Formation Mechanism, Structural, and Upconversion Properties of Alkaline Rare-Earth Fluoride Nanocrystals Doped with Yb3+/Er3+ Ions

Article abstract of DOI:10.1021/acs.inorgchem.8b00484

Ultrasmall (9-30 nm) Yb³⁺/Er³⁺-doped, upconverting alkaline rare-earth fluorides that are promising for future applications were synthesized by the microwave-assisted hydrothermal method. The formation mechanism was proposed, indicating the influence of the stability of metal ions complexes with ethylenediaminetetraacetic acid on the composition of the product and tendency to form M₂REF₇ (M_0.67RE_0.33F_2.33) cubic compounds in the M-RE-F systems. Their physicochemical properties (structure, morphology, and spectroscopic properties) are compared and discussed. The obtained nanoparticles exhibited emission of light in the visible spectra under excitation by 976 nm laser radiation. Excitation and emission spectra, luminescence decays, laser energy dependencies, and upconversion quantum yields were measured to determine the spectroscopic properties of prepared materials. The Yb³⁺/Er³⁺ pair of ions used as dopants was responsible for an intense yellowish-green emission. The upconversion quantum yields determined for the first time for M₂REF₇-based materials were 0.0192 ± 0.001% and 0.0176 ± 0.001% for Sr₂LuF₇:Yb³⁺,Er³⁺ and Ba₂LuF₇:Yb³⁺,Er³⁺ respectively, the two best emitting samples. These results indicated the prepared materials are good and promising alternatives for the most studied NaYF₄:Yb³⁺,Er³⁺ nanoparticles.

Full text of DOI:10.1021/acs.inorgchem.8b00484

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Formation Mechanism, Structural, and Upconversion Properties of
Alkaline Rare-Earth Fluoride Nanocrystals Doped With Yb3+/Er3+ Ions
Tomasz Grzyb* and Dominika Przybylska
Department of Rare Earths, Faculty of Chemistry, Adam Mickiewicz University in Poznan, Umultowska 89b, 61-614 Poznan, Poland
S
* Supporting Information
ABSTRACT: Ultrasmall (9−30 nm) Yb3+/Er3+-doped, upconverting alkaline
rare-earth fluorides that are promising for future applications were synthesized by
the microwave-assisted hydrothermal method. The formation mechanism was
proposed, indicating the influence of the stability of metal ions complexes with
ethylenediaminetetraacetic acid on the composition of the product and tendency
to form M2REF7 (M0.67RE0.33F2.33) cubic compounds in the M−RE−F systems.
Their physicochemical properties (structure, morphology, and spectroscopic
properties) are compared and discussed. The obtained nanoparticles exhibited
emission of light in the visible spectra under excitation by 976 nm laser radiation.
Excitation and emission spectra, luminescence decays, laser energy dependencies,
and upconversion quantum yields were measured to determine the spectroscopic
properties of prepared materials. The Yb3+/Er3+ pair of ions used as dopants was
responsible for an intense yellowish-green emission. The upconversion quantum
yields determined for the first time for M2REF7-based materials were 0.0192
0.001% and 0.0176
0.001% for
Sr2LuF7:Yb3+,Er3+ and Ba2LuF7:Yb3+,Er3+ respectively, the two best emitting samples. These results indicated the prepared
materials are good and promising alternatives for the most studied NaYF4:Yb3+,Er3+ nanoparticles.
high photochemical stability and resistance to photobleaching.
An additional advantage of inorganic NPs is also the fact that
their surface can be very easily modified, e.g., by using silica,
and further biologically functionalized.28−32
INTRODUCTION
■
In recent years, the properties of upconverting nanoparticles
(UCNPs) have been investigated very intensively because of
their potential applications. Particularly interesting are UCNPs
based on lanthanide ions (Ln3+).1−3 Knowledge of the
mechanisms responsible for the efficient upconversion process
is increasing dynamically.4−6 Furthermore, methods of nano-
particles (NPs) production are continuously developing as the
demand for smaller and more advanced NPs is growing.7,8
Ln3+ ions feature narrow-band absorption and emission that
are characteristic of these emitter ions, a large number of
excited states, long luminescence lifetimes, large Stokes shift
and upconversion (UC) phenomenon. The combination of
these properties allows use of the materials doped with Ln3+
ions in solar cells, lasers and fiber amplifiers, persistent
phosphors, and temperature/pressure sensors.9−16 Inorganic
nanomaterials doped with Ln3+ ions, excited under near-
infrared radiation (NIR), can be applied in bioimaging, cancer
treatment, or drug delivery.17−20 The use of UCNPs for cell
imaging ensures low autofluorescence and reduction of light
scattering generated by tissue.21−24 Moreover, UCNPs are
known to cause less photodamage to cells and usually have low
cytotoxicity.25−27
The most popular and commonly used inorganic host
materials for UC are fluorides. They have low phonon energy,
which reduces the possibility of the occurrence of nonradiative
relaxation, and high chemical stability. Nanomaterials based on
fluorides are characterized by a relatively high quantum yield
(QY) of luminescence.33−36 Nanocrystalline fluorides can be
synthesized by several methods, of which the most common is
the decomposition of product precursors (e.g., metal
trifluoroacetates) in high-boiling solvents, such as oleic acid
and octadecene.7,37,38 Other methods are based on the
precipitation reaction, also carried out under solvo- or
hydrothermal conditions.39−41
Recently, the NaYF4 matrix has been used most frequently in
research concerning UC systems.42,43 UCNPs based on this
compound are small, with high emission quantum yields (see
Table S2).43−46 However, growing interest in the application of
nanomaterials has created a need for new compounds with
bright emissions and effective methods for their synthesis.
