2 Sample Preparation and Electrical Characterization of SSFLC
2.2 Preparation of SSFLC Cells with and without nc-ZnO
The surface-stabilized ferroelectric liquid crystal samples used in this study were prepared with a procedure to be detailed in this section.
1. Substrate Cleaning: Two kinds of substrates were used to prepare SSFLC test cells. The ITO-coated fused silica substrates were used for optical measurements in the spectral range from UV to visible. For IR spectroscopic studies, we select ITO-coated CaF2 substrates. The substrates were cleaned with the following steps:
1. Soak the substrates in a diluted detergent aqueous solution for 15 min.
2. Wash the soaked substrates with the diluted detergent solution, then rinse with deionized (DI) water.
3. Put the washed substrates in a fresh detergent aqueous solution and ultrasonic treatment for 15 min.
4. Rinse every substrate with DI water, and then ultrasonic the substrates in clean DI water for 5 min.
5. Repeat the step 4.
6. Put as treated substrates into acetone and ultrasonic vibration treatment for 15 min.
7. Rinse the substrates with clean DI water. Put them into a fresh DI water bath to perform ultrasonic vibration treatment for 15 min. Repeat the step for two times.
8. Rinse the substrates with DI water and purge with nitrogen gas to dry the substrates.
9. Put the substrates into clean oven to bake at 100oC for 1hr.
2. Preparation of LC Alignment Layer: The selection of alignment material depends on the desired parameters of LC test cell such as pretilt angle, anchoring
energy, and residue charge on the surface of alignment layer. In this thesis study, a polyimide RN1182 from Nissan Chemical was used. We applied a rubbing process to produce the easy axis for LC alignment on RN1182-coated ITO substrates. The preparations of alignment layers are described in the following:
1. Wait for the temperature of RN1182 and its diluting solvent to warm up to room temperature. Dilute the RN1182 with the solvent to a concentration of 25% in weight.
2. Stir the diluted RN1182 solution for 2 hours at room temperature.
3. Put droplets of diluted solution of RN1182 on substrate surface. The amount of the solution deposited depends on the size of substrate.
4. Spread the alignment layer on the substrate with a spin coater. The spinning rate should be optimized based on the viscosity of the spreading solution. The spinning rate used for RN1182 is 300 rpm for 10 sec and then 3000 rpm for another 30 sec.
5. Soft bake the coated substrates in an oven at 80oC for 5 min after the spin coating procedure is completed.
6. Hard bake the coated substrates at 250oC for another 60 min.
7. Rub the coated substrates with a rubbing machine. The distance between the roller and the substrate is about 0.5 mm. Set the rotation speed of the roller at 1000 rpm and the translation speed of the substrates about 5 cm/min. Check the rotation direction of the roller to ensure sufficient relative velocity existing between roller and substrate.
3. FLC Preparation: To prepare FLC with an appropriate doping of nc-ZnO, the ferroelectric liquid crystal material FELIX -017/100 mixture from Clariant was used.
The phase transition sequences are I-N at 86oC-83oC, N-SA at 77oC, SA- SC at 73oC, S -X at -28oC. Two different procedures were used in order to find out the best
preparation approach of nc-ZnO-doped FLC:
A. Mix nc-ZnO Powder with FLC
1. Take a suitable amount of pure FLC, and then add several milligrams of nc-ZnO powder to the desired weight percentage. In this study, FELIX -017/100 doped with nc-ZnO powder to a weight percentage of 1.06% was used.
2. Put the mixture into an ultrasonic water bath at about 85oC to keep the FLC in the isotropic phase and performed ultrasonic treatment for 40 min.
3. Put the mixture in vacuum and wait for its temperature return to room temperature.
B. Mix FLC with nc-ZnO Ethanol Solution
1. Take an appropriate amount of pure FLC. Add several milligrams of TPM-caped nc-ZnO ethanol solution (what is the concentration of nc-ZnO in the ethanol solution) to a desired weight percentage. In this study, FLC FELIX -017/100 doped with nc-ZnO to a weight percentage of 1.086% was used.
2. Put the mixture into an ultrasonic water bath at about 85oC to keep the FLC in the isotropic phase and performed ultrasonic treatment for 5 min. Wait for the mixture returns to room temperature.
3. Put the mixture in vacuum line and purge it with nitrogen gas for 8 hours to drive the remaining ethanol out of the mixture.
4. Test Cell Assembly: The gap of the test cells used is controlled by silica balls with a diameter of 2 μm. The assembly of a test cell and liquid crystal filling are described as follows:
1. The silica balls were dispersed into a UV-curable gel NOA65 with a volume fraction of silica balls about 5% in the mixture.
2. Coat the spacer solution along the two parallel rims of one substrate.
3. Assemble the substrates into a test cell with a desired rubbing geometry. Adjust the cell gap to achieve the best gap uniformity.
4. Cure the UV gel by exposing the test cell with UV light.
5. Put the empty cell on a hot plate with a temperature of 87oC to keep the FLC used in the isotropic phase.
6. Put several droplets of FLC (or FLC doped with nc-ZnO) on the opening side of the empty test cell. Wait the test cell filled with FLC (nc-ZnO doped FLC) via capillary force.
7. Cool down the filled cell to room temperature slowly and then seal the opening sides of the cell with a quick-dry sealant.
8. Carefully remove the polyimide layer on the cell substrates to expose the ITO film and put on conducting cupper pads. Attach thin cupper wires to the conducting copper pads.