The authors study the effect of the spontaneous polarization (PS) of a ferroelectric liquid crystal mixture with compensated helix and of the thickness of the alignment layer (da) on the analog switching in a cell. The quality of analog switching is established in terms of its contrast ratio, texture in the dark state, and the electrostatic energy. The latter approximates to PS 2da for d

Manjuladevi, V.

Panarin, Yuri

Vij, J.K.

English

Arrow@TUDublin

Technological University Dublin ARROW@TU Dublin Articles School of Electrical and Electronic Engineering 2007 Experimental Study for the Conditions of Analog Switching in Ferroelectric Liquid Crystal Cells V. Manjuladevi Trinity College Dublin, Ireland Yuri Panarin Technological University Dublin, yuri.panarin@tudublin.ie J.K. Vij Trinity College Dublin, Ireland Follow this and additional works at: https://arrow.tudublin.ie/engscheleart2  Part of the Electrical and Computer Engineering Commons Recommended Citation Manjuladevi, V. et al. 2007. Experimental study for the conditions of analog switching in ferroelectric liquid crystal cells. Applied Physics Letters vol. 91. doi:10.1063/1.2767210 This Article is brought to you for free and open access by the School of Electrical and Electronic Engineering at ARROW@TU Dublin. It has been accepted for inclusion in Articles by an authorized administrator of ARROW@TU Dublin. For more information, please contact yvonne.desmond@tudublin.ie, arrow.admin@tudublin.ie, brian.widdis@tudublin.ie. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 License Experimental study for the conditions of analog switching in ferroelectricliquid crystal cellsV. Manjuladevi, Yu. P. Panarin,a and J. K. VijbDepartment of Electronic Engineering, Trinity College Dublin, University of Dublin, Dublin 2, IrelandReceived 17 April 2007; accepted 9 July 2007; published online 3 August 2007The authors study the effect of the spontaneous polarization PS of a ferroelectric liquid crystalmixture with compensated helix and of the thickness of the alignment layer da on the analogswitching in a cell. The quality of analog switching is established in terms of its contrast ratio,texture in the dark state, and the electrostatic energy. The latter approximates to PS2da for dadthickness of the ferroelectric liquid crystal cell layer. © 2007 American Institute of Physics.DOI: 10.1063/1.2767210The discovery of the electro-optic effect called surfacestabilized ferroelectric liquid crystal cell1 SSFLC led tolow-cost passive-matrix wide viewing angle displays as at-tractive alternatives to that of nematics. Though a number ofFLCs were designed and the addressing schemes advancedover a decade ago, a large-scale production of such micro-displays has only recently begun.2 The limiting reasons havebeen i a significant drop in the cost of thin film transistortechnology and ii an absence of the analog gray scale inFLCs. However, the discovery of the electro-optic effect,thresholdless switching3 also called “V-shaped switching” inSSFLCs, has added attraction to potential for such displaysin projections, viewfinders, and the switching devices usingR-G-B color sequential scanning due to gray scale and thefast switching. This effect shows smooth, domain-free de-pendence of the transmission on applied voltage.Although a number of V-shaped FLC displays have beenprototyped and developed,4–6 the physical interpretation ofthe relevant electro-optic effect has been a topic of debateover a decade. Originally, the V-shaped switching achievedin the “Tokyo mixture” was explained as due to frustrationbetween ferro- and antiferroelectricities,3 due to a ferrielec-tric phase7 with qT=2/3, and biaxial anchoring.8 Neverthe-less, V-shaped switching was also achieved in conventionalFLCs which offered a differing interpretation. Several theo-retical models9–13 sometimes controversial have been pro-posed to simulate conditions for observing this effect. One ofthe reasons for this controversy is the term “V shape.” This isbased on a particular observation of a smooth dependence ofthe transmittance on applied voltage. Similar response is alsoknown for other FLC structures: short-pitch helical cells,14chevron, and de Vries smectics.15 The conditions required foranalog switching have not been clearly established thoughthe importance of certain parameters such as a the largespontaneous polarization, b thick alignment layers, c rela-tively long or compensated helical pitch, d absence ofdomains during switching, and e existence of