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Abstract
A broadband transparent and flexible silver (Ag) mesh is presented experimentally for the first time for both efficient electromagnetic interference (EMI) shielding in the X band and high-quality free-space optical (FSO) communication. High transmission is achieved in a broad wavelength range of 0.4-2.0 µm. The transmittance of the Ag mesh relative to the substrate is around 92% and the sheet resistance is as low as 7.12 Ω/sq. The Ag mesh/polyethylene (PE) achieves a high average EMI shielding effectiveness (SE) of 28.8 dB in the X band with an overall transmittance of 80.9% at 550 nm. High-quality FSO communication with small power penalty is attributed to the high optical transmittance and the low haze at 1550 nm, superior to those of the Ag NW networks. With a polydimethylsiloxane (PDMS) coating, the average EMI SE is still up to 26.2 dB and the overall transmittance is increased to 84.5% at 550 nm due to antireflection. The FSO communication does not change much due to the nearly unchanged optical property at 1550 nm. Both the EMI shielding performance and the FSO communication function maintain after 2-hour chemical corrosions as well as after 1000 bending cycles and twisting. Our PDMS/Ag mesh/PE sandwiched film can be self-cleaned, suitable for outdoor applications.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
With the wide application of electronic devices, electromagnetic (EM) radiations are nearly ubiquitous, which can severely interfere sensitive electrical instruments and even make them malfunction [1–5]. Human and wildlife health can also be threatened after a long-term exposure to EM radiations [6–8]. Therefore, efficient shielding of EM interference (EMI) is highly desirable. For optoelectronic devices, the EMI shields over the optical windows or displays are simultaneously required to be highly transparent, but mainly in the visible regime [9–34]. Mechanical flexibility is another requirement for emerging flexible and wearable optoelectronics. In certain important military application scenarios, a shielded optoelectronic device, which is protected from releasing and receiving EM wave signals, still needs be communicated and controlled remotely. Such a third channel may be provided by optical communication, especially free space optical (FSO) communication, where a laser is utilized as an optical carrier to transmit information in vacuum or atmosphere. Due to the high brightness and directivity of the laser, FSO communication has the characteristics of long communication distance, high security, and robustness to EMI, etc. FSO communication systems are also easily deployable and can be rapidly installed in some emergency situations, very suitable for military applications [35,36]. To avoid the influence of the ambient light, FSO communication always works at near-infrared and mid-infrared wavelengths [37]. Obviously, conventional transparent EMI shields, providing only visible channels [9–34], cannot be applied to high-quality FSO communications. Existing EMI shields, which are transparent in the near-infrared and mid-infrared regimes, have not yet been explored for FSO communication [38,39].
Recently, various transparent EMI shields have been reported. Transparent conductive oxides (TCOs), e.g., the classical indium tin oxide (ITO), was employed as a transparent EMI shield because of its high transmittance and good conductivity [9,40]. However, TCOs are always fragile and not applicable to flexible or wearable optoelectronics. ITO also becomes increasingly expensive due to the scarcity of indium [40]. Carbon based nano-materials, e.g., graphene [10,11], reduced graphene oxides (rGOs) [12], carbon nanotubes (CNTs) [13], can form transparent and flexible conductive films and have been proposed to replace TCO EMI shields. A monolayer graphene has high optical transmittance of 97%, but the EMI shielding effectiveness (SE) is only 2.27 dB at 2.2-7 GHz [10]. A graphene/PET multilayer film shows an improved EMI SE at the expense of optical transmittance [11]. rGOs/PEI multilayer film [12] and CNTs covered with Ni-Pd nanoparticles [13] have low optical transmittance and their EMI SEs still cannot meet the basic requirement for EMI shielding applications (i.e., 25 dB) [5]. In contrast, metallic micro-/nano-structures, e.g., ultrathin metallic films [14,15], metallic nanowire (NW) networks [16–25], metallic micromeshes [26–34,38,39], are generally better in both optical transparency and EMI SE. For the ultrathin metallic films (with thicknesses below 15 nm), optical reflection is high [41] and TCO coatings are always necessary for antireflection, which however inevitably affect the mechanical flexibility [14,15]. Metallic NW networks, consisting of randomly and loosely distributed metallic NWs, are always weakly attached to the substrate and have large junction resistances between NWs, according to our previous experience [42–44]. Therefore, these random networks are always embedded in polymers [16–21] or modified with ferroferric oxides, rGOs or MXene [22–25], leading to complicated fabrication processes. The optical transparency may also be adversely affected by the modification layer [24]. Due to the strong light scattering of the NWs, high haze (over 10% in a broad wavelength range of 400-900 nm) is another issue for these networks [45,46]. The metallic micromeshes with continuous grid lines can balance the EMI SE and optical transmittance at a higher level by optimizing the opening ratio, the mesh shape and thickness, as well as the coating [26–34,38,39]. The transmission spectrum, determined by the opening ratio, can be extremely flat and broad, covering ultraviolet, visible and infrared wavelength ranges [39]. The optical haze can be even lower than 3.5% in the visible regime [34,39]. Metallic micromeshes, as a very promising transparent EMI shielding material, are also highly flexible [28,29,31,32,38,47].
