Chapter 2 Overview of Cholesteric Liquid Crystals
2.6 Summary
In this chapter, the basic optical properties are introduced and discussed for CLCs. It is known that different kind of nematic host is needed different operating method to drive it.
Then, the statements focus on the gray scale property and color shift for CLCs because it’s the major framework in the experiments. After that, in order to resolve the problem of color shift, the method of doping negative nematic material is proposed according to the Meyer’s
explanation. It can let the blue shift to be effectively minimized when the CLCs cells are driving.
Chapter 3
Measurement Systems
3.1 Introduction
In this chapter, the measurement setups used in the experiments were described in the following section. The ε of each sample is measured by Liquid Crystal Analysis System 3 (LCAS 3). The cell gap of the empty cell and the spectra of the reflective light intensity and color shift can be measured by using a spectrometer (Perkin Elmer Lambda 950) and the incident light is normal to the sample. The samples were driven by a waveform generator (WFG 600) under 1KHz square wave and parallel to the helix axis.
3.2 Spectrometer I. Introduction
The UV-VIS spectrometer. LAMBDA 950 (Perkin Elmer Lambda 950) is high performance between 185 nm and 3300 nm with the resolution ≦ 0.17 nm, and the spectrometer principles are double-beam, double mono-chromator, ratio recording spectrophotometers is as shown in Fig 3.1.
Fig. 3.1. The photograph of the spectrometer
The absorption, reflectance and transmittance of materials characterized with the LAMBDA 950 allows easy access to a wide variety of sampling accessories.
II. The Method for Cell Gap Measurement
For LC display, the thickness of a cell gap usually affects the optical properties. The cell gap of a LC display determines to a great extent the LCD’s electro-optical properties such as the contrast ratio, brightness, response time, and switching voltage, etc. It’s important to control the cell gap during the manufacturing process.
The concept of the measurement method is based on the interference pattern of the light reflected by a layer with two reflecting surfaces [26]. The situation diagram is shown in Fig. 3.2. It is defined that the coefficient of the reflection R1 as the ratio of the light reflected by surface 1 to the total incident light on surface 1. R2 is defined as the reflection coefficient of surface 2.
Fig. 3.2 Two reflecting surfaces separated by a layer causing light interference.
The dotted line indicates the first internal reflection [20]
If the total incident light is Icost and assume no absorption of light in surfaces 1 and 2,
1
2
1
2 cos
4 /
The periodic term in Eq. (3.3) causes an interference pattern.The periodicity of the reflected interference spectrum determines the optical thickness of the cell gap, ngapdgap.
If the 1 and 2 are two wavelengths showing extrema in Eq. (3.3), then
gap gapd
n . The sample data was shown in Fig.3.3 for 8.4um cell gap.
Fig.3.3. Example of a measurement
color shift can not be expressed perfectly by the reflection spectrum with the light source, the color shift is expressed by E*ab. The color shift, E*ab, is determined by the spectrometer.The color shift was measured in the CIE1976 LAB color space with standard D65 incident light in normal direction., because the lightness and chromaticity coordinates can represent the psychophysically perceived color shift [27]. The L*, a*, the b* are defined by
0
Here, X, Y, Z are the object goal tristimulus values, and X0, Y0, Z0 are the tristimulus value of the completely diffusely reflective surface. The L*, a*, and b* coordinates are used to construct a Cartesian color space as illustrated in Fig. 3.4.
Fig. 3.4. Three-dimensional representation of the CIELAB L*, a*, and b*
coordinates
The chromaticity coordinates difference, ab , is defined by:
(3.9)
The color difference, E*ab , is defined by:
(3.10)
Where L*, a*, and b* are the parameters of color differences when the CLCs light
3.3 Liquid Crystal Analysis System 3
The Liquid Crystal Analysis System 3 (LCAS 3) is a fully automated system, consisting of proprietary hardware and software, for measuring the physical parameters of ferroelectric and nematic liquid crystal materials. In this thesis, the major purpose is the dielectric anisotropic measured for the different CLC’s nematic host is as shown in Fig 3.4.
