With the dramatic increase in demand for portable electronic devices such as mobile phones, personal organisers, computers and games consoles there has recently been a significant amount of research undertaken with the aim to decrease the power consumption and thus increase the battery lifetime, which ultimately affects the portability of such a device. The general aim of current research is to reduce the operating voltages and minimise the time over which a voltage must be applied to the LCD. Many different areas of research have sprung up to tackle these problems and several devices have been created with varying degrees of effectiveness and success. One type of device which can give significant reduction in power consumption is the bistable display.

In a normal monostable LCD a layer of liquid crystal material is sandwiched between two glass or plastic substrates across which a voltage is applied independently to each pixel of the display. This applied electric field may alter the molecular configuration of the liquid crystal and thus alter the optical characteristics of the display. With the aid of optical filters this can enable the device to switch between a light and a dark state. When the voltage is removed the device switches back to its original and only stable state.

The construction of a bistable LCD is similar to that of a monostable LCD. However, in contrast to the monostable LCD, in a bistable device the two molecular configurations relating to the light and dark states are locally stable when the voltage is removed. Therefore, power is only needed to switch from one state to the other. For electronic devices in which the image or parts of the image remain in a fixed state for some time, this would dramatically reduce the power consumption of the device.

Possibly the most researched bistable LCD technology has been the ferroelectric SmC* LCD. These displays support two locally stable configuration states in which the optic axis is contained within the plane of the cell (thus leading to favourable optical characteristics). A permanent dipole allows fast ferroelectric switching between states, which is desirable to allow video rate addressing. The problem with these displays is that the shock resilience is very poor. With the slightest external pressure the delicate smectic layering structure is destroyed and the image is ruined.

In recent years alternative bistable LCDs have been investigated in order to combat this problem, These displays have all been based on the more traditional nematic LCD technologies that have been very successful in the display industry in the last 25 years. Using existing production methods and liquid crystal materials would aid the cost effectiveness of such new technologies. These new devices differ from the twisted and super twisted nematic displays commonly used, in that they usually contain a surface treatment or surface morphology that leads to bistability (examples of which are given below).

Surface controlled bistable nematic

One proposed liquid crystal display uses a simple construction to induce bistability. A liquid crystal is sandwiched between two substrates, one of which is given a weak planar anchoring while the other is treated to give strong planar or pre-tilted anchoring, as shown in Fig. 1(a).

Fig 1. When the electric field is applied the cell is essentially homeotropic at the lower boundary. After the field has been removed the molecules can relax to either the untwisted state (a) or the twisted state (c).

Without an electric field the director is either aligned in a roughly planar orientation throughout (Fig. 1(a)) or the director has a 180 degree twist as it moves from one surface to the other as shown in Fig. 1(c). The optical properties of each state are very different from each other when viewed between cross polarisers. To switch between these states the surface anchoring has to be broken with short electric field pulses. Applying the field normal to the plates causes the dielectric effect to reorient the bulk of the cell and break the surface anchoring resulting in a mainly homeotropic sample (Fig. 1(b)). The sample now has no memory at the lower surface as to its initial state. If the field is decreased slowly the untwisted state is achieved due to the elastic coupling between the two plate orientations. If however the field is turned off instantaneously a large hydrodynamic effect occurs causing the creation of a bent state which transforms spontaneously to the 180 degree twist state. Thus it is possible to switch between these two stable states.

ZBND

Another bistable LCD technology is the Zenithal Bistable Nematic Device (ZBND), also referred to as the ZBD, that makes use of a nonpolar substrate exhibiting a grating morphology and allows two distinct director structures.

Fig. 2. An example of the director profiles of the two stable states in a ZBND display. The molecules are sandwiched between a flat substrate at the top surface and a complex substrate~\cite{gerus2001} at the lower surface. The device can switch between the distorted state and the non-distorted state with the application of an electric field.

In Fig. 2 we can see an example of the two dimensional surface structure and director configuration of the two stable states. The second state retains its stability due to the formation of defects near the complex surface. These distortions serve to "lock" the director profile in its new configuration. To switch back and forth between these two states we ideally need to form and annihilate these defects.

\noindent The advantages of using the ZBND over other bistable devices is that the image is insensitive to variations of the panel, such as those caused by mechanical shock or due to temperature and cell gap changes. The use of gratings to achieve bistable surface alignment means that simple manufacturing techniques can be used (such as embossing) which is more suited for devices with plastic substrates. The switching properties within each pixel can be varied allowing a simple method of producing greyscale. It is this device that we intend to concentrate on modelling throughout this investigation.

PABN

Fig. 3. Picture of the ``post" stucture in PABN.[1]

In some cases it is required that one stable state be more favourable than the other. One main problem with most bistable devices is that it can be very difficult to give preferable treatment of either state. It has been suggested that more than just the normal two degrees of freedom from two-dimensional systems would be useful to help optimise optical switching properties. One example of this is when the surface structure itself consists of many isolated ``posts" around which the nematics arranges itself as shown in Fig. 3. This device is known as the Post Aligned Bistable Nematic (PABN). In the PABN the molecules are planarly anchored to the lower boundary of the cell and homeotropically anchored to the upper boundary. The molecules are anchored planarly to both the sides and top surface of the post and on the lower surface.

\noindent On the side of the posts the molecules are planarly anchored with respect to the post. Here the shape of the individual surface feature is the primary driving force behind the bistability. It also has the property that the posts can be rotated when designing the cell which causes both stable states to rotate by the same amount. This offers potentially greater flexibility in engineering alignment layers, as the direction of the alignment can be separately modified to optimise some required property.

Fig 4. Director configurations for various states of PABN. (a) View from above the post. (b) Directors are mainly planar around short post. (c) Different director configurations are produced by rotating the post. (d) The directors are mainly aligned vertically around a tall post.

The characteristics of the bistability can also be altered by the chosen height of these posts. For example, low posts favour planar alignment due to the proximity of the horizontal top of the post to the horizontal regions between the posts. Taller posts favour tilted states as the molecules are strongly influenced by the vertical sides of the posts when sandwiched between them (see Fig. 4).

\noindent With the posts there are a total of six potential surface alignment directions and as the molecules can either tilt positively or negatively away from the base this gives a total of \emph{eight} different bulk states. The liquid crystal alignment is governed by the competition between the alignment imposed by horizontal surfaces and the defects formed at the vertical edges. For intermediate post heights, both tilt profiles are feasible local energy minima of simliar levels. Flexoelectrical effects and the application of an electric field can produce switching.

References

Contents of above taken from "Flexoelectric switching in a bistable nematic device" - Thesis by Andrew Davidson.

[1] S. Kitson and A. Geisow, " Controllable alignment of nematic crystals around microscopic posts: Stabilization of multiple states", App Phys Lett, Vol 80, No. 19, 3635-3637, (2000).