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We demonstrate highly transparent and flexible polytetrafluoroethylene (PTFE) passivation of MoS2/Ag nanowire (Ag NW) electrodes for thin film heater (TFH).The electrical, optical, and mechanical properties of PTFE-coated MoS2/Ag NW electrodes were compared with bare MoS2/Ag NW electrodes to demonstrate efficient passivation of PTFE films before and after sputtering in a 85 °C–85% temperature relative humidity environment test.Furthermore, we investigated the performance of TFH with PTFE/MoS2/Ag NWs as a function of PTFE thickness from 50 to 200 nm.The saturation temperature (87.3 °C) of TFHs with PTFE/MoS2/Ag NW electrodes is higher than that of TFHs with bare MoS2/Ag NW electrodes (61.3 °C) even after a temperature relative humidity environment of 85 °C–85% tested, due to the effective passivation of the PTFE layer.This indicates that the transparent PTFE films prepared by the sputtering process provide effective film passivation for two-dimensional (2D) MoS2 and Ag NW hybrid electrodes under harsh environmental conditions.
Thin film heaters (TFH) used in smart windows in cars and smart buildings basically operate by Joule heating of electrodes when a direct current (DC) voltage is applied across electrodes 1, 2, 3, 4, 5, 6.Recently, high-performance TFHs have been used as heat sources for wearable devices, functional windows for smart buildings, and heaters for transparent windows in automobiles2,7,8,9,10,11.To improve the performance of TFH, it is imperative to develop high-quality transparent conductive electrodes (TCEs) with high electrical conductivity, high optical transparency, and excellent mechanical stability12,13,14,15.To date, Sn-doped In2O3 (ITO) prepared by physical vapor deposition or F-doped SnO2 (FTO) prepared by chemical vapor deposition have been mainly used as TCEs for rigid TFHs because of their high electrical conductivity and optical transparency16, 17, 18, 19, 20.However, due to the specific ceramic properties of typical ITO and FTO thin films, they suffer from key disadvantages such as high material cost and brittleness2,21.Therefore, in order to fabricate flexible transparent TFHs that can be applied to curved surfaces or wearable devices, it is necessary to develop flexible TCEs to replace current ITO and FTO electrodes.Conducting polymers, graphene, carbon nanotubes (CNTs), metal meshes, polymers, and oxides/metals/oxides (OMOs) have been reported as promising candidates to replace ITO and FTO thin films3,13,22 ,23,24,25.However, such alternative TCEs for commercial ITO and FTO electrodes still have limitations.In the case of conducting polymers, it suffers from key issues such as low electrical conductivity, chemical instability to oxygen environments, and easy degradation at high temperatures.Carbon-based conductive materials, such as CNTs and graphene, exhibit considerably high sheet resistance and are fabricated through complex processes.Both the metal mesh and the OMO electrodes require high fabrication costs because they are fabricated using a vacuum sputtering process.Another candidate to replace ITO and FTO electrodes is the Ag nanowire (Ag NW) network prepared by printing process, because the Ag NW network has low sheet resistance, high transmittance, excellent flexibility and simple fabrication process28, 29, 30, 31.However, Ag NWs are susceptible to external environment because Ag NWs are oxidatively degraded by ambient O2 and H2O32,33.Furthermore, Ag NW-based TFHs have lower operational stability against electrical and thermal stress during device operation34,35.To overcome this problem, the Ag NW network should be coated with additional functional materials, such as organic or inorganic layers 36 , 37 , 38 , 39 .Molybdenum disulfide (MoS2), a transition metal dichalcogenide, is basically a two-dimensional (2D) material that has been used in various applications due to its high specific surface area, large light absorption, and relatively high thermal stability. kinds of applications.,41,42,43,44,45.Therefore, the thermal dispersion of the Ag NW network can be enhanced and the thermal stress of the Ag NW junction can be reduced by coating a 2D MoS2 layer on the Ag NW junction.However, the 2D MoS2 layer is a highly hygroscopic material with high surface energy and thus very high absorption of oxygen and water46,47,48,49.Therefore, the 2D MoS2 layer on the Ag NW network would lead to the absorption of H2O or O2 molecules, which would lead to the degradation of the Ag NW network despite several advantages of the 2D MoS2 layer.With the initial expansion of the TFHs application market, the high stability and reliability of TCEs used in TFHs become more important.However, when MoS2/Ag NWs used in TFHs are exposed to external and harsh environments, such as variable external temperature, oxygen and H2O, the electrical and optical properties of MoS2/Ag NWs and the performance of TFHs will gradually degrade32, 50, 51, 52, 53, 54, 55.To overcome this problem, the operational stability of the MoS2/Ag NW structure can be enhanced by using a polytetrafluoroethylene (PTFE) coating as a passivation layer.Sputtered PTFE films are currently under investigation in many research areas, such as flexible solar cells, anti-icing glass, electromagnetic shielding, and TFH, due to its excellent hydrophobic, antireflection or passivation properties56,57,58,59.A PTFE layer with good temperature stability and high hydrophobicity can protect the MoS2/Ag NW electrode.However, to the best of our knowledge, there is no report on passivation of PTFE films for MoS2/Ag NW electrodes to improve the stability of MoS2/Ag NW-based TFHs.
