Three-Dimensional Nanoprinting via Scanning Probe Lithography

May 20, 2016 - †Department of Chemistry and ‡Biophysics Graduate Group, University of ... Department of Chemistry and Biochemistry, Florida State ...
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Three-Dimensional Nanoprinting via Scanning Probe Lithography-Delivered Layer-by-Layer Deposition Jianli Zhao,† Logan A. Swartz,‡ Wei-feng Lin,† Philip S. Schlenoff,† Jane Frommer,§ Joseph B. Schlenoff,# and Gang-yu Liu*,†,‡ †

Department of Chemistry and ‡Biophysics Graduate Group, University of California, Davis, California 95616, United States IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120, United States # Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States §

ABSTRACT: Three-dimensional (3D) printing has been a very active area of research and development due to its capability to produce 3D objects by design. Miniaturization and improvement of spatial resolution are major challenges in current 3D printing technology development. This work reports advances in miniaturizing 3D printing to the nanometer scale using scanning probe microscopy in conjunction with local material delivery. Using polyelectrolyte polymers and complexes, we have demonstrated the concept of layer-by-layer nanoprinting by design. Nanometer precision is achieved in all three dimensions, as well as in interlayer registry. The approach enables production of designed functional 3D materials with nanometer resolution and, as such, creates a platform for conducting scientific research in designed 3D nanoenvironments as well. In doing so, it enables production of nanomaterials and scaffolds for photonics devices, biomedicine, and tissue engineering. KEYWORDS: three-dimensional (3D) printing, nanostructure, layer-by-layer, polyelectrolyte, scanning probe microscopy (SPM), atomic force microscopy (AFM), scanning probe lithography (SPL) hree-dimensional (3D) printing, first reported in 1986 by Hull,1 has attracted much attention recently due to its capability to produce 3D objects by design.2,3 3D printers have produced impressive consumer products by custom design with sizes of micrometers or larger. Miniaturization of 3D printing is one of the frontier research and development areas in the 3D printing industry, motivated by the needs in advanced applications in tissue engineering,4 microfluidics,5 nanodevices, and biomaterials.6 Further miniaturization into the nanoscale (i.e., 3D nanoprinting) is the next “Holy Grail”.7 Motivation to attain 3D nanoprinting is driven by advanced applications including fabrication of nanoelectronic and photonic devices,8,9 quantum computing,10 new materials for controlling cellular function,11 stem-cellbased regenerative medicine and therapy,12,13 tissue engineering,14 and artificial organs,15 where the active structural units are of nanometer dimension. Much effort has been made toward this goal, such as direct deposition of small amounts of fluidic materials using a microsyringe7,16 and focused laserbeam-directed photopolymerization.2 Microsyringes enabled delivery of materials layer-by-layer (LbL), reaching feature size as small as hundreds of nanometers.7,16−18 These technologies enable printing of a wide range of materials into 3D patterns. However, further miniaturization is difficult due to the challenge of delivering minute amounts of material. Laserinduced polymerization has achieved LbL printing following

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custom design, with the smallest feature size of 9 nm reported via subdiffraction optical beam lithography.2,19,20 Further miniaturization is difficult due to the diffraction limit of optical-based technology. Additionally, materials are limited to tuned photopolymerizable media, and procedures require highpeak-power lasers. In principle, 3D nanoprinting should fulfill the following technical requirements: (a) genuine nanometer (preferably molecular level) precision in both positioning and material delivery; (b) accommodation of a wide range of functional materials; (c) 3D custom-design support; and (d) ease of use with practical throughput. Scanning probe microscopy (SPM), known for its high spatial accuracy in imaging and 2D nanolithography,21−24 represents a promising candidate for 3D nanoprinting. Prior efforts toward realizing 3D nanoprinting via scanning probe lithography (SPL) include dip-pen nanolithography in conjunction with Langmuir−Blodgett (LB) film formation.25 By stationary contact between an atomic force microscopy (AFM) tip and a substrate, a “three-layered cake” was produced with an overall height of 6 nm and a smallest layer diameter of 250 nm.25 AFM-based sculpting via thermal decomposition was also reported to produce a replica Received: February 14, 2016 Accepted: May 20, 2016 Published: May 20, 2016 5656

DOI: 10.1021/acsnano.6b01145 ACS Nano 2016, 10, 5656−5662

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Figure 1. SPL-based 3D nanoprinting enables construction of 3D nanolines. (A) Key steps for SPL-based 3D nanoprinting. (B) AFM topographic image of 3D nanoline array produced by 3D nanoprinting. Lines a, b, c, d, and e were printed by incremental numbers of passes: 10, 32, 64, 128, and 256, respectively. Printing was performed with an AFM load of 20 nN at 1 μm/s. (C) 3D view of (B). (D) Corresponding cursor profile as indicated by dotted line in (B).

