Liquid Crystal and their applications in Display

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Liquid Crystal and their applications in Display

• Chan Chun Lam

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Outline
1.Introduction (a) Brief history (b) LC
devices 2.Physics of LC (a) LC molecule (b)
Common LC mesophases (i) Nematic
(ii) Semectirc (iii) Cholesteric (c)
Texture and colors 3. Technology and application
(a) Electro-optic effects (b) Multiplexed
display (c) Merits and shortcomings
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HistoryThe discovery of Liquid Crystal

• In 1888
• Austrian botanicl physiologist
• Friedrich Reinitzer
• Extracting cholesterol
• At 145.5C it melted into a
• cloud liquid. At 178.5C,it melted
• again and became clear.

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LC devices
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Physics of LC

• LC molecules

Rod-shape Organic compound
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Physics of LC
the average alignment of the molecules
In solid state, molecules are highly ordered and
have little translational freedomThe
characteristic orientational order of the liquid
crystal state is between thesolid and liquid
phases and this is the origin of the term
mesogenic stateAn isotropic liquid has no
orientational order. Substances that aren't as
orderedas a solid, yet have some degree of
alignment are properly called liquid crystals
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Physics of LC

• Common LC mesophase

isotropic
nematic
smectirc
anisotropic
A liquid crystal is thermotropic if the order of
its components is changed by temperature.
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Physics of LC

• Structure
• Isotropic structure
• Nematic structure
• Smectirc structure
• Chiral structure

Smectirc structure
Cholesteric structure
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Nematic structure

• What is nematic?

The word nematic comes from Greek , which means
thread(?).
Nematic molecules
No positional order, long-range orientational
order
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Nematic structure

• Liquid crystal are anisotropic material
• The physical properties of the system vary with
  the average alignment with he director.
• If the alignment is large -gt very anisotropic
• If the alignment is small -gt almost isotropic
• If the temperature of the material is raised,
  causing transition to the back(isotropic)?

Nematic liquid crystal
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Smectirc structure

• What is smectirc?

The word smectirc comes from Greek, which means
soap.
Smectirc molecules
Well-defined layers Positionally ordered along
one direction Found at lower temperatures than
nematic
Smectirc A
Smectirc C
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Smectirc structure

• Molecules in this phase show a degree of
  translational order
• Molecules maintain the general orientational
  order of nematics
• Molecules tend to align themselves in layers
• Motion is restricted to within these layers
• Separate layers are observed to flow past each
  other
• Many compounds are observed to form more than one
  type of smectic structure

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Smectirc structure
Smectirc A

• Tic planctor is perpendicular to the smectic
  plane
• No particular postional order in the layer

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Smectirc structure
Smectirc C
Director of molecules is at a constant tilt
angle to the smectic plane
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Chiral structure

• What is chiral(???)?

Lack of inversion symmetry, only chiral molecules
can give such a structure
This structure often called as cholesteruc
structure because it was first observed
forcholesterol derivatives
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Chiral structure
Chiral phase molecules
This phase exhibits a twisting of the
moleculesperpendicular to the director, with the
molecular axis
Chiral pitch
Pitch is defined as the distance it rakes for
the Director to rotate one full turn the helix
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Chiral structure
Chiral pitch

• This pitch changes when the temperature is
  altered.
• A color will be reflected when the pitch is equal
  to the corresponding wavelength of light
• Change in temperature can modify the pitch length
  resulting in an alteration of the wavelength of
  reflection light
• Increasing the temperature -gt angle of the
  director change larger -gt tighten the pitch
• Decreasing the temperature -gt increase the pitch
  length
• This make it possible to make a liquid crystal
  thermometer or some sensors.detect flaws in
  circuit board connections, fluid flow patterns,
  condition of batteries, the presence of
  radiation...

