ANCHORING TRANSITIONS OF NEMATIC LIQUID CRYSTALS ON LARGE ANGLE DEPOSITED ...

139 Pages·2006·1.11 MB·English

ANCHORING TRANSITIONS OF NEMATIC LIQUID CRYSTALS ON
LARGE ANGLE DEPOSITED SILICON OXIDE THIN FILMS

A dissertation submitted
to Kent State University in partial
fulfillment of the requirements for the
Degree of Doctor of Philosophy

By
Cheng Chen
August 2006

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UMI Number: 3237852
UMI Microform3237852
Copyright2006 by ProQuest Information and Learning Company.
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ProQuest Information and Learning Company
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Dissertation written by
Cheng Chen
B.S., Peking University, China. 2001
Ph. D., Kent State University, 2006

Approved by

Chair, Doctoral Dissertation Committee
, Philip J. Bos, Professor of Chemical Physics Interdisciplinary Program
Members, Doctoral Dissertation Committee
, John L. West, Professor of Chemistry Department
, Deng-Ke Yang, Professor of Chemical Physics Interdisciplinary Program
, David W. Allender, Professor of Chemical Physics Interdisciplinary Program
, Kenneth K. Laali, Professor of Chemistry Department

Accepted by
, Oleg D. Lavrentovich, Director, Chemical Physics Interdisciplinary Program
, John R.D. Stalvey, Dean, College of Arts and Sciences
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TABLE OF CONTENTS

