Crackle template based metallic mesh with highly homogeneous light transmission for high-performance transparent EMI shielding

Our daily electromagnetic environment is becoming increasingly complex with the rapid development of consumer electronics and wireless communication technologies, which in turn necessitates the development of electromagnetic interference (EMI) shielding, especially for transparent components. We engineered a transparent EMI shielding film with crack-template based metallic mesh (CT-MM) that shows highly homogeneous light transmission and strong microwave shielding efficacy. The CT-MM film is fabricated using a cost-effective lift-off method based on a crackle template. It achieves a shielding effectiveness of ~26 dB, optical transmittance of ~91% and negligible impact on optical imaging performance. Moreover, high–quality CT-MM film is demonstrated on a large–calibre spherical surface. These excellent properties of CT-MM film, together with its advantages of facile large-area fabrication and scalability in processing on multi-shaped substrates, make CT-MM a powerful technology for transparent EMI shielding in practical applications.


Supplementary Note 1. Preparation of crackle emulsion
At present, researchers use a commercial nail for self-forming crackle templates. A relatively easier dissolving method can be applied to remove the sacrificial template at the last step compared with other approaches such as using the TiO 2 gel. 2,3 However, the unknown components or fillers within the commercial nail prevent us from getting metallic networks with good processibility, transmisivity and conductivity. An ideal emulsion should perform a homogeneous dispersion with specific rheological behavior and exhibit cracking property. It requires dispersion with reasonable viscosity (to facilitate the spreading of CE on large-scale) and suitable mixing ratio between hard monomers and soft monomers (to minimize the buckling of cracked cells), as well as rational coating technique that can avoid thickness non-uniformity and slowdown secondary flows induced by surface tension. Moreover, good drying strategy is also needed for cracking control.
Crackle-emulsion (CE) used in this research is a water-based acrylic dispersion, and it was synthesized by a traditional emulsion polymerization method. 4 In the CE preparation, AA and HPA were introduced as functional monomers to enhance the adhesivity and water solubility of CE. However, excessive AA and HPA would increase viscosity and particle size after polymerization, even cause large number of sediments that negatively influence the quality of CE. On the other hand, the amounts of emulsifier and initiator are also very important. Deficient amount of emulsifier or initiator will result in insufficient polymerization of monomers, while excessive emulsifier and initiator lead to fast polymerization; both situations produce poor CE.
Here, 3.4 wt% of AA, 6.7 wt% of HPA, 5.0 wt% of emulsifier and 0.5 wt% of initiator to the total amount of monomers were selected, and the emulsifier composed of 3 part of OP-10 and 2 S13 part of SDS was preferred. More importantly, the primary amount of hard monomer (MMA) and soft monomer (BA) essentially determined features of the CE solution. The total amount of monomers was adopted according to the final solid content needed, and a solid content of 40% of CE was selected for spray coating in our experiments. Taking CE3 (R MM-B =3) for example, specific ingredients for the preparation of CE3 are listed in Supplementary

Supplementary Note 2. Critical cracking thickness analysis
When a thin film of wet coating containing suspended colloidal particles is dried on a substrate, evaporation of the solvent concentrates the particles into a closed packed array. The film generally binds to the substrate and resists deformation in the transverse direction giving rise to transverse tensile stresses. 5 If the particles are hard, the film cracks to release the stresses. It has long been observed that the maximum crack-free thickness (i.e., critical thickness) for a film plays an important role in cracking analysis. The critical stress for cracking can be presented by the following equation 6 : Here, R is the particle radius, γ is the solvent-air interfacial tension, G is the shear modulus of the particles, M is the coordination number, φ rcp is the particle volume fraction at random close packing, and P max is the maximum attainable capillary pressure. The dimensionless capillary S14 pressure was determined as max ( ) / 2 1.2 0.08 P R γ − = ± using Kelvin equation 5 . Choosing a value of 1.15, the critical thickness can be reduced as:

Supplementary Note 3. Diffraction Calculation based on PSF
The diffraction characteristics of a regular metallic mesh structure were analyzed using a pupil function. The pupil function of a circular aperture square mesh can be expressed as the following equation: Where g is the metal line width, w is the line spacing, Ng refers to the sample diameter, which shows that the observed field strength is proportional to the Fourier transform of the aperture distribution. The diffraction intensity distribution of the observed field can further be evaluated as: Here, A 0 is the frequency spectrum of complex amplitude distribution of the aperture field.
Neglecting constant coefficients, the diffraction intensity distribution equals to the power spectrum of the aperture field distribution. S16

Supplementary Note 5. System SNR analysis of metallic mesh
In the imaging application, the flux contained in the central order (zeroth-order) represents the signal. The total flux reaching a given image point results from both the signal and background (noise). For a given point in the scene, the fraction of incident energy that is diffracted away from the central order into the background is obviously the total transmittance of the mesh (T opt ) minus the transmittance to the central order (T (0,0) ). If the scene is fairly uniform and extended, the unwanted background component will also be fairly uniform and extended across the image.
Thus, the ratio of the system signal-to-background level is