White-Light-Emitting Single Phosphors via Triply Doped LaF3

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White Light-Emitting Single Phosphors via Triply Doped LaF Nanoparticles Chantal Lorbeer, and Anja-Verena Mudring J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp312411f • Publication Date (Web): 07 May 2013 Downloaded from http://pubs.acs.org on May 15, 2013

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The Journal of Physical Chemistry

White Light-emitting Single Phosphors via Triply Doped LaF3 Nanoparticles Chantal Lorbeer and Anja-Verena Mudring* Ruhr-Universität Bochum, Inorganic Chemistry III, Materials Engineering and Characterization, Universitätsstr. 150, 44801 Bochum, Germany. E-mail: [email protected] KEYWORDS: White emission, ionic liquids, nanoparticles, phosphors, energy transfer ABSTRACT The production of high quality phosphors for white-emitting applications is an important goal for the settlement of LEDs in the market and households. Single phosphors directly yielding white emission are advantageous in comparison to a mixture of individual red, green and blue phosphors as these are hampered by re-absorption of the blue light. Here, a combined approach to uniform, nanoscale particles as single white-emitting phosphor is realized via an ionic liquidbased synthesis. LaF3 particles co-doped with various amounts of Tm3+, Tb3+ and Eu3+ were synthesized and their structural, morphological and optical properties studied. Small particles with a mean size of 6-8 nm were obtained with a narrow size distribution. At small dopant ion concentrations, white light emission close to standard daylight D65 is obtained under excitation of λex= 355 nm where simultaneous emissions of all three optically active ions occurs.

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INTRODUCTION The development of compact fluorescent lamps rendered them an energy-efficient and still comparatively cheap alternative to traditional lighting sources1, which caused the US, the EU and other countries to successively ban traditional incandescent lamps.2,3 Recently, a new technology entered the market, the cities and households: Solid state lighting, generally known as light-emitting diodes (LEDs). Although the advantages of this technology such as a long lifetime, the possibility of direct mounting on circuit boards and modest power consumption are clear, LEDs were used mainly for indicator applications or in remote controls4 in the past as only red and infrared LEDs were readily available. Apart from the fact that today LEDs have become more efficient, cheaper and brighter, the development of the blue LEDs led to a significant technological breakthrough and the establishment of LEDs as the new generation of lighting.5 However, white light-emitting diodes are still comparatively expensive and suffer from poor color rendering. Two approaches to achieve white light with LEDs are currently practiced. One is to combine three LEDs (a blue, a green and a red one) in a single device, to obtain white light through color addition. Problems arise due to the different lifetimes of the single LEDs, complex manufacturing and controlling constraints (for instance with dimming).6 The second approach concerns phosphor converted LEDs (pcLEDs), which bear a similarity to compact fluorescent lamps. A phosphor material converts the blue or UV light emitted by the diode to white light. This can be accomplished in a blue LED by combination with a yellow phosphor, wherein the transparency of the phosphor layer controls the final light color,7 or with the combination of a near UV LED and appropriate phosphor materials, which themselves create white light emission.4 The main difference and concurrently the main problem in comparison to compact fluorescent lamps is the excitation range of available LEDs.


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Albeit decades of research on (compact) fluorescent lamps led to phosphors with efficiencies close to unity, this is of little value for LEDs8 as they use light of lower wavelength for excitation (near UV or even blue light) and the quest for phosphors that can be excited by this radiation emerged only recently. Moreover, the well-established combination of three luminescent materials for conversion of high energy radiation into red, green and blue light, practiced in compact fluorescent lamps and yielding an overall white emission, is a complex task. With regard to the lower energy of excitation in LEDs, a three-phosphor-combination is hampered by a low luminescence efficiency as a result of strong reabsorption of the blue light by the green and red emitting phosphors.9 Another problem arises with the processing, packaging and assembly of the phosphor, which significantly influences the device performance:10,

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sedimentation and

unbalanced distribution of the three phosphors in the supporting material. Furthermore, the color rendering index and the color temperature need to be varied for a given application,12 which becomes easier if a single phosphor is present. A narrow particle size distribution and a careful grain size optimization can help to circumvent these problems.13,14 In addition, the cost can be significantly lowered by reducing the particle size as less material is required.