RESULTS AND DISCUSSION 3D Printing of Line Arrays. A well-known difficulty in using SPM to perform 3D nanoprinting is achieving precise vertical stacking of multiple passes along the surface normal while maintaining registry between each pass. The difficulty is caused mainly by the drift in probe position over time. Our initial attempt to reduce the drift was to deposit material lineby-line, instead of layer-by-layer, to minimize the time interval between each pass and thus the drift. In addition, PE complex formulation, loading, and deposition conditions were carefully selected to enable the line-by-line deposition with nanometer accuracy. Then, prior to imaging, the AFM probes were precoated with octadecyltrichlorosilane (OTS) self-assembled monolayers to minimize adhesion for high-resolution structural characterization. As shown in Figure 1A, a PE complex, defined as Ink I, was used by premixing 0.5 M poly(styrenesulfonate) (PSS, MW 75 kDa), 0.5 M poly(diallyldimethylammonium chloride) (PDAD, MW 450 kDa), and 1.8 M KBr in water. Ink I exhibits high viscosity, thus minimizing smearing during deposition. The addition of KBr enhances homogeneity.41 The coacervate, Ink I, has a viscosity of 105 ± 20 mPa s.44 A 20 μL droplet of Ink I was deposited on a clean Si(111) surface serving as an “ink reservoir”. To load the ink on the AFM tip, the AFM tip (AC-240, Olympus, spring constant 1.7 N/m) was then brought into contact with the edge of the droplet at a 20 nN load for 10 s. Next, the tip was lifted and moved to a designated location far from the inkwell, where the first pass, a 3 μm line, was completed at 20 nN and 1 μm/s. The tip was then stopped, kept under the same load, and allowed to reverse direction, scanning the second pass backward along the same linear trajectory with identical force

of the mountain the Matterhorn with a lateral resolution of 15 nm and an overall height of 20 nm.26 Mechanical shearing has also been performed, shaving away layers of multilayer thiols, creating a square-shaped hole with a maximum depth of 8 nm.27 2D nanolithography in conjunction with pattern transfer has also been practiced to build 3D nanostructures as defined by 2D bases.28−32 These efforts have paved the way for a critical technical development in SPM-based 3D printing, achieving genuine LbL 3D nanoprinting by design. The major challenges that remain for true 3D nanoprinting include (a) LbL deposition of materials following custom design in each layer; (b) a sufficient number of layers along the surface normal direction to form genuine 3D structures; (c) nanometer precision in all three dimensions and nanometer spatial registry between layers; (d) accommodation of a wide range of materials; and (e) practical throughput for both initial small-scale research and eventual industrial application. This work reports our approach to address the first four challenges. Using AFM and polyelectrolyte (PE) complexes, we have developed protocols to enable LbL deposition of PE complexes with nanometer precision in all three dimensions. Nanometer registry was demonstrated for a high number of passes and stacking of layers. Our approach enables production of designed geometries with nanometer precision. This work significantly advances the development of 3D nanoprinting technology and lays a solid foundation for 3D nanoprinting of functional materials. The materials used in our nanoprinting are PE materials, which have been widely used for 2D microlithography34−38 and for incorporation of functionalities within the PE matrix.39−43 5657

