The electro-optical and electrochromic properties of electrolyte-liquid crystaldispersionsDaniela Cupelli, Giovanni De Filpo, Giuseppe Chidichimo, and Fiore Pasquale Nicoletta Citation: Journal of Applied Physics 100, 024515 (2006); doi: 10.1063/1.2219696 View online: http://dx.doi.org/10.1063/1.2219696 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/100/2?ver=pdfcov Published by the AIP Publishing Advertisement:

[This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

192.109.140.188 On: Thu, 17 Oct 2013 03:29:52

JOURNAL OF APPLIED PHYSICS 100, 024515 �2006�

[This art

The electro-optical and electrochromic properties of electrolyte-liquidcrystal dispersions

Daniela Cupelli, Giovanni De Filpo, and Giuseppe ChidichimoDipartimento di Chimica, Università della Calabria, 87036 Rende (Cosenza), Italy

Fiore Pasquale Nicolettaa�

Dipartimento di Scienze Farmaceutiche, Università della Calabria, 87036 Rende (Cosenza), Italy

�Received 25 November 2005; accepted 14 May 2006; published online 31 July 2006�

Liquid crystals are known to exhibit a reversible color change by applying a direct current electricfield, if a small amount of quaternary ammonium salts is dissolved into them. Applications of suchan electrochromic liquid crystal cell have been proposed as interesting laser-addressed writing andimage storage devices. Liquid crystal dispersions are composite materials formed by liquid crystaldroplets embedded in either a polymer or a monomer matrix. Thin films of liquid crystal dispersionscan be turned from an opaque to a transparent state by application of a suitable alternating currentelectric field. Herein, we report our investigations on electrolyte-liquid crystal dispersions, whichshow independent electro-optical and electrochromic properties characterized by fast bleachingtimes. This cell involves the reorientation of liquid crystal molecules, trapped in droplets, for theelectro-optical changes from the opaque to transparent state and the formation of complexes at thecathode, between the positive ions of electrolyte and liquid crystal dispersed in the matrix, for theelectrochromic changes from the bleached to colored state. The device is able to change itselectro-optical transmittance within few milliseconds and its color within few seconds. © 2006American Institute of Physics. �DOI: 10.1063/1.2219696�

I. INTRODUCTION

Organic electrochromic materials are being widely in-vestigated for their variety of colors, which allow interestingapplications such as light shutters, smart windows, and ac-tive displays.1–6 These materials include bipyridilium sys-tems, conducting polymers, quinones, phthalocyanines,terephthalates, and cyanobiphenyls.1 These latter materialswith a small amount of electrolyte exhibit a reversible andintense color change, if a direct current �dc� electric field isapplied or removed.7 The interest in electrolyte doped cyano-biphenyls resides in the fact that they are components ofliquid crystal mixtures now widely used in electronic de-vices. When a dc electric field is applied to an electrochro-mic liquid crystal cell �ECLC�, i.e., a liquid crystal dopedwith small amount of electrolyte �e.g., tetrahexylammoniumiodide, THAI� sandwiched between two conductive glasssubstrates, colored species are produced at the cathode.

Nakamura et al.8 have proposed a mechanism for thecoloration in ECLCs, which involves, when the dc electricfield is turned on, the migration of positive ions of electro-lyte, THA+, to the cathode and the formation of coloredcharge transfer complexes between them and cyanobiphenylsreduced at the cathode. The bleaching occurs when the dcfield is removed and is attributed to the diffusion of halidemolecules, I2, produced by oxidation at the anode and theirreaction with colored species. The electrochemically inducedcolor depends on the type of used liquid crystal, i.e., on thetype of terminal group forming the component. In particular,4-cyano-4�-n-alkylbiphenyls and 4-cyano-4�-n-alkoxy-

