Combined Role of Well-Dispersed Aqueous Ag Ink and the Molecular Adhesive Layer in Inkjet Printing the Narrow and Highly Conductive Ag Features on a Glass Substrate Sunho Jeong,* Hae Chun Song, Won Woo Lee, Youngmin Choi, Sun Sook Lee, and Beyong-Hwan Ryu* DeVice Materials Research Center, Korea Research Institute of Chemical Technology, 19 Sinseongno, Yuseong, Daejeon 305-600, Korea ReceiVed: July 27, 2010; ReVised Manuscript ReceiVed: October 13, 2010 A novel inkjet printing procedure based on a well-dispersed aqueous Ag ink and a molecular adhesive layer is presented for the fabrication of highly conductive and narrow patterns exhibiting the excellent adhesion property on a glass substrate. The aqueous dispersion of silver nanoparticles is synthesized via a chemical reduction method in the aqueous medium in which an anionic polyelectrolyte is incorporated as both a capping agent and a dispersant. Owing to the electrosteric repulsion characteristic of the anionic polyelectrolyte, the prepared aqueous Ag ink exhibits the long-term dispersion stability. A molecular adhesive layer is deposited on a glass substrate using either aminopropyltriethoxysilane or mercaptopropyltriethoxysilane. The distinctive functional group of the molecular adhesive layer plays a critical role in the adhesion property as well as high contact angle appropriate for forming the narrow Ag patterns, which is achieved only with the aqueous ink. It is demonstrated that the resulting inkjet-printed Ag patterns with a line width of 45 µm exhibit the excellent adhesion property and resistivity as low as 3.7 µΩ · cm, after annealing at temperatures ranging from 200 to 300 °C. 1. Introduction In recent years, the direct printing of functional materials has attracted increasingly significant interest as large-area and low- cost processing techniques for the fabrication of conductive features on a transparent glass substrate in modern electronic, opto-electronic, and photovoltaic applications. Despite the wide adoption of photolithography in the microfabrication of conduc- tive features, its complicated process significantly increases manufacturing costs and poses major obstacles to realize modern large-area and inexpensive electronics. 1,2 In particular, inkjet printing, which can deliver precise quantities of materials to desired locations, has been regarded as an alternative to conventional lithography. 3-9 To date, Ag nanoparticles have been studied as promising functional materials for conductive inks since they exhibit high conductivity (∼10 5 S/cm), operational stability, and low- temperature processability. 4,5,8 As the size of the metal particles decreases below a few tens of nanometers, the melting point abruptly falls due to their high surface energy, and the sintering process, which is essential to form a conductive dense layer, takes place at a low temperature compatible to a glass substrate. Nevertheless, inkjet printing of conductive Ag ink on a glass substrates has been little studied because of its tendency to spread widely on a glass substrate and the poor adhesion property of printed Ag patterns. Recently, Jang et al. reported that the addition of glass frit, which can melt at elevated temperatures as high as 600 °C, enables the printed Ag patterns to adhere well to a glass substrate. 10 However, the melting of glass frit requires relatively high-temperature annealing, and the conductivity of printed Ag patterns significantly degrades the more that glass frit is added. In addition, the spreading problem was not resolved, resulting in the printing of the wide Ag pattern with a line width of 150 µm. To overcome such drawbacks, we have incorporated the molecular adhesive layer composed of self-assembled molecules and utilized DI water as a dispersant medium for Ag nanopar- ticles. The use of solvent with high surface tension as a dispersion medium aids in obtaining the liquid-solid interface with a high contact angle required to form narrow printed features. Recently, the salt-based aqueous ink was reported for printing Ag conductive lines on the polyimide substrate. However, despite optimization of process conditions, the conductivity of the Ag pattern was limited to 27 µΩ · cm. 11 In comparison with particle-based ink, the active material in a salt- based ink is partially composed of organic residues which should be thermally decomposed at elavated temperature, which deteriorates the microstructure of the printed pattern and in turn electrical property. To date, Ag nanoparticles have been, however, synthesized in the nonaqueous medium, 12-15 whereas noble metal nanoparticles such as Au, Pt, and Pd have been successfully synthesized in the aqueous medium. 