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3 STUDIES ON NON-CONTACT ALIGNMENT OF LIQUID CRYSTALS A dissertation submitted to Kent State University in partial fulfillment of the requirements for the degree o f Doctor o f Philosophy by Linli Su December 2002

4 UMI Number Copyright 2003 by Su, Linli All rights reserved. UMI (B) UMI Microform Copyright 2003 by ProQuest Information and Learning Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. ProQuest Information and Learning Company 300 North Zeeb Road P.O. Box 1346 Ann Arbor, Ml

5 Dissertation written by Linli Su B.Sc., Nanjing University, China, 1995 Ph.D., Kent State University, 2002 Approved by Co-advisor, Doctoral Dissertation Committee Co-advisor, Doctoral Dissertation Committee Members, Doctoral Dissertation Committee Accepted by, Chair, Department o f Chemistry, Dean, College of Arts and Sciences u

6 TABLE OF CONTENTS TABLE OF CONTENTS... iii LIST OF FIGURES...vi LIST OF TABLES...xvi ACKNOWLEDGEMENTS...xviii Chapter 1 Introduction...1 Chapter 2 Non-contact ALignment of Liquid Crystals, an Overview Photo-Alignment Ion Beam Alignment...24 Chapter 3 Study of Photo-alignment Using Adsorbed Dichroic D yes Experimental Setup Preparation of Polymer Films on ITO/glass Substrates Preparation of Dye Adsorbed Alignment Layer Observation of the Alignment Generation and Measurement of Pre-tilt Characterization of the Alignment Layer Thermal Studies of the Alignment Layer Results and Discussion... 42

7 3.2.1 Dichroic Dye structures and Photo-alignment Results UV-vis Spectroscopy Study Results and Alignment Mechanism Pre-tilt Angle Measurements Thermal Studies Conclusion Chapter 4 X-ray Photoelectron Spectroscopy Studies o f Photo-alignment Systems Introduction to X-ray Photoelectron Spectroscopy (XPS) and Angle Resolved XPS (ARXPS) Experimental Setup Results and Discussion Brilliant Yellow and PVA Alignment System Biphenol and PVA System Biphenyl and PVA System Liquid crystal 5CB and BY-PVA System Conclusion Chapter 5 Ion Beam Alignment of Liquid Crystals Experimental Setup Results and Discussion ARXPS Studies Polyimide PI Polystyrene iv

8 5.2.2 Polarized ATR-FTIR studies Polyimide PI Polystyrene Mechanism for Argon Ion Beam Alignment Conclusion Chapter 6 Summary and Future Directions Summary Future Directions Conclusion Appendix A Electro-optical Properties of Liquid Crystals in Infrared (IR) A.1 Introduction A.2 Experimental Setup A.3 Infrared Absorption o f Organic Compounds and the Removal of C-H Absorption Peak A.4 The Birefringence o f Liquid Crystals in IR A.5 Per-deuteration versus per-fluorination A.6 Database for the IR Absorption of Model Liquid Crystals A.7 Conclusion References v

9 LIST OF FIGURES Fig. 1.1 Illustration o f liquid crystal molecules in different phases... 3 Fig. 1.2 The bright state (left cell) and the dark state (right cell, with electric field applied) o f a twist nematic liquid crystal cell...5 Fig. 1.3 Detailed structure of a twisted nematic liquid crystal cell in a liquid crystal display. Each layer serves a specific function. The alignment layer is the one in contact with the liquid crystals... 6 Fig Reversible changes o f liquid crystal alignment modes induced by the photoisomerization of azobenzene units attached to a quartz substrate...13 Fig. 2.2 Reaction o f photo cross-linking between tw o polyvinylcinnamate molecules under linearly polarized UV exposure...14 Fig. 2.3 Cross-linking of photosensitive polymers with coumarin side chains exposed to polarized UV light. E indicates the polarization direction o f the PUV light...16 Fig. 2.4 Chemical structures of polyimide main chain (a) with different R groups. When R has the structure shown in (b), the polymer has a transition moment along the main chain. When R has the structure shown in (c), the polymer has a transition moment perpendicular to the main chain...17 Fig. 2.5 The transition between (a) homogeneous alignment and (b) homeotropic alignment o f liquid crystals as a result of the cis/trans isomerization of doped azo dyes under unpolarized UV light exposure vi

10 Fig. 2.6 Photo-induced reorientation o f liquid crystalline moiety attached to a photosensitive polymer. The co-operative action results in the alignment of liquid crystals perpendicular to the polarization direction o f the incident PUV light 21 Fig. 3.1 Setup for polarized UV exposure o f adsorbed dichroic dyes. Top: Illustration of a black mask used in the exposure. The white area is removed for PUV light to pass through, the black area is masked. Bottom: The setup for PUV exposure...30 Fig. 3.2 Illustration o f the cell using one photo-aligned substrate and one reference substrate made o f rubbed polyimide. The alignment direction of the photo-aligned substrate and the rubbed substrate are aligned 90 degrees to one another to form a twisted nematic (TN) cell for easy observation of the alignment effect. The completed liquid crystal cell is put in between a polarizer and an analyzer. The alignment effect can be observed by rotating the direction o f the analyzer while maintaining the direction o f the polarizer parallel with the rubbing direction o f the reference substrate...35 Fig. 3.3 Schematic representation on the generation o f pre-tilt. After double exposure, the pre-tilt direction is shown by the arrows at the bottom...37 Fig. 3.4 Illustration for an ECB (electrically controlled birefringence) cell used for pre-tilt measurement. Both substrates are photo exposed alignment layers using the same type of dichroic dyes and polymers...38 Fig. 3.5 Liquid crystal director configurations in an ECB (electrically controlled birefringence) cell with anti-parallel alignment...39

