Chapter 1 Introduction and Motivation
1.4 Motivation
When the CLCs cells are wanted to control the intensity of reflective light, it must be applied the field across the cells. It causes to the pitch contraction and the perturbations in the helix if the field parallel to the helical axis. Thus, the color of the reflective light would be changed and shifted to the shorter wavelengths. In order to solve this phenomenon, a study of molecular behaviors of CLCs is important for us to discover more information.
For the reason, the main objective of this thesis is to understand how the external field affects the helical axis, and propose a method based on these effects to enhance the stability of
Off state Glass
Alignment Polymer
On state Mirror
Incident Scattering Incident
Output
the helical axis. We infer the dielectric constant and the susceptibility act important parameters to influence the CLC’s behavior. Besides, a discussion of the results will be given in this thesis.
1.5 Organization of Thesis
In this thesis, the contents would be presented as following: the basic optical properties and the theories of field-induced wavelength shift for CLCs display are introduced in chapter 2. The measurement instruments which are used in experiments are described in chapter 3. The experiment and cell processes, results, characteristics measurements and performance evaluation including color shift, dielectric anisotropic, and spectrums that measured by spectrometer are in Chapter 4. Finally, the thesis concludes in Chapter 5 with a summary of the results and the future work for continued research on this subject.
Chapter 2
Overview of Cholesteric Liquid Crystals
2.1 Introduction
Cholesteric liquid crystals (CLCs) have a helical structure and the liquid crystal director axes twists around this helical axis as shown in Fig. 1.2. The unique optical properties of the cholesteric phase are recognized by both Reinitzer and Lehmann at the time of their early investigations which culminated in the discovery of the liquid crystalline state. CLCs possess a supermolecular periodic helical structure due to the chirality of molecules. The helical pitch of cholesteric can be the same order of magnitude as the wavelength of visible light. Thus, the Bragg reflection occurs. In this chapter, we consider the general optical properties of CLCs, such as the influences of chiral material, Bragg diffraction due to the periodical helical structure, and induced circular dichroism. The influences of external field are also discussed briefly in order to understand the phenomenon of color change effect.
2.2 Effect of Chiral Material
Chirality may cause an intrinsic helical structure of the director field as shown in Fig.
2.1. Instead of the uniform alignment of the director field occurring in the nematic phase, the respective chiral nematic phase exhibits a helical structure such like CLC. In the preparation of a CLCs mixture, the helical pitch is controlled by the concentration of the chiral material and the helical twisting power (HTP) of the chiral dopant. The HTP is defined as:
c P HTP
= 1 ⋅
, (2.1)
where c is the concentration of the chiral dopant in wt%, and P is helical pitch of the CLCs
mixture in [μm]. Both the magnitude and sign of HTP are meaningful. It is customary to assign a positive HTP and a negative HTP to materials that exhibit the right-handed helix, and the left-handed helix, respectively.
Fig. 2.1. Helical structures formed in chiral liquid crystal. (a) N* is the cholesteric phase, (b) BP is the blue phase, (c) SmC* is the chiral smectic-C, and (d) TGBA is the twist grain boundary phase [4].
2.3 Optical Properties of Cholesteric Liquid Crystals 2.3.1 Textures in Cholesteric Liquid Crystals
Several types of electro-optic effects have been observed in CLCs which depending on the surface treatment (boundary conditions), the helical pitch P, the thickness to pitch ratio d/P, the dielectric anisotropy Δε , the external field, and the frequency of the applied field.
The CLCs textures influence the electro-optical behaviors when the textures are changed. The typical cholesteric textures for the orientation of the liquid crystal are shown in Fig. 2.2. They are named the planar (Grandjean), focal conic, fingerprint or amorphous, and the isotropic (homeotropic), respectively.
a b c d e
Fig. 2.2. Textures in CLCs: (a) planar or Grandjean, (b) fingerprint, (c) focal conic, (d) isotropic or homeotropic, and (e) amorphous cholesteric texture [4].
In order to study the electro-optic effects, the CLCs are usually sandwiched between two parallel plates with transparent ITO electrodes. Many optical properties of CLCs are based on the different types of texture. It is a key helping for us to understand the applications of the CLC displays.