16,17 The Ag nanoparticles synthesized in organic medium are hardly dis- persed in the aqueous medium since most capping agents, which are used to control particle size and prevent interparticular agglomeration, are inactive in the aqueous medium. Therefore, for the development of aqueous Ag ink, the surface-capping molecules should be replaced with water-compatible molecues, or Ag nanoparticles should be synthesized in aqueous medium with capping molecules which can be active in aqueous medium. 18-20 In addition, the aqueous ink allows the use of polyelectrolyte as both a capping agent and a dispersant, which gives rise to electrosteric repulsion, 21 enabling a suspension with long-term dispersion stability. 22 In the case of nanoparticles synthesized in organic medium, most capping molecules exhibit relatively weak steric repulsion, so that the sophisticated design * To whom correspondence should be addressed. E-mail: [email protected](S.J.); [email protected] (B.-H.R.). J. Phys. Chem. C 2010, 114, 22277–22283 22277 10.1021/jp106994t 2010 American Chemical Society Published on Web 12/01/2010
Transcript
Combined Role of Well-Dispersed Aqueous Ag Ink and the Molecular Adhesive Layer inInkjet Printing the Narrow and Highly Conductive Ag Features on a Glass Substrate
Sunho Jeong,* Hae Chun Song, Won Woo Lee, Youngmin Choi, Sun Sook Lee, andBeyong-Hwan Ryu*DeVice Materials Research Center, Korea Research Institute of Chemical Technology, 19 Sinseongno, Yuseong,Daejeon 305-600, Korea
ReceiVed: July 27, 2010; ReVised Manuscript ReceiVed: October 13, 2010
A novel inkjet printing procedure based on a well-dispersed aqueous Ag ink and a molecular adhesive layeris presented for the fabrication of highly conductive and narrow patterns exhibiting the excellent adhesionproperty on a glass substrate. The aqueous dispersion of silver nanoparticles is synthesized via a chemicalreduction method in the aqueous medium in which an anionic polyelectrolyte is incorporated as both a cappingagent and a dispersant. Owing to the electrosteric repulsion characteristic of the anionic polyelectrolyte, theprepared aqueous Ag ink exhibits the long-term dispersion stability. A molecular adhesive layer is depositedon a glass substrate using either aminopropyltriethoxysilane or mercaptopropyltriethoxysilane. The distinctivefunctional group of the molecular adhesive layer plays a critical role in the adhesion property as well as highcontact angle appropriate for forming the narrow Ag patterns, which is achieved only with the aqueous ink.It is demonstrated that the resulting inkjet-printed Ag patterns with a line width of 45 µm exhibit the excellentadhesion property and resistivity as low as 3.7 µΩ · cm, after annealing at temperatures ranging from 200 to300 °C.
1. Introduction
In recent years, the direct printing of functional materials hasattracted increasingly significant interest as large-area and low-cost processing techniques for the fabrication of conductivefeatures on a transparent glass substrate in modern electronic,opto-electronic, and photovoltaic applications. Despite the wideadoption of photolithography in the microfabrication of conduc-tive features, its complicated process significantly increasesmanufacturing costs and poses major obstacles to realize modernlarge-area and inexpensive electronics.1,2 In particular, inkjetprinting, which can deliver precise quantities of materials todesired locations, has been regarded as an alternative toconventional lithography.3-9
To date, Ag nanoparticles have been studied as promisingfunctional materials for conductive inks since they exhibit highconductivity (∼105 S/cm), operational stability, and low-temperature processability.4,5,8 As the size of the metal particlesdecreases below a few tens of nanometers, the melting pointabruptly falls due to their high surface energy, and the sinteringprocess, which is essential to form a conductive dense layer,takes place at a low temperature compatible to a glass substrate.Nevertheless, inkjet printing of conductive Ag ink on a glasssubstrates has been little studied because of its tendency tospread widely on a glass substrate and the poor adhesionproperty of printed Ag patterns. Recently, Jang et al. reportedthat the addition of glass frit, which can melt at elevatedtemperatures as high as 600 °C, enables the printed Ag patternsto adhere well to a glass substrate.10 However, the melting ofglass frit requires relatively high-temperature annealing, and theconductivity of printed Ag patterns significantly degrades themore that glass frit is added. In addition, the spreading problem
was not resolved, resulting in the printing of the wide Ag patternwith a line width of 150 µm.