11 Fig. 3.6 Illustration of measuring the pre-tilt angle using the magnetic null method. Top: the alignment of liquid crystals molecules in a magnetic field. Bottom: The setup for magnetic null measurement...40 Fig. 3.7 Comparison of good, medium, and poor alignment effect. The mask pattern is shown above. The TN cells made of a photo-exposed substrate and a reference substrate are put in between a polarizer-analyzer pair. The polarizer is parallel to the rubbing direction of the reference substrate, and the analyzer is adjusted to show the darkest state o f the exposed area. The boundary between the exposed part and the masked part is shown with the dotted line...44 Fig. 3.8 The transformation between azo and hydrazone tautomers o f 2,2 - dihydroxyazobenzene...47 Fig. 3.9 Adsorbed dyes being washed off during the filling process. Far above: top view and side view of a test cell. Above: the filling process Fig UV-vis spectra of dichroic dyes in hexane solution and on PVA Fig The setup for acquiring polarized UV-vis spectra in parallel and perpendicular directions Fig Polarized UV-vis spectra of dichroic dyes 4,4 -dihydroxy azobenzene and azobenzene adsorbed on PVA before and after PUV exposure...54 Fig Illustration for anisotropic desorption o f dye molecules that align liquid crystals after Polarized UV exposure Fig Illustration for isotropic desorption of dye molecules that do not align liquid crystals after Polarized UV exposure...58 V lll

12 Fig Anisotropy change with PUV exposure time for 4,4 -dihydroxyazobenzene on PVA. Top graph: Change o f Apara and Aperp with exposure time. Bottom: change o f anisotropy R with exposure time Fig Anisotropy does not change with PUV exposure time for azobenzene on PVA. Top graph: Change of Apara and Aperp with exposure time. Bottom: change o f R with exposure time...62 Fig The conformation o f liquid crystals on a surface, (a) Homogeneous, (b) Homeotropic, and (c) with a pre-tilt angle a...64 Fig Brilliant yellow is a rod-like molecule. Its two hydroxyl end groups form strong hydrogen bonding with PVA surface. The molecule itself therefore prefers a flat conformation on the substrate surface...66 Fig Polarized UV-vis spectra o f the PUV aligned system 2244-PVA before and after heating at 150 C Fig Polarized UV-vis spectra o f the PUV aligned system 2244-PVA before and after heating at 50 C Fig Anisotropy o f the PUV exposed alignment layers remains stable under room temperature...71 Fig Change of Anisotropy (R) with heating time at 65 C and 90 C...73 Fig Decrease of anisotropy (R) o f the alignment surface after heating at different temperature...74 Fig Characteristic time o f the redistribution of dye molecules on the alignment surface with different heating temperatures...75 ix

13 Fig. 4.1 The generation o f photoelectron in X-ray photoelectron spectroscopy...81 Fig. 4.2 Illustration for non-destructive depth profiling using angle resolved XPS (ARXPS)...82 Fig. 4.3 High-resolution C Is spectra of dye brilliant yellow adsorbed on PVA at 90 take off angle, (a): C Is spectrum of brilliant yellow; (b): C Is spectrum of PVA; (c), C Is spectrum o f BY on PVA being curve fitted using the spectra o f pure components in (a) and (b). The extra peak is located at ev...88 Fig. 4.4 Change of ratio between individual components with take o ff angles for BY on PVA system...92 Fig. 4.5 Curve fitting o f the O Is high-resolution spectra of BY/PVA before PUV exposure at different take off angles. The take off angle is (a) 90 and (b) Fig. 4.6 Quantitative results of O Is curve fitting for BY/PVA system before PUV exposure. Plot uses actual percentage of individual components...97 Fig. 4.7 Quantitative results o f O Is curve fitting for BY/PVA system before PUV exposure. Plot uses normalized values for better observation o f the trend...98 Fig. 4.8 Layer structure o f adsorbed brilliant yellow on PVA. The LMI (hydrogen bond) region is in between the adsorbed dye brilliant yellow and polymer PVA...99 Fig. 4.9 Curve fitting of adsorbed 4,4 -biphenol on PVA Fig Change of biphenol/pva ratio with take off angles Fig Depth profile o f biphenol-pva alignment system Fig Curve fitting o f adsorbed biphenyl on PVA. Top graph: curve fitting. Bottom graph: change of biphenyl/pva ratio with take off angle x

14 Fig Change of biphenyl/pva ratio with take-off angle Fig The layer structure of biphenyl on PVA Fig The curve fitting of C Is high-resolution spectrum of 5CB-BY-PVA system. Spectra of pure compounds 5CB, brilliant yellow, and PVA are used as templates. The peak indicating intermolecular interaction between BY and PVA is also visible at ev Fig Change of percentage of each individual component with take off angle. Top graph is the actual percentage o f each component. The bottom graph magnifies the trend using arbitrary unit Fig Illustration for the multi-layered structure of 5CB/BY/PVA system Fig. 5.1 Chemical structures of polymer used for ion beam alignment, (a): polystyrene; and (b): polyimide PI Fig. 5.2 The setup for argon ion beam exposure o f alignment layers for liquid crystal Fig. 5.3 TN cells made from argon ion beam aligned polyimide PI 2555 and filled with 5CB. (a) Bright state; (b) dark state; (c) dark state; (d) bright state; and (e) illustration of the grid mask used in samples (c) and (d) Fig. 5.4 Change of the % decrease in oxygen and nitrogen content after ion beam exposure for 3 minutes with different take-off angle Fig. 5.5 Carbon Is high-resolution spectra of polyimide PI 2555 before (top graph) and after (bottom graph) argon ion beam exposure for 3 minutes at 90 take-off angle xi