Based on the above analysis, in this work, we present a broadband transparent and flexible silver (Ag) micromesh for both efficient EMI shielding in the X band and high-quality FSO communication. To the best of our knowledge, it is the first time for us to explore the optical communication function of the transparent EMI shield in an FSO communication system. Sandwiched between transparent polyethylene (PE) and polydimethylsiloxane (PDMS), the Ag micromesh also show excellent corrosion resistance and mechanical flexibility, promising for practical applications.
2. Experimental section
2.1 Preparation of Ag mesh/PE double-layer films
50-µm thick PE films were chosen as the flexible and transparent substrates for fabrication of Ag meshes. The substrates were first cleaned ultrasonically in acetone, isopropanol, and deionized water sequentially, each for 10 min, and then spin-coated a layer of about 1.5-µm thick photoresist (PR, AZ5214) at 3000 rpm for 29 s. The PR was solidified on a hot plate at 90 °C for 2 min and was subsequently exposed to ultraviolet (UV) light for 3 s through a photomask with predefined patterns. To reverse the pattern on PR, the samples were baked again at 120 °C for 2 min and exposed to the UV light under a flood-E mode for 25 s. After the development, undercut profiles of the PR were achieved, which were favorable for lift-off in the following step. On top of the patterned PR, Ag films with different thicknesses were deposited by a magnetron sputterer at a rate of 4.5 Å s-1 (Kurt J Lesker PVD75; DC power: 100 W, argon pressure: 3 mTorr, room temperature). A lift-off process was finally conducted. The samples were immersed into acetone, being shaken for 20 min and sonicated for 10 s to remove the PR and the Ag on top of it. The Ag directly on the substrate remained as uniform meshes. For comparison, Ag nanowire (NW) networks were fabricated by spray coating [44]. The Ag NWs (average diameter: 60 nm; average length: 20 µm) dispersed in ethanol (5 mg/mL) were sprayed on two pieces of PE substrates with doses of 20 µL and 50 µL, respectively. The spray nozzle was kept 10 cm above the substrate. Before spraying, the PE substrates were treated with air plasma for 10 min (Harrick Plasma, Plasma Cleaner PDC-002) to become hydrophilic for uniform distribution of the sprayed Ag NWs.
2.2 Preparation of polydimethylsiloxane (PDMS) protection coating
The PDMS elastomer was fabricated by mixing the pre-polymer with the cross-linker at a mass ratio of 10:1 (Sylgard 184 Silicone Elastomer, Dow Corning). The Ag mesh was protected by spin-coating a layer of PDMS elastomer on the top at 800 rpm for 2 min (∼ 60 µm in thickness). Finally, the whole devices were baked on a hot plate at 150 °C for 30 min.
