Liquid Crystal Devices

1. Liquid Crystal Devices Dr. Sally E. Day s.day@ee.ucl.ac.uk 1

2. Abstract: Liquid Crystals Displays (LCD) – very common, low power, light-weight displays, as well as larger area flat panel displays for monitors and 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 for the operation of liquid crystal devices including displays and non-display applications. 

3. Contents: • Structure-property relationships in liquid crystals • Phases of liquid crystals • Order parameter in liquid crystals • Anisotropy in a liquid: Dielectric, optical and viscoelastic properties • Molecular structure and 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 and other devices • Twisted nematic, In-plane switching, Vertically aligned nematic • Holograms and Beam steering • Micro and nano-structures and liquid crystals

4. C2H5 CN Structure property relationships Phases of liquid crystals • Liquid crystal materials are made of organic molecules. • But to understand the phase behaviour these can be considered as rods.

5. 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

6. 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

7. Structure property relationships Phases of liquid crystals • Other smectic phases have additional order within the layers • This order may be in the form of hexagonal packing • The phases can be identified by the patterns that form and 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

8. Structure property relationships Phases of liquid crystals – Discotic Liquid crystals • Disc shaped molecules are the basic building blocks, and the order can be in in terms of the orientation (nematic discotics) or in the form of columns. Nematic discotic

9. Columnar phase Structure property relationships Phases of liquid crystals – Discotic Liquid crystals • Disc shaped molecules are the basic building blocks, and the order can be in in terms of the orientation (nematic discotics) or in the form of columns. • The columns can then pack together to form a two dimensional crystalline array. • The columnar structure could be useful for 1-D conductors and semi-conductors and other properties along the columns. Nematic discotic Hexagonal Columnar phase

10. Structure property relationships Phases of liquid crystals Thermotropic liquid crystals phase forms as a function of temperature Lyotropic liquid crystals Phase forms as a function of concentration in a solvent

11. Structure property relationships Phases of liquid crystals – Lyotropic liquid crystals. • Lyotropic phases occur for molecules dissolved in a solution • Different phases occur with concentration • Often the solvent is water and the molecules have an hydrophilic end and an hydrophobic end (e.g. detergents with polar (hydrophilic) and non-polar (hydrophobic) end groups). • The lyotropic liquid crystals form many different phases, as with the thermotropic liquid crystals, but depending on concentration as well as temperature Hexagonal phase

12. 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 form metal nano-particles

13. Structure property relationships Phases of liquid crystals – Lyotropic liquid crystals. • 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 and pressures depending on the environment that the organism lives in and what is required water water Lamellar phase gel phase

14. C2H5 CN X Y F F F n CN CH2 CH2 CO N O CH CH N N O O N CH CH 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

15. C2H5 CN X Y F F F n CN CO CH2 CH2 N O CH CH N N O N CH CH 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

16. C2H5 CN X Y F F F n CN CO CH2 CH2 N O CH CH N N O N CH CH 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

17. C2H5 CN X Y F F F n CN CO CH2 CH2 N O CH CH N N O N CH CH 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

18. H13C6 C6H13 H H OH HO H3C CH3 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 for 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.

19. 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.

20. 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 performance of the displays and other devices that contain liquid crystals

21. q Order parameter n – the director • 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 for a liquid crystal; for a perfectly ordered crystal S = 1 and for an isotropic liquid S =0 • If the temperature is increased in a thermotropic liquid crystal, the molecules become more disordered and so the order parameter will reduce.

22. Anisotropy in a liquid • The order in the liquid allows the material to have different properties in different directions • In a liquid domains will form. Alignment methods will have to be used to obtain a uniform structure • Scattering of light at the domain boundaries give the bulk a ‘milky’ appearance

23. Splay, k11 Elastic properties • The molecules in the liquid crystal have a preferred orientation (the director) and 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 and is described by three elastic constants, k11, k22, k33. Bend, k33 Twist, k22

24. Dielectric properties • The electric permittivity of the liquid crystal is anisotropic • The permittivity is concerned with the polarisability of the material and the response of a material to an electric field. • D=eoerE

25. Dielectric properties • The electric permittivity of the liquid crystal is anisotropic • The permittivity is concerned with the polarisability of the material and the response of a material to an electric field. • D=eoerE The field will induce dipoles in the material, which will create a field inside. P the polarisation. D = eoE+P║=eoe║E E + - + - + - P + - + - + - + -

26. Dielectric properties • The electric permittivity of the liquid crystal is anisotropic • The permittivity is concerned with the polarisability of the material and the response of a material to an electric field. If the field direction changes then the size of the dipoles will be different in an anisotropic material D = eoE+P┴=eoe┴E + - + - E + - P + - + - + - + -

27. Measurement of permittivity • The permittivity is measured by making a capacitor filled with liquid crystal and 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 (whereDe = e║-e┴) Guard ring to avoid the effect of fringing fields. A Capacitance meter d

28. Permittivity, dielectric constants. Permittivity or dielectric constant, from capacitance measurements Reduced Temperature T/ TNI (TNI is the nematic to isotropic transition temperature)

29. Dielectric anisotropy and 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 and 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.

