Chapter 2
Review
Review
2.1. INTRODUCTION
Presently, nanoparticle research has gained significant attention as a newly
emerging field at the frontier of scientific research owing to their significant
technological importance. They provide challenges to both fundamental research
and breakthrough development of technologies in various areas such as electronics,
photonics, nanotechnology, display, lasing, detection, optical amplification,
fluorescent sensing to biomedical engineering, and environmental control. Besides
this, they are used as high-performance luminescent devices, catalysts and timeresolved fluorescence labels for biological detection based on their electronic,
optical and chemical properties [1-5]. Moreover, rare earth doped insulators have
been widely used as phosphors, scintillators and amplifiers for fiberoptic
communications due to their wide applications [6-7]. They were also extensively
applied in luminescence and display such as lighting; field emission display (FED),
cathode ray tubes (CRTs) and plasma display panels (PDPs) [8-12]. It is expected
that nanosized rare earth compounds can increase luminescent quantum efficiency
and display resolution.
Research of luminescent materials has several aspects. There is, for example, the
influence of sample preparation on the luminescence properties. Some materials are
required in the form of large crystals of high quality, others in the form of powders
with specific requirements on particle size distribution and morphology.
Additionally, insight into the physics of the luminescence properties is required to
obtain optimum properties in a controllable way. A special aspect is the way in
which the luminescence properties depend upon the chemical composition of the
material [13-15].
Furthermore, improved performance of lighting and display requires high quality of
phosphors for sufficient brightness. Hence, luminescence intensity is an important
character for phosphor. Sensitizers are useful if the activator ions cannot be
Ph.D. Thesis
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excited, e.g., because of forbidden transitions. In such cases, the exciting energy is
absorbed by the sensitizers and subsequently transferred to the activator ions.
In this review, we focus on the synthesis, characterization and important
developments in the field of lanthanide doped luminescent nanomaterials during
the past few decades and wish to demonstrate that the study of luminescent
nanomaterials is still challenging in spite of its long history.
The report for the development of luminescent phosphor can be dated as early as
1964 when A. K. Levine et al. [4] reported the development of a highly efficient
red-emitting
cathodoluminescent
phosphor,
europium-activated
yttrium
orthovanadate for use in color television. The phosphor was found to be far
superior in both color and brightness to silver-activated zinc cadmium sulphide,
which has been used universally in color picture tubes.
R.K. Datta [16] had successfully attempted to show that rare-earth oxides can be
activated by ions other than rare earths. They incorporated Bi3+ in yttrium,
gadolinium, and lanthanum oxides by solid state reaction. Probably a charge
transfer mechanism involving bismuth and oxygen introduces an absorption band
at about 3200 Å region. When incorporated in R2O3:Eu phosphors, Bi3+ functioned
as a as a sensitizer for Eu3+ emission under 3650 Å radiation. Energy absorbed by
bismuth- oxygen component was transferred by a radiationless process to Eu3+
resulting in the final emission from excited Eu3+ (5D0→7F2). However, under 2537
Å radiation Bi3+ in R2O3:Bi acted as a "quencher". Among the three oxide matrices
studied, the fluorescence of Bi was found to be most efficient in Y2O3 host.
Reflectance, emission, and excitation spectra of the phosphors were also discussed
with special reference to Y2O3:Bi and Y2O3:Eu:Bi.
J. Th. W. de Hair et al. [17] had discussed energy transfer from a sensitizer to an
activator via Gd3+ ions. This was done for the Bi3+→Dy3+ and Sb3+→Dy3+ transfer
in GdPO4, and for the Bi3+→Tb3+ transfer in GdB3O6. The phosphor
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Gd0.98Bi0.01Tb0.01B3O6 showed Tb3+ luminescence with a quantum efficiency of
80 % for excitation of Bi3+. After excitation of Bi3+, energy was transferred to Gd3+
ions. Then the energy might migrate from one Gd3+ to another until energy transfer
from Gd3+ to Tb3+ occurs. Compared to the energy transfer in other sensitized Tb3+
phosphors, this process had a high efficiency at relatively low activator
concentrations. The energy transfer from Bi3+ to Gd3+ and between Gd3+ ions
mutually had been investigated for Gd0.99-xLaxBi0.01B3O6. For x < 0.4 energy
transfer between Gd3+ ions was observed. It was reported that the quantum
efficiency of the Gd3+ luminescence for Bi3+ excitation of Gd0.59La0.4Bi0.01B3O6
amounts to 75 % and decreased for increasing Gd concentrations.
J. Hölsä et al. [18] studied the luminescence emission and UV-excitation properties
of LaOBr:Tb3+, LaOBr:Ce3+, and LaOBr:Tb3+,Ce3+ phosphors. The excitation of
Tb3+ ion gave a broad 4f→5d transition band at 254 nm and weaker 4f→4f
transition
lines
above
300
nm.
The
UV-excitation
and
emission
of
La0.995Ce0.005OBr at 290, 315, 355 (excitation) and 440 nm (emission) originated
from transitions between the 4f ground state and the four crystal field components
of the 5d 2D excited state. The sensitization of Tb3+ luminescence in LaOBr with
Ce3+ at varying concentrations was described and studied.
D. Hommel et al. [19] had studied ZnS bulk crystals grown by the chemical
transport method doped with rare earth impurities. The most critical point of the
whole sample preparation was determined to be the starting powder firing in a CS 2
stream. The latter influenced not only the RE3+ doping level in ZnS but also the
incorporation of the RE3+ impurities on an active luminescence site. The special
role of europium due to its 2+ charge state was interpreted on the basis of
photosensitive ESR measurements.
B.M.J. Smets [20] had studied phosphors based rare-earths, a new era in
fluorescent lighting. With the introduction of the tricolour lamp, containing a blend
of three phosphors each emitting in a narrow wavelength interval, an energy saving
Ph.D. Thesis
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Review
lamp with a good colour rendition became available. The phosphors used in the
tricolour lamps made a drastic reduction in lamp diameter feasible. This ultimately
resulted in the introduction of the compact fluorescent lamps. Super de Luxe
fluorescent lamps based on rare-earth phosphors was introduced which exhibited a
nearly continuous visible spectrum, resulting in a colour rendition far more
superior than the one of tricolour lamps. It was reported that rare-earth doped
phosphors made the realization of tricolour lamps feasible. The discovery that the
gadolinium sub-lattice can play an intermediate role in the energy transfer from a
sensitizer to an activator ion had given an impetus to the development of
gadolinium containing phosphors, which found application in the recent generation
of Super de Luxe lamps.
N. Hashimoto et al. [21] had reported improved light emission from (La,
Ce)PO4:Tb upon substitution of a small part of phosphate by borate or by doping
with thorium. They observed that emission intensity remains almost constant in the
temperature range from 20 to 350 oC. The reason for the improvement was
attributed to all of cerium becoming trivalent because of the presence of borate or
thorium.
T. Hatayama et al. [22] had investigated the luminescence properties of Li codoped ZnS:Tm phosphor. They observed very strong sharp lines of Tm. Blue
emission intensity was stronger as compared with that of infra red emission when
the co-doped Li concentration in ZnS:Tm was 0.1 at.%. They proposed that the role
of Li as to fill the defects in ZnS host and hence excitation energy transferred from
the ZnS host to dopant Tm3+ very easily.
