Page 493 - Book Hosokawa Nanoparticle Technology Handbook

11 DEVELOPMENT OF NEW PHOSPHORS APPLICATIONS
energy transfer process from the carboxylic acid to by heating at 600 C for 5 h. Higher temperature and
the Mn 2 ion. longer heating time increased particle size of the phos-
phors and photoluminescence intensity. The emission
2. Properties of rare earth nanophosphor spectrum is comparable with the conventional phos-
phor synthesized by the solid-state reaction. Wang et
However, these nanophosphors have a semiconduct- al. synthesized Y O :Tb nanophosphor by the combus-
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ing character even if they contain the localized emis- tion synthesis [16]. The particle size estimated by
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sion ions such as Mn . The mainstreams of current XRD pattern was 35–70 nm. It is an interesting feature
phosphor are the oxide phosphors including the rare that the emission intensity is increased by irradiating
earth ions as the emission center because of high the 250 nm UV light. It is considered by ESR meas-
luminous efficiency. When rare earth phosphor is urement that the behavior is due to the passivation of
activated by small-energy blue and near-UV light, the dangling bond on the surface. Tissue et al. reported
light absorption and emission occur around the emis- on the vapor-phase synthesis of Y O :Eu nanophos-
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sion ions. The semiconducting character of oxide host phor [17]. By CO laser heating under 10 and 400 Torr
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lattice was not reflected at all by the photolumines- nitrogen, 5 and 12 nm nanophosphor samples were
cence. There was the 4f orbital of rare earths located obtained, respectively. The particle size grew up to
near an atomic nucleus and comparatively covered by about twice by annealing for a long time at 800 C. The
an outside 5s orbital. Therefore, the electron of 4f crystal structure of 5 nm particle changed from mixed
orbital is hard to take a quantum effect for the nanopar- phase to cubic single phase and 13 nm particle did not
ticles. Recent results of rare earths doped nanophos- change the monoclinic structure. He et al. reported
phors are reviewed below. synthesis of Y O :Eu nanophosphor by wet chemical
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In 2000, Park et al. reported synthesis of synthesis [18]. Chloride solution of yttrium and
Y Al O :Tb nanosize phosphor (25–45 nm) from europium was mixed with butanol, which acted as a
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nitric acid solution by sol–gel method [10]. This sam- surfactant. The nanophosphor was obtained by adding
ple with heat treatment until 800 C was amorphous sodium carbonate to the solution and heat treatment at
and the excitation spectrum was different from that of 800 C. When butanol was added, the sample of a par-
the sample with high-temperature heating. From decay ticle size was smaller than that of the non-surfactant
time and excitation spectrum, they claimed that the sample.
nanophosphor sample was useful for plasma display. In 2004, Chander et al. synthesized the nanocrystal
Hasse et al. synthesized nanosize YVO and LaPO by of a long-persistence phosphor SrAl O :Eu, Dy by
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the hydrothermal technique. The sample was colloidal modified combustion method [19]. The nitrate solu-
solution and separated by centrifugation. The YVO 4 tion of starting materials was mixed with urea and
nanophosphor has the same crystal structure as its boric acid heated from 400 to 600 C. The nanophos-
bulk sample. On the other hand, monazite structure is phor sample of 50 nm or less was obtained after
observed for the nano LaPO . The particle size combustion reaction. However, the decay time is
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depends on pH value of the solution and other reaction shorter than that of the sample synthesized by a con-
parameters. In alkaline solution, the sample was fine ventional solid-phase reaction. Although Peng et al.
particles from 10 to 50 nm. In acid solution, morphol- also have synthesized the 25 nm phosphor by the
ogy of the sample was nanofiber [11, 12]. combustion synthesis method [20], the afterglow time
In 2001, Konrad synthesized 10 nm sized cubic was still shorter that of the conventional ones.
yttria phosphor by chemical vapor technique [13].
The crystallite size increased to 20 and 50 nm upon 3. Development trend of new nanophosphor
heating at 900 and 1,100 C, respectively, in air for 5 h.
Depending on the particle size, a broadening of Summarizing these, it has been observed that “lumi-
absorption edge and blue shift of photoluminescence nescence efficiency of fine(nano)particle phosphor is
spectra were found. The change of spectra was low”. Does the nanophosphor have practical use?
explained by the change of a configurational coordi- White LED application is one of the promising candi-
nate diagram. dates. Rayleigh scattering decreases in proportion to
In 2002, Lu et al. reported synthesis of small-size the sixth power of particle size. Therefore, the
LED phosphor Y Al O :Ce 3 by sol–gel method [14]. nanoparticles dispersed in resins scatter light less vig-
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Raw material aqueous solution reacts with urea in the orously and become transparent. Rare earth complex
presence of polyvinyl alcohol at 150 C. Gelling was phosphors are dispersed in the plastics [21]. Fukui et
achieved at 250 C. Although the heat treatment at al. reported nanocluster phosphors containing the rare
800–1,100 C yielded the nanosize phosphor, the emis- earth ions [22]. Isobe et al. reported the transparent
sion intensity was remarkably low compared with the phosphor sheet of nano ZnS:Mn and Y Al O :Ce dis-
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conventional bulk sample synthesized at 1,450 C. persed in the resins [23].
In 2003, Pan et al. reported the synthesis of a red On the other hand, there are some trials to improve
CaTiO :Pr 3 nanophosphor from polymer precursor the luminescence efficiency of the nanophosphors by
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[15]. The nanophosphor powder (10 nm) was obtained precise process control. Nishisu et al. reported the
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