In this study, we report the properties of MoS2-coated Ag NW electrodes and sputtered PTFE with excellent passivation films to protect the MoS2/Ag NW electrodes.The electrical, optical, and mechanical properties of the PTFE/MoS2/Ag NW electrode and the bare MoS2/Ag NW electrode were compared to confirm the effective passivation of the PTFE layer.To demonstrate the feasibility of the PTFE passivation layer, we compared the performance of flexible transparent TFHs with PTFE/MoS2/Ag NW electrodes and bare MoS2/Ag NW electrodes after testing in 85 °C–85% temperature relative humidity environment.Based on the properties of flexible and transparent TFHs, we propose the potential of using sputtered PTFE passivation layers on MoS2/Ag NW hybrid electrodes in smart window TFHs.
Figure 1a,b show the fabrication process of the slot-die coating of Ag NW films and the spin-coated 2D MoS2 layer on the Ag NW films.Furthermore, Figure 1c shows a schematic diagram of RF magnetron sputtering of PTFE thin films on MoS2/Ag NW samples using PTFE targets at room temperature.Specifically, we fabricated PTFE/MoS2/Ag NW samples according to the thickness of PTFE to compare the optimized passivation effect with electrical and optical properties.The samples are represented by different layers of bare Ag NWs (#1), MoS2/Ag NWs (#2), and PTFE/MoS2/Ag NWs as a function of PTFE thickness (#3: 50 nm, #4: 100 nm, #5 : 150 nm, #6: 200 nm), respectively.
Schematic illustration of the sequential fabrication of PTFE/MoS2/Ag NW films in the following order (a) slot-die coating of Ag NW films, (b) spin-coating of MoS2 crystals, and (c) RF magnetron sputtering using 4 inches. Poly Teflon target.
Figure 2a shows the sheet resistance and resistivity of the samples measured using Hall measurement.As the number of samples increased from #1 to #6, the sheet resistance increased from (28.2 to 49.6) Ohm/sq and the resistivity also increased from (1.69 to 61.1) × 10-5 Ω-cm.As the thickness of the PTFE film becomes thicker, the resistance increases because the MoS2 and PTFE layers have high resistivity.Figure 2b shows the optical transmittance of various samples depending on the wavelength range of (400 to 1200) nm.The bare Ag NW sample shows a high transmittance of 88.23% in the visible wavelength region of (400-800) nm.When each MoS2 layer was coated on Ag NWs, the light transmittance did not change due to the high light transmittance of 2D-MoS2.However, sputtering the PTFE layer on the MoS2/Ag NW sample resulted in a decrease in the optical mean transmittance in the wavelength range < 600 nm with increasing thickness of the PTFE layer.Table 1 summarizes the details of the electrical and optical properties of bare Ag NW, MoS2/Ag NW and PTFE/MoS2/Ag NW films with various PTFE thicknesses.To determine the optimal thickness of the sputtered PTFE layer, the figure of merit (FoM) value was calculated from the sheet resistance (Rs) and optical transmittance (Tav), as shown in Figure 2c, according to the following equation 60:
(a) Hall-measured sheet resistance and resistivity, (b) transmittance of different films at wavelengths (400-1200) nm.(c) Figures of merit calculated from sheet resistance and optical transmittance of different films.(d) Schematic diagram of the contact angle measurement system.(e) Calculated contact angles of bare Ag NW, MoS2/Ag NW and PTFE/MoS2/Ag NW samples using deionized water and diiodomethane.Insets show droplet shapes captured on different samples according to deionized water (top) and diiodomethane (bottom).(f) Calculated surface energies as a function of PTFE thickness for bare Ag NW, MoS2/Ag NW, and PTFE/MoS2/Ag NW samples.
