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Blue Green and Red Upconversion Emission from Lanthanide-Doped LuPO4 and YbPO4 Nanocrystals in a Transparent Colloidal Solution.

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Angewandte
Chemie
Upconversion of Photons
Blue, Green, and Red Upconversion Emission
from Lanthanide-Doped LuPO4 and YbPO4
Nanocrystals in a Transparent Colloidal
Solution**
Stephan Heer, Olaf Lehmann, Markus Haase, and
Hans-Ulrich Gdel*
We report the first observation of photon-upconversion in
transparent colloids, that is, the emission of visible (Vis) light
from a solution excited in the near-infrared (NIR). The
solutions displaying this multiphoton process consist of
transparent colloids of lanthanide-doped nanoparticles. Lanthanide-doped crystalline and amorphous materials play an
outstanding role in lighting, display, and solid-state laser
materials.[1–2] Many of these materials show high luminescence quantum yields and, since usually more than one
metastable excited state exists, multiple emissions are
observed.
In recent years, several research groups have reported on
the doping of nanocrystalline hosts with ions of the f elements.[3–15] Among these, ligand-capped nanoparticles are of
particular interest, as they combine the advantage of a rigid
crystal environment for the dopant ions with the advantage of
[*] Prof. Dr. H.-U. Gdel, S. Heer
Department of Chemistry and Biochemistry
University of Bern
3000 Bern 9 (Switzerland)
Fax: (+ 41) 31-631-4399
E-mail: hans-ulrich.guedel@iac.unibe.ch
O. Lehmann, Dr. M. Haase
Institute of Physical Chemistry
University of Hamburg
Bundesstrasse 45, 20146 Hamburg (Germany)
[**] We thank A. Kornowski and S. Bartholdi-Nawrath for the TEM
investigations and Dr. K. K?mpe for measuring X-ray powder
diffractograms. Part of this work was financially supported by the
Swiss National Science Foundation.
Angew. Chem. Int. Ed. 2003, 42, 3179 – 3182
DOI: 10.1002/anie.200351091
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3179
Communications
high solubility in liquids. We have previously shown that
ligand-capped nanocrystals of the lanthanide phosphates can
be prepared with narrow particle size distribution.[3–5, 15] As
colloidal solutions of these materials often display high
luminescence quantum yields of the dopant ions,[3, 4, 15] these
nanocrystalline systems may be considered as active components in future electroluminescent devices, for the labeling of
biomolecules, or for the amplification of signals transmitted
through fiber cables.[4, 15, 16]
Further possible applications can be expected to arise
from the phenomenon of photon upconversion, that is, the
emission of light at shorter wavelength than the excitation.
Upconversion emission has been observed and studied in
many doped bulk materials, mostly in oxides and halides. The
most efficient upconversion phosphors currently known are
based on the most frequently used upconversion ions Er3+
(for NIR to green) and Tm3+ (NIR to blue), often in
combination with Yb3+ as a sensitizer.[17] Applications range
from near-IR (NIR) detection[18] to imaging and lasers.[19] A
three-dimensional red-, green-, and blue-emitting display
based on upconversion processes in a glass has been
demonstrated.[20]
There have been a number of recent reports in the
literature about upconversion luminescence in nanomaterials.[21–24] All of these studies, however, have been performed
on dried powdered materials. The crucial test is to disperse
the particles in a fully transparent colloidal solution and then
to induce upconversion in the fluid state. This is a test of the
particle quality, but it is also a prerequisite for any imaging or
display applications based on the upconversion principle in a
fluid. Besides good crystallinity, a reasonable particle size
distribution is required, and the particles have to be properly
surface modified to ensure colloidal solubility. Herein we use
the well-established upconversion couples Yb3+/Tm3+ and
Yb3+/Er3+ to demonstrate the feasibility of inducing and
application of upconversion in solution.[17]
Ligand-capped lanthanide phosphate nanoparticles prepared by the method given below are readily soluble in
organic solvents and form colloidal solutions of well-separated particles.[3–5, 15] Figure 1 a displays micrographs of
LuPO4 :49 %Yb3+,1 %Tm3+ nanoparticles recorded by transmission electron microscopy (TEM). The sample shows the
narrow particle size distribution usually observed for this
preparation method as well as a mean particle diameter of
about 6–8 nm. Similar to YbPO4 :5 %Er3+ nanoparticles,[5] the
powder X-ray diffraction of LuPO4 :49 %Yb3+,1 %Tm3+ nanoparticles (Figure 1 b) display strongly broadened peaks at
positions in accord with the tetragonal xenotime phase known
from bulk YbPO4 and bulk LuPO4. A mean particle size of
about 7 nm is calculated from the peak width at half
maximum height by using the Debye–Scherrer formula
(shape parameter K = 1.0). This value is in accord with the
size determined from the TEM images.
