Gabriella
Tessitore
*a,
Anja-Verena
Mudring
b and
Karl W.
Krämer
a
aUniversity of Bern, Department of Chemistry and Biochemistry, Freiestrasse 3, 3012 Bern, Switzerland. E-mail: Gabriella.Tessitore@dcb.unibe.ch
bDepartment of Materials and Environmental Chemistry, Stockholm University, Svante Arrhenius väg 16 C, 106 91 Stockholm, Sweden
First published on 21st November 2017
Phase pure hexagonal β-NaGdF4:Eu3+,Er3+ samples of less than 10 nm in crystallite size were synthesized in anhydrous ethylene glycol within 24 hours at room temperature. The materials were characterized by powder X-ray diffraction, electron microscopy, and luminescence spectroscopy. After Gd3+ excitation at 273 nm Eu3+ luminescence from the 5DJ states dominates over Gd3+ 6PJ emission and only weak Er3+ emissions could be observed due to strong Gd→Eu and weak Gd→Er energy transfer. Excitation at 377 nm results in Er3+ 2H9/2 and Eu3+ 5DJ emissions. The concentration dependence of the Er3+ and Eu3+ luminescence intensities points to independent behavior of the dopant ions without Er→Eu or Eu→Er energy transfer. Eu3+ excitation at 394 nm results in Eu3+ 5DJ emissions only and confirms the absence of Eu→Er energy transfer. Eu3+ luminescence from the 5DJ states prevails for all investigated excitations. The optimum Eu3+ doping level is close to 5%. Significant Er3+ luminescence was observed for 377 nm excitation only. The strong 2H9/2 and weak 4S3/2 Er3+ emissions indicate small losses by multiphonon relaxation and a good quality of the nanomaterial. Er co-doping hinders the energy migration on the Gd3+ and Eu3+ sublattices which results in higher Gd3+/Eu3+ and Eu3+ 5D1/5D0 emission ratios.
Synthesis of phase pure β-NaREF4 nanomaterials is challenging. At high temperatures, β-NaREF4 crystallizes from solutions in a two-step process according to the Ostwald phase rule.8 First, metastable cubic α-NaREF4 is formed which may transform to the thermodynamically stable β-NaREF4 or persist as a metastable, kinetically stabilized phase. Luminescence from α-NaREF4 is significantly weaker than from β-NaREF4.9 The thermodynamic stability of β-NaREF4 decreases for smaller RE3+ ionic radii.10 Thus, the synthesis of highly luminescent material requires an accurate optimization in order to obtain phase pure β-NaREF4.
Several synthetic pathways are reported in the literature which show a high selectivity for the β-NaREF4 phase and a good size control of the nanoparticles.11–14 Most commonly, octadecene is used as a solvent with oleic acid or oleylamine as surfactants and the synthesis is performed at high temperatures. Recently, room temperature syntheses of fluoride nanoparticles were investigated.15–19 They provide several advantages since they can be performed in conventional solvents, e.g. water, ethylene glycol, or diethylene glycol, and require no heating. Several room temperature syntheses for phase pure β-NaREF4 nanoparticles were reported.15,16,19 However, the obtained particle size distribution is often broad due to a weak size control. With the help of surfactants it is possible to improve the size control. Ionic liquids were successfully employed in the room temperature synthesis of β-NaGdF4:Eu3+ nanoparticles with 2 nm crystallite size in aqueous solutions.16 A small crystallite size was recently achieved for the synthesis of β-NaGdF4:Eu3+ and Er3+ nanocrystalline samples in ethylene glycol19 and KGdF4:Ce3+,Ln3+ (Ln = Tb or Eu) nanoparticles in water and diethylene glycol solutions.18 The nanocrystallites obtained from both syntheses form aggregates smaller than 100 nm. The crystallite size is less than 10 nm without addition of further surfactants, e.g. ionic liquids. It was observed that the F− concentration from NH4F in the ethylene glycol synthesis affects the aggregate size.19 Here, we extend the room temperature synthesis in ethylene glycol,19 developed by us towards β-NaGdF4:Eu3+,Er3+ co-doped nanoparticles. The absence of water during the synthesis constitutes a distinct advantage for the synthesis of luminescence materials. It minimizes OH− impurities and the luminescence quenching resulting thereof. A synthesis under dry conditions is also beneficial for a good repeatability of the results, as suggested by comparison with the literature.19
