Article pubs.acs.org/cm
Synthesis Protocols for δ‑Doped NaYF4: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 NaYF4:Yb,Er, has been proposed for significantly enhancing the fluorescence efficiency of up-conversion phosphors. Theoretical calculations indicate that these new δ-doped NaYF4:Yb,Er structures will suppress the Yb3+-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 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 diffraction (XRD), energy dispersed spectroscopy (EDS) and optically by photoluminescence (PL) spectroscopy. Well-defined doping geometries of the order of 3−5 nm in width were clearly identified, both spatially and chemically. The upconverted emission spectra data were consistent with the theoretical predictions.
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INTRODUCTION Up-conversion phosphors (UCP) have attracted much attention in recent years, because their excellent properties allow the development of unique and effective optical devices such as solid-state lasers,1 optical-fiber-based telecommunications,2 lamps for illumination,3 flat-panel displays,1,4 optical storage,5 increasing the conversion efficiency of photovoltaic cells,7 and as biological labels.6 Compared to organic dyes and quantum dots, UC nanomaterials are significant and viable candidates because of their potential advantages: less photodamage to living organisms, weak background fluorescence, and deep detection range.8,9 However, despite many investigations, applications of UC nanocrystals are still limited by their low emission efficiency. Therefore, enhancing UC luminescence efficiencies, while challenging, will result if successful, in a wide range of applications. Among UC materials, hexagonal phase NaYF4 is reported as one of the most efficient hosts for infrared to visible photon upconversion when activated by Yb3+ and Er3+ ions.8 The luminescence mechanisms in NaYF4:Yb,Er are shown in Figure 1 and are as follows: When doped with Yb3+ and Er3+, the absorption of an infrared photon (976 nm) elevates the 4f electrons in the Yb3+ ion to the 2F5/2 level. They may then decay radiatively from this excited state back to the ground state, or they can transfer their energy to a nearby Er3+ ion. This energy transfer promotes the Er3+ ion from the 4I15/2 to the 4 I11/2 state or, if the 4I11/2 state is already populated, from the 4 I11/2 state to the 4F7/2 state, accomplishing the energy transfer up-conversion (ETU) process. The electrons in the Er3+4F7/2 state subsequently decay nonradiatively to a slightly lower © 2014 American Chemical Society
Figure 1. Energy level and transition scheme of UC spectra of Er3+ using Yb3+ as the sensitizer.
energy 4S3/2 state by a multiphonon relaxation process and then green light is emitted by the electronic transition from the 4S3/2 state to the ground state (Figure 1). Also, red and blue emissions, 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 the lattice as Er3+ ions.10 Received: July 13, 2013 Revised: February 8, 2014 Published: February 13, 2014 1770
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suppress the Yb3+-defect energy transfer rate while preserving or enhancing the Yb3+ to Er3+energy transfer rate. As mentioned previously, theoretical and experimental studies for an ideal activator system, such as Mn, were found to predict and confirm a ∼5 fold enhancement. However, for more complex activator ions the presence of manifolds of higher order energy states can introduce recombination by crossrelaxation that provides a fast nonradiative path for electron− hole recombination. A recent theoretical study showed that the cross-relaxation between Er3+ ions provides a major quenching mechanism for NaYF4:Yb,Er even in the low-dimensional doping geometries.22 This effect is very large, but it depends only on the separation between the two Er3+ ions and is rather insensitive to the environment. It is therefore possible that a combination of enhancement mechanisms, such as the plasmonic effect, in combination with δ doping, can provide pathways to enhance both the energy transfer rate from Yb to Er and the radiative recombination rate in Er beyond that possible in conventional material structures. If these effects result in the radiative decay rate becoming faster than the nonradiative rate then an enhancement in UC efficiency should result. In this paper, we therefore present the synthesis and characterizations of δ-doped UCP nanoparticles and the effects of low-dimensional doping on luminescence properties. The fabrication of nanoparticles with complex structures is a very significant challenge as is now being acknowledged and more openly discussed in the nanomaterials community.23,24 A wide range of variability can occur in colloidal synthesis unless very stringent protocols are followed. Without strong attention to detail, repeatability is hard to achieve from run-to-run and between different investigators. Thus, the fabrication of small core/shell and core/shell/shell structures is extremely difficult and additionally requires careful characterization. However, it is critically important to demonstrate that such structures can be grown, not only for this concept but also for the many heterostructures that will provide the foundation for the second generation of nanoparticle materials and devices. In the next section we briefly outline the theoretical developments and predictions developed by Yu et al.22 before describing the synthesis procedure and characterization of the δ-doped particles.