K. C. Mishra et al. [23] had studied the electronic structure and optical properties
of a red phosphor, Y2O3.Eu, using the first-principles molecular-orbital and bandstructure methods. Using the calculated one-electron energy levels, several
properties of the host material and properties of the phosphor based on hostimpurity interactions had been explained. However, it was found that the
Ph.D. Thesis
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Review
luminescence properties of Eu3+ require the spin-orbit effects to be included in the
computational methods.
W. Li et al. [24] had studied the fluorescence enhancement of Eu(III)thenoyltrifluoroacetone)-phenanthroline in micelle solution and Tb(III)-benzoic
acid in ethanol solution by adding other Ln(III) (Ln = Y, La, Gd, Lu) ions. From
fluorescence studies, it was found that the enhancement of Eu(III) luminescence is
due to energy transfer from the Ln(III) complex to the Eu(III) ion and the wrapping
effect of the Ln(III) complexes on the Eu(III) complex. Also, the enhanced Tb(III)
luminescence was attributed only to energy transfer from the Ln(III) complexes to
the Tb(III) complex.
Q. Su et al. [25] had prepared various borates, vanadates, niobates, antimonates,
titanates, zirconates and CaS doped with Dy3+ and reported the factors which have
an effect on the yellow-to-blue intensity ratio (Y/B) of Dy3+ emission are reported.
Y/B increased with decreasing Z/r or electronegativity of the next-neighbor
element M in the complex oxides Dy-O-M. The greater the degree of covalency
between Dy3+ and O2- , the greater Y/B is. When Dy3+ resides at a site with an
inverse centre and high symmetry, Dy3+ displays no luminescence. It seemed that
Y/B of Dy3+ located at a site deviated from an inverse centre is greater than that of
Dy3+ located at a site without an inverse centre. They reported that Y/B does not
vary much with the variation in concentration of Dy3+ when Dy3+ is substituted for
an element with the same valency, but it does depend on the concentration of Dy3+
when Dy3+is substituted for an element with a different valency in the matrix,
because defects were formed in this case.
The first report on nanosize luminescent materials was made by Bhargava et al.
[26] in 1994. They synthesized Mangenese doped nanocrystals of zinc sulphide
utilizing chemical precipitation method. The nanomaterials were found to have
luminescence efficiency of 18 % which has been explained on the basis of surface
passivation of the nanocrystals due to photopolymerization of the surfactant
Ph.D. Thesis
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Review
(methacrylic acid) used. The photoluminescent spectrum is found to shift slightly
and there occurs larger line width in the nanophosphor when compared to the bulk.
J. R. Peterson et al. [27] had studied the absorption and luminescence spectra from
SrB4O7:Tm2+ prepared in air at room temperature. Because thulium was introduced
as Tm3+ from Tm2O3, a valence change from Tm3+ to Tm2+ was observed.
Optimum production of Tm2+ ion occurred when the sample was heated in air at
650 oC. A broad emission band centered in the vicinity of 550 nm was observed
from the sample upon excitation at 457.9 nm. They suggested that it was due to
Tm2+ ion emission from the 5d band into the ground-state 4f level (2F7/2). Several
conditions promoting the reduction of Tm3+ ion in the sample were discussed. In
addition, to aid the reduction of Tm3+ ion, they had also prepared SrB4O7:Tm2+ in
Ar/H2 (4 %) atmosphere and compared the optical characteristics of Tm2+ ion in
these samples with those prepared in air.
O.A. Serra et al. [28] had studied the phenomenon of energy transfer between Ce3+
and Tb3+ in Tb3+ and/or Ce3+ supported on Y, A and ZSM-5 zeolites, silica gel
(SG) and silica gel functionalized with imidazole propyl (IPG). According to Tb3+
luminescence spectra and lifetime data, energy transfer from Ce3+ to Tb3+ occurred
in SG, IPG and mainly in Y zeolite. However, no evidence for Ce3+ to Tb3+ energy
transfer was detected in ZSM-5 and A zeolites.
J. Dexpert-Ghys et al. [29] investigated the luminescence of Eu3+ in two monazitetype orthophosphates: La0.98Eu0.02PO4 and EuPO4 by site-selective time-resolved
spectroscopy. Informations on the nature of perturbed sites in the doped phosphate
and on the localization of the europium on normal lanthanum sites or on perturbed
sites were discussed considering the crystal field parameters and the analysis of the
site-to-site energy transfers. Vibronic satellites were observed in both the doped
and the neat compounds and assigned to the coupling between the 5D0→7F0,1,2
transitions and the localized vibration modes of the (PO4)3- groups.
Ph.D. Thesis
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Review
E.T. Goldburt et al. [30] had reported that the luminescent efficiency in doped
nanocrystalline Y2O3:Tb phosphors and was found to increase with the decrease in
the particle size from 100 to 40 Å. The correlation was obtained from
microstructural studies performed using transmission electron microscopy and
luminescent measurements. The light output per Tb3+ ion in doped nanocrystalline
Y2O3:Tb3+ phosphors exceeded that in the standard LaOBr:Tb3+ phosphor.
K. Riwotzki et al. [31] had reported on wet-chemical synthesis (hydrothermal
method at 200 oC) and characterization of lanthanide (Eu, Sm and Dy) doped
YVO4 nanoparticles. The lanthanide ion substitutes for yttrium in the YVO4 lattice.
The particles were found to exhibit tetragonal zircon structure known for bulk
material. Upon UV excitation of the vanadate host, the energy transferred to the
lanthanide ion led to strong luminescence (f-f transitions). By analyzing line
splitting and intensity pattern in the luminescence spectrum of the europium-doped
sample, they verify that the dopant ions enter the same lattice site as in bulk
material despite the nanocrystalline nature of the sample and the low-temperature
synthesis. For YVO4:Eu nanoparticles a luminescence quantum yield of 15 % at
room-temperature was observed.
H. Meyssamy et al. [32] had also reported wet-chemical synthesis of doped
colloidal nanomaterials; particles and fibres of LaPO4:Eu, LaPO4:Ce and
LaPO4:Ce,Tb. The paper showed that the lanthanide-doped LaPO4 nanomaterials
can be prepared in the high temperature monazite phase and in two different
morphologies via a low temperature synthesis. Successful doping was evident from
the luminescence spectra of the materials and in particular, Eu3+ emission could be
used to show that the symmetry of the main dopant site is the same as in bulk
LaPO4.
K. Riwotzki et al. [33] had prepared nanocrystals of LaPO4:Eu and CePO4:Tb with
a mean particle size of 5 nm and a narrow size distribution
by reacting the
corresponding metal chlorides, phosphoric acid, and a base at 200 °C in
Ph.D. Thesis
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Review
tris(ethylhexyl) phosphate. Highly crystalline material was obtained (confirmed by
X-ray powder diffraction measurements and high-resolution transmission electron
microscopy). They reported successful doping with europium which was evident
from the splitting and the intensity pattern of the luminescence lines. Luminescence
lifetime measurements were used to confirm doping and energy transfer in both
materials. They also reported that colloidal solutions of CePO4:Tb exhibited an
overall luminescence quantum yield of 16 %.
