This content has been downloaded from IOPscience. Please scroll down to see the full text. Download details: IP Address: 93.180.53.211 This content was downloaded on 21/12/2013 at 18:46 Please note that terms and conditions apply. A new approach causing the patterns fabricated by silver nanoparticles to be conductive without sintering View the table of contents for this issue, or go to the journal homepage for more 2012 Nanotechnology 23 355304 (http://iopscience.iop.org/0957-4484/23/35/355304) Home Search Collections Journals About Contact us My IOPscience
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IP Address: 93.180.53.211
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A new approach causing the patterns fabricated by silver nanoparticles to be conductive
without sintering
View the table of contents for this issue, or go to the journal homepage for more
A new approach causing the patternsfabricated by silver nanoparticles to beconductive without sintering
Yao Tang, Wei He, Guoyun Zhou, Shouxu Wang, Xiaojian Yang,Zhihua Tao and Juncheng Zhou
State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Scienceand Technology of China, No. 4, Section 2, North Jianshe Road, Chengdu, Sichuan, 610054,People’s Republic of China
Received 11 March 2012, in final form 4 July 2012Published 15 August 2012Online at stacks.iop.org/Nano/23/355304
AbstractSilver nanoparticles (Ag NPs) with a mean size of about 90 nm were synthesized by polyolreduction of silver nitrate with ethylene glycol (EG) in the presence of poly(vinyl pyrrolidone)(PVP). The Ag NPs undergo a spontaneous coalescence in the presence of chloride ions evenwithout a traditional sintering process which occurs at a relatively high temperature. Suchbehavior can cause a rapid decrease in the resistivity of the patterns fabricated by Ag NPs.Conductive silver lines were successfully fabricated on FR-4 substrate using this method. Theresulting conductivity of the silver lines reached about 16% of the bulk silver value, whichenables fabrication of conductive patterns on some electronic devices.
(Some figures may appear in colour only in the online journal)
1. Introduction
In the past few years, increasing attention has been devoted tothe technology of printed electronics to pursue convenience,flexibility and low cost in many techniques, such as through-hole technology, radio frequency identification (RFID),encapsulation and fabrication of various devices [1–6].With these purposes, many new substrates such as glass,polymeric material, plastic packages and even paper haveemerged in those fields [7–12]. However, there are manyobstacles to direct fabrication of electric circuits on thosesubstrates with the dispersion of metallic nanostructure,because the metallic nanostructure fabricated on the substratesrequires an additional sintering process, which occurs at arelative high temperature, even above 200 ◦C, by conventionaltechnologies to achieve coalescence [12–15]. Generally, somenew substrates such as plastic and paper cannot sustain sucha temperature. The high temperature also causes oxidationof some metallic nanostructures, consequently raising theresistivity. So conventional sintering under high temperatureextremely limits the application of flexible electronic devices.
Many instrumentalities such as microwave sintering, laserradiation sintering and electrical sintering have appearedsubsequently to handle the problem [16–20]. But thesetechniques more or less unavoidably involve expensiveequipment, demanding experimental requirements or complexprocedures. Any of the described disadvantages greatlyobstructs the application of those instrumentalities into thecommercial process.
Fortunately, these problems can be settled by a specialsintering method. It was reported that the sintering processcould be carried out at room temperature and effectively avoidthese limitations [13, 21, 22]. The metallic NPs will suffer aspontaneous coalescence in the presence of so-called sinteringagent. The sintering agent could be a simple electrolyte suchas a halide which can destabilize the metal NPs dispersedin organic solvent and bring their accumulation. The keyto the success of the sintering method mainly relies ontwo prerequisites. One has been mentioned before, i.e. thesintering agent. The other is that the metallic NPs must bekept isolated before use. Some polymeric stabilizer couldabsorb on the surface of NPs preventing their agglomeration.
Figure 1. (a) TEM and (b) SEM images of monodisperse Ag NPs, (c) HRTEM image of the Ag NP, the inset shows the color change of thesolution during the syntheses (the numbers represents the reaction time).
It was reported that sodium polyacrylate (PAANa) is aneffectual stabilizing agent that can be utilized to encapsulatesilver NPs [12, 21]. After the polymer coating is detachedfrom the surface by the electrolyte, the Ag NPs begin tocoalesce together and grow larger, which of course is like aconventional sintering.
