Name: photoalignment of brilliant yellow films for fully tunable liquid crystal pretilt
Uploaded: 2022-03-16T18:36:02
Description: While the ability to pattern the azimuthal (i.e., in-plane) orientation of the LC director&#160;utilizing photoalignment films is well established, the ...

Colin McGinty acknowledges the postdoctoral National Research Council Associateship at the Naval Research Laboratory and funding from the Naval Research Laboratory Base Program.

1. Preparation of Brilliant Yellow solution
<ol>
	<li>Dissolve BY&#160;in dimethylformamide (DMF) at a concentration of 4% by weight. Vortex the mixture until BY powder is no longer visible. Pull the solution into a gas tight syringe and filter through a 0.2 &#181;m PTFE filter.</li>
</ol>
2. Cleaning of glass substrates
<ol>
	<li>Obtain appropriately sized substrates. All results in this paper were obtained using indium tin oxide (ITO)-coated glass substrates...

<ol>
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J., Hosting, L., Serati, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Wide-angle+nonmechanical+beam+steering+using+thin+liquid+crystal+polarization+gratings.">Wide-angle nonmechanical beam steering using thin liquid crystal polarization gratings.</a> Proceedings of SPIE. 7093, 709302 (2008).</li><li>Marrucci, L., Manzo, C., Paparo, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Optical+spin-to-orbital+angular+momentum+conversion+in+inhomogeneous+anisotropic+media.">Optical spin-to-orbital angular momentum conversion in inhomogeneous anisotropic media.</a> Physical Review Letters. 96 (16), 163905 (2006).</li><li>Peng, C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Controlling+placement+of+nonspherical+(boomerang)+colloids+in+nematic+cells+with+photopatterned+director.">Controlling placement of nonspherical (boomerang) colloids in nematic cells with photopatterned director.</a> Journal of Physics: Condensed Matter. 29 (1), 014005 (2017).</li><li>White, T. 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H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Voxelated+liquid+crystal+elastomers.">Voxelated liquid crystal elastomers.</a> Science. 347 (6225), 982-984 (2015).</li><li>Tseng, M. C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Tunable+lens+by+spatially+varying+liquid+crystal+pretilt+angles.">Tunable lens by spatially varying liquid crystal pretilt angles.</a> Journal of Applied Physics. 109 (8), 083109 (2011).</li><li>Uchida, T., Ohgawara, M., Wada, W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Liquid+crystal+orientation+on+the+surface+of+obliquely-evaporated+silicon+monoxide+with+homeotropic+surface+treatment.">Liquid crystal orientation on the surface of obliquely-evaporated silicon monoxide with homeotropic surface treatment.</a> Japanese Journal of Applied Physics. 19 (11), 2127 (1980).</li><li>Bezruchenko, V. S., Muravsky, A. A., Murauski, A. A., Stankevich, A. I., Mahilny, U. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Tunable+liquid+crystal+lens+based+on+pretilt+angle+gradient+alignment.">Tunable liquid crystal lens based on pretilt angle gradient alignment.</a> Molecular Crystals and Liquid Crystals. 626 (1), 222-228 (2016).</li><li>McGinty, C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Stable+azodye+photo-alignment+layer+for+liquid+crystal+devices+achieved+by+&amp;#34;turning+off&amp;#34;+dye+photosensitivity.">Stable azodye photo-alignment layer for liquid crystal devices achieved by &amp;#34;turning off&amp;#34; dye photosensitivity.</a> Journal of Applied Physics. 122 (20), 205301 (2017).</li><li>Chen, T., Chu, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Pretilt+angle+control+for+single-cell-gap+transflective+liquid+crystal+cells.">Pretilt angle control for single-cell-gap transflective liquid crystal cells.</a> Applied Physics Letters. 92 (9), 091102 (2008).</li><li>Fan, F., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Low+voltage+tunable+liquid+crystal+lens.">Low voltage tunable liquid crystal lens.</a> Optics Letters. 38 (20), 4116-4119 (2013).</li><li>Chigrinov, V. G., Kozenkov, V. M., Kwok, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Photoalignment of Liquid Crystalline Materials: Physics and Applications. , (2008).</li><li>West, J. L., Su, L., Reznikov, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Photo-alignment+using+adsorbed+dichroic+molecules.">Photo-alignment using adsorbed dichroic molecules.</a> Molecular Crystals and Liquid Crystals. 364 (1), 199-210 (2006).</li><li>Yaroshchuk, O., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Photoalignment+properties+of+brilliant+yellow+dye.">Photoalignment properties of brilliant yellow dye.</a> Proceedings of the 14th International Display Workshops. 7, 1665-1668 (2007).</li><li>Akiyama, H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Synthesis+and+properties+of+azo+dye+aligning+layers+for+liquid+crystal+cells.">Synthesis and properties of azo dye aligning layers for liquid crystal cells.</a> Liquid Crystals. 29 (10), 1321-1327 (2002).</li><li>McGinty, C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=tunable+liquid+crystal+pretilt+achieved+by+enhanced+out-of-plane+reorientation+of+azodye+thin+films.">tunable liquid crystal pretilt achieved by enhanced out-of-plane reorientation of azodye thin films.</a> Langmuir. 36 (29), 8554-8559 (2020).</li><li>McGinty, C., Kolackz, J., Spillmann, C. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Large+rewritable+liquid+crystal+pretilt+angle+by+in+situ+photoalignment+of+brilliant+yellow+films.">Large rewritable liquid crystal pretilt angle by in situ photoalignment of brilliant yellow films.</a> Applied Physics Letters. 119 (14), 141111 (2021).</li><li>Wang, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effects+of+humidity+and+surface+on+photoalignment+of+brilliant+yellow.">Effects of humidity and surface on photoalignment of brilliant yellow.</a> Liquid Crystals. 44 (5), 863-872 (2017).</li><li>Tseng, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Strengthening+of+liquid+crystal+photoalignment+on+azo+dye+films:+passivation+by+reactive+mesogens.">Strengthening of liquid crystal photoalignment on azo dye films: passivation by reactive mesogens.</a> RSC Advances. 6 (53), 48181-48188 (2016).</li><li>Berry, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+scalable+fabrication+process+for+liquid+crystal-based+uncooled+thermal+imagers.">A scalable fabrication process for liquid crystal-based uncooled thermal imagers.</a> Journal of Microelectromechanical Systems. 25 (3), 479-488 (2016).</li><li>McGinty, C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Highly+versatile+and+stable+photoalignment+process+for+AMLCDs.">Highly versatile and stable photoalignment process for AMLCDs.</a> SID Symposium Digest of Technical Papers. 49 (1), 375-377 (2018).</li></ol>

