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S1
Temporal Full-color Tuning through Non-Steady-State
Upconversion
Renren Deng, Fei Qin, Runfeng Chen, Wei Huang, Minghui Hong, Xiaogang Liu
Temporal full-colour tuning through non-steady-state upconversion
SUPPLEMENTARY INFORMATIONDOI: 10.1038/NNANO.2014.317
NATURE NANOTECHNOLOGY | www.nature.com/naturenanotechnology
© 2015 Macmillan Publishers Limited. All rights reserved
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Materials and Methods
Materials. Y(CH3CO2)3•xH2O (99.9%), Yb(CH3CO2)3•4H2O (99.9%), Er(CH3CO2)3•xH2O (99.9%),
Ce(CH3CO2)3•xH2O (99.9%), NaOH (98+%), NH4F (99+%), 1-octadecene (90%), oleic acid (90%), were
purchased from Sigma-Aldrich. Sylgard® 184 silicone elastomer kit used for preparation of
polydimethylsiloxane (PDMS) monoliths was purchased from Dow Corning. Unless otherwise noted, all
the chemicals were used without further purification.
Physical Measurements. Transmission electron microscopy (TEM) measurements and electron-
dispersive X-ray (EDX) spectrum were carried out on a JEM-2100F transmission electron microscope
(JEOL) operating at an acceleration voltage of 200 kV. Powder X-ray diffraction (XRD) data were
recorded on an ADDS wide-angle X-ray powder diffractometer with Cu K radiation (40 kV, 40 mA, λ =
1.54184 Å). Upconversion luminescence characterizations, including steady state/non-steady state
spectroscopic characterization, luminescence time decay measurements, and low-temperature
upconversion emission spectra, were obtained with a customized steady state and phosphorescence
lifetime spectrometer (FSP920, Edinburgh) equipped with either a 980 nm or 808 nm continues-wave
(CW) diode lasers as the excitation source. The modulated pulse laser excitation was obtained by
coupling a 980 nm-diode laser (FC-980 with TTL pulse mode, CNI) with either a mechanical optical
chopper (MC2000, Thorlabs) or a digital pulse generator (TGP110, TTi). The absolute upconversion
quantum yields were measured by the same spectrometer (FSP920, Edinburgh) coupled with an
integrating sphere (150 mm internal diameter and internally coated with barium sulfate) and a 980-nm
diode laser or an 808 nm diode laser. For low-temperature upconversion emission measurements, a
customized liquid helium cryostat (Optistat CF, Oxford Instruments) was equipped into the FSP920
spectrometer. For luminescence decay measurements, the effective lifetimes were determined by
0
0
)(1
dttII
eff
where I0 and I(t) represents the maximum luminescence intensity and luminescence intensity at time t
after cutoff of the excitation light, respectively. Digital photographs were taken with a Nikon D700 color
camera and a 750 nm short-pass filter was placed before the camera to cut the near-infrared excitation
light.
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Synthesis of core-shell nanocrystals with color-tunable upconversion. The upconversion nanocrystals
were synthesized using a modified wet-chemical procedure34-37. A layer-by-layer epitaxial growth method
was adopted for preparing the multilayered core-shell nanocrystals. NaYF4:Nd/Yb (20/20 mol%) core
nanocrystals were first synthesized and then used as seeds for epitaxial growth of four shell layers
containing different combinations of lanthanide dopants (Scheme S1).
Supplementary Scheme 1: Schematic procedure for the synthesis of core-shell nanocrystals with color-
tunable upconversion.
