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Nematic Liquid Crystal on a Two Dimensional Hexagonal Lattice and its Application.

Arslan Shehzad M, Hoang Tien D, Waqas Iqbal M, Eom J, Park JH, Hwang C, Seo Y - Sci Rep (2015)

Bottom Line: From the experimental data, it was found that there were 6 different alignment orientations of the liquid crystal molecules on a single crystal substrate.We explain this result considering the bending of the tail of the liquid crystal molecules.Using this anchoring effect with six accurate discrete angles, a novel non-volatile display can be developed with micron-scale pixel size, due to the molecular level accuracy of the alignment.

View Article: PubMed Central - PubMed

Affiliation: Faculty of Nanotechnology &Advanced Materials, HMC, and GRI, Sejong University, Seoul 143-747, South Korea.

ABSTRACT
We have studied the alignment of liquid crystal adsorbed onto graphene and hexagonal boron nitride by using a polarized optical microscope. From the experimental data, it was found that there were 6 different alignment orientations of the liquid crystal molecules on a single crystal substrate. This result has never been reported and is quite different from other previous results. As the hexagonal lattice has a threefold rotational symmetry, three different alignment orientations were expected, but our result seems counter-intuitive. We explain this result considering the bending of the tail of the liquid crystal molecules. Using this anchoring effect with six accurate discrete angles, a novel non-volatile display can be developed with micron-scale pixel size, due to the molecular level accuracy of the alignment.

No MeSH data available.


Schematics of liquid crystal alignment on hexagonal lattice.(a) Schematics of a LC molecule adhered on hexagonal lattice. The tail of the 5CB LC molecule is slightly titled (~8o) from the axis of the body composed of two benzene rings as shown in black line. As the molecule can be flipped 180o with the same probability, two different polarization directions are induced with ~15o deviation. As three armchair edge directions have two flipping degeneracy, 6-fold symmetry was found. (b) Schematics of LC cell with unidirectional rubbed layer. Direction of alignment of LC molecules on the hexagonal lattice (bottom layer) is selected among six preferred orientations. The unidirectional rubbed alignment layer is placed at the top, so that LC molecules will be twisted depending on the angle differences, and the twisted LC layer reflects different color depending on the twist rate.
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f3: Schematics of liquid crystal alignment on hexagonal lattice.(a) Schematics of a LC molecule adhered on hexagonal lattice. The tail of the 5CB LC molecule is slightly titled (~8o) from the axis of the body composed of two benzene rings as shown in black line. As the molecule can be flipped 180o with the same probability, two different polarization directions are induced with ~15o deviation. As three armchair edge directions have two flipping degeneracy, 6-fold symmetry was found. (b) Schematics of LC cell with unidirectional rubbed layer. Direction of alignment of LC molecules on the hexagonal lattice (bottom layer) is selected among six preferred orientations. The unidirectional rubbed alignment layer is placed at the top, so that LC molecules will be twisted depending on the angle differences, and the twisted LC layer reflects different color depending on the twist rate.

Mentions: Our result of the six aligning directions, instead of three, is different from recently published results13. One may consider the 3 zig-zagged edge directions additional to 3 arm-chair directions, but that would be double counting because they have 30o difference. In order to understand the nature of six aligning directions, a schematic drawing with a possible configuration is shown in Fig. 3(a). A benzene ring in the LC molecule (5CB) adsorbs on the substrate being coherently aligned11. Previously, it was suggested that van der Waals forces between alkyl groups plays a role in the alignment of LC molecules on the hexagonal substrate11. There are two different anchoring preferences, as the LC molecule can be flipped 180o. The tail of the molecule is slightly titled (~8o) from the hexagonal crystalline axis as shown in the black line in Fig. 3(a)16. When the molecule is flipped upside down, the tilt angle will be in the opposite direction where the difference between orientations would be about 15o. As all three different aligning orientations (arm-chair edge direction) have two degeneracy, there must be six directions. It was confirmed that the six peaks with equal spacing was not found when a different type of LC was used. (See Figure S10)


Nematic Liquid Crystal on a Two Dimensional Hexagonal Lattice and its Application.

