вход по аккаунту


Green-Emitting CePO4 TbLaPO4 CoreЦShell Nanoparticles with 70 Photoluminescence Quantum Yield.

код для вставкиСкачать
Core?Shell Particles
Green-Emitting CePO4 :Tb/LaPO4 Core?Shell
Nanoparticles with 70 % Photoluminescence
Quantum Yield**
Karsten Kmpe, Holger Borchert, Jrg Storz,
Arun Lobo, Sorin Adam, Thomas Mller, and
Markus Haase*
Luminescent nanocrystals may serve as active components in
light-emitting devices,[1a] low-threshold lasers,[1b] optical
amplifiers,[1c] and as markers for biomolecules,[1d] provided
that their photoluminescence quantum yield is high. In recent
years, strategies have been developed to significantly increase
the quantum yield of nanoparticles by suppressing energy-loss
processes at the particle surface. In the case of semiconducting nanoparticles, this is possibly by growing a crystalline shell
of a suitable large-bandgap material around each nanocrystal.[2] Examples of such core?shell systems are CdSe/CdS,[2a]
CdSe/ZnS,[2b] InAs/CdSe, InAs/ZnS,[2c] and InP/ZnS[2d] nano[*] Dr. M. Haase, Dr. K. Kmpe, H. Borchert, J. Storz
Institute of Physical Chemistry, University of Hamburg
Bundesstrasse 45, 20 146 Hamburg (Germany)
Fax: (+ 49) 40-42838-3452
Dr. A. Lobo, S. Adam, Dr. T. Mller
Nottkestrasse 85, 22 603 Hamburg (Germany)
[**] We thank A. Kornowski and S. Bartholdi-Nawrath for TEM investigations and Dr. LBhmann at Malvern Instruments GmbH for
dynamic light-scattering measurements. This work was supported
in part by the BMBF.
Angew. Chem. Int. Ed. 2003, 42, 5513 ?5516
DOI: 10.1002/anie.200351943
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
crystals. All these systems are based on two materials with
similar lattice constants to avoid the formation of defects at
the core?shell interface.
Another class of redispersible luminescent nanoparticles
is formed by lanthanide(iii)-doped large-bandgap materials
with compositions similar to those of solid-state laser
materials or of bulk phosphors used in lighting and display
applications.[3] Unlike most molecular lanthanide complex
compounds, the nanocrystals provide a rigid crystal environment for the dopant ions, resulting in a higher photoluminescence quantum yield of the latter. However, the quantum
yield of doped nanoparticles is usually lower than that of the
corresponding bulk material as a result of energy-transfer
processes to the surface through adjacent dopant ions or
because the luminescence of surface dopant ions is quenched.[3e, f, i] These processes could be suppressed if one were
able to grow a shell of an undoped material around each
doped nanoparticle, that is, a shell made up of a material
through which energy cannot be transferred. Herein we
report the successful synthesis of redispersible CePO4 :Tb/
LaPO4 core?shell particles, which to our knowledge is the first
realization of the core?shell principle for a nanocrystalline
phosphor material.
Figure 1 displays luminescence spectra of dilute colloidal
solutions of CePO4 :Tb particles and CePO4 :Tb/LaPO4 core?
shell particles, which have identical optical densities at the
Figure 1. Fluorescence spectra of colloidal solutions of CePO4 :Tb
nanoparticles (solid line) and of CePO4 :Tb/LaPO4 core?shell particles
(dashed line).
excitation wavelength (lexc. = 277 nm). The spectra consist
mainly of four groups of signals, which correspond to the f?f
transitions of the Tb3+ ions, and a weak band in the UV region
as a result of the d?f transitions of Ce3+.[3f,g] The photoluminescence quantum yield of the CePO4 :Tb core particles
in Figure 1 is 43 % for the terbium emission (53 % if the Ce
emission is taken into account) and depends on the surface
treatment of the particles and the solvent as described
earlier.[3f] Growth of the LaPO4 shell around these core
particles increases the quantum yield of the terbium emission
to 70 % and the total quantum yield to 80 %. These values are
obtained reproducibly and are not far from the quantum yield
of the bulk material, which is 86 % for the terbium emission
and 93 % for the total emission.[4]
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
TEM images of CePO4 :Tb nanoparticles before (upper
part) and after (lower part) growth of the LaPO4-shell are
shown in Figure 2. The mean particle diameter increases from
4?6 nm for the almost spherical CePO4 :Tb core-particles to 8?