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Texture and colors
LC have a wide variety of macroscopic texture
Factor affecting LC texture
Technique by which it is prepared Molecules
aligned perpendicular to the sample surfacewill
appear to be isotropic. Molecules aligned
parallel to the sample surfacewill appear to be
optically birefringent.
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Texture and colors
Birefringence in liquid crystal

• Birefringence double refraction (having two
  indices of refraction)
• Light polarized paralled to the director has a
  different index of refraction thanlight
  polarized perpendicular to the director

• The light being broken up into the ordinary ray
  and the extraordinary ray
• When the rays exit the birefringent material,
  they are recombined and thepolarization state
  has changed because of the phase difference

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Texture and colors
In a typical liquid crystal, the birefringence
and length are not constant overthe entire
sample. This means that some areas appear light
and others appeardark
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Texture and colors
LC have a wide variety of macroscopic texture
Factor affecting LC texture
Method by which the texture is observed Most
texture are observed easily in transmittedlight
with the sample between crossed
polarizers, Without the polarizers the texture
cannot be discerned. Cholesteric structure are
best seen in reflection using circularly
polarized light having the same rotational
sense As the sample.
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Texture and colors
Colors observed in a liquid crystal texture may
be caused by three mechanisms.
LC with chiral molecules will form visible Bragg
reflection
Colors also can be produced by the
interferenceof transmitted polarized light when
a birefringenttexture is placed between two
polarizers.
Dye molecules dissolved in LC molecules
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Technology and application
Electro-optic effects of LC
Electro-opitic effect is a change in the optical
properties of a material in response to an
electric field.
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Technology and application
How do LCD work?
Light polariztion

• Light is made out of pariticles called photons
• While moving, a photon vibrates in a plane
  whichis perpendicular to its direction
• The vibration direction is random for normal
  light
• To polarize light, you can use a polarization
  filter
• Two polarization filters placed after each other
  along the light with their admitting direction
  perpendicular to each other will not let any
  light through

Normal light
Polarized light
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Technology and application
Twisted nematic effect (TN-effect)?
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Technology and application
Twisted nematic effect (TN-effect)?
The LC molecules are twisted in the OFF-state.
It required two light-absorbing polarizer (P1,
P2)?
Twisted LC molecules is formed between two
glassplates(G), which are separated by several
micrometer wide spacers and coated with
transparent electrodesE1, E2.
In OFF state, the polarization of an incident
linearly polarized light wave is guided by the
LC helix. The transmitted wave may therefore
pass the second, crossed polarizer, P1. Appear
transparent.
Field-OFF state
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Technology and application
Twisted nematic effect (TN-effect)?
In ON state, the electrical field forces the
longmolecular axes to align in the field
direction(perpendicular to the electrodes). The
polarization of an incident light wave is not
affected by the vertically aligned C-molecules
andtherefore it cannot pass the second
polarizer.Appear dark.
Field-ON state
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Technology and application
Twisted nematic effect (TN-effect)?
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Technology and application
Twisted nematic effect (TN-effect)?
IN 1960s, the TN effect was against thedominant
trend in the small LC communuty. Enabling
bright Polaruzer-free High contrast Easy-to-manufa
cture Cost-effective solutions
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Technology and application
Multiplexed displays
Why need multiplexed displays?
For small numbers of pixels it is relatively easy
to adjust the timing of the signals and their
relative amplitudes in order to create
differences in voltage between different pixels
in the array, resulting in some pixels being ON
and others being OFF.When a large number of
pixels are needed in a display, it is not
technically possible to drive each directly since
then each pixel would require independent
electrodes. Instead, the display is multiplexed.
What is multiplexed displays?
In a multiplexed display, electrodes on one side
of the display are grouped and wired together
(columns), and each group gets its own voltage
source. On the other side, the electrodes are
also grouped (rows), with each group getting a
voltage sink. The groups are designed so each
pixel has a unique, unshared combination of
source and sink.
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Merits and shortcomings
Merits
Thin display device Low power consumption More
vibrant images Better contrast ratios
shortcomings
LCDs can have backlight bleed where light leaks
out and turns black into gray
LCDs have longer response time the plasma and CRT
counterparts, creatingvisible ghosting when
images rapidly change.
Some LCD TVs have significant input lag due to
slow video processing.
LCD panels using TN tend to have a limited
viewing angle relative to CRT andplasma displays.
Dead pixels can occur when the screen is damaged
or pressure is put upon thescreen
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QA