Tensile characteristics of metal nanoparticle films on flexible polymer substrates for printed

electronics applications

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IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 24 (2013) 085701 (7pp) doi:10.1088/0957-4484/24/8/085701

Tensile characteristics of metalnanoparticle films on flexible polymersubstrates for printed electronicsapplications

Sanghyeok Kim1, Sejeong Won1, Gi-Dong Sim, Inkyu Park andSoon-Bok Lee

Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology (KAIST),Daejeon, 305-701, Korea

E-mail: inkyu@kaist.ac.kr and sblee@kaist.ac.kr

Received 27 August 2012, in final form 12 December 2012Published 1 February 2013Online at stacks.iop.org/Nano/24/085701

AbstractMetal nanoparticle solutions are widely used for the fabrication of printed electronic devices.The mechanical properties of the solution-processed metal nanoparticle thin films are veryimportant for the robust and reliable operation of printed electronic devices. In this paper, wereport the tensile characteristics of silver nanoparticle (Ag NP) thin films on flexible polymersubstrates by observing the microstructures and measuring the electrical resistance undertensile strain. The effects of the annealing temperatures and periods of Ag NP thin films ontheir failure strains are explained with a microstructural investigation. The maximum failurestrain for Ag NP thin film was 6.6% after initial sintering at 150 ◦C for 30 min. Thermalannealing at higher temperatures for longer periods resulted in a reduction of the maximumfailure strain, presumably due to higher porosity and larger pore size. We also found thatsolution-processed Ag NP thin films have lower failure strains than those of electron beamevaporated Ag thin films due to their highly porous film morphologies.

S Online supplementary data available from stacks.iop.org/Nano/24/085701/mmedia

(Some figures may appear in colour only in the online journal)

1. Introduction

Recently, there has been growing interest in various solution-based direct printing processes using metal nanoparticle-based inks such as gold (Au), silver (Ag) or copper (Cu)nanoparticles (NPs) with diameters ranging from a few to tensof nanometers for the fabrication of microelectronic devices.Representative printing methods are gravure printing [1, 2],flexography printing [2, 3], nanoimprinting [4–8], transferpatterning [9] and inkjet printing [10–15]. These technologiesallow a simple fabrication process by means of all-solutionprocessing without conventional deposition processes such as

1 These authors equally contributed to this work.

sputtering or evaporation, which require expensive equipmentand tightly restricted vacuum conditions. Also, they havemany advantages such as low energy consumption, alow manufacturing cost, and broad substrate compatibility.For these reasons, direct printing methods are widelyused to manufacture micro/nano-scale metal electrodes andinterconnections in electronic devices.

Many previous studies of printed metal NP thin filmsand micropatterns focused on changes in the microstructureand electrical conductivity by different thermal annealingconditions [12–17]. However, the mechanical characteristicsof metal NP thin films fabricated by all-solution processesare very important because these devices often work undermechanical stresses caused by tension, bending and twisting,

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http://dx.doi.org/10.1088/0957-4484/24/8/085701mailto:inkyu@kaist.ac.krmailto:sblee@kaist.ac.krhttp://stacks.iop.org/Nano/24/085701http://stacks.iop.org/Nano/24/085701/mmedia

Nanotechnology 24 (2013) 085701 S Kim et al

Figure 1. Tensile testing of solution-processed Ag NP thin films: (a) schematic diagram of the tensile equipment, (b) real photograph of thetensile equipment with the sample loaded, (c) schematic diagram of the specimen before and after the tensile test and (d) real photographand SEM images of Ag NP thin film before and after the tensile test.

especially when the substrate is flexible. There have beensome studies for measuring the mechanical properties,such as the elastic modulus and indentation hardness ofAg NP thin film [18], the stiffness and elastic modulusof a free-standing gold nanoparticle membrane by thenanoindentation method [19] and the strain sensitivity of goldnanoparticle film by tension tests [20]. In other research, itwas demonstrated that both the elastic modulus and fatiguestrength of Ag NP thin films can be improved throughthe formation of a composite film with carbon nanotubes(CNTs) [21]. However, research on the stretchability ofsolution-processed NP thin films and comparisons with metalthin films fabricated by vacuum deposition processes has notbeen conducted thus far to the best of the authors’ knowledge.In addition, it is necessary to investigate the effects of theannealing temperature and period on the stretchability ofmetal NP thin films for the mechanically reliable operation offlexible electronic devices made of solution-processed metalNP thin films.

