JOURNAL OF APPLIED PHYSICS 107, 123108 共2010兲

Heat-driven and electric-field-driven bistable devices using dye-doped
nematic liquid crystals
Jin Ki Kim, Khoa Van Le, Surajit Dhara,a兲 Fumito Araoka, Ken Ishikawa, andHideo Takezoeb兲
Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguroku, Tokyo 152-8552, Japan

共Received 7 April 2010; accepted 10 May 2010; published online 18 June 2010兲
We have demonstrated memory and rewritable bistable devices based on an anchoring transition ofa nematic liquid crystal on a perfluoropolymer surface. Spontaneous orientation changes between
planar and homeotropic occur on cooling and heating with a large temperature hysteresis. Photo
共heat兲 addressing is possible from homeotropic to planar using dye-doped samples. For a coumarindye, photoaddressed images are preserved even after heating up the sample to the isotropic
temperature, whereas, for a 4-dicyanomethylene-2-methyl-6-共p-dimethylaminostyryl兲-4H-pyran dye
the images can be erased by decreasing the temperature out of the thermal hysteresis. Orientation
switching also occurs by applying an electric field with a response time of several milliseconds
depending on the field strength. © 2010 American Institute of Physics. 关doi:10.1063/1.3446826兴
I. INTRODUCTION

The stable orientation of liquid crystal 共LC兲 molecules isfacilitated by surfaces of LC cells constructed by parallel
plates. Commonly, planar 共long axes parallel to the substratesurface兲 and homeotropic 共long axes perpendicular to thesubstrate surface兲 alignments are used for various physicalmeasurements and device applications. In the absence of anelectric field, LCs with a particular surface condition orientwith the minimum elastic energy, typically uniform alignment. Most of LC devices are monostable, that is, LC molecules obtain an unstable orientation under the application ofan electric field and switch back to the stable orientation
when the electric field is terminated. In these devices, a continuous supply of power is required to maintain the unstablestate. By contrast, in bistable LC devices, there are twostable molecular orientations and one of them can be stabilized at a time by external triggering such as a pulsed electricfield. Once the desired orientation is obtained no continuous
power supply is needed to maintain that orientation. Suchbistable devices are useful for electronic devices, particularly
for E-book or smart window applications, in which the image has to be displayed for sufficiently long time. One of thewell-known examples of the bistable devices is a surfacestabilized ferroelectric LC cell,1 where molecules tilt right orleft in a smectic layer. Since molecules are parallel to surfaces in both states, so that bistability is realized.
For practical bistable LC devices, nematic 共N兲 LCs
would be an appropriate candidate in the sense that alignment characteristic is much better, i.e., stable due to theirsimple structure, than their counterparts. Generally, bistability in nematic-based LC devices can be achieved on various
surface conditions; for examples, surfaces exhibitinga兲

Present address: School of Physics, University of Hyderabad, Hyderabad500046, India.
b兲
Author to whom correspondence should be addressed. Electronic mail:takezoe.h.aa@m.titech.ac.jp.
0021-8979/2010/107共12兲/123108/4/$30.00

monostability2,3 or bistability,4–6 nonuniform surface alignment patterns,7–11 or specially treated surfaces that have twodimensional or three-dimensional micropatterns.12–17 Bistability being not directly related to surfaces was also proposedand demonstrated.18–20 Another totally different bistability
can be realized by equally stable planar and homotropicalignment. Actually, we recently reported a temperaturedriven discontinuous anchoring transition of a nematic LC
with a large thermal hysteresis region 共bistability兲 on a commercially available amorphous perfluoropolymer surface.21
Further we have also proposed and demonstrated a bistable
LC device in the dye-doped nematic LC system in which we
successfully stabilized bistable states, i.e., planar and homeotropic regions with high resolution by laser-lightirradiation.22 Here we report two different performances ofheat-driven bistable devices depending on kinds of dyes
doped; i.e., a memory device and a rewritable device. Finally
we demonstrate the performance of an electric-field-driven
bistable device.
II. EXPERIMENTAL PROCEDURE

