Orientation Control of Smectic Liquid Crystals via a Combination

Publication Date (Web): September 27, 2016 ... Indeed, a strong electric field (∼100 V) can make unidirectionally aligned LC molecules along with th...
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Orientation Control of Smectic Liquid Crystals via a Combination Method of Topographic Patterning and In-Plane Electric Field Application for a Linearly Polarized Illuminator Min-Jun Gim and Dong Ki Yoon* Graduate School of Nanoscience and Technology and KINC, KAIST, Daejeon 305-701, Republic of Korea S Supporting Information *

ABSTRACT: We dynamically controlled the configuration of layering structures built by smectic A liquid crystal molecules using the combination method of the microchannel confinement and the in-plane electric field to realize the linearly polarized illuminator and bistable structures. Once a mild in-plane electric field (∼30 V) is applied between polymeric walls, the layer configuration was changed from the toric focal conic domains to periodic zigzag patterns of alternatively packed focal conic domains. The transformed zigzag patterns maintained their structures even after turning off the applied electric fields, revealing the ability for use in a bistable memory device. Indeed, a strong electric field (∼100 V) can make unidirectionally aligned LC molecules along with the applied electric field via zigzag patterns, and electro-optical performance of resultant textures when the sample is mixed with fluorescent dyes was characterized to show a linearly polarized light illuminator. Our electric field in and on the confined geometries will be used in the fabrication of functional structures built by polar soft materials which can broaden applications in patterning platforms and efficient electro-optical devices in the near future. KEYWORDS: liquid crystal, smectic phase, topographic pattern, polarized fluorescence, in-plane electric field

1. INTRODUCTION Topographic patterning has been one of the most powerful tools for controlling the orientation of functional materials, including colloids, 1 block copolymers,2,3 liquid crystals (LCs),4,5 and various other types of inorganic materials.6 However, this tool cannot be easily used in dynamic optic or electronic devices because the external fields are not easy to combine with the topographic patterns. For the dynamic orientation control of soft materials, many methods have been introduced that use magnetic fields,7−9 electric fields,7,10 and light.11,12 Among these, the use of an electric field as a stimulus is the most effective (including cost-effective) and the easiest way because electrode fabrication techniques are widely known. This is the reason why the orientation control of LC molecules using an electric field has been extensively developed in the display industry.13,14 This is useful for controlling a conventional LC phase, e.g., a nematic (N) phase having a long-range orientation order. However, this method has been proven difficult to extend to other soft materials, although the control of ferroelectric LC materials has been introduced.15−17 Recently, we introduced a combination method using topographic patterns induced from DNA materials and in-plane switching (IPS) electrodes to control the LC alignment, in which rod-type LCs are well-aligned to the long axis of DNA chains with an inclined angle and are controlled by applying an electric field18 while control of the conventional N phase remains. © XXXX American Chemical Society

Currently, smectic LC phases are popular in the material science and nanotechnology fields because of their superior physical properties compared with those of the conventional N phase, including superfast response times,15−17 robust mechanical properties,4,19−22 and high orientational order parameters,23 which are important in finding potential applications beyond LC displays.4,20−22,24 Moreover, there is a great need to control smectic LC phases that are not easily controlled by conventional alignment methods.5,25−27 This orientation control problem results from the structural characteristics of the smectic LC phase, which has an orientation order as well as a translational order. One of the typical smectic LC phases is the smectic A phase (SmA), in which layer normal vector is mostly parallel to the molecular director (n).28 In the SmA phase, layers are incompressible and keep an interlayer space because the energy cost for compressing layers is much higher than that of layer bending, which allows only intermolecular splay deformation that corresponds to layer bending and expels twist and bend deformations.7 For example, under an antagonistic boundary condition made between planar and homeotropic treated substrates, LC molecules self-organize to form a negative Gaussian curvature of layers, which is called a toric focal conic domain (TFCD).29,30 This defect structure Received: August 26, 2016 Accepted: September 27, 2016

A

DOI: 10.1021/acsami.6b10762 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Schematic diagram of fabrication of PI microchannels on patterned ITO electrodes. (a) Channel geography of the Si microchannel (depth of 5 μm and width of 10 μm) was converted to the PDMS stamp. (b) The PDMS stamp was arrayed on patterned electrodes having 10 μm width and distance, where the rail of the channel contacts the space between electrodes. (c) PI solution was injected by capillary force into the microchannel, and then the PDMS stamp was pulled off after solvent evaporation at 90 °C. (d) The PI microchannel underwent thermal imidization at 200 °C. (e) LCs in the PI microchannels. (f) Optical microscopic image of the patterned ITO electrodes on a glass substrate. (g) Molecular structure of 8CB.

