-
Research ArticleFacile Synthesis of Yb3+- and Er3+-Codoped
LiGdF4 ColloidalNanocrystals with High-Quality Upconversion
Luminescence
Lu Zi,1 Dan Zhang,1 and Gejihu De 1,2,3
1College of Chemistry and Environment Science, Inner Mongolia
Normal University, Hohhot 010022, China2Physics and Chemistry of
Functional Materials, Inner Mongolia Key Laboratory, Hohhot 010022,
China3State Key Laboratory on Integrated Optoelectronics, Jilin
University, Changchun 130012, China
Correspondence should be addressed to Gejihu De;
[email protected]
Received 29 December 2018; Revised 14 March 2019; Accepted 14
April 2019; Published 16 May 2019
Academic Editor: Hiromasa Nishikiori
Copyright © 2019 Lu Zi et al. This is an open access article
distributed under the Creative Commons Attribution License,
whichpermits unrestricted use, distribution, and reproduction in
any medium, provided the original work is properly cited.
Herein, we synthesized high-quality colloidal nanocrystals of
Yb3+/Er3+-codoped LiGdF4 with intense green emission by using
afacile route and turning the associated reaction parameters.
Moreover, we probed the effects of reaction conditions
onnanocrystal properties (crystal structure, morphology, and
luminescence) and gained valuable mechanistic insights
intonucleation and growth processes. Sample purity was found to
depend on LiOH·H2O concentration, reaction temperature,and time,
which allowed us to manipulate sample purity and thus obtain
species ranging from mixtures of LiGdF4:Yb
3+/Er3+
with GdF3 to pure tetragonal-phase LiGdF4:Yb3+/Er3+.
Investigation of upconversion luminescence properties and the
luminescence lifetime of as-prepared samples revealed that
LiGdF4 is a promising host material for realizing the
desiredupconversion luminescence.
1. Introduction
Trivalent lanthanide ion- (Ln3+-) doped nanocrystals,which can
convert infrared radiation to visible lumines-cence, have attracted
much attention in view of theirexcellent luminescence properties
and unique applicationvalue [1–9]. The application fields are very
wide includingsolid-state lasers, 3D displays, solar cells,
photovoltaics,biological probe and label markers, and multimodal
bioi-maging [10–16]. Most of these nanocrystals correspondto
fluorides, which exhibit several advantages over otherhalides, such
as thermal and environmental stability, highrefractive index, high
transparency, low-frequency phonons,and lower emission threshold
[17–20]. Among fluorides,those based on Gd3+ have been intensively
researchedbecause of their excellent chemical and optical
properties[21–24]. Moreover, the energy gap between the 6P7/2
andthe 8S7/2 levels of Gd
3+ equals 32000 cm−1, which allowsGd3+ to be used as an
intermediary to promote fluorideenergy transfer and thus greatly
improve the efficiency ofupconversion luminescence [25, 26].
Much research has been performed on the
monodisperse,well-shaped, uniform-size, and phase-pure NaGdF4
nano-particles in recent decades [27–35]. For instance, Liu
andcoworkers prepared size-controllable, highly
monodisperse,oleate-capped NaGdF4:Yb,Er nanocrystals that can be
usedas biological probes for in vivo testing of tiny tumors
[36],while Johnson’s group showed that the regulation of
reactiontime and temperature allows the synthesis of
size-tunableand ultrasmall NaGdF4 nanoparticles (2.5-8.0 nm)
[37].There is no doubt that NaGdF4 is considered to be an
idealrigid host matrix for upconversion, and its synthesis
hastherefore become a research hotspot for the majority ofscholars.
However, LiGdF4 has been underexplored amongthe AGdF4 (A =Na+, K+,
Li+) hosts because of the difficultyof synthesizing pure
tetragonal-phase LiGdF4 nanocrystals.To the best of our knowledge,
LiGdF4 nanocrystals aremainly prepared using Czochralski, sol-gel,
or thermaldecomposition methods. For instance, Cornacchia
andcoworkers prepared LiGdF4:Tm
3+ single crystals utilizingthe Czochralski technique [38],
while Lepoutre andcoworkers prepared 90SiO210LiGd1-xEuxF4 (x = 0 or
0.05)
HindawiJournal of NanomaterialsVolume 2019, Article ID 3928526,
9 pageshttps://doi.org/10.1155/2019/3928526
http://orcid.org/0000-0003-1131-0533https://creativecommons.org/licenses/by/4.0/https://doi.org/10.1155/2019/3928526
-
composites using a sol-gel method [39]. Moreover, Xiong’sgroup
successfully synthesized LiGdF4 nanoparticles withdifferent Ca2+
contents using a thermal decompositionmethod, revealing that Ca2+
ions are vital to the successfulsynthesis of these nanoparticles
[40]. Na and coworkersprepared pure tetragonal-phase LiGdF4
upconversionnanophosphors doped with Y3+ by thermal decomposi-tion
in a methanol-LiOH·H2O-NH4F mixture and showedthat the orthorhombic
GdF3 phase was produced at Y
3+
doping degrees of 0-20mol% [41]. Initially, we tried toprepare
LiGdF4:Yb,Er nanocrystals by similar methods ofLiYF4:Yb,Er
nanocrystals used, while we did not get theresult we wanted. After
careful analysis, we speculate thatit may be caused by various
facts, such as chemical factoritself, equipment factor, and
experimental environmentalfactor. Furthermore, we noted that the
vast majority ofLiGdF4:Yb,Er nanocrystal syntheses employ solutions
ofoleic acid and 1-octadecenein methanol, which are
highlytoxic.
