Synthesis and Characterization of Upconversion Fluorescent
Hindawi Publishing Corporation
Journal of Nanomaterials
Volume2009,Article ID685624,7pages
doi:10.1155/2009/685624
Research Article
Synthesis and Characterization of Upconversion Fluorescent
Yb3+,Er3+Doped CsY2F7Nano-and Microcrystals
Helmut Sch¨a fer,Pavel Ptacek,Henning Eickmeier,and Markus Haase
Institute of Chemistry,University of Osnabr¨u ck,Barbarastrasse7,49069Osnabr¨u ck,Germany
Correspondence should be addressed to Helmut Sch¨a fer,helmut.schaefer@uos.de
Received21April2009;Accepted18July2009
Recommended by Kui Yu
CsY2F7:78%Y3+,20%Yb3+,2%Er3+nanocrystals with a mean diameter of approximately8nm were synthesized at185?C in the high boiling organic solvent N-(2-hydroxyethyl)-ethylenediamine(HEEDA)using ammonium?uoride,the rare earth chlorides and a solution of caesium alkoxide of N-(2-hydroxyethyl)-ethylenediamine in HEEDA.In parallel with this approach,a microwave assisted synthesis was carried out which forms nanocrystals of the same material,about50nm in size,in aqueous solution at 200?C/8bar starting from ammonium?uoride,the rare earth chlorides,and caesium?uoride.In case of the nanocrystals,derived from the HEEDA synthesis,TEM images reveal that the particles are separated but have a broad size distribution.Also an occurred heat-treatment of these nanocrystals(600?C for45minutes)led to bulk material which shows highly e?cient light emission upon continuous wave(CW)excitation at978nm.Besides the optical properties,the structure and the morphology of the three products were investigated by means of powder XRD and Rietveld method.
Copyright?2009Helmut Sch¨a fer et al.This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use,distribution,and reproduction in any medium,provided the original work is properly cited.
1.Introduction
In recent years,a broad range of applications,ranging from display devices[1],lasers[2],and biological imaging agents [3–9],which are based on luminescent nanocrystals have been reported.Especially a subgroup of these materials which are able to convert long wavelengths radiation,for example,infrared,into shorter wavelengths by so-called photon upconversion became popular.Excitation in the NIR has some advantages,it induces only a weak auto?uorescence background,avoids photodegradation in biotagging appli-cations,and hence increases the sensitivity of the method. Very well investigated is rare earth doped sodium yttrium ?uoride(NaYF4).Many reports are focused on the synthesis and investigation of(mainly Yb,Er doped)NaYF4.Synthesis procedures for lanthanide doped nanocrystals ofα-NaYF4 [10–19]andβ-NaYF4[11,12,18–24]have been developed. Very recently we investigated nanosized Yb3+,Er3+doped KYF4in detail.As a result of the occurred Fullprof re?nement we found that KYF4,generated in HEEDA at185?C, crystallizes in the cubic(alpha)NaYF4structure[25].We are interested in new upconversion?uorescent materials and had decided to investigate new caesium containing compounds.Ternary?uoride compounds of the type Cs x Yb y F z and Cs x Er y F z have been synthesized by solid state reactions or?uorination of solid compounds at high temperature and led to bulk material.For example Cs3.4Yb12F39.4and CsYb3F10were synthesized and investigated by Al′e onard et al.[26,27],respectively,Marsh[28]The structure of CsErF4was determined by Losch and Hebecker[29].Other groups have investigated the nonlinear optical properties of CsY2F7and CsGd2F7probes doped with di?erent rare earth ions in detail[30–33].The samples were prepared under very drastic conditions starting from the rare earth oxides and the alkalimetal?uorides in aqueous solutions [30–34].Schi?bauer et al.reported on the crystal structure and luminescence of Pr3+doped Cs2KYF6[35].Quaternary systems like Cs2NaLnF6were developed and investigated by Makhov et al.[36],respectively,Tanner et al.[37,38].
Anyway,as far as we know these materials neither had been produced in the nanoscale nor the upconversion prop-erties of Yb3+,Er3+codoped samples have been investigated.
