the wavelength range of 200–1100 nm. These optical investigations demonstrate the antireflection characteristics of the InGaAs nanowires. Furthermore, piezoelectric responses were collected from the top of the vertically standing InGaAs nanowires at five different points using piezoelectric force microscopy. The measured area for one point was about 50 nm × 50 nm, and the piezoelectric responses of one or two InGaAs nanowires per point were expected to be measured, as the growth direction was along with the polar c -axis [0001] direction.

In this paper, we present the design, fabrication, and performance evaluation of a polyimide-based flexible neural probe for the precise site stimulation and recording in the deep brain. The probe consists of five electrodes: one for stimulation, another for ground and the other three for recording electrodes. This probe is designed to be foldable, enabling easy insertion into the deep brain via temporary tungsten guide sticks. Because of its small cross-sectional area and the flexibility of the polyimide, the probe causes minimum damage to the neural tissue and does not show any evidence of serious immune reactions such as high density of macrophage or microglia. Around the simulation electrodes, an additional ground electrode prevents the stimulation of the undesired sites in the brain. To ensure we stimulate the target point specifically, for instance STh in this study, we confirm through both finite element analyses and in vitro tests. With the additional ground electrodes, we observe the leakage power decreased by about 80%. To check the performance of the probe, we demonstrate animal experiments using rats, and neural spike signals from STh in the 7-mm deep brain are successfully recorded after implantation.

The utilization of edge sites in two-dimensional materials including transition-metal dichalcogenides (TMDs) is an effective strategy to realize high-performance gas sensors because of their high catalytic activity. Herein, we demonstrate a facile strategy to synthesize the numerous edge sites of vertically aligned MoS2 and larger surface area via SiO2 nanorod (NRs) platforms for highly sensitive NO2 gas sensor. The SiO2 NRs encapsulated by MoS2 film with numerous edge sites and partially vertical-aligned regions synthesized using simple thermolysis process of [(NH4)2MoS4]. Especially, the vertically aligned MoS2 prepared on 500 nm thick SiO2 NRs (500MoS2) shows approximately 90 times higher gas-sensing response to 50 ppm NO2 at room temperature than the MoS2 film prepared on flat SiO2, and the theoretical detection limit is as low as ∼2.3 ppb. Additionally, it shows reliable operation with reversible response to NO2 gas without degradation at an operating temperature of 100 °C. The use of the proposed fa...

A K2-mNb2O6-m/2 single crystal with a pyrochlore phase formed when the Nb2O5 + x mol% KOH specimens with 0.6 ≤ x ≤ 1.2 were solvothermally heated at 230 °C for 24 h. They have an octahedral shape with a size of 100 μm, and the composition of this single crystal is close to K1.3Nb2O5.65. The single-crystal KNbO3 formed when the single-crystal K2-mNb2O6-m/2 was annealed at a temperature between 600 °C and 800 °C with K2CO3 powders. When annealing was conducted at 600 °C (or with a small amount of K2CO3), the KNbO3 single crystal has a rhombohedral structure that is stable at low temperatures (< − 10 °C). The formation of the rhombohedral KNbO3 structure can be explained by the presence of the K+ vacancies in the specimen. The KNbO3 single crystal with an orthorhombic structure formed when the K2-mNb2O6-m/2 single crystal was annealed at 800 °C with 20 wt% of K2CO3.

Recent research efforts show that piezoelectric polymers such as PVDF are competitive alternatives to the conventional piezoelectric ceramics. While possessing extraordinary toughness and fatigue resistance, however, piezoelectric polymers suffer from limited applications due to a large amount of deformation required for high power output. Here, we design and demonstrate a PVDF-based, high-power piezoelectric module installed on a local highway, for the first time. The module contains a bridge-type displacement amplification capability, demonstrating the 2.5 mm vertical displacement converted into 13 mm horizontal deformation, suitable for the uninterrupted driving experience. We provide the design guidelines and optimization strategies for the module, in terms of the piezoelectric power output. With 80 bimorph-shaped energy harvesters, the module achieves up to 16.5 W/m2 energy density when the test vehicle passes by at 80 km/h. The matching impedance decreases with vehicle speed, suggesting that different matching impedance should be used for highway and local roadways. The output power exhibits a linear relation to the vehicle speed and weight, implying the module’s potential application as a self-powered speed sensor. The study demonstrates that PVDF-based energy harvesters provide a competitive power output at small vertical displacements with relevant module design, making the tough piezoelectric materials suitable for efficient and durable roadway energy harvesting.

