MP5: Soft Matter: Physics of Liquid Crystals

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1 MP5: Soft Matter: Physics of Liquid Crystals 1 Objective In this experiment a liquid crystal display (LCD) is built and its functionality is tested. The light transmission as function of the applied voltage of the home-made LCD and a commercially available LCD is measured and compared. Furthermore the phase transition from the nematic phase to the isotropic phase is investigated. The temperature dependent birefringence in the nematic phase is a measure for the order within the liquid crystal. 2 Theory Please see literature attached in the folder: Introduction to Soft Matter Revised Version written by Ian W. Hamley; chapter 5 Liquid Crystals 2.1 Theory of liquid crystals Liquid crystals are a mesophase between crystalline solids and isotropic liquids. This means that liquid crystals have some properties of crystalline solids and some properties of liquids. Liquid crystals have direction-dependent properties like refractive index and dielectric constant which are typical of crystalline solids. But the fluidity and viscosity of liquid crystals show the behaviour of a liquid. The direction-dependence of the properties of the liquid in the mesophase is due to the orientational long range order of the molecules. This effect is caused by the shape anisotropy of the liquid crystal molecules, which are are rod-shaped (calamitic) or disc-shaped (discotic). Two prominent examples of liquid crystals with rod-shaped molecules are 4-Cyano-4'-pentylbiphenyl (5CB) and 4 -n-octyl-4-cyano-biphenyl (8CB). The structural formula of 5CB and 8CB and the one of discotic liquid crystal (Hexabenzocoronen) is shown in figure Liquid-crystalline phases figure 1 The phase transition of 5CB into the liquid-crystalline phase occurs at 22.5 C and the transition into the liquid-isotropic phase at 35 C. 5 CB is the liquid crystal which will be investigated in the course of this experiment. The different degrees of positional and orientational order are displayes in figure 2. In a crystalline solid there is a 3d order, in the smectic mesophase there is a 2d order and a disorder within the individual planes, in the nematic mesophase there is a 1d order (orientational long range order) and in the liquid-isotropic phase there is no type of order present. The degree of orientational long range order is expressed by the order parameter S, which is given by the average deviation from the preferential direction. S is 1, if all molecules have exactly the

2 same orientation and S becomes 0 if the liquid-isotropic phase is present. The state of S=0 is reached with increasing temperature as the parallel orientation of molecules is lost due to a change in entropy. figure 2 The temperature dependence of the order parameter of the nematic liquid crystal PAA is shown in the right-hand side of figure 3. In the left part of figure 3 the definition of the order parameter is demonstrated, where the director points in the preferential direction and θ is the angle between the prefered direction and the axes of the molecules. figure 3 At the critical temperature T c the order parameter discontinously drops to 0 and the anisotropic properties (e.g. birefringence) vanish and the appearance turns from a milky to a clear liquid. This temperature T c is called the clearing point. 2.3 Anisotropy of liquid crystals Materials are called birefringent if they split incident light into two beams which are polarised perpendicularly to each other. This effect is caused by the anisotropy of the refractive index. The two beams are classified by their polarisation relative to the optical axis, if the polarisation is

3 parallel to the optical axis the beam is called extraordinary and if the polarisation is perpendicular to the optical axis the beam is called ordinary. The velocity of propagation of these two beams is different due to the discrepancy in refractive indices. For 5CB at T=25 C the refractive index of the extraordinary beam is and the one of the ordinary beam is The birefringent character is only present in the nematic phase, if the liquid crystal is heated, the difference between the two refractive indices decreases and abruptly vanishes at the clearing point. The experimental setup and the refractive indices as function of temperature are shown in figure 4. figure 4 In analogy to the anisotropy of refractive indexes an anisotropy of dielectric constants exists in the nematic phase. This effect is caused by permanent electric dipole moment in direction of the molecule axis of rod-shaped molecules. This means that the molecules align parallel to an external electric field. This effect is made use of in the manufacture of LCD. The larger the anisotropy the faster the LCD can be switched. 2.4 Fréedericksz-effect An important requirement for the Fréedericksz-effect is that the parallel component of the dielectric constant is larger than the perpendicular component. The molecules of the liquid crystall have the same orientation and lie parallel to the interfaces. The Fréedericksz-effect describes the case that the molecul axes align themselves more in direction of the electric field with increasing field strength (applied voltage). The Fréedericksz-effect is depicted in figure 5. figure 5

