Spontaneous Additive Nano-Patterning from ... - ACS Publications

Subscriber access provided by NORTHERN KENTUCKY UNIV STEELY

Functional Nanostructured Materials (including low-D carbon)

Spontaneous Additive Nano-Patterning from Solution Route using Selective Wetting Hyeonho Jeong, Hanul Moon, Han-Jung Kim, Min Yoon, Chang-Goo Park, Yong Suk Oh, Hyung Jin Sung, Dae-Geun Choi, and Seunghyup Yoo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06538 • Publication Date (Web): 12 Jul 2018 Downloaded from http://pubs.acs.org on July 15, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Spontaneous Additive Nano-Patterning from Solution Route using Selective Wetting Hyeonho Jeong,#,† Hanul Moon,*,#,† Han-Jung Kim,‡,⊥ Min Yoon,§ Chang-Goo Park,‡ Yong Suk Oh,§ Hyung Jin Sung,§ Dae-Geun Choi,*,‡ and Seunghyup Yoo*,† †

School of Electrical Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, 34141, Republic of Korea

‡Nano-Mechanical

Systems Research Division, Korea Institute of Machinery and Materials (KIMM), Daejeon, 34103, Republic of Korea

§

Department of Mechanical Engineering, KAIST, Daejeon, 34141, Republic of Korea

KEYWORDS: nano-patterning, solution process, printing, selective wetting, plasmonic filter

ABSTRACT: Nano-patterns of functional materials have successfully led innovations in a wide range of fields, but further exploration of their full potential has often been limited due to complex and costinefficient patterning processes. We here propose an additive nano-patterning process of functional materials from solution route using selective wetting phenomenon. The proposed process can

1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 27

produce nano-patterns as narrow as 150 nm with high yield over large area at ultra-high process speed, that is the speed of solution dragging, of up to ca. 4.6 m·min-1. The process is highly versatile that it can utilize a wide range of solution materials, control vertical structures including pattern thickness and multi-stacks, and produce nano-patterns on various substrates with emerging form-factors such as foldability and disposability. The solution patterning in nano-scale by selective wetting is enabled by corresponding surface energy patterns in high contrast that are achieved by one step imprinting onto hydrophobic/hydrophilic bilayers. The mechanisms and control parameters for the solution patterning are revealed by fluid-dynamic simulation. With the aforementioned advantages, we demonstrate 25,400 pixel-per-inch light emitting pixel arrays and a plasmonic color filter of 10 cm × 10 cm area on a plastic substrate as potential applications.

1. INTRODUCTION Nano-patterning techniques or materials patterned in nanoscale have played a pivotal role in realization of new functionalities or enhancement of performance in a wide range of applications including electronics,1–3 optics,4–6 energetics,7,8 and biology.9,10 While cutting-edge lithographic processes based on deep UV or electron beams have successfully led the evolution of modern nano-patterning technology,3 their use has been limited to a few high-end applications due to high cost associated with sophisticated equipment and multi-step processes. In this respect, alternative nano-patterning methods were proposed such as simplified lithography techniques,11–13 nanotransfer methods,14,15 and capillary assembly.16 However, significant advances are still needed to realize a cost-effective and scalable nano-patterning process applicable to a wide range of functional materials with a high degree of freedom yet with a practicable throughput.


Page 3 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Additive patterning based on selective wetting phenomena defines patterns by confining liquid or “ink” inside hydrophilic regions surrounded or separated by hydrophobic areas.17,18 With its simplicity and compatibility with high-throughput volume processing, it is utilized for printing of mass media, as a form of offset printing, for instance. With the emergence of printed electronics, similar techniques have recently been used also for fabrication of micro-scale devices.19,20 Despite the popularity of these processes, however, it has been challenging to extend the application of selective wetting phenomenon into nano-scale patterning. This is because it is difficult to define, in a controlled manner, the nano-scale surface-energy patterns, which consist of hydrophilic and hydrophobic areas that are clearly distinguished from each other. Here we propose a facile solution-based route for additive nano-patterning by using the selective wetting phenomenon, enabled by nano-scale modulation of surface energy prepared via a one-step imprinting process onto bilayers consisting of a hydrophilic layer covered with a thin hydrophobic surface layer (Figure 1). In this scheme, imprinting forms nano-scale trenches and exposes hydrophilic parts along their vertical walls, which then makes a clear contrast in surface energy against the adjacent regions that remain hydrophobic. The resultant three-dimensional shapes, with spatially-modulated surface energy, are a key factor in successfully fabricating nano-patterns from solution materials. The proposed process produces uniform single- or multi-layer nano-patterns of various functional materials including metals, oxides, and organics over large area at high speed, and exhibits a compatibility with a wide range of substrates including but not limited to plastics and papers. Experimental results are supported by a fluid-dynamic simulation, which sheds a light on the operation mechanism and the key controlling parameters of the proposed process.



Figure

1.

