Page 587 - Book Hosokawa Nanoparticle Technology Handbook

33 DEVELOPMENT OF BRIGHT PHOSPHORS USING GLASSES APPLICATIONS
In the following are introduced the syntheses of
highly photoluminescent CdTe and ZnSe nanoparti-
cles and the preparation of glass phosphors incor-
porating those nanoparticles with three different Vis light
forms.

1. Syntheses of highly photoluminescent
semiconductor nanoparticles by an aqueous solution UV light
method
There is a general rule that the bandgap (Eg) of group
II–VI semiconductors becomes narrower as the con-
stituent atoms become heavier. In the case of 3 nm 4 nm
nanoparticles with diameters of ca. 2–10 nm, the Eg
is increased due to the quantum size effect compared Figure 33.1
with the bulk semiconductor, and it leads to various Aqueous solutions of CdTe nanoparticles having various
fluorescent colors reflecting small differences in the diameters between 3 and 4 nm. (Above) Colorless and
particle size. transparent appearance under visible light. (Below)
When the semiconductor nanoparticles are coated Photoluminescence under ultraviolet light (wavelength:
with sulfur- and phosphor-containing surfactants, the 365 nm).
PL efficiency improves. The uncoated nanoparticles
have many surface defects, which cause nonradiative the PL efficiencies more than 65% [6,7]. The ZnSe
deactivation after excitation. Also, such nanoparticles nanoparticles can be synthesized by using zinc per-
easily agglomerate, and the agglomeration causes chlorate, hydrogen selenide gas, and a surfactant
imperfect chemical bonds between nanoparticles. The such as TGA and thioglycerol (TG). A diluted aque-
coating with surfactants decreases the number of sur- ous solution of the ZnSe nanoparticles with a diam-
face defects on the nanoparticles and suppresses the eter of ca. 3 nm is almost colorless and transparent
agglomeration of the nanoparticles. Highly photolu- under visible light, and emits blue PL under ultravi-
minescent nanoparticles can be synthesized in organic olet light [8,9]. When the ZnSe nanoparticle core is
solutions [4] and aqueous solutions [5]. However, the coated with ZnS shell, the PL efficiency improves
organic solution route uses reactions of explosive up to ca. 40% [10,11].
materials at high temperatures that need complex, It is known that the PL intensity of an aqueous
expensive apparatus and devices, and the synthesized solution of semiconductor nanoparticles changes
nanoparticles tend to lose their PL quickly when con- with pH [12]. Zeta potential measurement of the
tacting with water. TGA-coated CdTe nanoparticles has shown that in
On the other hand, aqueous solution route enables
the syntheses of highly photoluminescent nanoparti- the acidic region, TGA loses the charge reflecting its
acid dissociation constant (pK ). Therefore the
a
cles under mild conditions, and the nanoparticles agglomeration of noncharged nanoparticles is pro-
obtained are stable in aqueous solution at least for moted, which gives rise to the decrease of PL inten-
about several days at room temperature. First, clus- sity at low pH [13].
ters of a group II–VI semiconductor are generated
by reacting the ion of a group VI element with an
alkaline solution of a group II element in the pres- 2. Preparation of glass phosphors incorporating
ence of a surfactant under inactive atmosphere at semiconductor nanoparticles by a sol–gel method
room temperature. Next, nanoparticles are grown by
refluxing the cluster solution. When reflux time is Previously, Mulvaney et al. tried to incorporate
prolonged, the particle size increases and PL wave- CdSe nanoparticles prepared by an organic solution
length shifts to longer wavelengths. The CdTe method in glass matrix by a sol–gel method [14].
nanoparticles can be synthesized by using cadmium However, their method does not seem to be appro-
perchlorate, hydrogen telluride gas, and a surfactant priate to obtain solidified glass phosphors with high
such as thioglycolic acid (TGA). The diluted aque- concentration of nanoparticles because the
ous solutions of the CdTe nanoparticles having hydrophobic CdSe nanoparticles used are not very
diameters of 3–4 nm are almost colorless and trans- compatible with the sol–gel process. By contrast, the
parent under visible light. They emit PL of green to authors have successfully prepared well-solidified
red colors depending on the particle size under ultra- glass phosphors having three forms, i.e., (a) bulk,
violet light (Fig. 33.1). The authors optimized the (b) small bead, and (c) thin film, by a sol–gel
conditions of synthesis of CdTe nanoparticles by method using water-dispersible semiconductor
reducing the amount of TGA, etc., and it increased nanoparticles.

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