To overcome such drawbacks, we have incorporated themolecular adhesive layer composed of self-assembled moleculesand utilized DI water as a dispersant medium for Ag nanopar-ticles. The use of solvent with high surface tension as adispersion medium aids in obtaining the liquid-solid interfacewith a high contact angle required to form narrow printedfeatures. Recently, the salt-based aqueous ink was reported forprinting Ag conductive lines on the polyimide substrate.However, despite optimization of process conditions, theconductivity of the Ag pattern was limited to 27 µΩ · cm.11 Incomparison with particle-based ink, the active material in a salt-based ink is partially composed of organic residues which shouldbe thermally decomposed at elavated temperature, whichdeteriorates the microstructure of the printed pattern and in turnelectrical property. To date, Ag nanoparticles have been,however, synthesized in the nonaqueous medium,12-15 whereasnoble metal nanoparticles such as Au, Pt, and Pd have beensuccessfully synthesized in the aqueous medium.16,17 The Agnanoparticles synthesized in organic medium are hardly dis-persed in the aqueous medium since most capping agents, whichare used to control particle size and prevent interparticularagglomeration, are inactive in the aqueous medium. Therefore,for the development of aqueous Ag ink, the surface-cappingmolecules should be replaced with water-compatible molecues,or Ag nanoparticles should be synthesized in aqueous mediumwith capping molecules which can be active in aqueousmedium.18-20 In addition, the aqueous ink allows the use ofpolyelectrolyte as both a capping agent and a dispersant, whichgives rise to electrosteric repulsion,21 enabling a suspension withlong-term dispersion stability.22 In the case of nanoparticlessynthesized in organic medium, most capping molecules exhibitrelatively weak steric repulsion, so that the sophisticated design
10.1021/jp106994t 2010 American Chemical SocietyPublished on Web 12/01/2010
on surface chemistry is essential to achieve the long-termdispersion stability.
In this study, we report remarkably well-dispersed aqueousAg ink containing the polyelectrolyte-capped Ag nanoparticlesand the noble way to enhance the adhesion property of printedAg patterns using the adhesive layer composed of self-assembledmolecules. We demonstrate that the combination of well-dispersed Ag ink and the molecular adhesive layer facilitatesthe inkjet-printed Ag patterns with excellent adhesion property,high conductivity, and narrow line width.
2. Experimental Section
2.1. Materials. All reagents, silver nitrate (AgNO3, 99%),sodium borohydride (NaBH4, 99%), poly(acrylic acid) (PAA,[CH2CH(CO2Na)]n, 35 wt % in H2O, Mw ) 15 000), ammoniumhydroxide (NH4OH, 25%), nitric acid (HNO3, 60%), 3-ami-nopropyltriethoxysilane (APTES, H2N(CH2)3Si(OC2H5)3,99%), and 3-mercaptopropyltriethoxysilane (MPTES, HS(CH2)3Si(OC2H5)3, 95%), were purchased from Aldrich and usedwithout additional purification. Hydrazine (NH2NH2, 80%) waspurchased from Daejung Chemicals & Metals and used withoutadditional purification.
2.2. Ink Preparation. Ag nanoparticles were synthesized viachemical reduction of Ag ions in DI water. To prevent theinterparticular agglomeration, poly(acrylic acid) (PAA) wasincorporated as a surface-capping molecule, and a mixture ofhydrazine and sodium borohydride was used as a reducing agent.Amounts of 25.5 g of Ag nitrate, 14.9 g of poly(acrylic acid),and 1.5 g of a mixture of hydrazine and sodium borohydridewere added into a three-neck, round-bottomed flask containing100 mL of DI water. The flask was fitted with a refluxcondenser. After reaction for 80 min at 25 °C, the synthesized
Ag nanoparticles were selectively separated by a centrifugationmethod, and obtained Ag nanoparticles were washed with DIwater by centrifugation for 40 min under 68 470g. For prepara-tion of the Ag conductive ink, Ag nanoparticles were dispersedin DI water with the solid loading of 25 wt %, and the pH ofaqueous Ag ink was adjusted by adding either diluted NH4OHor HNO3.