Schematic

illustration

of

the

overall

process

Page 4 of 27

consisting

of

3

steps;

hydrophobic/hydrophilic bilayer formation, imprinting including removal of mold, and solution deposition by dragging. A cross-sectional SEM image of the Cytop/NOA bilayer and a picture of the solution dragging step with a linear nozzle are depicted in left and right hand side, respectively.

2. RESULTS AND DISCUSSION 2.1. Additive Nano-Patterning Process using Selective Wetting. As a nano-imprintable bilayer, 1 μm-thick UV-curable resin (NOA series, Norland Product, Inc.) and 20 nm-thick perfluoropolymer (CytopTM, Asahi Glass, Inc.) are employed as the hydrophilic bottom and hydrophobic top layers, respectively. Their contact angles for water are ca. 50° and 110°, respectively (Figure S1 in Supporting Infomation). To ensure that imprinting processes can be performed afterwards, the bilayer must normally be formed with the NOA layer in its uncured liquid-like phase; however, it would be almost impossible to maintain the film integrity of the NOA layer, if not cured, during Cytop layer formation by spin-coating process. This problem of contradictory requirements can be circumvented by a brief pre-curing of the NOA layer, after which the top part of the NOA layer is stiffened to withstand the Cytop coating process, while most of the layer remains in soft phase and thus ready for imprinting. Via pre-curing with UV


Page 5 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


exposure of 0.58 J·cm-2 (wavelength of 365 nm), uniform Cytop/NOA bilayers are fabricated without any stain (Figure S2 in Supporting Information). A sharp interface between the Cytop and NOA layers is clearly visible, supporting the validity of the proposed remedy (Figure 1). The bilayers are then successfully imprinted at nano-scale with sharp edges through a UV-imprinting technique with perfluoropolyether (PFPE) molds. The NOA layer is hard-cured with plenty of UV exposure while the embossed parts of the mold are inserted into the Cytop/NOA bilayers by external pressure (Figure 1 and see MATERIALS AND METHODS Section for details). The resultant nano-engraved structure in the Cytop/NOA bilayers is almost identical to the patterns in an NOA single layer after the same imprinting process (Figure 2a and 2b). Subsequently, functional nano-patterns are prepared by dragging a solution or ink on top of the nano-imprinted bilayer surface (Figure 1). The solution can be a target material itself, or a solvent containing target materials or their precursors. Using a custom-made dragging device with a linear nozzle (Figure S3 in Supporting Information), Ag-ink was dragged while being dispensed on top of the bilayer surface that has imprinted nano-hole arrays of 500 nm diameter with 1 μm pitch and 500 nm depth (Figure 1 and Movie S1 in Supporting Information). An annealing process at 150 ºC for 1 hour then followed for sintering. The results show clear Ag nano-dot patterns lying exactly inside the engraved parts, virtually without any residue on the embossed hydrophobic parts (Figure 2c and 2d). It can be seen from the shape of the Ag confined in the engraved part that Ag covers both the bottom and the sidewall, with a clear boundary right below the top Cytop surface layer (Figure 2c). Upon comparison with the shape of the nano-imprinted bilayers shown in Figure 2b, what is shown in Figure 2c indicates that the hydrophobic Cytop layer is separated along the periphery of the engraved patterns, thus finally lying on the embossed top surface as well as on the bottom surface of the engraved patterns (Figure 2b). Consequently, the hydrophilic NOA surface



Page 6 of 27

is considered to get exposed along the sidewall of the engraved patterns. This originates from the imprinting depth, which is much larger than the thickness of the Cytop layer. In this way, the proposed approach results in a uniform nano-scale spatial modulation of the surface energy, and thus leads to Ag nano-patterns with significant yield over a relatively large area. (Figure 2d). It is also confirmed that these homogeneous, high quality nano-patterns can be obtained not only for dot arrays but also for line patterns of various sizes (Figure 2e and 2f).

Figure 2. a-c) Cross-sectional SEM images of an imprinted NOA layer (a), an imprinted Cytop/NOA bilayer (b), and an Ag pattern using Ag-ink (c). All patterns are fabricated for dot patterns of 500 nm diameter. d) A top-view SEM image of 500 nm-diameter dot arrays fabricated using Ag-ink, near the edge of the dragging trajectory. e-f) Top-view and cross-sectional SEM images of Ag dot arrays of which the diameters are 600, 360, and 250 nm (all scale bars denote 500 nm) (e), and Ag line arrays of which the widths are 600 and 250 nm with a period of 1 μm (f).