B.N. Mahalley et al. [34] had synthesized the doubly doped (Bi3+ and Eu3+) GdVO4
powder by hydrolyzed colloid reaction (HCR) technique and formation of material
is confirmed by XRD measurement. Uniform surface morphology was studied by
SEM measurement. The average particle size observed by SEM was about 1 m.
The Fritsch particle sizer was used to study the particle size distribution. It
distributed from 0.15 to 3.57 m. The small particle size (less than 5 m) and the
narrow particle size distribution are the necessary requirements of good phosphor
material. Photoluminescence result showed a narrow emission line of Eu3+ ion (4
nm FWHM) at 618 nm. The emission intensity was enhanced by a factor of five
with the addition of small amount of Bi3+. The emission bands of VO43− and Bi3+
partially overlapped with the excitation band of Eu3+. The process of energy
transfer from Bi3+ to Eu3+ was discussed and was found to depend strongly upon
the Bi3+ and Eu3+ concentrations, with a maximum for 0.2 mol % of Bi3+ and 3
mol % of Eu3+. It drastically decreased for higher concentrations. For
photoluminescent applications, the quantum efficiency (QE) of a phosphor material
is an important parameter. The QE of GdVO4:Bi; Eu(0.2, 3) was determined to be
76 %. Thus, the material was proposed as an efficient photoluminescent phosphor.
U. Rambabu et al. [35] reported the synthesis and fluorescence properties of Tb3+doped YPO4, LaPO4, GdPO4, (La,Gd)PO4, (La,Y)PO4 and (Gd,Y)PO4 powder
phosphors. Under UV-source these phosphors fluoresce in green color due to the
emission transition (5D4→7F5). FT-IR absorption spectra of such samples were
Ph.D. Thesis
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Review
recorded. Scanning electron microscopy investigations were carried out to
understand the surface morphological features and the grain size. The roomtemperature lifetime measurements were made for the different emission transitions
of these powder phosphors. X-ray energy dispersive analysis (EDAX) was also
performed.
L. Sun et al. [36] had investigated the effect of lithium introduction on the
luminescent properties of Y2O3:Eu in an attempt to improve the luminescent
intensity. They had studied the relationship between structures and optical
properties and found that Li+ ions doping effectively enhanced the luminescence
properties of Y2O3:Eu nanoparticles and did not change the cubic phase of the host.
They had obtained improved spherical morphology from the Li+ dopant.
K. Riwotzki et al. [37] had done their studies on the luminescence of pure and
europium-doped nanocrystalline YVO4 and YP0.95V0.05O4 and also the energy
transfer processes in such nanoparticles, by temperature dependent luminescence
spectroscopy and luminescence lifetime measurements. Their results indicated
thermally activated energy transfer between adjacent vanadate groups in YVO4 at
temperatures above 100 K, but energy transfer to europium seemed to take place
from direct vanadate neighbors only. In contrast to the luminescence decay of
europium, the kinetic of the vanadate luminescence depended strongly on the
choice of surface capping and solvent, indicating partial quenching of the vanadate
emission at surface sites. The strong competition with energy transfer to surface
sites seemed to be the reason for the absence of energy transfer to europium from
distant vanadate groups. The latter explained the low room-temperature quantum
yield of 15 % of YVO4:Eu colloids.
G.A. Hebbink et al. [38] had reported the synthesis of the first redispersible
lanthanide-doped nanoparticles that emit in the NIR (Near Infra Red) and also
showed that these particles can be easily incorporated in polymer materials. The
particles were found to have good processibility and thus the possibility to
Ph.D. Thesis
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incorporate them in polymer-based devices. The average lifetimes of the NIRemitting lanthanide(III) ions were upto 1.7 ms, making them of particular interest
as optical amplifiers and laser.
S.H. Byeon et al. [39] had investigated the influence of Li doping on the
crystallization behavior, morphology, and enhancement in photoemission intensity
of Gd2-xYxO3:Eu3+ solid-solution. They adopted the citrate route for the synthesis at
low temperature (650-850 oC) in order to maximize an effect of the Li component
which is volatile at elevated temperature. Firing the metal citrate precursor at
650 °C for 5 h was sufficient for the formation of 100 nm sized, non-aggregated,
and spherical Li-doped Gd2-xYxO3:Eu3+ particles. Such a temperature is reported to
be much lower than the typical solid-state reaction or spray pyrolysis temperature
(>1400 °C). Additional heat treatment up to 850 °C resulted in well-developed 500
nm sized pseudospherical particles whose PL brightness is close to ~150 % in
comparison with that of commercial red phosphor. Li-doped Gd2-xYxO3:Eu3+ might
appear to be a very promising red phosphor because the particle size and shape can
be controlled at low temperature (650-850 °C) by the change of Y or Li contents
without significant loss of brightness.
M. Yu et al. [40] had reported the fabrication of nanocrystalline YVO4:A (A= Eu3+,
Dy3+, Sm3+, Er3+) phosphor films and their patterning by a Pechini sol-gel process
combined with soft lithography. The results of XRD indicated that the films began
to crystallize at 400 °C and the crystallinity increased with the increase of
annealing temperatures. Transparent nonpatterned phosphor films were uniform
and crack-free, which mainly consisted of grains with an average size of 90 nm.
Patterned gel and crystalline phosphor film bands with different widths (5-60 µm)
were obtained. Significant shrinkage and a few defects were observed in the
patterned films during the heat treatment process. The doped rare earth ions (A)
showed their characteristic emission in crystalline YVO4 phosphor films because of
an efficient energy transfer from vanadate groups to them. The Sm3+ and Er3+ ions
Ph.D. Thesis
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also showed upconversion luminescence in a YVO4 film host. Both the lifetimes
and PL intensity of the rare earth ions increased with increasing annealing
temperature from 400 oC to 800 oC, and theoptimum concentration for Eu3+ was
determined to be 7 mol % and those for Dy3+, Sm3+, and Er3+ were 2 mol % of Y3+
in YVO4 films, respectively.
K. Kömpe et al. [41] had succeeded in preparing CePO4:Tb/LaPO4 core–shell
particles with a total quantum yield of 80 %. Till then, quantum yields this close to
the value of the bulk material were believed to be impossible for a nanocrystalline
phosphor material, as phosphors prepared by conventional solid-state methods
display lower quantum yields already when their grain size is smaller than about
0.5 m. Further, they found that the quantum yield of these nanoparticles can
probably be increased by optimizing the thickness of the shell and by employing
metal salts of very high purity.
The influence of Y doping on the crystallization behavior, structure transition and
the photoluminescence enhancement of Gd2O3:Eu(10 %) nanocrystals prepared by
combustion method had been investigated by L. Sun et al. [42]. The as-prepared
Gd2O3:Eu was in monoclinic phase. As x increased to 0.14, cubic phase could be
observed as well as monoclinic, and heavy Y doping (x40:40) changed the structure
to cubic instead of monoclinic. Though it was prominent that cubic Y2O3:Eu, as the
extreme case of Y substitution, had much stronger photoluminescence than monoclinic
Gd2O3:Eu in a same Eu3+-doping concentration, it was quite surprising that less Y
doping increased the emission of monoclinic Gd2O3:Eu. This effect was ascribed to
the crystal field difference induced by lattice distortion, which is a much more
significant behavior for the nanocrystals than the bulk.