Other fluorides, such as MxREyFz systems (where M = Ca, Sr,
Ba, and RE = Y, La, Gd, Lu) are relatively less studied. Only a
few reports about CaxREyFz and SrxREyFz UCNPs are
available.47−49 Similar compounds, like BaYF5 or BaGdF5,
The morphology and composition of NPs are sometimes
critical and often determine their applications. In nano-
medicine, the important feature is the small average size of
NPs, usually below 50 nm. Small NPs are noninvasive to cells
and may be distributed in organism through blood vessels.25,26
Another fundamental property of the described compounds is
Received: February 22, 2018
© XXXX American Chemical Society
A
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and ethanol several times. The final product was dried at 80 °C for 80
h.
have more often been the subject of studies revealing promising
upconversion properties.50,51 The current state of knowledge
about M2REF7-type compounds indicates that several of the
UC materials presented in this article, such as Ca2YF7,
Ca2GdF7, and Ba2LuF7, have not been as yet subjects of
literature reports. Other ones, such as Sr2LuF7, Ba2YF7, and
Ba2GdF7, have been more studied, but the upconversion
quantum yields (UCQY) in these systems have not been
reported to date.52−56
Synthesis of nanocrystalline MxREyFz fluorides is usually not
an easy process because of the diversity of their stoichiometry
and crystal structures.57−59 Very often, the resulting products
have more than one crystal phase. Fedorov et al. analyzed
BaF2−YF3 systems proving a nonequilibrium character of these
phases.60 Therefore, there is not possible to obtain barium−
yttrium fluorides with a specific composition, e.g., BaYF5.
Instead, these systems should be considered as Ba1−xRExF2+x
solid solutions.60 Despite this, there are some articles using the
BaYF5 formula.51,61−63 Although some of the mixed M2+ and
RE3+ fluorides have been investigated in recent years, their UC
properties have not been compared. What is more, there is a
high degree of uncertainty regarding the product composition
in published articles. Therefore, it is important to carefully
examine the properties of obtained MxREF2x+3 compounds and
determine their real stoichiometry. Our studies show that M−
RE−F systems are complex and require deep investigation.
Synthesis of NPs based on the compounds mentioned above
and analysis of their properties can expand knowledge of these
and similar systems, and also enlarge the base of inorganic
nanomaterials for UC applications.
Characterization. X-ray diffractograms (XRD) were recorded on a
Bruker AXS D8 Advance diffractometer in the Bragg−Brentano
geometry, with Cu Kα1 radiation, in the 2θ range from 6 to 60°. The
reference data were taken from The International Centre for
Diffraction Data (ICDD) database. Crystal cell volumes were
calculated using the Maud 2.33 software.64 The composition of the
prepared materials was analyzed by inductively coupled plasma-optical
emission spectrometer (Varian ICP-OES VISTA-MPX). Trans-
mission-electron-microscopy (TEM) images were recorded on an
FEI Tecnai G2 20 X-TWIN transmission electron microscope using an
accelerating voltage of 200 kV.
The luminescence characteristics (excitation, emission spectra,
luminescence decays, and power dependencies) of the prepared
samples were measured on a Photon Technology International
QuantaMaster 40 spectrofluorometer equipped with an Opotek Inc.
Opolette 355LD UVDM tunable laser, with a repetition rate of 20 Hz
and pulse length of 7 ns; a Hamamatsu R928 photomultiplier was used
as a detector and a Thorlabs FEL900 optical filter was used.
The absolute upconversion quantum yield (UCQY) measurements
usually involve the use of integrating sphere.43,44,65−67 In our studies a
barium sulfate-coated integrating sphere (80 mm diameter) from
Photon Technology International was employed. The integrating
sphere was mounted in a QuantaMaster 40 spectrofluorometer with
the entry and output ports located at 90° to each other. A continuous
Dragon Lasers DPSS 980 nm laser was used as the excitation source,
coupled to a 200 μm optical fiber and collimator. A S305C Compact
Thermal Power Sensor from Thorlabs was used to measure the power
of the laser beam. As a detector, a Princeton Instruments PIXIS:256E
Digital CCD Camera, equipped with an SP-2156 Imaging Spectro-
graph was applied, corrected for the instrumental response. All of the
powder samples were held in a white Teflon cuvette with a quartz
window, located in the center of the integrating sphere. Two filters,
Thorlabs NENIR30B and NENIR40B, were used in order to measure
the spectrum in the range covering the emission and excitation
wavelengths (500−1050 nm) and to avoid saturation of the CCD
camera. Measured spectra were corrected for the filters before the final
calculations. For more details see Supporting Information.
EXPERIMENTAL SECTION
■
Materials. Rare earth oxides: Y2O3, La2O3, Gd2O3, Er2O3, Yb2O3,
and Lu2O3 (99.99%, Stanford Materials, United States) were used as
the source of RE3+ (and Ln3+) ions. To obtain rare earth chloride
solutions at a concentration of 1 or 0.5 M, the oxides were dissolved
separately in hydrochloric acid (ultrapure, Sigma-Aldrich, 37%,
Poland). Ammonium fluoride (98+%, Sigma-Aldrich, Poland) was
used as the source of fluoride ions. Barium chloride dehydrate (Sigma-
Aldrich, 99+%, Poland), calcium chloride dihydrate (Sigma-Aldrich,
99+%, Poland), strontium chloride hexahydrate (Sigma-Aldrich, 99+%,
Poland), and ethylenediaminetetraacetic acid (EDTA) diammonium
salt hydrate (Sigma-Aldrich, 97%, Poland) were used as received
without further purification.
