Supporting Information
The Real Role of Active-shell in Enhancing the Luminescence of Lanthanides
Doped Nanomaterials
Fei Wu1,2, Xiaomin Liu1,a), Xianggui Kong1, Youlin Zhang1, Langping Tu1,2, Kai Liu1,2, Hong
Zhang3,b)
1State
Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and
Physics, Chinese Academy of Sciences, Changchun 130033, P. R. China.
2
Graduate University of the Chinese Academy of Sciences, Beijing 100049, P. R. China.
3Van’t
Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH
Amsterdam, The Netherlands.
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Reagents
YCl3•xH2O (99.9%), TbCl3•xH2O (99.9%), CeCl3•xH2O (99.9%), NaOH (98%), NH4F (98%),
1-octadecene (90%), oleic acid (90%) were purchased from Sigma-Aldrich. All chemicals were
used as received without further purification.
General procedure for the synthesis of core nanoparticles
In a typical procedure to the synthesis of NaYF4:Ce/Tb nanoparticles, LnCl3 (0.2 M, Ln = Y, Tb,
and Ce) was added to a 50-mL flask containing 3 mL of oleic acid and 7mL 1-octadecene . The
mixture was heated at 150 °C for 30 min to remove the water content from the LnCl3•xH2O.
After cooling down to 50 °C, 5 mL of methanol solution containing NH4F (1.36 mmol) and
NaOH (1 mmol) was added and the resultant solution was stirred for 30 min. After the methanol
was evaporated, the solution was heated to 305 °C under argon for 1.5 h and then cooled down to
room temperature. The resulting nanoparticles with a yield of 65 mg were precipitated by
addition of ethanol, collected by centrifugation at 6000 rpm for 5 min, washed with ethanol
several times, and re-dispersed in 6 mL of cyclohexane.
General procedure for the synthesis of core-shell nanoparticles
The NaYF4:Ce shell precursor was first prepared by mixing LnCl3 (0.2 M, Ln = Y and Ce), 3 mL
of oleic acid and 7ml 1-octadecene in a 50-mL flask followed by heating at 150 °C for 30 min.
After cooling down to 50 °C, NaYF4:Ce/Tb core nanoparticles (40 mg) dispersed in 2 mL of
cyclohexane were added along with a 5-mL methanol solution of NH4F (1.36 mmol) and NaOH
(1 mmol). The resulting mixture was stirred at 80 °C for 30 min to remove the methanol and
cyclohexane. Then the solution was heated to 305 °C under argon for 1 h and then cooled down
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to room temperature. The resulting nanoparticles were precipitated by addition of ethanol,
collected by centrifugation at 6000 rpm for 5 min, washed with ethanol several times, and
re-dispersed in 6 mL of cyclohexane.
Characterization
The structure and morphology of the nanoparticles were characterized by using a Brucker
D8-advance X-ray diffractometer (XRD) with Cu Ka radiation (λ= 1.5418 Å). Field emission
scanning electron microscopy (FESEM, Hitachi, S-4800) with an energy-dispersive X-ray
spectrometer (EDS). The transmission electron microscopy (TEM) was performed on a Tecnai
G2 F20 S-TWIN D573 electron microscope operated at 300 kV TEM. High-resolution STEM
was performed on an FEI Tecnai F20 transmission electron microscope operated at 200kV.
Ultraviolet-visible (UV-vis) absorption was measured at room temperature by a UV-3101
spectrophotometer. The fluorescent emission spectra were measured at room temperature by a
Hitachi F-4500 fluorescence spectrofluorimeter. The luminescence kinetics was recorded with a
500 MHz Tektronix digital oscilloscope and the excitation was realized by a nanosecond pulse
train at 260 nm from an optical parametric oscillator.
