Surface wrinkling method is used to fabricate a 1-dimensional nanostructure. The structure is transferred to an ultraviolet cured polymer which is used as an alignment layer. The anisotropic geometry serves as a guide for aligning liquid crystal molecules uniformly without defects. The TN-LC cell showed a successful LC switching, with a response time of 20.5 ms, and a threshold voltage of 2.00 V. It also exhibited high thermal stability above 180°C. The proposed UV-cured polymers with 1-D nano wrinkle geometry can be a candidate for alternative alignment techniques, for advanced liquid crystal devices with high thermal budgets.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Nanotechnology has contributed greatly to the development of modern technology. Nanostructure fabrication is an important technique that can be used to control the performance of functional devices such as optical [1–3], mechanical [3–5], electrical [4,5], and biological response [5,6] devices in vitro. The 1-dimensional (1-D) nanostructure has attracted attention because the functional surface or devices with anisotropic characteristics exhibit enhanced electrical performance [7,8] with distinct optical characteristics . Therefore, a simple and controllable nanostructure fabrication method is desirable in nanotechnology.
Surface wrinkling is based on a two-layer system consisting of a soft underlying substrate and a stiff-skin top layer . A mismatch of mechanical properties and the application of strain lead to surface undulation with systematically controlled periodicity, which is referred to as surface wrinkling. It has several advantages including fast and large patterning and easy control of the nanostructure orientation; hence, we can obtain a 1-D nanostructure simply by using surface wrinkling compared with conventional lithography such as photolithography or e-beam lithography [11,12].
Liquid crystal (LC) alignment is an important technique in liquid crystal display that uses anisotropic surface characteristics. The rubbing process is widely and conventionally used to align LCs . A roller covered with fabric causes friction in the underlying substrates, creating an anisotropic surface that can align liquid crystals on the surface. However, mechanical friction generates debris and local defects, which degrade the performance of the LC display [14,15]. For these limitations, many researches on the alternative alignment techniques has been investigated including photo polymer alignment [16,17], oblique deposition , and ion beam bombardment . These researches exhibited a high quality liquid crystal display with functions such as control over pretilt angle , and fast switching . However, a cost effective alignment technique with large area is required for mass production in industrial perspective. Moreover, there is an increased thermal budget requirement for advanced LC devices due to the internally generated heat resulting from the increased power consumption of backlight units with high brightness and numerous switching transistors. Therefore, it is also necessary to develop an alternative alignment technique for advanced LC devices.
Here, we present a LC alignment technique of a 1-D surface wrinkling pattern using ultraviolet nanoimprint lithography for twisted nematic LC display. Simple and controllable surface wrinkling was first fabricated, and the resulting pattern served as a master mold for transfer onto the ultraviolet curable polymer. LC displays need high thermal endurance because the displays consume a lot of power, which generates a lot of heat. Therefore, a UV curable polymer is a good choice for good thermal stability due to its intrinsic thermal endurance. By applying nanoimprint lithography, which has the advantages of a large process, high throughput, and cost-effectiveness, we demonstrate that a 1-D nanostructure can serve as a guide for aligning LC molecules. This alignment is investigated using a polarized optical microscope with the crystal rotation method.