In this paper, we present the tensile failure behavior ofAg NP thin films coated on the flexible polyimide substratesgiven the formation and growth of cracks under increasingamounts of tensile strain. The effects of changing the grainstructures of Ag NPs through the use of different annealingtemperatures and periods on the tensile behavior of thin filmare explained. Also, the failure strains of solution-processedAg NP thin films are compared with those of electron beam(e-beam) evaporated Ag thin films.

2. Experiment

A flexible polymer (polyimide) substrate with a thicknessof 25 µm was scribed using a cutting plotter (CE2000-120,Graphtec, Japan) for the specimen used in the tensile test. The

shape of the specimen was a slender rectangle with a lengthof 28 mm and a width of 1 mm. The Ag NP solution (DGP40LT-15C, Advanced Nano Products, Korea) was coated ontothe scribed polyimide substrate using a coating bar (D-Bar,TND System, Korea). The Ag NP solution initially filled inthe ∼10 µm deep grooves of the coating bar, after whichit was coated onto the substrate along the moving direction.Afterwards, the Ag NP thin film was sintered in a convectionoven at 150 ◦C for 30 min in order to remove any organicsolvent and to form a conductive metallic film. The Ag NPthin film samples were then annealed in a convection oven atdifferent temperatures (180 ◦C or 230 ◦C) for various periods(3, 6 and 9 h). This process resulted in Ag NP thin filmswith an average thickness of ∼500 nm regardless of theannealing temperature or period. As another set of samples,∼400 nm thick Ag film was deposited on polyimide substratesby e-beam evaporation at a rate of 1–2 Å s−1 and wasannealed at 150 ◦C or 220 ◦C for 2 h. After the annealingprocess of both solution-processed Ag NP thin films ande-beam evaporated Ag thin films, tensile tests were performedat a strain rate of 3.1 × 10−4 s−1 with a custom-madetensile tester with a displacement resolution of 10 µm, asshown in figures 1(a) and (b). Here, the strain rate wasmeasured by the displacement of two grips which were fixedonto two tips of the sample. During the tensile test, theelectrical resistance was measured in situ using a Keithley2000 multimeter with a four-point measurement setup. Thesurface morphologies and microstructures of the Ag thinfilms were observed with a scanning electron microscope(SEM). The area ratio of the pores and the pore size (i.e. thepore diameter) on the surface of the Ag NP thin film weremeasured via the image processing of SEM photographs usingImageJ R© software (National Institute of Health, USA) andMatrox Inspector R© (Matrox, Canada), respectively.

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Nanotechnology 24 (2013) 085701 S Kim et al

3. Results and discussions

A schematic and SEM images of crack propagation onthe Ag NP thin film by tensile loading are shown infigures 1(c) and (d). A crack was formed perpendicular tothe direction of tensile loading and propagated along the grainboundaries between Ag NPs. This phenomenon is consistentwith previous studies of the crack propagation behavior innanocrystalline metal thin films. In the work by Wang et al,the cracks which formed on a free-standing Au thin film wereeasily propagated along the grain boundary by an externaltensile load [22]. Also, Farkas et al observed intergranularcrack propagation in nanocrystalline nickel (Ni) by means ofan atomistic computer simulation [23].

