This paper proposes an approach for producing dual liquid crystal (LC) alignment configuration based on nanoparticle-doped polymer films. Experimental results indicate that illuminating a nanoparticle-doped pre-polymer film, coated onto a substrate with a homogeneous alignment layer, with unpolarized UV light through a photomask causes the polymerization of pre-polymer, ultimately generating homogeneous and vertical alignment layers in unpolymerized and polymerized regions, respectively. The dual LC alignment configuration of the homogeneous (vertical) and hybrid alignments can be achieved by combining the treated substrate with another substrate that has a homogeneous (vertical) alignment layer. Additionally, the applications of the dual LC alignment layer in phase gratings and transflective LC displays are demonstrated.
© 2011 OSA
The electro-optical characteristics of nanoparticle-doped liquid crystals (LCs), including improved nematic ordering and dielectric anisotropy , enhanced photoluminescence , fast response , reduced driving voltage , and spontaneously vertical LC alignment [5–8], have been thoroughly elucidated in recent decades. Of those studies, Jeng et al. initially developed an approach for producing a spontaneously vertical LC alignment in the entire LC cell by doping a unique nanoparticle material, polyhedral oligomeric silsesquioxanes (POSS), into LCs . This approach enables vertical LC alignment without coating conventional vertically aligned films. Recently, LC alignments, including homogeneous (planar) alignment, homeotropic (vertical) alignment, hybrid alignment, twisted nematics, and homogeneous (planar) alignment with pretilt angles, have been widely developed for fabrication in many LC devices. Moreover, binary LC alignments, known as dual LC alignment configurations and defined as two different common LC alignments in a single pixel or a specific area, have received considerable attention. An approach for achieving various binary LC alignments using a surface-treated alignment layer and a photoalignment film of the adsorption of azo dyes onto the polymer surface in azo dye-doped LCs was reported in our previous study [9,10]. In addition, Chen et al. described the feasibility of transforming the vertical LC alignment induced by POSS into homogeneous alignment by the photoalignment of colored dye-adsorption, subsequently producing dual-alignment configuration [7,8]. This method is applicable in fabricating many advanced LC devices, such as viewing-angle-dependent LCDs , transflective LCDs , and LC Fresnel lenses .
On the basis of the above-mentioned developments, the current study presents a simplified approach for producing a dual LC alignment layer based on nanoparticle-doped polymer films and its application in phase gratings and single-cell-gap transflective LCDs. The used material of nanoparticles is POSS [5–8]. Instead of adding POSS into LCs, however, we mix POSS with a pre-polymer and toluene into a solution, which is then spin-coated onto a substrate with a homogeneous alignment layer. A dual LC alignment layer, which consists of homogeneous and vertical alignment layers, can be formed by illuminating such a film with unpolarized UV light through a designed photomask. The polymerized nanoparticle-doped polymer film induces vertical alignment resulting from POSS. Combining another substrate with homogeneous/vertical alignment layers with the treated substrate can produce an LC sample with dual LC alignments. The proposed approach is the main focus of this paper. Aside from being a very simple approach, the proposed method does not limit the UV light used to polarized light. Furthermore, it can be adopted to create a patterned vertical LC alignment layer, implying that the vertical LC alignment layer can be fabricated selectively only onto the desired regions of the substrate with or without a pre-treated alignment layer. It is also applicable in demonstrating periodical LC phase gratings with vertical and hybrid LC alignments, and single-cell-gap transflective LCDs with homogeneous (hybrid) alignments in the transmissive (reflective) pixels. The fabrication processes and electro-optical characteristics of the fabricated phase grating and transflective LCDs are described below.