Figure 2. LC structures confined in the PI microchannel. (a) Schlieren texture of the NLC phase in the microchannel at 35 °C. (b) TFCDs of the SmA phase arraying along the channel at 28 °C. (c and d) Schematic diagrams of purple boxes in panels a and b, respectively. (c) Schlieren textures at NLC phase. (d) TFCDs at the SmA phase. The yellow surfaces are layers, and the green ellipsoids are LC molecules.

polarized optical microscopy (POM), fluorescent microscopy (FM), and UV−vis spectroscopy.

produces topographic characteristics and has localized defect points, which have been used in patterning tools,22 microlenses,20 and templates for particle manipulation.4 However, no one has succeeded yet in controlling confined TFCDs dynamically because the solid substrates are not easy to combine with electrode fabrication, although many attempts have been made to control TFCDs.4,31−34 Here, we fabricated polyimide (PI) microchannels placed on IPS electrodes using the technique of micromolding in capillaries (MIMIC)35 to control the SmA LC structures dynamically (Figure 1). In our platform, the microchannels can assist the formation of a uniform array of LC structures; here, in-plane electrodes are used to switch the molecular directors and the corresponding LC layered structures, which allows the curvature of smectics to be transformed into relatively flatlayered structures to show high polarization. These structural changes are reversible, allowing use of a linearly polarized illuminator when fluorescent dyes are mixed with LC materials. All of these experimental results were clearly observed with

2. RESULTS AND DISCUSSION To achieve this goal, we first used silicon microchannels 5 μm in depth and 10 μm in both width and separation to make a replica based on polydimethylsiloxane (PDMS) (Figure 1a). After removing the silicon master, we placed the PDMS mold on interdigitated electrodes made of indium tin oxide (ITO), which had the same in-plane dimensions as the mold (Figures 1b, f), in which ridge parts of the PDMS mold were in contact with the separation parts of the electrodes (Figure 1b). Then, we injected a PI precursor in solution into the grooves of the mold by capillary force, which was heated to 90 °C to evaporate the solvent (Figure 1c). Finally, we subjected the PI channels to a hard baking process after the PDMS mold was peeled from the ITO substrate, forming robust PI microwalls on the electrodes, which were exposed to air (Figure 1d). In this condition, the LC material, 8CB, was injected by capillary force B

DOI: 10.1021/acsami.6b10762 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 3. LC structures under the PI channel and the applied electric field. (a) Zigzag patterns under the electric field (30 V and 1 kHz). (b) POM image of the zigzag patterns after insertion of the first-order retardation plate (λ= 530 nm). Gray arrow inset of the image indicates the slow axis of the retardation plate. (c) Linearly oriented layers along the electric field direction (100 V and 1 kHz); inset is the POM image when the channel direction is parallel to the polarizer. Schematic diagrams of (d) the zigzag textures and (e) the linearly oriented layers, respectively.

in the isotropic state (∼45 °C), exhibiting N-SmA phases during cooling from the isotropic phase (Figures 1e, g). Even though the fabricated PI microwalls were shrunk during the solvent evaporation process, they are enough to give channel confinement into LCs (Figure S1). In the PI channels, there are four boundary conditions: one is air that gives the homeotropic anchoring condition, while the others can induce planar anchoring. Under this circumstance, topological defects in the N phase were generated to show typical Schlieren textures under the cross polarizers and to describe the corresponding molecular arrangement (Figures 2a, c).36 In the SmA phase, LC molecules formed the radial orientation on the bottom substrate to show Maltese cross patterns that represented TFCDs (Figures 2b, d).4,22 Further, the LC molecules were tangentially aligned from the bottom to the LC/air interface, in which each FCD owns the elliptical domain from the top view. Here, eccentricity (e) can be defined how much the ellipse is elongated, which is in the range of 0 ≤ e < 1, e =

(a 2 − b2) a2

oriented parallel or perpendicular to a polarizer or vertically to a substrate, whereas cyan blue or yellow appeared when the LC molecules were oriented parallel or perpendicular to the slow axis of the retardation plate.37 With this simple characterization technique, we found periodic blue and yellow lines in the zigzag patterns, in which the LC molecules were alternately arranged in the channel direction to form face-to-face closely packed FCDs (Figures 3b, d).34 During the transformation into zigzag patterns, the fringe electric field coming out from the electrode edge acted as a driving force to generate the tilted FCDs. The electric flux density near the edge of the electrode was higher than that in the other region, e.g., far from the electrode.38 This fringe field strongly generated the splay flexoelectric effect of the LC molecules at the electrode edge, which induced molecular elastic distortions and the flexoelectric polarization. According to Meyer’s notation, this polarization (Pf) is induced from two types of flexoelectric effects and the LC molecular director, as illustrated below:39 Pf = e1n(∇·n) − e3(∇ × n) × n