Pictures of the samples were taken using a Canon EOS 550D
camera under excitation by Opolette 355LD UVDM (∼10 mJ·cm−2)
or a Dragon Lasers DPSS 980 nm laser (∼20 W·cm−2) and with
Thorlabs FEL900 and FGS900-A optical filters (exposure time 2.5 s, f/
4, ISO-1600).
RESULTS AND DISCUSSION
■
Structure and Morphology. The MxREyFz fluorides are
known to crystallize as cubic, orthorhombic, hexagonal, or
monoclinic structures depending on the composition.57,59,68−70
The synthesized compounds crystallized as disordered, fluorite-
type, nonstoichiometric structures with a M0.67RE0.33F2.33
Synthesis. Nanoparticles showing UC based on M2REF7 matrices
(where M = Ca, Sr, Ba, and RE = Y, La, Gd, Lu) doped with Yb3+ and
Er3+ ions were synthesized using the microwave-assisted hydrothermal
method (Magnum II, Ertec). To obtain M2REF7:Yb3+,Er3+ (theoretical
concentrations were set to 18% and 2% mol), 6 mmol of EDTA
diammonium salt hydrate was dissolved in 20 mL of water, in a beaker
which was placed on a magnetic stirrer. This substance was used to
prevent precipitation of the product before applying the hydrothermal
conditions. Then, an aqueous solution of MCl2 (containing 3 mmol of
reactant) mixed earlier in a beaker with RECl3 and LnCl3 (Ln = Yb, Er;
total amount 3 mmol) with a specified concentration and
stoichiometric ratio (1:1 metal ions) was added to dissolved EDTA.
NH3(aq) (30%) was added dropwise upon vigorous stirring to obtain a
transparent solution until pH = 6 was reached. NH4F was used in the
synthesis as a source of fluoride ions: metal and fluoride ions were
mixed in a 1:5 ratio. During preparation of the solution, the pH was
monitored and maintained at 6 using NH3(aq). The transparent
solution with total volume of 70 mL was transferred into a Teflon-
lined autoclave (100 mL) and treated under hydrothermal conditions
for 4 h at 180 °C and 40 bar, with microwave heating. Afterward, the
obtained white precipitate was purified by centrifugation with water
composition per unit cell and Fm3m cubic space group.57,71
̅
According to Sorokin et al., these type of compounds crystallize
as solid solutions which remain metastable at low temper-
ature.71 The article uses the M2REF7 formula containing the
integer number of ions. Upon annealing, the fluorite structure
may order with formation of tetragonal M2REF7 phase.71 The
diffraction peaks of tetragonal phase are similar to those of the
pure cubic structure, Fm3m; however, according to literature,
̅
the compounds obtained by solid-state reaction may show very
weak additional superstructure reflections.72 Also splitting of
the main peaks of the fluorite-type lattice phase should be
observed.71 These minor peaks as well as splitting of XRD
peaks were not observed in the materials prepared, which
confirms the cubic structure. However, there are some articles
incorrectly reporting M2REF7-type compounds as tetragonal
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with similar XRD patterns to those presented in this
article.52,53,56,73
The crystal phases of all samples have been identified, and
the results are presented in Figure 1. The diffraction peaks are
compound, which resulted in a shift of the XRD peaks. Hence,
when the host compound consists of Ba2+ (r = 1.35 Å), the
calculated crystal cell volumes are the largest, and the shifts of
the XRD peaks with respect those in the reference pattern are
the highest. Analogously, the materials including Ca2+ ions with
the ionic radius smaller than that of the M2+ metal ions used (r
= 1.00 Å) had the smallest cell volumes from all products
obtained. In addition, differences between ionic radii of RE3+
ions caused similar changes, although to a smaller degree as the
differences were also lower: Y3+ (r = 0.9 Å), La3+ (r = 1.03 Å),
Gd3+ (r = 0.94 Å), and Lu3+ (r = 0.86) (ionic radius for the
coordination number, CN = 6). The size of nanocrystals,
calculated from the Scherrer equation, was independent of the
crystal cell volume.
ICP-OES analysis was performed to confirm the obtained
stoichiometry, and the results are presented in Table 1. Figure 1
presents the XRD patterns of the obtained compounds along
with sample labels according to their elemental composition.
On the basis of the elemental analysis, three different types of
structures were obtained: MREF5 (SrLaF5), M2REF7 (Ca2YF7,
Sr2YF7, Ba2YF7, Ba2LaF7, Ca2GdF7, Sr2GdF7, Ba2GdF7, Sr2LuF7,
Ba2LuF7, and M1−xRExF2+x for a structure containing Ca2+ and
Lu3+ ions with a very low content of lanthanide ions.
The complexity of the M−RE−F systems results from the
high capacity of the CaF2 fluorite-type structure for ion
exchange. The M2+ ions can be easily replaced by RE3+ ions,
which additionally are similar in ionic radius to those they
replace. The charge imbalance created after replacement can be
compensated by highly mobile F− ions. Hence, from the ideal
fluorite-type CaF2 structure, other structures can be delivered,
such as tetragonal-type distortion.74 However, in nanocrystal-
line materials in which a high proportion of ions are localized
Figure 1. XRD patterns of the samples synthesized by hydrothermal
method and the reference from the ICSD database (No. 190727).
broad, which indicates the small size of the nanocrystals (Table
1). The obtained patterns were compared to the standard data
for cubic Sr0.68Pr0.32F2.32 (ICSD No. 190727). The comparison
confirmed the single-phase structure of all the compounds.