Phosphor materials with Gd3+, Eu3+, and Er3+ are interesting in several aspects. LiGdF4:Eu3+20 and LiGdF4:Er3+21 are quantum cutters. For Gd3+ or Er3+ excitation, one high energy photon in the VUV can be converted in two lower energy photons. Also, β-NaGdF4:Eu3+ shows quantum cutting.16,22,23 However, the excitation energy of 6.1 eV is too low for Xe (7.3 eV) and too high for Hg discharge lamps (4.9 eV) which limits potential applications.24 For LiGdF4:Er3+ the dense energy level diagram of Er3+, the efficient energy migration between Gd3+ ions, and the weak absorption of the Gd3+ 6PJ→8S7/2 emission by Er3+ ions reduces the quantum cutting efficiency.21,25 Ternary systems (with respect to the rare earth ion) may enhance the energy transfer from Gd3+ ions. The best known nanomaterial is α-NaGdF4:1.5% Er3+,0.3% Tb3+.26 LiGdF4:Er3+,Tb3+ might be feasible as phosphor for Xe discharge lamps.24
The development of LEDs reduced the need for lamp phosphors.27–29 UV-A LEDs show efficient emission around 365 nm, which can excite white emission from Er3+, Eu3+ co-doped materials.30–32 Another pathway for white emission focuses on upconversion in Yb3+, Er3+, Eu3+ co-doped materials.33–38 Between 360–380 nm, Er3+ has a strong 4I15/2→4G11/2 absorption at 377 nm and a weaker 4I15/2→4G9/2 and 2KJ absorption at 364 nm,39 which may result in blue and green Er3+ emissions. In combination with red emission from the Eu3+ 5DJ states, white light may be obtained. β-NaGdF4:Eu3+,Er3+ co-doped nanoparticles can also be excited via Gd3+ at 273 nm. Here, we investigate the luminescence of β-NaGdF4:Eu3+,Er3+ co-doped nanoparticles upon UV excitation of Gd3+, Er3+, and Eu3+ ions.
Samples were characterized by powder XRD and transmission electron microscopy (TEM) with respect to phase purity and size, respectively. Luminescence and excitation spectra were measured from the powder samples, see Experimental section of ESI.†
The broad width of the XRD peaks indicates the nanometer particle size of the materials. From the line width the crystallite size of the β-NaGdF4:Eu3+,Er3+ samples was estimated by the Scherrer equation:
(1) |
TEM micrographs provide more details on the nanocrystal size and shape. Fig. 2 shows an overview on the particle size of β-NaGdF4:5% Eu3+. Fig. S3 of ESI† depicts TEM micrographs of further representative samples. As proven by XRD, the crystallite size is below 10 nm, see Fig. 2C. The nanocrystals form larger aggregates upon drying. Their size ranges between 50 and 100 nm, see Fig. 2. The aggregate size increases with the NH4F concentration from 3 to 9 wt% in the EG solutions.19 The optimum concentration of 9 wt% NH4F for a pure hexagonal phase and improved luminescence19 was used in this work for all samples.
Individual nanocrystals of 2–3 nm size are shown in Fig. 2C. The dopant concentration has no detectable influence on the crystallite size. When up to 5% Eu3+ ions substitute for Gd3+, only a negligible influence on the stability of the hexagonal phase is expected as their ionic radii are very close (1.120 Å and 1.107 Å, respectively, for a nine-fold coordination).41 The smaller Er3+ ions (1.062 Å)41 can induce a variation in crystallite size and shape due to the decreasing stability of the hexagonal phase along the trivalent rare earth cation series. However, within error limits, no change in crystallite size was detectable from the TEM micrographs of the β-NaGdF4:Eu3+,Er3+ nanocrystallites. Comparable aggregates of particles were observed in the room temperature synthesis of KGdF4:Ce3+/Ln3+ in water with diethylene glycol as co-solvent.18
After Gd3+ 8S7/2→6IJ excitation at 36630 cm−1 (273 nm), β-NaGdF4:1% Eu3+ shows the typical Eu3+ emission bands due to the 5DJ→7FJ transitions between 14000 and 19000 cm−1. The intensity of the Gd3+ 6P7/2→8S7/2 emission band at 32260 cm−1 (310 nm) is weak due to an efficient Gd3+→Eu3+ energy transfer.
