Core-shell metal fluoride nanoparticles: Via fluorolytic sol-gel synthesis-a fast and efficient construction kit

Article abstract of DOI:10.1039/c7tc01599b

An efficient, fast and easy construction kit using the fluorolytic sol-gel synthesis of rare-earth-doped alkaline earth fluoride core-shell nanoparticles at room temperature is presented, capable of synthesizing several hundred grams to kilograms of core-shell particles in one batch. We show ways for an effective design of energy transfer core-shell systems. Undoped metal fluoride shells rigorously shield a luminescent core from the surrounding solvent, resulting in higher quantum yields, longer lifetimes of the excited states, and finally a brighter luminescence. The heavy SrF₂ shields a luminescent core from the surrounding solvent three times more effectively than the light CaF₂. Energy transfer processes from core to shell are more efficient than vice versa, and hence, absorbing cores are more effective than absorbing shells. The application of these materials in the preparation of transparent tunable luminescent materials showing different luminescence colours upon different excitation wavelengths is demonstrated.

Full text of DOI:10.1039/c7tc01599b

View Article Online
View Journal
Journal of
M at erials Chemistry C
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: B. Ritter, P. Haida,
T. Krahl, G. Scholz and E. Kemnitz, J. Mater. Chem. C, 2017, DOI: 10.1039/C7TC01599B.
Volume
4
Number
1
7
January 2016 Pages 1–224
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Journal of
Materials Chemistry C
Materials for optical, magnetic and electronic devices
www.rsc.org/MaterialsC
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 2050-7526
PAPER
~
Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
rsc.li/materials-c

Page 1 of 11
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC01599B
Core-shell metal fluoride nanoparticles via fluorolytic sol-gel
synthesis – a fast and efficient construction kit
[a]
[a]
[a,b]
[a]
[a,b]
B. Ritter , P. Haida , T. Krahl , G. Scholz , E. Kemnitz*
[
a]
b]
Prof. Dr. Erhard Kemnitz, Humboldt-Universität zu Berlin, Institut für Chemie, Brook-Taylor- Str.
, D-12489 Berlin, Germany
2
[
Nanofluor GmbH, Rudower Chaussee 29, D-12489 Berlin, Germany
*
mail: erhard.kemnitz@chemie.hu-berlin.de, phone: +49 30 2093 7555, fax: +49 30 2093 7277
Abstract
An efficient fast and easy construction kit using the fluorolytic sol gel synthesis of rare earth doped
alkaline earth fluoride core-shell nanoparticles at room temperature is presented, capable of
synthesizing several hundred grams to kilograms of core-shell particles in one batch. We show ways
for an effective design of energy transfer core-shell systems. Undoped metal fluoride shells rigorously
shield a luminescent core from the surrounding solvent, resulting in higher quantum yields, longer
lifetimes of the excited states, and finally in brighter luminescence. The heavy SrF
2
shields three times
more effective than the light CaF . Energy transfer processes from core to shell are more efficient than
2
vice versa, and hence, absorbing cores are more effective than absorbing shells. The application of
these materials in the preparation of transparent tunable luminescent materials showing different
luminescence colours upon different excitation wavelengths is demonstrated.
Introduction
The search for luminescent nanomaterials is one of the major research areas and has gained enormous
1
, 2
3-5
attraction over recent years, resulting in applications in the field of solid-state lasers , photovoltaics
,
6
7-13
displays, LEDs , and biological imaging . Due to their high amount of closely contiguous transitions,
trivalent rare earth ions with their 4f electron configuration enable the excitement in the UV (down
conversion) or IR (up conversion) region with an emittance in the visible spectrum. Nano metal fluorides
Rare earth doped alkaline earth metal fluoride
nanoparticles are featured by their low toxicity11, 13, excellent photostability , and long transition lifetimes.
14, 15
are the most suitable host matrices for these ions.
5
In the past years, core-shell structured nanoparticles, possessing an inorganic core surrounded by an
inorganic shell, have been invented to reduce radiation free relaxations, to improve luminescent light
1
6-18
output, to increase quantum efficiency
, and to solve the problems from undesired energy transfers in
19, 20
multi rare earth doped systems which result in a divergent colour impression
. Many interesting core-
shell systems, including dual-mode particles capable of photon up- and downconversion simultaneously,
2
1-23
were synthesized during the last years.
limited due to their low accessibility.
Applications of luminescent metal fluoride nanoparticles is
2
4, 25
26, 27
28
Solvothermal
, as well as trifluoroacetate based routes and
co-precipitation29 are usually limited to amounts of a few grams of particles.
The fluorolytic sol-gel synthesis in general allows the up scaling to litre amounts of soles or several
3
0-32
kilograms of nano metal fluorides.
Using additional hydrolytic sol-gel processes allows the synthesis of
But until now, no synthesis approach has been reported that gives easy
access towards larger amounts of core-shell metal fluoride materials. Glycol and polyol mediated
SiO
2
shells around these cores.3
3, 34
32, 35-37
synthesis is an elegant way to stabilize the nanoparticles in situ.
Thus, we here report a new
synthesis approach, which allows the direct stepwise synthesis of different metal fluoride shells around an
existing core resulting in tailor-made particles. This method is capable of synthesizing kilogram amounts
of nanostructured core-shell particles in one batch at room temperature without further reprocessing like
centrifugation, filtration, thermal treatment etc.
The formation of core-shell structures is proven by NMR, DLS, TEM and photoluminescence. To
demonstrate the high potential of the new method in the synthesis of luminescent particles with different
3
+
3+
38-40
designs, the well-known energy transfer system Tb →Eu was chosen.
This allows deeper
quantitative insights in the rather poorly investigated energy transfer processes between core and shell or
between shell and shell, and into the shielding efficiency of different core materials.