To address this challenge, we have developed a suitablesynthetic
route to prepare high-quality LiGdF4:Yb/Er nano-crystals avoiding
extra ion doping and the use of methanol-LiOH·H2O-NH4F mixtures. An
improved thermal decom-position method is introduced in this paper.
Moreover,we determined optimal reaction conditions and
investi-gated the influence of reaction temperature, reaction
time,and LiOH·H2O content on the upconversion
luminescenceproperties and luminescence lifetimes.
2. Experimental
2.1. Materials. The synthesis was carried out using
standardoxygen-free procedures and commercially available
reagents.RE2O3 (RE = Gd
3+ 99 95%, Yb3+ 99.99%, and Er3+ 99.99%),CF3COOH (analytical
grade 99.0%), oleic acid (OA, analyti-cal grade), and LiOH·H2O
(>95.0%) were purchased fromSinopharm Chemical Reagent Beijing
Co. Ltd. Absoluteethanol (analytical grade > 99 7%), cyclohexane
(analyticalgrade 99.5%), and 1-octadecene (ODE, technical grade90%)
were purchased from Tianjin Chemical Co., Kemeng,and Sigma-Aldrich,
respectively. All chemicals were usedwithout further
purification.
2.2. Synthesis of Precursor Mixture. We improved andmodified
previously reported methods to synthesize tri-fluoroacetate
precursors [42–44]. Compared with thetraditional preparation
process of single trifluoroacetatesof the lanthanides [Ln(CF3COO)3,
Ln = Gd, Yb, and Er]and Li(CF3COO)3 samples, we are mixing the two
to pre-pare precursor mixture. As for reaction vessel, we choose
a100mL Teflon-lined autoclave to replace the traditional100mL
three-neck flask. The process was carried out viaadding Gd2O3
(0.78mmol), Yb2O3 (0.2mmol), Er2O3(0.02mmol), and a certain amount
of LiOH·H2O to the mix-ture solution of trifluoroacetic acid/water
(6mL : 6mL). Thisturbid solution was vigorously stirred and
transferred to a100mL Teflon-lined autoclave. Subsequently, the
emulsionwas heated and maintained at 80°C. After confinement for12
h, the solution was cooled to room temperature and
transferred to a 100mL three-neck flask and dryly heatedup at
60°C for evaporating excess CF3COOH/H2O. Then,the precursor mixture
was obtained. The profit about suchdoing is that the preparation
process will be safer and moreconvenient. It is mainly because a
mixed solution of metha-nol, LiOH·H2O, and NH4F for providing
Li
+ or other dopedions is required in the traditional preparation
process, whilenone of these is required in our preparation.
2.3. Synthesis of High-Quality Yb3+/Er3+-Codoped LiGdF4Colloidal
Nanocrystals. The high-quality Yb3+/Er3+-codopedLiGdF4 colloidal
nanocrystals were prepared by a thermaldecomposition route. The
process was carried out via adding15mL OA and 15mL ODE to precursor
mixture. The cloudysolution was vigorously stirred to yield a
transparent solu-tion. Subsequently, the transparent solution was
heated to110°C under vacuum condition for removing
needlesswater/oxygen. An hour later, the solution was heated
to310°C (or 280°C, 290°C, and 300°C) under an argon gasatmosphere
and maintained for a period of time (1 h, 2 h,3 h, 4 h, 5 h, or 6
h). The reaction system was cooled to roomtemperature and added
excess absolute ethanol to precipitateproducts. The as-synthesized
products were washed severaltimes with cyclohexane/ethanol (1 : 4)
mixed solution toremove the residue of organic ligands and other
mixtureson the products and isolated by centrifugation at 8000
rpmfor 3min. Finally, the products were dried under vacuum to60°C
for 12h to obtain a white powder sample for reserving.
3. Results and Discussions
3.1. Synthetic Procedure and Reaction Mechanism. Figure
1illustrates the synthesis of high-quality Yb3+/Er3+-codopedLiGdF4
colloidal nanocrystals, showing that it comprisedtwo steps, namely,
hydrothermal preparation of the precur-sor mixture, followed by
thermal decomposition to affordLiGdF4 colloidal nanocrystals.
To get better understanding, the nucleation and growthmechanisms
of LiGdF4:Yb0.2/Er0.02 colloidal nanocrystalsare speculated in
Scheme 1. Briefly, the process starts withthe cothermolysis of
Gd(CF3COO)3 and CF3COOLi in oleicacid and 1-octadecene systems.
When the reaction systemis heated to 100~120°C, trifluoroacetate
ligands are partiallyexchanged for oleic acid residues, and the C-F
bond of theformer is broken to release F- when the reaction
tempera-ture further increases to 250~330°C. Subsequently,
fluorideanions engage in fluorination and cleave Gd-OOCCF3 bondsto
promote nucleation, and the thus obtained crystal nucleiagglomerate
to form larger particles as the reaction pro-gresses. With the
increase of temperature, the growth rateof these nuclei crystal
nucleus increases and eventually formsnanocrystals [45–48].