The primary point of discussion for this method is the overall sensitivity of the substrate-BY-LC system to chosen materials as well as environmental conditions. Although both methods, exposure prior to filling with LC and exposure after filling with LC, are sensitive to environmental conditions, these can generally be accounted for and controlled. As noted in the protocol section, the humidity during the spin-coating step is a critical environmental factor that must be controlled to obtain high quality pretilt angles an...

Liquid crystal (LC) alignment layers are a fundamental component of a wide range of LC research. Conventional methods such as mechanical rubbing of polyimides have several drawbacks including static charge buildup, contamination by debris, and the high temperature imidization step. Additionally, the rubbing process impedes the ability to control and pattern the LC director at the LC-substrate interface. This is a requirement for several LC-related research areas including non-mechanical beam steering1, q-plates2, controlled placement of colloidal particles3, LC elastomer origami4, and control of active matter5.
Photoalignment is an alternative to mechanical rubbing which uses polarized light to manipulate an alignment layer and control the LC director at the LC-substrate interface. Azo-dye thin films are a promising photoalignment material because they provide strong anchoring energy, are easily patternable, and involve room-temperature processing. Polarization holography6, plasmonic metamasks7, and direct writing8 are methods which have been reported for patterning the azimuthal or in-plane axis of the LC director. Less attention has been given to the ability to control and pattern the pretilt or out-of-plane axis of the LC director. Some photoalignment methods have been reported for obtaining a tunable pretilt, but they typically involve a combination of homeotropic and planar alignment materials9,10, a combination of rubbing and photoalignment methods11, or a combination of a photoalignment film and a photocurable polymer layer12,13. A method for obtaining full control of the LC pretilt utilizing precise control over the irradiance doses of a cross-linking polymer has also been reported14.
The use of azo-dyes such as Brilliant Yellow (BY) and, especially, SD-1 as photoalignment films has been reported extensively15,16,17. Akiyama et al.18 reported a method for obtaining a tunable LC pretilt between 0&#176;-5.5&#176; utilizing a two-step exposure method for SD-1 photoalignment films. The first exposure is linearly polarized and encourages in-plane reorientation of SD-1 perpendicular to the polarization axis of the exposure. The second exposure is unpolarized and performed at oblique incidence to encourage out-of-plane reorientation of SD-1. Beyond the interesting research applications mentioned above, there is a fundamental need for a non-zero pretilt in any LC electro-optic device to prevent the appearance of reverse-tilt domains when switching the device; this was the original motivation for applying the method reported by Akiyama et al.18 to BY photoalignment films. When this method was optimized and applied to BY photoalignment films, it was found that a tunable pretilt angle over the range of 0&#176;-33&#176; was possible19; this represents a six-times increase over the previously reported magnitude for SD-1 and the largest tunable pretilt angle reported for a single photoalignment film in general. When the ability to re-write the pretilt angle in situ utilizing this method was investigated, it was found that larger pretilt angles could be generated and that BY photoalignment films were capable of providing homeotropic LC alignment20.