2.3. Preparation of the Glass Substrate with MolecularAdhesive Layer and Inkjet Printing. A bare glass substratewas cleaned subsequently with “Piranha solution” (H2O2:H2SO4
) 3:7), DI water, acetone, isopropyl alcohol, ethyl alcohol, andDI water and dried with an N2 stream. Either 3-aminopropyl-triethoxysilane (APTES) or 3-mercaptopropyltriethoxysilane(MPTES) was deposited by immersing a clean bare glasssubstrate in a mixture of silane, ethyl alcohol, and DI water.The volumeric ratio of ethyl alcohol to DI water was 19, andthe concentration of silane was 2 vol %. Then, the Ag conductiveink was printed on a bare, APTES-treated, and MPTES-treatedglass substrate. The substrate temperature was maintained at25 °C. The printer set up is composed of a drop-on-demandpiezoelectric inkjet nozzle manufactured by Microfab Technolo-gies, Inc. (Plano, TX), and the diameter of the orifice is 30 and50 µm. The inkjet-printed Ag nanoparticulate films wereannealed at various temperatures from 100 to 350 °C for 60min in ambient atmosphere.
2.4. Characterization. The size and shape of the synthesizedAg nanoparticles and the microstructures of Ag patternsannealed at different temperatures were observed by scanningelectron microscopy (SEM, JSM-6700, JEOL). The crystal andchemical structure of Ag nanoparticles were analyzed using anX-ray diffractometer (XRD, D/MAX-2200 V, Rigaku) andX-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo
Figure 1. (a) SEM image of particulate film composed of Ag nanoparticles. The inset is a TEM image of synthesized Ag nanoparticles. (b) Theparticle size distribution of synthesized Ag nanoparticles. The measured average particle size is 19.5 nm.
Figure 2. (a) XRD pattern and (b) XPS spectrum of synthesized Ag nanoparticles. Two peaks in the XPS spectrum, which is located at 368.1 and374.1 eV, correspond to Ag 3d5/2 and 3d3/2 binding energy, respectively. In comparison with Ag0 (368.3 and 374.3 eV), the peaks shift down tolower binding energy, indicating that the chemical environment around Ag atoms is changed due to capping molecules.
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Fisher Scientific). The XPS spectrum was collected usingmonochromatic Al KR radiation (1486.6 eV) in an ultrahighvacuum system with a base pressure of ∼10-10 Torr. The surfacecharge was measured with a zeta-potential analyzer (ELS-Z,Otsuka). Rheological behavior of prepared Ag conductive inkwas monitored using a modular compact rheometer (MCR 101,Anton Paar) at shear rates ranging from 10-1 to 103 s-1 and acapillary viscometer (Online viscometer VROC, Rheosense) atshear rates ranging from 103 to 105 s-1. The contact angle wasmeasured with a dynamic contact angle system (SEO 300, SEO),and the thermal decomposition behavior of the PAA adsorbedon the particles was monitored using thermal gravimetricanalysis (SDT2960, TA Instruments). The morphology andresistivity of the printed Ag patterns were analyzed using asurface profiler (Alpha-step IQ, KLATencor) and four-pointprobe station equipped with a semiconductor characterizationsystem (Keithley 4200, Keithley), respectively. The adhesionproperty of printed Ag lines was characterized according to theASTM D3359 standard test method based on tape testing.Printed lines with the length of 2 cm were cut 2 mm apart witha razor blade. A piece of 3M 610 tape was placed over theprepared sample and rubbed firmly with an eraser for goodcontact between the tape and sample. Then, the tape wasremoved by seizing the free end as close to an angle of 180° aspossible, and the fraction of the remaining area was inspectedby observing the resulting printed features using an opticalmicroscope.