Synthesis of NaYF4:Nd/Yb (20/20 mol%) core nanocrystals. In a typical experiment, an aqueous
solution (2 mL) containing Y(CH3CO2)3 (0.24 mmol), Nd(CH3CO2)3 (0.08 mmol), and Yb(CH3CO2)3 (0.08
mmol) was added into a mixture of oleic acid (3 mL) and 1-octadecene (7 mL) at room temperature. The
mixture was heated at 150 oC for 1h to form lanthanide-oleate complexes. Thereafter, the reaction
solution was cooled down to room temperature followed by addition of a methanol solution (6 mL)
containing NaOH (40 mg; 1 mmol) and NH4F (59.2 mg; 1.6 mmol). Subsequently, the mixture was
heated at 50 oC for 30 min under vigorous stirring. The temperature was then increased to 100 oC to
evaporate the methanol. After degassed for 10 min, the reaction mixture was heated to 290 oC at a
heating rate of 10 oC/min under an argon atmosphere. Upon completion of the reaction after 1.5 h, the
solution was cooled down to room temperature. The resulting nanocrystals were collected by
centrifugation, washed with a mixture of cyclohexane and absolute ethanol for several times, and re-
dispersed in cyclohexane (4 mL) prior to characterization and use for multi-layered shell growth.
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Synthesis of NaYF4:Nd/Yb@NaYF4:Yb/Tm (20/0.2 mol%) core-shell nanocrystals. To a 50 mL-flask
containing oleic acid (3 mL) and 1-octadecene (7 mL) was added an aqueous solution (2 mL) of
Y(CH3CO2)3 (0.319 mmol), Yb(CH3CO2)3 (0.08 mmol), and Tm(CH3CO2)3 (0.0008 mmol). The resulting
mixture was heated to 150 oC for 1 h to form yellowish lanthanide-oleate complexes, followed by cooling
of the solution to room temperature. Subsequently, a methanol solution (6 mL) of NaOH (40 mg; 1 mmol)
and NH4F (59.2 mg; 1.6 mmol) was added along with the as-prepared NaYF4:Nd/Yb core nanocrystals (4
mL in cyclohexane). The resulted mixture was stirred at 50 oC for 30 min followed by heating at 100 oC
and degassing for another 10 min to evaporate the methanol in the solution. Thereafter, the reaction
temperature was raised to 290 oC at a heating rate of 10 oC/min and kept for 2 h under an argon
atmosphere. The resulting core-shell nanoparicles were precipitated out by the addition of ethanol,
collected by centrifugation (6000 rpm for 3 min), washed with a mixture of cyclohexane/ethanol solvent
for several times, and re-dispersed in cyclohexane (4 mL).
Synthesis of NaYF4:Nd/Yb@NaYF4:Yb/Tm@NaYF4 tri-layered nanocrystals. The procedure for
preparing tri-layered core-shell nanocrystals is identical to that for bilayered core-shell nanocrystals. The
as-synthesized NaYF4:Nd/Yb@NaYF4:Yb/Tm core-shell nanocrystals (4 mL in cyclohexane) were used
as seeds to induce a subsequently epitaxial growth of the additional shell layer. Y(CH3CO2)3 (0.4 mmol)
was used as the shell precursor. The resulting tri-layered nanocrystals were dispersed in cyclohexane (4
mL) prior to characterization and use for further shell growth.
Synthesis of NaYF4:Nd/Yb@NaYF4:Yb/Tm@NaYF4@NaYF4:Yb/Ho/Ce (20/2/8 mol%) tetra-layered
core-shell nanocrystals. The procedure for synthesizing tetra-layered core-shell nanocrystals is identical
to the synthesis of tri-layered core-shell nanocrystals. Typically, an aqueous solution containing
Y(CH3CO2)3 (0.28 mmol), Yb(CH3CO2)3 (0.08 mmol), Ho(CH3CO2)3 (0.008 mmol), and Ce(CH3CO2)3 (0.032
mmol) were used as the lanthanide precursor for the additional shell growth of the tri-layered core-shell
nanocrystals. It should be noted that to minimize the blue emission of Tm3+ under 980 nm excitation
only a half-batch of the as-prepared NaYF4:Nd/Yb@NaYF4:Yb/Tm@NaYF4 nanocrystals (2 mL in
cyclohexane) was added as the seeds to induce the shell growth. The resulting core-shell nanocrystals
were stored in cyclohexane (4 mL) prior to characterization and further shell coating.
Synthesis of NaYF4:Nd/Yb@NaYF4:Yb/Tm@NaYF4@NaYF4:Yb/Ho/Ce@NaYF4 core-shell nanocrystals.