3.2. Structure and Morphology. Sample crystal structureswere
characterized by using a Rigaku Ultima IV X-raydiffractometer with
Cu Kα radiation (λ = 0 15406 nm) at200mA and 40 kV. XRD patterns
were recorded for driedpowders in the range of 2θ = 15°~80° at a
step size of8°/min. Figure 2 clearly demonstrates the growth
kinetics of
2 Journal of Nanomaterials
-
LiGdF4:Yb3+/Er3+ nanocrystals synthesized at different
reaction times. It is not hard to find that the diffractionpeaks
of samples obtained at Li+/Gd3+ = 3 and t = 1 h,2 h, and 3h
corresponded to a mixture of LiGdF4 (JCPDSNo. 27-1236) and GdF3
(JCPDS No. 49-1804), whereasthose obtained at Li+/Gd3+ = 3 and t =
4 h and 5h containLiGdF4 (JCPDS No. 27-1236). Hence, 4 or 5 h
was
concluded to be the optimal reaction times for synthesiz-ing
LiGdF4:Yb
3+/Er3+ nanocrystals.Next, we studied the optimum reaction
temperature for
preparing LiGdF4:Yb3+/Er3+ colloidal nanocrystals. Figure 3
clearly demonstrates the formation of LiGdF4 with GdF3at
Li+/Gd3+ = 3 and temperatures (T) of 280, 290, and300°C, revealing
that LiGdF4 nuclei were instantly formed
Gd2O3
Er2O3
Yb2O3 H2O
LiOH.H2O
CF3COO‒
Figure 1: Synthesis of LiGdF4:Yb3+/Er3+ colloidal
nanocrystals.
Gd3+
Li+
Li+
Li+Gd3+
Gd3+
F–
F–OA/ODE100 ~ 120 ºC
250 ~ 310 ºC
Aggregate Dissolve Grow
Nucleation
ODE
OA
CF3COO‒
Scheme 1: Schematic of the reaction mechanism.
3Journal of Nanomaterials
-
even at 280°C and that the growth rate of these nucleiincreased
with the increasing temperature. A balancebetween nucleation and
growth was established at 310°C,which was concluded to be the
optimum reaction temper-ature for the synthesized tetragonal
LiGdF4:Yb
3+/Er3+
nanocrystals. Finally, the optimum Li/RE ratio wasdetermined as
three, although the optimization of Li+
concentration proved to be very tortuous. Details of
thesynthetic process and parameters of
tetragonal-phaseLiGdF4:Yb
3+/Er3+ colloidal nanocrystals are given in thesupporting
information (Figure S1, S2, and Table S1). Thus,the above
experimental results suggest that the optimumconditions for
tetragonal LiGdF4:Yb
3+/Er3+ nanocrystalsynthesis were determined as Li+/Gd3+ = 3, t
= 4 h, andT = 310°C (Table S1).
Sample morphology was assessed by transmission elec-tron
microscopy (TEM) and high-resolution transmissionelectron
microscopy (HRTEM) with a FEI Tecnai G2 F20S-Twin transmission
electron microscope operating at200 kV. Samples for TEM imaging
were prepared by dryingnanocrystal dispersions in cyclohexane on
amorphouscarbon-coated Cu grids. Representative TEM micrographsof
as-synthesized LiGdF4 nanocrystals (relatively pure tetrag-onal
phase) are displayed in Figure 4. Figures 4(a)–4(d)
showlow-magnification TEM images, while Figures 5(a) and 5(b)show
high-magnification TEM images with related selected-area electron
diffraction patterns, revealing that the as-synthesized
LiGdF4:Yb
3+/Er3+ nanocrystals can withstandirradiation with high-energy
electrons. From the TEMimages, we can also see that most of the
samples are octahe-dron and sphere. And the average sizes of these
crystals focuson 137.7 nm nearby in Figure 4(e), while the
octahedralcrystals focus on 98 nm length × 98 nm length × 18 nm
thickness on average. HRTEM imaging revealed the presenceof
obvious lattice fringes, indicating the high crystallinity
ofindividual particles. The adjacent lattice spacing calculatedby
FFT analysis (~0.47 nm) was assigned to the (101) crystalplane of
tetragonal-phase LiGdF4, which confirmed thatas-synthesized LiGdF4
nanocrystals exhibited high crystal-linity and few defects.
3.3. Upconversion Luminescence. Upconversion emissionspectra
were recorded on a Hitachi F-4600 fluorescencespectrophotometer
(slit width = 5 0 nm, PMTvoltage = 700V,and λ = 400 – 700 nm) under
excitation with an adjustable980 nm NIR laser diode. Room
temperature upconversionemission spectra are obtained by drying the
nanocrystaldispersion in cyclohexane at a concentration of
2mg/mL.It is formed into a colloidal solution by dispersing
driedpowder in cyclohexane for several hours’ ultrasound.
It is well known that Yb3+- and Er3+-codoped rare earthfluorides
can exhibit strong upconversion luminescenceupon 980nm
near-infrared excitation, as observed hereinfor the colloidal
suspension of LiGdF4:Yb
3+/Er3+. Twovisible-light-region emission bands, positioned at
523 and543 nm (green upconversion luminescence) and 672nm(red
upconversion luminescence), were observed for allsamples in Figures
6(a) and 6(b). The above bands wereascribed to 2H11/2→
4I15/2,4S3/2→
4I15/2, and4F9/2→
4I15/2transitions, respectively. Meanwhile, we also find that
theintensity of upconversion luminescence increased withincreasing
reaction time, which was mainly attributed tothe concomitant
increase of tetragonal phase content.This phase could be obtained
in relatively pure form atLi+/Gd3+ = 3, t = 4 h, and T = 310°C,
while mixtures ofLiGdF4 with GdF3, exhibiting lower upconversion
lumines-cence intensities because of the presence of the latter
compo-nent and other impurities, were obtained otherwise. It
issurprising that there is a sudden drop in the
upconversionluminescence intensity of the sample, which is
synthesizedat Li+/Gd3+ = 3, t = 5 h, and T = 310°C. This finding
mightbe attributed to the fact that the content of GdF3 and
otherimpurities in the above sample exceeded that in the
optimalcondition sample.
