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Stable Aqueous Based Cu Nanoparticle Ink for Printing Well-Defined Highly Conductive Features on a Plastic Substrate Sunho Jeong,*,† Hae Chun Song,† Won Woo Lee,† Sun Sook Lee,† Youngmin Choi,† Wonil Son,‡ Eui Duk Kim,‡ Choon Hoon Paik,‡ Seok Heon Oh,‡ and Beyong-Hwan Ryu*,† †

Device Materials Research Center, Korea Research Institute of Chemical Technology, 19 Sinseongno, Yuseong-gu, Daejeon 305-600, Korea ‡ Nano R&D Group, Hanwha Chemical Research & Development Center, 6 Shinsung-dong, Yuseong-gu, Daejeon 305-804, Korea

bS Supporting Information ABSTRACT: With the aim of inkjet printing highly conductive and well-defined Cu features on plastic substrates, aqueous based Cu ink is prepared for the first time using water-soluble Cu nanoparticles with a very thin surface oxide layer. Owing to the specific properties, high surface tension and low boiling point, of water, the aqueous based Cu ink endows a variety of advantages over conventional Cu inks based on organic solvents in printing narrow conductive patterns without irregular morphologies. It is demonstrated how the design of aqueous based ink affects the basic properties of printed conductive features such as surface morphology, microstructure, conductivity, and line width. The long-term stability of aqueous based Cu ink against oxidation is analyzed through an X-ray photoelectron spectroscopy (XPS) based investigation on the evolution of the surface oxide layer in the aqueous based ink.

’ INTRODUCTION In recent years, various printing technologies and solutionprocessed functional materials to facilitate low cost flexible electronics have attracted increasingly significant interest for use in a wide range of applications.1-7 Among the printing techniques, inkjet printing is a particularly attractive technique, especially for the controlled solution deposition and delivery of precise quantities of functional materials to desired locations.8-13 To date, metal nanoparticles such as Au, Ag, and Cu have been studied as promising functional materials for conductive inks, since they exhibit high conductivity (∼105 S/cm) and operational stability as well as low temperature processability.14-21 As the size of the metal particles decreases to a few tens of nanometers, the melting point falls abruptly due to the high surface energy and the sintering process, which is essential to form a conductive dense layer, takes place at a low temperature compatible with the plastic substrate. In particular, copper is a good material as it is highly conductive but significantly cheaper than Au and Ag. However, owing to difficulty in synthesizing Cu nanoparticles with a minimal surface oxide layer and preventing post oxidation during additional processes (such as ink preparation, printing, and annealing), the inkjet printing of Cu nanoparticle based conductive inks has been seldom researched.14,22,23 Thus far, Cu nanoparticles for conductive inks have been primarily synthesized in organic solvents (nonaqueous medium) under an inert atmosphere to prevent undesirable oxidation14,23-28 and the resulting nanoparticles have been dispersed in organic solvents, since most capping agents used in organic solvents are active in the corresponding solvents. However, this conventional conductive Cu ink based on organic solvents with a low surface r 2011 American Chemical Society

tension has a limitation in obtaining high contact angles on specific substrates for printing narrow patterns as well as adjusting the hydrodynamic flow to prevent the formation of “coffeering patterns”. The formation of uniform surface morphology is crucial for applying the inkjet printing technology to flexible electronics, since many electronic applications require multilayer deposition and the surface morphology of the underlying layer critically influences the various properties of the upper layer. The printed Cu patterns reported to date in the literature are quite wide with line width of over 85 μm, and their morphology is not uniform.14,23 The optimal method for eliminating “coffee-ring patterns” is generating a Marangoni flow, which is induced by incorporating a solvent with a low boiling point and high surface tension.11,29-33 The magnitude of the Marangoni flow is proportional to the surface tension difference between solvents with different boiling points, which determines the surface morphology of the printed pattern. However, it is not easy to find organic solvents that are suitable for such physical properties, playing a role as dispersion medium for metal nanoparticles. In contrast, the aqueous solvent system allows these drawbacks to be easily overcome due to the following factors. (1) Water has a high surface tension (72.8 mN/m), and there are a variety of water-miscible solvents with different surface tensions. Thus, the surface tension of the aqueous solvent system can be adequately adjusted, yielding a high contact angle that is suitable for printing narrow patterns. (2) Water is an appropriate solvent Received: October 14, 2010 Revised: December 26, 2010 Published: February 21, 2011 3144

