APPLIED PHYSICS LETTERS 95, 063505 共2009兲

Bistable device using anchoring transition of nematic liquid crystals
Jin Ki Kim, Fumito Araoka, Soon Moon Jeong,a兲 Surajit Dhara,b兲 Ken Ishikawa, andHideo Takezoec兲
Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1 O-okayama,
Meguro-ku, Tokyo 152-8552, Japan

共Received 9 July 2009; accepted 21 July 2009; published online 11 August 2009兲
The authors have demonstrated a novel bistable memory device writable by a laser beam. Thedevice utilizes the hysteresis of a temperature-driven discontinuous anchoring transition 共orientation
change兲 in a dye-doped nematic liquid crystal. The laser light irradiation switches the stableorientation from homeotropic to planar in a liquid crystal on perfluoropolymer surface. The
thickness of recorded lines comes down to as low as ⬃20 ␮m. The recorded images were kept at
least for a day, i.e., memory effect. We also showed that the temperature range of the hysteresis waslowered down to room temperature using a binary mixture system. © 2009 American Institute ofPhysics. 关DOI: 10.1063/1.3202781兴
LC CCN-47 doped with C153 共0.5 wt %兲 was introduced
into the empty cells using capillary action in the isotropic
phase.
Let us first show the anchoring transition and explain theconcept of the bistable device using light-induced orientation
change. Figure 1共a兲 shows the variation in transmitted intensity under crossed polarizers as a function of temperature.
The measurement was made using a 545 nm light through arubbed cell where the rubbing direction makes 45° to thepolarizers. With decreasing temperature from the isotropic
phase, the transmittance due to birefringence appeared because of planar alignment. With further decreasing tempera&'(-./012344/056 8/.9* :0341;

Liquid crystals 共LCs兲 director can be aligned parallel
共planar or homogeneous alignment兲 or perpendicular 共homeotropic alignment兲 to substrate surfaces by means ofseveral techniques. One of the most commonly used is coating polymers such as polyimide. This technique is widelyused for commercial LC display devices and is very stable
and reliable. However, the spontaneous anchoring transition
in some combinations of polymer surfaces and LC molecules
are reported and explained on the basis of interplay between
them.1–6 It is sometimes possible to control or trigger
the orientational change by external stimuli such astemperature,7,8 electric field,9 and light.10,11 Among them,photoinduced orientational change was induced by geometrical change in surface azobenzene molecules, by which homeotropic to planar10 or in-plane molecular rotation11 occurs.
In our previous article,12 we have reported temperaturedriven anchoring transitions of LCs on a commercially available polymer surface, i.e., a discontinuous planar to homeotropic transition with a large hysteresis region, where bothorientations are stable 共bistable兲. In this letter, we demonstrate a memory device using the bistability. Planar regions
can be formed by a laser beam in a uniformly homeotropic
region and the regions are stable for a long time exhibiting amemory effect.
The nematic LC 4⬘-butyl-4-heptyl-bicyclohexyl-4carbonitrile 共CCN-47兲 was obtained from Merck Japan Ltd.,and used without any further purification. The LC exhibits
the following phase transitions: Cr 25.6 ° C SmA 28.2 ° C N
57.3 ° C I, and possesses a large negative dielectric anisotropy 共⌬␧ = −5.7 at 30 ° C兲.13,14 The laser dye coumarin 153
共C153兲 was purchased from Lambda physics. Perfluoropolymer poly 关perfluoro共4-vinyloxy-1-butene兲兴, known as CYTOP 共from Asahi Glass Co., Ltd兲, was spin coated onto glass
substrates and was cured for 30 min at 100 ° C. Empty cells
were fabricated by using two such glass substrates whichwere rubbed antiparallel to each other. Polymer beads wereused for spacers to make 5 ⫾ 0.3 ␮m thick cells. Then the$#%+%(*% ,("!

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Present address: Central Technical Research Laboratory, Nippon Oil Corporation, 8 Chidori-cho, Naka-ku, Yokohama 231-0815, Japan.
b兲Present address: School of Physics, University of Hyderabad, Hyderabad
500046, India.
c兲Electronic mail: takezoe.h.aa@m.titech.ac.jp.

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0$1%"2$)$%3FIG. 1. 共Color online兲 共a兲 Transmittance intensities of an orientated sampleof CCN-47 doped with C153 共0.5 wt %兲 under crossed polarizers as a function of temperature. 共b兲 Schematic illustration of light-induced anchoringtransition and bistability.

95, 063505-1© 2009 American Institute of Physics

Author complimentary copy. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

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Appl. Phys. Lett. 95, 063505 共2009兲

Kim et al.