Consequently, for Yb−Er activated phosphors, improving the up-conversion efficiency requires: (1) suppression of the direct IR emission by Yb3+ ions, (2) suppression of nonradiative relaxation process, which subsequently requires suppression of Yb3+-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+ occur between states of the same parity and that in a host lattice with inversion symmetry the transition becomes parity forbidden and consequently the transition rate is strongly suppressed. However, reduction of the radiative transition rate will simultaneously decrease the absorption strength and more importantly the energy transfer rate, according to the Dexter− Foster theory.11,12 Therefore, reducing the radiative transition rate by making it forbidden would not help improve upconversion efficiency. Another important consideration is that nanoscale upconversion crystals typically have a large surface area, and consequently, a high proportion of dopant ions is exposed to the dangling bonds and surface defects which promote surface recombination and luminescence quenching. The strong presence of dangling bonds and other defects on the surface of nanophosphors requires surface defect passivation, which up to now has been the most effective way to increase the luminescence efficiency.13−16 For UC NaYF4:Yb,Er nanocrystals, the introduction of an undoped shell around the doped nanocrystal can improve its luminescence efficiency by ∼4 times. In the core/shell structure, the dopant ions are confined in the interior core, and the defects and dangling bonds at the core−shell interface are minimized, which markedly suppresses the Yb3+-defect energy transfer near the surface. These results have been confirmed by Chow et al. and Yan et al., respectively.17,18 In fact, defects that provide nonradiative relaxation paths are unintentional and therefore are always distributed uniformly throughout the phosphor particle. As a result, the energy transfer processes between Yb3+ ions and defects are threedimensional (3D) in nature. So, the core/shell structure only eliminates surface defects from the “exterior” surface of the doped core and cannot reduce inner (core) defects and suppress the energy transfer from Yb3+ to inner defects in the nanocrystals. δ-Doping is a novel doping technique which incorporates dopant ions in two-dimensional planes. The depth profile of the dopant concentration is therefore a linear combination of δ functions from which the name, δ-doping originates. In semiconductors, δ-doping is used to create a potential well in which a quasi two-dimensional electron gas is formed. In phosphors, δ-doping is adopted to suppress energy transfer processes between luminescent ions and defects and thus to increase the luminescence efficiency of the phosphor. This technique was first applied to ZnS:Mn by Park et al.19−21 The results showed that a 5-fold increase was observed in the photoluminescence (PL) intensity for δ-doped ZnS:Mn under the same excitation conditions as in uniformly doped ZnS:Mn. We have, therefore, investigated the two-dimensional δ doping technique in the up-conversion phosphor NaYF4:Yb,Er. This required the development of unique nanostructures for optimizing the energy transfer processes and related upconversion phenomena. When the activator ions are not doped uniformly but in separate layers, the energy transfer process is confined in the two-dimensional doping layer, drastically modifying the interaction among activator ions, which could
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THEORETICAL ANALYSIS AND PREDICTIONS
For Yb and Er codoped NaYF4, the most general description of radiative transition rates is derived from Fermi’s golden rule:25 2 πω ⎛ E loc ⎞ 1 1 ⎜ ⎟ = τr εoκ ℏV ⎝ E ⎠ gi
2
∑ |⟨i|μe |f ⟩| g(ω)
(1)
where μe is the electric dipole operator and g(ω) is the photon density of states. The radiative transition rate is basically determined by the matrix element |⟨i|μe|f⟩|. Thus, it is possible to artificially modify the radiative transition rate by altering the site symmetry of the crystal. Thus, when the activator ions are not doped uniformly but confined in a two-dimensional doped layer, the energy transfer processes between activator ions and defects is drastically modified. The energy transfer processes for this structure has been well studied by Park et al. and, for completeness, is briefly described.26 The energy transfer process between Yb3+ ions is found by first considering the probability of finding a Yb3+ ion excited at time t: 1771
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N
