Transcript Liquid Crystals

Liquid Crystals & LCDs
They’re all around you
What are liquid crystals used for?
• Special types of liquid crystals, called lyotropic
liquid crystals, are used in biology for cell
membranes
• Liquid crystal’s main commercial use, though, is
for creating Liquid Crystal Displays – LCDs.
You may know them as TFT monitors, or simply
flatscreen monitors.
LCDs
• This presentation
will briefly
investigate how
LCDs operate with
liquid crystals
What are liquid crystals?
• Liquid crystals are often described at the 4th
state of matter between liquids and solids.
• They are closer to liquids than solids, as less
energy is required to change liquid crystals to
liquids than solids to liquid crystals.
• Thermotropic liquid crystals can behave like
solids at cold temperatures and behave like
liquids at warmer temperatures
Liquid crystal molecules
• Liquid crystal molecules are long and thin with a
rigid centre. They come in varying
arrangements, which are essential to their
functions.
The director
• Liquid crystals all have one thing in common: all
the molecules in their natural liquid crystal state
tend to point in one direction. This direction is
called the director. The amount the molecules
are oriented in this direction is referred to as the
liquid crystal’s orientational order
The 3 main types of liquid crystal
• Nematic – has orientational order
• Smectic – has orientational order & varying
degrees of positional order
We will focus on:• Twisted Nematic – layers of nematic liquid
crystal molecules
Twisted Nematic molecules
• Each 2-dimensional layer has
nematic liquid crystals (LCs)
aligned roughly with the director.
• However, each layer rotates the
director slightly as shown, so you
get an altering helical director
through the liquid crystal. The
distance for 1 entire director
rotation is the pitch.
• This is why these LCs are called
twisted.
Director alignment
• LC molecules have an intrinsic dipole
• Should we apply an electric field to the
molecule, the dipole will align along the field
• In this way, we are able to alter the director to
point in a specific direction
What happens in Twisted Nematics
• In Twisted Nematic LCs, this has the logical
effect of aligning all the directors in all the
layers, if the electric field is strong enough.
• RULE 1: The Twisted Nematic LC untwists
when electricity is applied.
• On its own, maybe this isn’t very useful.
However, LCs have another useful property to
do with optics.
Birefringence
• At a basic level, a refraction index simply
relates to the speed a wave will travel through a
substance in comparison to travelling without
impedance through nothing (a vacuum)
• Liquid crystals are birefringent
• This means they have two separate refraction
indices – one perpendicular and one parallel to
the director
Twisted Nematics and Light
• All EM waves are composed of perpendicular
electric and magnetic waves
• When circularly polarised light enters a TN LC,
the two components can be said to be left and
right circularly polarised; both components
rotate circularly in opposite directions.
Twisted Nematics and Light
• The two components will experience different
refraction indices, because both the two indices
of refraction are perpendicular to one another
and the two components of the wave are
perpendicular to one another.
• Therefore, by the very definition of refraction
indices, one wave will travel faster than the
other through the TN LC.
Twisted Nematics and Light
• When the components emerge, their sum gives
the final wave. As one of the components got
ahead of the other, the final wave will have
rotated through an angle.
• RULE 2: TN LCs rotate light through an
angle, directly depending on the depth of
the LC the light passes through
LCD Construction
• LCDs use rules 1 and 2 to operate.
• LCDs have two polarization plates – one at the
front, one at the rear. They are at 90° to each
other. There is a mirror behind the rear
polarization plate.
TN LCs in LCDs
• It should be apparent that, with both
polarization plates perpendicular, no light will
pass.
• TN LCs are then placed in between the two
plates. They are attached to both plates by
surface anchoring so they can’t move. The TN
LCs will be twisted by 90° (1/4 pitch) between
the two plates.
TN LCs in LCDs
• Linearly polarized light coming in from the first
polarization filter is a case of circularly polarized
light.
• The TN LCs will therefore turn this light by an
angle of 90°, so it passes through the second
polarization filter too. It is then reflected off of
the back mirror to make its way back through
the filters and into the eyes of the user, making
this LCD natively light instead of dark.
Adding the Electronics
• A conductive material is spread all over the
back polarization plate between the plate and
the TN LC.
• Two electronic contacts are placed on the front
polarization plate between the plate and the TN
LC.
• When an electric field is applied, the TN LC
untwists. Light is no longer twisted through 90°
and gets blocked by the second filter.
Putting it into Context
• If we repeat this situation over and over, we can
get a grid of TN LCs and electric contacts,
making up a screen, like that of a digital watch
or calculator.
• By altering the size and shape of the electronic
contacts, and using a circuit to control when the
LCs receive electricity through the contacts
(direct addressing), things like 7-segment
displays can be created.
7 Segment Displays
From elzet80.com
Modern LCD Screens
• LCD screens have too many picture elements
(pixels) – the small dots that make up an image
– to have them all controlled directly by wire
contacts.
• Modern LCD screens use a grid of wires which
connect to transistors at each and every pixel.
• The transistors act as switches, turning power
on and off to the TN LC below it.
Adding Colour
• Creating colour displays is quite easy. Colour
filters are put onto the front polarization filter
over the top of every filter.
• Three colour filters are used: red, green and
blue. It is by combining these colours in various
configurations that millions of unique colours
can be created for the user to see on their
screen.
LCDs versus CRT monitors
• CRT (Cathode Ray Tube) monitors are the
large, bulky, old-fashioned monitors that fire
photons at the screen. There are advantages
and disadvantages of using CRTs.
• Advantages of CRTs: price, colour range, no
dead pixels, no native resolution
• Advantages of LCDs: size, weight, power
consumption, no flicker, price (for passive
matrices).
Alternatives to Liquid Crystals
• Plasma displays – expensive, worse quality,
high power consumption, not as thin as LCDs.
• LED (dot matrix) displays – awful quality,
hardwired colours, unrealistic proposal for
monitors.
• OLED (Organic Light Emitting Diode) displays:
high quality, cheap, versatile, good power
efficiency. However, it’s an emerging
technology.