Biomimetic Turbinate-like Artificial Nose for Hydrogen Detection

Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 24386−24394

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Biomimetic Turbinate-like Artificial Nose for Hydrogen Detection Based on 3D Porous Laser-Induced Graphene Jianxiong Zhu,†,⊥ Minkyu Cho,†,⊥ Yutao Li,‡,⊥ Incheol Cho,† Ji-Hoon Suh,† Dionisio Del Orbe,† Yongrok Jeong,† Tian-Ling Ren,*,‡ and Inkyu Park*,† †

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Mechanical Engineering and KI for NanoCentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea ‡ Institute of Microelectronics, Tsinghua University, Beijing 100084, China S Supporting Information *

ABSTRACT: Inspired by the turbinate structure in the olfaction system of a dog, a biomimetic artificial nose based on 3D porous laser-induced graphene (LIG) decorated with palladium (Pd) nanoparticles (NPs) has been developed for room-temperature hydrogen (H2) detection. A 3D porous biomimetic turbinate-like network of graphene was synthesized by simply irradiating an infrared laser beam onto a polyimide substrate, which could further be transferred onto another flexible substrate such as polyethylene terephthalate (PET) to broaden its application. The sensing mechanism is based on the catalytic effect of the Pd NPs on the crystal defect of the biomimetic LIG turbinate-like microstructure, which allows facile adsorption and desorption of the nonpolar H2 molecules. The sensor demonstrated an approximately linear sensing response to H2 concentration. Compared to chemical vapor-deposited (CVD) graphene-based gas sensors, the biomimetic turbinate-like microstructure LIG-gas sensor showed ∼1 time higher sensing performance with much simpler and lower-cost fabrication. Furthermore, to expand the potential applications of the biomimetic sensor, we modulated the resistance of the biomimetic LIG sensor by varying laser sweeping gaps and also demonstrated a well-transferred LIG layer onto transparent substrates. Moreover, the LIG sensor showed good mechanical flexibility and robustness for potential wearable and flexible device applications. KEYWORDS: biomimetic sensor, 3D porous graphene, hydrogen sensor, laser-induced graphene, palladium nanoparticles

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INTRODUCTION Hydrogen (H2) is one of the most important energy resources due to its abundance, absence of hazardous byproducts during an energy generation process, and various industrial applications including hydrogen-cooled systems, petroleum refinement, and metallurgic processes.1−9 However, it is not only volatile and extremely flammable in nature with its concentration above 4% but also colorless and odorless and thereby cannot be detected by ordinary human senses. Thus, the development of a high-performance sensor for early and accurate detection of H2 leakage is essential for human safety and environmental protection. Over the past several decades, researchers have developed H2 sensors based on solid electrolytes, semiconducting metal oxide nanomaterials, carbon nanotubes, graphene, and so on.10−18 Among these sensing materials, graphene has a mechanically flexible 2D structure, and it has a high surface area (theoretical area of ∼2630 m2/g) and high electron mobility at room temperature (RT).19−26 However, despite the excellent gas sensing properties of graphene toward various polar molecules, it is insensitive to most nonpolar molecules such as H2. Therefore, noble metal catalysts coated onto graphene have been used for H2 sensing. © 2019 American Chemical Society

Among different kinds of noble metal catalysts, the palladium (Pd) nanoparticle (NP) has been demonstrated as a good H2 sensing material due to its high H2 solubility at RT.27−36 Johnson et al. reported chemical vapor deposition (CVD)based multilayer graphene/Pd for H2 sensing; however, the reported multilayer graphene nanoribbon was obtained through 800 °C thermal de-intercalation, the vacuum-filtration process, and anodic alumina (AAO) filter membranes, and therefore, it required a time-consuming and expensive process.30 Lupan et al. introduced ultralight aerographite microtubes for H2 sensing.35 However, it required a zinc oxide template in a CVD process at 760−900 °C for synthesis.32 All of these methods presented difficulty for material synthesis and thus restricted the development of graphene-based H2 sensing devices. On the other hand, laser-induced graphene (LIG) has been demonstrated as an effective method to directly synthesize porous 3D graphene on a polyimide (PI) substrate.37−41 Tour et al. first reported LIG from commercial Received: March 13, 2019 Accepted: June 13, 2019 Published: June 13, 2019 24386

DOI: 10.1021/acsami.9b04495 ACS Appl. Mater. Interfaces 2019, 11, 24386−24394

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ACS Applied Materials & Interfaces

Figure 1. Biomimetic turbinate-like structure of a dog’s nose: (a) an ordinary dog with a strong biological olfaction system, (b) CT scan image of a dog’s turbinate, and (c) a schematic diagram of a turbinate. (d) SEM image of the surface morphology of the biomimetic turbinate-like LIG structure and (e) cross-sectional image of the LIG structure.

