Chapter 2 Strong vertical alignment of liquid crystal on anodic aluminum oxide
2.4 Summaries
We investigated the alignment properties of AAO films prepared by two types of anodizing processes. In contrast to those anodized by the one-step process, the AAO films prepared by the two-step process exhibit regular hexagonal pore arrays. The diameter of pores of AAO films can be controlled easily by adjusting the anodizing voltage. For the two-step process, the diameter of pore is controlled from 17nm to 65nm. Using the nanoporous AAO film as the
alignment layer, excellent vertical alignment for the LC cell can be achieved.
The pretilt angle of the LC cell by using AAO film as an alignment layer is very close to 90°. The polar anchoring strength for this LC cell is about 15×10-6 J/m2 which is just a little weaker than DMOAP, commonly used vertical alignment layer. The AAO films, on the other hand, can be used without rubbing and easily scalable for industrial applications. Our studies further indicates that AAO films prepared by the one-step process, even though do not exhibit uniform array of pore, are as effective in alignment as those prepared by the two-step process. We also demonstrated that AAO films with smaller pore diameters exhibit higher anchoring strengths.
References
[1] T. Maeda and K. Hiroshima, “Vertically aligned nematic liquid crystal on anodic porous alumina,” Jpn. J. Appl. Phys. 43, L1004 (2004).
[2] T. Maeda and K. Hiroshima, “Tilted liquid crystal alignment on asymmertrically grooved porous alumina film,” Jpn. J. Appl. Phys. 44, L845 (2005).
[3] B. L. Van Horn, and H. H. Winter, “Analysis of the conoscopic measurement for uniaxial liquid-crystal tilt angles,” Applied Optics 40, 2089 (2001).
[4] Keithley Model 2400 Series SourceMeter® User’s Manual, Chapter 3 Basic Souce-Measure Operation, 3-3.
[5] T. J. Scheffer, and J. Nehring,” Accurate determination of liquid-crystal tilt bias angles,” J. Appl. Phys. 48, 1783 (1977).
[6] K. H. Yang, and C. Rosenblatt, “Determination of the anisotropic potential at the nematic liquid crystal-to-wall interface,” Appl. Phys. Lett. 43, 62 (1983).
Tables
Anodizing voltage Total charge
20 V 5.75 C
30 V 6.22 C
40 V 5.96 C
50 V 5.82 C
60 V 5.94 C
70 V 5.85 C
Table 2-1 The total charge of the one-step anodizing process for different anodizing voltage.
Anodizing voltage 1st step charge 2nd step charge Total charge
20 V 3.84 C 2.55 C 6.39 C
30 V 4.00 C 1.92 C 5.92 C
40 V 3.92 C 1.85 C 5.77 C
50 V 3.76 C 2.26 C 6.02 C
60 V 3.08 C 2.93 C 6.01 C
70 V 3.14 C 2.89 C 6.03 C
Table 2-2 The total charge of the two-step anodizing process for different anodizing voltage.
Figures
0 500 1000 1500 2000
0 5 10 15 20 25
Anodizing current (mA)
Time (s)
20V 1820s 30V 970s 40V 510s 50V 320s 60V 295s 70V 290s
Figure 2-1 The anodizing current characterization of the one-step process in different anodizing voltages.
0 500 1000 1500 2000
0 5 10 15 20 25
Anodizing current (mA)
Anodizing time (s)
20V 30V 40V 50V 60V 70V
Figure 2-2 The anodizing current characterization of the two-step process in different anodizing voltages. The dash and solid line are anodizing current characterization of the first and second step process, respectively.
Figure 2-3 FESEM images of AAO thin film by one-step process. The anodizing voltage is 20-70V.
40V 50V
20V 30V
Figure 2-3 (cont’d) FESEM images of AAO thin film by one-step process. The anodizing voltage is 20-70V.
60V 70V
Figure 2-4 FESEM images of AAO thin film by two-step process. The anodizing voltage is 20-70V.
20V 30V
40V 50V
Figure 2-4 (cont’d) FESEM images of AAO thin film by two-step process. The anodizing voltage is 20-70V.
60V 70V
20 30 40 50 60 70 0
10 20 30 40 50 60 70
80 1-Step
2-Step
The diameter of pore (nm)
Anodizing Voltage (V)
Figure 2-5 The relationship between the pore diameter and the anodizing voltage.
