nanoparticles. Here, the parabolic increase in the band gap with decreasing particle
size, as predicted by Eq. (9.6), is clearly visible. However, these data also show that
these phenomena are only of any importance when in the particle size range below
around 5 nm.
Figure 9.8 Energy bands in metals,
semiconductors, and insulators. Metals are
electric conductors because the highest
occupied band is not completely filled, allowing
movement of electrons, which is equivalent to
the possibility for electric conduction. In
semiconductors and insulators, the last
occupied band is filled completely; therefore,
electric conduction is impossible. However, in
semiconductors the energy gap is so narrow
that the thermal energy of some electrons is
sufficient to jump into the next band, creating
“holes” in the originally filled band and free
electrons in the next, initially empty, band.
Therefore, conduction of electricity is possible.
Additionally, the level of the Fermi energy E F
(the energy level of the least tightly bond
electron within a solid) is indicated.
Figure 9.9 Band gap energy E g for silicon and
germanium nanoparticles according to Khurgin
et al. [3]. Note the parabolic increase of the
band gap with decreasing particle size. These
data also show that the increase in band gap
width is important only for particles well below
5 nm diameter.
214j 9 Optical Properties of Nanoparticles
size, as predicted by Eq. (9.6), is clearly visible. However, these data also show that
these phenomena are only of any importance when in the particle size range below
around 5 nm.
Figure 9.8 Energy bands in metals,
semiconductors, and insulators. Metals are
electric conductors because the highest
occupied band is not completely filled, allowing
movement of electrons, which is equivalent to
the possibility for electric conduction. In
semiconductors and insulators, the last
occupied band is filled completely; therefore,
electric conduction is impossible. However, in
semiconductors the energy gap is so narrow
that the thermal energy of some electrons is
sufficient to jump into the next band, creating
“holes” in the originally filled band and free
electrons in the next, initially empty, band.
Therefore, conduction of electricity is possible.
Additionally, the level of the Fermi energy E F
(the energy level of the least tightly bond
electron within a solid) is indicated.
Figure 9.9 Band gap energy E g for silicon and
germanium nanoparticles according to Khurgin
et al. [3]. Note the parabolic increase of the
band gap with decreasing particle size. These
data also show that the increase in band gap
width is important only for particles well below
5 nm diameter.
214j 9 Optical Properties of Nanoparticles