Branched NaYF4 nanocrystals with luminescent properties

Article abstract of DOI:10.1021/ic700523x

In this article, branched NaYF₄ nanocrystals have been successfully synthesized via a simple hydrothermal method. On the basis of the analysis of HRTEM and TEM images, the growth modes of the branched structure and further branching behavior have been proposed. The up- and down-conversion luminescence of branched NaYF₄:Er³⁺/Yb³⁺ and NaYF₄:Eu³⁺ have been characterized. Multiarmed NaYF ₄ phosphors can be introduced into polystyrene to form composite luminescent polymers because of its special geometrical shape. In conclusion, the luminescent branched particles should be of wide potential application as building blocks in the future nanoscience and nanotechnology.

Full text of DOI:10.1021/ic700523x

Inorg. Chem. 2007, 46, 6050−6055
Branched NaYF Nanocrystals with Luminescent Properties
4
Xin Liang, Xun Wang,* Jing Zhuang, Qing Peng, and Yadong Li*
Department of Chemistry, Tsinghua UniVersity, Beijing, 100084, P. R. China
Received March 18, 2007
In this article, branched NaYF
On the basis of the analysis of HRTEM and TEM images, the growth modes of the branched structure and further
branching behavior have been proposed. The up- and down-conversion luminescence of branched NaYF
:Er3+/
phosphors can be introduced into polystyrene
4
nanocrystals have been successfully synthesized via a simple hydrothermal method.
4
Yb3 and NaYF :Eu3 have been characterized. Multiarmed NaYF
+
+
4
4
to form composite luminescent polymers because of its special geometrical shape. In conclusion, the luminescent
branched particles should be of wide potential application as building blocks in the future nanoscience and
nanotechnology.
Introduction
as CdS, CdSe, and ZnO, have been well researched for
9
-11
several years.
These kinds of branched structures share
Future nanodevices require a high diversity of building
blocks unique in both function and structure.1,2 Branched
structures on the nanometer scale have received much
consideration in recent years because of their intrinsic
electronic, magnetic, photonic, and catalytic properties and
their potential to be crucial building blocks for future
nanodevices by a “bottom-up” self-assembly process. The
appearance of branched structures greatly increases the
diversity of building blocks. Branching and further branching
of nanocrystals can effectively increase the structural diver-
sity of building blocks and create more complex structural
architectures.5
a typical character “polymorphism”. They usually have an
isotropic core and anisotropic arms, which require the
coexistence of two different kinds of crystal structures in
the same nanocrystals. The other kind of branched structure
that has been developed is noble metal branched nanocrystals
with no polymorphism. For example, branched gold nanoc-
rystals with face-centered cubic structure have been synthe-
sized by using CTAB as a regulating reagent in liquid
3
,4
6
medium. These different growth modes enabled the con-
trolled growth of nanocrystal building blocks with complex
topologies.
4
NaYF , an excellent luminescent host material, has many
Until now, branched nanocrystals of metal, metal oxide,
and semiconducting materials have been successfully
applications in lighting and display devices, optical telecom-
munications, and solid-state lasers because of its high
radiative emission rate, narrow emission bands, and stability
and durability under high temperature and intense excitation
6-17
synthesized.
Branched semiconducting nanocrystals, such
*
To whom correspondence should be addressed. E-mail: wangxun@
mail.tsinghua.edu.cn (X.W.); ydli@mail.tsinghua.edu.cn (Y.L.). Fax: (+86)-
0-62788765.
1
8-20
energy.
4
NaYF usually adopts face-centered cubic or
1
(
(
(
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6050 Inorganic Chemistry, Vol. 46, No. 15, 2007
10.1021/ic700523x CCC: $37.00
© 2007 American Chemical Society
Published on Web 06/29/2007

4
Branched NaYF Nanocrystals
Figure 1. (a and b) TEM images of obtained product. (c) XRD pattern of obtained product.
Figure 2. HRTEM images of (e) a single tetrapod, (a, b, d, and f) different parts of the single tetrapod (insert, corresponding Fuorier transform electron
diffraction pattern) and (c) a diagram illustrating the growth mode of the tetrapod.
hexagonal crystal structures, which indicate that these
luminescent materials may grow into branched morphologies
following either polymorphism or no polymorphism modes.
Since the luminescent properties of lanthanide-doped phos-
phors are host sensitive and can be tuned by adjusting the
crystal fields of the phosphors, synthesis of lanthanide-doped
4
NaYF phosphors with novel morphology, such as branched
structures, may lead to the discovery of novel size-depended
(
20) Xia, H. R.; Li, L. X.; Zhang, H. J.; Meng, X. L.; Zhu, L.; Yang, Z.
H.; Liu, X. S.; Wang, J. Y. J. Appl. Phys. 2000, 87, 269.
Inorganic Chemistry, Vol. 46, No. 15, 2007 6051