30. E E De = e║-e┴ < 0 De = e║-e┴ > 0 Dielectric anisotropy and 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

31. X Y F F F n CN CH2 CH2 CO N O CH CH N N O O N CH CH Influence of chemical structure on permittivity • Conjugation will increase the polarisability • Dipolar groups will increase the dipoles, either el or et depending on the position in the molecule n=1,2,3 X,Y CmH2m+1; CmH2m+1-O; CN etc Aromatic Aliphatic Hetrocyclic

32. X Y F F F n CN CH2 CH2 CO N O CH CH N N O O N CH CH Influence of chemical structure on permittivity • Conjugation will increase the polarisability • Dipolar groups will increase the dipoles, either el or et depending on the position in the molecule n=1,2,3 X,Y CmH2m+1; CmH2m+1-O; CN etc Aromatic Aliphatic Hetrocyclic

33. Permittivity as a function of frequency • As the frequency of the electric field is increased the permittivity will change. • At optical frequencies the dielectric anisotropy will be positive and can be related to the birefringence as follows: • n║2 = e║ and n┴2 =e┴ • Dn = n║ - n┴, the birefringence • The refractive index of the bulk depends on • the polarisability of the molecules • the order parameter as the temperature is increased the birefringence will reduce.

34. Measurement of refractive indices • The optical refractive indices can be obtained from Abbé refractometer measurements. • An aligned sample of liquid crystal is put onto a prism • The critical angle, qc, at which total internal reflection occurs is measured • By changing the polarisation of the light observed both refractive indices can be measured. qc qc

35. Refractive indices of liquid crystal Refractive indices, from Abbé refractometer measurements Reduced Temperature T/ TNI (TNI is the nematic to isotropic transition temperature)

36. CH2 CH2 CH CH CH CH Refractive indices of liquid crystal • The refractive index depends on the polarisability of the molecules and also depends on the order parameter • Polarisability varies with chemical group, increasing with increased conjugation • The birefringence is always positive, because there is no influence due to purely dipoles. increasing conjugation

37. Refractive indices of liquid crystal • The birefringence is critical to the optical properties of the liquid crystal and underlies many of the applications of liquid crystals. • By reorienting the liquid crystal the effective birefringence will change and so the optical properties will change

38. ne nx no Birefringence • Optically anisotropic materials have different optical properties depending on the polarisation of the light travelling through the material. • This is described by different refractive indices in the material. • For a uniaxial material such as liquid crystals there are two values for the refractive index. The refractive indices can be described by an optical indicatrix. Shown in the figure. q

39. Optical Indicatrix ne no

40. Optical Indicatrix ne q no

41. Optical Indicatrix ne q no

42. Optical Indicatrix ne nx q no

43. Optical Indicatrix ne nx q no

44. Optical Indicatrix The angle of incidence of the light may change ne nx q no

45. Optical Indicatrix ne nx q no

46. Optical Indicatrix The orientation of the optical indicatrix may change – this occurs when liquid crystals switch ne nx q no

47. l0 z x y Polarised light • Light is a transverse electromagnetic wave, • the electric field, the magnetic field and the direction of propagation are all at right angles to each other. • The wave is time varying • frequency given by n, • speed given by c= nlo in a vacuum • in a medium of refractive index n, the wavelength is changed by l=lo/n. A full analysis of polarised light must include both the electric and magnetic components of the light; this is particularly necessary when considering reflected components In transmissive optical systems the reflected light does not have to be considered in such detail and the light is considered only in terms of the electric components.

48. Polarised light Light can have two orthogonal states or polarisations. The waves can be written as follows: If f = 0 or 2p or an integral multiple then the light is plane polarised with The direction of propagation is taken to be along z These are waves travelling in the z-direction with a relative phase between them of f.

49. Circularly polarised light If the waves have equal amplitudes E0and the relative phase is -p/2 + 2mp (where m = 0, 1, 2, …..) then circularly polarised light is obtained.The components are then The intensity of the light is E.E = E02, a constant, but the direction of E is time varying and is rotating with angular frequency of w = 2pf. The light is described as circularly polarised – it can be right or left circularly polarised.

50. time y Ex component Ey component x Elliptical polarisation In a general case E0x  E0yand f has any value, the light is elliptically polarised. The electric vector E then rotates as a function of time and the amplitude varies as well. Linear and circular polarisations are special cases.