, where a and b are the lengths of

semimajor and semiminor axes, respectively. It is well-known that eccentricity varies with the anchoring conditions.29,33 In this experimental condition, the eccentricity of the domain boundary in the SmA phase was zero because of the absence of uniaxial orientation, leading to the generation of TFCDs.4,20−22,24,29,33 Once the uniaxial orientation was induced in our experimental system by applying an external force, the eccentricity was no longer zero, resulting in tilted FCDs or others.32,34 To realize this nonzero eccentricity in our system, we applied an in-plane electric field to the SmA phase. Under a certain condition of the electric field (30 V and 1 kHz), the TFCDs started to transform into tilted FCDs with zigzag optical textures, in which the FCDs were alternately arranged, which was reported in the SmA phase on the rubbed substrate (Figure 3a).34 To precisely analyze the molecular orientation in the zigzag patterns, we inserted a first-order retardation plate (λ = 530 nm) between the sample and an analyzer (Figure 3b). In the POM image, magenta indicates that the LC molecules were

(1)

where e1 and e3 are the splay and bend flexoelectric coefficients, respectively, and n is the molecular director. Here, we can ignore the bend flexoelectricity term in the SmA phase because of the expelled curl operator for the constant interlayer space.7 Thus, only divergence of the molecular distortion was considered to explain the zigzag patterns, in which the splay flexoelectricity mainly governs the formation of FCDs that had a tilted form, which were alternately packed together to show the zigzag patterns (Figure 3d). Indeed, these zigzag patterns were maintained even after the electric field was removed (Figure S2), revealing the bistability, which is essential to realize the optical memory.40 When a strong electric field (100 V and 1 kHz) was applied to the LC confined in the PI microchannels, the LC molecules were oriented uniaxially, which was parallel to the in-plane field between the electrodes (Figures 3c, e). Here, the flexoelectric effect near the electrode edge was ignored, considering the whole system, but the in-plain electric field-induced polarization (dielectric polarization) of LC molecules was much more dominant, resulting in the uniaxial C

DOI: 10.1021/acsami.6b10762 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 4. Response time of morphological change of the SmA LC structures between the zigzag patterns and the linearly oriented layers at 28 °C: (a) from the zigzag patterns to the linearly oriented layers after an applied 100 V and (b) from the linear orientation to the zigzag patterns after the electric field was turned off. The black lines are the fits, and the blue and red dots are experimentally recorded data.

Figure 5. Optical characterization of fluorescent LC layers doped with 0.02 wt % BTBP under the high electric field (100 V and 1 kHz) at 28 °C. FM images of the field-induced layer structures when the polarized laser is (a) perpendicular and (b) parallel to the field direction. (c) Fluorescence intensity as a function of the angle α between the polarized laser and the electric field.

there is no spontaneous polarization, unlike that with ferroelectric LCs.43 Moreover, the translational order in the SmA phase acts as a hurdle in reducing the response time compared with the N phase, which has only an orientation order. Furthermore, the slope in Figure 4a shows a rough increase as a function of time because the multidirectionally aligned LC molecules in the zigzag patterns experienced different torques from the applied electric field strength, resulting in the irregular electro-optical performance. When the electric field was turned off, the uniaxially arranged LC

orientation of the LC molecules. As expected from this, the optical retardation was maximized or minimized when the angle between the electric field and the polarizer was 45° or 0°, respectively (Figure 3c). The response time during the morphological change from the zigzag pattern to the linearly arranged structure was ∼154 ms (Figure 4a). This relatively slow reaction time was caused by the high viscosity and low dielectric constant of the SmA phase of 8CB.41,42 In the SmA phase, the dielectric characteristic governs the electric field-responsiveness property because D

DOI: 10.1021/acsami.6b10762 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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illuminator that has a relatively high degree of linear polarization (0.64). Dynamic control of the LC layered structures has not been studied much, although this is essential to find other LC-based applications beyond the display application.

