Semiconducting two-dimensional (2D) transition metal dichalcogenides (TMDCs) are promising gas-sensing materials due to their large surface-to-volume ratio. However, their poor gas-sensing performance resulting from the low response, incomplete recovery, and insufficient selectivity hinders the realization of high-performance 2D TMDC gas sensors. Here, we demonstrate the improvement of gas-sensing performance of large-area tungsten disulfide (WS) nanosheets through surface functionalization using Ag nanowires (NWs). Large-area WS nanosheets were synthesized through atomic layer deposition of WO followed by sulfurization. The pristine WS gas sensors exhibited a significant response to acetone and NO but an incomplete recovery in the case of NO sensing. After AgNW functionalization, the WS gas sensor showed dramatically improved response (667%) and recovery upon NO exposure. Our results establish that the proposed method is a promising strategy to improve 2D TMDC gas sensors.

Although there have been remarkable improvements in stretchable strain sensors, the development of strain sensors with scalable fabrication techniques and which both high sensitivity and stretchability simultaneously is still challenging. In this work, a stretchable strain sensor based on overlapped carbon nanotube (CNT) bundles coupled with a silicone elastomer is presented. The strain sensor with overlapped CNTs is prepared by synthesizing line‐patterned vertically aligned CNT bundles and rolling and transferring them to the silicone elastomer. With the sliding and disconnection of the overlapped CNTs, the strain sensor performs excellently with a broad sensing range (≥145% strain), ultrahigh sensitivity (gauge factor of 42 300 at a strain of 125–145%), high repeatability, and durability. The performance of the sensor is also tunable by controlling the overlapped area of CNT bundles. Detailed mechanisms of the sensor and its applications in human motion detection are also further investigated. With the novel structure and mechanism, the sensor can detect a wide range of strains with high sensitivity, demonstrating the potential for numerous applications including wearable healthcare devices.

detection; furthermore, mechanisms or devices to detect other stimuli, such as strain, temperature, and humidity, may be integrated. A tactile feedback system, integrated with a prosthetic hand, was demonstrated in 1974, [2] and since then, a variety of tactile sensors have been actively developed for use in various applications, such as touch screens and robotic hands. Flexible tactile sensors capable of measuring mechanical stimuli via physical contact have attracted significant attention in the field of human-interactive systems. The utilization of tactile information can complement vision and/or sound interaction and provide new functionalities. Recent advancements in micro/nanotechnology, material science, and information technology have resulted in the development of high-performance tactile sensors that reach and even surpass the tactile sensing ability of human skin. Here, important advances in flexible tactile sensors over recent years are summarized, from sensor designs to system-level applications. This review focuses on the representative strategies based on design and material configurations for improving key performance parameters including sensitivity, detection range/linearity, response time/hysteresis, spatial resolution/crosstalk, multidirectional force detection, and insensitivity to other stimuli. System-level integration for practical applications beyond conceptual prototypes and promising applications, such as artificial electronic skin for robotics and prosthetics, wearable controllers for electronics, and bidirectional communication tools, are also discussed. Finally, perspectives on issues regarding further advances are provided.

Resistive tactile sensors based on changes in contact area have been extensively explored for a variety of applications due to their outstanding pressure sensitivity compared to conventional tactile sensors. However, the development of tactile sensors with high sensitivity in a wide pressure range still remains a major challenge due to the trade‐off between sensitivity and linear detection range. Here, a tactile sensor comprising stacked carbon nanotubes and Ni‐fabrics is presented. The hierarchical structure of the fabrics facilitates a significant increase in contact area between them under pressure. Additionally, a multi‐layered structure that can provide more contact area and distribute stress to each layer further improves the sensitivity and linearity. Given these advantages, the sensor presents high sensitivity (26.13 kPa−1) over a wide pressure range (0.2–982 kPa), which is a significant enhancement compared with the results obtained in previous studies. The sensor also exhibits outstanding performances in terms of response time, repeatability, reproducibility, and flexibility. Furthermore, meaningful applications of the sensor, including wrist‐pulse‐signal analysis, flexible keyboards, and tactile interface, are successfully demonstrated. Based on the facile and scalable fabrication technique, the conceptually simple but powerful approach provides a promising strategy to realize next‐generation electronics.