2.3 Characterization
The morphology of the Ag meshes were inspected by both a microscope and a scanning electron microscope (SEM; Carl Zeiss Ultra 55). To avoid charging, a very thin layer of Au was sputtered onto the sample before SEM inspection. The transmissivity and haze spectra in the visible and near-infrared regimes were characterized by our home-built integrating sphere based spectrophotometer. To measure the transmissivity, we closed the outlet of the integrating sphere and recorded with photodetectors integrated on the integrating sphere the total light intensity transmitted through the sample, which was then divided by the input light intensity directly entering the integrating sphere without passing the sample, as the optical transmissivity. For the haze measurement, we measured the scattered and total transmitted light intensities when the outlet of the integrating sphere was opened and closed, respectively. Their ratio was calculated as the haze to evaluate the scattering property of the sample. The mid-infrared transmissivity spectra were measured by a Fourier transform infrared (FTIR) spectrometer (Bruker Vertex 70). The sheet resistances of the meshes were detected by a source meter (Keithley 2450) with two probes connecting to two pads on the sides [30]. The optical measurement of the Ag NW films was the same as that of the Ag mesh, while the sheet resistance was measured by a four-point probe method [42–44].
As shown in Fig. 1(a), for the EMI shielding measurement, the samples were cut into circles with diameters of 9 mm and inserted between two coaxial connectors APC-7 (Gwave Technology), which were connected to a vector network analyzer (Rohde&Schwarz, ZVA40) to record the S-parameters (S11 and S21). The EMI SE could be calculated using the following equations [4,5]:
Fig. 1. (a) Schematic of the EMI shielding measurement and picture of the APC-7 connectors. (b) Schematic of setup of the FSO communication system.
A simple FSO communication system schematically shown in Fig. 1(b) was built up to verify the feasibility of our samples to be simultaneously applied in this field. It consisted of a transmitter part and a receiver part, which were separated by a 50-cm long free-space channel. At the transmitter part, a continuous-wave laser source with linewidth of 100 kHz centered at 1550 nm was modulated using an intensity modulator to generate 1.25 Gbps on-off keying optical signals from a pseudo-random binary sequence with a length of (215-1). A variable optical attenuator (VOA) was used to tune the transmitted power. The modulated light was collimated by Collimator 1. After passing through the free-space channel where the sample was placed in the middle, the light was received by Collimator 2 at the receiver part. The received signals were first amplified by an erbium-doped fiber amplifier (EDFA) and then filtered by a band-pass filter (BPF) to remove low-frequency and high-frequency noises. Finally, the signals were detected by a photodetector (PD) and analyzed by a bit error rate (BER) analyzer. Here we only considered the FSO communication working at 1550 nm, mainly because of the very small attenuation of the laser signal in the atmosphere. On the other hand, the rapid development of EDFA, which has a good amplification performance near 1550 nm, make the FSO communication work the best at 1550 nm [37].
Chemical corrosion resistance was demonstrated by immersing the sample into water, 20 wt% KCl, 20 wt% HCl, 20 wt% KOH solutions for 2 hours, respectively. Mechanical flexibility tests were conducted by fixing our sample via two aluminum supports (one fixed, and the other moving) and bending it to the radius of 1.5 mm for 1500 cycles. The sample was also twisted by hand. After chemical immersion, bending and twisting, the EMI SE and the FSO communication quality were both characterized with the systems shown in Fig. 1. Contact angles were determined at room temperature by a contact-angle meter (SL2008 KINO, USA) using a 5-mL deionized water droplet.
3. Results and discussion
3.1 Optical and electrical characterizations of Ag mesh/PE double-layer films
According to the above method, we fabricate Ag meshes with different thicknesses on top of PE. The inset of Fig. 2(a) displays a piece of 6 × 6 cm2 Ag mesh sample with thickness, t, of 220 nm. It exhibits very good visible transparency and the bottom university logos can be clearly seen. From the zoomed-in microscopic image, square Ag grids are observed, periodically distributed on the substrate. The Ag grid lines are unbroken and self-connected. The period is measured as p ∼ 150 µm. The SEM image shows the details of a pair of crossed grid lines, whose linewidths are very uniform measured as w ∼ 6 µm, consistent with the design.
