Electronic Supplementary Material (ESI) for Physical Chemistry Chemical Physics.
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Supporting Information
Distinct
mechanisms
for
the
upconversion
of
NaYF4:Yb3+,Er3+ nanoparticles revealed by stimulated
emission depletion
Kyujin Shin,‡ Taeyoung Jung,‡ Eunsang Lee ‡ Gibok Lee, Yeongchang Goh, Junseok Heo,
Minhyuk Jung, Eun-Jung Jo, Hohjai Lee, Min-Gon Kim, and Kang Taek Lee*
Department of Chemistry, School of Physics and Chemistry, Gwangju Institute of Science and
Technology (GIST), Gwangju 61005, Korea
Corresponding Author
[email protected]
‡
Equally contributed authors.
1. Synthesis of β-NaYF4: 20 mol% Yb3+, 2 mol% Er3+ core/shell nanoparticles.
Materials. Ytterbium(III) acetate hydrate (99.9%), Yttrium(III) acetate hydrate (99.95%),
Erbium(III) acetate hydrate (99.9%), 1-octadecene (technical grade, 90%), oleic acid (technical
grade, 90%), ammonium fluoride (≥98%), sodium hydroxide (≥98%) were purchased from
Sigma-Aldrich. All reagents were used without further purification.
Synthesis of β-NaYF4:Yb3+, Er3+ core/shell nanoparticles. The NaYF4: Yb3+, Er3+
nanoparticles (UCNPs) were synthesized according to the previously reported procedure with
minor modification.1 Y(CH3COO)3•xH2O (0.798 mmol), Yb(CH3COO)3•xH2O (0.2 mmol)
and Er(CH3COO)3•xH2O (0.02 mmol) were added to a 100 mL three-neck round-bottom flask
containing oleic acid (6 mL) and 1-octadecene (15 mL). The solution was heated to 150°C with
stirring for 30 min and then cooled to room temperature. A mixture of ammonium fluoride (4.0
mmol) and sodium hydroxide (2.5 mmol) dissolved in methanol (10 mL) was added to the
reaction flask and stirred for 30 min at 50°C. The reaction mixture was heated to 100°C under
vacuum with stirring for 15 min to remove methanol. Subsequently, the reaction mixture was
heated to 300°C and kept at this temperature for 1 h under Ar. After cooling down to room
temperature, UCNPs were precipitated by adding ethanol and isolated by centrifugation. The
nanoparticles were washed several times with ethanol and then re-dispersed in hexane. The
ratio of lanthanide ions in the core UCNPs analyzed by ICP-MS (NexION 350D, Perkin-Elmer
SCIEX) is as follows;
Y: 78.57%, Yb: 19.28%, and Er: 2.15%
For hexagonal phase core/shell UCNPs, Y(CH3COO)3•xH2O (0.5 mmol) was added to a 100
mL three-neck round-bottom flask containing oleic acid (6 mL) and 1-octadecene (15 mL).
The solution was heated to 150°C with stirring for 30 min and then cooled to room temperature.
A solution of hexagonal phase core UCNPs in hexane (1 mmol) was added to the reaction flask.
The mixture was heated to 80°C with stirring for 30 min to remove hexane, and then cooled to
room temperature. A mixture of ammonium fluoride (2.0 mmol) and sodium hydroxide (1.25
mmol) dissolved in methanol (5 mL) was added to the reaction flask and stirred for 30 min at
50°C. The reaction mixture was heated to 100°C under vacuum with stirring for 15 min to
remove methanol. Subsequently, the reaction mixture was heated to 300°C and kept at this
temperature for 1 h under Ar. After cooling down to room temperature, core/shell UCNPs were
precipitated by adding ethanol and isolated by centrifugation. The nanoparticles were washed
several times with ethanol and then re-dispersed in hexane. Synthesized nanoparticles were
characterized by transmission electron microscope (TEM) (Tecnai G2 F30 S-Twin, Fei). The
size distributions of core/shell UCNPs were analyzed using ImageJ software (Fig. S2).
2. Experimental setup
Microscope setup (Fig. S3) for stimulated emission depletion of UCNPs was composed of
inverted microscope (IX73, Olympus), a 1540-nm IR diode laser (MDL-N-1532-1W, Laser
lab), a 980-nm NIR diode laser (1999CHP, 3SP Technologies), an electron multiplying charge
coupled device (EMCCD) camera (iXON3, Andor Technology), and a spectrometer
(QE65000, Ocean Optics). 980-nm laser beam was reflected by the dichroic mirror (ZT775sp2p-UF1, Chroma) and focused on UCNP samples by the objective lens (Apo N 60X/1.49 NA
oil immersion lens, Olympus). The 1540-nm laser beam was focused on the optical fiber by
the plano-convex lens (LA1805-C, f=30.0mm, Thorlabs) and collimated by the reflective
collimator (RC08SMA-P01, Thorlabs). This beam was focused on a specimen by the objective
lens (PlanC N 10X/0.25 NA lens, Olympus). Two laser beams were switched on and off by
shutters (SC10, Thorlabs). UCNPs emitted in the green and red region, and red emission was
depleted selectively by 1540-nm laser. Emission filter (ET750sp-2p8, Chroma) was used to
block 980-nm laser. Outside the microscope, since the green and red emission signals were
passed separately by band pass filters (ET665/70M-2p and ET535/70M, Chroma) and
background beam by two lasers was blocked by emission filter (ET700SP-298, Chroma).