Hollow-structured nanomaterials are presented as an outstanding sensing platform because of their unique combination of high porosity in both the micro- and nanoscale, their biocompatibility, and flexible template applicability. Herein, we introduce a bacterial skeleton method allowing for cost-effective fabrication with nanoscale precision. As a proof-of-concept, we fabricated a hollow SnO2 hemipill network (HSHN) and a hollow Pt-functionalized SnO2 hemipill network (HPN). A superior detecting capability of HPN toward acetone, a diabetes biomarker, was demonstrated at low concentration (200 ppb) under high humidity (RH 80%). The detection limit reaches 3.6 ppb, a level satisfying the minimum requirement for diabetes breath diagnosis. High selectivity of the HPN sensor against C6H6, C7H8, CO, and NO vapors is demonstrated using principal component analysis (PCA), suggesting new applications of HPN for human-activity monitoring and a personal healthcare tool for diagnosing diabetes. The skeleton method can...

We present a simple and facile method for producing high-performance hydrogen (H 2 ) sensors based on vertically ordered metal-oxide nanorods with a Pd films on a 4-inch SiO 2 /Si substrate by a glancing-angle deposition. Firstly, optimal density of nanorods was formed by changing an incident angle of vapor flux. Secondly, nanogaps between each nanorod were precisely controlled by manipulating thickness of Pd films. At room temperature in ambient air, 15-nm-thick Pd-coated SiO 2 nanorods showed the rapid on-off switches. The average response time was approximately 2.8 s (the longest response time: 5 s), and the recovery time was less than 1 s for 2%–0.8% H 2 . For 20-nm-thick Pd-coated SiO 2 nanorods, detection of limit was reduced to 10 ppm due to semi-on-off operation. The reproducibility of our approaches was investigated by fabricating the Pd-coated SnO 2 nanorods. They also exhibited the high H 2 sensing performance as Pd-coated SiO 2 nanorods. We strongly believe that high H 2 sensing performance of Pd nanogap controlled metal oxide nanorods provides a new perspective for room-temperature H 2 switches and sensors based on H 2 -induced lattice expansion.

Research on two-dimensional (2D) metal dichalcogenides is rapidly expanding owing to their unique characteristics that do not exist in bulk materials. The industrially compatible development of these emerging materials is indispensable to facilitate the transition of 2D metal dichalcogenides from the research stage to the practical industrial application stage. However, an industrially relevant method, i.e., the low-temperature synthesis of wafer-scale, continuous, and orientation-controlled 2D metal dichalcogenides, still remains a significant challenge. Here, we report the low-temperature (≤350 °C) synthesis of uniform and continuous n-type SnS2 thin films via the combination of atomic layer deposition (ALD) of tin oxides and subsequent sulfurization. Well-crystallized and aligned SnS2 layers parallel to the substrate are demonstrated through the phase engineering of the ALD-grown tin oxide and the substrate surface. The additional H2S plasma treatment at 300 °C leads to the formation of stoichiometric SnS2. The formation of conformal SnS2 layers over a three-dimensional undulating hole structure is confirmed, which reveals the potential for applications beyond the planar structured architecture. The present results could be a step toward the realization of 2D metal dichalcogenides in industry.

We report on InGaZnO (IGZO) thin film transistors (TFTs)-based bio-chemical sensors which can detect the chemical/biological species. As novel sensing platform, the IGZO TFT with Ag nanowire (NW) mesh showed pronounced output voltage changes responding to all analytes of H 2 O 2 , b- d -glucose, d -glucono-1,5-Lactione, and lactic acid, which are reproducible and reversible. Herein, porous Ag NW-functionalized top gate electrode plays a major role in sensing platform for enhanced sensing capability in aqueous medium. Moreover, these top gate geometry serve as a stable backplane for electrical modulation. As a result, analytes solutions become acidic or basic and such pH alterations induce significant turn-on voltage shifts on our devices. For implementation of a resistive load inverter, the output sensing voltage signals can be directly extracted, and such signals are reproducible and reversible. The proposed IGZO TFTs with Ag NW mesh top gate electrode based sensing platform pave the way for development of portable and reusable real-time non-destructive label-free chemical/biological sensors.

