Synthesis Protocols for δ Doped NaYF :Yb,Erecee.colorado.edu/~wpark/papers/Chem. Mater. 2014...

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Synthesis Protocols for δDoped NaYF 4 :Yb,Er Zhihua Li,* ,,W. Park, § G. Zorzetto, J.-S. Lemaire, and C. J. Summers* ,,College of Chemistry, Chemical Engineering and Materials Science, Key Laboratory of Molecular and Nano Probes, Shandong Normal University, Jinan 250014, China School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States § Department of Electrical and Computer Engineering, University of Colorado, Boulder, Colorado 80309, United States PhosphorTech Corporation, 3645 Kennesaw North Industrial Parkway, Georgia 30144, United States * S Supporting Information ABSTRACT: A novel structure, δ-doped NaYF 4 :Yb,Er, has been proposed for signicantly enhancing the uorescence eciency of up-conversion phosphors. Theoretical calculations indicate that these new δ-doped NaYF 4 :Yb,Er structures will suppress the Yb 3+ -defect energy transfer rate while eectively preserving or enhancing the Yb 3+ to Er 3+ energy transfer. To investigate this eect δ-doped NaYF 4 :Yb,Er nanocrystals have been synthesized according to the designed structure model and the prepared samples characterized physically by transmission electron microscopy (TEM), high-resolution TEM (HRTEM), X-ray diraction (XRD), energy dispersed spectroscopy (EDS) and optically by photoluminescence (PL) spectroscopy. Well-dened doping geometries of the order of 35 nm in width were clearly identied, both spatially and chemically. The up- converted emission spectra data were consistent with the theoretical predictions. INTRODUCTION Up-conversion phosphors (UCP) have attracted much attention in recent years, because their excellent properties allow the development of unique and eective optical devices such as solid-state lasers, 1 optical-ber-based telecommunica- tions, 2 lamps for illumination, 3 at-panel displays, 1,4 optical storage, 5 increasing the conversion eciency of photovoltaic cells, 7 and as biological labels. 6 Compared to organic dyes and quantum dots, UC nanomaterials are signicant and viable candidates because of their potential advantages: less photo- damage to living organisms, weak background uorescence, and deep detection range. 8,9 However, despite many investigations, applications of UC nanocrystals are still limited by their low emission eciency. Therefore, enhancing UC luminescence eciencies, while challenging, will result if successful, in a wide range of applications. Among UC materials, hexagonal phase NaYF 4 is reported as one of the most ecient hosts for infrared to visible photon up- conversion when activated by Yb 3+ and Er 3+ ions. 8 The luminescence mechanisms in NaYF 4 :Yb,Er are shown in Figure 1 and are as follows: When doped with Yb 3+ and Er 3+ , the absorption of an infrared photon (976 nm) elevates the 4f electrons in the Yb 3+ ion to the 2 F 5/2 level. They may then decay radiatively from this excited state back to the ground state, or they can transfer their energy to a nearby Er 3+ ion. This energy transfer promotes the Er 3+ ion from the 4 I 15/2 to the 4 I 11/2 state or, if the 4 I 11/2 state is already populated, from the 4 I 11/2 state to the 4 F 7/2 state, accomplishing the energy transfer up-conversion (ETU) process. The electrons in the Er 3+4 F 7/2 state subsequently decay nonradiatively to a slightly lower energy 4 S 3/2 state by a multiphonon relaxation process and then green light is emitted by the electronic transition from the 4 S 3/2 state to the ground state (Figure 1). Also, red and blue emissions, resulting from similar processes, can be observed. Usually, to ensure ecient infrared absorption and excitation (and thus a high electron population) of the Er 3+4 I 11/2 state, approximately 20 times as many Yb 3+ ions are added to the lattice as Er 3+ ions. 10 Received: July 13, 2013 Revised: February 8, 2014 Published: February 13, 2014 Figure 1. Energy level and transition scheme of UC spectra of Er 3+ using Yb 3+ as the sensitizer. Article pubs.acs.org/cm © 2014 American Chemical Society 1770 dx.doi.org/10.1021/cm4023425 | Chem. Mater. 2014, 26, 17701778

Transcript of Synthesis Protocols for δ Doped NaYF :Yb,Erecee.colorado.edu/~wpark/papers/Chem. Mater. 2014...

Synthesis Protocols for δ‑Doped NaYF4:Yb,ErZhihua Li,*,†,‡ W. Park,§ G. Zorzetto,‡ J.-S. Lemaire,‡ and C. J. Summers*,‡,∥

†College of Chemistry, Chemical Engineering and Materials Science, Key Laboratory of Molecular and Nano Probes, ShandongNormal University, Jinan 250014, China‡School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States§Department of Electrical and Computer Engineering, University of Colorado, Boulder, Colorado 80309, United States∥PhosphorTech Corporation, 3645 Kennesaw North Industrial Parkway, Georgia 30144, United States

*S Supporting Information

ABSTRACT: A novel structure, δ-doped NaYF4:Yb,Er, has been proposed forsignificantly enhancing the fluorescence efficiency of up-conversion phosphors. Theoreticalcalculations indicate that these new δ-doped NaYF4:Yb,Er structures will suppress theYb3+-defect energy transfer rate while effectively preserving or enhancing the Yb3+ to Er3+

energy transfer. To investigate this effect δ-doped NaYF4:Yb,Er nanocrystals have beensynthesized according to the designed structure model and the prepared samplescharacterized physically by transmission electron microscopy (TEM), high-resolutionTEM (HRTEM), X-ray diffraction (XRD), energy dispersed spectroscopy (EDS) andoptically by photoluminescence (PL) spectroscopy. Well-defined doping geometries of theorder of 3−5 nm in width were clearly identified, both spatially and chemically. The up-converted emission spectra data were consistent with the theoretical predictions.