Journal of Materials Chemistry C
Page 2 of 11
View Article Online
DOI: 10.1039/C7TC01599B
Experimental
Chemicals. Europium oxide (Eu
2
O
3
, 99.9%, REEtec Norway), terbium oxide (Tb
2
O
3
, 99.9%, Sigma
Aldrich), yttrium oxide (Y
2
O
3
, 99.9%, Sigma Aldrich), erbium oxide (Er
2
O
3
, 99.9%, Sigma Aldrich),
·5H O, AppliChem,
O, 98%, Dr. Paul Lohmann) and ethylene glycol
ytterbium oxide (Yb
2
O
3
, 99.9%, Sigma Aldrich), calcium lactate hydrate (Ca(OLac)
2
2
–
–
OLac = CH
3
CHOHCOO ), strontium lactate (Sr(OLac)
2
·xH
2
(
99% Sigma Aldrich) were used.
Rare earth metal acetates RE(OAc)
2 equivalents of 50% aqueous acetic acid. Rare earth acetates and alkaline earth lactates were dried
3
were obtained by reflux of the respective rare earth metal oxides with
1
under vacuum for 3 h at 150 °C. Water-free salts were obtained for rare earth acetates. Remaining water
in Ca and Sr lactates was analysed by thermogravimetry (0.42 for Ca, 0.80 for Sr). Methanolic HF solution
was formed by condensation of HF gas into water-free methanol to yield an approx. 15…20 M solution.
The exact HF concentration was determined by titration with NaOH using phenolphthalein as indicator.
CAUTION! HF is a hazardous agent and has to be used under restricted conditions only.
Material abbreviations. SrF
2
:Eu10 = Sr0.9Eu0.1
2.1, SrF :Eu5,Tb5 = Sr0.9Eu0.05Tb0.05
:Yb10,Er2 = Sr0.88Yb0.1Er0.02
F
2.1, SrF
2
:Eu20 = Sr0.8Eu0.2
F
2.2, SrF
2
:Eu30 = Sr0.7Eu0.3
F
2.1
2.3
,
,
SrF
SrF
2
:Tb10 = Sr0.9Tb0.1
F
2
F
2.1, Ca0.25Sr0.75 :Eu10 = Ca0.2Sr0.7Eu0.1
F
2
F
2
F
2.12
.
Sol-Gel Synthesis (core particles). The preparation is described exemplarily for the synthesis of 250 mL
of 0.2 M SrF :Eu10 (=i.e. Sr0.9Eu0.1 2.1): Eu(OAc) (1.646 g, 5.00 mmol) and Sr(OLac) ·0.8H O (12.609 g,
5.00 mmol) were dissolved in 240 mL ethylene glycol. Then, 6.41 mL (105 mmol) of a methanolic HF
2
F
3
2
2
4
solution (16.38 M) was added under vigorous stirring at room temperature. After some minutes a
transparent sol was received. The so-obtained sol was stirred overnight.
Sol-Gel Synthesis (core@shell particles). The preparation is described exemplarily for the synthesis of
250 mL of 0.2 M SrF
2 2 2
:Eu10@SrF (i.e. Sr0.9Eu0.1F2.1@SrF ): The sol containing the core particles was used,
Sr(OLac) ·0.8H O (14.010 g, 50.0 mmol) was added and dissolved in the sol. Additionally, 6.11 mL (100
2
2
mmol) of a methanolic HF solution (16.38 M) was added under vigorous stirring at room temperature.
After some minutes a transparent sol was received, which was still stirred overnight before further
investigations. Repeated application of this synthesis grows additional shells around the particles.
NMR. 19F NMR spectra were recorded on a. Bruker AVANCE 400 spectrometer (Larmor frequency: ν19F
76.4 MHz) using a 4 mm double-bearing magic angle spinning (MAS) probe (Bruker Biospin) in static
=
3
1
9
mode with 1024 accumulations and a π/2 pulse length of 3.6 ꢀs. F chemical shifts are referenced to
CCl
F. Existent background signals of 19F were suppressed with the application of a phase-cycled depth
3
pulse sequence according to Cory and Ritchey.
Dynamic light scattering (DLS). DLS measurements were performed by using a Malvern Zetasizer Nano
ZS instrument.
Transmission electron microscopy (TEM). TEM analysis has been carried out using a Philips CM200
–
4
6
LaB microscope operating at 200 kV. Sols were diluted with methanol to a concentration of 10 M prior
to the measurement. A few drops of the solution containing the nanoparticles were deposited on a carbon-
coated copper grid and left to dry for 1 hour prior to the inspection.
Luminescence measurements. Luminescence emission and excitation spectra as well as lifetimes were
recorded with a FluoroMax-4P from Horiba Jobin Yvon in 10 mm quartz cells. Luminescence lifetimes
were recorded in a single photon counting mode using the TCSPC accessory with a collecting time up to
175 ms. Upconversion spectra are obtained by excitation with an external 500 mW cw laser at 980 nm.
Fitting the luminescence lifetimes with a single monoexponential decay was not possible. This is typical
for nanoparticles, because luminescence centres in the middle or near the surface of the particle have
significantly different decay behaviour.4
1-43
Thus, lifetimes ꢀ were fitted using a q-exponential function
ꢁ
ꢁ
/(ꢁꢊꢅ)
ꢂ ꢄ (−ꢀ/ꢀ ) with ꢄ (ꢆ) = ꢇ1 + (1 − ꢈ)ꢆꢉ
. This function is a stretched exponential function, which is
ꢃ
ꢅ
ꢁ
ꢅ
capable to fit the luminescence decay of amorphous or nanoparticulate emitters with statistically
distributed lifetimes. The parameter q represents the degree of asymmetry or skewness of the
distribution of lifetimes. q was allowed to fit between 1.01 … 1.30. Higher values than 1.30 have no
ꢋ
physical sense, and for ꢈ → 1 this function approaches the normal exponential function ꢄ .
Quantum yields. Absorption spectra were obtained on a Specord 210 PLUS from Analytik Jena in 50 mm
quartz cells. Relative photoluminescence quantum yields ΦPL were determined employing organic dyes
with a known ΦPL as standards (Coumarin 102 in ethanol, ΦPL = 0.76). The uncertainty is ±10%.44-47