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Langmuir for creating a Marangoni flow, since the boiling point of water is 100 °C and most water-miscible solvents with boiling points higher than 100 °C have a much lower surface tension than water. (3) Water is a dispersion medium where the polyelectrolyte is active as a dispersant. Polyelectrolyte is ionized to produce charged polymers in water, which leads to superior dispersion stability based on electrosteric repulsion, involving a combination of electrostatic repulsion and steric repulsion.34 The degree of repulsion in electrosteric stabilization is much higher than that in a single electrostatic or steric stabilization. The dispersion mechanism of the ink based on organic solvents is primarily governed by a single steric repulsion, so that the sophisticated design on the surface chemistry is essential to achieve long-term dispersion stability. (4) An aqueous solvent system is costeffective and environmentally friendly. However, even with these advantages over organic solvent based inks, aqueous based Cu inks for inkjet printing have not yet been reported due to the enhanced surface oxidation of the copper nanoparticle in aqueous ink, despite that the synthesis of the Cu nanoparticles in water has recently been reported.35,36 The conductivity of a printed Cu pattern is highly dependent on the thickness of the surface oxide layer,14 and suppression of the formation of the surface oxide layer in aqueous synthesis in the level of electronic application is challenging. Recently, it was also reported that the aqueous synthesized Ag-capped Cu nanoparticles endowed oxidation stability due to the densely surrounding Ag shell layer. However, an additional synthesis step is required to introduce the shell layer, and the Ag shell layer is unstable under the elevated temperature required in the sintering process.16 In this study, the first aqueous based conductive Cu ink using water-soluble Cu nanoparticles is reported, and the aqueous based Cu ink for printing highly conductive patterns with uniform morphologies and narrow line width is designed based on the investigation of the influence of the solvent composition on the morphological and electrical properties of the printed pattern. In addition, the long-term stability of the prepared Cu ink is analyzed based on X-ray photoelectron spectroscopy (XPS) for observing the evolution of the surface oxide layer and the variation of conductivity of the printed Cu pattern.

’ MATERIALS AND METHODS Synthesis of Water-Soluble Cu Nanoparticle. Cu nanoparticles were synthesized via chemical reduction of Cu ions in toluene (C6H5CH3, Aldrich, anhydrous 99.8%) under inert atmosphere. To prevent interparticle agglomeration and surface oxidation, oleic acid (C18H34O2, Aldrich, 99%) was incorporated as a surface capping molecule and hydrazine (NH2NH2, Junsei, 98%) was used as a reducing agent. Amounts of 16.6 g of Cu acetate (Cu(CO2CH3)2, Aldrich, 98%), 10 g of oleic acid, and 16.6 g of hydrazine were added into a three-neck round-bottomed flask containing 100 mL of toluene. The flask was fitted with a reflux condenser and a mechanical stirrer. The solution was purged with nitrogen for at least 30 min and then heated to 100 °C. Reaction was continued for 120 min and then cooled to room temperature. Then the synthesized Cu nanoparticles were separated by centrifugation and washed with ethanol. The resulting Cu nanoparticles were well-dispersed in toluene but not in aqueous medium. To endow water-compatibility, we modified the surfaces of synthesized Cu nanoparticles by dipping the Cu nanoparticles in a methanol based solution in which 6 wt % of carboxyl-terminated anionic polyelectrolyte and 16 wt % of a mixture of polyoxylethylene oleylamine ether (Wako, Mw = 1,000) and oxalic acid (C2H2O4, Aldrich, 99%) are dissolved. After reaction for 30 min under nitrogen atmosphere, the surface-modified Cu

