From the theoryof liquid crystalsto LCD-displays

Nobel Price in Physics 1991: Pierre-Gilles de Gennes

Alexander KleinsorgeFHI Berlin, Dec. 7th 2004

Outline

• Survey of Liquid Crystals• Anisotropy properties• LC Displays• Other Applications

Complex Fluids

• broken symmetry, e.g. liquid crystals• slowly relaxing degrees of freedom:

e.g. polymers• heterogeneous sytems, e.g. colloids

(varnish,spumes,milk) – surface effects • modifiable basic modules, e.g.

miscellanea (red blood cells)

Liquid Crystals• state of matter between liquid and solid

( not isotropic, but liquid )• discovered by Friedrich Reinitzer in 1888

( first sample was cholesteryl benzonate )• 13000 mostly organic molecules are known• bacillary: W1-C6H4-M-C6H4-W2 (W..wing,M..middle)

Smectic A Mesophase (l)

and

Columnar Phase (r)

Types of Liquid Crystals (LC)• lyotropic LC: soap, biological membranes• thermotropic LC: LC-Phase exists in a range of

temperature between clarification and melting– sub-classification: nematic(normal,

cholesterolic,columnar-nematic), smectic (A..I), columnar(oblique angle,right a.,hexagonal)

• banana-phase becomes obsolete

isotropic nematic smecticdisordered +orientated +positioned

Degrees of Freedom Chirality

• in LCs: different d.o.f. melt at different phase-transition-temperatures

• LCs: position (3) + orientation (3)

• similar: Curie-temperature for spins

• chiral: molecules that aren’t its reflection

• asymmetric molecules with asymmetric C-atoms

• 2 enantiomorphs:distomer and eutomer

• racemic ( 1:1 mixture )

distomer - eutomer

Aminoacids in Life are counterclockwise

Chirality:

Eutomers / Distomers2001, Hazen+Filley: separation of aminoacids at calzit

LC-Examples

• cigar shaped:

• discotic:

• banana:

Director Orientation• director orientation minimizes the free energy• total Energy: ET= Eelast+Eelectric+Einterface

Eelast=

Einterface=

Eelectric=

Electric Anisotropy: ε• an example of anisotropic parameter is

the dielectric constant ε ; susceptibility• positive a.: along the molecule ε is larger

tend to align parallel to the E-field • negative anisotropy: mostly discotic

shaped molecules

switchable medium (applied voltage)

Light Wave - Photon• Maxwell Eq: E┴B┴k• n=c0/cm≥1, birefringence

occurs with: n( k/ ||k|| )

xB

E

Polarizer as filter, 2 per-pendicular P. block all light !Phase difference results in

changed polarization ellipse !

normal case

birefringence

Birefringence ∆n≠0• anisotropic refractive index n

rotation of polarization plane from light• happens with chiral molecules

(chiral: molecules that aren’t its reflection *)e.g. polysaccharides, liquid crystals, lactic acid, contergan, ..

• natural light is non-polarized (Boltzmann radiation)• polarizer only let pass one polarization (50%)

Waveguiding• waveguiding in

helical state,LC is sequenceof polarizers

cosn(90°/n), n>>100

Twisted Nematic LCD (TN-LCD)• multi layer: polarizer/analyzer, glass,

ITO capacitor, alignment layer (rubbing)• Normally White Mode• waveguiding• glass-balls used

as µm-spacer

• LCD mostly nematic

Twisted vs. Super Twisted90° light plane rotation / 270° l.p.r.

Gooch-Tarry-CurveT(retardation)

- Voltage: ∆VST< ∆VT<V !- AC to avoid electro-chemical processes

Color Pixel

R G B

unpolarized

white light

TFT (later)

orientation layer

orientation layer

color-filter

polarizer

glass-substrate

ITO-layer

liquid crystal

ITO-layer

glass-substrate

polarizer

Display Types• pure LCD (black/white, pocket calcu-

lator,clocks) – reflective ambient light, 2x turning

• DSTN, passive matrix display, column row grid (old laptops) – t.l. s.s.

• TFT, Thin Film Transistor, active matrixdisplay (Laptops >13“) – t.l. s.s.

• OLED, organic LED - matrix of self luminous pixels

• under develop.: SSFLC, ferroelectr. LCD(µs not ms),PDLC, polymer-dispersed LCDs

*t.l. s.s. = transmission light, separate source

Other Applications of LC, but Displaysmostly viscosity f(order)

• tearproof fibers (Kevlar, better than steel)• wearless brakes (>100x /sec)• thermography (optical, area)• adjustable damping• high-power

laser focusing

Nobel Prizes for Displays• Physics 1991:

Gennes; ( only 5+2 pages nobel lecture )

ordering of LC• Chemistry 2000:

Heeger, MacDiarmid, Shirakawa; conductive polymers (OLED)

• Chemistry 2001:Sharpless, Knowles, Noyori;chirally catalysis (LCD)

www.nobel.se

LC-Videos

Collapse Oscilation

http://liq-xtal.cwru.edu/Videos.htm

Notes• LCD $15b annual industry world-wide• References:

- http://www.elis.ugent.be/ELISgroups/lcd/lc/lc.html- http://www.presse-highlights.merck.de/

servlet/PB/show/1132890/Aktiv-Matrix_deutsch.pdf- http://www.mpip-mainz.mpg.de/~pleiner/papers/mkfa2.pdf- http://www.kth.se/fakulteter/tfy/kmf/lcd/lcd~1.htm- „Fachlexikon ABC Chemie", Ed.: Hans-Dieter Jakubke and

Hans Jeschkeit, Frankfurt am Main 1987, Band 1, p. 399-402