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Transparent carbon nanotube web structures with Ni-Pd nanoparticles for electromagnetic interference(EMI) shielding of advanced display devices
Journal Pre-proofs Full Length Article Transparent carbon nanotube web structures with Ni-Pd nanoparticles for electromagnetic interference(EMI) shielding of advanced display devices Jun-Beom Park, Hokyun Rho, An-Na Cha, Hyojung Bae, Sang Hyun Lee, Sang-Wan Ryu, Tak Jeong, Jun-Seok Ha PII: DOI: Reference:
S0169-4332(20)30501-8 https://doi.org/10.1016/j.apsusc.2020.145745 APSUSC 145745
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
14 January 2020 7 February 2020 11 February 2020
Please cite this article as: J-B. Park, H. Rho, A-N. Cha, H. Bae, S. Hyun Lee, S-W. Ryu, T. Jeong, J-S. Ha, Transparent carbon nanotube web structures with Ni-Pd nanoparticles for electromagnetic interference(EMI) shielding of advanced display devices, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc. 2020.145745
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2020 Published by Elsevier B.V.
Transparent carbon nanotube web structures with Ni-Pd nanoparticles for electromagnetic interference(EMI) shielding of advanced display devices Jun-Beom Park†a,b, Hokyun Rho†a, An-Na Chaa, Hyojung Baea,c, Sang Hyun Leea, Sang-Wan Ryuc, Tak Jeongb and Jun-Seok Ha*a,c aDepartment
of Advanced Chemicals & Engineering, Chonnam National University, Gwangju 61186, Republic of Korea bMicro
LED Research Center, Korea Photonics Technology Institute, Light Source Research Division, Gwangju 61007, Republic of Korea cOptoelectronics
Convergence Research Center, Chonnam National University, Gwangju 61186, Republic of Korea *Email: [email protected] † These
authors contributed equally to this work.
ABSTRACT Electromagnetic interference (EMI) has become an increasingly severe problem with the advancement in the development of electronic devices. In this study, an EMI shield was fabricated using a complex web structure made of Ni, Pd, and carbon nanotubes (CNTs). The Ni-Pd nanoparticles were evenly distributed on the surface of the CNTs. Samples with various thicknesses, specifically 10, 30, 50, and 100 nm, were fabricated and characterized. The results confirmed that the samples exhibited excellent and uniform electrical conductivity, flexibility, and transparency. In addition, the 100 nm thick shield demonstrated the capability of blocking 99.27% of the incident EMI and was successfully applied to a commercialized cell phone display. KEYWORDS: Electromagnetic interference, Pd-Ni Nanoparticles, Electroless plating, Carbon nanotubes, transparent, flexible display.
INTRODUCTION Electromagnetic pollution has been on the rise with the advancement in the development of electronic devices, creating problems such as negative impact on human health.1-4 According to a report by World Health Organization, electromagnetic waves generated from electronic devices such as mobile phones, computers, refrigerators, and TVs that are commonly used in every household can cause cancer in the human body.5 Furthermore, the development of advanced electronic devices such as wearable devices, which are in close contact with the human body could exacerbate this problem.6-8 An additional problem is the mutual interference of the electromagnetic waves within the electronic devices: electromagnetic interference (EMI) within a device can lead to its malfunction.9-11 Therefore, to solve these problems, research is being conducted to shield electromagnetic waves, and the most commonly used shielding material is metal.12-14 However, while vacuum-deposited metals have a good shielding effect owing to their high electrical conductivity, they are limited by their poor step coverage, low workability, and high cost.15-18 Research has been done to overcome this problem using composites of polymers and conductive fillers. AgNW / cellulose, one of the metal and polymer composites, reported a shielding performance of 48.6 dB at 160 µm thickness19, and about 65 dB at 1.5 mm thickness, Graphene Foam / PEDOT: PSS, one of the carbon fillers and polymer composites.20 In addition, shielding performance of about 45 decibels was reported for metal carbides (MXenes) at 1.5 micrometer thickness.21 However, these composites have the disadvantage of being difficult to apply to transparent devices such as displays because of their thick thickness and low light transmittance for sufficient shielding performance. In this study, a composite for electromagnetic shielding was fabricated using CNT, Ni, and Pd. We fabricated Ni-Pd CNT nanoplating (NP) layers by spraying to produce samples of various thicknesses, specifically 10, 30, 50, and 100 nm, and compared their optical, electrical, mechanical, and electromagnetic shielding properties to those of pure Ni films of the same thickness. Then, we applied the Ni-Pd CNT NP coating on the protective film of a cell phone to confirm its practical applicability.
