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Posted: Oct 24, 2017
Scientists use transparent graphene electrodes to cleanse ionic impurities in liquid crystal displays (LCDs)
(Nanowerk Spotlight) Image sticking phenomena in liquid crystal (LC) devices became obvious soon after the production of the first nematic LC displays and have been a concern ever since.
In a nematic LC display, electric fields are used to switch the pixels on and off by switching the nematic director from a planar to a vertical configuration. However, LCs generally contain free ions, which are generated as impurities during the synthesis process of the LC. Other external factors, such as the LC cellís polyimide alignment layers and the electrodes, can also inject free-ion impurities into the LC during the filling process.
In electro-optical LC displays (LCD), the presence of excess ionic impurities triggers several problems, such as slow responses and long-term image sticking effects. When an electric field is applied to switch the pixels, the positive and negative ions are separated at the opposite electrodes.
This ion-separation results in an internal electric field across the pixels in the opposite direction of the external applied field – which affects the switching behavior of the LC. After a long term, when the external field is removed, the internal filed due to the ion-separation remains present, leading to a reaming ghost image on the pixels – called image sticking.
Since image sticking is caused by the ion-transport, LCD manufacturers put in a lot of effort towards the purification of the LCs from unwanted ions. Also, there exists an important research direction to understand the ion transport phenomenon in an LC and the principles governing their subsequent impacts on the LCís electrical, mechanical, and electro-optical properties.
Associate Professor Rajratan Basu and his student Midshipman Andrew Lee in the Department of Physics at the United States Naval Academy have developed a method to reduce the presence of excess ionic impurities by using a graphene electrode in the LC cell.
In conventional LC cells, the two major components are the LC alignment layers and transmissive indium tin oxide (ITO) electrodes. The conventional LC alignment layer is a polyimide (PI)-coated surface where a unidirectional rubbing determines the nematic director profile of the LC in the cell.
"Graphene shows high optical transmittance and high electrical conductivity, and therefore, graphene can be used as transparent electrodes," Basu explains. "We report the fabrication of a graphene-ITO hybrid LC cell without using any additional PI alignment layer on the graphene-electrode side, and show that the free-ion concentration in the hybrid cell is ∼70% less than that in the conventional ITO-ITO LC cell."
"The LC molecules can anchor to the honeycomb pattern of graphene employing the π-π electron stacking," he continues. "Density-functional calculations suggest that this anchoring is reinforced with a binding energy of -2.0 eV by electrostatic energy due to a considerable amount of charge transfer from the LC molecule to the honeycomb pattern of the carbon atoms."
Figure 1: (a) A schematic representation of a conventional ITO-ITO LC cell containing a layer of ITO and a layer of polyimide with unidirectional rubbing on each glass slide. The small spheres represent the ions in the LC. Micrographs of E7 LC in the ITO-ITO cell under the crossed-polarized microscope with the director n at (b) 45° (bright) and (c) 0° (dark) with respect to the polarizer. The black dots in micrograph-(b) are 20 µm spacer particles. (d) The picture of a conventional ITO-ITO LC cell. (e) A schematic representation of the alignment of nematic LC molecules on graphene due to π-π electron stacking. The ellipsoids are LCs and the black honeycomb structure is the graphene surface. The LC molecular structure is shown in the ellipsoid on the graphene surface. The π-π electron stacking is illustrated by matching the LCís benzene rings on the graphene-honeycomb structure. (f) A schematic representation of a graphene-ITO hybrid cell, which contains a monolayer graphene-electrode on one side and a regular ITO-polyimide combination on the other side. It also shows some trapped ions on the GP-electrode, and therefore, fewer ions are present in the LC media. Micrographs of E7 LC in the graphene-ITO cell under the crossed-polarized microscope with n at (g) 45° (bright) and (h) 0° (dark) with respect to the polarizer. (i) The picture of a graphene-ITO LC cell. The white bar in micrograph-(c) presents 50 µm. (Image: Dr. Basu) (click on image to enlarge)
Figure 1(e) illustrates the π-π stacking interaction that arises due to the overlap of the LCís benzene rings on the GP-honeycomb structure. The LC can achieve a uniform planar-aligned state over a large-scale dimension on GP due to this strong π-π stacking interaction. Therefore, the GP-electrode at one side of the GP-ITO cell can function concurrently as the LC alignment layer as well. The GP-ITO LC cell is schematically presented in Fig. 1(f). Similar to the conventional cell, Fig. 1(g) and (h) present the micrographs of the LC in the GP-ITO cell, where n is at 45° (bright) and 0° (dark), respectively, with respect to the polarizer. These micrographs confirm the planar alignment of the LC in the GP-ITO cell. Figure 1(i) shows the picture of the GP-ITO cell.
"Several reports in the literature show that that the presence of carbon nanomaterials, such as carbon nanotubes, graphene, and fullerenes in the LC can significantly reduce the free-ion concentration by the ion-trapping process," Basu says. "In the GP-ITO hybrid cell, the GP-electrode is directly exposed to the LC, as there is no polyimide alignment substrate on GP. Therefore, the GP-electrode traps a significant amount of free ions and reduces the free-ion concentration in the hybrid cell."
The ion-trapping process is schematically presented in Fig. 1. The ITO-ITO cell in Fig. 1(a) shows the presence of free ions (both positive and negative) in the LC media. The GP-ITO cell in Fig. 1(f) shows the trapped ions on the GP-electrode, and therefore, fewer ions are present in the LC media.
Basu and Lee note that the reduction of ionic impurities leads to an improved orientational order of the LC in the hybrid cell. Thus, these results are important for developing novel methods of purifying LCs from excess ions without additional chemical synthesis.
This work was supported by the Office of Naval Research under Award No. N0001417WX01519.
Provided by the U.S. Naval Academy as a Nanowerk exclusive