Depending on the ions forming the material, crystal cell volume
(Table 1) changed in comparison to that for the reference
Table 1. Results of ICP-OES Analysis, the Stoichiometric Ratio between Alkaline Metal (Ca, Sr, Ba) and Rare Earth Ions,
Crystal Cell Volumes, and Nanocrystal Sizes Calculated Using the Scherrer Equation
metal ion
M2+ RE3+
concentration (% mol)
M2+
RE3+
Yb3+
Er3+
M2+:RE3+ ratio
sample name
crystal cell volume (Å3) nanocrystal size (nm)
Ca2+
Sr2+
Ba2+
70.0
74.2
66.6
64.1
66.4
69.3
30.0
24.6
33.4
33.6
33.6
28.9
2:0.9
2:0.7
2:1.0
2:1.1
2:1.0
2:0.9
Ca2YF7
170.38
169.47
187.40
186.65
210.78
211.15
16.4 1.6
13.7 1.6
16.1 0.6
16.3 0.8
19.7 1.7
19.7 1.8
1.0
2.0
1.6
0.2
0.3
0.3
Ca2YF7:3.9%Yb3+,0.6%Er3+
Y3+
Sr2YF7
Sr2YF7:5.6%Yb3+,0.9%Er3+
Ba2YF7
Ba2YF7:5.1%Yb3+,0.9%Er3+
Ca2+
Sr2+
52.6
55.3
67.1
63.3
67.0
73.3
71.7
60.4
74.0
72.6
89.6
86.4
72.3
67.8
66.4
66.1
47.4
44.2
32.9
36.2
33.0
25.6
28.3
37.1
26.0
25.1
10.4
9.6
2:1.8
2:1.6
2:1.0
2:1.2
2:1.0
2:0.7
2:0.8
2:1.3
2:0.7
2:0.8
2:0.2
2:0.3
2:0.8
2:1.0
2:1.0
2:1.0
SrLaF5
202.35
200.40
221.78
223.21
172.87
171.90
189.90
190.90
213.72
214.77
164.57
164.96
185.78
184.17
210.16
208.77
196.53
13.2 1.2
10.7 1.0
9.4 0.6
La3+
0.4
0.4
1.0
2.2
2.1
3.5
6.2
7.7
0.1
0.1
0.2
0.3
0.3
0.5
1.0
1.1
SrLaF5:0.8%Yb3+,0.2%Er3+
Ba2+
Ca2+
Sr2+
Ba2LaF7
Ba2LaF7:1.1%Yb3+,0.2%Er3+
Ca2GdF7
10.3 0.9
20.8 4.4
15.5 1.1
20.8 6.0
18.0 2.9
14.7 1.7
15.4 1.9
20.0 2.2
14.6 1.7
12.9 1.6
20.5 1.9
10.6 2.6
9.8 2.6
Ca2GdF7:3.7%Yb3+,0.7%Er3+
Gd3+
Sr2GdF7
Sr2GdF7:5.5%Yb3+,0.9%Er3+
Ba2GdF7
Ba2+
Ca2+
Sr2+
Ba2GdF7:7.5%Yb3+,1.1%Er3+
Ca0.9Lu0.1F2
Ca0.9Lu0.1F2:3.5%Yb3+,0.5%Er3+
Sr2LuF7
Lu3+
27.7
25.1
33.6
25.1
Sr2LuF7:19.3%Yb3+,3.0%Er3+
Ba2+
Ba2LuF7
Ba2LuF7:22.8%Yb3+,3.1%Er3+
Sr0.68Pr0.32F2.32
ICSD No. 190727
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on the surface (∼20% in nanocrystals with size around 8 nm),75
where the crystal structure is highly distorted and XRD peaks
are wide, the boundary between cubic and tetragonal structure
may be not clear.
ions) and why the fraction of precipitated Er3+ ions is always
larger than that of Yb3+ ions (Table 1). The dopant ions
precipitate, the last one during the reaction, first Er3+ and next
Yb3+ ions, except for syntheses of NPs containing Lu3+ ions.
This also explains the difference between Sr2LuF7- or Ba2LuF7-
based materials and the rest of products.
In the group of synthesized materials, three exceptions were
observed: in Ca2+- and La3+-containing mixtures, the product
was not formed in the applied conditions; Sr2+ and La3+ ions
precipitated as a SrLaF5 compound; Ca2+ and Lu3+ formed a
Ca0.9Lu0.1F2 compound. The explanation of these results is
unclear. It is worth noting that the difference between the ionic
radii of Ca2+ and La3+ is the smallest of the all used metal ions
as well as the difference in stability constant of EDTA
complexes. In addition, the differences in the size of M2+ and
RE3+ ions in the remaining two cases are one of the smallest.
This may be associated with the tendency to form the structure
closest to cubic CaF2.
The results of ICP-OES analysis typically indicated different
composition of products and lower dopant concentrations than
expected from stoichiometric calculations. Most of the products
were obtained with M2+ to RE3+ molar ratios of around 2:1,
despite the fact that MCl2 and RECl3 chlorides were added to
the solution at the 1:1 molar ratio. Also the ratios of RE3+ ions
forming host (Y3+, La3+, Gd3+, or Lu3+) to those used as
dopants (Yb3+ and Er3+) was not as expected from the
concentrations of the reagents. The data presented in Table 1
show that the highest percentage amount of Yb3+ and Er3+ ions
was found in the Ba2LuF7-based sample (22.8% and 3.04%,
respectively). The lowest efficiency of doping was observed for
SrLaF7:Yb3+,Er3+ sample in which the dopant concentrations
were 0.8% of Yb3+ and 0.2% of Er3+ ions.