Fig. 2. (a) Measured transmissivity spectra of the Ag mesh/PE double-layer films with different mesh thicknesses and the Ag NWs/PE double-layer films normalized to that of the PE substrate. The transmissivity of a 100-nm thick ITO layer at 550 nm [40] is indicated for comparison. Inset: Photograph and the zoomed-in microscopic image of our fabricated 220-nm thick Ag mesh on a 50 µm thick PE substrate, as well as the SEM image of the crossed grid lines. (b) Measured total transmissivity spectra of the samples without normalization. (c) Measured transmittance haze spectra of the Ag mesh/PE double-layer films with different mesh thicknesses and the Ag NWs/PE double-layer films. Insets: pictures of one Ag mesh/PE sample and one Ag NWs/PE sample. (d) Measured sheet resistances, Rsh, of the Ag meshes as a function of the mesh thickness. The Rsh values of the two Ag NW networks and the 100-nm thick ITO [40] are also indicated for comparison.
The measured optical transmissivity spectra of the Ag meshes and the Ag NW networks are shown in Fig. 2(a), which are normalized to that of the PE substrate. All the Ag meshes have very high optical transmittances. The spectra are insensitive to the wavelength or the mesh thickness. They are flat and very close to each other over a broadband wavelength range from 0.4 to 2.0 µm. In the visible wavelength range, the average transmissivity is up to 92.9%, which becomes 91.5% in the near-IR wavelength range of 0.78-2 µm. These transmissivity values match well with the opening ratio of the mesh defined by (p-w)2/p2, i.e., 92.2%. At 550 nm, the transmissivity reaches 93.7%, 1.7% higher than that of a 100-nm thick ITO layer [40]. Due to the optical reflection of the PE substrate, the Ag mesh/PE double-layer films show degraded transmissivities in the wavelength range of 0.4-2.0 µm but the average transmissivity is still up to 80.3% and 82.5% in the visible and near-IR regimes, respectively (Fig. 2(b)). In great contrast, the Ag NW networks show transmissivities quickly decreasing as the wavelength increases. The high-density Ag NW network fabricated with the dose of 50 µL has even lower transmissivity over the whole wavelength range.
The measured transmittance haze spectra in the visible and near-IR wavelength ranges in Fig. 2(c) illustrate that all the Ag mesh/PE double-layer films have lower haze values than the Ag NWs/PE samples, meaning weaker scattering of light. Therefore, the university logo under the Ag mesh/PE sample can be clearly seen, while the logo under the Ag NWs/PE sample looks obscure (inset of Fig. 2(c)). At 1550 nm, the transmittance is up to 92.3% and the haze is only 6.1% for the Ag mesh/PE samples, through which the optical signal will not be affected much (to be demonstrated in the FSO communication experiment below).
Figure 2(d) shows that the sheet resistance, Rsh, of the Ag mesh network decreases first quickly and then slowly with the rise of the mesh thickness. When t = 220 nm, the lowest Rsh = 7.12 Ω/sq is achieved, which outperforms that of ITO [40]. Even the denser Ag NW network fabricated with the dose of 50 µL cannot compete with it in terms of both optical transmittance and electrical conductivity. Based on these excellent optical and electrical properties of our Ag mesh/PE double-layer film, high-performance EMI shielding and high-quality FSO communication are likely to be achieved simultaneously.
3.2 High-performance EMI shielding and high-quality FSO communication of Ag mesh/PE double-layer films
Figure 3(a) depicts the measured SEtot in the X band of the Ag mesh/PE double-layer films with different mesh thicknesses and the Ag NWs/PE double-layer films fabricated with two different doses. It is seen that the thicker Ag meshes are more effective in EMI shielding. The SEtot spectra are approximately independent of the frequency. The averaged measured SEtot are plotted in Fig. 3(b), in comparison with the SEtot values theoretically calculated with Eq. (4) [14,17,18].
Fig. 3. (a) Measured SEtot, (b) average measured SEtot in the X band and theoretically calculated SEtot, measured (c) SEref and (d) SEabs, spectra of (e) reflectivity, R, and (f) absorptivity, A, for the Ag meshes with different thicknesses and Ag NW networks fabricated with doses of 20 and 50 µL on PE substrates.
