C hapter 4 Material Classes, structure, and properties
140
was originally absorbed. The light emitted from the material is a
mix of the transmitted and the reemitted wavelengths, and it is this
that gives it a characteristic color.
More specific control of color is possible by doping—the deliberate introduction of impurities that create a new energy level in the
band gap, as in Figure 4.70. Radiation is absorbed as before, but it
is now reemitted in two discrete steps as the electrons drop first into
the dopant level, emitting a photon of frequency ν 1 = ΔE 1 /h, and
from there back into the valence band, emitting a second photon of
energy ν 2 = ΔE 2 /h. Particularly pure colors are created when glasses
are doped with metal ions: copper gives blue; cobalt, violet; chromium, green; manganese, yellow.
fluorescence, phosphorescence,
and electroluminescence
Electrons can be excited into higher energy levels by incident
photons, provided they have sufficient energy. Energetic electrons,
like those of the electron beam of a cathode-ray tube, do the same.
In most materials the time delay before they drop back into lower
levels, reemitting the energy they captured, is extremely short, but in
some there is a delay. If, on dropping back, the photon they emit is
in the visible spectrum, the effect is that of luminescence; the material continues to glow even when the incident beam is removed.
When the time delay is fractions of seconds, it is called fluorescence,
used in fluorescent lighting, where it is excited by ultraviolet from
a gas discharge, and in TV tubes, where it is excited by the scanning
electron beam. When the time delay is longer, it is called phosphorescence; it is no longer useful for creating moving images but is used
instead for static displays, such as watch faces, where it is excited by
Figure 4.70
Impurities or dopants create energy levels in the
band gap. Electron transitions to and from the
dopant level emit photons of specific frequency
and color.
Ground
state
Fermi
level
Hole
Electrons
Empty
levels
(conduction
band)
Impurity or
dopant level
Photon in
Excited
state
Return to
donor level
Photon 2
out
Fermi
level
Fermi
level
∆E 2
∆E 1
Photon 1
out
Return to
ground state
140
was originally absorbed. The light emitted from the material is a
mix of the transmitted and the reemitted wavelengths, and it is this
that gives it a characteristic color.
More specific control of color is possible by doping—the deliberate introduction of impurities that create a new energy level in the
band gap, as in Figure 4.70. Radiation is absorbed as before, but it
is now reemitted in two discrete steps as the electrons drop first into
the dopant level, emitting a photon of frequency ν 1 = ΔE 1 /h, and
from there back into the valence band, emitting a second photon of
energy ν 2 = ΔE 2 /h. Particularly pure colors are created when glasses
are doped with metal ions: copper gives blue; cobalt, violet; chromium, green; manganese, yellow.
fluorescence, phosphorescence,
and electroluminescence
Electrons can be excited into higher energy levels by incident
photons, provided they have sufficient energy. Energetic electrons,
like those of the electron beam of a cathode-ray tube, do the same.
In most materials the time delay before they drop back into lower
levels, reemitting the energy they captured, is extremely short, but in
some there is a delay. If, on dropping back, the photon they emit is
in the visible spectrum, the effect is that of luminescence; the material continues to glow even when the incident beam is removed.
When the time delay is fractions of seconds, it is called fluorescence,
used in fluorescent lighting, where it is excited by ultraviolet from
a gas discharge, and in TV tubes, where it is excited by the scanning
electron beam. When the time delay is longer, it is called phosphorescence; it is no longer useful for creating moving images but is used
instead for static displays, such as watch faces, where it is excited by
Figure 4.70
Impurities or dopants create energy levels in the
band gap. Electron transitions to and from the
dopant level emit photons of specific frequency
and color.
Ground
state
Fermi
level
Hole
Electrons
Empty
levels
(conduction
band)
Impurity or
dopant level
Photon in
Excited
state
Return to
donor level
Photon 2
out
Fermi
level
Fermi
level
∆E 2
∆E 1
Photon 1
out
Return to
ground state