EXPERIMENTAL SECTION Fabrication of Ni-Pd CNT NP
The solution in which CNT was dispersed in deionized (DI) water (0.5%) and the solution of dispersed Pd2+ ions in a weak hydrochloric acid solvent (over 59.0%) were purchased from Ho-Seong Surface Technology, and the NiSO4 (22.3%) and (CH3)2NHBH3 solutions (5.3%) were purchased for Ni nanoplating (NP) from ShinPoong Metal Co., Ltd. First, the SWCNT solution and Pd2+ solution were mixed in the volume ratio of 3:1 to prepare a 40 mL solution mixture. To this, 50 mL of a solution mixture containing 10 μL each of NiSO4 and (CH3)2NHBH3 was added to prepare a total of 90 mL of Ni-Pd CNT NP solution. The NP solution was stirred for 10 min at 100 rpm and samples were prepared by coating this solution on a 2 inch sapphire wafer via spraying to various thicknesses, specifically 10, 30, 50, and 100 nm. The Ni-Pd CNT NP samples were dried at 90 ℃ in an oven for 60 min to improve adhesion. Finally, the Ni-Pd CNT NP samples were rinsed in DI water for 10 min to remove the chlorides and surfactants. Characterization of Ni-Pd CNT NP Samples prepared by the above method were observed using a field emission-scanning electron microscope (FE-SEM; S-4700, Hitachi), and the Ni-Pd alloyed metal particles were confirmed by a transmission electron microscope (TEM; JEM-2100F, JEOL KOREA LTD.). The EDS connected with the TEM was used to determine the weight percentage of the elements. The Raman spectra were obtained using a Tokyo instruments FLEX G micro Raman system at Energy Convergence Core Facility in Chonnam National University (CNU). High resolution X-ray diffraction, (HR-XRD; Empyrean Nano Edition, Malvern Panalytical) was used to observe the intensity by thickness. The transmittance of the region between 200 and 800 nm was measured using a SHIMADZU MPC-2200 system. The electrical conductivity of the samples of different thicknesses was measured using a 4-point probe (FPP-4000M, DASOL ENG) and plotted against the transmission data. To confirm the uniformity of the electrical conductivity, 4-probe mapping (FPP-RS 8, ASRMS-1000, DASOL ENG) was performed after spraying Ni-Pd CNT NP on a 5 × 5 cm PET substrate, and the results were imaged. A PDMS substrate with a size of 1 × 5 cm was fabricated, and sprayed with Ni-Pd CNT NP. Thereafter, a stretch test of up to 220% was performed to confirm its mechanical properties. A PET substrate of dimensions 1 × 5 cm was fabricated and subjected to 100,000 bending tests. EMI shielding properties of Ni-Pd CNT NP EMI shielding of 2-9 GHz was confirmed using a Keysight E5061B ENA vector network analyzer. The substrate used for the measurement was a round PET with a diameter of 16 mm, sprayed with a 100 nm thick
layer of Ni-Pd CNT NP. For comparison, a sample was prepared by depositing pure Ni using an e-beam evaporator. The absorptivity was calculated using the measured transmittance and reflectance, and the shielding percentage was calculated in dB (see Supporting Information). The Ni-Pd CNT NP was sprayed with a thickness of 100 nm on half of the commercialized cell-phone protective films. Thereafter, the E4407B system (Agilent Technologies) was used to map the EMI detected in the display and imaged.
RESULTS AND DISCUSSION We used an easy and simplistic procedure to fabricate a thin Ni-Pd CNT NP layer. This composite layer can be applied to a wide area by spraying. The solution used for spraying included CNTs containing Ni and Pd. The Ni-Pd CNT NP solution was prepared under optimal conditions (details are provided in the Supporting Data) and was used to coat a 2 inch sapphire wafer by a spray method, as shown in Figure 1a. For the SEM and EDS analyses, The Ni and Pd nanoparticles were uniformly coated on the surface of the CNTs, such that the Ni/Pd the ratio was approximately 3:1. The coatings of pure CNT and the NiPd CNT NP were analyzed by Raman spectroscopy, and the results are shown in Figure 1f. The typical Raman peaks (D-band at 1342 cm−2, G-band at 1583 cm−2, and two-dimensional (2D) modes at 2680 cm−2)22 are observed for the pure CNT. In contrast, for Ni-Pd CNT NP, an unprecedented peak (shoulder) is observed beside the G-peak; this is due to the surface charge and localized vibrational modes of the interactions that occur between Ni-Pd and the extended phonon modes of CNT.23 For a more precise confirmation, we performed XRD analysis on samples of various thicknesses, specifically 10, 30, 50, and 100 nm; as shown in Figure 1g, a peak is observed at 42.3. Generally, peaks of pure Pd and Ni are observed around 40 and 45, respectively, while in the case of alloys, the peaks are observed in between the two material peaks.24-26 Therefore, we assumed that Ni-Pd existed as an alloy metal. Moreover, the strength of the Ni-Pd peaks was observed to increase with their thickness. As before, samples of different thicknesses, specifically 10, 30, 50, and 100 nm were fabricated and the light transmittance of each sample was measured in the visible light region. the average transmittance was observed to be 96.7%, 90.3%, 81.7%, and 71.4% at 10, 30, 50, and 100 nm thickness, respectively. Furthermore, as shown in Figure 2b, the same sample set was used to confirm the electrical characteristics by measuring the sheet resistance of each sample and the light-blocking rate at 550 nm
