compartmentalized multistable liquid crystal alignment

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Compartmentalized Multistable Liquid CrystalAlignment

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By Jing Zhang,* Marius I. Boamfa, Alan E. Rowan, and Theo Rasing*
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[*] Prof. T. Rasing, J. Zhang, Prof. A. E. RowanInstitute for Molecules and MaterialsRadboud University NijmegenHeyendaalseweg 135, 6525 AJ Nijmegen (Netherlands)E-mail: [email protected]; [email protected]

Dr. M. I. BoamfaPhilips Research LaboratoriesProf Holstlaan 4 (WADp 114)5656 AA Eindhoven (The Netherlands)

DOI: 10.1002/adma.200903045

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Nematic liquid crystals (LCs) are anisotropic elastic fluids andtheir molecular orientation can be controlled by surfaces andexternal electric/magnetic fields. These properties make themhighly attractive for fabrication of tunable devices such asdisplays,[1] spatial light modulators,[2] dynamic lenses[3] andartificial muscles,[4] and for the manipulation of guest materialsas a tunable anisotropic host.[5–8] Here, we report a novel methodfor LC alignment based on micrometer-scale photolithographicpatterning of a self-assembled monolayer (SAM) coated onto anindium tin oxide (ITO) surface. We use closed vertically alignedLC-boundary walls, induced by the SAM surfaces, to controlplanar alignment of LCs on the isotropic ITO surfaces. Thissimple approach not only presents a new concept of planar LCalignment, which, so far, has been achieved only by nanome-ter-scale anisotropic surfaces, but also provides a compartmenta-lization of those planar-aligned LC domains. This compartmen-talization allows the planar domains to be switched independentlyby vertical electrical fields without the dynamic LC crosstalk(molecular orientation interference between neighboring switch-ing LC domains), a serious problem that is known to degradecurrent LC devices.[9–11] Due to the symmetry of the SAM pat-terns, our compartmentalized LC cells show multistable statesthat are tunable by an in-plane bias. The described method, ex-ploitingmature lithography techniques, is highly reliable and costeffective, and it opens new routes for the design and fabrication ofmultistable LC devices, switchable viewing angle displays, andtunable structures of guest materials within a LC host.

It is well known that vertical alignment of LCs can be inducedby long-alkyl-chain-terminated surfaces,[1] but achieving reliableand cost-effective planar alignment remains a challenge.[12]

Existing alignment methods, such as mechanical rubbing,photoalignment, and ion-beam bombardment, all rely onnanometer-scale topological or molecular/atomic anisotropy ofthe alignment surfaces, properties that are difficult to controluniformly on a macroscopic scale.[12–16] Alignment with micro-patterned isotropic surfaces, consisting of stripes of alternatingrandom planar- and vertical-aligning materials, has beensuggested as a more reliable and cost-effective technique.[13,17]

The LC alignment with those stripe patterns suffers, however,from random defects[13,17] due to the finite elastic correlationlength of LCs and the tilt degeneracy within the planar stripes. Tosolve these problems, it has been suggested to use obliqueillumination in combination with photoactive SAMs,[13] yet thisapproach relies again on molecular-level anisotropy of thesurfaces.

We use patterned surfaces with random planar-aligningsquares embedded in a vertical-aligning surface background.The closed vertically aligned LC boundaries induce well-definedplanar LC alignment on the random planar squares. Amacroscopic in-plane bias, such as a directional LC flow, removesthe tilt degeneracy. This simple approach enables not onlycontrolled planar alignment of LCs with lithography techniquesbut also compartmentalization of those planar-aligned LCdomains to avoid dynamic LC crosstalk during vertical switching.We demonstrate the potential of this method for deviceapplications by visual and quantitative electro-opticalmeasurements.

Figure 1 shows a scanning electron microscopy (SEM) imageof a patterned SAM-on-ITO surface (prepared from octadecyl-trimethoxysilane (OTMS) SAM that is patterned by deep-UVphotolysis, see the Experimental section for details). The brighterareas correspond to unexposed SAM (vertical-aligning surface)and the darker areas correspond to where the SAM is removed(random planar-aligning surface). The image contrast is due tothe conductivity difference between SAM and ITO. The patternsize corresponds well to the photolithography mask, which has aperiodicity of P¼ 42mm and an aperture width of W¼ 37mm.Removed SAM could be regrown by submerging the patternedsubstrates in a freshly prepared OTMS solution, suggesting that

Figure 1. SEM image of a patterned SAM on ITO.

