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Article

Electro-Optical Characteristics of Solution-Derived Zinc Oxide Film According to Number of Rubbing Iterations for Liquid Crystal Alignment

by
Hong-Gyu Park
1,2,
Jin-Ah Kim
2 and
Bong-Jin Ko
1,*
1
Department of Electrical, Electronic, Control Engineering, Changwon National University, 20-1, Changwondaehak-ro, Uichang-gu, Changwon 51140, Republic of Korea
2
Department of Smart Manufacturing Engineering, Changwon National University, 20-1, Changwondaehak-ro, Uichang-gu, Changwon 51140, Republic of Korea
*
Author to whom correspondence should be addressed.
Crystals 2022, 12(12), 1711; https://doi.org/10.3390/cryst12121711
Received: 26 October 2022 / Revised: 18 November 2022 / Accepted: 20 November 2022 / Published: 25 November 2022
(This article belongs to the Special Issue Liquid Crystals and Their Advanced Applications)
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Abstract

:
Zinc oxide (ZnO) films reportedly exhibit a rubbing effect for liquid crystal (LC) alignment. In this study, we investigated the LC alignment characteristics of solution-derived ZnO films according to the number of rubbing iterations. Uniform and homogeneous LC alignments were achieved on the rubbed ZnO films, regardless of the number of rubbing iterations. As the number of rubbing iterations increased, the surface energies of the rubbed ZnO films were similar to 42.20 mJ/m2, but the polar energy increased from 4.288 mJ/m2 to 6.470 mJ/m2. Additionally, the electro-optical characteristics of the twisted-nematic (TN) LC cells fabricated by rubbed ZnO films showed the best performance when the number of rubbing iterations was at five. By confirming that the ZnO film with improved physical, chemical, and electro-optical characteristics compared to the polyimide film achieved a perfect LC alignment through the conventional rubbing process, it indicates that the ZnO film can be an excellent substitute for the LC alignment film. In addition, it is expected that it can be applied to an LC-based virtual reality (VR)/augmented reality (AR) display system that requires a very fast response time through the excellent EO characteristics of the TN LC cell of the solution-derived ZnO film through the rubbing process.

1. Introduction

Lots of research on liquid crystal (LC)-based virtual reality (VR)/augmented reality (AR) displays or micro-displays requiring high-resolution and high-speed response characteristics has been recently studied [1,2]. Therefore, the research on LCs to improve their electro-optical (EO) properties is continuously being conducted.
The unidirectional orientation of liquid crystal (LC) molecules and a uniform LC alignment are key technologies in the manufacturing of LC displays (LCDs) [3,4,5,6]. Recently, research on improving the alignment and electro-optical properties of LC cells by dispersing nanoparticles in LCs and using nanoparticles as an alignment layer has been actively conducted [7,8,9,10,11,12]. To uniformly align the LC molecules in one direction, an LC alignment layer is commonly positioned on an indium tin oxide electrode. The LC alignment layer affects the electro-optical properties of the LCD applications; thus, organic/inorganic materials with excellent physicochemical properties are under investigation as a candidate group [13,14,15,16,17,18]. Conventional LC alignment layers typically use polyimide because of its high thermal and chemical stability. However, more recently, studies of high-k inorganic thin films (e.g., ZnO, Al2O3, TiO2, SnO2, and SiO2) as LC alignment layers have been actively explored to enhance the electro-optical properties of the devices [16,17,18,19,20,21,22,23]. Additionally, LC alignment processes such as rubbing, ultraviolet alignment, and ion beam alignment methods have been investigated for their potential use in uniform LC alignment in the organic/inorganic LC layer [13,14,15,16,17,18,19,20,21]. Among them, the rubbing method is one of the most widely used LC alignment methods. The reason for this is that the process is simple, and its orientation is possible in a large area. In addition, since expensive vacuum equipment is not required, it is cost-effective [22,23].
In this study, the effect of the number of rubbing iterations was investigated by applying a solution-derived ZnO thin film as an LC alignment layer. A sol-gel process was used to fabricate the ZnO thin film. Compared with the sputtering process, the solution process is more cost-effective because it involves spin coating without the use of a vacuum system [17,18]. Additionally, the ZnO thin film has a higher dielectric constant compared with the polyimide alignment film, which increases the effective voltage applied to the twisted-nematic (TN) LC cell and enables low-power driving [22]. In this study, we examined a ZnO thin film, as an inorganic thin film that is unlike polyimide, to observe the LC alignment effect with repeated rubbing.