■ INTRODUCTIONUp-conversion phosphors (UCP) have attracted muchattention in recent years, because their excellent propertiesallow the development of unique and effective optical devicessuch as solid-state lasers,1 optical-fiber-based telecommunica-tions,2 lamps for illumination,3 flat-panel displays,1,4 opticalstorage,5 increasing the conversion efficiency of photovoltaiccells,7 and as biological labels.6 Compared to organic dyes andquantum dots, UC nanomaterials are significant and viablecandidates because of their potential advantages: less photo-damage to living organisms, weak background fluorescence, anddeep detection range.8,9 However, despite many investigations,applications of UC nanocrystals are still limited by their lowemission efficiency. Therefore, enhancing UC luminescenceefficiencies, while challenging, will result if successful, in a widerange of applications.Among UC materials, hexagonal phase NaYF4 is reported as

one of the most efficient hosts for infrared to visible photon up-conversion when activated by Yb3+ and Er3+ ions.8 Theluminescence mechanisms in NaYF4:Yb,Er are shown in Figure1 and are as follows: When doped with Yb3+ and Er3+, theabsorption of an infrared photon (976 nm) elevates the 4felectrons in the Yb3+ ion to the 2F5/2 level. They may thendecay radiatively from this excited state back to the groundstate, or they can transfer their energy to a nearby Er3+ ion. Thisenergy transfer promotes the Er3+ ion from the 4I15/2 to the4I11/2 state or, if the 4I11/2 state is already populated, from the4I11/2 state to the 4F7/2 state, accomplishing the energy transferup-conversion (ETU) process. The electrons in the Er3+4F7/2state subsequently decay nonradiatively to a slightly lower

energy 4S3/2 state by a multiphonon relaxation process and thengreen light is emitted by the electronic transition from the 4S3/2state to the ground state (Figure 1). Also, red and blueemissions, resulting from similar processes, can be observed.Usually, to ensure efficient infrared absorption and excitation(and thus a high electron population) of the Er3+4I11/2 state,approximately 20 times as many Yb3+ ions are added to thelattice as Er3+ ions.10

Received: July 13, 2013Revised: February 8, 2014Published: February 13, 2014

Figure 1. Energy level and transition scheme of UC spectra of Er3+

using Yb3+ as the sensitizer.

Article

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© 2014 American Chemical Society 1770 dx.doi.org/10.1021/cm4023425 | Chem. Mater. 2014, 26, 1770−1778

Consequently, for Yb−Er activated phosphors, improving theup-conversion efficiency requires: (1) suppression of the directIR emission by Yb3+ ions, (2) suppression of nonradiativerelaxation process, which subsequently requires suppression ofYb3+-defect and Er3+-defect energy transfer processes, and (3)enhancement of energy transfer from Yb3+ to Er3+ ions.It is known that the 2F7/2 →

2F5/2 transitions in Yb3+ occurbetween states of the same parity and that in a host lattice withinversion symmetry the transition becomes parity forbiddenand consequently the transition rate is strongly suppressed.However, reduction of the radiative transition rate willsimultaneously decrease the absorption strength and moreimportantly the energy transfer rate, according to the Dexter−Foster theory.11,12 Therefore, reducing the radiative transitionrate by making it forbidden would not help improve up-conversion efficiency.Another important consideration is that nanoscale up-

conversion crystals typically have a large surface area, andconsequently, a high proportion of dopant ions is exposed tothe dangling bonds and surface defects which promote surfacerecombination and luminescence quenching. The strongpresence of dangling bonds and other defects on the surfaceof nanophosphors requires surface defect passivation, which upto now has been the most effective way to increase theluminescence efficiency.13−16 For UC NaYF4:Yb,Er nanocryst-als, the introduction of an undoped shell around the dopednanocrystal can improve its luminescence efficiency by ∼4times. In the core/shell structure, the dopant ions are confinedin the interior core, and the defects and dangling bonds at thecore−shell interface are minimized, which markedly suppressesthe Yb3+-defect energy transfer near the surface. These resultshave been confirmed by Chow et al. and Yan et al.,respectively.17,18

In fact, defects that provide nonradiative relaxation paths areunintentional and therefore are always distributed uniformlythroughout the phosphor particle. As a result, the energytransfer processes between Yb3+ ions and defects are three-dimensional (3D) in nature. So, the core/shell structure onlyeliminates surface defects from the “exterior” surface of thedoped core and cannot reduce inner (core) defects andsuppress the energy transfer from Yb3+ to inner defects in thenanocrystals.δ-Doping is a novel doping technique which incorporates

dopant ions in two-dimensional planes. The depth profile of thedopant concentration is therefore a linear combination of δfunctions from which the name, δ-doping originates. Insemiconductors, δ-doping is used to create a potential well inwhich a quasi two-dimensional electron gas is formed. Inphosphors, δ-doping is adopted to suppress energy transferprocesses between luminescent ions and defects and thus toincrease the luminescence efficiency of the phosphor. Thistechnique was first applied to ZnS:Mn by Park et al.19−21 Theresults showed that a 5-fold increase was observed in thephotoluminescence (PL) intensity for δ-doped ZnS:Mn underthe same excitation conditions as in uniformly doped ZnS:Mn.We have, therefore, investigated the two-dimensional δ dopingtechnique in the up-conversion phosphor NaYF4:Yb,Er. Thisrequired the development of unique nanostructures foroptimizing the energy transfer processes and related up-conversion phenomena. When the activator ions are not dopeduniformly but in separate layers, the energy transfer process isconfined in the two-dimensional doping layer, drasticallymodifying the interaction among activator ions, which could

suppress the Yb3+-defect energy transfer rate while preservingor enhancing the Yb3+ to Er3+energy transfer rate. Asmentioned previously, theoretical and experimental studiesfor an ideal activator system, such as Mn, were found to predictand confirm a ∼5 fold enhancement. However, for morecomplex activator ions the presence of manifolds of higherorder energy states can introduce recombination by cross-relaxation that provides a fast nonradiative path for electron−hole recombination. A recent theoretical study showed that thecross-relaxation between Er3+ ions provides a major quenchingmechanism for NaYF4:Yb,Er even in the low-dimensionaldoping geometries.22 This effect is very large, but it dependsonly on the separation between the two Er3+ ions and is ratherinsensitive to the environment. It is therefore possible that acombination of enhancement mechanisms, such as theplasmonic effect, in combination with δ doping, can providepathways to enhance both the energy transfer rate from Yb toEr and the radiative recombination rate in Er beyond thatpossible in conventional material structures. If these effectsresult in the radiative decay rate becoming faster than thenonradiative rate then an enhancement in UC efficiency shouldresult.In this paper, we therefore present the synthesis and

characterizations of δ-doped UCP nanoparticles and the effectsof low-dimensional doping on luminescence properties. Thefabrication of nanoparticles with complex structures is a verysignificant challenge as is now being acknowledged and moreopenly discussed in the nanomaterials community.23,24 A widerange of variability can occur in colloidal synthesis unless verystringent protocols are followed. Without strong attention todetail, repeatability is hard to achieve from run-to-run andbetween different investigators. Thus, the fabrication of smallcore/shell and core/shell/shell structures is extremely difficultand additionally requires careful characterization. However, it iscritically important to demonstrate that such structures can begrown, not only for this concept but also for the manyheterostructures that will provide the foundation for the secondgeneration of nanoparticle materials and devices. In the nextsection we briefly outline the theoretical developments andpredictions developed by Yu et al.22 before describing thesynthesis procedure and characterization of the δ-dopedparticles.

■ THEORETICAL ANALYSIS AND PREDICTIONS

For Yb and Er codoped NaYF4, the most general description ofradiative transition rates is derived from Fermi’s golden rule:25

∑τ

πωε κ

μ ω=ℏ

|⟨ | | ⟩|⎜ ⎟⎛⎝

⎞⎠V g

i f gE

E1 1

( )r o i

eloc

2 2

(1)

where μe is the electric dipole operator and g(ω) is the photondensity of states. The radiative transition rate is basicallydetermined by the matrix element |⟨i|μe|f⟩|. Thus, it is possibleto artificially modify the radiative transition rate by altering thesite symmetry of the crystal. Thus, when the activator ions arenot doped uniformly but confined in a two-dimensional dopedlayer, the energy transfer processes between activator ions anddefects is drastically modified. The energy transfer processes forthis structure has been well studied by Park et al. and, forcompleteness, is briefly described.26 The energy transferprocess between Yb3+ ions is found by first considering theprobability of finding a Yb3+ ion excited at time t:

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∏==

−P t( ) ek

Ntn r

1

( )k

(2)

where n(rk) is the transfer rate between Yb3+ ion pairs separated

by rk and k runs over all sites occupied by Yb3+ ions thatparticipate in the energy transfer. The ensemble average is

∫ϕ πρ= − −∞

−t rr( ) exp{ 4 d (1 e )}tn r

0

2 ( )(3)

where the donor ions are distributed in a three-dimensionalvolume and the form of n(rk) is determined by the nature of theinteraction between ions.In contrast, when the doping is confined in a two-

dimensional plane, eq 3 is modified to

∫ϕ πρ= − −∞

−t rr( ) exp{ 2 d (1 e )}tn r

0

( )(4)

Assuming multipolar interactions:

τ= = ···⎜ ⎟⎛

⎝⎞⎠n R

dR

s( )1

, 6, 8,s

(5)

where τ−1 is the transfer rate between a nearest neighbor pair, dthe nearest neighbor distance, and s = 6, 8, 10,... for dipole−dipole, dipole−quadrupole, quadrupole−quadrupole interac-tions, respectively. Integrating eq 3 and defining the hoppingtime between Yb3+ ions as, τhop, which is the average timeduring which the excitation resides on one Yb3+ ion beforehopping to another: Φ(τhop) = e−1, can be shown to result inhopping times between Yb3+ ions for 3D and 2D environments:

ττ

ρ=

Γ −π( )( )d 1s

shopYY

43

3 3 /3

(6)

where τYY is the transfer rate for nearest neighbor pairs and ρ isthe 3D density of Yb3+.

ττ

π σ=

Γ −( )( )d(2D)

1hop

s

sYY

2 2 /2

(7)

where σ is the 2D areal density.Direct comparisons between eqs 6 and 4 are made by

substituting σ = aρ, where a is the spacing between twoadjacent doped layers. If the coupling mechanism for energytransfer is electric dipole−dipole, the 2D transfer rate is greaterthan the 3D transfer rate by a factor ∼a3ρ,

ττ

ρ≅ a(3D)

(2D)1.4hop

hop

3

(8)

For the extreme case when activator ions substitute the entirelattice, the density is approximately d−3 and the ratio of 2D to3D energy transfer rates is ∼(a/d)3. For a lattice constant (0.5nm) typical of most oxide and halide lattices, we expect a 1000-fold increase in the energy transfer rate when a = 5 nm, whichfor a typical doping densities of 1−3%, results in anenhancement in the energy transfer rate of 10−30. This is aconsequence of the 2D confinement and the main conclusionof the theoretical analysis that the energy transfer rates amongYb3+ ions and between Yb3+ and Er3+ ions in the same dopingplane are enhanced proportionally to the total doping density.For practical doping conditions, we predict an order ofmagnitude increase in energy transfer rates, which directlytranslates to increased up-conversion efficiency. We note that

this analysis appears to indicate that the 2D energy transfer rateincreases monotonically as the spacing, a, between adjacentdoping planes as a consequence using the relation σ = aρ.Under this constraint, increasing the spacing, a, betweenadjacent doping planes means the 2D doping density σ shouldbe increased by the same factor to maintain the same effective3D doping density ρ. There will obviously be a practical limiton the achievable σ, which will determine the optimal value ofa.In contrast, defects that provide paths for nonradiative

relaxation are unintentional and always distributed uniformlythroughout the phosphor particle. As a result, the energytransfer processes between Yb3+ ions and defects are 3D innature and are strongly suppressed when Yb3+ ions are confinedin 2D planes. Detailed analysis of the activator-defect energytransfer for layered doping had been investigated by Park andindicates that in a layered 2D doped phosphor, the energytransfer rate to nonradiative defects is decreased due to thephysical separation between the activator ions and nonradiativedefects.26 This consequently leads to increased radiativeefficiency the improvement depending on the actual defectand doping densities and the nature of their coupling. However,because the activator ions are coupled through resonant energytransfer processes while activator-defect coupling is a non-resonance process, we expect a dramatic increase in theradiative efficiency. This has, in fact, been demonstrated inZnS:Mn phosphors where δ-doping was found to lead to 6-foldincrease in luminescence intensity under identical excitationcondition20,21,26

Neglecting the presence of competing recombinationmechanisms, such as cross-relaxation, we can use the energytransfer rates calculated from the above analysis in the rateequations to directly evaluate the up-converted luminescenceintensity possible in a 2D doped structure. Figure 2a shows theschematic of the codoping spherical surface, plane, and spherewith defects. For the relevant parameter values forNaYF4:Yb,Eu, the calculated upconversion efficiency is shownin Figure 2b. It is demonstrated that, at the same donor−donorand acceptor−acceptor distances, nanophosphors of 2Dgeometry can achieve higher upconversion efficiency thanthose of 3D geometry. The reason for this improvement is thatthe 2D geometry can greatly reduce the quenching effect bydefects.

■ DESIGN AND SYNTHESIS OF δ-DOPEDNAYF4:YB,ER NANOSTRUCTURES

From the theoretical model, the design required to test theproperties of a δ-doped up-conversion nanocrystal phosphor isillustrated in Figure 3. A center undoped core region of the hostmaterial (NaYF4) is surrounded by a conformal (Yb, Er) dopedlayer, which is then capped by an undoped layer of the hostmaterial to minimize nonradiative recombination at the surface.Thus, this nanoparticle structure requires the formation of a

very thin highly crystalline doped region and the elimination ofdefects along the two interfaces it makes with the undopedregions (core and capping layer), which will be achieved byadjustments to particle size and the annealing environment.The colloidal epitaxy approach to annular doping in thespherical system is controlled by the kinetics of heterogeneousnucleation. A shell of new material is adsorbed and atomicallyincorporated onto the surface of crystalline particles alreadypresent in solution and the quality of this incorporationdetermines the defect character at the interface between

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existing and new materials. Thus, a defect-free interface relieson excellent epitaxial growth of the new material which may beaccomplished by starting with core particles displaying highsurface energies. Although NaYF4 has a hexagonal structure, theimpact of the surface free energy (γ) on growth can be obtainedfrom analysis for a spherical particle as γ is directly related to itsradius (R) as described by the Gibbs free energy change (ΔG)for a system undergoing diffusion-limited growth,

π γ πΔ = − ΔG R R F443 V

2 3

where the ΔFV term characterizes the difference in free energybetween solvated and crystalline forms of the material. Asmaller NaYF4 core particle will have a greater surface-to-volume ratio and, therefore, display a much greater surface freeenergy, γ, and this should significantly enhance epitaxial growthand incorporation of both the NaYF4:Yb,Er (doped) andoutermost NaYF4 (undoped) shell layers. Thus, the kinetics ofsmall particle growth favors the formation of defect-freesurfaces and in principle should minimize the risk of etch-back. However, a concern in the synthesis of successive layergrowths, which must occur by the addition (injection) ofadditional precursors, is to prevent the nucleation of additionalparticles that will compete with heteroepitaxial growth on theexisting nanoparticles. Another important issue is thecleanliness of the process and to ensure that no NaYF4crystallites remain after synthesis the glassware was scannedby an IR (980 nm) that can detect any UC luminescence.

■ EXPERIMENTAL PROCEDURESAll of the commercially available reagents were purchased andunpurified. Rare-earth chlorides (Ln = Y, Yb, Er) (99.9%, Sigma), oleicacid (OA; 90%, Sigma), 1-octadecene (ODE; >90%, Sigma), absoluteethanol (>99.5%, Sigma), and cyclohexane (>90%, Sigma).

Synthesis of β-NaYF4 Core. For this initial step 0.195 g of YCl3was weighted and added to a mixed solution of 6 mL OA and 15 mLODE in a 50 mL flask. The mixture was stirred and heated to 160 °Cto form a homogeneous solution and then cooled to roomtemperature. Then, 10 mL of methanol solution containing NaOH(0.1 g) and NH4F (0.148 g) was slowly added into the flask and stirredfor 30 min. Subsequently, the solution was slowly heated to removemethanol, degassed at 100 °C for 10 min, and then heated to 320 °Cat a rate of 20 °C/min and maintained at this temperature for 60 minunder argon protection. After the solution had cooled naturally toroom temperature, nanocrystals were centrifuged from the solutionwith ethanol. The nanocrystals were precipitated without any sizeselection and washed several times with ethanol/cyclohexane (v/v =1:1), and could be easily redispersed in various nonpolar organicsolvents (e.g., hexane, cyclohexane, toluene).

Synthesis of NaYF4@NaYF4:Yb,Er Core/Shell Structure. Theas-synthesized NaYF4 nanoparticles were weighted to 0.1 g and placedinto a 50 mL two-mouth flask to act as the seed crystals for the growthof the doped layer. For this step the experimental procedure was thesame as the synthesis procedure described above for undoped NaYF4.However, a precursor mix of LnCl3 (Ln = Y, Yb, Er; Y:Yb:Er = 80:18:2in molar ratio) was used instead of the single YCl3 component and thetotal weight of the precursors was reduced by 4/5 of the previousquantity. The same method was used to wash the products.

Synthesis of NaYF4@NaYF4:Yb,Er@NaYF4 Multicoating Struc-ture. To form the second shell, the NaYF4@ NaYF4:Yb,Ernanocrystals were weighted to 0.1 g and used as the nucleationseeds for the grow of the undoped capping layer. The experimentalprocedure was the same as for the synthesis procedure of NaYF4 and

Figure 2. (a) Schematic of the codoping spherical surface, plane, andsphere with a random distribution of defects throughout the sample.(b) Upconversion efficiency varies as intensity of the incident light.dDD (dAA) is the distance between the nearest donors (acceptors), andddef is the distance between the nearest defects.

Figure 3. (a) Schematic of an annular-doped NaYF4:Yb,Er nano-phosphor system proposed for the fabrication of δ-doped samples; (b)Schematic of the different stages required for the synthesis of δ-dopedsamples showing (I) the crystalline core formation, (II) the first(doped) shell deposition, (III) the second shell deposition.

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NaYF4@NaYF4:Yb,Er. The only difference being that the dosage ofYCl3 was reduced to 0.097 g.Sample Characterization. Particle sizes and shapes were

characterized by transmission electron microscopy (TEM) (JEOL,100CX, Japan) and high-resolution transmission electron microscopy(HRTEM) (Hitachi HF2000, Japan). Samples were prepared bydrying a drop of nanocrystal dispersion in cyclohexane/toluene (1/1)on amorphous carbon coated copper grids. Conventional software wasused to determine the size, and the size distribution of a nanoparticlepopulation from the TEM images. The UC emission spectra weremeasured using a self-regulating spectrophotometer and a pulsed 980nm laser as the excitation source.

■ RESULTS AND DISCUSSIONFigure 4 shows TEM and HRTEM images and XRD patternsof the β-phase NaYF4 core synthesized as described in the

Experimental Procedures section. As can be observed, thenanoparticles were of similar shape and equal in size (Figure4a) with average “diameters” of 18 nm for the core synthesis.This value was obtained from the size distribution plots shownin Figure 2b, which also demonstrates a highly monodispersedpopulation of particles with a half-width of ±0.5 nm. TheHRTEM image displays an interplanar spacing of 0.52 nmcorresponding to the (101 0) plane of β-phase NaYF4 (Figure4c). Additionally, the XRD pattern (Figure 4d) indicates thatthe particles were of good quality and well-crystallized. All of

the diffraction peaks correspond to the data of the PDF 28-1192 index card. The calculated size of the core particles wasapproximately 18 nm according to the Debye−Scherrerequation, in excellent agreement with the TEM data. Theenergy-dispersive spectrometry (EDS) analysis spectra of thecore showed, as expected, the presence of Na, Y, and F (Figure8).The small size of the undoped core (∼18 nm), results in a

large surface-to-volume ratio and, therefore, the presence ofmany dangling bonds and defects that provide sufficient surfaceenergy for the epitaxial overgrowth of the doped layer. Figure 5

shows TEM and HRTEM images and XRD spectra of theNaYF4@NaYF4:Yb,Er (core/shell or C/S samples). The TEMimages show that the particles are very uniform in size and thatthe average size of the particles was about 25 ± 1 nm. Theinterface between core and shell can be observed distinctly.HRTEM image shows the interplanar spacing of 0.3 and 0.52nm corresponding to the (112 0) and (101 0) planes of β-phaseNaYF4 (Figure.3c), respectively. The thickness of shell wasabout 3.5 nm. The XRD pattern indicates the as-preparednanoparticles crystallized in the β-phase of NaYF4, and that thecalculated size of the C/S structure was 25.2 nm according tothe Debye−Scherrer equation, which again agrees very well

Figure 4. Physical characterization by TEM, HRTEM, and XRD ofNaYF4 core.

Figure 5. Physical characterization by TEM, HRTEM, and XRD of theNaYF4@NaYF4:Yb,Er (C/S) structure.

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with the particle size distribution data calculated from theresults of the images taken in the TEM study.Figure 6 shows the TEM and HRTEM images and the XRD

spectra of the final phase of the synthesis of the NaYF4@

NaYF4:Yb,Er@ NaYF4 (core/shell/shell or C/S/S samples). Ascan be observed the C/S/S nanoparticles that were preparedusing the above C/S nanoparticles as seeds now have a moreuniform spherical shape with average sizes of 46 nm and afwhm of ±2 nm. The HRTEM image shows an interplanarspacing of 0.52 nm corresponding to the (101 0) plane of β-phase NaYF4 (Figure.4c). Compared to the C/S samples, thesize of the C/S/S nanocrystals was ∼15 nm larger than that ofC/S particles indicating that the thickness of outer shell isabout 7.5 nm. The coating layer was too thick for the interfacebetween core and shell to be clearly observed. The XRDspectra indicate that the as-prepared nanoparticles crystallizedwith the β-phase of NaYF4. The calculated size of the C/Sparticles was 42.0 nm according to Debye−Scherrer equation,which agreed with the result from the TEM study. It wasnoticed that considerable peak sharpening of the XRD spectraoccurred in the transition from the C/S to the C/S/S structure,which indicated that the particles size gradually became larger.In all of the above synthesis processes no significant evidence

was found of the presence of small particles, indicating that noprenucleation occurred.The reaction temperature and reaction time are the major

parameters that markedly influence the synthesis reaction. Thereaction temperature determines if a chemical reaction can beinitiated. Experiment showed that the NaYF4 nanoparticleswere very difficult to obtain if the temperature was less than310 °C (Supporting Information, Figures S1, S2). However,when the temperature was much greater than 320 °C, thechemical reaction became difficult to control because thereaction rate was too fast (Supporting Information, Figures S3,S4). In our experiments, the reaction temperature was set at320 °C and the size and uniformity of the nanoparticles weredetermined by the reaction time. Normally, short reactiontimes (<30 min) lead to small particles with different sizes,whereas longer reaction times lead to larger particles which stillmaintained uniform size distribution. At the initiation of thereaction, numerous nuclei are formed under 320 °C. With theevolution of the reaction, the nuclei gradually grow larger andthe smaller particles dissolve into the solution and precipitateonto the larger particles, according to the crystallization rule,finally forming into uniform hexagonal platelets.In the coating experiment, the product formation was

influenced mainly by the seed washing process and reactiontime. The morphology of the particles was characterized byhexagonal platelets if the seeds were washed only in ethanol/cyclohexane. However, the shape of the particles became shortrod-like if washed using water and ethanol (SupportingInformation, Figure S5). In our synthesis route, the chemicalreaction was not totally completed because the yield of NaYF4was only about 53.1% (product washed by water and ethanol).Thus, NaF and NaCl remain as byproducts in the obtainedseeds solution after the synthesis experiments were stopped. So,NaCl was also one of the impurities existing in the obtainedseeds. In subsequent steps, NaF and NaCl would be removedfrom the seed solution if washed using water and ethanol,because NaF and NaCl are easily dissolved in water. However,if only ethanol was used to wash the seeds, NaF and NaClremained in the final products. In actuality, the shape of theproduct was determined ultimately by the NaF/Y3+ ratio for thefollowing reason. The structure of NaYF4 is depicted at Figure7, and the unit cell parameters of hexagonal β-NaYF4 are asfollows: a = b = 0.561280 nm, c = 0.333660 nm. According tothese parameters, the calculated density of Y3+ ion on differentface of crystal is ρ(0001) = 3.66/nm2, ρ(101 0) = ρ(0110) =5.39/nm2, and ρ(1120) = 3.08/nm2, respectively (SupportingInformation). The density of Y3+ on the (1010) plane is largerthan that on the (0001) plane. During the growth of NaYF4,

Figure 6. Physical characterization by TEM, HRTEM, and XRD of theNaYF4@NaYF4:Yb,Er@NaYF4 (C/S/S) structure.

Figure 7. Crystal structure of the hexagonal phase of NaYF4 showingthe atomic arrangement within a unit cell.

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the F− ion has two functions: on the one hand, the F− ion isone of the basic units for NaYF4 growth. So the growth of(101 0) plane needs more F− ions than that of (0001) planebecause of the larger density of Y3+ on the (101 0) plane. Thisleads to the concentration of the F− ions relatively lower nearthe interface of (101 0) plane than near the interface of (0001)plane. On the other hand, the F− ions are electron donors andwill coordinate with electron-poor metal atoms (Y3+) at thenanocrystal surface. So, the capping effect of F− on the (0001)plane is greater than it is on the (1010) plane. As a result, therelative growth rate in different directions would change withthe variation of the F− concentration, finally leading to differentcrystal shapes. In our synthetic route, since the seeds containNaF and NaCl, NaX (X = F, Cl) is in excess compared withLn3+ (Ln = Y, Yb, Er) ion, and the excess X− inevitably caps thecrystal surface due to the strong coordination effect between X−

and Ln3+. Experiments indicated that a higher ratio of NaF/Ln3+ seems in favor of the formation of hexagonal plates, whichwas line with the results of previous reports by Yi et al., Mai etal., and Qian et al.27 When they synthesized the NaYF4:Yb,Er@NaYF4 core/shell nanocrystals, Yi et al. had not washed thecores, so the unreacted NaF existed in the cores; Mai et al. justused ethanol to wash the cores, the unreacted NaF was also notremoved from the cores. However, their synthesized core/shellstructure of NaYF4:Yb,Er@NaYF4 nanocrystals were not rod-like but looked like hexagonal plates. The above results werenot in agreement with the reported papers.28 Why did thishappen? Detailed investigations are in progress.The size and uniformity of particles were also determined by

the reaction time of the coating process. When the reactiontime was less than 10 min, core/shell products with many verysmall particles were sometimes obtained. The particles wereseveral nanometers in size and appeared to be independentlynucleated particles. For longer reaction times, the very smallparticles dissolve into solution and then redeposit on thesurface of larger particles (core/shell particles) according to thecrystal growth rule. Normally, the particle size was uniformafter a reaction time of more than 10 min and the coating layerthickness increased gradually with prolonged reaction times.Thus, designed thickness of the coating layer can be achievedby controlling and adjusting the reaction time. The experimentsshowed that the smaller the core particles, the easier the coatingexperiment. Compared to large particles, smaller particles havea larger surface-to-volume ratio, and a higher density ofdangling bonds and defects on the surface, that is, have highersurface energy, which, as discussed previously, favors theepitaxial growth of the doped or undoped layer.Figure 8 shows the results of the EDS analysis of all the

sample structures. Due to the detection limitations of the EDStechnique, elements with small atomic weight (for example, Oand F) cannot be detected, and their compositions aredetermined by elemental analysis or XRD data. Theexperimental uncertainty of the EDS technique is about 3−5%. All the Cu peaks observed in the data are from the copperTEM grids. The top image shows the EDS data obtained fromthe undoped core and that there was no Yb and Er in thesample. The middle figure shows the EDS data of core/shellstructure, and clearly shows the presence of Yb and Er atomson the surface, confirming that the obtained sample was a core/shell structure with a NaYF4:Yb,Er shell. The last spectra showsthe measured EDS pattern for the core/shell/shell structureand clearly indicates that the surface of the core/shell/shellstructure particle has a far smaller concentration of Yb and Er

atoms on its surface. This indicates that the doping waspredominantly confined to the center region and that δ-dopedNaYF4:Yb,Er had been synthesized successfully.Figure 9 shows and compares the up-conversion lumines-

cence spectra and fluorescent intensity, respectively, ofNaYF4@NaYF4:Yb,Er (C/S) and NaYF4@NaYF4:Yb,Er@NaYF4 (C/S/S) when excited with a 980 nm laser diode. Asclearly observed, nanoparticles with the C/S/S structure have amuch higher quantum yield than those with the C/S structure.The up-conversion fluorescence enhancement is about 5 timesafter coating with an undoped NaYF4 shell. For very small sizedNaYF4@NaYF4:Yb,Er particles (about 25 nm), the doped layeris exposed to the surface where strong quenching due to surfacerecombination is expected. However, in the C/S/S architecture,the outermost shell serves as passivation layer, which decouplesthe dopant ions from the surface defects, resulting in enhancedluminescence. The spectra of the δ doped samples were thesame as for the conventional core/shell particles and the

Figure 8. EDS analysis of the core, core/shell, and core/shell/shellstructures of the synthesized samples, respectively.

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luminescent efficiency was higher. After rigorously accountingfor all possible losses, our data clearly show an increase in theefficiency of the δ-doped samples by ∼50%.In our experiments, NaYF4:Yb,Er@NaYF4 (C/S) NPs had

also been synthesized and characterized (Supporting Informa-tion, Figures S6 and S7). The PL intensity of NaYF4:Yb,Er@NaYF4 (C/S) compared with that of δ-doped NaYF4:Yb,Er (C/S/S) was carried out at the same conditions (SupportingInformation, Figure S9). The experiment showed that thefluorescence strength of δ-doped NaYF4:Yb,Er was greater thanthat of NaYF4:Yb,[email protected], in an initial study to explore the effect of Ag-

plasmon-enhanced up-conversion phosphors, a sample of the δ-doped material was mixed with different concentrations of Agnanoparticles, For this study, Ag nanoparticles approximately40 nm in diameter were mixed with a solution of UCPnanoparticles dispersed into cyclohexane (4 mL, 0.01 mol/L,particle size 45 nm). The effect of adding drops (Agnanopaticles disperse into cyclohexane to form 0.01 mol/Lsolution. A drop is approximately 0.2 mL) to the dispersion isshown in Figure 10. The spectra characteristic shows that with

the added Ag nanoparticle concentrations the spectral featuresremain the same and that a gain in luminescent intensity ofalmost an order of magnitude can be obtained. This resultshows the promise of combining δ-doping with the plasmonicenhancement effect to achieve significant gains in up-conversion efficiency.

■ SUMMARY AND CONCLUSIONS

In this work, we have developed a series of synthesis protocolsto prepare the sample set required to investigate a new concept,that of activator δ doping, for improving the efficiency ofphosphors, and in particular for up-conversion phosphors. Byrestricting the activator(s) to 2D planes, their interactions withdefects that are randomly distributed in 3D space areminimized, while leaving the energy transfer between activatorsunchanged. In idealized systems the 2D confinement ofactivators (sensitizer plus activator) predicts a factor of ∼5enhancement in the luminescence intensity. Specifically,theoretical analysis shows that a δ-doped NaYF4:Yb,Erstructure should suppresses the Yb3+-defect energy transferrate while effectively preserving or enhancing the Yb3+ toEr3+energy transfer rate. To be effective the width of the dopedlayer must be very thin <10 nm, and to achieve this, specialsynthesis protocols were developed. δ-doped NaYF4:Yb,Ernanocrystals were synthesized successfully, and the measuredoptical data were well in line with theoretical prediction. Forthese structures an increase in the photoluminescence intensityof a factor of 5.5 was obtained in comparison to a usual increaseof 3.5 when a doped core particle is capped. This is about∼50% higher, than the increase in intensity typically obtainedwhen a doped core particle is capped (passivated) by anundoped shell. An additionally consideration is that thisincrease is for samples where the doped layer has two interfaceswith the undoped host materialsthe core and the secondshell capping layer. Although passivation with undoped hostmaterials is well proven, we believe that the good performanceobtained from a thin two-dimensional doped layer exemplifiesthe success of the synthesis approach and the protocolsdeveloped and the potential of this concept and similarstructures in improving performance.Additionally, preliminary investigations showed that the up-

converted luminescence can be enhanced when mixed withsilver nanoparticles through the plasmonic interaction. Thus,both of these effects: plasmonic coupling and the realization ofspatially controlled doping, provides new approaches to theoptimization of phosphor performance and gives greaterflexibility in the choice of material systems available forlighting, displays and instrumentation. Finally, we believe thatthis work is important, as it demonstrates that high fidelitystructures can be grown not only for this concept but also forthe many heterostructures that will provide the foundation forthe second generation of nanoparticle materials and devicestructures.