To investigate the EMI shielding mechanism, the SEref and SEabs of the above samples were also measured and presented in Figs. 3(c) and 3(d), respectively. For a clearer presentation, the corresponding spectra of the reflectivity, R, and absorptivity, A, are plotted in Figs. 3(e) and 3(f), respectively. The Ag meshes show quite different shielding behaviors in the X band from the Ag NW networks. In the Ag NW networks, there are too many interfaces, leading to multiple reflections of EM waves and contributing to the absorption-dominant SEtot [18,20]. If the network is electrically more resistant, less EM waves will be reflected and more will enter the multiple-interfaced network, where the EM waves can be trapped and absorbed, and the induced currents can be more quickly dissipated as heat. Therefore, for the 20-µL Ag NW networks compared to the 50-µL sample, the A values increase and become almost equivalent to the R values (Figs. 3(e) and 3(f)). For the Ag meshes, as expected, SEref is high/low in the low/high frequency band (Fig. 3(c)) and SEabs behaves oppositely (Fig. 3(d)), leading to relatively flat spectra of SEtot for all the Ag mesh samples (Fig. 3(a)). Due the quite small meshes, whose periods are much smaller than the EM wavelengths, the R values of all samples are much larger than their A values (Fig. 3(e) and 3(f)), indicating clearly the reflection-dominant SEtot. Compared to the Ag NW networks, there are few interfaces in the Ag mesh samples. The incident EM waves cannot be trapped or absorbed in the meshes much thinner than the EM wavelengths (Fig. 3(f)). However, relatively high SEabs and A values are observed in Figs. 3(d) and 3(f). This is probably due to the in-plane currents induced by the APC-7 connectors where the sample is inserted for measurement. The currents ultimately dissipate as Joule heat. Nevertheless, the absorption is still quite low and does not affect the analysis of the shielding mechanism.
On the other hand, when the Ag mesh/PE double-layer films were applied in a free-space channel of a FSO communication system shown in Fig. 1(b), we measured the BER of the optical signal at 1550 nm versus the transmitted power and plotted it in Fig. 4. The results for the case with the Ag NWs/PE double-layer films applied in the free-space channel are also plotted in Fig. 4 for comparison. When there is no sample in the free-space channel, the BER normally decreases exponentially with the increase in transmitted power. When our Ag mesh/PE samples are inserted in the channel, the transmitted power must be increased to obtain equivalent BER values, because part of the light is reflected, absorbed, and even scattered by the grid lines. Therefore, the curves move to the right a little bit. Since all the Ag mesh samples have similarly high transmissivities and low hazes at the FSO wavelength of 1550 nm (Figs. 2(a) and 2(b)), the measured BER curves are very close to each other (Fig. 4). At BER = 10−5, the power penalty is merely ∼ 1.6 dBm for all the Ag mesh/PE samples. Due to the much lower transmissivity and higher hazes of the Ag NWs/PE samples (Figs. 2(a) and 2(b)), the BER curves for these cases shift more to the right with larger power penalty values, i.e., 3.4 and 4.1 dBm at BER = 10−5 for the Ag NWs fabricated with 20 and 50 µL, respectively (Fig. 4), meaning poorer communication quality. In a word, applying our Ag mesh/PE double-layer films to the FSO communication system will not affect the communication much, and instead will provide very good EMI shielding for the electrical devices in the system. This is of critical importance for many applications, where the devices can be effectively protected to prevent internal EM wave leakage and external EMI, and meanwhile maintain very good optical communication channels to the outside.
Fig. 4. The measured BER versus the transmitted optical power when the Ag mesh/PE double-layer films with different mesh thicknesses and the Ag NWs/PE films fabricated with Ag NWs doses of 20 and 50 µL are inserted to the free-space channel of the FSO communication system in comparison with the empty channel.
3.3 Chemical stability improvement with PDMS coating
It is known that stability is a big problem with Ag. It is not resistant to chemical corrosion and easy to oxidize even in air. In order to make our Ag mesh based EMI shields able to be stably applied in various environments, we coated the Ag mesh with a thin layer of PDMS. PDMS is nonabsorptive and highly transparent in a very broad wavelength range from visible to near-IR. It also has excellent chemical inertness, outstanding flexibility and elasticity. It is easy to prepare and handle. Therefore, it is expected to improve the stability of our Ag mesh based EMI shields, and meanwhile to keep the effective EMI shielding and high-quality FSO communication.