The nanoparticle and pre-polymer used in the experiment were POSS (from Aldrich, PSS-[3-(2-Aminoethyl)amino]propyl-Heptaisobutyl substituted) and NOA81 (from Norland Products, absorption spectrum from 320 to 380 nm), respectively. Both materials are inexpensive and easy to obtain. Figure 1 schematically depicts the fabrication processes of the nanoparticle-doped polymer film coated onto the substrate. Initially, the polymer film thickness was reduced by mixing NOA81, the solvent (toluene), and POSS at a weight ratio of 50:49.5:0.5 to form a solution. The solution was spin-coated onto an indium-tin-oxide (ITO)-coated glass substrate that was covered with a homogeneous alignment layer of rubbed poly(vinyl alcohol) (PVA). The direction of rubbing was along R (along the y-axis, as shown in Fig. 1). After being coated, the substrate was illuminated with unpolarized UV light (derived from an Hg-lamp, ~60 mW/cm2) for 5 min through a designed photomask that has transparent and opaque regions. The pre-polymers and POSS in the unpolymerized regions, as well as the toluene, were removed by immersing the substrate into a solvent of acetone and alcohol. The dual LC alignment layer, which consists of homogeneous and vertical alignment layers, was then generated. The former was produced by the polymerized nanoparticle-doped polymer film, while the latter was created by the pre-coated homogeneous alignment layer of rubbed PVA. Both of them are colorless. Additionally, the polymerized POSS-doped polymer film was examined using an atomic force microscope. The thickness of the film was measured to be ~5 μm, and the minor aggregation of POSS particles was observed. Notably, the size of the aggregated POSS on the polymer film is small (radius ~50-100 nm), which cannot disturb the orientation of LCs. The aggregation of POSS-doped polymer film is different from that of POSS-doped homeotropic LC device . The mechanism for the vertical alignment of the polymerized POSS-doped NOA81 film is described as follows. The functions of the nanoparticles used (POSS) in the polymerization process can be divided into three parts: initiators, cross-linking agents, and monomers [13–15]. In this study, the nanoparticles of POSS function as monomers, which can be mixed well with NOA81 and toluene, and then polymerized with NOA81 into a linear polymer chain by their single functional group. Finally, the POSS coated onto the film produces vertical alignment to the LCs. In addition, a separate experiment (data not shown) verified that the POSS-doped LCs cannot spontaneously provide vertical alignment onto a substrate that is covered with a polymerized polymer (NOA81) film. Restated, the POSS can be adsorbed onto the ITO layer, but not onto the polymerized NOA81 film, to induce vertical alignment to the LCs.
3. Results and discussion
The first demonstrated application is a periodic LC phase grating. The designed photomask used herein was a stripe-type photomask, and the demonstrated stripe-type dual LC alignments had vertical and hybrid alignments. The fabrication procedures are briefly described as follows. The empty cell, separated by two 12 μm-thick plastic spacers, consisted of a rubbed PVA-ITO substrate treated with stripe-type (spacing was 50 μm, and the direction was parallel to the rubbing direction, R) POSS-doped polymer films and one ITO-coated substrate that was covered with a vertical alignment layer formed by N, N-dimethyl-N-octadecyl-3-aminopropyltrimethoxy-ailyl chloride. The nematic LC (E7, Merck) was injected into the empty cell by capillary action. Finally, the stripe-type dual LC alignments were produced. Figures 2(a) –2(c) show images of the LC sample, observed under a cross-polarizer polarized optical microscope (POM), with the direction of rubbing at 0°, 45°, and 90° from the transmission axis of the polarizer (P).
Figure 3 plots the measured first-order diffraction efficiency (η1) as a function of the applied AC voltage of the fabricated LC phase gratings, probed using a polarized laser beam with its polarization parallel (blue) and perpendicular (pink) to the direction of rubbing (R, along the y-axis in the insets of Fig. 3). The probed laser beam polarized along the x-axis cannot experience any relative phase difference after passing through these two alignments, as schematically shown in inset (a) of Fig. 3. Accordingly, the η1 equals near zero, and is insensitive to the applied voltage. However, the probed laser beam polarized along the y-axis exhibits phase retardation as it passes through the region with hybrid alignment, and the η1 can be given as [16–18]Eq. (1) yields a maximum η1 of 0.4053. The η1 (~40%) of the third peak (at 3.5 V) is consistent with the theoretical value.
The second demonstrated application is a type of transflective LCD. In this experiment, one half of the photomask used was opaque, while the other half was a transparent region. The fabrication procedures are similar to those in the first application, except for the alignment layer of the other substrate. Briefly, the empty cell consisted of a treated substrate that has a dual LC alignment layer (vertical and homogeneous alignments), and one ITO-coated substrate covered with a homogeneous alignment layer of rubbed PVA (along the y-axis, as shown in Fig. 4 ). The dual LC alignments, which comprise the homogeneous (HA) and hybrid (HB) alignments, were used to examine the electro-optical characteristics of the transmissive and reflective pixels in the fabricated single-cell-gap transflective LCD. The optical path of the reflective pixel (hybrid alignment) was twice as long as that of the transmissive pixel (homogeneous alignment) by a reflector. Figure 4 schematically depicts the designed single-cell-gap transflective LCD. The produced LC sample with dual homogenous and hybrid LC alignments was sandwiched between two crossed polarizers. The angles between the transmission axes of the polarizers and R (along the y-axis) were + 45° and −45°. The LCs in the transmissive (T) and reflective (R) regions were aligned to be HA and HB alignments, respectively. The same phase retardation (2πdΔn/λ) in the T and R (with a reflector) regions was essential in achieving high optical performance of the transflective LCDs. Additionally, the optical path of the R region with a reflector was twice as long as that of the T region. In this design, the phase retardation for the bright state in the T region was set as λ/2 ( = π), while that in the R region was set as λ/4 ( = π/2). Therefore, the quarter-wave plate was set between the substrate and polarizer to optimize the bright and dark states.
