Journal ofMaterials Chemistry C

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View Article OnlineView Journal | View Issue

Crystal phase tra

aInstitute of Physics, Wroclaw University of

50-370 Wroclaw, Poland. E-mail:

podhorodecki@pwr.edu.pl; Tel: +48 71 320bCenter of Research Excellence in Renew

Petroleum and Minerals, PO Box: 1292, DhacAdvanced Nanofabrication, Imaging and Ch

University of Science and Technology, ThuwdAnalytical Chemistry Laboratory, King

Technology, Thuwal 23955-6900, Saudi Ara

† Electronic supplementary informationanalysis and absorbance spectra of LixNaspectra and FWHM of PL peaks. See DOI:

Cite this: J. Mater. Chem. C, 2014, 2,9911

Received 14th July 2014Accepted 21st September 2014

DOI: 10.1039/c4tc01539h

www.rsc.org/MaterialsC

This journal is © The Royal Society of C

nsition in LixNa1�xGdF4 solidsolution nanocrystals – tuning of opticalproperties†

M. Banski,*a M. Afzaal,b D. Cha,c X. Wang,c H. Tan,d J. Misiewicza

and A. Podhorodecki*a

The influence of precursor composition on the crystallization of LixNa1�xGdF4 is investigated and discussed.

Nanocrystals are prepared from the thermal decomposition of trifluoroacetates in the presence of

trioctylphosphine oxide to provide control over particle size. A crystal phase transition from hexagonal to

cubic and to tetragonal is observed by increasing lithium trifluoroacetate (Li-TFA) in the solution.

Controlling the composition of LixNa1�xGdF4 nanocrystals results in modified crystal field symmetry and

emission properties from doped europium (Eu3+) ions. We report that for lithium (Li+) substitution <15%,

the hexagonal crystal field is preferred, while the Eu3+ emission is already tuned, whereas at higher Li+

substitution, a phase change takes place and the number of crystalline matrix defects increases which is

reflected in the optical properties of Eu3+. From Eu3+ emission properties, the optimum Li+ content is

determined to be �6.2% in the prepared LixNa1�xGdF4 nanocrystals.

Introduction

Fluoride nanocrystals (NCs) doped with various lanthanide ionshave been the subject of recent scientic investigations, due totheir exceptional optical properties.1,2 Among them, sodiumyttrium uoride (NaYF4) and magnetic sodium gadoliniumuoride (NaGdF4) NCs are the leading candidates for the opti-cally active bio-markers.3,4 For this purpose, two lanthanidedoping strategies have been much proposed: (1) Stokes-shiedemission due to e.g. Eu3+ and/or Tb3+,5,6 and (2) anti-Stokes-shied emission due to e.g. up-converting Yb3+–Er3+ lanthanideions pair.7 The rst approach leads to temperature-calibratedemission which allows a bio-marker to be used as a nanoscale,optical thermometer.8 The second approach allows observationof an autouorescence-free emission in the visible range due toinfrared excitation which, moreover, results in extended pene-tration depth.9 The potential of up-converting NCs in imaginghas been recently demonstrated at a single particle level.10

Technology, Wybrzeze Wyspianskiego 27,

mateusz.banski@pwr.edu.pl; artur.p.

23 95

able Energy, King Fahd University of

hran, 31261, Saudi Arabia

aracterization Laboratory, King Abdullah

al 23955-6900, Saudi Arabia

Abdullah University of Science and

bia

(ESI) available: Results of ICP-AES

1�xGdF4 nanocrystals, and absorbance10.1039/c4tc01539h

hemistry 2014

Moreover, recent investigations have focused on the opticalmultiplexing required for e.g., data storage and security, probes,which can be achieved by tuning the emission color11 orlifetime.12,13

A new concept in the eld of upconversion efficiency waslately introduced by Wang et al. who proposed energy clusteringat a sublattice level for securing a four-photon-promoted violetupconversion with an unusual efficiency.14 Besides, trials ofmodication of the matrix structure are performed in order totune and maximize the optical output from the NCs. Yin et al.reported an enhanced red emission from GdF3:Yb

