X-RAY MICRODIFFRACTION STUDY OF THE HALF-V SHAPED SWITCHING LIQUID CRYSTAL

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1 Copyright JCPDS - International Centre for Diffraction Data 2004, Advances in X-ray Analysis, Volume X-RAY MICRODIFFRACTION STUDY OF THE HALF-V SHAPED SWITCHING LIQUID CRYSTAL Kazuhiro Takada 1, Takashi Noma 1, Takeshi Togano 1, Taihei Mukaide 1, and Atsuo Iida 2 1 Leading-Edge Technology Development Headquarters, Canon Inc., Morinosato-Wakamiya, Atsugi, Kanagawa , Japan 2 Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization, Oho, Tsukuba, Ibaraki , Japan ABSTRACT Local layer structures and their formation process in a half-v shaped switching ferroelectric liquid crystal (HV-FLC) were investigated by means of synchrotron X-ray microdiffraction. The HV-FLC is a FLC that has cholesteric - chiral smectic C (Ch - SmC) phase transition sequences. X-ray microdiffraction measurements revealed that the SmC phase in the HV-FLC was composed of asymmetric chevron and inclined-bookshelf structures. In addition, temperature-controlled X-ray diffraction measurements showed that the transient layer structures appeared during the Ch to SmC phase transition. INTRODUCTION Surface stabilized ferroelectric liquid crystal (SSFLC) cells, which have bistable characteristics, have been studied from the viewpoint of various applications, e.g., displays [1]. Recently, Asao et al. [2] proposed a monostable characteristic FLC which is realized using a liquid crystal which has a cholesteric chiral smectic C ( Ch SmC ) phase transition. Unlike FLC with SmA phase, an absence of the SmA phase meant that there were two possible directions with which the molecule could align. To select the one direction for molecule to align, it is necessary to apply the DC voltage during the phase transition from the Ch phase to the SmC phase. The electro-optical response of the HV-FLC under an AC voltage exhibited asymmetric characteristics between plus and minus voltages. Therefore, it is called the half-v shaped switching FLC (HV-FLC) mode. In addition, the HV-FLC has characteristics such as fast response and gray-scale control. Therefore, the HV-FLC mode is considered to be suitable for the display of moving pictures. The layer structure of a typical HV-FLC material has previously been measured by means of the X-ray diffraction technique

2 This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This document is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific community. All copyrights for the document are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website ICDD Website -

3 Copyright JCPDS - International Centre for Diffraction Data 2004, Advances in X-ray Analysis, Volume [3-4], and the previous study [3] has revealed that the layer structure was composed of the asymmetric chevron structure and inclined- bookshelf structure. In this paper, we measured the local layer structure of other HV-FLC materials and investigated how the layer structures were formed under phase transition, by means of the X-ray microdiffraction technique. EXPERIMENTAL The X-ray microdiffraction measurements were performed at the Photon Factory on BL-4A. The basic experimental setup is shown in Figure 1. Figure 1. Experimental setup for microdiffraction measurement of HV-FLC The synchrotron radiation from a bending magnet was monochromated by a synthetic multilayer mirror and focused to approximately 3 4 µm 2 or 5 3 µm 2 by a pair of elliptical mirrors (Kirkpatrick-Baez type) at the sample position [5]. The X-ray energy used was 8 kev (1.55Å). The angular divergence was approximately 1.0 mrad (0.06 ) in both horizontal and vertical directions. To detect the diffraction spot, we adopted a position-sensitive proportional counter (PSPC; Rigaku PSPC-5). The PSPC, placed 180 mm from the sample position, was set to the diffraction angle which corresponded to the smectic layer spacing (2θ B ). To describe the layer structure of the FLC, two parameters, ω and χ, are used. The parameter ω corresponds to the layer tilt angle from the surface normal, and χ corresponds to the layer tilt angle around the surface normal from the rubbing direction. The relationships between these two parameters and the layer structures are shown in Figure 2. The dotted line in Figure 2 is the Debye ring corresponding to the polycrystalline one. A diffraction spot appeared on the ring. If the layer was rotated along the χ-direction, the diffraction spot moved along the Debye ring. The χ-angle was calculated from the position of the PSPC.

4 Copyright JCPDS - International Centre for Diffraction Data 2004, Advances in X-ray Analysis, Volume Figure 2. Relationships between two parameters (ω and χ) of the layer structure and the layer structures of the diffraction spot of the HV-FLC. Layers in this figure are assumed to have a bookshelf structure. We measured a series of χ-profiles for various ω-angles (χ-ω profile) using a PSPC. The detailed χ- ω intensity distribution profiles revealed the layer structure of the HV-FLC. Using these two parameters, we can describe the layer structures exactly. The HV-FLC sample used in this study was ST211 that is originally blended at Canon Inc. The phase sequence on cooling was as follows: Isotropic phase(96.4 C) Ch(61.3 C) SmC(-20.9 C) Crystal The sample LC was sandwiched between two glass substrates. The inner side of each glass is coated with indium tin oxide (ITO) as an electrode and polyimide as an alignment film; their thicknesses were 70 nm and 10 nm, respectively. The gap between glasses was about 1.5 µm. RESULTS AND DISCUSSION Figures 3(a) and 3(b) are polarized optical micrographs of the typical two domains of the layers (TDL) cell and mono domain of the layers (MDL) cell. In the case of Figure 3(a), without DC voltage during the phase transition, we can observe the dark and bright regions. In Figure 3(b), we can observe only one region as a result of applying the DC voltage during the phase transition. To confirm the layer structure of each region in the TDL sample, we measured the χ-ω profiles by changing the analyzing position (7 points in 5µm steps). (a) (b)