TABLE OF CONTENTS...................................................................................................iii
LIST OF FIGURES............................................................................................................v
LIST OF TABLES...........................................................................................................xiii
ACKNOWLEDGEMENTS.............................................................................................xiv
Chapter 1 Introduction.............................................................................................................................1
1.1 Liquid Crystalline Materials...................................................................................1
1.2 Liquid Crystal Displays......................................................................................- 3 -
1.3 Liquid Crystal Alignment and the Method to Achieve the Same......................- 5 -
1.4 Overview of the Dissertation..............................................................................- 6 -
Chapter 2 Theory.......................................................................................................................................8
2.1 Introduction............................................................................................................8
2.2 Review of Previous Theories.................................................................................9
2.2.1 Short Range Interactions.....................................................................................9
2.2.2 Long Range van der Waals Potential................................................................10
2.2.3 Competition between Long Range and Short Range Forces.............................11
2.2.4 Topography.......................................................................................................12
2.3 Our Theory...........................................................................................................15
2.4 Summary..............................................................................................................22
Chapter 3 Physical-chemical properties of LAD-SiO thin films.............................................23
x
3.1 Introduction..........................................................................................................23
iii
3.2 Experimental Method...........................................................................................24
3.2.1 Inorganic Alignment Layer Preparation............................................................24
3.2.2 Thin Film Characterization Method..................................................................26
3.3 Experimental Results and Discussions.................................................................29
3.3.1 Surface Topography and Anisotropy................................................................29
3.3.2 Stoichiometry and Surface Properties...............................................................30
3.4 Summary..............................................................................................................36
Chapter 4 Anchoring Transitions on LAD-SiO Due to the Change in Liquid Crystal
x
Composition..............................................................................................................................................37
4.1 Introduction..........................................................................................................37
4.2 Experimental Methods.........................................................................................38
4.2.1 Materials............................................................................................................38
4.2.2 Sample Preparation...........................................................................................39
4.2.3 General Examination Methods and Definition for Alignment Quality.............39
4.2.4 Pretilt Measurement..........................................................................................40
4.2.5 Dielectric Anisotropy Measurement Method....................................................40
4.2.6 Birefringence Measurement Method.................................................................42
4.2.7 Electro-Optical Curve and Response Time Measurement Methods.................42
4.3 Experimental Results............................................................................................46
4.3.1 The Effect of Large Longitudinal Dipole..........................................................46
4.3.2 The Effect of Large Lateral Dipole...................................................................49
4.3.3 The effect of varying the molecular structure of the additives.........................55
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4.3.4 A Method to Make Improved Liquid Crystal Mixtures for Vertical Alignment
Applications...............................................................................................................60
4.4 Discussions...........................................................................................................67
4.4.1 The Effect of Large Longitudinal Dipole..........................................................67
4.4.2 The Effect of a Large Lateral Dipole................................................................68
4.4.3 The effect of molecular structure on liquid crystal anchoring on SiO .............70
x
4.5 Summary..............................................................................................................74
Chapter 5 Temperature Dependence of the Anchoring Transitions on LAD-SiO ..............76
x
5.1 Introduction..........................................................................................................76
5.2 Experimental Methods.........................................................................................77
5.2.1 Cell Preparation and Characterization...............................................................77
5.2.2 Surface Adsorption and Thermal Desorption....................................................77
5.3 Results..................................................................................................................80
5.3.1 Thermal Induced Anchoring Transitions..........................................................80
5.3.2 The Effect of Temperature on the Critical Concentration of 5CB....................80
5.3.3 Thermal Desorption...........................................................................................85
5.4 Discussions...........................................................................................................89
5.4.1 Thermal Induced Anchoring Transitions..........................................................89
5.4.2 The Effect of Temperature on the Critical Concentration of 5CB....................90
5.5 Summary..............................................................................................................95
Chapter 6 The Effect of LAD-SiO Thickness on Liquid Crystal Anchoring.......................96
x
6.1 Introduction..........................................................................................................96
v
6.2 Experimental Methods.........................................................................................97
6.2.1 LAD-SiO Sample Preparation.........................................................................97
x
6.2.2 Polyimide Sample Preparation..........................................................................97
6.2.3 Pretilt Measurement..........................................................................................98
6.3 Experimental Results............................................................................................98
6.3.1 The Effect of LAD-SiO Thickness on Liquid Crystal Alignment...................98
x
6.3.2 The Effect of LAD-SiO Thickness on the Critical Concentration of 5CB......99
x
6.3.3 Screening Effect..............................................................................................102
6.4 Discussions.........................................................................................................106
6.4.1 The Effect of LAD-SiO Thickness on the Alignment of Liquid Crystal.......106
x
6.4.2 The Effect of LAD-SiO Thickness on the Critical Concentration of 5CB....108
x
6.4.3 Screening Effect..............................................................................................113
6.5 Summary............................................................................................................114
Chapter 7 Conclusions and Suggestions for Future Work.........................................................115
7.1 Summary of Dissertation Work..........................................................................115
7.2 Conclusions........................................................................................................116
7.3 Suggestions for Future Work.............................................................................119

