Optical Materials 29 (2007) 999–1003 www.elsevier.com/locate/optmat
Luminescence and structural characterization of transparent nanostructured Eu3+-doped LaF3–SiO2 glass–ceramics prepared by sol–gel method A.C. Yanes a, J. Del-Castillo a, J. Me´ndez-Ramos b, V.D. Rodrı´guez M.E. Torres a, J. Arbiol c b
b,*
,
a Dpto. Fı´sica Ba´sica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain Dpto. Fı´sica Fundamental y Experimental, Electro´nica y Sistemas, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain c Dpto. Electro´nica, Universidad de Barcelona, 08028 Barcelona, Spain
Received 16 June 2005; accepted 27 February 2006 Available online 12 May 2006
Abstract Glass–ceramics with composition of 89.9SiO2–10LaF3–0.1EuF3 (mol%) were prepared by sol–gel method. LaF3 nanocrystals, precipitated by heat-treatments, were identified by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). Besides the well-known red emission of Eu3+ ions, higher energy emissions coming from upper-lying levels 5D1 and 5D2 can be observed at room temperature due to the very low phonon energies of the LaF3 matrix. Moreover, from site selective excitation and emission spectra in the temperature range from 13 to 300 K, it is concluded that the fraction of Eu3+ ions partitioned into LaF3 nanocrystals, substituting La3+ ions, is comparable to the one staying in the SiO2 glassy phase. 2006 Elsevier B.V. All rights reserved.
1. Introduction In the past few years, increasing interest on photonic devices has focused the attention in rare-earth (RE) doped nanomaterials, due to their size-induced structural, physical properties and potential applications in optoelectronic technology (optical communication, display phosphors, laser emitters, fluorescent markers, etc.) [1–4]. Oxyfluoride glass–ceramics represent a unique class of materials that combine the particular optical properties of RE ions in fluoride hosts with the elaboration and manipulation advantages of oxide glasses. Moreover they remain transparent due to the nano-scale of the fluoride crystals precipitated in the oxide glass matrix. Special attention has been paid to those glass–ceramics based on LaF3 nanocrystals *
Corresponding author. Tel.: +34 922 31 8304; fax: +34 922 25 69 73. E-mail addresses:
[email protected] (A.C. Yanes),
[email protected] (V.D. Rodrı´guez). 0925-3467/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2006.02.021
dispersed in an oxide glassy phase [5–8]. LaF3 is an excellent host material for various phosphors since it has considerable solubility for all RE ions and very low phonon energy (300–400 cm1). Thus the quenching of the excited states of the RE ions will be minimal [9–12]. On the other hand, Eu3+ ions have attracted considerable interest because of their potential technological applications [13–15]. The emission spectrum of Eu3+ shows bands ranging from visible to the near infrared; blue and green light emissions are very important for optoelectronics, high-density optical storage and medicine area. Colour displays need blue, green and red emitters. Accordingly, there is a need for materials generating multicolour visible output. Moreover, Eu3+ ions have been widely used as a probe to investigate local structure around RE ions in condensed matter [16]. Due to its particular energy level diagram, it is possible to perform site selective studies in order to discern between ions with different environments. The final environment of the optically active RE ions rules
A.C. Yanes et al. / Optical Materials 29 (2007) 999–1003
their optical properties and therefore their interest for optical devices. In Eu3+-doped oxide hosts, only red light emission coming from 5D0 energy level to lower 7FJ levels is observed. On the other hand, in a variety of fluoride glasses and crystals, higher energy fluorescence, originated from 5 D1, 5D2 and 5D3 levels, has been observed at room temperature [9]. In this work, we report the structural and luminescent properties of single Eu3+-doped oxyfluoride SiO2–LaF3 transparent glass–ceramics prepared by the sol–gel method. 