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Palladium (Pd) has been drawing increasing attention as a hydrogen (H

This study proposes a reliable and self-powered hydrogen (H2) gas sensor composed of a chemo-mechanically operating nanostructured film and photovoltaic cell. Specifically, the nanostructured film ...

This paper reports on the unprecedented 4 W power microelectromechanical systems (MEMS) relay with a contact- force-maximizing structure in a dual-contact material system. A MEMS relay with dual (hard and soft) contact materials is known to have high durability in hot-switching conditions, and the contact force is also considered to be the most important factor that contributes to an efficient reduction of the contact resistance. We devised a contact-force-maximizing structure with a plate-type suspended structure and in-plate springs and applied it to a dual-contact material system to achieve extremely low contact resistance. The fabricated MEMS relay successfully operated sequential switching between the dual contact materials in the contact-force-maximizing structure and thereby showed an extremely low contact resistance of 1.65 $\text{m}\Omega $ with a switching time of $7 {\mu \text {s}}$ at an operating voltage of 80 V. Thanks to the contact-force-maximizing structure with the dual-contact material system, the fabricated MEMS relay achieved a lifetime of up to $1.3\times 10^{\mathbf {3}}$ cycles in the hot-switching conditions of 4 W (10 V/400 mA) in an air atmosphere with a negligible variation of the contact resistance. This power level is the highest reported to date. [2020-0223]

Recently, copper oxide (CuO) has drawn much attention as a promising material in visible light photodetection with its advantages in ease of nanofabrication. CuO allows a variety of nanostructures to be explored to enhance the optoelectrical performance such as photogenerated carriers scattering and bandgap engineering. However, previous researches neglect in-depth analysis of CuO’s light interaction effects, restrictively using random orientation such as randomly arranged nanowires, single nanowires, and dispersed nanoparticles. Here, we demonstrate an ultra-high performance CuO visible light photodetector utilizing perfectly-aligned nanowire array structures. CuO nanowires with 300 nm-width critical dimension suppressed carrier transport in the dark state and enhanced the conversion of photons to carriers; additionally, the aligned arrangement of the nanowires with designed geometry improved the light absorption by means of the constructive interference effect. The proposed nanostructures provide advantages in terms of dark current, photocurrent, and response time, showing unprecedentedly high (state-of-the-art) optoelectronic performance, including high values of sensitivity (S = 172.21%), photo-responsivity (R = 16.03 A/W, λ = 535 nm), photo-detectivity (D* = 7.78 × 1011 Jones), rise/decay time (τr/τd = 0.31 s/1.21 s).

Gas sensors that can operate with sub-10 mW power consumption are in great demand in application where mobile devices are used to detect harmful gases in real time. Such low power gas sensors can be realized by employing miniaturized and air-suspended microheater platforms; however, a productive and reliable fabrication method should be developed for their practical use. Here, we present a batch-fabrication method based on a surface micro-machining process to uniformly and reliably produce the sub-10 mW power oerating gas sensors on an 8-inch wafer area. The fabricated gas sensors showed a reproducible and sensitive response to various concentrations of carbon monoxide gas (as low as 1 ppm) with 5.0 mW power operation, which is among the lowest power achieved for semiconducting metal-oxide gas sensors. In addition, the devices showed low sample-to-sample variation and high reliability in a long-term stability test, verifying their potential for practical use in mobile gas sensing applications.

Recently, copper oxide (CuO)-based visible-light photodetectors have attracted great interest due to their narrow bandgap (1.2 eV), low cost, and ease of fabrication. However, there has been insufficient theoretical analysis and study of CuO-based photodetectors, resulting in inferior performance in terms of responsivity, detectivity, and response speed. This work develops a method to enhance the performance of CuO photodetectors by engineering a grain structure based on a newly-developed theoretical model. In the developed theoretical grain-structure model, the grain size and the connections between grains are considered because they can strongly affect the optoelectronic characteristics of CuO photodetectors. Based upon the proposed model, the engineered CuO device achieves enhanced optoelectronic performance. The engineered device shows high responsivity of 15.3 A/W and detectivity of 1.08 × 1011 Jones, which are 18 and 50 times better than those of the unoptimized device, and also shows fast rising and decaying response speeds of 0.682 s and 1.77 s, respectively. In addition, the proposed method is suitable for the mass-production of performance-enhanced, reliable photodetectors. By using a conventional semiconductor fabrication process, a photodetector-array is demonstrated on a 4-inch wafer. The fabricated devices show uniform, high, and stable optoelectronic performance for a month.

