Flexible Embedded Metal Meshes by Sputter-Free Crack Lithography for Transparent Electrodes and Electromagnetic Interference Shielding

A facile and novel fabrication method is demonstrated for creating flexible poly(ethylene terephthalate) (PET)-embedded silver meshes using crack lithography, reactive ion etching (RIE), and reactive silver ink. The crack width and spacing in a waterborne acrylic emulsion polymer are controlled by the thickness of the polymer and the applied stress due to heating and evaporation. Our innovative fabrication technique eliminates the need for sputtering and ensures stronger adhesion of the metal meshes to the PET substrate. Crack trench depths over 5 μm and line widths under 5 μm have been achieved. As a transparent electrode, our flexible embedded Ag meshes exhibit a visible transmission of 91.3% and sheet resistance of 0.54 Ω/sq as well as 93.7% and 1.4 Ω/sq. This performance corresponds to figures of merit (σDC/σOP) of 7500 and 4070, respectively. For transparent electromagnetic interference (EMI) shielding, the metal meshes achieve a shielding efficiency (SE) of 42 dB with 91.3% visible transmission and an EMI SE of 37.4 dB with 93.7% visible transmission. We demonstrate the highest transparent electrode performance of crack lithography approaches in the literature and the highest flexible transparent EMI shielding performance of all fabrication approaches in the literature. These metal meshes may have applications in transparent electrodes, EMI shielding, solar cells, and organic light-emitting diodes.

Silver meshes provide for shielding through both reflection and absorption.Figure S4 shows the shielding efficiency contribution of the five samples, where the total shielding efficiency SE is a sum of the reflection shielding efficiency SE R and absorption efficiency SE A , SE = SE R + SE A , where all are evaluated in the 8 -18 GHz range.These shielding efficiencies are calculated using the following equations: where R rf denotes the reflection coefficient and where T rf denotes the transmission coefficient in the radio frequency.For the five samples, the reflection efficiencies SE R are 13.1, 13.0, 14.0, 13.The coefficients of transmission (T rf ), reflection (R rf ), and absorption (A rf ) represent the proportions of an incident electromagnetic wave that are transmitted, reflected, and absorbed, respectively.These coefficients satisfy the following equation due to conservation of energy: The coefficients T rf and R rf are determined from the scattering matrix elements and The reflection coefficients for the five samples are 0.95, 0.95, 0.96, 0.95, 0.95, respectively, and the corresponding absorption coefficients are 0.05, 0.05, 0.04, 0.05, 0.05, respectively.For the adhesion test, we prepared three smaller samples, each measuring 1 cm by 1 cm, cut from the larger silver-coated PET piece.The adhesion strength between the silver layer and the PET substrate was evaluated using a pull-off test.In this setup, rigid substrates were attached to both sides of the samples: one side to the PET and the other to the silver film, using superglue for a secure attachment (Figure S5 suggesting that the actual adhesion strength between the silver and PET might be higher than the measured value.This conclusion is further supported by the fact that the adhesion between the silver and PET was observed to be stronger than the adhesion between the PET and the glue, limiting our ability to precisely determine the ultimate adhesion strength of the silver-PET combination.

Figure S1 :
Figure S1: Depth of etched cracks in the five studied samples, determined by optical profilometry measurements, for different etch times of (a) 950, (b) 750, and (c) 650 seconds.

Figure S2 :
Figure S2: Probability density vs. width distribution for five fabricated samples , ranging from (a) to (e) for samples 1 through 5, respectively.

Figure S3 :
Figure S3: Centroid detection for each isolated cell analyzed through image processing for the five fabricated samples, ranging from (a) to (e) for samples 1 through 5, respectively.

Figure S4 :
Figure S4: (a) SE contribution of SE A and SE R .(b) Power coefficient for five samples in the radio frequency.

Figure S5 :
Figure S5: Characterizing of silver-PET adhesion test (a) schematic of the test set up (b) stress vs. displacement (c) optical images of the sample before and after the test.

Figure
FigureS5presents the results of the adhesion test performed on silver-coated PET (polyethylene terephthalate) samples.The process began with a 3 cm by 3 cm PET sample, onto which silver ink was spin-coated at a speed of 1000 rpm to ensure a uniform layer of silver film.This sample underwent a ramp-cure process identical to that used for our metal meshes.The curing procedure started at a temperature of 70 °C, increasing incrementally by 10 °C every 15 minutes, until reaching a final temperature of 110 °C.
(a)).The pull-off test was conducted at a steady rate of 0.1 mm/min.The stress-displacement data obtained from the pull-off tests (Figure S5(b)) indicated an average adhesion strength of 2.7 MPa, with a standard error of 0.2 MPa, based on three separate measurements.Notably, post-test examination of the interface (Figure S5(c)) revealed that the silver coating remained intact and undamaged,

Table S2 :
EMI shielding performance comparison between our metal meshes and those in the existing literature.

Table S2
shows the comparison of EMI shielding performance of our fabricated samples with other metal meshes in literature.The table is structured into three sections: the first segment presents samples derived from our research on PET (polyethylene terephthalate) substrates; the second section highlights metal meshes from other studies applied to PET substrates; and the third section centers on metal meshes employed with either glass or sapphire substrates.This table provides information regarding the material type, the substrate applied, light transmission rate at 550 nm wavelength, examined frequency range, and the average shielding efficiency (SE ave ).It is worth noting that the references studied in the literature have explored various frequency ranges and reported SE values in terms of maximum, minimum, or average.To maintain consistency throughout our paper, we have opted to specifically report the average SE within the frequency range of 8 -18 GHz.