Liang et al.
Figure 3. HRTEM images of (e) a single tripodlike nanocrystal, (a, c, d, f, g, and i) different parts of the tripodlike nanocrystal and a (b) digram illustrating
the growth mode of the tripodlike nanocrystal.
luminescent properties.21-25 In addition, branched nanostruc-
tures can be mixed with other materials to form multifunc-
Up-conversion fluorescent spectra were obtained with the LS-50B
fluorescence spectrophotometer (Perkin-Elmer Corp., Forster City,
CA) with an external 0-800 mW adjustable laser (980 nm, Beijing
Hi-Tech Optoelectronic Co., China) as the excitation source, instead
of the Xenon source in the spectrophotometer, and with an optic
fiber accessory.
tional complex materials2
6,27
because of its special geometri-
phosphors can
be introduced into polystyrene to form composite luminescent
polymers. Therefore, the synthesis of branched NaYF
cal shape. For example, multiarmed NaYF
4
4
nanocrystals should be of great significance for both research
and applications.
Results and Discussion
4
Branched NaYF nanocrystals have been fabricated by
Experimental Section
using CTAB as regulating agents to facilitate branched
growth in a methanol/water system. XRD analysis of
obtained products revealed that the product crystallized in a
In a typical synthesis, 3.0 g of CTAB, 20 mL of methanol, and
3 3
10 mL of water were mixed together; then, 1 mL of 0.5 M Y(NO )
4
cubic structure of NaYF . The XRD pattern shown in Figure
and 3 mL of 1 M NaF were added. Finally, the mixture was
transferred to a 40 mL autoclave, which was sealed and heated at
1c could be indexed as arising from the cubic phase (JCPDS
no. 77-2042) with lattice constant a ) 5.470 Å. TEM images
shown in Figure 1a and b displayed the branched structure
of the obtained nanocrystals. Careful observation revealed
that the morphologies of nanocrystals were diversity. It could
be seen that the nanocrystals were composed of some simple
multiarmed morphologies, such as tripod and tetrapod, and
other more complicated multibranched structures.
1
50 °C for about 24 h.
The sample obtained was characterized on a Bruker D8-Advance
X-ray powder diffractometer with Cu KR radiation (λ ) 1.5418
Å). The size and morphology of the nanocrystals were determined
by a JEOL JEM-1200EX transmission electron microscope (TEM)
at 100 kV and a Tecnai G2 F20 S-Twin high-resolution transmission
electron microscope (HRTEM) at 200 kV. Fluorescent spectra were
recorded with a Hitachi F-4500 fluorescence spectrophotometer.
A typical tetrapodlike nanocrystal is shown in Figure 2e.
HRTEM images (Figure 2a, b, d, and f) of different parts of
the nanocrystal provided more detailed information about
the branched structure. The lattice planes continuously
extending in each part of the nanocrystal showed the good
crystallinity of the nanocrystal. The distance of the lattice
planes extending on the same direction in each part of the
nanocrystal had been calculated to be the same (0.28 nm),
which could be assigned to the {200} planes of cubic phase
(
(
(
(
21) Menyuk, N.; Dwight, K.; Pierce, J. W. Appl. Phys. Lett. 1972, 21,
59.
22) Kano, T.; Suzuki, T.; Suzuki, A.; Minagawa, S. J. Electrochem. Soc.
973, 120, C87.
23) Martin, N.; Boutinaud, P.; Mahiou, R.; Cousseinsa, J.-C.; Bouderbal,
M. J. Mater. Chem. 1999, 9, 125.
1
1
24) Liang, L. F.; Wu, H.; Hu, H. L.; Wu, M. M.; Su, Q. J. Alloys Compd.
2004, 368, 94.
(
25) Yi, G. S.; Chow, G. M. AdV. Funct. Mater. 2006, 16, 2324-2329.
26) Zhang, H.; Cui, Z. C.; Wang, Y.; Zhang, K.; Ji, X. L.; L u¨ , C. L.;
Yang, B. AdV. Mater. 2003, 15, 777-780.
(
(27) Li, W. J.; Wang, X.; Li, Y. D. Chem. Commun. 2004, 164.
4
NaYF . It also revealed the branched nanocrystal was a single
6052 Inorganic Chemistry, Vol. 46, No. 15, 2007