2.3.2 Bragg Reflection
When the CLCs are in the planar texture, the circularly polarized light is reflected by Bragg theory because of their chiral and periodic structure. Selective reflection occurs when the white light is incident on a cholesteric cell. In various cholesteric systems, the period of the helical pitch varies by a wide range (about 0.1μm to several hundred μm). For the long pitch (low concentration chirality) P >>λ (where λ is the wavelength of light), the light propagation parallel to the helical axis may be described by a superposition of two eigenwaves having electric field vectors parallel and perpendicular to the director. The long pitch case was studied for the first time by C. Mauguin [5] [6].
For the short pitch (high concentration chirality), when λ and P are comparable, the eigenwaves become elliptical and circular in the limiting case. It is well known that the selection reflection occurs in the limiting case due to the Bragg diffraction at a wavelength λB :
ave
where m is the diffraction order, θ is angle of light incidence, and nave is the average refractive index of the CLC medium. There are some characteristics and restrictions for the light propagating along the helical axis as following:
Only the first order Bragg reflection is possible in this case. This is confirmed by experimental results and theoretical considerations [7]. According to Eq. (2.2), the maximum selective reflection occurs at the wavelength λB =naveP when the angle of incident light is zero. The spectral bandwidth of the selective reflection is Δλ=P⋅Δn, where Δn=ne−no is the birefringence of the nematic materials. Figure 2.3 presents the typical reflection spectrum for CLCs. The reflected and transmitted light is circularly polarized. It is noted for the reflected light that the sign of rotation coincides is the same with the sign of rotation of the cholesteric helix.
Fig. 2.3. Reflection spectrum is in the planar texture for CLCs.
2.3.3 Optical properties in different textures
CLCs are easy to change its texture by adding electric or magnetic field. When an electric field applied to the CLCs cell, a texture transition occurs to minimize the free energy system. The texture transition is strongly associated with alignment layer, dielectric
0
anisotropicΔε , field amplitude, and the frequency of the external field. The generally operating modes are described as following:
(i) When the dielectric anisotropyΔε>0:
Fig. 2.4. Schematic diagram for different states of the CLCs which dielectric anisotropy is positive. (a) Planar texture. (b) Focal conic texture. (c) Homeotropic texture.
When the CLCs are in the planar texture at zero field, the helical axis is perpendicular to the cell surface as shown in Fig. 2.4(a). It reflects the selective reflection light by Bragg diffraction. The texture would be switched into the focal conic texture when applied electric field V>=Vth. In the focal conic texture, the helical axis is more or less parallel to the surface as shown in Fig. 2.4(b). Incident light is diffracted or scattered in the forward direction and the material in this state has less reflective color appearance. It is noticed that CLCs exhibit two stable states at zero field. One of them is the planar state, and the other one is the focal conic state. So the CLCs material has the bistable property that is an advantage for display technology.
(ii) When the dielectric anisotropyΔε<0:
(a) Planar texture Incident light
(b) Focal conic texture
(c) Homeotropic texture
Focal conic texture Planar texture
Fig. 2.5. Schematic diagram of the states of the CLCs which the dielectric anisotropy is negative. (a) Initial texture in the planar state. (b) Initial texture in the focal conic state. (c) Planar texture. (d) Homeotropic texture.
No matter the CLCs cells are the in planar or focal conic state firstly as shown in Fig.
2.5(a) and (b), they have the same operating features when applied electric field. When an electric field E applied parallel to the axis h of the helix, it is a stable configuration and is shown in Fig. 2.5(c). In this instance the field only induces stabilization of the fluctuations. As a consequence, the order parameter is increased and displacement of the selective reflection maximum in the longwave spectral region (red shift) is observed.
Another situation is the electric field applied perpendicular to the axis of the helix as shown in Fig. 2.5(d). This situation was investigated experimentally and theoretically when the applied voltage V>>Vth [8]. The helix deformations and the threshold field are described the by the same expression as in the case of CLCs with dielectric anisotropyΔε >0, E || h,
~
~ Initial state
E
E
(a) (b)
(c) (d)
(defined below): threshold voltage that the CLCs switch from the planar texture to focal conic texture.
2.3.4 Gray Scale property of Cholesteric Liquid crystals
CLCs exhibit gray scale property because of their multi-domain structure if the planar texture and the focal conic texture are appeared at the same time. Starting with the imperfect planar texture, there are some domains can be switched to the focal conic texture when the threshold field is exceeded. The planar texture will break up into small domains and the incident is scattered [9] [10] as shown in Fig. 2.6. The reflective color will be decreased. Once a domain has been switched to the focal conic texture, it remains there even after the applied voltage is turned off because of the bistable property. The diagram of the gray scale states of a CLCs display is shown in Fig. 2.7. From right to left, the states are achieved by applying voltage pulses with increasing amplitude, and the reflectance. The domain is around 10 μm when CLCs are in the multi-domain and the domain structure cannot be observed by the naked eye. The typical pixel size for CLCs display is about 200 μm. It is misleading to call the CLCs multi-stable.