2. Materials and characterization
The UV curable polymer is composed of three different components as shown in Fig. 1(a)-1(c): dipentaerythritol hexaacrylate (DPEHA), tripropyleneglycol diacrylate (TPGDA), and 2-hydroxyethyl acrylate (2HEA). The three components are acrylic groups with six functionalities of DPEHA, two functionalities of TPGDA, and the monofunctionality of2HEA. A schematic of the 1-D nano wrinkle transfer process is shown in Fig. 1(d) and 1(e). The wrinkled template was fabricated by mechanically stretching polydimethylsiloxane (PDMS) in a parallel direction by 10%, followed by irradiation with an ion beam (IB) with the intensity of 1.8 keV for 60 sec on the surface of the stretched PDMS to release strain. As a result, a 1-D directional nano wrinkle was formed with pitch size of 980 nm. The pitch size can be controlled by irradiation time or intensity, which can modulate the skin layer thickness . The liquid UV curable polymer was dropped on a glass substrate and spin-coated at a rate of 3000 rpm for 30 sec. The wrinkled PDMS template was imprinted on the UV curable polymer. The imprinted UV curable polymer filled in the wrinkled nano-pattern in the PDMS template. UV light was irradiated on the imprinted sample with an energy density of 7.9 mW/cm2 for 3 min at wavelengths of 310 to 330 nm. Afterward, the liquid UV polymer was solidified and the wrinkle geometry was transferred onto the UV-cured polymer, and then peeled away. The upper and lower substrates with the 1-D wrinkle geometry pattern were assembled in two kinds of LC cells. One is fabricated in a parallel direction with a cell gap of 60 μm for alignment state evaluation including a polarized optical microscopy (POM) observation and measurement of the pretilt angle. The other is twisted nematic mode with a cell gap of 5 μm for a measurement of electro optical characteristics. The LC cells were filled with positive dielectric anisotropy LCs (Δε = 10.3 at 20°C and 1KHz, Δn = 0.111 at 20°C and wavelength of 589nm, IAN-5000XX T14, JNC Co.,Ltd.).
POM images were captured using BXP 51 (Olympus) to investigate the LC alignment state and thermal stability of the LC cell. The pretilt angle was measured via the crystal rotation method (TBA 107, Autronic). The electro-optical characteristics including response time and the transmittance vs applied voltage were measured via LCD evaluation system (LCMS-200 Sesim Photonics Technology) with applying a square wave voltage of 60 Hz. The beam size of light source (halogen lamp, 100W) is 2~3mm with a divergence of under 5°. Atomic force microscopy (AFM; XE-100, Park System) was used to confirm the 1-D nanostructure on the wrinkled and transferred UV-cured polymers. The chemical bonding states of PDMS, the wrinkle mold, and the UV-cured polymer were analyzed using X-ray photoelectron spectroscopy (XPS) (ES-CALAB 220i-XL, VG Scientific).
3. Results and discussion
To investigate the chemical composition of the wrinkle mold and UV-cured polymer, XPS analysis was conducted as shown in Fig. 2. The wrinkle mold was fabricated by Ion Beam irradiation on PDMS. The wide scan spectrum in XPS exhibited higher O 1s peaks for the wrinkle mold compared with that of PDMS. The ratios between the oxygen and silicon in the PDMS and wrinkle mold were 1.1 and 1.6, respectively. Ion beam irradiation created a highly oxidized new skin layer on the PDMS. The newly formed skin layer induced mechanical undulation on the underlying PDMS, thereby causing a wrinkle structure on the PDMS surface . In addition, the wide scan spectrum showed high oxygen content in the UV-cured polymer resulting from the presence of DPEHA, TPGDA, and 2HEA, which have high oxygen content. However, a Si 2p peak was observed in the wide scan spectrum with a bonding energy around 100 eV, even though the UV-cured polymer did not contain silicon atoms. This might be attributed to remnants of the wrinkle stamp on the UV-cured polymer after demolding. Figure 2(b) shows a high resolution C 1s spectrum for the UV-cured polymer containing a double peak. The main peak on the left side is composed of the main carbon and a carbon single bond at 285 eV, a carbon-oxygen single bond at 286.2 eV, and a small peak for a carbon oxygen double bond at 287.2 eV. The peak on the right side represents the carboxyl group (COOH) centered at 289 eV because the DPEHA, TPGDA, and 2HEA in the UV-cured polymer contain mainly carboxyl groups.