Page 3 of 11
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC01599B
Results and Discussion
Initially we proof via 19F NMR that the fluorolytic sol-gel synthesis is indeed capable of forming core-shell
structured nanoparticles. Rare earth metal free core-shell nanoparticles of the compositions
2 2 2 2 2 2 2 2
@CaF /SrF /CaF (core@shell/shell/shell) and CaF @SrF /CaF /SrF F
were synthesized. Static 19
SrF
NMR of these sols delivers liquid-like spectra due to the mobility of the particles in the liquid phase
Figure 1). The spectra of the core-shell particles (Figure 1A and 1B) show two distinct maxima, which are
slightly shifted to higher and lower fields compared to the typical signals of CaF and SrF (Figure 1D),
respectively. Additionally, a broad signal with chemical shifts between those extrema occurs,
corresponding to the spectrum of a true solid solution Ca0.5Sr0.5 (Figure 1C), but which is lacking in the
spectrum of a physical mixture of CaF and SrF particles (Figure 1D). Both, the new signal between the
two outer maxima and their already mentioned shifts is caused by the contact area between core and shell
(
2
2
F
2
2
2
2
+
2+
or between different shells, in which fluoride is coordinated simultaneously by Ca and Sr with different
Ca:Sr proportions ([FCa4-nSr
] (n = 0, 1, 2, 3, 4)). This results in five 19F signals, each of them corresponding
to one possible cation environment of the fluoride ion (Figure 1C).
CaF and SrF
domains can be clearly identified by 19F NMR.
n
3
2, 48
Thus, the contact area between
2
2
CaF
2
SrF
2
Ca0.5Sr0.5F
2
(
A)
(
(
B)
C)
(
D)
SrF2
CaF2
+
-40 -60 -80 -100 -120 -140 -160 -180 -200
δiso [ppm]
Figure 1. 19F NMR spectra of 0.2 M sols in ethylene glycol: (A) SrF
and (D) mixture of separately prepared CaF and SrF particles.
2 2 2 2 2 2 2 2
@CaF /SrF /CaF , (B) CaF @SrF /CaF /SrF ,
(
C) Ca0.5Sr0.5
F
2
2
2
This synthesis approach enables to synthesise huge amount of core and core-shell particles. As a prove of
concept we prepared 250 mL scale of 0.2 M SrF :Eu10@SrF and 0.2 M SrF :Tb10@SrF and 3 L scale of
.2 M CaF @SrF completely transparent and long time stable sols (Figure 2). Theoretically, there is no
2
2
2
2
0
2
2
synthetic limitation for higher amounts of sol.
Figure 2. Pictures at daylight (upper left) and under UV excitation (366 nm, lower left) of 250 mL scale of 0.2 M
SrF :Eu10 @SrF (left) and of 0.2 M SrF :Tb10@SrF (right) and on the right a 3 L scale of 0.2 M CaF @SrF (daylight).
2 2 2 2 2 2
Luminescent nanoparticles are obtained by synthesizing several non-luminescent shells around a core
SrF :Eu10 (i.e. Sr0.9Eu0.1
2.1), resulting in water-clear sols (Figure 3). The Eu3+ luminescence intensity
increases with every additional SrF shell (Figure 3A and D). The systematic growth of particle size with
2
F
2