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ACS Nano and speed. The tallest line in Figure 1B marked as “e” took 256 passes to complete. The load and printing speed were determined by systematic variation, balancing delivery efficiency and narrow line width. The capillary junction between the AFM tip and the oxide-covered silicon likely drives material delivery during the first pass, and interactions among PE complexes are responsible for material delivery during subsequent passes.45−47 Printed 3D structures were cured by immersion in water to extract salt ions, such as K+ and Br−, from the initial formulation. This step strengthens the electrostatic attraction between PSS and PDAD, stabilizing the structure for subsequent AFM characterization, shown in Figure 1A.41,44 The salt-free complexes have a modulus of at least 10 MPa, even when wet, when measured independently as a (300 nm thick) thin film48 or as (1 mm diameter) PSS/ PDAD rods.49 The five printed lines are parallel with respect to each other, as shown in Figure 1B. The height increases with increasing number of passes from 0.9 to 11.8 nm, measured from AFM topographic images using multiple cursor profiles as shown in Figure 1B and D. The line widths, characterized by full width at half-maximum (fwhm), measure 75, 80, 90, 88, and 90 nm, respectively, which indicates minimal broadening and high interpass registry even at 256 passes. The results demonstrate the ability to use SPM to perform 3D nanoprinting, at least for line-based structures. Another advantage of using our approach is the capability of characterizing these structures in situ and layer-by-layer. Polyelectrolyte materials have a wide range of elasticity and stability; therefore, using the same AFM instrument for characterization as printing, the structure can be monitored as it is deposited particularly for its timedependent behavior.50 The ability to print line-based 3D nanostructures by custom design is further demonstrated by production of four sets of nanocastle turrets at designated locations, shown in Figure 2.

measure similarly as the parapets, 2.2 nm in height and 145 nm in width. The delivery efficiency for Ink I, quantified as height versus number of passes, is plotted in Figure 3. The delivery efficiency

Figure 3. Plots of height (blue) and fwhm (red) of nanolines as a function of the number of passes. The heights of the printed structures were obtained from the cursor profiles crossing the lines and surrounding areas, with the uncertainty representing the roughness atop the lines. Dashed lines are guidelines to illustrate the trends.

is 0.035 nm/pass, defined as height increase caused by each printing pass, taken from the slope of the blue curve in Figure 3. This value is far less than the predicted height of a single PE layer, 0.6−2.3 nm/layer.28,40,51,52 This indicates the complexity of local PE chemistry at the nanometer scale, which significantly differs from the observed behaviors of dip-coated PE materials at the macroscopic scale. The local chemistry of nanoscaleconfined environments will play an important role in the future of 3D nanoprinting.53,54 This is seen further in the roughness, represented by the error bars in Figure 3. The degree of nonuniformity depends on the interplay between delivery efficiency, interactions among PE molecules, and interactions between PE and the AFM probes at the nanometer scale. The successful delivery method and mechanism in bulk and microprinting often become invalid at the nanometer scale. This is true for our 3D nanoprinting of PE complexes, which involves uneven electrostatic interactions,41 aggregation or entanglements of polymer molecules,50 and nonuniformity in local delivery,55 which increase the complexity in comparison to handling corresponding bulk materials. Figure 3 also reveals that beyond 64 passes and up to 512 passes the line width remains constant at 88 ± 5 nm, further indicating high interpass registry. Layer-by-Layer Printing of 3D Nanostructures. In order to improve delivery efficiency, we performed several variations on the ink formulation, loading, and printing protocols. The ink-loading process, in the protocols used in Figures 1, 2, and 3, was amended to a two-step process, as shown in Figure 4A. An AFM tip was first approached to a droplet of 0.1 M PDAD (450 kDa) aqueous solution, then lifted and dipped into a droplet of 0.1 M PSS (75 kDa) aqueous solution. The materials at the AFM probe apex, loaded by this sequential loading, are referred to as Ink II. In a sense this is a “non-equilibrium” toner since the individual polyelectrolyte

Figure 2. 3D nanoprinting enables construction of line-based structures by design. (A) AFM topographic image of four “nanocastle turrets” produced by 3D nanoprinting. Printing was performed at a delivery force of 20 nN and scanning speed of 1 μm/s. (B) 3D view of (A).

The printing was performed using the same ink formulation, loading, deposition, and imaging protocols as shown in Figure 1A. For each turret, four nanoparapets sit at the four corners where the walls perpendicularly cross. The walls of a, b, c, and d were printed line-by-line with 4, 40, 80, and 128 passes, respectively. For instance, the walls in structure “a” measure 0.3 nm in height and 88 nm in width, while the parapets at all four corners measure 0.3 nm in height above the wall and 85 nm in width. As each parapet was formed at the intersection of two walls, the height of a parapet above the wall equals the height of a wall. For structure “d”, the parapets measure 2.5 nm in height above the wall and 140 nm in width, and the walls below 5658