2. Materials and Methods

A ZnO thin film was deposited on an indium tin oxide-coated glass substrate using a sol-gel method. Prior to the deposition of the ZnO thin film, the indium tin oxide glass substrate was ultrasonically cleaned with acetone, methanol, and deionized water for 10 min each, then, it was dried with N2 gas. To generate the ZnO solution, zinc acetate dihydrate was dissolved in 2-methoxyethanol containing monoethanolamine as a stabilizer. After the ZnO solution had been stirred at 75 °C for 2 h, the temperature was increased to 200 °C and maintained for approximately 5 min until the solution became transparent. The transparent ZnO solution was applied to the indium tin oxide glass substrate for 1 min at a speed of 3000 rpm using a spin coating (SPIN-1200, MIDAS System Co., Daejeon, Republic of Korea) process. The residual volatile solvent of the ZnO solution was removed by heating it on a hotplate at a temperature of 200 °C for 10 min, then, it was annealed in an electric furnace at a temperature of 300 °C for 1 h to complete the ZnO thin film fabrication procedure. The ZnO thin film was rubbed 1, 3, 5, and 7 times by a rubbing machine that was wound with nylon cloth. Using the ZnO thin film produced through this process, an AP (anti-parallel) LC cell with a 60 μm cell gap and LC molecules which were arranged in reverse and parallel directions was fabricated to evaluate the LC alignment states such as the photomicrographs and pretilt angles. In addition, a TN LC cell having a 5 μm cell gap and twisted LC molecules was fabricated to evaluate electro-optical characteristics. To fabricate the respective LC and TN cells, a positive LC (Tc = 72 °C, Δε = 8.2, and Δn = 0.077; MJ001929; Merck KGaA, Darmstadt, Germany) was injected into the LC and TN cells at room temperature. Figure 1 shows the schematic diagram of the fabrication process for the AP and TN LC cells using the solution-derived ZnO films with a rubbing treatment.
Polarized optical microscopy (POM; BX53M, Olympus Corp., Tokyo, Japan) was used to evaluate the LC alignment and thermal stability of the ZnO thin film. To confirm the uniform alignment of the LC molecules on the ZnO LC alignment layer, the pretilt angle was measured using the crystal rotation method (TBA 107 Tilt-bias Angle Evaluation Device; Autronic-Melchers GmbH, Karlsruhe, Germany) [24]. The contact angle of the ZnO thin film was measured using a contact angle analyzer (Uni-Cam/A; GIT Software, San Francisco, CA, USA), and the surface energy was calculated. Finally, the electro-optical properties were evaluated by applying the ZnO thin film to the TN cell. The evaluation of electro-optical characteristics included the resolution of the voltage–transmittance (V–T) and response time (RT) using an LCD evaluation system (LCMS-200, Sesim, Uiwang, Republic of Korea). All of the measurements were conducted at room temperature.