The procedure for coating an inert NaYF4 layer onto the tetra-layered core-shell nanocrystals follows the
same protocol used for conventional core-shell nanocrystals. The pre-synthesized
NaYF4:Nd/Yb@NaYF4:Yb/Tm@NaYF4@NaYF4:Yb/Ho/Ce core-shell nanocrystals (4 mL in
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cyclohexane) were used as seeds and Y(CH3CO2)3 (0.4 mmol) was used as the shell precursor. The
resulting penta-layered core-shell upconversion nanocrystals were dispersed in cyclohexane (4 mL) prior
to characterization and use for volumetric 3D display.
Preparation of upconversion nanocrystal/PDMS composite monoliths. In a typical experiment,
Sylgard® 184 silicone elastomer base (15 mL), the curing agent (1.5 mL), and a cyclohexane solution of
upconversion nanocrystals (1.5 mL; 2 wt%) were mixed in a glass petri dish (50 mm x 15 mm) which was
used as a mould for the PDMS growth. The resulting jelly-like composites were thoroughly mixed
followed by degassing in a vacuum desiccator for 2 h to remove the air bubbles in the mixture.
Subsequently, the mixture was heated at 80 oC for 1 h. After cooling down to room temperature, the
nanocrystal/PDMS composite material can be released from the petri dish mould and used directly for
the volumetric 3D display.
Supplementary Scheme 2: Proposed upconversion mechanism in Yb-Ho-Ce tri-doped systems (Inset: a
schematic drawing of the penta-layered core-shell nanocrystals under investigation).
Upconversion mechanism investigations of the Ho3+ emission. The upconversion mechanism of the
Ho3+ emission in NaYF4:Nd/Yb@NaYF4:Yb/Tm@NaYF4@NaYF4:Yb/Ho/Ce@NaYF4 nanocrystals
was proposed in Scheme S2. The schematic upconversion process in Yb-Ho-Ce triply doped system can
be described by the rate equations of energy transfer38. According to the proposed energy transfer
upconversion process as shown in Scheme S2, the rate equations of each energy states are derived as
follows:
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0113344
0
8
53 :)I(Ho nnunwnwdt
dnYb
Eq. S1
11311201
1
7
53 :)I(Ho nnunwnnsdt
dnYbCe
Eq. S2
20121222011
2
6
53 :)I(Ho nnsnnunwnnudt
dnCeYbYb
Eq. S3
33402113
3
5
53 :)F(Ho nwnnsnnudt
dnCeYb
Eq. S4
40244212
4
2
5
4
53 :)S,F(Ho nnsnwnnudt
dnCeYb
Eq. S5
113212011110
1
5/2
23 :)F(Y nnunnunnunwIndt
dnb YbYbYbYbYbYbYb
Yb Eq. S6
dt
dn
dt
dnb YbYb 10
7/2
23 :)F(Y
Eq. S7
43210 nnnnnnHo Eq. S8
10 YbYbYb nnn Eq. S9
Where ni and wi (i = 0 to 4) represent the population densities and the intrinsic decay rates of the 5I8, 5I7,
5I6, 5F5 and 5F4/5S2 states of Ho3+ ions, respectively. ui (i = 1, 2, 3) is the upconversion energy transfer
rate from the 2F5/2 state of Yb3+ to the 5I8, 5I6 and 5I7 states of Ho3+, respectively. s1 and s2 represent the
Ho3+ to Ce3+ cross-relaxation processes [5I6 (Ho3+) + 2F5/2 (Ce3+) → 5I7 (Ho3+) + 2F7/2 (Ce3+)] and [5F4,5S2
(Ho3+) + 2F5/2 (Ce3+) → 5F5 (Ho3+) + 2F7/2 (Ce3+)], respectively. nYb0, nYb1, nCe0 and nCe1 are the population
densities of the ground states (0) and the excitation states (1) of Yb3+ and Ce3+. YbI refers to the
pumping rate of Yb3+ under the 980 nm excitation.