Journal of Materials Chemistry C
Page 4 of 11
View Article Online
DOI: 10.1039/C7TC01599B
increasing numbers of SrF
2
shells was confirmed by DLS (Figure 3B). The ternary system
Ca0.25Sr0.75F :Eu10 (i.e. Ca0.2Sr0.7Eu0.1F2.1) as core with three Ca0.25Sr0.75F shells behaves in the same way
2 2
(
cf. SI Figure S1 and Table S1).
Figure 3. Core-shell particles SrF
2
:Eu10@n×SrF
2
(n = 0…3). Upper left: Pictures at daylight and under UV excitation
3+
(
(
366 nm). (A) PL emission spectra (ex 393 nm). (B) DLS particle diameter (number), (C) Eu luminescence lifetime
em 590 nm), (D) luminescence intensity (ex 393 m), particles with different amounts of Eu doping are given for
3
+
comparison. PLQY: Photoluminescence quantum yield.
2 2
In both matrices (SrF and Ca0.25Sr0.75F ), a significant gain of luminescence intensity was found for the
first shell, which is another hint for a successful formation of core-shell structures. The stagnation of
intensity increase for the second shell is unusual. It was reconfirmed even by repetition of this synthesis
for several times. The third shell instead yields a further gain of intensity for both matrices (cf. SI Figure
S2). SrF
luminescent than SrF
2
:Eu10@3×SrF
2 2
is more than twice as luminescent as the pure core SrF :Eu10, and also more
2
:Eu20 (Figure 3D).
Luminescence lifetimes of both core-shell systems increase steadily with every additional shell (cf. Figure
and SI Table S2). This is a clear evidence for the successive formation of shells. The dramatic increase of
lifetime (307% for SrF :Eu10@3×SrF ) is an
important requirement for energy transfer processes, and especially up-conversion processes.
Furthermore, the photo luminescence quantum yields increase massively from 27.3% for SrF :Eu10 (core
:Eu10@2×SrF (two SrF
2
3
2
2 2 2
and 273% for Ca0.25Sr0.75F :Eu10@3×Ca0.25Sr0.75SrF
2
only) over 39.6% for SrF
shells).
2
:Eu10@1×SrF
2
(one SrF
2
shell) up to 53.7% for SrF
2
2
The increase of luminescence intensity, lifetime and quantum yield with each additional shell is caused by
the shielding effect, which decreases the non-radiative surface relaxation caused by OH groups of the
surrounding organic solvents.
TEM images of SrF
directly proving the increase of the particle size. The detected core size via DLS (7.8 nm) and TEM
9.1 nm) for CaF :Eu10 particles was used as starting values and compared with the calculated as well as
2 2 2
:Eu10 core particles and SrF :Eu10@3xSrF core-shell particles are shown in Figure 4,
(
2
experimentally detected diameters of prepared core-shell particles. As shown in Table S1, the
experimental and calculated data are in excellent agreement. Unfortunately, EDX mapping of such small
structures is nearly impossible due to the beam sensitivity of metal fluoride nanoparticles. Only some
4 4
groups manage to show this on related NaYF @NaGdF
particles.4
9, 50

Page 5 of 11
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC01599B
2 2 2
Figure 4. (A) TEM images of SrF :Eu10 and (B) SrF :Eu10@3xSrF (selected particles are surrounded by a black
lines).
The fluorolytic sol-gel synthesis is not limited to the same matrix material for core and shell. The effect of
the different shell materials CaF and SrF on the luminescence intensity and lifetime around a SrF :Eu10
core is shown in Figure 5 (cf. also SI Figure S2 and Table S2). The observed increase of intensity and
lifetime evidences the formation of core-shell particles also for CaF
2
2
2
2
.
7
6
5
4
3
2
2 shells
shell
2 shells
1 shell
1
1,6
1,4
1,2
1,0
(B)
(A)
core CaF2 SrF2
Shell matrix
core CaF2 SrF2
Shell matrix
Figure 5. (A) Luminescence intensity and (B) Eu3+ lifetimes of SrF
:Eu10 with different shell materials.
2
Noticeably, the CaF
phonon energy (CaF
2
shell shields the core less effectively than the SrF
2
shell. A shell material with a lower
-
1
-1
2
: 456 cm , SrF 366 cm ) is less susceptible to non-radiative relaxation. To
2
quantitatively evaluate the shielding ratio, the luminescence decay of the core material and non-radiative
energy loss into the shell material has to be calculated separately. This can be achieved by comparing
particles with a different position of the luminescent material. Particles with a luminescent core (2) show
longer lifetimes and slightly higher intensities than particles with a luminescent shell (3). Apparently,
non-radiative relaxation of Eu3+ in two different spatial directions causes a reduction of luminescence
3
+
intensity and lifetime as compared to Eu incorporated in the core (Figure 6).
Comparing particles (2) and (3), an “intrinsic lifetime” of the SrF :Eu10 core, i.e. the lifetime without
surface relaxation, can be estimated to be ≈11 ms (see SI for calculation details). Using this value, the non-
radiative relaxation constant from the core into the shell (none, CaF or SrF ) can be estimated. Results are
2
2
2
summarized in Table 1. Finally, a CaF shell reduces surface relaxation by 25% compared to the naked
core, while a SrF shell reduces surface relaxation by 75%. Thus, a SrF shell shields the particle three
times more effectively than a CaF
2
2
2
2
.