Page 7 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.2. Simulation to Verify Mechanisms and Major Process Parameters. To identify the mechanism and key parameters of the proposed nano-patterning process, a fluid-dynamic simulation was performed (see MATERIALS AND METHODS Section for details).21 In this simulation, a water droplet moves on top of a 200 nm × 200 nm trench, which corresponds to a replica of an engraved part formed upon imprinting (Figure 3a). The actual droplet movement resulting from solution dragging is mimicked in the simulation, with droplet movement induced by air-flow through a 3 μm-height “channel” in the front and at the rear of the droplet. The main parameters are (i) the surface energy of the hydrophobic and hydrophilic parts, and (ii) the dynamic characteristics of the solution under consideration. The surface energy is specified by the contact angles of the sidewalls (θside) and the bottom surface (θbottom) of the trench, and the top surface (θtop) (Figure 3a). θbottom and θtop are both determined by that of Cytop (= ca. 110º) used in this study, and θside is varied between 0 to 50º. Dynamic characteristics of the solution is represented by the capillary number (Ca), determined by the dynamic viscosity (μ), the flow velocity (Ս) of a liquid, and the surface tension between a liquid and a gas (σ), as Ca = μU/σ.22 In the simulation, a droplet moves through a channel from left to right at a steady speed Ս, which corresponds to the dragging speed (Udrag). When a part of the droplet meets the left top corner of the nano-trench (point ‘AL’ in Figure 3b), the liquid near the interface starts to spread along the hydrophilic sidewall, beginning to fill the nano-trench. The “filling” ends when a head part of the droplet (point ‘B’ in Figure 3b) touches the right top corner of the trench (point ‘AR’ in Figure 3b). The nano-patterning of the liquid is completed when the whole droplet passes over the trench and leaves behind a certain amount of its liquid inside the trench (Figure 3b and Movie S2 in Supporting Information).



Page 8 of 27

Figure 3. Simulation of the nano-patterning process of liquid using selective wetting. a) Schematic diagram of the simulation consisting of a micro-scale channel, a nano-scale square trench, and a water droplet moving through the channel with a speed of U. Surface contact angle distribution for the channel and the trench is also indicated. b-d) Patterning processes of a liquid shown near the nano-trench for three “θside, Ca” pairs, resulting in the values of FF of 72% (b), 52.6% (c), and 27.5% (d). e) FF values calculated for various θside and Ca. The conditions corresponding to Figure 3b, 2c, and 2d are indicated as “b”, “c”, and “c”.


Page 9 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


One should note that the filling factor (FF) − the ratio of the volume of filled liquid to the volume of a trench − is determined mainly by the competition between the lateral speed of a droplet along the top surface (Ulateral) and the liquid spreading speed into the trench (Uspread) (Figure 3b). If the head of the spreading liquid (point ‘C’ in Figure 3b) does not reach AR before point B of the droplet does, an air gap tends to remain inside the trench, limiting the amount of filled liquid and thus limiting FF. Establishing the hydrophilic sidewall is therefore essential in increasing FF because a more hydrophilic sidewall (lower θside) results in a higher value of Սspread and thus reduces the size of the air gap. Simulation results obtained by varying both θside and Ca, however, show that FF increases not only with low θside but also with low Ca (Figure 3c, 3d, and 3e, and Movie S3 and S4 in Supporting Information). It should be recalled that low Ca can be realized by a decrease in μ, an increase in σ, or a reduction in Udrag, which is proportional to Ulateral. These analytic results give us an insight into the proposed approach: i) a solution material with lower μ and higher σ is favorable for high FF, ii) with a given solution material, FF can be increased by decreasing θside or Udrag, and iii) to improve the solution deposition speed, i.e. Udrag while maintaining FF, one can use a favorable solution material, i.e. low μ and high σ, or decrease θside. The experimental results showing the decrease in the thickness of nano-dot patterns with the increase in Udrag of solutions are consistent with the relation between FF and Ca (Figure 4a). The range of Udrag that leads to an almost constant thickness in Figure 4a corresponds to the saturated value of FF, without air gap or with the least amount of air gap, and is established by sufficiently low Ca. Considering the volume reduction factor of the Ag-ink used in this work after sintering, which is ca. 80%,23 the 59 nm-thick Ag-patterns for the 250 nm-deep trench indicate that the saturated value of FF is slightly above 100% (Figure 4a and Figure S4 in Supporting Information).



Page 10 of 27

Figure 4. a) Thicknesses of Ag and InO nano-dot patterns from the Ag-ink and the sol-gel, respectively, according to various dragging speeds (Udrag). Insets are a snapshot of the Ag-ink dragging process with Udrag of 1.2 m·min-1 and a cross-sectional SEM image of the InO dot arrays fabricated with Udrag of 4.56 m·min-1. b-c) Side-view snapshots during the Ag-ink (b) and InO solgel (c) dragging with Udrag‘s of 1.2 and 4.56 m·min-1, respectively.