Pd nanowire-based H2 sensors have attracted significant attention because of their superior sensing performance. However, when exposed to H2 concentrations of more than 2%, Pd experiences volume expansion over 10%, resulting in a significant amount of mechanical stress. Thus, exposure to such high H2 concentrations causes physical destruction of Pd nanowires, such as breaks and peel-offs, leading to severe difficulty in the reliable detection of H2 over a wide concentration range. Here, we proposed a structural approach to resolve this issue by introducing a partially anchored Pd nanowire (PA-PdNW) structure. In this configuration, most of the structure was air-suspended, leaving a small portion anchored to the substrate. Air-suspension enabled PA-PdNW to expand freely, thus relieving the mechanical stress; therefore, the Pd nanowires could withstand numerous exposures to high H2 concentrations. To demonstrate the PA-PdNW structure, we developed a nano-fabrication method based on conventional semiconductor processes and successfully manufactured H2 sensor devices with uniform, perfectly aligned PA-PdNW arrays stably air-suspended with designed gaps from the substrate. The fabricated sensors achieved reliable detection of H2 in the 0.1%–3.9% concentration range with a significant resistance change. In addition, compared with fully anchored Pd nanowire (FA-PdNW) sensors, the PA-PdNW sensors showed superior durability, and the nanowires retained their initial structures even after 300 exposures to high H2 concentrations. Furthermore, it was confirmed that the PA-PdNW sensor can stably operate even in extremely humid environments at 85% relative humidity.

In this work, we present a compact self-powered wireless gas sensor node based on photovoltaic (PV) energy harvesting (EH). Instead of a bulky and power-hungry gas sensor with separate gas signal processing (GSP) circuits, a mm3-sized colorimetric sensor film is integrated with a PV cell, and the GSP function is seamlessly embedded within EH circuits. Also, a dual-input shared-inductor boost converter is used to improve the EH efficiency under gas exposure. Offset cancellation is performed in GSP circuits to provide accurate gas-sensing readout without any external trimming.

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.

This study proposes a reliable and self-powered hydrogen (H

This paper reports an unprecedented 4 W MEMS relay that utilizes a dual contact-material system and a contact force maximizing structure. The contact force-maximizing structure is designed to achieve extremely low contact resistance. So far, commercialized MEMS relays have achieved a power level of 0.03 W in hot-switching conditions. In this work, we achieved a MEMS relay with operation reliability up to $5.7\times 10^{4}$ cycles and $1.3\times 10^{3}$ cycles at 10 V/300 mA (3 W) and 10 V/400 mA (4 W) signals, respectively, in hot-switching conditions. This achievement was due to an extremely-low contact resistance of $1.65\ \mathrm{m}\Omega$ achieved by utilizing the proposed contact force maximizing structure coupled with dual contact materials.

Probing micro-bumps for pre-bond testing is an essential process to check for a known good die. In recent technologies, micro-bumps such those used in 3-D-IC are too small and dense, which gives the probing a new challenge. Moreover, developers are concerned that the tip ends of the micro-bumps are mechanically damaged during the pre-bond testing, which is detrimental for the post-process IC assembly. Thus, many low damage probing solutions have been developed, but they still inevitably damage the tip end of the micro-bumps when the conventional probing method, vertical contact, is used. In this paper, for the first time, we demonstrate lateral contact probing on 55- $\mu \text{m}$ pitch micro-bumps without any damage to the tip ends. We successfully realized the testing with monolithically fabricated probes by nickel electroplating with a high aspect ratio photoresist mold. The measured fatigue life of the fabricated probes was at least100 000 cycles. Furthermore, the measured current carrying capacity was more than 180 mA. Proving our concept, the contact test results on the micro-bumps showed no damage to the tip end, and the contact resistance was below 1.13 $ {\Omega }$ . Finally, the10 000 probes achieved a uniform 55- $\mu \text{m}$ pitch, which ensured the possibility in real testing. [2018-0041]