Following this concept we have found a new dispersionof Ag NPs which also can be utilized for fabrication ofconductive patterns without heating. Poly(vinyl pyrrolidone)(PVP) was used as capping agent to prevent the Ag NPsfrom aggregation. PVP can be detached from the surfaceof Ag NPs by chloride ions, causing their spontaneouscoalescence accompanied by Ostwald ripening. The feasiblemechanism is specified in section 3. In our work, silverpatterns fabricated with a dispersion of the synthesized AgNPs became conductive following activation in a sodiumchloride solution. The considerable conductivity of the silverpatterns reaches as high as 16% of that for bulk silver.
2. Experimental details
2.1. Materials
All of the chemical reagents used in the experiments werepurchased from commercial sources with analytical purityand used without further purification. Anhydrous ethyleneglycol (EG, 96%), silver nitrate (AgNO3, 99.8%), sodiumchloride (NaCl, 99%), PVP (MW ≈ 30 000) were purchasedfrom KELONG Chemical Company, Chengdu, China. FR-4epoxy class cloth substrates were offered by Zhuhai TopsunElectronic Technology Co. Ltd. Deionized water was obtainedthrough an ion-exchange resin system.
2.2. Synthesis of Ag NPs
Ag NPs were synthesized by the polyol reduction of AgNO3with EG in the presence of PVP. In a typical synthesis,0.1 g silver nitrate and 0.4 g PVP were dissolved in 20 mlEG and agitated vigorously for 15 min to provide adequatedissolution. Then the mixture was poured into a flask andheated at about 160 ◦C with continuous magnetic stirring for14 min. It was observed that the color of the mixture alteredin the sequence as listed below: colorless, yellow, orange,
black green and turbid dark green (inset of figure 1(c)).After that, the reaction mixture was cooled down to roomtemperature and diluted with deionized water. In order toremove the unreacted organic and metal salts, the mixturewas centrifuged at 8000 rpm for 15 min. The supernatantcontaining impurity was easily removed using a pipette. Thecentrifugation procedure could be repeated several times untilthe supernatant became transparent. Finally, the Ag NPs weredispersed in deionized water at a ratio of 25% according toweight for further use.
2.3. Fabrication and treatment of silver patterns
The dispersion of Ag NPs described above was ultrasonicallyshaken for 10 min before use to achieve uniformity. Then twolines of Ag NPs were directly printed on the FR-4 substratewith the dispersion by a screen printing machine. After drying,the silver patterns were placed in saturated sodium chloridesolution and ultrasonically shaken in chronological order(5 min, 10 min, 15 min, 20 min, 25 min, 30 min, 50 min,70 min) at room temperature so as to reveal the relationshipbetween resistivity and time of treatment. For each time, thepatterns were taken out, washed with deionized water twiceand the electrical resistivity measured. To test the reliabilityof the conductivity treated by this method directly, the twosilver lines printed on the FR-4 substrate were connectedin a simple electric circuit. Moreover, the critical thicknessand width of the printed silver lines fabricated under ourexperimental conditions were found to give a reference forpractical application.
2.4. Characterization
The scanning electron microscopy (SEM) and transmissionelectron microscopy (TEM) images were respectivelyobtained by a scanning electron microscope (FEI, inspect F)operated at 80 kV and a transmission electron microscope(JEOL, JEM-100CX) operated at 20 kV. High-resolutionTEM (HRTEM) was obtained by field emission transmissionelectron microscope (Tecnai, G2 F20 S-TWIN) operated at200 kV. XRD images were obtained by x-ray diffraction(X’ Pert Pro MPD with Cu Kα radiation, λ = 0.154 nm).Silver lines were printed by a semi-automatic screen printingmachine (Lenstar, BH-7010). Images of a microsection
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were taken by a metaloscope (Nikon, ME-600). The photosof the electrical circuit were taken by a digital camera(Fujifilm, FinePix Z3). Ultrasonic processing was realizedby an ultrasonic cleaner (As3120). The resistivity of thesilver pattern was detected by a four-point probe instrument(DP-SX1934) at room temperature.
