Understanding Energy Transfer Mechanisms for Tunable Emission of

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Understanding Energy Transfer Mechanisms for Tunable Emission of Yb -Er Codoped GdF Nanoparticles: ConcentrationDependent Luminescence by Near-Infrared and Violet Excitation 3+

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Dekang Xu, Chufeng Liu, Jiawei Yan, Shenghong Yang, and Yueli Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b00882 • Publication Date (Web): 03 Mar 2015 Downloaded from http://pubs.acs.org on March 12, 2015

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The Journal of Physical Chemistry

Understanding Energy Transfer Mechanisms for Tunable Emission of Yb3+-Er3+ Codoped GdF3 Nanoparticles: Concentration-Dependent Luminescence by Near-Infrared and Violet Excitation Dekang Xu,† Chufeng Liu,† Jiawei Yan,† Shenghong Yang,† and Yueli Zhang,*,†,‡ †

State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and

Engineering, Sun Yat-Sen University, Guangzhou 510275, People’s Republic of China ‡

State Key Laboratory of Crystal Material, Shandong University, Jinan 250100, People’s

Republic of China KEYWORDS: energy transfer mechanisms, concentration-dependent, luminescence, nanoparticles

ABSTRACT: Energy transfer (ET) is an important route to manage the population density of excited states, giving rise to spectrally tunable emission that is valuable for multicolor imaging and biological tracking. In this paper, a case study of GdF3 nanoparticles (NPs) codoped with

ACS Paragon Plus Environment

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Yb3+ and Er3+ was used to experimentally and theoretically investigate the ET mechanisms under near-infrared and violet excitation. Red-to-green ratio (RGR) is used as primary evaluating protocol and the power-dependent luminescence and Er3+ 4I13/2 luminescence behavior are used to identify the corresponding conjectures about ET mechanisms. Compared with the four common upconversion (UC) models, a joint effect of energy-back-transfer, multiphonon relaxation and linear decay depletion mechanisms for Er3+ 4I13/2 manifold was proposed for UC process based on UC spectra for samples with different dopants concentrations. Meanwhile, the varying RGR could also be observed from downshifting (DS) emission spectra. The ET mechanism for DS process, where three cross-relaxation processes coexisted including Yb3+ 2F5/2 manifold as energy in-transit state, was proposed for the first time. The findings are expected to provide an approach for understanding ET mechanisms in many Yb3+/Er3+ codoped UC and DS systems and enable spectrally tunable emission properties for applications that require precisely defined optical transitions.

INTRODUCTION Nanoparticles (NPs) doped with lanthanide ions have triggered intensive interest in recent years because of their prominent upconversion (UC) and down-shifting(DS)/-conversion (DC) luminescent properties. As eminent UC materials, they play a crucial role in the biomedical field such as biological labeling, sensing, imaging, detection, therapy and drug delivery.1-5 DS/DC property leads to applications such as organic light emitting diodes6-7 and photovoltaic.8-10 In all UC and DS/DC systems, energy transfer (ET) plays an indispensable role in manipulating the population of excited states of the doped lanthanide ions, which, hence, gives rise to the tunable luminescence.


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Multicolor tuning of UC emission can be realized by many strategies, such as binary lanthanide doping,11-13 ternary lanthanide doping,14-17 excitation modulation,18-20 and matrix modification21,22 and so on. Among which, the distribution of Yb3+/Ln3+ (Ln = Er, Ho, Tm) codoped in matrixes for UC luminescence (UCL) has attracted intense interests. Yb3+ ion is acting as an ideal sensitizer in efficiently transferring absorbed energy to the other Ln3+ ions, which has a higher extinction coefficient than other Ln3+ ions.23 Among those codopants, Er3+ ion is one popular UC dopant for two-color emission, which has similar excited states as Yb3+, i.e., Er3+ 4I11/2 manifold is closed to Yb3+ 2F5/2 manifold. Moreover, the excited state of Yb3+ has larger absorption cross-section than that of Er3+.24 Therefore, Yb3+/Er3+ can be efficient UC codopants in many host materials.25-31 Admittedly, the dopant (Yb3+/Er3+) concentration makes contribution to tuning visible UCL, where a cross-relaxation (CR)24,32-34 or energy-back-transfer (EBT)23,35,36 mechanism is predominant to manage the population density of corresponding intermediate states. Recently, it is found that the competition between UC and linear decay (LD) of Er3+ 4I13/2 manifold and 4I11/2 manifold is very important for determining power-dependent relationship of corresponding emission manifolds.37 A new UC mechanism with quantitative values for the rate constants involving Yb3+-to-Er3+ ET UC out of the green-emitting manifolds was proposed to account for all the observed optical transitions, which provides a powerful tool for predicting the optical properties of the most widely studied UC phosphor and can also be applied in luminescent materials with micro- and nano-scale.38 While many Yb3+/Er3+ codoped systems mainly focus on UC visible luminescence, there is only few reports on their DS property, such as codoping effect on visible emission under 484 nm excitation,24,33 DS visible emission in Yb3+/Er3+ codoped systems under a variety of excitation scenarios,39 and nearinfrared (NIR) emission under 378 nm excitation40 and so on.


