High-Temperature-Operable Electromechanical Computing Units Enabled by Aligned Carbon Nanotube Arrays.

Nano/micro-electromechanical (NEM/MEM) contact switches have great potential as energy-efficient and high-temperature-operable computing units to surmount those limitations of transistors. However, despite recent advances, the high-temperature operation of the mechanical switch is not fully stable nor repetitive due to the melting and softening of the contact material in the mechanical switch. Herein, MEM switches with carbon nanotube (CNT) arrays capable of operating at high temperatures are presented. In addition to the excellent thermal stability of CNT arrays, the absence of a melting point of CNTs allows the proposed switches to operate successfully at up to 550 °C, surpassing the maximum operating temperatures of state-of-the-art mechanical switches. The switches with CNTs also show a highly reliable contact lifetime of over 1 million cycles, even at a high temperature of 550 °C. Moreover, symmetrical pairs of normally open and normally closed MEM switches, whose interfaces are initially in contact and separated, respectively, are introduced. Consequently, the complementary inverters and logic gates operating at high temperatures can be easily configured such as NOT, NOR, and NAND gates. These switches and logic gates reveal the possibility for developing low-power, high-performance integrated circuits for high-temperature operations.

Mechanically resilient, alumina-reinforced carbon nanotube arrays for in-plane shock absorption in micromechanical devices.

Microelectromechanical systems (MEMS) are of considerable interest due to their compact size and low power consumption when used in modern electronics. MEMS devices intrinsically incorporate three-dimensional (3D) microstructures for their intended operations; however, these microstructures are easily broken by mechanical shocks accompanying high-magnitude transient acceleration, inducing device malfunction. Although various structural designs and materials have been proposed to overcome this limit, developing a shock absorber for easy integration into existing MEMS structures that effectively dissipates impact energy remains challenging. Here, a vertically aligned 3D nanocomposite based on ceramic-reinforced carbon nanotube (CNT) arrays is presented for in-plane shock-absorbing and energy dissipation around MEMS devices. This geometrically aligned composite consists of regionally-selective integrated CNT arrays and a subsequent atomically thick alumina layer coating, which serve as structural and reinforcing materials, respectively. The nanocomposite is integrated with the microstructure through a batch-fabrication process and remarkably improves the in-plane shock reliability of a designed movable structure over a wide acceleration range (0-12,000g). In addition, the enhanced shock reliability through the nanocomposite was experimentally verified through comparison with various control devices.

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.

Highly Sensitive Flexible Tactile Sensors in Wide Sensing Range Enabled by Hierarchical Topography of Biaxially Strained and Capillary-Densified Carbon Nanotube Bundles.

Flexible tactile sensors with high sensitivity have received considerable attention for their use in wearable electronics, human-machine interfaces, and health-monitoring devices. Although various micro/nanostructured materials are introduced for high-performance tactile sensors, simultaneously obtaining high sensitivity and a wide sensing range remains challenging. Here, a resistive tactile sensor is presented based on the hierarchical topography of carbon nanotubes (CNTs) prepared by a low-cost and straightforward manufacturing process. The 3D hierarchical structure of the CNTs over large areas is formed by transferring vertically aligned CNT bundles to a prestrained elastomer substrate and subsequently densifying them through capillary forming, providing a monotonic increase in the contact area as applied pressure. The deformable and hierarchical structure of CNTs allows the sensor to exhibit a wide sensing range (0-100 kPa), high sensitivity (141.72 kPa-1 ), and low detection limit (10 Pa). Additionally, the capillary-formed CNT structure results in increased durability of the sensor over repeated pressures. Based on these advantages, meaningful applications of tactile sensors, such as object recognition gloves and multidirectional force perceptions, are successfully realized. Given the scalable fabrication method, 3D hierarchically structured CNTs provide an essential step toward next-generation wearable devices.

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 Gold Nanoparticle-Carbon Nanotube Composite for Enhanced Contact Lifetime of Microelectromechanical Switches with Very Low Contact Resistance.

Electrical circuits require ideal switches with low power consumption for future electronic applications. However, transistors, the most developed electrical switches available currently, have certain fundamental limitations such as increased leakage current and limited subthreshold swing. To overcome these limitations, micromechanical switches have been extensively studied; however, it is challenging to develop micromechanical switches with high endurance and low contact resistance. This study demonstrates highly reliable microelectromechanical switches using nanocomposites. Nanocomposites consisting of gold nanoparticles (Au NPs) and carbon nanotubes (CNTs) are coated on contact electrodes as contact surfaces through a scalable and solution-based fabrication process. While deformable CNTs in the nanocomposite increase the effective contact area under mechanical loads, highly conductive Au NPs provide current paths with low contact resistance between CNTs. Given these advantages, the switches exhibit robust switching operations over 5 × 106 cycles under hot-switching conditions in air. The switches also show low contact resistance without subthreshold region, an extremely small leakage current, and a high on/off ratio.

Fabrication of fine-pored polydimethylsiloxane using an isopropyl alcohol and water mixture for adjustable mechanical, optical, and thermal properties.

Porous polydimethylsiloxane (PDMS) has garnered interest owing to its large inner surface area, high deformability, and lightweight, while possessing inherent properties, such as transparency, flexibility, cost-effectiveness, ease of fabrication, chemical/mechanical stability, and biocompatibility. For producing porous PDMS, gas foaming, sacrificial template, and emulsion template techniques have been used extensively. However, the aforementioned methods have difficulty in achieving submicron-sized inner pores, which is advantageous for improving flexibility and transparency. This study demonstrates a simple fabrication method for obtaining porous PDMS with fine pores partially down to the sub-micron scale. This is possible by the use of cheap, volatile, and easily accessible isopropyl alcohol (IPA) as a co-solvent in water and pre-PDMS emulsion. IPA shows an affinity towards both water and prepolymer, resulting in an increased distribution of small water particles inside PDMS before curing. These water particles evaporate while curing the prepolymer emulsion, thereby generating fine pores. The fine size and number density of pores are controlled by water and the added amount of IPA, resulting in adjustable mechanical, optical, and thermal properties of porous PDMS.

Light-assisted recovery of reacted MoS2 for reversible NO2 sensing at room temperature.

Two-dimensional (2D) nanomaterials have been extensively explored as promising candidates for gas sensing due to their high surface-to-volume ratio. Among many 2D nanomaterials, molybdenum disulfide (MoS2) is known to be functional in detecting harmful gases at room temperature; therefore, it has been actively studied as a gas sensing material. However, there has been a limitation in recovering the original signal from reacted MoS2 after exposure to the target gas. This work demonstrates the recovery of the initial resistance of reacted chemical vapor deposition-grown MoS2 by illuminating it with a UV light-emitting diode (LED). A novel mechanism involving photo-generated electron-hole pairs in MoS2 is proposed and experimentally verified. The fabricated sensor detects nitrogen dioxide (NO2) and distinguishes between concentrations from 1 to 10 ppm with the proposed recovery process. Reversible detection after repeated exposure to 5 ppm NO2 over eight cycles is achieved through UV-LED illumination for a short time during the recovery process, while the identical sensor without UV illumination shows a transitional response at each cycle. To apply a low cost gas sensing solution at room temperature, visible light LEDs are also used to recover the resistance of the reacted MoS2.

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.

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.