Chapter 1 Introduction
1.9 Overview of this work
φ . Here, K11 is a splay elastic constant of the liquid crystal.
On the other hand, J. M. Geary et al. suggested that liquid crystal molecules are anchored to buffed polymer chains of the polymer surfaces. The alignment of the liquid crystal molecules follows in an epitaxial manner.[26] The reorientation of polymers which gives the anisotropic characteristics caused by rubbing have been confirmed by optical retardation, IR absorption spectra and second harmonic generation. In view of problems mentioned above, studies on both the mechanism of the rubbing process and the development of other methods for liquid crystalline alignment have been carried out. Thus, novel alignment technologies are required that not only achieve increase in production yields, but also offer additional advantages, such as the convenience of multi-domain alignment.
1.9 Overview of this work
In this work, we develop two novel alignment methods based on the micro-morphology mechanism. The first alignment method is a novel vertical alignment method which is by using the porous anodic aluminum oxide (AAO) thin film as the alignment layer. The porous AAO thin film is formatted by anodizing the aluminum thin film in the acidic electrolyte. The detailed manufacture process is described in Appendix A. The morphology of porous AAO thin film surface is a nanopores array. It offers the liquid crystal molecules an excellent vertical alignment layer. The second alignment method is a homogenous alignment method on the grooving surface by using the nanoimprinting method.
In Chapter 2 and Chapter 3, we demonstrate the strong vertical alignment of liquid crystal on porous AAO thin film and also discuss the polar anchoring strength of the AAO thin film with different pores sizes and different aspect ratio, respectively. In detail, the measuring method of the polar anchoring strength by
using the magnetic field method is described in Appendix B.
In the past few years, our group has studied the optical constants of the liquid crystals in terahertz region[27][28] and developed several terahertz devices, such as the electrically controlled quarter and full wave plate[29][30] and the magnetically controlled phase shifter[31], based on the liquid crystal devices. In order to apply the AAO thin film as an alignment layer in terahertz region, we investigate the optical properties of the AAO thin film by using the terahertz time-domain spectroscopy in Chapter 3.
In Chapter 5, we demonstrate the preliminary work about the homogenous alignment on the flexible polydimethylsiloxane (PDMS) substrate. Here we use the glass substrates with the U-shaped groove as the imprinting mode. By using the nanoimprinting technology, the U-shaped groove can be imprinted on PDMS substrate. After O2 plasma treatment, the nematic liquid crystal can be aligned homogenously on the PDMS surface and parallel to the groove direction.
In order to investigate the homogenous alignment ability in the previous chapter, the azimuthal anchoring strength is a physical parameter to indicating the ability of the alignment layer. In Chapter 6, we develop a novel method to measure the pitch of the chiral nematic liquid crystal. By using this modified pitch to calculate the azimuthal anchoring strength, the error of the azimuthal anchoring strength can be reduced and become less than 1%.
Finally, in Chapter 7, we summarize all remarkable conclusions about each novel alignment method discussed in this thesis. Furthermore, we provide some interesting research topics for future work.
References
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Figures
Figure 1-1 Scheme of different type of liquid crystal.
Figure 1-2 Characteristic of the dielectric constant of 5CB. [3]
Nematic Smectic Cholesteric
Figure 1-3 Characteristic of the ordinary refractive index, the extraordinary refractive index and the birefringence of 5CB. ● is for 589 nm, and ○ is for 632.8 nm. Tc is the cleaning temperature.[5]
Figure 1-4 Typical orientations of liquid crystal molecules in different liquid crystal displays.
Homogenous Homeotropic
Twisted nematic Hybird
Homogenous
Homogenous
Homeotropic
Homeotropic
Homogenous
Homogenous
Homogenous
Homeotropic
Figure 1-5 Chemical formulas of DMOAP and MAP and the surfaces which result from their application to a substrate.[32]
Chapter 2
Strong vertical alignment of liquid crystal on anodic aluminum oxide film
with different pore size
2.1 Overview
Anodic Alumina Oxide (AAO) is an inorganic and porous material. It is almost transparent and colorless in visible region. Because of these properties, AAO film is a good candidate as the alignment layer of liquid crystal displays.
Appendix A.2 shows how to manufacture the porous AAO thin film. The mechanism of AAO formation is also discussed in Appendix A.4. Recently, T.
Maeda and K. Hiroshima had demonstrated the vertical alignment of liquid crystal molecules on AAO films. [1][2] In their work, the depth and the diameter of the nanopores in the AAO film are 100-150nm and 5-20nm, respectively. The measured pretilt angle and the observed conoscopic pattern of the liquid crystal cells indicate that the liquid crystal molecules are aligned vertically on AAO films.
