2-nm layer for the smallest particles and a 1.3-nm layer for the largest particles.
Thus, the explanation for different luminescence intensities must lie in the different
particle size. Since, at constant concentration, the number of small particles exceeds
that of the larger particles, the luminescence intensity is related to the particle
Figure 9.18 Photoluminescence of
PbS/polystyrene nanocomposites with particle
sizes from 6.1 to 7.9 nm [13]. The relationship
between particle size and composition is shown
in Table 9.1. (a) Emission of PbS/polystyrene
nanocomposites in the IR range excited by
532 nm photons. As expected from the theory
of quantum confinement, the wavelength of
emission in the IR decreases with decreasing
particle size. The explanation for the different
luminescence intensities is found in the
different particle size. As the luminescence
intensity is related to the number of particles, at
constant concentration, the number of small
particles exceeds that of the larger particles;
hence, the luminescence intensity increases
with decreasing particle size. (b) Emission of
PbS/polystyrene nanocomposites in the visible
range. The excitation wavelength for this
emission was 325 nm. The emission in the
visible range does not follow the general rule of
blue shift with decreasing particle size and there
is no simple explanation for this phenomenon.
For the intensity, the same consideration as in
the case of the IR emission may hold.
222j 9 Optical Properties of Nanoparticles