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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 hexagonal boron nitride surface. CVD grown hBN was transferred to silicon substrate.(a) Optical image of transferred hBN. Liquid crystal was then spin coated on the hBN. (b) POM image with cross-polarizer of hBN coated with liquid crystal. (c) Sample was rotated in clockwise direction, where different domains were marked on the images. (d) Intensity versus rotation-angle was plotted and fitted to extract the phases. (e) Histogram of orientation angle versus number of grains shows six discrete angles preferred.
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f2: Liquid crystal alignment on hexagonal boron nitride surface. CVD grown hBN was transferred to silicon substrate.(a) Optical image of transferred hBN. Liquid crystal was then spin coated on the hBN. (b) POM image with cross-polarizer of hBN coated with liquid crystal. (c) Sample was rotated in clockwise direction, where different domains were marked on the images. (d) Intensity versus rotation-angle was plotted and fitted to extract the phases. (e) Histogram of orientation angle versus number of grains shows six discrete angles preferred.

Mentions: The same experiment was performed on a different substrate. Highly oriented hBN was synthesized on a polished copper foil using the conventional CVD technique (See Methods). The copper foil was electro-polished and heated up to 900 °C, and borazine and hydrogen gases were used to synthesize hBN. (Supplementary Fig. S5). Grown hBN was then transferred to a silicon substrate using the standard wet transfer method. An optical image confirmed that BN was completely transferred to the silicon substrate (Fig. 2(a)). The Raman spectrum of the hBN was measured, which showed a broad peak at 1369 cm−1 (Supplementary Fig. S6(c)), confirming few-layer hBN was grown with high crystallinity. The surface roughness of transferred hBN was estimated using AFM, and the surface was found to be smooth with no remarkable features (Supplementary Fig. S6(d)). Liquid crystal was then spin-coated on the hBN, and POM with cross-polarizer was used to produce the image as shown in Fig. 2(b). This was the first observation of the LC alignment along the hBN crystallographic angle. This image shows continuously varying colors differently from the HOPG result, which is attributed to the non-uniformity of the thickness of LC. Due to the interference between the beams reflected from the top surfaces of LC and hBN, continuously changing colors appeared depending on the thickness. The sample was rotated in the clockwise direction, and different domains were marked as shown in Fig. 2(c). Intensity versus rotation angle was plotted and fitted to Eq.(1) in order to get the preferred orientation angle, as shown in Fig. 2(d). The number of domains having the same phases is shown in a histogram (Fig. 2(e)). It shows six discrete angles with 15o spacing, which were preferred for LC to anchor the hBN surface, as was the case with graphite. This also confirms that the behavior of six preferential orientations is not only for graphite but it occurs to other hexagonal lattices. It was established that this type of interaction is purely molecular instead of the surface morphology11. To confirm the molecular interaction between hBN and LC, an a small flake of hBN with a smooth surface was exfoliated and mounted on a silicon substrate and coated with LC. The POM image confirmed the presence of different domains with the molecular alignment of liquid crystals on hBN. The images of the rotated sample at different angles showed the same behavior, which confirms the LC-hBN alignment was mainly based on the molecular interaction, instead of the surface morphology like ripples or a grain boundary. (Supplementary, Fig. S7).


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 hexagonal boron nitride surface. CVD grown hBN was transferred to silicon substrate.(a) Optical image of transferred hBN. Liquid crystal was then spin coated on the hBN. (b) POM image with cross-polarizer of hBN coated with liquid crystal. (c) Sample was rotated in clockwise direction, where different domains were marked on the images. (d) Intensity versus rotation-angle was plotted and fitted to extract the phases. (e) Histogram of orientation angle versus number of grains shows six discrete angles preferred.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f2: Liquid crystal alignment on hexagonal boron nitride surface. CVD grown hBN was transferred to silicon substrate.(a) Optical image of transferred hBN. Liquid crystal was then spin coated on the hBN. (b) POM image with cross-polarizer of hBN coated with liquid crystal. (c) Sample was rotated in clockwise direction, where different domains were marked on the images. (d) Intensity versus rotation-angle was plotted and fitted to extract the phases. (e) Histogram of orientation angle versus number of grains shows six discrete angles preferred.
Mentions: The same experiment was performed on a different substrate. Highly oriented hBN was synthesized on a polished copper foil using the conventional CVD technique (See Methods). The copper foil was electro-polished and heated up to 900 °C, and borazine and hydrogen gases were used to synthesize hBN. (Supplementary Fig. S5). Grown hBN was then transferred to a silicon substrate using the standard wet transfer method. An optical image confirmed that BN was completely transferred to the silicon substrate (Fig. 2(a)). The Raman spectrum of the hBN was measured, which showed a broad peak at 1369 cm−1 (Supplementary Fig. S6(c)), confirming few-layer hBN was grown with high crystallinity. The surface roughness of transferred hBN was estimated using AFM, and the surface was found to be smooth with no remarkable features (Supplementary Fig. S6(d)). Liquid crystal was then spin-coated on the hBN, and POM with cross-polarizer was used to produce the image as shown in Fig. 2(b). This was the first observation of the LC alignment along the hBN crystallographic angle. This image shows continuously varying colors differently from the HOPG result, which is attributed to the non-uniformity of the thickness of LC. Due to the interference between the beams reflected from the top surfaces of LC and hBN, continuously changing colors appeared depending on the thickness. The sample was rotated in the clockwise direction, and different domains were marked as shown in Fig. 2(c). Intensity versus rotation angle was plotted and fitted to Eq.(1) in order to get the preferred orientation angle, as shown in Fig. 2(d). The number of domains having the same phases is shown in a histogram (Fig. 2(e)). It shows six discrete angles with 15o spacing, which were preferred for LC to anchor the hBN surface, as was the case with graphite. This also confirms that the behavior of six preferential orientations is not only for graphite but it occurs to other hexagonal lattices. It was established that this type of interaction is purely molecular instead of the surface morphology11. To confirm the molecular interaction between hBN and LC, an a small flake of hBN with a smooth surface was exfoliated and mounted on a silicon substrate and coated with LC. The POM image confirmed the presence of different domains with the molecular alignment of liquid crystals on hBN. The images of the rotated sample at different angles showed the same behavior, which confirms the LC-hBN alignment was mainly based on the molecular interaction, instead of the surface morphology like ripples or a grain boundary. (Supplementary, Fig. S7).

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.