Journal of Materials Chemistry C
Page 6 of 11
View Article Online
DOI: 10.1039/C7TC01599B
(1)
(2)
(3)
(4)
3+
LT Eu : 2.38
5.79
5.19
2.93 ms
SrF :Eu10
2
SrF2
CaF2
non-radiative relaxation
Figure 6. Variation of the position of the emitter in different core-shell particles. (1) SrF
SrF :Eu10@2×SrF (3) SrF @SrF :Eu10/2×SrF and (4) SrF :Eu10@2×CaF . Non-radiative relaxation is qualitatively
indicated by arrows. Core diameter for (1) (2) and (4) is 7.5 nm, for (3) it is 15.6 nm.
2
:Eu10 core only, (2)
2
2
,
2
2
2
2
2
,
2 2
and SrF shells compared to the naked core. τ
~ Eu3+ luminescence lifetime,
Table 1. Relative shielding effect of CaF
nr ~ non-radiative decay constant from shell into core, * Relative to naked core. Core diameter is 7.5 nm.
k
Relative non-
radiative
relaxation *
Particle
τ / ms
k
nr / ms-1
(
(
(
1) SrF
4) SrF
2) SrF
2
2
2
:Eu10 (core)
2.38
2.93
5.79
0.33
0.25
1
:Eu10@2×CaF
2
0.76
0.25
:Eu10@2×SrF
2
0.082
For an effective design of energy transfer core-shell systems, it would be important to know the efficiency
of the energy transfer between core and shell and vice versa. Therefore, the easy-to-handle energy
3
+
3+
38-40
3+
3+
transfer system Eu →Tb was chosen as a suitable probe system here.
Different Tb -Eu co-doped
3
+
3+
3+
particles were synthesised: particles containing only Tb or Eu , particles with at Eu containing core
and a Tb containing shell and vice versa, and Tb -Eu co-doped particles (Figure 7). When Tb and
Eu are in spatial proximity, energy transfer can occur from the excited states of Tb to Eu .
3
+
3+
3+
3+
3
+
3+
3+
The emission spectra of mixtures of particles doped with Eu3+ or Tb3+ (7) demonstrate that Eu3+ and Tb3+
can be selectively excited at 393 nm or 350 nm, respectively (cf. SI Figure S4). Only emission lines of the
3
+
excited ion are observed (and none of the other), and hence, no energy transfer processes between Eu
and Tb3 occur, when both rare earth ions are situated in different particles. This is a proof of the
+
protection of the doped cores caused by the undoped shells. It is in contrast to the known energy transfer
3
+
3+
38, 51
of physical mixtures of Tb and Eu doped core-only nanoparticles.
This behaviour is different, when Tb3+ and Eu3+ are doped into the same particle (Figure 7). Here, Tb can
3+
3
+
be selectively excited at 350 nm, and then transfer the absorbed energy to a neighbouring Eu (Figure 8)
resulting in a shift of the emission colour to more yellow. The efficiency of the energy transfer is measured
by the loss of intensity of the Tb emission lines and a gain of intensity of the Eu emission lines at 542
nm and 693 nm, respectively (cf. Figure 7A inset). The lines in-between overlap each other and cannot be
separately evaluated.
3
+
3+
3
+
The least efficient energy transfer was found for particle (8) caused by Tb positioned in the shell and
3
+
Eu in the core. Non-radiative relaxation always occurs as a concurrence process parallel to the energy
3
+
3+
transfer to Eu . In particle (8), Tb can undergo non-radiative relaxation in two spatial directions to the
core and the surrounding shell (Figure 8 right), resulting in a smaller fraction of energy transferred to
3
+
3+
Eu than in (9). The energy transfer in particle (9), in which Tb is located in the core, is more efficient,
3
+
because radiation-free relaxation of Tb occurs only unidirectional from the core to the first shell (Figure
right), which increases the Tb →Eu energy transfer intensity. Thus, the energy transfer from core to
3
+
3+
8
shell is more effective than from shell to core. For the design of tailor-made nanoparticles, an absorbing
core makes more sense than an absorbing shell.