Plasmonic nanocrystals, which exhibit extraordinary optical properties, are challenging to grow in selective positions with a cost-effective and high-throughput process. We demonstrate that plasmonic isotropic gold nanospheres (AuNSs) can be selectively synthesized on wafer-scale rigid and flexible substrates at room temperature by laser irradiation. First, we prepare gold nanocolumn (AuNC) thin films on sapphire and polydimethylsiloxane substrates with glancing angle deposition (GAD). Then, a KrF excimer laser is exposed at selected positions with a 24 ns pulse duration. Finally, highly isotropic AuNSs as plasmonic nanocrystals are synthesized at the targeted positions. We suggest that the formation of such isotropic AuNSs is caused by reshaping from the top of the AuNCs; this is verified by the temperature distribution in the AuNCs during laser irradiation through finite element method simulations. We further investigate the formation of AuNSs by varying the laser energy density and the kind of substrate. By using a simple mask process, we demonstrate patterning of the letters “KIST” via selectively grown AuNSs on a flexible substrate. The simple laser irradiation process on GAD-grown metal NC thin films is expected to be a promising method for scalable synthesis of plasmonic isotropic NSs at targeted positions with a rapid process and at room temperature.

Modern artificial neural network technology using a deterministic computing framework is faced with a critical challenge in dealing with massive data that are largely unstructured and ambiguous. This challenge demands the advances of an elementary physical device for tackling these uncertainties. Here, we designed and fabricated a SiOx nanorod memristive device by employing the glancing angle deposition (GLAD) technique, suggesting a controllable stochastic artificial neuron that can mimic the fundamental integrate-and-fire signaling and stochastic dynamics of a biological neuron. The nanorod structure provides the random distribution of multiple nanopores all across the active area, capable of forming a multitude of Si filaments at many SiOx nanorod edges after the electromigration process, leading to a stochastic switching event with very high dynamic range (≈5.15 × 1010 ) and low energy (≈4.06 pJ). Different probabilistic activation (ProbAct) functions in a sigmoid form are implemented, showing its controllability with low variation by manufacturing and electrical programming schemes. Furthermore, as an application prospect, based on the suggested memristive neuron, we demonstrated the self-resting neural operation with the local circuit configuration and revealed probabilistic Bayesian inferences for genetic regulatory networks with low normalized mean squared errors (≈2.41 × 10-2 ) and its robustness to the ProbAct variation.

We demonstrate a microscale vibration energy harvester exhibiting an ultra-low resonance frequency and high power density. A spiral shaped microelectromechanical system (MEMS) energy harvester was designed to harvest ambient vibrations at a low frequency ( $\mu \text{m}$ -thickness exhibiting remanent polarization of 36.2 $\mu \text{C}$ /cm2 and longitudinal piezoelectric constant of 155 pm/V was synthesized to achieve high efficiency mechanical to electrical conversion. The experimental results demonstrate an ultra-low natural frequency of 48 Hz for MEMS harvester. This is one of the lowest resonance frequency reported for the piezoelectric MEMS energy harvester. Further, the position of the natural frequency was controlled by modulating the number of spiral turns and weight of the proof mass. The vibration mode shape and stress distribution were validated through a finite element analysis. The maximum output power of 23.3 nW was obtained from the five turns spiral MEMS energy harvester excited at 0.25 g acceleration and 68Hz. The normalized area and the volumetric energy density were measured to be $5.04\times 10^{-4}~ \mu \text{W}$ /mm $^{2}~\cdot ~\text{g}^{2}~\cdot$ Hz and $4.92\times 10^{-2} ~ \mu \text{W}$ /mm $^{3}~\cdot ~\text{g}^{2}~\cdot$ Hz, respectively. [2017-0018]

Amorphous KNbO3 (KN) film containing KN nanocrystals was grown on TiN/SiO2/Si substrate at 350 °C. This KN film showed a dielectric constant (er) and a piezoelectric strain constant (d33) of 43 and 80 pm/V at 10 V, respectively, owing to the existence of KN nanocrystals. Piezoelectric nanogenerators (PNGs) were fabricated using KN films grown on the TiN/polyimide/poly(ethylene terephthalate) substrates. The PNG fabricated with the KN film grown at 350 °C showed an open-circuit output voltage of 2.5 V and a short-circuit current of 70 nA. The KN film grown at 350 °C exhibited a bipolar resistive switching behavior with good reliability characteristics that can be explained by the formation and rupture of the oxygen vacancy filaments. The KN resistive random access memory device powered by the KN PNG also showed promising resistive switching behavior. Moreover, the KN film shows good biocompatibility. Therefore, the KN film can be used for self-powered biomedical devices.

