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Controlling Smectic Liquid Crystal Defect Patterns by Physical Stamping-Assisted Domain Separation and Their Use as Templates for Quantum Dot Cluster Arrays Jong Min Ok, Yun Ho Kim, Tae Yong Lee, Hae-Wook Yoo, Kiok Kwon, Woo-Bin Jung, Shin-Hyun Kim, and Hee-Tae Jung Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03355 • Publication Date (Web): 21 Nov 2016 Downloaded from http://pubs.acs.org on November 24, 2016
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Controlling Smectic Liquid Crystal Defect Patterns by Physical Stamping-Assisted Domain Separation and Their Use as Templates for Quantum Dot Cluster Arrays Jong Min Ok,† Yun Ho Kim,‡ Tae Yong Lee, † Hae-Wook Yoo, † Kiok Kwon, † Woo-Bin Jung, † Shin-Hyun Kim† and Hee-Tae Jung*,† † KAIST Institute for Nanocentury (KINC) and Department of Chemical and Biomolecular Engineering (BK-21 plus), Korea Advanced Institute of Science and Technology, 291 Daehakro, Yuseong-gu, Daejeon, 34141, Korea ‡ Advanced Functional Materials Group, Korea Research Institute of Chemical Technology, Daejeon 34114, Korea *E-mail :
[email protected] KEYWORDS: Smectic liquid crystal, Domain separation, Toric focal conic domain, Quantum dot, Particle trapping
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ABSTRACT
Controlling the organization of self-assembling building blocks over a large area is crucial for lithographic tools based on the bottom-up approach. However, the fabrication of liquid crystal (LC) defect patterns with a particular ordering still remains a challenge because of the limited close-packed morphologies of LC defects. Here, we introduce a multiple-stamping domain separation method for the control of the dimensions and organization of LC defect structures. Pre-patterns with various grid shapes on planar polyimide (PI) surfaces were fabricated by pressing a line-shaped stamp into the PI surfaces in two different directions, and then used to prepare LC defect structures confined to these grid domains. The dimensions of the LC defect structures, namely the equilibrium diameter and the center to center spacing, are controlled by varying the line spacing of the stamps and the film thickness. A variety of arrangements of LC defects, including square, rhombic, hexagonal, and other oblique lattices, can be obtained by simply varying the stamping angle (Ω) between the first and second stamping directions. Furthermore, we demonstrate that the resulting controllable LC defect arrays can be used as templates for generating various patterns of nanoparticle clusters by trapping quantum dots (QDs) within the cores of the LC defects.
1. INTRODUCTION Self-assembled structures generated from soft building blocks have received significant attention in recent decades because of their potential applications as lithographic templates and opto-electronic devices.1-13 Block copolymers,1-4 spherical colloids,5-7 and surfactants8-9 have been used to fabricate specific self-assembled structures and control their periodic symmetries.
