Colour

Color:

1. Color is determined by sum of frequent transmitted and reemitted light from direction transition.
2. Small difference in composition can lead to large difference in appearance.

For example: cadminium sulphide, energy gap.

• Electron volts absorbs higher energy visible light (blue and violet). Lower the energy light red yellow orange is transmitted and gives its color.

3. Highly pure single crystal \(Al_2O_3\)(aluminum to 2% oxide) sapphire is colorless. When 0.5 % of chromium oxide (\(Cr_2O_3\)) is added the material looks red.

$$sapphire+Cr_2O_3\Rightarrow red$$

4. The chromium substitute for aluminum and introduced impurity levels in the band gaps of the sapphire. These levels gives strong absorption at 400 nm (green) and 600nm (blue) leaving only red to be transmitted.
5. A similar technique is used to glass on rotatary glass by adding impurities in molten state. i.e.

$$Cu^2+\Rightarrow blue-green $$

$$Cr^++\Rightarrow green$$

$$Co^2+\Rightarrow blue-violet$$

$$Mn^2+\Rightarrow yellow$$

6. If \(E_g<1.8 ev\)- full absorption of light color is black. For example: gallium arsenic and silicon

If \(E_g>3.1 ev\) no absorption, colorless diamond.

If 1.8<\(E_g\)<3.1 ev there is partial absorption the material appears colored.

Luminescence:

It is the process where material absorbs energy and then immediately emits visible or near visible radiation. It consists of electron excitation and then dropping down to lower energy state. Visible light has energy in between 1.8 ev to 3.1 ev i.e. 1.8 ev<\(h\nu\)<3.1 ev.

Fluorescence:

If the re-imitation of light \radiation occur within \(10^{-8}\) sec, after excitation the luminescence is called fluorescence. If it takes longer than \(10^{-8}\)sec, it is called phosphorescence. Special material cooled called phosphorous have capability of absorbing high energy radiation and spontaneously emitting low energy radiation.

Example:

Some sulphide, oxide and few organic materials have luminescence character. The intensity of luminescence character. The intensity of luminescence is given by,

$$I=I_\circ e^\frac{-t}{\tau}$$

Where,

\(I_\circ\)=initial intensity of luminescence

I=fraction of luminescence after time‘t’

\(\tau\)=relaxation time (constant for material)

Luminescence process is classified as photon luminescence, cathode luminescence, and electron luminescence based on energy source for electron excitation.

Photo luminescence:

Photo luminescence occur in fluorescence lamp are between electrons excite mercury in lamp to higher energy level. Electron falls back emitting light (UV light). Fluorescence light consist of glass housing coated on inside with specially prepare silicates. Ultraviolet light is generated within a tube from a mercury glow discharge. Which causes the coating to fluorescence and emit white light.

Here ultraviolet radiation from low pressure mercury are is converted to visible light by calcium haloshulphide phosphor.

In commercial lamp, about 205 of fluorine are replaced by chlorine.

Antimony ion provide a blue emission while manganese ion provides an orange red emission band.

Cathode luminescence

Cathode luminescence is produced by an energized cathode which generates a beam of electrons.

Application of this includes electron microscope, cathode ray oscilloscope, colored T.V.

Modern T.V. have very narrow about 0.25 mm wide vertical strips of red, green and blue emitting phosphorous deposited on inner surface of screen.

Commercial phosphorous for different colors are:

Red-yattrium oxy-suftide 3% chromium

Green- with \(Cu^+\) acceptor and \(Al^{3+}\) donor.

Blue-zinc sulphide (ZnS) with Ag acceptor and \(cl^-\) donor.

Elecro-luminescence

Elecro-luminescence occurs in device with p-n junction which are stimulated by an extremely applied voltage.

When a forward biased voltage is applied across the device electron and holes recombined at the junction and visible photons are emitted. These diodes are known s LED light emitting diode.

LED emit light of many color from red to violet depending on the composition of the semiconductor materials.

References:

Callister, W.D and D.G Rethwisch. Material Science and Engineering. 2nd. New Delhi: Wiley India, 2014.

Lindsay, S.M. Introduction of Nanoscience . New York : Oxford University Press, 2010.

Patton, W.J. Materials in industry . New Delhi : Prentice hall of India, 1975.

Poole, C.P. and F.J. Owens. Introduction To Nanotechnology. New Delhi: Wiley India , 2006.

Raghavan, V. Material Science and Engineering. 4th . New Delhi: Pretence-Hall of India, 2003.

Tiley, R.J.D. Understanding solids: The science of Materials. Engalnd : John wiley & Sons , 2004