Figure 3 shows the 3-D images, surface morphologies, and cross-sectional line profiles of the wrinkle structure (Fig. 3(a), 3(d), and 3(g)), the UV-cured polymer before imprint (Fig. 3(b), 3(e), and 3(h)), and the pattern-transferred UV-cured polymer (Fig. 3(c), 3(f), and 3(i)). We examined the 1-D topography of the wrinkle template and the transferred geometry of the UV-cured polymer. The 1-D structure serves as a director for aligning liquid crystal molecules in one direction similar to microgrooves. In addition, the profile showed a deformed wall on the UV-cured polymer after imprinting. The wrinkle pattern had an average width of 980 nm and an average height of 50 nm, whereas the pattern-transferred UV-cured polymer had an average width of 980 nm and an average height of 100 nm. During the curing process, an edge rim was generated at the side wall on the UV-cured polymer, which is a demolding issue in UV-cured polymers. This is attributed to the residual strain from the adhesion and the friction at the interface between the wrinkle PDMS and the UV-cured polymer during the curing process.
Figure 4 demonstrates the effect of the 1-D nanostructure on the homogeneous orientation of the LC molecules. In the POM images, a black image was observed between the intersectional polarizers where the direction of the nano-pattern was the same as the direction of the bottom polarizer. A white image was observed where the direction of the nano-pattern was at an angle of 45° relative to the direction of the bottom polarizer. When the LC cell was filled with positive LCs (Δε>0), the 1-D nanostructure induced a geometric restriction among the liquid crystal molecules and the surface. In addition, because the liquid crystal is elastic, the liquid crystal molecules align in a direction that minimizes elastic distortion. The geometric restriction of the 1-D nanostructure and the minimization of elastic distortion by the LC molecules guide the LCs near the surface of the 1-D nanosurface to align homogeneously along the direction of the 1-D structure. The LC molecules aligned near the surface propagate their state into the neighboring LCs in the bulk state along the same direction. Therefore, the geometric restriction of the 1-D nanostructure serves as a guide for orienting the LC molecules homogeneously. The fully filled LC molecules in the LC cell were aligned uniformly in one direction. As a result, the LC aligned in the same direction in the bottom polarizer can block the light passing through the LC cell from the upper crossed polarizer, resulting in a black image in the POM. In contrast, by rotating the LC cell by 45°, we obtained a white image in the POM, as the direction of the aligned liquid crystal was oblique between the intersectional polarizers. Since the LC molecules are birefringence materials, an obliquely aligned LC causes an optical path difference under the crossed polarizers, leading to circularly polarized light; hence, the light can pass through the intersectional polarizers.
To verify the homogeneous alignment of the LCs, the LC cell was evaluated using the crystal rotation method, which can be used to confirm the alignment state and calculate the pretilt angle (Fig. 5). The light beam irradiated to the LC cells normal to the plane and the oscillated transmittance was obtained by rotating the LC cells at a latitudinal direction. The blue line represents the simulated graph and the red line represents the experimental graph. A well-matched graph can be observed, which indicates that the liquid crystal molecules are well aligned and the pretilt angle can be calculated with high reliability. The pretilt angle of LCs is calculated by the crystal rotation method based on the phase retardation. The phase retardation was obtained by the anisotropic refractive index. Transmitted light is split into the ordinary and extraordinary components causing the phase retardation. The phase retardation of the irradiated beam can be expressed by Eq. (1) reported by Scheffer et al. 
Figure 6(a) and 6(b) showed the electro-optical characteristics of the response time and voltage-transmittance of TN cells composed of the wrinkle geometry of UV cured polymer and those of TN cells based on a rubbed PI for comparison. The TN cell is characterized by normally white mode, which means that the light transmits through the twisted LC molecules with homogeneous alignment and display the on state. When the external voltage is applied to LC cell, the LC molecules stand vertically along to the parallel direction of the electric field due to the positive permittivity. The light couldn’t transmit through the vertical LC molecules, which display OFF state. The rise time is defined by a response time of LCs switching on state to off state and the fall time is defined by a response time of LCs switching off state to on state. The response time of TN cell based on wrinkle geometry UV cured polymer showed a rise time of 7.38 ms and a fall time of 13.16 ms, whereas the PI showed a rise time of 3.40 ms and a fall time of 12.89 ms. The threshold voltage (a voltage at 90% transmittance) of the wrinkle geometry UV cured polymer was 2.00V, whereas that for rubbed PI was 1.89 V. The rubbed PI showed a better electro-optical performance, but the wrinkle geometry of UV cured polymer is successfully operated and showed a competitive performance.