Figure 2. (a) Fabrication process of the LIG-GS and TLIG-GS. (b) Schematic of Pd NPs on LIG-GS and TLIG-GS. (c) Transferred line pattern of 3D porous LIG. (d) Image of bendable TLIG-GS. (e) Morphology of TLIG and (f) cross-sectional image of TLIG. (g) Wide XPS scan of the LIG/ Pd film, (h) XPS scan of carbon, and (i) XPS scan of Pd.

polymers for supercapacitor application,37 which is a one-step method to produce porous graphene using a CO2 infrared laser. Compared to the synthesis of graphene using traditional CVD methods,42−48 the LIG method is capable of a roomtemperature process and direct patterning of graphene on a flexible polymer substrate in an ambient environment. Furthermore, it can generate 3D porous graphene micro-

structures with a larger surface area than those of a 2D graphene nanosheet. In sensory organs of animals, porous microstructures with large surface areas are very useful for enhancing the sensing performance. For example, in dogs’ noses, the turbinate serves as the most important odor perception structure. A major function of the turbinate is to increase the surface area of a dog’s internal nose. Another important role of the turbinate is 24387


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Figure 3. Schematic illustration of H2 sensing of the LIG/Pd sensor: (a) multilayer LIG, (b) LIG/Pd, and (c) H2 absorbed on LIG/Pd. (d) Crosssectional schematic image of the LIG/Pd in dry air, (e) catalytic reaction of H2 on LIG/Pd, and (f) reaction of H2 at a higher concentration. (g) Band energy analysis of LIG: Ef is the Fermi energy, Ec is the conduction energy, and Ev is the valence energy. (h) Band energy analysis of the H2 gas acting onto LIG.

to propel the respiratory air and carry more scent molecules toward the olfaction nerve receptors in nasal airways.49−52 Zhou et al. introduced artificial dog-nose NO2 sensors using a capillary structure and a crumpled structure of reduced graphene oxide nanosheets;53,54 however, the turbinate-like 3D structure using laser-induced graphene for a H2 sensor has not been explored. Inspired by the turbinate structure of biological olfaction in the dog’s nose, we took advantage of the LIG to produce a biomimetic artificial nose for H2 detection at RT. It is demonstrated that the biomimetic artificial nose can be electrically modulated by controlling the laser sweeping gaps and also can be transferred to transparent and flexible substrates for further applications. Compared to the existing CVD graphene-based gas sensor, we found that the biomimetic turbinate-like LIG-gas sensor showed ∼1 time higher sensing performance with a much simpler fabrication method. Moreover, the biomimetic artificial nose for H2 was integrated with a gas module for Internet of Things (IoT) applications.