wavelength. There is a tradeoff between the electrical conductivity and light-blocking rate: to obtain high electrical conductivity, the light transmittance is lowered, and to increase the transmittance, the electrical conductivity must be reduced. At 100 nm thickness, the sheet resistance was measured to be 87.26 (±12.07) Ω/sq, and the light-blocking rate at 550 nm wavelength was measured to be 27.24%. The uniformity of the sheet resistance was confirmed by 4-probe mapping and compared with that of a pure Ni layer. All samples were made of equal sizes, i.e., 50 × 50 mm and polyethylene terephthalate (PET) was used as the substrate. As shown in Figure 2c, Ni-Pd CNT NP exhibits better uniformity than pure Ni, which is clearly observed at 10 and 30 nm thicknesses; this is because the cross-network structure of the CNTs organically connects all areas. Meanwhile, the Ni-Pd on the surface lowers the contact resistance between the CNTs to obtain better electrical conductivity. Next, the flexibility of the samples was evaluated by tensile and bending tests. The specimens were prepared by depositing 100 nm thick Ni-Pd CNT NP and pure Ni using 10 × 50 × 1 mm sized Polydimethylsiloxane(PDMS) as a substrate. In the tensile test, the pure Ni was observed to fracture instantly as the tension was applied. However, Ni-Pd CNT NP did not disconnect even when elongated up to 200%. Next, to verify the durability of the sample, a bending test was carried out in which the PET substrate (10 × 50 mm), while the remaining conditions were the same as above. The measurement was performed 100,000 times under a bending radius of 30 mm. We observed that the resistance of pure Ni increased rapidly and was disconnected after approximately 10,000 times. On the other hand, the resistance of Ni-Pd CNT NP gradually increased but did not break. Thus, Ni-Pd CNT NP exhibited excellent flexibility owing to the cross-networking structure of the CNT as described above.27-28 Furthermore, when tensile force is applied to the sample, the CNT undergoes network reorientation in the direction of the tension as the slip occurs.29 This rearrangement in the CNT network results in flexibility, which complements the tension in the same direction, and thus, the sample does not completely disconnect. The Ni-Pd CNT NP layers of various thicknesses, specifically 10, 30, 50 and 100 nm, were each coated on a circular PET substrate (of 20 mm diameter and 0.5 mm thickness) and the EMI shielding efficiency (SE) was measured in the range of 8-13 GHz. The average EMI SE of Ni-Pd CNT NP was measured to be approximately 1.59, 4.43, 14.33, and 21.37 dB for coating thicknesses of 10, 30, 50, and 100 nm, respectively. These values are calculated as percentages of 30.79%, 63.92%, 96.30%, and 99.27%, respectively. The EMI SE of pure Ni was measured for comparison and was observed to be 6.59 dB at 100 nm thickness (Supporting Information).
The reason for this excellent EMI SE can be explained as follows. the reason is the conversion of electromagnetic waves into instantaneous surface current due to improved electrical conductivity.30-34 The electromagnetic waves incident on the Ni-Pd alloy are reflected according to Snell’s law of reflection (θi = θr),35 which is more accurately expressed as reproduction. When the electromagnetic waves collide with the Ni-Pd alloy, the energy is instantaneously converted into a surface current. Some of this surface current quickly moves toward the surrounding CNT, and is lost in the form of heat.36,37 The remaining current produces weakened electromagnetic waves at the same angle of incidence. Additionally, The incident electromagnetic wave decays exponentially inside the material, and is expressed as exp(-d/δ), where d is the thickness of the material and δ is the skin depth, which is a measure of the depth that electromagnetic waves can penetrate through a material; it is defined as the depth at which the intensity of the incident electromagnetic waves is reduced to 1/e.38,39 As the thickness of the material is much smaller than the skin depth (i.e., d << ) in this experiment, the electromagnetic shielding efficiency depends only on the electrical conductivity of the material regardless of the frequency of the incident electromagnetic waves.40,41 In addition, the magnetism of Ni attracts the surrounding electromagnetic waves toward itself, thereby providing more internal absorption opportunities, which in turn improves the EMI SE.42 A cell phone protective film of 100 nm thickness was prepared by spraying a solution of Ni-Pd CNT NP on a PET substrate (Figure 4a). The cell phone was observed to work properly even after the produced film was attached to its display; besides, the transparency of the display was also maintained. The EMI SE of the 2 GHz frequency region was mapped using a proximity sensor, which is an electromagnetic mapping device. The EMI was reduced to approximately 38.07% of its original intensity in areas coated with a 100 nm thick layer of Ni-Pd CNT NP. The reason for not achieving 99% SE was the open space conditions, wherein the EMI generated by the digital environments was always present. Thus, through this application, we confirmed that the Ni-Pd CNT NP film effectively blocks the electromagnetic waves generated from the electronic devices.