Long-pulse excitation (steady-state upconversion). When excited by a sufficiently long laser pulse,
the nanoparticles are likely to give rise to steady-state upconversion emission in which the decay of the
excited state and the energy transfer upconversion process occur at the same rate39. As the emission
intensity remains unaltered at the steady state, we should obtain a constant emission density of each
energy state at different time intervals as defined by
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0 143210 dt
dn
dt
dn
dt
dn
dt
dn
dt
dn
dt
dn Yb
Generally, the intrinsic decay rates (wi) at the intermediate states are much larger than the upconversion
energy transfer rates (ui). On the basis of this assumption combined with the rate equations (eq. S1-S9),
one can estimate the density of each energy state of the Ho3+ ion according to the following equations:
1
0121
0011
1)(
Yb
Ce
Ce nnsww
nnsun
Eq. S10
1
012
01
2 Yb
Ce
nnsw
nun
Eq. S11
3012
2
1
024
00221
1
00131
3)()( wnsw
n
nsw
nnsuu
w
nnsuun
Ce
Yb
Ce
CeCe
Eq. S12
2
1
012024
021
4))((
Yb
CeCe
nnswnsw
nuun
Eq. S13
Short-pulse excitation (non-steady-state upconversion). In contrast to steady-state upconversion
process, non-steady-state upconversion, which is characterized by different rates between the decay of
the excited state and the energy transfer upconversion process, can be achieved by a very short laser
pulse. At the non-steady state, the temporal dynamics of each excited state are a nonlinearly coupled
differential system with
0 dt
dni
As such, it is very difficult to obtain an explicit analytical solution according to the rate equations.
Alternatively, the temporal dependence of the population densities of each energy state can be calculated
with a Monte-Carlo simulation method using FORTRAN90 software40.
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S8
Supplementary Fig. 1: TEM images of NaYF4-based nanocrystals (left) and the corresponding size
distributions of the nanocrystals (right). (A) NaYF4:Nd/Yb core nanocrystals. (B)
NaYF4:Nd/Yb@NaYF4:Yb/Tm core-shell nanocrystals. (C) NaYF4:Nd/Yb@NaYF4:Yb/Tm@NaYF4 tri-
layered core-shell nanocrystals. (D) NaYF4:Nd/Yb@NaYF4:Yb/Tm@NaYF4@NaYF4:Yb/Ho/Ce tetra-
layered core-shell nanocrystals. (E) NaYF4:Nd/Yb@NaYF4:Yb/Tm@NaYF4@NaYF4:Yb/Ho/Ce@NaYF4
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penta-layered nanocrystals. The size distributions of the nanocrystals were calculated by counting over 300
particles recorded in the TEM images.
Supplementary Fig. 2: (A) XRD characterization of the as-prepared full-color tunable
NaYF4:Nd/Yb@NaYF4:Yb/Tm@NaYF4@NaYF4:Yb/Ho/Ce@NaYF4 core-shell upconversion nanocrystals
showing that all peaks can be well indexed in accordance with hexagonal-phase NaYF4 crystal structure
(Joint Committee on Powder Diffraction Standards file No. 16-0334). (B) EDX spectrum of the full-color
tunable core-shell nanocrystals. Note that the strong signal of Cu is from the copper TEM grid. The molar
ratios of Yb3+, Nd3+, and Ce3+ to the total amount of Ln(III) were determined to be 10.7 mol%, 2.25mol%,
and 2.31 mol%, respectively, which are close to the values of 11.4 mol%, 2.9 mol%, and 2.3 mol% pre-
designed in the core-shell structures.
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Supplementary Fig. 3: The luminescence decay profiles of 980 nm excitation obtained with different
pulse durations (200 s, 500 s, 1 ms, 2 ms, and 6 ms). Note that the width-tunable pulse excitation was
generated by coupling a 980 nm-diode laser with a digital pulse generator at fixed frequency of 100 Hz.
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Supplementary Fig. 4: Room-temperature (25 oC) upconversion emission spectra of the
NaYF4:Nd/Yb@NaYF4:Yb/Tm@NaYF4@NaYF4:Yb/Ho/Ce@NaYF4 core-shell nanocrystals under (A) a
dual-mode excitation with an 808-nm CW laser and a 980-nm pulsed laser (100 Hz, 400 s), (B) the
excitation of 808-nm CW laser alone, and (C) the dual-mode laser excitation at 808 nm (CW laser) and
980 nm (pulsed laser: 100Hz, 6 ms).