For direct Eu3+ 7F0→5L6 excitation at 25380 cm−1 (394 nm), β-NaGdF4:5% Eu3+ shows identical Eu3+ emissions as for Gd3+ excitation, see the traces on the left hand side of Fig. 3. The Eu3+ 5D0→7F2 emission at 16260 cm−1 (615 nm) has the strongest intensity. Emissions are detected from the 5D0 and 5D1 states. Most prominent are the 5D0→7FJ transitions with J = 1 and 4 at 16920 cm−1 (591 nm) and 14368 cm−1 (696 nm), respectively. Significantly less intensity is observed for the 5D1→7FJ transitions. They will be addressed in more detail in the following sections. The emission spectra are comparable to those reported in the literature for β-NaGdF4:Eu3+ nanoparticles9,16,22,42,43 and bulk material.6,23
For Gd3+ excitation at 273 nm, β-NaGdF4:1% Er3+ shows predominantly Gd3+ 6P7/2→8S7/2 emission at 32260 cm−1 (310 nm) and minor Er3+ emissions, see the right hand side of Fig. 3. The Gd3+→Er3+ energy transfer is less efficient than for Gd3+→Eu3+, in good agreement with reports in literature.20,21 The Er3+ emissions range from 17500 to 26500 cm−1. Er3+ 2P3/2→4I13/2 and 2P3/2→4I11/2 emissions are observed at 25063 cm−1 (399 nm) and 21368 cm−1 (468 nm), respectively. The Er3+ 2P3/2→4I15/2 transition is visible only as a shoulder of the Gd3+ 6P7/2→8S7/2 band. The band at 24510 cm−1 (408 nm) is assigned to the 2H9/2→4I15/2 transition, while only minor emissions from the 2H11/2 and 4S3/2 states are recorded at 18349 cm−1 (545 nm) and 19011 cm−1 (526 nm), respectively. A direct Er3+ excitation of β-NaGdF4:5% Er3+ into the 4G11/2 state at 26525 cm−1 (377 nm) results in stronger Er3+ emissions, see the green trace on the right hand side of Fig. 3. The 2H9/2→4I15/2 and 2H9/2→4I13/2 transitions show the highest intensities at 24510 cm−1 (408 nm) and 17986 cm−1 (556 nm), respectively. Also the green emissions 2H11/2 and 4S3/2→4I15/2 increase in intensity. For comparison, emission spectra of bulk β-NaGdF4:5% Er3+ with micrometer particle size are shown in Fig. S4 of ESI† for Gd3+ and Er3+ excitation, as well as normalized luminescence spectra of bulk β-NaGdF4 with 1% and 5% Er3+ in Fig. S5 (ESI†).
Fig. 4 shows excitation spectra of nanocrystalline β-NaGdF4:Eu3+ and Er3+ samples with 1% and 5% doping monitoring the Eu3+ and Er3+ emissions, respectively. Spectra are normalized to the Gd3+ 8S7/2→6IJ transition at 36630 cm−1 (273 nm). Absorption and emission via Eu3+ and Er3+ dopant bands increase with the doping level and allow a clear distinction from Gd3+ bands. In Fig. 4A the Eu3+ 5D0→7F2 emission at 16260 cm−1 (615 nm) is monitored. Bands are assigned to direct Eu3+ as well as Gd3+ excitations followed by Gd3+→Eu3+ energy transfer. Eu3+ excitation bands range from 24500 to 28000 cm−1 and 30000 to 35500 cm−1. The intensity of the Eu3+ emission roughly doubles when the doping is increased from 1% to 5% Eu3+, in good agreement with an enhanced absorption according to the Lambert–Beer law. The prominent Eu3+ 7F0→5L6 excitation is observed at 25380 cm−1 (394 nm). The strongest excitation band at 36630 cm−1 (273 nm) is attributed to the Gd3+ 8S7/2→6IJ transition. Weaker Gd3+ excitations are due to 8S7/2→6PJ transitions at 32468 cm−1 (308 nm) and 32787 cm−1 (305 nm), reflecting the lower absorption coefficients of 6PJ compared to 6IJ states.39
Normalized excitation spectra of the Er3+ 4S3/2→4I15/2 emission at 18349 cm−1 (545 nm) in β-NaGdF4 doped with 1 and 5% Er3+ are depicted in Fig. 4B. The low intensity of the Er3+ emission required a wider slit width which reduces the resolution of these spectra and results in broader peak width compared to Fig. 4A. The Er3+ excitation bands range from 26000 to 28000 cm−1. Further bands at higher energy were too weak to be detected. The two Er3+ excitations at 26525 cm−1 (377 nm) and 27473 cm−1 (364 nm) originate from 4I15/2→4GJ and 2KJ transitions with 4I15/2→4G11/2 being strongest. Increasing the doping from 1% to 5% Er3+ results in a distinct enhancement of the direct Er3+ excitations. The weaker emission from Er3+ compared to Eu3+ doped samples indicates smaller absorption coefficients, an increased non-radiative relaxation due to smaller energy gaps between the excited states of Er3+, and a less efficient energy transfer from Gd3+.
