The Interdigitated Electrode Array: IDA

1
The Interdigitated Electrode Array IDA
Top View
Cross Section
15?m
400Å
15?m
IR Beam

Fabrication of the IDA on IR-transparent ZnSe
windows ?Simultaneous investigation of
electrical and optical properties of material
confined within the IDA fingers.
2
ZnSe Cell Setup
Cell setup Material of interest sandwiched
between ZnSe and IDA fingers
ZnSe
IDA finger
Material Sandwiched between ZnSe and IDA fingers
3
Fabrication of IDA on ZnSe Photolithography
UV
IR Window
Spin Coat
Pattern
Photoresist
Mask
ZnSe
Photoresist
Photoresist
Develop
ZnSe
ZnSe
Unwanted photoresist and metal are removed by
sonicating in acetone
IDA Mask
ZnSe
4
IDA Cell
Metal Back-plate
Rear Gasket
IDA/ZnSe Windows
Teflon Spacer
Metal Spacer
Metal
Front-plate
Polarization of IR at normal incidence to IDA as
defined relative to electrode bands Parallel
Perpendicular
IDA Leads
BNC
connector
5
Liquid Crystal 4-pentyl-4-cyanobiphenyl 5CB
Pentyl Tail
Rigid Core
Liquid Crystal nematic 24.0-35.3
oC Anisotropic molecules (align via shearing)
No positional order- not a crystal
Preferred orientation- not an isotropic liquid
6
IR Absorbance Spectrum of 5CB
Spectral Mode Assignments for 5CB Frequency Assign
ment Frequency Assignment 2957 CH3
antisymmetric C-H stretch 2226 C º N
stretch 2926 CH2 antisymmetric C-H
stretch 1606 phenyl C-C stretch 2870 CH3
symmetric C-H stretch 1494 phenyl C-C
stretch 2857 CH2 symmetric C-H stretch
814 phenyl C-H out-of-plane deformation
T.I. Urano, H. Hamaguchi, Chem. Phys. Let. 1992,
195, 287-292.
7
Previous Studies IR Investigation of Bulk 5CB
IR Beam
Ge Window
Polarizer
Results Homogeneous to homeotropic
transition Liquid crystal reorients so the rigid
core is parallel to the applied electric field
2000V/cm required to induce reorientation of 5CB
(threshold voltage) Relaxation of the rigid
core took almost 100ms Step Scan Time Resolved
Spectroscopy (TRS) allowed real-time measurements
of the molecular reorientation
Nakano, Yokoyama, and Toriumi Appl. Spec. 1993,
47, 9, 1354-1366
8
The Freedericksz Transition
No electric field
V gt Vt
Competition between electric field alignment
and boundary alignment Above threshold
voltage, Vt, 5CB starts aligning parallel to the
electric field
9
Difference Spectrum of 5CB
10
Electric-Field Induced Reorientation of Rigid
Core 400 Å Film
Difference Peak at 2226 cm-1 2226 cm-1 is the
CN stretching mode Molecular axis reorients
parallel to the applied electric field Voltage
range 1-15 V
0.01
Increasing Voltage
?

11
Orientation of 5CB 400 Å Film
Peak Area vs. Applied Voltage
Orientation is maximum by around 15 volts
Growth in I? indicates orientation of 5CB
parallel to E-field I? remains constant,
indicating that molecules aligning with the field
are moving out of the transverse cell direction
I?
I?
12
Electric-Field Induced Reorientation of Rigid
Core 3000 Å Film
CN Stretch 2226 cm-1 Voltage range 1-15 V
0.01
?
Increasing Voltage
Absorbance
??
13
Orientation of 5CB 3000 Å Film
Peak Area vs. Applied Voltage
Orientation is maximized around 15 volts
Growth in I? indicates orientation of 5CB
parallel to E-field I? decreases, but not
commensurate with increase in I?
14
Orientation of Confined Liquid Crystal Response
to Applied Electric Field
Tilted rods represent an azimuthally averaged
orientation of tilts
15
Time-Resolved Scan (TRS) Data Collection
16
Time-Resolved Spectrum Response to Applied
E-field
17
Orientation During 50 ms Applied Pulse
Note Pulse is applied after a 25 ms settling time
18
Time-Resolved Orientation and Relaxation of 5CB
2226 cm-1 2870 cm-1 1494 cm-1
Intensity
19
Relaxation Aoff C0 exp-kt
C1
Orientation Aon C0
(1-exp-kt) C1
400 Å cell CN stretch
3000 Å cell CN stretch
20
Rate Constants for Orientation and Relaxation of
5CB
21
Rate Constants Across the CN Peak
400 Å Film
3000 Å Film
22
Bulk Nematic LC Between Crossed Polarizers
Dark director parallel or perpendicular to
linearly polarized incident light Point
defects Williams domains
23
Future Work
1. Confirm formation of thiol monolayer on
ZnSe a. XPS, IR, SIMS (preliminary work done) 2.
Patterning liquid crystals in nanoscale cavities
using SAMs a. Orientation in cavities b.
Molecular dynamics in applied E-field 3. Edge
effects decrease electrode spacing a. Initial
ordering, tilt angle b. TRS dynamic studies c.
SAM studies? d. Optical effects electrode
spacing at the mid-IR diffraction limit
24
Add XPS and IR spectra