Page 11 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.3. Throughput of the Process. Low throughput is one of the major limiting factors for most solution-based nano-patterning techniques reported to date. Low deposition speed of solution has dominantly limited the overall process speed. Even the best record reported to date is limited to 13.2 mm·min-1.14,16,24 In the proposed process, the speed of solution deposition is determined by the maximum Udrag that leads to proper pattern formation. As shown in Figure 4a, the proposed approach works even at a value of Udrag of up to ca. 0.5 m·min-1 for the Ag-ink; this value is already significantly higher than those of the other solution-based nano-patterning techniques. Beyond this speed, solution dragging resulted in the formation of discontinuous macroscopic Ag-ink droplets on the surface (Figure 4a and 4b, and Movie S5 in Supporting Information). Nevertheless, the process speed can further be improved by controlling the characteristics of the solution such that it may have higher σ, which would prevent a droplet from being broken into multiple droplets (Figure 4c). For instance, the water-based indium oxide (InO) sol-gel has σ of ca. 78.5 mN·m-1, which is much higher than that of the Ag-ink (= 22.8 mN·m-1).25 With this high σ, it was possible to prepare the same nano-dot arrays with InO without any formation of residual droplets, even at a value of Udrag as high as 4.56 m·min-1, which was the limit of the customized dragging machine used in this study (Figure 4a and 4c and Movie S6 in Supporting Information). This high speed is one of the major advantages of the proposed approach that originates from the selective wetting behavior, in which hydrophilic sidewalls promote the fast spreading of solution with Uspread that can match Udrag. This observation illustrates that the speed of the proposed process may be comparable to that of conventional roll-based printing processes.26 Hence the proposed process may be integrated with the roll-based nano-imprinting techniques reported by Ahn et al.27 for a full R2R deposition of functional nano-patterns. Nevertheless, it should be noted that the quoted speed is for definition of nano-patterns of the ink while they are still in solution phase. Actual



Page 12 of 27

speed of the whole process should consider the time to be spent for UV imprinting and various post-processing steps such as drying and sintering, etc.

2.4. Versatility in uisng Materials and Controllability of Vertical Structures. The surfaces of the NOA and Cytop layers are wettable and repellant, respectively, to a wide range of solvents, and thus the proposed approach can be applied to various kinds of inks. In addition to the Ag-ink and the InO sol-gel shown above (Figure 2 and Figure 5a), we have successfully applied the proposed approach to patterning an organic material consisting of tris[2-phenylpyridinatoC2,N]iridium (Ir(ppy)3) and poly(9-vinylcarbazole) (PVK) (PVK:Ir(ppy)3) using solutions based on organic solvents (Figure 5b). These results clearly demonstrate the versatility of the proposed process for applications requiring various materials including metals, oxides, or organics. For instance, Ir(ppy)3 has a photoluminescence property of green emission under UV exposure, and is often used as an emitting layer in solid-state electroluminescence devices, i.e. organic light emitting diodes (OLEDs).28 Nano-patterned arrays of PVK:Ir(ppy)3 with a diameter of 500 nm and a period of 1 μm could serve as light emitting pixel arrays with a high pixel density of ca. 25,400 pixels-per-inch (ppi), as shown in Figure 5c. In some applications, it is important to achieve a certain level of thickness for each pattern. In the proposed process, due to volume shrinkage of solution, the Ag patterns, for example, show a thickness of ca. 50 nm after sintering for a single deposition of the Ag-ink into the 230 nm-deep trenches (Figure 5d). The thickness of the nano-patterns may further be controlled by adjusting the density of the solutions; however, the density cannot be arbitrarily increased due to the solubility and viscosity issues. This problem could be easily solved in the proposed process via multiple deposition, provided that the hydrophobicity of the embossed parts is preserved after each cycle


Page 13 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of deposition and sintering;18 in such a case, target materials from the solution can be iteratively deposited in a self-aligned manner. Experiments done with the Ag-ink do indicate that such an approach is feasible; the thickness of the Ag patterns gradually increases from ca. 50 nm to ca. 160 nm via multiple depositions of up to 5 times using the same Ag-ink; even after this multiple depositions, no residue is found on the embossed hydrophobic parts, as can be confirmed in the cross-sectional profiles and surface topologies of Ag filled engraved dot patterns measured by atomic force microscopy (AFM) (Figure 5d). Furthermore, the self-aligned multiple deposition technique allows the formation of heterogeneous multi-layer structures as well. For example, selfaligned InO/Ag bilayer nano-patterns were successfully fabricated by sequential deposition of the Ag-ink and the InO sol-gel solution with an interim sintering step (Figure 5e). The ability to control the vertical structure, including the thickness and the multilayer configuration, greatly improves the validity of the proposed process for various applications. In the experimental and simulation results, it can be noted that it is indeed essential to have a trench structure in the proposed scheme, because this structure allows a sufficient amount of patterned material to be confined. If the selective wetting process was to be applied simply to a planar structure with nano-patterned surface energy but without a nano-trench, the fraction of solution that remained on the hydrophilic parts would be significantly small, as would be its solidified nano-pattern. Nevertheless, it is worthwhile to note that the bottom of the trench is still covered with a hydrophobic layer. For this reason, the cocentration of a solution or ink should be high enough to form patterns that fully cover the whole bottom surface inside the trench. If the solution or ink has too low concentration, the amount of solutes may be insufficient and may thus cover only a portion of the bottom surface, in particular, those adjacent to the hydrophilic sidewall.