3+–Er3+

up-converting NCs by Li+ doping.15 Then, Mao et al. observeda reduction of up-converting emission intensity fromNaYF4:Yb

3+–Er3+ NCs doped with Li+ ions.16 However, in thiscase, relatively large (several hundreds of nanometers)particles synthesized by a hydrothermal technique are investi-gated. On the other hand, 7 mol% of Li+ co-doping in smallNaGdF4:Yb

3+–Er3+ NCs (�20 nm) resulted in a signicantenhancement of green and red up-conversion emissionintensities (about 47 and 23 times, respectively).17 The authorsattributed the luminescence enhancement due to the distortion ofthe local asymmetry around Er3+ ions. Another mechanism foremission enhancement is proposed by Zhao et al.18 The authorshave commented that an improvement of 5 to 8 times in theemission intensity of 7mol% Li+ doped NaYF4:Yb

3+/Tm3+ NCs is aresult of improved sample crystallinity and distortion of the localsymmetry. It is also reported that due to 4 mol% Li+ doping, thequenching concentration of Er3+ in NaYF4:Er

3+–Yb3+ increasesfrom 2% to 4mol%.19Meanwhile, the green and red emissions areenhanced 2 and 3.3 times, respectively.

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Despite several reports on the tuning of up-conversionemission by Li+ doped ions, studies of Eu3+, which play a role oflocal crystal eld probes, in the LixNa1�xGdF4 NC matrix arelimited. The paramagnetic NaGdF4 matrix is of particularinterest, due to an increase in its magnetization aer Li+

doping.17 Moreover, the Gd3+–Eu3+ ion pair is well-known forefficient conversion of absorbed high-energy photons into twovisible photons (quantum cutting).20 Infact, the Li+ basedmatrixhas been already found to be the ideal candidate for thispurpose, and 175% efficiencies of the quantum cutting processare reported for the LiGdF4 system.21

Herein, we describe Eu3+ doped sub-10 nm LixNa1�xGdF4solid solution NCs prepared by a co-thermolysismethod. Initially, we prepare NCs in a wide compositional range(x¼ 0–1) evaluating the efficiency of Na+ by Li+ substitution. Theresulting particle sizes and crystal phase transitions areanalysed and evaluated. Lastly, we investigate the inuence ofLi+ ion doping on the optical properties of LixNa1�xGdF4including emission, excitation and energy transfer processes.

Experimental detailsChemicals

Sodium triuoroacetate Na(CF3COO) (98%) (NaTFA), lithiumtriuoroacetate Li(CF3COO) (95%) (LiTFA), europium(III) tri-uoroacetate trihydrate Eu(CF3COO)3$3H2O (98%) (EuTFA3),gadolinium(III) oxide (99.9%), triuoroacetic acid (99%) andtrioctylphosphine oxide (technical grade, 90%) (TOPO) werepurchased from Sigma-Aldrich Ltd. and used as received.Gadolinium(III) triuoroacetate (GdTFA3) was prepared fromgadolinium oxide and triuoroacetic acid according to theliterature method.22

Synthesis of LiGdF4–NaGdF4 nanocrystals

We carried out a series of reactions involving different relativeconcentrations of Na+ and Li+ precursors, to explore the crystalphase evolution of LixNa1�xGdF4 solid-solution colloidal NCs.

The precursors are utilized in a modied co-thermolysisapproach, originally proposed by Yan's group, for the prepara-tion of Eu3+ doped NaYF4 NCs.23 We utilized TOPO ligands asoriginally used by Shan et al.24 and reaction conditions, opti-mized by our group for obtaining ultra-small uoride NCs.25,26

The synthesis with a varying Li+/Na+ precursor ratio isperformed, while keeping the total amount of alkalineprecursors constant (1.0 mmol). For example, to prepareLi0.3Na0.7GdF4:Eu

3+(5%) NCs, a mixture of LiTFA (36.1 mg,0.3 mmol), NaTFA (95.2 mg, 0.7 mmol), GdTFA3 (188.8 mg,0.38 mmol), EuTFA3 (10.0 mg, 0.02 mmol) and TOPO (8 g,20.8 mmol) is placed in a 100ml three-neck round-bottom ask.The ask is placed on a heating mantle and heated up to 120 �Cunder vacuum for 30 min using standard Schlenk linetechniques. The temperature is increased to the desired growthtemperature (340 �C) within 10 min and heated for 60 min.

