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Alignment of liquid crystal molecules on solution-derived zinc-tin-oxide films via ion beam irradiation
Materials Chemistry and Physics 173 (2016) 186e191
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Alignment of liquid crystal molecules on solution-derived zinc-tinoxide films via ion beam irradiation Hong-Gyu Park a, Hae-Chang Jeong a, Ju Hwan Lee a, Sang Bok Jang a, Byeong-Yun Oh b, Jeong-Min Han c, Dae-Shik Seo a, * a Information Display Device Laboratory, Department of Electrical and Electronic Engineering, Yonsei University, 262 Seongsanno, Seodaemun-gu, Seoul, 120-749, Republic of Korea b ZeSHTech Co., Ltd., Business Incubator, Gwangju Institute of Science and Technology, 123, Cheomdangwagi-ro, Buk-gu, Gwangju, 500-712, Republic of Korea c Department of Electronic, Seoil University, Jungnang-gu, Seoul, 131-702, Republic of Korea
h i g h l i g h t s ZTO alignment films were deposited by a solution process on ITO-coated glass. Uniform and homogeneous LC alignment was achieved on the IB-irradiated ZTO surface. Oxidation of ZTO films was confirmed using FESEM and XPS analysis. Enhanced EO characteristics of ECB cells based on ZTO films were achieved.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 October 2015 Received in revised form 13 January 2016 Accepted 31 January 2016 Available online 9 February 2016
We present the characteristics of annealing temperature-dependent, zinc-tin-oxide (ZTO) films deposited by a solution process for application in liquid crystal displays (LCDs). ZTO surfaces supported homogeneously-aligned liquid crystal (LC) molecules based on an ion beam irradiation system. Uniform LC alignment and a precise pretilt angle were obtained at an annealing temperature greater than 300 C. The oxidation of ZTO films was confirmed using field-emission-scanning electron microscopy and X-ray photoelectron spectroscopy. The electro-optical characteristics of electrically controlled birefringence (ECB) cells based on the ZTO films were superior to those based on polyimide. Especially, IB-irradiated ZTO films exhibited superior performance with respect to response time. This result indicates that this approach will allow for the fabrication of advanced LCDs with high performance. © 2016 Elsevier B.V. All rights reserved.
Keywords: Liquid crystals Oxides Solegel growth Optical properties
1. Introduction Liquid crystal (LC) alignment is one of the most important techniques for LC applications and thus, studies have been consistently and intensively investigated. The conventional method is the rubbing process; however, it is accompanied by several drawbacks, such as accumulation of electrostatic charges, local defects, and the generation of debris, which is caused by mechanical contact between the fabric and film surface [1,2]. To overcome the disadvantages of conventional rubbed polyimide (PI), non-contact methods have been investigated for LC alignment including
* Corresponding author. E-mail address: [email protected] (D.-S. Seo). http://dx.doi.org/10.1016/j.matchemphys.2016.01.068 0254-0584/© 2016 Elsevier B.V. All rights reserved.
ultraviolet exposure [3e5], a nanocrystalline-induced self-alignment method [6,7], a wrinkle-induced nano-logging method [8], and ion-beam (IB) irradiation [1,2,9e14]. IB irradiation has especially attracted interest due to its controllability and reliability for LC applications, and application to oxide films. Because oxide films have diverse advantages for LC applications with electro-optical performance, such as a reduced threshold voltage, fast response of LC molecules, and hysteresis free characteristics, oxide films have recently attracted attention as candidate materials for LC cells based on conventional rubbed PI using IB irradiation. In addition, by using the solution process, the fabrication of thin oxide films including compound oxide materials can be achieved easily and cost effectively [15]. Zinc-tin-oxide (ZTO) from a solution process is a representative compound oxide material for semiconductor materials. Because
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Fig. 1. (a) UVeVis transmittance spectra of the solution-derived ZTO film and (b) Photomicrographs of LC cells based on ZTO films fabricated at various annealing temperatures. “A” denotes “analyzer” and “P” denotes “polarizer”.
ZTO is composed of heavy-metal cations with large radii, the energy band is dispersed by overlap between the adjacent orbitals, leading to a wide band gap (3.5 eV) that is associated with good transparency [16e18]. Moreover, ZTO film is well known for its chemical and physical durability. Therefore, thin films have been used for semiconductors without a passivation layer. In addition, ZTO film is characterized by a smooth surface morphology with a root-mean-square (RMS) roughness of less than 5 nm that is suitable for LC alignment in the range of 1e8 nm. These attractive attributes of ZTO films are expected to be useful as an LC alignment layer. In this study, we demonstrate applicable LC devices exhibiting effective switching performance of LC molecules on IB-irradiated ZTO films. Stable homogeneous LC alignment on IB-irradiated ZTO films was achieved by adjusting the annealing temperature. To confirm the alignment property, polarized optical microscope (POM) analysis and the crystal rotation method were conducted. Xray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were used to elucidate the mechanism of LC alignment on the thin films. The electro-optical properties of the electrically-controlled birefringence (ECB) cells were measured to confirm the switching performance of LC molecules on the IBirradiated ZTO films.
