Soonjae Pyo

A highly sensitive hydrogen sensor with gas selectivity using a PMMA membrane-coated Pd nanoparticle/single-layer graphene hybrid.

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%.

Development of a flexible three-axis tactile sensor based on screen-printed carbon nanotube-polymer composite

A flexible, three-axis carbon nanotube (CNT)–polymer composite-based tactile sensor is presented. The proposed sensor consists of a flexible substrate, four sensing cells, and a bump structure. A CNT–polydimethylsiloxane (PDMS) composite is produced by a solvent evaporation method, and thus, the CNTs are well-dispersed within the PDMS matrix. The composite is directly patterned onto a flexible substrate using a screen printing technique to fabricate a sensor with four sensing cells. When a force is applied on the bump, the magnitude and direction of force could be detected by comparing the changes in electrical resistance of each sensing cell caused by the piezoresistive effect of the composite. The experimentally verified sensing characteristics of the fabricated sensor exhibit a linear relationship between the resistance change and the applied force, and the measured sensitivities of the sensor for the normal and shear forces are 6.67 and 86.7%/N for forces up to 2.0 and 0.5 N, respectively. Experiments to verify the load-sensing repeatability show a maximum 2.00% deviation of the resistance change within the tested force range.

A High-Efficiency DC–DC Boost Converter for a Miniaturized Microbial Fuel Cell

This paper presents a high-efficiency dc-dc boost converter to interface a miniaturized 50 μL microbial fuel cell (MFC) having 1 cm2 vertically aligned carbon nanotube anode and 1 cm2 Cr/Au cathode. Geobacteraceae-enriched mixed bacterial culture in growth medium and 100 mM buffered ferricyanide solutions are used as the anolyte and catholyte, respectively. The miniaturized MFC produces up to approximately 10 μW with an output voltage of 0.4-0.7 V. Such low voltage, which is also load dependent, prevents the MFC to directly drive low power electronics. A pulse-frequency modulation type dc-dc converter in discontinuous conduction mode is designed and implemented to address the challenges and provides a load independent output voltage with high conversion efficiency. The fabricated dc-dc converter in UMC 0.18 μm has been tested with the MFC. At 0.9 V output, the converter has a peak efficiency of 85% with 9 μW load.

Heterogeneous Integration of Carbon‐Nanotube–Graphene for High‐Performance, Flexible, and Transparent Photodetectors

Low-dimensional carbon materials, such as semiconducting carbon nanotubes (CNTs), conducting graphene, and their hybrids, are of great interest as promising candidates for flexible, foldable, and transparent electronics. However, the development of highly photoresponsive, flexible, and transparent optoelectronics still remains limited due to their low absorbance and fast recombination rate of photoexcited charges, despite the considerable potential of photodetectors for future wearable and foldable devices. This work demonstrates a heterogeneous, all-carbon photodetector composed of graphene electrodes and porphyrin-interfaced single-walled CNTs (SWNTs) channel, exhibiting high photoresponse, flexibility, and full transparency across the device. The porphyrin molecules generate and transfer photoexcited holes to the SWNTs even under weak white light, resulting in significant improvement of photoresponsivity from negligible to 1.6 × 10-2 A W-1 . Simultaneously, the photodetector exhibits high flexibility allowing stable light detection under ≈50% strain (i.e., a bending radius of ≈350 µm), and retaining a sufficient transparency of ≈80% at 550 nm. Experimental demonstrations as a wearable sunlight sensor highlight the utility of the photodetector that can be conformally mounted on human skin and other curved surfaces without any mechanical and optical constraints. The heterogeneous integration of porphyrin-SWNT-graphene may provide a viable route to produce invisible, high-performance optoelectronic systems.

A highly sensitive flexible strain sensor based on the contact resistance change of carbon nanotube bundles.

A novel carbon nanotube (CNT)-based flexible strain sensor with the highest gauge factor of 4739 is presented. CNT-to-CNT contacts are fabricated on a pair of silicon electrodes fixed on a PDMS specimen for both flexibility and electrical connection. The strain is detected by the resistance change between facing CNT bundles. The proposed approach could be applied for diverse applications with a high gauge factor.