Structure and morphology
Bare core, core/inert-shell and core/active-shell NaYF4:Ce3+, Tb3+ NPs were synthesized and
their compositions, phase purities and morphologies were examined by TEM and XRD as shown
in figure S2 and figure S1. The diffraction peaks of the bare core NPs can be indexed as a pure
hexagonal phase of NaYF4. Good monodispersity of the NPs is witnessed by TEM where the
average diameter is estimated ~30 nm (figure S2a). The corresponding HRTEM image (insert in
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figure S2a) shows distinct lattice fringes with interplanar spacing of 0.31 nm ascribed to the (110)
plane of NaYF4, confirming that the bare core NPs have single crystalline feature. Taking these
30 nm NaYF4: 40% Ce3+, 15% Tb3+ NPs as the core, we have further constructed the core/shell
structured NPs following the similar procedure. Figure S2b and 2c are respectively the TEM
images of the NaYF4 : 40% Ce3+, 15% Tb3+ / NaYF4 core/inert-shell NPs (2b) and NaYF4 : 40%
Ce3+, 15% Tb3+ / NaYF4 : 15% Ce3+ core/active-shell NPs (2c), revealing that the crystal phase
remained pure hexagonal after shell coating. The size of core/shell structured NPs increased to
40 nm, corresponding to about 5 nm in shell thickness. In addition, the morphology turned from
sphere to hexagonal plate with monodispersity well maintained. The single-crystalline structure
without noticeable defects is guaranteed by the HRTEM image of core/active-shell NPs (insert in
figure S2c) with distinct lattice fringes of interplanar spacing of 0.51nm ascribed to the (100)
plane of NaYF4.
Despite the high resolution of HRTEM image, it is not easy from it to distinguish the core and
the active shell directly because they are very similar in crystalline structure and composition.
Therefore, the line scan of the elemental composition was performed with energy-dispersive
X-ray spectroscopy analysis conducted with STEM imaging on several randomly selected
nanoparticles. A higher Ce3+ concentration in the middle region of the nanocrystal was found that
is consistent with the designed compositions for the core/active-shell structure (figure S2d and
figure S3), confirming the spatial separation of the Ce3+ ions in the core and in the shell.
Determination of ηq
To minimize possible re-absorption,1 all sample solutions were sufficiently diluted. The
luminescence quantum efficiency was calculated from the equation ηsample = ηref
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(IsampleAref/IrefAsample),2 in which A is the absorption intensity and I is the emission intensity. The
quantum efficiency of quinine bisulfate is 54.6%.3
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References
1
S. Dhami, A. J. Demello, G. Rumbles, S. M. Bishop, D. Phillips and A. Beeby, Photochem.
Photobiol., 1995, 61, 341.
2
F. Wang, Y. Zhang, X. P. Fan and M. Q. Wang, J. Mater. Chem., 2006, 16, 1031.
3
W. H. Melhuish, J. Phys. Chem., 1961, 65, 229.
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Figure captions
Figure S1. XRD pattern of (a) core, (b) core/inert-shell and core/active-shell NaYF4:Ce3+,Tb3+
NPs.
Figure S2. TEM images of the (a) bare core NaYF4:Ce3+,Tb3+ NPs with HRTEM image (insert),
(b)
core/inert-shell
NaYF4:Ce3+,Tb3+/NaYF4
NPs,
(c)
core/active-shell
NaYF4:Ce3+,Tb3+/NaYF4:Ce3+ NPs with HRTEM image (insert) and (d) the line profile scan
conducted with STEM imaging (insert) on a core/active-shell NaYF4:40%Ce3+,15%Tb3+/
NaYF4:15%Ce3+ nanoparticle, indicating a higher Ce3+ concentration in the middle region of the
crystal that is consistent with the designed core/active-shell structure.
Figure S3. The energy-dispersive X-ray spectroscopy of the core/active-shell NPs under the
STEM pattern.
Figure S4. The excitation spectra of bare core NaYF4:Ce3+,Tb3+ NPs (red), core/inert-shell
NaYF4:Ce3+,Tb3+/NaYF4 NPs (blue) and core/active-shell NaYF4:Ce3+,Tb3+/NaYF4:Ce3+ NPs
(black) dispersed as a 1wt % colloid in cyclohexane.
Figure S5. Effect of Tb3+ and Ce3+ concentration (x) on the emission intensity of core-only
NaYF4 : Ce3+,Tb3+NPs. It can be seen from this figure that, (a) with increasing the emitter Tb3+
concentration, the intensity increases gradually and reaches the maximum at 15%, and then
decreases. Accordingly, (b) increasing the sensitizer Ce3+ concentration at constant concentration
of Tb3+ (15 %) will increase total emission intensity up to 40%, after which the intensity will
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decrease.
