In situ homeotropic alignment of nematic liquid crystals based on photoisomerization of azo-dye, physical adsorption of aggregates, and consequent topographical modification

Article abstract of DOI:10.1002/adma.201300730

In situ homeotropic alignment is achieved by photochromic trans- to cis-isomerization of an azo-dye doped in a nematic host. The augmented dipole moment of the cis-isomer formed under UV-irradiation expedites molecular assembly into crystalline aggregates. Subsequent deposition of the aggregates creates a roughened surface and induces an anchoring transition from the initial planar to a homeotropic alignment of the LCs.

Full text of DOI:10.1002/adma.201300730

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In Situ Homeotropic Alignment of Nematic Liquid Crystals
Based on Photoisomerization of Azo-Dye, Physical
Adsorption of Aggregates, and Consequent Topographical
Modification
Sudarshan Kundu, Myong-Hoon Lee, Seung Hee Lee,* and Shin-Woong Kang*
The control of the alignment of liquid crystal (LC) is a classic
but essential issue in the field of LC research. The fundamental
mechanism behind LC alignment has been a controversial argu-
ment for a long period of time since early twentieth century.
New techniques have been developed for various applications
of liquid crystals.[1] Both the physical interactions between an
elastic LC medium and a solid surface, and the chemical inter-
actions attributed to the epitaxial growth of LC order from the
chemically anisotropic surface are generally accepted for the
origin of LC alignment.[1,2] Either planar or homeotropic align-
ment, where the easy axis of a nematic director aligned in the
plane of a surface or normal to a surface, respectively, is an
essential requirement for various device applications. Espe-
cially, the techniques for a homeotropic alignment attract a great
attention because not only for scientific reason but also for high
end television applications of liquid crystal display (LCD).[1,3]
Although various methods such as a physical adsorption of
ionic surfactants, self-assembled monolayer, evaporation deposi-
tion of SiOx, 2D-topographic patterns have been reported, the
polyimide coating with a long hydrocarbon side chain is known
as the most reliable and cost-effective process.[1,3–6] Covalently
attached photochromic azo-benzene moiety on the side chain
polymers or glass surfaces can induce a homeotropic align-
ment and facilitate anchoring transition from the initial homeo-
tropic to the planar state upon irradiation of UV-light due to the
trans- to cis- isomerization of the azo-chromophore (so called,
command surface).[7] Photochromic azo-benzene derivatives
have also been used for a control of homogeneous planar and
homeotropic alignment.[8] W. M. Gibbons et al. have reported
the photoalignment of nematic LCs based on the photochromic
behavior of azo-compound. The molecular reorientation of elon-
gated dye molecules caused by intensive polarized light induces
an orientational anisotropy of dye molecules and is responsible
for homogeneous planar anchoring of LC molecules at the
surface. In this case, molecular reorientation of the azo-dye is a
primary factor rather than a tran-to-cis isomerization.[8] Ruslim
and Komitov reported a photoinduced homeotropic alignment
by using high content liquid crystalline azo-dye.[8] In this report,
a molecular adsorption of the dye due to a strong dipole-dipole
interaction and hydrogen bonding between dye-molecule and
polar solid surface were proposed. Subsequent steric interaction
between vertically aligned alkyl chain of the cis-form dye and
LC-molecule is a primary factor for LC alignment.[8]
The common feature of all these methods is a pretreatment
of substrates for LC alignment and subsequent LC loading. It
requires separate processes for alignment and injection of LCs.
It is very recent report that nano-particle doped LCs exhibit spon-
taneous homeotropic alignment in a confined cell without using
additional pretreatment for LC alignment.[9] Although it is a sig-
nificant advance for a cost-effective manufacture, there are crit-
ical hurdles to overcome such as notorious aggregation issues of
nano-sized particles, uniformity, and reliability of the alignment
due to a weak adsorption and migration of nano-particles.
Here, a completely different approach has been proposed.