X.C. Jiang et al. [43] had prepared well-crystallized YBO3:Eu3+ nanocrystals by a
mild hydrothermal method in the presence of urea, and a pure hexagonal phase
could be obtained at a low temperature of 200 oC only. The photoluminescence
spectra showed a remarkable improvement on the chromaticity as well as the
Ph.D. Thesis
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luminescent intensity, compared with the samples synthesized by solid-state
reaction (SR). The effects of the synthesis temperature, urea concentration, and the
doping concentration of Eu3+ on the crystallization and luminescent properties were
investigated. The results showed that both high temperature and low urea
concentration were favourable to the formation of YBO3:Eu3+, and the ratio of red
emission (5D0→7F2) to orange emission (5D0→7F1) increased with decreasing the
synthesis temperature and the urea concentration. Furthermore, the samples
exhibited a higher quenching concentration of Eu3+ in comparison with those
prepared by the SR, which was beneficial to further enhancing the luminescent
intensity. These synthesis-dependent phenomena were analyzed, and possible
explanations were proposed.
The plasma display panel (PDP) as a medium of large format TV has gained much
attention due to its high performance and scalability. The performance and life, or
stability, of a PDP is strongly related to the nature of phosphors. Life and
persistence are the main concerns in Mn-activated zinc silicate and bariumaluminate-based green phosphors. Therefore, R.P. Rao et al. [44] had reported the
preparation and characterization of terbium-activated-yttrium borate (YBO3:Tb3+),
gadolinium borate (Y,GdBO3:Tb3+), lanthanum phosphate (LaPO4:Tb3+), yttrium
phosphate (YPO4:Tb3+), and their blends with Mn-activated zinc silicate and
barium aluminate phosphors. Despite high quantum efficiency, Mn-activated
phosphors had a higher persistence (> 10 ms). The persistence of (10 %) Tb3+activated phosphors ranges from 8-10 ms, required for typical TV operation. AC
PDPs (42 in.) were made with conventional zinc silicate- and Tb-activated borate
phosphors. Their optical characteristics, as well as the life of the phosphors, were
studied over 2 years of continuous operation. The data revealed that borate-based
phosphor is stable and also improves the uniformity of discharge characteristics in
ac PDPs. To overcome the blue peak and take advantage of higher lifetime and
lower persistence of yttrium borate, blends of zinc silicate and yttrium borate or
barium aluminate and yttrium borate phosphors were studied and their luminescent
Ph.D. Thesis
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characteristics were presented. The advantage of using these blends in PDPs was
also discussed.
W. Bu et al. [45] had reported the emission of strong green light for Tb(III)-doped
cerium phosphate single-crystalline nanorods synthesized via a facile, Pluronic
P123 (EO20PO70EO20 Mav = 5800)-assisted, hydrothermal method. The surfactant
Pluronic P123 was found to play a crucial role in the enhanced photoluminescence
emission as compared with the same material but synthesized without the
surfactant. They considered smooth surface, well-defined facets, single crystalline
feature, uniform morphologies, and especially very narrow size distributions of 10–
12 nm of the Tb(III)-doped cerium phosphate single-crystalline nanorods as factors
responsible for the strong photoluminescence emission.
H. Zhang et al. [46] reported the photoluminescence (PL) properties of
nanocrystalline YVO4:Tm phosphor synthesized by the polymerizable complex
method based on the Pechini-type reaction. The results of XRD and TEM showed
that, high quality nanopowders with controlled morphology and microstructures
were prepared at a relatively low temperature about 700 oC. Upon ultra violet
excitation the vanadate host transferred energy to thulium ions efficiently and
strong blue emission (475 nm) assigned to 1G4→3H6 transmission was observed.
By analyzing excitation and emission spectra of thulium doped yttrium vanadate,
they had deduced the mechanism of the energy transfer between vanadate host and
thulium ions.
W. Fan et al. [47] had reported the synthesis of single-crystalline tetragonal LnVO4
(Ln = La, Nd, Sm, Eu, Dy) nanorods via a simple hydrothermal method, in the
absence of any surfactant or template using cheap and simple inorganic salts as raw
materials. They had shown that after the hydrothermal process, LaVO4 transformed
its crystal structure from monoclinic to tetragonal phase, but LnVO4 (Ln = Nd, Sm,
Eu, Dy) had not exhibited the structural change which could be associated with the
change of lanthanide ion radius. TEM and HRTEM results showed that the
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nanorods are pure, structurally uniform, single crystalline, and most of them are
free from dislocations. Further study revealed the nanorods grow along the [001]
direction. A possible growth mechanism of lanthanide orthovanadate nanorods was
also proposed. The advantages of this method for the nanorods synthesis were high
yield, low temperature and mild reaction conditions, which permit large scale
production at low cost.
J.W. Stouwdam et al. [48] had synthesized colloidal, organic solvent-soluble Ln3+doped LaVO4 nanoparticles by a precipitation reaction in the presence of
(C18H37O)2PS2- as ligand, that coordinates to the surface of the nanoparticles. The
materials are well soluble in chlorinated solvent such as chloroform. Energy
transfer of excited vanadate groups was observed for Ln3+ ions that emit in the
visible and the near-infrared (Eu3+, Tm3+, Nd3+, Er3+, Ho3+, Dy3+, Sm3+, Pr3+), thus
making it a very generic sensitization mechanism. They found that LaVO4
nanoparticles had a different crystal structure than bulk LaVO4 ones (xenotime
instead of monazite), similar to YVO4 nanoparticles. This xenotime crystal
structure resulted in a more asymmetric crystal field around the Ln3+ ions that is
advantageous to their luminescence, for it increases the radiative rate constant, thus
reducing quenching processes.
M. Yu et al. [49] had developed an effective and simple sol-gel process to deposit
YVO4:Eu3+ layers on SiO2 spheres. The obtained phosphors had spherical
morphology,
sub-micrometer
size,
and
narrow
size
distribution.
The
photoluminescence intensity of the core-shell phosphors could also be tuned by the
annealing temperature, PEG (polyethylene glycol) concentration in the precursor
solutions and the number of coatings. Photoluminescence intensity increased with
annealing temperature and the number of coatings. Optimum concentration for
PEG was determined to be 0.08 g/mL. They further claimed that the current
method could be extended to prepare other core-shell phosphors with homogeneous
morphology and decrease the cost of phosphors to some extent.
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The first rare-earth metal phosphate nanotubes made of CeP had been synthesized
by careful control of the composition of the reactants and the reaction temperature
as reported by C. Tang et al. [50]. Under post-heat treatments in a reduced
atmosphere, the tubular morphology was maintained up to about 900 oC. Further
increase in the temperature resulted in the formation of nanowires, and led to
valence change from +4 to +3 for the cerium ion. Strong blue and UV
luminescence was observed for the Ce3+/Ce4+ hybrid nanotubes and pure Ce3+
nanowires, respectively. Taking into account the short emission lifetime (a few ns)
of Ce3+, CeP one-dimensional nanostructures and, in particular, valence-hybrid
nanotubes developed are promising candidates for light-emission-diode lamps,
electroluminescence devices, non-mercury-fluorescent lamps, and plasma display
panels.
