Meniscus-on-Demand Parallel 3D Nanoprinting - ACS Nano (ACS

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Meniscus-on-Demand Parallel 3D Nanoprinting Mojun Chen, Zhaoyi Xu, Jung Hyun Kim, Seung Kwon Seol, and Ji Tae Kim ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b00706 • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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Meniscus-on-Demand Parallel 3D Nanoprinting Mojun Chen1, Zhaoyi Xu1, Jung Hyun Kim2,3, Seung Kwon Seol2,3, and Ji Tae Kim1,* 1

Department of Mechanical Engineering, The University of Hong Kong, Pokfulam road, Hong

Kong, China 2

Nano Hybrid Technology Research Center, Korea Electrotechnology Research Institute (KERI),

Changwon-si, Gyeongsangnam-do 51543, Republic of Korea 3

Electrical Functional Material Engineering, Korea University of Science and Technology (UST),

Changwon-si, Gyeongsangnam-do, 51543, Republic of Korea *Email: [email protected]

ABSTRACT Exploiting a femtoliter liquid meniscus formed on a nanopipette is a powerful approach to spatially control mass transfer or chemical reaction at the nanoscale. However, the insufficient reliability of techniques for the meniscus formation still restricts its practical use. We report on a noncontact, programmable method to produce a femtoliter liquid meniscus that is utilized for parallel three-dimensional (3D) nanoprinting. The method based on electrohydrodynamic dispensing enables one to create an ink meniscus at a pipette-substrate gap without physical contact and positional feedback. By guiding the meniscus under rapid evaporation of solvent in air, we successfully fabricate freestanding polymer 3D nanostructures. After a quantitative characterization of the experimental conditions, we show that we can use a multi-barrel pipette to achieve parallel fabrication process of clustered nanowires with precise placement. We expect this technique to advance productivity in nanoscale 3D printing.

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KEYWORDS: 3D printing, meniscus-guided fabrication, multi-barrel nanopipette, polymers, freestanding nanowires, electrohydrodynamic dispensing.


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Three-dimensional (3D) printing, a class of techniques known as additive manufacturing, is emerging as a disruptive technology that can transform our everyday life. 3D printing techniques can produce customized and complex objects via a simple manner, therefore they enable the fabrication of innovative products in electronics,1-5 robotics,6,7 medical engineering,811

architecture,12 and so on. However, relatively low spatial resolution and productivity and

limited material selection in conventional techniques such as fused deposition modeling (FDM) and stereo-lithography still restrict their practical applications.

One powerful approach to address these issues is the use of a nanopipette. Over the past decades, scientists have continually developed and utilized a glass nanopipette for diverse applications in condensed matter physics,13,14 chemistry,15-19 and biology.20-24 Because of its confined geometry that allows the precise spatiotemporal control of mass transfer and chemical reactions, a nanopipette enables local analysis and delivery of materials. Examples include surface patterning,16,25-27 mapping topography15/electrochemical reactivity,17,18 and singlemolecule detection22-24 /manipulation.13,14,28 Recently, the application of a nanopipette has also extended to 3D micro- and nanoprinting due to the capability of continuous material supply from its large reservoir.29-35

Two types of working mechanisms have been exploited in the nanopipette-assisted 3D printing: the uses of (1) a localized material flux through a nanopipette in solution29,30 and (2) a tiny liquid meniscus formed at a pipette-substrate gap in air.31-34 The former provides a high degree of control over shape, however, suffers from limited spatial resolution due to diffusional broadening in solution. Consequently, the diameter of grown structures tends to be larger than


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the nanopipette aperture. On the other hand, in the latter, material transfer is highly confined at the geometry of a femtoliter-sized meniscus made of liquid/air interface. In addition, the mechanical flexibility of the meniscus allows one to control and reduce the feature size down to sub-100 nm, smaller size than the pipette aperture, by a simple pulling process.32 However, one longstanding challenge remains. To date, the production of the meniscus has relied on either the physical contact of a fragile glass nanopipette to the substrate or the integrated positional feedback based on the detection of electrochemical current across the experimental system. The response of the positional feedback, when the meniscus is formed in air, requires careful and fast monitoring due to its drastic OFF/ON behavior, in contrast “gradual warning” in solution.27 In addition, the feedback system is not easy to apply for multiple nanopipette arrays due to alignment errors. Therefore, the production of the femtoliter meniscus with sufficient reliability remains a great challenge for scalable, high throughput 3D nanoprinting.