2. Experimental Silica glasses with composition of 89.9SiO2–10LaF3– 0.1EuF3 (mol%) were prepared by a sol–gel method following the procedure described by Fujihara et al. [5]. First, an ethanolic solution of tetraethoxysilane (TEOS) was hydrolyzed for 1 h at room temperature with water using acetic acid as a catalyst. The TEOS/ethanol/water/CH3COOH molar ratio was 1:4:10:0.5. On the other hand, La(CH3COO)3 Æ nH2O and Eu(CH3COO)3 Æ nH2O were dissolved in a CF3COOH and water solution, which was slowly mixed with the initial solution. Molar ratio of RE3+ (La3+ and Eu3+) to CF3COOH was 1:4. The resultant clear solution was stirred for 1 h at room temperature. A wet-gel was obtained by leaving the solution in a sealed container at 35 C for 1–2 weeks. After this step, evaporation for several weeks at 35 C was required in order to obtain dried samples, known as xerogels. At last, these xerogels were heat treated in air, at 800 and 1000 C, in order to precipitate nanocrystallites giving rise to transparent glass– ceramics. The same procedure was used to prepare SiO2 glass with 2.5 mol% of Eu3+ for comparison. X-ray analysis (XRD) was carried out by using a Cu anode (Cu Ka1,2) in the 10-85 2-Theta range. Transmission electron microscopy (TEM), high-resolution TEM (HRTEM) images as well as selected area electron diffraction (SAED) patterns were obtained using a JEOL 2010F microscope operated at 200 kV. This microscope is equipped with a field emission gun, which allowed us to achieve a point-to-point resolution of 0.19 nm and a resolution of 0.14 nm between lines. HRTEM images were studied by using the fast Fourier transform (FFT) of selected regions, obtaining power spectra patterns that were indexed using the GATAN digital micrograph software package. The specimens were prepared by dispersing the fine powder obtained by grinding the sample, using a mortar and pestle in hexane with ultrasonic agitation, and then, a droplet of the suspension was put on a copper holey carbon grid. The UV–VIS fluorescence signals were obtained by exciting the samples with light from a 300 W Xe arc lamp, passed through a 0.25 m double-grating monochromator and detecting with a 0.25 m monochromator with a photomultiplier. The spectra were corrected by the instrumental response. Low temperature spectra were acquired by using a closed-cycle He cryogenerator.
3. Results and discussion 3.1. Structural characterization A complete structural characterization by means of X-ray diffraction and HRTEM has been performed in these 89.9SiO2–10LaF3–0.1EuF3 (mol%) transparent glass– ceramics. X-ray diffraction patterns of as made, 800 and 1000 C annealed samples are presented in Fig. 1. It can be seen that the pre-heated gel is totally amorphous with no diffraction peaks of crystals, meanwhile peaks corresponding to hexagonal LaF3 nanocrystals are clearly observed in the samples heat treated at 800 and 1000 C (JPCD file 32-0483). No second crystalline phase was detected. From the half-width of the XRD peaks, the size of the nanocrystals in the obtained glass–ceramics was calculated by using Scherrer’s equation D ¼ Kk=b cos h
ð1Þ
where D is the crystal size, k the X-ray wavelength, h the diffraction angle, b the full-width at half-maximum (FWHM) of the diffraction peak and K a constant fixed by the instrument. The obtained results were 12 and 14 nm for glass–ceramics treated at 800 and 1000 C, respectively. A TEM bright-field micrograph is shown in Fig. 2(a). Nanoparticles homogeneously dispersed can be seen in dark contrast. The inset in Fig. 2(a) corresponds to the SAED pattern obtained in a 500 nm · 500 nm region of the SiO2 matrix. Pattern indexation confirms that the diffraction rings correspond to the LaF3 hexagonal phase. Using HRTEM the structure of the nanoparticles embedded in the SiO2 matrix was studied. A few crystalline nanoparticles can be detected in the HRTEM micrograph shown in Fig. 2(b). The right-top side inset displays the power spectrum obtained from the nanoparticle squared in black. Indexation of the spectrum spots clearly shows that the nanoparticle crystallized in the LaF3 phase (attending to the atomic distances and angles found). In
Intensity (arb. units)
1000
1000 ºC 800 ºC
as made 10
20
30
40
50
2θ (degrees)
60
70
80
Fig. 1. XRD patterns of as made and heat treated at 800 and 1000 C samples.