Although the FoM values ​​of the PTFE/MoS2/Ag NW samples were lower than those of the bare Ag NW or MoS2/Ag NW samples, all PTFE/MoS2/Ag NW samples showed similar FoM values, suggesting that the PTFE thickness does not affect the electrical or optical properties characteristics of the electrodes.Among the PTFE/MoS2/Ag NW samples, PTFE (100 nm)/MoS2/Ag NW showed the highest FoM value.To compare the surface properties of the samples, we measured the contact angle and surface energy of the samples.Figure 2d shows a schematic diagram of the contact angle measurement system using deionized water and diiodomethane droplets.The contact angle is calculated from the interface angle between the film and the liquid when the liquid is dropped onto the sample surface.Figure 2e shows the dependence of the contact angles of deionized water and diiodomethane droplets to calculate the surface energies of samples from #1 to #6.Table 2 summarizes the estimated contact angles and surface energies for different liquids.The contact angle of a droplet on the film surface is determined by the following Young’s equation:
where γLG is the free energy of the liquid-gas interface; γSG is the free energy of the interface between the solid and the gas; γSL is the free energy of the interface between the solid and the liquid, and θ is the contact angle.Each liquid/gas/solid interface free energy can determine the contact angle with the surface.The contact angle of bare silver NWs in deionized water droplets is 67.83°.When the MoS2 layer was coated on the Ag NWs, the contact angle decreased to 56.28° due to the high area ratio of 2D MoS2 with hydrophilic surface.This indicates that the 2D MoS2 coating cannot protect the Ag NW network from the external environment.To overcome this problem, we directly sputtered a PTFE layer as a passivation layer.As a result, as the PTFE thickness increases from (50 to 200) nm, the contact angle tends to increase slightly due to the hydrophobic surface of the PTFE membrane.In the case of the diiodomethane droplet, the contact angle of the bare Ag NW is 49.71°, and the angle of the MoS2 layer is further reduced to 36.16°.As mentioned above, when the PTFE layer was deposited, the contact angle gradually increased from (52.20 to 59.51)°.Therefore, the sputtered PTFE passivation layer changes the surface of the MoS2/Ag NW from hydrophilic to hydrophobic, which is beneficial to protect the MoS2/Ag NW electrode from environmental conditions.Figure 2f shows the surface energy values ​​calculated from the contact angles of the samples from deionized water and diiodomethane.The surface free energy is calculated using the Owens-Wendt method and can be calculated by the following equation:
in the equation.(3), \({\gamma }_{s}\) is the surface free energy, \({\gamma }_{s}^{d}\) is the dispersion component of the surface free energy, \( {\gamma }_{s}^{p}\) is the polar component of the surface free energy.Therefore, \({\gamma }_{s}^{d}\) and \({\gamma }_{s}^{p}\) are estimated using the following equations.(4) and (5), where \({\gamma }_{d}\) is the surface free energy of diiodomethane and \({\gamma }_{d}^{d}\) is the dispersive diiodine methane surface energy component, \({\gamma }_{d}^{p}\) is the polar component of water surface energy, \({\gamma }_{w}\) is the surface free energy of deionized water, \({\gamma }_{w}^{d}\) is the dispersive component of the surface free energy of deionized water, \({\theta }_{d}\) and \( {\theta }_{w} \) are the contact angles of diiodomethane and deionized water, respectively.As a result, the MoS2/Ag NW sample has the highest surface energy at 55.89 mJ.As the thickness of the PTFE layer increases, the surface energy of the samples decreases slightly.Therefore, this confirms that the PTFE layer can act as a passivation layer and can adequately withstand the external environment 63 , 64 .
To investigate the passivation effect of sputtered PTFE films, we performed external environmental testing on each sample, and Figure 3a shows a schematic diagram of the 85°C–85% temperature relative humidity environmental testing system.Figure 3b shows the change in sheet resistance of bare Ag NW, MoS2/Ag NW, and PTFE/MoS2/Ag NW samples in a temperature-relative humidity environment of 85 °C–85%.During environmental testing, the change in sheet resistance characteristics of each sample was measured by a four-point probe setup.Tests were performed every 10 hours, then sheet resistance was measured and repeated for 140 hours.In the case of bare Ag NWs, its sheet resistance increases due to the oxidation of Ag NWs, the adsorption of H2O, and the sulfidation of Ag NWs.Furthermore, the sheet resistance of the MoS2/Ag NW samples is significantly higher than that of the bare Ag NW samples.This clearly shows that the MoS2 layer is not suitable for the passivation layer of Ag NWs.The resistance change of the PTFE/MoS2/Ag NW samples was also evaluated as a function of test time.Compared with the bare Ag NW and MoS2/Ag NW samples, the sheet resistance of the PTFE-coated samples is almost unchanged.Due to the hydrophobicity and high thermal stability of the PTFE passivation layer, the resistance did not change significantly even after 140 h of testing in a temperature relative humidity environment of 85°C–85%.Figure 3c shows the change in optical mean transmittance in the