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Figure 2. Optical Microscopy images of LC aligned with a uniformSAM-on-ITO surface and different patterned SAM-on-ITO countersurfaces, viewed in white-light transmission between crossed polarizerand analyzer. a,a0) LC aligned with stripe pattern of periodicity P¼ 16.5mmand planar domain width W¼ 11.5mm. a) Viewed with boundary linesat 08 to the polarizer. a0) Viewed with boundary lines at 458 to thepolarizer. b,b0) Randomly distributed multistable alignment with s squarepattern of P¼ 16.5mm andW¼ 11.5mm. The four stable alignment statesare highlighted with red circles. b) Viewed with boundary lines at 08/908to the polarizer. b0) Viewed with boundary lines at 458 to the polarizer.c,c0) Flow-induced monodomain alignment with square pattern ofP¼ 16.5mm and W¼ 11.5mm, the flow direction is indicated with ared arrow. c) Viewed with boundary lines at 08/908 to the polarizer.c0) Viewed with boundary lines at 458 to the polarizer. d,e) Flow-inducedmonodomain alignment with larger square patterns viewed withboundary lines at 08/908 to the polarizer. d) P¼ 25mm and W¼ 19mm.e) P¼ 42mm and W¼ 37mm. f) Schematic representation (top viewand one diagonal cross-section) of one of the four stable states ofLC alignment.

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the complete alkyl chains of the exposed SAM are removed byphotolysis. This allows further chemical modification of thesurfaces by growing a different silane SAM in the previouslyphotoetched regions.[18]

LC cells with a cell gap of 4.0mm are fabricated using onepatterned and one nonpatterned SAM-on-ITO surface. Figure 2shows the LC (E7) alignment in these cells. On a stripe pattern theLC bulk is aligned parallel to the boundary lines with randomlydistributed point defects within the planar stripes (Fig. 2a and2a0). This is consistent with the results observed on patternedSAM-on-SiO2 substrates.

[13,17] On the square patterns, however,the LC alignment is different. The LCs on the planar squaresappear brightest when the boundary lines are at 08/908 withrespect to the polarizer (Fig. 2b) and they appear darkest when theboundary lines are at 458 with respect to the polarizer (Fig. 2b0).This suggests that the majority of the LC bulk on the planarsquares is aligned along the diagonal axes of the squares. Thedark corners in Figure 2b, which appear bright in Figure 2b0,indicate that the LC directors at those corners are aligned alongthe two orthogonal boundary lines. From these images, fourstable alignment states can be identified (Fig. 2b); the LC-directordistribution of one of the alignment states is schematicallypresented in Figure 2f. The remaining three states can begenerated by in-plane rotation of 908, 1808, and 2708 from thisalignment state. For a detailed LC-director distribution of thisalignment 3D computer simulations are required, however, aqualitative explanation can be given on the basis of minimizationof the total free energy, i.e., the sum of the surface-LC interfaceenergy and the LC-bulk elastic energy. Due to the isotropic natureof the material on the planar squares, LC directors will haverandom planar orientations on such surfaces. To minimize theinterface energies and the elastic-distortion energy introduced bythe vertical aligned LC boundaries, the LCs adopt a tilted planaralignment along one of the diagonal axes of the planar squares.The surface tilt angle u0 (Fig. 2f) is estimated to be�178 from theretardation of the cell (see the Supporting Information fordetails). Because of the symmetry of the boundary conditions, thefour alignment configurations sketched in Figure 2f are expectedto have the same free energy. Therefore, one of the stable statescan be favored by a slight in-plane bias, such as a directional flow(Fig. 2c and c0). This flow bias is induced by a directional capillaryflow of the LC molecules into the cell at about 80 8C (TIN¼ 60 8C,where TIN is the mematic-to-isotropic phase-transition tempera-ture) and immediately cooling down to room temperature. Themultidomain or monodomain LC cells are stable for at least3 months at room temperature and the alignment changes onlyafter being heated above the TIN¼ 60 8C and cooled down to roomtemperature, suggesting that the four alignment states are indeedstable states and separated by high-enough energy barriers. Thethermal stability of the alignment could be further improved byusing LCmaterials with higher TIN. Similar results were obtainedwith different planar domain sizes: 11.5mm, 19mm, and 37mm,as shown in Figure 2c–e. We expect that larger or smaller domainsizes are possible, depending on the elasticity of the LC materialsand their interface energies with the surface materials.

