Chemistry Of Liquid Crystal Displays Engineering Essay

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Nanoscience includes the study and creation of materials, devices, and systems by manipulating individual atoms and molecules. It describes research where the characteristic dimension of 1 or more planes is less than 100 nanometers i.e., 0.1 microns.

As we know that all manufactured products are made from atoms and the properties of those products depend on how atoms are arranged in them.

So if we can rearrange the atoms in:

Coal we get diamonds or graphite.

Sand (adding a few other trace elements) we get computer chips.

Dirt, water and air we can make potatoes.

Unlike their larger or "macro" counterparts, nanoparticles often display unique properties that may be of use for various applications.

Introduction to Liquid Crystals

The study of liquid crystals began (1888) when an Austrian botanist, Friedrich Reinitzer observed that a material known as cholesteryl benzoate had two distinct melting points. In his experiments, Reinitzer increased the temperature of a solid sample and watched the crystal change into a hazy liquid. As he increased the temperature further, the material changed again into a clear, transparent liquid. Because of this early work, Reinitzer is credited with discovering a new state of matter - the liquid crystal state.

What are Liquid Crystals?

There are solids which when heated undergo two sharp phase transformations one after the other. They first fuse sharply yielding turbid liquid and then again equally sharply at higher temperature yielding clear liquids.

These changes are exactly reversed on cooling at the same temperatures. The turbid liquid however show anisotropy (i.e., they have different physical properties in different directions). Anisotropy is observed particularly in their optical behavior.

Thus they exhibit double refractions and interference patterns in polarized light. True liquids, on the contrary, are isotropic (i.e., they have same properties in all directions).

Since anisotropic properties are associated with crystalline state, the turbid liquids are called liquid crystals. And as they are neither solids nor liquids, hence, the term mesomorphic state (meaning intermediate form, in Greek) probably fits the best. But liquid crystal also continues to be used even in present day literature.

Obviously, an essential requirement for mesomorphism to occur is that the molecule should be anisotropic in shape, like rod or a disc. Industrial Lubricants exist, more or less, in liquid crystalline, i.e., mesomorphic state. The proteins and fats also exist or get changed into this state before digestion and are thus easily assimilated into the body. This state is believed to play an important role in the nutritional and other processes.

Note the average alignment of the molecules for each phase in the following diagram.

It is sometimes difficult to determine whether a material is in a crystal or liquid crystal state.

Crystalline materials demonstrate long range periodic order in three dimensions.

By definition, an isotropic liquid has no orientational order.

Substances that are not ordered as solids, yet have some degree of alignment are properly called liquid crystals.

PHASES OF LIQUID CRYSTALS

There are two basic phases of Liquid Crystals:

NEMATIC SMECTIC

Liquid crystals act like polarizing filters. The orientationally ordered rod-like molecules in liquid crystals affect the polarization of the transmitted light.

What is Liquid Crystal Display…?

A process of video display which is based on liquid crystal layer sandwiched between two polarizing transparent glass layers. The crystals that are charged by transistors open and close to allow and block the passage of light. In a projection format, light is passed through the LCD panel, from an internal lamp, onto the screen and each crystal is represented by one pixel.

Types of LC Displays

There are the following types of Liquid Crystal Displays:

Activematrix liquid crystal display

Cholesteric liquid crystal display

Transflective liquid crystal display

ST liquid crystal display

Applications Of LCD

Types of Display Screens Used Today

There are two basic types of display screens used in today's time that are as following:

Cathode Ray Tube Display

Liquid Crystal Display

What is CRT Display…?

CRT Display is an analog display device which uses a vacuum tube that generates images, on a layer of phosphors driven by an electron beam, inside the tube.

How Does CRT Works…?

CRTs work by illuminating dots of phosphor on the screen by firing a beam of electrons from the back of the tube.

At the rear of the tube an emitter is present. A beam of electrons is sent towards the screen at the front of the tube, by the emitter. The beam of electrons is passed through a magnetic field which targets it at specific position on the screen, at the front of the tube. The screen in front of the tube is coated with phosphor which glows when it is struck by electron beam. The phosphor dots are arranged in small groups of red, green and blue and by combining these three primary colors, any color can be created.