Page 14 of 27

Figure 5. Process applicability to various solutions and controllability of vertical structures. a) Top-view and cross-sectional SEM images of the nano-dot arrays filled with InO with diameters of 250, 360, and 600 nm. b) Top-view SEM image of nano-dot arrays of 500 nm-diameter and 1 μm-period, near the edge of the dragging trajectory to fill PVK:Ir(ppy)3. c) And a picture of signatures drawn using PVK:Ir(ppy)3 dot pixels of 25,400 ppi on a glass substrate, under UV exposure (inset: a picture without UV exposure). d) Cross-sectional profiles and surface topologies of Ag filled engraved dot patterns measured by atomic force microscopy (AFM), according to the number of Ag-ink deposition times (see MATERIALS AND METHODS Section for details). e) Schematic illustration and SEM images of a self-aligned InO/Ag hetero-multilayer.


Page 15 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.5. Adaptability to Various Form-Factors and Potential Applications. For nano-patterns to be applied to a wide range of forth-coming applications, patterning techniques should be applicable to various substrates. Due to the simplicity and mild nature of the proposed process, we were able to fabricate nano-dot arrays on 200 μm-thick poly(ether sulfone) (PES) films (Figure 6a), ultrathin, 6 μm-thick, MylarTM substrates (Figure 6b), and copy papers (Figure 6c). Nano-patterns on a 6 μm-thick Mylar substrate were found to endure high mechanical deformation such as bending with small radius even below 100 μm, and thus can conformally adhere to a wrinkled surface such as a human palm (Figure 6b). Unconventional substrates like paper may already have micro-scale bumps on their surfaces;29 in such a case, the vertical dimension of the bumps may be larger than the depth of the imprinting, causing unwanted defects during the imprinting process. One possible strategy to resolve this problem, which does not compromise the merits of the proposed process, is to use a sufficiently thick hydrophilic layer to form a smooth surface onto which a sufficiently flat hydrophobic layer can be deposited. For example, nano-patterns were successfully fabricated onto a conventional copy paper with a ca. 10 μm-thick NOA layer (Figure 6c). The same process is applicable onto a banknote, for example, for anti-forgery, as nano-patterned structures cannot be reproduced with a copy machine or a printer. Thanks to the advantage of the proposed process at producing uniform nano-patterns over a large area, the method shown here may make more practically viable some advanced applications using nano-patterns that have so far been demonstrated only in a tiny area. For instance, plasmonic color filters consisting of periodic nano-scale metal patterns have attracted much attention due to their wide range of color tunability, and their bright and non-fading colors. These techniques have been intensively studied, especially for high-resolution image sensors,4 but studies aiming at large area applications such as displays have been rarely reported due to the lack of process scalability or the



Page 16 of 27

costly nature of the typical, large-area nano-patterning process.6 Using the proposed process and the Ag-ink, metallic-dot hexagonal arrays with a diameter of 150 nm and a period of 300 nm were fabricated on a plastic substrate, for use as a flexible band-stop filter with a low transmission at a wavelength of around 500 nm (Figure 7a). The results show that the color filter made using the proposed process exhibits a uniform magenta color and almost identical transmittance spectra over the area of 10 cm × 10 cm, thus proving the potential of the proposed method for large-area fabrication of nano-patterns (Figure 7a and 7b). This is, to the best of our knowledge, the largest plasmonic color filter demonstrated to date.6

Figure 6. a-c) Photographs and SEM images of nano-dot patterns on a 200 um-thick PES film (a), on a 6 um-thick Mylar film (b), and on a paper (c).


Page 17 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. a-b) Transmittance spectrum at four different positions (a) and a photograph (b) of a magenta-colored plasmonic filter, formed using 300 nm-period hexagonal Ag dot arrays over an area of 10 cm × 10 cm fabricated on a PES film. Insets in Figure 7a show a SEM image of the hexagonal dot arrays of 150 nm-diameter and 300 nm-period, and the four positions at which transmittance was measured.



Page 18 of 27

3. CONCLUSIONS A nano-patterning technique for functional materials was proposed from solution route using selective-wetting phenomena. With a smart yet simple method to prepare engraved hydrophilic patterns with a hydrophobic top surface, nano-patterns down to 150 nm were fabricated over large area with high yield and uniformity in a cost-effective way, at high process speed of up to 4.6 m·min-1. The process is highly versatile in that various materials can be nano-patterned with controllable vertical configurations on various substrates. The mechanisms were verified via fluiddynamic simulation, which revealed the major parameters influencing the process. With its simplicity and compatibility with a wide range of materials, the proposed nano-patterning process may open up new avenues to the nano-fabrication of functional materials in many emerging applications beyond lab-scale demonstrations.