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Figure 4. SPL-based 3D nanoprinting enables LbL construction of 3D nanostructures by custom design. (A) Key steps for SPL-based LbL 3D nanoprinting with sequential loading of PDAD and PSS onto an AFM probe prior to printing. (B) AFM topographic image of a square pyramid produced by LbL nanoprinting of 15 layers, each layer clearly visible. Printing was performed with Ink II (see text) with a delivery force of 20 nN over 20 min, each layer taking less than 2 min. (C) 3D view of (B). (D) 3D view of an AFM image of a nanosquare pyramid produced by LbL printing of three layers of Ink III (see text).

the same loading force. Nonequal components in PE mixtures typically lead to a less viscous liquid than in 1:1 mixing.41,58 In addition, the packing among positive and negative components is less dense due to the excess amount of PSS. As such, the PSS tends to favor separation to reach force balance, i.e., facilitating delivery.41,44 This could explain the higher delivery efficiency of Ink II in comparison to Ink I. An analogous approach has been reported previously using poly(acrylic acid) (PAA) and poly(ethylenimine) (PEI) with a 5.7:1 ratio to obtain suitable delivery viscosity for injection and formation of 3D microstructures.17,58 Ink I is a homogeneous complex of the two polyelectrolytes, which requires sufficient KBr to plasticize the complex to a liquid state. Each pass is a competition between deposition of material and redissolution/adsorption of previously deposited material to the AFM tip. Thus, net deposition efficiency is low. In contrast, the complex formed by sequential dipping of PSS then PDAD (Ink II) does not contain additional salt, i.e., KBr, and the complex formed in this “reactive” ink cannot be redissolved by subsequent passes.44,59 The 3D nanopyramid of Figure 4B and C exhibits high structural stability. Immersion in water for 2 weeks showed little deformation or swelling. Storing in a dust-free sealed container for seven months or treating by overnight baking at 200 °C resulted in no observable morphological changes. The high stability is consistent with the known chemical and structural stability of this class of materials.60 With true 3D nanoprinting demonstrated in the experiments shown in Figure 4B, we also tested the smoothness within each layer as a function of ink formulation. For example, the layers in Figure 4B exhibit granular features with lateral grain size of 0.2−0.5 nm. The roughness can be reduced by using PE components with lower molecular weight, i.e., smaller molecules. As shown in Figure 4D, a three-layer structure was

components were not premixed homogeneously before loading to the tip and delivering to surfaces. The components of Ink II, 0.1 M PSS and PDAD solutions, had viscosities of ∼3 and ∼ 15 mPa s, respectively. Both are more viscous than water (1 mPa s).56,57 Figure 4B and C depict a square pyramid constructed LbL with 15 layers using Ink II loaded only once onto the tip in the manner described above. The entire structure was printed within 20 min at a force of 20 nN. Each layer was designed as a 2D square. The smallest square of 500 nm × 500 nm was printed first at 1 μm/s, 64 lines per frame. Then, a concentric square sized 1250 nm × 1250 nm was printed to cover the first square. The above operation was iterated an additional 13 times, each time at a greater square size, to build the square pyramid. A total of 64 lines per frame were used for the first through the seventh operations, and 128 lines per frame were used for the eighth through the 15th operations. In the final product (Figure 4B and C), the last and widest deposited layer measures 11.5 μm × 11.5 μm, and the topmost layer measures 500 nm × 500 nm. The spatial accuracy is demonstrated by the near-perfect square geometry and the precise nanometer interlayer registry among all 15 layers. The height of each layer is 0.4 nm, much taller than 0.035 nm/pass of previous printing (Figure 3), indicating at least a 10-fold improvement in delivery efficiency. The higher delivery efficiency can be rationalized by the packing of ink materials at the apex of the AFM probe, as well as by the ink−surface interactions. Unlike Ink I or conventional PE systems that contain equal cationic and anionic components (PSS:PDAD = 1:1) and form densely packed PE complexes,45,46 Ink II does not follow the 1:1 stoichiometry due to different pick-up efficiencies in sequential loading. There is likely more PSS than PDAD, because the PSS was loaded atop the PDAD inked tip apex, i.e., facing a larger surface area under 5659