Fig. 2.6. The POM’s photograph of the planar texture is transferred to the focal conic texture when applied electric field.
Fig. 2.7. The diagram of the gray scale states of the CLCs displays [3].
2.4 Reflective Color for CLCs 2.4.1 Controlling Cholesteric Color
In the preceding section, the theories of reflective color for CLCs are discussed generally. The cholesteric color is determined by temperature, electric field, chiral doping ratio, etc. However, if the color is easily changed by the environment, it is not suitable for CLCs to be a communication of information [11].
The pitch length depends on the environment temperature because of the helical twisting power will be changed. It may cause the pitch length to become longer or shorter and
Planar texture
Oily streak and focal conic texture
Increasing voltage Focal conic texture with
minimum reflectance
Planar texture with minimum reflectance
lead the cholesteric reflected color to be different [12] [13]. Therefore, it is important that the CLCs material is not sensitive to the temperature and the color can be well controlled and fixed if it wants to use on the display applications.
The color change of the electric fields to CLCs was proposed by Harper, Hansen, and Schneeberger [14] [15]. It is also called the ETC (Electrically Tunable Color) [16]. In 1968, de Gennes [17] and Meyer [18] proposed that an electric or magnetic field perpendicular to the helical axis of a CLCs would elongate the pitch length and thereby modifying the reflected color according to the Bragg’s Law [19]. It is observed that the color change from blue to red (redshift) with increasing electric field which applied normal to the helical axis. In contrast, the effects observed from Harper change from red to blue (blueshift) with increasing electric field which applied parallel to the helical axis. The helical axis will be disturbed or unwind and resulting in an aligned homeotropic configuration finally.
2.4.2 Blueshift
When the CLCs displays are drove, the blueshift is always occurred if the electric field parallels to the helix. It is defined for the short pitch, a shift of the selective reflection peak to shorter wavelengths can be observed. It is assumed that above a threshold field, a conical deformation of the planar texture leads to a contraction of the pitch and thus central wavelength λB is shifted to shorter area. However, it is shown that the blueshift of the selection reflection results from a helix distortion of the texture greater than from a pitch contraction. It causes a larger angle with the incident light and the blueshift can be observed given by Eq. (2.2). In this thesis, the main purpose is to resolve the blueshift when the CLCs cells are driving. This problem will be improved from the arguments originally advanced by Meyer.
2.5 Proposed Method to Resolve Blueshift 2.5.1 Effects of Electric Field on CLCs
Electric field acts on the anisotropic of the electric susceptibility exert torques within a LC which may compete with the elastic torques determining its internal structure. R. B.
Meyer was proposed [18] [20] [21] and use the Frank’s theory of curvature elasticity in molecularly uniaxial LC is used and calculates the effects of external field on the helical structure of cholesteric materials.
Frank’s theory assumes that in cholesteric materials only one molecular axis is aligned.
They are equivalent to nematic materials on a small scale. A unit vector L is defined which is parallel to the axis of molecular alignment at any point. Frank derives an expression for the elastic free energy density in a cholesteric substance:
∫
∇ + ⋅∇× + + ×∇× uniform helix which minimizes the elastic free energy. An L defines the helix in a right-handed (x, y, z) coordinate system isL=(cosq0z,sinq0z,0), (2.5)
Since there is local cylindrical symmetry about L, the electric susceptibility has only two components, χ|| and χ⊥, parallel and perpendicular to L, respectively. Assuming that the system is neither ferroelectric nor ferromagnetic, the field (F) energy density is given by
( ( ) ( ) )
The general procedure for calculating the effect of the field is to find the structure, characterized by L as a function of position in the sample, which minimizes the total free energy.
G=
∫
sample(gE+gF)dV , (2.7)The field dependence of the structure is in general a perturbation of the helix at low fields, followed by complete breakdown of the helix at some critical field.