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However, there is no systematic study to give a clear picture about dynamic behaviors of the above ET processes or compare the difference of CR and EBT mechanisms for Yb3+/Er3+ codoped systems. Besides, most UC mechanisms are proposed in high Yb3+ dose materials based on UC as dominant depletion for Er3+ 4I13/2 manifold. Moreover, it still remains questionable about the effects of dopants concentrations on the DS visible luminescence properties under violet excitation and the detailed ET mechanism has yet to be decided. Herein, GdF3 is used as a case study, and their luminescence behaviors and UC ET mechanisms with different Yb3+ and Er3+ dose are explored by adopting rate equations based on four common ET models. According to our experimental results, however, a more detailed UC ET mechanism is proposed rather than the four common models. In addition, a particular ET mechanism for DS process is also proposed. Red-to-green ratio (RGR) versus dopant concentration is regarded as a primary evaluating protocol for determining the ET mechanisms in both processes. Power-dependent luminescence and Er3+ 4I13/2 luminescence behavior are also used to verify the above conjectures. EXPERIMENTAL DETAILS Rate Equation Model. Since the emission intensity of a radiative manifold is proportional to the product of the population of the emitting state and the rate constant for transition, a set of coupled differential equations is used to describe instantaneous populations and ET between Yb3+ and Er3+ manifolds.13 Base on the steady-state rate equation model, the population Ni of the lanthanide manifold i over time in both UC and DS processes is determined by incoming and outgoing rates of ET, multiphonon relaxation (MPR) and radiative transition dN i ET NR NR = ∑ (ω ET ji ,lk N j N l − ωij , kl N i N k ) + ( ωi +1,i N i +1 − ωi ,i −1 N i ) − Ai N i dt ij ,kl

(1)


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ωijET, kl is the ET parameter for donor i to j transition and the acceptor k to l transition. ωiNR ,i −1 is the nonradiative MPR rate from the manifold i to the next lower-lying manifold i-1. Ai is the radiative rate of manifold i. Since the steady-state equations are used only to determine the relationship between the population of the given manifold and the dopant concentration (N0 and NYb0(1)), the exact values of all the rate constants, i.e., Ai, ωijET, kl and ωiNR ,i −1 , are not required.

The primary evaluating protocol, RGR, is defined as

RGR =

IR IG

(2)

For experimental results, IR and IG are the integrated areas from 600 to 700 nm and 500 to 600 nm, respectively. Synthetic Procedure. All the reagents are analytical grade and used without further purification. Yb3+, Er3+ codoped GdF3 NPs were prepared by a simple hydrothermal synthesis using citric acid as the chelating agent. In a typical process, Gd2O3, Yb2O3, Er2O3 (total mole is 1 mmol) were dissolved with nitric acid and heated at 80 oC until excess nitric acid vaporized. Then citric acid was added to the mixture and stirred for 15 min. NH4HF2 solution was added dropwise while stirring to form a white suspension. Finally, the suspension was transferred into a 60 mL Teflon-lined stainless steel autoclaves and heated at 180 oC for 2 days. The NPs were isolated by centrifugation. The final product was collected by washing with ethanol several times and drying in air at 70 oC over night. For tunable luminescence, samples doped with different Er3+ and Yb3+ concentration were prepared in a similar way. The molar ratio of Gd3+:Yb3+:Er3+ was (0.8 – x%):0.2:x% (x = 1, 2, 3,