2. Experimental ZTO films were prepared by a solution method. A 0.1 M ZTO solution was prepared for the alignment layer by dissolving zinc acetate dihydrate [Zn(CH3COO)2$2H2O] and tin(II) chloride [SnCl2] in a solvent of 2-methoxyethanol (2 ME). A few drops of acetic acid and mono-ethanolamine (MEA) were added for stability and homogeneity of the composite solution. The solution was stirred at 75 C for 3 h using a hot plate with a magnetic stirrer. The obtained homogeneous solution was aged for 1 day at room temperature. Indium-tin-oxide (ITO)-coated glass was used as the substrate. Prior to deposition of the ZTO films on the substrate, the ITO-glass substrates were subject to a typical cleaning process of electronic devices. The substrate was cleaned using ultrasonic vibration with acetone, methanol, and deionized water for 10 min each and then dried with N2 gas. The prepared ZTO solution was spin-coated on the cleaned substrate, where the spin rate was 3000 rpm and the time duration was 30 s. The coated substrate was prebaked at 100 C for 10 min and then annealed at 100, 200, 300, 400, and 500 C for 1 h on a hot plate. A Duo PI Gatron ion beam system was used for IB irradiation. The ZTO film on the substrate was exposed to Arþ IB plasma at various intensities of 1700 eV for 2 min. Using a Faraday cup system, the current density of the IB of positively-charged particles was
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Fig. 2. (a) Transmittance versus incident angle of the LC molecules on the ZTO films fabricated at annealing temperatures of 200 and 400 C. (b) Pretilt angles of LC molecules on the IB-irradiated ZTO films at various annealing temperatures.
Fig. 3. FESEM images of solution-derived ZTO films at various annealing temperatures.
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morphology was confirmed using a field-emission-scanning electron microscope (FESEM; S-4300SE, Hitachi). The chemical bonding of the ZTO films treated by IB irradiation was analyzed using XPS (ES-CALAB 220i-XL, VG Scientific). ECB mode cells with a 2.5 mm cell gap were fabricated to measure the electro-optical properties, including voltage-transmittance (V-T) and response-time (RT), indicating the possibility of LCD application. 3. Results and discussion
Fig. 4. XPS spectra of the (a) Zn 2p, (b) Sn 3d, and (c) O 1s core levels of IB-irradiated ZTO films fabricated at various annealing temperatures.
measured to be 1.1 mA cm2. The LC cells were fabricated based on the IB-treated ZTO films in an antiparallel configuration with a cell gap of 60 mm. The LC cells based on IB-treated ZTO films were filled with a positive LC (MJ001929, ne ¼ 1.5859. no ¼ 1.4872 and Dε ¼ 8.2; Merck). A polarized optical microscopy (POM) image was captured via a BXP 51 (Olympus). The optical properties of the ZTO films were confirmed by UVevisible near-infrared (UVeVISeNIR) spectrometric measurements (V-650, JASCO). The oscillation of the transmittance was measured by rotating the LC cell via TBA 107 (Autronic) for calculating the pretilt angle of the LCs. The surface
The optical property and pretilt angle of the LC alignment for the ZTO thin films were measured to confirm the practical application of the LC devices. ZTO is known to have good transparency, due to its large band gap. Fig. 1(a) shows UV/VIS transmittances over the wavelength range 250e850 nm for solution-derived ZTO films on glass substrate. The average transmittances of the solution-derived ZTO film and as-glass substrate over the wavelength range 420e780 nm were 83.3% and 83.5%, respectively. Thus, solutionderived ZTO films are strong candidates for alignment layers of LC devices, without loss of transparency. Fig. 1(b) shows POM images of LC cells based on IB-irradiated ZTO films at different annealing temperatures. From the POM images, locally-aligned LC molecules on the ZTO surface were observed at a relatively low annealing temperature less than 300 C. In this case, a unidirectional schlieren texture was observed, which was attributed to the capillary forces from LC injection, but a slight leakage was also observed over the whole area on the LC cells and was not suitable for industrial LC devices. In contrast, uniform and unidirectional alignment of LC molecules was observed on the ZTO surface at an annealing temperature greater than 300 C, as with conventional rubbed PI. Fig. 2 shows the transmittance of each antiparallel LC cell based on IB-irradiated ZTO films with a latitudinal rotation of ±70 and the calculated pretilt angles of those from oscillation curves via the crystal rotation method. The oscillation of the transmittance was measured by LC cell rotation. As shown in Fig. 2(a), the blue and red lines of the oscillation curves represent simulated and experimental curves, respectively. If two lines are matched, the LC molecules are well aligned and can be used to calculate the pretilt angles with high reliability. Alternatively, differences between the two lines indicate that the thin films did not induce uniform alignment of the LC molecules uni-directionally and the LCs did not have a stable pretilt angle. The standard for determining the wellmatched oscillation curves is the point in which the error value is less than 0.01. At an annealing temperature of 200 C, a significant difference between two curves was observed with an error rate of 0.037, illustrating that the IB-irradiated thin film at an annealing temperature of 200 C did not induce uniform LC alignment. In contrast, at an annealing temperature of 400 C, LC molecules had stable pretilt angles from nearly identical curves with an error rate of 0.001. These two different results were consistent with the POM analyses. The calculated pretilt angles of the LC molecules on the ZTO thin films are shown in Fig. 2(b). In addition to the aforementioned POM analysis that resulted in a uniform alignment state at an annealing temperature greater than 300 C, pretilt angles were also obtained at annealing temperatures greater than 300 C. The calculated pretilt angles of each sample at 300, 400, and 500 C were 0.01, 0.31, and 0.21, respectively, which shows a state of homogeneously-aligned LC molecules, and the standard deviations were 0.002, 0.005, and 0.004, respectively. The distributions of pretilt angles were obtained from 10 measurements on each sample. A very low standard deviation indicates that the LC molecules on the thin films were well aligned over the entire surface. The results from the POM analysis and pretilt angles confirmed the uniformity and stability of homogeneous LC alignment on IBirradiated ZTO films at an annealing temperature greater than
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Fig. 5. The EO characteristics of ECB cells. (a) V-T curves, (b) switching voltage, (c) RT curves, and (d) total RT values.