The piezoelectric properties of (1-x-y)PbZrO3-xPbTiO3-yPb(Ni1/3Nb2/3)O3 ceramics were investigated. Specimens with a large Pb(Ni1/3Nb2/3)O3 content, which have compositions close to the triple point, show small g33 and d33 × g33 values because of their large eT33/e0. These values increased with a decrease in y (amount of Pb(Ni1/3Nb2/3)O3) and the specimen with x = 0.39 and y = 0.29 showed the largest g33 of 43 × 10−3 V·m/N and d33 × g33 of 25.2 × 10−12 m2/N. Cantilever-type energy harvesters were fabricated using specimens with 0.38 ≤ x ≤ 0.41 and y = 0.29. The output power densities of the energy harvesters were related to the d31 × g31 × k312 value of the piezoelectric ceramics. The energy harvester fabricated using a specimen with x = 0.39 and y = 0.29, which has a maximum d31 × g31 × k312 value, showed the maximum output power density of 1.01 mW/cm3.

Two-dimensional (2-D) metal chalcogenides have received great attention because of their unique properties, which are different from bulk materials. A challenge in implementing 2-D metal chalcogenides in emerging devices is to prepare a well-crystallized layer over large areas at temperatures compatible with current fabrication processes. Tin monosulfide, a p-type layered semiconductor with a high hole mobility, is a promising candidate for realizing large-area growth at low temperatures because of its low melting point. However, tin sulfides exist in two notable crystalline phases, SnS and SnS2. Therefore, it is imperative to control the oxidation state of Sn to achieve a pure SnS film. Here, the synthesis of SnS thin films by atomic-layer-deposition (ALD) is demonstrated using bis(1-dimethylamino-2-methyl-2-propoxy)tin(II) and H2S as Sn and S sources, respectively, over a wide temperature window (90–240 °C). Impurities such as carbon, oxygen, and nitrogen were negligibly detected. The morphological evol...

We report the highly sensitive and selective downsized gas sensors for the IoT application. Sensing areas of Pt-interdigitated electrodes (IDEs) were varied to investigate the relation between sensing materials and electrodes. In2O3 nanocolumns were deposited on the pre-patterned Pt IDEs using glancing angle deposition (GLAD). The effect of the interface resistance between electrodes and sensing materials, and the intergrain resistance between nanocolumns is analyzed by linear regression at different sensing area and incident angle of GLAD. In2O3 (angle: 85°, sensing area: 0.3 mm × 0.3 mm) nanocolumns with double Schottky barriers show the highest response and selectivity with fast response time of 10 s to VOCs among the samples fabricated in this study. The analysis reveals that the intrinsic response of In2O3 (angle: 85°, sensing area: 0.3 mm × 0.3 mm) nanocolumns are dominantly affected by intergrain resistance, resulting in a high response. Our demonstration for the fundamental aspect of downsizing gas sensor makes an important contribution to the chemical sensor field with broad interest.

Rhombohedral 0.69Pb(Zr0.47Ti0.53)-0.31Pb(Zn0.6Ni0.4)NbO3 (PZT-PZNN) ceramics were textured using 10.0 vol. % BaTiO3 (BT) platelets along the direction at 950°C with a high Lotgering factor of 95.3%. BT platelets did not react with the PZT-PZNN ceramics, and the textured PZT-PZNN ceramic had a tetragonal structure. The PZT-PZNN ceramics exhibited a strain of 0.174% with a piezoelectric strain constant (d*33) of 580 pC/N at 3.0 kV/mm. The textured PZT-PZNN ceramic showed an increased strain of 0.276% and d*33 of 920 pC/N at 3.0 kV/mm, which can be explained by the domain rotation. However, the d33 values of the textured specimens are smaller than those of the untextured specimens because of the small remanent polarization and relative dielectric constant of BT platelets. The textured PZT-PZNN ceramic synthesized in this work can be used for piezoelectric multilayer actuators because of its large strain and low sintering temperature.