Fig. 3.5. The photograph of the Liquid Crystal Analysis System
The LC cell can be considered a parallel-plate capacitor and follow the capacitor equation in the electromagnetic as defined:
d
C S
. (3.10)where S is the area of the two parallel conducting plates separated by a uniform distance d , and is the dielectric constant. In a nematic single crystal, the static dielectric constants ||
and are existed simultaneously. The LCAS 3 system will measure the capacitor value of
the cell and calculate the dielectric constant (|| and ) value of the material. The capacitance subtracted from the results, and the LCAS 3 will yield incorrect results.
(b)For measuring the parallel dielectric constant, the LCAS 3 will measure the C||
automatically. According to the Eq. (3.10), the value is determined as:
capacitance. It can also obtain the perpendicular dielectric constant ||
using the same
perpendicular dielectric constant have no units.
(c) The dielectric anisotropy is the difference between the parallel and the perpendicular dielectric constants as given by:
|| . (3.13)
where it is possible that the value of the dielectric anisotropy is positive or negative. It depends on the character of the LC material.
The LCAS 3 system is useful for the LC technologies. It can measure almost kinds of parameters for LC material, such as specific resistivity, threshold voltage, dielectric anisotropy, elastic constant, voltage holding ratio, and rotational viscosity, etc.
Chapter 4
Experiments and Results
4.1 Introduction
Cholesteric Liquid Crystals (CLCs) have the severe color shift (blue shift)
phenomenon which depends on the applied voltage. It causes the selective reflection peak to shift to the shorter wavelength and the reflection color is turned. The display images will be distorted and it’s quality become much badly. This phenomenon is a serious hindrance to its potential display applications. In this study, the host dielectric anisotropic (ε) and
birefringence (n) were fine-tuned by mixing negative and positive nematic LCs based on the Helfrich deformation. The blue shift has been modeled by Meyer. The experimental results show that the blue shift can be effectively minimized.
As mentioned in the section 2.5, the will become higher gradually with doping VA material. This method helps to increase the stability of helical axis when the CLCs cells are driving. Different mixing ratio of CLC’s host is discussed in this chapter and the best dopant ratio will be chosen to the balance of the gray scale property and the low color shift character.
4.2 Display Cell preparation
The CLCs null cells are prepared by our-self in the clean room. We start to fabricate normal steps of the CLCs cell process. The detail steps to produce the prototype are listed below.
Fig. 4.1. CLCs cell processes for our experiments
(1) At first the ITO substrates were shocked 5 minute with detergent in the ultrasonic vibration equipment. Then, the substrates were rubbed with detergent by hands carefully, and then the substrates were washed with DI water until the water flowing along the surface smoothly. All cleaned substrates were put into a holder into DI water and shaken 30 minute by the ultrasonic. Then, the ITO substrates were blew with nitrogen gas to remove the DI water and then baked for 30 minutes at 110oC.
(2) The substrates were explored by UV-ozone for 20 minutes before coating the PI alignment layer for better adhesion of alignment layer. Put the clean ITO substrate into the
spin coater. Drop the solution of alignment material and wetting for 30 second (make sure the solution to cover the whole substrate), then spin. The parameters of spin-coating are shown as Table 4.1.
Table 4.1. parameters for spin-coating of alignment meterial
When the polyimide is coated, the substrates are needed to bake for 1 hour at 200oC. The polyimide (PIA-X201-G01) is from Chisso. According to the spin rate, the thickness of 50wt% polyimide is about 350Å .
(3) Using the rubbing machine to rub the substrates. Because of the material of alignment layer is the same to every substrate, the rubbing strength will not change to get the parameters.
The rubbing strength is different depending on the LC material, alignment layer, and pretilt angle. The rubbing condition are shown as Table 4.2.
Table 4.2. Rubbing condition for PI alignment layer
PI alignment layer
the top plate and press it. The cell was assembled such that the rubbing directions of the layers were anti-parallel. Next, because the UV glue is sensitive to the UV light, take the cell under a UV lamp for 5 minutes to cure the glues, and the empty cells are finished.