CONCLUSIONS
In this study, we successfully fabricated a thin layer of Ni-Pd CNT by a simplistic method. The Ni-Pd metal was uniformly grown on the surface of each CNT in the form of granules. The Ni-Pd CNT solution was prepared by nanoplating under the specified conditions and sprayed onto a substrate. The thin Ni-Pd CNT NP layer with a thickness of 100 nm exhibited a transmittance of 71.4% in the visible region. The layer exhibited high uniformity compared with the thin Ni layer deposited by an electronbeam evaporator; and its excellent mechanical properties were confirmed in the tensile and bending tests. Moreover, with only 100 nm thickness, it exhibited a high performance with an EMI SE of 99.47%. The cell phone protective film made with the Ni-Pd CNT NP reduced the EMI from the front of the cell phone to approximately 38.07%. Thus, this technology offers the advantages of high electrical conductivity, EMI SE, and transparency. In addition, it can also be rapidly industrialized owing to its high processability. Therefore, this technology can not only be used to shield EMI but also be applied to a wide range of fields such as energy storage, capacitors, solar cells, flexible devices, and nanoscale devices.
ACKNOWLEDGMENT This research was supported by the Priority Research Centers Program (2018R1A6A1A03024334), and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (NRF-2018R1D1A1B07051009). This research was supported by Basic Science Research Capacity Enhancement Project through Korea Basic Science Institute (National research Facilities and Equipment Center) grant funded by the Ministry of Education.(grant No. 2019R1A6C1010024)
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Figure captions Figure 1. a) Schematic of the method used for spraying Ni-Pd CNT NP. b) Sapphire wafer sprayed with a 100 nm thick layer of Ni-Pd CNT NP. c,d) SEM images of the Ni-Pd CNT NP layer. e) EDS analysis of the Ni-Pd CNT NP layer. f) Raman spectra of Ni-Pd CNT NP and CNT. g) XRD analysis of the Ni-Pd CNT NP layers with thicknesses of 10, 30, 50, and 100 nm Figure 2. a) Transmittance in the visible, UV, and IR regions of the Ni-Pd CNT NP layers with thicknesses of 10, 30, 50, and 100 nm. b) Variation of sheet resistance and transmittance at 550 nm wavelength with increasing thickness of the Ni-Pd CNT NP layer. c) Mapping image of the conductivity of pure Ni and Ni-Pd CNT NP layers of varying thicknesses. d) Tensile test of Ni-Pd CNT NP and pure Ni. e) Graph of the bending test to verify bending durability Figure 3. a) Graph of the EMI SE for Ni-Pd CNT NP layers of thicknesses 10, 30, 50, and 100 nm. b) Graph of EMI SE (dB) converted to percentage. c,d) Variations in the EMI reflection and absorption with thickness of the layers Figure 4. a) Image of the sprayed Ni-Pd CNT NP layer of 100 nm thickness on a PET protective film (left). Image showing the operation and transparency of the cell phone after attaching the protective film (middle). Image of the intensity analysis of the EMI generated from the cell phone (right). b) Image of the location-wise intensity analysis of the EMI generated from the cell phone. c) EMI detection graph with and without the Ni-Pd CNT NP layer.
Figure 1
Figure 2
Figure 3
Figure 4
Graphical abstract
Highlights Cross-network structure of the CNTs organically connects all areas. As the thickness of the material is much smaller than the skin depth, the EMI SE depends only on the electrical conductivity regardless of the frequency. Surface current quickly moves toward the surrounding CNT, and is lost in the form of heat. When the electromagnetic waves collide with the Ni-Pd alloy, the energy is instantaneously converted into a surface current.
Declaration of interests
☐
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
This research was supported by the Priority Research Centers Program (2018R1A6A1A03024334), and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (NRF-2018R1D1A1B07051009). This research was supported by Basic Science Research Capacity Enhancement Project through Korea Basic Science Institute (National research Facilities and Equipment Center) grant funded by the Ministry of Education.(grant No. 2019R1A6C1010024)
And we don’t have potential competing interests with personal relationships.