An innovative plastic deformation method is proposed to tune metal-based MEMS devices after fabrication. The plastic deformation is a well-known failure mechanism in MEMS devices, which includes a phenomenon that the devices do not return original position after repeated actuation. Here, in contrast to this common experience, we exploit the plastic deformation for the first time to tune the performance of MEMS devices, by producing deliberate, controlled plastic deformation with the help of Joule heating. The proposed plastic deformation method is precisely and predictably controlled by the model we developed here, and it was successfully demonstrated and evaluated using a fabricated MEMS device. Unlike conventional plastic deformation methods, the proposed method was accomplished solely by electrical control, which means that the method can be employed in integrated devices. Tuning resolution was demonstrated at a 10-nm level, which is remarkably smaller than that of conventional plastic deformation methods, where the tuning resolution is typically several micrometers. We confirmed that the proposed method does not deteriorate the basic performance of the fabricated MEMS devices. Finally, we proposed and demonstrated a method for calibrating MEMS devices after fabrication and suggested an interesting application, which is recovering the performance of aged MEMS devices. [2017-0129]

A 4-terminal microelectromechanical systems (MEMS) relay has long been sought for signal switching applications because it allows de-coupling of the actuation (body and gate) and signal (source and drain) components unlike a 3-terminal MEMS relay. However, the contact resistance still remains a serious problem in current 4-terminal MEMS relays compared with the 3-terminal MEMS relays. In this paper, using theoretical analysis, we first determined that the two-contact design, which was the most common design in the 4-terminal MEMS relay, was the origin of the high contact resistance. We subsequently developed a 4-terminal MEMS relay with a novel single-contact design. The fabricated 4-terminal MEMS relay with the single-contact design exhibited a contact resistance of 18 $\text{m}\Omega $ . To the best of our knowledge, this result is the lowest value among 4-terminal MEMS relays. In addition, we demonstrated that the two-contacts are the origin of the high contact resistance using measurement. The relay operated up to $1.1\times 10^{6}$ cycles at 1 V / 50 mA in air and hot switching conditions. Finally, the proposed 4-terminal MEMS relay was hermetically packaged for use in commercial applications. [2017-0183]

High contact force in a microelectromechanical systems (MEMS) relay is known to be the most important factor in achieving low contact resistance. Achieving a high contact force in an electrostatically-actuated MEMS relay usually requires high actuation voltage to exert a high electrostatic force. In this paper, for the first time, we demonstrate the successful use of a mechanical adhesion force as a high contact force for realizing an extremely low contact resistance, even at a low actuation voltage of 3.8 V. To maximize the mechanical adhesion, we designed a suitably-large contact area (100 μm × 100 μm) in the MEMS relay and a detachment spring structure to separate the two electrodes that were in contact with one another by only the mechanical adhesion force, thereby enabling the MEMS switch to perform multiple switching operations. The proposed MEMS relay exhibited an extremely low contact resistance of 4.9 mΩ with only the mechanical adhesion force after we applied an actuation voltage of 3.8 V, and it achieved a retention time of 105 s because of stable mechanical contact. Through this experiment, we verified that the mechanical adhesion force can provide an efficient source for generating high contact force in the MEMS relay.

