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