15, 16

However,

nanoparticles suffer from defects and contaminations arising from large surface areas, which could lead to quenching of the luminescence. Thus, taking phosphors to the nanoscale is a tightrope walk and difficult to accomplish. The aim of this study is to combine the approach on single phase phosphors for white-emitting applications and nanosized materials for improved processing. For this purpose, an ionic liquidmediated synthesis route to pure nanofluorides was used, wherein the ionic liquid acts as solvent, stabilizer and reagent (scheme 1).17,18


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Scheme 1. Synthesis procedure. Here a combination of the luminescent ions Tm3+, Tb3+ and Eu3+ as emitters of blue, green and red light doped into a matrix of LaF3 was chosen for white light generation. Lanthanide ions have been proven to be highly advantageous for luminescent applications due to the shielding of the 4f orbitals and the corresponding energy levels, which are hardly influenced by the surroundings of the lanthanide ions, which usually leads to high color purity.19,20,21 As energy transfer between the different optically active ions could lead to quenching and thus color changes,22 samples with various degrees of doping were prepared in order to examine the structural, morphological and luminescent properties of these materials. Although the lanthanide ions themselves suffer from poor absorption coefficients and it is advisable to improve this by sensitizers, the present investigation allows studying how the three ions affect each other in the single phosphor and whether sufficiently luminescent particles can be realized at the nanoscale by utilizing this approach.23,24,25 EXPERIMENTAL SECTION Syntheses were carried out using standard Schlenk and argon glove box techniques. The ionic liquid

1-butyl-3-methylimidazolium

tetrafluoroborate

([C4mim][BF4]),

was

synthesized

according to a common literature procedure.26 1-methylimidazole (99 %, Sigma Aldrich),


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chlorobutane (99 %, Acros Organics), acetonitrile (99.5 %, J.T. Baker), sodium tetrafluoroborate (98 %, Aldrich) and dichloromethane (99.9 %, Fisher Scientific) were used as received. For nanoparticle synthesis, the starting materials lanthanum acetate sesquihydrate (99.9 %, ABCR), thulium acetate tetrahydrate (99.9 %, ABCR), terbium acetate tetrahydrate (99.9 %, ABCR), europium acetate hydrate (99.9 %, ABCR), ethylene glycol (99%, J.T. Baker) and ethanol (p.a., Riedel de Häen) were used. Nanoparticle synthesis. In a typical reaction, appropriate amounts of the lanthanum acetate sesquihydrate, thulium acetate tetrahydrate, europium acetate hydrate and terbium acetate tetrahydrate (see SI) were mixed in 0.5 ml ethylene glycole and added to an excess of [C4mim][BF4] (6 ml). The reaction mixture was heated to 120 °C with a single mode microwave operating at 2455 MHz (CEM Discover, Kamp-Lintfort, D). After reaching the reaction temperature, the reaction mixture was kept for additional 10 min at 120°C under continuous high stirring and temperature, pressure and microwave power control before cooling to room temperature by means of pressurized air. The obtained colloidal solution was centrifuged and the reaction product was washed several times with an ethanol/dichloromethane mixture. Finally, the colorless powder was dried at 70 °C. Quantitative yields of 96-99 % were obtained. Characterization. Powder X-ray diffraction measurements were carried out on a G670 diffractometer with an image plate detector (Huber, Rimsting, D) operating with Mo Kα radiation. For particle size determination according to the Scherrer equation, the (103), (111) and (101) reflections were used. These reflections were fitted with the program Origin and the center and FWHM of the fit were taken for the calculation. The error of the fit calculated with Origin was taken to determine the estimated standard deviations. The error of the method itself (for instance the influence of strain) was not taken into account. A Tecnai G2 20 X-Twin transmission


