Luminescent properties of Nd3+-doped LaF3 core/shell nanoparticles with enhanced near infrared (NIR) emission

Article abstract of DOI:10.1016/j.cplett.2010.05.070

A kind of Nd³⁺-doped LaF₃ nanoparticles with core/shell structure were synthesized via a co-precipitation method. The possible formation mechanisms for the core/shell architectures were presented. The X-ray diffraction (XRD) patterns indicated that the obtained nanoparticles exhibit hexagonal structure. In comparison with LaF₃:Nd core nanoparticles, the fluorescence intensity and lifetime were increased by 140% and 150% respectively in LaF₃:Nd/LaF₃ core/shell nanoparticles. The obvious features are mainly attributed to the formation of LaF₃ shell which effectively protected the Nd³⁺ ions from surrounding influence. The results show that rare-earth-doped nanoparticles with core/shell structure have potential applications in biological labeling and light-emitting devices.

Full text of DOI:10.1016/j.cplett.2010.05.070

Chemical Physics Letters 494 (2010) 60–63
Contents lists available at ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Luminescent properties of Nd3+-doped LaF3 core/shell nanoparticles
with enhanced near infrared (NIR) emission
a,c,
Xiaoxia Cui a,b, Jiangbo She a,b, Chao Gao a, Kai Cui a,b, Chaoqi Hou a,b, Wei Wei a,c, , Bo Peng
*
**
a State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Science (CAS), Xi’an, Shaanxi 710119, PR China
b Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China
c Institute of Advanced Materials, Nanjing University of Posts and Telecommunications, Nanjing 210003, PR China
a r t i c l e i n f o
a b s t r a c t
A kind of Nd3+-doped LaF3 nanoparticles with core/shell structure were synthesized via a co-precipitation
method. The possible formation mechanisms for the core/shell architectures were presented. The X-ray
diffraction (XRD) patterns indicated that the obtained nanoparticles exhibit hexagonal structure. In com-
parison with LaF3:Nd core nanoparticles, the fluorescence intensity and lifetime were increased by 140%
and 150% respectively in LaF3:Nd/LaF3 core/shell nanoparticles. The obvious features are mainly attrib-
uted to the formation of LaF3 shell which effectively protected the Nd3+ ions from surrounding influence.
The results show that rare-earth-doped nanoparticles with core/shell structure have potential applica-
tions in biological labeling and light-emitting devices.
Article history:
Received 25 March 2010
In final form 24 May 2010
Available online 27 May 2010
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
LaF3 [9–12] have been introduced to shield the lanthanide ion.
These core/shell nanoparticles, in which the core is doped with
Neodymium ions (Nd3+) doped nano-materials have a variety of
applications in the optical cells window and the optical telecom-
munication window due to their near infrared emissions [1–3]. Re-
cently, rare-earth (RE) fluorides have become the research focus
because of their very low vibrational energies; this advantage will
minimize the quenching effects of the excited states of the rare-