this analysis appears to indicate that the 2D energy transfer rate increases monotonically as the spacing, a, between adjacent doping planes as a consequence using the relation σ = aρ. Under this constraint, increasing the spacing, a, between adjacent doping planes means the 2D doping density σ should be increased by the same factor to maintain the same effective 3D doping density ρ. There will obviously be a practical limit on the achievable σ, which will determine the optimal value of a. In contrast, defects that provide paths for nonradiative relaxation are unintentional and always distributed uniformly throughout the phosphor particle. As a result, the energy transfer processes between Yb3+ ions and defects are 3D in nature and are strongly suppressed when Yb3+ ions are confined in 2D planes. Detailed analysis of the activator-defect energy transfer for layered doping had been investigated by Park and indicates that in a layered 2D doped phosphor, the energy transfer rate to nonradiative defects is decreased due to the physical separation between the activator ions and nonradiative defects.26 This consequently leads to increased radiative efficiency the improvement depending on the actual defect and doping densities and the nature of their coupling. However, because the activator ions are coupled through resonant energy transfer processes while activator-defect coupling is a nonresonance process, we expect a dramatic increase in the radiative efficiency. This has, in fact, been demonstrated in ZnS:Mn phosphors where δ-doping was found to lead to 6-fold increase in luminescence intensity under identical excitation condition20,21,26 Neglecting the presence of competing recombination mechanisms, such as cross-relaxation, we can use the energy transfer rates calculated from the above analysis in the rate equations to directly evaluate the up-converted luminescence intensity possible in a 2D doped structure. Figure 2a shows the schematic of the codoping spherical surface, plane, and sphere with defects. For the relevant parameter values for NaYF4:Yb,Eu, the calculated upconversion efficiency is shown in Figure 2b. It is demonstrated that, at the same donor−donor and acceptor−acceptor distances, nanophosphors of 2D geometry can achieve higher upconversion efficiency than those of 3D geometry. The reason for this improvement is that the 2D geometry can greatly reduce the quenching effect by defects.
∏ e−tn(r )
P(t ) =
k
(2)
k=1 3+
where n(rk) is the transfer rate between Yb ion pairs separated by rk and k runs over all sites occupied by Yb3+ ions that participate in the energy transfer. The ensemble average is ϕ(t ) = exp{− 4πρ
∫0
∞
drr 2(1 − e−tn(r))}
(3)
where the donor ions are distributed in a three-dimensional volume and the form of n(rk) is determined by the nature of the interaction between ions. In contrast, when the doping is confined in a twodimensional plane, eq 3 is modified to ϕ(t ) = exp{− 2πρ
∫0
∞
drr(1 − e−tn(r))}
(4)
Assuming multipolar interactions: 1⎛d ⎞ ⎜ ⎟ , τ ⎝R⎠ s
n(R ) =
s = 6, 8, ···
(5)
−1
where τ is the transfer rate between a nearest neighbor pair, d the nearest neighbor distance, and s = 6, 8, 10,... for dipole− dipole, dipole−quadrupole, quadrupole−quadrupole interactions, respectively. Integrating eq 3 and defining the hopping time between Yb3+ ions as, τhop, which is the average time during which the excitation resides on one Yb3+ ion before hopping to another: Φ(τhop) = e−1, can be shown to result in hopping times between Yb3+ ions for 3D and 2D environments: τYY τhop = s /3 4π 3 3 d ρΓ 1 − s 3
(
))
(
(6)
where τYY is the transfer rate for nearest neighbor pairs and ρ is the 3D density of Yb3+. τYY τhop(2D) = s /2 2 πd 2σ Γ 1 − s
(
(
))
(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 two adjacent doped layers. If the coupling mechanism for energy transfer is electric dipole−dipole, the 2D transfer rate is greater than the 3D transfer rate by a factor ∼a3ρ, τhop(3D) τhop(2D)
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DESIGN AND SYNTHESIS OF δ-DOPED NAYF4:YB,ER NANOSTRUCTURES From the theoretical model, the design required to test the properties of a δ-doped up-conversion nanocrystal phosphor is illustrated in Figure 3. A center undoped core region of the host material (NaYF4) is surrounded by a conformal (Yb, Er) doped layer, which is then capped by an undoped layer of the host material 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 of defects along the two interfaces it makes with the undoped regions (core and capping layer), which will be achieved by adjustments to particle size and the annealing environment. The colloidal epitaxy approach to annular doping in the spherical system is controlled by the kinetics of heterogeneous nucleation. A shell of new material is adsorbed and atomically incorporated onto the surface of crystalline particles already present in solution and the quality of this incorporation determines the defect character at the interface between