In addition, it might be that the growth rate of LiGdF4crystal
nucleus reached the maximum at 4 h, while thereaction time that
further increased to 5 h might lead toexcessive surfactant and
activator ions accumulate on thecrystal surface. All of these cause
fluorescence quenching ofthe sample, which is synthesized at
Li+/Gd3+ = 3, t = 5 h,and T = 310°C. The clear contrast figures of
emissionintensity and reaction time are provided in the
supportinginformation (Figure S3 and Table S2). To gain
furtherinsights into the upconversion emission process,
weinvestigated the dependence of upconversion emissionintensity on
excitation power adopting the log Iem ∝ logInex relation for data
analysis in Figure 6(c). We regard thetetragonal LiGdF4
nanocrystals as example to illuminate theupconversion emission
process. The slopes of Gaussianfunction-based log-log fits were
determined as 2.11 and1.95 for the dominant green emissions at 523
and 543 nm,respectively, which illustrated that these emissions
involve a
15 20 25 30 35 40 45 50 55 60
JCPDS No.49-1804JCPDS No.27-1236
Inte
nsity
(a.u
.)
2θ/(°)
1h
2h
3h
4h
5h
Figure 2: XRD patterns of LiGdF4:Yb3+/Er3+ colloidal
nanocrystals,
which are synthesized at Li+/Gd3+ = 3 and t = 1 h, 2 h, 3 h, 4
h,and 5 h.
4 Journal of Nanomaterials
-
two-photon process. For red emission, the correspondingslope was
obtained as 2.37, and the same conclusion wasdrawn. It corresponds
with the analysis of upconversionemissions mechanism.
Figure 7 schematically illustrates energy transfer
andupconversion emission processes occurring at an
excitationwavelength of 980nm and a pump power density of55mW/cm2.
Under continuous excitation at 980nm photon,sensitizer Yb3+ ions
can be excited from the 2F7/2 groundstate to the 2F5/2 state.
Subsequently, the latter states decayback to the former, and the
released energy is captured bynearby Er3+ ions, which are excited
from the 4I15/2 groundstate into the 4I11/2 state. Further energy
capture by Er
3+ ionsin the 4I11/2 excited state results in the population of
a higher-lying 4F7/2 state that can relax to the
2H11/2 and4S3/2 levels
(nonradiatively) and to the 4I15/2 level (radiatively)
withdominant 2H11/2→
4I15/2 and4S3/2→
4I15/2 transitions result-ing in green emission. Alternatively,
Er3+ ions in the 4I11/2excited state can nonradiatively relax to
the 4I13/2 state andcapture further energy to populate a
higher-lying 4F9/2 statethat subsequently radiatively relaxes to
the 4I15/2 level withdominant 4F9/2→
4I15/2 transitions resulting in red emis-sions. Notably,
as-synthesized LiGdF4 nanocrystals not onlyshowed higher
upconversion emission intensity but alsolonger luminescence
lifetime (Figure 8 and Figure S4).
The decay of upconversion luminescence was recordedat room
temperature using a lifetime fluorescence spec-trometer (Delta Flex
TCSPC system, Horiba Scientific,Scotland) equipped with an
adjustable pulse laser as excita-tion source (slit width = 16 nm,
wavelength = 980 nm, pulsewidth = 100 ns, and output power =
100Hz). The obtaineddecay curves were fitted by a
single-exponential function,
and the effective UCL lifetime of nonexponential decay
wascalculated as
τeff =1
Imax
∞
0I t dt, 1
where Imax is the maximum upconversion luminescenceintensity and
I t is the upconversion luminescence intensityas a function of time
[49, 50]. The UCL lifetimes of the 2H11/2state, determined by
monitoring Er3+ emission at 523nmunder 980nm, were found to
increase with a reaction timeof up to 4 h, equalling 369.30 (2 h),
663.23 (3 h), 745.74μs(4 h), and 540.45μs (5 h), respectively
(Figure 8). This resultconfirmed that the sample synthesized at
Li+/Gd3+ = 3, t =4 h, and T = 310°C exhibited good photostability
because ofthe relatively pure tetragonal phase. In other samples,
thepresence of impurity ingredients led to the rapid migrationof
energy to lattice defects or surface quenchers,
inducingluminescence quenching. This behaviour was consistentwith
the trend of upconversion emission intensity inFigure 6(a).
Finally, we measured the lifetimes of 2H11/2,4S3/2, and
4F9/2 states of Er3+ under 980nm excitation in
LiGdF4:Yb3+/Er3+ nanocrystals, synthesized at Li+/Gd3+ = 3,
t = 4 h, and T = 310°C (Figure S4), revealing increases
from663.23μs (523nm) and 665.54μs (543 nm) to 678.41μs(672 nm),
respectively.
4. Conclusion
We have successfully improved and modified previouslyreported
methods to synthesize high-quality Yb3+/Er3+-
15 20 25 30 35 40 45 50 55 60
Orthorhombic GdF3
Inte
nsity
(a.u
.)
280°C
290°C
300°C
310°C
JCPDS No.49-1804JCPDS No.27-1236
2θ/(°)
Tetragonal LiGdF4
Figure 3: XRD patterns of LiGdF4:Yb3+/Er3+ colloidal
nanocrystals, which are synthesized at Li+/Gd3+ = 3 and T = 280°C,
290°C, 300°C,
and 310°C.
5Journal of Nanomaterials
-
200 nm
(a)
200 nm
(b)
100 nm
(c)
50 nm
(d)
100 120 140 160 1800
1
2
3
4
5
6
7 Min/nm 100.5Max/nm 172.8Mean/nm 137.7
Num
ber o
f par
ticle
s
Particle diameter (nm)
(e)
Figure 4: (a–d) TEM images of LiGdF4:Yb3+/Er3+ nanocrystals,
which are synthesized at Li+/Gd3+ = 3, t = 4 h, and T = 310°C. (e)
Grain size
distribution histograms of Figure 4(a).