The surface morphologies and microstructures of AgNP thin films annealed at different temperatures for variousperiods are shown in figure 2. After removing the solvent(methyl alcohol) of Ag NP ink and forming a solid thin filmby an initial sintering process at 150 ◦C for 30 min, the AgNP thin film showed a porous and granular structure withan average grain size of 25–30 nm. Further annealing at180 ◦C resulted in the aggregation and grain growth of AgNPs. At this temperature, the NP aggregation became moreobvious as the annealing period was increased (3, 6 and 9 h).This phenomenon is usually caused by the decomposition oforganic shells that were originally coated onto the surfaceof Ag NPs as a surfactant [13, 14]. However, when the AgNP thin films were annealed at 230 ◦C, most NP aggregationand grain growth appeared to have occurred during the firstthree hours of annealing. At this temperature, no considerablechanges in the microstructures (e.g. the grain size, porosityand pore size) arose after 6 and 9 h of annealing, as comparedto the result at 3 h. Another notable fact is that Ag NP thinfilms annealed at 180 and 230 ◦C exhibited greater porosity,larger individual pores, and aggregation of the NPs comparedto the initially sintered films (at 150 ◦C for 30 min). The arearatio of the pores of the initially sintered Ag NP thin filmwas only 2%. The area ratio of the pores was continuouslyincreased by annealing at higher temperatures and for longerperiods of time. Annealing at 230 ◦C for 9 h increased theporosity up to 5.2%, which is 2.6 times higher than thatof the initially sintered Ag NP thin film. We found thatthe material of the organic shell surrounding Ag NPs ispolyvinylpyrrolidone (PVP) by Fourier transform infraredspectroscopy (FTIR) analysis, as shown in figure S1 of thesupplementary material (available at stacks.iop.org/Nano/24/085701/mmedia). The melting point of PVP is 150–180 ◦C.Therefore, organic shells began to decompose during theannealing process at temperatures higher than 150 ◦C. Duringthe thermal annealing process, a close-packed structure of theindividual NPs is broken but larger agglomerates are formedby the merging of NPs, increasing the size of individual pores.At the same time, the removal of organic shells results inthe increase of porosity. Furthermore, a major difference inthe thermal expansion coefficients of the polyimide substrate(αpolyimide = 55 × 10−6 ◦C−1) and the Ag NP thin film(αAg NP ∼ 1.9×10−6 ◦C−1 [18]) induced large thermal stressin the NP thin film during the thermal annealing process. As a

Figure 2. SEM images of surface morphologies of Ag NP thinfilms on flexible polyimide film after an annealing process atdifferent temperatures for various periods of time.

result, larger pores and more initial cracks were generated onthe Ag NP thin film by annealing at higher temperatures.

Assuming a constant electrical resistivity and Poissonratio of 0.5 (i.e. no volume change by stress) during thedeformation of the thin film, the ideal curve for the relativeelectrical resistance upon an increasing amount of tensilestrain satisfies the following equation [24–27]

R/R0 = (L/L0)2. (1)

Here R is the electrical resistance of a thin metal film stretchedto length L. R0 and L0 are respectively the initial resistanceand length of the metal thin film. The failure strain was definedas the strain at which the measured resistance of a specimendeviated from the theoretical curve (1) by more than 5%.Previous studies [26, 27] verified that cracks typically startwhen there is a 5% deviation of the measured resistance fromthe theoretical curve. This phenomenon was also confirmedin this study. The initiation and growth of cracks result in anincrease in the electrical resistivity, thus leading to a deviationfrom the theoretical curve based on the assumption of constantresistivity.

The resistance–elongation curves of Ag NP thin filmsannealed at 180 and 230 ◦C for 3, 6 and 9 h are shownin figures 3(a)–(c). The resistance–elongation curve for theinitially sintered NP film has also been inserted into all of thefigures for comparison. The failure strains of the Ag NP thinfilms are summarized in figure 3(d). The maximum failurestrain was 6.6% (standard deviation (SD) = 0.3%) after initialsintering without an additional thermal annealing process ata higher temperature. On the other hand, the failure strainsdecreased to 4.6–5.4% and 3.8–4.9% after annealing at 180 ◦Cand 230 ◦C, respectively. Although the statistical significanceis low due to the large standard deviations, the general trendshows that the failure stains decrease by annealing at a highertemperature. It is generally known that the grains of metal thinfilms grow during the thermal annealing process [24]. In thepresent work, as shown in figure 2, Ag NPs did not growcontinuously in proportion to the annealing periods duringthe annealing process at 230 ◦C. At this temperature, the

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Nanotechnology 24 (2013) 085701 S Kim et al

Figure 3. Resistance–elongation curves of Ag NP thin film on a polyimide substrate annealed at different temperatures (180 ◦C and 230 ◦C)for (a) 3 h, (b) 6 h and (c) 9 h. (d) Failure strains according to various annealing conditions, (e) pore size of annealed Ag NP thin films and(f) electrical resistivity of Ag NP thin films annealed at 100–230 ◦C for 1 h.

grain growth appeared to be stabilized after 3 h of annealing.However, the connectivity between Ag NPs was continuouslyimproved during the annealing process at 180 ◦C and 230 ◦C.These phenomena resulted in not only larger pore sizes butalso larger area ratios of the pores (i.e. porosity) during thegrain growth and aggregation of Ag NPs in the annealingprocess.

