LIQUID

1. LIQUID CRYSTALS by Claudia Weinberg

2. OUTLINE • What are Liquid Crystals? • Brief History of Liquid Crystals • Liquid Crystals Characteristics • Why are Liquid Crystals so Interesting? • What is a Liquid Crystal Display (LCD)? • Other Applications of Liquid Crystals • Summary

3. Reinitzer suggested this cloudy liquid is a new phase of matter, the Liquid Crystal phase: an intermediate phase having the ordering properties of solids but flowing like liquids Solid Phase Liquid Phase Liquid Crystal Phase Fixed position Fixed orientation Random position Random orientation What are Liquid Crystals? 1888 - the Austrian botanist Friedrich Reinitzer observed that cholesteryl benzoate had two distinct melting points: a. the crystal changed into a cloudy liquid b. increasing further the temperature, the material changed again into a clear, transparent liquid

4. Brief History of Liquid Crystals 1900s - George Freidel conducted many experiments on liquid crystals and explained the orienting effect of electric fields and the presence of defects in liquid crystals 1922 - World War II - Oseen and Zöcher developed a mathematical basis for the study of liquid crystals 1968 – First LCD was demonstrated by scientists from Radio Corporation of America 1972 - First active-matrix LCD panel was produced in USA by Peter Brody 2007 - First double-sided LCD panel and the World's slimmest LCD panel are produced by Samsung Electronics.

5. Rod-like Discotic shaped Banana shaped Liquid Crystals characteristics • Composed of moderate size organic molecules • Rod-like molecular structure, less usual Discotic shaped and Banana shaped • Thermotropic(temperature dependent) orLyotropic “water loving”(concentration/thickness and temperature dependent)

6. Light propagation in an isotropic medium Light propagation in a birefringent medium Liquid Crystals characteristics • Orientational order – molecules pointing in the same direction, called director • Birefringent: two different refractive indices for the director direction and the perpendicular to director direction • Easily polarizable substituents: weak electric field suffices to align the director with the applied field

7. Order Parameter of Liquid Crystals To quantify how much order is present an order parameter S is defined: * theta is defined as the angle between the director and the long axis of each molecule: • For isotropic average cosine=0, therefore S=0, indeed no order • For perfect crystalline S=1 • For liquid crystal, typical values of S are 0.3 - 0.9, function of temperature A typical S graph for a nematic liquid crystal:

8. Liquid Crystals Phases Crystalline Smectic Nematic Liquid Greek word for “soap” Greek word for “thread” High orientational order Some positional order, layers High orientational order Random positional order

9. Liquid Crystals Phases A special class of nematic liquid crystals is called chiral nematic, the molecules are not symmetric when reflected and lie next to each other in a slightly skewed orientation – the Cholesteric Phase The director is helical through the material, this structure representing key to the operation of Twisted Nematic Liquid Crystal Displays Chiral nematic There are also smectic liquid crystals who present chirality, where the tilted director rotates from layer to layer forming a helical structure Chiral smectic

10. Why are LC so interesting? Two unusual phenomena liquid crystals experience made them interesting for technological uses: • Reorientation of molecules in an electric field: the ability of the director to align along an external applied field, resulting in permanent electric dipoles Situation in electric field Result electric field Original orientation Result strong electric field Importance: switchable medium by varying the applied field

11. L A liquid crystal layer between crossed polarizers • Opticalbirefringence: results from LC’s anisotropy nature and changes the polarization state of the light(as light polarized parallel to the director has a different index of refraction, than light polarized perpendicular to the director) Polarizer with vertical transmission axis Crossed polarizers Polarizer with horizontal transmission axis Importance: modifying and controlling the polarization state of the light after the liquid crystal layer by varying thickness and n Typical nematic n difference

12. Birefringence is temperature dependent since it results from LC’s anisotropy which shows a strong temperature dependence (vanishing at the nematic phase) Example: Microscope picture of a nematic liquid crystal, taken between crossed polarizers. The light and dark areas denote regions of differing director orientation, birefringence, and length.

13. What is a Liquid Crystal Display? An LCD consists primarily of two glass plates with a thin film of liquid crystal material between them (thickness 6-8 um) and transparent electrodes to apply an electric field The most common LCD is twisted nematic display: crossed polarizers alignment layers on top and bottom substrate rubbed perpendicular to each other: the director in the left pixel makes a 90° twist from bottom to top  “Twisted Nematic LCD” transparent electrodes deposited on the glass (for control of the reorientation of the director)

14. How does it work? LEFT PIXEL: light enters the liquid crystal layer with a polarization parallel to the bottom director, follows the rotation of the director, is transmitted through the top polarizer and the pixel is bright

15. How does it work? RIGHT PIXEL: voltage is applied between the two electrode layers, the liquid crystal director is reoriented and the polarization of the light will no longer rotate through the liquid crystal layer, the light is absorbed at the top polarizer and the pixel is dark Note: Parallel polarizers will reverse the states Color LCD: same technique, each pixel being divided into 3 cells - Red, Green, Blue (which yield many possible colors)