Piezoelectric knocking sensors with a dense microstructure were fabricated at 960°C for 2 hours using various CuO-added (Na0.5K0.5)NbO3 (NKN)-based piezoelectric ceramics. The practical sensitivity (SP) of the knocking sensor, which is the ability to detect the knocking of a car engine, was influenced by the g33 × kp value of the piezoelectric ceramics, indicating that the g33 × kp can be considered a figure of merit of the piezoelectric ceramics used in the knocking sensor. The knocking sensor synthesized using the CuO-added 0.95(Na0.5K0.5)(Nb0.95Sb0.05)O3–0.05CaTiO3 (CNKNS–CT) ceramic, which showed a g33 of 25.7 Vm/N and kp of 0.46, exhibited a high SP of 119 mV/g at the resonance frequency. The SP of the commercial knocking sensor, which was synthesized using the Pb(Zr,Ti)O3 (PZT)-based ceramic, was 112 mV/g at the resonance frequency. Hence, the knocking sensor fabricated using the CNKNS–CT piezoelectric ceramic can be used to replace the commercial PZT-based knocking sensor.

Interest in energy harvesters has grown rapidly over the last decade. The research effort in large-scale energy harvesting has mainly focused on piezoelectric ceramic based devices, due to its high piezoelectric constants. In this study, we demonstrate a piezoelectric energy harvester module based on polyvinylidene fluoride (PVDF) polymer for roadway applications. Flexible energy harvesters are fabricated with PVDF films and it exhibited stable performance and durability over the repeated number of bending cycles. In order to structurally optimize the design, finite element analysis was performed on two possible module configuration, with detailed input conditions on how the flexible energy harvester must be bent. A piezoelectric energy harvester module is then constructed with the fabricated unit energy harvesters inserted in the vertical direction, with initial radii of curvature as high as possible. The module was tested with a model mobile load system (MMLS3) and exhibited up to 200 mW instantaneous power output across a 40 kΩ resistor. The power output scaled linearly with the number of parallel connected harvesters. The calculated power density at this impedance reaches up to 8.9 W/m2, suggesting that the flexible energy harvesters based on the piezoelectric polymers may provide energy density as high as those based on piezoelectric ceramics.

Touch screens have become an inherent part of the user interface in many electronics applications such as smartphones. The two types of developed touch sensors, the resistive and capacitive sensing devices, may face several difficulties when applied to flexible device applications such as touch signals arising from bending motions. In this study, we assess the feasibility of flexible touch sensors based on piezoelectric PbZr0.52Ti0.48O3 (PZT) thin films. Piezoelectric ceramic based flexible touch sensors possess unique advantages including scalable fabrication, fast response time, durability, and being self-powered. A demonstration device has been fabricated with a sandwich structure consisting of Pt electrode/functional PZT/Pt electrode/flexible substrate structure using laser lift-off (LLO) method. In order to anneal the functional PZT layer at high temperature (600 °C), the device was first fabricated on the sapphire substrate and transferred via melting sacrificial PZT layer with an excimer laser. We demonstrate the detection of x- and y-axis touch location via piezoelectric materials and confirm that the flexible piezoelectric touch sensors can distinguish between touch-induced and bending-induced signals via signal location, signal shape, and duration time. A notable feature of this fabrication technique involves its possibility to be fabricated in high resolution. This device may potentially achieve high resolution with suitable fabrication techniques, thus, providing the possibility for the next generation touch sensors.

We fabricated polycrystalline yttrium-iron-garnet (Y3Fe5O12, YIG) samples using the sol-gel synthesis method to develop an energy harvesting material based on the spin Seebeck effect. We confirmed that crystallization occurred during calcination at 850 °C and that only the polycrystalline YIG structure was formed. We found that a sintering process at 1400 °C not only increased the size of the YIG particles and the densification of their microstructure but also enhanced their saturation magnetization (Ms) and dramatically reduced their coercivity (Hc). A mechanical pressing process was carried out between the calcination and sintering treatments to prepare a free-standing YIG fillet sample. We found that Ms was enhanced by almost three times without an associated reduction in Hc. We found that mechanical pressing assists the oxidation reaction during the subsequent sintering process by increasing the surface energy and densification before the heat treatment.

