Highly Conductive Fractal-structured Silver Particles for Preparing Flexible Printed Circuits via Screen Printing

Sufeng Zhang,Chen Hua,Bin He,Yongwei Li,Qiusheng Zhou,Pengbing Chang,Xuxu Hu,Lina Liu

1. Shaanxi Provincial Key Lab of Papermaking Technology and Specialty Paper Development,National Demonstration Center for Experimental Light Chemistry Engineering Education, Key Lab of Paper Based Functional Materials of China National Light Industry, Shaanxi University of Science and Technology,Xi'an,Shaanxi Province,710021,China

2. Key Lab of Pulp and Paper Science & Technology of Ministry of Education/Shandong Province,Qilu University of Technology (Shandong Academy of Sciences), Ji'nan, Shandong Province,250353,China

Abstract: Fractal-structured silver particles (FSSPs) are conductive materials with a micron-scale trunk and nanoscale branches,and are characterized with high electrical conductivity and high connectivity. In this study, FSSPs were added to an aqueous additive solution for synthesizing a conductive ink,which was used to prepare two types of printing electrodes via screen printing. The first type included two flexible printed electrodes (FPEs): an FPE on a polyethylene terephthalate (PET) film and an FPE on paper. The second one was a polydimethylsiloxane (PDMS)-embedded FPE. The PETbased FPE exhibited high electrochemical stability when its sheet resistance was 0.38 Ω/sq for a 50% (w/w) content of FSSPs in the prepared conductive ink. Moreover, the embedded FPE demonstrated excellent mechanical properties and high chemical stability. In addition, the embedded structure was endowed with stretchability, which is important for different devices,such as flexible biomedical sensors and flexible electronics.

Keywords: fractal-structure silver particles; screen printing; embedded electrodes; stretchability

1 lntroduction

Recently, printed electronics technology has been garnering extensive attention related to the preparation of flexible electrodes, owing to the existence of versatile, low-cost, and environmentally friendly materials[1-3]. Moreover, flexible electrodes are very promising for applications in next-generation devices[4-8],such as light-emitting diodes (LEDs)[9-10], radio-frequency identification tags[11], conductive pattern-based devices[12-14], sensors[15-18], and supercapacitors[19-20].However, to realize low-cost and large-scale production of printed electronic devices,two key issues need to be solved. One is related to using appropriate conductive materials for preparing low-cost highly conductive inks, while the other is linked to a convenient and efficient printing method.

Conductive inks based on metal particles (e.g., gold and silver) are widely used in integrated circuit printing,owing to their excellent electrical conductivity[21-25]. However, the high cost of gold and other noble metals limits their large-scale applicability.Silver has attracted extensive attention because of its relatively low price, stability, as well as the shape and size tunability of its samples/particles. Silver particles can be tuned to particular structures, such as zerodimensional nanospheres, one dimensional (1D) silver nanowires[26-32], two-dimensional (2D) nanosheets[33],and three-dimensional(3D)nanoplates[34].The electrical conductivity of the host ink largely depends on the spatial distribution of silver fillers, which are ideally dispersed in the form of an isotropic conductive network, in the additive solution during the ink formation. Most of the above-mentioned regular-shape silver particles (1D, 2D) need to contact each other to form a uniform conductive network, which increases the process/product cost. In addition, these regularshape silver particles are forcefully rearranged during the printing process, resulting in an anisotropic film conductivity; that is, the resulting film will have excellent conductivity in one direction and no conductivity in the other direction, thus limiting its application. Fractal-structured silver particles (FSSPs)have special 3D structures with a multimodal distribution of micro- and nano-sized particles[35-36].Such particles are advantageous for forming a uniform conductive network in an additive solution because of their spatial distribution in the matrix (resulting in a large number of contact points); therefore, only a small amount of these particles needs to be added to the ink,keeping the product cost low. Moreover, the nanoscale tip of the fractal structure can be sintered at a lower temperature to save energy during the drying process[37].