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5) for different Er3+ dose and (0.98 – y%):y%:0.02 (y = 10, 20, 40, 60) for different Yb3+ dose, respectively. Materials Characterization. The phase and structure of the as-prepared products were measured by the D-Max 2200VPC X-ray Diffractometer (XRD), with Cu Κα radiation and scan step 8 o/min. Morphologies were characterized by FEI Tecnai G2 Spirit transmission electron microscopy (TEM), operated at 120 kV. UC and DS photoluminescence spectra were performed at room temperature on the Edinburgh Instrument FLSP920 combined fluorescence lifetime and steady-state fluorescence spectrometer equipped with a 2 W 980 nm laser diode (spot area is about 0.05 cm2) and 150 W Xenon lamp, respectively. RESULTS & DISCUSSION Structure and Morphology. Figure 1(a) shows the XRD patterns of the GdF3:20%Yb3+/2%Er3+ NPs. All the diffraction peaks of the XRD patterns are well indexed, respectively, to the orthorhombic GdF3 phase (JCPDS 012-0788, with space group Pnma). No other phases or impurities diffraction peaks can be observed, demonstrating its high crystalline purity. The average crystal size is about 37 nm, as calculated by the Debye-Scherrer formula. Figure 1(b) shows TEM images of 20%Yb3+/2%Er3+ codoped GdF3 NPs. The morphology of the particles is ellipse-like shape with average diameter about 36 nm, which is considered approximately to the calculated value from the XRD patterns. And the aggregation of nanoparticles is somehow to a degree, which is similar to the reported GdF3 nanoparticles.41 Moreover, no obvious changes in size can be observed along with the increasing Er3+ or Yb3+ concentration and all the Yb3+ ions are doped into the crystal structure well (More details as discussed in Supporting Information (SI) Figure S1 and S2).


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Figure 1 (a) XRD patterns for samples GdF3:0.2Yb3+/0.02Er3+ and the standard data (JCPDS No.12-0788) for GdF3 as reference. (b) TEM image of the as-prepared sample. (Scale bar = 100 nm) Upconversion Mechanism. Figure 2(a) shows the UC spectra of Yb3+/Er3+ codoped GdF3 with different ratios of Er3+ excited by 980 nm NIR lasers. Two typical emission bands can be observed, with emission peaks centered at 523/545 nm assigned to Er3+ 4H13/2/4S3/2 → 4I15/2 transitions and peak centered at 660 nm assigned to Er3+ 4F9/2 → 4I15/2 transition, respectively. It has also revealed that the emission intensity varies with the increase of Er3+, reaching the maximum at 2% Er3+ concentration, which is in accordance with the previous reports.15,42 Additionally, the intensity ratio of 4F9/2 manifold to 2H11/2/4S3/2 manifolds (that is, RGR) hardly changes, indicating Er3+ dose has nothing to do with the spectral tuning of UC visible emissions.


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Figure 2 (a) UCL spectra of GdF3:20%Yb3+/x%Er3+ (x = 1, 2, 3, 5) NPs excited by 980 nm laser with 20 W/cm-2 power density. (b) UCL spectra of GdF3:y%Yb3+/2%Er3+ (y = 10, 20, 40 and 60) NPs excited by 980 nm laser with 20 W/cm-2 power density. The spectra are normalized at Er3+ 4

F9/2 → 4I15/2 transition. Insets show RGR as a function of different dopant concentrations. The UC spectra of Yb3+/Er3+ codoped GdF3 with different Yb3+ dose are exhibited in Figure

2(b). Yb3+ ion plays a role as a sensitizer, and due to a large cross-section of 2F5/2 manifold, it absorbs the NIR photons more efficiently than Er3+ 4I11/2 manifold. After the excitation of the Yb3+, energies can be transferred to the ground state of Er3+ through the ET process 2F5/2 (Yb) + 4

I15/2 (Er) → 2F7/2 (Yb) + 4I11/2 (Er), thus populating the 4I11/2 manifold. Generally two photons

would be involved within green and red UC emissions. For green emission, 4F7/2 manifold is populated through energy transfer upconversion (ETU) process by absorbing another photon from 4I11/2 manifold. Then green emission generates after MPR from 4F7/2 manifold to 4H13/2 manifold and 4S3/2 manifold. For red emission, there are three possible ways to populate 4F9/2 manifold: (i) First 4F9/2 manifold can be directly populated through MPR process of 2H11/2/4S3/2 manifolds, which can easily be bridged by high phonon-energy groups (OH- of 3400 cm-1 and C=O of 1500 cm-1) attached on the surface of NPs33 though there is a large energy gap between 4

S3/2 manifold and 4F9/2 manifold (∆E ~ 3128 cm-1), which is about 10 times of optical phonon

energy of GdF3 (~ 300 cm-1). Another two ways are that 4F9/2 manifold is populated by 4I13/2 manifold absorbing another NIR photon after (ii) MPR process of 4I11/2 → 4I13/2 or (iii) energyback-transfer (EBT) from 2H11/2/4S3/2 manifolds to 4I13/2 manifold. One can observe the broadened red emission peaks from Fig. 2(b), which may be resulted from the crystal field effect with heavier Yb3+ replacing Gd3+ in orthorhombic structure that causes the Stark splitting for the red emission peaks.


