Variable liquid crystal pretilt angles generated by photoalignment in homeotropically aligned azo dye-doped liquid crystals

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Variable liquid crystal pretilt angles generated by photoalignment in homeotropically

aligned azo dye-doped liquid crystals

Andy Ying-Guey Fuh, Cheng-Kai Liu, Ko-Ting Cheng, Chi-Lung Ting, Che-Chang Chen, Paul Chang-Po Chao, and Hsu-Kuan Hsu

Citation: Applied Physics Letters 95, 161104 (2009); doi: 10.1063/1.3253413 View online: http://dx.doi.org/10.1063/1.3253413

View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/95/16?ver=pdfcov Published by the AIP Publishing

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Variable liquid crystal pretilt angles generated by photoalignment

in homeotropically aligned azo dye-doped liquid crystals

Andy Ying-Guey Fuh,1,2,3 Cheng-Kai Liu,1Ko-Ting Cheng,1,a兲Chi-Lung Ting,2 Che-Chang Chen,2Paul Chang-Po Chao,4and Hsu-Kuan Hsu5

1

Department of Physics, National Cheng Kung University, Tainan 701, Taiwan 2

Institute of Electro-optical Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan

3

Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 701, Taiwan 4

Department of Electrical and Control Engineering, National Chiao Tung University, Hsinchu 300, Taiwan 5

Chi-Mei Optoelectronics Corporation, Tainan 741, Taiwan

共Received 1 July 2009; accepted 30 September 2009; published online 21 October 2009兲

This letter demonstrates the feasibility of producing variable liquid crystal共LC兲 pretilt angles using light-induced ripple structures 共LIRSs兲 in homeotropically aligned azo dye-doped liquid crystals 共ADDLCs兲. Illuminating homeotropically aligned ADDLCs with a linearly polarized light for a suitable period yields the LIRSs which provide LCs an anisotropic homogeneous anchoring force. Experimentally, the effective alignment force produced by the homeotropic alignment layer and the LIRSs determines the LC pretilt angle共24° to 63.5°兲, defined from the normal to the substrate. A no-bias pi cell for liquid crystal displays is demonstrated using this approach. © 2009 American Institute of Physics.关doi:10.1063/1.3253413兴

The use of liquid crystals 共LCs兲 requires a surface LC alignment on the substrate. Most LC devices are based on either homogeneous or homeotropic alignment. Recently, controlling intermediate pretilt angles of LCs has attracted increasing interest because of its potential use in the no-bias pi cell for liquid crystal displays.1–3Accordingly, various ap-proaches for generating LC pretilt angles, from about 0° to 90°, have been reported.3–9 Some of these methods raise many technical disadvantages, such as the need for mechani-cal rubbing and complexity. By overcoming these shortcom-ings, the noncontact approach for photoalignment in LCs, therefore, has become increasingly important. Photoalign-ment by photoisomerization using the light-induced adsorp-tion of azo dye molecules, such as methyl red共MR兲 doped in LCs, has been extensively discussed.10–16 Notably, MR ad-sorption markedly depends on the substrate surface.15

Another photoalignment approach, which involves the light-induced ripple structures 共LIRSs兲 on the adsorbed azo dyes in ADDLCs, has also been reported.12–15The previous letters reported by the authors have demonstrated that the morphologies of the LIRSs in ADDLCs depend markedly on the intensity and the wavelength of the impinging light, the period of illumination, and the ambient temperature.12–15 These related works have focused on the periodicity, the am-plitude, the orientation, and the homogeneously anchoring force exerted by the developed LIRSs.

This letter presents a method for fabricating LC cells with various LC pretilt angles by LIRSs in homeotropically aligned ADDLCs. The formed LC pretilt angle, defined in this case the angle made between the LC director and the surface normal of the substrate, can be controlled from about 24° to 63.5°. Experimentally, the amplitude of the formed LIRSs increases with the period of illumination. Based on the Berreman theory,17–19 the unidirectional anchoring force exerted by the LIRSs is proportional to the amplitude of the

LIRSs. Additionally, based on the dual easy axis model,20the combination of the constant homeotropic alignment force ex-erted by the initially homeotropic alignment layer and the variable homogeneous forces exerted by the LIRSs deter-mines the final LC pretilt angles. Restated, a homeotropically aligned ADDLC sample becomes a hybrid one 共inset in Fig. 3兲. Additionally, a no-bias pi cell is demonstrated using this approach.

