Improved photo- and chemical-responses of graphene via porphyrin-functionalization for flexible, transparent, and sensitive sensors

Pyo, Soonjae,Choi, Jungwook,Kim, Jongbaeg IOP 2019 Nanotechnology Vol.30 No.21

<P>The functionalization of graphene with organic molecules is beneficial for the realization of high-performance graphene sensors because functionalization can provide enhanced functionalities beyond the properties of pristine graphene. Although various types of sensors based on organic-graphene hybrids have been developed, the functionalization processes have poor thickness-controllability/reliability or require post-processing, and sensor applications rely on conventional, rigid substrates such as SiO<SUB>2</SUB>/Si. Here, a flexible and transparent metalloporphyrin (MPP)-graphene hybrid for sensitive UV detection and chemical sensing is demonstrated. MPP, which provides strong light absorption, redox chemistry, and catalytic activity, is simply deposited onto graphene via one-step evaporation. Optical and electronic characterizations confirm that the graphene is successfully functionalized by MPP while maintaining its outstanding electronic properties. The MPP-functionalization greatly improves the photo- and chemical-sensing performances of the graphene, resulting in over 200% enhanced sensitivities for both UV light (365 nm) and toluene. Simultaneously, the MPP-graphene sensor exhibits no considerable change in electrical resistance under bending conditions, and remarkable optical transmittance in the visible range. On the basis of the excellent performances of the MPP-graphene hybrid, including high sensitivities, flexibility, transparency, and the ease and cost-effectiveness of the MPP-functionalization, it will be a promising candidate for flexible and transparent sensor applications.</P>

Sensitivity enhancement in photoionization detector using microelectrodes with integrated 1D nanostructures

Pyo, Soonjae,Lee, Kyounghoon,Noh, Taegyoon,Jo, Eunhwan,Kim, Jongbaeg Elsevier 2019 Sensors and actuators. B Chemical Vol.288 No.-

<P><B>Abstract</B></P> <P>Photoionization detectors (PIDs) that use high-energy photons to ionize gas molecules have attracted considerable attention as volatile organic compound (VOC) sensors owing to their high sensitivity, selectivity, and reliability. Recently, miniaturized PIDs have been developed to further improve the sensing performance. However, most studies have focused solely on the miniaturization of the ionization chamber, although the electrode is an important factor for determining the sensitivity and operation voltage. This work demonstrates a PID composed of Si microelectrodes fabricated via microfabrication with one-dimensional (1D) nanostructures coated on them. The interdigitated microelectrodes offer a larger surface area capable of enhanced capturing of positively charged ions and electrons compared to typical mm-scale electrodes, resulting in the successful detection of 500 ppb toluene at an applied voltage of 0.2 V. Furthermore, taking advantage of the outstanding surface-to-volume ratio of 1D nanostructures, Ag nanowire-coated microelectrodes and carbon nanotube (CNT)-coated microelectrodes exhibit 38% and 76% improvement in sensitivity, respectively, than the bare Si microelectrodes. The CNT-coated microelectrodes show superior linearity and repeatability at toluene concentrations from 0.1 to 1.0 ppm. The proposed PID will be a promising candidate for portable VOC sensors.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Photoionization detector based on 1D-nanostructure-coated microelectrodes was proposed. </LI> <LI> Proposed photoionization detector was able to detect 500 ppb of toluene at an applied voltage of 0.2 V. </LI> <LI> Ag-NW-coated and CNT-coated electrodes provided sensitivity enhancement of 38 and 76%, respectively, than bare Si electrodes. </LI> <LI> CNT-coated electrodes exhibited excellent linearity and repeatability. </LI> </UL> </P>

Frequency Up-Conversion Hybrid Energy Harvester Combining Piezoelectric and Electromagnetic Transduction Mechanisms

Soonjae Pyo,Dae-Sung Kwon,Hee-Jin Ko,Youngkee Eun,Jongbaeg Kim 한국정밀공학회 2022 International Journal of Precision Engineering and Vol.9 No.1

A hybrid energy harvester with frequency up-conversion structures is proposed. The harvester achieves a high power output by utilizing both piezoelectric and electromagnetic transduction mechanisms. The harvester comprises a flexible substrate and two (internal and external) cantilevers. The internal and external cantilevers used for piezoelectric and electromagnetic conversion, respectively, are arranged such that the piezoelectric internal cantilever can vibrate with a large displacement to produce high output power. We use a frequency up-conversion method to convert the bending of the harvester into the vibration of the structure so that the harvester can generate energy even from the mechanical motion with an extremely low frequency. Two harvester configurations are investigated to validate the effect of the relative positions of the coil and magnet on the output voltage of the harvester. The maximum power output of the hybrid harvester is 7.38 mW, with outputs of 1.35 and 6.03 mW for piezoelectric and electromagnetic conversion, respectively.

인간-기계 상호작용을 위한 촉각 시스템 기술

표순재(Soonjae Pyo) Korean Society for Precision Engineering 2023 한국정밀공학회 학술발표대회 논문집 Vol.2023 No.5

Impact Ionization Induced by Accelerated Photoelectrons for Wide-Range and Highly Sensitive Detection of Volatile Organic Compounds at Room Temperature

Kang, Yunsung,Pyo, Soonjae,Jeong, Han-Il,Lee, Kyounghoon,Baek, Dae-Hyun,Kim, Jongbaeg American Chemical Society 2019 ACS APPLIED MATERIALS & INTERFACES Vol.11 No.22