This paper describes how a full range of LC pretilt angles can be obtained from a single photoalignment film by exposing BY&#160;photoalignment films to obliquely-incident unpolarized light. Large pretilt angles (30&#176;-90&#176;) are accessed by performing an in situ oblique exposure in the presence of LC material20. The pretilt is controlled by the incident angle of the oblique exposure and the lower limit is determined by the index of refraction of the substrate, as well as the BY photoalignment film. This method demonstrates that BY photoalignment films can be used to obtain both homeotropic and planar LC alignment. Additionally, the ability to re-write the LC pretilt is inherent since the oblique exposure is performed in situ. Smaller pretilt angles are accessed by exposing BY photoalignment films to obliquely-incident unpolarized light in the absence of LC material; this method is similar to the one originally reported by Akiyama et al.18. Simple patterning of the director is accomplished with both methods through the use of photomasks. The combination of these two exposure methods allows BY photoalignment films to be used to obtain and simply pattern the pretilt of the LC director at the substrate. The simplicity and low cost of the method makes it an ideal candidate for LC research and applications which benefit&#160;from 3D control of the LC director at the substrate.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>415 nm LED</td>
			<td>ThorLabs</td>
			<td>SOLIS-415C</td>
			<td></td>
		</tr>
		<tr>
			<td>450 nm LED</td>
			<td>Luxeon</td>
			<td>SP-03-V4</td>
			<td>Luxeon TriStar LED Module with 3 LXML-PR02-A900 Rebel LEDs. 448 nm.</td>
		</tr>
		<tr>
			<td>Brilliant Yellow</td>
			<td>Sigma Aldrich</td>
			<td>201375-25G</td>
			<td>Dye Content &#62;= 50%. Also called Direct Yellow 4.</td>
		</tr>
		<tr>
			<td>Cleaning Solution</td>
			<td>International Products Corporation</td>
			<td>M-9050-12</td>
			<td>Micro 90 Concentrated Cleaning Solution</td>
		</tr>
		<tr>
			<td>Dimethylformamide</td>
			<td>Sigma Aldrich</td>
			<td>227056-1L</td>
			<td>N,N-Dimethylformamide anhydrous, 99.8%</td>
		</tr>
		<tr>
			<td>E7</td>
			<td>Merck Licristal</td>
			<td>28656</td>
			<td></td>
		</tr>
		<tr>
			<td>Isopropanol</td>
			<td>Fisher Scientific</td>
			<td>AC184130010</td>
			<td></td>
		</tr>
		<tr>
			<td>Indium Tin Oxide coated glass</td>
			<td>Colorado Concept Coatings</td>
			<td></td>
			<td>0.43&#34; x 14&#34; x 14&#34; sheets, 80-90 ohms</td>
		</tr>
		<tr>
			<td>Nebulizer</td>
			<td>3M</td>
			<td>FT-13</td>
			<td></td>
		</tr>
		<tr>
			<td>Optical Adhesive</td>
			<td>Norland</td>
			<td>NOA 65</td>
			<td></td>
		</tr>
		<tr>
			<td>PTFE Filter</td>
			<td>Pall Life Sciences</td>
			<td>2400</td>
			<td>Acrodisc Syringe Filter 0.2 micron</td>
		</tr>
		<tr>
			<td>Scriber</td>
			<td>Delphi Glass</td>
			<td>5426</td>
			<td>Beetle Bits Cutting System</td>
		</tr>
		<tr>
			<td>Silica Spacers</td>
			<td>Sekisui Chemical Company Ltd</td>
			<td>SP-205</td>
			<td>Sekisui Fine Chemical Division</td>
		</tr>
		<tr>
			<td>Spin coater</td>
			<td>Specialty Coating Systems</td>
			<td>SCS 6800</td>
			<td></td>
		</tr>
		<tr>
			<td>Ultrasonic Cleaner</td>
			<td>Branson</td>
			<td>Model 2800</td>
			<td>Available from several distributors.</td>
		</tr>
		<tr>
			<td>UV LED</td>
			<td>Electro-Lite</td>
			<td>72005</td>
			<td></td>
		</tr>
		<tr>
			<td>UV/Ozone Cleaner</td>
			<td>Ossila</td>
			<td>L2002A2-UK</td>
			<td></td>
		</tr>
		<tr>
			<td>Vacuum Mat</td>
			<td>Barant Co.</td>
			<td>M14 309</td>
			<td>For Assembly of LC Cells</td>
		</tr>
		<tr>
			<td>Vacuum Pump</td>
			<td>Barant Co.</td>
			<td>400-2901</td>
			<td></td>
		</tr>
	</tbody>
</table>

