LCDs operate in either dynamic or bistable mode. Dynamic-mode LCDs produce high-quality images, but require the presence of an electric field. However, for portable devices such as laptop computers and mobile phones, power consumption becomes a critical consideration. One solution involves bistable LCDs, which have two stable states—both operating without application of an electric field—with different optical properties. Unfortunately, most bistable LCDs do not have gray-scale rendering capability.1 Dual-operation-mode LCDs, which can be operated in power-saving bistable mode or used to display high-quality images in dynamic mode, have been proposed.2 However, current-generation dual-mode displays have slow response times of ~15.8ms and 2s in dynamic and bistable modes, respectively. In addition, they are not multistable.
Liquid-crystal (LC) silica dispersions have attracted significant research input for reasons of both academic interest and technology development.3 Hydrophilic silica nanoparticles (with sizes of ∼10nm) form agglomerate networks in the LC mixture. The hydroxyl groups covering the nanoparticles and the LC's polar nature result in homeotropic LC alignment (i.e., perpendicular to the silica surface), while the LC director in the void volume is oriented parallel to the silica strands. Scattering-type bistable devices fabricated using nematic-silica (loosely parallel) suspension have been demonstrated.3 More recently, electrophoretically controlled (i.e., driven by an external electric field) nematic-silica suspension was investigated.4 The doped-silica nanoparticles result in polarity-controlled multistable switching in conventional hybrid-aligned nematic (HAN) cells. Doping nanoparticles in the LC host also reduces the dielectric relaxation time of the LC mixture, in turn reducing the cell's response time.5
We recently developed a new dual-operation-mode LCD using silica-nanoparticle-doped HAN cells6 (see Figure 1). The charged nanoparticles move electrophoretically toward the planar side of the cell: see Figure 1(a). The accumulated nanoparticles create agglomerate networks to stabilize the LCs in a homeotropic state under DC voltage excitation, which is retained after switching off the voltage. In addition, a pulse voltage with opposite polarity moves the accumulated nanoparticles away from the cell's planar side, thus erasing its memory state. However, under AC voltage, the electrophoretic motion of the doped nanoparticles is suppressed: see Figure 1(b). The cell functions as a conventional LCD, returning to its homeoplanar state after the AC voltage is switched off.
Figure 1. Schematic representation of the dual-operation-mode LCD in (a) multistable and (b) dynamic modes.
Figure 2. Electro-optical response of the silica-nanoparticle-doped HAN cell under AC and DC pulse-voltage excitation. The doped-nanoparticle concentration is 1% by weight. The dotted line indicates zero intensity for the optical response curve.
Figure 2 displays the electro-optical response of our new LCD. With 1kHz AC pulse-voltage excitation, it works like a conventional LC cell with fast response and low operating voltage. The cell's transmittance varies with the amplitude of the applied voltage. On the other hand, under DC pulse-voltage excitation, the cell behaves as a multistable device. Figure 3 shows multistable images of the cell, while videos demonstrating the device's dual-operation mode are available online.7,8
Figure 3. Multistable images of the cell for (a) 8 and (b) 30V DC pulse-voltage excitation.
In conclusion, we have demonstrated a new dual-operation-mode LCD device. In the (1.5% by weight) nanoparticle-doped cell, the measured response time in dynamic mode is ∼12ms, while that in multistable mode is ~500ms. Its memory state can be retained for one month. We are currently working on improving the response time in multistable mode. Our recent results suggest that it can be reduced to ~30ms.
The author would like to thank the National Science Council of Taiwan and Chi Mei Optoelectronics for financial support.
Graduate Institute of Photonics
National Changhua University of Education
Changhua City, Taiwan