X. Wu et al. [51] had synthesized rodlike, olivelike, pineapplelike, and particlelike
nanocrystals of theYVO4:Eu (5 at. % Eu) by a hydrothermal reaction with different
conditions, respectively. The paper reported that their UV-vis absorption peaks
appeared at 305, 308, 285, and 280 nm, respectively, and the absorption peaks shift
to higher energy as the size of the particles decreases. Compared with other-shape
nanocrystals, the luminescence intensity of the olivelike nanocrystals was
enhanced. It suggested that the function-improved materials could be obtained by
tailoring the size and shape of theYVO4:Eu nanostructures.
L. Zhu et al. [52] had established a simple, efficient and quick method for the
synthesis of CePO4:Tb nanorods and CePO4:Tb/LaPO4 core/shell nanorods via
ultrasound irradiation of inorganic salt aqueous solution under ambient conditions
for 2 h. TEM micrographs showed that all of the as-prepared cerium phosphate
products had rod-like shape, and a relatively high degree of crystallinity and
uniformity. HRTEM micrographs and SAED results proved that these nanorods are
single crystalline in nature. The emission intensity and lifetime of the
CePO4:Tb/LaPO4 core/shell nanorods increased significantly with respect to those
Ph.D. Thesis
Page 43
Review
of CePO4:Tb core nanorods under the same conditions. They observed a substantial
reduction in reaction time as well as temperature compared with the hydrothermal
process.
S.D. Han et al. [53] had reported the synthesis of dysprosium-activated yttrium
vanadate phosphor at 500 oC by combustion method using urea as fuel and metal
nitrates as precursor. The phosphors were treated at different temperatures from
700 to 1100 oC for 2-3 h to get better luminescent properties. The characteristic
emission peaks of Dy3+ due to the transitions of 4F9/2→6H15/2 at 483 nm and
4
F9/2→6H13/2 at 573 nm were observed in the emission spectra. Scanning electron
microscopy (SEM) investigations were carried out to understand surface
morphological features and the particle size of the phosphor. They had checked the
uniformity of phase of Dy3+ doped YVO4 phosphor by X-ray diffraction (XRD)
technique. In addition, the dependence of the luminescence intensity on Dy3+
concentrations and effect of heat treatment on the particle size of the phosphor had
also been discussed.
N. Wang et al. [54] had synthesized single-crystalline lanthanum orthovanadate
(LaVO4) nanorods by an ethylenediaminetetraacetic acid (EDTA)-mediated
hydrothermal method. The experimental results illustrated that the morphologies
and microstructures of LaVO4 nanorods were influenced by EDTA and pH value of
the precursor solution. The magnetic properties measurements showed that the
magnetocrystalline anisotropy was greatly strengthened by the strong effect of the
one-dimensional anisotropy, and the magnetic properties of LaVO4 nanorods are
better than that of LaVO4 nanoparticles. They had also discussed the growth
mechanism of LaVO4 nanorods.
J. Liu et al. [55] had reported the preparation of Ln3+-doped LaVO4
nanocrystals(NCs) by a precipitation reaction in the presence of oleic acid as the
ligand, which coordinated to the surface of the NCs. The materials were very
soluble in non-polar solvents such as cyclohexane. They reported that the NCs
Ph.D. Thesis
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Review
could self-assemble to an ordered array by the interaction of oleic acid molecules
on the crystal surface, and they exhibited luminescence typical of doped ions.
L. Yu et al. [56] had reported the synthesis of Ce3+ and Tb3+ co-activated LaPO4
nanowires (NWs) by hydrothermal method and studied in contrast to corresponding
micrometer rods (MRs). They detected the electronic transition rate of Ce3+ and
Tb3+ in NWs had only a little variation in comparison with that in MRs, and energy
transfer
(ET) rate and efficiency of Ce3+→Tb3+ in NWs reduced. They
interestingly observed that the brightness for 5D4→7F5 via ET of Ce3+→Tb3+ in
NWs increased several times than that in MRs. This was attributed to the decreased
energy loss in excited states being higher than 5D4 of Tb3+ ions due to hindrance of
boundary. They had also discussed the energy-transfer process from Ce3+ to Tb3+ in
YPO4:Ce3+,Tb3+ nanoparticles in detail. Further, they claimed that the phosphors
could be potentially used as labels for biological molecules.
J. Ma et al. [57] had synthesized LaBO3 bundle-like nanorods that are selfassembled through a novel oxide-hydrothermal method. In this route,
stoichiometric amounts of La2O3 and boric oxide B2O3 were mixed in proper
amounts of water in a surfactant/ligand-free sealed hydrothermal system. The
nanobundles are de-agglomerated to nanorods by tiny Sm3+ replacing La3+. Asprepared aragonite-type LaBO3 nanorods are fairly controlled to have a high purity,
homogeneous, and bundled morphology composed of numerous nanorods with
diameter ca. 90 nm. The formation processes and mechanics of LaBO3
nanobundles and nanorods were investigated. The room-temperature fluorescence
properties of various Sm3+ doped LaBO3 were investigated.
Y. Wang et al. [58] had reported the preparation of monoclinic LnPO4:Tb,Bi (Ln =
La,Gd) phosphors by hydrothermal reaction and investigated their luminescent
properties under ultraviolet (UV) and vacuum ultraviolet (VUV) excitation. They
reported that LaPO4:Tb,Bi phosphor and GdPO4:Tb phosphor showed the strongest
emission intensity under 254 and 147nm excitation, respectively, because of the
Ph.D. Thesis
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Review
different energy transfer models. In UV region, Bi3+ absorbed most energy then
transferred to Tb3+, but in VUV region it was the host which absorbed most energy
and transferred to Tb3+.
One-dimensional
YVO4:Ln
and
Y(V,P)O4:Ln
nanofibers
and
quasi-one-
dimensional YVO4:Ln microbelts (Ln = Eu3+, Sm3+, Dy3+) had been prepared by a
combination method of sol-gel process and electrospinning by Z. Hou et al. [59].
The paper reported that due to an efficient energy transfer from vanadate groups to
dopants,YVO4:Ln phosphors showed their strong characteristic emission under
ultraviolet excitation (280 nm) and low-voltage electron beam excitation (1-3 kV).
The energy transfer process was further studied by the time-resolved emission
spectra as well as kinetic decay curves of Eu3+ upon excitation into the VO43- ion.
Furthermore, the PL emission color of YVO4:Ln nanofibers could be tuned from
blue to green, orange-red, and red easily by partial replacement VO43- with PO43and changing the doping concentrations of Ln, making the materials capable of
potential applications in fluorescent lamps and field emission displays (FEDs).
S. Takeshita et al. [60] had reported the preparation of YVO4:Bi3+,Eu3+
nanophosphors by the citrate-assisted low-temperature wet chemical synthesis.