Jun-Beom Park: Writhing-Original Draft, Conceptualization, Formal analysis Hokyun Rho: Writhing-Original Draft, Visualization, Conceptualization, Formal analysis An-Na Cha: Formal analysis, Formal analysis Hyojung Bae: Formal analysis, Investigation Sang Hyun Lee: Resources, Formal analysis Sang-Wan Ryu: Resources, Formal analysis Tak Jeong: Resources, Funding acquisition Jun-Seok Ha: Project administration, Methodology, Writing - Review & Editing
S0169-4332(20)30501-8 https://doi.org/10.1016/j.apsusc.2020.145745 APSUSC 145745
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
14 January 2020 7 February 2020 11 February 2020
Please cite this article as: J-B. Park, H. Rho, A-N. Cha, H. Bae, S. Hyun Lee, S-W. Ryu, T. Jeong, J-S. Ha, Transparent carbon nanotube web structures with Ni-Pd nanoparticles for electromagnetic interference(EMI) shielding of advanced display devices, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc. 2020.145745
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2020 Published by Elsevier B.V.
Transparent carbon nanotube web structures with Ni-Pd nanoparticles for electromagnetic interference(EMI) shielding of advanced display devices Jun-Beom Park†a,b, Hokyun Rho†a, An-Na Chaa, Hyojung Baea,c, Sang Hyun Leea, Sang-Wan Ryuc, Tak Jeongb and Jun-Seok Ha*a,c aDepartment
of Advanced Chemicals & Engineering, Chonnam National University, Gwangju 61186, Republic of Korea bMicro
LED Research Center, Korea Photonics Technology Institute, Light Source Research Division, Gwangju 61007, Republic of Korea cOptoelectronics
Convergence Research Center, Chonnam National University, Gwangju 61186, Republic of Korea *Email: [email protected] † These
authors contributed equally to this work.
ABSTRACT Electromagnetic interference (EMI) has become an increasingly severe problem with the advancement in the development of electronic devices. In this study, an EMI shield was fabricated using a complex web structure made of Ni, Pd, and carbon nanotubes (CNTs). The Ni-Pd nanoparticles were evenly distributed on the surface of the CNTs. Samples with various thicknesses, specifically 10, 30, 50, and 100 nm, were fabricated and characterized. The results confirmed that the samples exhibited excellent and uniform electrical conductivity, flexibility, and transparency. In addition, the 100 nm thick shield demonstrated the capability of blocking 99.27% of the incident EMI and was successfully applied to a commercialized cell phone display. KEYWORDS: Electromagnetic interference, Pd-Ni Nanoparticles, Electroless plating, Carbon nanotubes, transparent, flexible display.
INTRODUCTION Electromagnetic pollution has been on the rise with the advancement in the development of electronic devices, creating problems such as negative impact on human health.1-4 According to a report by World Health Organization, electromagnetic waves generated from electronic devices such as mobile phones, computers, refrigerators, and TVs that are commonly used in every household can cause cancer in the human body.5 Furthermore, the development of advanced electronic devices such as wearable devices, which are in close contact with the human body could exacerbate this problem.6-8 An additional problem is the mutual interference of the electromagnetic waves within the electronic devices: electromagnetic interference (EMI) within a device can lead to its malfunction.9-11 Therefore, to solve these problems, research is being conducted to shield electromagnetic waves, and the most commonly used shielding material is metal.12-14 However, while vacuum-deposited metals have a good shielding effect owing to their high electrical conductivity, they are limited by their poor step coverage, low workability, and high cost.15-18 Research has been done to overcome this problem using composites of polymers and conductive fillers. AgNW / cellulose, one of the metal and polymer composites, reported a shielding performance of 48.6 dB at 160 µm thickness19, and about 65 dB at 1.5 mm thickness, Graphene Foam / PEDOT: PSS, one of the carbon fillers and polymer composites.20 In addition, shielding performance of about 45 decibels was reported for metal carbides (MXenes) at 1.5 micrometer thickness.21 However, these composites have the disadvantage of being difficult to apply to transparent devices such as displays because of their thick thickness and low light transmittance for sufficient shielding performance. In this study, a composite for electromagnetic shielding was fabricated using CNT, Ni, and Pd. We fabricated Ni-Pd CNT nanoplating (NP) layers by spraying to produce samples of various thicknesses, specifically 10, 30, 50, and 100 nm, and compared their optical, electrical, mechanical, and electromagnetic shielding properties to those of pure Ni films of the same thickness. Then, we applied the Ni-Pd CNT NP coating on the protective film of a cell phone to confirm its practical applicability.