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One can also find that red emission intensity compared to green one is improved by the increasing Yb3+ dose. According to many previous reports, the population processes of 4I13/2 and 4

F9/2 manifolds are considered to be the key role in red UC emission for high Yb3+ concentration.

As is known to all, cross-relaxation (CR) is dependent on the distance between activators. Since Yb3+ ion radii (0.868 Å) is smaller than that of Gd3+ ion (0.935 Å), which leads to the reduced lattice constants of GdF3 NPs with increasing Yb3+ dose and thus shorten the average distance between Er3+ ions. Additionally, the increasing Er3+ dose also undoubtedly diminishes average distance between Er3+ ions. Thus, CR process is considered to be a possible mechanism for the red emission enhancement. There are generally three possible ET mechanisms involving CR process that account for such phenomenon, which is illustrated in Figure 3. According to Capobianco et al.,24 the striking red enhancement was observed due to a CR process 4F7/2 (Er) + 4

I11/2 (Er) → 4F9/2 (Er) + 4F9/2 (Er) that directly populates 4F9/2 state and thus enhanced the red

emission, denoted as ET1. ET2 process was proposed by D. Gao et al.,32 which was derived from CR process 4S3/2 (Er) + 4I13/2 (Er) → 4F9/2 (Er) + 4I11/2 (Er) and then promoted the red emission and quenched the green emission. ET3 process was proposed by P. Salas et al.,43 where the enhanced red emission was mainly ascribed to CR 4S3/2 (Er) + 4I15/2 (Er) → 4I9/2 (Er) + 4I1132 (Er) and subsequent ETU I13/2 (Er) + 2F5/2 (Yb) → 4F9/2 (Er) + 2F7/2 (Yb). Recently, an ET mechanism, where an efficient EBT process 4F7/2 (Er) + 2F7/2 (Yb) → 4I13/2 (Er) + 2F5/2 (Yb) and subsequent ETU process 4I13/2 (Er) + 2F5/2 (Yb) → 4F9/2 (Er) + 2F7/2 (Yb) ascribe to the green and red spectral tailoring, is proposed by Zhang et al.44,45 and developed by Cao’s team23 and Jeong’s team.46 EBT process is becoming more prominent due to the reduced average distance between Yb3+ and Er3+ ions, which is also illustrated in Figure 3. In this article, the above four ET mechanisms are


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discussed systematically. To validate the mechanism, the theoretical models are established for the four ET mechanisms using steady-state rate equations, as listed in SI Table S1.

Figure 3 Proposed ET scheme inducing tunable emission in Yb3+-Er3+ ion pair. Solid lines, dashed lines and dotted lines represent radiative transition, energy transfer and multiphonon relaxation, respectively. In the following discussion, the radiation rates of 4I13/2 and 4I11/2(9/2) manifolds can be neglected compared to the UC rates from these manifolds due to their long lifetimes (A1 and A2(2’)). Moreover, it is also rational to omit ETU rate from 4I13/2 manifold to 4F9/2 manifold because CR process is considered predominant in ET1 and ET2. Under this situation, the ratio of N3 to N4 is also obtained (that is, RGR), as listed in SI Table S1. In ET1, RGR is related to N0 rather than NYb1(NYb0), showing that RGR varies along with Er3+ concentration instead of the increasing Yb3+

concentration, and thus suggesting that ET1 is not the main ET mechanism. In ET2, RGR shows the inverse proportional relationship to the Yb3+ concentration, which also indicates ET2 is not applicable in explaining the enhancing red emission with increasing Yb3+ concentration. In the third ET mechanism, RGR is positive proportional to Er3+ concentration, which, similar with