A variety of methods have been developed to confer high electrical conductivity to traditional flexible printed circuits[38-39]. Among them, screen printing has been widely studied, owing to its simple operation and suitability for industrial production[40-42]. Some researchers have screen-printed conductive graphene inks on polyethylene terephthalate (PET) and paper substrates[43]. The screen-printing method has been demonstrated to be an efficient and low-cost tool for synthesizing printed circuits, but the pretreatment of graphene is a complex process, and graphene is more expensive than silver, copper, and other metal nanoparticles[44]. Moreover, FSSP patterns can be screen-printed on a range of substrates (e.g., paper and PET). These substrates have good stability, printability,and relatively low price, and are suitable for low-cost mass production. In our previous study, we reported a coating method for the formation of conductive networks composed of FSSPs on a PET film. The obtained composite was then embedded into a polydimethylsiloxane (PDMS) elastomer, and the final product was used as a flexible electrode[45]. The asprepared electrode demonstrated excellent tensile properties, but the coating method and uniform distribution of FSSPs required a complex process.

In the current work, FSSPs were synthesized and used as a filler material for a conductive ink, which was subsequently screen-printed on substrates to form on-surface patterns for flexible printed electrodes(FPEs) and circuits.This process has the advantages of simplicity, versatility, efficiency, and low-cost production of printed electronics. A mixture of hydroxypropyl methylcellulose, fluoride ion surfactant,and MO-2170-type defoamer was used as the aqueous ink additive. The FSSP-based conductive ink was printed on two different substrate materials: PET and commercial paper. Furthermore, the FPE of electrode/PET was embedded in a PDMS elastomer to enhance the mechanical flexibility of the circuit. The experiments performed on both samples under different external conditions, such as stretching, curling, and bending, under several operation cycles indicated the product feasibility for industrial commercialization.

2 Experimental

2.1 Materials

Silver nitrate (AgNO3, ≥99.8%) was purchased from Tianjin Damao Chemical Reagent Factory (Tianjin,China). A hydroxylamine solution (NH2OH, 50% in water) was obtained from Alfa Aesar Chemicals Co.,Ltd. (Germany). The PDMS prepolymer and Sylgard 184 (curing agent) were purchased from Dow Corning(USA). Hydroxypropyl methylcellulose (C56H108O30, 2%viscosity) was obtained from Aladdin (China).Propylene glycol, fluoride ion surfactant, isopropyl alcohol,and an antifoaming agent were purchased from Aladdin (China). All chemicals were of analytical grade and used as received.

2.2 Preparation of conductive ink

FSSPs were synthesized according to a previous study[45].First,AgNO3(50 mL,0.06 mol/L)and NH2OH(50 mL,0.24 mol/L) solutions were simultaneously added to a 150 mL three-neck round-bottom flask at a speed of 3-5 mL/min, and were strongly stirred to obtain FSSPs. The product was then washed several times with deionized water and anhydrous ethanol. Finally,the product was dried in a vacuum oven at 60℃for 5 h.Hydroxypropyl methylcellulose, deionized water,propylene glycol, the fluoride ion surfactant, and the antifoaming agent were stirred at room temperature for 6 h to obtain an additive solution. Subsequently,FSSPs, the additive solution, and isopropyl alcohol were mixed uniformly by ultrasonic treatment for 15 min and stirred at 500 r/min for 30 min. Finally, the excess solvent in the mixture was removed by vacuum heat treatment at 70℃, to obtain a stable conductive ink.

2.3 Preparation of PET-and paper-based FPEs

First, clean paper and PET film were prepared. The PET film was cleaned with anhydrous ethanol to remove surface stains and dried to serve as a substrate for the printed electrodes. Second, the previously prepared conductive ink was added to a customized screen-printing plate (I-shaped). Using a scraper, the ink was evenly transmitted through the plate mesh to the substrate (paper and PET). The ink/PET and ink/paper films were placed in an oven at 60℃for 5 min.Then,PET-based and paper-based FPEs were obtained.