(5) The cell gap of the empty cells is measured by spectrometer (Perkin Elmer Lambda 950) using interferometric method. The details are explained in chapter 3.2.
(6) Heat up the CLCs material to isotropic and inject it from the edge of the cell by using capillarity until the CLCs is full of the gap. Then annealing and cooling the CLCs cells continuously in order to let the alignment of molecules much better.
(7) Soldering the wire at the ITO contact on the CLCs cells, and then the cells are available for measuring the electro-optical properties.
4. 3 Materials and CLCs Mixtures
Seven different nematic hosts from Merck were prepared to study the color shift in CLCs: pure LXX-06 1153, LXX-06 1153-60 mixtures, pure MLC 1744, MLC 1744-55 mixtures, MLC2048, BL036, and negative nematic LC (MJ-041937). The chiral dopant, DBD ( (s)-dioctan-2-yl biphenyl-4, 4'-dicarboxylate) (Fig 4.2), were mixed with nematic hosts to have a selective reflection at 555±5 nm.
COOC*H(CH3)C6H13 (s)+C6H13C*H(CH3)OOC
Fig. 4.2. Chemical structure of chiral dopants
4.4 Properties of the Cholesteric Liquid Crystal Hosts
The Δn of MLC 2048, LXX-06 1153, MLC 1744, and BL036 are 0.2214, 0.1621, 0.0905, and 0.267, respectively. As a result, the reflection bandwidths are 62, 48, 30, and 72 nm, respectively. The narrower the reflective bandwidth, the higher the color saturation. The Δε of aforementioned LCs are 2.82, 11.1, 7.8 and 16.4, respectively. The ratio of VA in the LXX-06 1153-60 and MLC 1744-55 mixtures were 60wt% and 55wt%, respectively. They were prepared specially for Δε closed to 1 for evaluating the blue shift suppression [28, 29, 30] . And the ratio of MLC 2048, LXX-06 1153, LXX-06 1153-60, MLC 1744, MLC 1744-55 and BL036 for the selective reflection at 555±5 nm are 14.73, 15, 14.91, 14.48, 15.23 and 16.91wt%, respectively. The properties of nematic hosts properties were listed in Table 4.3.
Table 4.3. Properties of the synthesized cholesteric liquid crystals.
* Measured at 589 nm
Different values of ε show the different degrees of tilt or unwinding to the CLCs molecules when applying operating field. The different properties for each CLCs cells will be
4.5. Measurement form Spectrometer I. Introduction
Spectrometer is usually used for CLCs displays measurement. It can cover all visible light wavelengths and help us to gauge the various properties in CLCs cells. combining with the waveform generator in order to observe the different characteristics for each CLC cell when applied electric field. The results will be discussed.
II. Observation from Spectrometer
The reflectance spectrums are measured in order to see the color shift and the gray scale properties for different mixing ratio of CLCs cells. The samples were driven by a waveform generator (WFG 600) under 1KHz square wave and parallel to the helix axis. The refractive oil (n=1.56) is used here to adjust the inner reflection which comes from the refractive index mismatch of CLCs and ITO glass. The results are shown in Fig. 4.3. and Fig.
4.4.
Fig. 4.3. Reflective spectra for 1153 mixtures for 555nm green
∆n=0.1621
∆ε=11.1
∆n=0.1213
∆ε=1.57
Colors start to be shift to blue and reflectance reduces severely after 8.6V and 20.8V in 1153 and 1153-60 mixtures, respectively. The blue shift is suppressed in 1153-60 is shown in Fig. 4.3. The blue shift is suppressed in order to avoid effect in the Δn for the Δn is not the same, the 1744 mixtures substitute.
Fig. 4.4. Reflective spectra for 1744 mixtures for 555nm green
Colors start to be shift to blue and reflectance reduces severely after 9.4V and 20.8V in 1744 and 1744-55, respectively. The blue shift is also suppressed when the Δn are the same in 0.09.
