LaVo4: Eu Phosphor Films with Enhanced Eu Solubility Page: 3 of 3
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the zircon and monazite structures, and Eu is distributed
to both phases.
Unlike that of the polycrystalline pellet, the spectrum of
the LaVO4:Eu (10 mol%) film showed almost identical fea-
tures to that of the LaVO4:Eu (1 mol%) reference. More-
over, the PL intensity was almost ten times as large as that
of the LaVO4:Eu (1 mol%) film - it is proportional to the
total number of Eu3+ ions, which is a noteworthy difference
from the case of the phase-separated bulk pellet, where the
PL intensity was smaller than expected from the increase
of activator ions. Since Eu substitutes at the La site in
LaVO4:Eu (1 mol%) with a single monazite phase, this cor-
respondence of the PL profile and intensity indicates that
the 10 mol% of Eu is fully incorporated at the La site in the
monazite LaVO4 films. Segregation of Eu in other forms,
for example Eu203 was unlikely, because they have PL fea-
tures different from that of monazite LaVO4:Eu. In ad-
dition, within the possible chemical compounds, Eu3+ can
only be deactivated in V compounds with V 3d electrons
due to their excitation energy below that of Eu3+, which
was ruled out by XAS. Therefore, segregated phases incor-
porating dark Eu could be excluded.
It is striking that LaVO4:Eu (10 mol%) consisted of a
single monazite phase in the PLD grown films, even though
the Eu concentration exceeded the bulk solubility in LaVO4.
LaVO3 and EuVO3 have the same perovskite structure ,
resulting in complete solubility in the epitaxial structures,
and moderate annealing at a temperature far lower than
the melting point does not induce significant migration of
atoms. Another aspect of the lack of phase separation is the
small volume of the films. Crystal phase separation kinetics
often depends on the crystal size [11, 20], and in thin films
phase separation can be suppressed, because the energy gain
by phase separation is small compared to the large grains
obtained by bulk solid state reaction.
To study the excitation mechanism of the activator
ions, the photoluminescence excitation (PLE) spectra were
recorded from the annealed LaVO4:Eu (10 mol%) and
YVO4:Eu (10 mol%) films, using a grating monochrome-
ter with a Xe lamp for the excitation ([Fig. 3(c)]). The
LaVO4:Eu (10 mol%) film showed a larger tail of PLE spec-
tra below the bandgap. This can be understood based on
the difference between the direct excitation of the activa-
tors, and the indirect process via energy transfer from the
excited host material [1, 21]. When they are excited by light
with energy higher than the bandgap, 3.8 eV (A = 326 nm)
in LaVO4 and 3.7 eV (A = 335 nm) in YVO4 , due to
the stronger energy transfer from VO4 to Eu, YVO4 shows
stronger PL, which was indeed observed when excited by
A = 266 nm light. If the excitation photon energy is smaller
than the host bandgap, however, the optical penetration
depth in the host material is much larger than the film
thickness (40 nm) and the indirect process is suppressed.
Therefore, the LaVO4:Eu films showed stronger emission
due to its direct excitation process in this regime.
In summary, structural conversion of PLD-grown RVO3
epitaxial thin films to RVO4 by postannealing was con-
firmed. Using this technique, phosphor RVO4:Eu films were
fabricated. The PL spectrum of the LaVO4:Eu (10 mol%)
thin films showed identical shape as that of the 1 mol%
doped film and its intensity was proportional to the Eu dop-
ing concentration, indicating the enhancement of the solid
solubility limit of Eu in the films up to as high as 10 mol%.
Especially when excited by photons below the bandgap, the
PL intensity exceeded that of the YVO4:Eu (10 mol%) with
the same thickness, reflecting the stronger direct emission
of Eu via dipole transitions. Epitaxial growth of RVO3 per-
ovskite films before postannealing played a key role in the
structural control, by prefixing the substituting site in fully
incorporated structures, and this technique may provide a
general approach for enhancing phosphor films.
The authors appreciate L. Fitting-Kourkoutis and D. A.
Muller for helpful discussions. H. Y. H. acknowledges sup-
port from the Department of Energy, Office of Basic Energy
Sciences, Division of Materials Sciences and Engineering,
under contract DE
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Hwang, Harold. LaVo4: Eu Phosphor Films with Enhanced Eu Solubility, article, August 11, 2011; United States. (digital.library.unt.edu/ark:/67531/metadc841170/m1/3/: accessed December 15, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.