Supplementary Table 1: Chromaticity coordinates (CIE, 1931) obtained by using the as-
prepared NaYF4:Nd/Yb@NaYF4:Yb/Tm@NaYF4@NaYF4:Yb/Ho/Ce@NaYF4 core-shell
nanocrystals under dual-beam excitation (808 nm CW laser and 980 nm pulsed laser).
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980 nm Excitation 808 nm Excitation x y
200 s – 0.325 0.649
500 s – 0.397 0.585
1 ms – 0.437 0.544
2 ms – 0.485 0.492
6 ms – 0.586 0.392
400 s CW 0.288 0.460
– CW 0.161 0.140
6 ms CW 0.354 0.271
Red Emission of Ho3+(5F5→5I8) 0.723 0.277
Green Emission of Ho3+(5F4,5S2→
5I8) 0.245 0.740
Blue Emission of Tm3+(1G4→3H6) 0.112 0.089
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Supplementary Fig. 5: Control experiments showing the comparison of room-temperature (25 oC)
upconversion emission spectra between the penta-layered
NaYF4:Nd/Yb@NaYF4:Yb/Tm@NaYF4@NaYF4:Yb/Ho/Ce@NaYF4 core-shell nanocrystals and bilayered
core-shell nanocrystals homogeneously doped with Nd3+, Yb3+, Tm3+, Ho3+, and Ce3+ in the core under (A)
980 nm CW laser excitation, and (B) 808 nm CW laser excitation.
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Supplementary Fig. 6: Proposed steady-state blue (left) and green/red (right) upconversion mechanisms
in the NaYF4:Nd/Yb@NaYF4:Yb/Tm@NaYF4@NaYF4:Yb/Ho/Ce@NaYF4 nanocrystals. The dashed-dotted,
dashed and full arrows represent photon excitation, energy transfer, and emission processes, respectively.
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Supplementary Fig. 7: (A) Upconversion emission spectrum of NaYF4:Yb/Ho/Ce@NaYF4 nanocrystals
recorded at 20 K under a CW laser excitation at 980 nm. (B) Temporal dependence of Ho3+ emission at
549 nm under 980 nm pulsed excitation (pulse width: 17 ms) at 20 K. (C) Proposed upconversion emission
mechanism of the NaYF4:Yb/Ho/Ce@NaYF4 nanocrystals at ultralow temperatures. Note that the energy
transfer from Ho3+ to Ce3+ requires the assistance of phonon energy in the crystal lattice to overcome the
energy gap (E = 570 cm-1) between the 5I6 state of Ho3+ and the 2F5/2 state of Ce3+. Low temperature (e.g.
20 K) can suppress the phonon energy of the nanocrystals and minimize the energy transfer from Ho3+ to
Ce3+, thus resulting in a significant decrease in the intensity of red emission (5F5→ 5I8) at 646 nm as shown
in A (41).
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Supplementary Fig. 8: (A-C) Room-temperature (25 oC) upconversion luminescence decay curves of
Ho3+ emission at 541 nm, Ho3+ emission at 646 nm, and Yb3+ emission at 985 nm in the
NaYF4:Nd/Yb@NaYF4:Yb/Tm@NaYF4@NaYF4:Yb/Ho/Ce@NaYF4 nanocrystals under pulsed laser
excitation (200 s) at 980 nm. (D) The luminescence decay profile of the pulsed excitation at 980 nm
confirms the pulse width of 200 s.
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Supplementary Fig. 9: (A-C) Upconversion luminescence decay curves of Ho3+ emission at 541 nm,
Ho3+ emission at 646 nm, and Yb3+ emission at 985 nm, respectively, under pulsed laser excitation (200 s)
at 980 nm. (D) Time-dependent intensity profiles of Ho3+ emissions at 541 nm and 646 nm under pulsed
laser excitation (10 ms) at 980 nm in the NaYF4:Nd/Yb@NaYF4:Yb/Tm@NaYF4@NaYF4:Yb/Ho@NaYF4
nanocrystals. The luminescence lifetime of the nanocrystals was measured at room temperature (25 oC). It
should be noted that without the cerium doping the phenomenon of non-steady-state upconversion is not
significant as evident by the time-dependent intensity profiles of Ho3+ emissions shown in D.