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Table 1. Physical Properties Such as Surface Tension, Boiling Point, and Viscosity for Conductive Cu Inks and Various Solvents boiling

surface tension

viscosity

point (°C)

(mN/m)

(mPa 3 s)

DI water

100

72.8

1

2-methoxyethanol

125

27.2

1.6

glycerol

182

64

ethylene glycol EG-free Cu ink

198

48.5 41.2

19 4.8

EG-added Cu ink

853.1

37.3

5.3

toluene

110

27.9

0.6

hexane

69

18.4

0.3

253

26.6

2.2

tetradecane

nanoparticles were separated by centrifugation and washed with methanol, and the resulting Cu nanoparticles were easily well-dispersed in DI water.

Preparation of Aqueous Based Cu Nanoparticle Ink and Its Printing on a Plastic Substrate. For preparation of the Cu conductive ink, the obtained Cu nanoparticles were dispersed in DI water, and 2-methoxyethanol (C3H8O2, Aldrich, 99%) and glycerol (C3H5(OH)3, Aldrich, >99.5%) were added to reduce the surface tension and evaporation rate of the aqueous based Cu ink, respectively. The possible solvent compositions of 2-methoxyethanol and glycerol were 20-60 and 5 vol%, respectively. The solvent composition of Cu ink, denoted as EG-free ink in this study, was DI water:2-methxyethanol: glycerol = 75:20:5 vol%. In another case, ethylene glycol (EG, C2H6O2, Aldrich, 99%) was additionally incorporated as another cosolvent. The possible solvent compositions of 2-methoxyethanol, glycerol, and ethylene glycol were 10-25, 5, and 10-45 vol%, respectively. The solvent composition of Cu ink, denoted as EG-added ink in this study, was DI water:2-methoxyethanol:glycerol:ethylene glycol = 50:20:5:25 vol%. The viscosity and surface tension of both EG-free and EG-added ink are summarized in Table 1. The solid loading for both inks was 40 wt %. Then the prepared inks were subjected to ball milling for 1 h, and the Cu conductive ink was printed on a polyimide substrate (Kapton, Dupont) using an inkjet printer. Since both Cu inks exhibit the contact angle of 40-45° appropriate for the narrow and uniform printed patterns, the surface of the polyimide substrate was not modified. The substrate temperature was maintained at 25 °C. The printer setup is composed of a drop-on-demand piezoelectric inkjet nozzle manufactured by Microfab Technologies, Inc. (Plano, TX), and a nozzle with an orifice diameter of 30 μm was used. The inkjet printed Cu nanoparticulate films were annealed for 30 min at various temperatures from 100 to 350 °C under vacuum. The annealing time is also an important factor for the conductivity of the printed conductive patterns. However, in order to study the influence of annealing temperature on the electrical performance of the printed Cu patterns and rule out the effect of annealing time, we annealed the printed Cu pattern for 30 min. Characterizations. The size and shape of the synthesized Cu nanoparticles and the microstructures of the Cu patterns annealed at different temperatures were observed by scanning electron microscopy (SEM, JSM-6700, JEOL) and transmission electron microscopy (TEM, JEM-4010, JEOL). The crystal structure of Cu nanoparticles was analyzed using an X-ray diffractometer (D/MAX-2200 V, Rigaku), and chemical structural analyses of Cu nanoparticles and printed Cu patterns were performed with X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Fisher Scientific). The contact angle was measured with a dynamic contact angle system (SEO 300, SEO), and the thermal behavior of Cu nanoparticles was monitored using thermal gravimetric analysis (SDT2960, TA Instruments) with the heating rate of 5 °C/min. 3145

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Figure 1. (a) SEM and (b) TEM images of the synthesized watersoluble Cu nanoparticles. The dashed lines in the TEM image indicate the amorphous-like layer, which is composed of the surface oxide layer and capping molecules. The morphology and resistivity of the printed Cu patterns were analyzed using laser scanning confocal microscopy (LEXT OLS3000, Olympus) and a four point probe station equipped with a semiconductor characterization system (Keithley 4200, Keithley), respectively.