Aer heating, the reaction solution is cooled to 70 �C and anexcess of dry methanol is added to precipitate the NC product.The solutions are centrifuged at 10 000 rpm for 10 min and the

9912 | J. Mater. Chem. C, 2014, 2, 9911–9917

resulting NCs are easily dispersed in non-polar solvents, e.g.cyclohexane. The precipitation/dissolution procedure isrepeated at least three times to remove any excess TOPO orunreacted precursors. Finally, the NCs are dispersed in cyclo-hexane and further centrifuged to remove any insolubleproduct(s).

Structural and optical characterization

The chemical composition of nanocrystals are measured byinductively coupled plasma atomic emission spectroscopy (ICP-AES) using a Perkin Elmer's Optima 8300 DV spectrometer. Amicrowave digestion method (CEMMars 5) with HCl (1 ml) andHNO3 (4 ml) held at 200 �C for 15 min is used to prepare thesamples. X-ray powder diffraction patterns are recorded on aBruker D8 diffractometer (Cu-Ka) at a scanning rate of 0.5�

min�1 from 20� to 70�. The transmission electron microscopy(TEM) images are examined with a FEI titan super twin at 300kV. The samples are dispersed in ethanol by sonication and adrop is placed on a copper grid and dried in air. Nanocrystallinedomain sizes (D) are calculated using the Scherrer equation,D ¼ 0.89l/(B cos q). Here, D is the domain size to bedetermined, l is the X-ray wavelength, B is the width of thediffraction peak of interest, and q is the angle of thecorresponding diffraction peak. Photon TechnologyInternational Inc. systems equipped with a ash xenonlamp and a strobe detector (both coupled with mono-chromators) are used to observe photoluminescence (PL) andphotoluminescence excitation (PLE) spectra. The absorptionspectra are measured on a JASCO V-570 spectrophotometer.

Results and discussion

The exact amount of Li+/Na+ ratios in LixNa1�xGdF4 NCs isimportant for working out the effect of substitution on theproperties of materials. Inductively coupled plasma-opticalemission spectroscopy (ICP-AES) is used to determine theamount of Li+ incorporated within the NCs (y) and arecompared with the total amount of Li-TFA precursor usedduring the synthesis (x). From Fig. S1,† it is clear that Na+ ispreferred over Li+ and the resulting Li+ concentration inprepared samples is signicantly lower than expected values(dashed line). It is noteworthy that the total concentration ofmonovalent cation precursors, ([Na-TFA] + [Li-TFA]), is 2.5 timesgreater than [Gd-TFA] + [Eu-TFA], in all the experiments. Evenfor x ¼ 0.5, the nal NCs could be theoretically synthesizedwithout any Li+ substitution. However, the ICP-AES analysesindicate that Li+ incorporates into the NCs even for smaller xvalues. When the value of x is 0.1–0.5, �6–14% of Na+ ions aresubstituted by Li+. The efficiency of substitution increasessignicantly for higher x values (>0.6) and reaches up to 62% asx ¼ 0.9 approaches. A pure LiGdF4 sample is also prepared forcomparison purposes.