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ET1, indicates ET3 has nothing to do with the increase of red emission. In our fourth case, the ratio has a positive proportional relationship with Yb3+ concentration, which is exactly in agreement with the experimental result shown in Figure 2(b), indicating that EBT and subsequent ETU processes contribute significantly to the enhancement of red emission. In Zhang’s report,44 CR process could not be the prominent tool for inducing saturation of Er3+ 4I13/2 manifold. In our case, CR process produces a factor of ωCN0 in ET1/ET3, where the increase of Er3+ concentration should make a contribution to the enhancement of red emission. As reflected in the inset of Figure 2(a), variation of Er3+ concentrations does not affect the RGR, which, in some degree, strengthens the point view of Zhang’s group that CR process is not as important as EBT process. However, EBT process can lead to the saturation of Er3+ 4I13/2 manifold, which means that the 4I13/2 emission is not related to pump power, i.e., INIR ~ ρ0 (see summary in Table 1). In our results, the saturation behavior of 4I13/2 manifold can be observed, which, however, reveals that 4I13/2 emission is still proportional to pump power. The quasi-quadratic powerdependence of 4I13/2 manifold emission in low Yb3+ dose (10%) and quasi-linear powerdependence in high Yb3+ dose (60%) samples can be observed from Figure 4. Hence, only EBT process is inadequate to explain the results in our case. According to a recent study,37 the competition between UC and LD for 4I13/2 manifold is very influential for the power-dependent luminescence. As the above four ET mechanisms are based on the UC as depletion mechanism for 4I13/2 manifold, it is reasonable to assume LD to be dominant depletion for 4I13/2 manifold due to the low intrinsic phonon-energy matrix.47 In addition, due to the adhesion of carboxyl and hydroxyl groups on the surface of NPs, MPR is also important, i.e., MPR from 2H11/2/4S3/2 manifolds to 4F9/2 manifold and MPR from 4I11/2 manifold to 4I13/2 manifold. Therefore, ET5 is put forward, as illustrated in Figure 3. By solving the corresponding rate equations (seen Table


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S1, ET5), RGR factor of A1ω43 + ω1ωb NYb 0 NYb1 is obtained, elucidating that RGR is proportional to A1 A3

Yb3+ concentration and pump power. To compare the two mechanisms (ET4 and ET5), the corresponding power-dependent luminescence relationship is summarized in Table 1 (More details see SI, the interpretation for Table 1 and Table S1). As can be seen from Table 1, the slope value n for power-dependent NIR (4I13/2) luminescence is 1 for low Yb3+ dose and 0 for high Yb3+ dose in ET4, and 1 to 2 for low Yb3+ dose and 0 to 1 for high Yb3+ dose in ET5. Apparently, one can see that the findings of NIR emission behaviors from ET5 are in good agreement with results in Figure 4.

Figure 4 Power-dependent Er3+ NIR (4I13/2) emission spectra of GdF3:2%Er3+ with (a) 10% and (b) 60% Yb3+ under 980 nm excitation with 20 W/cm-2 power density. The experimental data were fitted well with linear relationship. Table 1 Power-dependent luminescence for ET4 and ET5. The situations are discussed with low Yb3+ dose (LD as dominant depletion for 4I11/2 manifold) and high Yb3+ dose (UC as dominant depletion for 4I11/2 manifold)

Dominant Depletion 4

I11/2

4

I13/2

Power-Dependent Luminescence N1 (4I13/2)

N3 (4F9/2)

N4 (2H112/4S3/2)


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LD

UC

ρ1

ρ2

ρ2

UC

UC

ρ0

ρ1

ρ1

LD

LD

aρ1 + bρ2

aρ2 + bρ3

ρ2

UC

LD

aρ0 + bρ1

aρ1 + bρ2

ρ1

ET4

ET5

To get deeper insight of the UCL, power-dependent luminescence intensities for green and red emissions are investigated. As shown in Figure 5, all the logarithmic luminescence intensities show perfect linearity with the logarithmic power density. Apparently, the value of n is tapering off along with Yb3+ concentration in the range of 1 and 2, which is primarily ascribed to the competition between LD and UC processes for the depletion of the intermediate excited states.47 As shown in Table 1, with low Yb3+ dose, the population of red-emitting manifold and greenemitting manifolds have quasi-quadratic relationships with pump power, i.e., N3 ~ aρ2 + bρ3 and N4 ~ ρ2, which corresponds to Figure 5(a) where n is 2.2 for red emission and 2.08 for green

emission. With higher Yb3+ (60%) concentration, N3 ~ aρ1 + bρ2 and N4 ~ ρ1 are obtained, which corresponds to the decreasing slope for red and green emissions in Figure 5(b-d).