They also showed that the pretilt angle can be changed by anodizing the pre-rubbing aluminum film. In this work, we use both one-step and two-step process to produce non-uniform and uniform porous AAO thin film, respectively.
By using different anodizing voltages, we can control the pore sizes of the AAO films. We examined the relationship between the polar anchoring strength and the anodizing voltage. All these different AAO films are good vertical alignment layers. It is shown, however, that the anchoring strength depends on anodizing condition. This is due to the voltage-dependent pore sizes of the AAO films.
2.2 Experimental procedures
The porous AAO is formed by anodizing aluminum thin film or aluminum
foil in acid electrolytes. By using one-step process or two-step process, it is easy to form crack-like or hexagonal porous AAO structure. The pore sizes are controllable by varying the anodizing voltage. For the two-step process, the thickness of AAO thin film can be controlled by changing the anodizing time of the first step. The remaining curved aluminum thin film can form the uniform porous AAO thin film. The more detail manufacture description is shown in Appendix A.2.
By using the field emission scanning electron microscope (FESEM:
HITACHI S-4700i), we can observe the morphology and the nanostructure of the porous AAO thin film. All AAO thin films are evaporated thin platinum or gold layer to reduce charging effects. The thickness of AAO thin film can be measured by examining the cross-view of the AAO FESEM image. By using the image processing program, ImageJ, we can analysis the FESEM image to determine the average pore size, the average AAO wall area, and the density of pores. The software introduction and the analysis algorithm of ImageJ are shown in Appendix C.
In order to use the AAO thin film as the alignment layer, the transmittance of AAO thin films in visible region are very important. The transmittance of AAO thin films are taken by using a UV-Visible spectrometer (Oceanoptics, model ISS-UV-VIS and USB2000) with air as the reference.
Finally, we need to investigate the alignment characterization of the AAO thin film. The investigative cell is made by putting together a pair of glass substrates with the AAO film face to face. The nematic liquid crystal, 5CB (Merck) was filled into the cell in the isotropic phase (above 36°C). The thickness of liquid crystal layers is 23 μm. The liquid crystal alignment in the cell is examined with a polarizing microscope. The cell is put between a pair of crossed polarizers. By using the polarizing microscope, we can examine the transmitted image and the conoscopic image. The conoscopy is a useful optical method to analyze the alignment of the uniaxial crystals by observing the interference
image.[3]
By applying the magnetic field on the vertical alignment cell, the polar anchoring strength can be determined from the transmittance, which is a function of applied magnetic field above the threshold magnetic field. The polar anchoring strength characterizes the surface energy ability out of the substrate plane. The theory and analysis method of the polar anchoring strength are discussed in Appendix B.
2.3 Results and discussions
2.3.1 Anodizing current characterization
By using the function of the power supply (Keithley Instruments Inc. Model 2410 General-Purpose SourceMeter), we can record the anodizing current during the anodizing process. For the anodizing current characterization analysis, the thickness of the aluminum thin film is 500 nm. The anodizing current characterization is shown in Figure 2-1 and Figure 2-2 for one-step and two-step process, respectively. For both one-step and two-step process, the anodizing process with lower anodizing voltage has lower anodizing current and longer anodizing time. According to the manual of 2410 SourceMeter, the maximum power is 22W. The range of operation is 21V at 1.05A or 1100V at 21mA.[4]
Because the range of operation is 21V for 20V, the capability of anodizing current can be 1.05A. When the anodizing voltage is larger than 20V, the range of operation is 1100V, the capability of anodizing current can only be 21mA. In Figure 2-1 and Figure 2-2, it is obviously that the anodizing current is saturated at 21mA for V≥50V. In Figure 2-2, the dash and solid line are the anodizing current characterization of the first and second step process of two-step process, respectively.
For more investigations, we can integrate the anodizing current by the anodizing time. The integrated result is the total charge of the anodizing process.
Table 2-1 shows the total charge of the one-step anodizing process for different anodizing voltage. It clearly shows the total charge is around 6 Coulomb regardless the anodizing voltage. According to the mechanism, the total charge is proportional to the thickness and the anodized area of the aluminum thin film.
Table 2-2 shows the charge of first step and second step in the two-step process for different anodizing voltage. The total charges are also around 6 Coulomb for different anodizing voltage, even the first step or the second step is different anodizing duration.
2.3.2 Morphology of the anodic aluminum oxide surface
Figure 2-3 shows the FESEM image of the AAO film formed by using the one-step process. There were only fine crack-like structures connecting irregular small pores on the surface of the AAO film. When the anodized voltage was varied from 20 to 70V, the widths of the pores remained almost the same, about 5-15nm. By using the two-step process, the surface of the AAO film had regular pores as shown in Figure 2-4. These pores were self-assembled into hexagons.