300 C. Fig. 3 illustrates the SEM micrographs of the surfaces of the IBirradiated ZTO films annealed at 100, 200, 300, 400, and 500 C. These micrographs reveal that the annealing temperature partially affects the surface morphology of the ZTO films via two behaviors. In the first behavior, the surface morphology became smooth with an increase in annealing temperature up to 300 C. Because the boiling points of the solvent and organic additive are less than or equal to 200 C, the solvent and organic additive of the stabilizer gradually evaporated up to their boiling points. Thus, unstably distributed aggregations and particles on the surface below 200 C were attributed to residual solvent and organic additive that had not evaporated and remained on the surface. Subsequently, at 300 C, a perfectly smooth surface was observed after removing the residual solvent. ZTO films from the solution process are preferred for the buffer layer in semiconductors because of the smooth surface. The second behavior was observed using SEM in which the surface morphology becomes rough with an annealing temperature greater than 300 C. Chemical decomposition originating from the metal acetates of the ZTO precursor occurred between 300 and 500 C in the solution process. During this decomposition process, weight loss was caused by the temperature. The physical collision between Arþ and the decomposed ZTO surface induces a defect on the surface and provides nucleation sites for aggregation. Thus, a smooth surface at 300 C originating from the removal of residual solvent translated to evenly distributed particles with a relatively uniform size. To further investigate the effect of LC alignment on the IBirradiated ZTO films, XPS analysis was conducted because LC alignment is commonly affected by the chemical composition of the surface alignment. The Zn 2p, Sn 3d, and O 1s core-level spectra and binding energies that were referenced to the neutral adventitious C 1s peak defined at 284.6 eV are shown in Fig. 4. Fig. 4(a) shows XPS spectra for the Zn 2p3/2 and Zn 2p1/2 peaks. At an annealing temperature of 100 C, ZTO films were not properly formed due to the presence of residual solvents. The
measured Zn 2p3/2 and Zn 2p1/2 photoelectron peaks were at 1022.7 ± 0.1 eV and 1045.7 ± 0.1 eV with various annealing temperature. The peaks of Zn 2p3/2 and Zn 2p1/2 were above those of conventional ZnO, it demonstrated Zn was in oxidation state. The binding energies of the Zn 2p3/2 and Zn 2p1/2 peaks on conventional ZnO surface were centered at 1020.8 ± 0.1 and 1043.8 ± 0.1 eV, respectively [19]. With increasing the annealing temperature, a shift of the Zn 2p binding energy in the direction of the higher energy side existed compared to conventional ZnO [19], which was attributed to Zn oxidation state increase. This result indicates that the ZTO films were fully oxidized and gradually grew at an annealing temperature greater than 300 C. The XPS spectra for the Sn 3d showed a similar tendency with that of Zn 2p, as shown in Fig. 4(b). The binding energy of Sn 3d5/2 was centered at 486.5 eV, indicating a fully oxidized Sn irrespective of the annealing temperature [7]. At an annealing temperature of 100 C, the Sn component was formed better than the Zn component in the ZTO film because solubility of the Sn component was superior to that of the Zn component. However, its peak intensity considerably increased at an annealing temperature greater than 400 C as the ZTO films were fully oxidized. A peak shift in the O 1s peak was also observed as a function of the annealing temperature, as shown in Fig. 4(c). This result demonstrates that the ZTO films were gradually oxidized as the annealing temperature increased. Based on the previous studies, IB bombardment on the SnO2 surfaces strongly induced a surface transformation, irrespective of the initial state of the SnO2 film [20]. Similarly, anisotropic characteristics were induced by IB irradiation on the ZTO surfaces. The uniform LC alignment on the IB-irradiated ZTO films was attributed to van der Waals interactions between the LC molecules and the ZTO films due to the anisotropic characteristics. To confirm the possibility for practical application of LC devices, the electro-optical properties, including V-T and RT characteristics, were measured with ECB cells fabricated based on the IB-irradiated ZTO film. At annealing temperature of 300 C and 400 C, ECB cells with IB-irradiated ZTO films unusually operated with optical
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bounce; hence, the electro-optical properties of ECB cell with the IB-irradiated ZTO films at an annealing temperature of 500 C were measured. Moreover, ECB cells assembled with conventional rubbed PI and IB-irradiated PI were fabricated for comparison. Stable voltage-transmittance curves of ECB cells based on IB-irradiated ZTO films were obtained, as shown in Fig. 5(a). The threshold voltages at 90% transmittance of each ECB cell with rubbed PI, IBirradiated PI, and IB-irradiated ZTO films were 2.23, 1.77, and 1.66 V, respectively. Reduced threshold voltages were obtained on IB-irradiated thin films. The rubbing process is a mechanical contact method, and thus it can create electrostatic charge on the surface by contact of the fabric and film surface, with accumulation over the entire surfaces. Because accumulated charges on the surface can affect the capacitance of the layers, increased threshold voltages of the rubbed PI were obtained compared to those of IBirradiated thin film, whereas because IB-irradiated thin films overcome these charge accumulation issues, reduced threshold voltages were obtained. As shown in Fig. 5(b), the switching voltage of the LC molecules was obtained from the V-T curves by subtracting the 10% from the 90% threshold voltage. The switching voltage is associated with the intensity of the digital image, i.e., the gray scale. A large switching voltage can be realized as many shades of gray between black and white, and thus can produce a large amount of digital information. ECB cells based on PI films exhibit similar switching voltages irrespective of the method for alignment treatment; ECB cells with rubbed PI and IB-irradiated PI were 1.10 and 1.06 V, respectively. The switching voltage of ECB cells with IBirradiated ZTO films was 2.03 V, which is larger than for PI. This large switching voltage of the LC molecules on IB-irradiated ZTO films has the advantage of intensive digital information for display devices. Fig. 5(c) shows RT curves of the ECB cells fabricated with the three different films. Conventional-rubbed PI had a response time of 17.29 ms (rise and fall times of 3.00 and 14.29 ms, respectively). IB-irradiated PI had a response time of 16.30 ms (rise and fall times of 3.40 and12.90 ms, respectively). IB-irradiated ZTO films yielded the fastest response of LC molecules at 6.60 ms, with rise and fall times of 4.03 and 2.57 ms, respectively, despite the higher rise time compared to the other conditions. In addition, the ECB cells with IB-irradiated ZTO films exhibited an enhanced electrooptical performance compared to that with IB-irradiated La2O3 films, which has a threshold voltage of 1.39 V and a response time of 7.74 ms [21]. These effective electro-optical performances of IBirradiated ZTO films, including V-T and R-T characteristics, demonstrate the possibility for advanced LC applications that may be a substitute for conventional rubbed PI cells of ECB cells.