300 400 500 600 700 800
0 20 40 60 80 100
Transmittance (%)
Wavelength (nm)
substrate 20V 30V 40V 50V 60V 70V
Figure 2-6 Transmittance of and the substrates with AAO films with anodized at different voltages.
Anodizing
voltage 0° 45°
1 step 20 V
1 step 30 V
1 step 40 V
1 step 50 V
1 step 60 V
1 step 70 V
Figure 2-7 Polarizing microscopic images of the liquid crystal cells with the AAO thin film manufactured by using one-step process.
P A P
A
P A
P A P
A
P A P
A
P A
P A
P A
P A
P A
Anodizing
voltage 0° 45°
2 step 20 V
2 step 30 V
2 step 40 V
2 step 50 V
2 step 60 V
2 step 70 V
Figure 2-8 Polarizing microscopic images of the liquid crystal cells with the AAO thin film manufactured by using two-step process.
P A P
A
P A
P A P
A
P A P
A
P A
P A
P A
P A
P A
Anodizing
voltage One-step Two-step
20 V
30 V
40 V
50 V
60 V
70 V
Figure 2-9 Conoscopic images of the liquid crystal cell with the AAO thin film manufactured by using one-step and two-step process.
20 30 40 50 60 70 88
89 90 91
92 1-step
2-step
Pretilt angle (deg.)
Anodizing voltage (V)
Figure 2-10 Pretilt angles of the liquid crystal cell with the AAO thin film manufactured by using one-step and two-step process with different anodizing voltage.
20 30 40 50 60 0
5 10 15 20 25 30
Polar anchoring strength
(
10-6 J/m2 )
Anodizing voltage (V)
one-step process two-step process
Figure 2-11 The relationship between the polar anchoring strength and the anodizing voltage.
10 20 30 40 50 60 70
0 5 10 15 20 25 30
Polar anchoring strength
(
10-6 J/m2 )
The diameter of pore (nm)
one-step process two-step process
Figure 2-12 The relationship between the polar anchoring strength and the diameter of pore.
Figure 2-13 Scheme of the possible alignment mechanism.
Smaller pore sizes larger pore sizes
Porous AAO thin film Liquid crystal
molecules
Chapter 3
The alignment properties of liquid crystal on anodic aluminum oxide film
with different aspect ratio
3.1 Overview
In Chapter 2, we have demonstrated the strong vertical alignment of liquid crystal on Anodic Alumina Oxide (AAO) thin film with different pores size. By using the porous AAO film as the alignment layer, excellent vertical alignment for the LC cell can be achieved. In order to investigate the alignment mechanism, there are more variables discussed in the following chapter.
3.2 Experimental procedures
In this chapter, we only use the one-step porous AAO thin film. The detail manufacture description is also shown in Appendix A.2. Figure 3-1 shows the scheme of the experimental procedure for the etched AAO thin film with different etching time. Here we only consider 40V one-step process case, and fix the anodizing voltage. After the one-step process finished, the substrate with the one-step AAO thin film was immersed in a mixture of chromic acid (1.5 wt%
H2CrO4) and phosphoric acid (6 wt% H3PO4) at 60°C. It is the same etching solution used in the two-step process. Here, we control the etching time, between 1.5 minutes and 6.0 minutes. The etching process not only removes the formed AAO from top to bottom, but also etches the wall of the AAO cylinder. By this process, we can reduce the thickness of the AAO thin film and expand the diameter of the pores of AAO thin film.
To investigate the alignment properties of liquid crystal, the morphology and the nanostructure of the porous AAO thin film, the transmittance of AAO thin
films in visible region, the alignment characterization of the AAO thin film, and the polar anchoring strength are measured for the AAO thin film with different aspect ratio. All of these measurements are taken by the same equipments and the same method as those described in Section 2.2.