3. Result and discussion
3.1. Properties of silver nanoparticles
Monodisperse Ag NPs with controlled spherical shape weresuccessfully prepared by solution-phase polyol synthesis.Though there are many physical parameters that can be usedto control the morphology and property of nanostructure,such as pressure, viscosity and temperature [23, 24], heretemperature was considered the key factor to form sphericalAg NPs. Xia and his members have sucessfully synthesizedAg nanostructures with various controlled shapes [25]. It wasfound in most of their syntheses of silver nanostructures thata higher temperature provided a smaller nanostructure yieldbecause the growth speed of NPs rises at higher temperaturefavoring the nucleation of Ag seeds. The molar quantity ofthe reagent is fixed. The more seeds generated at the initialstage, the smaller particle size that appears in final product.Furthermore, silver nitrate and PVP dissolved in EG serveas salt precursor and capping agent, respectively [26]. Thereductant is continuously generated in situ upon heating ofthe EG. With the protection of PVP, Ag seeds efficiently avoidagglomeration. Moreover, under such a high temperature thesmall discrepancy of growth speed on different facets can beeliminated leading to the non-oriented growth of Ag NPs.The inset of figure 1(c) indirectly reflects the whole reactioncourse. A set of samples were extracted at various stagesduring the reaction from the beginning to 12 min. It can beclearly seen that the color of the reactant mixture changedfrom colorless to turbid dark green. Different colors justindicate different growth stages of the silver nanostructure.Much research has revealed that silver seeds with size about5–10 nm in solution will reflect a light yellow color as oursample in 2 min reaction [27–29]. With the growth of thesilver seed, the color of the mixture gradually becomes darkand turbid because the large particles can reflect and scattermore light than small ones. In a time as short as 12 min, theNPs have grown up to 90 nm and the color of the mixture stopschanging with more time, indicating the termination. Underthis condition, spherical Ag NPs with sizes about 90 nm wereproduced well.
Figure 1(a) shows a TEM image of the sample, whichcontains a spherical particle with mean size about 90 nm.The uniform size is attributed to the parallel growth speedof most Ag seeds. With PVP capping on the surface, eachNP separated from the others can be easily dispersed insolution. It not difficult to find in figure 1(a) that most ofthe NPs are single. Compared with the TEM image, the SEMimage (figure 1(b)) shows the structure of concentrated NPswhich was obtained by air-drying a droplet of the prepareddispersion of Ag NPs. Although it seems as though all the NPs
are accumulating together, the clear boundary between NPsshows the division. That means the NPs cannot interact witheach other even if they are put together. More detail is revealedby the HRTEM image (figure 1(c)). Any twin boundary isfound in the figure suggesting the Ag NPs are single crystal.The bright amorphous layer with thickness about 1 nm alongthe edge of the Ag NP is most probably the PVP, which isjust consistent with the explanation of Younan Xia [25]. Itis also the reason why the Ag NPs cannot interact with eachother. Because the organic layers coordinate on the surface,acting as insulating barrier, the conduction of electricitybetween Ag NPs is impossible. In this situation electrons alsomay transport through neighboring NPs by the tunnel effect.However, from the view of statistics it contributes nothing tothe electrical conduction of the whole system. Actually, theresistivity of the structure in figure 1(b) is infinity, as we willdiscuss later. In order to make the fabricated patterns of thedispersion of Ag NPs conductive, the organic protective covershould be detached.
3.2. The detachment of PVP and the changes appended to theAg NPs
Sintering at above 200 ◦C is the conventional way to removethe organic layers and also to cause the coalescence of AgNPs leading to a conductive structure. Here a new approach,which gives the same result, is introduced to avoid hightemperature. As all we know, small NPs have a large surfaceenergy. Once PVP is absorbed on the surface of NPs, it canrapidly lower the energy of the NPs and stabilize them. Ifthere is no capping on the surface, NPs will contact each otherand undergo an Ostwald ripening process and thus becomelarger [30]. Ostwald ripening is a spontaneous process thatoccurs by dissolution of small crystals and the redeposition ofthe dissolved parts on the surfaces of larger crystals; due tolarger crystals being more energetically favored than smallercrystals. The coalescence mechanism, which can be achievedat room temperature, mainly relies on this point. If PVP isdesorbed from the surface of Ag NPs by other small atoms orions, the surface energy of the NPs may rise again, leading tothe Ostwald ripening process. It is well known that halogenshave a very strong interaction with silver. Chloride ions werefound to be competent for the detachment of PVP.
The detachment of PVP most probably undergoes thestages shown in figure 2. PVP is absorbed on the surface ofAg NPs mostly through an oxygen atom of the carboxylategroup (and probably a nitrogen atom of the pyrrolidylnitrogen group) (figure 2(a)) [31–33]. Because of the strongerinteractions between silver and chloride ions, such desorptionis realized by chloride ions spontaneously replacing thecarboxylate or pyrrolidyl nitrogen at the same site, as shownin figure 2(b). After that, only the chloride ions are absorbedon the surface of Ag NPs, greatly broadening the specificsurface and consequently increasing the energy of the NPs(figure 2(c)). The neighboring Ag NPs with increased energywill naturally assemble together to lower the total surfaceenergy and achieve a stable state.
