From Liquid Crystals to LCDs From Liquid Crystals to

10/27/2012

Department of Photonics and Display Institute

From Liquid Crystals to LCDs -1

Dr. Huang‐Ming Philip Chen (陳皇銘) [email protected]

NCTU/Display Institute NCTU/Display Institute

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Dr.Dr. Huang‐Ming Philip Chen Huang-Ming Philip Chen Huang-

OUTLINE  The history of liquid crystals researches  What are liquid crystals?  Electooptical behaviors    

1st class

Nematics Chiral-Nematics Smectic C* (Ferroelectric) Discotics

 Major LC Modes for Display  Fast Response LC Modes

NCTU/Display Institute

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2nd class

Dr. HuangHuang-Ming Philip Chen

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History shows that lyotropic liquid crystals History shows that lyotropic were discovered around 30 years earlier In the early 1850’s , Virchow described a soft, floating substance from nerve core he named “Myelin”

In 1857, Metteneheimer discovered that Myelin was birefringent

• Physician, pathologist, scientist, and social politician


•

Physician, opthamologist, microscopist

•

Knighted for his work in anatomy, physiology, pathology, clinical medicine and public welfare

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In 1888, Friedrich In 1888, Friedrich Reinitzer Reinitzer observed “striking and marvelous apparitions” in his samples cholesteryl acetate and benzoate prepared from cholesterol

Friedrich Reinitzer (1857‐1927) Lecturer in Botany German University of Prague

“ . . .The substance exhibits two melting points, if one may say so. At 145.5o , it melts to a turbid but absolutely fluid liquid which becomes suddenly clear not until 178.8o . On cooling, violet and blue colors appear which quickly vanish with the sample leaving lactescently turbid but fluid. On further cooling bid b fl id f h li the violet and blue colors reappear, but very soon the sample solidifies forming a white crystalline mass” . . . *

Heating

Cooling

*excerpt from a 16 page letter written to Prof. Otto Lehmann on March 14,1888


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Lehmann confirmed the existence of a “flowing, soft crystalline state ” in soft crystalline state ” in Reinitzer’s Reinitzer’s samples Lehmann writes to Reinitzer on August 20,1889: “ … And so my new results confirm your already in good time declared view, that the “Griess” (the substance that causes the turbidness) consists of very soft crystals, that are to be considered as a physically isomeric modification of the substance. It is absolutely homogeneous, and another liquid‐ as you assumed formerly‐ is not present. . . . . . . It is of a high interest for the physicist that crystals exist that are of such a considerable softness that one could almost call them liquid.”

Otto Lehmann

Friedrich Reinitzer

Lehmann’s article entitled “Uber Fliessende Krystalle” (On Floating Crystals) submitted to Zeitschrifte fur Physikalishe Chemie on August 30, 1889 is the first published work to introduce the concept of a “liquid crystal” 5 NCTU/Display Institute


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In 1889, In 1889, Gatterman Gatterman and and Ritschke Ritschke synthesized the first LCs not first LCs not derived from natural products derived from natural products

O RO

N N

OR

Azoxy ethers

• Observed two phase transitions and “floating turbid melts” under the microscope • Published the synthesis details on June 11,1890 • Lehmann confirms behavior, and terms them , “Krystalline Flussigkeiten”* because their viscosity was much lower than the cholesterics Ludwig Gatterman (1860‐1920) Assistant Professor of Chemistry Heidelberg University NCTU/Display Institute

• Lehmann publishes “The Structure of Crystalline Liquids” on March 24, 1890

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The chemist Daniel The chemist Daniel Vorlander Vorlander dominated the LC dominated the LC research for research for three decades beginning in 1905 three decades beginning in 1905 • In 1902, he found that one of the compounds he had synthesized was also a liquid crystal (smectic) O H5C2

O

O O

N N

O

C2H5

• As principal teacher of organic chemistry for all science students in Halle, he held each lecture as a “special event” with visual demonstrations • Was Was the first to study the influence of molecular the first to study the influence of molecular structure on the appearance of liquid crystalline phases Daniel Vorlander (1867‐1941) “Ordinarius” fur Chemie University of Halle NCTU/Display Institute

• Developed synthesis of “homologous series” of compounds for studying structure‐property relationships 7


Georges Friedel develops a classification system Georges Friedel for liquid crystal phases in 1922 • The son of the famous chemist Charles Friedel • Described liquid crystal phases as “crystalline‐ amorphous” rather than “solid‐liquid” • Coined the term “mesomorphic” to describe liquid crystals as a new state of matter