The crystal phase analysis is conducted using X-ray powderdiffraction (XRD) and compared with the corresponding inter-national centre for diffraction data standard patterns (ICDD no:27-0699, 27-0697, 27-1236 for cubic NaGdF4, hexagonal NaGdF4

This journal is © The Royal Society of Chemistry 2014

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and tetragonal LiGdF4, respectively) (Fig. 1). Based on ourprevious results, NaGdF4 NCs prepared in TOPO result in a purehexagonal phase under various reaction conditions,25,26

whereas, LiGdF4 NCs crystallize in a thermodynamically stabletetragonal phase.27 Thus, a crystal phase transition is expectedby increasing the Li+ content within the NC matrix.28 Based onthe XRD results, it is observed that the hexagonal phase ofLixNa1�xGdF4 NCs is conserved for x in the range between 0 and0.5. For higher x values, transition from the hexagonal to cubicphase takes place. For x ¼ 0.7, a mixture of both cubic andhexagonal phases is seen. The pure cubic phase is determinedfrom the XRD pattern for x ¼ 0.8. While for x¼ 0.9, a mixture ofboth cubic and tetragonal phases is identied. In general,increasing the Li-TFA precursor concentration in the reactionmixture leads to crystal phase transitions in the order ofhexagonal to cubic and to tetragonal. Moreover, these transi-tions are in relation with increased Li+ ions concentration in theLixNa1�xGdF4 NC samples.

One key advantage of utilizing TOPO during the synthesis isthe resulting sub-10 nm particle sizes.25 This is highly favored by

Fig. 1 XRD patterns of resulting LiGdF4–NaGdF4 nanocrystals andcomparison made with corresponding ICDD values.

This journal is © The Royal Society of Chemistry 2014

many applications in particular for biomarkers.29 In the inves-tigated sample, an average particle size (�5 nm) for NaGdF4 NCsis calculated from XRD broadening using the Scherrer equation.For LixNa1�xGdF4 samples, the diffracted peaks initially becamenarrow as the amount of Li+ ion is increased from x ¼ 0 to 0.3aer which diffraction patterns (x > 0.3) broadened again. Itimplies that the synthesis involving a higher concentration ofLi-TFA not only induces cation substitution, but also affectsparticle size as well.

The effect of Li+ concentration on the particle size is evidentfrom transmission electron microscopy (TEM) images (Fig. 2).Highly crystallized (indicated by lattice fringes) and mono-dispersed NCs are clearly visible in the given images. For everysample, a size distribution based on at least 150 NCs is calcu-lated and the results are shown in Fig. 3. The average particlesizes initially increase as the value of x goes from 0 to 0.2 andthen decrease for higher Li+ concentration. Two samples (x ¼0.7 and 0.9) are out of the trend as their diameters are bigger.Interestingly, both of these samples are also a mixture of twophases as shown by their corresponding XRD patterns. Thus, wewill not consider these two samples during our optical investi-gations, due to difficulties in determining the factor(s) govern-ing the property changes. Note that the average diameters ofsamples prepared with x ¼ 0, 0.8 and 1.0, which correspond tohexagonal, cubic and tetragonal phases, are equal within theexperimental error (�3.5 � 0.5). Thus, we excluded the sizeeffect as a factor governing the optical properties of NCs.

On the one hand, the substitution of at least 14% of Na+ byLi+ ions is required to induce the crystal phase transition. Onthe other, a minimal amount of smaller Li+ ions substitutingbigger Na+ ions cause crystal symmetry distortion in the NaGdF4matrix.18,28,30 In the results, the crystal eld in the vicinity oflanthanide ions (e.g. Eu+3) is changed and their optical prop-erties are altered. The net result is that Li+ substitution affectsboth the excitation and emission properties of doped NCs.

Fig. S2† shows a series of absorption spectra for doped andundoped samples. Two peaks present at 272 and 395 nm are aresult of optically active Gd3+ and Eu3+, respectively. It isimportant that the relative Eu3+–Gd3+ ratio of absorption peakintegrals is almost constant (�0.071 � 0.016) for all the inves-tigated samples, which corresponds to less than 1.2% deviationof Eu3+ doping (Fig. S3†). We have made an assumption thatdespite of Li+/Na+ variations in the matrix, the percentage ofEu3+ doping is almost unchanged in the NCs.

In Fig. S2,† the absorbance of TOPO ligands is also presentedfor clarication purposes. The difference between the absorp-tion of pure TOPO and TOPO-caped NCs in an �230–350 nmrange is assigned to charge transfer (CT) transition. This is dueto Eu3+ at the NC surface interacting with oxygen at the ligandsterminal.31,32 A small contribution can also arise from theabsorption at crystal matrix defects.