visible wavelength range between (400 and 800) nm for various samples during 85°C–85% temperature relative humidity environmental testing.The passivation test was carried out for a total of 140 h, and the transmittance of all samples was measured every 70 h.Figure S1 shows the optical transmittance in the (400-800) nm region for each sample, depending on the environmental test.Before the passivation test for 140 h, the average transmittance of the bare Ag NW samples in the visible wavelength region was 88.23%.Thereafter, the average light transmittance of bare Ag NWs slightly decreased to 79.38% after 140 h passivation test.Furthermore, when MoS2 was coated on the Ag NW samples, the optical average transmittance in the visible wavelength region decreased from (73.69 to 67.60)% during harsh environment testing.This confirms that the optical transmittance is severely decreased due to the influence of the hygroscopicity of the 2D MoS2 layer.In the case of PTFE/MoS2/Ag NW samples, due to the hydrophobicity and thermal stability of PTFE, the transmittance did not decrease significantly after the passivation test regardless of the thickness of PTFE.To study the chemical composition of the MoS2 layer on Ag NW and the PTFE layer on MoS2/Ag NW, the changes of core energy level spectra before and after 140 °C 85 °C–85% relative humidity environmental test were examined by XPS analysis.H.We calibrated all binding energies of the peaks by the C(carbon) 1s peak at 284.8 eV.Figures 3d and e show the XPS results of the O 1s and C 1s peaks of MoS2 layer on Ag NW film and the C 1s peak of PTFE layer on MoS2/Ag NW film before and after the temperature-relative humidity test, which were fitted by Gaussian function.The position of the O 1s core peak at 532.0 eV is deconvoluted into two other chemical binding energy peaks in the lattice oxygen (Mo-O bonding) (OI) peak at 530.8 eV, and adsorbed oxygen (chemisorbed oxygen) and –OH group) (OII) peak at 532.3 eV65.Due to the hygroscopicity of MoS2, the peak area ratio of adsorbed oxygen is relatively increased compared to lattice oxygen.To calculate the change in OI and OII ratios, the peak area ratio of OII/(OI + OII) was evaluated; the peak area ratio of O 1s at MoS2 increased from (61.2 to 82.0)%.The C 1s spectrum was deconvoluted into three peaks for carbon-carbon (CC) bonds with a CI peak at 284.6 eV, carbon-hydrogen (CH) bonds and carbon-oxygen (CO) bonds with a CII peak at 285.9 eV.) bonding of the CIII peak at 287.6 eV.The area ratio of the CII and CIII peaks increased relative to the CI peak due to the high humidity and temperature in the harsh environment test.Therefore, the MoS2 layer is not suitable as a passivation layer for certain harsh environments.Finally, the resistance of the MoS2/Ag NW samples increased during the temperature and humidity environment testing.The C 1s spectrum of the PTFE layer was deconvoluted into distinct peaks consisting of distinct groups: peak CC bonding at 284.6 eV; C-CFn bonding peak at 286.6 eV; CF bonding peak at 287.2 eV; CF bonding peak at 289.1 eV CF-CF bonding peak at 291.2 eV; CF3 bonding peak at 293.3 eV49.In particular, there is a carbon-oxygen (CO) single bond peak at 285.9 eV and a carbon=oxygen (C=O) double bond peak at 288.6 eV.After the external environment test, the area ratios of CC, CO, and C=O peaks to PTFE C 1s hardly increased compared with those before the test.This indicates that due to the passivation of PTFE, due to the interaction of CF bonds 66, 67, 68, almost no oxidation and adsorption functional groups on the PTFE surface are observed in the CF groups.Therefore, the sputtered PTFE film can serve as an effective passivation layer to stabilize TFH even in harsh external environments.
(a) Schematic diagram of the 85 °C–85% temperature and humidity environment test system.(b) Changes in sheet resistance and (c) optical mean transmittance in the visible region (400 to 800) nm obtained from bare Ag NW, MoS2/Ag NW, and PTFE/MoS2/Ag NW samples obtained from temperature-humidity environmental tests.XPS analysis revealed that in the 2D MoS2 thin film and sputtered C 1s spectra, the O 1s core peak was deconvoluted into two adsorbed oxygen and lattice oxygen, and the C 1s peak was deconvoluted into three CC, CH and CO peaks PTFE layers on MoS2/Ag NW films (d) before and (e) after 85 °C–85% temperature relative humidity environmental testing.
HR-TEM was used to study the microstructure of Ag NW, MoS2 and PTFE layers after external environmental testing at 85 °C–85% to verify the passivation effect of PTFE layers.Figure 4a shows the cross-sectional images and EDS mapping images of bare MoS2/Ag NW electrodes after 85 °C–85% external ambient testing.This clearly shows that the oxygen adsorption increases in harsh environments due to the hygroscopicity of the 2D MoS2 layer.In addition, Mo and S elements were slightly dispersed due to exposure to high temperature conditions.Figure 4b shows the cross-sectional image and EDS mapping image of the PTFE/MoS2/Ag NW electrode.Unlike the bare MoS2/Ag NW electrode, the passivation of the PTFE layer reduces the adsorption of the MoS2 layer, and the thermal stability of the PTFE layer also slightly reduces the dispersion of Mo and S elements.Therefore, the sputtered PTFE layer effectively protects the MoS2/Ag NW electrode from the external environment.
