Flexible and ultra-thin silver films with superior electromagnetic interference shielding performance via spin-coating s ilver metal organic decomposition ink

Herein, we presented a clean, green and economical method to prepare flexible and ultrathin silver films for electromagnetic interference (EMI) shielding by thermal decomposition of metal organic decomposition (MOD) ink. The non-particulate ink synthesized by complexing silver oxalate with 1,2-diaminopropane in ethanol solvent can be easily decomposed to pure silver at a quite low temperature in air. The efficient electrical transport network constructed by interconnected nanoparticles of the silver film results in the outstanding electrical properties and shielding performance, with an absorption-dominated mechanism. Curing the ink at 130 ℃ for 30 min, a more effective network structure for electron transport can be obtained. By spin coating deposition method, the silver film with only a 200 nm thickness has an ultrahigh conductivity of 8.46  10 6 S·m -1 and excellent shielding effectiveness of 51.1 dB at 10.3 GHz in X band, which is the highest shielding effectiveness of the film with such thin thickness so far. The factors affecting EMI shielding performance of silver films were elucidated, including heat treatment process, the ink concentration and spin coating times, and the relationship between the microstructure and properties of the film was established. This work indicated that silver metal organic decomposition inks will be a favorable choice for industrial production of EMI shielding applications.

Finally, the relationships between the microstructure, electrical properties and electromagnetic shielding properties of silver films were established.

Materials
Silver nitrate was purchased from Sino-platinum Metals Co. Ltd. Sodium oxalate and ethanol were purchased from Sinopharm Chemical Reagent Co. Ltd. 1,2-diaminopropane and ethyl cellulose were purchased from Aladdin Industrial Corp. All the reagents were analytical grade and used without further purification. Polyethylene terephthalate films (PET) with a thickness of 0.4 m used here were purchased from Du Pont Co Ltd.

Preparation of silver MOD inks and films
Firstly, silver oxalate powder was synthesized by ion-exchange reaction between silver nitrate and sodium oxalate, according to a procedure in our previous work 24 . Then the mixture of ethanol, 1,2-diaminopropane, ethyl cellulose ethoce and silver oxalate powder was constantly stirred until all the mass was fully dissolved and a transparent ink was obtained.
Finally, the ink was filtered through a 0.22 m syringe filter and then stored in a refrigerator for the following use. For preparing different concentrations of ink, we dissolved different amounts of silver oxalate powder in 1 ml ethanol. For example, 1 mmol silver oxalate powder was prepared for the 1 M ink.
The 3 cm3 cm PET films used as the substrates were cleaned with ethanol and deionized water, followed by O 2 plasma treatment to achieve hydrophilic surfaces 26 . The prepared MOD ink was spin-coated on the pretreated PET substrate at 3000 rpm for 20 s to get the wet film.
Then wet films were thermally treated on a hotplate at different temperatures and time. For films with multilayers, spin-coating and thermal treatment were repeated for several times and the repeated times corresponded to the film layers.

Characterization
Thermogravimetric analysis and differential scanning calorimetry (TG-DSC) were carried where R, A, T represent the coefficients of reflected power, absorbed power, transmitted power, respectively. SE R , SE A , SE T represent reflection shielding effectiveness, absorption shielding effectiveness and total shielding effectiveness.