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 Sm-A 28.2 ° C N
57.3 ° C Iso, and possesses a large negative dielectric anisotropy 共⌬␧ = −5.7 at 30 ° C兲. Here Cr, Sm-A, and Iso stands
for crystal, smectic-A, and isotropic phases. The laser dyes,
coumarin 153 共C153兲 and 4-dicyanomethylene-2-methyl-6共p-dimethylaminostyryl兲-4H-pyran 共DCM兲, were purchasedfrom Lambda Physik and Exciton, respectively. Amorphous
perfluoropolymer, poly 关perfluoro 共4-vinyloxy-1-butene兲兴,
known as CYTOP 共from Asahi Glass Co., Ltd.兲, was spincoated onto glass substrates and was cured for 30 min at
100 ° C. Empty cells were fabricated by using two such glass
substrates which were rubbed antiparallel to each other.
Polymer beads were used for spacers to make 5 ⫾ 0.3 ␮m107, 123108-1© 2010 American Institute of Physics

Downloaded 24 Jun 2010 to 202.41.85.22. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jsp

J. Appl. Phys. 107, 123108 共2010兲

Kim et al.

thick cells. Then CCN-47 doped with 0.5 wt % C153 or
DCM was introduced into the empty cells using capillaryaction in the isotropic phase.
The texture was observed under a polarizing microscope
共Nikon, OPTIPHOT-POL兲 equipped with a temperaturecontrolled Mettler-Toledo FP90 hot stage. The cooling andheating rate was ⫾2 ° C / min. As for light source, we employed a laser diode, LQC405-360C 共Newport兲, whose
wavelength 共405 nm兲 matches the absorption peaks of bothdyes 共C153, DCM兲 used. For C153-doped and DCM-doped
samples, we used 1.2 kW/ mm2 at 51 ° C and 4.2 kW/ mm2
at 48 ° C. For voltage dependent polarizing optical microscopy 共POM兲 images of the switching cell, rectangular voltage with a frequency of 10 kHz was applied.
III. RESULTS AND DISCUSSIONSIn our previous study, we used C153-doped CCN-47.
Here we show transmittance of DCM-doped CCN-47 withvarying temperature, as shown in Fig. 1. The measurement
method is the same as that described in our previous paper.22
As observed in Fig. 1共a兲, we confirmed that a DCM-doped
sample also exhibits a discontinuous anchoring transition
with a considerable width of hysteresis, the same as in C153doped one. Temperatures in parentheses are for a C153doped CCN-47 sample. By scanning a focused laser beam on
the cells, which was heated from temperatures a little below
the upper anchoring transition temperature 共50.5 ° C for
DCM and 53.5 ° C for C153兲, we can record lines if the local
heating by laser irradiation exceeds the upper anchoring transition temperature, as shown at the top of Figs. 1共b兲 and 1共c兲,for DCM and C153, respectively. Dark background andbright lines correspond to homeotropic and planar orientations, respectively.
Photomicrographs display the temperature dependent
evolution of recorded lines for temperature cycles down below the lower anchoring transition temperature. In DCMdoped CCN-47, erasing and rewriting are possible, as shownin Fig. 1共b兲; i.e., anchoring transition from planar to homeotropic starts to occur at 43 ° C, and completed at 41 ° C. The
homeotropic state obtained at 41 ° C was well preserved up
to 48 ° C during heating process, which made it possible tooverwrite another pattern. We could rewrite tiny characters“LC” at 48 ° C in the same spot, in which two lines werepreviously recorded. Thus rewritability is clearly shown; different kinds of patterns were recordable repeatedly using thehysteresis cycle. Totally different performance was obtainedin C153-doped sample, as shown in Fig. 1共c兲. The recorded
patterns remained unchanged even at 43 ° C, well below thelower anchoring transition temperature, and after heating up
to the original temperature 51 ° C. Thus, expected anchoringtransition from planar to homeotropic state did not occur; therecorded patterns were memorized irrespective of temperature change. Namely, the LC director seems to firmly stick tothe surface once the laser-light-induced planar state is stabilized.
To understand the different behavior in DCM-doped andC153-doped samples, texture evolution by raising the temperature up to the isotropic phase was examined. The transi-

Transmitta
ance (arb. unitss)123108-2

14

CoolingC
li
Heating

(a)

12

o

44 C
o
( 47.5 C )10o50.5 Co( 53.5 C )

8
6
4
o

o

42.5 Co( 46 C )2

49.5 Co( 52.5 C )

o

57 C
o
( 57 C )

0
38

40

42

44

46

48

50

52

o

54

56

58

60

62

Temperature ( C)