10 nm for the slightly elongated CePO4 :Tb/LaPO4 core?shell
Figure 2. TEM micrographs of a) CePO4 :Tb nanoparticles and
b) CePO4 :Tb/LaPO4 core?shell particles. Insets: High-resolution
images of nanoparticles.
particles. High-resolution images (insets) exhibit lattice
fringes for most particles and show that each particle consists
of a single crystalline domain. As the lattice constants of
monazite-type CePO4 and LaPO4 are virtually identical, the
core and the shell of these particles cannot be distinguished in
the TEM images. These observations are in accord with the
powder X-ray diffraction (XRD) data of both samples
(Figure 3). The Debye?Scherrer approximation was used to
calculate mean particle sizes of 5 and 8 nm for the core
and the core?shell particles, respectively. However, the widths
of the reflections of the elongated CePO4 :Tb/LaPO4 core?
shell particles show some anisotropy, which is most pronounced for the 200 reflection. The width of this peak
corresponds to a particle size of approximately 11 nm, which
indicates that the elongated shape of the nanoparticles is
caused by preferential growth along the a axis of the unit cell.
The CePO4 :Tb core nanoparticles as well as the CePO4 :Tb/
LaPO4 core?shell particles display almost identical XRD
Figure 3. Top: XRD patterns of CePO4 :Tb core particles after a) 2 h
and b) 16 h of heating at 200 8C. Bottom: XRD patterns of CePO4 :Tb/
LaPO4 core?shell particles after c) 2 h and d) 16 h of heating at 200 8C.
Angew. Chem. Int. Ed. 2003, 42, 5513 ?5516
patterns after 2 h and after 16 h of growth at 200 8C. This
result indicates that the reactants initially present in solution
are consumed within 2 h and that the resulting particles do not
grow further by Ostwald ripening, that is, by exchange of
material among the particles. The absence of Ostwald
ripening is a considerable advantage in the synthesis of
core?shell particles, as dissolution of the smaller particles
during Ostwald ripening leads finally to an intermixing of the
core and the shell materials.
In fact, X-ray photoelectron spectroscopy (XPS) measurements performed with varying excitation energy provides
further evidence that a shell was grown on the core particles
and that no intermixing of lanthanum and cerium phosphate
took place (Figure 4). If a photoelectron is generated by an X-
Figure 4. XPS data for the CePO4 :Tb/LaPO4 core?shell particles: experimental intensity ratio of the Ce and La 3d peaks versus X-ray photon
energy (dots with error bars). Expected pattern for a mixed phosphate
of composition La3Ce0.75Tb0.25(PO4)4 (dashed line) and for core?shell
particles with a 5-nm CePO4 :Tb core and 1.13-nm-thick LaPO4 shell
(solid line).
ray photon in the core of a core?shell nanoparticle, the
probability of its escape through the shell scales and its
detection depends on its kinetic energy.[5] If a synchrotron is
used as the X-ray source, the kinetic energy of the photoelectrons can be adjusted by choosing a suitable X-ray photon
energy (hn) according to Ekin = hn Ebind (Ebind is the electronbinding energy in the solid). The average shell thickness can
then be extracted from the dependence of the experimental
XPS peak intensities on the X-ray photon energy.[5] The
simulation shows that the intensities of the XPS signals (data
points in Figure 4) cannot be explained by the formation of a
mixed phosphate (La,Ce,Tb)PO4 (dashed line in Figure 4). If,
on the other hand, a CePO4 :Tb core particle with a diameter
of 5 nm is assumed (see TEM images in Figure 2) a good fit to
the measured data is obtained for a LaPO4 shell thickness of
1.13 0.05 nm. This value is in reasonable agreement with the
expected value of 1.5 nm, the difference presumably being
due to the slight elongation of the particles and local
variations of the shell thickness which could not be taken
into account.
Finally, we determined the particle-size distribution of the
core and the core?shell particles in a colloidal solution in
methanol (1 wt %) by dynamic light-scattering measurements
Angew. Chem. Int. Ed. 2003, 42, 5513 ?5516
Figure 5. Semilogarithmic plot of the number-averaged particle size
distribution curves of CePO4 :Tb nanoparticles (solid line) and
CePO4 :Tb/LaPO4 core?shell particles (broken line) in a colloidal
solution (1 wt %), determined by dynamic light scattering.