10 nm
(a)
5 nm
(101)
d = 0. 472
nm
(b)
Figure 5: HRTEM images of LiGdF4:Yb3+/Er3+ nanocrystals, which
are synthesized at Li+/Gd3+ = 3, t = 4 h, and T = 310°C. The insets
of (a)
and (b) correspond with Fourier transform electron diffraction
pattern.
6 Journal of Nanomaterials
-
codoped LiGdF4 colloidal nanocrystals with intense
greenemission. Specifically, the above synthesis involved
thehydrothermal preparation of trifluoroacetate
precursorsGd(CF3COO)3 and CF3COOLi that were subsequently
ther-molyzed to afford the desired nanocrystals. Importantly,
theadopted approach obviated the need for additional ion dop-ing
and the use of toxic methanol-LiOH·H2O-NH4Fmixture.Studies on the
impact of LiOH·H2O concentration, reactiontemperature, and time on
the upconversion luminescenceof nanocrystal samples showed that
relatively phase-puretetragonal LiGdF4 nanocrystals could be
obtained underoptimal conditions (Li+/Gd3+ = 3, t = 4 h, and T =
310°C).Moreover, as-synthesized LiGdF4:Yb
3+/Er3+ nanocrystals
400 450 500 550 600 650 7000
1000
2000
3000
Wavelength (nm)
Inte
nsity
(a.u
.)
1h2h
4h5h
3h
(a)
400 450 500 550Wavelength (nm)
600 650 7000
100
200
300
400
500
600
700
800
Inte
nsity
(a.u
.)
ErH
3+2
4:
11/2
15/2
I
ErS
3+4
4:
3/2
15/2
I
ErF
3+4
4:
9/2
15/2
I
(b)
Slope = 2.11 ± 0.06
Slope = 1.95 ± 0.04
Slope = 2.37 ± 0.03
0.2 0.25 0.3 0.35 0.4 0.45
lg[in
tens
ity (a
.u.)]
lg[laser power (mW/cm2)]
(c)
Figure 6: (a) Room temperature upconversion emission spectraof
LiGdF4:Yb
3+/Er3+ nanocrystals, which are synthesized at Li+/Gd3+ = 3 and
t = 1 h, 2 h, 3 h, 4 h, and 5 h. (b) Room temperatureupconversion
emission spectra of LiGdF4:Yb
3+/Er3+ nanocrystals,which are synthesized at Li+/Gd3+ = 3, t =
4 h, and T = 310°C.(c) Log-log plot of power dependence of the
upconversionemissions intensity for LiGdF4:Yb
3+/Er3+ nanocrystals at Li+/Gd3+ = 3, t = 4 h, and T =
310°C.
0
5
10
15
20
25
2F7/2
2F5/2
Yb3+ Er3+
4I15/2
4I13/2
4I11/2
4I9/2
4F9/2
4F7/22H11/24S3/2
Ener
gy (1
03 cm
–1)
Figure 7: Energy level diagrams of upconversion emissions
fromYb3+ to Er3+ in LiGdF4:Yb
3+/Er3+ nanocrystals under 980 nmpump power excitation.
0 300 600 900 1200 1500 1800 2100 2400 2700
1
10
Inte
nsity
(a.u
.)
Time (�휇s)
�휆ex = 980 nm
Er3+ 2H11/24H15/2
2h3h4h5h
Figure 8: UCL decays from 2H11/2 of Er3+ by monitoring the
Er3+
emission at 523 nm under 980 nm excitation in
LiGdF4:Yb3+/Er3+
nanocrystals, which are synthesized at Li+/Gd3+ = 3, T =
310°C,and t = 2 h, 3 h, 4 h, and 5 h.
7Journal of Nanomaterials
-
not only showed a stronger upconversion emission intensitybut
also featured a longer luminescence lifetime. This workpaves the
way to the broad utilization of LiGdF4, whichis viewed as an ideal
alternative matrix material, sincethe ionic radius of Li+ is much
smaller than that of Na+.Herein, this research work will be
indispensable for furtherfollow-up study.
Data Availability
All data are obtained through our own experiments. Thepublic
database is not used in this article. If the reader needsthe data
in this article, he can contact the correspondingauthor.
Conflicts of Interest
The authors declare that they have no conflict of interest.
Acknowledgments
We gratefully acknowledge professor Gejihu De for guid-ing and
the testing Center of Inner Mongolia University.This work is
supported by the Postgraduate ScientificResearch Innovation
Foundation of Inner Mongolia Uni-versity (Grant No. CXJJS15082),
Open Fund of the StateKey Laboratory on Integrated Optoelectronics
(Grant No.IOSKL2013KF08), National Science Foundation of
China(Grant No. 21261016), and Talents Project Inner Mongoliaof
China (Grant No. CYYC5026).
Supplementary Materials
Supplementary material including X-ray diffraction patternsof
as-prepared samples, decay curves of LiGdF4:Yb
3+/Er3+
nanocrystals, and discussion of influencing factors (such
astemperature, time, and emission intensity).
(SupplementaryMaterials)
References
[1] Z. L. Wang, H. L. W. Chan, H. L. Li, and J. H. Hao,
“Highlyefficient low-voltage cathodoluminescence of LaF3:Ln
3+
(Ln=Eu3+,Ce3+,Tb3+) spherical particles,” Applied
PhysicsLetters, vol. 93, no. 14, article 141106, 2008.
[2] F. Zhang and D. Zhao, “Fabrication of ordered
magnetite-doped rare earth fluoride nanotube arrays by
nanocrystalself-assembly,”Nano Research, vol. 2, no. 4, pp.
292–305, 2009.