’ RESULTS AND DISCUSSION Synthesis of Air-Stable and Water-Soluble Cu Nanoparticles. To synthesize the water-soluble Cu nanoparticles, the

surface capping molecules, which are compatible with H2O, should surround the Cu nanoparticles. The capping molecules act as a dispersant for preventing interparticle agglomeration and as a protecting layer for preventing oxidation of the surface Cu atom. The facile way to obtain water-soluble Cu nanoparticles is by synthesizing the Cu nanoparticles in an aqueous medium with the appropriate capping molecules. However, Cu is easily oxidized in aqueous medium, even at room temperature. If the proper protection layer against oxidation is not incorporated during the synthesis procedure or the surface capping kinetics are not sophisticatedly controlled, the Cu nuclei are oxidized prior to the surface capping by the protection layer. Alternatively, in this study, the Cu nanoparticles were synthesized in an organic solvent medium where the oxidation of Cu nanoparticles is suppressed to some extent, and then the surfaces of the resulting Cu nanoparticles were functionalized with water-soluble capping molecules. As shown in Figure 1, the size of the synthesized Cu nanoparticle is ∼40 nm, and the Cu nanoparticles are surrounded by a 4 nm thick amorphous layer. The formation of a thin oxide layer is inevitable (see the phase diagram shown in Figure S1 in the Supporting Information), and the oxide is not crystallized without an additional annealing step; thus, it is speculated that the amorphous layer is composed of a surface oxide layer as well as capping molecules. For more in-depth investigation on how thick surface oxide layer is present, XPS analysis (Figure 2a) was performed. According to the X-ray diffraction (XRD) analysis, it appears that the synthesized Cu nanoparticles consist of a pure metal phase (Supporting Information Figure S2). However, the present surface oxide layer is amorphous and the volume of surface oxide layer is quite small; thus, XRD is not suitable for analyzing the surface oxide layer. In the XPS spectra of Cu 2p3/2, the peaks centered at ∼932.1, 932.6, and 934.6 eV are assigned to Cu, Cu2O, and CuO, respectively. Since CuO is a thermodynamically stable phase and it has been reported that CuO is partially converted to Cu2O under X-ray irradiation during XPS analysis, the observed Cu2O is not believed to be an inherent phase.37,38 The Cu2O phase can be stable when the particle size is smaller

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Figure 2. (a) XPS Cu 2p spectra and (b) XPS based semiquantitative analysis of the Cu nanoparticles stored for the indicated number of days in ambient air. CuxO denotes the oxide phase including both CuO and Cu2O.

than 25 nm.39,40 However, considering the size range of the Cu nanoparticles synthesized in this study, it could be assumed that a size-induced distinct phenomenon does not occur. Based on the semiquantitative analysis of each phase (Figure 2b), the surface oxide layer exists with an atomic fraction as small as 0.15. Taking into consideration the measurement depth of the XPS analysis, it is presumed that an ultrathin surface oxide layer is present and most of the amorphous layer volume consists of capping molecules. The atomic fraction of CuxO does not significantly vary even after exposure in air for 1 month, indicative of its longterm antioxidation stability in air. At the beginning, the atomic fraction of the surface oxide increased slightly and then reached a plateau. This indicates that the resulting surface oxide layer acts as another passivation layer against oxidation together with the capping molecules which effectively reduces the susceptibility of the Cu atom to oxidation. Preparation of Conductive Cu Nanoparticle Ink. In preparing the aqueous ink using water-soluble Cu nanoparticles, the design of a solvent composition that can meet various requirements, such as stable jetting behavior, narrow line width, uniform surface morphology, and high conductivity, is crucial. First of all, for a stable jetting characteristic, the surface energy of the aqueous based ink should decrease to prevent the formation of satellites and the evaporation rate needs to be suppressed to prevent the nozzle from clogging due to solvent evaporation along the nozzle orifice. In this study, 2-methoxyethanol was incorporated as a cosolvent in order to adjust the surface energy of the aqueous ink and glycerol to suppress the evaporation rate (this ink is denoted as EG-free ink). Other alcohols and watermiscible solvents with low vapor pressures can also be used. The physical properties of the prepared Cu ink and the various solvents used in this study are summarized in Table 1. Surface morphology and conductivity also strongly depend on the physical properties of the cosolvent added to the aqueous solvent system (i.e., the hydrodynamic motions of the solvents in the droplet deposited on the substrate). During evaporation of the solvents incorporated in the printed droplet, while the solvent molecules that evaporated at the center of the droplet are reabsorbed, the solvent molecules from the edge can escape easily without the reabsorption process. This enhanced evaporation rate around the edge leads to an outward convective flow that compensates the liquid removed by evaporation and in turn transports the suspended particles to the edge region.41-43 When using a mixture of low and high boiling point liquids, 3146