16. Addressing Pixels Addressing is the process by which pixels are turned on and off in order to create an image. Two main types of addressing: • Direct addressing: convenient for displays with only a few elements that have to be activated. Each pixel in the display has its own drive circuit and a microprocessor must individually apply a voltage to each element. Ex: 7-segment LCD found in wristwatches, calculators

17. Multiplex addressing: reduces the complexity of the circuitry when a larger number of pixels are involved. Addressing pixels by rows and columns instead of each element being driven separately. Active matrix –eachpixeladdressed using a thin film transistor (TFT) acting as a switch (column voltage felt only at row addressed) Passive matrix – pixel addressed by a set of multiplexed transparent electrodes, perpendicular to one another, above and below the liquid crystal layer in a row and column formation • superior picture quality and viewing characteristics • high switching speed (few msec) • more power requirements • expensive to fabricate • less expensive • low power requirements • low cost • suffer from reduced contrast • effective viewing angle is small • need low switching speed (150msec) to address all the pixels

18. Passive matrix analysis When addressed, a voltage is applied, the pixel has a short turn-on time (molecules align so pixel is opaque) When the voltage is removed the pixel behaves similar to a discharging capacitor, slowly turning off and becoming clear As long as the time to scan the entire matrix is shorter than the turn-off time, a multiple pixel image can be displayed Row not addressed: OFF=0V Column addressed: -1V=ON, 0V=OFF (by other row) Row addressed: ON=+1V Column : -1V=ON, 0V=OFF Pixel ON average voltage: [2+(N-1)/2]/N Pixel OFF average voltage: [1+(N-1)/2]/N Selection ratio (contrast): (N+3)/(N+1) for SR=1.01: Nmax=200 rows for SR=1.05: Nmax=60 rows

19. Improvements in passive matrix displays Super-Twisted Nematic LCD Twisted Nematic: differences in the OFF and ON voltages must be very small when addressing many pixels with a multiplexing scheme Difficult to achieve with TN Super-Twisted Nematic: director rotates through an angle of 270° Differences in the OFF and ON voltages is smaller, MUXing up to 500 rows LCD Sharpness = V90-V10

20. Dual Scan Super-Twisted LCD Two separate displays placed adjacent and scanned simultaneously by separate electronics  ½ scanning for each Double Cell Super-Twisted LCD Two cells placed on top of eachother with opposite helical directions  undo the disturbance caused by circular birefringence  improved contrast, true B&W. “Active” addressing Several rows are addressed simultaneously in different patterns corresponding to orthogonal functions.

21. Active matrix displays An active matrix display contains, besides polarizing sheets and liquid crystal cells, a matrix of thin-film transistors (TFTs) which act as a switch for each pixel and actively maintain the pixel state

22. Active matrix displays Results: • much brighter, sharper display (than a passive matrix of the same size) • contrast at least 40:1 • viewing-angle is improved (45°) • active matrix NxM requires only N+M connectors  switching time is reduced to few msec There are many different active matrix displays on the market, each associated with a specific physical display resolution (the number of pixels on the entire screen)

23. Active matrix displays Video Graphics Array Extended Graphics Array Widescreen Super Extended Graphics Array

24. Active matrix displays Pixels can be square or rectangular, depending on aspect ratio. An example of pixel shape affecting "resolution" or perceived sharpness: if you have two similar displays, both the same picture height and both having a 720×576 pixel array but one a 16:9 wide screen, the other a 4:3 screen and you display the same 720×576 4:3 picture on both, apart from the picture being stretched on the 16:9 display, the 4:3 screen will look the sharper.

25. Display Lighting • In order for a display to show information, it must have a light source • Displays using only ambient light employ a reflective surface mounted behind the display - most calculators and watches are like this not very bright because the light must pass through multiple polarizers which severely cut down on the intensity of the light  • Displays using back lighting system: light bulbs mounted behind and at the edges of the display brighter displays but more power intensive  

26. Commercial LCDs

27. Other Applications of Liquid Crystals • Thermometers for circuit boards, battery condition testers • Shampoo and body wash • Paints: Mercedes offers to paint your car with a paint made of LC so as the car passes you it will appear to shift color, due to change in viewing angle • Solitary wave propagation: A high intensity laser beam injected in a LC can produce a local reorientation of the director molecules so light produces it's own waveguide andstays confined in a narrow beamAddressable liquid crystal waveguide to switch light between several optical fibers

28. Liquid crystal solar cell: the Liquid Crystal Semiconductor and the creation of a band gap similar to semiconductors can lead to new types of solar cells

29. Hot Spot Detection: locating heat sources associated with the electrical failure of a device; LC can easily detect a point source of 1mW and under optimal conditions even 1uW. The mottled, colorful area is the natural appearance of a thin film of liquid crystal viewed under cross polarized conditions. The pulsing dark spot indicates where 1 to 2 mW of power from abnormal leakage heats the defect area above 29°C (characteristic for this LC) 

30. Summary • Liquid Crystals – Brief History, Characteristics, Phases • Why are Liquid Crystals so Interesting – Molecules Reorientation in an Electric Field • LCDs: How does it Work, Addressing pixels, Passive Matrix vs. Active Matrix • Other Applications of Liquid Crystals

31. QUESTIONS ?