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4. Conclusions In this study, we investigated the characteristics of IB-irradiated ZTO films fabricated by a solution process for use as LC alignment layers. Uniform LC alignment was achieved at an annealing temperature greater than 300 C. As the annealing temperature increased, the ZTO films were oxidized, which affected the uniform LC alignment. In addition, the LC alignment on the ZTO surfaces was also attributed to surface reformation induced by IB irradiation. Moreover, superior EO characteristics were observed for ECB cells fabricated from the ZTO films, showing the remarkable potential for LC applications. References [1] P. Chaudhari, J. Lacey, J. Doyle, E. Galligan, S.C.A. Lien, A. Callegary, G. Hougham, N.D. Lang, P.S. Andry, R. John, K.H. Yang, M. Lu, C. Cai, J. Speidell, S. Purushothaman, J. Ritsko, M. Samant, J. Stӧhr, Y. Nakagawa, Y. Katoh, Y. Saitoh, K. Sakai, H. Satoh, S. Odahara, H. Nakano, J. Nakagaki, Y. Shiota, Nature 411 (2001) 56e59. €hr, M.G. Samant, J. Lüning, A.C. Callegari, P. Chaudhari, J.P. Doyle, [2] J. Sto J.A. Lacey, S.A. Lien, S. Purushothaman, J.L. Speidell, Science 292 (2001) 2299e2302. [3] M. Schadt, K. Schmitt, V. Kozinkov, V. Chigrinov, Jpn. J. Appl. Phys. 31 (1992) 2155e2164. [4] P.J. Shannon, W.M. Gibbons, S.T. Sun, Nature 368 (1994) 532e533. [5] J.Y.L. Ho, V.G. Chigrinov, H.S. Kwok, Appl. Phys. Lett. 90 (2007) 243506. [6] S.Y. Kim, H.-G. Park, M.-J. Cho, H.-C. Jeong, D.-S. Seo, Liq. Cryst. 41 (2014) 940e945. [7] H.-G. Park, H.-C. Jeong, J.-J. Han, S.Y. Kim, J.-H. Kim, D.-H. Kim, D.-S. Seo, J. Mater. Chem. C 2 (2014) 3960e3964. [8] H.-C. Jeong, H.-G. Park, J.H. Lee, Y.H. Jung, S.B. Jang, D.-S. Seo, Sci. Rep. 5 (2015) 8641. [9] H.-G. Park, Y.-H. Kim, B.-Y. Oh, W.-K. Lee, B.-Y. Kim, D.-S. Seo, J.-Y. Hwang, Appl. Phys. Lett. 93 (2008) 233507. [10] S.-W. Hwang, J.-H. Seo, T.-H. Yoon, J.C. Kim, Thin Solid Films 519 (2010) 885e889. [11] P.K. Son, S.-W. Choi, Surf. Interface Anal. 44 (2012) 763e767. [12] S.S. Chae, B.H. Hwang, W.S. Jang, J.Y. Oh, J.H. Park, S.J. Lee, K.M. Song, H.K. Baik, Soft Matter 8 (2012) 1437e1442. [13] J.J. Lee, H.-G. Park, J.-J. Han, D.-H. Kim, D.-S. Seo, J. Mater. Chem. C 1 (2013) 6824e6828. [14] Y.-G. Lee, H.-G. Park, H.-C. Jeong, J.H. Lee, G.-S. Heo, D.-S. Seo, Opt. Express 23 (2015) 17290e17300. [15] S.-J. Ding, C. Zhu, M.-F. Li, D.W. Zhang, Appl. Phys. Lett. 87 (2005) 053501. [16] H.Q. Chiang, J.F. Wager, R.L. Hoffman, J. Jeong, D.A. Keszler, Appl. Phys. Lett. 86 (2005) 013503. [17] D. Hong, J.F. Wager, J. Vac. Sci. Technol. B 23 (2005) L25eL27. [18] S.-J. Seo, C.G. Choi, Y.H. Hwang, B.-S. Bae, J. Phys. D. Appl. Phys. 42 (2009) 035106. [19] P.S. Venkatesh, S. Balakumar, K. Jeganathan, RSC Adv. 4 (2014) 5030e5035. [20] H.-G. Park, H.-C. Jeong, T.-K. Park, D.-S. Seo, RSC Adv. 5 (2015) 1918e1922. [21] J.-W. Lee, H.-G. Park, H.-C. Jeong, D.-S. Seo, ECS Solid State Lett. 4 (2015) R13eR16.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Alignment of liquid crystal molecules on solution-derived zinc-tinoxide films via ion beam irradiation Hong-Gyu Park a, Hae-Chang Jeong a, Ju Hwan Lee a, Sang Bok Jang a, Byeong-Yun Oh b, Jeong-Min Han c, Dae-Shik Seo a, * a Information Display Device Laboratory, Department of Electrical and Electronic Engineering, Yonsei University, 262 Seongsanno, Seodaemun-gu, Seoul, 120-749, Republic of Korea b ZeSHTech Co., Ltd., Business Incubator, Gwangju Institute of Science and Technology, 123, Cheomdangwagi-ro, Buk-gu, Gwangju, 500-712, Republic of Korea c Department of Electronic, Seoil University, Jungnang-gu, Seoul, 131-702, Republic of Korea
h i g h l i g h t s ZTO alignment films were deposited by a solution process on ITO-coated glass. Uniform and homogeneous LC alignment was achieved on the IB-irradiated ZTO surface. Oxidation of ZTO films was confirmed using FESEM and XPS analysis. Enhanced EO characteristics of ECB cells based on ZTO films were achieved.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 October 2015 Received in revised form 13 January 2016 Accepted 31 January 2016 Available online 9 February 2016