A nanoparticle has been reported as a promising lubricant to increase the durability of microelectromechanical systems (MEMS) switches, but there remains much room for development and scientific observations. In this paper, we present the fabrication and research platform for a nanoparticle lubricated MEMS switch; we measured its characteristics and investigated the mechanisms behind it. In the measurements, gold nanoparticle lubrication demonstrated an increased average adhesion force from 16.38 to 23.68 $\mu \text{N}$ by 44.51% and a decreased average electrical contact resistance from 211.20 to 49.85 $\text{m}\Omega $ by 76.40%. In a hot switching mode lifetime test, the gold nanoparticle lubricated MEMS switch exhibited extension of reliability from $5.13\times 10^{4}$ to $4.75\times 10^{5}$ cycles, which corresponds to 9.26-fold increase. We attribute this remarkable enhancement to a mechanical stress release via lateral deformation of the nanoparticle and refreshment of the contact spots owing to movement of nanoparticles. [2017-0127]

The fabrication of a highly ordered array of single-crystalline nanostructures prepared from solution-phase or vapor-phase synthesis methods is extremely challenging due to multiple difficulties of spatial arrangement and control of crystallographic orientation. Herein, we introduce a nanotransplantation printing (NTPP) technique for the reliable fabrication, transfer, and arrangement of single-crystalline Si nanowires (NWs) on diverse substrates. NTPP entails (1) formation of nanoscale etch mask patterns on conventional low-cost Si via nanotransfer printing, (2) two-step combinatorial plasma etching for defining Si NWs, and (3) detachment and transfer of the NWs onto various receiver substrates using an infiltration-type polymeric transfer medium and a solvent-assisted adhesion switching mechanism. Using this approach, high-quality, highly ordered Si NWs can be formed on almost any type of surface including flexible plastic substrates, biological surfaces, and deep-trench structures. Moreover, NTPP provides controllability of the crystallographic orientation of NWs, which is confirmed by the successful generation of (100)- and (110)-oriented Si NWs with different properties. The outstanding electrical properties of the NWs were confirmed by fabricating and characterizing Schottky junction field-effect transistors. Furthermore, exploiting the highly flexible nature of the NWs, a high-performance piezoresistive strain sensor, with a high gauge factor over 200 was realized.

We present a fast, energy-efficient nano-thermomechanical encoding scheme for digital information storage and retrieval. Digital encoding processes are conducted by the bistable electrothermal actuation of a scalable nanobridge device. The electrothermal energy is highly concentrated by enhanced electron/phonon scattering and heat insulation in a sub-100 nm metallic layer. The efficient conversion of electrothermal energy into mechanical strain allows digital switching and programming processes within 60 ns at 0.75 V with a programming energy of only 54 pJ. Furthermore, this encoding scheme together with the thermally robust design enables data retention at temperatures up to 400 °C. These results suggest that the proposed nano-thermomechanical encoding method could contribute to low-power electronics and robust information storage/retrieval systems.

Herein, we report a triboelectric nanogenerator (TENG) with improved performance using extremely-long sub-50 nm grating patterns with high aspect ratio (4,000,000:1) fabricated by the multistep pattern downscaling (MS-PaD) method, which is advantageous in terms of fast throughput, high resolution, cost-effectiveness, industrial compatibility, and large-area scalability in nanofabrication. The intensely-arrayed nanograting pattern is replicated to the flexible substrate on the 8-in. wafer-scale without any defects for the nanograting-assisted TENG (NGTENG). As the well-aligned nanoscale morphology in sub-50 nm grating leads to efficient triboelectric charges, the NGTENG shows an outstanding performance enhancement by a factor of 200 compared to non-patterned surface during repetitive contact electrification. To demonstrate capability of self-powered electronics, the output power from the NGTENG is directly used for charging a commercial capacitor and turning on the LEDs. In addition, theoretical study of triboelectric signals according to nanoscale-tuned metal electrodes were investigated to improve the effective output and robustness of triboelectric device.

Nanowire (NW) transfer technology has provided promising strategies to realize future flexible materials and electronics. Using this technology, geometrically controlled, high-quality NW arrays can now be obtained easily on various flexible substrates with high throughput. However, it is still challenging to extend this technology to a wide range of high-performance device applications because its limited temperature tolerance precludes the use of high-temperature annealing, which is essential for NW crystallization and functionalization. A pulsed laser technique has been developed to anneal NWs in the presence of a flexible substrate; however, the induced temperature is not high enough to improve the properties of materials such as ceramics and semiconductors. Here, we present a versatile nanotransfer method that is applicable to NWs that require high-temperature annealing. To successfully anneal NWs during their transfer, the developed fabrication method involves sequential removal of a nanoscale sacrificial layer. Using this method, we first produce an ultralong, perfectly aligned polycrystalline barium titanate (BaTiO