hysteresisinverse frequency have been known. A combination of therequirements, however, allows us to “localize” the term Vshape and assign it to a nonuniform splayed structure. Such astructure might be realized with a combination of certainconditions, e.g., a large p /d ratio,11,16 where p and d arethe polar and the nonpolar parts of the anchoring energy. Theeffect of large p /d is increased by high PS values and theuse of thick alignment layers17 da. Moreover the splay struc-ture is formed exclusively by large PS and thick da. A pre-liminary experimental study showed18 that the originallybistable cells became thresholdless or monostable with anincrease in da up to 1 m and the conventional hysteresis atmoderate frequencies i.e., f1 Hz inversed at frequencieslower than the hysteresis-inverse frequency f i. According tothe “electrostatic” model used here, PS creates an internalelectric field which is proportional to both PS and da. Thisinternal field interacts with the spontaneous polarizationmaking the uniform up and down states unstable. There-fore, for sufficient large PS and da the only stable state in thecell is that of the splayed structure, because the spontaneouspolarization vectors are opposite thus canceling out witheach other. The internal field consequently is zero in theabsence of the external field. Recently O’Callaghan19showed quantitatively that V-shaped switching occurs whenthe electrostatic energy in the cell sufficiently exceeds sur-face anchoring energies, the latter expected to be of the orderof 1 erg/cm2.In this letter, we provide an evidence for the conditionsof V-shaped switching in SSFLCs while examining the elec-trostatic model. According to this model, different param-eters PS, da, pitch, and anchoring energy account forV-shaped structure/switching. Here we vary PS and da inde-pendently of each other. In our study a series of FLC mix-tures with PS ranging from 24 to 96 nC/cm2 and compen-sated helix p0→ are designed. A second set of theexperiments is designed to study the effect of the thick-ness of the alignment layers; these are varied from40 to 400 nm. We address a number of problems includingthe term V-shape switching. The FLC mixtures used in thestudy show simple phase transition sequencesIs-SmA-SmC*-Cr. This rules out other possible physicalmechanisms for the V-shape switching such as de Vries15 andferri- and/or antiferroelectric phases.3,7 The helical pitch ofthe studied mixture is rather long compared to the cell thick-ness to guarantee the absence of the helix and consequentlythe deformed helix ferroelectric DHF mode14 is ruled outas the reason for switching. Only two FLC structures may bepresent in these conditions: chevron structure and electro-statically stabilized splay twist structure.9–11 The transitionfrom chevron to the monostable splay structure may occur atsome critical value of the electrostatic energy depending onaAlso at: School of Electronic Engineering, DIT, Dublin 8, Ireland; elec-tronic mail: yuri.panarin@dit.iebElectronic mail: jvij@tcd.ieAPPLIED PHYSICS LETTERS 91, 052911 20070003-6951/2007/915/052911/3/$23.00 © 2007 American Institute of Physics91, 052911-1Downloaded 03 Aug 2007 to 134.226.86.54. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jspboth PS and da. In practice, however, this transition is broadand has been discussed in detail16 where they find that forPS100 nC/cm2, the effect of chevron is negligible with aconsequential transition to the monostable state. Therefore itis important to introduce a quantitative parameter character-izing the quality of V-shape switching. Normally the extinc-tion angles of up and down states in the chevron cell arerather low 3°–5° thus the electro-optical response maythough appear similar to V shape at zero applied voltage, thechevron cell consists of ideally 50/50% mixtures of up anddown domains and the overall structure must show lowerextinction than the monostable splay cell. Therefore in ourstudy we utilized the contrast ratio CR as a qualitative pa-rameter for characterizing the quality of the V-shape switch-ing and transition from chevron to splayed structures. Thesecond, rather useful measure of the V-shape quality is justthe texture of the cell at zero applied voltage obtained usinga polarizing microscope.We prepared a series of FLC mixtures with compensatedhelix helix free with PS varying from 96 to 24 nC/cm2by varying the concentration of two optically active dopants,with the same sign of PS but opposite signs of the helicalpitch p0. For