4. MATERIALS AND METHODS 4.1. Materials used for the Nano-Patterning Process. The materials used in the hydrophobic/ hydrophilic bilayer structure were a UV-curable resin (Norland Optical Adhesive 61; NOA61, Norland Products) and an amorphous fluoropolymer (Cytop; CTL-809M, Asahi glass). CTL809M was diluted using its solvent, CT-solv180 (Asahi glass), at a ratio of 1:10 in volume for the 20 nm-thick Cytop layers. For a thick NOA layer on a paper, NOA63 with a high viscosity of 2000 cps was employed. Three kinds of solutions were prepared to fill the engraved nano-patterns in the Cytop/NOA bilayers with Ag, indium oxide (InO), and a photoluminescent organic material (PVK:Ir(ppy)3). For Ag, a nano-particle type Ag ink (NPS-J, Harima Chemicals) was used. This ink is known to consist of 62~67 wt% silver nano-particles dispersed in tetradecane; particle diameter is in the


Page 19 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


range of 8~15 nm. For InO, the precursor solution was prepared by dissolving indium nitrate hydrate (In(NO3)3∙xH2O, Sigma Aldrich) in DI water to a density of 5 M based on sol-gel chemistry. The solution for the photoluminescent organic material was prepared by dissolving 0.1 wt% mixture of tris[2-phenylpyridinato-C2,N]iridium (Ir(ppy)3, Nichem) and Poly(9-vinylcarbazole) (PVK, Sigma Aldrich) in chlorobenzene (CHROMASOLV® for HPLC 99.9%, Sigma Aldrich), in which the ratio of Ir(ppy)3:PVK is 8:100 in weight. For sintering of the solutions to be functional nano-patterns in solid phase, the substrates with nano-patterned solution were baked on a hotplate at 150 ºC for 30 min for the Ag-ink, at 100 ºC for 1 hour for the InO sol-gel, and at 60 ºC for 30 min for the PVK:Ir(ppy)3 solution. 4.2. Preparation of Substrates. Polished silicon wafers and glass wafers were used as purchased (Namkang Hi-tech Co. Ltd. and Corning, respectively). 200 um-thick PES films (SCL200, icomponents Inc.), 6 um-thick Mylar films (DuPont Teijin Films), and conventional photo papers were prepared as substrates at proper sizes by cutting and sticking them onto the carrier glasses, using polydimethylsiloxane (PDMS) films for the plastics and double-sided carbon tape for the papers. 4.3. Preparation of Imprinting Mold. Silicon masters that have inverse replica patterns of the imprinting molds, thus are direct replicas of the imprinted hydrophobic/hydrophilic patterns, were fabricated by photolithography (KrF Scanner S203-B by Nikon Inc.) and dry etching processes. Using a mixture solution consisting of perfluoropolyether (PFPE, Fluorolink MD-700, Solvay), 2,2-dimethoxy-2-phenylacetophenone, and 1-hydroxycyclohexylphenylketone (Sigma Aldrich), the imprinting mold of PFPE was reversely replicated from the silicon master. The PFPE solution was dropped on the silicon mater and a polyester terephthalate (PET) film was used to cover the top; then, the solution was uniformly spread and filled the nano-patterns in the silicon master by a



Page 20 of 27

press-roll process on the top of the PET film. Finally, the PFPE solution was cured by UV exposure of 18.36 mW·cm-2 for 5 minutes through the PET film; the completed PFPE mold sticking on the PET film was detached from the silicon master. Prior to replication using the PFPE solution, the silicon masters were immersed in the mixture solution, Nano-stripTM (Cyantek), consisting of sulfuric acid and hydro-peroxide, for 30 minutes for cleaning. Then, self-assembled monolayer (SAM) treatment followed, during which the silicon masters were put into an 80˚C oven for an hour with a few drops of the SAM material of Trichloro(1H,1H,2H,2H-perfluorooctyl)silane (Sigma Aldrich). The SAM treatment is necessary to guarantee that the PFPE molds detach from the silicon masters without defects after UV-curing. 4.4. UV Nano-Imprinting Process. In an imprint chamber, substrates coated with UV-curable hydrophobic/hydrophilic bilayers (Cyotp/NOA) were engraved by the PFPE imprinting mold with a pressure of 3 bar; then, the bilayers were hardened by UV exposure of 5.8 J·cm-2 while maintaining the pressure. For uniform imprinting with high-yield, the “imprinting mold/UVcurable bilayer/substrate” stack was covered with a thin and soft plastic film, and evacuated to remove air bubbles within the stack and the cover-film; then, air pressure was applied to the top of the cover-film. 4.5. Solution Dragging Process. A substrate having imprinted hydrophobic/hydrophilic patterns and a dot- or linear-shaped nozzle at the end of a syringe were placed vertically close to each other at a distance of few millimeters. And a droplet was formed between the surface and nozzle by pushing a solution out from the syringe. Then, the droplet was dragged by moving the substrate in the direction opposite to the dragging direction, at a controlled speed. 4.6. Analysis Techniques. Nano-patterns were observed using a field-emission scanning electron microscope (Sirion FE-SEM, FEI) and an atomic force microscope (Nanosurf Nanite AFM &