2.4 Preparation of PDMS-embedded FPE

PDMS was selected as the substrate to endow the prepared electrode/PET system with flexibility and extensibility. First, the PDMS prepolymer was mixed with a curing agent at a weight ratio of 10∶1. The mixture was slowly poured onto the surface of the PET pre-substrate to completely cover the printed electrode,and the PET/electrode/PDMS film was formed. The prepared PET/electrode/PDMS film was placed in an oven at 80℃for 4 h. The PET film was then stripped off to form a PDMS-embedded FPE.

2.5 Measurement and characterization

The FSSPs morphologies were characterized by scanning electron microscopy (SEM, Hitachi S-4800,Japan) and transmission electron microscopy (TEM,FEI Tecnai G20, USA). The crystal properties of the FSSPs were determined via X-ray diffraction (XRD,PANalytical X'Pert PRO, German). The sheet resistances of the FPEs were recorded using the fourprobe method (RST-9). The FPE surface morphology was observed via field-emission scanning electron microscopy. A Keithley source meter (model: 2400,USA) was used for detecting the change in electrode resistance during the mechanical deformation of the printed electrodes, and the KH-01 stepper motor controller and a supporting mobile control platform were used for controlling the mechanical deformation of the printed electrodes.

3 Results and discussion

3.1 Fabrication process of FPEs and embedded FPE

The embedded FPE preparation process is illustrated in Scheme 1. The conductive ink composed of FSSPs, a solvent, and additives was coated onto a screenprinting plate, with the PET film or paper as a presubstrate.After scraping,screen-printed electrodes with FSSPs on the PET film and paper(i.e.,PET-and paperbased FPEs) were obtained. Because the FPE prepared with paper as the substrate was difficult to be peeled off, the FPE prepared with the PET film as the substrate was selected to prepare the embedded FPE.The PDMS solution and curing agent at a mass ratio of 10∶1 were poured slowly to completely cover the printed electrode. After the electrode solidified, the PET film was peeled off and the electrode/PDMS was obtained.

3.2 Structural characterization of FSSPs

Fig.1 presents the SEM images of the FSSPs,the highresolution transmission electron microscopy (HRTEM)image of the FSSP tip, and the XRD patterns of the prepared FSSPs. All of the FSSPs had a threedimensional structure (Fig. 1(a)), indicating good consistency. As seen from the enlarged view of the FSSPs (Fig. 1(b)), many nanoscale tips occurred on the main stem of the FSSP microstructures. The length of the micron-sized branches was approximately 1-2 μm,and that of the nano-sized branches was approximately 200-300 nm. HRTEM image of the obtained FSSPs(Fig. 1(c)) revealed their growth direction. The lattice spacing was 0.241 nm, corresponding to the (111)plane of silver; that is, the elemental silver deposition on the (111) crystal plane caused the FSSP branches to grow along the (111) crystal plane. The XRD pattern(Fig. 1(d)) exhibited a strong diffraction peak at 2θ=38.14° and four weak diffraction peaks at 2θ= 44.34°,64.46°,77.43°,and 81.58°,corresponding to the crystal planes of (111), (200), (220), (311), and (222)[45],repectively.All of the diffraction peaks were indexed to the plane of the face-centered cubic (fcc) crystalline structure of silver (JCPD card No. 04-0783). No other diffraction peaks were present in the XRD pattern,indicating that the FSSPs had high purity.

Scheme 1 Fabrication process of PET-based FPE and PDMS-embedded FPE

3.3 Sheet resistance and electrical resistance of PETbased and PDMS-embedded FPEs

The sheet resistance (Rs) and electrical resistance of FPEs are important parameters for characterizing the electrical properties of the final product. To study the effect of the FSSPs content on the ink conductive properties, four conductive inks were prepared with different FSSPs contents: 30%, 40%, 50%, and 60%.Square electrodes (50 mm × 50 mm) were prepared on the PET film by screen printing, and these electrodes were examined using the four-probe method.