With the development of high-quality display applications, it is essential to remedy all thermal issues. For example, advanced displays with high brightness and high resolution include numerous switching components and a bright backlight, which generates heat. The generated heat can deteriorate the component materials and reduce the lifetime of devices; hence, embedded heat dissipating layers or components in the device are required, which are costly to develop and fabricate. The thermal stability was investigated via POM for the liquid crystal cells as shown in Fig. 7. POM can be used to evaluate the stability of the initial orientation of the liquid crystal molecules. The liquid crystals in a liquid crystal cell based on the 1-D patterned UV-cured polymer maintain their initial orientation above 180°C, which reflects good thermal stability compared to the thermal stability of a conventional rubbed PI liquid crystal cell (100°C) . The UV-cured polymer has good thermal stability due to dense crosslinking resulting from UV irradiation.
We successfully demonstrated the high thermal stability of a liquid crystal device using UV-cured polymers with 1-D nano wrinkle geometry. The geometry was fabricated by mechanical-stretching surface wrinkling. The geometry was transferred by UV imprint lithography onto a UV-cured polymer. Liquid crystal molecules were directed by the 1-D nanostructure to align in one direction. Uniform liquid crystal alignment is caused by the geometric restriction of the surface and liquid crystal molecules to minimize the elastic distortion of the liquid crystal molecules. The LC cell in TN mode showed a successful operation. The rise time has 7.38 ms and the fall time has 13.16 ms. The threshold voltage is 2.00 V. In addition, the UV-cured polymer has thermal endurance; hence, the liquid crystal molecules can hold their initial orientation even above 180°C. Therefore, UV-cured polymers with 1-D nano wrinkle geometry can be a candidate for alternative alignment liquid crystal techniques, for advanced liquid crystal devices with high thermal budgets.
National Research Foundation of Korea (Grant No. 2019-11-0042).
2. C. Zhang, P. Yi, L. Peng, and J. Ni, “Optimization and continuous fabrication of moth-eye nanostructure array on flexible polyethylene terephthalate substrate towards broadband antireflection,” Appl. Opt. 56(10), 2901–2907 (2017). [CrossRef] [PubMed]
3. L. Gao, Y. Zhang, H. Zhang, S. Doshay, X. Xie, H. Luo, D. Shah, Y. Shi, S. Xu, H. Fang, J. A. Fan, P. Nordlander, Y. Huang, and J. A. Rogers, “Optics and nonlinear buckling mechanics in large-area, highly stretchable arrays of plasmonic nanostructures,” ACS Nano 9(6), 5968–5975 (2015). [CrossRef] [PubMed]
4. D. Y. Khang, H. Jiang, Y. Huang, and J. A. Rogers, “A stretchable form of single-crystal silicon for high-performance electronics on rubber substrates,” Science 311(5758), 208–212 (2006). [CrossRef] [PubMed]
6. N. Gadegaard, E. Martines, M. O. Riehle, K. Seunarine, and C. D. W. Wilkinson, “Applications of nano-patterning to tissue engineering,” Microelectron. Eng. 83(4–9), 1577–1581 (2006). [CrossRef]