Page 7 of 11
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC01599B
(7) = mixture of (5) and (6)
(
5)
(6)
(8)
(9)
(10)
+
3
+
LT Tb :
5.17
3.11
2.92
2.17 ms
SrF2
SrF :Eu10
SrF :Tb10
SrF :Eu5,Tb5
2
2
2
Tb 542 nm
Eu 698 nm
(
(
(
(
7)
8)
9)
10)
(B)
(
A)
(7) (8) (9) (10)
3
35 340 345 350 355
Tb
Eu
4
50 500 550 600 650 700 750 300 325 350 375 400 425
Wavelength / nm Wavelength / nm
3+
3+
3+
Figure 7. Upper: Different core-shell particles for energy transfer Tb →Eu (below: lifetime of Tb ). Particle
diameter is 7 ± 0.5 nm. (5) SrF
SrF :Eu10@SrF :Tb10/SrF (9) SrF :Tb10@SrF
particles. (A) PL emission (ex 350 nm @Tb ), inset shows the band intensities of significant Tb and Eu emission;(B)
2
:Eu10@2×SrF
2
,
(6) SrF
2
:Tb10@2×SrF
2
,
(7) mixture of (5) and (6), (8)
2
2
2
,
2
2
:Eu10/SrF
2
,
(10) SrF
2
:Eu5,Tb5@2×SrF
2
. Lower: PL spectra of these
3+
3+
excitation (em 698 nm @Eu ), inset shows the selected energy transfer region. All spectra are normalized to the rare
earth content.
As expected, the most efficient energy transfer can be found for particle (10) with Tb3+ and Eu3+ co-doped
in the core. In this system, both ions are in the closest proximity of all investigated energy transfer
particles.
3
+
Additionally, the energy transfer is clearly observed in the excitation spectra (Eu at 698 nm) by an
3
+
increase of intensity in the region of 350 nm (cf. Figure 7B inset). Eu cannot be excited at 350 nm, but
3
+
3+
Tb can. Hence, the increase of signal at 350 nm in the Eu excitation spectra is caused by an increasing
3
+
3+
3+
efficiency of the energy transfer Tb →Eu . Contrary, the excitation spectra of Tb measured at 542 nm
do not show any additional bands and especially, no intensity at 393 nm (cf. SI figure S5). Thus, no energy
transfer occurs from Eu to Tb .
3
+
3+
Interestingly, the luminescence emission spectra excited at 393 nm of these particles. i.e. upon selective
3
+
excitation of Eu (cf. SI S6), show an increase of luminescence intensity and lifetimes for particles (8) and
10) compared to (7), and a decrease for particle (9). This increase for particle (8) and (10) is due to the
fact that a Tb containing SrF matrix in the core or the shell has a lower phonon energy than pure SrF
366 cm ) due to the high mass of Tb, and hence, non-radiative relaxation is further decreased. Particles
9) exhibit the previously discussed increase of non-radiative relaxation of Eu3+ in two different spatial
directions (cf. Figure 6).
(
2
2
-
1 52
(
(
ET
(
8)
9)
621
584
542
490
693
648
615
590
579
(
3+
3+
ET Tb ꢀEu
Tb3+
Eu3+
non-rad. relaxation
3
+
3+
Figure 8. Left: Schematic representation of the energy transfer Tb →Eu . Right: Different relaxation processes
between core and shell in particles (8) and (9)
.

Journal of Materials Chemistry C
Page 8 of 11
View Article Online
DOI: 10.1039/C7TC01599B
An exciting application of these particles can be seen as filler for transparent luminescent composite
materials. Transparent PMMA containing Eu and Tb doped SrF -nanoparticles (5) and (6) at the same
time show different emission colours depending on the excitation wavelength, in line with the selective
2
3
+
3+
excitation of Tb or Eu (Figure 9). Photon upconversion upon excitation at 980 nm is also possible using
3
+
3+
2 2 2
Yb and Er co-doped SrF particles, SrF :Yb10,Er2@3xSrF .
Figure 9. (A) Photographs of 10 g of core-shell particles. Transparent polymer composite of (B) mixture (50:50)
SrF :Eu10@3×SrF + SrF :Tb10@3×SrF (5 m-% PMMA, left: at room light, right: excited at 393 nm and 350 nm) and
C) SrF :Yb10,Er2@3×SrF (2.5 m-% PMMA, left: at room light, right: excited via 980 nm laser, cw 500 mW), (D) PL
2
2
2
2
(
2
2
emission spectrum of C.
Conclusion
It is demonstrated that the fluorolytic sol-gel synthesis in ethylene glycol is a fast and efficient approach
for the synthesis of core-shell structured metal fluoride nanoparticles, which can be easily upscaled to
litre amounts, i.e. up to a few kilograms amount of nanoparticles. In addition we demonstrate the possible
energy transfers between core and shell, and illustrate strategies to reduce or increase energy transfers,
and to reach long transition lifetimes for efficient luminescence of nanoparticles. Energy transfer from
core to shell is more effective than shell to core.
Acknowledgement
This work was financed by the graduate school GRK 1582 “Fluorine as a Key Element” by the Deutsche
Forschungsgemeinschaft (DFG). We thank Dr. Knut Rurack and his group of BAM's Division 1.9 for
support with the quantum yield determinations.
References
1.
2.
3.
4.
5.
H. Zhu, X. Chen, L. M. Jin, Q. J. Wang, F. Wang and S. F. Yu, ACS Nano, 2013, 7,
1420-11426.
Y. K. Voronko, A. A. Sobol, A. Y. Karasik, N. A. Eskov, P. A. Rabochkina and S. N.
Ushakov, Opt. Mater., 2002, 20, 197-209.
H.-Q. Wang, M. Batentschuk, A. Osvet, L. Pinna and C. J. Brabec, Adv. Mater., 2011,
1
23, 2675-2680.
Z. Q. Li, X. D. Li, Q. Q. Liu, X. H. Chen, Z. Sun, C. Liu, X. J. Ye and S. M. Huang,
Nanotechnology, 2012, 23, 025402.
A. D. Ostrowski, E. M. Chan, D. J. Gargas, E. M. Katz, G. Han, P. J. Schuck, D. J.
Milliron and B. E. Cohen, ACS Nano, 2012, 6, 2686-2692.
6
7
.
.
K. Korthout, P. F. Smet and D. Poelman, Appl. Phys. Lett., 2011, 98, 261919.
Y.-F. Wang, G.-Y. Liu, L.-D. Sun, J.-W. Xiao, J.-C. Zhou and C.-H. Yan, ACS Nano, 2013,
7
, 7200-7206.