4
Branched NaYF Nanocrystals
Figure 4. TEM images and XRD patterns of Branched structures of NaLnF4 (Ln ) Er, Tm, Yb) synthesized via the same synthetic route for branched
NaYF4 nanocrystals. (a) TEM image of branched NaErF4 nanocrystals. (b) TEM image of branched NaTmF4 nanocrystals. (c) TEM image of branched
NaYbF4 nanocrystals. (d) XRD patterns of branched structures of NaLnF4 (Ln ) Er, Tm, Yb).
crystal. The observation and analysis of the lattice planes
also reveal the epitaxial growth direction is along <110>
of different parts of the multiarmed nanocrystal and HRTEM
of arms of the branched nanocrystal also revealed the
preferential growth direction of other arms was along the
[ 1h 01] and [01 1h ] planes (Supporting Information S5). The
(111) plane was found to be perpendicular, by calculation,
to the plane defined by [ 1h 10], [ 1h 01], and [01 1h ]. Therefore,
the tripodlike nanocrystal was flat on its (111) plan with three
arms stretching in the direction of [ 1h 10], [ 1h 01], and [01 1h ].
It is worth noting that multibranching behavior was quite
common in our system. As shown in Figure 3e, the tripodlike
nanocrystal had obviously further branching at the end of
each arm. For the polymorphism-branched structure, such
as CdS and CdSe, the precondition of further branching was
nucleation of a cubic phase after initial arm growth to lead
another period of arm growth, which was hard to control in
(see Supporting Information). The inserts showed that the
corresponding Fuorier transform electron diffraction patterns
of the nanocrystal with spots indexed as face-centered cubic
structure. From the SAED, it can be seen that the growth
directions of the four arms are consistent with the directions
of some diffractional dots, whose orientation index can be
appointed to [ 2h 2h 0], [ 2h 20], [2 2h 0], and [220], respectively.
Therefore, it could be concluded that the epitaxial growth
direction of the four arms is along [ 1h 1h 0], [ 1h 10], [1 1h 0], and
[110] planes, respectively. The nanocrystal could be viewed
like as being flat on its (001) plan with four arms stretching
in the direction of [ 1h 1h 0], [1 1h 0], [ 1h 10], and [110]. The result
was in agreement with with the geometry of the tetrapod.
A similar analysis had been carried out on a tripodlike
nanocrystal. As shown in Figure 3, the lattice planes
extending in each part of the nanocrystal were continuous,
and the distance of the exposed lattice planes of nanocrystal
extending on the same direction had been calculated to be
the same (0.31 nm, {111} crystal planes). The Fourier
transform electron diffraction pattern of Figure 3c indicated
the longitudinal axis of the arm was [ 1h 10] (the growth
direction of this arm is parallel to the direction of diffractional
dot [ 1h 10]). The analysis on the electron diffraction patterns
4
both kinetics and thermodynamics. For NaYF branched
structures, both the core and arms belonged to the cubic
phase; therefore, further branching could be easily initiated
by the outer environment, such as the surfactant, micelles,
regulating reagents, the concentration of reactants, and so on.
The formation of branched structure was believed to have
close relationship with the high concentration of CTAB in
the reaction system. When the other experimental parameters
4
were kept unchanged, only spheres or rods of NaYF were
formed in absence of CTAB. The mixture of a few
Inorganic Chemistry, Vol. 46, No. 15, 2007 6053