EXPERIMENTAL SECTION Fabrication of Ni-Pd CNT NP
The solution in which CNT was dispersed in deionized (DI) water (0.5%) and the solution of dispersed Pd2+ ions in a weak hydrochloric acid solvent (over 59.0%) were purchased from Ho-Seong Surface Technology, and the NiSO4 (22.3%) and (CH3)2NHBH3 solutions (5.3%) were purchased for Ni nanoplating (NP) from ShinPoong Metal Co., Ltd. First, the SWCNT solution and Pd2+ solution were mixed in the volume ratio of 3:1 to prepare a 40 mL solution mixture. To this, 50 mL of a solution mixture containing 10 μL each of NiSO4 and (CH3)2NHBH3 was added to prepare a total of 90 mL of Ni-Pd CNT NP solution. The NP solution was stirred for 10 min at 100 rpm and samples were prepared by coating this solution on a 2 inch sapphire wafer via spraying to various thicknesses, specifically 10, 30, 50, and 100 nm. The Ni-Pd CNT NP samples were dried at 90 ℃ in an oven for 60 min to improve adhesion. Finally, the Ni-Pd CNT NP samples were rinsed in DI water for 10 min to remove the chlorides and surfactants. Characterization of Ni-Pd CNT NP Samples prepared by the above method were observed using a field emission-scanning electron microscope (FE-SEM; S-4700, Hitachi), and the Ni-Pd alloyed metal particles were confirmed by a transmission electron microscope (TEM; JEM-2100F, JEOL KOREA LTD.). The EDS connected with the TEM was used to determine the weight percentage of the elements. The Raman spectra were obtained using a Tokyo instruments FLEX G micro Raman system at Energy Convergence Core Facility in Chonnam National University (CNU). High resolution X-ray diffraction, (HR-XRD; Empyrean Nano Edition, Malvern Panalytical) was used to observe the intensity by thickness. The transmittance of the region between 200 and 800 nm was measured using a SHIMADZU MPC-2200 system. The electrical conductivity of the samples of different thicknesses was measured using a 4-point probe (FPP-4000M, DASOL ENG) and plotted against the transmission data. To confirm the uniformity of the electrical conductivity, 4-probe mapping (FPP-RS 8, ASRMS-1000, DASOL ENG) was performed after spraying Ni-Pd CNT NP on a 5 × 5 cm PET substrate, and the results were imaged. A PDMS substrate with a size of 1 × 5 cm was fabricated, and sprayed with Ni-Pd CNT NP. Thereafter, a stretch test of up to 220% was performed to confirm its mechanical properties. A PET substrate of dimensions 1 × 5 cm was fabricated and subjected to 100,000 bending tests. EMI shielding properties of Ni-Pd CNT NP EMI shielding of 2-9 GHz was confirmed using a Keysight E5061B ENA vector network analyzer. The substrate used for the measurement was a round PET with a diameter of 16 mm, sprayed with a 100 nm thick
layer of Ni-Pd CNT NP. For comparison, a sample was prepared by depositing pure Ni using an e-beam evaporator. The absorptivity was calculated using the measured transmittance and reflectance, and the shielding percentage was calculated in dB (see Supporting Information). The Ni-Pd CNT NP was sprayed with a thickness of 100 nm on half of the commercialized cell-phone protective films. Thereafter, the E4407B system (Agilent Technologies) was used to map the EMI detected in the display and imaged.
RESULTS AND DISCUSSION We used an easy and simplistic procedure to fabricate a thin Ni-Pd CNT NP layer. This composite layer can be applied to a wide area by spraying. The solution used for spraying included CNTs containing Ni and Pd. The Ni-Pd CNT NP solution was prepared under optimal conditions (details are provided in the Supporting Data) and was used to coat a 2 inch sapphire wafer by a spray method, as shown in Figure 1a. For the SEM and EDS analyses, The Ni and Pd nanoparticles were uniformly coated on the surface of the CNTs, such that the Ni/Pd the ratio was approximately 3:1. The coatings of pure CNT and the NiPd CNT NP were analyzed by Raman spectroscopy, and the results are shown in Figure 1f. The typical Raman peaks (D-band at 1342 cm−2, G-band at 1583 cm−2, and two-dimensional (2D) modes at 2680 cm−2)22 are observed for the pure CNT. In contrast, for Ni-Pd CNT NP, an unprecedented peak (shoulder) is observed beside the G-peak; this is due to the surface charge and localized vibrational modes of the interactions that occur between Ni-Pd and the extended phonon modes of CNT.23 For a more precise confirmation, we performed XRD analysis on samples of various thicknesses, specifically 10, 30, 50, and 100 nm; as shown in Figure 1g, a peak is observed at 42.3. Generally, peaks of pure Pd and Ni are observed around 40 and 45, respectively, while in the case of alloys, the peaks are observed in between the two material peaks.24-26 Therefore, we assumed that Ni-Pd existed as an alloy metal. Moreover, the strength of the Ni-Pd peaks was observed to increase with their thickness. As before, samples of different thicknesses, specifically 10, 30, 50, and 100 nm were fabricated and the light transmittance of each sample was measured in the visible light region. the average transmittance was observed to be 96.7%, 90.3%, 81.7%, and 71.4% at 10, 30, 50, and 100 nm thickness, respectively. Furthermore, as shown in Figure 2b, the same sample set was used to confirm the electrical characteristics by measuring the sheet resistance of each sample and the light-blocking rate at 550 nm