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electron microscope (Fei, Hillsboro, USA) operating at 200 kV acceleration voltage and equipped with an EDAX detector was used for determination of particle size, morphology and energy-dispersive X-ray microanalysis. For sample preparation, a small amount of the sample was added to 6 ml of ethanol and subsequently dispersed in an ultrasonic bath for 30 min. The suspension of the sample in ethanol was dropped on a copper grid coated with carbon and then dried in air. Measurements of the size of the synthesized particles on different overview TEM images of the border of smaller and larger agglomerates was undertaken with the program ES Vision (Fei, Hillsboro, USA) and yielded the particle size distribution. Luminescence measurements were carried out on a Fluorolog FL 3-22 spectrometer (Horiba JobinYvon, Unterhachingen, D) equipped with a continuous xenon lamp for steady state measurements and a pulsed xenon lamp for lifetime determinations. The signal is detected by a photomultiplier. For measurement, powdered samples were filled in silica tubes and carefully positioned in the incoming beam in the sample chamber. The lifetimes were determined with the method “decay by delay”. The lifetimes were fitted two-exponentially and average lifetimes

were calculated according to a common procedure for nanoparticles.27,28,29,30,31 Estimated errors were retrieved from the fitting of the decay curves.

RESULTS AND DISCUSSION LaF3 particles co-doped with different amounts of Tm3+, Tb3+ and Eu3+ were synthesized ranging from low to high overall dopant concentrations (0.75 to 16 %) and from low to high


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single dopant ion concentrations (0.25 to 10 %). The exact doping level and the respective sample identifier are given in Table 1. Table 1. Sample identifier of LaF3: Tm3+, Eu3+ and Tb3+ and respective molar dopant ion concentrations. Particle sizes were determined from the powder X-ray diffraction pattern (SI-Fig. 1) according to the Scherrer equation. Sample identifier

Tm3+ /%

Eu3+ /%

Tb3+ /%

Particle size / nm

a b c d e f g h i j k l m n

0.25 1 1 0.5 0.5 1 1 1 5 5 10 5 10 5

0.25 1 0.5 1 0.5 5 5 10 1 1 1 5 5 10

0.25 1 0.5 0.5 1 5 10 5 5 10 5 1 1 1

6.4 ± 0.8 6.9 ± 0.9 7.3 ± 0.9 7.8 ± 1.0 6.9 ± 0.9 8.1 ± 1.0 7.1 ± 0.9 6.9 ± 0.9 7.5 ± 0.9 6.3 ± 0.8 6.2 ± 0.8 5.9 ± 0.7 6.4 ± 0.8 5.9 ± 0.7

Powder X-ray diffraction. The powder X-ray diffraction patterns of all samples are shown in the SI. All synthesized samples crystallized phase purely in the tysonite structure (LaF3, P-3c1). The broadness of the reflections reveals the small size of the particles. The crystallite size was calculated according to the Scherrer equation (table 1). Independent of the dopant concentration, the particle size is equal in the error margin and was calculated to be in the range of 6-8 nm. Thus, the particles are smaller than 10 nm, a size-regime were truly nanosize-related properties can be observed. There are only few reports on nanofluorides with sizes below 10 nm.32,33 Most


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of the literature on easy, cheap and environmentally friendly preparation routes report particles with a size above 100 nm.34,35,36 Transmission electron microscopy. To study the morphology of the synthesized particles, transmission electron microscopy (TEM) has been carried out. Representative TEM micrographs of samples b and h are displayed in Figure 1.

Figure 1. Representative TEM micrographs and particle size distributions of LaF3: Tm, Eu, Tb (for doping of sample b and h see Table 1). High resolution image (right, sample h) reveal crystalline growth via lattice fringes. ACS Paragon Plus Environment

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The LaF3 particles with low overall dopant concentration (1 % Tm, Tb, Eu each, sample b) are small with an average size of 7.6 ± 1.4 nm. These smaller particles cluster in a column-like fashion to larger agglomerates. This clustering is a result of the washing process, after the stabilizing ionic liquid is removed. The LaF3 particles co-doped with a higher overall dopant concentration (1 % Tm, 10 % Eu, 5 % Tb, sample h) consist of similarly small particles with a mean size of 8.3 ± 1.4 nm. Albeit the clustering tendency is less pronounced, column-like agglomerates can be observed as well. A high resolution micrograph of the latter reveals crystalline growth by showing clear and well-grown lattice fringes. Although the particle sizes determined by TEM are slightly larger than those calculated from the powder X-ray diffraction pattern, the values are in good agreement in frame of the error margin and clearly prove the existence of sub-10 nm crystallites. Optical properties. To evaluate if the synthesized particles are suitable as a single phosphor for white emitting applications, the luminescent properties of the variously doped samples were analyzed. The emission and excitation spectra of the LaF3 particles equally co-doped with 1 % Tm, Tb and Eu are shown in Figure 2. Excited with λex= 393 nm, only (orange-red) Eu3+ emission occurs observable by the naked eye. The emission consists of the 5D0 → 7FJ (J=1-4) and 5D1 → 7FJ’ (J=0-3) transitions as denoted. Amongst the lanthanides, Eu3+ is considered as a structural probe.37 The asymmetry ratio between the 5D0 → 7F1 (magnetic-dipole) and 5D0 → 7F2 (electric dipole, hypersensitive) transition yields information about the local symmetry of the Eu3+ ion. In the emission spectrum of LaF3: 1 % Tm, 1 % Tb, 1 % Eu, the 5D0 → 7F1 transition is dominant, which is in agreement with other reports and mirrors the expected site symmetry of Eu3+ in LaF3.38,39,40


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Figure 2. Room temperature excitation (left) and emission spectra (right) of the LaF3: 1 % Tm, 1 % Eu, 1 % Tb particles (sample b). When excited with λex= 378 nm, Tb3+ transitions (5D4 → 7FJ, J=6-3) become visible in the emission spectrum, in addition to the Eu3+ transition, with the main terbium emission centered at 541 nm, and thus, in the green region of the electromagnetic spectrum. Because of the overlap of the Eu3+ and Tb3+ transitions, the single ion contributions cannot be analyzed. However, judged from the main emissions at 541 nm (Tb3+, 5D4 → 7F5) and 592 nm (Eu3+, 5D0 → 7F1), the Eu3+ emission is prominent, albeit the Tb3+ emission is of only marginally less intensity. Finally, when excited with λex= 355 nm, emission of all three dopant ions occurs. The Tm3+ emission originates from the 1D2 → 3F4 transition and is centered in the blue region of light at 449 nm. The latter represents the most intense transition under excitation of 355 nm, which is nearly equal to the intensity of the most intense Tb3+ emission, whereas the Eu3+ emission intensity is significantly reduced to about half of the Tb3+ intensity. The excitation spectra of the three main emission bands monitored at 449 nm, 540 nm and 590 nm for Tm3+, Tb3+ and Eu3+ emission, respectively, consist of the typical f-f transitions of the corresponding ion. Monitoring the Tb3+ and Tm3+ emission, no energy transfer from one ion to another was detected, whereas both ions can be excited within the range of 350-360 nm with a broad excitation source (± 30 nm around the excitation maximum). As the Tm3+ excitation is empty above 360 nm, no emission of the latter


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can be observed when excited with 378 nm or 393 nm. The same holds true for Tb3+ emission when excited with λex= 393 nm, so that only Eu3+ is obtained. However, in the excitation spectrum monitoring the Eu3+ emission at 590 nm, there are contributions of Tb3+ transitions (evidently particularly in the region around 350 nm). A schematic energy level diagram involving Tm3+, Tb3+ and Eu3+ absorption, nonradiative relaxation and processes leading to the blue, green and red emission is given in figure 3.

Figure 3. Schematic diagram of the Tm3+, Tb3+ and Eu3+ energy levels and the processes leading to blue, green and red emission. ACS Paragon Plus Environment

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After clearly identifying the contribution of each ion to the emission, LaF3 samples co-doped with different amounts and ratios of the optically active ions are studied under excitation of λex= 393 nm (only Eu3+), 378 nm (Eu3+ and Tb3+) and 355 nm (Eu3+, Tb3+ and Tm3+). Low overall dopant concentration. When the overall dopant concentration is kept low (below 2 %, samples a-e), the ratio of the optically active ions and possible quenching mechanisms play a major role in the explanation of the luminescent characteristics. The Eu3+ emission spectra (λex= 393 nm) are displayed in Figure 4.

5

5

7

D0 → FJ

λex= 393 nm 7

D1 → FJ'

a

Intensity / a.u.