a�

Electronic mail: fiore.nicoletta@unical.it

0021-8979/2006/100�2�/024515/5/$23.00 100, 0245

icle is copyrighted as indicated in the abstract. Reuse of AIP content is sub

192.109.140.188 On: Thu,

biphenyls �nCB and nOCB of general formulaCnH2n+1C6H4C6H4CN and CnH2n+1OC6H4C6H4CN, respec-tively� and chiral nematic biphenyls �such as C15 and CB15of molecular formula C2H5CH�CH3�CH2OC6H4C6H4CNand C2H5CH�CH3�CHC6H4C6H4CN� give colored cells,which appear bluish green to the eye. On the contrary,alkoxy-benzylidene cyanoanilines �general formulaCnH2n+1OC6H4CHNC6H4CN� give an orange coloration. Ifa eutectic mixture is used, the resulting color will bej thesuperimposition of all single colorations due to eachliquid crystal component. The main drawback is that thecomposition of most eutectic mixtures is unknown, beingunder patent. So it is not possible to predict the final colorof cells. The compositions of E7 nematic liquid crystalare 51 wt. % 5CB, 25 wt. % 7CB, 16 wt. % 8OCB, and8 wt. % 5CT �a cyanoterphenyl of molecular formulaC5H11C6H4C6H4C6H4CN� and, experimentally, give rise to agreen coloration. E49 nematic mixture, which has chemicalphysical properties similar to those of E7 but unknown com-position, gives a yellowish green color upon application of adc electric field, if it has been doped with ammonium salts.The origin of electrochromism can be attributed to the for-mation of colored complexes between the phenyl rings andammonium ions, as reported by Nakamura et al.9 In thatwork, the authors investigated the electrochromic behaviorof organic materials with relatively simple molecular struc-ture. All compounds had a common phenyl ring and differentterminal groups. They found that the electrochemically in-duced color depends upon the type of terminal groups andtheir position �para, meta, ortho� in the compounds.

The electrochromism is observed both in liquid crystal-

line state as well as in the isotropic state. Nematic, smectic,

© 2006 American Institute of Physics15-1

ject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

17 Oct 2013 03:29:52

024515-2 Cupelli et al. J. Appl. Phys. 100, 024515 �2006�

[This art

and cholesteric ECLCs have been investigated7 even if theirelectrochromic properties are lowered by resistivity increaseand texture changes due to electrohydrodynamic instabilities.Nevertheless a laser addressed smectic liquid crystal lightvalve with electrochromic properties has been proposed.10

The study of time dependence of the coloration andbleaching after the application and removal of the dc fieldled to rather simple equations, which well fitted the experi-mental results. In particular, the coloration density increaseslinearly with the current, and the dominant electrochemicalreaction for bleaching is a first order one.

Liquid crystal dispersions are composite materials con-sisting of micrometer sized liquid crystal droplets dispersedeither in a solid polymer matrix11 �polymer dispersed liquidcrystals, PDLCs� or in a viscous oligomer matrix12 �liquidcrystal emulsions, LCEs�. Liquid crystal dispersions can beturned from an opaque to an optically transparent state byapplication of a suitable alternating current �ac� electric field.The operation principle is the electrically driven reorienta-tion of liquid crystal directors, from their structures and av-erage orientations taken within the droplets, along a prefer-ential direction parallel to the external field. Uponapplication of a sufficient electric field, the reorientation ofliquid crystal droplets leads to a transparent state. An exactmatching between the liquid crystal ordinary refractive indexof liquid crystal droplets and polymer matrix one is desirableeven if not absolutely necessary. PDLCs and LCEs have be-come a recent focus of research for application such as spa-tial light modulators, switchable shutters, and displays.13 Inthe past it has been demonstrated that polymer dispersedcholesteric liquid crystals can be used in electro-optical colordisplays14–17 as their helical texture selectively reflects a spe-cific wavelength of light associated with the cholesteric he-lical pitch, when light propagates parallel to the helicalaxis.18

More recently, we have shown that liquid crystal disper-sions hosting electrochromic guest molecules are able to giveboth an independent and fast switching from a scatteringopaque state to a transmissive transparent state, owing toliquid crystal director reorientation, and a color change, dueto electrochromic reactions occurring at the electrodes.19,20

Both changes are obtained with rather fast switching andrelaxation times: few milliseconds and few seconds for theelectro-optical and electrochromic responses, respectively.Bifunctional devices that possess both the electrically con-trolled scattering and electrochromic properties are importantin practical applications.