We report a novel technique for a uniform, reliable, and cost-
effective method for a homeotropic LC alignment. By using
a small amount of azo-dye doped to a nematic host (i.e.,
alignment agent and LC host contained in the same vessel,
so called “one-bottle approach”), high quality homeptropic
alignment is achieved without any pretreatment of substrates
for LC alignment and without compromising physical prop-
erties of a host LC. In situ photoirradiation of the dye-doped
nematic host, confined in a thin cell, results in a spontaneous
homeotropic alignment of the nematic host. In this report, we
demonstrate the in situ homeotropic alignment of nematic
liquid crystal and corresponding mechanism behind the phe-
nomenon. In addition to the optical and electro-optical char-
acterizations, atomic force microscopy (AFM), field emission
scanning electron microscopy (FE-SEM), energy dispersive
spectrometer (EDS), UV–vis spectroscopy, Fourier transform
Infrared spectroscopy (FT-IR), particle size measurement tech-
niques are employed for the study. We strongly believe that
a novel concept of “one-bottle approach” effectuated by in
situ process for homeotropic LC alignment provides signifi-
cant advantages over the conventional methods for LC align-
ment. Furthermore, the proposed mechanism will provide a
new insight on liquid crystal alignment which would lead to
exciting researches in future.
Dr. S. Kundu, Prof. S. H. Lee, Prof. S.-W. Kang
Department of BIN Fusion Technology
Chonbuk National University
Jeonju, 561-756, Korea
E-mail: swkang@jbnu.ac.kr
Prof. M.-H. Lee
The Graduate School of Flexible and Printable Electronics
Chonbuk National University
Jeonju, 561-756, Korea
E-mail: lsh1@jbnu.ac.kr
Our new approach begins with a nematic LC mixture doped
with a small amount of photochromic azo-dye. Two different
DOI: 10.1002/adma.201300730
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compounds with an azo-benzene component are used inde-
under crossed polarizers. The cell exhibits a planar alignment
through a whole active area as shown in Figure 1a. The UV-irra-
diated area turns into a dark circle as seen in Figure 1b. The cor-
responding polarized optical microscopy (POM) and conoscopy
images in Figure 1c and 1d clearly illustrate the transition from
a random planar to homeotropic alignment upon UV-exposure.
Essentially the same anchoring transition is observed for dif-
ferent concentrations of the Dye-2 ranging 0.1 to 1.0 wt%. The
effective concentration is very low so that the physical proper-
ties of a host LC, initially optimized for device applications,
are not hampered by the dopant. The electro-optic responses
shown in Figure 1e and 1f present a typical behavior of the
vertically aligned nematic LC cell which is dominantly adapted
for LCD TVs. Both voltage-to-transmittance curve (V–T) and
grey-to-gray response time for both rising and decaying exhibit
accomplished characteristics. The homeotropic state induced by
UV-irradiation shows a good temperature stability. It retains a
dark homeotropic state after heating the cell above nematic-to-
isotropic transition temperature at 100 °C for
pendently as
a dopant: 4-hydroxy-4′-buthyl diazobenzene
(Dye-1, see Supporting Information, Figure S1) and relatively
more elongated and symmetric bis(4-azobenzoic acid-4′-n-
octyloxyphenyl ester) (Dye-2, Figure 3a, see also Supporting
Information, Figure S1). The Dye-1 crystal melts to isotropic
liquid at 82.4 °C without showing mesomorphic phase. The
Dye-2 exhibits Smectic A phase from 152.0 °C and melts to
isotropic liquid at 302.0 °C. 1H-NMR spectroscopy data and
phase sequences of the dye molecules are shown in the Sup-
porting Information. Indium-tin-oxide (ITO) coated glass sub-
strates were used without any treatment for LC alignment.
Both electro-optic (E.O.) LC cells loaded with different dye-
doped mixtures exhibit an initial random planar alignment as
expected. Subsequent UV-irradiation with a photomask results
in a homeotropic alignment for the Dye-2 mixture while no
significant change is observed for the Dye-1 mixture. Figure 1a
and 1b display macroscopic images of the cell with the Dye-2
24 hours. The anchoring transition caused
by UV-light is irreversible under visible light
(430–450 nm wavelength) irradiation. It is
also noteworthy that the Dye-2 doped mix-
ture with LC cells consisted of bare-glass,
polyimide, or polystyrene coated substrates
exhibit essentially the same anchoring transi-
tion phenomenon.