Herein,two synthesis routes are presented,which are suitable in order to achieve rare earth doped crystalline material of CsY2F7in a particle size ranging from less than10nm to more than10μm.The optical properties
were investigated as well as the structural properties based on powder di?ractometry and Rietveld re?nements.As the luminescence of the synthesized nanomaterial was very weak the heat-treated sample,consisting of micrometer-sized grains,turned to be a quite good upconversion emitter.
2.Experimental Section
2.1.Synthesis of the Yb3+,Er3+Doped CsY2F7Particles in HEEDA.CsY2F7:20%Yb,2%Er nanocrystals were pre-pared in the coordinating solvent N-(2-hydroxyethyl)-ethylenediamine(HEEDA)similar to the method described previously[39].The following three solutions were used in the synthesis.
(A)Solution of the Lanthanide Chlorides:a clear solution of3.55g(11.7mmol)of YCl3·6H2O(99.9%,Treibacher Industries), 1.16g(3mmol)of YbCl3·6H2O(99.9%, Treibacher Industries),and0.115g of(0.3mmol)ErCl3·6H2O(99.9%,Treibacher Industries)in about25mL of methanol was combined with25mL of N-(2-hydroxyethyl)-ethylenediamine(99%,Sigma Aldrich).The methanol was removed with a rotary evaporator,and the water was distilled o?in high vacuum at75?C.The remaining slightly cloudy suspension was allowed to cool down to60?C and kept at this temperature under dry nitrogen.
(B)Preparation of the Caesium Alkoxide Solution:a solu-tion of the caesium alkoxide of N-(2-hydroxyethyl)-ethylenediamine(HEEDA)was prepared by dissolving1g (7.5mmol)of caesium metal(Sigma Aldrich)in10mL of HEEDA at20?C under dry nitrogen.
(C)Preparation of the Fluoride Containing Solution:1.55g (42mmol)of NH4F(98%Fluka)were dissolved in about 25mL of methanol and combined with25mL of HEEDA. The methanol was removed with a rotary evaporator at45?C and subsequently in high vacuum at60?C.The remaining solution was kept at45?C under dry nitrogen.
Solution A and solution B were combined and heated to60?C.Subsequently,the?uoride-containing solution(C) which had a temperature of45?C was added under stirring and the resulting mixture was degassed at80?C under vacuum.The reaction mixture was heated to185?C under dry nitrogen and kept at this temperature for13hours. After the transparent solution had cooled down to room temperature,the nanocrystals were precipitated by adding a mixture of200mL of water and200mL of propanol. The precipitate was separated by centrifugation and washed several times by repeatedly resuspending the solid in2-propanol and centrifuging the https://www.wendangku.net/doc/90760890.html,ually,the puri?ed precipitate was directly redispersed in methanol without drying the powder(described hereafter).For XRD measurements the precipitate was dried in air(white powder, yield:2.77g(77%)).
2.2.Heat Treatment of the Particles.the particles were heated under air for45minutes at600?C.2.
3.Microwave Assisted Synthesis.A solution containing 2.36g(7.8mmol)of YCl3·6H2O(99.9%,Treibacher Indus-tries),0.78g(2mmol)of YCl3·6H2O(99.9%,Treibacher Industries),76mg(0.2mmol)of ErCl3·6H2O(99.9%, Treibacher Industries),1.48g(40mmol)of NH4F(98% Fluka)and52mL water was?lled in the80mL reaction vessel of the microwave system.Subsequently,the amount of0.89g (5mmol)CsF was added and the process was started.The maximum radiation power was set to300W,the maximum temperature was set to200?C,and the maximum pressure was set to13.8bar.After radiation for2hours the mixture was allowed to cool down.The precipitate was separated by centrifugation and washed several times with water.For XRD measurements the precipitate was dried in air(white powder, yield:3.45g(71.8%)).