15 Fig. 5.6 Oxygen Is high-resolution spectra o f polyimide PI 2555 before (top graph) and after (bottom graph) argon ion beam exposure for 3 minutes at 90 take-off angle Fig. 5.7 Nitrogen Is high-resolution spectra o f polyimide PI 2555 before (top graph) and after (bottom graph) argon ion beam exposure for 10 minutes at 90 take-off angle Fig. 5.8 Carbon Is high-resolution spectra o f polystyrene before (top graph) and after (bottom graph) argon ion beam exposure for 10 minutes at 90 take-off angle Fig. 5.9 Illustration on the principle of attenuated total reflection Fig Illustration for s-polarized light Fig Polarized ATR-FTIR spectra of polyimide PI 2555 before and after argon ion beam exposure for 3 minutes. Top curve: spectrum after exposure in the parallel direction to the ion beam; middle curve: spectrum after exposure in the perpendicular direction to the ion beam; and bottom curve: spectrum before exposure Fig ATR-FTIR dichroic spectrum of polyimide PI 2555 after ion beam exposure for 3 minutes Fig Polarized ATR-FTIR spectra of polystyrene before and after argon ion beam exposure for 3 minutes. Top curve: spectrum before exposure; middle curve: spectrum after exposure in the parallel direction to the ion beam; and bottom curve: spectrum after exposure in the perpendicular direction to the ion beam xu

16 Fig The ratio of ATR-FTIR spectra of polystyrene in the parallel and the perpendicular to ion beam direction after exposure for 10 minutes Fig Illustration o f ion beam bombards polymer at different incident angles 90 and Fig Generation o f anisotropy for polyimide PI 2555 exposed by argon ion beam. The top graph shows isotropic distribution of the polymer main chains before exposure. The bottom graph shows excess amount o f polymer main chains parallel to ion beam direction after exposure Fig Generation of anisotropy for polystyrene exposed by argon ion beam. The top graph shows isotropic distribution of the phenyl rings before exposure. The bottom graph shows excess amount o f phenyl rings parallel to ion beam direction after exposure Fig. A. 1 Illustration of the liquid crystal cell for IR measurements Fig. A.2 Chemical structures o f (a) D-8CB, (b) 8CB, (c) decafluorobiphenyl, (d) biphenyl, (e) 5CB, (f) 50CB, and (g) ML Fig. A.3 The absorption spectrum o f liquid crystal 5CB in the infrared region. Cell thickness: 6 micron. Measured under room temperature when 5CB is in its nematic phase Fig. A.4 The IR absorption spectra o f (a) benzene, and (b) benzene-d6. The Y-axis is absorbance in arbitrary unit xiii

17 Fig. A.5 The polarizer/lc cell/analyzer setup for the scanning wavelength method. Both the liquid crystal cell and the analyzer can be rotated to change the angle between them and the polarizer Fig. A.6 The birefringence of liquid crystal 5CB in the inbared Fig. A.7 The birefringence of liquid crystal 50CB in the infrared Fig. A.8 IR spectra o f D-8CB (top curve) and 8CB (bottom curve) Fig. A.9 Birefringence o f liquid crystals 8CB and D-8CB in the infrared Fig. A. 10 IR spectra o f biphenyl (top curve) and decafluorobiphenyl (bottom curve) Fig. A.l 1 Polarized IR transmission spectra for liquid crystal 5CB for alignment direction parallel and perpendicular to the polarizer. Cell thickness: 5.5 micron. Spectra were measured at 24 C Fig. A. 12 Polarized IR transmission spectra for liquid crystal 50CB for alignment direction parallel and perpendicular to the polarizer. Cell thickness: 4.7 micron. Spectra were measured at 60 C Fig. A. 13 Polarized IR transmission spectra for liquid crystal PCH5 for alignment direction parallel and perpendicular to the polarizer. Cell thickness: 4.76 micron. Spectra were measured at 45 C Fig. A. 14 Polarized IR transmission spectra for liquid crystal MBBA for alignment direction parallel and perpendicular to the polarizer. Cell thickness: 6.4 micron. Spectra were measured at 24 C xiv

18 Fig. A. 15 Polarized IR transmission spectra for liquid crystal RC2593C for alignment direction parallel and perpendicular to the polarizer. Cell thickness: 4.63 micron. Spectra were measured at 65 C Fig. A.16 Polarized IR transmission spectra for liquid crystal LCI13561C for alignment direction parallel and perpendicular to the polarizer. Cell thickness: 4.76 micron. Spectra were measured at 65 C xv