The switching of multidomain and monodomain aligned LCcells under vertical electric fields is shown in Figure 3. The colorchange under different voltages is due to the wavelengthdependent retardation of the LC cells. The absence of

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Figure 3. Switching ofmultidomain (left) andmonodomain (right) alignedLC cells (P¼ 25mm and W¼ 19mm) under vertical electric field. They areviewed in white-light transmission between crossed polarizer and analyzerand with boundary lines at 08/908 to the polarizer.

Figure 4. Electro-optical switching of three square-pattern aligned LC cellsin comparison with the conventional TN-cell and hybrid cells. Cell gaps thatare not mentioned in the figures are all 4.0mm. a) Transmission–voltagecurves. b) Response curves. c) Relaxation curves.

transmission change in the dark boundary areas confirms that thevertically aligned LCs do not switch under vertical electric fields.Thus, these LCs serve as boundary walls between the bright LCdomains and allow them to switch without interfering with eachother. This provides a simple solution to the serious problem offlow-induced dynamic LC crosstalk in existing pixelated LCdevices, which has recently been shown to degrade the dynamiccontrast and switching speed during pixelated switching ofmultidomain and monodomain LC devices.[9–11,19] Our multi-domain or monodomain LC cells do not change their initialalignment after they have been switched on and off for >106

times with up to 10V, supporting again the multistable nature ofthis alignment.

Quantitative electro-optical measurements on our compart-mentalized LC cells and a comparison with two rubbed-polyimidealigned cells are shown in Figure 4. All cells are filled with thesame LC mixture E7. The twisted nematic cell (TN-cell) consistsof two rubbed-polyimide surfaces with rubbing directions rotated908 from one to the other. The reference hybrid cell has the samevertical-aligning surface as our cells but with rubbed polyimideas the planar-aligning surface. Our cells show similar character-istics as the reference hybrid cell: no threshold voltage and a veryfast response (<1ms). This suggests that our cells switch likecompartmentalized hybrid cells, consistent with the schematicdrawing in Figure 2f. The 4-mm-thick hybrid cell (dashed red linein Fig. 4a), however, shows higher retardation than our cells,indicating that the tilt angle at the planar surface of our cells islarger than that of the reference hybrid cells. This could be due toa lower polar anchoring energy of our planar surface or to theextra LC-boundary walls in our cells. As monotonic transmis-sion–voltage dependence is required for displays, a hybrid cellwith a smaller (3.5mm) cell gap is used for the switching-speedmeasurements. Compared to the TN-cell and hybrid cell, our cells


require considerably lower operating voltages in the intermediatetransmission range (Fig. 4a), greatly reducing the powerconsumption. Our cells also show much faster response andrelaxation speeds than the TN-cells, but a slower relaxation than


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the reference hybrid cell (Fig. 4b and 4c). This difference inrelaxation time, toff, arises from the smaller cell gap d(3.5mmcompared to the 4.0mm of our cell) of the reference hybrid cell(toff/ d2)[20] and the absence of domain walls in the nonpixelatedswitching of the hybrid cell. The small peak in the relaxation curveof the TN-cell (Fig. 4c) is caused by the well-known back-floweffect.[21] Our compartmentalized LC cells with different domainsizes show very similar switching voltages and response times(Fig. 4a and 4b). This indicates that the electric field dominatesthe response process. There is, however, a clear increase in therelaxation time with decreasing domain size (Fig. 4c), which isattributed to the increased elastic distortion energy caused by thevertically aligned LC walls. This is consistent with the fact thatdomain walls that are temporarily created during pixelatedswitching of conventional LC devices can slow down theswitching speed.[9–11,19] The contrast ratio (CR) of our compart-mentalized cells is dependent on operating voltage, initiallystored elastic energy, surface anchoring energy, and viewingangle. CR increases with operating voltage. At a practical voltageof 5 V and a vertical viewing angle, the CRs of our compart-mentalized cells with domain sizes of 37, 19, and 11.5mm are500:1, 700:1, and 1400:1, respectively. This indicates that cellswith smaller domains have a higher stored elastic energy at thebright state and require less electric energy to switch to the darkstate. Therefore, CR could be increased by using LC materialswith higher elastic constants or by using planar-aligning surfacematerials with lower polar-anchoring energy. Because theretardation of a monodomain cell is strongly angle dependent,the CR will decrease at wider viewing angles. This could beimproved by using multidomain cells, with each pixel consistingof four axially symmetric stable alignment states.