Figure 3(e) shows the diameters of pores as measuredby the image processing of the SEM photographs shown infigure 2. The average pore size was 45.9 nm (SD = 24.1 nm)for the specimen that was initially sintered at 150 ◦C for30 min. For the specimens additionally annealed at 230 ◦C,the pore size increased to 54.6 nm (SD = 28.7 nm) after3 h of annealing and to 67.5 nm (SD = 32.6 nm) after 9 hof annealing. The porosity and pore size of the thin filmconsiderably affected the initiation and growth of cracks.In the work by Gerard et al, the authors demonstrated thatcrack initiation was observed adjacent to pores and that thepore-induced strain concentration accelerated the initiationof micro-cracks [28]. In the work by Lee et al, a lowersurface porosity by oxygen-pressure-controlled annealing ofa composite film composed of Ag NPs and carbon nanotube(CNT) resulted in a higher elastic modulus and yield strengthcompared to those of a composite film annealed under anambient air condition [21]. Therefore, in the present work,larger pores and greater porosity could accelerate the initiationand growth of cracks, resulting in lower failure strains of AgNP thin films annealed at higher temperatures. Also, the stressconcentration factor increases with growing length of pore.The stress concentration factor is expressed as follows

Kt = 1+ 2√

a/ρ. (2)

In this equation, Kt, 2a and ρ are the stress concentrationfactor, length of major axis of the pore and tip radius of

the pore, respectively. In the present case, pores can beconsidered as pre-cracks. The Ag NP thin film annealed athigher temperature shows the increase of both the pore size2a and tip radius ρ. However, the increase rate of the poresize is much higher than that of the tip radius. Therefore, thestress concentration factor Kt increases by annealing at highertemperature, causing an early failure under tensile stress (Seefigure S2 in the supplementary information available at stacks.iop.org/Nano/24/085701/mmedia).

Figure 3(f) shows the electrical resistivities of Ag NP thinfilm samples annealed at various temperatures (100–230 ◦C)for 1 h. After the annealing process at 100 ◦C without an initialsintering step, the resistivity was 6.15 × 10−7 m (SD =0.35× 10−7 m), which is 38 times higher than that of bulksilver (1.6 × 10−8 m). This resulted from the incompleteremoval of solvents and insufficient connections between AgNPs in the film. However, the resistivity of Ag NP thin filmdecreased steeply to 2.35×10−7 m (SD= 0.15×10−7 m)after annealing at 150 ◦C. After annealing at 230 ◦C, theresistivity was measured as 1.25 × 10−7 m (SD =0.25 × 10−7 m), which is only eight times higher thanthat of bulk Ag. This trend in the electrical conductivitywith higher annealing temperatures is consistent with thefindings in the literature [12–16]. However, as explainedabove, we found that the stretchability of Ag NP thin filmdegrades with higher annealing temperatures. An annealingprocess at a higher temperature could not improve both theelectrical and mechanical tensile properties. In other words,the electrical properties can be improved by annealing athigher temperatures, but only with the sacrifice of mechanicalstretchability.

The SEM images of Ag NP thin films annealed at varioustemperatures and stretched by 5% and 20% strains are shown

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Nanotechnology 24 (2013) 085701 S Kim et al

Figure 4. SEM images of cracks formed by tensile loading with 5% and 20% strain rates for Ag NP thin films annealed under variousprocess conditions: ((a) 150 ◦C for 30 min, (b) 180 ◦C for 3 h and (c) 230 ◦C for 3 h).

in figure 4. As mentioned above, we found that the initiallysintered Ag NP thin film has a failure strain of 6.6%. Thisis consistent with the SEM observation in which cracks werefound not at ε = 5% but at ε = 20% (figure 4(a)). The failurestrains for Ag NP thin films annealed at 180 ◦C and 230 ◦Cfor 3 h were 5.4% and 3.8%, respectively. The SEM images ofthese films show that small and short cracks were created aftertension by 5% (figures 4(b) and (c)). The thin film annealedat 230 ◦C exhibits a higher number density and larger cracksthan that annealed at 180 ◦C after extending by ε = 5%. Thisphenomenon is consistent with the fact that the failure strainis lower for the sample annealed at 230 ◦C (3.8%) than for thesample annealed at 180 ◦C (5.4%).

