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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: [email protected] and [email protected]
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,
10957-4484/13/085701+07$33.00 c© 2013 IOP Publishing Ltd Printed
in the UK & the USA
http://dx.doi.org/10.1088/0957-4484/24/8/085701mailto:[email protected]:[email protected]://stacks.iop.org/Nano/24/085701http://stacks.iop.org/Nano/24/085701/mmedia
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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.
2
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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
4
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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