As the reaction conditions and reagent compositions were
the same for each synthesis, the final product stoichiometry and
composition were determined by other factors. The differences
in ionic radii between metal ions,76 solubilities of metal
fluorides,77,78 and, the most important factor, stability of the
complexes with EDTA are known to influence the properties of
the materials obtained (see Table S1 in Supporting Information
for appropriate data).79 The proposed product formation
mechanism is presented in Figure 2. Alkaline metals, M2+, form
In our opinion, the information mentioned above is critical in
designing the properties of fluoride nanocrystals and planning
their syntheses. A similar mechanism can be extended to
systems containing other chelating agents, such as citric acid or
polyethylene glycol.
Morphology of the products was determined by analyzing
XRD patterns and TEM images. The characteristics of the XRD
patterns indicate the nanocrystallinity of the products. Average
nanocrystal sizes, calculated using the Scherrer equation,81 vary
in the range 9−21 nm and are displayed in Table 1. The TEM
images together with size distribution histograms are presented
in Figure 3. They show a very similar, quasi-spherical shape of
the nanocrystals as well as their tendency to agglomerate.
However, some single nanocrystals can still be observed,
proving that they are not strongly connected. The
Ba2LaF7:Yb3+,Er3+ sample (Figure 3f) presented the smallest
average particle size, estimated at approximately 9 nm, but the
nanocrystal sizes of the other products were also small. The
sample was composed of Ba2LaF7 nanocrystals making small
clusters of several nanocrystals. Such behavior was not observed
in other materials. The determined nanocrystal dimensions are
comparable with those calculated from the Scherrer equation
except for Ba2YF7:Yb3+,Er3+. All nanocrystals exhibited a high
degree of crystallinity.
Figure 2. Proposition of mechanism of nanocrystal growth in
hydrothermal conditions.
Spectroscopic Properties. Upconversion properties were
studied under excitation by a pulsed laser (λex = 976 nm).
Emission and excitation spectra are presented in Figure 4.
Luminescence decays were also recorded for each transition
band as well as the dependencies of the emission intensity on
laser energy were obtained. The calculated emission lifetimes
and the number of photons involved in the upconversion
process are presented in Table 2. Experimental data
(luminescence decays and dependencies of emission intensities
on laser energy) are presented in the Supporting Information
(Figures S3 and S4).
complexes with EDTA that are less stable than the complexes
of RE3+ ions. An equilibrium between [M-EDTA]2+, [RE-
EDTA]+, M2+, RE3+, and EDTA4+ individuals is established
after mixing all of the reagents. The RE3+ ions are in larger
fraction complexed in [RE-EDTA]+ than M2+ in [M-EDTA]2+;
therefore, the ratio between these ions is higher than expected.
Additionally, after the reaction starts, [M-EDTA]2+ complexes
first dissociate (Step 2 in the mechanism) as they are less stable
and as the result of EDTA decomposition.80 Afterward, [RE-
EDTA]+ begins to dissociate, which explains why the
concentration of RE3+ ions in the prepared materials is much
lower than expected from the amounts of reagent salts added.
After these two stages, the equilibrium between M2+, RE3+, and
their complexes is established, and precipitation of the products
begins (Step 4). The final step is nucleation followed by growth
of nanoparticles (Step 5). This simplified mechanism is the
only proposition not verified experimentally.
Figure 4 shows that the spectra of all samples show similar
emission and excitation peaks with quite different intensities.
The highest emission intensities were recorded for
Sr2LuF7:Yb3+,Er3+ and Ba2LuF7:Yb3+,Er3+ samples. Also the
emission intensity of Ba2GdF7:Yb3+,Er3+ was relatively high.
The other materials showed low emission or their emission was
even almost quenched, like, e.g., for Ca2YF7:Yb3+,Er3+.
The excitation spectra observed in Figure 4 show a narrow
The proposed mechanism explains why most of the
synthesized materials contained lower concentrations of Yb3+
and Er3+ dopants than expected (20% of Yb3+ and 2% of Er3+
2
band with the maximum at 976 nm, characteristic of the F7/2
2
→ F5/2 transition of Yb3+ ions. These ions participate in the
D
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energy transfer process (as sensitizers) to Er3+ ions, which
results in photon upconversion.
The intensity of UC is connected with the concentration of
Yb3+ ions. The samples in which the Yb3+ ion concentration is
high also show intense emission, which may be expected as
Yb3+ ions absorb exciting radiation. Analysis of the sample
composition is crucial for the appropriate interpretation of
spectroscopic results. The presented results indicate that
preparation of a series of samples, keeping the same conditions
in the synthesis, does not necessarily lead to the desired
composition of each product.
4
4
In the emission spectra, the S3/2 → I15/2 transition (with a
maximum at 536 nm) was observed as the most intense for all
samples. Therefore, the color of the samples luminescence was
green. The ratio between the intensities of transitions for the
obtained nanopowders was very similar (see Figure S2). Four
2
4
characteristic transitions of Er3+ ions: H9/2 → I15/2 (∼406
2
4
4
4
nm), H11/2 → I15/2 (∼520 nm), S3/2 → I15/2 (∼540 nm),
4
4
and F9/2 → I15/2 (∼650 nm) were recorded only for some of
the materials obtained: Sr2LuF7:Yb3+,Er3+, Ba2LuF7:Yb3+,Er3+,
Ba2 GdF7 :Yb3 + ,Er3 +
,
Ca0 . 9 Lu0 . 1 F2 :Yb3 + ,Er3 +
,
and
Ca2GdF7:Yb3+,Er3+, which is related to the tendency of the
2H9/2 4I15/2 level to be quenched by a cross-relaxation
→
process, or a noneffective, multiphonon de-excitation process.82
The upconversion quantum yields determined for the best
emitting samples are presented in Figure 5. The calculated
values were the highest for Sr2LuF7:Yb3+,Er3+ and
Ba2LuF7:Yb3+,Er3+ the two best emitting samples (see also
Table S2 and Figure 4). In these samples a saturation effect was
observed, visible as a plateau-like region at higher power
densities. The observed saturation may be explained by
excitation trapping into intermediate state involved in UC
4
mechanism. For Er3+ it is long-lived (bottleneck) I13/2 excited
state being unavailable for further excitation transfer at higher
power densities.10,83
UCQYs are rarely published owing to the difficulties in their
determination.36,43,66,83−89 The obtained values are usually low,
in comparison to those for downshifting phosphors. The
highest published values do not exceed several percent, and
they were determined only for the core/shell samples, prepared
by the thermal decomposition method.43,89,90 UCQY for a
sample prepared by the hydrothermal method had been
reported previously only once by Balabhadra et al.66
Figure 3. TEM images and nanocrystal size distributions of
hydrothermally synthesized samples: (a) Ca2GdF7:Yb3+,Er3+, (b)
Ca0.9Lu0.1F2:Yb3+,Er3+, (c) Sr2GdF7:Yb3+,Er3+, (d) Sr2LuF7:Yb3+, Er3+,
(e) Ba2YF7:Yb3+,Er3+, (f) Ba2LaF7:Yb3+, Er3+.