wavelength. There is a tradeoff between the electrical conductivity and light-blocking rate: to obtain high electrical conductivity, the light transmittance is lowered, and to increase the transmittance, the electrical conductivity must be reduced. At 100 nm thickness, the sheet resistance was measured to be 87.26 (±12.07) Ω/sq, and the light-blocking rate at 550 nm wavelength was measured to be 27.24%. The uniformity of the sheet resistance was confirmed by 4-probe mapping and compared with that of a pure Ni layer. All samples were made of equal sizes, i.e., 50 × 50 mm and polyethylene terephthalate (PET) was used as the substrate. As shown in Figure 2c, Ni-Pd CNT NP exhibits better uniformity than pure Ni, which is clearly observed at 10 and 30 nm thicknesses; this is because the cross-network structure of the CNTs organically connects all areas. Meanwhile, the Ni-Pd on the surface lowers the contact resistance between the CNTs to obtain better electrical conductivity. Next, the flexibility of the samples was evaluated by tensile and bending tests. The specimens were prepared by depositing 100 nm thick Ni-Pd CNT NP and pure Ni using 10 × 50 × 1 mm sized Polydimethylsiloxane(PDMS) as a substrate. In the tensile test, the pure Ni was observed to fracture instantly as the tension was applied. However, Ni-Pd CNT NP did not disconnect even when elongated up to 200%. Next, to verify the durability of the sample, a bending test was carried out in which the PET substrate (10 × 50 mm), while the remaining conditions were the same as above. The measurement was performed 100,000 times under a bending radius of 30 mm. We observed that the resistance of pure Ni increased rapidly and was disconnected after approximately 10,000 times. On the other hand, the resistance of Ni-Pd CNT NP gradually increased but did not break. Thus, Ni-Pd CNT NP exhibited excellent flexibility owing to the cross-networking structure of the CNT as described above.27-28 Furthermore, when tensile force is applied to the sample, the CNT undergoes network reorientation in the direction of the tension as the slip occurs.29 This rearrangement in the CNT network results in flexibility, which complements the tension in the same direction, and thus, the sample does not completely disconnect. The Ni-Pd CNT NP layers of various thicknesses, specifically 10, 30, 50 and 100 nm, were each coated on a circular PET substrate (of 20 mm diameter and 0.5 mm thickness) and the EMI shielding efficiency (SE) was measured in the range of 8-13 GHz. The average EMI SE of Ni-Pd CNT NP was measured to be approximately 1.59, 4.43, 14.33, and 21.37 dB for coating thicknesses of 10, 30, 50, and 100 nm, respectively. These values are calculated as percentages of 30.79%, 63.92%, 96.30%, and 99.27%, respectively. The EMI SE of pure Ni was measured for comparison and was observed to be 6.59 dB at 100 nm thickness (Supporting Information).
The reason for this excellent EMI SE can be explained as follows. the reason is the conversion of electromagnetic waves into instantaneous surface current due to improved electrical conductivity.30-34 The electromagnetic waves incident on the Ni-Pd alloy are reflected according to Snell’s law of reflection (θi = θr),35 which is more accurately expressed as reproduction. When the electromagnetic waves collide with the Ni-Pd alloy, the energy is instantaneously converted into a surface current. Some of this surface current quickly moves toward the surrounding CNT, and is lost in the form of heat.36,37 The remaining current produces weakened electromagnetic waves at the same angle of incidence. Additionally, The incident electromagnetic wave decays exponentially inside the material, and is expressed as exp(-d/δ), where d is the thickness of the material and δ is the skin depth, which is a measure of the depth that electromagnetic waves can penetrate through a material; it is defined as the depth at which the intensity of the incident electromagnetic waves is reduced to 1/e.38,39 As the thickness of the material is much smaller than the skin depth (i.e., d << ) in this experiment, the electromagnetic shielding efficiency depends only on the electrical conductivity of the material regardless of the frequency of the incident electromagnetic waves.40,41 In addition, the magnetism of Ni attracts the surrounding electromagnetic waves toward itself, thereby providing more internal absorption opportunities, which in turn improves the EMI SE.42 A cell phone protective film of 100 nm thickness was prepared by spraying a solution of Ni-Pd CNT NP on a PET substrate (Figure 4a). The cell phone was observed to work properly even after the produced film was attached to its display; besides, the transparency of the display was also maintained. The EMI SE of the 2 GHz frequency region was mapped using a proximity sensor, which is an electromagnetic mapping device. The EMI was reduced to approximately 38.07% of its original intensity in areas coated with a 100 nm thick layer of Ni-Pd CNT NP. The reason for not achieving 99% SE was the open space conditions, wherein the EMI generated by the digital environments was always present. Thus, through this application, we confirmed that the Ni-Pd CNT NP film effectively blocks the electromagnetic waves generated from the electronic devices.