We present the characteristics of annealing temperature-dependent, zinc-tin-oxide (ZTO) films deposited by a solution process for application in liquid crystal displays (LCDs). ZTO surfaces supported homogeneously-aligned liquid crystal (LC) molecules based on an ion beam irradiation system. Uniform LC alignment and a precise pretilt angle were obtained at an annealing temperature greater than 300 C. The oxidation of ZTO films was confirmed using field-emission-scanning electron microscopy and X-ray photoelectron spectroscopy. The electro-optical characteristics of electrically controlled birefringence (ECB) cells based on the ZTO films were superior to those based on polyimide. Especially, IB-irradiated ZTO films exhibited superior performance with respect to response time. This result indicates that this approach will allow for the fabrication of advanced LCDs with high performance. © 2016 Elsevier B.V. All rights reserved.
Keywords: Liquid crystals Oxides Solegel growth Optical properties
1. Introduction Liquid crystal (LC) alignment is one of the most important techniques for LC applications and thus, studies have been consistently and intensively investigated. The conventional method is the rubbing process; however, it is accompanied by several drawbacks, such as accumulation of electrostatic charges, local defects, and the generation of debris, which is caused by mechanical contact between the fabric and film surface [1,2]. To overcome the disadvantages of conventional rubbed polyimide (PI), non-contact methods have been investigated for LC alignment including
* Corresponding author. E-mail address: [email protected] (D.-S. Seo). http://dx.doi.org/10.1016/j.matchemphys.2016.01.068 0254-0584/© 2016 Elsevier B.V. All rights reserved.
ultraviolet exposure [3e5], a nanocrystalline-induced self-alignment method [6,7], a wrinkle-induced nano-logging method [8], and ion-beam (IB) irradiation [1,2,9e14]. IB irradiation has especially attracted interest due to its controllability and reliability for LC applications, and application to oxide films. Because oxide films have diverse advantages for LC applications with electro-optical performance, such as a reduced threshold voltage, fast response of LC molecules, and hysteresis free characteristics, oxide films have recently attracted attention as candidate materials for LC cells based on conventional rubbed PI using IB irradiation. In addition, by using the solution process, the fabrication of thin oxide films including compound oxide materials can be achieved easily and cost effectively [15]. Zinc-tin-oxide (ZTO) from a solution process is a representative compound oxide material for semiconductor materials. Because
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Fig. 1. (a) UVeVis transmittance spectra of the solution-derived ZTO film and (b) Photomicrographs of LC cells based on ZTO films fabricated at various annealing temperatures. “A” denotes “analyzer” and “P” denotes “polarizer”.
ZTO is composed of heavy-metal cations with large radii, the energy band is dispersed by overlap between the adjacent orbitals, leading to a wide band gap (3.5 eV) that is associated with good transparency [16e18]. Moreover, ZTO film is well known for its chemical and physical durability. Therefore, thin films have been used for semiconductors without a passivation layer. In addition, ZTO film is characterized by a smooth surface morphology with a root-mean-square (RMS) roughness of less than 5 nm that is suitable for LC alignment in the range of 1e8 nm. These attractive attributes of ZTO films are expected to be useful as an LC alignment layer. In this study, we demonstrate applicable LC devices exhibiting effective switching performance of LC molecules on IB-irradiated ZTO films. Stable homogeneous LC alignment on IB-irradiated ZTO films was achieved by adjusting the annealing temperature. To confirm the alignment property, polarized optical microscope (POM) analysis and the crystal rotation method were conducted. Xray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were used to elucidate the mechanism of LC alignment on the thin films. The electro-optical properties of the electrically-controlled birefringence (ECB) cells were measured to confirm the switching performance of LC molecules on the IBirradiated ZTO films.
