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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.


Liquid crystal alignment on HOPG surface. HOPG was exfoliated and transferred to a glass substrate. Liquid crystal was then spin-coated on the same sample.(a) Optical microscope image of HOPG without polarizer (b) POM with cross-polarizer image shows LC domains with clear boundaries. (c) The sample was rotated in clockwise direction. The domain marked was dark at 0o, which became bright at 30o. (d) Intensity versus rotation angle was plotted and fitted into a sinusoidal function. (e) The phases for more domains were measure, and the number of domains with the same phase was plotted.
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f1: Liquid crystal alignment on HOPG surface. HOPG was exfoliated and transferred to a glass substrate. Liquid crystal was then spin-coated on the same sample.(a) Optical microscope image of HOPG without polarizer (b) POM with cross-polarizer image shows LC domains with clear boundaries. (c) The sample was rotated in clockwise direction. The domain marked was dark at 0o, which became bright at 30o. (d) Intensity versus rotation angle was plotted and fitted into a sinusoidal function. (e) The phases for more domains were measure, and the number of domains with the same phase was plotted.

Mentions: Nematic liquid crystal (4-pentyl-4-cyano-biphenyl, 5CB) with a clearing point of 33 °C was used to visualize the domains of transferred films. LC was spin-coated on HOPG. The sample was then heated above the clearing point to the isotropic phase and then slowly cooled down to the nematic phase. Exfoliated graphite was marked with mechanical scratches and observed with an optical microscope (Fig. 1(a)). A polarized optical microscope (POM) with perpendicular alignment between polarizer and analyzer was used to study the interaction between graphene and liquid crystal. The POM image showed different alignments of LC on the single crystal graphite surface (Fig. 1(b)). The LC domains having the same contrast were observed, and domain boundaries were clear under POM. It was known that the LC domain has strong correlation with grain of CVD grown graphene. Grains of CVD grown graphene on copper foil were also observed using the same liquid crystals (Supplementary Fig. S3). As LC domains have different polarization angle, they have different brightness. The sample was rotated in the clockwise direction, and the POM images were taken as shown in Fig. 1(c). Consecutive 25 images at different angles were taken to complete the full rotation. The brightness change as a function of the angle was observed for more than 20 domains. A representative domain was marked in Fig. 1(d), where one can see that the brightness was oscillated as a sinusoidal function of the rotation angle. The brightness of each grain was calculated using MATLAB (See Methods). The calculated intensity for the domain was plotted with rotation angle, as shown in Fig. 1(e), and it was observed that four cycles were repeated within the whole rotation. Fitting was done using the following equation in order to get the phase or orientation angle of liquid crystal:


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)

Liquid crystal alignment on HOPG surface. HOPG was exfoliated and transferred to a glass substrate. Liquid crystal was then spin-coated on the same sample.(a) Optical microscope image of HOPG without polarizer (b) POM with cross-polarizer image shows LC domains with clear boundaries. (c) The sample was rotated in clockwise direction. The domain marked was dark at 0o, which became bright at 30o. (d) Intensity versus rotation angle was plotted and fitted into a sinusoidal function. (e) The phases for more domains were measure, and the number of domains with the same phase was plotted.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f1: Liquid crystal alignment on HOPG surface. HOPG was exfoliated and transferred to a glass substrate. Liquid crystal was then spin-coated on the same sample.(a) Optical microscope image of HOPG without polarizer (b) POM with cross-polarizer image shows LC domains with clear boundaries. (c) The sample was rotated in clockwise direction. The domain marked was dark at 0o, which became bright at 30o. (d) Intensity versus rotation angle was plotted and fitted into a sinusoidal function. (e) The phases for more domains were measure, and the number of domains with the same phase was plotted.
Mentions: Nematic liquid crystal (4-pentyl-4-cyano-biphenyl, 5CB) with a clearing point of 33 °C was used to visualize the domains of transferred films. LC was spin-coated on HOPG. The sample was then heated above the clearing point to the isotropic phase and then slowly cooled down to the nematic phase. Exfoliated graphite was marked with mechanical scratches and observed with an optical microscope (Fig. 1(a)). A polarized optical microscope (POM) with perpendicular alignment between polarizer and analyzer was used to study the interaction between graphene and liquid crystal. The POM image showed different alignments of LC on the single crystal graphite surface (Fig. 1(b)). The LC domains having the same contrast were observed, and domain boundaries were clear under POM. It was known that the LC domain has strong correlation with grain of CVD grown graphene. Grains of CVD grown graphene on copper foil were also observed using the same liquid crystals (Supplementary Fig. S3). As LC domains have different polarization angle, they have different brightness. The sample was rotated in the clockwise direction, and the POM images were taken as shown in Fig. 1(c). Consecutive 25 images at different angles were taken to complete the full rotation. The brightness change as a function of the angle was observed for more than 20 domains. A representative domain was marked in Fig. 1(d), where one can see that the brightness was oscillated as a sinusoidal function of the rotation angle. The brightness of each grain was calculated using MATLAB (See Methods). The calculated intensity for the domain was plotted with rotation angle, as shown in Fig. 1(e), and it was observed that four cycles were repeated within the whole rotation. Fitting was done using the following equation in order to get the phase or orientation angle of liquid crystal:

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.