EMCCD camera and spectrometer could detect each frequency region selectively by band pass
filter. EMCCD camera detected magnified images by a set of plano-convex lens. The emission
spectrum of UCNPs around 1540 nm was obtained by FLS980 Spectrometer (Edinburgh
Instruments).
3. Emission spectra and emission pathways of UCNPs
The emission spectra of UCNPs were measured by the imaging setup shown in Fig. S3.
Emission spectra of UCNPs were obtained by the spectrometer (QE65000, Ocean Optics)
under 980-nm and 1540-nm illumination (Fig. S4). UCNPs shows red and green emission
spectrum under 1540-nm excitation by multi-photon process.2,3 UCNP films were prepared on
the coverslip by spin coating. The energy diagrams of the emission pathway of UCNPs are
shown in Fig. S1, where the pathways A (green emission, Fig. S1a) and B (red emission, Fig.
S1b) were designated as in our previous study.4
(b)
Pathway A
4F
Pathway B
4F
7/2
2H
11/2
7/2
2H
11/2
4S
2F
7/2
Yb3+
Yb3+
4F
4I
2F
5/2
15/2
Er3+
11/2
4I
2F
7/2
4I
Yb3+
3/2
9/2
13/2
655 nm
13/2
4I
980 nm
11/2
4I
540, 525 nm
655 nm
980 nm
980 nm
4I
9/2
980 nm
2F
5/2
4S
Energy
4F
3/2
Energy
(a)
Yb3+
15/2
Er3+
Fig. S1 (a) The photophysical pathway A for the green (4I15/2  4I11/2  4F7/2  (2H11/2 or 4S3/2)
 4I15/2) and red (4I15/2  4I11/2  4F7/2  4S3/2  4F9/2  4I15/2) emission. (b) The photophysical
pathway B for red emission (4I15/2  4I11/2  4I13/2  4F9/2  4I15/2). It turned out that the red
emission originated mostly from the pathway B.
(a)
(b)
Number of UCNPs
60
50
40
30
20
10
0
25
26
27
28
29
30
31
Diameter (nm) 
Fig. S2 (a) The TEM images of core/shell UCNPs (NaYF4:Yb3+,Er3+/NaYF4). (b) The size
distribution of UCNPs obtained from 264 single core/shell UCNPs. The Gaussian fit (black
curve) for the distribution indicates that the average diameter of UCNPs is 28.51 ± 0.78 nm.
Reflective Collimator
DM : Dichroic Mirror
M : Mirror
BPF : Bandpass Filter
BS : Beam Splitter
L : Lens
Optical Fiber
10X Olympus Objective Lens
NA 0.25
L
Sample
Shutter
Shutter
Spectrometer
1540nm CW
Laser
60X Olympus Objective Lens
(Oil Immersion)
NA 1.49
980nm CW
Laser
DM
EMCCD
M
L (75 mm)
L (50 mm)
BS
BPF
Fig. S3 Schematic diagram of the setup for the measurement of UCNP images and spectra. The
980-nm CW laser (1999CHP, 3SP Technologies) was directed to the inverted microscope
(IX73, Olympus) by a dichroic mirror (ZT775sp-2p-UF1, Chroma). The 1540-nm CW laser
(MDL-N-1532-1W, Laser lab) is focused on the optical fiber and excites the sample from the
upper side. Both the 980-nm and 1540-nm lasers were blocked by the shutter (SC10, Thorlabs)
in a controlled manner. The emission from the UCNPs was divided by a beam splitter and
detected simultaneously by the spectrometer (QE65000, Ocean Optics) and EMCCD camera
(iXON3, Andor Technology), respectively. We used the band-pass filter (ET665/70M-2p or
ET535/70M, Chroma) for selective detection of red or green emission.
350
40000
980 nm excitation
1540 nm excitation
1540 nm excitation
300
Intensity (AU)
Intensity (AU)
50000
30000
250
200
150
100
50
0
450
500
550
600
650
700
750
Wavelength (nm)
20000
10000
0
450
500
550
600
650
700
750
Wavelength (nm)
Fig. S4 The emission spectrum of UCNP obtained by 980-nm excitation alone (black line).
The “non-sensitized” emission spectrum of the same sample upon 1540-nm excitation alone
(red line and inset). Even with the high power of the 1540-nm laser (1 W) compared to that of
the 980-nm laser (230 mW), the signal level of the former was negligible.
Mov. 1 The emission from the laser beams propagating through the colloidal UCNP solution.
We turned on 1540-nm CW IR laser in order to induce ED. The yellowish green emission
(red + green) turned green due to the depletion of the red emission by the IR laser (1540 nm).
By turning off the IR laser, the yellowish green emission recovered and such switching is the
evidence for the stimulated emission depletion.
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