≅ 1.4a3ρ (8)
For the extreme case when activator ions substitute the entire lattice, the density is approximately d−3 and the ratio of 2D to 3D energy transfer rates is ∼(a/d)3. For a lattice constant (0.5 nm) typical of most oxide and halide lattices, we expect a 1000fold increase in the energy transfer rate when a = 5 nm, which for a typical doping densities of 1−3%, results in an enhancement in the energy transfer rate of 10−30. This is a consequence of the 2D confinement and the main conclusion of the theoretical analysis that the energy transfer rates among Yb3+ ions and between Yb3+ and Er3+ ions in the same doping plane are enhanced proportionally to the total doping density. For practical doping conditions, we predict an order of magnitude increase in energy transfer rates, which directly translates to increased up-conversion efficiency. We note that 1772
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Figure 3. (a) Schematic of an annular-doped NaYF4:Yb,Er nanophosphor system proposed for the fabrication of δ-doped samples; (b) Schematic of the different stages required for the synthesis of δ-doped samples showing (I) the crystalline core formation, (II) the first (doped) shell deposition, (III) the second shell deposition.
Figure 2. (a) Schematic of the codoping spherical surface, plane, and sphere 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), and ddef is the distance between the nearest defects.
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All of the commercially available reagents were purchased and unpurified. Rare-earth chlorides (Ln = Y, Yb, Er) (99.9%, Sigma), oleic acid (OA; 90%, Sigma), 1-octadecene (ODE; >90%, Sigma), absolute ethanol (>99.5%, Sigma), and cyclohexane (>90%, Sigma). Synthesis of β-NaYF4 Core. For this initial step 0.195 g of YCl3 was weighted and added to a mixed solution of 6 mL OA and 15 mL ODE in a 50 mL flask. The mixture was stirred and heated to 160 °C to form a homogeneous solution and then cooled to room temperature. Then, 10 mL of methanol solution containing NaOH (0.1 g) and NH4F (0.148 g) was slowly added into the flask and stirred for 30 min. Subsequently, the solution was slowly heated to remove methanol, degassed at 100 °C for 10 min, and then heated to 320 °C at a rate of 20 °C/min and maintained at this temperature for 60 min under argon protection. After the solution had cooled naturally to room temperature, nanocrystals were centrifuged from the solution with ethanol. The nanocrystals were precipitated without any size selection and washed several times with ethanol/cyclohexane (v/v = 1:1), and could be easily redispersed in various nonpolar organic solvents (e.g., hexane, cyclohexane, toluene). Synthesis of NaYF4@NaYF4:Yb,Er Core/Shell Structure. The as-synthesized NaYF4 nanoparticles were weighted to 0.1 g and placed into a 50 mL two-mouth flask to act as the seed crystals for the growth of the doped layer. For this step the experimental procedure was the same as the synthesis procedure described above for undoped NaYF4. However, a precursor mix of LnCl3 (Ln = Y, Yb, Er; Y:Yb:Er = 80:18:2 in molar ratio) was used instead of the single YCl3 component and the total weight of the precursors was reduced by 4/5 of the previous quantity. The same method was used to wash the products. Synthesis of NaYF4@NaYF4:Yb,Er@NaYF4 Multicoating Structure. To form the second shell, the NaYF4@ NaYF4:Yb,Er nanocrystals were weighted to 0.1 g and used as the nucleation seeds for the grow of the undoped capping layer. The experimental procedure was the same as for the synthesis procedure of NaYF4 and
existing and new materials. Thus, a defect-free interface relies on excellent epitaxial growth of the new material which may be accomplished by starting with core particles displaying high surface energies. Although NaYF4 has a hexagonal structure, the impact of the surface free energy (γ) on growth can be obtained from analysis for a spherical particle as γ is directly related to its radius (R) as described by the Gibbs free energy change (ΔG) for a system undergoing diffusion-limited growth, ΔG = 4πR2γ −
EXPERIMENTAL PROCEDURES
4 3 πR ΔFV 3