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Supplementary Fig. 10: Calculated (A) and measured (B) temporal dependence of the Ho3+ emissions at
green (541 nm, 5F4/5S2→5I8) and red (646 nm, 5F5→5I8) wavelength regions from the
NaYF4:Nd/Yb@NaYF4:Yb/Tm@NaYF4@NaYF4:Yb/Ho@NaYF4 nanocrystals without Ce3+ doping.
Calculated (C) and measured (D) temporal dependence of the Ho3+ emissions at 541 and 646 nm in the
NaYF4:Nd/Yb@NaYF4:Yb/Tm@NaYF4@NaYF4:Yb/Ho/Ce@NaYF4 nanocrystals with Ce3+ doped in the
third shell layer of the nanocrystal. The temporal dependence curves were measured by exciting the
nanocrystals with a pulsed laser (25 Hz; 10 ms) at 980 nm at room temperature (25 oC). To simulate the
temporal dependent changes of the population densities at the 5F5 and 5F4/5S2 states of Ho3+ basing on the
measured radiative lifetimes, we assume that the excitation flux of the nanocrystals remains constant. As
shown in A-D, the results simulated by using the rate equations match well with the experimental results,
strongly suggesting the existence of non-steady-state energy transfer mechanism. Notably, the cerium
doping clearly accelerates the non-steady-state process, largely owing to the rate increase in pumping the
5F5 state of Ho3+.
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Supplementary Fig. 11: Experimental setup for full-color volumetric 3D display. In this system, two
diode lasers (980 nm and 808 nm) were aligned and directed into a fast scanning 3D galvanometer. A
digital-pulse generator was used to modulate the 980 nm diode laser to generate pulsed laser beams at 980
nm with a fixed frequency of 100 Hz and variable pulse width (from 200 s to 6 ms). The scanning of the
laser beams was controlled using Cyberlease scanning software (IDI Laser).
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Supplementary Fig. 12: (A) Photograph showing the dimension and shape of a PDMS monolith
composed of 0.2 wt% NaYF4:Nd/Yb@NaYF4:Yb/Tm@NaYF4@NaYF4:Yb/Ho/Ce@NaYF4 core-shell
nanocrystals. (B-F) Luminescent images showing the full-color volumetric 3D display in the
PDMS/nanocrystal composite via computer-controlled NIR laser scanning.
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Supplementary Fig. 13: Room-temperature (25 oC) upconversion emission spectra and luminescent
photographs of conventional blue, green, red upconversion nanocrystals of (A) NaYF4:Yb/Tm (49/1 mol%)
(ref. 42), (B) NaYF4:Yb/Er (20/2 mol%) (ref. 35), and (C) KMnF3:Yb/Er (18/2 mol%) (ref. 43). (D)
Upconversion spectrum and corresponding luminescent photograph of the sample containing randomly
mixed blue, green, red upconversion nanocrystals. The luminescent spectra and photographs are recorded
under 980 nm excitation using a CW diode laser.
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Supplementary Table 2: Absolute upconversion quantum yields of the as-prepared multilayer
NaYF4:Nd/Yb@NaYF4:Yb/Tm@NaYF4@NaYF4:Yb/Ho/Ce@NaYF4 nanocrystals.
Quantum Yield
(%)
Excitation
Wavelength (nm)
Excitation Power
Density (Wcm-2)
0.13* 980 100
0.09 808 250
*The measured value of quantum yield is comparable to the quantum yield reported for conventional core-
shell NaYF4:Yb/Er nanocrystals (0.3%) (ref. 44).
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Supplementary Fig. 14: Excitation power density dependence of the upconverted Tm3+ emission at 474
nm (1G4→3H6) in NaYF4:Nd/Yb@NaYF4:Yb/Tm@NaYF4@NaYF4:Yb/Ho/Ce@NaYF4 core-shell
nanocrystals under CW laser excitation at 808 nm.
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