X-ray di?raction data were recorded at room temper-ature on an X’Pert Pro Di?ractometer(Panalytical)with Bragg-Brentano geometry using CuKα1radiation(40kV, 40mA)λ=1.5406?A.The average apparent crystallite size as well as the lattice parameters are evaluated by structure pro?le re?nements of X-ray powder di?raction data col-lected at constant step in scattering angle2θusing Fullprof program[40](version February.2007.LLB,Juan Rodr′?guez-Carvajal,Saclay,France).The Y2O3powder standard was used to determine the instrumental resolution function of the di?ractometer.Emission spectra of colloidal solutions of the nanocrystals and of the pure crystals were measured with a Fluorolog3–22spectrometer(Jobin Yvon)combined with a continuous wave978nm laser diode(LYPE30-SG-WL978-F400).Quartz cuvettes(Hellma,QX)containing solutions of the samples or tubes with powder samples were placed inside the spectrometer and excited by the978nm light via an optical?ber.All spectra were corrected for the sensitivity of the monochromators and the detection system. The upconversion emission spectra of powder samples were measured with the same instrument but in front-face geometry.TEM images were taken with a200kV JEOL JEM-2100microscope equipped with a charged-coupled device-(CCD-)camera(Gatan).The microwave synthesis was performed in a CEM Discover,Kamp-Lintfort,Germany.
3.Results and Discussion
Figure1presents the measured powder di?raction patterns together with the Rietveld re?nements and di?raction lines of KEr2F7(PDF01-086-2454,ICSD040450).Using orthorhombic KEr2F7(space group Pnam,K+and Er3+ replaced by Cs+and Y3+,resp.)as structural model for our samples leads to minimal di?erences between observed and calculated X-ray powder di?raction pro?les.This result can be easily explained by the?ndings of Karbowiak et al.
[41],who found CsGd2F7to be isostructural with KEr2F7 (orthorhombic,space group Pna21).In the case of bulk CsY2F7(see Figure1(b)),the structure was indexed with following lattice constants a=12.30?A,b=13.56?A,and c=7.83?A,and we deduced an estimated particle size of>10μm.Additionally,a small trace of Y2O3was observed (see the second group of Bragg lines under line1b),revealed after the heat treatment of nanocrystalline sample(HEEDA
I n t e n s i t y (a .u .)
2θ (°)
Figure 1:(a):A line pattern of orthorhombic KEr 2F 7?le number PDF 28-01-086-2454(b):Observed X-ray powder di ?raction pro?le of the heat treated sample of CsY 2F 7:78%Y,20%Yb,2.0%Er (grey line),Rietveld ?t (black line)and residuum,(c):observed X-ray powder di ?raction pro?le of the microwave generated sample of CsY 2F 7:78%Y,20%Yb,2.0%Er (grey line),Rietveld ?t (black line)and residuum,(d):observed X-ray powder di ?raction pro?le of CsY 2F 7:78%Y,20%Yb,2.0%Er generated in HEEDA (grey line),Rietveld ?t (black line)and residuum.
synthesis)under air.Powder pattern of the nanocrystalline sample gained from the microwave synthesis is presented in Figure 1(c)with Rietveld re?nement yielding a value of 50nm for the average apparent crystallite size and the
lattice parameters a =12.11?A,
b =13.52?A,and
c =7.84?A.The TEM images reveale
d a broad siz
e distribution from which an averaged particle size in the same range (～50nm)can be extracted (Figure 2).In case o
f the second nanocrystalline sample (Figure 1(d)),the best agreement between observed and calculated pro?les was obtained to the prede?nition of elongated particles in 001direction.Assumin
g elongated particles,the di ?ractogram is well ?tted
by Rietveld method yielding a value of a =12.37?A,
b =13.66?A,
and c =7.82?A for the orthorhombic unit cell and a mean crystallite size of 10×5nm,a value which is in accord with the size distribution observed in TEM images of the particles (Figure 3).Obviously some particles are not single crystallites and contain more than one crystallite and hence the averaged particle size is higher than the corresponding Figure 2:TEM image of CsY 2F 7:78%Y,20%Yb,2.0%Er nanocrystals generated in the microwave synthesis apparatus.
50 nm
Figure 3:TEM image of CsY 2F 7:78%Y,20%Yb,2.0%Er nanocrystals prepared in HEEDA.
size of the crystallites.It should be remembered at this point that there is a limit to the amount of information that can be retrieved from a powder di ?raction pattern and,therefore,we cannot completely assume at this time that the structure of our CsY 2F 7samples is totally identical to orthorhombic KEr 2F 7.