3. Results and Discussion
3.1. Preparation of Well-Dispersed Aqueous Ag Ink andInkjet Printing on a Bare Glass Substrate. We synthesizedAg nanoparticles by reducing the Ag ions in the aqueousmedium in which poly(acrylic acid) (PAA) was dissolved. Asshown in Figure 1, the size of the synthesized Ag nanoparticleswas measured as ∼20 nm with a small deviation, and thenanosized monodispersed particles allowed the formation of ahighly packed particulate film. In addition, according to XRDand XPS analysis, it is revealed that the synthesized Agnanoparticles consist of a pure Ag phase without secondaryphases such as byproduct or oxide phases, as shown in Figure2.
In our synthesis method, PAA plays a critical role asa capping agent to control particle size and shape as well as adispersant to give rise to electrosteric repulsion. PAA is apolymer with an ionizable functional group (COONa), whichdissociates to produce charged polymers. These charged poly-mers lead to electrosteric repulsion, involving a combinationof electrostatic repulsion and steric repulsion. The degree ofrepulsion in electrosteric stabilization is much higher than thatin a single electrostatic or steric stabilization. The dissociationrate is strongly dependent on the pH of the aqueous medium. Ithas been known that the fraction of the dissociated group is∼1 in the aqueous medium with pH > 6, and the degree ofdissociation gradually diminishes as the pH of the aqueousmedium decreases (the degree of dissociation is nearly zero inthe aqueous medium with pH < 3).18 We measured the zeta-potential to analyze the surface charge induced by the dissociatedgroups and its dependence on pH of the aqueous medium (Figure3a). As expected, the high zeta-potential greater than -50 mVevolved at pH > 6, and the zeta-potential decreased slightly atpH of 4.8 due to the slightly incomplete dissociation; the surfacecharge was nearly zero at a pH of 2.5. Thus, it is speculatedthat the dispersion of Ag nanoparticles in the aqueous mediumwith pH > 4.8 would be governed by strong electrostericrepulsion, yielding the superior dispersion stability.
As shown in Figure 3b, PAA-capped Ag nanoparticlesdispersed well in the aqueous medium with pH > 4.8 withoutincorporating any additional dispersants. The dispersion stabilityof the prepared ink has been monitored for more than twomonths, and no precipitates have been observed yet. Furtherinvestigation on long-term dispersion stability is ongoing. Onthe contrary, Ag nanoparticles in ink prepared using the aqueousmedium with pH of 2.5 were precipitated 120 min after inkpreparation. This different dispersion characteristic was alsoconfirmed in rheological behavior. As shown in Figure 3c, Agink prepared using the aqueous medium with pH > 4.8 exhibitedNewtonian behavior with a shear rate of 105 s-1 (since the shearrate around 104∼105 s-1 is applied in the nozzle during inkjet
Figure 3. (a) Zeta-potential of Ag nanoparticles as a function of pHof an aqueous medium. (b) Gravitational sedimentation test for Ag inksprepared using the aqueous medium with different pH. This test wascarried out to determine the degree of particle agglomeration byobserving the volumes of suspension zone. As the particles in theunstable suspension agglomerate and settle, the volume of the suspen-sion zone decreases. (c) Rheological behavior of Ag inks prepared usingthe aqueous medium with different pH. The viscosity of ink containingagglomerates (pH ∼ 2.5) was not measured at shear rates ranging from103 to 105 s-1 due to capillary-tube clogging. In the case of well-dispersed Ag ink (pH > 4.8), reliable viscosity data were not obtainedat a low shear rate below 10 s-1 due to the extremely low torque value.
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printing, the monitoring on rheological property in this rangeis very important), indicative of a well-dispersed suspension.If particle aggregation occurs, the inks shows shear-thinningbehavior, which is characterized by a gradual decrease inapparent viscosity with increasing shear rate.23 The increase inthe shear rate breaks down the aggregates, which leads to areduction in the amount of solvent immobilized by the particles,thus lowering the apparent viscosity of the system.24 Shear-thinning behavior could be observed in well-dispersed suspen-sions containing asymmetric particles. However, since thesynthesized Ag nanoparticles were spherical in shape, it is
presumed that the shear-thinning behavior is attributed only tothe formation of agglomerates.