(b)

(c)

48 oC

51 oC
Rubbing

Rubbing

43 oC

49 oC

41 oC

46.5
6.5 oC

48 oC

43 oC
200μm

200μm

FIG. 1. 共Color online兲 共a兲 Temperature dependence of transmittance showing anchoring transition in DCM-doped CCN-47. Note the existence of temperature hysteresis 共bistability兲 with a wide temperature range. C153-doped
CCN-47 exhibits the similar behavior with slightly different anchoring transition temperatures, which are given in parentheses. 共b兲 POM images showing a rewritable bistable device using DCM-doped CCN-47; i.e., cooled
from 48 to 41 ° C, then heated up to 48 ° C and rewrote LC. 共c兲 POM
images showing a memory bistable device using C153-doped CCN-47; i.e.,cooled from 51 to 43 ° C. Heating from 43 to 51 ° C exhibited the sametextures.

tions from homeotropic to planar alignment occurred at
50.8 ° C. As expected, the textures of DCM-doped CCN-47
were very uniform without any trace of stripes when heated
up to 55 ° C, as observed in Fig. 2共a兲. The uniform texture
was maintained even after cooled down to 44 ° C from isotropic 共60 ° C兲. With further decreasing temperature below
44 ° C, the anchoring transition to homeotropic alignment
共dark part兲 was initiated at 43.9 ° C and was completed withfurther decreasing temperature. By contrast, in C153-doped
sample, homeotropic orientation indicated by dark region began to turn into planar 共53.7 ° C兲. However, an afterimage
recorded at 51 ° C was preserved even at 56 ° C, as shown in
Fig. 2共b兲. The afterimage could not be erased even after heating up the cell to the isotropic phase, and reappeared when

Downloaded 24 Jun 2010 to 202.41.85.22. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jsp

J. Appl. Phys. 107, 123108 共2010兲

Kim et al.

(a)

(b)

Rubbing
51 oC

50.8 oC

Heating

55 oC

Isotropic

Field off

200m

3 V p-p

3 V p-p

Rubbing

5 V p-p

5 V p-p

56 oC

R bbi
Rubbing

60 oC

Surface

200m

200m

(a)

Surface LCs
Bulk LCs

0V

53.7 oC

60 oC

Isotropic

Rubbing

(c)

(b)

5

0
0.0

-5
-10

0

10

20

30

Time (ms)47.5 oC

FIG. 2. 共Color online兲 POM images of LC textures as observed above upper
anchoring transition temperature in 共a兲 DCM-doped sample and 共b兲 C153doped sample.

the sample was cooled down to the nematic phase 共55 ° C兲and even down to 49 ° C. When the temperature of thesample was brought to close to the lower transition temperature 共47.5 ° C兲, the planar alignment changed into homeotropic one only at the background part. The texture evolutions
with further decreasing temperature below 47.5 ° C exactly
followed the situations as already described in Fig. 1共c兲. As
inferred from Fig. 2共b兲, dye adsorption to the surface is most
likely to be responsible for temperature-insensitive permanent planar orientation in C153-doped samples. Actually dye
aggregation at the surface has been reported earlier.23,24
However, DCM has no contribution to the surface but acts asa heat source for the recording process.
So far we have discussed heat-induced switching of the
LC molecular orientation. The orientation change can beachieved from homeotropic to planar by applying an electricfield between substrate surfaces, since the LC director isforced to be perpendicular to the field because of negative
dielectric anisotropy, as schematically shown in the middleof Fig. 3共a兲. Since molecules are homeotropically anchoredto surfaces, the field-induced orientation change first occurs
in bulk at a field 共3 Vp-p兲 for the Frederiks transition. However, this distortion is so small as not to influence the deviation of surface molecules from vertical direction. With further increasing voltage 共5 Vp-p兲, the distortion of the bulk
molecules as well as the surface molecules becomes larger.
When switched off, most of the bulk molecules return tohomeotropic state, so do the surface molecules by elastic
torque. However, some of the surface molecules stay in pla-

40

50

60

70

Elec
ctric field ( V / Pm )10610 V p-p

1.5
10
1.0
5
0.5

0
0.0

-5
-0.5
-4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Time (ms)Transmmittance (arb. units)