3.3 Results and discussions
3.3.1 Morphology of the anodic aluminum oxide surface
Figure 3-2 shows the FESEM image of the etched AAO thin film with different etching time. The top view images are shown in left column, and the side view images are shown in right column. When the etching time is 0 minute, the process is just a one-step process, and the FESEM image also shows the fine crack-like structures connecting irregular small pores. After immersing the one-step AAO thin film in the etching solution, the nanoporous structure is appeared in FESEM image. Figure 3-3 shows the relationship between the diameters of the pores and the etching time. The pore sizes of the first and second manufacture AAO thin films are consistent. If the etching time is less than 3.0 minute, the pore sizes are almost proportional to etching time. When the AAO thin films immerse in the acid solution longer, the pore sizes become larger. That is because the AAO wall between two nanopores can be etched by the acid mixture. If the etching time is longer than 3.0 minute, the diameters of pores slightly decrease from 92nm to 81nm. At 3.0 minute, the FESEM image shows a different image. There are some clusters of nanorods with some hexagonal curve on the bottom of these clusters. The walls of AAO are etched too much and collapse because of the thinner walls. According to the side view image, it is easy to measure the thickness of the etched AAO thin film. Figure 3-4 shows the thickness of the etched AAO thin film with different etching time. Between 1.5 and 3.0 minute, the thickness remains around 450 nm. The thickness has a sudden drop at 3.5 minute. When the etching time is longer than 3.5 minute, the thickness
becomes less than 50 nm. According to the FESEM image for longer than 3.5 minute, most of the porous AAO arrays collapse and just remain the hexagonal curve surface. These rough surfaces contain thousand of nanotips, instead of the hexagonal pores array.
3.3.2 Transmittance of the anodic aluminum oxide layer
Figure 3-5 shows the transmittance of the AAO films on the ITO glass substrate as a function of wavelength from 300 to 1000 nm. The cut off wavelength at 350nm is due to absorption of the ITO glass substrate. For all etched AAO films with different etching time, the transmittance is about 65%
over this spectral range. In comparison, the transmittance of the substrates with ITO thin film on the back side is around 80%. The ripples in the spectral transmittance for the AAO films with the etching time between 0.0 and 3.0 minute, for which the thickness are around 450 nm, are attributed to the interference effects of the films. Here the transmittance of the etched AAO thin film shows the same behavior. It is highly transparent in the visible region.
3.3.3 Alignment characterization
The alignment characterizations can be investigated by using the liquid crystal cell. The manufacture process and the structure of the test liquid crystal cell are the same as those in section 2.3.4. The test liquid crystal cell is a sandwiched cell with the etched AAO thin film as the alignment layer. The cell is filled into the nematic liquid crystal, 5CB (from Merck co.), 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 the nematic phase (room temperature, around 25°C).
In Figure 3-6, we show the polarizing microscopic images of the liquid crystal cells with the etched AAO alignment thin film. 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. According to the discussion of the alignment characteristic in section 2.3.4, the AAO thin film can provide the vertical alignment for the nematic liquid crystal. 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 3-6 shows only three cells, 2.0 min., 2.5 min. and 3.0 min., have the dark state in both 0° and 45°. It means only the AAO thin films manufacturing by these three conditions can perform as the vertical alignment layers. If the etching time is 0 min., 3.5 min., 4.0 min., 5.0 min., and 6.0 min., the polarizing microscopic images show the random textures that mean the alignment of liquid crystal molecules is random.
When the etching time is 1.5 min., the polarizing images show a dark state in 0° and a white state in 45°. It shows the homogenous alignment characteristic. It is different from the other cells. According to the FESEM image of this condition, it still has the hexagonal pore array. This performance is unusual. In order to investigate this unusual behavior, the conoscopic image is taken. Figure 3-7 shows the conoscopic image of the same liquid crystal cells with the etched AAO alignment layers. It shows that the conoscopic images of the 2.5 min., 3.0 min., and 3.5 min., are the cross texture. The cross or parabolic textures show that the liquid crystal cell was vertically or homogenously aligned, respectively. The other images do not show any regular pattern. For 1.5 min., it is neither the cross texture nor the parabolic texture. Therefore, the alignment mechanism for 1.5 min. is not clear, yet.
3.3.4 Polar anchoring strength analysis
According to the previous section, we already knew that the etched AAO thin film with the etching time from 2.0 min. to 3.0 min. can vertically align nematic liquid crystal. By measuring the polar anchoring strength, the alignment ability of the etched AAO thin film can be characterized. The polar anchoring
strength is measured by using the magnetic field method.[2] Figure 3-8 show the polar anchoring strengths of the liquid crystal cell cells are plotted versus the etching time from 2.0 min. to 3.0 min.. The data dots in Figure 3-8 are the average anchoring strength, and the error is the standard deviation. The different data dots with the same etching time are the polar anchoring strength in different liquid crystal cell with the AAO alignment layer with the same etching time. It clearly shows the etched AAO thin films with shorter etching time have stronger polar anchoring strength. Table 3-1 summarizes the diameter of pores, the thickness of the etched AAO thin film, and the aspect ratio with different etching time. The aspect ratio is the thickness of the etched AAO thin film divided by the diameter of pores. According to the relationship between the etching time and the aspect ratio, shown in Table 3-1, it is easy to show the relationship between the polar anchoring strength and the aspect ratio in Figure 3-9. This relationship is almost linear. The etched AAO thin film with higher aspect ratio has higher polar anchoring strength.