3. Results

Figure 2 shows the POM images of the solution-derived ZnO thin film according to the number of rubbing iterations. The magnification used for taking POM images was 100×. As the number of rubbing iterations increases, the number of defects caused by the rubbing treatment increases in the POM images. However, these defects are difficult to identify with the naked eye. Compared with the randomly aligned sample without the rubbing treatment, the clean and uniform POM image indicates that the LC molecules were arranged in one direction due to the rubbing treatment [25]. Additionally, clean and uniform POM images were confirmed in all of the AP LC cells, regardless of the number of rubbing iterations.
Figure 3 shows the measurement of the pretilt angle of the LC molecules on a ZnO thin film uniformly aligned through the rubbing process. As shown in the inset of Figure 3, the pretilt angle refers to the angle between the axis of the LC molecules and the LC alignment film. All of the LC molecules must have a constant pretilt angle to operate in the same direction according to the applied voltage [19]. The pretilt angle can be measured by the crystal rotation method, which calculating the phase difference change of the measured transmittance according to the change of the incident angle when light passes through the AP LC cell [24]. As shown in Figure 3a–d, the pretilt angle was measured by rotating the cell from −70° to +70° in the latitude direction. The blue line represents the simulated curve calculated from the birefringence information of the well-aligned LC data, and the red line represents the actual measured curve. As the number of rubbing iterations increased to one, three, five, and seven, the mean pretilt angles of LC molecules on the ZnO thin film were 0.61°, 0.67°, 0.10°, and 0.02°, respectively. The LC alignment of the ZnO thin film was confirmed with a stable pretilt angle of <1°, regardless of the number of rubbing iterations. Additionally, when the rubbing iteration number was five or seven, the error value decreased, depending on the measurement position.
Figure 4 shows the contact angle and the calculated surface energy of the solution-derived ZnO thin film according to the number of rubbing iterations. The contact angle was measured using deionized water and diiodomethane. As the number of rubbing iterations increased, the contact angle of deionized water decreased from 78.8° to 74.6°, and the contact angle of diiodomethane increased from 42.2° to 47.4°. The change in the contact angle of the ZnO thin film indicates that the ZnO thin film gradually became hydrophobic as the number of rubbing iterations increased. The surface energy values using the Owen–Wendt formula confirmed this tendency [26]. As the number of rubbing iterations increased, the surface energy value showed a similar trend at approximately 42.20 mJ/m2; however, the polar energy value increased from 4.288 to 6.470 mJ/m2. As the number of rubbing iterations increased, the surface energy value showed a similar trend at approximately 42.20 mJ/m2; however, the polar energy value increased from 4.288 to 6.470 mJ/m2. These results show that rubbing iterations affected the contribution of electrostatic energy to the free energy density of the ZnO thin films. However, it is difficult to conclude that the insignificant difference contributed to the LC alignment on the ZnO thin film [20]. On the other hand, it seems that the change in the distribution of electrostatic energy due to the rubbing iterations made the LC molecules more strongly anchored on the ZnO thin film surface, resulting in a low pre-tilt angle [27,28,29,30,31,32,33].
The above results revealed the LC alignment characteristics of the solution-derived ZnO thin film as a function of rubbing iteration number. The applicability of rubbing solution-derived ZnO thin film on LCD application devices was investigated by evaluating the electro-optical properties of the TN LC cells, including the V–T and RT characteristics. Figure 5 shows the V–T and RT characteristics of a TN LC cell using a rubbing-treated solution-derived ZnO thin film and a graph summarizing these results. The electro-optical characteristics of the TN LC cells were measured in the normally white state in which the polarizing plates were alternately attached to both sides of the TN LC cell. The V–T characteristic curves in Figure 5a were obtained by measuring the transmittance in the TN LC cells while increasing the voltage from 0 V to 6 V by 0.2 V increments. As shown in Figure 5a, the threshold voltages (at a transmittance rate of 90%) of the TN cell using the ZnO thin film were 1.72, 1.60, 1.54, and 1.65 V as the number of rubbing iterations increased to one, three, five, and seven, respectively. The lowest threshold voltage among the TN cells was observed when the rubbing iteration number was five. Figure 5b shows the RT characteristics of the TN cell using the solution-derived ZnO thin film with respect to the rubbing iteration number. The rise time is the time that it takes to change from 90% transmittance to 10% transmittance when the voltage is increased from 0 V to 6 V, whereas the fall time is the time that it takes to change from 10% transmittance to 90% transmittance when the voltage is decreased from 6 V to 0 V. The RT is the sum of the rise time and fall time. The RTs were 11.42, 10.66, 8.24, and 9.12 ms as the number of rubbing iterations increased to one, three, five, and seven, respectively. The optimal RT characteristics were observed with five rubbing iterations. In particular, the respective rise and fall times of 2.02 and 6.22 ms were superior to the rise and fall times of the other TN cells. In addition, compared to the RT of the conventional PI-based TN LC cell (20.08 ms), the TN LC cell using the rubbing-treated solution-derived ZnO films shows a very fast RT at half of that in [22]. Moreover, it is expected that it can be applied to an LC-based VR/AR display system which requires a very fast RT through the excellent EO characteristics of the TN LC cell of the solution-derived ZnO film through the rubbing process. Figure 5c and Table 1 summarizes the measurement results of the electro-optical properties of the TN cell using the ZnO thin film.

4. Conclusions

In this study, the solution-derived ZnO thin films were validated as an alternative to the LC alignment films. Excellent performance was verified through the evaluation of the electro-optical characteristics by the application to a TN cell, which is an LCD device. POM images and pretilt angle measurements of the rubbing-treated ZnO thin film confirmed a uniform LC alignment in one direction. All of the films showed uniform LC alignment characteristics, regardless of the number of rubbing iterations; however, the pretilt angle decreased as the number of rubbing iterations increased. The measurements of the contact angle and surface energy revealed that the number of rubbing iterations influenced the chemical affinity of the ZnO thin film and modified the pretilt angle. The evaluation of the V–T and RT characteristics of the TN cell showed that the optimal threshold voltage of 1.54 V and the fastest response time of 8.24 ms were obtained when the rubbing iteration number was five. Thus, the solution-derived ZnO inorganic alignment film showed the best performance when the rubbing iteration number was five. Finally, these results suggest that it can be applied to LC-based VR/AR display systems that require high-speed response characteristics in the near future.