Page 21 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C3000 controller). For precise observation of the nano-engraved patterns with sharp sidewalls by AFM, an inspection tip with an ultra-sharp end, whose diameter is below 15 nm, was used. The contact angles of the Cytop and NOA layers and the Cytop/NOA bilayer were measured using the contact angle analyzer (General type Phoenix 300, Surface Electro Optics). The transmittance of the plasmonic color filters was measured with a spectrophotometer (Lambda 950 UV/Vis/NIR spectrophotometer, Perkin Elmer). Surface tension was measured by a pendent drop method and calculation from the shape of a droplet with Young-Laplace equation using an optical tensiometer (Attension Theta optical tensiometer, Biolin Scientific). 4.7. Computational-Fluid-Dynamics (CFD) Simulation. The fluid calculation in the laminar flow range was carried out by finite element modelling using a commercial software (CFD-ACE+, ESI group). The shape of the interface between the liquid and air is constructed using the volume of fluid method (VOF),30 with piecewise linear interface calculation (PLIC).31

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI:. Surface contact angles of the materials used to define hydrophilic and hydrophobic parts; precuring condition for NOA layers prior to Cytop coating; solution dragging machine; CFD simulation for a case resulting in “zero” air gap inside a nano-squire trench (PDF). AUTHOR INFORMATION Corresponding Author



Page 22 of 27

*E-mail: [email protected] (H.M.) *E-mail: [email protected] (D.-G.C.) *E-mail: [email protected] (S.Y.) Present Addresses ⊥

H.-J.K. is currently working in Center for Integrated Smart Sensors (CISS), Korea Advanced

Institute of Science and Technology (KAIST), Daejeon, 34141, Republic of Korea. Author Contributions H.J., H.M., H.-J.K., D.-G.C., and S.Y. designed the nano-imprinting process of hydrophobic/hydrophilic bilayers and the solution dragging process. H.J. and H.M. designed the nano-patterns, and carried out the experiments of fabricating and characterizing the nano-patterns and their applications. H.-J.K., C.-G. Park and D.-G.C. designed and performed the processes to make the imprinting molds. H.J., H.M., M.Y., Y.S.O., H.J.S., and S.Y. designed the fluid-dynamic simulation and analyzed the results thereof. H.J., H.M., D.-G.C., and S.Y. wrote the majority of the manuscript. All authors have contributed and given approval to the final version of the manuscript. #H.J. and H.M. contributed equally as main authors. *H.M., D.-G.C., and S.Y. contributed equally as corresponding authors. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Center for Advanced Soft-Electronics (CASE2013M3A6A5073175)

and

the

Center

for

Advanced

Meta-Materials

(CAMM-


Page 23 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2014M3A6B3063707) as Global Frontier Projects, and by Nano Material Technology Development Program (NRF-2016M3A7B4910631) funded by the Ministry of Science, ICT and Future Planning. REFERENCES (1) Wallraff, G. M.; Hinsberg, W. D. Lithographic Imaging Techniques for the Formation of Nanoscopic Features. Chem. Rev. 1999, 99, 1801–1822. (2) Plummer, J. D. Silicon VLSI Technology: Fundamentals, Practice and Modeling; Pearson Education; Hudson, New York, USA, 2009. (3) Pease, R. F.; Chou, S. Y. Lithography and other Patterning Techniques for Future Electronics. Proc. IEEE 2008, 96, 1–17. (4) Kristensen, A.; Yang, J. K. W.; Bozhevolnyi, S. I.; Link, S.; Nordlander, P.; Halas, N. J.; Mortensen, N. A. Plasmonic Colour Generation. Nat. Rev. Mater. 2016, 2, 16088. (5) Shrestha, V. R.; Lee, S-S. Kim; E.-S.; Choi, D.-Y. Aluminum Plasmonics based Highly Transmissive Polarization-independent Subtractive Color Filters Exploiting A Nanopatch Array. Nano Lett. 2014, 14, 6672–6678. (6) Do, Y. S.; Park, J. H.; Hwang, B. Y.; Lee, S.-M.; Ju, B.-K.; Choi, K. C. Plasmonic Color Filter and its Fabrication for Large‐Area Applications. Adv. Opt. Mater. 2013, 1, 133–138. (7) Boukai, A. I.; Bunimovich, Y.; Tahir-kheli, J.; Yu, J.-K.; Goddard, W. A.; Heath, J. R. Silicon Nanowires as Efficient Thermoelectric Materials. Nature 2008, 451, 168–171.