The most possible process causing the coalescence ofAg NPs is suggested in figure 3. At the initial stage the
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Figure 2. Illustration of the feasible process by which PVP repeated units are detached by chloride ions.
Figure 3. Schematic illustration of the changes undergone by Ag NPs activated by the appearance of chloride ions.
Figure 4. SEM images of Ag NPs fabricated on FR-4 substrates which were treated with sodium chloride solution for (a) 0 min, (b) 30 minand (c) 70 min, respectively.
Ag NPs deposit on the substrate. With the protection ofPVP they are isolated from each other. Once chloride ionsapproach the Ag NPs, the PVP layers are detached fromthe surface as explained above. The Ag NPs with expandedspecific surface will spontaneously assemble together drivenby surface tension [34]. The assembly, coalescence andrecrystallization of NPs can lower the total energy of thewhole system to bring about a stable state. Eventually, mostof the coalesced Ag NPs will continue to suffer Ostwaldripening. Many NPs grow by ingesting the dissolved parts ofthe small particles in this step. Finally, both large and smallNPs will appear in the product. The detached PVP depositedon the substrate can provide an adhesive force that permits thesilver patterns to sustain a certain distortion.
The changes of the silver nanostructure fabricated on theFR-4 substrate, which was triggered by the appearance ofchloride ions, were recorded by the SEM images as shownin figure 4. Initially, the Ag NPs were uniformly deposited onthe substrate with PVP capping on the surface after drying(figure 4(a)). Being treated with saturated sodium chloridesolution for 30 min the silver patterns have suffered manychanges in structure. It can be seen from figure 4(b) that mostof the Ag NPs have aggregated into a block; the boundariesbetween some neighboring Ag NPs become vague; and the
well-packed structure has converted into an irregular ruggedstructure with many voids in it. Because PVP was detached bychloride ions being eliminated from the structure, large spacesoccupied by PVP were released at this time, resulting in theshrinkage of the entire system. In this change most of the AgNPs have contacted with each other with a large surface areathat made electronic transport between particles possible. Inthe macroscopic view, the silver pattern has been transformedinto an electrical conductor. The nanostructure shown in figure4(c), which was obtained by characterizing the sample aftercontinued treatment, gives another interesting result. Manynew small Ag NPs with size less than 20 nm appeared in thestructure, which could be reasonably explained by Ostwaldripening. It is not surprising that many NPs with growth arefound in the image. The coalescence of Ag NPs has alsotaken place. The evidence is that some NPs have mergedtogether. The coalescence and growth of Ag NPs could makethe structure more conductive, but those processes will takea long time. In order to reveal the relationship between thoseparameters and find the most efficient time of treatment, morecharacterizations have been performed.
Figure 5(a) shows the XRD image of samples which wereprepared by air-drying the dispersion of Ag NPs before andafter treatment, respectively. It shows five diffraction peaks
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Figure 5. (a) XRD image of Ag NPs, (black) before treatment and ( red) treated with sodium chloride solution for about 70 min; (b) thephoto of two silver lines fabricated on the FR-4 substrate by Ag NPs using screen printing, the inset shows the microsection image of thelines; (c) the lines were connect into a simple circuit.
at 2θ of 38.2◦, 44.4◦, 64.5◦, 77.5◦, and 81.6◦ in the image,respectively indexed as (111), (200), (220), (311) and (222)(JCPDS file No. 04-0783) reflections of the face-centeredcubic (fcc) structure of silver. The clear peaks in the profileof the initial sample indicate it had a structure identical topure crystalline silver. However, after treatment, the intensityof x-ray diffraction profile has evidently dropped, indicatinga degraded crystalline quality of the NPs. The reason mainlylies in the fact that many voids have been generated in thestructure after treatment. The residua of chemical reagentwhich cannot be completely washed out also lowers therelative intensity of the diffraction profile. It is noted infigure 5(a) that the XRD peaks of the treated sample becomeslightly broader than before. This fact just proves that AgNPs have undergone the Ostwald ripening process during thetreatment. As explained above, Ostwald ripening could yieldmany new smaller Ag NPs, as shown in figure 4(c). Accordingto Scherrer’s equation, the particle size and the width of XRDpeaks have a reciprocal relationship, i.e. the smaller NPs leadto broader peaks [35]. The problem is that many NPs havinggrown larger than before in the sample should lead to narrowerpeaks in the profile. In fact this change is masked by thewidened XRD peaks which are formed by the minified NPs.