Here we spin-coated a 60-µm thick PDMS film on the 220-nm thick Ag mesh, forming a PDMS/Ag mesh/PE three-layer configuration. The mesh was completely covered with the PDMS for sufficient protection. Because of the good transparency, the PDMS coating almost does not affect the overall optical transmissivity of the Ag mesh/PE double-layer film much, and instead, the transmissivity is improved a little bit in the wavelength range shorter than about 1.6 µm. This is probably attributed to the antireflection of the PDMS coating (Fig. 5(a)). The average visible transmissivity is increased to 84.2% and the average near-IR transmissivity does not change much, when the PDMS coating is applied. The measured contact angle is 84.5° and 96° for the PE substrate and PDMS coating, respectively (insert of Fig. 5(a)), showing excellent hydrophobicity and self-cleaning ability of our EMI shielding film. Therefore, application of our sample in a humid environment is not a big concern.
Fig. 5. (a) Measured optical transmissivity spectra of Ag mesh/PE double-layer, PDMS/Ag mesh/PE three-layer films. Insert: measured contact angles of the PE substrate and the PDMS coating. Measured (b) SEtot, (c) SEref and (d) SEabs of the Ag mesh/PE double-layer and PDMS/Ag mesh/PE three-layer films. SEtot of the PDMS/PE double-layer film is also measured and plotted in (b) for comparison. The corresponding spectra of (e) reflectivity, R, and (f) absorptivity, A, are inserted in (c) and (d), respectively.
As expected, the PDMS/PE double-layer film has nearly no shielding effects in the frequency band of 8-12 GHz (Fig. 5(b)). Therefore, the Ag mesh sandwiched between PDMS and PE is the main reason for the high EMI SE. Interestingly, it is found that the SEtot for the PDMS/Ag mesh/PE three-layer sample becomes a little bit lower than that for the Ag mesh/PE double-layer sample in a broad frequency range from 8 to 11.2 GHz. From SEref (Fig. 5(c)), it is seen that the PDMS helps minimize the impedance mismatch between Ag mesh to the free space in the range of 8-9.6 GHz, where the SEref values become much lower than those for the sample without the PDMS coating. With more unreflected EM waves, the SEabs or A values are enhanced in the similar range, as shown in Fig. 5(d). The R peak around 8.7 GHz in the inset of Fig. 5(c) means perfect impedance match. The EM wave may be trapped in the PDMS between the conductive Ag mesh and the APC-7 connector used for measurement, resulting in an A peak in the inset of Fig. 5(d). Due to the sufficiently thin Ag mesh, the coupled EM waves cannot be fully absorbed, resulting in a little bit more transmission through the three-layer sample and thus a little bit lower SEtot compared with the Ag mesh/PE double-layer sample.
To verify the chemical stability of the coated Ag mesh, the Ag mesh/PE samples and the PDMS/Ag mesh/PE samples were immersed into four different chemical solutions, each for 2 hours. The chemical solutions were pure deionized water, 20 wt% KCl, 20 wt% HCl, 20 wt% KOH solutions, respectively. Subsequently, their EMI shielding performances and the FSO communication qualities were characterized and plotted in Fig. 6(a) and 6(b), respectively. The EMI SEtot spectra (Fig. 6(a)) and the BER curves (Fig. 6(b)) kept almost the same for all the PDMS/Ag mesh/PE samples before and after being treated chemically. This means that the Ag mesh is well maintained. In contrast, the EMI SE became weakened (Fig. 6(a)) and the FSO communication quality became better with left-shifted BER curves (Fig. 6(b)) for the uncoated Ag mesh/PE samples after being treated in water and KCl solution, meaning partial removal of the Ag mesh; while the Ag mesh completely disappeared after being taken out of the HCl and KOH solutions, whose EMI SE and the BER were not measured. These experiments fully prove that the Ag mesh coated with the PDMS coating can be well protected and become quite resistant to various chemical corrosion.
Fig. 6. Measured (a) SEtot and (b) BER of the PDMS/Ag mesh/PE and the Ag mesh/PE samples before and after being immersed in water, 20 wt% KCl, 20 wt% HCl, 20 wt% KOH solutions for 2 hours.