0.5

10 V p-p
7 V p-p
5 V p-p
4 V p-pElecctric field ( V / Pm )

49 oC

1.0

-0.5
-20

43.9 oC

(d)
0.8

15

15
1.5

Transmmittance (arb. units)

48 oC

7 V p-p

55 oCTransmittance (arb. units)

52 oC

Cooling fro
om isotropic

7 V p-p

4

0.6

2
0.4
0
0.2

Elec
ctric field ( V / Pm )123108-3

-2
0.0

-0.2
-2

-4

0

2

4

6

8

-6
10

Time (ms)FIG. 3. 共Color online兲 共a兲 Schematic illustration of the electric-field-induced
LC molecule reorientation by application of an electric field 共middle兲 and
POM images of LC textures during 共left兲 and after 共right兲 the application ofan electric field at 51 ° C. 共b兲 Electro-optical response curves of an LC cellby application of a single bunch of rectangular voltage of 10, 7, 5, and4 Vp-p 共from left to right兲 during 40 ms. 共c兲 The rise process in the electrooptical response in an enlarged time scale. 共d兲 The response by applying asingle rectangular reverse pulse of 10 Vp-p.

nar state due to nonuniformity of the cell surface. All of thesurface molecules eventually become stabilized as planarstate at 7 Vp-p being applied and removed.
The situation described above was actually realized using a C153-doped sample, as shown in both sides of Fig.
3共a兲. In the absence of a field, the equilibrium texture of thecell at 51 ° C was homeotropic. When a voltage exceeding
the Frederiks transition is applied to the cell 共3 Vp-p兲, thebulk LC molecules started to reorient with the director perpendicular to the field. When the field is turned off, theyrelax back to the original 共homeotropic兲 state. With 5 Vp-p
being applied, the bulk molecules are more deviated from thevertical direction, which results in higher birefringence.
When the field is removed, the majority of the homogeneousstate relaxes into homeotropic, whereas a tiny minority doesnot. Above 7 Vp-p, presumably being capable of overcoming
the energy barrier between two stable states, surface anchoring state also changed so that back switching did not occur.
In this way, all the experimental observation is consistent tothe cartoons illustrated in the middle of Fig. 3共a兲.
Figures 3共b兲 and 3共c兲 show transmittance changes infield-induced anchoring transitions in long and short time
ranges, respectively. Here we applied a bunch of ac rectangular voltage 共1 ms pulse duration 共500 Hz兲, 40 ms bunch
duration兲. The transmittance changed from dark to bright under 7 and 10 Vp-p, whereas it increased first and decreasedback under 4 and 5 Vp-p 关Fig. 3共b兲兴, being consistent withthe results shown in Fig. 3共a兲. With increasing voltage, an
induction period becomes shorter and the transmittance rise
becomes faster, that is more clearly seen in Fig. 3共c兲. At

Downloaded 24 Jun 2010 to 202.41.85.22. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jsp

123108-4

J. Appl. Phys. 107, 123108 共2010兲

Kim et al.

10 Vp-p, a fast rise in the range of 1 ms is followed by a slow
process. The fast process is also seen at 5 Vp-p, although it is
not very fast because of a low voltage but about 10 ms.
These switching behaviors suggest that the fast and slow
components are due to the molecular reorientation in thebulk and at the surfaces, respectively. To confirm this, weapplied a single rectangular reverse pulse of 10 Vp-p. We
found that fast switching occurred transiently but returnedback to the original dark state, as shown in Fig. 3共d兲. Thiscan be possible only when the surface orientation remains
unchanged. After application of 10 Vp-p to a small electroderegion, perfect planar alignment was realized as a bright region in a dark homeotropic region 共not shown兲. The image
was kept for a long time showing memory effect. In addition,
the planar region was reproducible; i.e., the bright region
disappeared as the temperature decreased out of the bistableregion and it appeared again by applying 10 Vp-p within thehysteresis region.
IV. CONCLUSIONS

We have demonstrated the performance of bistable devices based on the anchoring transition in dye-doped
CCN-47 on amorphous perfluoropolymer. Three types of devices were demonstrated, namely, heat- 共light-兲 driven
memory and rewritable devices, and electric-field-driven rewritable devices. In heat-induced switching, the temperature
evolution of the planar regions was highly related to the dyes
chosen. For DCM-doped samples, rewritable devices are
possible, whereas C153-doped samples provide memory devices. The planar regions can be memorized in C153-doped
samples because of adsorption of dyes and associated surface
modification. In an electric field-induced switching, 7 Vp-p
was high enough to induce full reorientation, although theapplication of a low voltage such as 5 Vp-p reoriented themolecules only in the bulk.