Here, we develop a noncontact, on-demand method that combines dispensing and guiding an ink meniscus by a nanopipette to realize parallel 3D nanoprinting. An electrohydrodynamic dispensing successfully produces a femtoliter-sized liquid meniscus at a gap between a glass nanopipette and a conductive substrate by applying a voltage of ~ 102 V without any physical contact or positional feedback. The formed meniscus can write various 3D nanostructures under rapid evaporation of solvent in ambient air. In this work, freestanding nanostructures of polymers are successfully fabricated with precise positioning. Furthermore, our method can use a multibarrel nanopipette to realize parallel 3D printing without significant changes in the experimental arrangement, resulting in the improvement of productivity at a low cost. To prove the printability in diverse materials, we test different materials such as poly(3,4-ethylenedioxythiophene)


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polystyrene sulfonate (PEDOT:PSS), polypyrrole (PPy), PEDOT:PSS/PPy composite, graphene oxide/polyvinylpyrrolidone (GO/PVP) composite, and non-conductive polystyrene (PS) (Supporting Information, Figure S1).

RESULTS AND DISCUSSION Figure 1 depicts the experimental procedure of our meniscus-on-demand 3D printing. First, an ink-filled glass nanopipette is placed at a separation of few micrometers from a silicon (Si) substrate (Figure 1a). The separation distance is precisely controlled by using a 3-axis linear motorized stage with sub-micrometer displacement precision. When a voltage in the range of ~ 102 V with a programmed pulse width is applied to the substrate, the electrostatic force (attractive) generates between the ink and substrate (Figure 1b), resulting in the formation of a femtoliter-volume ink meniscus at a pipette-substrate gap (Figure 1c) via noncontact electrohydrodynamic dispensing manner. The role of the formed meniscus is to deliver materials at confined region. Guiding the meniscus in three-dimension under rapid solidification by evaporation of solvent results in creation of 3D nanostructures with programmed shapes as shown in Figure 1d.

Real-time optical imaging confirms the concept of printing process (described in the METHODS). We image the existence of the meniscus by detecting scattered light in real-time, enabling quantitative investigation of the printing process. A heat-pulled glass nanopipette filled with PEDOT:PSS conjugated polymer ink is placed at a pipette-substrate gap of 3 µm (Figure 2a; Supporting Information, Movie S1). The PEDOT:PSS is dissolved in water/ethanol mixture (the volume ratio of PEDOT:PSS/water/ethanol is 1:30:30) with engineered surface


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tension, , γ ~ 28.88 mN/m, which is much lower than water (γ = 72.86 mN/m at 293 K), and viscosity, η ~ 1.1 mPa·s to achieve sufficient consistency in electrohydrodynamic dispensing. Note that such low viscosity of the ink may help to prevent clogging of a nanopipette. The average diameter of the nanopipette opening is ~ 600 nm shown as a field-emission scanning electron microscopy (FE-SEM) image in the inset of Figure 2a. A PEDOT:PSS ink meniscus is formed at the pipette-substrate gap when a voltage of 360 V is applied for 1.5 seconds (Figure 2b). The meniscus is mechanically stable and its extremely high surface-area-to-volume ratio allows rapid evaporation of solvent in ambient air, resulting in solidification. Figure 2c shows continuous solidification process of the meniscus by moving down the substrate with a constant speed of 5.0 µm/s. As a result, a straight PEDOT:PSS nanowire with 600 nm diameter is successfully obtained (Figure 2d). This overall process does not require any positional feedback system or physical contact of the pipette to the substrate. We believe this noncontact, feedbackfree approach would be able to improve productivity at a low cost. Traditional electrohydrodynamic jetting – another noncontact nanoprinting method – essentially requires electrostatic autofocusing by electrically conductive deposits (e.g. metals) in order to achieve out-of-plane growth, therefore the printable materials are limited.35 On the other hand, our meniscus-guiding approach enables continuous and high aspect ratio nanomaterial growth of diverse materials from polymers,32 carbon nanomaterials,34 to metals.33 Furthermore, this method can control nanowire’s diameter down to sub-100 nm by instant variation of the pulling speed, as successfully demonstrated in our previous works.32 However, further downsizing driven by fast pulling could be restricted by a Plateau-Rayleigh instability.