The samples were doped with the sensitizer/activator ion couple Yb 3+/Er 3+and the emission behaviour upon excitation in the NIR was investigated.Figure 4shows the light emission of a 1wt.-%colloidal solution of microwave generated CsY 2F 7:Yb,Er particles in methanol upon exci-tation in the NIR.The laser power was about 30W mm ?2,the overall laser power was 4.5W.The emitted light appears pale green to the eye.The corresponding emission spectrum can be taken from Figure 5.The spectrum is similar to those of Yb,Er doped NaYF 4but there are di ?erences concerning to the splittings of the emission lines which are not yet understood.Generally,rare earth ?uorides doped with the Yb 3+and Er 3+ion couple show light-emission mainly in the
Figure 4:Image of the upconversion luminescence in 1wt.-%coll-oidal solutions of CsY 2F 7:78%Y,20%Yb,2.0%Er nanocrystals in methanol.Excitation at 978nm with a power density of about 30W mm ?2.Overall laser power:4.5W.The laser is positioned on the right side.
400500600700
Wavelength (nm)
I n t e n s i t y (a .u .)
Figure 5:Emission spectrum of a 1wt.%colloidal solution of CsY 2F 7:78%Y,20%Yb,2.0%Er nanocrystals in methanol.
green (Er 3+2H 11/2,4S 3/2→4I 15/2)and red (Er 3+4F 9/2→4I 15/2)spectral region after excitation in the NIR.The green to red ratio (GRR),de?ned as the intensity ratio between the emission bands centered at about 550nm and 670nm,depends on the particle size,the crystallographic phase as well as on the doping concentration.A GRR value of about 0.3can be extracted from the spectrum measured in solution.Mai et al.obtained a very high GRR value of 30for hexagonal phase NaYF 4:Yb,Er nanocrystals,the highest GRR value reported to our knowledge [42].For cubic NaYF 4and cubic KYF 4we determined values of about 0.2[39],respectively 0.26[25].Very recently we found a value of 0.6in case of hexagonal NaYF 4[24].
Unfortunately the luminescence e ?ciency of the nano-particles received from the HEEDA synthesis was
extremely
Wavelength (nm)
Figure 6:Emission spectra of the pure crystals of (a):CsY 2F 7:78%Y,20%Yb,2.0%Er nanocrystals generated in HEEDA,(b):CsY 2F 7:78%Y,20%Yb,2.0%Er nanocrystals generated in the microwave synthesis apparatus.(c):CsY 2F 7:78%Y,20%Yb,2.0%Er nanocrystals generated in HEEDA and heat treated for 45minutes at 600?C,(d):Hexagonal bulk NaYF 4:18%Yb 3+,2.0%Er 3+.
low.Even at high laser power (>6W)it was not possible to detect visible light emission in transparent solutions of the nanocrystals (～1wt.%)in methanol under excitation in the NIR.The dried powder sample however showed visible light emission upon excitation in the NIR at a laser power of about 3W.
In order to evaluate the upconversion quality of the particles,the samples were compared with other well known upconversion phosphors.We used hexagonal bulk NaYF 4:
18%Yb 3+,2%Er 3+synthesized by Kr¨
a mer (University of Bern)as reference substance.Figure 6presents the emission spectra of the two nanocrystalline samples (a,b)together with the heat treated sample (c)and the reference substance (d)obtained after excitation at 978nm with a laser power of about 3.5W.The green to red ratios di ?er distinctly.The corresponding GRR values are 0.2(nanocrystals from HEEDA synthesis),0.24(nanocrystals from the microwave assisted synthesis),respectively,0.05in case of the heat treated nanocrystals.Figure 7,displaying double logarithmic plots of the emitted light intensity versus the power density of the exciting light,shows the upconversion e ?ciency of the synthesized probes in detail.