photoalignment of brilliant yellow films for fully tunable liquid crystal pretilt

While the ability to pattern the azimuthal (i.e., in-plane) orientation of the LC director&#160;utilizing photoalignment films is well established, the ability to pattern the polar orientation, or pretilt angle, remains limited. Most reported methods for obtaining large, tunable pretilt angles with photoalignment materials require modification of the chemical structure, combinations of materials, or expensive equipment and set-ups with low scalability. To date, methods which utilize a single photoalignment material can only access a limited range of pretilt angles up to approximately 10&#176;. Access to the full range of pretilt angles from 0&#176;-90&#176; is demonstrated here by utilizing oblique exposures of Brilliant Yellow (BY) photoalignment films. Smaller pretilt angles (between 0&#176;-30&#176;) are obtained by utilizing an unpolarized oblique exposure prior to LC fill. Larger pretilt angles (between 30&#176;-90&#176;) are obtained utilizing an in situ unpolarized oblique exposure after LC fill. The ability to rewrite the LC pretilt is inherent in the latter method. Simple patterns are generated utilizing photomasks during the oblique exposure. The work establishes BY as an ideal photoalignment material for research applications which would benefit from full control of the LC director orientation at the LC-substrate interface. These applications include non-mechanical beam-steering, q-plates, controlled placement of colloidal particles, LC elastomer origami, and general patterning and control of active matter.

Exposure of BY film prior to filling for smaller range of pretilt angles. For exposure prior to filling, the magnitude of the pretilt angle is most easily controlled by modulating the duration of the oblique exposure. Longer oblique exposures result in a larger degree of out-of-plane reorientation in the BY film. A bench top spectrometer with a linear polarizer in the beam path is used to collect absorbance spectra from the BY-coated substrates after exposure; by rotating the polarizer, spectra can be co...

The ability to pattern the liquid crystal (LC) pretilt angle at the LC-substrate interface with a single photoalignment material remains limited. The protocol here presents a method for accessing a large range of pretilt angles utilizing oblique exposures of brilliant yellow photoalignment films.

Photoalignment of Brilliant Yellow Films for Fully Tunable Liquid Crystal Pretilt

While the ability to pattern the azimuthal (i.e., in-plane) orientation of the LC director&#160;utilizing photoalignment films is well established, the ...