■ ASSOCIATED CONTENT

*S Supporting InformationSynthesis of β-NaYF4 core, NaYF4@NaYF4:Yb,Er C/Sstructure, NaYF4:Yb,Er, NaYF4:Yb,Er@NaYF4 (C/S), and δ-doped NaYF4:Yb,Er (C/S/S). PL intensities. Calculation of Y3+

ion density. This material is available free of charge via theInternet at http://pubs.acs.org.

Figure 9. PL spectra of NaYF4@NaYF4:Yb,Er (C/S) and NaYF4@NaYF4:Yb,Er@NaYF4 (C/S/S) nanoparticles, respectively. (PL excitedwith a 980 nm laser diode).

Figure 10. PL spectra of NaYF4@NaYF4:Yb,Er@NaYF4 (C/S/S)nanoparticles mixed with different ratios of Ag nanoparticles. (PLexcited with a 980 nm laser diode.)

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■ AUTHOR INFORMATIONCorresponding Authors*E-mail: [email protected].*E-mail: [email protected] authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe are very grateful for Prof. Qiyue Shao for taking some of thePL measurements. This work was supported in part by an SBIRDOE contract: Award No. DE-FG02-08ER85146. Prof. ZhihuaLi acknowledges support from the Visiting Scholar Council ofChina.

■ REFERENCES(1) Downing, E.; Hesselink, L.; Ralson, J.; MacFarlane, R. Science1996, 272, 1185.(2) Denjake, M. J.; Samson, B. Mater. Res. Soc. Bull. 1999, 8, 39.(3) Bergh, A. A.; Dean, P. Light Emitting Diodes; Clarendon Press:Oxford, U.K., 1976; Ch. 4, p 343.(4) Lodahl, P.; vanDriel, A. F.; Nikolaev, I. S.; Irman, A.; Overgaag,K.; Vanmaekelbergh, D.; Vos, W. L. Nature 2004, 430, 654.(5) Cheben, P.; del Monte, F.; Worsfold, D. J.; Carlsson, D. J.;Grover, C. P; Mackenzie, J. D. Nature 2000, 408, 64.(6) Millar, W. N.; Casida, L. E. Can. J. Microbiol. 1970, 16, 305.(7) Yi, G. S.; Lu, H. C.; Zhao, S. Y.; Ge, Y.; Yang, W. J.; Chen, D. P.;Guo, L. H. Nano Lett. 2004, 4, 2191.(8) Suyver, J. F.; Aebischer, A.; Biner, D.; Gerner, P.; Grimm, J.;Heer, S.; Kra mer, K. W.; Reinhard, C.; Gu del, H. U. Opt. Mater.2005, 27, 1111.(9) Wang, F.; Tan, W.; Zhang, Y.; Fan, X.; Wang, M. Nanotechnology2006, 17, R1.(10) Kano, T.; Suzuki, T.; Suzuki, A.; Minagawa, S. J. Electrochem. Soc.1973, 120, C87.(11) Forster, T. Ann. Physik 1948, 437, 55.(12) Dexter, D. L. J. Chem. Phys. 1953, 21, 836.(13) Grundemann, M.; et al. Phys. Rev. Lett. 1995, 74, 4043.(14) Moerner, W. E. Science 1994, 265, 46.(15) Trautman, J. K.; Macklin, J. J.; Brus, L. E.; Betzig, E. Nature1994, 369, 40.(16) Mews, A.; Eychmuller, A.; Giersig, M.; Schooss, D.; Weller, H. J.Phys. Chem. 1994, 98, 934.(17) Yi, G-S; Chow, G.-M. Chem. Mater. 2007, 19, 341−343.(18) Mai, H.; Zhang, Y.; Sun, L.; Yan, C. J. Phys. Chem. C 2007, 111,13721−13729.(19) Schon, S.; Park, C. W.; Yang, T.; Wagner, B. K.; Summers, C. J.J. Cryst. Growth 1997, 175/176, 598.(20) Park, W.; Jones, T. C.; Tong, W.; Schon, S.; Chaichimansour,M.; Wagner, B. K.; Summers, C. J. J. Appl. Phys. 1998, 84, 6852.(21) Park, W.; Jones, T. C.; Schon, S.; Chaichimansour, M.; Wagner,B. K.; Summers, C. J. J. Cryst. Growth 1998, 184/185, 1123.(22) Yu, X.; Summers, C. J.; Park, W. J. Appl. Phys. 2012, 111,073524.(23) Chan, E. M.; Xu, C.; Mao, A. W.; Han, G.; Owen, J. S.; Cohen,B. E.; Milliron, D. J. Nano Lett. 2010, 10, 1874−1885.(24) Kruger, M. MRS Fall Meeting Proceedings 2011, 1284.(25) Henderson, B.; Imbusch, G. F. Optical Spectroscopy of InorganicSolids; Clarendon Press: Oxford, 1989.(26) Park, W. Ph.D. thesis, Georgia Institute of Technology, 1997.(27) (a) Yi, G.-S.; Chow, G.-M. Chem. Mater. 2007, 19, 341. (b) Mai,H.; Zhang, Y.; Sun, L.; Yan, C. J. Phys. Chem. C 2007, 111, 13721.(c) Qian, H.; Zhang, Y. Langmuir 2008, 24, 12123−12125.(28) (a) Liang, X.; Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. Adv. Funct.Mater. 2007, 17, 2757−2765. (b) Mai, H.; Zhang, Y.; Si, R.; Yan, Z.;Sun, L.; You, L.; Yan, C. J. Am. Chem. Soc. 2006, 128, 6426−6438.

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