Such well-dispersed ink allowed the use of a nozzle with asmall orifice diameter, which aids in forming the narrow printedpatterns. If the metal nanoparticles are not well-dispersed, it isdifficult to eject the stable droplets through the nozzle with anorifice diameter smaller than 30 µm since the nozzle is easilyclogged, and even if the droplet is successfully ejected, itsstability would be poor. The image and dimensions of dropletsejected through a nozzle with orifice diameters of 50 and 30µm are shown in Figure S1 and Table S1 (Supporting Informa-
Figure 4. Thermal gravimetric analysis curves of (a) PAA and (b-d) Ag nanoparticle: (b) thermal behavior as a function of temperature rangingfrom 25 to 400 °C, (c) thermal behavior as a function of annealing time at 200 °C, and (d) thermal behavior as a function of annealing time at 200°C. The number in red in (c,d) indicates the fraction of decomposed PAA.
Figure 5. (a) Molecular structure of APTES and MPTES. (b) Comparison of adhesion property (based on ASTM D3359 tape test) of Ag linepatterns printed on bare, MPTES-treated, and APTES-treated glass substrates. The number on top of the bar indicates the annealing temperature.The orifice diameter of the nozzle used here was 30 µm.
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tion). The line widths of Ag patterns printed using nozzles withdiameters of 50 and 30 µm are 100 and 65 µm, respectively(Figure S2, Supporting Information), which represents that theline width of printed patterns linearly decreases as a functionof the volume of the jetted droplets.
The as-printed Ag pattern consists of resistive particulatelayers, which are converted to conductive dense layers by athermally activated sintering process. The sintering process isalso involved in the temperature-dependent decomposition ofPAA adsorbed to Ag nanoparticles since the sintering processoccurs by diffusion of Ag atoms into the boundary area betweenneighboring particles and capping molecules act as a barrierlayer against atom diffusion. According to the analysis on athermal decomposition of PAA (Figure 4a), the weight lossbelow 140 °C is attributed to the evaporation of solvent (DIwater) and adsorbed solvent molecules, and the weight lossabove 400 °C is assigned to the thermal decomposition of PAA.Therefore, when the surface of Ag nanoparticles was surroundedby PAA, it is suggested that the solvent molecules adsorbed toPAA present in the surface of the Ag nanoparticle evaporatebelow 140 °C, and the weight loss above 140 °C is mainly dueto the thermal decomposition of PAA, as shown in Figure 4b.The shift of temperature at which PAA is decomposed resultsfrom the catalytic effect of the Ag nanoparticle.25,26 On the basisof the fraction of decomposed PAA at 200 and 250 °C (thisvalue was calculated using the ratio of weight loss at a specifictemperature, obtained from either Figure 4c or 4d, to total weightloss observed in Figure 4b), it is believed that the thermaldecomposition of PAA surrounding Ag nanoparticles partiallytook place at 200 °C and was completed at 250 °C, whichindicates that interparticular growth begins at 200 °C. This isin line with the fact that the resistivity of the printed Ag patterndrastically decreased at 200 °C to 4.1 µΩ · cm (Figure S3,Supporting Information). However, the printed Ag patternannealed at 200 °C completely peeled off after the adhesiontest (see Experimental Section for details).