Page 9 of 11
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC01599B
8
.
.
G. Chen, J. Shen, T. Y. Ohulchanskyy, N. J. Patel, A. Kutikov, Z. Li, J. Song, R. K.
Pandey, H. Ågren, P. N. Prasad and G. Han, ACS Nano, 2012, , 8280-8287.
Z. Zhao, Y. Han, C. Lin, D. Hu, F. Wang, X. Chen, Z. Chen and N. Zheng, Chem. Asian
J., 2012, , 830-837.
6
9
7
1
1
0.
1.
T. R. Hinklin, S. C. Rand and R. M. Laine, Adv. Mater., 2008, 20, 1270-1273.
N.-N. Dong, M. Pedroni, F. Piccinelli, G. Conti, A. Sbarbati, J. E. Ramírez-Hernández,
L. M. Maestro, M. C. Iglesias-de la Cruz, F. Sanz-Rodriguez, A. Juarranz, F. Chen, F.
Vetrone, J. A. Capobianco, J. G. Solé, M. Bettinelli, D. Jaque and A. Speghini, ACS
Nano, 2011,
L. Prodi, E. Rampazzo, F. Rastrelli, A. Speghini and N. Zaccheroni, Chem. Soc. Rev.
015, 44, 4922-4952.
S. Sasidharan, A. Jayasree, S. Fazal, M. Koyakutty, S. V. Nair and D. Menon,
Biomater. Sci. UK, 2013, , 294-305.
M. Haase and H. Schäfer, Angew. Chem. Int. Ed., 2011, 50, 5808-5829.
F. Wang and X. Liu, Chem. Soc. Rev., 2009, 38, 976-989.
5, 8665-8671.
1
2.
3.
,
2
1
1
14.
15.
16.
17.
G.-S. Yi and G.-M. Chow, Chem. Mater., 2007, 19, 341-343.
J.-C. Boyer, J. Gagnon, L. A. Cuccia and J. A. Capobianco, Chem. Mater., 2007, 19
,
3358-3360.
18.
F. Vetrone, R. Naccache, V. Mahalingam, C. G. Morgan and J. A. Capobianco, Adv.
Funct. Mater., 2009, 19, 2924-2929.
1
2
9.
0.
S. Y. Kim, Y.-H. Won and H. S. Jang, Sci. Rep. UK, 2015,
F. Wang, R. Deng, J. Wang, Q. Wang, Y. Han, H. Zhu, X. Chen and X. Liu, Nat. Mater.
011, 10, 968-973.
B. Zhou, L. Tao, Y. Chai, S. P. Lau, Q. Zhang and Y. H. Tsang, Angew. Chem. Int. Ed.
016, 55, 12356-12360.
5, 7866.
,
,
2
21.
2
2
2
2
2
2
2.
3.
4.
5.
6.
X. Huang, Dyes Pigment., 2016, 130, 99-105.
M. Ding, Y. Li, D. Chen, H. Lu, J. Xi and Z. Ji, J. Alloys Compd., 2016, 658, 952-960.
J. Zhou, Q. Liu, W. Feng, Y. Sun and F. Li, Chem. Rev., 2015, 115, 395-465.
S. Gai, C. Li, P. Yang and J. Lin, Chem. Rev., 2014, 114, 2343-2389.
X. Xie, N. Gao, R. Deng, Q. Sun, Q.-H. Xu and X. Liu, J. Am. Chem. Soc., 2013, 135
2608-12611.
N. Kumam, N. P. Singh, L. P. Singh and S. K. Srivastava, Nanoscale Res. Lett., 2015,
, 347.
,
1
27.
28.
29.
10
X. Wu, Y. Zhang, K. Takle, O. Bilsel, Z. Li, H. Lee, Z. Zhang, D. Li, W. Fan, C. Duan, E.
M. Chan, C. Lois, Y. Xiang and G. Han, ACS Nano, 2016, 10, 1060-1066.
W. Zheng, S. Zhou, Z. Chen, P. Hu, Y. Liu, D. Tu, H. Zhu, R. Li, M. Huang and X. Chen,
Angew. Chem., 2013, 125, 6803-6808.
3
3
0.
1.
E. Kemnitz and J. Noack, Dalton Trans., 2015, 44, 19411-19431.
T. Krahl, Bro, K. Scheurell, B. Lintner and E. Kemnitz, J. Mater. Chem. C, 2016,
4,
1
454-1466.
32.
B. Ritter, P. Haida, F. Fink, T. Krahl, K. Gawlitza, K. Rurack and E. Kemnitz, Dalton
Trans., 2017, 46, 2925-2936.
33.
34.
35.
36.
37.
W. Yu, X. Wang, N. Chen, G. Du and W. Gui, CrystEngComm, 2014, 16, 3214-3221.
Y. Liu, K. Ai, J. Liu, Q. Yuan, Y. He and L. Lu, Adv. Healthc. Mater., 2012,
C. Feldmann, Adv. Funct. Mater., 2003, 13, 101-107.
1, 461-466.
C. Feldmann, M. Roming and K. Trampert, Small, 2006, 2, 1248-1250.
F. Waltz, M. A. Swider, P. Hoyer, T. Hassel, M. Erne, K. Möhwald, M. Adlung, A.
Feldhoff, C. Wickleder, F.-W. Bach and P. Behrens, J. Mater. Sci., 2012, 47, 176-
183.