Fig.1 (a,b)SEM images of the prepared FSSPs;(c)HRTEM image of the FSSP tip;(d)XRD patterns of the prepared FSSPs

Fig.2 (a)Rs of the PET-based FPEs with different FSSPs contents;(b)photographs of PDMS-embedded FPE and paper-based and PET-based FPEs

Fig. 2(a) shows theRsof the PET-based FPEs with different FSSP contents. TheRsvalues of the PETbased FPEs with 30%, 40%, 50%, and 60% FSSPs were 2.67, 0.83, 0.38, and 0.23 Ω/sq, respectively. TheRsof the PET-based FPE significantly decreased as the content of FSSPs increased from 30% to 40%. This was owing to the excess addition of FSSPs, which caused a non-uniform distribution of particles on the prepared electrode.As the content of FSSPs in the ink increased to 60%, theRsof the PET-based FPE also decreased; however, the change inRswas not as significant as that corresponding to the increase in the FSSPs content from 30% to 40%.Thus, the conductive network of the electrode was already dense and formed a good conductive path, even for the FSSPs contents under 40%. In addition, when the FSSPs content was 60%, the high solid content affected the printing process by shielding a uniform distribution of the particles on the PET-based FPE. Hence, the FSSPs content of 50% was optimal for conductive ink preparation, yielding the best electronic performance of the electrode. Fig. 2(b) shows the PDMS-embedded FPE and FPEs on commercial paper and PET substrates(the FSSP content was 50%).

The SEM images of the PDMS-embedded FPE are shown in Fig. 3(a) and Fig. 3(b). Evidently, half of the FSSPs are embedded in the PDMS substrate, thus ensuring good mechanical and electrical stability of the electrode.The SEM images of the paper-based FPE are shown in Fig. 3(c) and Fig. 3(d). The paper contains cellulose fibers with rough and porous surfaces,creating a more adherent surface for the conductive ink. Because of the good printability of the ink on the paper surface, theRsof the printing electrode was lower than that for the PET surface. According to the four-point probe method, when the FSSPs content was 50%,theRsof the paper-based FPE was 0.28 Ω/sq,and that of the PET-based FPE was 0.38 Ω/sq.

Fig. 4(a) shows the electrical resistance versus the width of the paper-based FPE,for the FSSPs content of 50%. The inset shows the printed electrodes with the widths of 0.5, 1, 2, 3, and 4 mm, but with the same length of 5 cm. The electrical resistance of the electrodes decreased with increasing width of the paperbased FPE. When the line width of the printed electrode was only 0.5 mm, the electrical resistance of the electrode was 19.7 Ω. Interestingly, when the printed electrode width was increased to 4 mm, the electrical resistance was lowered to 2.64 Ω. This finding can be explained by the formulaR=ρL/Sof resistance. With an increase in the printing line width,the cross-sectional area (S) increases, and because the length (L) and resistivity (ρ) remain unchanged, the electrical resistance (R) decreases. The relative conductivity (σ/σ0) results (Fig. 4(b)) demonstrate that the width of the printed electrode does not strongly affect its conductivity. The results show that FSSPs in the prepared conductive ink had excellent dispersivity and that the electrodes had good replicability,suggesting their suitability for industrial production.

Fig.3 SEM images of(a,b)PDMS-embedded FPE;(c,d)paper-based FPE

3.4 Effect of bending strain on the electrical properties of PET-based and PDMS-embedded FPEs

From the applicability viewpoint, conductive inks are mainly used in flexible devices, such as flexible sensors or organic LEDs (OLEDs). Thus, the printing layer must have a certain bending ability to ensure the stability of the use process. Electronic devices should maintain their electronic transmission capacity at all times, when subjected to bending at varying degrees.The change in the relative electrical resistance(R/R0)of PET-based FPE and PDMS-embedded FPE during 100 bending cycles and the bending angle of the electrode at 180°, for the FSSPs content of 50%, are shown in Fig. 5(a) and Fig. 5(b). The printed electrode with the width of 2 mm and the length of 4 cm was chosen for the bending test. Starting at the bending angle of 180°,the resistance of the PET-based FPE during the bending cycles was lower than the initial one.This was because the tested printed electrodes were manufactured by extrusion stress. After five bending cycles, theR/R0values of the PET-based FPE in the bending and releasing states decreased to 0.95 and 1.03. After 100 bending cycles, theR/R0values of the PET-based FPE in the bending state and the releasing state were 0.87 and 0.97, respectively.After 100 cycles, theR/R0of the electrode differed little from the one in the initial state,suggesting that the electrode resistance did not change significantly under tensile stress; thus, the sample was concluded to have good mechanical stability.