A more complete analysis of Eu3+ excitation is carried out byrecording photoluminescence excitation (PLE) spectra as shownin Fig. 4a. It is evident that the most intense emission of Eu3+

ions observed for a 272 nm excitation wavelength correspondsto 8S7/2–

6IJ transition of Gd3+ ions. Several less intensive exci-tation bands for Gd3+ and Eu3+ are also present in the spectra.

J. Mater. Chem. C, 2014, 2, 9911–9917 | 9913

Fig. 2 TEM images of LixNa1�xGdF4 nanocrystals (a) x ¼ 0, (b) x ¼ 0.1, (c) x ¼ 0.2, (d) x¼ 0.3, (e) x ¼ 0.5, (f) x ¼ 0.8, (g) x ¼ 0.9, and (h) x ¼ 1.0. Theinset shows their corresponding HR-TEM images.

Fig. 3 Average diameter of NCs calculated from TEM as a function of x– fraction of the Li-TFA precursor. The corresponding crystal phasesare marked as shaded areas: right inclined lines – hexagonal, verticallines – cubic, and left inclined lines – tetragonal.

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More importantly, the intensity of the band at 272 nm (Gd3+)compared to the band at 395 nm (Eu3+) initially increases (x ¼0–0.3) and then decreases for higher Li+ loadings (Fig. 4c). Then,a drop in the Gd3+–Eu3+ integrated PLE intensity ratio is noticedfor x$ 0.5, which expresses the reduction in Gd3+ to Eu3+ energytransfer (ET) efficiency. It is consistent with a previous obser-vation which showed a decrease in the inter-ionic interactionbetween neighbouring Er3+ ions in the presence of Li+ ions inthe NaGdF4 matrix.17

Moreover, a broad band between 250 and 330 nm (shadedarea) can be distinguished from PLE spectra for the selectedsamples. This band seems to be completely absent for low Li+

doping. However, its intensity increases and becomes signi-cant for samples crystallized in cubic and tetragonal phases.

9914 | J. Mater. Chem. C, 2014, 2, 9911–9917

Most probably, this band originates from the absorption at thedefect states followed by energy transfer to lanthanide ions(defects/Gd3+ integrated PLE intensity ratio are presented inFig. 4c). The same defect states are also probably responsible fora new channel of Gd3+ relaxation and restricted Gd3+ to Eu3+

energy transfer, as discussed above.The symmetry distortion of the hexagonal phase can be

discussed based on the interpretation of photoluminescence(PL) data. In Fig. 4b, the PL spectra for all the samples using anexcitation energy of 272 nm are presented. Two main transi-tions at �590 and �612 nm correspond to 5D0–

7F1 (magneticdipole type – MD) and 5D0–

7F2 (electric dipole type – ED),respectively. A blue-shi of ED transition is clearly visible withincreased Li+ doping and can be attributed to the sensitivenature of ED transitions. This is conrmed by the5D0–

7F2/5D0–

7F1 (ED/MD) integrated intensity ratio given inFig. 4d. A monotonic increase in the non-centrosymetric char-acter of the crystal eld is determined for NCs doped with Li+

ions. Then, it decreases for pure tetragonal LiGdF4 NCs. Theseresults indicate that the crystal eld distortion in the hexagonalphase is primarily related to the amount of Li+ substituted inthe NCs. The reduced local crystal eld symmetry around up-converting ions is already reported by Zhao et al. and Chenget al. for Tm3+ and Er3+ ions, respectively.17,18 The deviation ofspectroscopic sites symmetry from that of crystallographic sitesin disordered structures was recently explained by Tu et al. forthe Eu3+ doped ions in the NaYF4 matrix.33 The proposedexplanation is consistent with our ndings on the role ofsmaller Li+ cations substituting Na+ in LixNa1�xGdF4 NCs.