Herein, we report our results devoted to fabrication andcharacterization of a bifunctional device based onelectrolyte-liquid crystal dispersions �ELCDs�, in which theliquid crystal is responsible of both color and transmittancechanges. Both relaxation times remain almost unchangedwith respect to our previous bifunctional devices, but thepreparation process is extremely simplified by using all com-mercially available materials and offer, in addition, a great

variety of colors.icle is copyrighted as indicated in the abstract. Reuse of AIP content is sub

192.109.140.188 On: Thu,

II. EXPERIMENTAL PART

PDLC films were prepared by a thermally orpolymerization-induced phase separation process. LCEswere prepared by mixing liquid crystals and bisphenol Aglycerolate diacrylate in different weight ratios.12 The nem-atic liquid crystals used in this work were eutectic mixturesof cyanobiphenyls �E7, E49, CB15, and ZLI4788-000 fromMerck and TN10427 from Rolic�. Thermoplastic matriceswere poly�methyl methacrylate�, poly�isobutyl methacry-late�, and poly�vinyl butyral� from Aldrich. PDLCs were ob-tained via polymerization-induced phase separation startingfrom LCE doped with small amounts of either thermal or UVinitiators �2 wt. % azoisobutyronitrile from Aldrich and2 wt. % Irgacure 651 from Ciba, respectively�.

Ammonium salts �tetrabutylammonium tetrafluorobo-rate, tetrabutylammonium hexafluorophosphate, tetrabuty-lammonium perchlorate, octadecyltrimethylammonium bro-mide, and didodecyldimethylammonium bromide all fromAldrich� were dissolved in different weight percentages�from 1% to 25%� in propylene carbonate �Aldrich�. Thechoice of either a particular polymer or ammonium salt canproduce a change in the intensity and shade of the devicecolor. Bifunctional liquid crystal dispersions were preparedin vials by mixing the appropriate amounts of polymer oroligomer �30–74 wt. % �, liquid crystal �60–25 wt. % �, andelectrolyte solution �10–1 wt. % � in a common solvent. Af-ter solvent evaporation, a small amount of mixture was sand-wiched between transparent conductive substrates. The cellgap was 40 �m. Cells were heated to about 100 °C and,then, cooled at a controlled rate untill room temperature inorder to induce phase separation. Alternatively, cells, con-taining polymerization initiators, were either UV light ex-posed for 15 min �average power of 10 mW/cm2� or heatedto 70 °C for 2 h. The electro-optical properties of sampleswere measured with the optical setup previously reported.21

Spectroelectrochemistry was performed with a YASCOV550 UV-vis spectrometer.

In this paper we report results concerning poly�methylmethacrylate� based samples. In particular, either E7 or E49was used, in the weight ratio polymer:liquid crystal=1:1, forthe preparation of PDLCs, which were doped with differentpercentages �2.5, 5, and 10 wt. %, 5 wt. % if not clearlywritten� of an electrolyte solution �tetrabutylammonium tet-rafluoroborate 10 wt. % in propylene carbonate�.

III. RESULTS AND DISCUSSION

If the liquid crystal concentration in ELCD is kept belowthe onset of phase separation �around 25–30 wt. % for poly-�methyl methacrylate� based devices, small variations de-pend on the different liquid crystal solubilities�, films appearto be transparent and uncolored. They do not possess electro-optical properties as liquid crystal is dispersed in the matrixand is not phase separated in droplets. Nevertheless, theyacquire different colors as a function of the used liquid crys-tal if a dc electric voltage �2 V� is applied as reported in Fig.1. The differences in the absorption can come from the dif-ferent compositions of used eutectic mixtures �E7, E49, and

ZLI 4788-000� and from the particular chemical structure ofject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

17 Oct 2013 03:29:52

024515-3 Cupelli et al. J. Appl. Phys. 100, 024515 �2006�

[This art

CB15 �the presence of a chiral center�. These films are reallyelectrochromic polymeric solid films. They are self-supporting and can be subsequently laminated between con-ductive substrates. Their self-supporting nature providesmany benefits to the electrochromic devices manufacturedwithin, including the enhancement of processibility and,from a safety perspective, in the event that electrochromicdevices should break or become damaged, the polymer ma-trix is able to impede seepage in the environment. For thesake of completeness, samples are not “perfectly sealed” bypolymer matrix and liquid crystal can seep out of �or otherliquids can seep into� the cells over time.

The increase of liquid crystal concentration causes phaseseparation. Liquid crystal droplets form and increase theirnumber and size. Electrolyte-liquid crystal dispersions ap-pear opaque and uncolored when no field is applied. Theopaque state is a consequence of the random distribution ofliquid crystal directors that scatter light. If an ac externalfield is applied �drive frequency �drive=1 kHz�, liquid crystaldirectors will reorient along the field direction, the light scat-tering will be largely reduced, if the match condition is sat-isfied, and the device will appear transparent.13 The fielddependent transmittance of a typical electrolyte-E49 liquidcrystal dispersion �E49-ELCD� is reported in Fig. 2. There isno particular effect due to the presence of electrolyte withrespect to conventional PDLCs.