[3] S. Fan, S.Wang,W. Xu, M. Li, H. Sun, and L. Hu,
“Enormouslyenhanced upconversion emission in
β-NaYF4:20Yb,2Ermicrocrystals via Na+ ion exchange,” Journal of
MaterialsScience, vol. 52, no. 2, pp. 869–877, 2017.
[4] F. Vetrone, R. Naccache, A. Zamarrón et al.,
“Temperaturesensing using fluorescent nanothermometers,” ACS
Nano,vol. 4, no. 6, pp. 3254–3258, 2010.
[5] V. Mahalingam, F. Mangiarini, F. Vetrone et al.,
“Brightwhite upconversion emission from
Tm3+/Yb3+/Er3+-dopedLu3Ga5O12 nanocrystals,” The Journal of
Physical ChemistryC, vol. 112, no. 46, pp. 17745–17749, 2008.
[6] X. Zou, M. Xu, W. Yuan et al., “A water-dispersible
dye-sensitized upconversion nanocomposite modified with
phos-phatidylcholine for lymphatic imaging,” Chemical
Communi-cations, vol. 52, no. 91, pp. 13389–13392, 2016.
[7] W. Yang, X. Li, D. Chi, H. Zhang, and X. Liu,
“Lanthanide-doped upconversion materials: emerging applications
forphotovoltaics and photocatalysis,” Nanotechnology, vol. 25,no.
48, article 482001, 2014.
[8] C. Zhang, H. P. Zhou, L. Y. Liao et al.,
“Luminescencemodulation of ordered upconversion nanopatterns by a
photo-chromic diarylethene: rewritable optical storage with
nonde-structive readout,” Advanced Materials, vol. 22, no. 5,pp.
633–637, 2010.
[9] T. Zako, H. Nagata, N. Terada, M. Sakono, K. Soga, andM.
Maeda, “Improvement of dispersion stability and char-acterization
of upconversion nanophosphors covalentlymodified with PEG as a
fluorescence bioimaging probe,”Journal of Materials Science, vol.
43, no. 15, pp. 5325–5330, 2008.
[10] F. Wang and X. Liu, “Recent advances in the chemistry
oflanthanide-doped upconversion nanocrystals,” ChemicalSociety
Reviews, vol. 38, no. 4, pp. 976–989, 2009.
[11] H. Guo, Z. Li, H. Qian, Y. Hu, and I. N. Muhammad,
“Seed-mediated synthesis of NaY F4:Y b, Er/NaGdF4 nanocrystalswith
improved upconversion fluorescence and MR
relaxivity,”Nanotechnology, vol. 21, no. 12, article 125602,
2010.
[12] S. V. Eliseeva and J. C. G. Bünzli, “Lanthanide
luminescencefor functional materials and bio-sciences,” Chemical
SocietyReviews, vol. 39, no. 1, pp. 189–227, 2010.
[13] B. M. Tissue, “Synthesis and luminescence of lanthanide
ionsin nanoscale insulating hosts,” Chemistry of Materials,vol. 10,
no. 10, pp. 2837–2845, 1998.
[14] J. Kim, Y. Piao, and T. Hyeon, “Multifunctional
nanostruc-tured materials for multimodal imaging, and
simultaneousimaging and therapy,” Chemical Society Reviews, vol.
38,no. 2, pp. 372–390, 2009.
[15] X. Liu, X. Kong, Y. Zhang et al., “Breakthrough in
con-centration quenching threshold of upconversion lumines-cence
via spatial separation of the emitter doping areafor
bio-applications,” Chemical Communications, vol. 47,no. 43, pp.
11957–11959, 2011.
[16] Y. Liu, Y. Chen, Y. Lin, Q. Tan, Z. Luo, and Y. Huang,
“Energytransfer in Yb3+–Er3+-codoped bismuth borate glasses,”
Jour-nal of the Optical Society of America B, vol. 24, no. 5,pp.
1046–1052, 2007.
[17] F. Wang, J. Wang, J. Xu, X. Xue, H. Chen, and X. Liu,
“Tunableupconversion emissions from lanthanide-doped monodis-perse
β-NaYF4 nanoparticles,” Spectroscopy Letters, vol. 43,no. 5, pp.
400–405, 2010.
[18] E. van der Kolk, P. Dorenbos, K. Krämer, D. Biner, and H.
U.Güdel, “High-resolution luminescence spectroscopy study
ofdown-conversion routes inNaGdF4:Nd
3+ and NaGdF4:Tm3+
using synchrotron radiation,” Physical Review B, vol. 77,no. 12,
article 125110, 2008.
[19] C. Li and J. Lin, “Rare earth fluoride
nano-/microcrystals:synthesis, surface modification and
application,” Journal ofMaterials Chemistry, vol. 20, no. 33, pp.
6831–6847, 2010.
[20] P. R. Diamente, M. Raudsepp, and F. C. J. M. van
Veggel,“Dispersible Tm3+-doped nanoparticles that exhibit
strong1.47μm photoluminescence,” Advanced Functional Materials,vol.
17, no. 3, pp. 363–368, 2007.
8 Journal of Nanomaterials
http://downloads.hindawi.com/journals/jnm/2019/3928526.f1.dochttp://downloads.hindawi.com/journals/jnm/2019/3928526.f1.doc
-
[21] R. T. Wegh, H. Donker, K. D. Oskam, and A.
Meijerink,“Visible quantum cutting in LiGdF4:Eu
3+ through downcon-version,” Science, vol. 283, no. 5402, pp.
663–666, 1999.
[22] F. You, S. Huang, S. Liu, and Y. Tao, “VUV excited
lumines-cence of MGdF4:Eu
3+ (M=Na, K, NH4),” Journal of Lumines-cence, vol. 110, no. 3,
pp. 95–99, 2004.