Figure S2a, and its higher magnification image is shown in Figure S2b. Figure 2a depicts the fabrication process of an LIG-gas sensor (LIG-GS) on a PI substrate and the transferred LIG-gas sensor (TLIG-GS) on a PET substrate. to provide the sensitivity and selectivity to the H2 gas, Pd NPs were coated onto the surface of the entire device using the e-beam evaporation process (2 nm thickness of Pd film deposition). The thickness of Pd NPs56 was 2 nm, generating no electrical conduction on the other part of our device (Figure S4). The sensing component for the H2 gas sensor was LIG with coated Pd NPs, not the nonconductive component (i.e., Pd NPs). Figure 2b illustrates the schematic of Pd NPs on LIG. Figure 2c illustrates the transferred line pattern to show the transferred LIG with an optical microscope image. Figure 2d depicts a bending state of the biomimetic TLIG-GS, which was used to demonstrate its flexibility. The turbinate-like LIG microstructure transferred onto the PET substrate after UV light exposure is shown in Figure 2e, and the cross-sectional image of the transferred LIG (TLIG) is displayed in Figure 2f. By comparing the microstructure of as-grown LIG and TLIG in Figure 1d and Figure 2e, we concluded that the LIG was transferred well to the PET substrate, maintaining its original microstructure. The SEM images show that the LIG demonstrated a much higher 3D surface area than 2D flat carbon or CVD-based graphene materials. X-ray photoelectron spectroscopy (XPS) measurements of the TLIG/Pd were performed to investigate C 1s and Pd 3d, as shown in Figure 2g−i. The high-resolution spectra of the C 1s peak with four components, namely, C−C (284.5 eV), C−O (286.3 eV), CO (288.7 eV), and OCO (291.6 eV), could be observed in Figure 2h. The XPS spectra of the Pd 3d regions of Pd on the LIG are shown in Figure 2i. The Pd 3d5/2 states at 340.6 eV and Pd 3d3/2 states at 346.2 eV indicate a small positive value relative to pure polycrystalline Pd. It is interesting that CVD graphene on silicon is known to exhibit a p-type semiconducting behavior;24,45 however, the produced LIG was not pure graphene (2.3% yields) but contained both graphite and graphene.55 This LIG contained considerable oxygen and nitrogen atoms shown in the XPS data (Figure S5),

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RESULTS AND DISCUSSION Figure 1a shows an ordinary dog that owns a strong biological olfaction system. The air flows into the dog’s nostril and quickly reaches the prolonged turbinate bones during the inhalation process. Figure 1b depicts a computed tomography (CT) image of a common turbinate structure that is located laterally in the nasal cavity of the dog’s nose. The schematic diagram of the turbinate-like structure in the dog is shown in Figure 1c for a clearer view. The detailed CT scan process of a dog’s head is shown both in Figure S1 and Video S1. The role of the turbinate in a dog’s nose is to increase the surface area of the olfaction system, which results in more scent molecules toward olfaction nerve receptors of the nasal airways. Figure 1d depicts the surface morphology of a porous biological turbinate-like LIG structure resulting from the rapid liberation of gaseous products, and Figure 1e shows its cross-sectional image on a PI film. The formation of this turbinate-like structure can be explained by the dramatically high thermal expansion due to localized laser irradiation onto the PI film. The LIG formed along the laser sweeping line is shown in 24388


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Figure 4. Effect of the laser sweeping parameter: (a) laser sweeping pattern; (b) cross-sectional microstructure of the laser sweep where the yellow line means the current flow trace; (c) photographs of the TLIG-GS, d = 0.22 mm and d = 0.15 mm; and (d) schematic of the laser sweep and current flow direction. (e) I−V characteristics of the LIG/Pd and TLIG/Pd devices.

which most likely provided the LIG an n-type behavior.57−59 The characterization of the energy dispersive X-ray spectroscopy (EDS) of the TLIG/Pd is shown in Figure S6 of the Supporting Information, which reveals the existence of Pd NPs in two different locations. Figure S7a displays the X-ray diffraction (XRD) of patterned LIG/Pd with an intense peak centered at 2θ = 26.3°, giving an interlayer spacing (Ic) of ∼3.3858 Å at the (002) plane by calculating with JADE 6 software (Material Data Inc., California, United States). Figure S7b depicts the Raman spectroscopy of 3D LIG with three prominent peaks: the D peak at 1350 cm−1 induced by defects or bent sp2 carbon bonds, the first-order allowed G peak at 1580 cm−1, and the 2D peak at 2700 cm−1 originating from second-order zone-boundary phonons. The 2D band profile was typical for graphite consisting of randomly stacked graphene layers. The LIG structures were investigated by transmission electron microscopy (TEM) in Figure S8a,b, which reveals multilayer graphene. The average lattice space of ∼3.4 Å corresponds to the distance between two neighboring (002) planes in graphitic materials. The aberration-corrected scanning TEM image shows the unusual ultrapolycrystalline feature of LIG with disordered-grain boundaries. The hexagon lattices of graphene can be observed in Figure S8c. The schematic diagrams of atomic structures of the LIG and LIG/Pd are depicted in Figure 3a,b, respectively. Figure 3c further shows the schematic diagram of the atomic structure of the LIG/Pd when they react with H2 molecules. When the H2 concentration increases (see Figure 3d−f) at RT, it causes structural changes in Pd with H2 adsorption. During H2 detection, the LIG acts as a conductive path between silver electrodes, and the nonpolar molecules of H2 interact with both Pd NPs on the surface of LIG and on graphene crystal defects in LIG. The adsorption of H2 molecules on Pd NPs results in resistance change due to the electrons transferred in defect holes of LIG. The n-type LIG graphene on the polymer is shown in Figure 3g−h. This n-type behavior contributes to the polarization of defects during the adsorption of gas molecules, which significantly alters the electrical conduction

