temporal full-colour tuning through non-steady- temporal ... · temporal full-colour tuning through...

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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

http://dx.doi.org/10.1038/nnano.2014.317

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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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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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