19 LIST OF TABLES Table 3.1 Chemical structures for most of the organic compounds involved in this research (Part 1 ) Table 3.2 Chemical structures for most of the organic compounds involved in this research (Part 2)...32 Table 3.2 Chemical structures for most of the organic compounds involved in this research (Part 3). * 2244 is the abbreviation for 2,2 -[4-(4-aminophenylazo)- phenyliminojdiethanol...33 Table 3.4 List o f some of the dichroic dyes with their structures and alignment effect Table 4.1 Quantification data for curve fitting of high-resolution C Is spectra of BY on PVA. The table shows the percentage o f each component with change of TO A Table 4.2 Quantification data for curve fitting o f high-resolution C Is spectra of BY on PVA. The ratio between individual components changes with TOA Table 4.3 Quantification data for curve fitting o f high-resolution O Is spectra of BY-PVA Table 4.4 Quantitative information o f C Is high-resolution spectra o f biphenol-pva system Table 4.5 Quantitative information of the C Is high-resolution spectra of biphenyl-pva system xvi

20 Table 4.6 The percentage of each component in the 5CB/BY/PVA system with change o f take off angle Table 5.1 Alignment directions o f liquid crystal 5CB for polyimide PI 2555 and polystyrene under different alignment conditions Table 5.2 Quantitative data for survey spectra o f polyimide PI 2555 before and after argon ion beam exposure for 3 minutes at different take off angles Table 5.3 Quantitative data for carbon Is high-resolution spectra o f polystyrene before and after argon ion beam exposure for 10 minutes at 90 and 30 take-off angles Table A.l Comparison between the birefringence values of liquid crystal 8CB and highbirefringence liquid crystal ML1 in the infrared Table A.2 Phase characteristics of liquid crystals 8CB and D-8CB. K is the crystalline phase, S A is the smectic A phase, N is the nematic pahse, and I is the isotropic phase o f the liquid crystal. The temperatures shown are the transition temperature between these phases Table A.3 Average birefringence values o f liquid crystal 8CB and its perdeuterated counterpart D-8Cb in certain regions o f infrared excluding the absoprtion peaks Table A.4 The transmission database for some model liquid crystal compounds. Listed here are values averaged for the particular wavelength regions xvii

21 ACKNOWLEDGEMENTS This dissertation, and the research work it based on, would not have been possible without my advisors: Dr. John L. West, and Dr. Julia E. Fulghum. I have been very fortunate to receive twice what a good advisor and mentor can offer to a graduate student. Their knowledge and enthusiasm have always been a constant source of motivation during the years of my research and study. Their wisdom has guided me through one of the most important stages o f my life. I want to give special thanks to Dr. Yuri Reznikov, who has given me precious advice on so many things. My appreciation also goes to Dr. Philip J. Bos, who has been a great source of knowledge. I would also like to thank my committee members for their willingness to participate and for their valuable insights. I consider myself very lucky to have the opportunity to work with these top scientists in their fields, and to have learned so much from them. Special thanks go to members of the West Lab and Fulghum Lab. They have been a great joy to work and study with. I want to thank particularly Dr. Kateryna Artyushkova. She is not only a good friend but an outstanding scientist. Also, faculty and staff members of both the Chemistry Department and the Liquid Crystal Institute have been very helpful and supportive to me. Last, but not the least, I would like to thank my husband, Chenhui Wang, for his support to me, as always. xviii

22 Funding resources provided for this research are: AirForce DAGSI SN-AFIT- 9903, NSF ALCOM DMR , NSF CHE , Keck Foundation, and CRDF UP1-2121A.

23 Chapter 1 Introduction When Reinizer discovered the liquid crystalline phase in the late 19th century, he probably could not have imagined the popularity liquid crystals enjoy today. In fact, liquid crystalline materials did not attract much attention until after liquid crystal displays were invented in the middle part of the last century. Today, the liquid crystal display (LCD) dominates in the flat panel display industry. Liquid crystal related research and applications are carried out in many areas including display, telecommunication, medical, and military applications. Liquid crystals have properties intermediate between an isotropic liquid and wellordered crystals. An isotropic liquid has no order, while a crystal has long-range order. Liquid crystalline materials have liquid-like fluidity, but at the same time maintain a certain short-range order. The liquid crystalline phase does not possess long-range positional order, but does have long-range orientational order.111this gives liquid crystals many special properties that make them all the more interesting and intriguing to scientists. Liquid crystal molecules are anisotropic. It is this anisotropy that promotes the orientational order. There are many types of liquid crystals. A thermotropic liquid crystal forms liquid crystalline phases by itself, and its liquid crystalline phases change with 1

24 2 temperature. A lyotropic liquid crystal only forms liquid crystalline phases when dissolved in a certain solvent, and its liquid crystalline phases change with the concentration o f the solution and with the temperature. Thermotropic liquid crystals are the most widely studied and used liquid crystals. Thermotropic liquid crystals can have a variety of different phases, such as: the smectic and nematic phase, etc. Many compounds exhibit one or more of the liquid crystalline phases. A comparison between the crystalline, smectic, nematic, and isotropic phases is shown in figure In the crystalline phase, each molecule has a fixed position. In the smectic phase, a layer structure is preserved while molecules within each layer have some freedom of movement. In the nematic phase, the layer structure is destroyed and the molecules flow like a liquid, but maintain their orientational order. In the isotropic phase, all molecules can randomly move about and the liquid has no order. When liquid crystals come in contact with another phase o f matter, the order of the interface has a significant influence on the orientation of the liquid crystals. The alignment of liquid crystals at an interface is generally produced by controlling the surface order o f the interface between the liquid crystal and the substrate. This interface layer is called the alignment layer. Understanding and controlling the alignment of liquid crystals is a critical area of research involving liquid crystals as almost all of the practical applications using liquid crystals rely on a controlled alignment. The quality of alignment directly affects the performance and durability o f liquid crystal devices. Alignment of liquid crystals on an ordered surface was first discovered by Mauguin in the early 20lh century. Mauguin successfully aligned liquid crystals on a glass

25 Crystalline phase Smectic phase IIIIIIH llllllll IIIIIIH tl II $5 IV \W i l/h N \ \M /\N iw'l/ * = * \m i ( / l,w Isotropic phase Nematic phase Figure 1.1 Illustration of liquid crystal molecules in different phases.