earth ions [4]. However, the near infrared (NIR) emitting rare-earth
ions such as Nd3+ and Er3+ can be quenched efficiently when they
are incorporated into nanoparticles [5]. It may be explained that a
large surface-to-volume ratio of nanoparticles cause a number of
dopant ions to reside on the nanoparticles surface, which form
the surface defects and hamper the optical efficiency. On top of
that, the surface ions would adsorb some groups such as O–H
and C–H which also noticeably quench the emissions of lumines-
cence ions [6,7]. Similar effect has been observed in the LaF3:Nd
nanoparticles reported in our previously work [8], although the
nanoparticles surface were modified by oleic acid, the O–H groups
were not eliminated completely. In order to reduce these influence,
the passivated inorganic shells including NaYF4, SiO2, LaPO4 and
the lanthanide ion and the shell is not doped, could confine the ex-
cited state well away from the surface and in this way reduce sur-
face quenching. An appropriate shell should possess two features
to improve the emission properties: (1) low phonon energy and
environmental stability to minimize the nonradiative quench
[13]; (2) similar crystalline structure between the shell and core
to decrease lattice mismatch [14]. LaF3 as doping host meets these
requirements due to its low vibration energies and superior optical
transparency in the 0.2–1.1 lm region.
Rare-earth fluoride (REF3) core/shell nanocrystals have been
successfully prepared by the thermolysis of organometallic pre-
cursors [15] and hydrothermal synthesis [16]. However, these
methods usually require high temperature and high pressure,
which may lead to the formation of the no-doped nanoparticles
and the relatively large diameter, limiting their applications in
biological labeling. Water–ethanol–ammonium di-n-octadecyldi-
thioposhate (ADDP) mixing system with a facile precipitation
technology was first developed by Ballato’s group to synthesize
core/shell nanoparticles [17]. This strategy is a green method
without high temperature decomposition of rare-earth salts. In
this work, we modified the method to synthesize a kind of
LaF3:Nd/LaF3 core/shell nanoparticles. This architecture contrib-
uted to significant enhancement of fluorescence intensity and life-
time, which implied that the LaF3 shell with low vibration energy
has protected the emission of Nd3+ centers. It provides a useful
way to improve luminescent properties of Nd3+-doped LaF3
nanoparticles.
* Corresponding author at: State Key Laboratory of Transient Optics and
Photonics, Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of
Science (CAS), Xi’an, Shaanxi 710119, PR China. Fax: +86 25 83535502.
** Corresponding author at: State Key Laboratory of Transient Optics and
Photonics, Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of
Science (CAS), Xi’an, Shaanxi 710119, PR China. Fax: +86 25 83535502.
E-mail addresses: iamwwei@njupt.edu.cn (W. Wei), bpeng@opt.ac.cn (B. Peng).
0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2010.05.070