ACKNOWLEDGMENTS

We sincerely acknowledge Dr. S. M. Jeong, Mr. H. J.
Yun, and Dr. H. Choi for experimental advice and assistanceon carrying out the experiments.
N. A. Clark and S. T. Lagerwall, Appl. Phys. Lett. 36, 899 共1980兲.
I. Dozov, M. Nobili, and G. Durand, Appl. Phys. Lett. 70, 1179 共1997兲.
3
L. A. Parry-Jones, E. G. Edwards, S. J. Elston, and C. V. Brown, Appl.
Phys. Lett. 82, 1476 共2003兲.
4
A. J. Davidson and N. J. Mottram, Phys. Rev. E 65, 051710 共2002兲.
5
R. Barberi, M. Giocondo, and G. Durand, Appl. Phys. Lett. 60, 1085
共1992兲.
6
R. Barberi, J. J. Bonvent, M. Giocondo, M. Iovane, and A. L. AlexeIonescu, J. Appl. Phys. 84, 1321 共1998兲.
7
J. Cheng, R. N. Thurston, G. D. Boyd, and R. B. Meyer, Appl. Phys. Lett.
40, 1007 共1982兲.
8
G. D. Boyd, J. Cheng, and P. D. T. Ngo, Appl. Phys. Lett. 36, 556 共1980兲.
9
R. Barberi, M. Giocondo, J. Li, R. Bartolino, I. Dozov, and G. Durand,
Appl. Phys. Lett. 71, 3495 共1997兲.
10
F. Schmid and D. Cheung, Europhys. Lett. 76, 243 共2006兲.
11
L. Harnau, S. Kondrat, and A. Poniewierski, Phys. Rev. E 72, 011701
共2005兲.
12
S. Kitson and A. Geisow, Appl. Phys. Lett. 80, 3635 共2002兲.
13
L. A. Parry-Jones and S. J. Elston, J. Appl. Phys. 97, 093515 共2005兲.
14
J. S. Gwag, J. Fukuda, M. Yoneya, and H. Yokoyama, Appl. Phys. Lett.
91, 073504 共2007兲.
15
J. S. Gwag, J.-H. Kim, M. Yoneya, and H. Yokoyama, Appl. Phys. Lett.
92, 153110 共2008兲.
16
Y. W. Yi, M. Nakata, A. R. Martin, and N. A. Clark, Appl. Phys. Lett. 90,
163510 共2007兲.
17
C. G. Jhun, C. P. Chen, T.-H. Yoon, and J. C. Kim, Jpn. J. Appl. Phys.,
Part 1 45, 5117 共2006兲.
18
D.-K. Yang, J. L. West, L.-C. Chien, and J. W. Doane, J. Appl. Phys. 76,
1331 共1994兲.
19
B. Wen and C. Rosenblatt, Phys. Rev. Lett. 89, 195505 共2002兲.
20
Y. Yin, S. V. Shiyanovskii, and O. D. Lavrentovich, Phys. Rev. Lett. 98,
097801 共2007兲.
21
S. Dhara, J. K. Kim, S. M. Jeong, R. Kogo, F. Araoka, K. Ishikawa, and H.
Takezoe, Phys. Rev. E 79, 060701共R兲 共2009兲.
22
J. K. Kim, F. Araoka, S. M. Jeong, S. Dhara, K. Ishikawa, and H. Takezoe,
Appl. Phys. Lett. 95, 063505 共2009兲.
23
D. Voloschenko and O. D. Lavrentovich, J. Appl. Phys. 86, 4843 共1999兲.
24
C.-R. Lee, T.-L. Fu, K.-T. Cheng, T.-S. Mo, and A. Y.-G. Fuh, Phys. Rev.
E 69, 031704 共2004兲.
1
2

Downloaded 24 Jun 2010 to 202.41.85.22. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jsp