3.3.5 Possible alignment mechanism
In Section 2.3.6, the possible alignment mechanism of AAO thin film has been discussed. For further investigation in this chapter, both homogenous and homeotropic alignment are observed in the liquid crystal cell with AAO alignment layers. The alignment ability does not only depend on the pore sizes but also on the aspect ratio. The AAO thin films with higher aspect ratio have stronger polar anchoring strength. Here, the modified possible mechanism of liquid crystal alignment is discussed as follows. Figure 3-10 shows the scheme of the modified possible alignment mechanism. The AAO thin films with smaller pore sizes and higher aspect ratio have stronger polar anchoring strength.
The aspect ratio R is defined as thickness/diameter, l 2r . Considering a perfect cylinder, the total area of the wall is 2πrl, where r and l are the
radius and depth of the cylinder, respectively. The total area can be rewritten as R
r2
4π . Assuming the pore sizes are constant, the porous AAO thin films with higher aspect ratio have larger surface area at out-of-plane direction. The liquid crystal molecules are affected by the AAO surface at not only in-plane but also out-of-plane direction. Therefore, the alignments of liquid crystal molecules depend on the ratio of the in-plane area to out-of-plane area of the porous AAO surface. The surface property of the AAO material is dominated the alignment property, homogenous alignment. The porous structures modify the alignment ability. For porous AAO thin films which contain high aspect ratio pores array, the area at out-of-plane direction is much larger than that at in-plane direction.
The liquid crystal molecules are aligned homeotropically by the out-of plane surfaces. Resulting in the AAO thin films are as the vertical alignment layer. It is the most possible alignment mechanism of liquid crystal on the porous AAO thin film.
3.4 Summaries
We investigated the alignment properties of the etched AAO films prepared by etching the one-step AAO thin film with different etching time. Because the etching process is perpendicular to the AAO surface, both the thickness and the pores diameter of the etched AAO thin film can be modified, and the aspect ratio is controllable. Both homeotropic and homogenous alignment properties are observed on the etched AAO thin films. When the etched walls of the AAO pores are too thin, the wall will collapse, and the hexagonal pores array will be removed. The hexagonal curve surface can not perform as the alignment layer.
According to our experiments, the AAO thin film with higher aspect ratio has higher polar anchoring strength. The further research about the mechanism of the alignment on the etched AAO thin film is under processed. We need more evidences to understand the alignment properties and the alignment mechanism
of the AAO thin film.
References
[1] T. Maeda and K. Hiroshima, “Vertically aligned nematic liquid crystal on anodic porous alumina,” Jpn. J. Appl. Phys. 43, L1004 (2004).
[2] K. H. Yang, and C. Rosenblatt, “Determination of the anisotropic potential at the nematic liquid crystal-to-wall interface,” Appl. Phys. Lett. 43, 62 (1983).
Tables
Etching time (min.)
diameter (nm)
thickness (nm)
aspect ratio
0.0 13 657 50.54
1.5 60 403 6.72
2.0 65 456 7.02
2.5 84 415 4.94
3.0 92 435 4.73
3.5 88 48 0.55
4.0 80 42 0.53
5.0 83 37 0.45
6.0 85 48 0.56
Table 3-1 The diameter of pores, the thickness of the etched AAO thin film, and the aspect ratio with different etching time.
Figures
Figure 3-1 Scheme of the experimental procedures for the etched AAO thin films with different etching time.
Longer etching time
Shorter etching time Glass
Al
Only 1st anodization
Glass Al2O3
Al2O3
Glass Al2O3
Glass
Figure 3-2 FESEM images of the one-step AAO thin film with different etching time. The images in left column are the top view of these thin films. The images in the right column are the side view of these thin films.