Figure 5(a) plots the transmittance versus the voltage (T-V) curves of the T (pink) and R (blue) regions. A linearly polarized green probe laser beam, derived from a diode pumped solid state laser (λ = 532 nm), is normally incident in the LC sample placed between two polarizers with the transmission axis at ± 45° with respect to R. The substrate without a nanoparticle-doped polymer film was placed in a position facing the incident light. When the AC (1 KHz) voltage was applied, the transmittance varies with an increase in the applied voltage. According to the electrically controlled birefringence (ECB) mode LCD, the transmittance observed under the cross-polarizers (T⊥) can be given by T⊥ = sin2 2βsin2(δ/2), where β and δ denote the angle between the polarization direction of the incident light and the rubbing direction, and the phase retardation [2πd(neff-no)/λ], respectively . In this case, T⊥ equals sin2(δ/2) when β is set to 45°. Therefore, the maximum (minimum) transmittance in Fig. 5(a) implies that the phase retardation equals (2n + 1)π [(nπ)], where n represents an integer. With detailed computation, the phase retardation versus voltage (δ-V) curves of the T and R regions were obtained, as shown in Fig. 5(b). At the applied voltage of V = 0, the phase retardation in the T (R) region is around 11.8 π (5.7 π). The phase retardation in these two regions decreases when the applied voltage increases, ultimately reaches nearly zero. Additionally, the ratio of phase retardation in the R region to that in T region at an applied voltage higher than 1.8 V is around 0.5. The bright, dark, and grayscale states that originated from the phase retardation between 0 and π can be obtained by applying an AC voltage ranging from 6 to 14 V in the LCDs.
Figure 6 presents the images of the T (left-hand side) and R (right-hand side) regions (no reflector) in the transflective LCDs observed under crossed polarizers by applying AC (1 KHz) voltages of 6, 8, 10, 12, and 14 V. The transmittances in both the T and R regions decrease with an increase in the AC voltage. The transmittances consist of the measured transmittance and computed phase retardation, as shown in Figs. 5(a) and 5(b). The bright (dark) state can be achieved when applying an AC voltage of 6 (14) V. When assembling the reflector in the R region, the reflectance of the R region should be identical to the transmittance of the T region owing to the equal phase retardations. Moreover, no boundary between the T and R regions is found at V = 14 V because the LCs were aligned parallel to the normal of the substrate. A single-cell-gap transflective LCD with HA (HB) alignments in the T (R) region is achieved.
In conclusion, the study demonstrated the feasibility of producing a dual LC alignment layer using nanoparticle-doped polymer films and its applications in phase gratings and single-cell-gap transflective LCDs. Experimental results indicate that the UV-cured nanoparticle-doped polymer film can induce vertical alignment onto the substrate. In other words, a patterned vertical LC alignment layer can be easily obtained using the proposed approach. In addition, dual LC alignment layers can also be produced by UV curing the nanoparticle-doped pre-polymer film coated onto a substrate with a homogeneous alignment layer through a designed photomask, and combining it with another substrate covered with a homogeneous/vertical alignment layer. An LC phase grating, comprising periodic vertical and hybrid LC alignments, can be fabricated using this approach. The diffraction efficiency and its electro-optical properties were examined, and are consistent with the theoretical values. Furthermore, a single-cell-gap transflective LCD consisting of a homogeneous alignment region (T) and a hybrid alignment region (R) can be demonstrated using this approach. The optimized phase retardations of the T (λ/2) and R regions (λ/4) are achieved by applying an AC voltage. The bright, dark, and grayscale states in both the T and R regions can also be operated electrically. The proposed approach is highly promising for use in fabricating colorless optical devices, including LC Fresnel lenses, viewing-angle-dependent LCDs, and so on [10–12].
The authors thank the National Science Council (NSC) of Taiwan for financially supporting this research under Grant No. NSC 98-2112-M-006-001-MY3 and NSC 99-2112-M-006-002-MY3. This work is partially supported by Advanced Optoelectronic Technology Center.
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