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Figure 5 Double logarithmic relationship of red and green luminescence intensities versus power density of GdF3:2%Er3+ NPs codoped with (a) 10%, (b) 20%, (c) 40% and (d) 60% Yb3+. Additionally, the RGR factor, A1ω43 + ω1ωb NYb 0 NYb1 , is related to Yb3+ concentration and pump A1 A3

power. Therefore, RGR should be elevated along with the increasing pump power. Hence, the relationship between RGR and excitation power density is studied. As shown in SI Figure S3, RGR indeed increases with excitation power density, which is similar to the previous report38,48 and to some extent verifies our conjecture above. The ratio increases slightly in low Yb3+ dose samples; however, in higher Yb3+ dose samples, the ratio grows significantly and the value varies from several to dozen, which again demonstrates RGR is related to Yb3+ concentration and excitation power density. Therefore, the UCL spectra can also be greatly tuned by excitation power density.


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In a word, it is thus clear that our experimental results conform to the ET5 model, where ET mechanism is mainly due to the joint effect of efficient EBT, MPR and LD depletion for 4I13/2 manifold. There may be some reasons. First, Yb3+ ions have large absorption cross-section than Er3+ ions at 980 nm. Second, due to the intrinsic low phonon energy of our samples, the radiative transition can be predominant, especially Er3+ 4I13/2 NIR emission. Third, some high phononenergy groups attach to the surface of NPs, leading to the MPR process. Additionally, the energy mismatch of EBT process is adequate for our samples and become more prominent with increasing Yb3+ concentration. Downshifting Mechanism. The excitation and DS luminescence (DSL) spectra of GdF3:20%Yb3+/x%Er3+ (x = 1, 2, 3 and 5) NPs were shown in Figure 6. Figure 6(a) shows two kinds of absorption bands, consisting of Gd3+ 8S7/2 → 6IJ/2 transition and Er3+ 4G11/2 → 4I15/2 transition. However, the absorption intensity of Er3+ 4G11/2 → 4I15/2 transition is much stronger than that of Gd3+ 8S7/2 → 6IJ/2 transition, indicating direct excitation of Er3+ is much more efficient than ET between Gd3+ and Er3+. Figure 6(b) is the normalized emission spectra of Yb3+/Er3+ codoped GdF3 with different Er3+ concentration excited by 378 nm. Two emission bands are observed, where the red peak intensity is much stronger than the green one. Unlike the UC process, Er3+ dose enables the enhancing RGR in DS process. This relationship can be well fitted with quadratic polynomial function, as illustrated in Figure 6(c). Moreover, the NIR emission increases comprehensively with respect to red emission with increasing Er3+ dose (see SI Figure S4), which indicates a CR process occurs regarded to green-emitting (2H11/2/4S3/2) manifolds and 4I13/2 manifold, depopulating green-emitting manifolds and populating NIR manifold.


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Figure 6 (a) Excitation spectra of GdF3:20%Yb3+/2%Er3+ NPs monitored at Er3+ 4F9/2 → 4I15/2 transition. (b) DSL spectra of GdF3:20%Yb3+ NPs codoped with 1%, 2%, 3% and 5% Er3+ concentration under 378 nm excitation. The spectra are normalized at Er3+ 4F9/2 → 4I15/2 transition. Inset shows enlarged green emission band. (c) RGR as a function of Er3+ concentrations. (d) DSL spectra of GdF3:2%Er3+ NPs codoped with 10%, 20%, 40% and 60% Yb3+ concentration under 378 nm excitation. All spectra are normalized at Er3+ 4F9/2 → 4I15/2 transition. Inset shows the magnification of Er3+ 4I13/2 NIR emission. Figure 6(d) shows the luminescence spectra of GdF3:y%Yb3+/2%Er3+ (y = 10, 20, 40, 60) NPs under 378 nm excitation. Green emission decreases so fast that only singular red emission band is shown in samples with 40% and 60% Yb3+. The striking RGR indicates the key role of Yb3+ ions in tuning the populations of Er3+ ions between 2H11/2/4S3/2 manifolds and 4F9/2 manifold. According to the previous report,39 a CR process should occur between Er3+ and Yb3+ and there exists Yb3+ 2F5/2 emission under 4G11/2 excitation. However, no emission around 1 µm is observed under 378 nm excitation (see SI Figure S5), indicating the Yb3+ 2F5/2 manifold is acting as energy in-transit rather than energy transfer. In a previous report,13 Tm3+ 3F4 manifold participates in storing energy from Er3+ green-emitting manifolds and then donating all of its energy to Er3+ 4I11/2 manifold to populate Er3+ red-emitting 4F9/2 manifold. Similarly, energy in-