characteristics of LIG. The charge transfer between the metal and graphene depends on the difference between the Fermi level of the metal and the charge neutrality level of graphene. When the H2 gas reacts with the Pd NPs, the Fermi energy level of Pd goes down, and this increases the charge carrier density in the LIG, decreasing its electrical resistance. The H2 response of the sensor involving Pd is governed by the spillover effect depicted in Figure 3d−f. The dissociative chemisorption of H2 onto the Pd NPs and their subsequent transfer to the adjoining adsorbate (i.e., graphene) upon saturation enables the sensitivity of the LIG sensor to H2 molecules.15,21 H2 is incorporated onto the Pd lattice in the form of PdHx, and this lowers the work function of Pd, resulting in the transfer of electrons from Pd to graphene underneath. These reactions in the adsorption and desorption processes of H2 with Pd can be described as follows16,33 adsorption: 3H 2 ↔ 6[H] ; 6[H] + 2Pd + O2 ↔ 2Pd/H + 2H 2O desorption: O2 + 2Pd/H + 2[H] → 2Pd + 2H 2O

(1) (2)

During the H2 adsorption process, the oxygen molecules combine with dispersed H2, and Pd is converted to Pd/H. The H2 atoms react with both Pd NPs and oxygen molecules. The response of the LIG-GS is obtained by the resistance variation with different H2 concentrations. To evaluate the gas sensing performance, the response of the Pd-coated LIG-GS is defined as follows S=

ΔR × 100% R0

(3)

where R0 and ΔR represent the initial resistance measured under the carrier gas and the resistance change recorded after the sensor was exposed to H2, respectively. The effect of the laser sweeping gap on the sensor performance was experimentally investigated as follows. Figure 4a,b depicts the top and cross-sectional views of the laser 24389


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Figure 5. Sensor response to H2 at RT conditions. (a) Surface morphology of LIG microstructure before transfer and (b) after transfer. (c) LIG-GS and (d) TLIG-GS. The device initial resistance R0 in the diagram was used for a better gas response observation with and without Pd NPs, and the laser sweeping line was 0.15 mm in both LIG-GS and TLIG-GS. (e) Response vs H2 concentrations.

Figure 6. Response of TLIG-GS to the H2 gas: (a) without bending, (b) with a bending radius of 3 cm, and (c) with a bending radius of 2 cm. (d) Response vs H2 concentration with different bending states. (e) Image of a gas sensor system for a test. (f) Schematic diagram of the gas sensor system by integration of TLIG-GS and as a sensing module and (g) measured data of H2 sensing experiment from the sensor module. 24390