3.2. Inkjet Printing of Well-Dispersed Aqueous Ag Inkon a Glass Substrate with a Molecular Adhesive Layer. Toimprove the adhesion property, we introduced the molecularadhesive layer via depositing either aminopropyltriethoxylsilane(APTES) or mercaptopropyltriethoxysilane (MPTES), whichconsists of three alkoxy groups and one distinctive functionalgroup. The molecular structures of APTES and MPTES aredepicted in Figure 5a. Alkoxy groups react with the hydroxylgroups present in the surface of the glass substrate, and eitherthe amine group in APTES or the thiol group in MPTES ischemically bonded to the Ag nanoparticle. It has been well-known that the thiol group is strongly chemisorbed on the Ag
surface by the formation of a covalent-like bond between thesilver and the sulfur atoms, and the binding of a silvernanoparticle to an amine group arises from the silver’s abilityto donate electrons to the nitrogen’s antibonding orbital via πback bonding.27-32 As shown in Figure 5b, the adhesion propertyof the printed Ag pattern after annealing at 200 °C wasdrastically improved on ATPES-treated glass. However, in thecase of MPTES-treated glass, the adhesion property of theprinted Ag pattern appeared to be relatively poor and thermallyvulnerable above 250 °C. While Ag printed patterns adheredwell to the amine-functionalized glass substrate even afterannealing at 300 °C, the adhesion property of Ag patterns printedon the thiol-functionalized glass substrate significantly deterio-rated after annealing at 250 °C. This different behavior wasbelieved to relate to molecular-structural thermal stability ofthe adhesive layer, so that we analyzed the variation of contactangles as a function of annealing temperature to elucidate theorigin for this different behavior (Figure 6). The contact angleof DI water on the amine-functionalized glass substrate did notvary significantly at temperatures ranging from 25 to 300 °Cand abruptly fell after annealing at 350 °C, which is inaccordance with the adhesion property tendency. Therefore, itis presumed that the degradation of the adhesion property inthe Ag patterns printed on the amine-functionalized glasssubstrate was due to the thermal decomposition of APTES. Onthe contrary, in the case of the thiol-functionalized glasssubstrate, the variation in the contact angles and adhesionproperties as a function of annealing temperature did not agreewell with each other. While the contact angle abruptly fell afterannealing at 300 °C, the adhesion property degraded afterannealing at 200 °C. It has been reported that a desorption of
Figure 6. Variations in the contact angle of DI water on MPTES-treated and APTES-treated glass substrate as a function of annealingtemperature.
Figure 7. (a) Resistivity of Ag line patterns as a function of annealingtemperature. The orifice diameter of the nozzle used here was 30 µm.(b) SEM images showing the microstructural evolution of Ag linepatterns annealed at the indicated temperatures. The scale bar is 1 µm.The insets are high-magnification SEM images, and the scale bar is500 nm.
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alkanethiolate adsorbed on Au takes place through cleavage ofthe Au-S bond over the range of 170-220 °C,33-36 and mostthiol-derived self-assembled molecules supported on silver showsome qualitative resemblance to properties observed in assemblyon gold.27-29 In addition, it was reported that the benzenethi-olates are desorbed from the silver surface below 200 °C, notundergoing the structural changes.37 Thus, it is speculated thatthe cleavage of the Ag-S bond took place around 200 °C priorto the thermal decomposition of the MPTES itself at 300 °C,resulting in a relatively poor adhesion property at 200 °C andcomplete peel-off at 250 °C. This implies that the thiol-functionalized adhesive layer is not acceptable as an adhesivelayer for conductive printed features activated by annealingabove 200 °C.
Figure 7a shows the variations in resistivity of the Ag patternprinted on the APTES-treated glass as a function of annealingtemperature. As the annealing temperature increases to 200 °C,the resistivity of the printed pattern drastically decreased,exhibiting resistivity as low as 3.7 µΩ · cm, and the resistivitydid not change at temperatures ranging from 200 to 300 °C.This dependence of resistivity on annealing temperature isassociated with the aforementioned thermally driven sinteringprocess, which was confirmed by the structural evolution as afunction of annealing temperature (Figure 7b). The individuallyisolated Ag nanoparticles were interconnected by a sinteringprocess at 200 °C at which the resistivity reaches the valuecomparable with the resistivity of bulk Ag. The resistivity of3.7 µΩ · cm was 2.3 times larger than the resistivity of bulkAg, which is attributed to the presence of small voids generallyobserved in the printed patterns. It should be noted that giventhe resistivity of Ag patterns printed on a bare glass substratethe amine-functionalized adhesive layer did not deteriorate theelectrical property of the upper conductive line. However,resistivity abruptly increased after annealing at 350 °C, whichresults from the microstructural collapse caused by the thermaldecomposition of the underlying self-assembled adhesive layer(Figure 8).