Our alignment surfaces (patterned SAM-on-ITO) are stableagainst chemicals (detergent, acetone, and water), heat (up to120 8C), and UV light (>300 nm), fulfilling the requirements forbasic device applications. For applications where a highvoltage-holding ratio is required, similar photolithography orprinting techniques can be used for other materials, such as acombination of specially developed planar- and vertical-aligningpolyimides. Since the photolithography facility is already used inthe current thin-film transistor (TFT) production,[20] ouralignment technique can be easily integrated into LC-devicemass-production lines, yielding a reduced total production cost.For independent pixelated switching and maximum aperture ofthe backlight in actual LC devices, the dark, vertically aligned LCarea can be aligned or directly produced on top of the TFTs tooverlap with the nontransparent matrices of the pixelated TFTelectrodes.[20]

In conclusion, we have demonstrated a potentially cost-effective and reliable method for compartmentalized LCalignment. LC cells fabricated with this method show superiorelectro-optical performance compared to conventional TN-cells:lower operating voltages, faster switching speeds and noflow-induced dynamic cross-talk between neighboring switchingLC domains. The pattern-symmetry-induced multistability that isintrinsic to our method opens new routes for the design andfabrication of numerous LC devices, such as multistable LCdevices (by using in-plane electrodes) and switchable view-ing-angle displays (by using vertical electrodes for display andin-plane electrodes for viewing-angle switching).

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Experimental

Alignment Surface Preparation: The ITO substrates were initially cleaned andtreated with UV–ozone to oxidize the surface layer. The oxidized surfaceprovides a high density of reactive hydroxyl groups for the self-assembly ofsilane monolayers [22,23]. The substrates were then submerged in a freshlyprepared OTMS solution in air–moisture saturated toluene. To growuniform SAMs, the optimal condition was found at low silaneconcentrations at corresponding temperatures, similar as another silanesystem that we have recently studied [24]. Under the condition of 0.5–1mMOTMS concentration at room temperature (20� 1 8C), no anhydroussolvent or environment is needed and uniform SAMs can be reproduciblyproduced under a wide range of humidity (30–80%) within 2 h. Theresulting OTMSmonolayer was patterned viamask photolithography usingan argon fluoride excimer laser (193 nm) to remove the exposed parts ofthe SAM. Similar results were obtained with a collimated 200WHg(Xe) arclamp. After UV exposure, the patterned surfaces were rinsed to remove thephotocleaved products (see the Supporting Information for details).

Cell Fabrication: The LC cells were fabricated with a UV-curable glue(NOA65, Norland), using embedded particles (Licristar, Merck) to controlthe cell gap. The cells were filled with LC materials E7 (Merck) by capillaryforce at�80 8C (TIN¼ 60 8C). The filled cells were kept at�80 8C for 10minbefore they was cooled down to room temperature to avoid flow alignment.The cooled cells were finally sealed with epoxy glue (Epoxy Rapid, Bison).

Electro-optical Characterization: The electro-optical switching curveswere measured at normal incident angle with red light (l¼ 633 nm) intransmission. The cells were mounted between crossed polarizer andanalyzer. The compartmentalized LC cells and the TN-cell are measuredwith the boundary lines and with the rubbing directions respectively at 08/908 to the polarizer (their extinction ratio is <5� 10�5). The hybrid cellswere measured with the rubbing direction at 458 to the polarizer. Acontinuous and 200ms 2 kHz square wave were used for the static anddynamic switching measurements respectively.

Acknowledgements

The authors thank Dr. D. K. G. de Boer, and Prof. D. J. Broer for usefuldiscussions and C. N. van Dijk, Dr. N. Dam, A. F. van Etteger, and A.Toonen for technical help. Part of this work was supported by the DutchScience Foundation (NWO) and the EU-ITN network HIERARCHY.Supporting Information is available online from Wiley InterScience orfrom the author.

Received: September 4, 2009

Revised: September 19, 2009

Published online: December 15, 2009

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compartmentalized multistable liquid crystal alignment

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