Without following this general procedure, in simple situations, it can guess at the form of the perturbation in material far from surfaces. For example, consider the uniform helix of Eq. (2.5), and apply a F =Fx. Assuming that Lz remains 0, the general form of the perturbed helix is
L=(cosθ,sinθ,0),θ =tz+ f(z), (2.8)
in which f(0)=0, and f has period π/q. By varying the integral G, one can find the different equation for f. Solve it, one can then find the q which minimizes G for a given value of the field. The pitch of the helix can often be observed as the color of light reflected by the helical structure. The result in this case, to lowest order approximation, is
] boundary condition will maintain the axis of the helix parallel to the field. Then a likely form
of perturbation, if χ|| >χ⊥, is to rotate L at every point through an angle ρ toward the field;
L=(cosρcosqz,cosρsinqz,sinρ), (2.10)
In this case, the perturbation involves both bending and torsional strains, and depends on the relative magnitudes of the moduli of these strain, k33 and k22, respectively. If k33 ≧ k22,
For k33 ＞ k22, the breakdown will involve hysteresis around this field value, since high energy configurations exist between the unperturbed helix and the breakdown configuration. For k33 ＜ k22, there is a range of field for which ρ and q change continuously:
Below this range, there is no perturbation, and above it there is complete breakdown.
2.5.2 Propose Method to Improvement Blueshift
It has also been observed that in some cases, application of a large dc electric field perpendicular to a thin film of cholesteric substance in the planar texture, rather than changing the wavelength of light reflected by the helical structure, increases its intensity greatly.
This observation can be explained by the present theory by assuming thatχ⊥ >χ||. It makes the unperturbed uniform helix of Eq. (2.5), with F = Fz, the lowest energy structure for the system. Since the planar texture already satisfies field merely eliminates existing perturbations in the helical axis, making it more perfect, and thus intensifying the reflected light.
According to the above explanations, it must find a method to let the χ⊥ >χ|| in CLC’s nematic host and the reflective color will be uniform when CLCs are driving. Here we use the relative permittivity to substitution the electric susceptibility is given by:
e
r χ
ε = 1+ , (2.14)
It uses the Eq. (2.14) to substitute the Eq. (2.11) and the critical field becomes proportional to the (1 /ε|| −ε⊥)1/2 if the influences of the elastic modulus of κ22 is neglected.
The diagram of electric susceptibility for LC molecular is shown in Fig. 2.8. It is obviously that whenε⊥ >ε||, the blue shift is avoided by applying the electric field which is parallel to the helical axis. According to the above explanations to lower the blueshift in CLCs, a negative dielectric anisotropic (Δε = ε||−ε⊥ <0) nematic LC material would be doped appropriately into the positive nematic host in CLCs. We infer that it can increase the stability for the pitch and make the color more uniform when the electric field is applied.
Fig. 2.8. The diagram of electric susceptibility for LC molecular.
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 methods to drive it.
Then, the statements focus on the gray scale property and color shift for CLCs because it’s the major framework in this experiments. After that, in order to resolve the issue of blueshift, the method of doping negative nematic material is proposed according to the Meyer’s explanation.
It can make the reflective color more uniform when the CLCs cells are driving.
χ⊥
χ||
z
Chapter 3
Measurement Systems
3.1 Introduction
In this chapter, the measurement systems and the cell fabrication process are described.
We use the hydrochloric acid to etch the ITO substrates to enhance the stability when the cell is applied electric field. The spectrometer is used to measure the cell gap and the spectrums of transmission light intensity of the cell. The ConoScope is used to measure the chromaticity and the tristimulus values of the cell.
3.2 Spectrometer 3.2.1 Introduction
The UV-VIS spectrometer is as shown in Fig 3.1. LAMBDA 650 PerKin Elmer is high performance between 190 nm and 900 nm with the resolution ≦ 0.17 nm, and the spectrometer principles are double-beam, double mono-chromator, ratio recording spectrophotometers.
Fig. 3.1. The photograph of the spectrometer.
The absorption, reflectance and transmittance of materials characterized with the LAMBDA 650, and the large sample compartment allows easy access to a wide variety of sampling accessories.
3.2.2 The Method for Cell Gap Measurement
For LC display technologies, the distance of cell gap usually affects the optical properties. The cell gap of a LC display determines to a great extent the electro-optical properties such as the contrast ratio, response time, and switching voltage, etc. It’s important to control the cell gap during the manufacturing process.
The basic concept of the measurement method is based on the interference pattern of the light reflected by a layer with two reflecting surfaces [22]. The situation diagram is shown in Fig. 3.1. 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 the light interference. The dotted line indicates the first internal reflection [16].
If the total incident light is I =cosωt and assume no absorption of light in surfaces 1 and 2, the total reflected light R can be wrote as