3.3. Role of Aqueous Ag Ink in Printing the NarrowConductive Features. In addition to its role in improving theadhesion property, the molecular adhesive layer facilitated anarrow printed pattern. Once the self-assembled adhesive layeris deposited, the surface of the glass substrate is functionalizedby either an amine or a thiol group, which leads to the lowersurface energy compared with a hydroxyl-terminated baresubstrate. The surface energy of the top layer, which is in contactwith Ag ink during the printing process, predominantly deter-mines the contact angle of the Ag ink. After the deposition ofthe self-assembled adhesive layer, the contact angle effectivelyincreased above 40° (Figure 9). In general, the high contact anglebelow 50° facilitates the formation of the narrow printed patternsince the diameter of the printed dots diminishes inversely withthe contact angle and dots printed on the substrate with a contact
angle above 50° cannot merge uniformly into a line feature.Note that other self-assembled adhesive layers do not enhancethe adhesion property because of the lack of chemical-bondingnature with Ag atoms and cannot also satisfy the contact-anglecriterion for narrow printed features (for instance, the contactangle of Ag ink on the glass substrate treated with hexameth-yldisilazane and octadecyltrichlorosilane was 75° and 98°,respectively). As shown in Figure 10, the narrow inkjet printedAg feature with the line width of 45 µm was obtained onAPTES-treated glass, and the complex Ag patterns were alsosuccessfully printed on a large-area APTES-treated glasssubstrate. On the other hand, in the case of nonaqueous ink,this distinctive role of self-assembled adhesive layer is notactive. Organic solvents such as ethanol, toluene, hexane,chlorobenzene, and tetradecane, which are commonly used innonaqueous metal inks, have a low surface tension (the surfacetension of each solvent is summarized in Table 1), so that thesubstrate surface energy is not capable of acting as a determiningfactor for the contact angle. As shown in Figure 9, allnonaqueous solvents tested in this study showed extremelyhydrophilic wetting behavior.
Figure 8. Cross-sectional profiles of (a) nonannealed Ag line pattern and Ag line patterns annealed at (b) 250 and (c) 350 °C. All line patternswere printed on APTES-treated glass substrate, and the orifice diameter of the nozzle was 30 µm.
Figure 9. Contact angle of Ag ink and several solvents on bare,MPTES-treated, and APTES-treated glass substrates. The inset imagesshow the wettability of Ag ink on each substrate.
Figure 10. Optical microscope image for Ag complex patterns printedon the large-area APTES-treated glass substrate. The orifice diameterof the nozzle was 30 µm, and the dot spacing was 40 µm. The insetshows an optical microscope image for a printed Ag single line.
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To the best of our knowledge, this study represents the firstattempt to directly write highly conductive and narrow Agpatterns that adhere well to a glass substrate at a low temper-ature. The noble way described here, which is based on thecombination of a self-assembled adhesive layer and remarkablywell-dispersed aqueous Ag ink, is expected to provide aconvenient and low-cost method for fabricating conductivefeatures which can be adopted in various fields including modernelectronics, opto-electronics, and photovoltaic applications.
4. Conclusions
We prepared aqueous Ag ink with long-term dispersionstability using monodispersed Ag nanoparticles synthesized inthe aqueous medium in which PAA was incorporated as acapping agent as well as a dispersant. We also deposited theself-assembled adhesive layer on a glass substrate to enhancethe adhesion property of printed Ag features, and it wasdemonstrated that the amine-functionalized adhesive layer actedwell as a thermally stable adhesive layer at temperatures rangingfrom 25 to 300 °C. The combination of aqueous ink andmolecular adhesive layer enabled the narrow printed featuresand superior adhesion property without degrading the electricalproperty of the printed Ag features, resulting in a line width of45 µm and resistivity as low as 3.7 µΩ · cm.
Acknowledgment. This study was supported by a grant(B551179-08-03-00) from the cooperative R&D Program fundedby the Korea Research Council Industrial Science and Technol-ogy and carried out for the Direct Nano Patterning Project(TS091-45) supported by the Ministry of Knowledge Economyunder the National Strategic Technology Program.
Supporting Information Available: The images and physi-cal dimensions of jetted droplets, images of the Ag patternprinted on a bare glass substrate, and resistivity variation as afunction of annealing temperature. This material is availablefree of charge via the Internet at http://pubs.acs.org.
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