Liquid Crystal Devices Dr. Sally E. Day s.day@ee.ucl.ac.uk Abstract: Liquid Crys

Liquid Crystal Devices Dr. Sally E. Day s.day@ee.ucl.ac.uk Abstract: Liquid Crys www.phwiki.com

Liquid Crystal Devices Dr. Sally E. Day s.day@ee.ucl.ac.uk Abstract: Liquid Crys

Beamish, Jeff, Meteorologist has reference to this Academic Journal, PHwiki organized this Journal Liquid Crystal Devices Dr. Sally E. Day s.day@ee.ucl.ac.uk Abstract: Liquid Crystals Displays (LCD) – very common, low power, light-weight displays, as well as larger area flat panel displays as long as monitors in addition to TV applications. Liquid Crystals have a remarkable electro-optic coefficient, a large birefringence is switched with a very low voltage. Newer displays require complex structures with careful control of small features in the liquid crystal. This makes them of interest in other applications besides displays. This tutorial will cover the physical properties essential as long as the operation of liquid crystal devices including displays in addition to non-display applications. Contents: Structure-property relationships in liquid crystals Phases of liquid crystals Order parameter in liquid crystals Anisotropy in a liquid: Dielectric, optical in addition to viscoelastic properties Molecular structure in addition to influence on the physical properties Optical properties of liquid crystals Birefringence Polarisation of light Control of polarisation Structure of liquid crystal devices (LCD) Alignment Basic construction of LCDs Optical properties of display in addition to other devices Twisted nematic, In-plane switching, Vertically aligned nematic Holograms in addition to Beam steering Micro in addition to nano-structures in addition to liquid crystals

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Structure property relationships Phases of liquid crystals Liquid crystal materials are made of organic molecules. But to underst in addition to the phase behaviour these can be considered as rods. Structure property relationships Phases of liquid crystals Liquid crystals are liquids, but have some additional order associated with them, which is crystalline like. The simplest is the nematic phase:- the rods align in a particular direction, but have no positional order. Nematic liquid crystals are ‘milky’ looking liquids Structure property relationships Phases of liquid crystals Smectic phases have the additional order of layers, but they are not precise layers, but ‘density waves’ In addition to layering, there may be some other order, e.g. tilting within the layer. Smectic liquid crystals tend to be ‘wax’ like substances

Structure property relationships Phases of liquid crystals Other smectic phases have additional order within the layers This order may be in the as long as m of hexagonal packing The phases can be identified by the patterns that as long as m in addition to can be seen using a polarising microscope, or by X-ray scattering. The order between the molecules can also be seen by NMR Some of the polarising microscope images can be seen at http://reynolds.ph.man.ac.uk/~dierking/gallery/gallery1.html Structure property relationships Phases of liquid crystals – Discotic Liquid crystals Disc shaped molecules are the basic building blocks, in addition to the order can be in in terms of the orientation (nematic discotics) or in the as long as m of columns. Nematic discotic Structure property relationships Phases of liquid crystals – Discotic Liquid crystals Disc shaped molecules are the basic building blocks, in addition to the order can be in in terms of the orientation (nematic discotics) or in the as long as m of columns. The columns can then pack together to as long as m a two dimensional crystalline array. The columnar structure could be useful as long as 1-D conductors in addition to semi-conductors in addition to other properties along the columns. Nematic discotic Hexagonal Columnar phase

Structure property relationships Phases of liquid crystals Thermotropic liquid crystals phase as long as ms as a function of temperature Lyotropic liquid crystals Phase as long as ms as a function of concentration in a solvent Lyotropic phases occur as long as molecules dissolved in a solution Different phases occur with concentration Often the solvent is water in addition to the molecules have an hydrophilic end in addition to an hydrophobic end (e.g. detergents with polar (hydrophilic) in addition to non-polar (hydrophobic) end groups). The lyotropic liquid crystals as long as m many different phases, as with the thermotropic liquid crystals, but depending on concentration as well as temperature Structure property relationships Phases of liquid crystals – Lyotropic liquid crystals. Hexagonal phase Liquid crystal templates Lyotropic liquid crystal structures can be converted to solid structures using the sol-gel process to give silicates with the same structure as the liquid crystal phase. Other methods can be used to as long as m metal nano-particles

The lamellar phases are found in cell membranes This allows a liquid environment to exist, so transporting material around, but with a layer which controls the transport of material across the layer An example of the phase transition is from the lamellar liquid crystal phase to a gel phase, sometimes an undesirable transition. This transition occurs at different temperatures in addition to pressures depending on the environment that the organism lives in in addition to what is required Structure property relationships Phases of liquid crystals – Lyotropic liquid crystals. water Lamellar phase water gel phase Chemical structure of liquid crystal molecules Cyano biphenyl, shown above was the first stable liquid crystal developed at Hull University Chemistry Dept. – enabled the LCD industry to develop. Generally the rod shaped molecules can have the following structure: n=1,2,3 X,Y CmH2m+1; CmH2m+1-O; CN etc Aromatic Aliphatic Hetrocyclic Chemical structure of liquid crystal molecules Cyano biphenyl, shown above was the first stable liquid crystal developed at Hull University Chemistry Dept. – enabled the LCD industry to develop. Generally the rod shaped molecules can have the following structure: n=1,2,3 X,Y CmH2m+1; CmH2m+1-O; CN etc Aromatic Aliphatic Hetrocyclic

