Facile Synthesis of Yb3+- and Er3+-Codoped LiGdF ...Research Article Facile Synthesis of Yb3+- and Er3+-Codoped LiGdF 4 Colloidal Nanocrystals with High-Quality Upconversion Luminescence

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.


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.


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.


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.


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)

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