2. Experimental ZTO films were prepared by a solution method. A 0.1 M ZTO solution was prepared for the alignment layer by dissolving zinc acetate dihydrate [Zn(CH3COO)2$2H2O] and tin(II) chloride [SnCl2] in a solvent of 2-methoxyethanol (2 ME). A few drops of acetic acid and mono-ethanolamine (MEA) were added for stability and homogeneity of the composite solution. The solution was stirred at 75 C for 3 h using a hot plate with a magnetic stirrer. The obtained homogeneous solution was aged for 1 day at room temperature. Indium-tin-oxide (ITO)-coated glass was used as the substrate. Prior to deposition of the ZTO films on the substrate, the ITO-glass substrates were subject to a typical cleaning process of electronic devices. The substrate was cleaned using ultrasonic vibration with acetone, methanol, and deionized water for 10 min each and then dried with N2 gas. The prepared ZTO solution was spin-coated on the cleaned substrate, where the spin rate was 3000 rpm and the time duration was 30 s. The coated substrate was prebaked at 100 C for 10 min and then annealed at 100, 200, 300, 400, and 500 C for 1 h on a hot plate. A Duo PI Gatron ion beam system was used for IB irradiation. The ZTO film on the substrate was exposed to Arþ IB plasma at various intensities of 1700 eV for 2 min. Using a Faraday cup system, the current density of the IB of positively-charged particles was
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Fig. 2. (a) Transmittance versus incident angle of the LC molecules on the ZTO films fabricated at annealing temperatures of 200 and 400 C. (b) Pretilt angles of LC molecules on the IB-irradiated ZTO films at various annealing temperatures.
Fig. 3. FESEM images of solution-derived ZTO films at various annealing temperatures.
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morphology was confirmed using a field-emission-scanning electron microscope (FESEM; S-4300SE, Hitachi). The chemical bonding of the ZTO films treated by IB irradiation was analyzed using XPS (ES-CALAB 220i-XL, VG Scientific). ECB mode cells with a 2.5 mm cell gap were fabricated to measure the electro-optical properties, including voltage-transmittance (V-T) and response-time (RT), indicating the possibility of LCD application. 3. Results and discussion
Fig. 4. XPS spectra of the (a) Zn 2p, (b) Sn 3d, and (c) O 1s core levels of IB-irradiated ZTO films fabricated at various annealing temperatures.
measured to be 1.1 mA cm2. The LC cells were fabricated based on the IB-treated ZTO films in an antiparallel configuration with a cell gap of 60 mm. The LC cells based on IB-treated ZTO films were filled with a positive LC (MJ001929, ne ¼ 1.5859. no ¼ 1.4872 and Dε ¼ 8.2; Merck). A polarized optical microscopy (POM) image was captured via a BXP 51 (Olympus). The optical properties of the ZTO films were confirmed by UVevisible near-infrared (UVeVISeNIR) spectrometric measurements (V-650, JASCO). The oscillation of the transmittance was measured by rotating the LC cell via TBA 107 (Autronic) for calculating the pretilt angle of the LCs. The surface
The optical property and pretilt angle of the LC alignment for the ZTO thin films were measured to confirm the practical application of the LC devices. ZTO is known to have good transparency, due to its large band gap. Fig. 1(a) shows UV/VIS transmittances over the wavelength range 250e850 nm for solution-derived ZTO films on glass substrate. The average transmittances of the solution-derived ZTO film and as-glass substrate over the wavelength range 420e780 nm were 83.3% and 83.5%, respectively. Thus, solutionderived ZTO films are strong candidates for alignment layers of LC devices, without loss of transparency. Fig. 1(b) shows POM images of LC cells based on IB-irradiated ZTO films at different annealing temperatures. From the POM images, locally-aligned LC molecules on the ZTO surface were observed at a relatively low annealing temperature less than 300 C. In this case, a unidirectional schlieren texture was observed, which was attributed to the capillary forces from LC injection, but a slight leakage was also observed over the whole area on the LC cells and was not suitable for industrial LC devices. In contrast, uniform and unidirectional alignment of LC molecules was observed on the ZTO surface at an annealing temperature greater than 300 C, as with conventional rubbed PI. Fig. 2 shows the transmittance of each antiparallel LC cell based on IB-irradiated ZTO films with a latitudinal rotation of ±70 and the calculated pretilt angles of those from oscillation curves via the crystal rotation method. The oscillation of the transmittance was measured by LC cell rotation. As shown in Fig. 2(a), the blue and red lines of the oscillation curves represent simulated and experimental curves, respectively. If two lines are matched, the LC molecules are well aligned and can be used to calculate the pretilt angles with high reliability. Alternatively, differences between the two lines indicate that the thin films did not induce uniform alignment of the LC molecules uni-directionally and the LCs did not have a stable pretilt angle. The standard for determining the wellmatched oscillation curves is the point in which the error value is less than 0.01. At an annealing temperature of 200 C, a significant difference between two curves was observed with an error rate of 0.037, illustrating that the IB-irradiated thin film at an annealing temperature of 200 C did not induce uniform LC alignment. In contrast, at an annealing temperature of 400 C, LC molecules had stable pretilt angles from nearly identical curves with an error rate of 0.001. These two different results were consistent with the POM analyses. The calculated pretilt angles of the LC molecules on the ZTO thin films are shown in Fig. 2(b). In addition to the aforementioned POM analysis that resulted in a uniform alignment state at an annealing temperature greater than 300 C, pretilt angles were also obtained at annealing temperatures greater than 300 C. The calculated pretilt angles of each sample at 300, 400, and 500 C were 0.01, 0.31, and 0.21, respectively, which shows a state of homogeneously-aligned LC molecules, and the standard deviations were 0.002, 0.005, and 0.004, respectively. The distributions of pretilt angles were obtained from 10 measurements on each sample. A very low standard deviation indicates that the LC molecules on the thin films were well aligned over the entire surface. The results from the POM analysis and pretilt angles confirmed the uniformity and stability of homogeneous LC alignment on IBirradiated ZTO films at an annealing temperature greater than
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Fig. 5. The EO characteristics of ECB cells. (a) V-T curves, (b) switching voltage, (c) RT curves, and (d) total RT values.