Journal of Materials Chemistry C
Page 10 of 11
View Article Online
DOI: 10.1039/C7TC01599B
3
8.
9.
M. Back, R. Marin, M. Franceschin, N. Sfar Hancha, F. Enrichi, E. Trave and S.
Polizzi, J. Mater. Chem. C, 2016, , 1906-1913.
M. Ding, H. Zhang, D. Chen, Q. H. Junhua Xi and Z. Ji, J. Alloys Compd., 2016, 672
17-124.
S. Y. Kim, Y.-H. Won and H. S. Jang, Sci. Rep. UK, 2015,
C. Caron, D. Boudreau and A. M. Ritcey, J. Mater. Chem. C, 2015,
4
3
,
1
40.
41.
42.
5, 7866.
3
, 9955-9963.
A. Podhorodecki, M. Banski, J. Misiewicz, M. Afzaal, P. O'Brien, D. Cha and X. Wang,
J. Mater. Chem., 2012, 22, 5356-5361.
4
3.
4.
B. Ritter, P. Haida, F. Fink, T. Krahl, K. Gawlitza, K. Rurack, G. Scholz and E.
Kemnitz, Dalton Trans., 2017, 46, 2925-2936.
D. Giaume, V. Buissette, K. Lahlil, T. Gacoin, J. P. Boilot, D. Casanova, E.
4
Beaurepaire, M. P. Sauviat and A. Alexandrou, Prog. Solid State Chem., 2005, 33
9-106.
B. Mutelet, P. Perriat, G. Ledoux, D. Amans, F. Lux, O. Tillement, C. Billotey, M.
,
9
45.
Janier, C. Villiers, R. Bazzi, S. Roux, G. Lu, Q. Gong and M. Martini, J. Appl. Phys.
011, 110, 094317.
R. E. Kellogg and R. G. Bennett, J. Chem. Phys., 1964, 41, 3042-3045.
K. Rurack and M. Spieles, Anal. Chem., 2011, 83, 1232-1242.
,
2
46.
47.
48.
M. Heise, G. Scholz, A. Düvel, P. Heitjans and E. Kemnitz, Solid State Sci., 2016, 60
,
6
5-74.
K. A. Abel, J.-C. Boyer and F. C. J. M. v. Veggel, J. Am. Chem. Soc., 2009, 131, 14644-
4645.
K. A. Abel, J.-C. Boyer, C. M. Andrei and F. C. J. M. van Veggel, J. Phys. Chem. Lett.
011, , 185-189.
B. Ritter, T. Krahl, K. Rurack and E. Kemnitz, J. Mater. Chem. C, 2014,
613.
I. Richman, J. Chem. Phys., 1964, 41, 2836-2837.
49.
50.
51.
52.
1
,
2
2
2
, 8607-
8

Page 11 of 11
Journal of Materials Chemistry C
(7)
(8)
(9)
(10)
(
(8)
7)
(A)
Tb 542 nm
Eu 698 nm
ex 350 nm
(9)
(10)
(7) (8) (9) (10)
/
(
Tb
SrF
2
SrF
2
:Eu10
Eu
SrF
2
:Tb10
SrF :Eu5,Tb5
2
450 500 550 600 650 700 750
Wavelength / nm