To understand the origin of anchoring
transition effect, we have performed several
experiments by using 0.5 wt% Dye-2 doped
samples. First, the homeotropic samples
after UV-irradiation have been immersed
into excess amount of hexane to selectively
remove LC host. After complete removal of
LCs, a few different fresh LCs, without a dye
additive, have been reloaded. All LC mix-
tures, regardless of positive or negative die-
lectric anisotropy, show essentially the same
alignment of LC molecules as in the initial
irradiated cell (see Supporting Information,
Figure S2). Therefore, it is evident that the
anchoring transition occurs through the sur-
face-induced phenomenon during the UV-
irradiation and the modified surface is stable
against solvent treatment.
Second, we have investigated inner sur-
faces of the cell after removal of LCs by using
AFM, FE-SEM, and EDS. Figure 2 represents
selected AFM and SEM images for the top
and bottom surfaces of UV-irradiated area and
pristine ITO-surface. As seen in Figure 2a–c
and d–f, the microscopic structures are
formed on both top and bottom inner sur-
faces in the exposed area. Relatively thicker
Figure 1. Depolarized optical images and electro-optical response of the Dye-2 doped nematic
mixture: a,b) the macroscopic figures before (a) and after (b) UV-irradiation with a mask;
c,d) the corresponding microscopic images exhibiting random planar (c) and homeotropic
(d) states and (inset) a conoscopic figure for the exposed area. The UV-exposed area is marked
by a red circle. e,f) The voltage–transmission (V–T) curve and grey-to-grey response time for
both rising and decaying measured from 4 μm thick E.O. cell, respectively.
layer with a rough surface has been deposited
on near UV-side (top) surface compared to the
opposite surface. This is presumably due to
the intensity gradient of UV-light inside the
LC cell. For the bottom surface, approximately
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Figure 2. AFM and FE-SEM micrographs of the inner surfaces of the UV-irradiated LC cell and bare ITO glass: (a–c) represent the AFM image, a 3D
view of the surface, and the FE-SEM image from the top substrate of the UV-irradiated region. d–i) The corresponding images for the bottom substrate
of the UV-irradiated area (d–f) and the ITO-surface of the unexposed area (g–i).
micrometer sized flower-like structure is observed. The inco-
herent structures are formed on a thin continuous layer. In both
cases, the surface topography becomes much rougher than the
pristine ITO-surface. The scale of the z-axis in Figure 2b and
2e is at least 50 times larger than that in Figure 2h. The corre-
sponding values of root mean square (RMS) roughness are 22.8,
54.4, and 0.6 nm, respectively (see Supporting Information,
Figure S3 for the statistical data of each AFM image). We conjec-
ture that the roughened surfaces formed by the deposition of dye
molecules are responsible for a homeotropic alignment of LCs.
The EDS data collected from each surface show a clear dif-
ference. The existence of a carbon (C) and nitrogen (N) atoms
is evident for the top and bottom surfaces of UV-irradiated sub-
strates while no subsistence of C and N atoms is observed for
a clean ITO-surface (see Table S1–3 in the Supporting Infor-
mation for details). Therefore, it is reasonable to conclude that
the microscopic structures on the UV-irradiated surface are put
together from the Dye-2 molecules. The dye molecules form
agglomerates and deposit on the inner surface of the LC cell.
The structures on FE-SEM images in Figure 2c and 2f are more
perceptible and provide more detailed information. The square-
shape morphology strongly indicates a crystalline nature of the
dye agglomerates assembled on a continuous layer of azo-dye
molecules. Additional SEM images are presented in Figure S4
in the Supporting Information, for more information.