Results and discussion
3.1 Effects of thermal treatment on films microstructure and properties Fig. 1a and Fig. 1b show the images of the prepared silver MOD ink and the silver film.
The ink with 24.5 wt% of solid content exhibits a clear and transparent appearance with no particles, as also confirmed by Uv-Vis absorption analysis (Fig. S1). The PET substrates spincoated with silver ink and heat-treated at 130℃ show luster of silver metal and were flexible with bending. According to previous works, heat treatment process plays an important role on the microstructure and properties of silver films derived from the MOD inks 24, 29-32 . Prior to draft the thermal treatment process, the thermal decomposition behaviors of the silver oxalate-1,2-diaminopropane ink were investigated. TG-DSC thermal analysis was carried out and the results were shown in Fig. 1c. The thermal decomposition process of the ink ended before 170 ℃ and can be divided into three steps. The first step is from room temperature to 105 ℃, during which the mass fraction of the ink decreased by ~40%, accompanied with an obvious endothermic peak at 97.7 ℃ in the DSC curve. This decomposition process is caused by the volatilization of solvent. The second decomposition process started from 105 ℃ and ended to 135 ℃, with a 20% mass loss and an endothermic peak at 115.5 ℃ in the DSC curve. This step is corresponding to the decomposition of silver-amine complex and the volatilization of 1,2diaminopropane. In the last step, from 135 ℃ to 152 ℃, the mass fraction of the ink decreased to 17.3%, and a strong endothermic peak appeared at 149.5 ℃ in the DSC curve, corresponding to the decomposition of the remaining organic matter in the ink. Finally, the mass was stable at 16.9%, representing the mass fraction of silver in the ink. Accordingly, the temperature range of heat treatment process for the ink is set as 90~170 ℃ and six films heat-treated at different temperatures were fabricated. corresponds to the C-H stretching vibration of organic solvent 24 . The peaks between 1760 cm -turned into a strong and sharp peak at 3306 cm -1 , which is the vibration peak of amine group 24 , indicating that the ethanol solvent had volatilized and 1,2-diaminopropane remained in the decomposition product of the ink. A large number of sharp peaks appear at 1760 ~ 1240 cm -1 , also indicating that 1,2-diaminopropane and oxalate were remained and wrapped on the surface of silver nanoparticles. The small peak at 2210 cm -1 , corresponding to carbon oxide 35 , is likely that oxalate partially decomposed into CO 2 and adsorbed on the decomposition products. As the temperature was raised to 100 ℃, the peak at 3306 cm -1 disappeared, indicating that 1,2 propanediamine had volatilized. In the local expansion of FTIR spectrum (shown as Fig. S2), two weak peaks are found at 1383 cm -1 and 2168 cm -1 , corresponding to symmetric stretching vibration of carboxylate 36 and CO adsorbed on the product surface, respectively. With the increase of temperature, the intensity of two remaining peaks decreased gradually. When the temperature was raised to 170 ℃, no peak was found. Therefore, the FTIR results, being consistent with the TG-DSC analysis, suggest that the residual organic matter decomposes more thoroughly with the increase of curing temperature and there is almost no organic matter in the film at 170 ℃.
is longer than 30 min. In consideration of the energy-saving synthesis of silver films with high shielding performance, the appropriate curing process is set as 130 ℃ for 30 min. The low curing temperatures make it possible that our ink can be applied to various flexible substrates.
Many printing and coating technologies can be adopted for this ink to deposit the silver film or patterns, which make the fabrication process more easily and more economical. In this experiment, we select spin coating method to fabricate more uniform films. By this fabrication method, the silver film possessed a high shielding effectiveness of 49.68 dB with the thickness of 400 nm. To clarify the outstanding EMI shielding performance of the ultra-thin silver films in this work, a theoretical analysis was carried out according to eq. 7 for electrically thin materials 37, 38 : where f is the frequency and  is the magnetic permeability. In this experiment, the film obtained at 170 ℃ for 30 min has the highest conductivity of 8.7610 6 S·m -1 , and its calculated skin depth is 1.8 m, whereas its thickness is only 400 nm. Accordingly, eq 7 can be applied to all the fabricated silver films to estimate their theoretical SE and the results are presented in Fig. 3d. It can be seen that the measured SE is very close to the theoretical value, which suggests that the excellent EMI shielding performance of silver films benefits from its ultrahigh conductivity. An efficient electronic transport network structure of interconnected silver nanoparticles has been established in the film properly cured. Moreover, a theoretical guidance of eq 7. for conductive films is that the EMI shielding effectiveness is determined by the film sheet resistance.
It is well known that shielding materials weaken the penetration ability of electromagnetic waves by the means of reflection and absorption. The interaction mechanism between incident electromagnetic wave and the shielding material was illustrated in Fig. 3e. The power coefficients R, A and T (shown as Fig. 3e) represent the respectively ratio of the power of reflected, absorbed and transmitted electromagnetic waves to the incident electromagnetic waves, calculated from S parameters using eq 1~3.  respectively. Accordingly, the shielding mechanism of the silver films is mainly dominated by absorption. The network structure providing large amounts of interfaces among nanoparticles maybe the reason for the absorption-dominated shielding mechanism. The incident electromagnetic waves are reflected and scattered in the pores formed among nanoparticles, resulting in an effective absorption loss, as schematically shown in Fig. 3e.

Effects of MOD silver ink concentration on film microstructures and properties
Generally, the thickness of shielding materials will make an influence on its shielding

Effects of spin coating layers on film microstructures and properties
We spin coated several times to obtain multilayer films with different thickness. The four inks with different concentrations were used to prepare the corresponding single layer and  1 M films with 1 layer (g1~g3), 3 layers (h1~h3), 6 layers (i1~i3).
Then we explore the relationship between the surface roughness, sheet resistance and resistivity. According to SEM images, due to the uniform morphology of 0.25 M films, the roughness can be detected by atomic force microscope, as shown in Fig. 7a and Fig. S4.
However, considering the uneven surfaces of other concentration films and the big variety of the roughness in different micro regions, the laser confocal microscope was used to observe the large area region (Fig. S5) and obtain the average surface roughness, as shown in Fig. 7b~d. Moreover, the increase of multilayer film thickness and the introduction of pores between interlayers will also make the transmission path of incident electromagnetic waves become longer and the multiple reflected loss will be enhanced, which will further improve the electromagnetic shielding effectiveness. From the above analysis, it can be concluded that the shielding effectiveness of silver films fabricated by this method is related to the sheet resistance. Multilayers will affect the roughness and thickness of films, which will further affect the sheet resistance and the shielding

Conclusion
In this study, flexible silver films with ultrathin thickness and high shielding performance were fabricated from silver oxalate-1,2-diaminopropane ink. The efficient electron transport network structure of films constructed by interconnected silver nanoparticles brings about ultrahigh conductivity and excellent shielding effectiveness. Large amounts of interfaces among nanoparticles determine the absorption dominated shielding mechanism of silver films.
Thermal treatment process has influences on decomposition degree of silver amine complex and microstructure of films, and then affect conductivity and shielding effectiveness. A suitable thermal treatment for silver oxalate-1,2-diaminopropane ink is at 130 ℃, for 30 min. The electromagnetic shielding performance of silver films by this method is determined by sheet resistance, which is influenced by the roughness and thickness of films. By adjusting ink concentrations and film layers, the obtained films are with a large range of roughness and thickness, resulting in different shielding performances. Considering the ultrathin characterization, the single layer film prepared from 0.5 M ink was the best film, with a high EMI shielding effectiveness of 51.1 dB and just only a 200 nm thickness. Consequently, the silver MOD ink is promising to be applied to industrial production for EMI shielding films.