X. Cui et al. / Chemical Physics Letters 494 (2010) 60–63
61
reaction system alternatively, the nanoparticles surface exhibits
excessive La3+ or FÀ ions in turn, which bond additive counter ions
via the electrostatic attraction. Therefore, the additive ions were
grafted on the core surface to form shell layer instead of individual
nanoparticles.
2. Experimental
The LaF3:Nd3+/LaF3 core/shell nanoparticles were synthesized
as follows [17]. NH4F (8 mmol) and oleic acid (OA) surfactant
(0.6 mmol) were firstly dissolved in 70 mL ethanol/water (1:1),
and then the solution was heated to 75 °C. Aqueous solution
(4.0 mL) with La(NO3)3Á6H2O (3.6 mmol) and Nd(NO3)3Á6H2O
(0.4 mmol) were added dropwise into the NH4F solution to form
the core particles. After 30 min, LaF3 shell was grown by the alter-
nating addition in 10 parts of a 2.5 mL aqueous NH4F solution
(4.0 mmol) and a 4.0 mL aqueous Ln(NO3)3 solution (2.0 mmol).
The result solution was stirred at 65 °C for 2 h and allowed cooling
to room temperature. The products were centrifugated and washed
by ethanol and water, the resultant powder was dried over P2O5 for
2 days under vacuum.
The morphologies of the samples were characterized by JEOL
JEM-3010 transmission electron microscope (TEM) under a work-
ing voltage of 300 kV. The X-ray diffraction (XRD) was measured
on a Rigaku Dmax-2400 X-ray powder diffractometer with graph-
ite monochromatized Cu K
spectra were obtained by Axis Ultra Spectrometer (Kratos, UK)
using monochromatic Al radiation (150 W, 15 KV, and
3.2. Morphology, structure and surface composition
Fig. 2 shows the TEM images of core and core/shell nanoparti-
cles. The average diameters of these nanoparticles are 9.6 and
11.1 nm respectively. The thickness of the shells is about 1.5 nm.
No small free-standing nanoparticles are inlayed on the large ones
in TEM images of the core/shell nanoparticles, indicating that all of
the added monomer had grown on the existing core nanoparticles
and no new nucleation had occurred. Because the core and the
shell have similar electron density and lattice parameters, they
cannot be clearly distinguished [18].
Fig. 3 shows the XRD patterns of LaF3:Nd/LaF3 core/shell and
LaF3:Nd nanoparticles. All of the diffraction peaks were indexed
to the hexagonal LaF3 crystal structure (JCPDS: 32–0483). No other
peaks were observed in the patterns, revealing that the products
were single. By means of Debye–Scherer equation, the average
sizes of core and core/shell nanoparticles were estimated to be
about 9.2 and 11.3 nm, respectively. These results were in good
agreement with the TEM image. Compared with the core nanopar-
ticles, the peak intensities of core/shell nanoparticles increased
obviously, which demonstrate that the formation of LaF3 shell
reduced the surface defects successfully.
a radiation (k = 0.15405 nm). The XPS
K
a
1486 eV), and the vacuum in the spectrometer is 10À9 Torr. Photo-
luminescence emission spectra were recorded on a Zolix Omini-k
300 spectrophotometer pumped by a laser diode at 800 nm. Lumi-
nescence decay times were measured by modulating the laser with
a mechanical chopper, and the signal was collected and analyzed
by a 300 MHz Tektronix oscilloscope (Model 3032B).
In order to confirm that the LaF3:Nd core nanoparticles have
been coated by LaF3 shell successfully; the surface compositions
3. Results and discussion
3.1. Formation mechanism of core/shell nanoparticles
Fig. 1 is a scheme diagram of the formation mechanism of the
LaF3:Nd/LaF3 core/shell nanoparticles. The modified method is to
grow the shell layer around the core by the alternate addition of
small portions of the shell reagents.
At the beginning of the core formation, the surfactant and
excessive amounts of FÀ ions were introduced. When the rare-
earth ions were added dropwise into the solution, LnF3 nuclei
was generated by the chemical reaction between Ln3+ and FÀ.
Meanwhile, the OA acted as surfactant kept the core nanoparticles
growing gradually. The excessive FÀ ions were absorbed on the
core surface, then the nanoparticles exhibit negative charge sur-
face. That makes the coordination of positive charge La3+ ions
possible.
Fig. 2. TEM images of the as-prepared core LaF3:Nd (a) and LaF3:Nd/LaF3 core/shell
samples (b).
At the second step, the reaction temperature was adjusted to
65 °C to slow the reaction rate, avoiding additional nucleation to
form un-doped nanoparticles. When the shell solution containing
La3+ ions was added, the La3+ interacted with the FÀ ions gradually.
Accordingly, the excessive La3+ ions were located at the nanoparti-
cles surface, leading to a positive charge surface. When the FÀ ions
were added subsequently, there is competition between the OA
molecules and fluoride ions. The Ln3+ ions first react with FÀ ions
instead of OA molecules on account of its smaller radius, Thus,
when the solutions containing La3+ and FÀ ions were added into
Fig. 1. A scheme diagram of the formation mechanism for the LaF3:Nd/LaF3 core/
Fig. 3. XRD patterns of standard data for LaF3 (a) (PDF card No. 32-0483), LaF3:Nd
shell nanoparticles.
core nanoparticles (b) and LaF3:Nd/LaF3 core/shell nanoparticles (c).