7. J. Kim, M.-S. Lee, S. Jeon, M. Kim, S. Kim, K. Kim, F. Bien, S. Y. Hong, and J.-U. Park, “Highly transparent and stretchable field-effect transistor sensors using graphene-nanowire hybrid nanostructures,” Adv. Mater. 27(21), 3292–3297 (2015). [CrossRef] [PubMed]
8. S. G. Lee, H. Kim, H. H. Choi, H. Bong, Y. D. Park, W. H. Lee, and K. Cho, “Evaporation-induced self-alignment and transfer of semiconductor nanowires by wrinkled elastomeric templates,” Adv. Mater. 25(15), 2162–2166 (2013). [CrossRef] [PubMed]
9. S. Camelio, D. Babonneau, D. Lantiat, L. Simonot, and F. Pailloux, “Anisotropic optical properties of silver nanoparticle arrays on rippled dielectric surfaces produced by low-energy ion erosion,” Phys. Rev. B Condens. Matter Mater. Phys. 80(15), 155434 (2009). [CrossRef]
11. W.-K. Lee, C. J. Engel, M. D. Huntington, J. Hu, and T. W. Odom, “Controlled three-dimensional hierarchical structuring by memory-based, sequential wrinkling,” Nano Lett. 15(8), 5624–5629 (2015). [CrossRef] [PubMed]
12. H.-C. Jeong, H.-G. Park, Y. H. Jung, J. H. Lee, B.-Y. Oh, and D.-S. Seo, “Tailoring the orientation and periodicity of wrinkles using ion-beam bombardment,” Langmuir 32(28), 7138–7143 (2016). [CrossRef] [PubMed]
13. M. F. Toney, T. P. Russell, J. A. Logan, H. Kikuchi, J. M. Sands, and S. K. Kumar, “Near-surface alignment of polymers in rubbed films,” Nature 374(6524), 709–711 (1995). [CrossRef]
14. P. J. Shannon, W. M. Gibbons, and S. T. Sun, “Patterned optical properties in photopolymerized surface-aligned liquid-crystal films,” Nature 368(6471), 532–533 (1994). [CrossRef]
15. J. Stöhr, M. G. Samant, J. Lüning, A. C. Callegari, P. Chaudhari, J. P. Doyle, J. A. Lacey, S. A. Lien, S. Purushothaman, and J. L. Speidell, “Liquid crystal alignment on carbonaceous surfaces with orientational order,” Science 292(5525), 2299–2302 (2001). [CrossRef] [PubMed]
16. Y. Reznikov, O. Ostroverkhova, K. D. Singer, J. H. Kim, S. Kumar, O. Lavrentovich, B. Wang, and J. L. West, “Photoalignment of liquid crystals by liquid crystals,” Phys. Rev. Lett. 84(9), 1930–1933 (2000). [CrossRef] [PubMed]
17. C.-J. Hsu, B.-L. Chen, and C.-Y. Huang, “Controlling liquid crystal pretilt angle with photocurable prepolymer and vertically aligned substrate,” Opt. Express 24(2), 1463–1471 (2016). [CrossRef] [PubMed]
18. J. L. Janning, “Thin film surface orientation for liquid crystals,” Appl. Phys. Lett. 21(4), 173–174 (1972). [CrossRef]
19. H.-C. Jeong, B.-Y. Oh, H.-G. Park, J. H. Lee, Y. H. Jung, S. B. Jang, and D.-S. Seo, “Superior switching behavior of liquid crystals on surface-modified compound oxide films,” Opt. Mater . 50(Part B), 104–109 (2015).
20. T. J. Scheffer and J. Nehring, “Accurate determination of liquid-crystal tilt bias angles,” J. Appl. Phys. 48(5), 1783–1792 (1977). [CrossRef]
21. W. K. Lee, Y. S. Choi, Y. G. Kang, J. Sung, D. S. Seo, and C. Park, “Super-fast switching of twisted nematic liquid crystals on 2D single wall carbon nanotube networks,” Adv. Funct. Mater. 21(20), 3843–3850 (2011). [CrossRef]