The most often studied NaYF4:Yb3+,Er3+ nanoparticles,
which showed one of the brightest emissions, revealed
UCQY from 0.0022% to 0.32% under 980 nm laser irradiation,
depending on the concentration of dopant ions, synthesis
procedure, and particle size.36,44,65,67,90 Comparison of UCQYs
determined for samples doped with Yb3+ and Er3+ presented in
Supporting Information (Table S2) shows that Sr2LuF7 and
Ba2LuF7-based samples are capable of efficient emission, in
some cases with higher quantum yield than for the most
studied and best known NaYF4:Yb3+,Er3+ nanoparticles. It
should be emphasized that the materials prepared by us were
obtained by the hydrothermal method based on water solution,
in contrast to most of the other reports. UCQY values
measured in nonwater solutions are usually higher.46 Also
reported power densities, used for UCQYs measurements, were
usually higher than applied in this article.
Figure 4. Upconversion luminescence (solid line) and excitation
spectra of synthesized materials under pulsed excitation source (at 15
mJ·cm−2).
Because of the nonlinear nature of UC, the QYs of this
process are strongly dependent on the excitation den-
sity.12,66,46,91 Furthermore, properties such as laser beam
shape can also influence UCQYs.43 Therefore, UC properties
E
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Table 2. Emission Lifetimes Calculated on the Basis of Luminescence Decays and the Number of Photons Determined from
a
Dependencies of Luminescence Intensity on Laser Energy
emission lifetimes (μs)
number of photons
4
2
4
4
4
4
4
2
4
2
4
4
4
4
4
sample name
2H9/2 → I15/2 H11/2 → I15/2 S3/2 → I15/2 F9/2 → I15/2 H11/2 → I15/2 H11/2 → I15/2 S3/2 → I15/2 F9/2 → I15/2
Ca2YF7:3.9%Yb3+,0.6%Er3+
Sr2YF7:5.6%Yb3+,0.9%Er3+
Ba2YF7:5.1%Yb3+,0.9%Er3+
SrLaF5:0.8%Yb3+,0.2%Er3+
Ba2LaF7:1.1%Yb3+,0.2%Er3+
Ca2GdF7:3.7%Yb3+,0.7%Er3+
Sr2GdF7:5.5%Yb3+,0.9%Er3+
Ba2GdF7:7.5%Yb3+,1.1%Er3+
7.5
20.1
73.4
22.4
27.7
285.5
18.1
104.1
28.3
8.4
20.3
10.9
65.6
1.6
1.1
1.8
1.2
1.6
1.5
1.1
1.9
1.3
1.6
1.5
2.3
0.75
1.5
1.8
1.5
1.8
1.6
1.2
1.9
74.4
107.7
35.5
336.7
95.2
28.8
144.9
521.2
48.1
1.3
1.6
1.4
1.8
1.4
142.7
271.5
19.9
1.6
80.1
20.4
108.9
27.2
334.0
61.7
1.1
1.5
Ca0.9Lu0.1F2:3.5%Yb3+,0.5%
Er3+
Sr2LuF7:19.3%Yb3+,3.0%Er3+
Ba2LuF7:22.8%Yb3+,3.1%Er3+
65.6
63.5
103.2
91.1
106.9
93.2
225.2
202.4
2.5
1.5
1.8
1.6
1.9
1.7
1.9
a
Experimental data in Supporting Information, Figures S3 and S4.
6. The pulsed excitation results mostly in a two-photon process
(Table 2 and Figure S4), populating green- and red-emitting
Figure 6. Upconversion luminescence images under NIR excitation
(pulsed and continuous lasers).
Er3+ energy levels similarly, whereas continuous laser excitation
may lead to a saturation effect, which was most noticeable for
the green emission band at higher power densities.43
Figure 5. Upconversion quantum yields calculated for the best
emitting samples.
Luminescence decays measured for the obtained samples
were used for calculation of luminescence lifetimes and are
collected in Table 2 (for emission decays see Figure S3). From
among all obtained matrices doped with Yb3+/Er3+ ions, the
longest emission was found for Ca2GdF7 (between 142.7 and
521.2 μs, depending on the transition band). Long lifetimes
were also calculated for Ba2YF7 (74.4−336.7 μs) and Ba2GdF7-
based materials (80.1−334.0 μs). The shortest (7.5−10.9 μs)
emission was recorded for Ca2YF7:Yb3+,Er3+, which may be
connected to the very low content of dopant ions in this matrix.