Fig.4 (a)Electrical resistance versus the width of the paper-based FPE(FSSPs content 50%),the inset shows the printed electrodes;(b)relative conductivity versus the width of the paper-based FPE

Fig.5 Dependence of relative electrical resistance on the number of bending cycles,for(a)the PET-based FPE and(b)the PDMS-embedded FPE(in both cases,the FSSPs content was 50%)

Similar results were also obtained for the PDMSembedded FPE (Fig. 5(b)). After five bending cycles,theR/R0values of the PDMS-embedded FPE in the bending and releasing states decreased to 1.18 and 1.08. After 100 bending cycles, theR/R0values of the PDMS-embedded FPE in the bening and releasing states stabilized at 1.10 and 1.01, respectively. These results suggest a uniform distribution of the FSSPs in the additive solution; hence, throughout all bending cycles, the FPE resistance in the releasing state was close to the initial resistance. However, theR/R0values of both electrodes in the bending state were different.In the bending state, theR/R0value of the PET-based FPE decreased, whereas that of the PDMS-embedded FPE decreased in the first 10 cycles and then stablized.The results confirm that the different flexible substrates(i. e., PET and PDMS) yield different bending properties of the FPEs prepared under the same conditions.

Fig.6 (a)Dependence of the relative electrical resistance of the PDMS-embedded FPE on the number of stretching cycles;(b)dependence of the relative electrical resistance of the PDMS-embedded FPE on strain,under stretching and releasing actions

3.5 Effect of tensile strain on the electrical properties of the PDMS-embedded FPE

The electrical performance of flexible electrodes under stress provides an important indication about their stability during operation.Fig.6 shows the change ofR/R0value of the PDMS-embedded FPE during 100 stretching cycles, in which the tensile stress of the stretching-releasing cycle is 30%. Fig. 6(a) shows the variation inR/R0of the PDMS-embedded FPE, along a series of stretching-releasing cycles. After the first stretching-releasing cycle, theR/R0values of the electrode were 5.82 and 1.51, in the stretching and releasing states, respectively. As the number of stretching cycles increased, theR/R0values of the PDMS-embedded FPE in the stretching and releasing states remained nearly unchanged.After 100 stretching cycles, theR/R0values of the PDMS-embedded FPE in the stretching and releasing states were 5.89 and 1.56,respectively. These data demonstrate the remarkable electrical and mechanical properties of the FPE under tensile conditions and many use cycles, indicating their feasibility for practical applications. In the stretching state, as the tensile strain of the PDMS-embedded FPE increased from 0 to 70%, in steps of 10%, theR/R0value of the PDMS-embedded FPE also increased (Fig.6(b)). In the releasing stage, as the strain was reduced from 70% to 60%, 50%, 40%, 30%, 20%, 10%, and 0,theR/R0value of the PDMS-embedded FPE correspondingly decreased from 24.2% to 17.8%,12.3%, 7.7%, 4.7%, 3.0%, 2.2%, and 1.9%,respectively. The above behavior occurred because during the stretching process, as the tensile stress increased, the ink layer was subjected to the tensile force, causing the position of FSSPs to shift. As a result, the dendrites on the FSSP surface could not establish full contacts for forming a conductive network. With the reduction in the tensile stress, the dendrites of the FSSP were able to recontact, to form a conductive network, thereby reducing the resistance.However,owing to the excessive mechanical stress,the conductive film did not return to its original state, even in the complete absence of external force; thus, the resistance was not be restored to the original state(before the tensile strain application).

