Photoalignment of Brilliant Yellow Films for Fully Tunable Liquid Crystal Pretilt

Summary

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.

Abstract

While the ability to pattern the azimuthal (i.e., in-plane) orientation of the LC director 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°. Access to the full range of pretilt angles from 0°-90° is demonstrated here by utilizing oblique exposures of Brilliant Yellow (BY) photoalignment films. Smaller pretilt angles (between 0°-30°) are obtained by utilizing an unpolarized oblique exposure prior to LC fill. Larger pretilt angles (between 30°-90°) 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.

Introduction

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 materials^9,10, a combination of rubbing and photoalignment methods¹¹, or a combination of a photoalignment film and a photocurable polymer layer^12,13. 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 extensively^15,16,17. Akiyama et al.¹⁸ reported a method for obtaining a tunable LC pretilt between 0°-5.5° 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°-33° 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 photoalignment films to obliquely-incident unpolarized light. Large pretilt angles (30°-90°) 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 from 3D control of the LC director at the substrate.

Protocol

1. Preparation of Brilliant Yellow solution

1. Dissolve BY 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 µm PTFE filter.

2. Cleaning of glass substrates

1. Obtain appropriately sized substrates. All results in this paper were obtained using indium tin oxide (ITO)-coated glass substrates scribed from a larger sheet into 2 cm x 2.5 cm pieces.
2. Fill the ultrasonic bath with 1% concentrated cleaning solution and deionized water. Adjust the cleaning time to 15 min using appropriate buttons on the panel. Place substrates in the ultrasonic bath and press the start button. Perform ultrasonic cleaning at room temperature.
3. After 15 min, remove substrates from the ultrasonic bath and rinse thoroughly with deionized water until beads no longer form on the substrate surface. Then rinse thoroughly with 100% isopropanol. Place substrates in an oven at 100 °C until dry.
4. After removing the substrates from oven, place them in a UV/Ozone cleaning system with the substrates' sides facing upward, intended to be coated in the system. Here, the ITO-coated surface was always facing upward in the UV/Ozone cleaning system. Set the system to run for 15 min.

3. Depositing Brilliant Yellow photoalignment film

1. Place the cleaned substrates onto an appropriately sized vacuum chuck in a spin-coating system so that the face of the substrate that was facing upward in the UV/Ozone cleaning system is still facing up.
2. Dispense 75 mL of the BY solution onto the substrates. This volume of solution should completely wet the 2 cm x 2.5 cm glass substrates without any manual spreading.
   NOTE: If the solution does not wet the substrate completely, the spatial uniformity of the resulting film will be compromised. Longer UV/Ozone cleaning may be required to solve this issue.
3. Immediately after dispensing the BY solution, begin the spin-coating process; ensure the vacuum pump is switched on and then hit the start button. The spin-coating should ramp up to 3000 rpm for 5 s, and then hold at 3000 rpm for 30 s. During this step the humidity must be controlled to below ~45% to avoid reported complications²¹.
   ​NOTE: The substrate was not heated after spin-coating for solvent evaporation. Whilst developing this process, it was found this step had no impact on film quality and obtainable results.

4. Assembly of liquid crystal cells

1. Place spacers of desired size into a nebulizer. In this paper, all experiments were performed with 5 µm diameter silica sphere spacers.
2. Place the substrates in a box with the BY-coated face facing upward. Cut a hole in the box and insert the nebulizer. Use a bulb to disperse spacers within the box. Dispense spacers onto half of the coated substrates.
3. Apply an optical adhesive to substrates as follows. Dispense the optical adhesive into a small dish and then dip a razor blade into the optical adhesive so that a small line of adhesive runs along the entire edge of the razor.
4. Carefully touch this edge to the substrate to create a uniform glue line. Since these cells will eventually be filled by capillary action, apply adhesive only along two edges of the substrate. In all cases, apply glue along the longer (2.5 cm) edges of the substrates.
   NOTE: This method requires practice; use of a commercial dispenser is ideal if one is available. Due to the manual application, the thickness of the glue lines vary slightly across samples.
5. Sandwich substrates together such that the BY-coated sides are facing each other. The substrates should be offset by about 1 mm so that there is a ledge from where the LC can be filled into the cell.
6. Pull the substrates onto the spacers. To do this, use a metal plate with holes drilled into it and which is hooked up to a vacuum pump. Place the assembled cell onto the metal plate and switch on the vacuum. Place a piece of cellophane over the sandwich cell and plate so that a vacuum is created and the substrates are pulled onto the spacers.
7. Observe the interference colors to determine whether the substrates are on the spacers and if the cell-gap is uniform. If the interference colors do not appear uniform, apply gentle manual pressure to the cell to improve the gap uniformity.
8. Use a UV LED system to cure the optical adhesive. Operate the UV LED (central wavelength 365 nm) at maximum power for a total of 120 s to cure the adhesive (60 s for each glue line).
   ​NOTE: Do not apply any seal to the samples used here. For BY it was found that it is unnecessary to mask the regions of the cell where glue is not present. If a UV-sensitive material such as SD-1 were to be used, masking would likely be critical.