The off state transmittance is around 1% and reaches

FIG. 1. Absorbance through electrolyte-liquid crystal saturated dispersionsin their dc on state with different liquid crystals: �a� CB15, �b� E7, �c� E49,and �d� ZLI4788-000.

FIG. 2. Transmittance dependence on ac electric field ��drive=1 kHz�through bifunctional electrolyte-E49 liquid crystal dispersions for differentelectrolyte solution �ES� loadings: �a� ES=2.5%, �b� ES=5%, �c� ES

=10%, and �d� ES=2.5%, with a 2 V dc simultaneous excitation.

icle is copyrighted as indicated in the abstract. Reuse of AIP content is sub

192.109.140.188 On: Thu,

80% value when electric field strength lower than 3 V �m−1

is applied �the acceptance angle is equal to 2.5°�. It is impor-tant to note that, increasing the electrolyte concentration, theresponse of transmittance to the applied field becomessharper and shifts towards lower switching fields. This is inagreement with previous works reporting similar effects dueto an increase of polymer matrix conductivity.22 Another pos-sible explanation for these experimental results �i.e., thefilms respond at lower electric fields with increasing dopinglevels� could be a decrease in the clearing temperature ofliquid crystal and a consequent reduction of its elastic con-stants due to the carrier solvent �propylene carbonate�. Opti-cal observations in E7 based PDLCs, with 10 wt. % of elec-trolyte solution, showed appreciable changes in the clearingtemperature of liquid crystal �56 °C rather than 60 °C�.

If a simultaneous dc excitation �2 V� is provided to thesamples, their T vs E curves are slightly right shifted due tothe presence of small depolarization fields. As an example,curve a changes to curve d in Fig. 2, if one supplies a dcfield.

If the external field is removed, the restoring forces act-ing at droplet interfaces will cause the random distribution ofdirector orientations within few milliseconds. The electro-optical response of an electrolyte-E49 liquid crystal disper-sion is reported in Fig. 3. The driving field is a square waveat the drive frequency of 1 kHz and with a rms drive strengthof 3 V �m−1. Both the rise and the decay times are of theorder of few milliseconds. Such values are similar to thoseshown by conventional PDLCs. No appreciable changes inthe response times are observed if a simultaneous dc excita-tion is provided �e.g., in Fig. 3 curve a changes to curve d,which results less noisy�.

The increase of electrolyte concentration in samplescauses a decrease in the rise and decay times, due to a fasteronset and depletion of the local fields acting on droplets.22

FIG. 3. Electro-optical response of bifunctional electrolyte-E49 liquid crys-tal dispersions for different electrolyte solution �ES� loadings: �a� ES=2.5%, �b� ES=5%, �c� ES=10%, and �d� ES=2.5%, with a 2 V dc simul-taneous excitation.

The optical properties of electrolyte-liquid crystal dis-ject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

17 Oct 2013 03:29:52

024515-4 Cupelli et al. J. Appl. Phys. 100, 024515 �2006�

[This art

persions were examined by spectroelectrochemistry over thewhole visible range after the application of a dc field.

Figure 4 shows the changes in the steady-state absor-bance for E49-ELCDs containing different electrolyte con-centrations. The absorbance and, consequently, the samplecoloration increase as the electrolyte concentration increases.If the coloration process is observed at the optical micro-scope along cross sections of thick samples �2 mm�, it isevident that the colored region grows at the cathode as afunction of dc current time and electrolyte concentration. Asa consequence, the coloration is due to the increase in layerthickness of colored complexes at the cathode.

The change in the absorbance for samples of Fig. 4 dur-ing a coloration and bleaching sequence is reported in Fig. 5.

When a dc current is applied to cells, colored complexesare produced at the cathode. The absorbance reaches a pla-teau value within few seconds �12–16 s� depending on elec-trolyte concentration and dc current application time. Uponfield removal the color spontaneously bleaches within some

FIG. 4. Spectroelectrochemical UV-vis transmittance through bifunctionalelectrolyte-E49 liquid crystal dispersions for different electrolyte solution�ES� loadings: �a� ES=2.5%, �b� ES=5%, and �c� ES=10%.