Advantages of CRT

Viewing Angle

Due to the glowing dots being near to the surface of glass, each dot is equally bright from any viewing angle.

b.) color fidelity

As each dot is self-illuminating, uneven lighting or viewing angles would not cause color variations across the surface of the screen.

No "dead pixels"

CRT's don't suffer from the problem of "dead pixels" that effect Plasma and LCD displays because the process of laying down the phosphor dots is very simple.

Disadvantages of CRT

Weight

As the CRT screens get larger, their glass must get thicker to withstand the pressure of the vacuum inside which would result in televisions weighing hundreds of pounds.

b.) Phosphor Burn-in

The phosphor compounds which actually create the visible light in the CRT display become less luminous with use. This is one of the main reasons for why an old television set look dim. If the display is consistently used to show the same image for a long period of time, the image can get "burned in" to the screen and it appears as a darkened image whenever the display is turned on.

This is a common issue in cases when the display is connected to a computer where the common screen elements such as menus or windows may remain on the screen for a long period of time.

While the new sets use a no. of techniques to reduce the occurrence of burn-in drastically, the overall brightness of the display still diminishes over time.

c.) Size

CRT displays are limited to size around 45 inches. They are also deeper than almost all other displays because the electron gun is placed far enough behind the screen such that the entire surface can be hit by it. Earlier the electron gun had a sweep angle of 90 degrees but in the late 1990's many CRT's started using electron guns with 100 degrees arc, which allowed shorter tubes to be used.

How Does Liquid Crystal Display Works…?

The liquid crystal display works by shining a constant, pure white light, first through a liquid crystal "shutter" and then through a colored filter to create each dot (pixel) on the display. Each pixel is actually made up of three of these dots of red, green, and blue, each.

The liquid crystal "shutters" first pass the polarizing the light in one direction and then pass it through a layer of liquid crystal.

The liquid crystals are used to rotate the polarization of the light passing through them, when the current is applied and the amount of rotation is controlled by varying the current.

The light is then passed through a color filter for the removal of other components of the white light, leaving only the desired colors.

The colored (and polarized) light is then passed through a final polarizing filter which is rotated 90 degrees with respect to the original polarizing filter. If the light had not been rotated, then no light would've passed through the second polarizing filter and the dot would appear dark.

If the light had been fully rotated, then it would pass through the filter at maximum intensity.

Advantages of LCD

The advantages of a liquid crystal display are:

Flat

Due to the absence of any large tube, the LCD is much thinner than a CRT display.

Lightweight

Because it requires a source of light and the lightweight LCD shutters, the liquid crystal display is much lighter than CRT displays.

Low Power Consumption

Although the light source is required to run at maximum power over the entire surface of the display, LCD's generally use lesser energy to run than CRT displays. Nowadays LCD displays use LED backlights use less energy than any other display.

Disadvantages of LCD

The disadvantages of the Liquid crystal display are:

Viewing Angle

Because of the color filters and liquid crystals sitting between the viewer and the light source, each pixel on an LCD display exists in a small "box" that prevents light from bleeding from one pixel to another. The "walls" of these boxes block the line-of-sight between the light source and the viewer when viewed at an angle which reduces the perceived brightness of that pixel. Newer and more expensive displays are using shallower boxes to minimize this issue.

Response Time

Earlier, LCD displays had been plagued by slow response times because of the liquid crystals taking time to change state which results in "blurring" or "streaking" of fast-moving images.

Newer models do not suffer from this display "lag" as much.

Color fidelity

LCD displays use a single light source that eliminates each pixel in the display evenly. It is very difficult to manufacture a light source which is evenly lit across its entire surface.

Inexpensive LCD's often have uneven lighting across the display. In addition, on larger displays, the viewer's angle from the display may vary quite a bit from the center to the edges of the screen. Because of this the pixels near the edges appear darker because they are not being viewed head-on.

Contrast

As the light source is always "on" behind every dot of the LCD, it is difficult to completely block the light at a given pixel and achieve true black.

LCD Construction & Nomenclature

1.) F substrate (glass)

2.) Terminal

3.) Segment electrode

4.) Common electrode

5.) B substrate (glass)

6.) Upper polarizing plate

7.) Orientation layer

8.) Sealant

9.) LC (liquid crystal)

10.) Conducting material

11.) Sealant

12.) Inlet

13.) Viewing area

14.) Lower polarizing plate, or lower polarizing plate and reflecting plate

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