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Three-dimensional Printing of Silver Microarchitectures Using Newtonian Nanoparticle Inks Sanghyeon Lee,†,§,⊥ Jung Hyun Kim,†,⊥ Muhammad Wajahat,†,‡ Hwakyung Jeong,† Won Suk Chang,†,§ Sung Ho Cho,§ Ji Tae Kim,*,∥ and Seung Kwon Seol*,†,‡ †
Nano Hybrid Technology Research Center, Korea Electrotechnology Research Institute (KERI), Changwon-si, Gyeongsangnam-do 51543, Republic of Korea ‡ Electrical Functional Material Engineering, Korea University of Science and Technology (UST), Changwon-si, Gyeongsangnam-do 51543, Republic of Korea § Department of Electronics and Computer Engineering, Hanyang University, Seoul 04763, Republic of Korea ∥ Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, China S Supporting Information *
ABSTRACT: Although three-dimensional (3D) printing has recently emerged as a technology to potentially bring about the next industrial revolution, the limited selection of usable materials restricts its use to simple prototyping. In particular, metallic 3D printing with submicrometer spatial resolution is essential for the realization of 3D-printed electronics. Herein, a meniscus-guided 3D printing method that exploits a low-viscosity (∼7 mPa·s) silver nanoparticle (AgNP) ink meniscus with Newtonian fluid characteristics (which is compatible with conventional inkjet printers) to fabricate 3D silver microarchitectures is reported. Poly(acrylic acid)-capped AgNP ink that exhibits a continuous ink flow through a confined nozzle without aggregation is designed in this study. Guiding the ink meniscus with controlled direction and speed enables both vertical pulling and layer-by-layer processing, resulting in the creation of 3D microobjects with designed shapes other than those for simple wiring. Various highly conductive (>104 S·cm−1) 3D metallic patterns are demonstrated for applications in electronic devices. This research is expected to widen the range of materials that can be employed in 3D printing technology, with the aim of moving 3D printing beyond prototyping and into real manufacturing platforms for future electronics. KEYWORDS: 3D printing, 3D-printed electronics, meniscus-guided printing, silver microarchitecture, Newtonian fluid ink
1. INTRODUCTION “Printed electronics” is the term used to describe electronic circuits and devices, such as radio-frequency identification tag antennas, sensors, transistors, batteries, and conductive traces, that are manufactured via conventional printing techniques.1−7 Techniques such as screen, gravure, and inkjet printing offer rapid and inexpensive ways of fabricating electrical circuits on flexible substrates.8−11 To realize multifunction printed electronics, various nanoparticles (NPs) made up of ceramics, polymers, or metals have been used as printing inks. In particular, silver NPs (AgNPs) have emerged as a promising material for electrically conductive micropatterns and nanopatterns, which are essential components for electronic circuits and devices.12−17 However, such patterns are still limited to two-dimensional forms when printed using gravure or inkjet techniques. Three-dimensional (3D) printing, also known as additive manufacturing, is emerging as an innovative technique to create complex objects via a simple and rapid approach.18 Recently, 3D printing has resulted in a new device concept called “3Dprinted electronics”.19−22 Although the advantages offered by © 2017 American Chemical Society
3D electronic systems, such as enhanced integration density and the creation of new functions, are well known, practical realization still faces a number of technological challenges involving complicated and expensive fabrication procedures. Three-dimensional printing methods based on layer-by-layer material addition could potentially provide a direct and less constrained way for realizing 3D electronic devices. However, because most efforts in 3D printing research have been devoted to the study of sufficient mechanical strengths in 3D-printed structures, new methods and materials are still in great demand for real 3D electronics. Conventional methods, such as stereolithography (SLA), fused deposition modeling (FDM), and selective laser sintering (SLS), restrict their use to only a few material types, with coarse spatial precision.23−27 Ladd et al. demonstrated 3D printing of conductive structures composed of a low-viscosity liquid metal at room temperature.28 However, this method has disadvantages, including difficulties in the Received: February 21, 2017 Accepted: May 16, 2017 Published: May 25, 2017 18918
DOI: 10.1021/acsami.7b02581 ACS Appl. Mater. Interfaces 2017, 9, 18918−18924
Research Article
ACS Applied Materials & Interfaces
and pulling rate during the 3D printing were accurately controlled using three-axis stepping motors with 250 nm positioning accuracy. The growth process was observed in situ using a high-resolution monitoring system consisting of an optical lens (200×) and a chargecoupled device camera (Spot RT Xplore). Platinum-coated Si wafers were used as substrates for the Ag structures. The as-printed Ag structures were annealed at 250 °C in air. 2.4. Characterization of Ag Microarchitectures. The microscopic characteristics of the Ag structures were analyzed by field emission scanning electron microscopy (FE-SEM; Hitachi S-4800). The electrical resistivity values of the individual Ag microbridges were measured using a two-probe method, with a Keithley 2612A instrument, at room temperature. A Ag interconnector was connected to the light-emitting diodes (LEDs) (any vendor, 1005 smd LED) by consecutive bridges with different arch radii.