It is worth mentioning that for samples Sr2LuF7:Yb3+,Er3+ and
Ba2LuF7:Yb3+,Er3+, which exhibited the strongest emission of
light from all samples, the lifetimes were relatively short
(Sr2LuF7:Yb3+,Er3+ τ = 225.25 μs, Ba2LuF7: Yb3+,Er3+ τ =
202.41 μs for the F9/2 → I15/2 transition). The observed
spectroscopic properties lead us to assume that the differences
between samples are not only a result of their composition but
also other factors. The EDTA adsorbed during the synthesis as
well as the inhomogeneous distribution of dopant ions may also
influence the upconversion process and luminescence decays.93
This effect is especially visible in host materials in which
alkaline M2+ ions are replaced by rare earth ions, RE3+,
requiring different charge compensation.
are sensitive to instrumentation, which makes it difficult to
compare properties of the same materials in different setups.90
The determined UCQYs were lower than for the best-known
emitters. However, the emissions QY of Sr2LuF7:Yb3+,Er3+ and
Ba2Lu7:Yb3+,Er3+ NPs are still at a satisfactory level, comparable
with some of the reports for NaREF4-type materials. This
makes the obtained materials look promising for upconversion
applications. Moreover, the M2REF7-type materials seem to be
excellent candidates for synthesis by the thermal decomposition
method.
The small difference in UCQY values between the two the
best emitting materials results from different nanocrystals size,
which are smaller in Ba2Lu7:Yb3+,Er3+ (see Table 1). The
relatively low QY values of the remaining M2+-RE3+ systems
resulted from the low dopant concentrations. Also the synthesis
method used, which requires the presence of water, may be
responsible for low UCQY as the −OH oscillators quench the
higher excited states of Ln3+ ions.92 Other factors affecting
UCQY are nonradiative processes between Ln3+ dopants, such
as cross-relaxation and down-shifting emission of Yb3+ and Er3+
ions (above 1000 nm).90 The dependence of QYs on the type
of host used was similar as that presented in Figure 4.
The upconversion properties of the prepared materials were
highly dependent on the excitation source, as shown in Figure
4
4
For all samples, the longest decay times were measured for
4
4
the F9/2 → I15/2 transition. It may be caused by the necessity
F
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of the cross-relaxation process preceding the feeding of the
4F9/2 excited state or even a different excitation mechanism.6
According to Xiang et al.,94,95 if the decay time of the transition
achieved by three-photon absorption followed by relaxation
from higher excited states.
On the basis of the obtained values of the slopes, which are
greater than 1, it can be concluded that for the other matrices a
two-photon process also occurred to excite Er3+ ions into their
4
4F9/2 → I15/2 is almost twice as long (or even more) than the
4
4
4S3/2 → I15/2 transition, the excitation to the F9/2 level may
4
4
2H11/2, S3/2, and F9/2 levels. For the SrLaF5 sample, only two
4
occur from the I13/2 excited state, which is directly populated
2
4
4
4
by energy transfer from Yb3+ ions: I13/2 (Er3+) + F5/2 (Yb3+)
4
2
transitions were measured: H11/2 → I15/2 and S3/2 → I15/2
,
to F9/2 (Er3+) + F7/2 (Yb3+). The long rise time for samples
that exhibited good emission (Yb3+- and Er3+-doped: Ca2GdF7,
Ca0.9Lu0.1F2, Sr2GdF7, Sr2LuF7, Ba2YF7, Ba2GdF7, Ba2LuF7
hosts) can also be explained by this mechanism. If the
excitation pathway occurs by multiphonon relaxation from
4
2
for which the slope values were 1.22 and 0.75. This confirmed
the occurrence of highly quenching processes in this sample
and explained the very low emission.
Figure 7 shows the UC mechanism occurring in the obtained
alkaline rare-earth fluoride matrices doped with Yb3+ and Er3+
ions. In these materials, the Yb3+ ions acted as sensitizers
because of their simple energy level structure and high
absorption coefficient in the NIR region. In turn, Er3+ ions
acted as energy acceptors and luminescence activators. Yb3+
ions can be excited by radiation with a wavelength of around
976 nm, which provides a transition from the 2F7/2 ground state
2H11/2 to F9/2, emission lifetime related to F9/2 → I15/2
transition should be similar to 4S3/2 → 4I15/2, which is true only
for Ca2YF7 sample.
4
4
4
Dependencies of the luminescence intensity on the laser
energy were also measured to better understand the mechanism
of the UC process. They are presented in Supporting
Information, Figure S4. At low power densities, the number
of photons involved in the upconversion phenomena is equal to
the slope coefficient in linear regressions obtained from
excitation power dependencies. The determined numbers of
photons involved in the upconversion mechanism are collected
in Table 2.
2
4
to the F5/2 excitation state, having similar energy to the I11/2
excited state of Er3+ ions.53 Therefore, energy can be relatively
easily transferred to the neighboring Er3+ ions, raising them to
4
the I11/2 excited state level or, owing to the relaxation
4
processes, to the I13/2 excited state. Depending on the energy
of Er3+ ions, two or three photons are required to observe green
2
4
The numbers of photons determined from the experimental
data suggest that at least three photons are required to excite
emission from the H11/2 and S3/2 levels. Er3+ ions, in their
4I11/2 excited state, can absorb the second photon from Yb3+
Er3+ ions to the 2H9/2 level, and two photons to achieve 2H11/2
,
2
ions, which promotes them to the H11/2 excited state.
4
4S3/2, and F9/2 excited states. The schematic energy diagram
Furthermore, neighboring Er3+ ions may act as sensitizers,
transferring their energy to the Er3+ emitter.97 From the 2H11/2
excited state, green emission may occur, or relaxation to the
presented in Figure 7 describes the proposed energy transfer
4S3/2 excited state and finally green emission (4S3/2 → I15/2
4
transition).