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ACS Applied Materials & Interfaces sweeps onto the PI film, respectively. The electrical current flows in the transverse direction of laser sweeping. To investigate the electronic properties of LIG/Pd and TLIG/ Pd devices and the effect of the laser sweeping gap (Figure 4c,d), their current−voltage (I−V) characteristics were measured as shown in Figure 4e. We found that the conductance of the LIG/Pd device was ∼20% higher than that of the TLIG/Pd device. It was also demonstrated that the conductance of the LIG can be controlled by the gap of the laser sweeping lines. The controllability of the load resistance of the LIG-GS can be potentially used for various sensor interfaces requiring different load resistances. The gas responses of the LIG-GS and TLIG-GS are plotted in Figure 5c−e. The flat curves in the figure indicate that the pristine LIG without Pd decoration has no response to the H2 gas. More interestingly, we found that the shapes of sensing response curves for the LIG-GS and TLIG-GS devices are different. The cause of this variation is presumed to be the generation of cracks (flakes) in the graphene microstructure during the process of mechanical transfer as shown in Figure 5a,b. The TLIG-GS showed an ∼20% lower sensitivity in average than the LIG-GS in Figure 5e. It is also presumed that the loss of sensitivity is due to the cracks in the graphene microstructure during the process of mechanical transfer, as shown in Figure 5a,b. The real-time measurement of the TLIG-GS is shown in Video S2 of the Supporting Information. Figure S9 was used to explain why we chose 2 nm Pd thicknesses for the surface functionalization of LIG. Figure S10 depicts the turbinate microstructure LIG presenting 2−3 times higher sensing performance than underpowered “flat LIG”. Figure S11 shows the repeated gas sensor experiment for both the LIG-GS and TLIG-GS. We observed that both sensors showed a good sensing performance during the H2 sensing experiment. To investigate the selectivity of the TLIG-GS, NO2 and NH3 gases were used for the measurement at RT (Figure S12, Supporting Information). It was found that the LIG-GS had no response to those gases. Because H2 reacts selectively to Pd forming PdHx, the fabricated Pd sensors have high selectivity to the H2 gas. The permissible exposure limits (PELs) set by the Occupational Safety and Health Administration (OSHA) for the test gases were 5 ppm for NO2 and 25 ppm for NH3. The tested concentration for H2 for comparison was much higher than the concentration ranges for other test gases, but considering the lower flammability limit of H2 (4%), our gas exhibited good selectivity over the other test gases in practical applications. In the LIG/Pd H2 sensor, Pd NPs provided a highly selective catalytic reaction to H2 only. NO2 and NH3 reacted neither with LIG nor with a Pd catalyst from our observation. To demonstrate the mechanical flexibility of the TLIG-GS, the gas sensing tests in different bending deformation states were conducted as shown in Figure 6a−c. The TLIG-GS was fixed on 3D-printed half-cylindrical convex blocks with three different radii of curvature ρ: ∼∞, 3 cm, and 2 cm. The reason for the increased resistance baseline with a large bending state was due to the tensile strain under bending. Nevertheless, we found that the TLIG-GS presented good gas sensing performances with different bending states, as shown in Figure 6d. As shown in Figure S13, there was no evident degradation after long-term reservation (a month), and this reflects the long-term durability of the TLIG-GS sensor. The mechanical durability of the TLIG-GS is demonstrated in Figure S14, showing the results before and after 100,000

bending cycles (∼1.5 cm maximum radius of curvature and ∼0.11 Hz). The video recording during the mechanical bending is shown in Video S3 of the Supporting Information. Table S1 (Supporting Information) shows the advantages and disadvantages in the state-of-the-art graphene-based H2 gas sensors. Even though the graphene-based H2 gas sensor can achieve higher sensitivity in refs 30 and 32, the hightemperature environment could result in an unavoidable explosion of H2, which is not suitable for the real application. Compared to the graphene-based sensors in refs 34 and 35, we found that the TLIG-GS showed a relatively higher sensing performance with a much simpler and low-cost fabrication method. To show practical applications of TLIG-GS, a gas sensing module60 was integrated with the TLIG-GS (Figure 6e−g). This gas module was made on a printed circuit board through the combination of analog/digital signal processing and a highly reconfigurable circuit structure, which together can be directly applied to IoT use. The schematic diagram and real images of the gas sensing system are shown in Figure 6e, and the data collected by the sensing module is presented in Figure 6g. It was demonstrated that the gas response with the integration system works well based on the repeated sensing tests to 2% H2 gas.

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CONCLUSIONS This study demonstrated the development of a biomimetic artificial nose for RT H2 sensing based on the laser-induced 3D porous turbinate-like network structure of graphene. This graphene network was made using a one-step laser-induced method on a PI substrate and also could be transferred onto a transparent PET film. To broaden its applications, a modulated resistance of the LIG-GS was demonstrated by controlling the laser sweeping gaps, and a viable transfer method of LIG on a transparent flexible substrate for H 2 gas sensing was introduced. The reported sensor revealed a good response to the H2 gas with excellent selectivity against other interfering gases. Furthermore, it showed good mechanical flexibility and robustness in H2 sensing application. The study of these 3D porous turbinate-like networks of LIG for a transferable and flexible H2 gas sensor not only provides a deep understanding and broadens the applications of the biomimetic artificial nose based on LIG but also provides a facile fabrication method of graphene-based gas sensors and low-cost devices toward IoT applications.