fabrication of delicate structures and the high cost of liquid metals. The use of functional NP inks has opened new avenues for realizing 3D-printed electronics. For example, an extrusionbased 3D printing method29−39 using a pressurized nozzle to write highly viscous inks on substrates or in midair has demonstrated diverse functional 3D systems on the microscale for electronics,33−35 photonics,36,37 and biomedicine.38,39 Because this extrusion method utilizes the precisely controlled rheological properties of inks with a high viscosity (∼105 mPa· s) for retaining 3D features,33−35,40 a high pressure must be persistently applied across a fine nozzle during the printing process to prevent clogging, potentially limiting the possible print sizes. In this work, we report on a simple and field-free meniscusguided method for printing 3D metallic microarchitectures, using a Newtonian AgNP ink. The method uses a fluid-like ink meniscus formed on a micronozzle to transport AgNPs onto target positions, and rapid solidification induced by precipitation and evaporation creates 3D structures on the microscale without the need to apply pressure. To realize continuous ink flow through a confined nozzle geometry, we designed poly(acrylic acid)-capped AgNP (PAA-AgNP) ink, with Newtonian fluid behavior and a low viscosity of ∼7 mPa·s, synthesized by microwave rapid heating. We printed highly conductive 3D microstructures (up to 104 S·cm−1) by programmed guiding of the meniscus, using horizontal and vertical pulling processes, with the aid of post-thermal treatment. Successful proofs of concept, such as freestanding pillar arrays, pyramids, and colosseum structures, are demonstrated herein.
3. RESULTS AND DISCUSSION Figure 1a depicts meniscus-guided 3D printing of a Ag micropyramid. When the nozzle made contact with the substrate in the initial printing stage, an ink meniscus
2. EXPERIMENTAL SECTION 2.1. Synthesis and Characterization of AgNPs. AgNPs were synthesized by dissolving 0.025 g of poly(acrylic acid) (PAA) solution (25 wt % polymer in water, molecular weight = 50 000 g·mol−1; Polysciences Inc.) and 2 g of diethanolamine (DEA; Sigma-Aldrich) in 2.5 mL of distilled water while stirring for 2 h in a water bath at room temperature. A silver nitrate solution (1 g of AgNO3 in 1 mL of distilled water; Deajung Chem.) was added to this solution, and the solution mixture was heated using a microwave synthesizer (CEM Discover system; CEM Corp.). The solution temperature was held at 75 °C for 60 s using continuous power at 2.45 GHz (which can supply a maximum power of 200 W), and the temperature was maintained at 60 °C for 1 h using variable power values of 1−5 W. The synthesized AgNPs were concentrated by titrating 50 mL of ethanol and centrifuging at 9000 rpm for 20 min. Coagulated AgNPs were dried at 40 °C for 8 h to eliminate residual ethanol and solvent. UV−vis absorption spectra of the synthesized AgNPs were recorded using a spectrophotometer (Agilent Cary 5000; Agilent Tech.), and the morphology and size of the AgNPs were measured by transmission electron microscopy (TEM, JEM-2000FXII; JEOL Ltd.). 2.2. Preparation and Characterization of PAA-Ag Inks. PAAAg inks were prepared by dilution of different concentrations of PAAAgNP (5, 10, 15, 20, 25 wt %) in distilled water. The ink viscosity was characterized using a cone-and-plate rheometer (MCR102; Anton Paar) and at a shear rate range of 101−103 s−1. 2.3. Three-dimensional Printing of Ag Microarchitectures. Three-dimensional Ag microarchitectures were fabricated at the meniscus formed at the tip of a micronozzle filled with an aqueous ink (25 wt % PAA-AgNP). Glass micronozzles with opening diameters (Dn) of approximately 5 and 15 μm were obtained using a nozzle puller (P-97; Sutter Instruments). The ink was filled from the back of the micronozzle and drawn to the front tip by capillary forces without applied pressure. The initial gap between the nozzle and the substrate was set as 1 μm for all results discussed here. The micronozzle position