26 4 surface by rubbing the glass with a piece of paperj31 Since then numerous methods and materials have been developed to control alignment. A typical liquid crystal display (LCD) utilizes the concept of a twisted nematic (TN) cell. A TN cell can appear bright, dark or in between to display the intended information. The bright and dark states of a TN cell are determined by the setup o f the cell and the electric field applied to it. The working mechanism of a TN cell is illustrated in figure 1.2. The alignment layers at the top and the bottom of the cell are set to be perpendicular to each other. The liquid crystals in the cell are aligned according to the orientation direction of the alignment layer. The polarizer transforms the incoming light into linearly polarized light when it enters the liquid crystal cell. In the cell on the left side of figure 1.2, the polarization direction of the light is guided by the liquid crystals and is rotated 90 degrees when exiting the top of the cell. At this point the polarized light is parallel to the analyzer and can pass through. Therefore the TN cell in this configuration appears bright. If an electric filed is applied to the cell as shown in the cell on the right side, the liquid crystal molecules in the cell align with the electric field. In this configuration the liquid crystals can no longer rotate the polarization direction o f the light. The exiting polarized light is perpendicular to the analyzer and the TN cell appears dark. An actual TN cell (figure 1.3) consists o f many layers that serve different functions. The alignment layer is critical in determining the performance of the liquid crystal display, however.

27 X Analyzer Alignment layer Liquid crystals Alignment layer 11 I I Polarizer Light Bright state Dark state Figure 1.2 The bright state (left cell) and the dark state (right cell, with electric field applied) of a twist nematic liquid crystal cell.

28 6 Glass substrate Barrier layer ITO electrode Insulating layer Polarizer Insulating layer ITO electrod Barrier laye^f Glass substrati Polarizer Reflector Liquid Crystal Polarization directum directum lignment layer Figure 1.3 Detailed structure of a twisted nematic liquid crystal cell in a liquid crystal display. Each layer serves a specific function. The alignment layer is the one in contact with the liquid crystals.

29 7 Liquid crystals can be aligned on smooth surfaces, grooved surfaces, and chemically treated surfaces. A comprehensive review of alignment methods of liquid crystals before 1980s is provided by Cognard.[4' Different materials can be used as alignment layers. For inorganic alignment materials, vacuum deposition o f SiOx is a popular method mostly used in research labs. It is generally not used in mass production due to several drawbacks. SiOx is a brittle material, and the deposition conditions are unsuitable for processing large substrates. For organic alignment layers, a variety of polymeric materials can be used. Polyimides are the most popular alignment material for both laboratory and commercial usage. Currently mechanical rubbing of a polyimide film is the most widely used method for commercial preparation of liquid crystal displays. The alignment direction can be either parallel or perpendicular to the rubbing direction, depending on the type o f polymer used. Rubbing, as a contact alignment method, can introduce a variety o f problems that will not be apparent until after the liquid crystal displays are fabricated. These problems include the introduction of dust particles from the fabric materials used for rubbing, static charges generated when the fabric material comes in contact with the polymer, and formation of scratches on the polymer surface during rubbing. These defects undermine the quality of the display and increase the cost for manufacturing. Therefore, non-contact alignment has become an intensively pursued field due to the overwhelming advantages it has over conventional methods to create surface ordering of liquid crystals. Non-contact alignment completely eliminates these problems created by the contact of the rubbing media with the polymer surface while producing the same or

30 8 better alignment effect than rubbing. In addition, non-contact methods are more versatile. Different energy sources, different substrate materials (organic and inorganic), and different exposure configurations can be explored to optimize the alignment. In this dissertation, two major aspects of non-contact alignment are studied: photo-alignment and ion beam alignment. The study o f photo-alignment focuses on polarized UV exposure of adsorbed dichroic dyes on polymer films to align liquid crystals. Extensive studies were carried out on the generation of photo-alignment, including variation of the materials and parameters to improve alignment control, and investigation o f the alignment mechanism using a variety of analytical methods, as described below. Liquid crystal alignment resulting from Ar+ ion beam exposure of diamond-like carbon (DLC) and polyimide surfaces was first reported by scientists at IBM.[S1 However, the mechanism for this type of alignment is still not fully understood. My study of ion beam alignment focused on using an Ar+ ion beam to bombard various polymer substrates. Different polymer materials and exposure parameters are tested to optimize the alignment effect. The mechanism of this type of alignment was studied using various analytical techniques. Analytical techniques involved in this study include Polarized UV-Vis Spectroscopy, X-ray Photoelectron Spectroscopy (XPS), and Attenuated Total Reflection Fourier Transformed Infrared Spectroscopy (ATR-FTIR). Polarized UV-vis Spectroscopy provides information on the anisotropy of the alignment layer before and after polarized UV exposure. It is especially suitable for dichroic dyes which generally have strong