Author Contributions

Conceptualization, methodology, writing—original draft preparation, H.-G.P.; validation, formal analysis, investigation, J.-A.K.; supervision, project administration, B.-J.K.; funding acquisition, H.-G.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2022-0121) and by the MSIT (Ministry of Science and ICT), Korea, under the Innovative Human Resource Development for Local Intellectualization support program (IITP-2022-RS-2022-00156361) supervised by the IITP (Institute for Information & communications Technology Planning & Evaluation).

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Crystals 12 01711 g001 550
Figure 1. Schematic for fabricating the AP (anti-parallel) and TN (twisted-nematic) LC cell using the solution-derived ZnO films with rubbing treatment. AP LC cell and TN LC cell are to confirm the liquid crystal alignment state and electro-optical characteristics, respectively.
Figure 1. Schematic for fabricating the AP (anti-parallel) and TN (twisted-nematic) LC cell using the solution-derived ZnO films with rubbing treatment. AP LC cell and TN LC cell are to confirm the liquid crystal alignment state and electro-optical characteristics, respectively.
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Crystals 12 01711 g002 550
Figure 2. Photomicrographs of AP LC cells with solution-derived ZnO films as a function of rubbing iteration number. A and P represent the analyzer and polarizer axes, respectively.
Figure 2. Photomicrographs of AP LC cells with solution-derived ZnO films as a function of rubbing iteration number. A and P represent the analyzer and polarizer axes, respectively.
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Figure 3. (a–d) Pretilt angles of LC molecules on solution-derived ZnO films in AP LC cells as a function of rubbing iteration number and (e) calculated pretilt angles of the LC molecules.
Figure 3. (a–d) Pretilt angles of LC molecules on solution-derived ZnO films in AP LC cells as a function of rubbing iteration number and (e) calculated pretilt angles of the LC molecules.
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Crystals 12 01711 g004 550
Figure 4. (a) Contact angles and (b) surface energies of solution-derived ZnO films as a function of rubbing iteration number.
Figure 4. (a) Contact angles and (b) surface energies of solution-derived ZnO films as a function of rubbing iteration number.
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Figure 5. (a) Voltage–transmittance (V–T) (b) response time and (c) change trend of threshold voltage and total response time characteristics of TN LC cells with solution-derived ZnO films as a function of rubbing iteration number.
Figure 5. (a) Voltage–transmittance (V–T) (b) response time and (c) change trend of threshold voltage and total response time characteristics of TN LC cells with solution-derived ZnO films as a function of rubbing iteration number.
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Table
Table 1. Voltage–transmittance (V–T) and response time (RT) characteristics of TN LC cells with rubbed ZnO films as a function of rubbing iteration number.
Table 1. Voltage–transmittance (V–T) and response time (RT) characteristics of TN LC cells with rubbed ZnO films as a function of rubbing iteration number.
SampleV–T (V)RT (ms)
Rubbing Iteration NumberThreshold VoltageRise TimeFall TimeTotal RT
11.723.557.8711.42
31.603.457.2110.66
51.542.026.228.24
71.652.226.909.12
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Park, H.-G.; Kim, J.-A.; Ko, B.-J. Electro-Optical Characteristics of Solution-Derived Zinc Oxide Film According to Number of Rubbing Iterations for Liquid Crystal Alignment. Crystals 2022, 12, 1711. https://doi.org/10.3390/cryst12121711

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Park H-G, Kim J-A, Ko B-J. Electro-Optical Characteristics of Solution-Derived Zinc Oxide Film According to Number of Rubbing Iterations for Liquid Crystal Alignment. Crystals. 2022; 12(12):1711. https://doi.org/10.3390/cryst12121711

Chicago/Turabian Style

Park, Hong-Gyu, Jin-Ah Kim, and Bong-Jin Ko. 2022. "Electro-Optical Characteristics of Solution-Derived Zinc Oxide Film According to Number of Rubbing Iterations for Liquid Crystal Alignment" Crystals 12, no. 12: 1711. https://doi.org/10.3390/cryst12121711

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