Figure 2 a shows a survey absorption spectrum of a
transparent
colloidal
solution
with
6.7 wt %
of
YbPO4 :5 %Er3+ nanocrystals in chloroform. It is dominated
by the strong 2F7/2 !2F5/2 absorption band of Yb3+ centered at
about 10 500 cm1 in the NIR region. The weak absorption
lines in the visible region are readily assigned to f-f transitions
3180
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 1. a) TEM micrograph of LuPO4 :49 %Yb3+,1 %Tm3+ nanocrystals. Inset: High-resolution image of a nanocrystal. b) Powder XRD
pattern of LuPO4 :49 %Yb3+,1 %Tm3+ nanocrystals. The line spectrum
corresponds to the literature data of bulk LuPO4 (PDF No. 84–337
reference pattern, body-centered tetragonal, space group I41/amd).
Figure 2. a) Absorption spectrum and assignments of the
YbPO4 :5 %Er3+ colloid. b) Upconversion luminescence spectrum and
assignments after excitation at 10 230 cm1 (see arrow) with a laser
power of 300 mW.
of the Er3+ ion as shown in the Figure. Figure 2 b shows the
upconversion luminescence spectrum of the same solution
obtained by exciting into the prominent absorption peak of
the Yb3+ ion at 10 230 cm1 with 300-mW laser power (arrow
in Figure 2 a). This luminescence is visible to the eye and
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Angew. Chem. Int. Ed. 2003, 42, 3179 – 3182
Angewandte
Chemie
materials containing Er3+ and Tm3+ ions, respectively, and
we can expect analogous upconversion mechanisms. In our
YbPO4 :5 %Er3+ sample the host lattice itself acts as a
sensitizer, and the Yb3+ excitations can be considered as
excitonic. In a first nonradiative energy transfer step an
2
F5/2 excitation of the Yb3+ ion (about 10 230 cm1) is transferred to a Er3+ ion to create a 4I11/2 excited state. A second
NIR photon takes the Er3+ ion to a higher energetic state,
whose energy lies in the visible region. Emission from 2H11/2
and 4S3/2 leads to the dominant green luminescence between
18 000 cm1 and 19 300 cm1. The red emission centered at
15 200 cm1 arises from the 4F9/2 !4I15/2 transition of the
Er3+ ion. In macrocrystalline Yb3+/Er3+ upconversion materials the ratio of green to red emission intensity can be tuned by
varying the concentration of the Yb3+ and Er3+ ions. We
would expect similar behavior in nanomaterials.
The LuPO4 :49 %Yb3,1 %Tm3+ system shows a richer
upconversion scheme (Figure 4). Up to four 2F5/2 excitations
of the Yb3+ ions (at about 10 230 cm1) can be injected
Figure 3. a) As in Figure 2 b), but for LuPO4 :49 %Yb3+,1 %Tm3+.
b) Power dependence of the LuPO4 :49 %Yb3+,1 %Tm3+ upconversion
intensity of the bands at 22 000 cm1 (*, 1D2 !3F4), 21 000 cm1
(&,1G4 !3H6), 14 000 cm1 (~, 3F3 !3F4), and 10 000 cm1 (^, Yb3+
2
F5/2 !2F7/2 of Yb3+).
appears green. Figure 3 a shows the upconversion luminescence spectrum of a transparent colloidal solution with
6.7 wt % of LuPO4 :49 %Yb3+,1 %Tm3+ nanocrystals in
chloroform after 300-mW excitation of the 2F7/2 !2F5/2 transition of the Yb3+ ion at 10 230 cm1 (arrow in Figure 3 a). The
upconversion luminecence appears blue and red. The upconversion excitation spectra of both samples were found to
correspond to the 2F7/2 !2F5/2 absorption band of the Yb3+ ion
(Figure 2 a).
The remarkable result of this study is the observation—by
eye and by spectroscopy—of upconversion luminescence in
the fluid state, which to our knowledge is without precedent.
Our own attempts to induce upconversion in molecular
encapsulation complexes of Er3+ and Tm3+ in solution have
failed.[25] We ascribe the observation of upconversion in our
solutions to our nanoparticles having a high crystal quality.
Lanthanide ions at the crystal surface are not likely to show
upconversion, because of their proximity to the high energy
NH and CH vibrational oscillators of the dodecylamine
protecting layer and the solvent. These vibrational oscillators
are very efficient quenchers of the metastable states in
molecular complexes of lanthanides. We, therefore, attribute
the observed upconversion to Er3+ and Tm3+ ions replacing
Yb3+ and Lu3+ ions, respectively, inside the nanocrystals.