300 C. Fig. 3 illustrates the SEM micrographs of the surfaces of the IBirradiated ZTO films annealed at 100, 200, 300, 400, and 500 C. These micrographs reveal that the annealing temperature partially affects the surface morphology of the ZTO films via two behaviors. In the first behavior, the surface morphology became smooth with an increase in annealing temperature up to 300 C. Because the boiling points of the solvent and organic additive are less than or equal to 200 C, the solvent and organic additive of the stabilizer gradually evaporated up to their boiling points. Thus, unstably distributed aggregations and particles on the surface below 200 C were attributed to residual solvent and organic additive that had not evaporated and remained on the surface. Subsequently, at 300 C, a perfectly smooth surface was observed after removing the residual solvent. ZTO films from the solution process are preferred for the buffer layer in semiconductors because of the smooth surface. The second behavior was observed using SEM in which the surface morphology becomes rough with an annealing temperature greater than 300 C. Chemical decomposition originating from the metal acetates of the ZTO precursor occurred between 300 and 500 C in the solution process. During this decomposition process, weight loss was caused by the temperature. The physical collision between Arþ and the decomposed ZTO surface induces a defect on the surface and provides nucleation sites for aggregation. Thus, a smooth surface at 300 C originating from the removal of residual solvent translated to evenly distributed particles with a relatively uniform size. To further investigate the effect of LC alignment on the IBirradiated ZTO films, XPS analysis was conducted because LC alignment is commonly affected by the chemical composition of the surface alignment. The Zn 2p, Sn 3d, and O 1s core-level spectra and binding energies that were referenced to the neutral adventitious C 1s peak defined at 284.6 eV are shown in Fig. 4. Fig. 4(a) shows XPS spectra for the Zn 2p3/2 and Zn 2p1/2 peaks. At an annealing temperature of 100 C, ZTO films were not properly formed due to the presence of residual solvents. The
measured Zn 2p3/2 and Zn 2p1/2 photoelectron peaks were at 1022.7 ± 0.1 eV and 1045.7 ± 0.1 eV with various annealing temperature. The peaks of Zn 2p3/2 and Zn 2p1/2 were above those of conventional ZnO, it demonstrated Zn was in oxidation state. The binding energies of the Zn 2p3/2 and Zn 2p1/2 peaks on conventional ZnO surface were centered at 1020.8 ± 0.1 and 1043.8 ± 0.1 eV, respectively [19]. With increasing the annealing temperature, a shift of the Zn 2p binding energy in the direction of the higher energy side existed compared to conventional ZnO [19], which was attributed to Zn oxidation state increase. This result indicates that the ZTO films were fully oxidized and gradually grew at an annealing temperature greater than 300 C. The XPS spectra for the Sn 3d showed a similar tendency with that of Zn 2p, as shown in Fig. 4(b). The binding energy of Sn 3d5/2 was centered at 486.5 eV, indicating a fully oxidized Sn irrespective of the annealing temperature [7]. At an annealing temperature of 100 C, the Sn component was formed better than the Zn component in the ZTO film because solubility of the Sn component was superior to that of the Zn component. However, its peak intensity considerably increased at an annealing temperature greater than 400 C as the ZTO films were fully oxidized. A peak shift in the O 1s peak was also observed as a function of the annealing temperature, as shown in Fig. 4(c). This result demonstrates that the ZTO films were gradually oxidized as the annealing temperature increased. Based on the previous studies, IB bombardment on the SnO2 surfaces strongly induced a surface transformation, irrespective of the initial state of the SnO2 film [20]. Similarly, anisotropic characteristics were induced by IB irradiation on the ZTO surfaces. The uniform LC alignment on the IB-irradiated ZTO films was attributed to van der Waals interactions between the LC molecules and the ZTO films due to the anisotropic characteristics. To confirm the possibility for practical application of LC devices, the electro-optical properties, including V-T and RT characteristics, were measured with ECB cells fabricated based on the IB-irradiated ZTO film. At annealing temperature of 300 C and 400 C, ECB cells with IB-irradiated ZTO films unusually operated with optical
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bounce; hence, the electro-optical properties of ECB cell with the IB-irradiated ZTO films at an annealing temperature of 500 C were measured. Moreover, ECB cells assembled with conventional rubbed PI and IB-irradiated PI were fabricated for comparison. Stable voltage-transmittance curves of ECB cells based on IB-irradiated ZTO films were obtained, as shown in Fig. 5(a). The threshold voltages at 90% transmittance of each ECB cell with rubbed PI, IBirradiated PI, and IB-irradiated ZTO films were 2.23, 1.77, and 1.66 V, respectively. Reduced threshold voltages were obtained on IB-irradiated thin films. The rubbing process is a mechanical contact method, and thus it can create electrostatic charge on the surface by contact of the fabric and film surface, with accumulation over the entire surfaces. Because accumulated charges on the surface can affect the capacitance of the layers, increased threshold voltages of the rubbed PI were obtained compared to those of IBirradiated thin film, whereas because IB-irradiated thin films overcome these charge accumulation issues, reduced threshold voltages were obtained. As shown in Fig. 5(b), the switching voltage of the LC molecules was obtained from the V-T curves by subtracting the 10% from the 90% threshold voltage. The switching voltage is associated with the intensity of the digital image, i.e., the gray scale. A large switching voltage can be realized as many shades of gray between black and white, and thus can produce a large amount of digital information. ECB cells based on PI films exhibit similar switching voltages irrespective of the method for alignment treatment; ECB cells with rubbed PI and IB-irradiated PI were 1.10 and 1.06 V, respectively. The switching voltage of ECB cells with IBirradiated ZTO films was 2.03 V, which is larger than for PI. This large switching voltage of the LC molecules on IB-irradiated ZTO films has the advantage of intensive digital information for display devices. Fig. 5(c) shows RT curves of the ECB cells fabricated with the three different films. Conventional-rubbed PI had a response time of 17.29 ms (rise and fall times of 3.00 and 14.29 ms, respectively). IB-irradiated PI had a response time of 16.30 ms (rise and fall times of 3.40 and12.90 ms, respectively). IB-irradiated ZTO films yielded the fastest response of LC molecules at 6.60 ms, with rise and fall times of 4.03 and 2.57 ms, respectively, despite the higher rise time compared to the other conditions. In addition, the ECB cells with IB-irradiated ZTO films exhibited an enhanced electrooptical performance compared to that with IB-irradiated La2O3 films, which has a threshold voltage of 1.39 V and a response time of 7.74 ms [21]. These effective electro-optical performances of IBirradiated ZTO films, including V-T and R-T characteristics, demonstrate the possibility for advanced LC applications that may be a substitute for conventional rubbed PI cells of ECB cells.