The arising question is how the Dye-2 molecules are put
together into crystalline aggregates and deposited on the sur-
faces. To investigate this object, FTIR spectra have been taken
for a melt film of the Dye-2 on a silicon wafer and a deposited
layer on a silicon wafer prepared in a LC host under UV-irradi-
ation. The results shown in Figure S5 in the Supporting Infor-
mation are virtually the same which may indicate no chemical
reaction such as polymerization or degradation of the dye mol-
ecules during the crystal formation. It is also confirmed that
the crystal can be melted by heating and UV–vis absorption
spectrum changes as a result of it. Based on these results, we
conjecture that the dye molecule retains its identity and pho-
tochromic isomerization is responsible for the crystallization
of the dye molecules. As well known, the azo-benzene chromo-
phore exhibits tans-cis isomerization as represented in Figure 3a
for the Dye-2 molecule. In general, the process is reversible. It
is open reported that the dipole moment (represented by the
blue arrows) in cis-configuration is several folds larger than that
in trans-form due to the broken symmetry. As a result of the
augmented dipole moment, the cis-form dye molecules pack
together, form crystalline agglomerates, and deposit on the sur-
face.[10,11] It should be noted that the relatively smaller Dye-1
molecules in both trans- and cis-form dissolve very well in the
LC host. Therefore, it cannot form solid aggregates at a low con-
centration and be deposited on the surface, and consequently
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blue shift of absorption peaks for crystalline chromophore has
been previously reported in ref.[12] UV-light induced trans- to cis-
isomerization increases a population of cis-isomer and conse-
quently expedites aggregation to form a crystal. This inference
is corroborated by the experiment of particle size analysis per-
formed before and after UV-exposure. The accelarated forma-
tion of azo-dye aggregates in cis-configuration is also recently
reported in ref.[10,11]
Interestingly, in our case, the cis-form crystals are not con-
verted to trans-isomer by the visible light irradiation. However,
thermal treatment to its melting temperature instigates relaxa-
tion back to the trans-isomer. The UV–vis spectra measured
after melting the cis-form crystal clearly demonstrate a thermal
conversion from cis- to trans-isomer. The solid aggregates
readily dissolve in solvent after annealing the crystal at above
melting temperature (180 °C for 1 hour). The UV–vis absorp-
tion spectrum taken at room temperature after annealing and
dissolving in chloroform shows a complete recovery of the
absorption peak at 322 nm, corresponding to the absorption
of trans-isomer (see Supporting Information, Figure S6). This
substantiates our assumption for the photoisomerization initi-
ated assembly of the dye molecules rather than the polymeri-
zation induced phase separation. This is further corroborated
by the particle size analysis performed before and after the UV-
exposure of Dye-2 solution in chloroform. One hour UV-irradi-
ation to a fresh solution with no measurable particles cultivates
a more light scattering state due to the particle formation. The
average particle size is 2.8 μm with a count rate of 606 kcps (see
Supporting Information, Figure S7 for details). Therefore, it
is evident that the agglomeration of cis-form isomers develops
prior to a deposition at the solid surface.
Based on the results discussed above, we conclude that the
trans- to cis-isomerization and remarkably larger dipole moment
of the cis-isomer are the primary factor for a crystallization of
azo-Dye-2 molecules under UV-exposure. The resulting dye
agglomerates formed in a bulk LC under UV-light irradiation
are drawn by the liquid-solid interface (i.e., so-called “Pickering
effect”) and self-assemble into a unique crystal structure at the
solid surface. This significantly alters topography of surfaces
and form roughened surfaces as seen by the AFM and FE-SEM
images in Figure 2. The topographical modification of a sur-
face, induced by the azo-dye doped LC mixture under UV-irra-
diation, plays a critical role for the anchoring transition from
the initial planar to homeotropic state of host liquid crystals.
The crucial question is why LC molecules anchor vertically
at the dye deposited surfaces. It is well known and generally
accepted that one dimensional topographic pattern such as
a grooved surface aligns LC director parallel to the smoother
direction (i.e., along the groove direction).[2] In this case, mainly
physical interaction between elastic LC medium and patterned
solid surface is the primary driving force for a uniform LC
alignment in the plane of the solid surface. The 2D-topographic
patterns have also been reported for a homeotropic alignment.
The 2D-structured surfaces with pillars, posts and wells can
align LC director vertical to the surface.[6]
Figure 3. Molecular structure, trans- to cis-isomerization of the Dye-2
molecule, and corresponding UV–vis absorption spectra as a function of
UV-irradiation time: a) molecular structures of the Dye-2 in its trans- and
cis-configuration and isomerization process. The magnitude of the dipole
moment is qualitatively proportional to the length of the blue arrows.
b) The UV–vis absorption spectra of the 0.01 mM Dye-2 in chloroform
solution with respect to the UV-irradiation time. The UV-irradiation time
is indicated in the legend.
cannot induce the anchoring transition as mentioned earlier.