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Figure 3. Confocal microscope images showing the surface morphology and SEM images representing the microstructure of Cu features printed using (a) EG-free ink and (b) EG-added ink. (c) Photograph of the complex Cu patterns printed using EG-added ink, and (d) contact angle of the Cu inks and various solvents on a polyimide substrate. All printed lines were annealed at 250 °C.

the solvent composition at the contact line will shift toward a higher fraction of the high boiling point solvent than in the bulk due to the fast evaporation of the low boiling point solvent at the edges. This compositional gradient induces a Marangoni flow from the regions with low surface tensions to those with high surface tensions.11,29-33 In the case of the 2-methoxyethanol added aqueous solvent system, the boiling point (or vapor pressure) of the main solvent and cosolvent is similar, so that the compositional gradient along the droplet surface does not significantly occur and the aforementioned outward convective flow is dominant, which results in an irregular surface morphology (Figure 3a). However, when ethylene glycol (EG) is added as a cosolvent, the Marangoni flow is induced from the center to the edge because the low boiling point solvent (water) has a higher surface tension than the high boiling point solvent (EG) (this ink is denoted as EG-added ink). As long as the boiling point is higher and surface tension is lower than that of DI water, other watermiscible cosolvents can also be added. The optimum amount of cosolvent varies depending on the solvents, since the magnitude of the Marangoni flow is primarily determined by the viscosity of solvent mixture and the surface tension difference between each solvent.11,29-33 This inward Marangoni flow is opposite to the outward convective flow, and the migration of suspending particles toward the droplet edge is suppressed. As shown in Figure 3b, the surface morphology is dramatically improved with a line width as narrow as 45 μm by simply adjusting the solvent composition, and complex Cu patterns were successfully printed on a large-area polyimide substrate (Figure 3c). All conductive Cu patterns printed using the inks tested in this study (the composition of EG: 15-45 vol %) showed the narrow and uniform morphologies, which indicates 15 vol % of EG is enough to develop the inward Marangoni flow. This narrow Cu feature

reveals that water is an appropriate primary solvent for obtaining a narrow printed pattern, since the liquid-solid interface with a high contact angle is easily achieved due to its high surface tension. In general, a high contact angle below 50° facilitates the formation of the narrow printed pattern. The diameter of the printed dots diminishes inversely with the contact angle and the dots printed on the substrate with a contact angle above 50° cannot merge uniformly into a line feature. In contrast, the organic solvents, which have been commonly used in conventional conductive inks, have a low surface energy and exhibit extremely hydrophilic wetting behavior, as shown in Figure 3d. The introduction of the inward flow in the droplet also endows additional controllability on the line width of the printed pattern. When the outward convective flow is dominant, evaporation proceeds with an accompanying decrease in the contact angle, but with almost no change in the contact radius,41 which results in a wide pattern. However, when the additional inward flow compensates for the outward convective flow, the contact line recedes freely with a volumetric shrinkage during the evaporation of solvent,43 enabling the formation of a narrow pattern. According to the contact angles of the EG-free and EG-added Cu inks (EG-free Cu ink, 50°; EG-added Cu ink, 45°), the line width of the patterns printed using both inks is expected to be almost identical. However, the EG-added ink, where an inward flow takes place, yielded a much narrower conductive Cu pattern, as shown in Figure S3 in the Supporting Information. The line widths of the patterns printed using the EG-added and EG-free ink are 45 and 100 μm, respectively. Proper control of the suspended particle’s migration also affects the electrical properties of the printed conductive pattern itself. Whereas the resistivity of the Cu pattern printed using the EG-added ink was measured to be 11 μΩ 3 cm after annealing at 250 °C, the resistivity of the Cu pattern obtained from the 3147

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Figure 4. Resistivity variation of the Cu films either printed or casted using the EG-added ink as a function of the annealing temperature. The as-synthesized Cu nanoparticles were used for ink formulation.

