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A highly sensitive NO2 gas sensor of p-type SnS operating at room temperature is developed using crosslinked SnS nanoplates self-formed only on SiO2 nanorods, without an additional patterning process.

Atomic layer deposition of MoS2 reveals stepwise growth – ‘stop and go’ growth of MoS2 – behavior where the surface migration energy of the adsorbates is significantly enhanced.

Acoustic energy transfer using ferroelectrically augmented triboelectric receivers can efficiently deliver energy to implantable medical devices, marine cable operation sensors, and electronic devices with electromagnetic interference shielding cases.

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.

Artificial Adaptive and Maladaptive Sensory Receptors In article number 2103484, Chong‐Yun Kang, Jung Ho Yoon, and co‐workers implement artificial adaptive and maladaptive sensory receptors using a diffusive memristor as a biological counterpart of the somatosensory system in all vertebrates. A new class of artificial receptor underlying surfacedominated switching dynamics successfully mimics key parameters of the biological receptor, including short‐latency, selective response, and tunability. [Image: see text]

Nonlinear mono-stable and multi-stable piezoelectric energy harvesters have attracted a lot of attention owing to their broadband frequency spectra and excellent energy harvesting performance. Herein, two types of nonlinear mono-stable, bi-stable, tri-stable, and quad-stable piezoelectric energy harvesters using cantilever structure and magnetic interaction are compared and analyzed. Based on the magnetizing current method, the magnetic force equations are obtained. Calculation results demonstrate that the stability of these harvesters is dependent on the equivalent linear elastic force and the vertical magnetic force. The equilibrium point occurs when the equivalent linear elastic force equals to the vertical magnetic force. The relationship between the number of stable equilibrium points E S and the number of the intersections of the two force curves N I is that E S= (N I + 1)/2. Experiments are carried out to verify the equivalent linear elastic force, vertical magnetic force, and the number of stable equilibrium points of the fabricated prototypes. The experimental results are consistent with the calculated results, which verifies the correctness of the stability mechanism. Moreover, it is found that the stability mechanism is also applicable to the harvesters with more stable equilibrium points, such as penta-stable and hexa-stable harvesters. This work reveals the stability mechanism of nonlinear mono-stable and multi-stable energy harvesters using cantilever structure and magnetic interaction, and provides technical methods for the design of multi-stable energy harvesters.

An innovative autonomous resonance-tuning (ART) energy harvester is reported that utilizes adaptive clamping systems driven by intrinsic mechanical mechanisms without outsourcing additional energy. The adaptive clamping system modulates the natural frequency of the harvester's main beam (MB) by adjusting the clamping position of the MB. The pulling force induced by the resonance vibration of the tuning beam (TB) provides the driving force for operating the adaptive clamp. The ART mechanism is possible by matching the natural frequencies of the TB and clamped MB. Detailed evaluations are conducted on the optimization of the adaptive clamp tolerance and TB design to increase the pulling force. The energy harvester exhibits an ultrawide resonance bandwidth of over 30 Hz in the commonly accessible low vibration frequency range (100 Hz) owing to the ART function. The practical feasibility is demonstrated by evaluating the ART performance under both frequency and acceleration-variant conditions and powering a location tracking sensor.

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 SiO

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.

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.