• Used the Greek terms “nematic” (“thread” ) and smectic (“soap”) Georges Friedel (1865‐1933) University of Strasbourg

instead of “schlierien” and ”rod” to classify non‐ chiral liquid crystal phases

Friedel did not take the possibility of smectic phase polymorphism into account NCTU/Display Institute

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The 1960’s mark the beginning of a period of “re‐‐awakening” of interest in liquid crystals “re 1950‐1970

• Extensive synthesis work by George Gray (University of Hull) and researchers at the Halle school under the direction of Sackmann yielded a host of new compounds for experimentation of physical and optical properties • The First International Liquid Crystal Conference was held at Kent State University in 1965, with an attendance of 90 delegates • By the Fourth ILCC in held in Stockholm in 1972, device applications were being discussed and presented openly

Thermal mapping using CLC was Thermal mapping using CLC was the first practical the first practical application application •

Initial research begun at Westinghouse Electric by J. L. Fergason in the late 1950’s

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First applications were for non‐destructive thermal testing of turbine blades for cracks and defects

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Later applications in the field of biomedicine included placental location, mammography, and study of blood flow


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The invention of the “twisted The invention of the “twisted nematic nematic” LC display ” LC display marks the birth of the modern LCD industry 1970~1980

• Helfrich and Schadt (Europe) and Fergason (USA) apply separately for patents on the twisted nematic (TN) display concept

ITO‐coated substrate

• LCD manufacturing for watch and calculator displays begins in the U.S. (RCA and ILIXCO)

Alignment layer

• The US was the dominant manufacturer of LCD displays d i thi during this period i d

Twisted Nematic (TN) Polarizer

Analyzer

Acts as a voltage‐controlled polarization rotator (light valve)

• Japanese companies acquired the technology through licensing agreements


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R.B. Meyer observed the first evidence of ferroelectric switching in the chiral switching in the chiral smectic C* phase 1970~1980

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Observed field‐linear helix unwinding in the chiral smectic C material DOBAMBC

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Postulated that three elements were required for ferroelectric effects to be seen in liquid crystals:

Thick

 Molecular tilt (produced by alkyl tail and small transverse dipole moment  A chiral center (noncentrosymmetric)  Large transverse dipole moment coupled rigidly to the chiral center


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Discotic liquid crystals represent the first mesophases liquid crystals represent the first mesophases obtained “by rational design” 1970~1980

•

Hexahydroxbenzene‐n‐alkanoates first synthesized by Chandresekhar’s

research group in 1977

Nematic discotic phase

Columnar phase

Applications in optical storage media, displays, nonlinear optics, and as anisotropic semiconductors have been explored NCTU/Display Institute

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Ferroelectric LC’s brought the promise of “bistable Ferroelectric LC’s brought the promise of “bistable switching and broke the millisecond response time barrier 1980~1990 • Surface‐Stabilized Ferroelectric Liquid Crystal (SSFLC) device ( Clark and Lagerwall, 1980) V‐shape

1988

Half V‐shape T=0

T=1

Analyzer

Analyzer

FLC Cell

FLC Cell

Polarizer

Polarizer

EZ0

Helix‐unwinding by surface forces and thin cell spacing NCTU/Display Institute

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The discovery of antiferroelectric LC phases by Fukuda The discovery of antiferroelectric LC phases by Fukuda opened new possibilities for device applications 1990~2000

The energy barrier between the ferroelectric and antiferroelectric states is very small and molecular switching occurs at very low voltages

Ferroelectric LC Citizen's 5.5-Inch Antiferroelectric LCD


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Antiferroelectric LC ~1989 Denso's 17‐, 6‐Inch Antiferroelectric LCD (98’)


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Banana‐shaped Banana‐ shaped mesogens mesogens show ferroelectric properties even though the constituent molecules are even though the constituent molecules are achiral achiral 1990~2000

• Banana LC’s were first reported by Vorlander as “bad rods”

MHOBOW

• Antiferroelectric switching demonstrated by Takazoe in 1996 • MHOBOW synthesized by the Walba group (U. Colorado) (2002) • ODBP mesogens (biaxial nematics) synthesized by Madsen in 2004 Because they are noncentrosymmetric (C2 symmetry), banana LC’s can show nonlinear optical properties (SHG) NCTU/Display Institute

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Banana mesogens meet the C2 symmetry requirement for Banana mesogens ferroelectricity without a without a chiral chiral center 2000 above

Molecules are not superimposable on their mirror image! NCTU/Display Institute

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Nematic liquid crystal displays have become ubiquitous in the flat‐‐panel display market flat 2000 above

From a “niche” research area in 1950 to a multi‐billion dollar industry today! NCTU/Display Institute

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What is Liquid Crystal?