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b

c

d

e

500

550

600

650

700

Wavelength / nm

Figure 4. Room temperature emission spectra of LaF3: Tm, Eu, Tb (samples a-e) excited with λex= 393 nm. The emission resulting from the 5D0 → 7FJ (J=1-4) transitions of Eu3+ is equal in all samples without any significant change in the asymmetry ratio proving a similar environment surrounding the Eu3+ ion in the LaF3 host matrix. Though, the ratio of the emission originating ACS Paragon Plus Environment

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from the 5D0 and 5D1 levels differs with varying dopant ion concentrations and is particularly dependent on the amount of Tb3+. Energy transfer between Tb3+ and Eu3+ is a well-known phenomenon. The energy transfer is due to 5D4(Tb) + 7F0 (Eu) → 7F4 (Tb) + 5D0 (Eu) processes.41 In presence of Tb3+, nonradiative relaxation in Eu3+ from 5D1 to 5D0 excited state is favored. The particles co-doped with only small amounts of Tb3+ and Eu3+ (sample c) exhibit the highest intensity of emission originating from 5D1 (about 50 % of the 5D0 originated intensity), whereas with a Tb3+ concentration of 1 % and a Eu3+ concentration of only 0.5 % (sample e) the intensities of the 5D1 → 7FJ’ transitions are significantly reduced, thus implying stronger nonradiative relaxation of Eu3+ via Tb3+ in this material. A further treatment of the energy transfer applying kinetics can be found below. The emission spectra obtained upon excitation with λex= 378 nm (SI-Fig. 2) do not change significantly apart from the different intensity ratio of the Eu3+ 5D1 and 5D0 emission as discussed above. The relative intensity of the Tb3+ transition at 541 nm is about 85 % in comparison to the Eu3+ transition at 590 nm for the samples a-e. The emission spectra of samples a-e excited with λex= 355 nm are displayed in Figure 5. The most intense Tm3+, Tb3+ and Eu3+ transitions are marked in blue, green and red, respectively. Increasing the overall dopant ion concentration from 0.75 % to 3 % but keeping the ratio of Tm3+:Tb3+:Eu3+ constant (1:1:1) does not alter the emission spectrum significantly. The overall emission color was calculated according to CIE color space with color coordinates of 0.331 (x)/ 0.332 (y) and 0.317 (x)/ 0.332 (y) for samples a and b, respectively, which is close to standard D65 daylight (with coordinates of 0.3129 (x)/ 0.3292 (y) 42). The color coordinates of samples ae are listed in the SI and displayed in a color coordinate diagram in Figure 5.


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Figure 5. Room temperature emission spectra of LaF3: Tm, Eu, Tb particles (samples a-e, left) excited with λex= 355 nm and corresponding color coordinates diagram (right).

When the Tm3+ concentration is doubled in comparison to Tb3+ and Eu3+ (sample c), the emission at 449 nm is enhanced leading to a slightly more bluish white in comparison with the equally doped particles. An increased Tb3+ concentration (1 %) in comparison to Tm3+ and Eu3+ (0.5 % each, sample e) leads to dominant Tb3+ transitions, the Tm3+ and Eu3+ transitions are reduced to less than 50 % of the green Tb3+ emission centered at 541 nm. As a result, the emission color is calculated to a more yellowish appearance located in the warm white region of the color space. In sample d with 1 % Eu3+, the strongest relative intensity belongs to the Eu3+ emission (in comparison to Tb3+) whereas the Tm3+ emission is rather weak leading to an overall yellow emission color. Indeed, the correlated color temperature calculated according to


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McCamy43 and Hernández-Andrés44 given in Table 2 indicate the warmest white for sample e (4991 K), whereas the color temperature of sample c (6678 K) lies in the cold white region close to standard daylight at noon (D65, 6500 K).42 Sample d with an emission color in the yellow region is, hence, in the warm region (3309 K). Thus, by doping LaF3 with appropriate amounts of Eu3+, Tb3+ and Tm3+ it is possible to prepare white light emitting nanophosphors. Table 2. Correlated color temperatures (CCT) of LaF3: Tm, Tb, Eu material calculated according to McCamy43 and Hernández-Andrés44.