5. Linear exposure of Brilliant Yellow film

1. Use a wire-grid polarizer to linearly polarize the output from a collimated LED light source.
2. Set the intensity of the polarized output to at least 25 mW/cm². To do this, turn the knob which controls the output power of the LED and place the detector at the intended location of the sample in the exposure setup. Ensure that the entire aperture is filled for an intensity measurement, and measure the intensity with an optical power meter.
   NOTE: LEDs with central wavelengths of both 450 nm and 415 nm have been used successfully for this step. For the exposure steps described below, use only the 415 nm LED.
3. Place the substrate such that the plane of the substrate is perpendicular to the direction in which the collimated output is traveling. The polarization axis of the light should be in the plane of the substrate but perpendicular to the long edge of the substrate.
4. Expose the assembled cell to the polarized output for 5 min. Using this protocol, dye film order parameters of ~0.8 should be easily obtainable; see Discussion for more details.

6. Oblique exposure of BY film for lower range of pretilt angles

1. If a smaller pretilt angle is desired (between 0°-30°), perform an oblique incident exposure with unpolarized light. If a larger pretilt angle is desired (between 30°-90°), skip this step and proceed directly to step 7.
   1. Ensure the output from the LED is unpolarized by measuring the intensity of the light source with an optical power meter while rotating the polarizer from the previous exposure step. Rotate the polarizer manually in 10° increments and note the measured intensity at each step. There should be little change in the measured intensity at each polarizer orientation for unpolarized light.
   2. Remove the polarizer from the path of the collimated output and ensure that the intensity of the output is 75 mW/cm² by adjusting the knob which controls the output power. To measure the intensity, place the detector of the meter at the intended location of the sample in the exposure system, and ensure that the collimated output makes a normal incidence with the detector. Measure the intensity with an optical power meter.
2. Place the assembled cell in the path of the collimated output. Rotate the sample about one of the short edges so that the surface normal makes an angle of 15° with the direction in which the collimated output is traveling.
   NOTE: This can also be thought of as rotation about the polarization axis of the linear exposure from the previous step; this ensures that the incidence plane of the light is parallel to the alignment induced by the linear exposure. The surface normal is defined as the vector that is perpendicular to the plane of the substrate and points outward from either face of the assembled sandwich cell.
3. Expose the assembled cell to the unpolarized output for up to 20 min depending on the desired pretilt angle. Larger pretilt angles require larger exposures, and this dependence is described in Figure 1 and Figure 2.

7. Filling of liquid crystal cell

1. Fill the assembled cells with LC material in the isotropic phase as described below. The LC material used for all results presented was nematic LC mixture E7.
   1. Place the assembled cells on a heat stage at room temperature and ramp the temperature up to 20 °C above the nematic-isotropic phase transition temperature of the material. In the case of E7, ramp the temperature to 80 °C.
   2. Once the heat stage has reached the temperature, dispense LC material onto the ledge of the sample and allow the material to fill the cell through capillary action. Allow filled LC samples to cool down slowly below the nematic-isotropic transition temperature.

8. Oblique exposure of BY film for larger range of pretilt angles

1. Perform an oblique incident exposure with unpolarized light to obtain larger pretilt angles between 30°-90° as described below.
   1. Ensure the output from the LED is unpolarized by measuring the intensity of the light source while rotating the polarizer from the previous exposure step. There should be little change in the measured intensity if light is unpolarized. Remove the polarizer from the path of the collimated output and ensure that the intensity of the output is 75 mW/cm² by adjusting the knob which controls the output power and measuring with an optical power meter.
   2. Place the assembled cell in the path of the collimated output. Rotate the sample about one of the short edges so that the surface normal makes an angle with the direction in which the collimated output is traveling. The angle value is determined by the magnitude of the desired pretilt angle. Larger pretilt angles require a smaller angle between the surface normal and the direction of the collimated output (i.e., closer to normal incidence). The dependence of the LC pretilt on this angle is described in Figure 3.
      NOTE: This can also be thought of as rotation about the polarization axis of the linear exposure from the previous step; this ensures that the incidence plane of the light is parallel to the alignment induced by the linear exposure. The surface normal is defined as the vector that is perpendicular to the plane of the substrate and points outward from either face of the assembled sandwich cell.

Results

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...

Discussion

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...

Materials

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