Excitation into the 4F9/2 level of Er3+ ions may be realized in
several different ways. The first possible pathway is a simple
absorption of two photons as a result of energy transfer from
Yb3+ ions, forming Er3+ ions in their 2H11/2 excited state. Then,
4
relaxation to the F9/2 state takes place followed by red
emission. However, at this point it needs to mentioned that the
proposed pathway is not always realized. Other mechanisms
may also be proposed to explain the origin of the red emission,
including cross-relaxation between Er3+ ions, which depends on
the Er3+ concentration.6 Another possible pathway requires
4
absorption of a photon from an excited Yb3+ ion to the I11/2
4
level of the Er3+ ion, then relaxation to the I13/2 state, from
which the second absorbed photon results in the Er3+ ion in its
4F9/2 excited state. Then the emission of red light can be
observed.
Figure 7. Schematic energy levels diagram for Yb3+ and Er3+ ion-doped
samples.
Another explanation of the origin of the red emission and, in
general, the UC mechanism at low Er3+ concentrations has
been given by Berry et al.6 In the proposed mechanism, they
4
proved that red (4F9/2 → I15/2) UC is preceded by a three-
4
photon excitation process to G/2K states through the green
4
processes occurring in the obtained materials. The H9/2
→
4
2
4I15/2 transition was recorded only for four obtained samples,
but, unfortunately, the determined slope values were much
lower than the theoretical ones. It is caused by nonradiative
deactivation processes, such as cross-relaxation between Er3+
ions, and the saturation effect,96−98 which appear at high laser
emitting S3/2 and H11/2 states. Multiphonon relaxation from
the Er3+ higher-excited states may lead to the blue-emitting
state, H9/2, or by back energy transfer to Yb3+ ions to red
2
6
4
4
emitting F9/2. The long rise and decay time of the F9/2
→
4I15/2 transition are explained by the fact that the S3/2 and
4
2H11/2 states are the initial states for the energy transfer from
energies. For Ba2GdF7, Ba2YF7, and Ca2GdF7-based materials,
2
Yb3+ ions, leading to excitation of the Er3+ ions to their G/2K
4
the calculated slope values are close to two for the H11/2
→
4I15/2, 4S3/2 → 4I15/2, and 4F9/2 → 4I15/2 transitions (preceded by
states.6
two-photon excitation).51 Interestingly, for the
2
4
The H9/2 → I15/2 transition must be preceded by three-
Ba2YF7:Yb3+,Er3+ sample, the S3/2 excited state can also be
photon absorption. The third photon may be absorbed by Er3+
4
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Inorganic Chemistry
Article
4
ions in their F9/2 excited state achieved by two pathways
ASSOCIATED CONTENT
■
mentioned above. However, absorption from the 2H11/2 excited
state is also possible, as mentioned above. Both possible
mechanisms lead to the emission of blue light.
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b00484.
CONCLUSIONS
Table S1: Solubility of metal ions fluorides and stability
constants of M2+ and RE3+ complexes with EDTA; Table
S2: Upconversion quantum yields of noncore/shell
nanoparticles with similar size to the described in the
article products; Figure S1: Schematic representation of
system setup used to measure upconversion quantum
yields; Figure S2: Integral emission intensities of the red
and the green bands, and the ratio between them
calculated for the obtained samples; Figure S3:
Upconversion luminescence decays; Figure S4: Compar-
ison of upconversion luminescence dependencies on
laser energy) (PDF)
■
Alkaline, M2+, and rare earth, RE3+, fluorides as well as their
Yb3+/Er3+-doped counterparts were synthesized by the hydro-
thermal method. Three different M2+ ions, Ca2+, Sr2+ and Ba2+,
and four RE3+ ions, Y3+, La3+, Gd3+, and Lu3, were used in order
to obtain upconverting materials. The synthesized compounds
crystallized with M0.67RE0.33F2.33 composition in a cubic crystal
cell. The mechanism of material formation was studied,
indicating the influence of the stability of the metal ion
EDTA complexes on the final product composition. The
tendency to getting lower than expected concentrations of RE3+
metal ions forming complexes with EDTA of high stability was
observed. Comparison of the XRD patterns to the references
confirmed the formation of nanocrystals of cubic structure.
Nanocrystal sizes determined from XRD patterns were in the
range of 10−21 nm. Similar results were obtained by analyzing
TEM images, which revealed sizes in the range of 9−37 nm.
All of the Yb3+- and Er3+-doped materials exhibited UC
phenomenon. The most intense emission under 976 nm
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: tgrzyb@amu.edu.pl.
ORCID
Tomasz Grzyb: 0000-0002-2947-6366
Notes
excitation was observed for Sr2 LuF7 :Yb3 + ,Er3 +
,
The authors declare no competing financial interest.
Ba2LuF7:Yb3+,Er3+, and Ba2GdF7:Yb3+,Er3+ samples. Lumines-
cence lifetimes were in the range of 7.5−521 μs, depending on
the sample, and indicating the complexity of the obtained
systems and energy processes occurred. Energy transfer
between Yb3+ and Er3+ ions was also analyzed by measuring
the dependence of the integral luminescence intensity on the
excitation laser energy. Analysis of experimental data revealed
the occurrence of the two- and three-photon processes,
preceding the observed upconversion luminescence. In
addition, the presence of quenching phenomena was noticed.
The UCQY measured for the two the best emitting samples
ACKNOWLEDGMENTS
■
Funding for this research was provided by the National Science
Centre, Poland (Grant No. DEC-2011/03/D/ST5/05701 and
DEC-2016/22/E/ST5/00016).
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