Figure S6. Decay curves of Tb3+ luminescence (542 nm) in the bare core NPs (black), inert-shell
coated NPs (blue) and active-shell coated NPs (red).
Figure S7. SEM images of the (a) bare core NaYF4:Yb3+,Er3+ NPs, (b) core/inert-shell NaYF4:
20% Yb3+, 2%Er3+/NaYF4 NPs, (c) core/active-shell NaYF4: 20% Yb3+, 2%Er3+/NaYF4: 5%Yb3+
NPs, (d) core/active-shell NaYF4: 20% Yb3+, 2%Er3+/NaYF4: 10%Yb3+ NPs (e) core/active-shell
NaYF4: 20% Yb3+, 2%Er3+/NaYF4: 20%Yb3+ NPs (f) core/active-shell NaYF4: 20% Yb3+,
2%Er3+/NaYF4: 40%Yb3+ NPs.
Figure S8. Upconversion photoluminescence spectra of the NaYF4:Yb3+,Er3+ / NaYF4: x%Yb3+
colloidal solutions (0.1M).
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Figure S1
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Figure S2.
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Figure S3
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Figure S4
The excitation spectra monitored with the 542 nm emission (5D4-7F5) of Tb3+ consists of a
strong band with a maximum at 260 nm, which correspond to the transitions from the ground
state 2F5/2 of Ce3+ to the different components of the excited Ce3+ 5d states split by the crystal
field. The excitation intensity of the active-shell NPs is the strongest, which is consistent with the
results obtained from the absorption spectra and verifies that the core/active-shell NPs have the
highest absorption efficiency ηa. Excitation into the Ce3+ band at 260 nm yields strong emission
of Tb3+ (450-650 nm), indicating that an energy transfer from Ce3+ to Tb3+ occurs in the
nanoparticles. The emission of Tb3+ is due to transitions between the excited 5D4 state and the 7FJ
(J = 6-3) ground states of Tb3+ ions. Further, the Ce3+ doped in the active shell afford additional
energy to Tb3+, which is also responsible for populating the emitting (5D4) state as shown insert
in figure 2b, resulting in the significant improvement of the emission and absorption intensities.
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Figure S5
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Figure S6
We have tracked the temporal behavior of the green emission of Tb3+, i.e. transition 5D4
→7F5 (~542nm), under the excitation of 260 nm. The decay curves can be reasonably well fitted
with an exponential function. The lifetime of Tb3+ changes from 2.6 ms in bare core NPs, 5.4 ms
in core/active-shell structure to 5.9 ms in core/inert-shell structure, which tells us that both active
and inert shell can effectively minimize luminescence quenching from the surface states of the
particles.
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Figure S7
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Figure S8
We have employed the NaYF4:Ce3+,Tb3+ system as the target for the sake of simplicity to
validate the analysis and conclusions. Because the luminescence quantum efficiency ηq can be
accurately calculated in this down-conversion system, which offers a convenient way to
comment on the active-shell effects on ηeff , ηa and ηq. In the upconversion scenario, the energy
transfer kinetic process from the sensitizer to emitter is much more complicated, however, still
can be described as the schemes shown in figure 1. The relationship among the ηeff , ηa and ηq is
similar. Therefore, we can extend the conclusions to the upconversion scenario. Core/active-shell
NaYF4:Yb3+,Er3+ / NaYF4: x% Yb3+ upconversion nanoparticles were prepared (see figure S7)
and the dependence of the upconversion luminescence intensity on the doping concentration (x)
of Yb3+ ions in the active shell was investigated. Figure S8 exhibits the upconversion emission
spectra of the NaYF4:Yb3+,Er3+ / NaYF4: x%Yb3+ colloidal solutions with the same concentration
of Er3+ ion under 980 nm laser excitation. It can be found that the upconversion luminescence
intensity reaches a maximum value at x = 5% and then decreases with increasing Yb3+ content
(x), even lower than that of core/inert-shell NPs, illustrating that in the upconversion scenario,
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for the best luminescence performance, doping concentration of sensitizer Yb3+ in the active
shell is 5% instead of 20%, which was reported to be the optimal doping concentration for Yb3+
in the active shell. For this reason, it can be conclude that in the upconversion scenario, the
optimal doping concentration of sensitizer in the active shell should also be less than that in the
core.
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