Tunable pretilt angles based on
nanoparticles-doped planar liquid-crystal cells
Shie-Chang Jeng,1Shug-June Hwang,2,*and Chen-Yu Yang21
Institute of Photonics and Communications, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan
2
Department of Electro-Optical Engineering and Optoelectronics Research Center, National United University, Miaoli 360, Taiwan
*Corresponding author: [email protected]
Received October 23, 2008; revised January 7, 2009; accepted January 7, 2009; posted January 15, 2009 (Doc. ID 103091); published February 10, 2009
The nanoparticles-induced vertical alignment technique was applied to generate variable liquid-crystal pretilt angles based on doping different concentrations of polyhedral oligomeric silsesquioxane (POSS) nano-particles in the planar-aligned liquid crystal cells. Competition between the homogeneously aligned polyim-ide layer and POSS-induced spontaneous vertical alignment domain generated the variable pretilt angle. Experimental results demonstrated that the pretilt anglepis a function of the doped POSS concentration and can be controlled continuously over the range of0 °⬍p⬍90°. © 2009 Optical Society of America
OCIS codes: 230.3720, 160.3710.
Some applications of liquid crystal (LC), such as no-bias bend (NBB) displays [1], bistable bend-splay dis-plays [2], and bistable super-twisted nematic dis-plays [3], which require the intermediate pretilt angle alignment of nematic LC, have recently re-ceived much attention. Among them, the develop-ment of an NBB LCD with very fast total response times of less than 1 ms [1] has the most important applications. As the NBB LCD has a sufficiently large pretilt angle (p, always ⬎45°) [1], it operates between the bend deformation (B state) at low volt-age and the homeotropic state at high voltvolt-age. There-fore, no backflow occurred during switching, and the response time was very fast. However, the pretilt angles of the planar and homeotropic alignment of the conventional LC displays produced by mechani-cally rubbing the polyimide (PI) layer were less than 10° from the horizontal and the vertical directions, respectively. To achieve these novel applications re-quires addressing the well-known challenge of tuning the intermediate pretilt angle alignment of the nem-atic LC. Indeed, this challenge has attracted the in-terest of researchers seeking to develop technologies for a continuously controllable pretilt angle of nem-atic LC alignment [4–7]. Several techniques have al-ready been proposed to produce an intermediate pretilt angle, such as including mixing the vertical and planar PIs [4], mechanically rubbing PI with a long alkyl side chain [5], exposing vertically aligned PI film to UV light [6], using dual alignment layers [7], and forming a polymer network on the substrate surface to sustain the predetermined alignment of LC molecules by the application of curing voltage.
In our previous study, nanoparticles of polyhedral oligomeric silsesquioxanes (POSSs) doped in an LC cell were observed to spontaneously produce vertical alignment in LC cells without the conventional verti-cal alignment layers [8–10]. In practice, the nanoparticles-induced vertical alignment (NIVA) phenomenon results from the adsorption of the nano-particles on the inner surfaces of the indium-tin
ox-ide (ITO) glass substrates. The aptitude of vertical alignment induced by POSS nanoparticles was found to be significantly influenced by the doped POSS con-centrations [10]. According to the exceptional NIVA behavior, the current study proposed a simple tech-nique based on doping a minute amount of nanopar-ticles in a homogeneously aligned LC cell and subse-quently demonstrated the tuning of the LC pretilt angle. The basic idea of the controllable pretilt angles is to form a random distribution of POSS nanopar-ticle domains with spontaneously induced vertical alignment on the rubbed homogeneously aligned PI materials. The competition between the nanoparticle-induced vertical alignment domains and horizontal alignment PI will result in the LC molecules realigning themselves to achieve a uniform pretilt angle near the alignment surface.
In the experiment, parallel-aligned cells were cre-ated using the two ITO-cocre-ated substrates, which were separated by 5m spacers and treated with conventional horizontal PI layers, providing a low pretilt angle of about 2° for LC molecules. Various concentrations of propyl-hepta-isobutyl-substituted POSS (aminoethyl-aminopropylisobutyl-POSS, Ald-rich) in positive nematic LC (E7, ⌬=14.1, ⬜= 5.2; Merck) were prepared using ultrasound. The LC mix-ture was then injected into the empty homogeneous aligned cells through capillary action. During the in-jection of the mixture, the sample was heated to a temperature above the clearing point to avoid non-uniform injection. The LC cell was annealed at 120° C after injection. In this Letter we discuss the control of the pretilt angle by adjusting the amount of dopant of POSS nanoparticles in the LC cell. As the resultant alignment of the LC molecules near the alignment layer is based on elastic energy minimiza-tion [11] and the nanostructure of the alignment lay-ers, the nanoparticle modification allows the LC sur-face pretilt angle of PI film to be continuously altered by doping different POSS concentrations in the homogeneously aligned cell. Figure1provides
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also consistent with the results of Figs.2and Fig.3. Therefore, POSS material doped in a parallel-aligned LC can successfully modify the LC alignment. In ad-dition, as the LC molecules with a higher pretilt angle can realign more effortlessly toward the elec-tric field, a lower threshold voltage is achieved.
To verify the technique outlined in the previous paragraph, one p⬃3° antiparallel-aligned LC cell without POSS dopant and one p⬃65° antiparallel-aligned LC cell with 0.8 wt. % POSS dopant were fab-ricated for comparison. The switching behaviors of the antiparallel-aligned LC cells under different ap-plied voltages and finally relaxing the bias voltage are shown in Figs. 5(a) and Fig. 5(b), respectively. The transmission becomes totally dark when apply-ing 10 V, reachapply-ing the homeotropic state. The transi-tion from the bend state to the homeotropic state and the subsequent return from the homeotropic state to the initial bend state occur immediately for the
p⬃65° antiparallel-aligned LC cell, which is an NBB cell. No intermediate transition modes occur for the p⬃65° NBB LC cell when compared with the p⬃3° optically compensated bend LC cell. As a re-sult, our proposed method stabilizes the bend state deformation of an NBB cell even at zero bias voltage. The electro-optical property and reliability of the NBB-LCD will be studied in the near future.
This work has presented a simple method for ob-taining the continuous control of an LC pretilt angle from a planar to a homeotropic state and generated an NBB LC cell with p⬃65°. Based on the experi-mental results, the pretilt angle of the LC molecules can be continuously controlled by adjusting the POSS concentration in a homogeneously aligned cell. In ad-dition, the magnitude of the pretilt anglepproduced in the alignment layer can be continuously controlled over a range of 0 °⬍p⬍90°. Therefore, continuously controllable LC pretilt angles can be easily achieved. We would like to acknowledge the funding of the research by the National Science Council of Taiwan (NSCT) 97-2221-E-239-013 and sincerely appreciate Electronics and Optoelectronics Research Laborato-ries, Industrial Technology Research Institute, Tai-wan, for supplying the essential materials and assis-tance in this study.
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Fig. 5. (Color online) Polarizing microscope photographs of (a) conventional OCB cell and (b) NBB cell under differ-ent applied voltages: 0, 2.5, 10 V, and finally the voltage released.