According to cross-sectional observation by the FESEM, the actual thicknesses of AAO films prepared by the one-step process were 415±15 nm, while those by the two-step process were 210±30nm. In Figure 2-5, we show the relationship between the diameter of pores and the anodizing voltage. The pore size is determined by measuring some pores in the SEM image directly. The data dots in Figure 2-5 are the average size of these pores and the error bars are the standard deviation of these pores.
The diameters of pores for the two-step process increased linearly from 15 to 50nm as the anodizing voltage was increased from 20V to 50V. For V > 50V, the diameter of pore was saturated at a value around 65nm. The saturated values for pore diameters were observed for both cases, nonetheless. According to the anodizing current characterizations, we note that the anodizing current was saturated at 21mA, the current limit of the power supply, when V > 50V. If the
anodizing current is saturated at the current limitation, the power supply will operate in constant current mode, and the anodizing voltage will be limited by the maximum current, 21mA for Model 2410, when V > 50V. In contrast, the diameters of pores for AAO prepared by the one-step process were small and did not exhibit apparent trend when the anodizing voltage was varied in this range.
2.3.3 Transmittance of the anodic aluminum oxide layer
Figure 2-6 shows the transmittance of the AAO films on the ITO glass substrate as a function of wavelength from 300 to 800 nm. The cut off wavelength at 350nm is due to absorption of the ITO glass substrate. For the AAO films anodized at a bias voltage from 20V to 40V, the transmittance is about 65% over this spectral range. In comparison, the transmittance of the substrates with the ITO thin film on the back side is around 80%. For AAO films anodized at a bias voltage from 50V to 70V, the transmittance reduces to about 55%. Ripples in the spectral transmittance for the AAO films, for which the thickness varies from 300 to 500 nm, are attributed to the interference effects of the films. The spectral transmittance of the substrate is relatively smooth, because the ITO-film is very thin, 50-100 nm in thickness. Our data showed that the AAO film is highly transparent in the visible region and is a very good candidate of alignment layer in LCD applications.
2.3.4 Alignment characterization
In this section, the alignment characterizations are investigated by using the liquid crystal cell. By putting together a pair of glass substrates with the AAO alignment layer face to face, the cell with cell gap of 23μm was made. The nematic liquid crystal, 5CB (Merck) was filled into the cell in the isotropic phase (above 36°C). The liquid crystal alignment in the cell was examined with a polarizing microscope in microscopic mode and conoscopic mode.
In Figure 2-7, we show the polarizing microscopic images of the liquid crystal cells with the AAO alignment thin film manufactured by using one-step process. The liquid crystal cells are observed in a pair of crossed polarizers. The micrographs are taken in two orientations of the cell, 45° with respect to each other. Regardless of the anodizing voltages, all of the liquid crystal cells show the dark state in both 0° and 45°. The dark state observed for both cases indicates that vertical alignment of liquid crystal was achieved. Figure 2-8 shows the polarizing microscopic images of the liquid crystal cells with the AAO alignment thin film manufactured by using two-step process. No matter what the anodizing voltage is, all of these images in Figure 2-8 are the dark state. That is mean all of the AAO thin film with two-step process are good vertical alignment.
Figure 2-9 shows the conoscopic image of the same liquid crystal cells with both one-step and two-step AAO alignment layers. It obviously shows that all conoscopic images are the cross texture. The cross texture also shows that the liquid crystal cell was vertically aligned.[3] Further, in order to confirm the vertical alignment, the pretilt angles of the liquid crystal cells were measured by using the crystal rotation method [5]. Figure 2-10 shows the pretilt angle of the liquid crystal cell with the AAO alignment layer. For all of these cells with anodizing voltage between 20 and 70V, the pretilt angles were between 89.5° and 90.0°. There is no significant difference between the samples prepared with one-step or two-step processes.
2.3.5 Polar anchoring strength analysis
According to the previous section, we already knew that the AAO thin film can vertically align nematic liquid crystal. In order to know the alignment ability of the AAO thin film and compare to the other traditional vertical alignment layer, N, N-dimethyl-N-octadecyl-3-amino-propyl-trimeth oxysilyl chloride (DMOAP) , we made a few liquid crystal cells with the same AAO alignment layer, and
measured the polar anchoring strength of these samples by using the magnetic field method [6]. The polar anchoring strengths of the liquid crystal cells are plotted versus the anodizing voltages from 20V to 60V as shown in Figure 2-10.
The data dots in Figure 2-11 are the average anchoring strength, and the error is the standard deviation. For AAO films anodized at different voltages, the
The data dots in Figure 2-11 are the average anchoring strength, and the error is the standard deviation. For AAO films anodized at different voltages, the