FIG. 5. Changes in optical absorbance during a coloration and bleachingsequence for electrolyte-E49 liquid crystal dispersions with different elec-trolyte solution �ES� loadings: �a� ES=2.5%, �b� ES=5%, and �c� ES

=10%.

icle is copyrighted as indicated in the abstract. Reuse of AIP content is sub

192.109.140.188 On: Thu,

seconds �4–5 s� depending on electrolyte concentrations. Itis important to note that short bleaching times can be ob-tained by shortening the cells.

The choice of halide counterions does not affect thecolor of cells. Similar colorations are obtained by using qua-ternary ammonium salts with different halides. On the con-trary, the type of liquid crystal used in the formulations givesdifferent absorption spectra and, consequently, different col-ors to the films as shown in Fig. 6 for E7 �curve a� and E49�curve b�.

Following the mechanism of coloration and bleachingproposed by Nakamura et al.8 for the electrochromism ofliquid crystal materials, the application of a dc current to anELCD causes the migration of ammonium ions A+ to thecathode, where the reaction between A+ and liquid crystal�LC�,

A+ + LC + e− → A*LC�colored� ,

takes place and gives a colored complex.At the same time halide molecules, Ha2, are produced at

the anode. These last molecules diffuse and react with thecolored species according to the following reaction:

2A*LC + Ha2 → 2�A+ + LC� + 2Ha−�uncolored� ,

determining the bleaching of the sample when the dc exter-nal field is removed.

The combined electro-optical and electrochromic behav-iors of E49-ELCDs were examined by spectroelectrochemis-try by applying different electric fields, as shown in Fig. 7.The four distinct states are �a� opaque and uncolored, whenno field is applied; �b� opaque and colored, if a dc electricfield is turned on; �c� transparent and uncolored, when an acelectric field is applied; and �d� transparent and colored, ifboth the ac and dc electric fields are present.

The opaque state is due to light scattering by liquid crys-tal droplets, and the transparent one is attributed to liquidcrystal director reorientations as previously reported in otherbifunctional devices.19,20 Color is a consequence of the elec-trochromic reactions, which take place at the cathode be-tween ammonium ions and liquid crystal. The pictures of thefour states are reported in Fig. 8 for an E49-ELCD. Even ifthe migration of ions to electrodes is the basic physical prin-

19,20

FIG. 6. Spectroelectrochemical UV-vis transmittance through bifunctionalelectrolyte-liquid crystal dispersions for different kinds of liquid crystals: �a�E7, and �b� E49. Sample �a� looks green and �b� dark yellow.

ciple as previously reported, different reactions takeject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

17 Oct 2013 03:29:52

024515-5 Cupelli et al. J. Appl. Phys. 100, 024515 �2006�

[This art

place, i.e., the formation of a colored complex rather than theoxidation and/or reduction of components. In addition, thisdevice results to be extremely simplified as the ammoniumsalts play a double role of chromogenic material and conduc-tivity enhancer, and the liquid crystal, which is usually anisotropic plasticizer dispersed in the matrix, is used as sec-ond chromogenic molecule. Consequently, the cell does notneed conductive oligomers ad hoc synthesized and redoxcouples. Since a high conductivity often degrades the stabil-ity or the lifetime of PDLCs �e.g., by “discoloring” the in-dium tin oxide �ITO� conductive substrates� the electrolyteconcentration should be carefully determined depending onthe particular application.

IV. CONCLUSIONS

We have shown that electrolyte-liquid crystal dispersionsrepresent a class of materials, which are able to combine theelectro-optical properties of liquid crystal dispersions and theelectrochromic properties of electrolyte-liquid crystal mix-tures. The proposed device is able to change its electro-

FIG. 7. The four states of a bifunctional electrolyte-E49 liquid crystal dis-persion: �a� opaque and uncolored, �b� opaque and colored, �c� transparentand uncolored, and �d� transparent and colored.