EG-free ink was 38 μΩ 3 cm. The as-printed Cu pattern is composed of individually isolated particles, which are converted to a continuous film by annealing under vacuum, reduction, or inert atmosphere. This thermally induced sintering process occurs via diffusion of the surface atoms into an interparticular junction, and the packing structure of the nanoparticles after drying determines the interparticular junction density, which is directly related to the conductivity after the sintering process. For the printed pattern with an irregular surface morphology, the overall packing structure is poor due to the uneven lateral distribution of the particles by migration toward the droplet edge, so that the probability for the formation of interparticular connections via the sintering process would be low, resulting in a relatively porous microstructure. As shown in the SEM images in Figure 3, the microstructures of the conductive pattern printed using the EG-free ink is much more porous, compared with the pattern printed using the EG-added ink. In addition, there are more locally agglomerated structures and largely porous microstructures in the center region of the pattern printed using the EG-free ink, where the particles move away due to the outward hydrodynamic flow, which reveals that the overall microstructure of the printed pattern is highly influenced by the particles’ migration during the solvent evaporation in the printed droplet. Figure 4 shows the variations in the resistivity of the Cu patterns printed using the EG-added ink as a function of the annealing temperature. As the annealing temperature increased to 250 °C, the resistivity of the printed pattern drastically decreased and the resistivity did not change at temperatures from 250 to 350 °C. This temperature dependence of the resistivity on the annealing temperature is associated with thermal decomposition of the capping molecules adsorbed to Cu surface, which partially proceeds at 250 °C and is complete above 300 °C as shown in Figure S4 in the Supporting Information. Note that the resistivity of the printed pattern is almost identical to that of the casted film. While the solvents in the casted film are uniformly dried along the overall wet film surface, the solvent drying in the inkjet printed pattern is dynamically varied depending on the solvent composition, as mentioned above. If the hydrodynamic flow of the solvents in the printed pattern is not adequately adjusted, the packing structure of the particles worsens, resulting in a relatively resistive printed pattern. Thus, it is believed that the resistivity of 11 μΩ 3 cm is the best value that can be obtained using Cu nanoparticles synthesized in this study and all parameters in ink preparation are well optimized.

Figure 5. (a) XPS Cu 2p spectra, (b) XPS based semiquantitative analysis, and (c) resistivity variation of the Cu particulate film printed using the EG-added ink as a function of the Cu nanoparticles’ residence time in aqueous based ink. The annealing was performed at 250 °C.

Long-Term Stability of Conductive Cu Nanoparticle Ink. The long-term antioxidation stability of Cu nanoparticles in aqueous based ink was analyzed based on an XPS analysis of the Cu film prepared using Cu nanoparticles stored in aqueous based ink for different time frames up to 1 month (Figure 5a). According to the semiquantitative analysis shown in Figure 5b, the fraction of Cu and CuxO did not vary for 1 month, which means the formation of an additional oxide layer in the aqueous medium is prevented by a proper protection layer composed of capping molecules and a surface oxide layer. The long-term antioxidation stability was also confirmed by measuring the resistivity of the Cu film prepared using Cu nanoparticles stored in aqueous based ink for different numbers of days. The resistivity of the printed pattern at a given annealing temperature is dependent on the particle size, volume fraction of the surface oxide, and packing structure of the particles influenced by the dispersion stability and hydrodynamic flow in the printed droplet. Among these factors, the time-dependent factors are the dispersion stability and volume fraction of the surface oxide. However, the Cu conductive ink prepared in this study exhibits long-term dispersion stability over 2 months. As described previously, the dispersion stability in the aqueous ink could be easily obtained by choosing the proper dispersant that can give rise to electrosteric repulsion. To monitor the long-term dispersion stability, a sedimentation test was performed and the solid loading of the Cu nanoparticles suspended in ink was analyzed. No precipitates were observed for 2 months, and the solid loading was measured to be 38.8 wt % 2 months after the ink preparation (initial solid loading was 40 wt %). Therefore, the critical factor for long-term stability in resistivity is believed to be the evolution of an additional oxide layer in the ink. If the watersoluble Cu nanoparticle is not entirely protected from the surroundings, the oxidation of the surface Cu atom might proceed in a kinetically controlled manner, so that the conductivity of the 3148