In these reports, a signicant increase of the up-conversionemission is observed for 7 mol% of Li+ doping and it wasassigned not only due to the modication of the local crystal

This journal is © The Royal Society of Chemistry 2014

Fig. 4 PLE (a) and PL (b) spectra of LixNa1�xGdF4 nanocrystals. Emission wavelength for PLE was 612 nm, whereas PL spectra were recorded witha 272 nm excitation wavelength and their intensity were normalized at 590 nm. Ratios of Gd3+ (272 nm) to Eu3+ (395 nm) and defects to Gd3+

integrated intensity PLE bands (c). Ratios of 5D0–7F2/

5D0–7F1 and

5D1–7F2/

5D0–7F1 PL integrated intensity bands (d).

Fig. 5 Effective decay times (seff) and stretching parameters (b) (a)fitted for PL decays of LixNa1�xGdF4 NCs (lexc ¼ 272 nm and lem ¼

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eld,17 but also due to improved crystallinity.18 In our case, bycomparing defects/Gd3+ and 5D0–

7F2/5D0–

7F1 integrated inten-sity ratios, (Fig. 4c and d) it is noted that both changes follow avery similar trend from x ¼ 0–0.9. The contribution from defectstates increases as the non-centrosymetric character of thecrystal eld rises. The smaller ionic radius of Li+ is the mostprobable cause of these defects states.34 A signicant stress inthe crystal matrix is induced due to cationic radii inconsistency,which can be relaxed by vacant or excessive F� or O� ions. Thehypothesis that oxygen is incorporated into the crystal matrixto balance the Li+ substitution is supported by the broadband in PLE spectra (250–330 nm). Such bands were alreadyfound to be present in oxygen stabilized cubic NaYF4:Eu

3+ andNaGdF4:Eu

3+ NCs.25,35

In PL spectra, additional optical transitions are present atshorter wavelengths, which originate from the upper excitedstate (5D1 and 5D2) transitions. These transitions are observedwhen nonradiative recombination is inefficient to relax Eu3+

ions to the lowest excited state (5D0).31 In the investigatedsystems, the ratio of 5D1–

7F2/5D0–

7F1 intensity transitionsdecreases with increasing Li+ content and a signicant drop isobserved when the hexagonal phase is converted to cubic and/ortetragonal phase (above x ¼ 0.5) (Fig. 4d). In Fig. 4d, it can alsobeen seen that the 5D1–

7F2/5D0–

7F1 ratio follows an oppositetrend to the crystal eld dependent 5D0–

7F2/5D0–

7F1 (ED/MD)ratio. Crystal lattice defects may impact on both 5D1–

7F2/5D0–

7F1and 5D0–

7F2/5D0–

7F1 ratios due to the inuence on the rate ofnon-radiative recombination and the local crystal eld. Thus,these results conrm the importance of matrix defects and theirinuence on the optical properties of Eu3+ with increasing Li+

substitution.To further analyze the effect of Li+ substitution on the optical

properties, PL decay measurements are performed under direct

This journal is © The Royal Society of Chemistry 2014

(lexc ¼ 395 nm) and indirect (through Gd3+ at lexc ¼ 272 nm)excitation of Eu3+ ions (Fig. 5). The experimental data are ttedwith a stretched exponential function, taking into account the

612 nm) (b).

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Fig. 6 Photoluminescence quantum yield (PL QY) as a function ofprecursor composition (x).

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non-homogeneity of the emitting centers which result in thedistribution of PL decay times.36

Fig. 5b shows the results of PL decay in the case of an indi-rect Eu3+ excitation. An initial PL increase is clearly visible forNaGdF4 NCs and vanishes aer Li+ doping levels are increased.Assuming that the Eu3+ concentration is constant within allinvestigated LixNa1�xGdF4 samples, which is justied by a smallvariation in the Eu3+–Gd3+ absorption peak ratio (Fig. S3†), weexclude cross-relaxation as a mechanism responsible for fastdepopulation of the 5D1 state and the lling 5D0 state. Moreover,if cross-relaxation is present, it will inuence both rise anddecay times to a much bigger extent, as already shown.31

Another possible mechanism is related to the matrix defects,which can act as nonradiative recombination centers, leading toquenching of excited states.37 Oxygen impurities in our matrixsuit well to this hypothesis as less massive atoms, typically,show local vibrational modes of higher frequency.38 Theobserved shortening of rise and decay times of the 5D0 state is ingood agreement with the defect state contribution whichincreases signicantly for x > 0.5. This result is also in-line witha decrease in the 5D1–

7F2/5D0–

7F1 ratio, conrming the defect-enhanced recombination from the higher excited states.