3.4 Excellent mechanical flexibility
Our PDMS/Ag mesh/PE three-layer sample demonstrates excellent mechanical flexibility. There was nearly no change in EMI SE for the sample either after being bent with a radius of ∼1.5 mm for 1000 cycles (Fig. 7(a)) or after being rotated and twisted into a small pipe shape (Fig. 7(b)). The samples subjected to bending and twisting also had almost the same optical transmissivity at 1550 nm and the measured BER curves did not change much compared to the curve for the sample without bending nor twisting (Fig. 7(c)). Such mechanical flexibility and stability illustrate the good continuity of the Ag mesh and its good adhesion to the PE substrate with the coverage of the 60-µm thick PDMS film.
Fig. 7. Measured SEtot of the PDMS/Ag mesh/PE three-layer film (a) before and after being bent at a radius of ∼1.5 mm for 1000 cycles (inset: the bent sample fixed to two holders), (b) before and after being rotated and twisted into a small pipe shape (inset: the twisted sample). (c) Measured BER vs. the transmitted optical power of the PDMS/Ag mesh/PE sample before, after bending for 1000 cycles and after being twisted severely.
3.5 Extended EMI shielding application demonstration
Our transparent PDMS/Ag mesh/PE three-layer film does not only have very good EMI shielding performance in the X band as shown in Fig. 5, but also can shield low-frequency signals. When a mobile phone in an aluminium box was covered with our shielding film, the orignally strong 4 G signal disappreared totally, as demonstrated in Fig. 8. The high transparency of our shielding film allows us to see the texts on the screen clearly. Extended applications of our three-layer film in the low-frequency band can be thus expected.
Fig. 8. The 4 G signal of a mobile phone in an aluminiu box (a) without and (b) with our PDMS/Ag mesh/PE three-layer transparent EMI shielding film as an optical window.
4. Conclusion
In conclusion, we first present experimentally a broadband transparent and flexible Ag micromesh for both efficient EMI shielding in the X band and high-quality FSO communication. High transmission is achieved in a broad wavelength range of 0.4-2.0 µm. The transmittance of the Ag micromesh relative to the PE substrate is around 92% over the whole wavelength range, and the sheet resistance is as low as 7.12 Ω/sq. The Ag mesh/PE double-layer sample achieves an overall transmittance as high as 80.9% at 550 nm and a high average EMI SE of 28.8 dB from 8 to 12 GHz and. At 1550 nm, the high optical transmittance and the low haze allows a high-quality FSO communication with small power penalty, superior to those of the Ag NW networks. With a PDMS coating, the average EMI SE is still as large as 26.2 dB and the overall transmittance is increased to 84.5% at 550 nm due to antireflection. The FSO communication does not change much due to the nearly unchanged transmittance and haze at 1550 nm. From Table 1, it is seen that in terms of both optical transparency and EMI SE, our Ag mesh/PE double-layer and PDMS/Ag mesh/PE three-layer samples are comparable and even superior to most of the previously-reported transparent EMI shields. Both the EMI shielding performance and the FSO communication function maintain after 2-hour chemical corrosions as well as after 1000 bending cycles and twisting. It is also found that our PDMS/Ag mesh/PE sandwiched film has a self-cleaning capability, suitable for outdoor applications. We believe that our work is of great military significance. Besides, extended applications are also potential because our shielding film also shields EM signals in the 4 G low-frequency band.
Table 1. Comparison of our transparent EMI shielding films with various previously-reported counterparts.
Note that Ag mesh is transparent in an extremely broad wavelength range, i.e., 0.4-20 µm (Fig. 9). If the PE substrate or the PDMS coating can be made transparent in the mid-infrared regime, the FSO communication of the Ag mesh/PE double-layer or the PDMS/Ag mesh/PE three-layer could be extended to the long wavelengths, where atmospheric interferences can be reduced greatly, and even higher communication quality could be achieved [48,49].
Fig. 9. The transmissivity spectra of PE and PDMS, as well as the transmissivity spectum of the 220-nm thick Ag mesh normalized to that of the PE substrate.
Funding
National Key Research and Development Program of China (2017YFA0205700); National Natural Science Foundation of China (61307078, 91833303); Ningbo Science and Technology Project (2018B10093).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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