Ultrafast (∼0.6 s), Robust, and Highly Linear Hydrogen Detection up to 10% Using Fully Suspended Pure Pd Nanowire.

The high explosiveness of hydrogen gas in the air necessitates prompt detection in settings where hydrogen is used. For this reason, hydrogen sensors are required to offer rapid detection and possess superior sensing characteristics in terms of measurement range, linearity, selectivity, lifetime, and environment insensitivity according to the publicized protocol. However, previous approaches have only partially achieved the standardized requirements and have been limited in their capability to develop reliable materials for spatially accessible systems. Here, an electrical hydrogen sensor with an ultrafast response (∼0.6 s) satisfying all demands for hydrogen detection is demonstrated. Tailoring structural engineering based on the reaction kinetics of hydrogen and palladium, an optimized heating architecture that thermally activates fully suspended palladium (Pd) nanowires at a uniform temperature is designed. The developed Pd nanostructure, at a designated temperature distribution, rapidly reacts with hydrogen, enabling a hysteresis-free response from 0.1% to 10% and durable characteristics in mechanical shock and repetitive operation (>10,000 cycles). Moreover, the device selectively detects hydrogen without performance degradation in humid or carbon-based interfering gas circumstances. Finally, to verify spatial accessibility, the wireless hydrogen detection system has been demonstrated, detecting and reporting hydrogen leakage in real-time within just 1 s.

Formation of sub-100-nm suspended nanowires with various materials using thermally adjusted electrospun nanofibers as templates.

The air suspension and location specification properties of nanowires are crucial factors for optimizing nanowires in electronic devices and suppressing undesirable interactions with substrates. Although various strategies have been proposed to fabricate suspended nanowires, placing a nanowire in desired microstructures without material constraints or high-temperature processes remains a challenge. In this study, suspended nanowires were formed using a thermally aggregated electrospun polymer as a template. An elaborately designed microstructure enables an electrospun fiber template to be formed at the desired location during thermal treatment. Moreover, the desired thickness of the nanowires is easily controlled with the electrospun fiber templates, resulting in the parallel formation of suspended nanowires that are less than 100 nm thick. Furthermore, this approach facilitates the formation of suspended nanowires with various materials. This is accomplished by evaporating various materials onto the electrospun fiber template and by removing the template. Palladium, copper, tungsten oxide (WO3), and tin oxide nanowires are formed as examples to demonstrate the advantage of this approach in terms of nanowire material selection. Hydrogen (H2) and nitrogen dioxide (NO2) gas sensors comprising palladium and tungsten oxide, respectively, are demonstrated as exemplary devices of the proposed method.

Location-specific fabrication of suspended nanowires using electrospun fibers on designed microstructure.

While there have been remarkable improvements in the fabrication of suspended nanowires, placing a single nanowire at the desired location remains to be a challenging task. In this study, a simple method is proposed to fabricate suspended nanowires at desired locations using an electrospinning process and a designed microstructure. Using electrospun polymer fibers on the designed microstructure as a sacrificial template, various materials are deposited on it, and the electrospun fibers are selectively removed, leaving only nanowires of the deposited material. After the polymer fibers are removed, the remaining metal fibers agglomerate into a single nanowire. Throughout this process, including the removal of the polymer fibers, the samples are not exposed to high temperatures or chemicals, thereby allowing the formation of nanowires without oxidation or contamination. The diameter of the nanowire can be controlled in the electrospinning process, and a suspended Pd nanowire with a minimum diameter of 100 nm is fabricated. Additionally, a suspended single Pd nanowire-based H2gas sensor fabricated using the proposed process exhibits a highly sensitive response to H2gas.

Integration of a Carbon Nanotube Network on a Microelectromechanical Switch for Ultralong Contact Lifetime.

Micro-/nanoelectromechanical (MEM/NEM) switches have been extensively studied to address the limitations of transistors, such as the increased standby power consumption and performance dependence on temperature and radiation. However, their lifetimes are limited owing to the degradation of the contact surfaces. Even though several materials and structural designs have been recently developed to improve the lifetime, the production of a microswitch that is compatible with a complementary metal-oxide semiconductor (CMOS) with a long lifetime remains a significant challenge. We demonstrate a vertically actuated MEM switch with extremely high reliability by integrating a carbon nanotube (CNT) network on a gold electrode as the contact material using a low-temperature, CMOS-compatible solution process. In addition to their outstanding mechanical and electrical properties of CNTs, their deformability dramatically increases the effective contact area of the switch, thus resulting in the extension of the lifetime. The CNT-coated MEM switch exhibits a lifetime that is more than 7 × 108 cycles when operated in hot-switching conditions, which is 1.9 × 104 times longer than that of a control device without CNTs. The switch also shows an excellent switching performance, including a low electrical resistance, high on/off ratio, and an extremely small off-state current.

Humidity-resistant triboelectric energy harvester using electrospun PVDF/PU nanofibers for flexibility and air permeability.

We present a triboelectric energy harvester fabricated with a simple electrospinning process of polyvinylidene fluoride/polyurethane polymers on conductive fabric. This electrospinning process provides higher electrical power output and hydrophobicity driven humidity resistance compared to flat polymer energy harvesters. By using conductive fabric as collector and electrode, the device could retain air permeability and flexibility. The triboelectric energy harvester exhibits a high open-circuit voltage of 45.1 V (at a compressive contact force of 20 N and relative humidity (RH) of 20%), humidity resistance (maintains about 40% of the open-circuit voltage at RH of 80%) and air permeability without deteriorating the air permeability of the fabric. Its durability was tested and shows no significant degradation of electrical output throughout 324,000 cycles of operation. This work suggests an approach for human energy harvesting in textile form with electrospun nanofibers as the contact surfaces of a triboelectric energy harvester.

Ultrasensitive Strain Sensor Based on Separation of Overlapped Carbon Nanotubes.

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.

Using confined self-adjusting carbon nanotube arrays as high-sensitivity displacement sensing element.

Displacement sensing is a fundamental process in mechanical sensors such as force sensors, pressure sensors, accelerometers, and gyroscopes. Advanced techniques utilizing nanomaterials have attracted considerable attention in the drive to enhance the process. In this paper, we propose a novel and highly sensitive device for detecting small displacements. The device utilizes the changes in contact resistance between two sets of vertically aligned carbon nanotube (CNT) arrays, the growth of which was confined to enable their facile and reliable integration with fully fabricated microstructures. Using the displacement transduction of the proposed device, we successfully demonstrated a 3-axis wide bandwidth accelerometer, which was experimentally confirmed to be highly sensitive compared to conventional piezoresistive sensors. Through a test involving 1.2 million cycles of displacement transductions, the contact resistance of the CNT arrays was proved to be excellently stable, which was a consequence of the high electrical stability and mechanical durability of the CNTs.