The observed upconversion spectra in Figures 2 a and 3 a
are similar to the corresponding spectra of solid oxide
Angew. Chem. Int. Ed. 2003, 42, 3179 – 3182
Figure 4. Energy level and upconversion scheme for the
LuPO4 :49 %Yb3+,1 %Tm3+ system. Full, dotted, and curly arrows indicate radiative, nonradiative energy transfer, and multiphonon relaxation
processes, respectively.
nonradiatively into the Tm3+ ion to excite it up to the
D2 level in the near-UV region. This result is in good
agreement with our experimental results. We observe upconversion emission from four different levels, with the dominant
blue band around 21 000 cm1 assigned to the 1G4 !3H6
transition of the Tm3+ ion. The assignments in Figure 3 a,
which are spectroscopically straightforward from energy
considerations, are nicely confirmed by the experimental
power dependencies of the respective upconversion emission
1
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2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3181
Communications
intensities. Figure 3 b shows different power dependences for
the various emissions. The slopes of 2.9, 2.2, 1.3, and 0.7
derived from the experimental data in this double logarithmic
representation are in good agreement with the 4-, 3-, 2-, and
1-photon excitation processes, respectively, described in
Ref. [26] and shown schematically in Figure 4.
We have demonstrated the feasibility of exciting blue,
green, and red light emission in transparent colloidal solutions
by upconversion excitation in the near-IR region. We consider
this work as a first and important step towards the development of materials with considerable application potential for
luminescent markers, imaging, and displays in fluid media.
Extremely powerful and cheap semiconductor light sources
are available for the spectral region around 980 nm, and thus
upconversion excitation of visible luminescence would be an
elegant alternative for such applications. The overall efficiency of the upconversion processes in our samples is still
very poor compared with the best upconversion phosphors in
the solid state. There is plenty of scope for improvement,
however, and we are engaged in a research program in this
direction. One apparent direction is the reduction of the
phonon energies of the host lattice to reduce multiphonon
relaxation processes and thus increase the lifetimes of the
metastable states involved in upconversion. Another direction is the protection of the nanocrystals from high-energy
vibrational oscillators by growing a shell of an inert material
on the surface of the particles.
Experimental Section
Er3+-doped LuPO4 and YbPO4 nanoparticles were prepared as
described recently.[5] Ligand-capped LuPO4 :Yb,Tm nanoparticles
were synthesized analogously by treating hydrated lutetium chloride
(5 mmol), hydrated ytterbium chloride (4.9 mmol), and hydrated
thulium chloride (0.1 mmol) with phosphoric acid in a high-boiling
mixture of the coordinating solvents tributylphosphate, trihexylamine, and diphenyl ether. The nanoparticles were surface-modified
by dissolving LuPO4 :49 %Yb3+, 1 %Tm3+ or YbPO4 :5 %Er3+
(500 mg) in dodecylamine (5 g) at 100 8C under nitrogen to increase
their solubility in chloroform.[5] After one hour at 100 8C, the solution
was allowed to cool to about 50 8C and subsequently mixed with
toluene (5–10 mL). The nanoparticles were precipitated from this
solution by adding methanol (20–30 mL), the precipitate was
separated by centrifugation, washed three times with methanol, and
finally dried at room temperature.
Samples used in this study were prepared by dissolving the
nanoparticle powder (300 mg) in CHCl3 (3.00 mL, Fluka, 99.5 %).
This led to an almost transparent dispersion with a concentration of
6.7 wt %. This was passed through a 0.2 mm filter to give an optically
transparent dispersion. Absorption spectra were measured on a
Cary 5E (Varian) spectrometer in cuvettes of 1-cm path length
(Hellma, QX). Luminescence spectra were measured with the same
samples in the same cuvettes as the absorption spectra. They were
excited by an Nd:YVO4 laser (Spectra Physics Millennia XS-FRU,
second harmonic) pumped tunable Ti:sapphire laser (Spectra Physics
3900S) in the NIR region. The luminescence was dispersed by a 0.85m double monochromator (Spex 1402) using gratings with 1200 lines
per mm (blaze angle optimal for gratings 500 nm) after detection by a
cooled photomultiplyer (Hamamatsu 3310–01) using a photon count-
3182
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ing system (Stanford Research SR 400). All the spectra were
corrected for the sensitivity of the monochromator and the detection
system as well as for the refractive index of air (vacuum correction).
They are represented as photon flux versus wavenumbers. In all the
experiments the excitation laser was focused with a lens of f = 53 mm.
X-ray diffraction patterns of powder samples were recorded with a
Philips X'pert system. High-resolution electron micrographs of the
particles were taken using a Philips CM 300 UT transmission electron
microscope equipped with a CCD-camera (Gatan).
Received: February 3, 2003 [Z51091]
.
Keywords: colloids · lanthanides · luminescence · nanocrystals ·
upconversion
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