molecules spontaneously turned back to form zigzag patterns with the curvature of layers because of the confined geometries (Figure S3). Moreover, this process showed quite a slow response time of ∼3 s (Figure 4b) because the LC molecules had to spend more time rotating, bending, and forming layering structures (FCDs) during the return to zigzag patterns. Although the response time was slow, this is the first work that realized the control of layering topographic patterns that have been used in many applications, which can lead to a widening of the application field of smectic LCs. For example, the uniaxially or alternately arranged LC molecules responding to the electric field can be used in an optical-phase retardation film or a polarized light illuminator.17 To demonstrate the potential application of our system, here a linearly polarized illuminator, we characterized the electrooptical performance (Figure 5). For this, an LC sample was doped with 0.02 wt % of a fluorescent dye molecule, N,N′bis(2,5-di-tert-butylphenyl)-3,4,9,10-perylenedicarboximide (BTBP, Aldrich), because 8CB molecules do not have an intrinsic fluorescent property. The fluorescent dyes can exhibit a linear dichroism because of their anisotropic shape, in which the molecular long axis is parallel with the molecular transition dipole moment for the fluorescence. Thus, the fluorescent intensity can be varied with an angle (α) between molecular director and the linearly polarized excitation laser. Furthermore, the unidirectionally aligned mixture sample could also emit linearly polarized light because the fluorescent dye molecules tend to be aligned parallel with the LC director. Under a high electric field (100 V and 1 kHz), the mixture that had layering structures showed a uniaxial orientation and produced high fluorescent dichroism. In detail, the fluorescent LC layered structures showed the maximum emission when the direction of the applied electric field was parallel to the polarization axis of the linearly polarized excitation laser (Figure 5b), and vice versa (Figure 5a), because the dipole axis for the fluorescence in BTBP as well as the LC molecular director n are parallel to the direction of the applied electric field. Moreover, the fluorescence was modulated with varying α (Figure 5c), in which the fluorescent intensity decreases from α = 0−90°. On the basis of the fluorescent spectra, we obtained the degree of linear polarization (ρ) using the following equation:44 ρ = (I − I⊥)/(I + I⊥)

4. EXPERIMENTAL SECTION PDMS Stamp Preparation. For the master mold, a Si microchannel was used,4 whose depth, width, separation, and length were 5, 10, 10 μm, and 10 mm, respectively. After the Si microchannel was modified by a silane reagent (tridecafluoro-1,1,2,2-tetrahydrooctyltrichloro-silane, Gelest, Inc.), we poured a PDMS slurry mixed in a 10/ 1 weight ratio (PDMS monomer (Sylgard 184A, Sewang Hitech)/ cross-linking agent (Sylgard 184B, Sewang Hitech)) onto the substrate. Then, it was placed in a vacuum chamber for 2 h at 25 °C to remove the air bubbles, and after a thermal curing process at 60 °C for 4 h, the PDMS stamp was made. Microchannel Fabrication on a Patterned Electrode. The IPS cell with a 10 μm electrode width and a 10 μm space was prepared;17 then, the PDMS stamp was placed on the interdigitated electrodes where the trench of the PDMS stamp contacted the electrodes in parallel, and the wall of the PDMS stamp was positioned in the space between the electrodes. Then, a PI solution was injected into the trench using capillary force. Then, the solvent in the PI solution was removed at 90 °C for 10 min; after the PDMS stamp was carefully removed from the substrate, the PI microchannels on the substrate were heated to 200 °C for 2 h to cure the sample. Characterization. To control the temperature, we used a heating stage (LINKAM LTS350) with a temperature controller (LINKAM TMS94). The electric field was generated using a function generator (33210A, Agilent) and a voltage amplifier (A400, FLC Electronics). The electro-optic signal was recorded using an oscilloscope (DSOX2012A, Keysight). The polarized optical textures of the SmA LC phase were investigated using POM (LV100POL, Nikon). Fluorescent images were observed by FM (C2 plus, Nikon) with a 488 nm excitation laser (Coherent). Fluorescence spectra were recorded using a USB-2000+ spectrometer (Ocean Optics) with a solid-state light source at 475 nm (SPECTRA X, Lumencor).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b10762. SEM images of fabricated PI walls and POM images of zigzag patterns with/without electric field (PDF) Video of switching smectic layers under applied electric field (AVI)

(2)

where I∥ and I⊥ are the fluorescent intensities when α is 0° and 90°, respectively. In our case, ρ was 0.64, which was slightly higher than that of the nematic LC phase,45 which means that the smectic-based illuminator can be applied to a linearly polarized light illumination system.

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AUTHOR INFORMATION

Corresponding Author

3. CONCLUSIONS We fabricated PI microchannels placed on IPS electrodes to control the LC layered structures. When no electric field was applied, the LC molecules in the SmA phase formed TFCDs that had curvatures. Once a relatively mild electric field was applied, periodic zigzag patterns were generated due to the flexoelectric effect at the edge of the electrodes. Under a strong electric field, the formation of curvatures was restricted to form uniaxially oriented LC molecules that were parallel to the inplane field direction. The zigzags and the uniaxially arranged domains were reversely changed by varying the electric field in seconds (Movie S1). The resultant unidirectionally aligned LC structures can be used to fabricate a linearly polarized light

*E-mail: [email protected]. Author Contributions

M.-J.G. and D.K.Y. designed the research, analyzed the data, and wrote the manuscript; M.-J.G. performed the experiments. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from the National Research Foundation (NRF) funded by the Korean Government (2015K1A3A7A08071737 and 2014M3C1A3052537). E

DOI: 10.1021/acsami.6b10762 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.6b10762 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.6b10762 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX