Controlling energy transfer between multiple dopantswithin a single nanoparticleJeffrey R. DiMaio*†, Clement Sabatier‡, Baris Kokuoz*, and John Ballato*

*Center for Optical Materials Science and Engineering, School of Materials Science and Engineering, Advanced Materials Research Laboratory,Clemson University, Anderson, SC 29625; and ‡Ecole Nationale Superieure de Physique de Strasbourg, Parc d’Innovation, BoulevardSebastien Brant BP 10413, 67412 Illkirch Cedex, France

Communicated by Anthony E. Siegman, Stanford University, Stanford, CA, December 19, 2007 (received for review May 8, 2006)

Complex core-shell architectures are implemented within LaF3

nanoparticles to allow for a tailored degree of energy transfer (ET)between different rare earth dopants. By constraining specificdopants to individual shells, their relative distance to one anothercan be carefully controlled. Core-shell LaF3 nanoparticles dopedwith Tb3� and Eu3� and consisting of up to four layers weresynthesized with an outer diameter of �10 nm. It is found that byvarying the thicknesses of an undoped layer between a Tb3�-doped layer and a Eu3�-doped layer, the degree of ET can beengineered to allow for zero, partial, or total ET from a donor ionto an acceptor ion. More specifically, the ratio of the intensities ofthe 541-nm Tb3� and 590 nm Eu3� peaks was tailored from <0.2 to�2.4 without changing the overall composition of the particles butonly by changing the internal structure. Further, the emissionspectrum of a blend of singly doped nanoparticles is shown to beequivalent to the spectra of co-doped particles when a core-shellconfiguration that restricts ET is used. Beyond simply controllingET, which can be limiting when designing materials for opticalapplications, this approach can be used to obtain truly engineeredspectral features from nanoparticles and composites made fromthem. Further, it allows for a single excitation source to yieldmultiple discrete emissions from numerous lanthanide dopantsthat heretofore would have been quenched in a more conventionalactive optical material.

core-shell � rare earth

The past 10 years have witnessed an increase in research onrare earth-doped nanoparticles (NP) because of the numer-

ous applications to which they may be applied. The rare earthions, typically trivalent, although sometimes di- or tetravalent,can exhibit luminescent emissions ranging from 172 nm to �7�m providing that they are doped into a host of high intrinsictransparency and low vibrational energy. Because of such abroad spectral range of potential emissions, active NPs are beingconsidered for practical use in LEDs (1), solar cell energyconversion (2, 3), lasers and amplifiers (4), and biologicalassaying (5, 6).

Heavy metal halide crystals are known to be excellent hostmaterials for rare earth ions because of their intrinsically lowphonon energies. Unfortunately, they also tend to be hygro-scopic, brittle, and not thermally robust. One solution to thisproblem was found in the development of glass-ceramics, wherehalide nanocrystallites are nucleated in an oxide glass matrix.Some of the optically active rare earth (RE) ions partition intothe crystallites and emit from lower vibrational energy hostnanocrystals with improved quantum efficiencies than if theywere emitting from the oxide host matrix (7). For example, LaF3nanocrystals were precipitated from oxyfluoride compositionsby Dejneka (8) to create glasses that were more easily processedthan fluoride glasses while maintaining their superior opticalproperties over oxide glasses (8). While this approach is bene-ficial for improving the emission properties of a glass andmaintaining processability, it has two major limitations. The firstlimitation of the glass-ceramics relates to the necessity to develop

a specific glass composition that can form the desired crystallites.The second limitation is that discrete independent emissionsfrom multiple dopants still would be quenched because of energytransfer (ET), because one cannot control the partitioning ofdifferent rare earths into the precipitated nanocrystallinephases.

A solution to these issues can be found in the synthesis of NPsfollowed by the incorporation of the NPs into a processable hostmatrix. Because this process requires the growth of the particlesindependently from the composite matrix into which the parti-cles will be incorporated, the NP composition is not dictated bythe matrix composition. As discussed above, ET places a limi-tation on the ability to realize tailored independent emissionsfrom a co-doped material. Ideally, one would be able add thespecific spectrum of an ion and place that ion into a given hostwith no regard for any other lanthanides that were also dopedinto the host. In this way, ultimate tailorability could be achievedby the type and amount of lanthanides in the host. This problemcan be solved by constraining the lanthanide ions to the separateregions of the NPs so that the ions do not interact with oneanother. Using NPs singly doped with different lanthanides, onecan produce a composite structure that has many types of NPswith the lanthanides constrained to the host particle. This wasoriginally suggested by Ballato and Riman in 1996 as a means forproducing broadband amplifiers and volumetric displays (9, 10).More recently, white-light emission through up-conversion of a980-nm pump using three types of lanthanide-doped NPs (Tm�3,Er�3, and Eu�3) has been achieved (1). All researchers to datehave concluded that the only means of producing materials withmultiple lanthanides and no ET is through the use of NPs eachdoped with a particular lanthanide. In this way, the use of NPshas an advantage over the glass-ceramic.

The approach taken in this work allows multiple lanthanidesto be incorporated into a single NP by using complex core-shellarchitectures. More specifically, LaF3 is chosen because in bothbulk (11, 12) and NP form (13–15) it is a well studied opticalmaterial that possesses low vibrational energies, high thermaland chemical stability, and high solid-solubility for opticallyactive rare earth (co)dopants. The method of synthesis used hereis an extension of the procedure developed by Dang et al. (13)and modified by Stouwdam and van Veggel (14). It has beenobserved that lanthanide emissions are quenched when incor-porated into NPs. This has been postulated to arise from the highsurface area of NPs (16), in which a large fraction of the dopantions reside in surface sites and are prone to quenching fromadsorbed species such as OH (17). To mitigate this issue, passiveLaF3 shells are grown around the lanthanide-doped core (re-

Author contributions: J.R.D. and J.B. designed research; J.R.D., C.S., and B.K. performedresearch; J.R.D. and B.K. analyzed data; and J.R.D. and J.B. wrote the paper.

The authors declare no conflict of interest.

Freely available online through the PNAS open access option.

†To whom correspondence should be sent at the present address: Tetramer Technologies,657 South Mechanic Street, Pendleton, SC 29670. E-mail: dimaio@clemson.edu.

© 2008 by The National Academy of Sciences of the USA

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ferred to within this text as ‘‘simple core-shell’’ NPs). NPs grownin this way have been shown to exhibit radiative quantumefficiency and lifetime values that approach those of the bulkLaF3 (15, 18). To make complex core-shell structures, we haveextended Stouwdam and van Veggel’s method for producingsimple core-shell particles to produce multiple concentric shells.LaF3 NPs doped with Eu3� and Tb3� were synthesized withvarying core/shell architectures. Characterization using x-raydiffraction (not shown) and high-resolution TEM proved theNPs to be crystalline LaF3 with an average particle size of �10nm. For detailed methods used for the analysis of the complexcore-shell NPs, the reader is referred to ref. 19. Fig. 1 shows themultiple crystalline faces of a single NP. Doping concentrationsof the lanthanides into the LaF3 NPs were determined to beacceptably close to the targeted 1:1 level (Eu/Tb � 1.20 � 0.09)by using x-ray fluorescence.

Results and DiscussionTerbium (Tb3�) and europium (Eu3�) were chosen as dopantsto study the degree of ET within a single NP because both exhibit

a strong, easily measured, visible photoluminescence and Tb3�

is known to act as a sensitizer to Eu3� (20–22). The excitationspectra of the 541-nm Tb�3 and 590-nm Eu�3 emissions forseparate individually doped LaF3 NPs (i.e., Tb.2La.8F3 andEu.2La.8F3) were measured and are shown in Fig. 2a. Anexcitation wavelength of 375 nm was chosen for the emissionspectra because it would excite both the Tb.2La.8F3 andEu.2La.8F3 NP as well as a 1:1 blend. It can be seen in Fig. 2b thatthe luminescence spectra of the mixed NP solution is approxi-mately the sum of the peak intensities of the two ‘‘pure’’ NPemissions. When Tb�3and Eu�3 are co-doped into a particle, theTb�3 ion transfers its energy to the Eu�3 ion. Fig. 2c shows the5D4(Tb) 3 5D1(Eu) ET path that usually results in a greatlydiminished Tb�3 emission. The presence of the 541-nm peak inFig. 2b indicates that there is no significant ET between lan-thanide ions when the singularly doped NPs are blended. Thiscorroborates the original postulation of Ballato and Riman, aswell as the work of Stouwdam and van Veggel on individualsingularly doped NP blends.

There are numerous applications that would benefit from thisapproach to isolating lanthanide-doped NPs within a composite.However, there also are some disadvantages. One has to be surethat there is appropriate mixedness of the NPs within thecomposite because there is no guarantee that the particles areevenly dispersed. If one type of NP tends to agglomerate orsegregate more than another, the ratios of the different lan-thanides could be altered during processing and yield spatiallyinhomogeneous optical properties. Most importantly, if therewas a desired degree of ET, there would be no control over itbetween the different NPs. While separate NPs for differentlanthanide ions allow for the elimination of ET, a tailored degreeof ET between the Eu3� and Tb3� can be controlled continu-ously by using a complex core-shell architecture where individualdopants are confined to layers (or shells) of a single NP.

In this work, LaF3 NPs were grown with a Eu�3 core and threeadditional outer shells. These shells were either undoped LaF3or Tb�3-doped LaF3. Fig. 3 shows models of the four differentcore-shell architectures that were developed. The first structure(Fig. 3a) is the simple core-shell NP. In this particle, the outer(passive) LaF3 shell acts to prevent quenching by the host matrix(either a solvent or some material matrix). The three additionalconstructions (Fig. 3 b–d) show complex core-shell structureswith varying relative thicknesses of the undoped LaF3 layerintermediate between the inner and outer doped shells. Thislayer serves to control the ET between doped layers by sepa-rating the ions by a prescribed distance. The thickness of thislayer was calculated by assuming that the ratio of the volume of

Fig. 1. High-resolution TEM image of complex core-shell LaF3 NPs.

Fig. 2. Excitation and emission spectra for Tb3�- and Eu3�-doped LaF3 NPs. (a) Excitation scan for Tb:LaF3 and Eu:LaF3 measuring the 541- and 590-nm peaks,respectively. (b) Emission spectra for the singularly doped NPs and a mix of the NPs. (c) Energy level diagrams (22) for Tb3� and Eu3� showing the excitation,radiative emission transitions, and energy transfer (E.T.) path in a co-doped sensitized system.

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material added for the growth of a layer was equal to the ratioof the volume for each layer. This method has been shown toaccurately predict the core-shell structures in doped LaF3 NPs(19). With the ratio of the volumes of each shell, the averageparticle size is used to calculate the total volume of an averageparticle and from this the average thickness of each shell.

The degree of ET between the Eu3� and Tb3� ions can becontrolled by spatially confining the dopant ions to a specific shellwithin the individual NP. The two extreme case of controlling ETare that of a co-doped homogeneous single NP where ET ismaximized and that of two different individually doped homoge-neous NPs that are separated in a matrix from one another whereET is minimized (approaching zero). For ease of comparisonbetween samples, the total volume of precursors was kept constant,so that only the internal layer thickness of the particles was varied.It is expected that the degree of ET between ions should decreaseas one increases the thickness of an intervening layer. In the caseof a Eu3�:LaF3 core and a Tb3�:LaF3 shell, the spectra for theseparticles are shown in Fig. 4 for excitation at � � 350 nm. Thisexcitation wavelength was chosen because Tb3� has a strongabsorption whereas excitation by Eu�3 is negligible; hence, it allowsfor the direct inference of ET.

It can be seen in the co-doped simple core-shell NPs that thereis a strong emission peak at 590 and 615 nm. These emissionscorrespond to the 5D03 7F1 and 5D03 7F2 transitions of Eu3�,respectively, and confirm the Tb3�-to-Eu3� ET that arises whenEu3� and Tb3� are co-doped together. These samples wereexcited at 480 nm (data not shown), which corresponds to the5D4(Tb) absorption and the same high-intensity emissions fromthe Eu3� where observed. The emission indicates that ET occurs

from the 5D4(Tb) to the 5D1(Eu) through a phonon relaxation asindicated in Fig. 3c (20).

In the case of the complex core-shell NPs with no intermediatepassive LaF3 shell (Fig. 3b), a Eu.2La.8F3 core is grown with aTb.2La.8F3 shell surrounding it. The peak intensities of the 5Do3 7FJ emissions can be seen in Fig. 4 to decrease. To test theeffects that are due to the dopant’s position in the core-shellarchitecture, NPs then were synthesized that had a Tb�3 in thecore and a Eu�3 in the shells versus Eu�3 in the core and Tb�3

in the shells. The emission spectra of these two types of NPs werefound to be the same (data not shown). It can therefore beconcluded that there are no major effects due to the order of adopant’s position within a NP but only due to a dopant’s positionwith respect to another dopant.

Complex core-shell NPs with an �1-nm-thick LaF3 shell grownbetween the lanthanide-doped layers (Fig. 3c) exhibited a 60%decrease in the 5Do3 7FJ intensity. The thickness of the LaF3 layerseparating the Eu.2La.8F3 core and Tb.2La.8F-doped shell wasincreased to �2 nm (Fig. 3d), which resulted in a decrease of theintegrated intensity of the 5Do3 7FJ emissions by �90%. It can beseen that there is a substantial decrease in the Eu3� luminescenceas the intermediate shell thickness approaches 2 nm. At thisthickness, the Eu3� emissions are only slightly more intense than theemission spectrum of a blend of singularly doped particles, indi-cating that there is a small amount of ET. This suggests that tominimize the ET, a shell slightly thicker than �2 nm is needed inthis particular case of Eu3�/Tb3� ET.

A more thorough spectroscopic characterization of the NPscan be accomplished by taking emission spectra at multipleexcitation wavelengths to form two-dimensional excitation/emission landscapes as shown in Fig. 5. These emission maps areconvenient for comparing particles with multiple dopants in acore-shell architecture because the excitation spectra are asimportant as the emission spectra when looking at ET. Forexample, in Fig. 5b, negligible emissions are observed for Eu3�

when it is excited below 355 nm. When the Eu3� is co-doped withTb3�, the emission map (Fig. 5c) of simple core-shell co-dopedparticles matches very closely that of Fig. 5b with the exceptionthat the 590- and 612-nm emissions from Eu3� now occur whenexcited with 350-nm light. There is also a low-intensity emissionof the Tb3� at 540 nm. In contrast, Fig. 5d shows that a mix ofEu3�-doped NPs and Tb3�-doped NPs in equal portions isapproximately a summation of Fig. 5 a and b, indicating verylittle ET.

Using core-shell architectures, a single NP can be engineeredto exhibit varying degree of ET between dopants. Fig. 5e showsthe emission map of NPs where the different dopants areconstrained to separate shells, but those separate shells are indirect contact with one another (equivalent to the Fig. 3bmodel). It can be seen that there is a significant increase in theintensity of the 540-nm Tb3� emission in comparison with theco-doped LaF3. If an undoped LaF3 layer with an �2-nmthickness (Fig. 3d) is now placed between the two dopant shells,the emission map shown in Fig. 5f is very similar to the emission

b c da

Fig. 3. Different core-shell architectures are used to control ET in the NP systems. Red and green correspond to Eu3�- and Tb3�-doped layers, respectively. (a)A simple core-shell structure is used to protect the lanthanides from quenching. Complex core-shell architectures with no LaF3 layer (b), one LaF3 layer (c), andtwo LaF3 (d) layers are used for controlling the degree of ET.

Fig. 4. Effect of changing the core-shell architecture on the emission spectraEu3�- and Tb3�-doped NPs. Emission spectra are normalized to the 543-nmpeak of Tb3�.

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map for the mixed NPs (Fig. 5d). This indicates that the �2-nmshell is sufficiently thick so as to significantly reduce ET betweenlayers. As such, NPs with multiple dopants constrained to shellsseparated by a shell of �2 nm could be used instead of multipleparticles with single dopants.

ConclusionsIn conclusion, multiply doped core-shell NPs have been synthe-sized that permit new strategies for spectrally designing lumi-nescent materials. As such, this introduces an enabling technol-ogy that will allow for advances in many fields of photonics by

controlling the ET between lanthanides through shell positionsand thicknesses. Using core-shell particles with upwards of threeshells, the ratio of the 540-nm Tb3� peak to that of the 590-nmEu3� peak has been varied from 0.2 to 2.4 in the emission spectraof the NPs. This variation in emission spectra was accomplishedwithout changing the overall composition or external dimensionsof the particles but only the internal structure. An averagecalculated shell thickness of �2 nm is needed to produceemission spectra equivalent to the spectrum of a blend ofsingularly doped NPs. Beyond simply varying the emissionspectra, the excitation spectra of rare earths can be altered by

Fig. 5. Excitation/emission maps of core-shell NPs where the darker regions correspond to high luminescence intensities. (a and b) Simple core-shell structuresof Tb3�- and Eu3�-doped LaF3, respectively. (c) Simple core-shell structure with a co-doped Tb3� and Eu3� core. (d) 1:1 blend of the NPs in a and b. (e) Complexcore-shell structure with a Eu3�-doped core and a Tb3�-doped shell. ( f) Complex core-shell structure with a Eu3�-doped core, �2-nm LaF3 passive layer,Tb3�-doped shell, and LaF3 passive shell.

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allowing rare earths to act as sensitizers and emitters. It can beimagined that these highly structured particles will have uses inwhite-light emitters, multicolor displays, lasers, and broadbandamplifiers where multiple emission are desired from a singleexcitation wavelength.

Materials and MethodsA solution of 614 mg of ammonium di-n-octadecyl dithiophosphate (ADDP)and 126 mg of NH4F in 70 ml of ethanol/water was heated to 75°C. A 2-mlaqueous solution with total molar Ln(NO3)3 concentration of 1.33 mmol wasthen added dropwise to the stirring fluoride solution to form the core of theparticles. After stirring for 10 min, the first shell was grown by the alternatingaddition in 10 parts of a 2-ml aqueous NH4F (126 mg) solution and a 2-mlaqueous Ln(NO3)3 solution with total molar concentration of 1.33 mmol. Thecomposition of the Ln(NO3)3 solution will be the composition of the shell. Inthis work, Eu(NO3)3 and/or Tb(NO3)3 were used as dopants at 20 mol %concentrations (i.e., Eu.2La.8F3 and Tb.2La.8F). This process was repeated foreach shell. After the addition of the last shell, the solution was then stirred foran additional 2 h and cooled to room temperature. After cooling, the particleswere cleaned by washing in ethanol and water followed by dispersing in 5 mlof dichloromethane and precipitating with the addition of 20 ml of ethanol.

The resultant powder was dried for 2 days over P2O5 in a desiccator. Theparticles were dispersed in tetrahydrafuran for measurements.

Photoluminescence measurements were performed with a Jobin YvonFluorolog-3 spectrofluorometer with a double grating configuration with anexcitation bandpass of 2 nm and a scan rate of 60 nm/min. TEM was performedon a Hitachi H9500 operating at an acceleration voltage of 300 kV. X-raydiffraction was performed with a Scintag XDS 4000 by using Cu K� radiation.X-ray fluorescence was performed with a Thermo Noran QuanX EC energydispersive x-ray fluorescence spectrometer by using a fundamental parame-ters model to quantify the data. The La, Eu, and Tb were measured at 20 kVwith a Pd filter in air. P and S were measured at 8 kV with no filter in air. Datawere acquired for 100 sec (at 50% dead time) for each excitation condition.LaF3, Eu2O3, Tb4O7, NH4H2PO4, and (NH4)2SO4 were used as peak shape char-acterization standards for the fundamental parameters model. Average par-ticle size was determined through XRD peaks and analysis of TEM images asdescribed in ref. 19.

ACKNOWLEDGMENTS. We thank Dr. JoAn Hudson of the Clemson UniversityElectron Microscope Laboratory for high-resolution TEM imaging. This workwas supported by the U.S. Army through the South Carolina Research Author-ity (Subrecipient Agreement 2001-509, Task Order 0007, Active CoatingsTechnology program).

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