electro-optical studies, planar cells were pre-pared using the indium tin oxide coated glass plates andpolyimide RN1175 as the alignment layer. The alignmentlayer thickness was varied from 40 to 400 nm using the spincoating procedure. One side rubbed cells with thickness inthe range of 1.4–1.8 m were prepared. The cell was filledwith the material in the isotropic phase and then cooleddown to room temperature. The rubbing direction of the cellsubstrate was adjusted parallel to one of the crossed polariz-ers of the microscope. A triangular voltage was applied atvarious frequencies and the transmittance measured using aphotodiode attached to the microscope. The electro-optic re-sponse at various frequencies was recorded using an oscillo-scope. The images in the dark state were recorded using adigital camera interfaced with the computer.The effect of the spontaneous polarization on the appear-ance of the V-shaped switching was studied by measuring theelectro-optic response of the FLC mixtures with differentPS. The helical pitch of the mixtures remains rather long10 m to guarantee the surface stabilized helix-freestructure. The smectic tilt angle at room temperature of allthe mixtures lies in the range of 26°–27°. The electro-opticresponse of the material with PS=96 nC/cm2 with variousthicknesses of the alignment layer are shown in Fig. 1a.Figure 1b shows the electro-optic response of the variouscells with FLC mixtures with PS ranging from96 to 24 nC/cm2 with da400 nm. The response isV-shaped switching at a frequency of 3 Hz in cells with daranging from 200 to 400 nm. The texture is uniformly blackwith a single domain as seen in Fig. 2a. Though the re-sponse of the cell with da of 200 nm looks V-shaped, the CRdrops from 92 to 33 due to the stripe-shaped domains appear-ing in the texture in Fig. 2b. V-shaped switching is nolonger observed for da100 nm and below. With a changein PS from 96 to 73 nC/cm2 the response remains V-shaped-like but the CR drops again to only 16 due to the presence ofdomains, as shown in Fig. 2c. Finally, for the FLC cell with24 nC/cm2 the response is no longer V shaped.Figures 3a and 3b show the dependence of the hys-teresis width or coercive voltage as a function of frequency.At higher frequencies the electro-optic response of all cellsshows positive hysteresis due to the dynamic delay inswitching. At low frequencies, the electro-optic responseshows negative or inversed hysteresis due to a differencebetween the applied and internal electric fields.15,17,18 Theinternal or actual electric field Et inside a FLC cell is asum of the field due to the applied voltage and the field dueto surface charges distributed between the two electrodes ofthe cell and can be written as17Et = −Vta2da + da+2tda2da + da, 1where Vt is the voltage of the triangular wave appliedacross the FLC cell,  and a are dielectric permittivities ofthe LC and alignment layers, respectively, and d and da arethe thicknesses of the LC cell and of the alignment layers,respectively. The sign in the electric field arises from thisbeing the negative gradient of the potential function. t isFIG. 1. Electro-optic response of FLC mixture: a PS=96 nC/cm2 andvarying thickness of alignment layers; and b da400 nm and varying PS.K here is the contrast ratio.FIG. 2. Color online Photographs of the cell texturesat zero applied field: a PS=96 nC/cm2, da400 nm;b PS=96 nC/cm2, da200 nm; and c PS=24 nC/cm2, da400 nm. The linear dimension of thetexture image horizontally is 120 m.052911-2 Manjuladevi, Panarin, and Vij Appl. Phys. Lett. 91, 052911 2007Downloaded 03 Aug 2007 to 134.226.86.54. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jspthe effective surface charge density that accumulates at theinterface between the alignment layer and FLC ast = ion + Pscos 1 + cos 2 , 2ion is the charge density due to the ions and the second termis due to the polarization charge density, 1 and 2 are theazimuthal angles of the directors measured with respect tothe two aligning surfaces; these being zero for the directorslying along the aligning surfaces. The effect of ions on theappearance of the low frequency hysteresis has beenexamined.15,20 For higher frequencies, the field induced bythe ions enhances the externally applied field, and at lowfrequencies it diminishes this field. Hence the internal fieldcan be either greater or lower than the external field leadingto a change in the coercive force.15 The frequency range ofthe hysteresis-free V-shaped switching can be increased con-siderably