Magnified cross-sectional images and EDS mapping images of O, S, Mo, and Ag elements of (a) MoS2/Ag NW and (b) PTFE (100 nm)/MoS2/Ag NW films after ambient treatment obtained from HR-TEM Tested at 85 °C–85% temperature relative humidity for 140 hours.
Figure 5a shows the bending test steps of bare Ag NWs, MoS2/Ag NWs and PTFE/MoS2/Ag NWs on PET substrates with dimensions of 1.5 cm × 6.0 cm, as a function of bending radius, using a bending test system.In the outer bending test, the bending radius of the film decreases with increasing mechanical stress.Figure 5b shows the resistance changes according to the outer bend test of bare Ag NWs, MoS2/Ag NWs and PTFE/MoS2/Ag NWs, respectively.Particularly large resistance changes can occur when tensile stress is applied to the film during outer bending.The change in resistance (\(\Delta R)\) is defined as the following equation, where the initial resistance is \({R}_{0}\) and the resistance is \(\left(R\right)\) with the bending radius and change.
(a) Photograph of a working bend test system estimating the critical radius from the bend radius.(b) Outer critical bend radius results and (c) inner critical bend radius results for the bare Ag NW, MoS2/Ag NW, and PTFE (100 nm)/MoS2/Ag NW samples.
Furthermore, the critical radius is defined as the point at which the resistance change rapidly increases as the bending radius decreases.The critical radius of the bare silver NW sample when bent out is 3 mm.In the case of bare Ag NWs, it is easily isolated due to the tensile stress exerted on the Ag NWs.However, MoS2/Ag NWs have a critical radius of 2 mm, showing a slightly lower radius than bare Ag NWs.The 2D MoS2 layer-coated Ag NWs can uniformly cover the Ag nanowires and junctions, and can play a role in relieving the mechanical stress imposed on the film.This is related to the durability of the wire-to-wire junction that determines the conductivity of Ag NWs.In addition, the critical radius of the PTFE/MoS2/Ag NW sample is also 2 mm, indicating that the sputtered PTFE layer does not affect the mechanical flexibility of the MoS2/Ag NW electrode.Figure 5c shows the resistance of bare Ag NWs, MoS2/Ag NWs and PTFE/MoS2/Ag NWs as a function of internal bending tests.In the case of inward bending, a compressive stress was applied to the film, but the change of the electrical properties of the film was smaller than that of the tensile stress.As a result, all samples exhibited a critical radius of 1 mm and exhibited smaller electrical changes compared to the outer bending.
Figure 6 shows the resistance changes of bare Ag NW, MoS2/Ag NW, and PTFE/MoS2/Ag NW samples after 10,000 cycles under outer and inner bending with a fixed bending radius of 15 mm.In the case of the bare Ag NW sample in Fig. 6a, the resistance of the bare Ag NW sample tends to increase with increasing external bending cycles.The mechanical stress repeatedly applied to the Ag NWs during the external bending fatigue test resulted in the degradation of the Ag NW network.However, when the MoS2 and PTFE layers were coated as shown in Fig. 6b,c, the resistance did not change regardless of the bending mode.This proves that the additional coating can improve the durability and flexibility of Ag NW electrodes.The mechanical bending test results confirmed that the electrical stability of Ag NW electrodes could be improved by coating with MoS2 and PTFE.The right side of Figure 6 shows the surface FE-SEM images of bare Ag NW, MoS2/Ag NW, and PTFE/MoS2/Ag NW samples after outer (left) and inner (right) fatigue cycling tests.After fatigue testing, the SEM images of the external fatigue bending cycles of the bare Ag NW samples show the dissociation of the film and the separation of the Ag NWs because there is no coating film that can relieve the mechanical stress of the Ag NWs.However, even after 10,000 cycles of fatigue testing, the overcoat of the 2D MoS2 layer still produced the same surface SEM images.Furthermore, it shows that the Ag NW junction is well maintained without any cracks or nanowire disconnection.The surface SEM images of the PTFE/MoS2/Ag NW films also showed a well-connected Ag NW network after 10,000 fatigue tests, just like the MoS2/Ag NW samples.Therefore, this confirms that the MoS2 coating and sputtered PTFE layer on the Ag NW electrode enhanced the mechanical flexibility and stability of the Ag NW network through the bridging effect of 2D MoS2 and the coating of the flexible PTFE layer.Therefore, the overcoat of 2D MoS2 is beneficial for flexibility, while the sputtered PTFE layer is beneficial for passivation of MoS2/Ag NW electrodes.