4 2.5 Waveguiding-effect and Schadt-Helrich-effect In twisted nematic layers of liquid crystal molecules the waveguiding-effect can appear. This effect means that the polarisation direction of the linearly polarised light follows the direction of the director. The Schadt-Helrich-effect is a special case of the waveguiding-effect, which is of particular importance for the function of LCD. In the case of the Schadt-Helrich-effect the director is turned by exactly 90 so that the light is transmitted through the crossed polarisation filters of a twisted nematic panel. The liquid crystal molecules are align parallel to the interfaces (electrodes). 2.6 Function of polarisation filters Polarisation filters can be made of foils consisting of long-chained hydrocarbon molecules. In the course of the production the foils are stretched and the molecules align themselves in direction of elongation. The foils are then dipped into a solution containing iodine, this process is similar to doping in semiconductor processing. The iodine atoms attach themselves to the foil and contribute conducting electrons. These electrons are free to move in direction of the molecule chains, but not perpendicular to them. This means that incident light is absorbed by the electrons if the electric field vector of the light wave is parallel to the molecule chain. The transmission direction is perpendicular to the molecule chain. As the electrons are not free to move in this direction, light with a polarisation perpendicular to the molecule chain, can travel through the polarisation filter without loss of intensity. 3 Setup and function of a LCD For the function of a LCD it is made use of the dielectric and optical anisotropy of the liquid crystal. The most common setup of a LCD is a twisted nematic panel. These displays have a 10 µm thick layer of a twisted nematic liquid crystal between to parallel glass plates. Two stripes of polymer foil at the edges serve as spacers to secure a constant distance between the two glass slabs. The glass slabs are coated with an electrically conductive and optically transparent layer (ITO layer=indium tin oxide) on the side facing the liquid crystal. This is important to exploit the dielectric anisotropy. In order to align the liquid crystal molecules along the glass slabes, the plates are structured by means of grooves. The grooves on the top and the bottom glass slab are perpendicular so that the twisted structure of the liquid crystal molecules is guaranteed. On the outer faces of the glass slabs polarisation foils are attached. The transmission directions of these foils are perpendicular to each other, this is important for the function of the Schadt-Helfricheffect. The LCD works as follows: non-polarised white light is linearly polarised at the first polarisation filter and its polarisation is turned by 90 within the liquid crystal due to the Schadt-Helrich-effect. So the light can then pass the second polarisation filter in the case of zero external voltage. If a voltage is applied between the two electrodes (ITO-cover) the molecules start to align themselves parallel to the electric field and leave the twisted arrangement. The polarisation of the light is not turned any longer and the transmission through the second filter is hindered. The setup of an LCD is schematically depicted in figure 6, the left plot shows the case of no external voltage applied (transmission high) whereas the right plot shows the case of an applied external voltage (transmission low). The manufacture of the home-made display is quite simple and described here in brief. A pattern is etched into the ITO-layer with a photo-lithographic process. Therefore a thin layer of photoresist is spincoated onto the ITO and then dried on a heating plate. The desired pattern is burned into the photoresist by exposing the glass slab to UV-radiation. Shadowing the UV-light with a metal cover defines the shape of the pattern. The exposed photoresist layer is then developed in a special solution. The pattern which is not covered by photoresist anymore is then exposed to concentrated hydrochloric acid in order to remove the ITO-layer. In a following step the remaining photoresist is

5 removed with aceton. In order to prepare the glass slabs with grooves the plates are rubbed over optical paper. The spacers are cut from polymer foils with a scalpel. The glass slabs are then placed upon another with the spacers inbetween and the ITO-layers facing each other and the grooves oriented in perpendicular directions. Two edges of the rectangle where the glass slabs overlap are glued twogether with two-component adhesive. If the two-component adhesive is hardended the liquid crystal is injected into the display with a syringe over the non glued edges. After that the two remaining edges are sealed with two-component adhesive. The final step is the sticking on the polarisation filters with transmission directions crossed. figure 6 The function of the LCD is checked with a simple test. Without external voltage supply the display is transparent, but when an AC voltage of 4 to 5 V is applied to the electrodes the display turns dark. Only the etched pattern stays transparent. The transmission of the display as function of the applied voltage is measured with a photodiode. This curve is called the electro-optical chararsteristic of a LCD and it is compared to the transmission of a commercially available display. In addition to that the degree of long range order as function of temperature is characterised by the experimental setup in figure 4. 4 Questions These questions are to be answered in the preliminary discussion and in the experimental report. 4.1 What is the meaning of positional and orientational long range order? Compare an ideal crystal with a liquid crystal. 4.2 Why is a laser beam split into two beams when passing a wedge-shaped liquid crystal. Explain the phenomen of birefringence. What would happen if the shape of the liquid crystal is parallel instead of wedge-shaped. 4.3 Which structural changes happen when the temperature of the liquid crystal in increased above the clearing point. And how do these changes affect the electrical and optical properties? 4.4 Why are grooves rubbed into the ITO-layer and why are the grooves perpendicular to each other? Why are the spacers between the glass slabs important? 4.5 Why is an AC voltage applied to the LCD and not a DC voltage? 4.6 How are LCD used for the display of colours and how are different colours displayed? 4.7 How is a pn-junction employed to serve as a photodiode? 4.8 How is the photocurrent normalised to a transmission value ranging from 0 to 1?

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