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One important question – how can a meniscus be obtained consistently? – motivates us to quantitatively investigate the experimental conditions. First, we investigate the dependence of the meniscus formation on the electrical energy when a DC voltage is applied. Figure 3a shows the dependence of the formation of a PEDOT:PSS ink meniscus on the interplay between applied voltage, V, and pulse length, t, at a constant pipette-substrate gap of 3 µm. The solid blue circles indicate the success cases of the meniscus formation, whereas the hollow circles indicate the absence of the meniscus. The blue solid fit indicates a threshold of the meniscus formation, corresponding to a functional dependence of V ~ t-0.5. This experimental result is well matched to the supplied electrical energy described36 as follows, = ⁄

(1)

Where is the supplied electrical energy, is the voltage, is the pulse length, and is the electrical resistance of the liquid ink which is assumed to be constant during the meniscus formation. This investigation shows the existence of minimum required electrical energy for the meniscus formation. In practical point of view, higher voltage and longer pulse length are effective. The force balance acting on the meniscus also needs to be considered. Note that the meniscus formation occurs only if electrostatic force is sufficient for overcoming surface tension of the ink. No meniscus formation was observed below a voltage of 350 V regardless of the pulse length in this experimental condition.

At this point, it is necessary to consider allowable pipette-substrate gaps. The sufficient range of the pipette-substrate gap for the meniscus formation would tackle a number of technological issues related to inaccuracies of motorized system and nanopipette alignment, which are critical for parallel configuration. We experimentally investigate attainable maximum


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pipette-substrate gaps allowing the meniscus formation. Figure 3b shows the dependence of the meniscus formation on the pipette-substrate gap at different DC voltages for sufficiently long t > ~ 10 sec. The meniscus formation relies on the electric field applied on the ink, determined by the interplay between applied voltages and pipette-substrate gaps. The red solid fit shows the dependence of the maximum gaps for the meniscus formation on applied voltages. The meniscus is consistently formed below the maximum gaps. As the voltage increases from 240 to 560 V, the maximum gap linearly increases from 1.275 to 6 µm, which is ~ 10 times larger than the pipette opening of 0.6 µm, providing extra tolerance in parallel meniscus formation by multiple nanopipettes. The obtained meniscus instantly retains its shape due to solidification by solvent evaporation. Our noncontact approach mitigates the need for highly precise positioning and feedback system in the nanopipette-assisted 3D printing techniques. Furthermore, the approach demonstrates parallel meniscus-guided 3D printing at the nanoscale (Figures 4 and 5).

Utilization of a produced meniscus by the noncontact manner facilitates omnidirectional nanoprinting. First, Figures 4a-c prove the high reproducibility in the fabrication. A set of 25 freestanding PPy nanowires with 25 µm spacing was successfully fabricated on a Si substrate (Figure 4a). Each grown nanowire exhibits a high aspect ratio of ~ 15, a uniform diameter of ~ 560 nm along the vertical axis, and a smooth surface (Figure 4b). The histogram in Figure 4c represents the size distribution of the nanowires (demonstrated in Figure 4a) fabricated under the same experimental condition. The average diameter of the body obtained is 560 nm with relative standard deviation of only 2.9 % and the foot size is 994 nm with 3.8 % relative standard deviation. No nanopipette breakage occurred during the whole 3D fabrication process despite the absence of any positional feedback system.


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Figures 4d and e show printing of GO/PVP composite by lateral guiding process. A mesh-like GO/PVP structure with 600 nm diameter and 25 µm distance is fabricated by programmed movement of the stage, as shown in Figure 4d. In addition, a fabricated nanowall with ~ 600 nm width and ~ 4200 nm height, consisting of 7 layers of GO/PVP nanowires (Figure 4e; Supporting Information, Figure S3), demonstrates an example of nanoscale layerby-layer additive manufacturing.

To prove the ability of our technique to extend into parallel 3D nanoprinting aiming at high throughput fabrication, we employ a three-barrel nanopipette as shown in Figure 5a. The use of multi-barrel pipettes takes advantages in two aspects: improvements of (1) printing speed and (2) integration density without a significant change in the printing machine. Single-pulsed voltage of 440 V for 3.0 sec simultaneously produces three individual menisci of the PEDOT:PSS/PPy composite ink. Subsequent pulling process results in the fabrication of three clustered nanowires with elliptical shape, as demonstrated in Figure 5b (the real-time printing process is described in Supporting Information, Movie S2). The overall fabrication time of these three nanowires is the same as that of the single one, resulting in the improvement of printing speed three times. Interestingly, a surface-to-surface distance of the adjacent freestanding nanowires obtained is ~ 680 nm that is smaller than the wire size of ~ 900 nm (Figure 5c). It is not easy to attain such narrow distance with high integration density in the serial printing using a single nanopipette because the pipette’s tapered geometry may bother approaching to the grown nanowire. In addition, the precise scanning capability in our nanopipette-assisted technique successfully produces an array of these nanowire clusters as


