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 orig....

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.......

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