The upconversion e ?ciency of pure crystals of bulk hexagonal NaYF 4:Yb,Er is,depending on the laser power,1-2orders of magnitudes higher than for the heat treated sample.For example at 4W laser power the emission intensity of the reference phosphor is about 12times as high as that of the particles from the HEEDA synthesis treated at 600?C for 45minutes.The e ?ciency ratio between the reference material and the microwave sample is at 4W laser-power 345.As mentioned before the nanomaterial generated in the HEEDA synthesis is a very weak upconversion emitter.The upconversion e ?ciency of the bulk reference material is approximately 1.5×105times higher than for the nanomaterial generated in HEEDA.The result of this comparison was a little bit disappointing but nevertheless it
104
105
106107
108
109I n t e n s i t y (a . u .)
Laser power (W)
Figure 7:Double logarithmic plots of the emitted light intensity versus the power density of the exciting light for pure crystals of (a):CsY 2F 7:78%Y,20%Yb,2.0%Er generated in HEEDA,(b):CsY 2F 7:78%Y,20%Yb,2.0%Er generated in the microwave synthesis apparatus.(c):CsY 2F 7:78%Y,20%Yb,2.0%Er generated in HEEDA and heat treated for 45minutes at 600?C,(d):Hexagonal bulk NaYF 4:18%Yb 3+,2%Er 3+.
Wavenumber (cm ?1)
3000
2000
1000
(c)
(b)
(a)
T r a n s m i s s i o n
Figure 8:Infrared spectrum of (a):CsY 2F 7:78%Y,20%Yb,2.0%Er generated in the microwave synthesis apparatus,(b):CsY 2F 7:78%Y,20%Yb,2.0%Er generated in HEEDA,(c):the solvent N -(2-hydroxyethyl)-ethylenediamine (HEEDA).
is noteworthy to keep in mind that the microwave generated material is about 430times more e ?cient than the material generated in the organic solvent HEEDA.To sum up we can say that the upconversion e ?ciency of the synthesized probes increases with increasing particle size.This ?nding is not surprising and can be easily explained by the increment of the surface to volume ration with decreasing particle size.It is well known that the quantum yield of luminescent nanoparticles is strongly a ?ected by surface properties of the particles.In the case of lanthanide doped materials the quantum yield and,hence,also the upconversion e ?ciency can be strongly reduced if OH,NH 2,or other groups with vibrational modes of high energy are located in close
proximity to the lanthanide ions.Therefore,the surface ligands and the solvent strongly a ?ect the optical properties of these nanomaterials.In case of high surface to volume ratio (small particles)this lowering e ?ect is of course quite strong,vice versa.The used solvent HEEDA contains a lot of these hampering molecule fragments and hence the probe generate in this solvent shows the lowest UC e ?ciency.The presence of NH 2and OH groups of HEEDA molecules had been proven by FTIR.Figure 8shows the results of the FTIR measurements.The curve (c)belongs to the solvent HEEDA,the curve (b)belongs to the particles gained from the HEEDA synthesis and the curve (a)belongs to the sample from the microwave assisted synthesis.HEEDA molecules on the surface in combination with a very high surface to volume ratio explain the very low upconversion e ?ciency of the particles gained from the HEEDA synthesis.
4.Conclusion
We demonstrated simple routes for the synthesis of nano-and microcrystalline Yb 3+,Er 3+doped CsY 2F 7.Depending on the starting material and the reaction conditions the particle sizes vary.Whereas the reaction carried out in water staring from CsF lead to bigger particles,the reaction carried out in HEEDA starting from the corresponding Cs HEEDA alkoxide lead to real nanosized material.So the nature of the caesium source seems to in?uence the particle growth which we do not understand up to now.Fullprof re?nements of the powder di ?raction data of the probes led to the conclusion that CsY 2F 7,generated in a way presented in this article,crystallizes in an orthorhombic structure known from KEr 2F 7.The average particle sizes,which were also extracted from these Rietveld re?nements,are in accordance with the corresponding values derived from the TEM images.The samples showed visible upconversion emission upon excitation in the NIR.Nevertheless,in comparison with the most e ?cient upconverion phosphor known today,hexagonal bulk NaYF 4:18%Yb 3+,2%Er 3+,the upconversion e ?ciency especially of the sample generated in HEEDA,was very low.
Acknowledgment
The authors are grateful to Dr.Karl Kr¨a mer (University of Bern)for synthesizing the bulk reference material.
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