During the synthesis of Bi2Ge3O9 ceramics using Bi2O3 + 3GeO2 powders, the Bi4Ge3O12 phase was formed at low temperature (≤800 °C). Bi4Ge3O12 preferentially adopted GeO2-excess phase, and this phase was consistently present in the sintered Bi2Ge3O9 ceramic as a secondary phase. Therefore, Bi4Ge3O12 powder was first calcined and subsequently reacted with GeO2 powder to obtain the pure Bi2Ge3O9 ceramic through the following reaction: 1/2Bi4Ge3O12 + 3/2GeO2 → Bi2Ge3O9. Formation of the Bi2Ge3O9 phase was initiated at temperature of 850 °C. The pure Bi2Ge3O9 ceramic sintered at 875 °C for 8 h had a dense microstructure with an average grain size of 2.7 μm. Furthermore, the pure Bi2Ge3O9 ceramic exhibited promising microwave dielectric properties for the advanced ceramic substrate: er = 9.7, Q × f = 48,573 GHz and τf = −29.5 ppm/°C.

Dense Bi 4 Ge 3 O 12 ceramics sintered at 850 °C had a relative permittivity ( e r ) of 15.7, temperature coefficient of resonant frequency ( τ f ) of −24.2 ppm/°C, and quality factor ( Q × f ) of 28,361 GHz. However, because of the existence of Bi 12 GeO 20 as a secondary phase, the microwave dielectric properties of Bi 4 Ge 3 O 12 would be degraded. Therefore, to avoid the formation of the Bi 12 GeO 20 secondary phase, Bi 2 O 3 -deficient Bi 4- x Ge 3 O 12-1.5 x ceramics with 0.1≤ x ≤0.6 were sintered at 850 °C. A single phase of Bi 3.6 Ge 3 O 11.4 ( x =0.4) ceramic sintered at 850 °C for 5 h without any secondary phase exhibited suitable microwave dielectric properties for a ceramic substrate: e r =14.9, τ f =−9.5 ppm/°C, and Q × f =53,277 GHz.

Noble metal oxides, such as RuO2, have received attention as capacitor electrodes in dynamic random access memories (DRAMs). Noble metal oxides generally have a high work function compared to noble metals and enhance the crystallinity of dielectric materials grown on them, resulting in a lower leakage current and higher dielectric constants. Despite these advantages, noble metal oxides are easily reduced during the dielectric film, such as TiO2, growth on top or by annealing under a forming gas atmosphere, degrading the capacitor performance. In this work, Ta-doped SnO2 is suggested as a potential capacitor electrode for DRAMs. Ta-Doped SnO2 films have a high work function, comparable to that of RuO2, and induce the formation of a high-temperature phase with a high dielectric constant, namely rutile TiO2, at low temperatures. More importantly, the Ta-doped SnO2 films show suitable structural and chemical stabilities, even after annealing at 400 °C under a forming gas atmosphere. RuO2 films, on the other hand, turn into a mixture of RuO2 and Ru after annealing under the same conditions. These findings suggest that Ta-doped SnO2 could serve as capacitor electrodes in next-generation DRAMs.

Two-dimensional (2D) metal dichalcogenides have drawn considerable interest because they offer possibilities for the implementation of emerging electronics. The emerging electronics are moving toward two major directions: vertical expansion of device space and flexibility. However, the development of a synthesis method for 2D metal dichalcogenides that meets all the requirements remains a significant challenge. Here, we propose a promising method for wafer-scale, conformal, and low-temperature (≤240 °C) synthesis of single-phase SnS2 via the atomic layer deposition technique. There is a trade-off relationship between the crystallinity and orientation preference of SnS2, which is efficiently eliminated by the two-step growth occurring at different temperatures. Consequently, the van der Waals layers of the highly crystalline SnS2 are parallel to the substrate. Thin-film transistors (TFTs) comprising the SnS2 layer show reasonable electrical performances (field-effect mobility: ∼0.8 cm2 V-1 s-1 and on/off ratio: ∼106), which are comparable to that of a single-crystal SnS2 flake. Moreover, we demonstrate nonplanar and flexible TFTs to identify the feasibility of the implementation of future electronics. Both the diagonal-structured TFT and flexible TFT fabricated without a transfer process show electrical performances comparable to those of rigid and planar TFTs. Particularly, the flexible TFT does not exhibit substantial degradation even after 2000 bending cycles. Our work would provide decisive opportunities for the implementation of future electronic devices utilizing 2D metal chalcogenides.