Page 24 of 27

(8) Chan, C. K.; Peng, H.; Liu, G.; McIlwrath, K.; Zhang, X. F.; Huggins, R. A.; Cui, Y. HighPerformance Lithium Battery Anodes using Silicon Nanowires. Nat. Nanotechnol. 2008, 3, 31–35. (9) Han, H.; Kim, J.; Shin, H. S.; Song, J. Y.; Lee, W. Air‐Bridged Ohmic Contact on Vertically Aligned Si Nanowire Arrays: Application to Molecule Sensors. Adv. Mater. 2012, 24, 2284–2288. (10) Jeong, C. K.; Jin, H. M.; Ahn, J.-H.; Park, T. J.; Yoo, H. G.; Koo, M.; Choi, Y.-K.; Kim, S. O.; Lee, K. J. Electrical Biomolecule Detection using Nanopatterned Silicon via Block Copolymer Lithography. Small 2014, 10, 337–343. (11) Qin, D.; Xia, Y.; Whitesides, G. M. Soft Lithography for Micro-and Nanoscale Patterning. Nat. Protoc. 2010, 5, 491–502. (12) Guo, L. J. Nanoimprint Lithography: Methods and Material Requirements. Adv. Mater. 2007, 19, 495–513. (13) Suh, K. Y.; Lee, H. H. Capillary Force Lithography: Large-Area Patterning, SelfOrganization, and Anisotropic Dewetting. Adv. Funct. Mater. 2002,12, 405–413. (14) Melosh, N. A.; Boukai, A.; Diana, F.; Gerardot, B.; Badolato, A.; Petroff, P. M.; Heath, J. R. Ultrahigh-Density Nanowire Lattices and Circuits. Science 2003, 300, 112–115. (15) Hwang, J. K.; Cho, S.; Dang, J. M.; Kwak, E. B.; Song, K.; Moon, J.; Sung, M. M. Direct Nanoprinting by Liquid-Bridge-Mediated Nanotransfer Moulding. Nat. Nanotechnol. 2010, 5, 742–748. (16) Kraus, T.; Malaquin, L.; Schmid, H.; Riess, W.; Spencer, N. D.; Wolf, H. Nanoparticle Printing with Single-Particle Resolution. Nat. Nanotechnol. 2007, 2, 570-576.


Page 25 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(17) Sirringhaus, H.; Kawase, T.; Friend, R. H.; Shimoda, T.; Inbasekaran, M.; Wu, W.; Woo, E. P. High-Resolution Inkjet Printing of All-Polymer Transistor Circuits. Science 2000, 290, 2123– 2126. (18) Moon, H.; Kim, M.; Yoo, S. Bilayer Source/Drain Electrodes Self-Aligned With Fluoropolymer Dielectrics for Stable High-Performance Organic TFTs. IEEE Elect. Dev. Letters 2011, 32, 1137–1139. (19) Liu, X.; Kanehara, M.; Liu, C.; Sakamoto, K.; Yasuda, T.; Takeya, J.; Minari, T. Spontaneous Patterning of High‐Resolution Electronics via Parallel Vacuum Ultraviolet. Adv. Mater. 2016, 28, 6568-6573. (20) Na, Y. J.; Lee, S.-W.; Choi, W.; Kim, S.-J.; Lee, S.-D. Multidimensional Generation of Combinatorial Organic Arrays by Selective Wetting Inscription. Adv. Mater. 2009, 21, 537-540. (21) Blazek, J. Computational Fluid Dynamics: Principles and Applications; Elsevier; Amsterdam, Netherland, 2005. (22) Reddy, S.; Schunk, P. R.; Bonnecaze, R. T. Dynamics of Low Capillary Number Interfaces Moving through Sharp Features. Phys. Fluids 2005, 17, 112104. (23) Harima Chemicals, “Nanopaste® series” NPS-J brochure; Jan. 2016 [Retrieved 26. Jun. 2017]. (24) Kang, J.; Park, C.-G.; Lee, S.-H.; Cho, C.; Choi, D.-G.; Lee, J.-Y. Fabrication of High Aspect Ratio Nanogrid Transparent Electrodes via Capillary Assembly of Ag Nanoparticles. Nanoscale 2016, 8, 11217–11223.



Page 26 of 27

(25) Hwang, Y. H.; Seo, J.-S.; Yun, J. M.; Park, H.; Yang, S.; Ko Park, S.-H.; Bae, B.-S. An ‘Aqueous Route’ for the Fabrication of Low-Temperature-Processable Oxide Flexible Transparent Thin-Film Transistors on Plastic Substrates. NPG Asia Mater. 2013, 5, e45. (26) Søndergaard, R. R.; Hosel, M.; Krebs, F. C. Roll‐To‐Roll Fabrication of Large Area Functional Organic Materials. J. Polym. Sci. Part B: Polym. Phys. 2013, 51(1), 16–34. (27) Ahn, S. H.; Guo, L. J. Large-Area Roll-to-Roll and Roll-to-Plate Nanoimprint Lithography: A Step toward High-Throughput Application of Continuous Nanoimprinting. ACS NANO 2009, 3, 2304–2310. (28) Blados, M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.; Forrest, S. R. Very HighEfficiency Green Organic Light-Emitting Devices based on Electrophosphorescence. Appl. Phys. Lett. 1999, 75, 4–6. (29) Lee, S.; Seong, H.; Im, S. G.; Moon, H.; Yoo, S. Organic Flash Memory on Various Flexible Substrates for Foldable and Disposable Electronics. Nat. Commun. 2017, 8, 725. (30) Hirt, C. W.; Nichols, B. D. Volume of Fluid (VOF) Method for the Dynamics of Free Boundaries. J. Comput. Phys. 1981, 39, 201–225. (31) Rider, W. J.; Kothe, D. B. Reconstructing Volume Tracking. J. Comput. Phys. 1998, 141, 112–152.


Page 27 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For Table of Contents Only


Spontaneous Additive Nano-Patterning from ... - ACS Publications

Recommend Documents