Dynamic fatigue testing of external and internal bending cycles of (a) bare Ag NW, (b) MoS2/Ag NW and (c) PTFE (100 nm)/MoS2/Ag NW samples with 10,000 cycles.The right side of the figure shows surface SEM images of the samples after external (left) and internal (right) fatigue testing.
To demonstrate the feasibility of sputtering PTFE layers with excellent passivation effect, we fabricated PTFE/MoS2/Ag NW-based-TFHs with hydrophobic passivated PTFE layers and compared their properties with MoS2/Ag NW-based and bare Ag The performance of NW- is compared.Based on TFH in harsh environments.Figure 7a shows a schematic diagram of the fabrication process of TFHs using PTFE/MoS2/Ag NW electrodes.We examined the performance of TFH using a temperature measurement system with thermocouples mounted on conductive films as a function of input DC voltage.Figure 7b shows a possible Joule heating mechanism for transparent TFH.A current (I) flows through a conductive film (2D MoS2/Ag NW) that generates Joule heating, and its magnitude can be expressed as proportional to the product of \({I}^{2}\), resistance \( R\) and time \(t\)69.Furthermore, the heat dissipation generated around the conductive film can be explained by conduction, convection in the air, and radiation mechanisms70.If the heat loss due to conduction effects to the substrate is neglected, the thermal convection effect becomes the dominant effect of heat dissipation, which has the following equation 71:
(a) Schematic diagram of the fabrication process of PTFE/MoS2/Ag NW TFH; the bottom shows the thermocouple used to analyze the saturation temperature of TFH as a function of DC voltage.(b) Schematic illustration of the Joule heating mechanism of TFH when a DC voltage is applied to TFH.(c) Temperature profiles of TFH fabricated on bare Ag NWs, and (d) MoS2/Ag NWs.(e) PTFE (100 nm)/MoS2/Ag NW electrode during 85 °C–85% temperature relative humidity environmental testing.
The conductive film and substrate are represented by subscripts 1 and 2, respectively, where \(m\) is the mass of the material, \(c\) is the specific heat capacity, \(h\) is the convective heat transfer coefficient – efficient, \(A\) is the heating area; \(\sigma\) is the Stefan-Boltzmann constant, \(\varepsilon\) is the emissivity of the conductive film, \(T\left(t\right)\) is the estimated temperature as a function of time, and \({T}_{0}\) is the initial temperature at ambient conditions.It is important to reduce heat dissipation by reducing thermal convection effects and to increase the saturation temperature when the TFH operates at low voltages.To calculate the saturation temperature of the conductive film as a function of DC voltage, it can be defined by the following equation 72:
where \({h}_{conv}\) is the convective heat transfer coefficient and \({A}_{conv}\) is the surface area where convection occurs.Also, \({T}_{s}\) is the saturation temperature and \({T}_{0}\) is the initial temperature.As a result, the sheet resistance of the film is lower and the saturation temperature value is higher.Figure 7c–e show the temperature profiles of transparent TFH with bare Ag NW, MoS2/Ag NW and PTFE (100 nm)/MoS2/Ag NW tested in 85 °C–85% temperature relative humidity environment as a function of input DC voltage .Figure S2 shows the performance of TFHs as a function of (50, 150 and 200) nm PTFE layer thickness.Furthermore, Figure S3 shows the performance of TFH, which can reach the maximum saturation temperature, depending on the applied DC voltage and the calculated operating power value when the same 6 V is applied.The power \(P\) is expressed according to the resistance value of the conductive film based on the equation \(P={V}^{2}/R\).Table 3 summarizes the saturation temperature of TFH for 140 hours of environmental testing for different electrodes at specific DC voltages.The left side of Fig. 7c shows that the saturation temperature of bare Ag NW-based TFH at 6 V DC is 94.7 °C.However, when the DC voltage is higher than 7 V, the degradation of TFH occurs at more than 100 degrees Celsius.Figure 7d shows the TFH performance of MoS2/Ag NW electrodes as a function of input DC voltage.MoS2/Ag NWs result in electrodes with higher sheet resistance than bare Ag NW electrodes due to the presence of a high-resistance 2D MoS2 capping layer on the Ag NWs.Despite the higher sheet resistance, the MoS2/Ag NW-based TFH shows a higher saturation temperature of 114.1 °C, even at a higher DC voltage of 9 V.Because the MoS2 layer can fully disperse the thermal stress of the Ag wire-wire junction, the MoS2/Ag NW based TFH can reach a higher saturation temperature than the bare Ag NW TFH.Figure 7e shows the performance of TFH fabricated on PTFE/MoS2/Ag NW electrodes as a function of input DC voltage.This indicates a saturation temperature of 95.5 °C at 9 V.Also, TFH based on PTFE/MoS2/Ag NW shows lower saturation temperature voltage than MoS2/Ag NW sample under the same applied DC due to the insulating PTFE passivation layer.The right panels of Figure 7c-e show the temperature profiles of TFH after 85°C-85% testing (70 and 140) hours.The right side of Figure 7c shows the performance of the bare Ag NW-based TFH after 85 °C–85% ambient testing.Therefore, after 140 hours of ambient testing, the saturation temperature at 7 V is 55.4 °C.This degradation of Ag NW-based TFH can be explained by the oxidation and sulfidation of the Ag NW network when exposed to external humid environment at high temperature.In addition, the wire-to-wire connector is susceptible to harsh environments, which reduces the operational stability of the TFH.The right side of Fig. 7d shows the temperature profile of MoS2/Ag NW-based TFH after 85 °C–85% ambient testing.After exposure to harsh environments, TFH showed a saturation temperature of 61.3 °C at the same 9 V voltage due to increased sheet resistance.In particular, the hygroscopic and oxidative properties of the 2D MoS2 layer in harsh environments mainly affect the degradation characteristics of TFHs.However, the right side of Fig. 7e shows that the TFH based on PTFE/MoS2/Ag NW reaches a saturation temperature of 87.3 °C at 9 V even after harsh environmental testing.Furthermore, we explain the lifetime extension of PTFE/MoS2/AgNW TFH using the linear Arrhenius curve shown below4.