FIG. 8. �Color online� A picture of the four states of a bifunctionalelectrolyte-E49 liquid crystal dispersion: �a� opaque and uncolored, �b�opaque and colored, �c� transparent and uncolored, and �d� transparent andcolored.

icle is copyrighted as indicated in the abstract. Reuse of AIP content is sub

192.109.140.188 On: Thu,

optical transmittance within few milliseconds and its colorwithin few seconds. The films represent a simplification inthe preparation of bifunctional devices, as they do not needthe use of any particular chemical, such as conductive oligo-mers, and of other electrochromic molecules, such as violo-gens. The matrix conductivity is opportunely increased bythe electrolyte solution. The liquid crystal dispersed in thematrix, which is generally considered a “lost” material, actsas chromogenic molecule. Consequently, ELCDs could offerinteresting contributions in the color display production.Work is in progress in order to obtain ELCDs operating in areverse mode, i.e., transparent in the off state and opaquewhen an ac electric field is applied.

ACKNOWLEDGMENT

MIUR, the Italian Ministry for University, is acknowl-edged for financial supports �Grant Nos. Ex60% andPRIN2005�.

1P. M. S. Monk, R. J. Mortimer, and D. R. Rosseinsky, Electro-chromism:Fundamentals and Applications �VCH Inc., Weinheim, 1995�.

2G. G. Wallace, G. M. Spinks, L. A. P. Kane-Maguire, and P. R. Teasdale,Conductive Electroactive Polymers, 2nd ed. �CRC, Boca Raton, 2003�.

3S. A. Sapp, G. A. Sotzing, and J. R. Reynolds, Chem. Mater. 10, 2101�1998�.

4D. R. Rosseinsky and R. J. Mortimer, Adv. Mater. �Weinheim, Ger.� 13,783 �2001�.

5H. Meng, D. Tucker, S. Chaffins, Y. Chien, R. Helgeson, B. Dunn, and F.Wudl, Adv. Mater. �Weinheim, Ger.� 15, 146 �2003�.

6A. A. Argun, et al. Chem. Mater. 16, 440 �2004�.7K. Nakamura, S. Kaneko, Y. Ito, H. Hirabayashi, and K. Ogura, J. Appl.Phys. 53, 1792 �1982�.

8K. Nakamura, K. Nakada, Y. Ito, and E. Koishi, J. Appl. Phys. 57, 135�1985�.

9K. Nakamura, Y. Oda, T. Sekikawa, and M. Sugimato, Jpn. J. Appl. Phys.,Part 1 26, 931 �1987�.

10K. Ogura, H. Hiarbayashi, A. Uejima, and K. Nakamura, Jpn. J. Appl.Phys., Part 1 21, 969 �1982�; K. Nakamura, Y. Oda, and M. Sugimato, J.Appl. Phys. 59, 593 �1986�.

11L. Fergason, U.S. Patent No. 4,435,047 �6 March 1984�.12G. De Filpo, J. Lanzo, F. P. Nicoletta, and G. Chidichimo, J. Appl. Phys.

84, 3581 �1998�; G. De Filpo, F. P. Nicoletta, J. Lanzo, and G. Chid-ichimo, ibid. 85, 2894 �1999�.

13P. S. Drzaic, Liquid Crystal Dispersions �World Scientific, Singapore,1995�.

14P. P. Crooker and D. K. Yang, Appl. Phys. Lett. 57, 2529 �1990�.15J. L. West, R. B. Akins, J. Fracl, and J. W. Doane, Appl. Phys. Lett. 63,

1471 �1993�.16C. Binet, M. Mitov, A. Boudet, M. Mauzac, and P. Sopena, Liq. Cryst. 26,

1735 �1999�.17G. De Filpo, F. P. Nicoletta, and G. Chidichimo, Adv. Mater. �Weinheim,

Ger.� 17, 1150 �2005�.18N. Tamaoki, Adv. Mater. �Weinheim, Ger.� 13, 1135 �2001�.19F. P. Nicoletta, D. Cupelli, G. De Filpo, and G. Chidichimo, Appl. Phys.

Lett. 84, 4260 �2004�.20F. P. Nicoletta, G. Chidichimo, D. Cupelli, G. De Filpo, M. De Benedittis,

B. Gabriele, G. Salerno, and A. Fazio, Adv. Funct. Mater. 15, 995 �2005�.21G. Chidichimo, Z. Huang, C. Caruso, G. De Filpo, and F. P. Nicoletta,

Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 299, 379 �1997�.22D. Cupelli, F. P. Nicoletta, G. De Filpo, G. Chidichimo, A. Fazio, B.

Gabriele, and G. Salerno, Appl. Phys. Lett. 85, 3292 �2004�.

ject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

17 Oct 2013 03:29:52