0 min
1.5 min
2.0 min 2.0 min
1.5 min
Figure 3-2 (cont’d) FESEM images of the one-step AAO thin film with different etching time. The images in left column are the top view of these thin films. The images in the right column are the side view of these thin films.
2.5 min
3.0 min
3.5 min 3.5 min
3.0 min 2.5 min
Figure 3-2 (cont’d) FESEM images of the one-step AAO thin film with different etching time. The images in left column are the top view of these thin films. The images in the right column are the side view of these thin films.
4.0 min
5.0 min
6.0 min 6.0 min
5.0 min 4.0 min
0 1 2 3 4 5 6 7 0
10 20 30 40 50 60 70 80 90 100
1st manufacture 2nd manufacture
The diameter of pores (nm)
Etching time (min.)
Figure 3-3 The relationship between the diameter of pores and the etching time.
0 1 2 3 4 5 6 7
0 100 200 300 400 500 600 700 800
Thickness of the etched AAO film (nm)
Etching time (min.)
Figure 3-4 Thickness of the etched AAO thin films with different etching time.
200 400 600 800 1000 0
10 20 30 40 50 60 70 80 90 100
Transmittance (%)
Wavelength (nm)
1.5 min.
2.0 min.
2.5 min.
3.0 min.
3.5 min.
4.0 min.
5.0 min.
6.0 min.
only ITO 0 min.
Figure 3-5 Transmittance of the ITO substrates with the etched AAO films with different etching time.
Etching
time 0° 45°
0 min
1.5 min
2.0 min
2.5 min
3.0min
Figure 3-6 Polarizing microscopic images of the liquid crystal cells with the etched AAO thin film as the alignment layer.
P A P
A
P A
P A P
A
P A
P A
P A
P A
P A
Etching
time 0° 45°
3.5 min
4.0 min
5.0 min
6.0 min
Figure 3-6 (cont’d) The polarizing microscopic images of the liquid crystal cells with the etched AAO thin film as the alignment layer.
P A P
A
P A
P A P
A
P A
P A
P A
Etching time
Etching time
0.0 min 3.5 min
1.5 min 4.0 min
2.0 min 5.0 min
2.5 min 6.0 min
3.0 min
Figure 3-7 The conoscopic images of the liquid crystal cell with the etched AAO thin film with different etching time.
1.5 2.0 2.5 3.0 3.5 0
1 2 3 4 5 6 7 8 9 10
Polar anchoring strength (10-5 J/m2 )
Etching time (min)
Figure 3-8 The relationship between the polar anchoring strength and the etching time.
4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 0
1 2 3 4 5 6 7 8 9 10
Polar anchoring strength (10-5 J/m2 )
Aspect ratio
Figure 3-9 The relationship between the polar anchoring strength and the aspect ratio.
Figure 3-10 Scheme of the possible alignment mechanism.
Smaller pore sizes larger pore sizes
Porous AAO thin film Liquid crystal
molecules
2r l
Chapter 4
The optical constants and birefringence of the anodic aluminum oxide
in terahertz frequency range
4.1 Overview
Terahertz (THz) waves are the electromagnetic waves sent at frequencies in the terahertz range. It is normally used for the region of the electromagnetic spectrum between 0.1 THz (1011 Hz) and 10 THz (1013 Hz), corresponding to the sub-millimeter wavelength range between 3 mm and 30 μm. 1 THz is corresponding to 300 μm. Figure 4-1 shows the spectrum of electromagnetic wave.[1] It shows that the THz waves are between the microwave and infrared optical bands. Because of the development of the efficient emitters and detectors, there are a lot of industrial applications in each of the spectral regimes. Before mid-1980s, the efficient THz generators and detectors are short. Most THz sources are low brightness emitters and with narrow band. In order to detect the
Terahertz (THz) waves are the electromagnetic waves sent at frequencies in the terahertz range. It is normally used for the region of the electromagnetic spectrum between 0.1 THz (1011 Hz) and 10 THz (1013 Hz), corresponding to the sub-millimeter wavelength range between 3 mm and 30 μm. 1 THz is corresponding to 300 μm. Figure 4-1 shows the spectrum of electromagnetic wave.[1] It shows that the THz waves are between the microwave and infrared optical bands. Because of the development of the efficient emitters and detectors, there are a lot of industrial applications in each of the spectral regimes. Before mid-1980s, the efficient THz generators and detectors are short. Most THz sources are low brightness emitters and with narrow band. In order to detect the