CCT / K McCamy

CCT / K Hernández

5567 6258 6678 3309 4991 2510 2166 1749 6094 5627 6973 3164 5520 5520

5583 6255 6696 3280 4959 2392 1957 1290 6021 5534 6915 3299 5732 5764

High overall dopant concentration. At higher overall dopant concentrations of 11 or even 16 % (samples f-n), quenching processes become more likely.45 This is evidenced by the emission spectra excited with λex= 393 nm and shown in Figure 6.


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λex= 393 nm

f g

Intensity / a.u.

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h i j l m n

500

550

600

650

700

Wavelength / nm

Figure 6. Room temperature emission spectra of LaF3: Tm, Eu, Tb (samples f-n) excited with λex= 393 nm. In the samples co-doped with more than 1 % Tb3+ (samples f-j), the emission arising from the 5

D1 → 7FJ’ transitions is almost completely quenched. In contrast to that, the presence of a high

Tm3+ concentration does not seem to affect this emission negatively (samples l-n). With regard to the asymmetry ratio, the relative intensity between the 5D0 → 7F1 and 5D0 → 7F2 transition of Eu3+ is not significantly influenced by the dopant ion concentrations pointing to a similar environment of the Eu3+ ion in all particles. In the emission spectra excited with λex= 378 nm (SI), a dependence on the dopant ion concentration can be stated. Dependent on which ion exhibits the smallest concentration (1 %) and irrespective of the other dopant concentrations, three kinds of spectra are obtained. At first, a low Tm3+ concentration (samples f-h) yields strong Eu3+ emission in addition to small ACS Paragon Plus Environment

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contributions arising from radiative transitions of the Tb3+ ion. However, the Eu3+ 5D1 → 7FJ’ transitions, as stated above, are almost completely quenched. In the emission spectra of the particles co-doped with 1 % Eu3+ (samples i-k) the Tb3+ emission is dominant, whereas the Eu3+ emission is weak. This probably originates from the relative amounts of optically active ions present. A higher concentration of one optically active ion gives rise to a higher contribution to the total emitted light. Similarly, in the particles co-doped with 1 % Tb3+ there is a lack of Tb3+ emission. The Eu3+ emission originating from both the 5D0 and 5D1 level is strong, supporting the previously stated explanation. When excited with λex= 355 nm (Fig. 6), a similar trend regarding the Tb3+ and Eu3+ emission intensity is observed. Moreover, the Tm3+ emission is weak with a Tm3+ concentration of 1 % (sample f-h). This leads to an orange emission of these phosphors as shown in the color coordinates diagram in Figure 7. Correspondingly, the correlated color temperature (Table 2) is even beyond the warm white region with 2510 K, 2166 K and even 1749 K for samples f, g and h, respectively. In the samples co-doped with 1 % Eu3+ (samples i-k), the Tm3+ emission is significantly enhanced and its intensity ranges from 25-40 % of the most intense Tb3+ transition. Together with the weak Eu3+ emission, a turquoise to greenish emission color is gained with correlated color temperatures between 5600-6970 K. On the contrary, Tm3+ emission represents the most intense transition when the Tb3+ concentration is kept at 1 % (samples l-n). However, the lack of the green Tb3+ emission yields color coordinates in the pinkish region.


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Figure 7. Room temperature emission spectra of LaF3: Tm, Eu, Tb particles (samples f-n, left) excited with λex= 355 nm and corresponding color coordinates diagram (right). White light emission has been previously reported for LaF3 core-shell particles, wherein the respective dopant ions Tm3+, Tb3+ and Eu3+ were confined in different shells.32 Tuning the shell thickness is claimed to control the degree of energy transfer. However, it was not possible to prevent effectively the energy transfer from one ion to the other by selective ion doping in different shells. But, in fact, the energy transfer between Eu3+ and Tb3+ is absolutely necessary for white light emission rendering the core-shell approach futile. Hence, our one-step synthesis route to LaF3:Tm, Tb, Eu particles is an improved, cheap and facile method. In comparison to the three-step aqueous solution based route presented by DiMaio et al., where warm white emission is detected, we obtained white emission close to daylight at noon.