The intensity distribution among the Eu3+ emissions is close to identical for all the spectra shown in Fig. 5. Small variations of the 5D1vs.5D0 emission intensities are discussed in more detail with Fig. 6. Most obvious in Fig. 5 is a strong decrease of the Gd3+ emission intensity with increasing Eu3+ doping. The changes of the luminescence intensities with doping are resolved in Fig. 7.
Fig. 6 Expanded Eu3+ luminescence spectra from Fig. 5. Spectra are normalized to the Eu3+ 5D0→7F2 emission at 615 nm (16260 cm−1), marked by an asterisk. |
Fig. 7 Integrated relative intensity of the Eu3+ 5D0→7F2 (red square), Gd3+ 6PJ→8S7/2 (purple circle), and Er3+ 2P3/2→4I13/2 (green triangle) luminescence of the β-NaGdF4:x% Eu3+,y% Er3+ nanocrystalline samples shown in Fig. 5. The empty (filled) symbols refer to 1% (5%) total doping with x% Eu3+ and y = 1 − x (y = 5 − x)% Er3+. Gd3+ was excited at 273 nm. |
The Eu3+ emissions from the 5D0 and 5D1 to the 7FJ states are labelled in Fig. 6. The 5D1→7FJ transitions with J = 0, 1, 2, and 3 are also clearly assigned. They correspond to the bands at 19011 cm−1 (526 nm), 18657 cm−1 (536 nm), 18018 cm−1 (555 nm), and 17123 cm−1 (584 nm). Further weaker bands are the 5D0→7F0 and 5D0→7F3 transitions at 17270 cm−1 (579 nm) and 15385 cm−1 (650 nm), respectively, and the 5D1→7F4 transition at 15950 cm−1 (627 nm). Much lower intensities show the 5D2→7FJ transitions with J = 0, 2, and 3 which correspond to bands at 21505 cm−1 (465 nm), 20492 cm−1 (488 nm), and 19608 cm−1 (510 nm). The energy gap of 1740 cm−1 between the 5D1 and 5D0 states corresponds to 5 times the effective phonon energy of the β-NaGdF4 host lattice. This gives rise to a competition between radiative and non-radiative relaxation from the 5D1 state.
Interestingly, the relative emission from the 5D1vs. the 5D0 state decreases with increasing Eu3+ doping, as previously observed for β-NaGdF4:Eu3+ samples.6,9,19 Higher Eu3+ concentrations enhance the energy migration between Eu3+ ions. Cross relaxation rates may increase between 5DJ and 7FJ states and reduce the population of higher 5DJ states in favour of 5D0. On the contrary, the co-doping with Er3+ ions increases the relative 5D1 emission. The Er3+ ions hinder the energy migration on the Eu3+ sublattice and retain a higher 5D1 population.
Normalized integrated intensities are reported in Fig. 7 for the Gd3+ 6PJ→8S7/2, Eu3+ 5D0→7F2, and Er3+ 2P3/2→4I13/2 emissions upon Gd3+ excitation at 273 nm. These transitions were chosen avoiding any overlap of emissions from different ions. The Gd3+ and Eu3+ intensities are displayed vs. the Eu3+ concentration on the bottom axis. Since the Er3+ emission is much weaker than the others, its intensity is displayed ten-fold expanded on the right hand axis vs. the Er3+ concentration on the top axis. The normalized integrated intensities in Fig. 7 were obtained from the luminescence spectra in Fig. S6 of ESI† which correspond to the normalized emission spectra in Fig. 5. The integrated intensities are displayed on a logarithmic scale in Fig. S7 of ESI.† According to the Lambert–Beer law, the absorbance A of a sample is proportional to the absorption coefficient ε, the concentration c, and the sample thickness d. In our standard luminescence setup, the excitation energy and d are fixed. The absorption coefficients ε of the rare earth ions are determined by the host lattice β-NaREF4. Therefore, the logarithm of the absorbed number of photons is proportional to the concentration of the respective ion. Ideally, the absorption results in the emission of a photon from the same ion. Any deviation indicates additional decay pathways such as energy transfer or multiphonon relaxation.