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Liquid crystals are a form of condensed matter intermediate in order between the solid and liquid states “Liquid crystal”

Crystalline solid

Isotropic liquid

Decreasing molecular order • Common materials that are liquid crystals include cholesteryl esters, lecithin, DNA, cellulose, and graphite • Classified into different categories depending on their orientational (long‐range) and positional (short‐range) order NCTU/Display Institute

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Terminology Liquid crystal phases: also called Mesophases (meso‐ from the Greek meso which means in between)

Thermotropic (熱致型): Liquid crystal molecules which exhibit temperature dependent liquid crystalline behavior • Enantiotropic(互變轉移型) : same mesophases reversibly upon heating and cooling • Monotropic(單變轉移型) M t i (單變轉移型) : mesomorphism upon cooling only hi li l Lyotropic (溶致型): materials in which liquid crystalline properties appear induced by the presence of a solvent, with mesophases depending on solvent concentration, as well as temperature NCTU/Display Institute

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Molecular shapes and their effects on molecular assembly nematic, smectic, chiral nematic

Calamitic (rod‐like) mesogens

Disk‐like

discotic

Bend‐shape

banana

Board‐like

sandic


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Calamitic (rod (rod‐‐like) like) mesogens mesogens are the best known and most widely occurring class of liquid crystals . . . .

• High orientational order • Random positional order Cholesteric

Nematic

• Both orientational and positional order

Smectic Smectic C* NCTU/Display Institute

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“Optically active” compound into an LC matrix producing mesophases with a helical structure Nematic

Non‐chiral

Chiral nematic (cholesteric) Chiral

* Photo Adopted from “Organic Chemistry‐Biological Approach”

* Derived from the Greek term “chiros” (hand) NCTU/Display Institute


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LC materials dependent on orientation…

Isotropic

Anisotropic

Physical and optical properties are invariant with orientation

Physical and optical properties depend on orientation

n = optical anisotropy (birefringence) = ne ‐ no  = dielectric anisotropy = ll ‐  ┴

Molecular reorientation induced by electromagnetic and optical fields can produce large effects NCTU/Display Institute

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Nematic Liquid Crystals


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Basic LC Optics An incoming light beam is split into two components, one that follows Snell’s law of refraction, called the y y ordinary ray, and one that does not, called the extraordinary ray.

Principle

The phase difference, , is

Illustration of the indicatrix of a uniaxial, optically positive (left) and optically negative (right) material. NCTU/Display Institute

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 is the angle between the optic axis and the direction of light propagation Dr. HuangHuang-Ming Philip Chen

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Light propagation in Light propagation in Nematics Nematics Uniaxial

Biaxial

n1K22


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Order Parameter Related Parameters

Nomenclature Parameter Elastic Constant K11

()

S2 n   

Birefringence Dielectric Anisotropy Magnetic Anisotropy Viscosity Anisotropy


proportional to

S S S S


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Classes of Materials Small molecules : CH3CH2CH2CH2CH2

CN

Polymers : Main chain

Side chain

CH2

CH O

n

(CH2)5


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CN


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Examples


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Material requirements for AM AM--LCDs


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Examples


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Chiral Nematic Liquid Crystals


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Selective Reflection

Left-Handed Circularlyy Polarixed Light, = R

Unpolarized Incident Light

substrate CLC film substrate

Unpolarized Light, =  R


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Right-Handed Circularly Polarixed Light, = R


Electric field vector in CLC The handedness of light can be defined based on the observer’s position. We adopted the definition where the We adopted the definition where the observer looking into light source observer looking into light source..

Clockwise Right‐handed Right‐

Counter‐‐clockwise Counter Left‐‐handed Left

Clockwise : right‐‐handed circularly polarized light Clockwise : right NCTU/Display Institute

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Light propagation in CLCs Light ‘sees’ constant refractive index : transmitted

See constant refractive index

See periodic refractive index change See periodic refractive index change

reflected Light ‘sees’ constant Light ‘sees’ constant changed refractive index : reflected NCTU/Display Institute

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Optical properties

The optic axis of nematics is defined as:

n = ne,n‐no,n

The optic axis of CLC is defined to lie along its helectrical axis: ne, ch = no, n no, ch = [1/2(ne, n2 + no, n2)]1/2


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Pitch Determination Grandjean Wedge

Picture from Texture of liquid Crystal

Cano geometry between a planar substrate and a lens

SEM

Each black or white strip is p/2 p/2 NCTU/Display Institute


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Structure and selective reflection property of CLCs 0.30

c = p n

1/2 p

n= O.D.