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Teflon container (100 mL) containing 200 μL of OTS, then heated in an oven at 70−80 °C for 1 h.71 Upon removal from the oven, each cantilever was rinsed with ethanol for 10 s and dried in N2. All 3D nanostructures were immersed in water for curing, followed by AFM imaging using these modified AFM probes. In aqueous media, tapping mode was utilized at 80% damping, while contact mode imaging was acquired at a load of 10−20 nN. Images were acquired with scanning speeds in the range 3−25 μm/s. The AFM images were acquired and analyzed using Asylum MFP-3D software developed on the Igor Pro 6.34 platform.

constructed using Ink III, i.e., sequential loading of 0.1 M PDAD (8.5 kDa) and 0.1 M PSS (75 kDa) aqueous solutions. The 0.1 M PDAD and PSS solutions exhibited viscosities of ∼15 and ∼3 mPa s, respectively.56,57 Printing was performed under the same conditions as Figure 4B. The roughness of each layer was reduced to less than 0.2 nm. The top layer measures 0.4 nm in height and 80 nm in lateral dimensions, revealing the capability of SPL-based 3D printing to construct 3D objects of nanometer feature size in all three dimensions.

CONCLUSIONS Using polyelectrolyte materials and AFM-based delivery methodologies, we have made important advances in the technology development of 3D nanoprinting. LbL production of desired geometries is clearly demonstrated. Nanometer precision is achieved in all three dimensions. The structures produced have exhibited remarkable stability. This approach is highly versatile because PE-based materials enable a wide range of functionalities to be incorporated, such as proteins,61,62 nanoparticles,63,64 dyes,65,66 and DNA.67,68 This investigation also revealed similarities and differences in the formation of PEbased thin films in the nanometer environment compared to conventional PE coatings. Work is in progress to explore the rich surface chemistry, rheological properties, and delivery efficacy in the 3D nanoenvironment, as well as to improve throughput by using arrays of tips and high-speed AFM.

AUTHOR INFORMATION Corresponding Author

*Phone (G.-y. Liu): (530) 754-9678. Fax: (530) 754-8557. Email: [email protected]. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We dedicate this work to our friend and coauthor, the late Philip S. Schlenoff, whose dedication and contribution to science serve as an inspiration to us all. We thank R. Stevens at CDI for helpful discussions. S. Stagner’s careful proofreading of the manuscript is appreciated. This work was supported by the Gordon and Betty Moore Foundation, the National Science Foundation (CHE-1413708), and UC Davis.

METHODS Materials. PSS (MW = 75 kDa), PDAD (MW = 450 and 8.5 kDa), and KBr (>99%) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Sulfuric acid (95.0%), hydrogen peroxide (30% aqueous solution), and ammonium hydroxide (30% aqueous solution) were purchased from EMD Chemicals (Gibbstown, NJ, USA). OTS was purchased from Gelest (Morrisville, PA, USA). Ethanol (99.99%) was purchased from Gold Shield Chemical Co. (Hayward, CA, USA). Water (≥18.2 MΩ) was generated from a Milli-Q system (Q-GARD 2, Millipore, Billerica, MA, USA). Polished silicon wafers, Si(111) doped with boron, were purchased from Virginia Semiconductor Inc. (Fredericksburg, VA, USA). Nitrogen gas (99.999%) was purchased from Praxair, Inc. (Danbury, CT, USA). All the materials were used without further treatment unless described specifically. Preparation of Silicon Substrate. Polished silicon wafers were used as substrates and cleaned following previously reported protocols.69,70 In brief, substrates were cleaned by immersion in piranha solution for 1 h, rinsed with copious quantities of ultrapure water, and then immersed in basic bath at 70 °C for 1 h. Piranha solution is a mixture of sulfuric acid and hydrogen peroxide at a (v/v) ratio of 3:1. It is highly corrosive and should be handled carefully. Basic bath is a mixture of ammonium hydroxide, hydrogen peroxide, and water at a (v/v) ratio of 5:1:1. Substrates were then rinsed with copious quantities of ultrapure water and dried in nitrogen gas. AFM Imaging. 3D nanostructures were characterized by an atomic force microscope (MFP-3D, Oxford Instrument, Santa Barbara, CA, USA). Silicon probes, AC 240-TS (Olympus America, Central Valley, PA, USA), were used for nanofabrication. The force constant measures 1.7 N/m and has a resonant frequency of 70 kHz. For imaging of 3D nanostructures, these probes were coated with OTS shortly before imaging. In brief, cantilevers were placed into a sealed

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DOI: 10.1021/acsnano.6b01145 ACS Nano 2016, 10, 5656−5662