Fitting parameters obtained for PL decays are presented inFig. 5a. An effective PL decay time (seff) varies with Li+ content.There are two distinguishable areas. When seff is longer andshorter than 4.5 ms, it corresponds to hexagonal and cubic/tetragonal phases, respectively. We also notice that seff is inde-pendent of the excitation wavelength (395 nm and 272 nm).

The b parameter is close to 1 for all samples in the hexagonalphase (x # 0.5). For higher x values, b gradually decreases to<0.9. The inhomogeneous crystal eld induced by Li+ ions,related crystal matrix defects, and smaller particle sizes aremost probably responsible for the results. Moreover for x$ 0.5,we observe an increased difference in the b value determinedfor direct (395 nm) and indirect (272 nm) excitation. Thisconrms that energy transfer from Gd3+ to Eu3+ is inuenced bythe Li+ substitution, as stated above.

The PL quantum yield (QY) is one of the most importantparameter for evaluating the optical quality of light emitters.We calculated the PL QY of the NaGdF4:Eu

3+ sample to be 41%using Rhodamine 6G as the reference. Then, PL QYs of the othersamples were calculated by comparison integrated PL intensityand absorption at an excitation wavelength (272 nm) (Fig. 6).The PL QY is found to be almost the same for both NaGdF4 andLiGdF4 NCs. For solid-solution NCs, PL QYs vary with compo-sition and reach a maximum (56%) at x ¼ 0.3. This samplecrystallizes in the pure hexagonal phase, with an averagediameter of 9.2 � 2.0 nm. From ICP-AES analysis, 6.2% of Li+

substitution takes place in the sample. It shows a minimumnumber of defects (oxygen impurity) and low probability ofnonradiative relaxation which is indicated by the intenseemission from the 5D1 state. Due to crystal eld distortion, theintensity of ED transition at 612 nm is increased and seff isdecreased as compared to lithium-free samples. Moreover, themost efficient Gd3+ to Eu3+ energy transfer is observed for thissample as well. In summary, the sample obtained from0.3 mmol of Li-TFA and 0.7 mmol of Na-TFA precursor

9916 | J. Mater. Chem. C, 2014, 2, 9911–9917

composition yields the optimum NC matrix for the opticallyactive Eu3+ ions, amongst all the investigated samples.

Conclusions

We investigated the effect of precursor composition on thecrystallization of LixNa1�xGdF4 solid solution NCs. Byincreasing Li+ ions within the NC matrix, a crystal phase tran-sition from hexagonal to cubic and to tetragonal is observed.Crystal phase transition is accompanied by incorporation ofoxygen related matrix defects. Thus, controlling the composi-tion of LixNa1�xGdF4 NCs leads to changes in the crystal eldsymmetry and the nonradiative recombination inuence on theemission properties of Eu3+ ions. Finally, the highest Eu3+

photoluminescence quantum yield (56%) is calculated for thesample containing 6.2% of Li+ cations obtained from 0.3 mmolof Li-TFA and 0.7 mmol of Na-TFA alkaline precursors.

Acknowledgements

The authors would like to thank the National Centre forResearch and Development for their nancial support underthe LIDER project no. 014/L-2/10. MB would like to acknowledgeThe Iuventus Plus program (no. IP2011 001271) and Foundationfor Polish Science (FNP) “Start” program for the nancialsupport. MA wishes to acknowledge NSTIP strategic technolo-gies program number (12-ENE3204-04) for the nancialsupport.

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