The ADDLC composite used herein was prepared by mixing 99 wt % nematic LC 共E7, from Merck, clearing temperature ⬃61 °C兲 with 1 wt % azo dye 共MR, from Aldrich兲. The dichroic ratio of MR is approximately six for visible light.14 To promote an empty cell with homeo-tropic alignment, a film, N, N-dimethyl-N-octadecyl-3-aminoprophyltrimethoxy-ailyl chloride 共DMOAP兲, was coated onto two indium-tin-oxide-coated glass slides that were separated by two 27 ␮m-thick plastic spacers. The ho-mogeneously mixed compound was then injected into an empty cell by capillary action. Additionally, to increase the absorbance of green light by MR in a homeotropically aligned ADDLC cell, the temperature of the sample was maintained at 55 ° C during illumination.

Figures1共a兲and1共b兲present the experimental setups for forming the LIRSs by the photoalignment in homeotropically aligned ADDLCs and for measuring the T-V curve of the formed hybrid LC cell, respectively. As presented in Fig. 1共a兲, a linearly polarized green laser beam共EGalong y-axis,

from an Ar+ laser, ␭G= 514.5 nm兲, with an intensity of

⬃280 mW/cm2_{, was adopted as a pump beam, which was}

normally incident 共along z-axis兲 onto the sample from one substrate, referred as the command surface 共SC兲. The other

was called reference surface 共SR兲. After ADDLCs had been

pumped by the green laser beam for the specified periods, LIRSs with various amplitudes were formed on SC. Finally,

the generated LIRSs on SCand the DMOAP alignment layer

on both SR and SC yielded hybrid LC cells with various

pretilt angles. As presented in Fig. 1共b兲, a red probe beam

a兲_{Electronic mail: [email protected].}

APPLIED PHYSICS LETTERS 95, 161104共2009兲

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from a He-Ne laser 共␭=632.8 nm兲, with an intensity of 1.2 mW/cm2_{, E}

R, linearly polarized at an angle 45° with

respect to EG, was normally incident onto the ADDLC

sample. The transmission of the sample under the application of an ac 共1 KHz兲 voltage was measured and fitted with the simulated result by 1D-DIMOS software to obtain the pretilt angles.8

The approach for generating various LC pretilt angles presented herein involves setting the period of illumination at 15, 20, 25, 40, 60, 90, and 180 min. LIRSs are not ob-served experimentally when the period of illumination is less than 15 min in this case. Extending the period of illumination yields the LIRSs and produces an anisotropic homogeneous anchoring force that aligns LCs. The combination of the DMOAP film, which results in the vertical alignment, and the LIRSs, which are associated with homogeneous align-ment, enables various pretilt angles to be achieved. Figure2 plots the measured T-V curves of the ADDLC samples that were illuminated for 25 and 90 min, together with the simu-lated curves. Clearly, the experimental results qualitatively fit the simulated ones well, such that the pretilt angles of the 25- and 90-min-illumination samples are determined to be around 43.5° and 56°, respectively.

Figure3plots the measured pretilt angle as a function of the period of illumination with green laser beam. The LC pretilt angles, ranging from 24° to 63.5°, increase with the period of illumination. The results in Fig. 3 show that the rate of increase in the pretilt angles upon irradiation with

green laser beam is initially large and almost saturates at 40 min. Because the interference between the impinging polar-ized light and the light scattered from the surface is the key to produce the LIRSs, and since the transmittance 共scatter-ing兲 of the impinging light, caused by the azo dye-adsorbed layer, is verified to increase共decrease兲 as the period of illu-mination in the late stage of the photoalignment process increases.15 Notably, according to our previous study,21 the adsorbed dyes are stable at room temperature, but can be thermally erased and optically rewritten.

The morphologies of the SR, and the formed LIRSs on

S_Cof the 90-min-illumination sample are analyzed using an atomic force microscope 共AFM兲. Figure 4共a兲 presents the two-dimensional 共2D兲 AFM image of the SR. Clearly, the