When the colloidal solution was aged at 60 oC, the crystalline YVO4:Bi3+,
Eu3+nanorods were formed from the amorphous gel precursors. The nanophosphors
emitted red through energy transfer from Bi3+ to Eu3+ under near-UV-light
excitation. The emission intensity increases with increasing the fraction of the
crystalline phase during aging. The excitation peak corresponding to Bi3+–V5+
charge transfer relative to those of O2-–V5+ and O2-–Eu3+ charge transfers gradually
becomes strong until the completion of the crystallization, although the contents of
individual Bi3+ and Eu3+ ions incorporated into YVO4 were kept constant. When
the aging was continued after the completion of the crystallization, the content of
incorporated Bi3+ gradually increased, and hence the emission intensity decreased
as a result of the energy migration among Bi3+ ions. These results suggested that in
Ph.D. Thesis
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Review
addition to the fraction of the crystalline phase and the contents of incorporated
Bi3+ and Eu3+ ions, the local chemical states around Bi3+ play significant roles in
photoluminescence properties.
J. Ma et al. [61] had successfully obtained pure monoclinic (m-) and tetragonal (t-)
LaVO4 nanorods via a facile oxides-hydrothermal method, in which V2O5 and
La2O3 bulk powders were directly utilized as precursors without pre-treatment.
Ethylenediaminetetraacetic disodium salt (EDTA) was found to be a key factor for
synthesizing t-LaVO4. The FTIR spectra of VO4 around 800 cm-1 were suggested
as an effective auxiliary means to identify the crystal phase of LaVO4. UV–Visible
spectra of LaVO4 nanomaterials are obvious blue shift compared with the bulk mLaVO4 materials. The different photoluminescent properties of Eu3+ doped m- and
t-LaVO4 are demonstrated. A dissolution–precipitation mechanism was mainly
responsible for the anisotropic morphology and phase control evolution of the
LaVO4 nanocrystals. The oxides-hydrothermal system is also applicable to prepare
other pure LnVO4 (Ln3+:Nd3+, Y3+, Sm3+) and doped LnVO4 nanomaterials.
H. Zhu et al. [62] had successfully deposited YBO3:Eu3+/Tb3+ nanocrystalline thin
films onto quartz glass substrates by Pechini sol–gel dip-coating method, using
rare- earth nitrates and boric acid as starting materials. Characterization of the
samples revealed that the films were composed of spherical YBO3:Eu3+/Tb3+
nanocrystals with average grain size of 80 nm. The YBO3:Eu3+ film exhibited
strong orange emission at 595 nm and red emission at 615 nm, which were,
respectively ascribed to the (5D0→7F1) and (5D0→7F2) transitions of Eu3+. The
YBO3:Tb3+ film showed dominant green emission at 545 nm due to the 5D4→7F5
transition of Tb3+.
L. Chen et al. [63] had reported the synthesis of Bi3+, Eu3+ and/or Tb3+ co-doped
YBO3 phosphors by the oxalate deposition assisted solid state reaction method, and
their spectroscopic properties were investigated using a synchrotron radiation
instrument with an emphasis on excitation, electronic transition and energy transfer
Ph.D. Thesis
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Review
from Bi3+ to Eu3+/Tb3+. They observed that the residual emission of Bi3+ only
occurs in the Bi3+ and Eu3+ co-doped system but not in the YBO3: Bi3+,Tb3+
system. These phenomena were reported to be apparently caused by the differences
in the energy transfer between Bi3+→Eu3+ and Bi3+→Tb3+, which hide the
substantial differences between the interaction of Bi3+ and Eu3+/Tb3+ and the
energy level match (spectra overlap). The physical mechanism involved in them
was also discussed.
A.K. Gulnar et al. [64] had reported the synthesis and characterization of highly
crystalline CePO4, CePO4:Tb3+ and CePO4:Dy3+ nanoleaves with monoclinic
structure and dispersible in solvents such as water and methanol were prepared by a
low temperature synthesis. The nanoleaves were incorporated into silica sols by the
sol-gel method and were found to exhibit improved luminescence properties
compared to silica sols directly doped with lanthanide ions. The observed
difference in the luminescent properties of nanoleaves incorporated into silica sols
and silica sols directly incorporated with lanthanide ions had been explained based
on the different extent of energy transfer from Ce3+ to Tb3+/Dy3+ ions. The different
extent of quenching of the excited-state of lanthanide ions due to OH groups from
Si-OH linkages and water molecules, taking place in nanoleaves incorporated into
silica sols compared to silica sols directly doped with lanthanide ions, was also
responsible for the improvement in the luminescence properties. The study would
be quite relevant for developing biosensors for studying enzyme and protein
activities based on luminescent silica sols.
G. Jia et al. [65] had synthesized well-dispersed YVO4:Ln3+ (Ln = Eu, Dy, and Sm)
nanocrystals with uniform morphology and size via a facile solvothermal route.
XRD results demonstrated that all of the three samples can be well indexed to the
pure tetragonal phase of YVO4, indicating that the Eu3+, Dy3+, and Sm3+ had been
effectively doped into the host lattices of YVO4. TEM study showed that the YVO4
nanocrystals exhibit ellipsoid shape and a mean size of about 20 nm (in good
Ph.D. Thesis
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Review
agreement with the estimation of XRD results). The YVO4:Ln3+ (Ln = Eu, Dy, and
Sm) nanocrystals showed strong light emissions with different colors coming from
different Ln3+ ions under ultraviolet excitation or low-voltage electron beams
excitation, which might find potential applications in the fields such as light
emitting phosphors, advanced flat panel display, field emission display devices or
biological labelling.
B. Yan et al. [66] had engaged a modified hydrothermal process in the synthesis of
LaVO4:Eu3+ nanophosphor. All kinds of inorganic salts (solid state hydrated rare
earth nitrates and NH4VO3) and precipitation reagents (ammonia and urea) were
mixed to form the solid state precursors instead of general aqueous solution
systems, and a little amount water existed in the hydrothermal reaction systems.
Both X-ray powder diffraction (XRD) and transmission scanning electronic
microscope (TEM) showed that the uniform microstructure with the particle size of
around 60 nm and the product from ammonia possesses the higher phase purity
than that from urea. LaVO4:Eu3+ showed a strong red emission at 617 nm
originating from the 5D0→7F2 hypersensitive transition of Eu3+ ion. Especially the
LaVO4:Eu3+ nanophosphor from ammonia presented the more excellent
photoluminescent property (lifetime and quantum efficiency) than that from urea.
Kumar et al. [67] had reported that enhancing the optical emission of cerium oxide
nanoparticles is essential for potential biomedical applications. They employed a
simple chemical precipitation technique to synthesize europium-doped cerium
oxide nanostructures to enhance the emission properties. Structural and optical
properties showed an acute dependence on the concentration of oxygen ion
vacancy and trivalent cerium, which, in turn, could be modified by dopant
concentration and the annealing temperature. Results from X-ray photoelectron
spectroscopy showed an increase in tetravalent cerium concentration to 85 % on
annealing at 900 oC. The concentration of oxygen ion vacancy increased from
1.7x1020 cm-3 to 4.1x1020 cm-3 with the increase in dopant concentration.
Ph.D. Thesis
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Review
Maximum emission at room temperature was obtained for 15 mol % Eu-doped
ceria, which improved with annealing temperature. In their paper, the role of
oxygen ion vacancies and trivalent cerium in modifying the emission properties
were discussed.