We compared the failure strains of solution-processedAg NP thin films with those of e-beam evaporated Ag thinfilms. The resistance–elongation curves and SEM images ofsurfaces of various specimens after tensile tests are shownin figure 5. The failure strains of e-beam evaporated films

annealed at 150 ◦C and 220 ◦C for 2 h were 10% (SD = 1.8%)and 15.5% (SD = 0.9%), respectively, as shown in table 1.These values were about two–four times larger than those ofthe solution-processed and annealed Ag NP thin films. Also,the failure strains of the e-beam evaporated Ag films wereimproved by the annealing process at a higher temperature, asopposed to the solution-processed Ag NP thin films. As shownin figure 5(b), the surface morphology and microstructure ofthe e-beam evaporated Ag films were denser with larger grainsize and fewer inter-grain pores than the solution-processedAg NP thin films. These conditions resulted in mechanicallyrobust metal films with higher failure strains. This shows thatsolution-processed Ag NP thin films are mechanically weakerwith poorer stretchability than e-beam evaporated Ag thinfilms due to the significant difference in the microstructures.

In summary, we investigated the mechanical tensilecharacteristics of solution-processed Ag NP thin films onflexible polyimide substrates by observing the microstructures

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Nanotechnology 24 (2013) 085701 S Kim et al

Table 1. The failure strains of solution-processed Ag NP thin films and electron beam evaporated Ag thin films as annealed at the indicatedtemperatures and periods of time.

Solution-processed Ag NP thin film

E-beamevaporated Ag

thin film

Annealing condition Sintering (150 ◦C, 30 min) 180◦C 230 ◦C 150 ◦C 220 ◦C

(3 h) (3 h) (2 h) (2 h)

Failure strain 6.6% 5.4% 3.8% 10% 15.5%(standard deviation) (0.3%) (0.7%) (0.6%) (1.8%) (0.6%)

Figure 5. (a) The resistance–elongation curves of Ag NP thin filmcoated using a coating bar and Ag thin film deposited by electronbeam evaporation. The failure strains for solution-processed Ag NPthin films and e-beam evaporated Ag thin films ranged from 3.8% to6.6% and from 10% to 15.5%, respectively. (b) SEM images ofcracks on the surface of tensile test specimens with 20% strain.

and measuring the electrical resistance under tensile strain.The effects of the annealing temperature and period onthe microstructure and failure strain were investigated. Amaximum failure strain of 6.6% was obtained from aspecimen initially sintered at 150 ◦C, and the failure strainswere reduced by additional annealing at higher temperaturesand for longer periods of time. Although the electricalconductivity of Ag NP thin film was increased monotonicallyby annealing at higher temperatures, the stretchability ofthe film was worsened. Therefore, it is necessary to chooseappropriate annealing temperatures and periods to achievesuitable levels of both electrical and mechanical propertiesof solution-processed Ag NP thin films. Ag NP thin filmsshowed porous and granular microstructures as compared toe-beam evaporated Ag films with higher density and lessporosity, resulting in lower failure strains than those of e-beam

evaporated Ag films. It is believed that this work can providea better understanding of the mechanical characteristics ofsolution-processed metal NP thin films under tensile loadingfor various electronics applications and that the results herecan serve as a cornerstone for the design of fabricationprocesses of printed electronic devices with better mechanicalreliability.

Acknowledgments

This research was supported by the Fundamental R&DProgram for Core Technology of Materials funded bythe Ministry of Knowledge Economy (K0006028) and bythe Mid-career Research Program (Key Research) (2011-0027669) through the National Research Foundation of Korea(NRF) funded by the Korean government.

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Tensile characteristics of metal nanoparticle films on flexible polymer substrates for printed electronics applicationsIntroductionExperimentResults and discussionsAcknowledgmentsReferences