LIRSs were not formed on S_R, because most of the azo dyes in the sample are diffused toward the impinging light and adsorbed on SC. Figures 4共b兲 and 4共c兲present the 2D and

three-dimensional 共3D兲 AFM images, respectively, of S_C. Refer to Ref.14, the spacing共⌳兲 of the formed LIRSs in an ADDLC system is ␭/n cos␪, where␭ and␪ are the wave-length in vacuum and the angle of incidence of the imping-ing light, respectively. n is the refractive index of the mate-rial. Substituting ␭=514.5 nm, n⬃1.6, and ␪= 0° into the equation yields a spacing of ⬃320 nm, which agrees with that measured from Figs. 4共b兲 and4共c兲. The average ampli-tudes of the LIRSs, generated by irradiation with an Ar+_laser

beam for 15, 20, 25, 40, 60, 90, and 180 min are 80, 90, 100, 110, 120, 140, and 150 nm, respectively. The average ampli-tude of the LIRSs is proportional to the period of

illumina-FIG. 3. 共Color online兲 Pretilt angle as a function of the period of illumina-tion. The inset defines the pretilt angle.

FIG. 1. 共Color online兲 Experimental setups for 共a兲 forming LIRSs in ADDLCs; 共b兲 measuring the T-V curve of samples. P, A, M, BE, AP, and D represent polarizer, analyzer, mirror, beam expander, aperture, and detector, respectively.

FIG. 2. 共Color online兲 The simulated 共green diamonds兲关blue dots兴 and experimental 共purple squares兲关yellow triangles兴 T-V curves of ADDLC sample illuminated for 25关90兴 mins.

161104-2 Fuh et al. Appl. Phys. Lett. 95, 161104共2009兲

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tion. Furthermore, according to Fig. 3, the achieved pretilt angle is proportional to the average amplitude of the LIRSs. The anchoring energy共W兲 provided by LIRSs is estimated to be,17–19

W = 2␲3A2K33sin4␾/⌳3

冑

cos2␾+共K33/K11兲sin2␾, 共1兲

where A, K33 共K11兲, and ␾ are, respectively, the amplitude,

the bend 共splay兲 elastic constant of the used LC, and the angle between the ripple direction and the director at the other substrate. Substituting A, given above 共80–140 nm兲, ⌳⬃320 nm, K33⬃19.5 pN, K11⬃12 pN, and ␾= 90° into

Eq.共1兲 yields W of the order of 10−4 _J/m2_{, which is a}

typi-cal value of anchoring energy for aligning LC.22The combi-nation of the initial alignment layer and the variable LIRSs determines the final LC pretilt angles. Hence, the value of LC pretilt angle is proportional to the anchoring energy.

This approach was adopted to fabricate two substrates with a pretilt angle 40° 共⬃50° from the surface兲 to show a no-bias pi cell. A pi cell is more stable in the bend state than in the splay state when the pretilt angle exceeds 47° from the surface of the substrate.1–3 Experimentally, two ADDLC samples were separately irradiated for about 25 min using the experimental setup that is presented in Fig.1共a兲, and then carefully separated into two SC and two SR substrates. A

no-bias pi cell was fabricated using these two SCsubstrates.

Additionally, a homogeneous cell with pretilt angle 40° was constructed as well. The cell gap of these two cells was ⬃7 ␮m. Figure 5 shows both the experimental and the simulated T-V curves of the no-bias pi cell and the homoge-neous cell. The experimental results are consistent with the simulated results obtained using 1D-DIMOSsoftware.

In conclusion, this letter demonstrates the feasibility of obtaining variable LC pretilt angles by forming LIRSs in homeotropically aligned ADDLCs. The combination of the fixed alignment force due to the homeotropic alignment layer and the variable one induced by LIRSs offers an approach to achieve various LC pretilt angles from 24° to 63.5°. A no-bias pi cell is demonstrated using this approach. It is believed that, based on dual easy axis model, larger pretilt angles can be achieved using suitably weaker homeotropic treatments. An experiment is underway to verify this approach.

The authors would like to thank the National Science Council 共NSC兲 of Taiwan for financially supporting this

re-search under Grant No. NSC 95-2112-M-006-022-MY3. Ad-ditionally, this work is partially supported by Chi Mei Opto-electronics as well.

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FIG. 5. 共Color online兲 The simulated 共green diamonds兲 and experimental 共purple squares兲 T-V curves of homogeneous cell with pretilt angle of 40° and the simulated共blue dots兲 and experimental 共yellow triangles兲 T-V curves of no-bias pi cell.

FIG. 4. 共Color online兲 AFM images of the 90-min-illumination sample. 共a兲 2D AFM image of SR,共b兲 2D, and 共c兲 3D AFM images of formed LIRSs.

161104-3 Fuh et al. Appl. Phys. Lett. 95, 161104共2009兲