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LIST OF FIGURES

Figure 1: Illustration of Dubois-Violette and de Gennes’ model in which long range van
der Waals torque prefers planar alignment while short range forces prefer homeotropic
alignment................................................................................................................................................14
Figure 2: The preference in LC orientation by long-range/short-range forces......................18
Figure 3: The Working Principle of AFM........................................................................................28
Figure 4: The working principle of XPS..........................................................................................28
Figure 5: AFM images of LAD-SiO thermally evaporated at a medium angle. (a): 10µm
x
x 10µm tapping mode 3D image (b): 5µm x 5µm tapping mode 3D image (c): 3µm x
3µm contact mode 2D image of friction (d): Cross-section analysis....................................32
Figure 6: (a): RMS Roughness of LAD-SiO surface as a function of layer thickness (b):
x
Anisotropy in surface roughness as a function of layer thickness.........................................33
Figure 7: XPS spectrum of thermally evaporated LAD-SiO and e-beam evaporated
x
LAD- SiO , measured at 45º take-off angle. Atomic ratio of Si and O of the sample can
2
be calculated from the corresponding area of the peak. Signal of carbon is from the
residual of CO or hydrocarbon contaminations on the sample surface..............................34
2
Figure 8: XPS spectrum analysis of silicon (Si2p) in (a) e-beam evaporated LAD- SiO
2
and (b) thermally evaporated LAD-SiO . The blue line is the characteristic peak of Si in
x
SiO ; The cyanic line is the characteristic peak of Si in SiO; The magenta line is the
2
characteristic peak of Si in Si crystal; The black line is the measured Si peak; The red
line is the synthetic peak based on characteristic Si peak in SiO , SiO and Si crystal....35
2
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Figure 9: Chemical structure of (a) 5CB and (b) C3.....................................................................45
Figure 10: Anchoring transitions from parallel to homeotropic to parallel again as the
concentration of 5CB in the mixture with LC1 decreases. From top left to bottom right:
pure 5CB, 50% 5CB, 25% 5CB, 10% 5CB, 5% 5CB, and pure LC1. Photo taken with
cells placed between crossed polarizers on a light table..........................................................47
Figure 11: Anchoring transitions of liquid crystal mixtures (5CB/LC1) on LAD-SiO due
x
to the change of the ratio of two components..............................................................................48
Figure 12: The addition of C3 into LC2 leads to an anchoring transition of liquid crystal
on LAD-SiO from homeotropic to planar...................................................................................51
x
Figure 13: The addition of 5CB into the mixture of C3 and LC2 causes an anchoring
transition from planar to homeotropic on LAD-SiO ................................................................52
x
Figure 14: The correlation between the concentration of C3 and the critical amount of
5CB that is needed to maintain homeotropic alignment of C3/5CB/LC2 mixture on
LAD-SiO ...............................................................................................................................................53
x
Figure 15: On E-beam evaporated SiO , more C3 is needed than on thermally evaporated
2
SiO to cause its mixture with LC2 to change from homeotropic alignment to planar
x
alignment................................................................................................................................................54
Figure 16: Alignment of mixtures with different additives of LC1 on LAD-SiO ,
x
photographed between crossed polarizers on a light table. From top left to bottom right
cells are filled with: LC1; 10%C5-Ph-Ph-CN (5CB); 10%C5-Ph-Ph-O-C2; 5% C5-Ph-
Ph-Br, 10% C3-Cyclohexyl-Ph-O-C2 (PCH302); 10% C5-Ph-Ph; 10%C6-Ph-Ph-C5..58
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Figure 17: The effect of cyano groups on the liquid crystal anchoring on LAD-SiOx. Left:
20% C7-Cyclohexyl-Ph-CN; Right: 5% C3 (C3- Cyclohexyl-COO-Ph(-2CN)-O-C2)..59
Figure 18: The addition of 5CB enables the mixture of LC2 and C3 to obtain uniform
vertical alignment on LAD-SiO with a greater negative dielectric anisotropy................62
x
Figure 19: The addition of 5CB also allows higher birefringence of the LC2/C3 mixture
to be used for vertical alignment applications on LAD-SiO ..................................................63
x
Figure 20: E-O curves of two identical LCoS devices filled with LC2 and improved
mixtures (88% LC2, 10% C3 and 2% 5CB) respectively........................................................64
Figure 21: Time response curves of two identical LCoS devices that used LAD-SiO as
x
alignment layers and were filled with LC2 and improved mixtures (88% LC2, 10% C3
and 2% 5CB) respectively.................................................................................................................65
Figure 22: The addition of small amount of 5CB into a LC that has a large negative
dielectric anisotropy also helps to produce uniform vertical alignment on polyimide
alignment layers. Photo of SE-7511 coated cells purchased from EHC with ITO
patterns. Left cell was filled with LC1. Right cell was filled with 10% 5CB +90% LC1.
...................................................................................................................................................................66
Figure 23: Dielectric anisotropy of 5CB/LCI mixtures as a function of 5CB concentration
...................................................................................................................................................................71
Figure 24: A cartoon showing the effect of adding 5CB into LC1. Green and orange rods
represent LC1 and 5CB molecules respectively. The blue surface represents the LAD-
SiO ..........................................................................................................................................................72
x
Figure 25: A cartoon that shows the interaction between the LAD-SiO and the cyano
x
groups......................................................................................................................................................73
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