Figure 1. Meniscus-guided 3D printing in a layer-by-layer manner. (a) Schematic illustration of layer-by-layer 3D printing of Ag micropyramids by exploiting a low-viscosity (∼7 mPa·s) liquid-ink meniscus. A localized PAA-AgNPs additive process in the meniscus and a rapid solidification of the meniscus induced by evaporation allows for the formation of freestanding structures. (b) A micropyramid of 45 μm height is fabricated using this meniscus-guided printing method (υh = 5 μm·s−1, Dn = 10 μm). 18919
DOI: 10.1021/acsami.7b02581 ACS Appl. Mater. Interfaces 2017, 9, 18918−18924
Research Article
ACS Applied Materials & Interfaces
Figure 2. Microwave-assisted synthesis of PAA-AgNPs. (a) Schematic diagram showing spontaneous microwave-assisted synthesis of PAA-AgNPs. (b) TEM image of PAA-AgNPs obtained with 60 min reaction time. Inset: image of PAA-AgNP pellets with a high concentration. (c) Graph showing detailed parameters of the microwave-assisted synthesis. A 200 W microwave pulse is required to rapidly increase the reaction temperature up to 75 °C. After the overheating process, a constant temperature of 60 °C is maintained by applying additional microwave pulses of 1−5 W. (d) UV−vis absorption spectra of PAA-AgNPs synthesized, with different reaction times of 0, 30, and 60 min.
groups (COO−), was to prevent nanoparticle aggregation by a combination of electrostatic and steric repulsions.44−46 The detailed procedure for ink preparation is described in the Experimental Section. We designed polydispersed PAA-AgNPs for highly conductive ink, as size polydispersity leads to more efficient packing, resulting in enhanced electrical conductivity of the printed structures.47,48 A TEM image in Figure 2b shows that the size distribution of the synthesized particles from a simple one-step process is broad (10−100 nm), and their average size is approximately 20 nm. Figure 2c shows the profile of the applied microwave power (blue line) and the corresponding temperature of the solution containing a mixture of AgNO3, DEA, and PAA in water. A single microwave pulse (200 W power) was applied for the nucleation of AgNPs while rapidly increasing the solution temperature to 75 °C (ramping rate: 50 °C·min−1). Subsequently, the nuclei were grown at a constant temperature of 60 °C obtained using applied microwave pulses 1−5 W in power. UV−vis absorption spectra (Figure 2d) show the growth of AgNPs as a function of reaction time. As the reaction time increases from 30 to 60 min, the absorption intensity increases; the maximum peak position, λmax, shifts from 420 to 408 nm, and a shoulder peak appears at ∼490 nm. This indicates Ostwald ripening during growth, wherein larger