Recently, geometrically structured nanomaterials have received great attention due to their unique physical and chemical properties, which originate from the geometric variation in such materials. Indeed, the use of various geometrically structured nanomaterials has been actively reported in enhanced-performance devices in a wide range of applications. Recent significant progress in the development of geometrically structured nanomaterials and associated devices is summarized. First, a brief introduction of advanced nanofabrication methods that enable the fabrication of various geometrically structured nanomaterials is given, and then the performance enhancements achieved in devices utilizing these nanomaterials, namely, i) physical and gas nanosensors, ii) nanoelectromechanical devices, and iii) nanosieves are described. For the device applications, a systematic summary of their structures, working mechanisms, fabrication methods, and output performance is provided. Particular focus is given to how device performance can be enhanced through the geometric structures of the nanomaterials. Finally, perspectives on the development of novel nanomaterial structures and associated devices are presented.

Pd nanowire-based H

This paper first reports a highly reliable self-powered hydrogen (H 2 ) sensor employing a palladium (Pd)-based nano-transducer. The developed sensor is based on the principle of a novel chemomechanical mechanism of the Pd nano-transducer exploiting a solar cell. We theoretically and experimentally demonstrated that the proposed device can achieve highly durable operation for a wide range of H 2 concentrations with remarkable sensitivity (3.1% at 2%-H 2 ) and response time (111 s at 2%-H 2 ) without external power. Significantly, the proposed sensor, which has a novel sensing mechanism, maintains high sensing performance for more than 150 cycles under various H 2 conditions (0.5 to 2%).

A major obstacle to the industrial application of nanowires is difficulty in fabricating nanowire devices with uniform performance. In this study, we newly developed a wafer-level uniformly size-controlled nanowire fabrication method for reliable demonstration of a high-yield, uniform nano-sensor device. We show that the developed fabrication method can be used to simply fabricate remarkably small nanowires (sub-50 nm) with a wide material selection. Importantly, the fabricated nanowires have ultra-high size uniformity on the 4-inch wafer. Finally, we demonstrate nanowire-based hydrogen (H 2 ) sensors using the developed method. Owing to the geometric uniformity of the fabricated nanowires, the sensor devices achieved highly uniform sensitivity of 10.9±0.5 % at a 2% H 2 concentration.

This work demonstrates a highly reliable microelectromechanical (MEM) switch with high switching performance by integrating gold (Au)-decorated carbon nanotube (CNT) network in the contact area. By decorating the CNTs with Au nanoparticles (Au-NPs), we obtained several times extension in the lifetime of the MEM switch with low contact resistance. We verified that the extended lifetime and the low contact resistance originate from the deformable CNTs under mechanical load and the Au NPs with high conductivity at the contact-interface. Our switch also revealed low adhesion force at the contact-interface, which results in highly repeatable and stable actuation voltages at both turn-on and off, whereas typical MEM switches with high adhesion force show irreversible stiction or unstable turn-off voltage. The proposed switch exhibited 1.9 times longer lifetime under hot-switching conditions, compared to previously studied MEM switches based on Au-Au contact.

This paper reports a high-sensitivity wearable transparent pressure sensor by maximizing the percolation effect using a sub-100 nm nano-valley. The secret to achieving high sensitivity is to make the nano-valley as narrow as possible to maximize the percolation effect (stress concentration effect). The narrow nano-valley pressure sensor showed a remarkable 400% enhancement in sensitivity compared to a sensor with no nano-valley while maintaining a high transmittance of 83%. This sensitivity value is higher than those of the previous wearable transparent pressure sensors. The fabricated sensor also showed high durability with low variation of 1.2% in repeated pressing of up to 100,000 times under 100 kPa pressure.

