Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

GTBL042-19 GTBL042-Callister-v2 September 17, 2007 17:39

Revised Pages

19.12 Photoconductivity • 775

For intrinsic polymers (without additives and impurities), the degree of translu-

cency is influenced primarily by the extent of crystallinity. Some scattering of visible

light occurs at the boundaries between crystalline and amorphous regions, again as a

result of different indices of refraction. For highly crystalline specimens, this degree

of scattering is extensive, which leads to translucency, and, in some instances, even

opacity. Highly amorphous polymers are completely transparent.

Applications of Optical

Phenomena

19.11 LUMINESCENCE

Some materials are capable of absorbing energy and then reemitting visible light

luminescence in a phenomenon calledluminescence.Photons of emitted light are generated from

electron transitions in the solid. Energy is absorbed when an electron is promoted to

an excited energy state; visible light is emitted when it falls back to a lower energy

state if 1.8 eV<hν<3.1 eV. The absorbed energy may be supplied as higher-energy

electromagnetic radiation (causing valence band–conduction band transitions, Fig-

ure 19.6a) such as ultraviolet light, or other sources such as high energy electrons,

or by heat, mechanical, or chemical energy. Furthermore, luminescence is classified

according to the magnitude of the delay time between absorption and reemission

events. If reemission occurs for times much less than one second, the phenomenon

fluorescence is termedfluorescence;for longer times, it is calledphosphorescence.A number of

phosphorescence

materials can be made to fluoresce or phosphoresce; these include some sulfides,

oxides, tungstates, and a few organic materials. Ordinarily, pure materials do not dis-

play these phenomena, and to induce them, impurities in controlled concentrations

must be added.

Luminescence has a number of commercial applications. Fluorescent lamps con-

sist of a glass housing, coated on the inside with specially prepared tungstates or sil-

icates. Ultraviolet light is generated within the tube from a mercury glow discharge,

which causes the coating to fluoresce and emit white light. The picture viewed on a

television screen (cathode ray tube screen) is the product of luminescence. The inside

of the screen is coated with a material that fluoresces as an electron beam inside the

picture tube very rapidly traverses the screen. Detection of x-rays andγ-rays is also

possible; certain phosphors emit visible light or glow when introduced into a beam

of the radiation that is otherwise invisible.

19.12 PHOTOCONDUCTIVITY

The conductivity of semiconducting materials depends on the number of free elec-

trons in the conduction band and also the number of holes in the valence band,

according to Equation 12.13. Thermal energy associated with lattice vibrations can

promote electron excitations in which free electrons and/or holes are created, as

described in Section 12.6. Additional charge carriers may be generated as a conse-

quence of photon-induced electron transitions in which light is absorbed; the atten-

photoconductivity dant increase in conductivity is calledphotoconductivity.Thus, when a specimen of

a photoconductive material is illuminated, the conductivity increases.

This phenomenon is utilized in photographic light meters. A photoinduced cur-

rent is measured, and its magnitude is a direct function of the intensity of the incident

light radiation, or the rate at which the photons of light strike the photoconductive

material. Of course, visible light radiation must induce electronic transitions in the

photoconductive material; cadmium sulfide is commonly utilized in light meters.