With increasing Eu3+ doping, the Gd3+ emission intensity (purple circle) is strongly reduced and the Eu3+ emission (red square) increases due to Gd3+→Eu3+ energy transfer, see Fig. 7. The Eu3+ emission increases up to 5% doping where it almost reaches a plateau. The optimum Eu3+ doping is expected in the vicinity of 5%. For 10% Eu3+ doping the emission intensity is already lower than for 5% due to concentration quenching.19 The total Gd3+ and Eu3+ emission is approximately constant, as shown in Fig. S7 (ESI†). Sample to sample variations of the total luminescence intensity due to experimental errors are strongly reduced in the normalized data of Fig. 7, which yields clear trends along the series. The Er3+ emission (green triangle) increases with Er3+ concentration, but it is much weaker than the Gd3+ and Eu3+ emissions. Both the Gd3+→Er3+ and Eu3+→Er3+ energy transfers are inefficient for 273 nm excitation. However, Er3+ co-doping has another important effect: the relative Gd3+vs. Eu3+ emission is higher in the presence of Er3+ than without. A comparison of the spectra in Fig. 5 reveals a much stronger relative Gd3+ emission for β-NaGdF4:1% Eu3+,4% Er3+ (Fig. 5B, green trace) than for β-NaGdF4:1% Eu3+ (Fig. 5A, black trace). In Fig. 7 the Gd3+/Eu3+ emission intensity ratios of these samples are about 45/55 and 10/90, respectively. Er3+ acts as a neutral scatterer, as discussed above for the Eu3+ 5D1vs.5D0 emission ratio. Co-doping reduces the energy migration on the Gd3+ and Eu3+ sublattices, decreases the Gd3+→Eu3+ energy transfer, and therefore retains the excitation energy on Gd3+. An optically inactive ion, such as Y3+, is likely to yield comparable results.
Fig. 9 Integrated intensity of the Eu3+ 5D0→7F2 (red square) and Er3+ 2H9/2→4I15/2 (green triangle) luminescence of β-NaGdF4:x% Eu3+,y% Er3+ nanocrystalline samples shown in Fig. S8 (ESI†). The empty (filled) symbols refer to a 1% (5%) total doping for x% Eu3+ and y = 1 − x (y = 5 − x)% Er3+. Er3+ was excited at 377 nm (26525 cm−1). The integrated intensity is reported on a logarithmic scale. |
Er3+ excitation at 377 nm results in strong Er3+ emission from the 2H9/2, 2H11/2, and 4S3/2 states, see Fig. 8. A minor emission at 19881 cm−1 (503 nm) is ascribed to the Er3+ 4G11/2→4I13/2 transition. At 377 nm Eu3+ is also directly excited, see Fig. 3. The resulting Eu3+ 5DJ→7FJ emissions are shown on an expanded scale in Fig. S9 (ESI†). A comparison with Fig. 6 reveals the additional overlapping Er3+ emissions labelled a–c in Fig. S9 (ESI†).
Since the luminescence spectra in Fig. 8 are normalized to the Eu3+ 5D0→7F2 emission, the strongly varying intensity of the Er3+ emissions allow a conclusive assignment of the Er3+ states. Their energies are reported in Table 1. The Eu3+ energies for β-NaGdF4:Eu3+ nanocrystallites were reported elsewhere.18 Both Er3+ and Eu3+ emission intensities increase with doping. On the logarithmic scale of Fig. 9, they show a close to linear dependence on the concentration. This indicates the absence of significant energy transfer between Er3+ and Eu3+ ions. Both dopant ions behave rather independently and their luminescence intensity correlates with the respective absorption. For the same doping level, the Eu3+ 5D0→7F2 emission shows an approximately five-times higher intensity than the Er3+ 2H9/2→4I15/2 emission upon excitation at 377 nm.
State | Energy [cm−1] |
---|---|
2P3/2 | 31587 |
4G9/2, 2KJ | 27473 |
4G11/2 | 26525 |
2H9/2 | 24510 |
2H11/2 | 19011 |
4S3/2 | 18349 |
4I11/2 | 10219 |
4I13/2 | 6524 |
4I15/2 | 0 |
In contrast to our observations in β-NaGdF4:Eu3+,Er3+, efficient Er3+→Eu3+ energy transfer was reported in NaLa(WO4)2:x% Eu3+,3% Er3+ phosphors upon excitation at 378 nm with variable Eu3+ doping.31 Increasing the Eu3+ concentration enhanced the Eu3+ and quenched the Er3+ luminescence.
Footnote |
† Electronic supplementary information (ESI) available: Synthesis, additional TEM micrographs, XRD patterns, thermogravimetric analysis, luminescence spectra, and integrated emissions. See DOI: 10.1039/c7nj03242k |
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018 |