[23] R. Naccache, F. Vetrone, V. Mahalingam, L. A. Cuccia, andJ.
A. Capobianco, “Controlled synthesis and water dispersibil-ity of
hexagonal phase NaGdF4:Ho
3+/Yb3+ nanoparticles,”Chemistry of Materials, vol. 21, no. 4,
pp. 717–723, 2009.
[24] T. Li, S. Liu, H. Zhang, E. Wang, L. Song, and P.
Wang,“Ultraviolet upconversion luminescence in Y2O3:Yb
3+,Tm3+
nanocrystals and its application in photocatalysis,” Journal
ofMaterials Science, vol. 46, no. 9, pp. 2882–2886, 2011.
[25] Y. Wu, C. Li, D. Yang, and J. Lin, “Rare earth β-NaGdF4
fluo-rides with multiform morphologies: hydrothermal synthesisand
luminescent properties,” Journal of Colloid and InterfaceScience,
vol. 354, no. 2, pp. 429–436, 2011.
[26] H. Dong, L. D. Sun, and C. H. Yan, “Energy transfer in
lantha-nide upconversion studies for extended optical
applications,”Chemical Society Reviews, vol. 44, no. 6, pp.
1608–1634, 2015.
[27] C. De Nadaï, A. Demourgues, L. Lozano, P. Gravereau, andJ.
Grannec, “Structural investigations of new copper
fluoridesNaRECu2F8 (RE
3+=Sm3+, Eu3+, Gd3+, Y3+, Er3+, Yb3+),” Jour-nal of Materials
Chemistry, vol. 8, no. 11, pp. 2487–2491, 1998.
[28] H. X. Mai, Y.W. Zhang, R. Si et al., “High-quality sodium
rare-earth fluoride nanocrystals: controlled synthesis and
opticalproperties,” Journal of the American Chemical Society,vol.
128, no. 19, pp. 6426–6436, 2006.
[29] J. Y. Park, M. J. Baek, E. S. Choi et al., “Paramagnetic
ultrasmallgadolinium oxide nanoparticles as advanced T1 MRI
contrastagent: account for large longitudinal relaxivity, optimal
parti-cle diameter, and in vivo T1 MR images,” ACS Nano, vol. 3,no.
11, pp. 3663–3669, 2009.
[30] R. Naccache, P. Chevallier, J. Lagueux et al., “High
relaxivitiesand strong vascular signal enhancement for NaGdF4
nanopar-ticles designed for dual MR/optical imaging,”
AdvancedHealthcare Materials, vol. 2, no. 11, pp. 1478–1488,
2013.
[31] R. Lv, S. Gai, Y. Dai, N. Niu, F. He, and P. Yang,
“Highlyuniform hollow GdF3 spheres: controllable synthesis,
tunedluminescence, and drug-release properties,” ACS
AppliedMaterials & Interfaces, vol. 5, no. 21, pp. 10806–10818,
2013.
[32] H. Chen, X. Li, F. Liu, H. Zhang, and Z.Wang, “Renal
clearablepeptide functionalized NaGdF4 nanodots for
high-efficiencytracking orthotopic colorectal tumor in mouse,”
MolecularPharmaceutics, vol. 14, no. 9, pp. 3134–3141, 2017.
[33] M. Ahrén, L. Selegård, A. Klasson et al., “Synthesis and
charac-terization of PEGylated Gd2O3 nanoparticles for MRI
contrastenhancement,” Langmuir, vol. 26, no. 8, pp. 5753–5762,
2010.
[34] S. Dühnen, T. Rinkel, and M. Haase, “Size control of
nearlymonodisperse β-NaGdF4 particles prepared from smallα-NaGdF4
nanocrystals,” Chemistry of Materials, vol. 27,no. 11, pp.
4033–4039, 2015.
[35] J. L. Bridot, A. C. Faure, S. Laurent et al., “Hybrid
gadoliniumoxide nanoparticles: multimodal contrast agents for in
vivoimaging,” Journal of the American Chemical Society, vol.
129,no. 16, pp. 5076–5084, 2007.
[36] C. Liu, Z. Gao, J. Zeng et al., “Magnetic/upconversion
fluores-cent NaGdF4:Yb,Er nanoparticle-based dual-modal
molecularprobes for imaging tiny tumors in vivo,” ACS Nano, vol.
7,no. 8, pp. 7227–7240, 2013.
[37] N. J. J. Johnson, W. Oakden, G. J. Stanisz, R. Scott
Prosser, andF. C. J. M. van Veggel, “Size-tunable, ultrasmall
NaGdF4 nano-particles: insights into their T1 MRI contrast
enhancement,”Chemistry of Materials, vol. 23, no. 16, pp.
3714–3722, 2011.
[38] F. Cornacchia, A. Di Lieto, and M. Tonelli,
“LiGdF4:Tm3+:
spectroscopy and diode-pumped laser experiments,” AppliedPhysics
B, vol. 96, no. 2-3, pp. 363–368, 2009.
[39] S. Lepoutre, D. Boyer, S. Fujihara, and R. Mahiou,
“Structuraland optical characterizations of sol–gel based
composites con-stituted of LiGdF4:Eu
3+ nanocrystallites dispersed into a silicamatrix,” Journal of
Materials Chemistry, vol. 19, no. 18,pp. 2784–2788, 2009.
[40] Z. Xiong, Y. Yang, and Y. Wang, “Enhanced
upconversionluminescence and tuned red-to-green emission ratio of
LiGdF4nanocrystals via Ca2+ doping,” RSC Advances, vol. 6, no.
79,pp. 75664–75668, 2016.