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shown in Figure 5d. Our result proposes that integrated nanopipette arrays (Note that sevenbarrel glass capillaries are currently commercialized.) could also be utilized for individually addressable multi-material, parallel 3D nanoprinting without the need for elaborate alignment and positional feedback. CONCLUSION In this work, we developed a meniscus-on-demand parallel 3D nanoprinting for freestanding nanowire arrays. Successful combination of the electrohydrodynamic dispensing and the meniscus guiding provided a noncontact, feedback-free, highly reproducible protocol to produce a femtoliter ink meniscus for fabricating various freestanding nanostructures. The application of our method could also extend into diverse materials at the micro and nanoscale if printing ink is engineered, as demonstrated in our previous works, for example, graphene,34 carbon nanotubes,5 and metals.33 We expect this work to provide a minimalist and cost-effective manner to improve the productivity in 3D printing, paving the way for becoming a realmanufacturing platform beyond prototyping. Although we focused on the development of the nanopipette-assisted fabrication process, our experimental platform could be used for exploring a number of fundamental questions related to nanoscopic evaporation dynamics,37 catalytic activity,17 and topography.

METHODS Sample preparation. Borosilicate glass nanopipettes (World Precision Instruments) with ~ 600 nm diameter were fabricated by a programmed heat-pulling process (P-97 Flaming/Brown Micropipette Puller, Sutter Instrument). The pipette diameters were characterized by a fieldemission scanning electron microscope (FE-SEM) (LEO 1530, Zeiss) installed at the HKU


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electron microscope unit. The nanopipettes and silicon (Si) wafers were cleaned by rinsing with acetone, isopropyl alcohol, and deionized water under sonication for 5 minutes each and an O2 plasma process for 5 minutes. For the conjugated polymer printing inks with engineered viscosity of a few mPa·s and surface tension of 25 ~ 30 mN/m, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) (Sigma Aldrich), polypyrrole (PPy) (Sigma Aldrich), and PEDOT:PSS/PPy composite in water/ethanol mixture were prepared. The volume ratio of polymer/water/ethanol in the inks was 1:30:30. Graphene oxide/polyvinylpyrrolidone (GO/PVP) ink was prepared by mixing GO (1 wt %) and PVP (10 wt %, Mw=10,000, Aldrich) in deionized water. The GO was prepared from natural graphite (Alfa Aesar, 99.999% purity, –200 mesh) by a modified Hummers method.38 The viscosity of the inks was characterized using a rheometer (MCR 302, Anton Paar) and the surface tension was measured by the pendant drop method. Three-dimensional printing. The position and speed of an ink-filled nanopipette in air were controlled with a 3-axis stepping motorized stage (NanoMax 343, Thorlabs) with submicrometer displacement capability. At a pipette-substrate gap of a few µm, a voltage in the range of ~ 102 V was applied to the Si substrate using a pulse generator (NI USB-6212, National Instruments) with an amplifier (AMJ-2B10, Matsusada Precision Inc.) for electrohydrodynamic dispensing of an ink meniscus. The printing process was imaged in real-time using a side-view optical microscope consisting of a long working distance objective (50×, 0.55 NA, Mitutoyo Plan Apo) and a CCD camera (DCC1545M, Thorlabs). The fabricated freestanding polymer nanowires were characterized using an FE-SEM.


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SUPPORTING INFORMATION Optical micrographs and FE-SEM images of PS printing, FE-SEM images of glass nanopipettes, an optical micrograph of layer-by-layer GO/PVP printing (PDF) Movie S1: Meniscus-on-demand 3D printing of a PEDOT:PSS nanowire (AVI) Movie S2: Parallel 3D printing of PEDOT:PSS/PPy composite nanowires (AVI) Movie S3: Meniscus-on-demand 3D printing of a graphene oxide-PVP composite nanowire (AVI)

ACKNOWLEDGMENTS This work is supported by the Early Career Scheme (HKU 27207517) from the Research Grants Council of Hong Kong, the Seed Fund for Basic Research from the University of Hong Kong (201611159002), and Korea Electrotechnology Research Institute (KERI) Primary research program through the National Research Council of Science & Technology (NST) funded from the Ministry of Science and ICT (No. 18-12-N0101-17). We thank A. Shum for experimental support.


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Hu, J.; Yu, M. F. Meniscus-Confined Three-Dimensional Electrodeposition for Direct Writing of Wire Bonds. Science 2010, 329, 313-316.