(Figure 5). From the full width at half maximum of the
distribution curves, particle sizes of 3.5?4.5 nm and of
about 7?10 nm are derived for the CePO4 :Tb core nanoparticles and CePO4 :Tb/LaPO4 core?shell particles, respectively. These values are similar to those derived from the
TEM images, which indicates that the 1-wt % colloidal
solutions consist mainly of well-separated particles.
In summary, we succeeded in preparing CePO4 :Tb/LaPO4
core?shell particles with a total quantum yield of 80 %. Until
recently, quantum yields this close to the value of the bulk
material were believed to be impossible for a nanocrystalline
phosphor material, because phosphors prepared by conventional solid-state methods display lower quantum yields
already when their grain size is smaller than about 0.5 mm.
The quantum yield of these nanoparticles can probably be
increased further by optimizing the thickness of the shell and
by employing metal salts of very high purity.
Experimental Section
The CePO4 :Tb core nanoparticles were prepared as described
previously by using hydrated metal chlorides with a purity of
99.9 %:[6] CeCl3�H2O (2.66 g, 7.5 mmol) and TbCl3�H2O (0.93 g,
2.5 mmol) were dissolved in methanol (10 mL). The clear solution
was mixed with tributylphosphate (10.98 mL, 40 mmol), and the
methanol was removed on a rotary evaporator. Diphenyl ether
(30 mL) was added, and the released water content was removed
under vacuum at 50?80 8C. Subsequently, trihexylamine (10.15 mL,
30 mmol) and a solution of dry phosphoric acid in dihexyl ether (2 m ;
7.0 mL) were added, and the mixture was heated to 200 8C under dry
nitrogen. After 2 h, one third of the solution was removed, cooled to
50 8C, diluted with methanol (200 mL), and purified by diafiltration
(5000-Dalton filter, Millipore; Diafiltration cell (Berghof); solvent:
methanol). The nanoparticles were isolated as a white powder (0.8 g)
by removing the methanol on a rotary evaporator.
After an additional 14 h of heating at 200 8C, one half of the
remaining reaction solution was removed and treated in the same
way. Colloidal solutions of the resulting white powder (yield: 0.8 g)
were used for the measurements on CePO4 :Tb core nanoparticles.
Synthesis of core?shell nanoparticles: Phosphoric acid in dihexyl
ether (2 m ; 12.7 mmol) was added to the transparent solution
remaining in the reaction vessel (i.e., one third of the initial content)
at 20 8C. The mixture was stirred vigorously and heated to 200 8C. A
solution of LaCl3�H2O (3.89 g, 10 mmol) in methanol was mixed
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
with tributylphosphate (10.98 mL, 40 mmol), the methanol was
removed by distillation, and the residue was taken up in diphenyl
ether (30 mL). After the water had been removed by vacuum
distillation as described above, the solution was mixed with trihexylamine (10.15 mL, 30 mmol) and added to the reaction mixture
through a dropping funnel within 2 h. After an additional 2 h of
heating at 200 8C, half of the solution was removed, purified by
diafiltration, and the nanoparticles were isolated as described above
(white powder, 1.0 g yield). The remaining solution was treated in the
same way after 14 h of further heating at 200 8C (white powder; 1.4 g
yield) and used as core?shell particles.
Dilute colloidal solutions were obtained by redispersing the
nanoparticles in methanol and were transparent. Transparent methanolic solutions of higher concentration were obtained by the
addition of a few drops of a solution of tetra-n-butylammonium
hydroxide in methanol (25 wt %).
Powder XRD measurements were performed on a Philips X'pert
system, TEM investigations on a Philips CM 300 UT transmission
electron microscope, and dynamic light-scattering measurements on a
Malvern HPPS system. The absorption and luminescence spectra
were recorded in cuvettes (1 cm path length; Hellma, QS-series) on a
Cary 500 Scan UV/vis spectrometer (Varian) and on a Fluorolog-03
spectrometer (Jobin-Yvon), respectively.
Photoluminescence quantum yields of colloidal solutions were
measured by comparing their integrated emission to the emission of a
solution of Rhodamin 6G (Lambda Physik, laser grade; solution in
spectroscopic grade absolute ethanol; quantum yield 95 %[7]), which
has the same optical density of 0.1 at the excitation wavelength
(277 nm). A dilution series of the dye showed that the quantum yield
at this concentration was 3 % lower than in more dilute solutions. A
comparison of the absorption and luminescence excitation spectrum
between 250 and 550 nm gave no indication for a notable dependence
of the quantum yield on the excitation wavelength.
