Page 588 - Book Hosokawa Nanoparticle Technology Handbook

APPLICATIONS 33 DEVELOPMENT OF BRIGHT PHOSPHORS USING GLASSES
(1) Preparation of bulk glass phosphors micelles. Bawendi et al. prepared photoluminescent
Sol–gel derived bulk glass phosphors incorporating small glass beads by forming thin silica layer contain-
semiconductor nanoparticles can be prepared by ing semiconductor nanoparticles by a sol–gel method
hydrolysis of alkoxysilane or alkoxide in solutions onto the surface of a silica bead without containing
mixed with aqueous solutions of semiconductor nanoparticles [19]. Alivisatos et al. reported a com-
nanoparticles [6,15,16]. When using alkoxysilane plex chemical synthetic method to deposit mono- and
such as 3-aminopropyltrimethoxysilane (APS) having multilayers of silica onto the surface of a single semi-
an amino group that adsorbs TGA on the surface of conductor nanoparticle [20]. Compared with those
nanoparticles, the agglomeration of the nanoparticles methods, inverse micelle-based methods have advan-
was suppressed and led to bright glass phosphors. It tages such as good controllability of glass bead size
was possible to incorporate CdTe nanoparticles up to and nanoparticle concentration in a glass bead. The
the concentration of ca. 2 10 4 mol/l in the bulk following two preparation methods have been devel-
glass. The obtained bulk glass phosphors showed PL oped by the authors using inverse micelles.
spectra similar to those of the aqueous solutions of In the first preparation method, an inverse micelle
nanoparticles, and exhibited PL efficiencies higher of an ionic surfactant Aerosol OT (AOT) (sodium
than 40%. These glass phosphors retained PL effi- bis(2-ethylhexyl) sulfosuccinate) is formed in a
ciencies of higher than 30% even after 6 months of hydrophobic organic solvent isooctane. Then aque-
preparation, demonstrating superior long-term stabil- ous solution of CdTe nanoparticles is mixed to
ity [6,16]. In a similar manner, blue-emitting bulk generate inverse micelles that involve small droplets
glass phosphors incorporating ZnSe nanoparticles can of the nanoparticle solution. Afterwards, an alkoxide
be prepared. It is also possible to fabricate bulk glass tetraethoxysilane (TEOS) is added, and the solution is
phosphors having various shapes and PL colors under stirred for 1–3 days in order to complete the hydroly-
atmospheric pressure and at room temperature, by sis of TEOS. As a result, small glass bead phosphors
applying the sol solutions to a patterned substrate are obtained [17]. The small glass beads emit PL both
(Fig. 33.2). in the isooctane solution and in the powder form
(Fig. 33.3). The PL and absorption spectra of the
(2) Preparation of small glass bead phosphors nanoparticles were almost unchanged before and after
Small glass bead phosphors incorporating semicon- the incorporation into small glass beads. However,
ductor nanoparticles can be prepared by sol–gel reac- electron microscopic observation revealed that the
tion in the small space inside inverse micelles [17,18]. nanoparticles with diameters of ca. 3 nm were fixed
The agglomeration of nanoparticles is suppressed by in the vicinity of the outer surface of the glass beads
involving water-dispersible semiconductor nanoparti- whose diameters were around 25 nm. It was specu-
cles in the individual inverse micelle. On the other lated that the nanoparticles were pushed out of the sil-
hand, a few methods have been reported to prepare ica network structure that developed slowly during the
small glass bead phosphors without using inverse sol–gel reaction. The PL efficiencies of thus obtained
small glass beads were ca. 10% at most. Such rela-
tively low PL efficiencies were thought to be due to
possible deterioration of nanoparticles during the long
stirring in preparation.
In order to obtain brighter glass beads, the authors
have developed the second inverse-micelle-based
preparation method with shorter reaction time. In
this method, an aqueous solution containing

Figure 33.2 Powder
(Above) Photoluminescence of bulk glass phosphors
incorporating CdTe and ZnSe nanoparticles. (Below)
Multi-color photoluminescence of prototype indicator Figure 33.3
using bulk glass phosphors incorporating CdTe and ZnSe Photoluminescence of small glass beads incorporating
nanoparticles (Left: under visible light. Right: under CdTe nanoparticles. (Left: Isooctane solution of the glass
ultraviolet light (wavelength: 365 nm)). beads. Right: Powder of the glass beads.)

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