CONCLUSIONS
In this study, we successfully fabricated a thin layer of Ni-Pd CNT by a simplistic method. The Ni-Pd metal was uniformly grown on the surface of each CNT in the form of granules. The Ni-Pd CNT solution was prepared by nanoplating under the specified conditions and sprayed onto a substrate. The thin Ni-Pd CNT NP layer with a thickness of 100 nm exhibited a transmittance of 71.4% in the visible region. The layer exhibited high uniformity compared with the thin Ni layer deposited by an electronbeam evaporator; and its excellent mechanical properties were confirmed in the tensile and bending tests. Moreover, with only 100 nm thickness, it exhibited a high performance with an EMI SE of 99.47%. The cell phone protective film made with the Ni-Pd CNT NP reduced the EMI from the front of the cell phone to approximately 38.07%. Thus, this technology offers the advantages of high electrical conductivity, EMI SE, and transparency. In addition, it can also be rapidly industrialized owing to its high processability. Therefore, this technology can not only be used to shield EMI but also be applied to a wide range of fields such as energy storage, capacitors, solar cells, flexible devices, and nanoscale devices.
ACKNOWLEDGMENT This research was supported by the Priority Research Centers Program (2018R1A6A1A03024334), and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (NRF-2018R1D1A1B07051009). This research was supported by Basic Science Research Capacity Enhancement Project through Korea Basic Science Institute (National research Facilities and Equipment Center) grant funded by the Ministry of Education.(grant No. 2019R1A6C1010024)
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Figure captions Figure 1. a) Schematic of the method used for spraying Ni-Pd CNT NP. b) Sapphire wafer sprayed with a 100 nm thick layer of Ni-Pd CNT NP. c,d) SEM images of the Ni-Pd CNT NP layer. e) EDS analysis of the Ni-Pd CNT NP layer. f) Raman spectra of Ni-Pd CNT NP and CNT. g) XRD analysis of the Ni-Pd CNT NP layers with thicknesses of 10, 30, 50, and 100 nm Figure 2. a) Transmittance in the visible, UV, and IR regions of the Ni-Pd CNT NP layers with thicknesses of 10, 30, 50, and 100 nm. b) Variation of sheet resistance and transmittance at 550 nm wavelength with increasing thickness of the Ni-Pd CNT NP layer. c) Mapping image of the conductivity of pure Ni and Ni-Pd CNT NP layers of varying thicknesses. d) Tensile test of Ni-Pd CNT NP and pure Ni. e) Graph of the bending test to verify bending durability Figure 3. a) Graph of the EMI SE for Ni-Pd CNT NP layers of thicknesses 10, 30, 50, and 100 nm. b) Graph of EMI SE (dB) converted to percentage. c,d) Variations in the EMI reflection and absorption with thickness of the layers Figure 4. a) Image of the sprayed Ni-Pd CNT NP layer of 100 nm thickness on a PET protective film (left). Image showing the operation and transparency of the cell phone after attaching the protective film (middle). Image of the intensity analysis of the EMI generated from the cell phone (right). b) Image of the location-wise intensity analysis of the EMI generated from the cell phone. c) EMI detection graph with and without the Ni-Pd CNT NP layer.
Figure 1
Figure 2
Figure 3
Figure 4
Graphical abstract
Highlights Cross-network structure of the CNTs organically connects all areas. As the thickness of the material is much smaller than the skin depth, the EMI SE depends only on the electrical conductivity regardless of the frequency. Surface current quickly moves toward the surrounding CNT, and is lost in the form of heat. When the electromagnetic waves collide with the Ni-Pd alloy, the energy is instantaneously converted into a surface current.
Declaration of interests
☐
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
This research was supported by the Priority Research Centers Program (2018R1A6A1A03024334), and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (NRF-2018R1D1A1B07051009). This research was supported by Basic Science Research Capacity Enhancement Project through Korea Basic Science Institute (National research Facilities and Equipment Center) grant funded by the Ministry of Education.(grant No. 2019R1A6C1010024)
And we don’t have potential competing interests with personal relationships.
Jun-Beom Park: Writhing-Original Draft, Conceptualization, Formal analysis Hokyun Rho: Writhing-Original Draft, Visualization, Conceptualization, Formal analysis An-Na Cha: Formal analysis, Formal analysis Hyojung Bae: Formal analysis, Investigation Sang Hyun Lee: Resources, Formal analysis Sang-Wan Ryu: Resources, Formal analysis Tak Jeong: Resources, Funding acquisition Jun-Seok Ha: Project administration, Methodology, Writing - Review & Editing