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AB ,5FIG. 2. 共Color online兲 POM images of LC textures showing anchoringtransitions in the cooling 共left兲 and heating 共right兲 processes.

ture in the nematic phase the transmittance abruptly decreased to zero at 47.5 ° C, indicating a discontinuouschange in LC director 共anchoring transition兲 from planar tohomeotropic. Upon heating back, the transmittance suddenly
increased at 52.5 ° C with a large hysteresis of ⬃5 ° C. The
presence of the hysteresis region with a considerable width isa key requirement for the bistable device. First we keep ourcell at a homeotropic anchoring state 共␣ state兲, as shown inFig. 1共b兲 共left兲. By irradiating a part of the cell, dye molecules absorb the light energy, resulting in a certain temperature rise within a region of beam size in the cell. If thetemperature rise is above the upper anchoring transition temperature, the irradiated part of the sample changes its orientation into planar anchoring 共␤ state兲. Because of bistability,
the planar and homeotropic states coexist stably, as shown inFig. 1共b兲 共right兲.
The process of anchoring transition in the dye-doped
CCN-47 is shown in Fig. 2. In our previous paper, we reported the transition process in an unrubbed cell, where planar orientation appeared with schlieren texture. In the presentcase, planar orientation appears as a uniform bright region
because of uniform orientation to the rubbing direction. On
cooling 共left row兲 after the transition from the isotropic
phase, planar texture was preserved down to 47.5 ° C. On
further cooling 共47.2 ° C兲 a dark domain appeared, propagated, and finally coalesced into one dark domain at 45 ° C,indicating a homeotropic orientation. On heating 共right row兲the transition to the planar from homeotropic anchoring took
place. Noticeable observation was that the transition duringcooling and heating processes propagated along the rubbing
direction, whereas the transition was associated with rounddomains in unrubbed cells.

FIG. 3. 共Color online兲 共a兲 POM image of recorded line patterns. 共b兲 POM
image of recorded LC pattern.

Now we describe the performance of recording by laser
light. We employed a laser diode, LQC405-360C 共a productof Newport兲, whose wavelength 共405 nm兲 well matches theabsorption peak of the dye 共C153兲 used. The laser power
before hitting the sample was set to 1.2 kW/ mm2 by neutral
density filters. The beam was focused on the sample cellusing a lens of a focal length of f = 2.5 cm. The sample cell
was laterally transferred by a computer-controlled XY stage.
Before irradiation, the cell was cooled down to room temperature from isotropic state in order to obtain complete homeotropic alignment, then heated up to 51 ° C, which is in
the bistable region, as shown in Fig. 1共a兲. Parallel lines wererecorded along the rubbing direction using a single scan withdifferent speeds, 0.2, 0.5, and 0.8 mm/s. Polarizing optical
microscopic 共POM兲 images of the recorded lines are shownin Fig. 3共a兲, where the rubbing direction makes about 45°
with respect to the polarizers. The generated line widths wereabout 60, 40, and 20 ␮m depending on the scanning speed,0.2, 0.5, and 0.8 mm/s, respectively. Thinner uniform line
less than 20 ␮m was hard to obtain maybe due to the intrinsic shape of Gaussian beam and/or heat dissipation of LC.
We confirmed that both 共bistable兲 anchoring states remained
unchanged at least for a day. Figure 3共b兲 shows an example
of recorded image of tiny characters “LC” observed under apolarizing optical microscope.
It is necessary to decrease the temperature range of thehysteresis to room temperature in order to make the device
practical. For that purpose, we prepared binary mixtures ofCCN-47 and p-cyanophenyl p-n-heptylbenzoate 共CP7B兲.
CP7B shows the nematic-isotropic transition at 55.6 ° C, so
that we can expect a lower anchoring transition temperatureby mixing with CP7B. We confirmed that the binary mixtures exhibit the first order anchoring transition with hysteresis 共between triangular and circular symbols兲 up to the concentration less than 37.2 wt % of CP7B, as shown in Fig. 4.
We found that the anchoring transition temperature as well as
the clearing temperature 共square symbols兲 decreases with increasing CP7B amount. About 35 wt % of CP7B, we canbring the anchoring transition range down to room temperature. This result indicates the possibility of bistable devices
at ambient temperature.
In conclusion, we reported memory and display devices
utilizing bistability associated with anchoring transition ofAuthor complimentary copy. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

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Appl. Phys. Lett. 95, 063505 共2009兲

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We found that the phase transition temperatures and anchoring transitiontemperatures in our previous paper 共Ref. 12兲 were not correct. This is dueto the mistake of temperature calibration.
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We sincerely acknowledge Merck Japan Ltd. forsupplying a valuable liquid crystal, CCN-47.

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FIG. 4. 共Color online兲 Transition temperatures as a function of the concentration of CP7B in the CCN-47/CP7B binary mixture system. Iso and N
stand for isotropic and nematic phases, respectively. The area between triangular and circular symbols represents bistable regime, i.e., both planar andhomeotropic alignments are stable.

LC materials on perfluoropolymer-coated substrates. We
demonstrated the laser-light-driven recording on the cell. The
bistable temperature region was brought down to room temperature using a binary mixture for room-temperature devices.

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