J.-P. Boilot, Chem. Mater. 2000, 12, 1090; f) H. Meyssamy, K.
Riwotzki, A. Kornowski, S. Naused, M. Haase, J. Phys. Chem. B
2000, 104, 2824; g) K. Riwotzki, H. Meyssamy, H. Schnablegger,
A. Kornowski, M. Haase, Angew. Chem. 2001, 113, 574; Angew.
Chem. Int. Ed. 2001, 40, 573; h) Z. Wie, L. Sun, C. Liao, C. Yan,
Sh. Huang, Appl. Phys. Lett. 2002, 80, 1447; i) G. A. Hebbink,
J. W. Stouwdam, D. N. Reinhoudt, F. J. M. van Veggel, Adv.
Mater. 2002, 14, 1147.
B. M. J. Smets, Mater. Chem. Phys. 1987, 16, 283.
H. Borchert, S. Haubold, M. Haase, H. Weller, C. McGinley, M.
Riedler, T. MNller, Nano Lett. 2002, 2, 151.
O. Lehmann, H. Meyssamy, K. KNmpe, H. Schnablegger, M.
Haase, J. Phys. Chem. B 2003, 107, 7449.
R. F. Kubin, A. N. Fletcher, J. Lumin. 1982, 27, 455.
Received: May 21, 2003 [Z51943]
Keywords: colloids � fluorescence � lanthanides � luminescence �
[1] a) V. L. Colvin, M. C. Schlamp, A. P. Alivisatos, Nature 1994, 370,
354; N. Tessler, V. Medvedev, M. Kazes, S. H. Kan, U. Banin,
Science 2002, 295, 1506; B. O. Dabousi, M. G. Bawendi, O.
Onitsuka, M. F. Rubner, Appl. Phys. Lett. 1995, 66, 1316; b) V. I.
Klimov, A. A. Mikhailovsky, S. Xu, J. A. Hollingsworth, C. A.
Leatherdale, H. J. Eisler, M. G. Bawendi, Science 2000, 290, 314;
c) M. T. Harrison, S. V. Kershaw, M. G. Burt, A. L. Rogach, A.
Kornowski, A. EychmLller, H. Weller, Pure Appl. Chem. 2000, 72,
314; d) B. Dubertret, P. Skourides, D. J. Norris, V. Noireaux, A. H.
Brivanlou, A. Libchaber, Science 2002, 298, 1759; M. P. Bruchez,
M. Moronne, P. Gin, S. Weiss, A. P. Alivisatos, Science 1998, 281,
[2] a) X. Peng, M. C. Schlamp, A. V. Kadavanich, A. P. Alivisatos, J.
Am. Chem. Soc. 1997, 119, 7019; b) B. O. Dabousi, J. RodriguezViejo, F. V. Mikulec, J. R. Heine, H. Mattoussi, R. Ober, K. F.
Jensen, M. Bawendi, J. Phys. Chem. B 1997, 101, 9463; D. V.
Talapin, A. L. Rogach, A. Kornowski, M. Haase, H. Weller, Nano
Lett. 2001, 1, 207; c) Y. W. Cao, U. Banin, J. Am. Chem. Soc. 2000,
122, 9692; d) S. Haubold, M. Haase, A. Kornowski, H. Weller,
ChemPhysChem 2001, 5, 331.
[3] a) M. Yin, W. Zhang, S. Xia, J.-C. Krupa, J. Lumin. 1996, 68, 335;
b) B. Tissue, Chem. Mater. 1998, 10, 2837; K. Riwotzki, M. Haase,
J. Phys. Chem. B 1998, 102, 10 129; c) Y. L. Soo, S. W. Huang,
Z. H. Ming, Y. H. Kao, G. C. Smith, E. Goldburt, R. Hodel, B.
Kulkarni, J. V. D. Veliadis, R. N. Bhargava, J. Appl. Phys. 1998, 83,
5404; d) H. Meyssamy, K. Riwotzki, A. Kornowski, S. Naused, M.
Haase, Adv. Mater. 1999, 11, 840; e) A. Huignard, Th. Gacoin,
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2003, 42, 5513 ?5516
Без категории
Размер файла
121 Кб
greek, coreцshell, quantum, emitting, photoluminescence, cepo4, yield, tblapo4, nanoparticles
Пожаловаться на содержимое документа