Zenithal alignment of liquid crystal on homeotropic polyimide film irradiated by ion beam
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We investigate the pretilt characteristics of a nematic liquid crystal [LC] in terms of ion beam exposure conditions on the homeotropic polyimide alignment layer. The pretilt angle of LCs in the case of high-energy ion beam treatment was decreased considerably almost the same to that of the homogenous alignment layer though we used homeotropic polyimide film at first. Increasing irradiating energy, we could control the pretilt from 90° to 1° with several steps. We believe that this is because the side chain with hydrophobicity in the used polyimide is broken by ion beam exposure. To confirm it, contact angle measurement was carried out. With this result, we can easily control the LC pretilt in the pixel with appropriate exposure conditions which is critical to achieve excellent electrooptic characteristics and good image quality.
KeywordsContact Angle Polyimide Dynamic Contact Angle Alignment Layer Pretilt Angle
In practical applications of liquid crystals [LCs], the study of the LC alignment near the surface including in-plane and out-of-plane directions is crucial. Various alignment techniques have been introduced to create physical/chemical anisotropy on the surface of the alignment layer [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]. The most representative skill used in industry is the rubbing method which has high productivity. However, since rubbing is a contact method, it often produces a fine scratch on the alignment layer during the process, and it results in several disadvantages, such as the rubbing mura at a dark level of liquid crystal displays [LCDs].
To overcome such problems, various alternative non-contact methods are suggested for high-resolution LCDs such as photo-alignment technique with ultraviolet [UV] light and ion beam technique [1, 7]. The polarized UV irradiation technique has many merits such as a clear image without the mura at a black level and an easy creation of multi-domain in a pixel. However, it also has a problem such as an image-sticking issue due to a low-surface anchoring strength.
Since the extremely collimated ion beam to confirm a high linear motion can induce an excellent anisotropy on the polyimide surface by selective destruction of π-bonding which plays an important role in the alignment of LCs, it could be a good option of non-contact surface modification method [2, 10]. Also, this method is relatively free of the image-sticking problem since it generates a strong anchoring strength like the rubbing method. Therefore, we can conclude that an appropriate LC pretilt control technique considering LCD modes is essential to obtain better electrooptical characteristics of LCDs and to develop new LC device applications.
In this paper, we investigate the pretilt characteristics of a nematic LC in terms of ion beam irradiation conditions using homeotropic polyimide alignment layer. The modification of LC pretilt with different ion beam exposure parameters was studied experimentally. As a result, we successfully changed the pretilt angle from 90° to 1° with several sub-steps.
After coating and baking, this homeotropic polyimide film was bombarded by an argon ion beam to change the LC aligning properties of the surface. In this experiment, various ion beam exposure conditions such as the irradiation energy, exposure angle, exposure time, and current density are modulated to examine how the surface property is changed according to them. As an ion source, a cold hollow cathode [CHC] is used to yield a highly collimated ion beam. In order to collimate the ion beam, two perforated grids are used as electro-focusing lenses. The CHC represents a separate cooled chamber which is equipped with a magnetic system and is connected to a discharge chamber through an orifice. Argon gas feeding into the ion source is carried out through the CHC only.
In order to investigate the pretilt angle of treated polyimide surfaces, we had fabricated several LC cells by using the substrates with different ion beam exposure conditions. The cell was filled with a nematic LC (MLC-6610 from Merck KGaA, Darmstadt, Germany) with negative dielectric anisotropy, and the cell gap was maintained by 12-μm glass spacers. The LC injection was carried out at room temperature. The pretilt angle was measured by the extended crystal rotation method which can measure a wider range of the pretilt [12, 13].
Results and discussion
where θ is the contact angle. As known in Equation 2, when the liquid is water with chemical polarity, the lower the contact angle is, the higher the surface energy and the hydrophilic property are. Meanwhile, the higher the contact angle is, the lower the surface energy and the hydrophilic property are.
In this measurement, we used distilled water having chemical polarity as a liquid. The ion beam-treated polyimide AL-00010 substrate of which the ion beam conditions are ion beam energy of 300 eV, ion beam current density of 50 μA/cm2, exposure angle of 30°, and exposure time of 30 s was used as a sample. The ion beam condition is enough to modify the surface as shown in Figure 2. As a comparison, the sample with untreated polyimide AL-00010 substrate was examined.
In summary, we investigated the LC pretilt property according to the ion beam exposure conditions on a homeotropic LC alignment. The LCs on a higher energy ion beam-treated surface is aligned almost homogenously. We assumed that it may be caused by the breaking of the side chain with hydrophobic property in the used polyimide by ion beam exposure. It was confirmed from the result of the contact angle measurement. LC pretilt which may create new LCD applications with excellent electrooptic characteristics and good image quality can be controlled by an ion beam condition with proper ion beam parameters.
This work was supported by a grant from the National Research Foundation of Korea (NRF) funded by the Korean government (Grant No. 2011-0002447) and a grant from the Human Resources Development Program (R&D Workforce Cultivation Track for Solar Cell Materials and Processes) of Korea Institute of Energy Technology Evaluation and Planning (KETEP) (No. 20114010100580) funded by the Korean government's Ministry of Knowledge Economy.
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