Arslan Shehzad M, Hoang Tien D, Waqas Iqbal M, Eom J, Park JH, Hwang C, Seo Y - Sci Rep (2015)

Schematics of liquid crystal alignment on hexagonal lattice.(a) Schematics of a LC molecule adhered on hexagonal lattice. The tail of the 5CB LC molecule is slightly titled (~8o) from the axis of the body composed of two benzene rings as shown in black line. As the molecule can be flipped 180o with the same probability, two different polarization directions are induced with ~15o deviation. As three armchair edge directions have two flipping degeneracy, 6-fold symmetry was found. (b) Schematics of LC cell with unidirectional rubbed layer. Direction of alignment of LC molecules on the hexagonal lattice (bottom layer) is selected among six preferred orientations. The unidirectional rubbed alignment layer is placed at the top, so that LC molecules will be twisted depending on the angle differences, and the twisted LC layer reflects different color depending on the twist rate.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4542626&req=5

f3: Schematics of liquid crystal alignment on hexagonal lattice.(a) Schematics of a LC molecule adhered on hexagonal lattice. The tail of the 5CB LC molecule is slightly titled (~8o) from the axis of the body composed of two benzene rings as shown in black line. As the molecule can be flipped 180o with the same probability, two different polarization directions are induced with ~15o deviation. As three armchair edge directions have two flipping degeneracy, 6-fold symmetry was found. (b) Schematics of LC cell with unidirectional rubbed layer. Direction of alignment of LC molecules on the hexagonal lattice (bottom layer) is selected among six preferred orientations. The unidirectional rubbed alignment layer is placed at the top, so that LC molecules will be twisted depending on the angle differences, and the twisted LC layer reflects different color depending on the twist rate.
Mentions: Our result of the six aligning directions, instead of three, is different from recently published results13. One may consider the 3 zig-zagged edge directions additional to 3 arm-chair directions, but that would be double counting because they have 30o difference. In order to understand the nature of six aligning directions, a schematic drawing with a possible configuration is shown in Fig. 3(a). A benzene ring in the LC molecule (5CB) adsorbs on the substrate being coherently aligned11. Previously, it was suggested that van der Waals forces between alkyl groups plays a role in the alignment of LC molecules on the hexagonal substrate11. There are two different anchoring preferences, as the LC molecule can be flipped 180o. The tail of the molecule is slightly titled (~8o) from the hexagonal crystalline axis as shown in the black line in Fig. 3(a)16. When the molecule is flipped upside down, the tilt angle will be in the opposite direction where the difference between orientations would be about 15o. As all three different aligning orientations (arm-chair edge direction) have two degeneracy, there must be six directions. It was confirmed that the six peaks with equal spacing was not found when a different type of LC was used. (See Figure S10)

Bottom Line: From the experimental data, it was found that there were 6 different alignment orientations of the liquid crystal molecules on a single crystal substrate.We explain this result considering the bending of the tail of the liquid crystal molecules.Using this anchoring effect with six accurate discrete angles, a novel non-volatile display can be developed with micron-scale pixel size, due to the molecular level accuracy of the alignment.

View Article: PubMed Central - PubMed

Affiliation: Faculty of Nanotechnology &Advanced Materials, HMC, and GRI, Sejong University, Seoul 143-747, South Korea.

ABSTRACT
We have studied the alignment of liquid crystal adsorbed onto graphene and hexagonal boron nitride by using a polarized optical microscope. From the experimental data, it was found that there were 6 different alignment orientations of the liquid crystal molecules on a single crystal substrate. This result has never been reported and is quite different from other previous results. As the hexagonal lattice has a threefold rotational symmetry, three different alignment orientations were expected, but our result seems counter-intuitive. We explain this result considering the bending of the tail of the liquid crystal molecules. Using this anchoring effect with six accurate discrete angles, a novel non-volatile display can be developed with micron-scale pixel size, due to the molecular level accuracy of the alignment.

No MeSH data available.