Lecture handout including QS - Department of Materials Science ...

BH14 Course B: Materials for Devices BH14
In another example of a chiral molecule, the steepness of the twist decreases (i.e. the pitch increases)
as m increases, and the chiral centre (the asymmetric, or ‘handed’ part of the molecule) moves away
from the mesogenic core (the ‘stiff’ part of the molecule):
mesogenic core
chiral centre
i.e. a long molecule will be ‘less’ asymmetric, have little angular twist with its neighbours, &
therefore have a long pitch.
- a shorter molecule → strongly asymmetric → steeper twist (larger angle between neighbours), &
Liquid Crystal Displays
hence a short pitch.
The director of a nematic LC can be encouraged to lie along a particular direction by creating grooves
on a surface in contact with it (this is generally done with an aligned polymer coating; but in one of
the samples in BP1 parallel scratches were made with fine emery paper). The LC molecules will tend
to lie along the length of the grooves. If the LC is sandwiched between 2 glass plates, with grooves at
90° to each other, the molecules (and hence the director) will twist across the sandwich, resulting in a
precisely defined twisted nematic structure:
Viewed from the top
In order to achieve this a small amount of chiral nematic dopant is generally added to the LC.
Polarized light entering the bottom of the
sandwich has its plane of polarization twisted
through 90° as it reaches the top. If the
polarizer and analyzer are parallel to the
grooves at the bottom and the top, respectively,
light will be transmitted through the LC cell:
the ON state.