[41] H. Na, J. S. Jeong, H. J. Chang et al., “Facile synthesis
of intensegreen light emitting LiGdF4:Yb,Er-based upconversion
bipyra-midal nanocrystals and their polymer composites,”
Nanoscale,vol. 6, no. 13, pp. 7461–7468, 2014.
[42] G. S. Yi, W. B. Lee, and G. M. Chow, “Synthesis of
LiYF4,BaYF5, and NaLaF4 optical nanocrystals,” Journal
ofNanoscience and Nanotechnology, vol. 7, no. 8, pp. 2790–2794,
2007.
[43] H. T. Wong, F. Vetrone, R. Naccache, H. L. W. Chan,J. Hao,
and J. A. Capobianco, “Water dispersible ultra-small
multifunctional KGdF4:Tm
3+, Yb3+ nanoparticles withnear-infrared to near-infrared
upconversion,” Journal ofMaterials Science, vol. 21, no. 41, pp.
16589–16596, 2011.
[44] Q. Cheng, J. Sui, and W. Cai, “Enhanced upconversion
emis-sion in Yb3+ and Er3+ codoped NaGdF4 nanocrystals by
intro-ducing Li+ ions,” Nanoscale, vol. 4, no. 3, pp. 779–784,
2012.
[45] M. Li, H. Schnablegger, and S. Mann, “Coupled synthesis
andself-assembly of nanoparticles to give structures with
con-trolled organization,” Nature, vol. 402, no. 6760, pp. 393–395,
1999.
[46] J. Shin, J. H. Kyhm, A. R. Hong et al., “Multicolor
tunableupconversion luminescence from sensitized seed-mediatedgrown
LiGdF4:Yb,Tm-based core/triple-shell nanophosphorsfor transparent
displays,” Chemistry of Materials, vol. 30,no. 23, pp. 8457–8464,
2018.
[47] Y.-P. Du, Y.-W. Zhang, L.-D. Sun, and C.-H. Yan,
“Opticallyactive uniform potassium and lithium rare earth
fluoridenanocrystals derived from metal trifluroacetate
precursors,”Dalton Transactions, no. 40, pp. 8574–8581, 2009.
[48] M. R. Jones, K. D. Osberg, R. J. Macfarlane, M. R.
Langille, andC. A. Mirkin, “Templated techniques for the synthesis
andassembly of plasmonic nanostructures,” Chemical Reviews,vol.
111, no. 6, pp. 3736–3827, 2011.
[49] J. Xie, X. Xie, C. Mi et al., “Controlled synthesis,
evolutionmechanisms, and luminescent properties of ScFx:Ln (x
=2.76, 3) nanocrystals,” Chemistry of Materials, vol. 29, no.
22,pp. 9758–9766, 2017.
[50] H. Dong, L. D. Sun, W. Feng, Y. Gu, F. Li, and C. H.
Yan,“Versatile spectral and lifetime multiplexing nanoplatformwith
excitation orthogonalized upconversion luminescence,”ACS Nano, vol.
11, no. 3, pp. 3289–3297, 2017.
9Journal of Nanomaterials
-
CorrosionInternational Journal of
Hindawiwww.hindawi.com Volume 2018
Advances in
Materials Science and EngineeringHindawiwww.hindawi.com Volume
2018
Hindawiwww.hindawi.com Volume 2018
Journal of
Chemistry
Analytical ChemistryInternational Journal of
Hindawiwww.hindawi.com Volume 2018
Scienti�caHindawiwww.hindawi.com Volume 2018
Polymer ScienceInternational Journal of
Hindawiwww.hindawi.com Volume 2018
Hindawiwww.hindawi.com Volume 2018
Advances in Condensed Matter Physics
Hindawiwww.hindawi.com Volume 2018
International Journal of
BiomaterialsHindawiwww.hindawi.com
Journal ofEngineeringVolume 2018
Applied ChemistryJournal of
Hindawiwww.hindawi.com Volume 2018
NanotechnologyHindawiwww.hindawi.com Volume 2018
Journal of
Hindawiwww.hindawi.com Volume 2018
High Energy PhysicsAdvances in
Hindawi Publishing Corporation http://www.hindawi.com Volume
2013Hindawiwww.hindawi.com
The Scientific World Journal
Volume 2018
TribologyAdvances in
Hindawiwww.hindawi.com Volume 2018
Hindawiwww.hindawi.com Volume 2018
ChemistryAdvances in
Hindawiwww.hindawi.com Volume 2018
Advances inPhysical Chemistry
Hindawiwww.hindawi.com Volume 2018
BioMed Research InternationalMaterials
Journal of
Hindawiwww.hindawi.com Volume 2018
Na
nom
ate
ria
ls
Hindawiwww.hindawi.com Volume 2018
Journal ofNanomaterials
Submit your manuscripts atwww.hindawi.com
https://www.hindawi.com/journals/ijc/https://www.hindawi.com/journals/amse/https://www.hindawi.com/journals/jchem/https://www.hindawi.com/journals/ijac/https://www.hindawi.com/journals/scientifica/https://www.hindawi.com/journals/ijps/https://www.hindawi.com/journals/acmp/https://www.hindawi.com/journals/ijbm/https://www.hindawi.com/journals/je/https://www.hindawi.com/journals/jac/https://www.hindawi.com/journals/jnt/https://www.hindawi.com/journals/ahep/https://www.hindawi.com/journals/tswj/https://www.hindawi.com/journals/at/https://www.hindawi.com/journals/ac/https://www.hindawi.com/journals/apc/https://www.hindawi.com/journals/bmri/https://www.hindawi.com/journals/jma/https://www.hindawi.com/journals/jnm/https://www.hindawi.com/https://www.hindawi.com/