31 9 absorption in the visible and UV regions. However, UV-vis spectroscopy is a bulk analytical technique that cannot distinguish surface chemistry from the bulk. XPS, a surface sensitive technique, provides elemental and chemical information on the changes of the alignment layer associated with either photo-alignment or ion beam alignment. The sampling depth of XPS is generally about 10 nanometers. Angle resolved XPS (ARXPS) probes the depth profile of the alignment surface non-destructively, providing important information on the ordering of multi-layer alignment. ATR-FTIR has a sampling depth of about 0.5 ~ 1 micron. The ATR-FTIR spectra reveal changes in the functional group chemistry at or near the sample surface. When ATR-FTIR is coupled with a polarizer, surface anisotropy, which is considered to be directly related to the alignment of liquid crystals, can be characterized. The pre-tilt angle is one important characteristic involved with liquid crystals alignment. A pre-tilt angle is defined as the angle between the director o f a liquid crystal molecule and the alignment surface. It is essential to optimizing the electro-optic properties o f liquid crystal cells. Generation and control o f pre-tilt angles in both photoalignment and ion beam alignment were studied. The details regarding investigations on pre-tilt angles are discussed in the following chapters. The electro-optical properties of liquid crystals in the infrared is also investigated and is discussed in Appendix I. Liquid crystals, similar to other organic molecules, have strong absorptions in the infrared region. The structure-property relationships of liquid crystals are probed in order to design and develop liquid crystals that have no absorption

32 10 peaks in the technologically important 2-5 micron region. The birefringence of liquid crystals in the IR is also studied.

33 C h a p te r 2 Non-contact Alignment of Liquid Crystals, an Overview As mentioned in the previous chapter, rubbing of a polyimide film is the most popular method used by liquid crystal display manufacturers in recent years. Mechanical rubbing of polyimide creates strong alignment o f liquid crystals. However, the many disadvantages associated with rubbing induced scientists to search for alternative alignment methods that align liquid crystals free of these deficiencies. Non-contact alignment, as opposed to methods requiring contact between the rubbing cloth and the polymer surface, has been extensively researched in recent years. 2.1 Photo-alignment Photo-alignment is one of the most important methods of non-contact alignment of liquid crystals, providing a promising substitute for conventional rubbing to align liquid crystals. Photo-alignment uses polarized light to generate anisotropy on the substrate surface. As a non-contact method, photo-alignment has many advantages over the conventional rubbing method. It produces homogeneous, reproducible alignment on a truly molecular level. Alignment directions and pre-tilt angle can be easily adjusted using different materials and photo exposure conditions. 11

34 12 Photo-induced alignment was first reported by Ichimura in 1988J6 He chemically attached a monolayer o f an alkylsilyl group modified with azobenzene to a quartz plate. This azobenzene monolayer undergoes cis/trans isomerization when exposed to alternating UV and visible light. Nematic liquid crystals are aligned homeotropically (alignment direction vertical to the substrate) when azobenzene molecules are in the trans state. When azobenzene molecules are in the cis state, liquid crystals are aligned homogeneously (planar alignment). The transformation is shown in figure 2.1. Ichimura utilized unpolarized light to generate photo-alignment. Later on, others presented work on the generation of alignment using polarized UV (PUV) light exposure. PUV exposure rapidly became a common method of photo-alignment. Comprehensive reviews on photo-alignment o f liquid crystals are provided by Ichimura and 0 Neill.[7,f81 Although materials may vary, the basic concept is the same for different photoalignment techniques. A substrate coated with light sensitive materials is exposed to polarized light, resulting in a surface that aligns liquid crystals either parallel or perpendicular to the polarization direction of the radiation. Vertical alignment (homeotropic) of liquid crystals is obtained in some cases. For those materials that align liquid crystals parallel to the polarized light source direction, placing the substrate at an oblique angle to the source may produce a certain pre-tilt. For those materials that align liquid crystals perpendicular to the polarization direction of the radiation source, a second oblique exposure with the polarization direction rotated 90 degrees is needed to create pre-tilt angles. Alignment surface with unidirectional pre-tilt angles for liquid crystal

35 13 Liquid crystals 365 nm < trans nm cis Azobenzene monolayer Fig. 2.1 Reversible changes of liquid crystal alignment modes induced by the photoisomerization o f azobenzene units attached to a quartz substrate.131

36 14 / 21\ i c~chr. / H H 2 \ frc7. Polarized UV exposur^ t Fig. 2.2 Reaction of photo cross-linking between two polyvinylcinnamate molecules rgl under linearly polarized UV exposure.

37 15 molecules help to optimize the electro-optical response of the liquid crystal cell. Generally, photo-induced anisotropy that leads to the alignment of liquid crystals can be assigned into three major categories: photo-induced cross-linking, photo-induced decomposition and photo-induced isomerization. For photo-induced cross-linking, polyvinyl cinnamate (PVCN) appeared to be one o f the most interesting alignment materials under polarized UV light exposure. Reznikov, Schadt, and Chigrinov et al [91[10J[I11 proposed that the (2+2) cyclo-addition of PVCN under polarized UV light results in the anisotropic depletion of the cinnamate side chain (Figure 2.2). The alignment direction of both the unreacted side chains, and the principle photoproduct, align liquid crystals perpendicular to the PUV direction. PVCN did not become a commercial product due to its poor thermal stability. The anisotropy generated by photo-exposure either decreases or disappears after prolonged exposure at elevated temperatures. Photosensitive polymers other than polyvinyl cinnamate have also been studied. Ichimura et al [12 studied polymethacrylates with coumarin side chains which also undergo [2+2] cyclo-addition under UV exposure. The reaction mechanism of a photosensitive polymer with coumarin side chains is shown in figure The thermal stability of these coumarin type polymers is generally better than that o f PVCN. Photo-induced decomposition can also produce alignment. This type of alignment was first reported by Hasagawa and Taira in 1995 who used polarized UV light exposure of polyimide.1131 The polyimide was originally used as the polymer o f choice for rubbed alignment in the liquid crystal display industry due to its good thermal and chemical stability. Before PUV exposure, the polyimide chains are randomly distributed.