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transit, in our paper, is defined as energy from Er3+ stimulated 4G11/2 manifold stores in Yb3+ 2F5/2 manifold through a CR process and nonradiatively transfers all the energy to Er3+ 4I13/2 manifold, which subsequently participates in populating 4F9/2 manifold and thus leads to the severe depopulation of 2H11/2/4S3/2 manifolds and fast population of 4F9/2 manifold. This is reasonable because Yb3+ ions have large absorption cross-section and the reduced average distance between Yb3+ and Er3+ enables energy to efficiently transfer between them. Additionally, one can find out that 4I13/2 emission rapidly diminishes with respect to 4F9/2 emission with increasing Yb3+ dose (as seen from NIR emission in Figure 6(d)), implying the fast depopulation of 4I13/2 manifold. This result confirms the above “energy in-transit” conjecture that the Yb3+ 2F5/2 manifold donates all of the stored energy to Er3+ 4I13/2 manifold, which then populates Er3+ 4F9/2 manifold through ETU process. Therefore, an ET mechanism for DSL is proposed as shown in Figure 7. Besides, another reasonable CR process (ET2 in Figure 7) is possible due to the near-zero energy mismatch between 4G11/2 → 4F9/2 and 4I15/2 → 4I11/2, which also manages to populate 4F9/2 manifold.


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Figure 7 Proposed DS ET scheme in Yb3+-Er3+ ion pair. Solid lines, dashed lines and dotted lines represent radiative transition, energy transfer and multiphonon relaxation processes, respectively. To get deeper information about such ET model, rate equations are established to analyze the DSL process, which can be expressed as dN1 = ωC 2 N 0 N 4 − ω1 NYb1 N1 − A1 N1 dt

(3)

dN 2 = ωC1 N 0 N 5 − A2 N 2 dt

(4)

dN 2' = ωC 2 N 0 N 4 − A2' N 2' dt

(5)

dN 3 = ω1 NYb1 N1 + ωC 3 NYb 0 N 5 + ωC1 N 0 N 5 − A3 N 3 dt

(6)


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dN 4 = ω54 N 5 − ωC 2 N 0 N 4 − A4 N 4 dt

(7)

dN 5 = σρ N 0 − ωC1 N 0 N 5 − ωC 2 N 0 N 4 − ωC 3 NYb 0 N 5 − ω54 N 5 dt

(8)

dNYb1 = ωC 3 NYb 0 N 5 − ω1 NYb1 N1 dt

(9)

where ωC1, ωC2 and ωC3 are CR rates of Er3+ 4G11/2 + Er3+ 4I15/2 → Er3+ 4F9/2 + Er3+ 4I11/2, Er3+ 2

H11/2/4S3/2 + Er3+ 4I15/2 → Er3+ 4I9/2 + Er3+ 4I13/2 and Er3+ 4G11/2 + Yb3+ 2F7/2 → Er3+ 4F9/2 + Yb3+

2

F5/2, respectively. ω54 is MPR rate from Er3+ 4G11/2 manifold to Er3+ 2H11/2/4S3/2 manifolds. The

red and green emissions that we care about are obtained by solving the above equations

N3 =

N4 =

(ωC 2 N0 + A4 )(ωC1 N0 + 2ωC 3 NYb0 ) σρ N0

∝ρ

(10)

ω54σρ N0 ∝ρ ωC1ωC 2 N + ωC1 A4 N0 + 2ωC 2ω54 N0 + A4ω54 + ωC 2ωC 3 N0 NYb 0 + A4ωC 3 NYb 0

(11)

A3 (ωC1ωC 2 N + ωC1 A4 N0 + 2ωC 2ω54 N 0 + A4ω54 + ωC 2ωC 3 N 0 NYb 0 + A4ωC 3 NYb 0 ) 2 0

2 0

The results indicate that both red and green emissions would increase linearly with excitation power. Moreover, by dividing N3 with N4, RGR can be obtained

I R N3 (ωC 2 N0 + A4 )(ωC1 N0 + 2ωC 3 NYb 0 ) ~ = IG N 4 A3ω54

(12)

This RGR increases quadratically with N0 (Er3+ concentration), and is proportional to NYb0 (Yb3+ concentration), which can be well observed in Figure 6(c) and 6(d). In order to verify the correctness of the proposed ET model, the power dependence curves for red emission established in the double logarithmic scale. As shown in Figure 8, the slope values of power dependence curve for red emission are close to 1 in different Er3+ codoped samples, while the values are also close to 1 in samples with increasing Yb3+ dose. The results for low dopants dose can be well explained in SI Figure S6. However, the results for high dopants dose reveal that the red DS


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emission also bears the linear relationship of excitation power, which is also in accordance with the calculation results above. Hence, the DS mechanism for the case of GdF3:Yb3+/Er3+ NPs is due to three CR processes, including Yb3+ 2F5/2 manifold as energy in-transit state.