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EXPERIMENTAL SECTION

Fabrication of LIG. The turbinate-like graphene microstructure was synthesized directly on a PI film through consumer-grade CO2 infrared laser equipment at a power of 50 W (MYL-0705, MyCNC Company, Korea). The wavelength of this CO2 infrared laser was ∼10.6 μm with a pulse duration of ∼14 μs. The black color in Figure 2c is patterned graphene that was synthesized by the LIG process. Even with producing a large number (more than 60 LIG devices) of 3D porous graphene devices, the fabrication time for those devices is within ∼5 min (Supporting Information, Figure S3a). Fabrication of TLIG-GS. For broader applications in flexible sensors, we demonstrated a transferable graphene-based gas sensor device on a transparent PET or thinner PI substrate (Figure S3b,c, Supporting Information). A sticky SU-8 photoresist was used as a transfer adhesive, which was spin-coated onto the PET film by ∼10 μm of thickness. Afterward, the SU-8 coated PET film and the patterned LIG on the PI film were stacked together followed by UV exposure for 30 s. Finally, the transfer of the LIG layer was completed after peeling off the PI substrate from the PET film. Pd NPs were 24391


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ACS Applied Materials & Interfaces coated onto the surface of the device using e-beam evaporation of the Pd thin film by 2 nm to provide the sensitivity to the H2 gas. Finally, the Ag paste was added onto the LIG film as a metal electrode. Figure 2d illustrates the transferred logo pattern of “KAIST”. The nanostructure of LIG can be found in Figure S3d in the Supporting Information. Material Characteristics. The surface morphology of the 3D porous turbinate-like LIG was characterized by a scanning electron microscope (SEM, SIRON-100, FEI Corp., Netherlands). X-ray photoelectron spectroscopy of the pristine TLIG and TLIG/Pd was performed using equipment (XPS, K-Alpha, Thermo VG Scientific, Pittsburgh, USA). Gas Platform and Measurement. Figure S15 (Supporting Information) depicts the setup for H2 gas sensing measurements. Three mass flow controllers (MFCs) were connected to the gas measurement chamber to provide the inlet gases. The ratio and flow rate of the gas, including H2, N2, and O2, were accurately controlled using MFCs. H2 in the carrier gases was injected into the gas measurement chamber through a mixer joint. Two signal wires in the chamber were used to connect the LIG-GS on a home-made probe station. The probe station was made from two copper probes and a substrate holder to connect the LIG-GS with a Keithley 2400B source meter. The H2 sensing characteristics of the LIG-GS were measured by using this two-probe station with the mentioned source meter to record the resistance change during the measurement. The computer was used to adjust the MFCs and collect the measurement data automatically.

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convergence technology development program for the bionic arm through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (no. 2017M3C1B2085318). We also thank Hojung Choi and Youngwon Lee (College of Veterinary Medicine, Chungnam National University, Korea) who provided CT images of a dog’s head.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b04495. CT scan of an ordinary dog head to show the turbinate structure in dog nose; description of the materials; measurement results and analysis of the device; comparison of the advantage of graphene-based gas sensor in our work and the current state-of-the-art ones (PDF) CT scan of an ordinary dog’s head to show the turbinate structure in the dog’s nose (AVI) Measurement test (AVI) Mechanical reliability test (AVI)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (T.-L.R.). *E-mail: [email protected] (I.P.). ORCID

Minkyu Cho: 0000-0002-0006-2063 Inkyu Park: 0000-0001-5761-7739 Author Contributions ⊥

J.Z., M.C., and Y.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Multi-Ministry Collaborative R&D Program (Development of Techniques for Identification and Analysis of Gas Molecules to Protect Against Toxic Substances) through the National Research Foundation of Korea (NRF) funded by KNPA, MSIT, MOTIE, ME, and NFA (no. NRF-2017M3D9A1073863), the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (no. 2018R1A2B2004910), and the 24392


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