Liang et al.
Figure 6. (a) Digital camera image of NaYF4 and polystyrene composite.
b) Emission spectra of NaYF4:Er3 /Yb
+
3+
phosphor and polystyrene
(
composite under a 980 nm laser source.
Figure 5. (a) Emission spectra of branched NaYF4:Eu3+ phosphor; excited
wavelength ) 375 nm. Inset: Digital camera image of NaYF4:Eu3
phosphor under ultraviolet radiation. (b) Emission spectra of branched
earth elements, such as Er, Tm, and Yb, have great analogy
with the crystal structure of NaYF . Branched structures of
NaLnF (Ln ) Er, Tm, Yb) have been successfully synthe-
sized by using the same synthetic route for branched NaYF
nanocrystals. (Figure 4) However, for the larger lanthanide
ions (La, Pr, Sm), only LnF was obtained in our synthetic
+
4
3
+
3+
NaYF4:Er /Yb phosphor under 980 nm laser source.
4
4
multiarmed nanocrystals and a majority nanorods formed at
the low concentration of CTAB. The adsorption effect of
CTAB and methanol on different crystal planes and the
influence of micelles formed by CTAB were believed to be
main factors to influence the formation of the branched
structures. When ethanol was used to replace methanol, a
mixture of only multiarmed and rodlike nanocrystals was
obtained. This phenomenon revealed that MeOH can favor
the formation of multiarmed nanocrystals. Similar phenom-
ena have been observed by Chen et al. in the synthesis of a
3
condition. And TEM observation of the products shows that
there were no branched structures formed. (Figure S2) The
results further confirmed the crystal structure of cubic NaYF
is important for the formation of branched structure.
4
4
As an outstanding host material, NaYF nanocrystals doped
with lanthanide ions can bring various luminescent properties.
3+
The most frequently used down-conversion ion Eu and up-
3
+
3+
conversion ion Er /Yb were selected to dope into the
branched NaYF nanocrystals to study the luminescent
behavior of branched NaYF phosphor. Doped samples were
prepared by the same synthetic procedure, except for adding
6
face-centered cubic Au branched structure in CTAB system.
4
In addition, a mixture of multiarmed nanocrystals and
nanorods can be obtained by adjusting the synthetic condi-
tions (protracting the reaction time or increasing the input
of NaF). SAED patterns and XRD pattern analysis revealed
4
3
+
3+
3+
3+
10% (total molar ration) Yb and Er or Eu , in Y at
the beginning stage. The morphology of doped NaYF had
no obvious difference with undoped NaYF branched nanoc-
rystals. Red light could be observed by naked eyes when
4
that the rods belonged to hexagonal phase of NaYF
4
, while
4
the multiarmed nanocrystals adopted a cubic phase. (Figure
S1) This phenomenon further indicated that the growth of
branched cubic-phase NaYF nanocrystals should follow the
4
no polymorphism mode, similar to that of the multiarmed
Au nanocrystals.
For the similarity of lanthanide elements on the ion radii,
3
+
the NaYF4:Eu nanocrystals were excited by ultraviolet
radiation. Digital camera image of luminescence from
3
+
NaYF4:Eu was shown in the inset image of Figure 5a.
3+
The emission spectra of NaYF4:Eu phosphor was recorded
from 500 to 750 nm. The emission peaks could be assigned
5
7
3+
4
the crystal structure of NaLnF , especially for the heavy rare
to the typical D
0
- F (J ) 1, 2, 3, 4) transition of Eu ion.
J
6054 Inorganic Chemistry, Vol. 46, No. 15, 2007

4
Branched NaYF Nanocrystals
5
-7F
The peak located at 590 nm (corresponding to D
magnetic dipole transition) is stronger than the peak located
0
1
Conclusion
5
7
In conclusion, branched nanocrystals of cubic NaYF
4
at 614 nm (corresponding to D
0
- F
2
electric dipole transi-
tion). As the relative strength of these two typical peaks of
Eu ion was strongly depended on the crystal field, it could
be figured out the high symmetry of the crystal field Eu
ion located, which was according with the crystal structure
of the branched NaYF nanocrystals.
Figure 5b showed the up-conversion emission spectra of
4
:Yb /Er under a 980 nm laser source. The peaks
at 545 and 648 nm could be assigned to the typical S3/2
have been successfully synthesized via a simple hydrothermal
method. The growth modes of the branched structure and
further branching behavior have been discussed. The lumi-
nescence of lanthanide ion-doped branched NaYF
characterized. An example of branched NaYF
the polymer composite was depicted. The branched particles
should be of wide potential application as building blocks
in future nanoscience and nanotechnology.
3+
3
+
4
have been
applied in
4
4
3+
3+
NaYF
4
-
4
4
4
I
15/2 and F9/2- I15/2 transitions, respectively. The peak at
Acknowledgment. This work was supported by NSFC
5
45 nm was much stronger than the 648 nm emission, indica-
(
20501013, 90606006), the Foundation for the Author of
3+
3+
ting good monochromaticity of the NaYF4:Er /Yb phos-
phor.
National Excellent Doctoral Dissertation of P. R. China, the
Program for New Century Excellent Talents of the Chinese
Ministry of Education, and the State Key Project of
Fundamental Research for Nanoscience and Nanotechnology
The branched nanocrystals with luminescence properties
were good candidates to be compounded with other materials
to form multifunctional complex materials for its unique
(2006CB932300).
3+
3+
4
structure. Here, branched NaYF :Er /Yb phosphors have
been introduced into polystyrene to develop a novel lumi-
nescence lanthanide-doped polymer. Figure 6a showed a
Supporting Information Available: TEM images and
XRD patterns of multiarmed nanocrystals, nanorods, and pro-
ducts with larger lanthanide ions, SEM image of the composite,
HRTEM images of a single tetrapod, and discussions. This
material is available free of charge via the Internet at http://
pubs.acs.org.
digital camera image of NaYF
The up-conversion luminescent of NaYF
4
and polystyrene composite.
is maintained in
4
the composite. Under a 980 laser source, the composite can
emit green lights (Figure 6b) which can be observed by the
naked eye. (Detailed information provided in Figure S3.)
IC700523X
Inorganic Chemistry, Vol. 46, No. 15, 2007 6055