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4. Conclusions In this study, we investigated the characteristics of IB-irradiated ZTO films fabricated by a solution process for use as LC alignment layers. Uniform LC alignment was achieved at an annealing temperature greater than 300 C. As the annealing temperature increased, the ZTO films were oxidized, which affected the uniform LC alignment. In addition, the LC alignment on the ZTO surfaces was also attributed to surface reformation induced by IB irradiation. Moreover, superior EO characteristics were observed for ECB cells fabricated from the ZTO films, showing the remarkable potential for LC applications. References [1] P. Chaudhari, J. Lacey, J. Doyle, E. Galligan, S.C.A. Lien, A. Callegary, G. Hougham, N.D. Lang, P.S. Andry, R. John, K.H. Yang, M. Lu, C. Cai, J. Speidell, S. Purushothaman, J. Ritsko, M. Samant, J. Stӧhr, Y. Nakagawa, Y. Katoh, Y. Saitoh, K. Sakai, H. Satoh, S. Odahara, H. Nakano, J. Nakagaki, Y. Shiota, Nature 411 (2001) 56e59. €hr, M.G. Samant, J. Lüning, A.C. Callegari, P. Chaudhari, J.P. Doyle, [2] J. Sto J.A. Lacey, S.A. Lien, S. Purushothaman, J.L. Speidell, Science 292 (2001) 2299e2302. [3] M. Schadt, K. Schmitt, V. Kozinkov, V. Chigrinov, Jpn. J. Appl. Phys. 31 (1992) 2155e2164. [4] P.J. Shannon, W.M. Gibbons, S.T. Sun, Nature 368 (1994) 532e533. [5] J.Y.L. Ho, V.G. Chigrinov, H.S. Kwok, Appl. Phys. Lett. 90 (2007) 243506. [6] S.Y. Kim, H.-G. Park, M.-J. Cho, H.-C. Jeong, D.-S. Seo, Liq. Cryst. 41 (2014) 940e945. [7] H.-G. Park, H.-C. Jeong, J.-J. Han, S.Y. Kim, J.-H. Kim, D.-H. Kim, D.-S. Seo, J. Mater. Chem. C 2 (2014) 3960e3964. [8] H.-C. Jeong, H.-G. Park, J.H. Lee, Y.H. Jung, S.B. Jang, D.-S. Seo, Sci. Rep. 5 (2015) 8641. [9] H.-G. Park, Y.-H. Kim, B.-Y. Oh, W.-K. Lee, B.-Y. Kim, D.-S. Seo, J.-Y. Hwang, Appl. Phys. Lett. 93 (2008) 233507. [10] S.-W. Hwang, J.-H. Seo, T.-H. Yoon, J.C. Kim, Thin Solid Films 519 (2010) 885e889. [11] P.K. Son, S.-W. Choi, Surf. Interface Anal. 44 (2012) 763e767. [12] S.S. Chae, B.H. Hwang, W.S. Jang, J.Y. Oh, J.H. Park, S.J. Lee, K.M. Song, H.K. Baik, Soft Matter 8 (2012) 1437e1442. [13] J.J. Lee, H.-G. Park, J.-J. Han, D.-H. Kim, D.-S. Seo, J. Mater. Chem. C 1 (2013) 6824e6828. [14] Y.-G. Lee, H.-G. Park, H.-C. Jeong, J.H. Lee, G.-S. Heo, D.-S. Seo, Opt. Express 23 (2015) 17290e17300. [15] S.-J. Ding, C. Zhu, M.-F. Li, D.W. Zhang, Appl. Phys. Lett. 87 (2005) 053501. [16] H.Q. Chiang, J.F. Wager, R.L. Hoffman, J. Jeong, D.A. Keszler, Appl. Phys. Lett. 86 (2005) 013503. [17] D. Hong, J.F. Wager, J. Vac. Sci. Technol. B 23 (2005) L25eL27. [18] S.-J. Seo, C.G. Choi, Y.H. Hwang, B.-S. Bae, J. Phys. D. Appl. Phys. 42 (2009) 035106. [19] P.S. Venkatesh, S. Balakumar, K. Jeganathan, RSC Adv. 4 (2014) 5030e5035. [20] H.-G. Park, H.-C. Jeong, T.-K. Park, D.-S. Seo, RSC Adv. 5 (2015) 1918e1922. [21] J.-W. Lee, H.-G. Park, H.-C. Jeong, D.-S. Seo, ECS Solid State Lett. 4 (2015) R13eR16.