62
X. Cui et al. / Chemical Physics Letters 494 (2010) 60–63
of the samples were investigated by XPS analysis (as shown in
Fig. 4). For LaF3:Nd nanoparticles, the obvious peaks of visible Nd
3d (binding energy = 1000 eV), La 3d (binding energy = 836 eV)
and F 1s (binding energy = 635 eV) were observed. For LaF3:Nd/
LaF3 core/shell structure, the sharp and enhanced peaks of La 3d
and F 1s are observed obviously, and the intensity of peak of Nd
3d was weakened. This is consistent with the proposed structure
where the LaF3 shell encapsulated the LaF3:Nd core. The oxygen
and carbon detected in the XPS measurement may come from
the atmosphere. Furthermore, low amounts of oxygen and carbon
are frequently observed in the XPS spectra.
3.3. Photoluminescent properties
Fig. 5 shows the emission spectra of LaF3:Nd/LaF3 core/shell and
LaF3:Nd nanoparticles under excitation at 800 nm. The emission
peaks at 880, 1053, and 1330 nm correspond to the transitions
4
Fig. 6. Decay curves of the F3/2
?
4I11/2 transition for LaF3:Nd core and LaF3:Nd/
LaF3 core/shell nanoparticles (Nd10 mol%). The samples were excited at 800 nm and
emission was monitored at 1053 nm at room temperature.
4
4
4
from F3/2 to I9/2,
4I11/2 and I13/2, respectively. The 4F3/2 ? 4I11/2
is a typical magnetic dipole transition, generally investigated and
applied in near-IR. For LaF3:Nd/LaF3 core/shell nanoparticles, the
shapes of the emission patterns are similar to that of LaF3:Nd nano-
particles, while the emission intensity was increased by about
140% at 1053 nm. The ratio is much higher than the previous re-
ported [19].
Decay curves for the core and core/shell nanoparticles are
shown in Fig. 6. The fluorescence lifetime (
s) was measured to be
56 s for LaF3:Nd core and 140 s for LaF3:Nd/LaF3 core/shell
l
l
nanoparticles by fitting the decay curves with double exponential
function. Compared with the core nanoparticles, the fluorescence
lifetime of the core/shell nanoparticles was increased by 150%.
The increase ratio is much bigger than that of literature [9].
This remarkable enhancement can be explained that when the
shell-ion solution was added into the core solution, there is compe-
tition between the organic molecules (OA) and FÀ ions. The OA has
little chance to bond with Ln3+ ions adsorbed on the core surface,
avoiding the quenching of –CH3, –COOÀ groups. After the shell
layer formed, the coordination of OA to the lanthanide ions on
the shell surface increased the distance between neighbouring
nanoparticles, reducing the nonradiative pathways and suppress-
ing the quenching in the energy transfer process. Besides, in the
proposed core/shell structure, the light-emitting center ions
(Nd3+) located on the core surface own fewer dangling bonds,
thereby reducing the surface defects. Amounts of nonradiative cen-
ters existing on the core surface are reduced and the emitting cen-
ters were less sensitive to surface quencher. The enhanced
fluorescence intensity and lifetime indicate that the strategy is
an effective way to improve luminescent properties of LaF3:Ln3+
nanoparticles.
Fig. 4. XPS spectra of the LaF3:Nd core (A) and LaF3:Nd/LaF3 core/shell (B)
nanoparticles.
4. Conclusions
In summary, we have successfully synthesized LaF3:Nd/LaF3
core/shell nanoparticles via a co-precipitation method. The fluores-
cence intensity and lifetime were increased by 140% and 150%
respectively compared to LaF3:Nd nanoparticles. It can be ascribed
to the formation of shell layer which provides a physical barrier be-
tween the optically active core and the surrounding medium,
reducing the nonradiative pathways. Moreover, the shell further
provides an efficient passivation of the surface trap states, and
gives rise to a strongly enhanced fluorescence intensity and life-
time. The advantage of this facile method appears to provide an
effective way to improve the luminescence properties for other
rare-earth fluoride compounds.
Acknowledgments
This work was financially supported by the National Natural
Science Foundation of China (No. 60977023) and one Hundred Tal-
ents Programs of the Chinese Academy of Sciences.
Fig. 5. Emission spectra of LaF3:Nd/LaF3 core/shell and LaF3:Nd (Nd10 mol%)
nanoparticles. The peaks at 880, 1053, and 1330 nm correspond to the transitions
4
4
4
from F3/2 to I9/2,
4I11/2 and I13/2, respectively.

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