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Recently, topological defects in liquid crystals (LCs), which arise due to the distortion and spatial discontinuity in molecular orientation that result from specific surface anchoring and external stimuli, have been recognized as new soft building blocks.13-26 In contrast to other soft building blocks, those based on LC defects have many advantages including short fabrication times, facile control by varying the external environment, and a wide spectrum of feature sizes ranging from micrometers to sub-micrometers.13, 21 Further, such structures are formed rapidly via reversible and non-covalent interactions between LC molecules and can easily be generated by controlling the temperature and the surface anchoring; this mechanism is very simple and cost-effective, and so is suitable for the mass production of soft building blocks.13, 21 In addition, various candidates based on the numerous types of LC materials including rod-like and bent-core molecules, columnar supramolecules, and lyotropic LCs, can in principle be used in LC-defectbased lithography.13 To date, however, only a few highly ordered LC defect structures, such as periodic line patterns of cylindrical linear domains (LDs),14-17 square patterns of parabolic focal conic domains (PFCDs)18, 19 and hexagonal patterns of toric focal conic domains (TFCDs),20-23 have been utilized in this way because of thermodynamic limitations on LC defect morphologies. For example, one-dimensional periodic LD defect structures were prepared on crystalline substrates,14, 15 namely MoS2 and mica, and unidirectional rubbed surfaces16, 17 and square arrays of parabolic FCDs were produced by dilatation and shear flow.19 Highly periodic hexagonal arrays of TFCDs were obtained by confinement within channels20-22 and polymer coating.23, 27 As for other soft building blocks, however, this new type of soft-building block will only be recognized as a practical lithographic tool once the range of defect structures is substantially
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extended and lithographic applications such as particle trapping,16, 21 photonic masking27 and pattern fixation28 are demonstrated. In this study, significant progress was made toward overcoming the limitations imposed by previous LC-defect-based self-assemblies. A variety of TFCD structures, including square, rhombic, hexagonal, and other oblique lattices, can be simply created with a multiple-stampingassisted domain separation technique. A stamp of parallel lines is pressed into a polyimide (PI) surface in two different directions; this method can be used to prepare various pre-patterns. After the deposition of LC thin films onto such stamped PI substrates, grid domains can be generated according to the pre-patterns in the nematic phase and then converted into various types of TFCD arrays with confinement in the domains by cooling to the smectic A (SmA) phase. In addition, quantum dots (QDs) can be selectively trapped into the resulting topological dimple structure of the TFCDs by making use of two significant effects, namely geometric confinement and energy minimization, to produce square, rhombic, and hexagonal patterns of QD clusters embedded in the topological LC defects. We believe that this simple approach opens up new lithographic applications based on liquid crystal defect structures.
2. RESULTS AND DISCUSSION
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Figure 1. (a-e) A schematic representation of multiple stamping and square TFCD array fabrication. (a) Preparation of the polymeric stamp by molding it with V-shaped micro-channels. (b-c) Stamping of the PI-P coated glass. (d) Assembly of the cell consisting of a PI-H plate on top and the stamped PI-P substrate. (e) Generation of a square TFCD array in the cell. (f) Molecular structure and phase transition temperature of the commercial SmA LC (8CB). (g) AFM images and line profile of a 90°-stamped PI-P substrate pressed with a stamp with a line spacing of 10 µm. (h) POM images of a square array of TFCDs in the smectic phase with a film thickness of 7 µm. The white arrows indicate the stamping directions (scale bar: 100 µm).
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Figures 1a-e illustrate the overall process for the fabrication of TFCD arrays. In order to prepare periodic pre-patterns of parallel lines in a PI substrate, a V-shaped linear polymeric stamp prepared via the replica molding (Figure 1a) of a UV-curable polymer (NOA63) from a V-shaped linear channel (see the SEM images in Figure S1) is used. In fact, we tested stamps with other shapes, such as a rectangular-shaped polymeric stamp. Unfortunately, however, it was difficult to apply uniform pressure on the contact area with the rectangular-shaped stamp, so somewhat dislocated arrangements of LC defects were produced (Figures S2 and S3). The polymeric line stamp is pressed into a glass substrate coated with PI-P (polyimide inducing planar alignment in a two-directional stamping process (Figures 1b and c). Antagonistic boundary conditions, namely tangential (planar) alignment at the bottom surface and vertical (homeotropic) alignment at the top surface, are necessary to generate the TFCDs by bending the smectic layers into straight lines passing through the centers of circular defect lines to minimize the total energy.29-32 Then, a cell is assembled consisting of glass coated with PI-H (polyimide inducing homeotropic alignment) as the top surface in order to produce a constant film thickness (Figure 1d). A commercial SmA LC material, 8CB (4’-n-octyl-4-cyano-biphenyl), which contains an alkyl-terminated chain and a biphenyl group, is then injected into the cell (Figure 1f). The cell is heated above the isotropic temperature (45°C) to spread the LC material, and then cooled to the SmA phase (28.5°C) through the nematic phase to produce a highly periodic square TFCD array, as shown in Figure 1e. Note that multiple stamping of the PI-P substrate at various angles can be employed to prepare other TFCD arrays. Figure 1g shows an atomic force microscopy (AFM) image and corresponding height profile of a square grid pre-pattern on a PI-P substrate with an area of 30 × 30 µm2. Pressing a sharp line stamp with 10 µm line spacing on the PI-P surface has produced linear ridges with periodic
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intervals of approximately 10 µm. A height of the ridges on the stamped regions is approximately 1-3nm higher than that of un-stamped area. The square grid pattern is well formed by two perpendicular sets of periodic lines; note that the periodic lines generated by the first stamp are retained during the second stamping in the orthogonal direction. Figure 1h displays a polarized optical microscopy (POM) image of the 8CB SmA phase with a film thickness of 7 µm on the stamped substrate. A highly ordered square array of TFCDs with uniform size is evident over the entire PI grid substrate surface after cooling from the isotropic phase (45°C) to the SmA phase (28.5°C). The characteristic Maltese cross texture indicates that this is a typical TFCD structure composed of two defect lines, i.e. a circular line and a straight line passing through the center of the circle, with the smectic layers curved towards the center of concentric TFCDs. Thus, the alignment of the LCs is radial from the center of the circular defects. Outside the TFCDs, the LC molecules are vertically aligned in the cell; these regions appear dark under crossed polarization. The average diameter of the TFCDs is approximately 6.3 µm and their center to center spacing is 10 µm, which is identical to the distance between the parallel lines of the stamp. In order to understand the formation of this highly periodic TFCD grid pattern on the PI grid pre-pattern, we investigated the changes in the orientation of the 8CB molecules during the isotropic-nematic-SmA phase transition by monitoring the texture. Figures 2a–e show a series of POM images of 8CB and expected schematic illustrations of progressive change of the LC orientation in the PI grid substrate during cooling (-5°C/min) from the isotropic to the SmA phase via the nematic phase. The completely dark texture in the isotropic phase under the crossed polarizer is characteristic of optical isotropy because of randomly aligned LC molecules (Figure 2a). Although height of grid pre-pattern is very small (1~3 nm), square domains were generated
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by different alignment of LC molecules above the stamped region and a bright brownish texture appeared within the square domains below the isotropic-nematic transition temperature (40.5°C) (Figure 2b). When the sample is rotated by 45°, the brownish color in the square domains becomes darker, which implies that most LC molecules near the center of the square are aligned in one of the polarizer directions (Figure S4). Schematic diagrams of the LC director field within one square are shown in Figure 2b (bottom). The director at the center of each square is aligned along one of the diagonal directions of the square and gradually becomes parallel to the boundary near the square edges.
Figure 2. Texture changes and schematic diagrams of 8CB during the transition between the isotropic (45°C) phase and the SmA phase (28.5°C) on a 90°-stamped substrate with a film thickness of 7 μm. POM textures (top), schematic diagrams of the alignment of LC molecules or FCD structures with the smectic layers in cross-section along a diagonal line in the square domain (middle) and within each square (bottom) for (a) the isotropic phase (45°C), (b) the nematic phase (35°C), and the smectic phase at (c) 33.5°C, (d) 32°C, and (e) 28.5°C. The white arrows indicate the stamping directions (scale bar: 20 µm).
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During the transition between the nematic and SmA phases, a typical focal conic domain (FCD) texture consisting of an ellipse and a confocal hyperbola line appears within the square domains.30-32 The equidistant smectic layers of the FCDs are wrapped around the ellipse line and curved along the hyperbola line with a negative Gaussian curvature, which can be described in terms of the eccentricity of an ellipse ( = 1 − ⁄ with a semimajor axis a and a semiminor axis b, 0