38 16 Figure 2.3 Cross-linking of photosensitive polymers with coumarin side chains exposed to polarized UV light.181 E indicates the polarization direction o f the PUV light.

39 17 (a) N R- (b) (c) Figure 2.4 Chemical structures of polyimide main chain (a) with different R groups. When R has the structure shown in (b), the polymer has a transition moment along the main chain. When R has the structure shown in (c), the polymer has a transition moment perpendicular to the main chain.[u1

40 18 Spectroscopic studies indicate that linearly polarized light induces preferred decomposition of the polymer chains aligned parallel to the polarization direction of the PUV light.[l41tl5]. The direction o f the transition moment in the polyimide determines photoalignment behavior. Assorted moieties with transition moments along the main chain o f the polymer structure are selectively destroyed during exposure. The decomposition products are also randomly distributed on the surface. The PUV exposed surface has a preferred distribution of unreacted polymer chains aligning liquid crystals perpendicular to the PUV polarization direction. Polyimides with their transition moments aligned perpendicular to the main chain align liquid crystals parallel to the PUV direction instead. [l61 Examples of polyimide with transition moments both along and perpendicular to the main chain are shown in figure 2.4. If the R group in the polyimide has the structure shown in (b), the transition moment of the polymer is along the main chain. In this case liquid crystals will align parallel to the rubbing direction but perpendicular to the PUV polarization direction. If the R group in the polyimide has the structure shown in (c), the transition moment of the polymer is perpendicular to the main chain. Liquid crystals will then align perpendicular to the rubbing direction but parallel to the PUV polarization direction. The pre-tilt angle of PUV exposed polyimides depends on both the polymer structure and exposure conditions. For example, changing the fluorine content of the polyimide is one way to vary the pre-tilt angle. Studies show that the pre-tilt angle can be varied from 0 (planar alignment, or homogeneous alignment) to 90 (vertical alignment,

41 19 or homeotropic alignment) depending on the temperature o f the imidization process and the exposure conditions.1171 Spectroscopic evidence indicates that oblique exposure o f the polyimide film breaks the in-plane degeneracy of the directors, therefore creating pre-tilt in a certain direction. Some other polymers also align liquid crystals after PUV exposure. For example, polarized deep UV exposed polystyrene aligns liquid crystals parallel to the polarization direction. It was believed that polarized UV exposure induces selective bond breaking of the phenyl rings, which results in a preferred distribution o f the residual phenyl groups parallel to the polarization direction of the UV light. It is this anisotropy that results in liquid crystals alignment parallel to the PUV direction Alignment realized by photo-induced decomposition has certain disadvantages. Photo-degradation products tend to negatively affect the performance of liquid crystal displays. Electro-optical measurements have confirmed that a net negative charge developed on the polyimide surface, which may result from acidic decomposition products.1201 This will result in image sticking and display flicker. Last, but not the least type o f photo-alignment, is photo-induced isomerization. Photo-induced isomerization o f azo dyes has long been a popular field in photoalignment studies. Azo dyes exist in two isomeric forms: cis and trans. The transition between the cis and trans isomers of an azo dye is called the isomerization process. This is a reversible process realized by using light of different wavelengths or heat. Generally some o f the physical properties, such as polarity and molecular size, are slightly different

42 20 (a) Liquid crystal Azo dye in trans form (b) V Azo dye in cis form Fig. 2.5 The transition between (a) homogeneous alignment and (b) homeotropic alignment o f liquid crystals as a result of the cis/trans isomerization of doped azo dyes under unpolarized UV light exposure [20]

43 21 Polarized UV light direction cr Photochromic moiety Liquid crystalline moiety Figure 2.6 Photo-induced reorientation of liquid crystalline moiety attached to a photosensitive polymer. The co-operative action results in the alignment of liquid crystals perpendicular to the polarization direction of the incident PUV light.[25'

44 22 for the two isomers of an azo dye. These changes in physical properties may change LC alignment properties. For photo-alignment, azo dyes are generally not used alone. They are incorporated as part of a multi-component alignment layer. There are many ways to introduce azo dyes into the alignment system. Azo dyes can be mixed directly into liquid crystals to create a guest-host system. Various alignment effects can be achieved using different host (liquid crystals) and guest (azo dye) materials. The alignment direction o f the liquid crystals is perpendicular to the polarization direction o f the laser beam in one case.1211 A homogeneous-homeotropic transition can also be achieved by cis/trans isomerization of the doped azo dyes under unpolarized UV light exposure. The cis isomer promotes homeotropic alignment of liquid crystals while the trans isomer promotes homogeneous alignment.1221 An illustration o f this transition is shown in figure 2.5. Alignment layers can also be formed from azo dyes covalently attached to a substrate surface as the alignment layers. Azobenzene chromophores can be attached onto a modified quartz surface through addition reactions.1231 One example is discussed earlier in this chapter. In-plane reorientation of liquid crystal molecules can be realized by irradiation o f linearly polarized visible light.1241 Self-assembled monolayers of azo dyes can be deposited by the Langmuir Blodgett technique using hydrogen bonding to attach the monolayers to the surface.1251 Homogeneous alignment can be produced by polarized light exposure. Polymer films with azobenzene as side groups can also be used as alignment layers.1261 The polymer is spin-coated onto a substrate and then exposed to linear