In order to discover the most efficient time of thetreatment, the electrical resistivities of silver lines fabricatedby dispersions of Ag NPs were intentionally recorded duringthe whole treatment, as shown in figure 5(b). It can be seenthat the electrical resistivity drops rapidly at the initial stageand slows down after 30 min. Because most of the PVPcapping layers are detached as soon as the treatment startsthat leads to the contact of NPs over a large surface area,thus lowering the resistivity sharply. Later, the contact of NPsbecomes saturated and the primary driving force to decreasethe resistivity is provided by the coalescence and growth ofNPs. But this is a relatively slower process, consequentlyretarding the descending tendency of resistivity. Figure 5(b)reveals the most efficient time of treatment is about 30 min.The lowest electrical resistivity of the silver lines obtained byour method is 9.91 × 10−6 � cm, which was achieved after70 min treatment. Though it seems the electrical resistivitycan be further lowered by prolonging the treatment, it is very
time consuming. In any case, the resistivity is now low enoughto be used in some real applications.
An experiment was carried out to test the applicabilityof our method. Two silver lines were fabricated on a FR-4substrate with the dispersion of Ag NPs described in section2. After drying, the silver patterns were treated with sodiumchloride solution at room temperature for about 30 min.Figure 6(a) shows the photograph of the two sintered silverlines. It can be seen that the silver patterns adhering on theflexible FR-4 substrate can sustain a certain distortion. Asimple circuit is made up of the two lines and a light-emittingdiode (LED), which was lit using a lithium battery thusindicating the possibility of a real application of the methods(figure 6(b)). The mean thickness of the line is about 0.41 µm,as shown in the microsection image (bottom of figure 5(a)),and the mean width is about 1 mm. The two parameters aregiven as a reference of the real condition. In order to findthe critical film thickness and a critical printed line widthof our experiment, the thinnest silver line with tapered widthwas printed on a FR-4 substrate by screen printing. The meanthickness of the line is about 0.40 µm. Being limited by ourequipment it cannot be made thinner. As shown at the bottomright of figure 6(c), some rifts appear at the end of the silverline that could lead to the failure of electrical conduction.It is caused by the low concentration of Ag NPs in sucha narrow width. On the other side, the mean width of thecontinuous section of the silver line is about 0.23 mm, asshown at the bottom left of figure 6(c). It means 0.23 mm isthe critical width which can be printed under our experimentalconditions. The resistivity of the continuous section of thesilver line is about 9.43× 10−5 � cm, which was obtained bytreating the line with sodium chloride solution for one hour.We believe, if more advanced experimental conditions permit,the result will be better.
4. Conclusions
Ag NPs with average size about 90 nm were prepared throughreduction of silver nitrate by EG in the presence of PVP.PVP acted both as a protective agent and stabilizing agent
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Figure 6. (a) The photo of two silver lines fabricated with a dispersion of Ag NPs on a FR-4 substrate by screen printing. The bottom of (a)shows the microsection image of the lines. (b) The lines were connected into a simple circuit. (c) Photo of the thinnest silver line obtainedby our method with slightly tapered width at the end. The images at the bottom of (c) show partial enlarged details of the silver line whichwere taken by a metaloscope.
in the syntheses of Ag NPs. A stable colloidal solution ofAg NPs was prepared, which was successfully utilized infabrication of silver patterns on FR-4 substrate by screenprinting. After treating with sodium chloride solution at roomtemperature, the Ag particles coalesce together following thedetachment of PVP. This behavior of Ag NPs will make thesilver patterns conductive. The lowest electrical resistivityof the silver patterns obtained by our method is 9.91 ×10−6 � cm, which could be utilized in some electronicdevices. The critical thickness and critical width were foundunder our experimental conditions respectively to be 0.40 µmand 0.23 mm, below which the conductivity function of theprinted line fails.
Acknowledgments
We would like to thank Sichuan University for the SEM,HRTEM and XRD measurements. We would also like toacknowledge the Chinese Academy of Sciences, ChengduBranch for the TEM test. This work is supported by the MajorScientific and Technological Projects of Guangdong Province(2011A090200017).
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