The stress inevitably imposed during electrochemical reactions is expected to fundamentally affect the electrochemistry, phase behavior and morphology of electrodes in service. Here, we show a strong stress-composition coupling in lithium binary alloys during the lithiation of tin-tin oxide core-shell nanoparticles. Using in situ graphene liquid cell electron microscopy imaging, we visualise the generation of a non-uniform composition field in the nanoparticles during lithiation. Stress models based on density functional theory calculations show that the composition gradient is proportional to the applied stress. Based on this coupling, we demonstrate that we can directionally control the lithium distribution by applying different stresses to lithium alloy materials. Our results provide insights into stress-lithium electrochemistry coupling at the nanoscale and suggest potential applications of lithium alloy nanoparticles., Electrochemical reactions can generate stresses that detrimentally affect battery electrodes. Here, the authors directly image the lithiation of core-shell tin-based nanoparticles and show that lithium can be redistributed by applying stress and change the nanoparticle composition.

The development of high performance gas sensors that operate at room temperature has attracted considerable attention. Unfortunately, the conventional mechanism of chemiresistive sensors is restricted at room temperature by insufficient reaction energy with target molecules. Herein, novel strategy for room temperature gas sensors is reported using an ionic-activated sensing mechanism. The investigation reveals that a hydroxide layer is developed by the applied voltages on the SnO2 surface in the presence of humidity, leading to increased electrical conductivity. Surprisingly, the experimental results indicate ideal sensing behavior at room temperature for NO2 detection with sub-parts-per-trillion (132.3 ppt) detection and fast recovery (25.7 s) to 5 ppm NO2 under humid conditions. The ionic-activated sensing mechanism is proposed as a cascade process involving the formation of ionic conduction, reaction with a target gas, and demonstrates the novelty of the approach. It is believed that the results presented will open new pathways as a promising method for room temperature gas sensors.

Morphological evolution accompanying a surface roughening and preferred orientation is an effective way to realize a high-performance gas sensor because of its significant potential as a chemical catalyst through chemical potentials and atomic energy states. In this work, we investigated a heterojunction of double-side-W-decorated NiO nanoigloos fabricated through radio frequency sputtering and a soft-template method. Interestingly, a morphological evolution characterized by a pyramidal rough surface and the preferred orientation of the (111) plane was observed upon decorating the bare NiO nanoigloos with W. The underlying mechanism of the morphological evolution was precisely demonstrated based on the van der Drift competitive growth model originating from the oxygen transport and chemical strain in the lattice. The gas sensing properties of W-decorated NiO show an excellent NO2 response and selectivity when compared to other gases. In addition, high response stability was evaluated under interference ga...

Semiconducting gas sensors that operated at room-temperature (around 25 °C) have been studied to monitor target gases. Furthermore, a self-powered gas sensor with no reliance on a battery or external power source was highly demanded to effectively reduce the power consumption and apply for practical environments. In this study, we designed the self-powered gas sensors by connecting ionic-activated semiconducting sensors, a promising candidate for room-temperature operation, with an oval-shaped piezoelectric generator, and light-emitting diode alarm. The effect of the divided voltage from the energy generator to the sensor on the sensing performance was analyzed. The self-powered sensor has an extremely low detection limit (4.32 ppb NO2) with rapid and constant recovery (6 s) regardless of the NO2 concentration. Additionally, the practical applicability was confirmed by the light-emitting diode alarms according to the NO2 concentration in the atmosphere.

One of the well-known pathways toward low power consuming chemoresistive gas sensors is the utilization of 2-dimensional materials. Especially, transition metal dichalcogenides (TMDs), which are usually atomically thin semiconductors, have a notable characteristic of their highly reactive edge sites. The edge sites of TMDs having high d-orbital electron density can serve as highly favorable chemically active sites for direct interaction with target gas molecules. In this study, WS2 was synthesized on highly porous SiO2 nanorods template to have numerous edge-exposed WS2 flakes in a limited active area taking advantage of 1-dimensional nanostructures with extremely high surface-to-volume ratio. The fabricated WS2 on 1D nanostructures exhibited a gas response of 151.2 % toward 5 ppm NO2, which has not been reported in performance-wise at room temperature to the best of the author’s knowledge. Density functional theory calculations theoretically supported the highly sensitive and selective NO2 detection with a theoretical detection limit of 13.726 ppb.