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Lifetimes. The Eu3+ (5D0) and Tb3+ (5D4) lifetimes are listed in Table 3. The Eu3+ lifetimes vary from 10.8 to 4.3 ms. This is in agreement with LaF3:Eu single crystals, where a lifetime of 6.7 ms was obtained.46 The Tb3+ lifetimes are situated in the range from 9.2 to 5.5 ms, which is slightly longer than reported before for LaF3:Tb nanoparticles (4 ms).47 Table 3.Room temperature Eu3+ (λex= 393 nm, λem= 590 nm) and Tb3+ (λex= 378 nm, λem= 540 nm) lifetimes for various LaF3: Tm, Tb, Eu samples.


Eu3+lifetime / ms

Tb3+lifetime / ms

10.8 ±0.1 10.1 ±0.1 7.4 ±0.1 7.0 ±0.1 10.2 ±0.1 9.2 ±0.1 7.6 ±0.1 7.8 ±0.1 4.6 ±0.1 4.3 ±0.1 8.9 ±0.1 9.0 ±0.1 8.2 ±0.1

9.2 ±0.1 8.7 ±0.1 7.5 ±0.1 5.7 ±0.1 9.0 ±0.1 6.9 ±0.1 5.5 ±0.1 6.6 ±0.1 6.2 ±0.1 6.9 ±0.1 6.8 ±0.1 -

Though, long lifetimes were previously reported for nanosized particles and are one of the few size-related properties found in insulating nanomaterials.48 Herein, there are two major issues which influence the lifetime of the synthesized particles. In Figure 8, the lifetime is plotted against the overall dopant ion concentration (sum of the concentrations of Tm3+, Tb3+ and Eu3+).


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Figure 8. Eu3+ (above) and Tb3+ (below) lifetimes for LaF3: Tm, Eu, Tb with various rare earth ion doping concentrations dependent on the overall dopant ion concentration (Ln= Tm, Eu, Tb) and the ratio of the respective ion, for which the lifetime was measured, and the overall dopant ions, Eu3+/Ln3+ and Tb3+/Ln3+, respectively. The error of the lifetime measurements amounts to 0.1 ms. For both Eu3+ and Tb3+, a strong decrease in the luminescent lifetimes is observed when the overall dopant ion concentration is increased (due to concentration quenching). In addition, the Eu3+ lifetime depends on the percentage of Eu3+ in comparison to the other optically active ions Tm3+ and Tb3+. With a larger relative Eu3+ content, a longer lifetime is obtained. This implies that probably nonradiative relaxation involving the levels of the other lanthanides takes place, which is in agreement with the lack of Eu3+ → Ln3+ energy transfer leading to emission of the


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Ln3+ ions Tb3+ or Tm3+. The effect is more pronounced when the overall dopant ion concentration is 11 % (rather than 16 %). In case of the Tb3+ lifetime, there is no significant effect of the Tb3+/Ln3+ ratio. However, there is energy transfer from Tb3+ → Eu3+ leading to Eu3+ emission, which explains why the Tb3+/Ln3+ ratio has no significant impact. Tb-Eu energy transfer.The importance of energy migration may be judged by the 5D4 lifetimes of Tb3+. However, the variation of the 5D4 lifetime as a function of the Tb3+ dopant concentration does not follow a trend because it is strongly dependent on the other dopant ion concentrations. Thus, the determination of the energy migration between Tb3+ ions in the LaF3:Tm, Tb, Eu codoped particles is not straightforward. For a more comprehensive analysis, LaF3 particles singly doped with different concentrations of Tb3+ have been synthesized. The lifetimes are illustrated in figure 9 and listed in table 4 (

0).

As expected, the lifetime decreases

with increasing Tb3+ concentration. Though, the lifetime of the 10 % Tb3+ co-doped sample still amounts to 7.7 ms indicating energy migration to be less important. This is exactly what we wanted to achieve and what is useful for a single white-emitting phosphor.