A. Bao et al. [68] had reported an efficient process for preparing monodispersed
SiO2@Y0.95Eu0.05VO4 core–shell phosphors using a simple citrate sol–gel method
and without the use of surface-coupling silane agents or large stabilizers. The XRD
results demonstrated that the Y0.95Eu0.05VO4 particles crystallization on the surface
of SiO2 annealing at 800 oC was perfect and the crystallinity increases with raising
the annealing temperature. The obtained core–shell phosphors had a near perfect
spherical shape with narrow size distribution (average size ca. 500 nm and an
average thickness of ~ 50 nm), were not agglomerated, and had a smooth surface.
The thickness of the YVO4:Eu3+ shells on the SiO2 cores could be easily tailored by
changing the mass ratio of shell to core (W = [YVO4]/[SiO2]) (~ 50 nm for W =
30 %). The Eu3+ showed a strong PL luminescence (dominated by 5D0 - 7F2 red
emission at 618 nm) under the excitation of 320 nm UV light. PL intensity of Eu3+
increased with annealing temperature and values of W.
B. Wang et al. [69] Ca3Al6Si2O16: Ce3+, Tb3+ phosphors had been prepared by sol–
gel method. The structure and photoluminescence properties were studied. The
results indicated that the single-phased Ca3Al6Si2O16 phosphors crystallize at
1,000 oC for 2 h in conventional furnace. With appropriate concentrations of Ce3+
and Tb3+ ions into Ca3Al6Si2O16 matrix, these materials exhibited blue phosphors
and white light under ultraviolet radiation. White-light emission could be achieved
because of a 400 nm emission ascribed to transitions of Ce3+ ions and three sharp
peaks at 487, 543, 585 nm, respectively, resulting from transitions of Tb3+ ions.
A.H. Krumpel et al. [70] had compiled and analyzed optical and structural
properties of lanthanide doped non-metal oxides of the form APO4:Ln3+ with A a
rare earth and of transition metal oxides with formula ABO4:Ln3+ with B a
Ph.D. Thesis
Page 50
Review
transition
metal.
Their
main
objective
was
to
understand
better
the
interrelationships between the band gap energy, the O2-→Ln3+ charge transfer
energy, and the Ln3+→B5+ inter-valence charge transfer energy. Various models
exist for each of these three types of electron transitions in inorganic compounds
that appear highly related to each other. When properly interpreted, these optically
excited transitions provided the locations of the lanthanide electron donating and
electron accepting states relative to the conduction band and the valence band of
the hosting compound. These locations in turn determined the luminescent
properties and charge carrier trapping properties of that host. Hence, they suggested
that understanding the relationship between the different types of charge transfer
processes and its implication for lanthanide level location in the band gap is of
technological interest.
J. Fang et al. [71] had reported that CePO4 nanorods decorated with quantum
dots(QDs) (QDs@CePO4) can be prepared in a sequential, aqueous procedure
under continuous flow using a rotator tube processor and a narrow channel reactor.
The emission the QD@CePO4 is tunable from green to red by simply adjusting the
feeding rate, which in turn regulates the particle size of the QDs. The Ce3+ ions in
the QDs@CePO4 served as an efficient fluorescence resonance energy transfer
(FRET) donor, effectively enlarging the Stokes shift of the QDs.
J. Zhang et al. [72] had hydrothermally synthesized nanostructured tetragonal
LaVO4 with sheaf-like and prickly spherical morphologies with the assistance of
ethylenediaminetetraacetic acid (EDTA). EDTA not only acted as a chelating
reagent to facilitate the formation of t-LaVO4, but also as a surface capping agent
to adhere to the newly created surface and to promote the crystal splitting. t-LaVO4
nanostructures with different morphologies were achieved by adjusting the molar
ratio of EDTA/La3+, the concentration of La3+, and the total volume of the reaction
mixture, which resulted in the change of the crystal growth rate. Nanostructured
Ph.D. Thesis
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Review
Eu3+-doped t-LaVO4 was also synthesized and showed intense red emission under
near UV-light excitation.
Y. He et al. [73] Y0.99-xPO4:0.01Dy3+, xBi3+ (x = 0, 0.01, 0.05, 0.10, 0.15, 0.20 and
0.25) phosphors had been synthesized by a modified chemical co precipitation
method using urea as a pH value regulator. XRD results showed that the samples
have only single tetragonal structure when x ≤ 0.15, but extraneous BiPO4 phase
appeared besides major tetragonal phase when x ≥ 0.20. The crystallinity of the
samples was found to improve with increasing Bi3+ ion concentration from 0 to 15
mol %, and then decreased for higher concentrations associated with increasing
BiPO4 phase. Photoluminescence excitation spectra results showed that the
phosphor can be efficiently excited by ultraviolet light from 250 to 400 nm
including four peaks at 294, 326, 352 and 365 nm Emission spectra exhibited
strong blue emission (483 nm) and another strong yellow emission (574 nm). When
the Bi3+ ion concentration reached 1mol %, the intensity of excitation and emission
spectra increased evidently. In addition, the yellow-to-blue emission intensity ratio
(IY/IB) is strongly related to the excitation wavelength and not to the Bi3+ ion
concentration.
Z. Xu et al. [74] had successfully synthesized lanthanide orthovanadate LnVO4 (Ln
= La to Lu) nano-/microcrystals with multiform crystal structures (monoclinic and
tetragonal)
and
microdoughnut
morphologies
and
spherical
(separated
aggregates)
nanoparticles,
by
a
facile,
micropancake,
effective,
and
environmentally friendly hydrothermal method. The experimental results indicated
that the use of the organic additive trisodium citrate (Cit3-) had an obvious impact
on the morphologies of the products. They had presented the possible formation
mechanisms
for
LnVO4
nano-/microcrystals
with
diverse
well-defined
morphologies in detail. Additionally, they systematically investigated the
luminescent properties of the LuVO4:Ln3+ (Ln = Eu, Sm and Dy). Due to an
efficient energy transfer from vanadate groups to the dopants, LuVO4:Ln3+ (Ln =
Ph.D. Thesis
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Review
Eu, Sm and Dy) phosphors showed the strong characteristic emission of Ln 3+ under
ultraviolet excitation and low-voltage electron beam excitation. Furthermore, the
PL emission color of LuVO4:Ln3+ (Ln = Eu, Sm and Dy) phosphors could be tuned
from blue to red, orange-red, and green easily by partial replacement VO43- with
PO43- and changing the doping concentrations (x) of Ln3+.
P. Huang et al. [75] had also reported the Na2EDTA-assisted synthesis of the
lanthanide (Ln3+) ion doped one-dimensional YVO4 nanobelts and nanorods by a
facile hydrothermal route. The morphology-dependent luminescent behaviors of
Eu3+:YVO4 were systematically investigated. Under a single wavelength UV light
excitation, the Ln3+:YVO4 (Ln = Nd, Sm, Eu, Dy, Ho, Er, Tm, or Yb) nanorods
exhibited visible to near-infrared (NIR) multicolor tunable luminescence via
efficient host sensitization, while no emissions were detected for the Ce3+, Pr3+, or
Tb3+ doped samples. Based on the results, possible mechanisms depicting the
YVO4 host sensitizing or quenching of lanthanide emissions were proposed.
N. Xie et al. [76] proposed luminescent lanthanide nanocrystals (NCs) to be a
promising new class of fluorescent labeling agents due to their attractive optical
and chemical features including low toxicity, wide photoluminescence (PL)
emission and high resistance to photobleaching. They had reported an ionic-liquidinduced synthesis of Ce1−xTbxF3 nanoparticle via utilizing a capillary micro reactor.