45 23 polarized UV light. Homogeneous alignment is achieved, but it is not very stable. Liquid crystals can also be attached to a photosensitive polymer. The azobenzene moiety of the polymer chain undergoes cis/trans isomerization when exposed to polarized UV light. The isomerization-induced reorientation o f the photochromic part causes the reorientation of the liquid crystal units.1271 An example o f this kind of co-operative interaction is shown in figure 2.6. Generally, the photo-induced cis/trans isomerization of the azo dyes incorporated in the alignment layer changes the volume, shape, conformation or alignment direction, subsequently resulting in a preferred alignment direction for the liquid crystals. In most cases the alignment can be reversed through exposure to light of different wavelengths or to heat. Most of the photo-sensitive materials studied in photo-alignment of liquid crystals so far undergo certain chemical changes after UV exposure. However, photo-induced reorientation is also possible for producing alignment of liquid crystals. One interesting phenomenon of photo-alignment o f liquid crystals involve photoinduced reorientation was recently discovered by Reznikov et al.1281 Under polarized UV exposure, an alignment layer consisting o f adsorbed liquid crystal 5CB on fused quartz substrate can align liquid crystal 5CB perpendicular to the polarization direction of the PUV light. 5CB molecules have strong dichroic absorption of the polarized UV light. Their studies show that the alignment is likely to be produced by light induced reorientation of 5CB in the plane of the substrate. Upon PUV exposure, 5CB molecules are re-oriented to a direction perpendicular to the polarization direction o f the UV light. Due

46 24 to the structural characteristics of 5CB, there is no cis/trans isomerization involved in this process. Photo-alignment o f liquid crystals provides enormous versatility in the control of the alignment effects. Numerous types of alignment materials and methods are available for choosing the best conditions for specific liquid crystals. A significant advantage for photo-alignment is its low-temperature operating ability, which is particularly suitable for plastic substrates. Together with its non-contact nature, photo-alignment has a great prospective as the alignment method of choice for liquid crystal applications. 2.2 Ion Beam Alignment Argon ion beam alignment of liquid crystals, as the latest non-contact technique, has attracted a lot o f attention in recent years. Sun Z.M. et al first studied the molecular orientation of the liquid crystal 5CB on various inorganic substrates eroded obliquely by an Ar+ beam in 1994.[29] They used a normally evaporated SiOx layer which is isotropic, and produced no preferred alignment of 5CB. This SiOx layer is then sputtered by an 8 kev Ar+ beam at grazing incidence. They found that 5CB is homogeneously aligned on the ion-sputtered surface without pre-tilt. However, planar alignment is generated only when the ion beam approaches the surface at an oblique angle. When the SiOx surface is exposed to an ion beam at a normal angle, liquid crystals align in a radial pattern. The authors suggest that this pattern is formed by the flow of the ions deviating from the center of the focus when the flux of the ions hits the substrate. However, SiOx deposition

47 25 is not considered the best way to generate alignment due to its disadvantages discussed previously. More recently, scientists at IBM used an argon ion beam to bombard various inorganic and organic substrates to align liquid crystals.t30 [3,1[321 Low energy neutral atomic and ion beams were selected (with energy ranging from 75eV to 500eV). They tried a variety of optically transparent and insulating films such as glass, A I 2O 3, and Diamond like carbon (DLC), in addition to many different types of polymers. DLC and polyimides are the materials of choice. They were able to vary the pre-tilt angle within 10 degrees by adjusting the incidence angle, type of alignment materials, and the exposure time of the ion beam. Ultra-high resolution liquid crystal displays fabricated by this type o f alignment method were also demonstrated by IBM at the Society of Information Display 2001 convention.1331 Ion beam exposure is able to produce liquid crystal alignment layers with desirable properties such as high uniformity, low defects, and the technique can be easily adapted for mass production. The interaction between argon ion beams and polymers has been studied extensively.1341 It is known that ion beam exposure leads to irreversible changes in the polymer structures. Ion beams affect the exposed polymer through the transfer of energy from the ions to the substrate, resulting in bond scission and bond formation. The actual results depend on the structure of the polymers involved. Various analytical techniques have been applied to study the surface changes of different polymers after argon ion beam exposure.[351[361[371 However, the mechanism by which the ion beam exposed polymer develops anisotropy and aligns liquid crystals was not clearly understood.

48 26 In 2001, J. Stohr et al used near edge X-ray absorption spectroscopy (NEXAFS) to study this process.138 From the NEXAFS results, they concluded that the orientational order of the bombarded surface is created by the preferential bond breaking and bond formation relative to the ion beam direction. However, NEXAFS does not provide readily interpretable information on the chemical changes produced in the alignment layer by the ion beam, which is very important for a thorough understanding of the interaction between the ion beam and the polymer substrate during exposure. X-ray photoelectron spectroscopy (XPS) appears to be the best choice to provide chemical and elemental analysis of the ion-bombarded surface. For polymer systems, polarized infrared attenuated total reflection (ATR-FTIR) provides orientational and chemical information o f the polymer surface, which is directly related to the alignment mechanism. Comprehensive characterization based on information obtained by these techniques shines light on the mechanism of ion beam alignment on both microscopic and macroscopic levels. Better control of the alignment properties can be achieved through advanced understanding o f the mechanism.