where tf is the run-to-failure time of TFH, A is the prefactor, Ea is the activation energy, k is the Boltzmann constant, T is the absolute temperature, and AF is the acceleration factor at various temperatures.Using the above equation, we compared the failure times of PTFE/MoS2/Ag NW TFH and bare Ag NW TFH, as shown in Figure S4.Compared to the bare Ag NW-based TFHs, the PTFE/MoS2/Ag NW TFHs showed longer failure times even after harsh environmental tests due to the passivation effect of the PTFE film.This indicates that the sputtered PTFE layer effectively inhibits the oxidation or vulcanization of the hygroscopic MoS2/Ag NW electrode under high temperature and high humidity conditions.Due to the effective passivation of the PTFE layer, the conductivity of the MoS2/Ag NW electrode can be successfully maintained even in harsh environments.As a result, passivating the PTFE layer resulted in a consistent saturation temperature of the TFH performance, similar to that of the fabricated samples.
To study the operational stability of Ag NW-based TFHs, we conducted operational stability tests on TFHs.Furthermore, the insets in each panel of Fig. 8 show the IR images when the TFH reaches the saturation temperature, measured using an IR camera.Figure 8a shows the repeated temperature profiles of bare Ag NW-based, MoS2/Ag NW-based, and PTFE/MoS2/Ag NW-based TFHs tested over a wide temperature range to evaluate the repeated on/off characteristics during 10 cycles.This clearly shows that the PTFE/MoS2/Ag NW-based TFH exhibits superior working stability at high saturation temperatures in repeated on/off states compared to bare Ag NW-based and MoS2/Ag NW-based TFH.Figure 8b also shows the step test of a sample of TFHs using successively applied different DC voltages without a cooling step.Both Ag NW-based and MoS2/Ag NW TFH exhibit unstable cooling characteristics after saturation temperature.Unlike the PTFE/MoS2/Ag NW-based TFH, there is no isothermal step due to the inappropriate thermal dispersion of the bare Ag NW-based and 2D MoS2/Ag NW-based electrodes.The PTFE/MoS2/Ag NW TFH exhibits stable operation stability even after the saturation temperature because the MoS2 and PTFE layers relieve the thermal stress of the Ag NW junction.The outstanding performance and stability of the PTFE/MoS2/Ag NW TFH demonstrate that the sputtered PTFE layer provides efficient thin film passivation for the fabrication of high-performance transparent and flexible TFH for next-generation smart windows.
(a) Repeated cycling test of TFH within 6000 s and (b) step test of TFH within 3000 s depending on the applied DC voltage for bare Ag NW, MoS2/Ag NW and PTFE/MoS2/Ag NW electrodes.The inset IR image shows the saturation temperature reached by TFH.
We investigated the feasibility of sputtered PTFE films as passivation layers for 2D MoS2/Ag NW electrodes to protect them from harsh external environments and provide the operational stability of TFH due to the high hydrophobicity and thermal properties of the PTFE layers.The performance of bare Ag NW-based TFHs degrades at over-DC voltages because the Ag NW junctions are degraded by thermal stress.Furthermore, the oxidation and sulfidation of the Ag NW network at high operating temperature lead to the degradation of TFH performance.Although the 2D MoS2 nanosheet coating improves the thermal stability of the Ag NW electrode due to the dispersion of Joule heat at the wire junction, the hygroscopic 2D MoS2 leads to the absorption of H2O molecules and O2, thereby reducing the 2D MoS2/Ag NW Electrodes. Therefore, by sputtering PTFE thin films on 2D MoS2/Ag NWs, we demonstrate high-quality PTFE/MoS2/Ag NW electrodes, TFHs with high performance and Operational stability.The TFH based on 2D MoS2/Ag NW shows stable temperature profile and repeated on/off characteristics even after 85°C–85% temperature relative humidity environmental testing, which is due to the effective passivation of bare Ag by the PTFE layer. TFH based on NW and 2D MoS2/Ag NW.Therefore, this undoubtedly indicates that the high-quality PTFE films prepared by the sputtering process provide effective film passivation for 2D MoS2 and Ag NW hybrid electrodes against external environmental conditions for advanced smart windows.