Ionic liquid-[bmim]BF4 acted as both a fluoride source and stabilizing solvent
during the reaction, which was shown to be a key factor that governs luminescence
intensity of the obtained nanoparticles. The luminescent properties could be greatly
improved by optimizing the volume percentage of [bmim]BF4. Furthermore, the
reaction temperature exerted an influence on the properties of the prepared
samples. Experimental results showed that the colloidal solutions of Tb3+-doped
CeF3 NCs exhibit the characteristic emission of Ce3+ 5d–4f and Tb3+ 5D4–7FJ (J =
6–3) transitions with 5D4–7F5 green emission at 542 nm as the strongest peak. The
as-prepared samples were found dispersible in water with the quantum yield (in
Ph.D. Thesis
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aqueous solution) as 12 %, which indicated a potential application on biolabels,
light-emitting diodes (LEDs) and redox luminescent switches.
W. Xu et al. [77] had reported recently that the irradiation of ultraviolet (UV) light
usually induces the photodegradation of organic photovoltaic materials, leading to
the significant degeneration of organic photovoltaic (OPV) solar cells. In view of
this, they designed UV to visible photo-conversion YVO4:Eu3+, Bi3+ nano-films for
the first time. These nano-films could not only filter out the harmful UV light to
improve the lifetime of OPV, but also enhanced the power conversion efficiency
(PCE). In their paper, they presented the preparation, structure and excellent
photoconversion properties of Y0.97-xVO4:0.03Eu3+, xBi3+ (0 ≤ x ≤ 0.4) microsized
powders and the nano-film converters fabricated by laser ablation. Furthermore, the
photo-stability of P3HT(poly(3-hexylthiophene)) film with the converter was
investigated. It is exciting to observe that the degradation of P3HT thin-film could
be effectively retarded with the converter, about 3 fold compared to that of the
glass reference. The photoconverter from UV to visible had the potential to be
practically used in future OPV solar cells devices.
X. Junjie et al. [78] had synthesized YPO4:Eu3+ phosphors by solution coprecipitation method assisted by urea in the precursor reaction solution. X-ray
diffraction spectral analysis showed that the samples synthesized with urea had
smaller particle size and lower crystallinity than those samples synthesized without
urea. Moreover, the calculated strain result indicated that the Eu3+ site in the former
exhibited lower crystal field symmetry than that in the latter. Hence, the influence
of crystal field symmetry dominated luminescence efficiency rather than
crystallinity because the luminescence intensity observed in Eu0.05Y0.95PO4
synthesized with 1.0 g urea was six-fold higher than that of the as-synthesized
sample. With increased concentration of Eu3+ ion, the concentration of Eu3+ ion
exceeded 12 mol % due to concentration quenching. The optimal condition for
YPO4:Eu3+ phosphor was Eu0.12Y0.88PO4 with 1.0 g urea added in the precursor.
Ph.D. Thesis
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Review
The luminescence intensity of the optimal condition was again enhanced 1.6-fold
relative to that of Eu0.05Y0.95PO4 synthesized with 1.0 g urea.
Very recently, H. Yu et al. [79] had fabricated singly distributed YBO3:Eu
nanofibers with an average diameter of around 120 nm using the electrospinning
technique. The luminescent properties of the YBO3:Eu nanofibers were studied
relative to the corresponding bulk material. The location of the charge transfer band
in the excitation spectra showed a slight blue shift in the nanofibers compared with
the bulk material. In the emission spectra, the ratio of the red emission at 611 nm to
the orange emission at 591 nm (R/O value) in the nanofibers increased slightly, in
contrast to the bulk, indicating that improved chromaticity can be obtained from
YBO3:Eu nanofibers. The high color-rendering index obtained from them implies
that these novel luminescent fibers could be used as potential candidates for
nanodevices.
J. Thakur et al. [80] had synthesized Eu3+ and Tb3+ doped in nanocrystalline InBO3,
GdBO3, and LaBO3 having three different morphs of calcite (CaCO3) such as
calcite, vaterite, and aragonite, respectively, by glycine–nitrate combustion method.
Luminescence due to Eu3+ and Tb3+ doped individually as well as simultaneously
in these three different morphs of calcite were investigated and compared. Also the
effect of concentration of dopant ions on the luminescence was studied. The
highest photoluminescence emission intensity was observed for RE0.05M0.95BO3
(RE = Eu3+, Tb3+, M = In, Gd, La) samples. A further increased in doping led to
concentration quenching of the luminescence. In case of the co-doped borates, the
energy transfer between the co-doped rare earth ions was influenced by the host
crystal structure. This study revealed that there is remarkable effect of the crystal
structure of host and concentration of dopant ions on the luminescence.
R.F. Wei et al. [81] had reported the preparation of highly transparent Ce3+, Tb3+
co-doped sodium silicate glasses by melt-quenching technique. The luminescent
properties of Ce3+, Tb3+ single-doped and co-doped glasses, energy transfer process
Ph.D. Thesis
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Review
from Ce3+ to Tb3+ were systemically investigated through excitation spectra,
emission spectra, decay curves, and energy level diagram. Tuning the content of
Tb3+ could generate the varied hues from blue to white and eventually to yellowish
green. Their results suggested that Ce3+, Tb3+ co-doped sodium silicate glasses
could be used as converting phosphors for near-ultraviolet LED chips to generate
W-LEDs.
A systematic study of the preparation of lanthanide (Eu3+, Tb3+, Sm3+ and Dy3+)
phosphors was carried out by Q. Wang et al. [82] and luminescent materials (red,
green, orange-red and blue-yellow) were successfully achieved at low temperature
(70 oC) in 45 min, using simultaneous supersonic and microwave irradiation.
Scanning and transmission electron microscopic images indicated that onedimensional nanorods with diameters of 10–20 nm and lengths of up to 200 nm
were formed. The phosphors were entrapped in cellulose gels and the
corresponding hydrogels were also strongly emissive.
J.C. Batista et al. [83] had prepared nanosized rare earth phosphovanadate
phosphors (Y(P,V)O4:Eu3+) by applying the organic–inorganic polymeric
precursors methodology. Luminescent powders with tetragonal structure and
different vanadate concentrations (0 %, 1 %, 5 %, 10 %, 20 %, 50 %, and 100 %,
with regard to the phosphate content) were then obtained for evaluation of their
structural and spectroscopic properties. The solids exhibited very intense 5D0→7FJ
Eu3+ transitions, and it was possible to control the luminescent characteristics, such
as excitation maximum, lifetime and emission colour, through the vanadium (V)
concentration. The observed luminescent properties correlated to the characteristics
of the chemical environments around the Eu3+ ions with respect to the composition
of the phosphovanadates. The Eu3+ luminescence spectroscopy results indicated
that the presence of larger vanadium (V) amounts in the phosphate host lattice led
to more covalent and polarizable chemical environments. So, besides allowing for
control of the luminescent properties of the solids, the variation in the vanadate
Ph.D. Thesis
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Review
concentration in the obtained YPO4:Eu3+ phosphors, enabled the establishment of a
strict correlation between the observable spectroscopic features and the chemical
characteristics of the powders.
Ph.D. Thesis
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Review
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