This paper reports a Pd hydrogen gas sensor that uses perfectly aligned uniform nanogap arrays. Depending on the nanogap size, the sensor can differentiate hydrogen concentration thanks to the volume expansion of Pd. The fabrication method of the sensor is based on conventional semiconductor processes; thus, perfectly aligned uniform nanogap arrays can be easily obtained. The standard deviation of the measured nanogap widths was less than 5 nm, and this level of uniformity has not been previously demonstrated in Pd-switching-type sensors. The fabricated sensors with the uniform nanogap array showed abrupt switching characteristics when they were exposed to H2 concentrations over a certain threshold level. Furthermore, based on the unprecedented high uniformity of the nanogap widths, a fine modulation of the threshold level of the H2 concentrations was firstly demonstrated.

Recently, flexible electronics have attracted significant attention as they can be integrated on diverse platforms from curved to flexible surfaces. As flexible electronics are used on a curved surface of wearable or manufacturing devices for health and system monitoring, the working environment of such applications forces electronics to be exposed to diverse stimuli such as deformation, temperature, humidity, and gas, resulting in performance changes. Therefore, rather than research on improving the specific performance of electronics, research on maintaining a stable performance in various environmental stimuli has been receiving tremendous interest. Reflecting the latest research trends, this paper introduces efforts in structural designs heading for both improving and maintaining the performance of flexible electronics in diverse environmental stimuli. Firstly, we will sequentially explain the geometric and structural designs introduced for achieving (a) reliable electronics insensitive to undesired mechanical stimuli, (b) reliable electronics in harsh environments, and (c) flexible electrodes. Also, (d) diverse applications of reliable and flexible electronics are introduced. Finally, a perspective on reliable and flexible electronic devices has been presented for suggesting next-generation research.

The electrostatic actuation method has not been used widely in bimorph cantilevers despite its advantage of large displacement and low power consumption, mostly owing to the fact that it is limited to just two stable positions: 1) initial curled; and 2) final flat positions. In this paper, we first demonstrate a bimorph cantilever actuator having multiple steady-state positions in between those two boundary positions, accomplished by simply dividing the bottom electrode. We found that the number of steady-state positions and the steady-state tip heights can be controlled by the design of the bottom electrode. In our test experiment, the steady-state bimorph cantilever tip heights were 48 $\mu \text{m}$ at 0 V, 28 $\mu \text{m}$ at 21 V, 7 $\mu \text{m}$ at 25 V, and 0 $\mu \text{m}$ (flat) at 29 V, respectively. In order to relieve low reliability of the electrostatic bimorph cantilever, we devised non-contact actuation method and demonstrated that the lifetime of the electrostatic bimorph actuator greatly increased. This paper will pave the way to utilize bimorph cantilever actuators with greater versatility. [2016-0081]

This paper reports a highly reliable electrostatic microelectromechanical systems (MEMS) relay for high-power switching applications. The main proposal to elevate reliability is to reduce thermal damage in the contact area. Since a contact resistance is the key parameter determining the amount of Joule-heating and the corresponding thermal damage, we devised a unique spring structure to maximize the contact force (resulting in a low contact resistance) using a reasonable actuation voltage named a two-step spring system. Another important feature was applied to alleviate Joule-heating, which is to use an insulator having high thermal conductivity to dissipate the generated heat efficiently, named a heat sink insulator. The fabricated MEMS relay exhibited 2 $\text{m}\Omega $ in contact resistance, which is the lowest level reported so far with an actuation voltage of 45 V. Reliability was remarkably enhanced over ten times by the heat sink insulator. Consequently, by applying these two approaches simultaneously, the fabricated MEMS relay was successfully operated up to the $5.3 \times 10^{6}$ cycles at 1 V/200 mA in ambient air and hot switching condition, which is the highest reliability reported at that power level. [2015-0237]

In article number 2004448, Jun‐Bo Yoon and co‐workers propose the concept of two distinctive nanowire arrays with an ‘air‐bridge’ structure, with the first of the two nanowire arrays acting as a heater and the second nanowire array as the sensing material. The developed configuration greatly suppresses beam conduction loss to the substrate, overcoming the fundamental power consumption limits of conventional microheaters.