Figure 8 Double logarithmic plots of red emission intensities versus excitation power of 379 nm with different (a) Er3+ and (b) Yb3+ dose. CONCLUSIONS In summary, ET mechanisms for Yb3+/Er3+ codoped GdF3 NPs are investigated. Orthohombic GdF3:Yb3+/Er3+ with ellipse-shaped NPs are confirmed by XRD and TEM, respectively. In order to determine the influence of dopants concentration on luminescence properties, the corresponding UCL and DSL spectra of GdF3:Yb3+/Er3+ NPs with a variety of Yb3+ and Er3+ performed are performed under NIR and violet excitation, respectively. Emission spectra under NIR excitation reveal the vital role of Yb3+ in spectral tuning. Power-dependent visible and NIR luminescence results demonstrate the importance of depletion mechanism for Er3+ 4I13/2 manifold. By analyzing corresponding rate equations, the UC mechanism for tunable emission is attributed to the joint effect of EBT, MPR and LD depletion mechanisms for 4I13/2 intermediate state. On the other hand, the relative luminescence intensity of Er3+ 4I13/2 emission to 4F9/2


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emission and power-dependent red emission conclude that the DS mechanism for tunable emission is due to a joint effect of three CR mechanisms, including Yb3+ 2F5/2 manifold as energy in-transit state. It is anticipated that the results will further provide valuable information for understanding ET mechanisms in spectrally tuning emission UC and DS/DC systems. ASSCOCIATED CONTENT Supporting Information XRD patterns of GdF3:Yb3+/Er3+ with different Er3+ and Yb3+ dose. EDX spectra of GdF3:Yb/Er with different Yb3+ dose. Table for five models and corresponding rate equations and the related discussion. Power dependence of RGR for different Yb3+ dose. Er3+ 4I13/2 NIR emission spectra with respect to red emission for different Er3+ concentration. Yb3+ 2F5/2 NIR excitation and emission spectra for GdF3:Yb3+/Er3+. DS ET mechanism for Yb3+-Er3+ in low dopants concentration and the related discussion. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail address: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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This work was supported by the National Natural Science Foundation of China under Grant No. 61176010 and No. 61172027, Guangdong Natural Science Foundation under Grant No. 1414050000317, the Research Foundation of IARC-SYSU under Grant No. IARC 2014-09. REFERENCES (1) Wang, F.; Banerjee, D.; Liu, Y.; Chen, X.; Liu, X. Upconversion Nanoparticles in Biological Labeling, Imaging, and Therapy. Analyst 2010, 135, 1839-1854. (2) Wang, F.; Liu, X. Recent Advances in the Chemistry of Lanthanide-Doped Upconversion Nanocrystals. Chem. Soc. Rev. 2009, 38, 976-989. (3) Lin, M.; Zhao, Y.; Wang, S.; Liu, M.; Duan, Z.; Chen, Y.; Li, F.; Xu, F.; Lu, T. Recent Advances in Synthesis and Surface Modification of Lanthanide-Doped Upconversion Nanoparticles for Biomedical Applications. Biotechnol. Adv. 2012, 30, 1551-1561. (4) Chatterjee, D. K.; Gnanasammandhan, M. K.; Zhang, Y. Small Upconverting Fluorescent Nanoparticles for Biomedical Applications. Small 2010, 6, 2781-2795. (5) Wang, L. Y.; Yan, R. X.; Hao, Z. Y.; Wang, L.; Zeng, J. H.; Bao, J.; Wang, X.; Peng, Q.; Li, Y. D. Fluorescence Resonant Energy Transfer Biosensor Based on Upconversion-Luminescent Nanoparticles. Angew. Chem. Int. Ed. 2005, 44, 6054-6057. (6) Bunzli, J. C. G.; Comby, S.; Chauvin, A. S.; Vandevyver, C. D. B. New Opportunities for Lanthanide Luminescence. J. Rare Earth. 2007, 25, 257-274. (7) So, F.; Krummacher, B.; Mathai, M. K.; Poplavskyy, D.; Choulis, S. A.; Choong, V. E. Recent Progress in Solution Processable Organic Light Emitting Devices. J. Appl. Phys. 2007, 102, 091101.


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Table of Contents Graphic


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Understanding Energy Transfer Mechanisms for Tunable Emission of

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