∆n=0.0905
∆ε=7.8
∆n=0.0903
∆ε=1.3
4.6 Color Characterization
The CIE 1976 (L* a* b*) color space, abbreviated CIELAB, is used here to help us to calculate the quantities of color shift under electric field. It would be the reference color when the cells are in 0V and calculate the color shift E*ab. Fig. 4.5(a)-(c) show the changes of L, ab and E*ab under applied voltage. There are three distinguishable stages in Fig.
4.5(a), (b), and (c). The intensity of CLCs’ selective reflection drops quickly when the applied voltage exceeds the threshold voltage, where the CLC’s plannar structure converts into focal conic state. The light quantity, Y, has strong influence on L*. The change of CLCs selective reflection intensity is not significant in the later focal conic state. The chromaticity coordinates (a*, b*), instead, are strongly altered with increased voltage. The cell reaches the homeotropic state when the voltage is further increased. Take 1744-55 as an example: 0-20V is the planar texture, 20-50V is the focal conic texture, and 50-60V is the homeotropic texture.
The E*ab has large differences between plannar to homeotropic state for 1744 and 1744-55 mixtures. When the E*ab is increased to 14 for 1744and 1744-55 mixtures, the driving voltage need to be increased 7.3V and 25V, respectively are shown in Fig. 4.5. It is known that the color shift is resolved obviously by doping negative nematic LC to lower the value of
ε. The results of color difference E*ab are improved as shown in Fig. 4.5.
On the other hand, the E*ab is affected by light quantity. And light quantity is associated with reflective bandwidth (λ) in CLCs. And reflective bandwidth is proportional to birefringence (n). The values of n are 0.2214 and 0.0903 for 2048 and 1744-55, and the values of E*ab are decreased from 66 to 40.9 are shown in Tab. 4.5.
(a)
(b)
(c)
Table 4.4. The Color difference’s value for each CLC samples
Although the blue shift phenomenon is inevitable in CLC displays, this study shows that low ∆n CLC mixture, 1153-60, 1744, and 1744-55 mixtures have smaller E*ab, and the low ε CLC mixture , 1153-60, 1744-55,and 2048 mixtures can suppress blue shift. The improvement is verified by using the perceptually uniform CIELAB metric.
4.7 Summary
If the ChLCD is only driven between planar and focal conic textures to produce
“dark” and “bright” states, then an ideal material should have a large L and a large E*ab
when driven by a low voltage such as BL036. In addition, to produce smooth grayscales within the focal conic texture, an ideal material should have mimimal L and ab variation to maintain its minimal E*ab variation to avoid color shift. Therefore, according to the results, the 1744-55 stands out because of its low L for brighter image and E*ab for less color shift.
As a result, the best dopant ratio will be chosen to the balance of the gray scale property and the low color shift character.
Chapter 5 Conclusions
5.1 Summary
The ChLCD is suitable for large-sized low-power, bi-stable applications, but its color shift is not acceptable when color accuracy is demanded. Based on Meyer’s explanations, we proposed doping negative nematic material into positive nematic host in CLCs cells appropriately to cope with voltage-dependent color shift. We synthesized different materials and found the best one with minimal color shift when CLCs cells were driving. This study showed that low ∆n CLC mixture, 1153-60, 1744, and 1744-55 mixtures have smaller E*ab, and the low ε CLC mixture , 1153-60, 1744-55, and 2048 mixtures can suppress blue shift.
The improvement is verified by using the perceptually uniform CIELAB metric.
5.2 Future Works
Although the color shift is lowed successfully in our experiments, but the high Voltage and low reflective intensity are big problem. If the CLCs materials are wanted to use for display applications. It must find and try to get this ratio depends on the industrial requirements when the dielectric anisotropy ε and ∆n is lower.
The color shift phenomenon can also be used for others applications. It can get a multicolor display which combines with the photo-curing and electrically induced color shift [31] [32]. But the issue is the reflective intensity decreased seriously when the color shift is occurred.
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