On the other hand, the Dye-2 molecules have limited solubility
in solvents. No more than ≈2.0 wt% dissolve in a host LC due
to its symmetric molecular structure with a large elongation of
a hard-core. It seems that a blue shift of the UV–vis absorption
peak with a comparatively narrower band as seen in Figure 3b
is attributed to the limited solubility. In this case, the increased
dipole moment (approximately up to 4 Debye) can facilitate
crystalline agglomeration of cis-form molecules.[11] Figure 3b
presents the UV–vis absorption changes of a 0.01 mM Dye-2
solution in chloroform with respect to UV-irradiation time. The
fresh solution shows a strong peak at 322 nm and weak peak
at 469 nm corresponding to the π-π∗ transition of trans-form
isomer and n-π∗ transition of cis-form isomer, respectively[12]
Upon UV-irradiation the strong peak rapidly diminishes and a
new peak aries at 289 nm attributed to the side by side stacking
of cis-isomers due to a strong dipole moment. This peak is
intensified by the prolonged exposure time. The significant
In our case of dye deposition, presumably the same phys-
ical interaction is the primary driving force for a homeotropic
alignment rather than the chemical interaction stemming from
the chemical anisotropy of a surface. In general, calamitic LC
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molecules prefer to anchor tangentially on the smooth surface
as on the ITO-surface illustrated in Figure 4a. No specific in-
plane orientation of the director (represented by a green arrow)
is preferred if the surface anisotropy is not imposed by external
treatments. However, the in-plane alignment of a LC director in
any direction on the rough surface causes a high elastic distor-
tion of LC medium. Figure 4b presents a cross-sectional pro-
file of the dye deposited surface (illustrated by orange color),
planar anchored LC molecules at the surface, and consequent
large elastic deformation of LCs. The LC molecules align tan-
gential to the white lines. The color contour in blue represents
a degree of elastic deformation of the LC medium. The darker
indicates the higher distortion and the lightest corresponds to
a minimum elastic energy. To avoid a highly distorted state at
a rough surface, the LC molecules prefer to align vertically to
the surface to minimize elastic distortion and subsequently
minimize total free energy of the system. Figure 4c shows a
homeotropic alignment of LC director at a rough surface and
minimized elastic deformation of LC molecules as represented
by lightest blue. The elastic energy of a LC medium competes
with the interaction energy between LCs and surface. For a
planar anchoring on smooth surfaces, the interaction energy
efficiently overcomes imperceptible elastic deformation. For a
rough surface, however, elastic energy predominates and thus
the director escapes to a vertical direction. For a planar align-
ment on the rough surface, the elastic energy critically depends
on roughness of surface especially the average height of the
peaks and the average inter-distance between peaks. Accord-
ingly, degree of roughness, height-to-distance ratio, molecular
dimension and elastic constant of LCs are crucial parameters
to determine an anchoring condition for either planar or ver-
tical alignment of LC director. Although the topography of
dye deposited surfaces observed in our study is irregular, the
height-to-distance ratio seems to satisfy the conditions for a
homeotropic alignment of conventional LCs.
We have demonstrated reliable, practical and cost-effective
“one-bottle approach” for a homeotropic alignment of nematic
liquid crystals. In situ homeotropic alignment is achieved by
photochromic trans- to cis-isomerization of the azo-dye doped in
a nematic host. The augmented dipole moment of a cis-isomer
formed under UV-irradiation expedites a molecular assembly
into crystalline aggregates. Subsequent deposition of the aggre-
gates creates roughened surface and induces anchoring transi-
tion from the initial planar to homeotropic alignment of LCs.
In our method, it is experimentally supported that the surface
modification is achieved through an interfacial self-assembly
of the dye aggregate instead of molecular adsorption due to a
dipole-dipole interaction or hydrogen bonding between solid
surface and LC molecules. The offered method provides a
stable LC alignment which is an essential requirement for
device applications. The alignment is unwavering against tem-
perature, light and chemical treatment and therefore practi-
cally permanent for a device implementation. We propose that
the origin of anchoring transition is attributed to the physical
interaction between elastic LC medium and roughened sur-
face rather than a chemical interaction due to the anisotropy
in molecular orientation at the surface. Our work will provide
a new insight on liquid crystal alignment which would lead to
exciting researches in future. Simple but reliable “one-bottle
approach” not only provides an intuitive understanding for LC
alignment at surfaces but also is a promising technique for
practical device applications.
