This concept makes use of the vast amount of organic lumophore molecules
available. The ceramic core is selected according to the properties demanded and, as
shown later, it may even be used to add an additional physical property besides
luminescence to the particle. The polymer coating at the outer surface is selected
according to the demands with respect to the surroundings during application. In
general, PMMA is used for coating, but in order to obtain a highly hydrophobic layer
fluorinated aliphatic compounds are used, while for hydrophilic layers PHPMA is
selected. The best results are obtained using a monolayer of lumophores; moreover,
to minimize parasitic absorption in the coating the outer polymer layer is made as
thin as possible, usually less than 1 nm.
A typical spectrum of such a composite is shown in Figure 9.43a. For these
particles, iron oxide was used as the ceramic core, PMMA as the outer coating, and
pyrene was applied as the lumophore. The emission spectrum of pure solid pyrene
is also plotted in Figure 9.43a. Clearly, the spectrum of the nanocomposite is quite
similar to that of the pure lumophore, with just a slight blue shift of about 15 nm;
more prominently, the spectrum of the composite has lost the fine structure
characteristic of pyrene. In contrast to pyrene, the situation with composites
containing anthracene as lumophore is more complicated. Figure 9.43b shows
the spectra of pure anthracene and a composite using hafnia as ceramic core. As
shown in Figure 9.43b, anthracene shows a more structured spectrum as compared
to pyrene. Each of the maxima in the spectrum can be associated with a vibration
mode. On comparison, the spectra of pure anthracene and the anthracene/hafnia
composite are essentially identical, except for the strongest line of the composite
(positioned between vibration modes 5 and 6), which does not appear in the pure
anthracene spectrum.
As described for the luminescent oxide/PMMA nanocomposites, there is a strong
interaction between the oxide core and the lumophore coating. The influence of the
ceramic core (which is selected according to its interaction with the exciting UV
radiation) on the luminescence intensity of different oxide/pyrene/PMMA nanocomposite powders is shown in Figure 9.44. Absorption in the range of the 325-nm
radiation used for excitation is increased (see Figure 9.44), from silica (SiO 2 ) over
alumina (Al 2 O 3 ) and zirconia (ZrO 2 ) to hafnia (HfO 2 ), with the highest absorption.
In all of these examples, the particle size and coverage of the surface of the oxide
kernels with lumophore were kept constant.
The data in Figure 9.45 display maximum photoluminescence intensity for the
different ceramic cores in Figure 9.44 versus absorbance at the excitation wavelength of 325 nm. As shown in Figure 9.22, for the PMMA/ceramic composites
there is a strong correlation between luminescence intensity and UV absorption in
the ceramic core. The intensities of the pyrene luminescence as a function of the
ceramic core (which are visible in Figures 9.44 and 9.45) reflect the interaction of the
lumophore and the core. This suggests a mechanism of excitation of the lumophore
where the photon is absorbed in the ceramic core, after which the excitation is
transferred to the lumophore.
The combination of inorganic nanoparticles with organic lumophores may be
exploited for further modifications of the emission spectrum.
9.6 Special Luminescent Nanocomposites j245
available. The ceramic core is selected according to the properties demanded and, as
shown later, it may even be used to add an additional physical property besides
luminescence to the particle. The polymer coating at the outer surface is selected
according to the demands with respect to the surroundings during application. In
general, PMMA is used for coating, but in order to obtain a highly hydrophobic layer
fluorinated aliphatic compounds are used, while for hydrophilic layers PHPMA is
selected. The best results are obtained using a monolayer of lumophores; moreover,
to minimize parasitic absorption in the coating the outer polymer layer is made as
thin as possible, usually less than 1 nm.
A typical spectrum of such a composite is shown in Figure 9.43a. For these
particles, iron oxide was used as the ceramic core, PMMA as the outer coating, and
pyrene was applied as the lumophore. The emission spectrum of pure solid pyrene
is also plotted in Figure 9.43a. Clearly, the spectrum of the nanocomposite is quite
similar to that of the pure lumophore, with just a slight blue shift of about 15 nm;
more prominently, the spectrum of the composite has lost the fine structure
characteristic of pyrene. In contrast to pyrene, the situation with composites
containing anthracene as lumophore is more complicated. Figure 9.43b shows
the spectra of pure anthracene and a composite using hafnia as ceramic core. As
shown in Figure 9.43b, anthracene shows a more structured spectrum as compared
to pyrene. Each of the maxima in the spectrum can be associated with a vibration
mode. On comparison, the spectra of pure anthracene and the anthracene/hafnia
composite are essentially identical, except for the strongest line of the composite
(positioned between vibration modes 5 and 6), which does not appear in the pure
anthracene spectrum.
As described for the luminescent oxide/PMMA nanocomposites, there is a strong
interaction between the oxide core and the lumophore coating. The influence of the
ceramic core (which is selected according to its interaction with the exciting UV
radiation) on the luminescence intensity of different oxide/pyrene/PMMA nanocomposite powders is shown in Figure 9.44. Absorption in the range of the 325-nm
radiation used for excitation is increased (see Figure 9.44), from silica (SiO 2 ) over
alumina (Al 2 O 3 ) and zirconia (ZrO 2 ) to hafnia (HfO 2 ), with the highest absorption.
In all of these examples, the particle size and coverage of the surface of the oxide
kernels with lumophore were kept constant.
The data in Figure 9.45 display maximum photoluminescence intensity for the
different ceramic cores in Figure 9.44 versus absorbance at the excitation wavelength of 325 nm. As shown in Figure 9.22, for the PMMA/ceramic composites
there is a strong correlation between luminescence intensity and UV absorption in
the ceramic core. The intensities of the pyrene luminescence as a function of the
ceramic core (which are visible in Figures 9.44 and 9.45) reflect the interaction of the
lumophore and the core. This suggests a mechanism of excitation of the lumophore
where the photon is absorbed in the ceramic core, after which the excitation is
transferred to the lumophore.
The combination of inorganic nanoparticles with organic lumophores may be
exploited for further modifications of the emission spectrum.
9.6 Special Luminescent Nanocomposites j245