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Kim, J. T.; Seol, S. K.; Pyo, J.; Lee, J. S.; Je, J. H.; Margaritondo, G. Three-Dimensional Writing of Conducting Polymer Nanowire Arrays by Meniscus-Guided Polymerization. Adv. Mater. 2011, 23, 1968-1970.

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Lee, S.; Kim, J. H.; Wajahat, M.; Jeong, H.; Chang, W. S.; Cho, S. H.; Kim, J. T.; Seol, S. K. Three-dimensional Printing of Silver Microarchitectures Using Newtonian Nanoparticle Inks. ACS Appl. Mater. Interfaces 2017, 9, 18918-18924.


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Kim, J. H.; Chang, W. S.; Kim, D.; Yang, J. R.; Han, J. T.; Lee, G. W.; Kim, J. T.; Seol, S. K. 3D Printing of Reduced Graphene Oxide Nanowires. Adv. Mater. 2015, 27, 157-161.

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ACS Nano

Figure 1. Schematic illustration of noncontact, meniscus-on-demand three-dimensional (3D) nanoprinting. (a) An ink-filled glass nanopipette (diameter: ~ 600 nm) is approached the vicinity of a silicon (Si) substrate with a gap of ~ µm. (b) When a high voltage is applied to the Si substrate, an attractive electrostatic force between the charged substrate and the ink is generated, (c) resulting in a femtoliter-sized ink meniscus formed at the pipette-substrate gap without physical contact. (d) Guiding the ink meniscus in three-dimension under rapid evaporation results in creation of 3D nanostructures.


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Figure 2. Real-time optical micrographs of the printing process. (a) The PEDOT:PSS filled glass nanopipette is placed with a nozzle-substrate distance of 3 µm. The distance is estimated as a half distance between a pipette and its mirror image. (inset) A field emission scanning electron microscope (FE-SEM) image of a glass nanopipette with a diameter of ~ 600 nm (scale bar: 500 nm). (b) An ink meniscus is formed by applying a voltage of 360 V for 1.5 sec. (c) The formed meniscus is solidified by rapid evaporation of solvent and a freestanding PEDOT:PSS nanowire is produced by pulling down the substrate with a constant speed of 5 µm/s (scale bar: 10 µm). (d) An FE-SEM image of the freestanding PEDOT:PSS nanowire (scale bar: 1 µm).


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ACS Nano

Figure 3. Phase diagrams for the meniscus formation. (a) Dependence of meniscus formation on the interplay between applied voltage, V, and pulse length, t, with a constant pipette-substrate gap of 3 µm when a DC voltage is applied. The blue zone indicates the success of the meniscusformation, whereas the white zone indicates the absence of meniscus. The blue solid fit corresponds to a functional dependence of V ~ t-0.5. (b) Dependence of meniscus formation on the interplay between pipette-substrate gap and applied voltage (t > 10 s). The red zone describes meniscus-formation. As the applied voltage increases, the attainable nozzle-substrate distance for meniscus-formation increases linearly. Circles represents experimental data (filled circles: meniscus formation, empty circles: no meniscus formation). For this study, PEDOT:PSS/solvent mixture with controlled surface tension of , γ ~ 28.88 mN/m is used.


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Figure 4. Omnidirectional nanoprinting. (a-c) Meniscus-on-demand vertical printing of PPy nanowires. (a) An array of 25 vertical PPy nanowires fabricated with high reproducibility (scale bar: 10 µm), and (b) magnified view of one of the grown wires (scale bar: 1 µm). (c) Histogram shows the average diameters of the wire body and foot are 560 nm (SD: 16.3 nm) and 995 nm (SD: 37.4 nm), respectively. (d-e) Layer-by-layer printing of GO/PVP nanowires. (d) A meshlike structure fabricated on a planar surface (scale bar: 10 µm), and (e) a nanowall (width: ~ 600 nm, height: ~ 4200 nm) made up of 7 layers of GO/PVP nanowires (scale bar: 2 µm).


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ACS Nano

Figure 5. Multi-barrel nanopipettes for the meniscus-on-demand printing. (a) An FE-SEM micrograph shows the aperture of a three-barrel glass nanopipette (scale bar: 2 µm). Each barrel is filled with liquid ink and create menisci individually. (b) An optical micrograph shows the parallel printing of PEDOT:PSS/PPy composite nanowires (scale bar: 10 µm).

(c) Three

freestanding nanowires with designed shape and arrangement fabricated by a single process (scale bar: 2 µm). (d) An array of three-clustered nanowires with 25 µm step (scale bar: 10 µm).


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Meniscus-on-Demand Parallel 3D Nanoprinting - ACS Nano (ACS

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