10.5 10.0

Lifetime / ms

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


9.5 9.0 8.5 8.0 7.5 0

2

4

6

8

10

Tb concentration / %

Figure 9. Tb3+ 5D4 lifetimes of LaF3:Tb3+ particles dependent on the doping concentration. ACS Paragon Plus Environment

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Table 4. Tb3+5D4 lifetimes of LaF3:Tb, Eu (τ) and LaF3:Tb (τ0) particles and calculated Tb-Eu energy transfer efficiency ET. Tb3+ /%

Eu3+ /%

τ (Tb3+)

τ0 (Tb3+)

ET / %

0.25 1 0.5 1 5 5 10 5 10

0.25 0.5 0.5 0.5 5 1 5 10 1

10.2 ±0.1 8.8 ±0.1 8.9 ±0.1 8.7 ±0.1 5.6 ±0.1 6.1 ±0.1 4.5 ±0.1 3.7 ±0.1 6.3 ±0.1

10.2 ±0.1 9.5 ±0.1 9.9 ±0.1 9.5 ±0.1 8.7 ±0.1 8.7 ±0.1 7.7 ±0.1 8.7 ±0.1 7.7 ±0.1

0.4 6.9 10.4 7.9 35.9 30.2 41.6 57.7 18.3

Furthermore, the energy transfer efficiency of Tb-Eu energy transfer can be calculated according to49,50 ET(Tb-Eu) = 1-(τ/τ0) Although it is questionable whether one can accurately apply this equation to our triply doped LaF3 particles, the evaluation of the obtained qualtitative trend could yield valuable information. With increasing Tb3+ concentration, but invariant Eu3+ concentration, the 5D4 lifetime of Tb3+ decreases, which indicates a larger energy transfer probability with larger Tb3+ concentrations. This is in agreement with the expectations, as more Tb3+ ions are in close proximity to Eu3+ ions and thus can transfer their energy. To further study the energy transfer between Tb and Eu, we synthesized LaF3:Tb, Eu particles without Tm3+ (Table 4), with concentrations analogous to that of the LaF3:Tm, Tb, Eu particles.


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For these samples, which exhibit similar emission spectra and lifetimes as the triply codoped samples with corresponding concentrations, the energy transfer efficiency was calculated. The highest energy transfer rate is 58 % with concentrations of 5 % Tb and 10 % Eu. Generally, higher overall concentrations of Tb3+ and Eu3+ are expected to yield a better energy transfer as the probability of Tb3+ being in the neighborhood of Eu3+ is strongly enhanced. At low Tb3+/Eu3+ concentrations, the energy transfer efficiency was calculated to be between 0-10 %. Indeed, there is no evident energy transfer in case of 0.25 % codoping. CONCLUSION The synthesis of uniform 6-8 nm sized nanophosphor particles was successful. A white emission close to standard D65 daylight is obtained under excitation of λex= 355 nm at quite small rare earth dopant ion concentrations (1 % Eu, 1 % Tb, 1 % Tm).

At larger Tb3+

concentrations, the Eu3+ 5D1 emission is weakened due to energy transfer from Tb3+ to the 5D0 level of Eu3+. A small concentration of one optically active ion in comparison to the other two ions leads to a comparatively weak emission intensity of the respective ion. As a result, a nonwhite emission complementary to the missing emission color is obtained.

ASSOCIATED CONTENT Supporting Information. Powder X-ray diffraction, amounts of starting materials, additional emission spectra and color coordinates. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author *Prof. Dr. Anja-VerenaMudring, e-mail: [email protected]

ACKNOWLEDGMENT AVM would like to acknowledge support from the European Research Council through an ERC Starting Grant (EMIL, contract no. 200475), the Fonds der ChemischenIndustrie through a Dozentenstipendium as well as a Doktorandenstipendium for CL.

SUPPORTING INFORMATION AVAILABLE Powder X-ray diffraction, amounts of starting materials, additional emission spectra and color coordinates. This information is available free of charge via the Internet at http://pubs.acs.org.

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A combined approach to uniform, nanoscale (6-8 nm) particles as single white-emitting phosphor was made possible by an ionic liquid-based synthesis approach. LaF3 particles codoped with Tm3+, Tb3+ and Eu3+ exhibit a white light emission close to standard daylight (D65) under excitation with UV light.


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White-Light-Emitting Single Phosphors via Triply Doped LaF3

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