by connecting the FLC cell through the capacitor12but in the case of pulse switching, the frequency is not of anysignificance. From the results presented in Figs. 1–3, we ob-serve that the V-shaped switching is distorted by decreasingthe CR on lowering either PS or by reducing da. These ob-servations are explained in terms of the “electrostatic”model.9,10 Using Eq. 2, the electrostatic energy can be cal-culated. O’Callaghan19 discussed its importance and found itas W0=daPS2 /a1+2da /ad. This approximates to PS2da /afor dad. In order to get W0 in erg/cm2, we need to multiplythis expression by 4	,  and a are then dimensionless num-bers, thickness and PS are in CGS units. At some criticalvalue of the electrostatic energy W0*, it exceeds the elasticenergy of the splay deformations and then the monostabledomainless splay structure is realized. The CR is shown asa function of W0 in Fig. 4. The CR decreases by decreasingW0 and for value 4 erg/cm2, the CR is reduced due to theappearance of the switching up and down domains and thesample is no longer monostable.The experimental results presented in this letter supportthe electrostatic,9–11 and related models17,18 according towhich both PS and da thickness of the alignment layer playimportant roles in giving V-shaped switching structure. Weobserve that this switching is distorted by lowering either Psor da. The CR is reduced by lowering W0 and for W0=W0*4 erg/cm2 due to the switching up and down domainsappearing in the texture where the cell is no longermonostable. Therefore this value of W0* is the critical valuebetween the bistable and the monostable V shaped struc-tures and it presumably exceeds the surface anchoring energyas predicted.19 It would also be important to study the effectof the helical pitch and of the cell thickness. These are theother two important parameters in addition to PS and da forrealizing a high-quality V-shaped switching.The authors thank J. K. Song for useful discussions andNissan Chemicals for gifting us RN1175.1N. A. Clark and S. T. Lagerwall, Appl. Phys. Lett. 36, 899 1980.2http://www.displaytech.com/company.html3S. Inui, N. Iimura, T. Suzuki, H. Iwane, K. Miyachi, Y. Takanishi, and A.Fukuda, J. Mater. Chem. 6, 671 1996.4T. Saishu, K. Takatoh, R. Iida, H. Nagata, and Y. Mori, SID Int. Symp.Digest Tech. Papers 1996, 703.5T. Yoshida, T. Tanaka, J. Ogura, H. Wakai, and H. Aoki, SID Int. Symp.Digest Tech. Papers 1997, 841.6R. Hasegawa, H. Fujiwara, H. Nagata, Y. Hara, T. Saishu, R. Fukushima,M. Akiyama, H. Okumura, and K. Takatoh, J. Soc. Inf. Disp. 9, 1072001.7M. Takeuchi, K. Chao, T. Ando, T. Matsumoto, A. Fukuda, and M.Yamashita, Ferroelectrics 246, 1 2000.8S. S. Seomun, J. K. Vij, N. Hayashi, T. Kato, and A. Fukuda, Appl. Phys.Lett. 79, 940 2001.9P. Rudquist, J. P. F. Lagerwall, M. Buivydas, F. Gouda, S. T. Lagerwall, N.A. Clark, J. E. Maclennan, R. Shao, D. A. Coleman, S. Bardon, T. Bellini,D. R. Link, G. Natale, M. A. Glaser, D. M. Walba, M. D. Wand, and X.-H.Chen, J. Mater. Chem. 9, 1257 1999.10N. A. Clark, D. Coleman, and J. E. Maclennan, Liq. Cryst. 27, 9852000.11M. Copic, J. E. Maclennan, and N. A. Clark, Phys. Rev. E 65, 0217082002.12L. M. Blinov, S. P. Palto, F. V. Podgornov, H. Moritake, and W. Haase,Liq. Cryst. 31, 61 2004.13S. Tzeng, T. Y. Lin, R. H. Haung, J. Wu, and S. Wu, Phys. Rev. E 70,011712 2004.14B. I. Ostrovsky and V. G. Chigrinov, Kristallografiya 25, 560 1980.15Yu. P. Panarin, V. Panov, O. E. Kalinovskaya, and J. K. Vij, Ferroelectrics246, 35 2000.16N. Vaupotič and M. Čopič, Phys. Rev. E 68, 061705 2003.17K. H. Yang, A. Lien, and T. C. Chieu, Jpn. J. Appl. Phys. 27, 2022 1988.18Yu. P. Panarin, Ph.D. thesis, Organic Intermediates Dyes Institute, Mos-cow, 1993; Yu. P. Panarin, Mols. Mats. 2, 137 1993.19M. J. O’Callaghan, Phys. Rev. E 67, 011710 2003.20M. Čopič, J. E. Maclennan, and N. A. Clark, Phys. Rev. E 63, 0317032001.FIG. 3. Variation of coercive field in both cases a and b as a function offrequency.FIG. 4. Color online Contrast ratio as function of electrostatic energy W0for =6 and a=3.052911-3 Manjuladevi, Panarin, and Vij Appl. Phys. Lett. 91, 052911 2007Downloaded 03 Aug 2007 to 134.226.86.54. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

Experimental Study for the Conditions of Analog Switching in Ferroelectric Liquid Crystal Cells

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Experimental Study for the Conditions of Analog Switching in Ferroelectric Liquid Crystal Cells

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