Figure 4. Graphical illustration of the LC alignment at solid surfaces with
different roughnesses: a) planar aligment on a smooth surface, b) planar
alignment with a high elastic distortion, and c) homeotropic alignment
with a minimum distortion on a rough surface. The LC molecules align
tangentially to the white lines, and the average nematic director is indi-
cated by the green arrow. The cross-section of each surface is shown as an
orange color and the color contour in blue represents the degree of elastic
deformation of LC molecules. The darker color indicates higher distortion
and the lightest color corresponds to the minimum elastic energy.
Experimental Section
Materials: Nematic liquid crystals with either positive (E7, Merck)
or negative (LC-1 and LC-2, Merck) dielectric anisotropy are used as
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a host liquid crystal. The specifications of LC material are provided in
the supporting information. Two photochromic azo-dye molecules,
4-hydroxy-4′-buthyl diazobenzene (Dye-1 shown in Supporting
Information, Figure S1) and bis(4-azobenzoic acid-4’-n-octyloxyphenyl
ester) (Figure 3a), are independently used as a dopant. To obtain a
homogeneous mixture, 0.1, 0.3, 0.5 and 1.0 wt% dye-molecule added
separately to the LC host is melted by stirring at a few degrees above the
nematic-to-isotropic transition temperature (TNI) of host LCs.
Technologies in Korea for their kind support of LC materials and useful
discussions.
Received: February 14, 2013
Published online:
[1] M. Hasegawa, in Alignment Technologies and Applications of Liquid
Crystal Devices, (Eds: K. Takatoh, M. Hasegawa, M. Koden, N. Otoh,
R. Hasegawa, M. Sakamoto), Taylor and Francis, New York, USA,
2005, Ch. 2 and Ch. 3.
[2] a) D. W. Berreman, Phys. Rev. Lett. 1972, 28, 1683; b) S. Kumar,
J.-H. Kim, Y. Shi, Phys. Rev. Lett. 2005, 94, 077803; c) J. Fukuda,
M. Yoena, H. Yokoyama, Phys. Rev. Lett. 2007, 98, 187803;
d) J. Fukuda, M. Yoena, H. Yokoyama, Phys. Rev. Lett. 2007, 99,
139902.
[3] K.-H. Kim, J.-K Song, NPG Asia Mater. 2009, 1, 29.
[4] a) V. K. Gupta, N. L. Abbott, Science 1997, 276, 1533; b) R. R. Shah,
N. L. Abbott, Science 2001, 293, 1296.
[5] a) S. M. Malone, D. K. Schwartz, Langmuir 2008, 24, 9790;
b) V. K. Gupta, N. L. Abbott, Phys. Rev. E 1996, 54, R4540.
[6] a) Y. Yi, G. Lombardo, N. Ashby, R. Barberi, J. E. Maclennan,
N. A. Clark, Phy. Rev. E 2009, 79, 041701; b) S. Kiston, A. Geisow,
Appl. Phys. Lett. 2002, 80, 3635; c) S. Lazarouk, A. Muravski,
D. Sasinovich, V. Chigrinov, S. S. Kwok, Jpn. J. Appl. Phys. 2007, 46,
6889.
Sample Preparation: To fabricate electro-optic LC cells, either bare
ITO-coated glass or ITO-coated glass with a conventional polyimide
deposition were used as substrates. The cell gap was maintained by
10 μm thick tape spacers or silica-ball spacers with 4 μm diameter. The
dye-doped mixtures were loaded into the cells with a capillary action at
5 degree above the TNI and slowly cooled to an ambient temperature.
The planar aligned nematic cells were covered with a black mask
with a 5 mm circular hole and subsequently exposed to UV-light with
500 mW cm−2 intensity for 30 minutes. The Spot Cure Model SP-9
(Ushio Inc.) was used for UV-source and a mirror was kept beneath
the cell during the vertical irradiation. To selectively dissolve the LC
host, the cells were immersed in excess amount of hexane for 24 hours
and dismantled to examine the surface with the AFM, FE-SEM, EDS,
FTIR and UV–vis spectroscopy. For particle size analysis and UV–vis
spectroscopy, various concentrations of dye-solutions in chloroform or
liquid crystal were prepared. The drop cast films of dye in chloroform
solution and melt films of solid dye on the quartz plate are also
prepared for the UV–vis spectroscopy. The hybrid cells with quartz plate
and silicon wafer were fabricated for the UV–vis and FTIR spectroscopy.