MoS2 nanosheets were prepared by electrochemical exfoliation.MoS2 crystals (purchased from HQ Graphene) were fixed with alligator clips as the cathode and placed with a graphite rod as the counter electrode.Tetraheptylammonium bromide as an intercalating agent was dissolved in acetonitrile at a concentration of 5 mg/mL.Electrochemical reactions were carried out at an applied voltage of 7 V for 1 hour.After the reaction, MoS2 crystals were washed with ethanol and sonicated in 0.2 M polyvinylpyrrolidone in dimethylformamide (DMF) solution for 30 min.To remove unexfoliated crystals, the prepared dispersion was centrifuged at 4000 rpm for 10 minutes.DMF was exchanged with isopropanol for spin coating.This MoS2 solution was spin-coated at 2500 rpm for 40 s, and Ag NW films fabricated by a roll-to-roll (RTR) slot die coating system on polyethylene terephthalate (PET) substrates Repeat 2 times.Toray Advanced Materials Korea Corporation).
Deposited using 4 inch PTFE film.PTFE targets (PTFE: 95 wt%, carbon nanotubes: 5 wt%) were passed through a radio frequency magnetron sputtering system.PTFE films were deposited under the conditions of constant RF power of 150 W, working pressure of 4 mTorr, and argon flow of 20 sccm.To improve the uniformity of the PTFE film, the substrate was rotated at a constant speed of 15 rpm and held with the cathode gun tilted at 30°.PTFE thin films were deposited on MoS2/Ag NW samples, ranging from (50 to 200) nm as a function of PTFE thickness, respectively.
In addition, electrodes were deposited under a 8.0 × 10 Torr substrate by a DC magnetron sputtering system using a 4-inch AgPdCu target (APC; Ag: 99.90 wt%, Pd: 0.05 wt%, Cu: 0.05 wt%; Dasom RMS) pressure.The APC electrodes were coated at a constant DC power of 100 W, a working pressure of 1 mTorr, and an argon flow of 20 sccm.In addition, all samples had the same deposition time of 250 seconds.We prepared a 2.5 cm × 2.5 cm Ag NW thin film sample with 0.6 cm × 0.6 cm APC electrodes at both ends of the conductive sample.For APC deposition, a PET mask pattern with dimensions of 2.5 cm × 1.3 cm was attached to each sample consisting of bare Ag NWs, MoS2/Ag NWs, and PTFE/MoS2/Ag NWs.The upper limit of the measurement temperature was set to 120°C in consideration of the temperature range of the measurement equipment and the physical properties of the PET substrate.The applied voltage was maintained for 400 seconds until the saturation temperature was reached.The saturation temperature was brought back to the initial temperature by switching off the voltage for 200 s, and the test was repeated by slightly increasing the input voltage.
By UV/Vis spectrometer (UV540, Unicam), four-point probe (FPP-HS8, DASOL ENG) and Hall measurement (HMS-4000AM, Ecopia).To analyze the mechanical properties of different samples, we use a bending test system (JIRBT-620, JUNIL TECH) to study the critical radius and fatigue tests.A field emission scanning electron microscope (FE-SEM: JSM-7600F, JEOL) was used to examine the surface morphology of the samples.High-resolution transmission electron microscopy (HR-TEM: JEM-2100F, JEOL) was used to analyze the interface and cross-sectional images between Ag NWs and MoS2 or PTFE.To calculate the surface energies of bare Ag NWs, MoS2/Ag NWs and PTFE/MoS2/Ag NWs, we used contact angle measurements (Phoenix-MT(A), SEO CO) using liquid droplets of deionized water and diiodomethane.To evaluate the performance of the TFH, a temperature measurement system (McScience) used a contact thermocouple and an infrared camera (A35sc, FLIR, Wilsonville) and a thermal imager (FLIR ONE Pro) to check the temperature of the sample on the prepared Keithley 2634B of several DC voltages.For the stability testing of the fabricated TFHs, environmental tests were performed at 85°C–85% temperature relative humidity for 140 hours using a low temperature and humidity chamber (TH3-KE (desktop), JEIO TECH).
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