5 Copyright JCPDS - International Centre for Diffraction Data 2004, Advances in X-ray Analysis, Volume µm 100µm Figure 3. Polarized optical micrographs of the (a) TDL and (b) MDL cells. The scanning direction is from the arrow point in Figure 3(a) downwards. Figures 4(a) 4(c) correspond to the typical χ-ω profiles measured at the 1st, 4th, and 7th points of the line scan in Figure 3(a). The dotted lines in these figures show the center of the PSPC which represents χ=0. The darker regions correspond to the higher diffracted intensity. From these profiles, the dark and bright regions have different layer structures that have reverse χ directions. The bright regions in Figure 3(a) have the same ω and χ distributions. -30 (a) (b) (c) ω (deg) χ(deg) χ(deg) χ(deg) Figure 4. χ-ω profiles at the line scan of the TDL cell. (a) is the 1st, (b) is the 4th and (c) is the 7th position of the line scan shown in Figure 3. (a) and (c) correspond to the bright regions and (b) corresponds to the dark region in Figure 3(a). The X-ray beam size was 3 4 µm 2. Figure 5(a) is the series of ω-integrated profiles corresponding to the peak which is observed around ω=-18 in Figure 4(a), and Figure 5(b) is the series of ω-integrated profiles corresponding to the peak which is observed around ω=0 in Figure 4(a). From the profiles of Figures 5(a) and 5(b), we can easily understand that the angle χ was reversed between the bright and dark regions. For the MDL cell (Figure 3(b)), a similar χ-ω profile to that in Figure 4 was obtained. From these results, it was evident that the basic structures of the

6 Copyright JCPDS - International Centre for Diffraction Data 2004, Advances in X-ray Analysis, Volume HV-FLC (ST-211) were also composed of the asymmetric chevron structure and inclined-bookshelf structures, similarly to other HV-FLC [2]. (a) (b) X-ray Intesity (a.u.) X-ray Intesity (a.u.) χ(deg) χ(deg) Figure 5. Series of ω-integrated profiles. (a) Integration range is from 22 to -15 at angle ω of the line scan. (b) Integration range is from 10 to 1 at angle ω of the line scan. The scan step was 5µm and the collection time was 20s. The scanning direction is from the arrow point in Figure 3(a), downwards. The arrow in Figure 5 corresponds to the scanning direction. The sign denotes the dark region in Figure 3(a) The role of the electric field was to select only one direction from two possible directions. We also derived the cone angle of this material by applying the same relationship as that expressed in eq. (4) of ref. [2]. The cone angle of ST-211 was estimated to be about The differences between cone angles seemed to depend on the differences between materials. The other important factor which must be considered for the FLC is to know how the layers are formed and whether the layer structure at the SmC phase is formed immediately below the transition temperature or not. Therefore, we measured the χ-ω profiles while decreasing the temperature from the Ch phase to the SmC phase. Figures 6(a) 6(c) show the typical χ-ω profiles obtained at the (a) transition temperature (Tc), (b) Tc-2 C and (c) Tc-12 C, respectively. The dotted lines in these figures show χ=-20. At around the phase transition temperature, we observed the simple spot in the χ-ω profile (Figure 6(a)). This profile means that only one layer structure formed at the beginning. At 2 C below Tc during cooling, the intensity was distributed in the ω and χ directions (Figure 6(b)). During cooling,

7 Copyright JCPDS - International Centre for Diffraction Data 2004, Advances in X-ray Analysis, Volume the transient layer structure split into two layer structures, and these two layer structures remained at room temperature. -30 (a) (b) (c) ω (deg) χ(deg) χ(deg) χ(deg) Figure 6. χ-ω profiles of the temperature controlled measurements. (a) T=Tc, (b) T=Tc-2 C and (c) T=Tc-12 C. The X-ray beam size was 5 3 µm 2. CONCLUSION From the results of the temperature controlled X-ray diffraction measurements, we can understand that the layer structure at the SmC phase was not formed from the beginning. In the layer formation process, the transient layer structure formed first, around the phase transition and then the transient layer structure converted to another layer structure while bending in the ω and χ directions. To analyze the layer structures of FLC cells, it was important to use an X-ray microbeam because it was able to separate an area with a 5µm or better resolution. REFERENCES [1] N. A. Clark and S. T. Lagerwall: Appl.Phys.Lett. 36(1980)899. [2] Y.Asao, T.Togano, M.Terada, T.Moriyama, S.Nakamura and J.Iba: Jpn. J. Appl. Phys. 38 (1999) [3] K.Takada, T.Noma, T.Togano, T.Mukaide and A.Iida: Jpn. J. Appl. Phys. 42 (2003) [4] R.Hasegawa and T.Takatou: The Japanese Liquid Crystal Society Annual Meetings [5] A.Iida and T.Noma: Nucl.Instrum. Methods B82 (1993) 129.