<P>Ionization-based volatile organic compound (VOC) sensors that use photons or electrons operating at room temperature have attracted considerable attention as a promising alternative to conventional metal oxide-based sensors that require high temperature for sensing function. However, the photoionization sensors cannot ionize many gas species for their limited photon energy, and field emission-based ionization sensors that rely on the breakdown voltage of specific gas species in a pure state may not tell different concentration. This work demonstrates the detection of VOCs using impact ionization induced by accelerated photoelectrons. Although the photoelectrons emitted by relatively low photon energy typically have insufficient kinetic energy to cause impact ionization, in this approach, they are accelerated between microgap electrodes to enhance their kinetic energy such that the impact ionization of VOCs can be achieved. The demonstrated gas sensor sensitively detects toluene concentration in a wide range from 1000 ppm to 100 ppb with fast response and recovery time at room temperature. Additionally, diverse VOC species including benzene, <I>p</I>-xylene, and even acetylene with high ionization energy can be detected. The proposed method could be a viable solution for VOC sensors with low cost, scalable producibility, and high performance.</P> [FIG OMISSION]</BR>

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

Kang, Yunsung,Pyo, Soonjae,Jo, Eunhwan,Kim, Jongbaeg IOP 2019 Nanotechnology Vol.30 No.35

<P>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 (MoS<SUB>2</SUB>) 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 MoS<SUB>2</SUB> after exposure to the target gas. This work demonstrates the recovery of the initial resistance of reacted chemical vapor deposition-grown MoS<SUB>2</SUB> by illuminating it with a UV light-emitting diode (LED). A novel mechanism involving photo-generated electron–hole pairs in MoS<SUB>2</SUB> is proposed and experimentally verified. The fabricated sensor detects nitrogen dioxide (NO<SUB>2</SUB>) and distinguishes between concentrations from 1 to 10 ppm with the proposed recovery process. Reversible detection after repeated exposure to 5 ppm NO<SUB>2</SUB> 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 MoS<SUB>2</SUB>.</P>

Mechanical Metamaterials for Sensor and Actuator Applications

Keun Park,Soonjae Pyo 한국정밀공학회 2024 International Journal of Precision Engineering and Vol.11 No.1

Mechanical metamaterials (MMs) have emerged as a promising class of engineered materials, distinguished by their unique mechanical behaviors derived from architectural design rather than inherent composition. Recent advancements in the design, structural analysis, and manufacturing technology of architectured MMs have resulted in the development of high-performance sensors and actuators with new functionalities. In this review, important advances in MM-based sensors and actuators over recent years are summarized, from structural designs to fabrication and applications. We briefly outline the fundamental mechanical characteristics of key MMs, such as auxetic-, chiral-, pentamode-, origami-, and kirigami-based MMs, with an emphasis on the principles that drive their unique mechanical behaviors. We also discuss various fabrication methods employed to realize theoretical MM designs. These methods encompass additive manufacturing, subtractive manufacturing, micro-molding/casting, and other fabrication techniques that enable the production of complex MM structures. Moreover, we comprehensively explore the applications of MMs in sensors by categorizing their implementations as strain, pressure, and multimodal sensors, highlighting their superior performances compared to conventional devices. Additionally, we examine MM-based mechanical, pneumatic, thermal, and other types of actuators, showcasing their enhanced capabilities and functionalities. Finally, we provide perspectives on current issues that must be addressed to fully leverage the potential of MM in this field. By summarizing recent advances in MM-based sensors and actuators, this review paper aims to provide a comprehensive understanding of the design, fabrication, and applications of MMs in the context of mechanical engineering.

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

Lee, Jae-Ik,Pyo, Soonjae,Kim, Min-Ook,Kim, Jongbaeg IOP Pub 2018 Nanotechnology Vol.29 No.5

<P>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.</P>

Flexible and multi-directional piezoelectric energy harvester for self-powered human motion sensor

Kim, Min-Ook,Pyo, Soonjae,Oh, Yongkeun,Kang, Yunsung,Cho, Kyung-Ho,Choi, Jungwook,Kim, Jongbaeg Institute of Physics Publishing 2018 Smart materials & structures Vol.27 No.3

<P>A flexible piezoelectric strain energy harvester that is responsive to multi-directional input forces produced by various human motions is proposed. The structure of the harvester, which includes a polydimethylsiloxane (PDMS) bump, facilitates the effective conversion of strain energy, produced by input forces applied in random directions, into electrical energy. The structural design of the PDMS bump and frame as well as the slits in the piezoelectric polyvinylidene fluoride (PVDF) film provide mechanical flexibility and enhance the strain induced in the PVDF film under input forces applied at various angles. The amount and direction of the strain induced in PVDF can be changed by the direction of the applied force; thus, the generated output power can be varied. The measured maximum output peak voltage is 1.75, 1.29, and 0.98 V when an input force of 4 N (2 Hz) is applied at angles of 0°, 45°, and 90°, and the corresponding maximum output power is 0.064, 0.026, and 0.02 <I>μ</I>W, respectively. Moreover, the harvester stably generates output voltage over 1.4?×?10<SUP>4</SUP> cycles. Thus, the proposed harvester successfully identifies and converts strain energy produced by multi-directional input forces by various human motions into electrical energy. We demonstrate the potential utility of the proposed flexible energy harvester as a self-powered human motion sensor for wireless healthcare systems.</P>

동적 광 패터닝에 기반한 전기 촉각 자극 제어

김원도(Wondo Kim),표순재(Soonjae Pyo),이재용(Jaeyong Lee),최규준(Gyujun Choi),심상준(Sang Jun Sim),이재익(Jae-Ik Lee),김진혁(Jin Hyuk Kim),유호동(Ho Dong Yu),이태훈(Tae Hoon Lee),김종백(Jongbaeg Kim) 대한기계학회 2018 대한기계학회 춘추학술대회 Vol.2018 No.12