3.6 Effect of the tape-stripping test on the electrical properties of PET-based and PDMS-embedded FPEs

The abrasion resistance of the ink layer on the electrode surface importantly determines the printing ink quality and substrate appropriateness. Poor resistance of the ink layer during abrasion can strongly affect some physical parameters of FPEs (e. g.,electrical conductivity). Fig. 7(a) presents the variation in theR/R0of the PET-based FPE along the tapestripping cycles. The tested PET-based FPE was fixed on a platform, and both ends of the electrode were connected to a copper wire. Then, the PET-based FPE was subjected to a peeling test using the tape-stripping method. TheR/R0of the FPE during the tape-stripping test increased with increasing the number of tapestripping cycles. The changes in theR/R0of the FPE were not significant after a small number of tapestripping cycles. TheR/R0of the electrode slowly increased,reaching 1.29 after 115 tape-stripping cycles.After a larger number of tape-stripping cycles, theR/R0of the PET-based FPE increased rapidly until falling after the 127-th test. This falloff may be explained by the damage to the ink electrode owing to the stripping process. Finally, the PET-based FPE lost its conductivity when its conductive network was completely destroyed. Fig. 7(b) shows the variation in theR/R0of the PDMS-embedded FPE during the 100 tape-stripping cycles. During the tape-stripping test, we observed a small variation in theR/R0(within 0.2).The PET-based FPE underwent multiple cycles under the same peeling condition, which gradually increased the amount of the conductive filler that fell off the electrode, thus gradually increasing the FPE resistance and eventually resulting in the FPE failure. In contrast,the PDMS-embedded FPE was fixed with PDMS,which imparted robustness to the conductive layer under extreme environmental conditions.

3.7 Application of paper-based FPEs in flexible circuits

To demonstrate a potential application of paper-based FPEs,a flexible circuit was screen-printed on paper.An LED lamp on a flexible printed circuit with an external power supply of 3 V was used; thus, the electrical performance of the printed circuit could be visualized(Fig. 8). Fig. 8(a)-Fig. 8(d) shows the versatility of the considered circuit,with respect to different deformation conditions:(a)initial state,(b)bending,(c)folding,and(d) squeezing[46-47]. The LED emitted light under various external operation conditions; thus, the considered printed circuit exhibited excellent electrical conductivity,flexibility,and mechanical stability.

Fig.7 Dependence of the relative electrical resistance on the number of tape-stripping cycles,for(a)the PET-based FPE and(b)the PDMS-embedded FPE

Fig.8 A paper-based FPE circuit subjected to various deformation conditions:(a)initial state,(b)bending state,(c)folding state,and(d)squeezing state

4 Conclusions

In this paper, conductive ink was prepared by using fractal-structured silver particles (FSSPs), and printed on two different substrates: polyethylene terephthalate(PET) and commercial paper to prepare PET-based flexible printed electrodes (FPE) and paper-based FPE.The results showed that when the content of FSSPs in conductive ink was 50%, the sheet resistance of PETbased FPE was 0.38 Ω/sq. And it was found that the paper-based FPE had excellent conductivity and could be used in flexible circuits. The PET-based FPE was then embedded into polydimethylsiloxane (PDMS)substrate to prepare PDMS-embedded FPE. The experimental results showed that the PDMS-embedded FPE had good stability under 30% tensile stress and after 100 stretching-releasing cycles. Finally, the bending cycle test and tape-stripping test of the PDMSembedded FPE were carried out, and the results showed that the PDMS-embedded FPE had good adhesion with the substrate and good conductivity stability. Our simple approach validates the fabrication of high-performance flexible printed circuits with high electrical conductivity, stretchability, and excellent mechanical resistance.

Acknowledgments

This work was supported by the International Joint Research Center for Biomass Chemistry and Materials,Shaanxi International Science and Technology Cooperation Base (2018GHJD-19), the Shaanxi Key Industry Innovation Chain Projects (2020ZDLGY11-03), the Science and Technology Plan of Weiyang District of Xi'an (201910), the Scientific Research Plan Projects of Shaanxi Education Department(19JK0131).The project was also supported by the Foundation of Key Laboratory of Pulp and Paper Science and Technology of the Ministry of Education/Shandong Province of China(KF201814).

Author Contributions

All authors contributed to the preparation of this manuscript.All authors have approved the final version of the manuscript.

Conflict of lnterest

The authors declare that they have no conflict of interests.