where the ΔFV term characterizes the difference in free energy between solvated and crystalline forms of the material. A smaller NaYF4 core particle will have a greater surface-tovolume ratio and, therefore, display a much greater surface free energy, γ, and this should significantly enhance epitaxial growth and incorporation of both the NaYF4:Yb,Er (doped) and outermost NaYF4 (undoped) shell layers. Thus, the kinetics of small particle growth favors the formation of defect-free surfaces and in principle should minimize the risk of etchback. However, a concern in the synthesis of successive layer growths, which must occur by the addition (injection) of additional precursors, is to prevent the nucleation of additional particles that will compete with heteroepitaxial growth on the existing nanoparticles. Another important issue is the cleanliness of the process and to ensure that no NaYF4 crystallites remain after synthesis the glassware was scanned by an IR (980 nm) that can detect any UC luminescence. 1773
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NaYF4@NaYF4:Yb,Er. The only difference being that the dosage of YCl3 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 by drying a drop of nanocrystal dispersion in cyclohexane/toluene (1/1) on amorphous carbon coated copper grids. Conventional software was used to determine the size, and the size distribution of a nanoparticle population from the TEM images. The UC emission spectra were measured using a self-regulating spectrophotometer and a pulsed 980 nm laser as the excitation source.
the diffraction peaks correspond to the data of the PDF 281192 index card. The calculated size of the core particles was approximately 18 nm according to the Debye−Scherrer equation, in excellent agreement with the TEM data. The energy-dispersive spectrometry (EDS) analysis spectra of the core showed, as expected, the presence of Na, Y, and F (Figure 8). The small size of the undoped core (∼18 nm), results in a large surface-to-volume ratio and, therefore, the presence of many dangling bonds and defects that provide sufficient surface energy for the epitaxial overgrowth of the doped layer. Figure 5
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RESULTS AND DISCUSSION Figure 4 shows TEM and HRTEM images and XRD patterns of the β-phase NaYF4 core synthesized as described in the
Figure 5. Physical characterization by TEM, HRTEM, and XRD of the NaYF4@NaYF4:Yb,Er (C/S) structure. Figure 4. Physical characterization by TEM, HRTEM, and XRD of NaYF4 core.
shows TEM and HRTEM images and XRD spectra of the NaYF4@NaYF4:Yb,Er (core/shell or C/S samples). The TEM images show that the particles are very uniform in size and that the average size of the particles was about 25 ± 1 nm. The interface between core and shell can be observed distinctly. HRTEM image shows the interplanar spacing of 0.3 and 0.52 nm corresponding to the (112̅0) and (101̅0) planes of β-phase NaYF4 (Figure.3c), respectively. The thickness of shell was about 3.5 nm. The XRD pattern indicates the as-prepared nanoparticles crystallized in the β-phase of NaYF4, and that the calculated size of the C/S structure was 25.2 nm according to the Debye−Scherrer equation, which again agrees very well
Experimental Procedures section. As can be observed, the nanoparticles were of similar shape and equal in size (Figure 4a) with average “diameters” of 18 nm for the core synthesis. This value was obtained from the size distribution plots shown in Figure 2b, which also demonstrates a highly monodispersed population of particles with a half-width of ±0.5 nm. The HRTEM image displays an interplanar spacing of 0.52 nm corresponding to the (101̅0) plane of β-phase NaYF4 (Figure 4c). Additionally, the XRD pattern (Figure 4d) indicates that the particles were of good quality and well-crystallized. All of 1774
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was found of the presence of small particles, indicating that no prenucleation occurred. The reaction temperature and reaction time are the major parameters that markedly influence the synthesis reaction. The reaction temperature determines if a chemical reaction can be initiated. Experiment showed that the NaYF4 nanoparticles were very difficult to obtain if the temperature was less than 310 °C (Supporting Information, Figures S1, S2). However, when the temperature was much greater than 320 °C, the chemical reaction became difficult to control because the reaction rate was too fast (Supporting Information, Figures S3, S4). In our experiments, the reaction temperature was set at 320 °C and the size and uniformity of the nanoparticles were determined by the reaction time. Normally, short reaction times (