containing PAA-AgNPs was created outside its opening. The meniscus height was decided by a gap between the nozzle and the substrate. A liquid-ink meniscus created between the micronozzle and the substrate acts as a pen. Then, the PAAAgNPs are deposited in the meniscus, with precisely controlled volumes. Guiding the meniscus in three dimensions by a programmed motion of the nozzle creates 3D-deposited layers of PAA-AgNPs. Note that the evaporation of water (solvent) at the micrometer-sized meniscus, with a high surface/volume ratio plays an important role in the printing process. The evaporation induces not only transport of PAA-AgNPs from the nozzle to the meniscus but also rapid solidification, enabling continuous printing in three dimensions. Figure 1b shows an FE-SEM image of a hollow 3D-printed Ag micropyramid. The pyramid (45 μm height, 83.6° inclined angle) was printed in a layer-by-layer manner by overlaying, using repetitive horizontal pulling (υh = 5 μm·s−1) of the micronozzle, following the path of the blue arrow depicted in the inset of Figure 1b. For layerby-layer deposition, the nozzle was alternately pulled rightward and leftward, with a precise vertical movement (∼1 μm) of the nozzle between each pull. The PAA-AgNPs were synthesized by a microwave-assisted method41−43 that enables rapid production, as shown in Figure 2a. The role of PAA, with its negatively charged functional 18920
DOI: 10.1021/acsami.7b02581 ACS Appl. Mater. Interfaces 2017, 9, 18918−18924
Research Article
ACS Applied Materials & Interfaces particles increase in size at the expense of smaller particles. The TEM images in Figure S1 (Supporting Information (SI)) also show the difference in the size distribution between reaction times of 30 and 60 min. The inset of Figure 2b shows an optical image of concentrated pellets of the synthesized PAA-AgNPs obtained by a drying process at 30 °C for 12 h. To achieve uniform printing with continuous ink flow through a confined micronozzle geometry, engineering of the rheological properties of the nanoparticle ink is required. To this end, we designed a printable ink composed of the synthesized PAA-AgNPs and water. Figure 3 describes the rheological characteristics of inks having different PAA-AgNP concentrations. First, the viscosity of the ink remains constant as the shear rate increases from 0 to 103 s−1 (Figure 3a). As the PAA-AgNP concentration increases from 5 to 25 wt %, the viscosity increases from 1.4 to 6.8 mPa·s. The linear dependence of the ink’s shear stress on the shear rate between 0 and 103 s−1 indicates that the ink is a well-dispersed suspension with Newtonian characteristics (Figure 3b). Figure 3c shows that the regime of our ink’s viscosity at a shear rate of 200 s−1 is positioned near that of conventional inkjet printing ink, differing from other 3D printing nanoparticle inks with typical viscosity of ∼105 mPa·s. It is noteworthy that our meniscus-guided method is potentially capable of utilizing various types of inkjet printing inks for multiple-material 3D printing by simply modulating the ink formulation. The electrical properties of 3D-printed microstructures are very important for electronic device applications. Figure 4a shows the dependence of the electrical resistivity of Ag bridges on the thermal treatment employed. In general, a Newtonian fluid ink with low viscosity is regarded as being unfit for 3D printing because the ink immediately spreads upon exiting the nozzle and lacks self-support capacity. Conversely, we successfully printed 5 μm diameter Ag microbridges (Dn = 5 μm, vertical pulling rate (υv) = 10 μm·s−1) between platinum electrodes with 10 μm insulating gaps, using our designed inks, with different concentrations of PAA-AgNPs (5, 10, 15, 20, 25 wt %; see inset of Figure 4a). An annealing temperature of 250 °C is appropriate for both the decomposition of PAA15,49,50 and sintering of AgNPs.15,44,50 The printed 3D objects maintained their shapes, exhibiting a total volumetric shrinkage of ∼20% after thermal treatment (250 °C for 1 h in air). The resistivity of all printed bridges drastically decreased from 10° to