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Langmuir printed pattern gradually deteriorates. However, as shown in Figure 5c, the resistivity was not significantly changed as a function of the Cu nanoparticles’ residence time in aqueous based ink, which is in line with the aforementioned fact that Cu nanoparticles are fairly stable in aqueous medium. This indicates that the aqueous based Cu ink prepared in this study exhibits excellent stability against surface oxidation and dispersion stability, facilitating well-defined highly conductive Cu printed patterns. This study represents the first attempt to directly inkjet print aqueous based conductive Cu ink, which allows highly conductive complex Cu patterns with narrow line widths and well controlled surface morphologies. This cost-effective, environmentally friendly, and high performance conductive Cu ink is expected to provide a convenient and low cost method for fabricating conductive features that can be adopted in various fields including flexible electronics, modern electronics, optoelectronics, and photovoltaic applications.

’ CONCLUSION The first aqueous based functional Cu ink was prepared using air-stable water-soluble Cu nanoparticles with ultrathin surface oxide layers, and the solvent composition of aqueous Cu ink was tailored for obtaining stable jetting behavior, narrow line width, uniform surface morphology, and high conductivity. By doing so, the highly conductive complex Cu features with a resistivity as low as 11 μΩ 3 cm, a line width as narrow as 45 μm, and a well controlled surface morphology were successfully inkjet printed on a flexible substrate. In addition, it was demonstrated that Cu nanoparticles are stable against surface oxidation in the aqueous based ink. ’ ASSOCIATED CONTENT

bS

Supporting Information. Thermodynamic diagram of Cu, Cu2O, and CuO as a function of oxygen partial pressure and temperature, XRD analysis of water-soluble Cu nanoparticles, optical microscope images of Cu conductive patterns, and thermal gravimetric analysis curves of Cu nanoparticles. This material is available free of charge via Internet at http://pubs. acs.org.

’ AUTHOR INFORMATION *E-mail: [email protected] (S.J.); [email protected] (B.-H.R.).

’ ACKNOWLEDGMENT This work has been supported by a grant from the Industrial Source Technology Development Program funded by the Ministry of Knowledge and Economy, Republic of Korea. ’ REFERENCES (1) Park, H. J.; Kang, M.-G.; Ahn, S. H.; Guo, L. J. Adv. Mater. 2010, 22, E247. (2) Chou, W.-Y.; Chang, M.-H.; Cheng, H.-L.; Yu, S.-P.; Lee, Y.-C.; Chiu, C.-Y.; Lee, C.-Y.; Shu, D.-Y. Appl. Phys. Lett. 2010, 96, 083305. (3) Duan, X.; Zhao, Y.; Perl, A.; Berenschot, E.; Reinhoudt, D. N.; Huskens, J. Adv. Funct. Mater. 2010, 20, 663. (4) Jeong, S.; Lee, S.; Kim, D.; Shin, H.; Moon, J. J. Phys. Chem. C 2007, 111, 16083. (5) Liu, P.; Wu, Y.; Li, Y.; Ong, B. S.; Zhu, S. J. Am. Chem. Soc. 2006, 128, 4554. (6) Li, Y.; Wu, Y.; Ong, B. S. J. Am. Chem. Soc. 2005, 127, 3266.

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