Download presentation
Presentation is loading. Please wait.
1
Surface Anchoring
2
Goals Surface Alignment Homeotropic Alignment Homogeneous Alignment
Weak Anchoring Deposition Methods Non-Rubbed Methods
3
Surface Anchoring There is really no rule of thumb when it
comes to surfaces ! It is a very empirical area
4
Surface: Microscope Slide
Courtesy of the NSF ALCOM Center: Presented at 1992 ALCOM Symposium by H. Vithana and D. Johnson
5
Surface: Optically Flat Glass
Courtesy of the NSF ALCOM Center: Presented at 1992 ALCOM Symposium by H. Vithana and D. Johnson
6
Surface: ITO coated display glass
Courtesy of the NSF ALCOM Center: Presented at 1992 ALCOM Symposium by H. Vithana and D. Johnson
7
Surface Anchoring Alignment at surfaces propagates over macroscopic distances microgrooved surface - homogeneous alignment (//) rubbed polyimide ensemble of chains - homeotropic alignment () surfactant or silane
8
Homeotropic Anchoring
Varying the pretilt angle for homeotropic anchoring n n=16 Aliphatic Chain Tilt Angle from Length (n=) Normal (qo) o o o o 9 22o 8 30o 7 55o 6 70o n=7 N 55o n
9
Surface: Silane Surfactant on Glass
Courtesy of the NSF ALCOM Center: Presented at 1992 ALCOM Symposium by H. Vithana and D. Johnson
10
Surface: DMOAP on ITO Courtesy of the NSF ALCOM Center:
Presented at 1992 ALCOM Symposium by H. Vithana and D. Johnson
11
Surface: Lecithin Surfactant on Glass
Courtesy of the NSF ALCOM Center: Presented at 1992 ALCOM Symposium by H. Vithana and D. Johnson
12
Surface Rubbing Technique
Glass plate coated with an organic layer Cloth
13
Surface Rubbing Technique: Groovy
director field profile to ridges z Scenario #1: to ridges Scenario #2: // to ridges
14
Surface Rubbing Technique: Groovy
Shape of profile Minimum elastic energy achieved under this condition Solution to above equation
15
Surface Rubbing Technique: Groovy
16
Surface Anchoring q f N polar anchoring Wq n surface azimuthal
Wf Wq,f is energy needed to move director n from its easy axis Strong anchoring J/m2 Weak anchoring J/m2
17
Surface Anchoring Measurement
VSAT Threshold Condition Voltage Intensity VTH time Saturation Condition
18
Effects of Anchoring Energy
Consider electrically controllable birefringence Weak anchoring results in: Lower threshold Rise time decreases Fall time increases Parameter Threshold Fall time Rise time Strong Anchoring Weak Anchoring
19
Weak Anchoring Energy Example:
Threshold d=5 mm, K=10-11 N, W=5x10-6 J/m2, De=10
20
Weak Anchoring Energy Example:
Fall Time d=5 mm, K=10-11 N, W=5x10-6 J/m2, De=10
21
Weak Anchoring Energy Example:
Rise Time d=5 mm, K=10-11 N, W=5x10-6 J/m2, De=10
22
Tilt Angle Measurement
photodiode with I/U converter HeNe - laser pinhole pinhole polarizer polarizer rotation stage with sample holder Q Intensity qo Io q1 I1 q2 I2 qn In . . IEEE-bus rotation stage controller digital multimeter
23
Tilt Angle: Crystal Rotation Method
Z j d a Z y x
24
Example Tilt Angle Transmission -60 -17 30 Angle of incidence y
25
Why Study Tilt Angle E Tilt angle stops reverse tilt disinclination
Simplest picture - disclination E Can influence your contrast Simple ECB Cell where qo is the pretilt angle
26
Surface Rubbing Technique
Rotation speed e.g. 405 mm/sec Rubbing Cloth Rotor Substrate Stage Moving speed e.g. 6 mm/sec
27
Surface Rubbing Technique: Rubbing Length
The rubbing length L is the total length of the rubbing fiber in contact with a certain point of the substrate N = 2 (number of rubbings) L = 11 mm (contact length); pile impression = 0.3mm r = 50 mm (radius of the rubbing wheel) n = 190 rpm (rotation speed) V = 25 mm/s (speed of the glass plate) L Rubbing length
28
Surface Rubbing Technique:
Tilt Angle vs. Rubbing Length 8 Example plot to demonstrate general dependence Licralign SN (STN LCD) 6 Tilt Angle (degrees) 4 Licralign 4200 (for AM LCD) 2 1 2 3 4 5 6 7 8 (mm) Rubbing Length L 103
29
Surface Rubbing Technique
4 Increasing pressure
30
Surface: Unrubbed Poyimide on
ITO Display Glass Courtesy of the NSF ALCOM Center: Presented at 1992 ALCOM Symposium by H. Vithana and D. Johnson
31
Surface: Rubbed Polyimde on ITO
Display Glass Courtesy of the NSF ALCOM Center: Presented at 1992 ALCOM Symposium by H. Vithana and D. Johnson
32
SiO Deposition Technique
33
Surface: SiOx at 60o on Glass
Courtesy of the NSF ALCOM Center: Presented at 1992 ALCOM Symposium by H. Vithana and D. Johnson
34
Surface: SiOx at 70o on Glass
Courtesy of the NSF ALCOM Center: Presented at 1992 ALCOM Symposium by H. Vithana and D. Johnson
35
Surface: SiOx at 80o on Glass
Courtesy of the NSF ALCOM Center: Presented at 1992 ALCOM Symposium by H. Vithana and D. Johnson
36
( )n ( )n ( )n Non-Rubbed Alignment O O UV O O O
Schadt (Jap. J. App. Phys. 31, 2155 (1992)) showed that Poly Vinyl 4-MethoxyCinnamate that has been exposed polymerized with polarized UV light, will align liquid crystals perpendicular to the UV polarization axis O ( )n ( )n O UV O O O ( )n 2+2 cycloaddition of cinnomoyle groups of the cinnamic acid side chains
37
Non-Rubbed Alignment: Top View
UV LC Alignment
38
Non-Rubbed Alignment: Cis-Trans
Gibbons et al, Nature Vol. 351, page 49 (1991).
39
Summary Surface Alignment Homeotropic Alignment Homogeneous Alignment Weak Anchoring Deposition Methods Non-Rubbed Methods
Similar presentations