A polymer membrane-coated palladium (Pd) nanoparticle (NP)/single-layer graphene (SLG) hybrid sensor was fabricated for highly sensitive hydrogen gas (H2) sensing with gas selectivity. Pd NPs were deposited on SLG via the galvanic displacement reaction between graphene-buffered copper (Cu) and Pd ion. During the galvanic displacement reaction, graphene was used as a buffer layer, which transports electrons from Cu for Pd to nucleate on the SLG surface. The deposited Pd NPs on the SLG surface were well-distributed with high uniformity and low defects. The Pd NP/SLG hybrid was then coated with polymer membrane layer for the selective filtration of H2. Because of the selective H2 filtration effect of the polymer membrane layer, the sensor had no responses to methane, carbon monoxide, or nitrogen dioxide gas. On the contrary, the PMMA/Pd NP/SLG hybrid sensor exhibited a good response to exposure to 2% H2: on average, 66.37% response within 1.81 min and recovery within 5.52 min. In addition, reliable and repeatable sensing behaviors were obtained when the sensor was exposed to different H2 concentrations ranging from 0.025 to 2%.

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optical transparency, and mechanical flexibility of the sensor are considered as essential requirements for use in future tactile sensing applications. To fulfill these demands, a broad range of materials, fabrication processes, and structural designs of the tactile sensor have been developed, [8] while sensing principles are mainly classified as either resistive [9][10][11] or capacitive types. [12][13][14] Many of resistive sensors use nanomaterial-embedded composites and exploit changes in contact resistance between the nanomaterials in the composite matrix (such as elastomer) under pressure loading, showing improved pressure sensitivity and mechanical flexibility compared to silicon-or metal-based piezoresistive sensors. [15] However, resistive tactile sensors suffer from signal drift due to temperature changes and require high power consumption. In addition, complicated circuit arrangement for multipoint recognition is regarded as a drawback to be addressed. [7,16] Compared to resistive tactile sensing mechanisms, capacitive tactile sensors have advantages in terms of temperature independence, low power consumption, stability against long-term signal drift, and easy multipoint recognition by simple assembly of row and column electrodes. [16,17] In general, the structure of a capacitive sensor consists of two parallel electrodes with a dielectric layer between them. Highly compressible dielectric materials are essential to achieve high sensitivity; a lower Young's modulus of the dielectric leads to greater deformation when pressure is applied to the sensor, resulting in a larger change in capacitance. Accordingly, considerable efforts have been devoted to using elastomers with low Young's modulus as dielectric materials, including polydimethylsiloxane (PDMS), [18] polyurethane, [19] or Ecoflex. [12] However, these low-modulus elastomers also tend to have high viscoelasticity, slowing their response and relaxation times. [20] To overcome this limitation and further improve the sensitivity, a few tactile sensors make use of the strategy of structuring the dielectric layer by fabricating a microstructured surface in an orderly fashion. [7,20,21] The microstructured dielectric layer led to much higher sensitivity and faster response/relaxation time by allowing a larger deformation compared to conventional capacitive sensors with a plain dielectric layer under equal applied pressure. Nevertheless, The development of sensitive, flexible, and transparent tactile sensors is of great interest for next-generation flexible displays and human-machine interfaces. Although a few materials and structural designs have been previously developed for high-performance tactile sensors, achieving flexibility, full transparency, and highly sensitive multipoint recognition without crosstalk remains a significant challenge for such systems. This work demonstrates a capacitive tactile sensor composed of two sets of facing graphene electrodes separated by spacers, which forms an air dielectric between them. The air gap facilitates mor...

Self-powered energy harvesters utilizing triboelectric effect and electrostatic induction have been widely studied, leading in the materials viewpoint to numerous material pairs for facile charge separation upon repetitive contacts with elaborate topological structures. Here, we present a simple but robust triboelectric platform based on a molecularly engineered surface triboelectric nanogenerator by self-assembled monolayers (METS). Triboelectric surface charge density of a substrate was readily controlled by the variation of end-functional groups of self-assembled monolayers (SAMs). In particular, by employing fluorine terminated SAMs, we are able to develop a METS with the maximum open circuit voltage and short circuit current of 105 V and 27 μA, respectively, under relatively gentle mechanical contacts with the 3N vertical force at 1.25 Hz. The power density of the device was 1.8 W/m 2 at the load resistance of 10 MΩ more than 60 times greater than that of an unmodified dielectric/Al device. Moreover, our approach with SAMs was extended to various types of surfaces including fabrics of silk, cotton, and poly(ethylene terephthalate) (PET) and a PET film, and the results of singlefriction-surface triboelectric nanogenerators with these materials offers a facile and universal guideline for designing triboelectic materials.