Development of flexible tactile sensor based on contact resistance of integrated carbon nanotubes

We have developed a novel three dimensional tactile sensor based on vertically aligned carbon nanotubes. The carbon nanotubes were directly synthesized on silicon microstructures and these CNTs-on-microstructures were integrated to flexible polydimethylsiloxane layers. Each tactile sensor has four sensing parts and the direction of force can be detected by monitoring the increase or decrease of electrical resistance in each sensing part. High gauge factor up to 272 and fast response less than 10 ms have been experimentally verified from the presented tactile sensor. The deviated contact resistance change from the initial value was less than 3% after repeated force input of 15 mN for 180,000 cycles.

Multidirectional flexible force sensors based on confined, self-adjusting carbon nanotube arrays

We demonstrate a highly sensitive force sensor based on self-adjusting carbon nanotube (CNT) arrays. Aligned CNT arrays are directly synthesized on silicon microstructures by a space-confined growth technique which enables a facile self-adjusting contact. To afford flexibility and softness, the patterned microstructures with the integrated CNTs are embedded in polydimethylsiloxane structures. The sensing mechanism is based on variations in the contact resistance between the facing CNT arrays under the applied force. By finite element analysis, proper dimensions and positions for each component are determined. Further, high sensitivities up to 15.05%/mN of the proposed sensors were confirmed experimentally. Multidirectional sensing capability could also be achieved by designing multiple sets of sensing elements in a single sensor. The sensors show long-term operational stability, owing to the unique properties of the constituent CNTs, such as outstanding mechanical durability and elasticity.

Length controlled in-plane synthesis of aligned carbon nanotube array by micromechanical spring

We have demonstrated length-controllable in-plane synthesis of aligned carbon nanotube (CNT) array on microfabricated structures using micromechanical springs. The micromechanical spring provides precise compressive stress during the chemical vapor deposition process for CNT growth. Different loading results in different final length of the CNT array, as well as different alignment and defectiveness of the individual CNT. The length and the alignment of CNT array according to the spring stiffness were measured inside scanning electron microscope. The defectiveness of CNT was examined using micro-Raman spectroscopy, and the intensity ratio between D- and G-band was analyzed.

Fabrication of carbon nanotube-coated fabric for highly sensitive pressure sensor

We have developed a highly sensitive pressure sensor using carbon nanotube (CNT)-coated polyester fabric. With simple and cost-effective dip-coating process using CNT ink, we produced conductive polyester fabric, and its resistance could be controlled easily by modulating dipping number. When the pressure is applied, the highly porous structure of the fabric allows the dramatic improvement of the mechanical contacts between fibers, leading to more electrical contacts in the CNT networks. The fabric also showed high mechanical flexibility and robustness against bending, without any significant electrical degradation. We observed a decrease in resistance of the fabricated sensor under external pressure and the maximum sensitivity of the sensor reached ∼10.63%/kPa for pressure up to 10kPa, which is two orders of magnitude higher than that of the existing CNT-polymer composite-based pressure sensors. The proposed sensor could serve as a flexible and cheap pressure sensing solution for wearable electronics or human-machine interface.

Flexible and transparent NO 2 sensor using functionalized MoS 2 with light-enhanced response

We have developed a flexible and transparent NO<inf>2</inf> sensor based on SnO<inf>2</inf>-functionalized MoS<inf>2</inf> and presented its visible light-assisted gas sensing response for the first time. A mechanism of NO<inf>2</inf> detection assisted by visible light was proposed and experimentally verified. The fabricated sensor was able to detect NO<inf>2</inf> at various concentrations ranging from 9 down to 3 ppm at room temperature and under visible light illumination. We observed sensitive decrease in the current at the exposure to NO<inf>2</inf> while the pristine MoS<inf>2</inf>-based sensor did not show any significant response.