Measurements: POM and conoscopic images were taken using an
Nikon Eclipse LV 100 POL polarizing optical microscope equipped with a
Nikon DS-Ri1 CCD camera and an Instec STC 200 temperature controller
with Instec HCS 402 hot stage. Atomic force microscopy was performed
on SPM Nano Focus n-Tracer. FE-SEM and EDS were carried out using
the Hitachi S-4800 high resolution SEM. The UV–vis absorption spectra
of the dye solutions in LC or chloroform were measured by the UV–vis
spectrophotometer (Jasco, ARSN-733). For particle size analyses, count
rate and particle size of the dye-solutions in chloroform contained in a
quartz cuvette were measured as a function of UV-irradiation time using
a Brook Haven BioPlus particle size analyzer (Brookhaven Inc.). FTIR
spectra of azo-dye deposited on silicon wafer were recorded prior to
and after UV-irradiation using FT-IR 4200 (Jasco, Japan). Electro-optical
switching behaviors are characterized by LCMS-200 (Sesim Photonics
Technology, Korea).
[7] K. Ichimura, Chem. Rev. 2000, 100, 1847.
[8] W. M. Gibbons, P. J. Shannon, S.-T. Sun, B. J. Swetlin, Nature 1991,
351; b) C. Ruslim, L. Komitov, Y. Matsuzawa, K. Ichimura, Jpn. J.
Appl. Phys. 2000, 39, L104; c) L. Komitov, K. Ichimura, A. Strigazzi,
Liq. Cryst. 2000, 27, 51.
[9] a) S.-C. Jeng, C.-W. Kuo, S.-L Wang, C.-C. Liao, Appl. Phys. Lett.
2007, 91, 061112; b) H. Qi, B. Kinkead, T. Hegmann, Adv. Funct.
Mater. 2008, 18, 212; c) D. Zhau, W. Zhou, X. Cui, L. Guo,
H. Yang, Adv. Mater. 2011, 23, 5779; d) L. Wang, W. He, X. Xiao,
F. Meng, Y. Zhang, P. Yang, L. Wang, J. Xiao, H. Yang, Y. Lu, Small
2012, 8, 2189; e) L. Wang, W. He, X. Xiao, M. Wang, M. Wang,
P. Yang, Z. Zhou, H. Yang, H. Yu, Y. Lu, J. Mater. Chem. 2012, 22,
19629.
[10] a) M. Grzelczak, J. Vermant, E. M. Frust, L. M. Liz-Marzan, ACS
Nano 2010, 4, 3591; b) M. Fialkowski, K. J. M. Bishop, R. Klajn,
S. K. Smoukov, C. J. Campbell, B. A. Grzybowski, J. Phys. Chem. B
2006, 110, 2482.
[11] a) X. Tong, G. Wang, Y. Zhao, J. Am. Chem. Soc. 2006, 128, 8746;
b) X. Tong, M. Pelletier, A. Lasia, Y. Zhao, Angew. Chem. Int. Ed.
2008, 47, 3596; c) H. S. Lim, W. H. Lee, S. G. Lee, D. Lee, S. Jeon,
K. Cho, Chem. Commun. 2010, 46, 4336; d) K. Ichimura, S. Oh,
M. Nakagawa, Science 2000, 288, 1624.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
[12] a) Z. Sekkat, J. Wood, W. Knoll, J. Phys. Chem. 1995, 99, 17226;
b) H. H. Jaffe, S. J. Yeh, R. W. Gardener, J. Mol. Spectrosc. 1958, 2,
120; c) T. Seki, K. Ichimura, Thin Solid Films 1989, 179, 77; d) T. Sato,
Y. Ozaki, K. Iriyama, Langmuir 1994, 10, 2363; e) Y. Wang, Q. Li, Adv.
Mater. 2012, 24, 1926; f) Q. Li, Y. Li, J. Ma, D.-K. Yang, T. J. White,
T. J. Bunning, Adv. Mater. 2011, 23, 5069; g) Y. Wang, A. Urbas,
Q. Li, J. Am. Chem. Soc. 2012, 134, 3342.
Acknowledgements
This research was supported by the World Class University program
funded by the Ministry of Education, Science and Technology through
the National Research Foundation of Korea (R31-20029). The authors
thank Dr. Yong-Kuk Yun and Dr. Dong-Mee Song from Merck Advanced
©
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2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2013,
DOI: 10.1002/adma.201300730