0.20

n  c = n

ne + n o 2

 n = ne n o

  = FWHM

0.10

  2  



 n  ave 

 n  ave    

1 sin  i sin  s     sin 1   c  nave p cos  sin 1   i: incident; s: scattering angles Ref: J. L. Fergason; Mol. Cryst 1, pp 293‐‐307, (1996) Ref: J. L. Fergason; Mol. Cryst 1, pp 293

for normal incidence: i and s = 0

600

 R800  nm ,

1000

1200

If thickness of film much greater than p, the bandwidth can be approximated as following:

c  nave p

 

And bandwidth of light, , centered about c is:

  np  ne ,n  no ,n  p NCTU/Display Institute

0.00 400

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n c nave Dr. HuangHuang-Ming Philip Chen

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Reflectivity for CLCs

J. C. Lee, et. al. J. Appl. Phys 68 J. C. Lee, et. al. J. Appl. Phys 68, pp6523 , pp6523‐‐6525 NCTU/Display Institute


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Manipulate Selective Reflection by Adjusting Pitch 4 m

4 m

100

8 m

80

80

60

60 100 / 0

76 / 24

66 / 34

40

40

20

20

0 300 NCTU/Display Institute

500

700 900 1100 W avelength, nm 50

1300

Reflectance, %

T ransmittance, %

100

0 1500 Dr. HuangHuang-Ming Philip Chen

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Cholesteric LCs as Reflecters LCs as Reflecters Cholesteric Bistable Displays

Kodak’s flexible display

Kent Display Inc. http://www.kentdisplays.com/corporate/print/B‐&‐W‐ Chlolesteric‐LCD.htm


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Smectic Liquid Crystals


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Smectic Mesophase

Textures of Liquid Crystals, Ch 1, Wiley NCTU/Display Institute

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The Hexatic The Hexatic Smectic Phases ( Phases (SmB SmB, , SmI SmI, , SmF SmF))

Model structure of (a) the orthogonal Sm B phase, (a) the orthogonal Sm B phase, (b) the Sm I phase being tilted towards the apex of the hexagon, (c) the Sm F phase being tilted towards the side of the hexagon. (The soft crystal B, J, and K phases have a similar structure, only with positional order being l long range.) )


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Smectic A and C

SmA

S C SmC Chiral dopant

Chiral dopant

SmA* NCTU/Display Institute

SmC* 55


Ferroelectricity (鐵電性鐵電性))

Chiralitiy


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Spontaneous Polarization (PS) R. B. Meyer and coworkers* in 1975, where it was deduced from symmetry considerations that all chiral tilted smectic phases exhibit a (local) spontaneous polarization PS, and thus show pyroelectricity. * R. B. Meyer, L. Liebert, L. Strzelecki, P. Keller, J. Phys. Lett., 36, (1975), L69.

In the case where this spontaneous polarization can be reoriented between two stable states by application of an electric field, we call : ferroelectricity.

Note: Solid ferroelectric materials :102~103 nC/cm2, Δn= 0 FLC: 0~102 nC/cm2, Δn≠0 NCTU/Display Institute

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The Ferrolectricity The Ferrolectricity of Smectic of Smectic C*


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Sign of Spontaneous Polarization

“━” compound

  z n



“+” compound



Ps

Ps

J. W. Goodby J. Am. Chem. SOC., 1986, 108, 4729‐4735 NCTU/Display Institute

J. Am. Chem. Soc. 1986, 108, 4736‐4742 Dr. HuangHuang-Ming Philip Chen

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Measuring Spontaneous Polarization In addition to the electrooptic response, the reversal of the direction of the spontaneous polarization also causes a characteristic current response, as shown in following figure for an applied triangular electric response, as shown in following figure for an applied triangular electric field, which can be used to determine the value of Ps*. *K. Miyasato, S. Abe, H. Takezoe, A. Fukuda, E. Kuze, Jpn. J. Appl. Phys. Lett., 22, (1983), L661.

Typical polarization reversal current response (solid line) to an applied electric triangular field (dashed line). Integration of the peak area allows Integration of the peak area allows a determination of the spontaneous polarization PS.


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Ferroelectric Liquid Crystals

• The spontaneous polarization (Ps) is primarily dependent on a coupling between the local diploes at, or in the asymmetric centers of the molecules.

R. B. Meyer, L. Liebert, L. Strzelecki, P. Keller, J. Phys. Lett. NCTU/Display Institute 61 36, L69 (1975)


Surface Stabilized Ferroelectric Liquid Crystal (SSFLC) In 1980, Clark and Lagerwall demonstrated that confinement of a SmC* liquid crystal to cells with a gap smaller than the pitch of the helical superstructure (d