Chemical structure of liquid crystal molecules Cyano biphenyl, shown above was the first stable liquid crystal developed at Hull University Chemistry Dept. – enabled the LCD industry to develop. Generally the rod shaped molecules can have the following structure: n=1,2,3 X,Y CmH2m+1; CmH2m+1-O; CN etc Aromatic Aliphatic Hetrocyclic Chemical structure of liquid crystal molecules Cyano biphenyl, shown above was the first stable liquid crystal developed at Hull University Chemistry Dept. – enabled the LCD industry to develop. Generally the rod shaped molecules can have the following structure: n=1,2,3 X,Y CmH2m+1; CmH2m+1-O; CN etc Aromatic Aliphatic Hetrocyclic Chemical structure of liquid crystal molecules Chirality is an important property of some of the molecules: A chiral molecule cannot be superimposed on its mirror image. The carbon centre of the molecules below is the chiral centre. The enantiomers are identical except as long as the way in which they are arranged in space. Solutions or mixtures containing chiral molecules will rotate the plane of polarisation of light travelling through: Optical activity. A racemic mixture has equal amounts of each enantiomer. Synthesis of chiral compounds must be carried out carefully to make sure that a racemic mixture is not obtained.

Chiral liquid crystals The chiral nematic (Cholesteric) liquid crystal phase is a nematic phase, but the average direction of the molecules rotates through the material. Chemical structure of liquid crystal molecules The different chemical groups affect the physical properties in many ways, some important effects are as follows Phase transition temperatures Dielectric properties Optical properties Visco-elastic properties Ferroelectric, flexoelectric coefficients Chirality These physical properties in turn affect the per as long as mance of the displays in addition to other devices that contain liquid crystals Order parameter The order parameter is the degree to which the individual molecules align with the average direction. It is defined in terms of the angle that the molecules make with n, the vector describing the average direction An important property of this vector is that n = – n The order parameter (S) is typically S 0.65 as long as a liquid crystal; as long as a perfectly ordered crystal S = 1 in addition to as long as an isotropic liquid S = 0 If the temperature is increased in a thermotropic liquid crystal, the molecules become more disordered in addition to so the order parameter will reduce. n – the director

Anisotropy in a liquid The order in the liquid allows the material to have different properties in different directions In a liquid domains will as long as m. Alignment methods will have to be used to obtain a uni as long as m structure Scattering of light at the domain boundaries give the bulk a ‘milky’ appearance Elastic properties The molecules in the liquid crystal have a preferred orientation (the director) in addition to as a result if there is a distortion in the structure then there is an elastic energy associated with the distortion The elastic energy is anisotropic in addition to is described by three elastic constants, k11, k22, k33. Bend, k33 Twist, k22 Dielectric properties The electric permittivity of the liquid crystal is anisotropic The permittivity is concerned with the polarisability of the material in addition to the response of a material to an electric field. D=eoerE

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Dielectric properties The electric permittivity of the liquid crystal is anisotropic The permittivity is concerned with the polarisability of the material in addition to the response of a material to an electric field. D=eoerE + – + – + – + – + – + – + – E The field will induce dipoles in the material, which will create a field inside. P the polarisation. D = eoE+P = eoeE P Dielectric properties The electric permittivity of the liquid crystal is anisotropic The permittivity is concerned with the polarisability of the material in addition to the response of a material to an electric field. + – + – + – + – + – + – + – E If the field direction changes then the size of the dipoles will be different in an anisotropic material D = eoE+P = eoeE P Measurement of permittivity The permittivity is measured by making a capacitor filled with liquid crystal in addition to measuring the capacitance. The two values are measured by orienting the liquid crystal in two directions The anisotropy in the liquid crystal has values in the range from -10 De 40 in mixtures (where De = e-e) Capacitance meter Guard ring to avoid the effect of fringing fields. A d

Permittivity, dielectric constants. Reduced Temperature T/ TNI (TNI is the nematic to isotropic transition temperature) Permittivity or dielectric constant, from capacitance measurements Dielectric anisotropy in addition to electric fields in a liquid crystal. When an electric field is applied the energy can be minimised by reorientation of the liquid crystal, because it is a liquid. the stored energy of a parallel plate capacitor is: So W is minimised by making the dielectric constant as large as possible. Note: this is not the effect of a dipole in addition to does not depend on the polarity (sign) of the field A liquid crystal responds to the average (r.m.s) value of the electric field. Dielectric anisotropy in addition to electric fields in a liquid crystal. With positive dielectric anisotropy the director will line up with the electric field With negative dielectric anisotropy the director will line up perpendicular to the electric field

Summary Liquid crystals give electrically switchable anisotropic properties The organic molecules can be synthesized to have many different properties in addition to mixture as long as mulation allows greater flexibility in addition to control Optical anisotropy can be switched so as to modulate the polarisation of light, or the phase of light passing through the layer Displays are the most common application, but there are many different configurations depending on the application requirements High resolution structures in the liquid crystal allow them to be used as diffraction or holographic displays Future applications in photonics may be based on the use of particles or dopants in the liquid crystal Switching Between the Stable States via defect as long as mation along post. Intermediate, defect state. The Flexoelectric effect responsible as long as switching between the stable states. Side view Top view Post Post Post Post

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