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1001
Fig. 2. (a) TEM bright-field image shows the presence of LaF3 nanoparticles (dark contrast). The inset corresponds to the SAED pattern and shows that nanoparticles crystallize in the LaF3 hexagonal phase. (b) HRTEM micrograph where LaF3 nanocrystals are observed. The right-top side inset corresponds to the power spectrum obtained from the region squared in black. The right-bottom inset corresponds to magnified detail of the same blacksquared region showing the crystalline pattern of a LaF3 nanoparticle.
fact, the atomic planes observed are the (1 2 1) and the (2 1 1), which are placed in the [1 0 0] LaF3 zone axis direction. In addition, a higher magnified detail of the black-squared region is shown in the right-bottom inset of Fig. 2(b). The atomic planes and the corresponding ˚ ) have been also labelled. atomic plane distances (3.23 A
Fig. 4 shows room and low temperature excitation spectra of the glass–ceramics, heat-treated at 800 C. Different results are obtained by detecting at 590 (5D0 ! 7F1) and 613 nm (5D0 ! 7F2). These excitation spectra are composed of well-resolved characteristics peaks of Eu3+, which are assigned to transitions coming from the ground level
3.2. Luminescence 393 nm
An extensive spectroscopic study in the 89.9SiO2– 10LaF3–0.1EuF3 (mol%) transparent glass–ceramics has been carried out. Excitation and emission spectra, both at room and low temperature, have been obtained. Energy level diagram of Eu3+ ions along with main transitions are indicated in Fig. 3.
Energy (103 cm-1)
25
20
15
D 5 4 5 5 G ,L L 2-6 7 5 6 D 5 3 D 5 2 D 5 1 D0
5
D4
5
5
GJ , L7
L6 5 5
D3
5
D2
D1
300 K
Intensity (arb. units)
5
590 nm
396 nm 5
100 K 13 K
613 nm
300 K
100 K
13 K
10
300
350
400
450
500
550
Wavelength (nm) 7
5
0
F6
5 4 3 2 1 0
Fig. 3. Eu3+ ion energy level diagram with main transitions indicated.
Fig. 4. Low (100 and 13 K) and room (300 K) temperature excitation spectra of glass–ceramic heat treated at 800 C detecting at 590 and 613 nm. The transitions come from the ground level 7F0 to the levels indicated in the figure. Additional lines coming from thermal population of the 7F1 level are observed at room temperature. Double peak corresponding to different environments of Eu3+ ions, at 393 and 396 nm, is arrowed.
1002
A.C. Yanes et al. / Optical Materials 29 (2007) 999–1003
7
5
7 5
7
D1 F0-4
F0
7
F1
7
F2
7
F3
7
F4 300 K
5
7
D 2 F3
396 nm
D0
(a)
100 K
13 K
393 nm
(b) Intensity (arb. units)
F0 to the excited levels indicated in the figure. At room temperature, the 7F1 level is thermally populated, in addition to the 7F0 level. This is the cause of some satellite lines on the long wavelength side of the 7F0 ! 5DJ transitions, observed at room temperature but not at low temperature. In agreement with this, additional peaks were observed at room temperature, as the 535 nm peak assigned to the 7 F1 ! 5D1 transition. There is an outstanding double peak at 393–396 nm, which is more clearly observed by detecting at 590 nm. It should be noticed that the two components are observed even at low temperature, which indicates that the double peak is not due to thermal population. In order to investigate the origin of this double peak, emission spectra were obtained with different excitation wavelengths. Fig. 5(a) and (b) show room and low temperature emission spectra of glass–ceramics heat treated at 800 C obtained by exciting at 393 and 396 nm. All these spectra present sharp lines characteristics of Eu3+ in like-crystalline environments, which are associated with the Eu3+ transitions from the upper-lying 5D2 and 5D1 levels (at 465–510 and 525–625 nm, respectively) and from the 5D0 level (at 575–700 nm) to 7FJ levels, as labelled in the figure. The relative intensity of the emissions from the 5D1,2 levels is higher with excitation at 396 nm than at 393 nm. Moreover, the emission from these levels is favoured when the temperature is lowered due to the decrease of non-radiative rates. The observed dependence of the emission spectra on the excitation wavelength indicates the presence of Eu3+ ions in different environments. In Eu3+-doped silicate glasses, the high vibration stretching energy of Si–O bonds about 1100 cm1 causes very fast non-radiative decays to the 5D0 level from the upper-lying levels. As a consequence, only emission from the 5D0 level is observed in these glasses. This is confirmed in Fig. 5(c) where emission spectra of as made 89.9SiO2– 10LaF3–0.1EuF3 glassy sample and SiO2:Eu3+ sol–gel glass heat treated at 800 C have been included for comparison. Accordingly, the presence of higher energy emissions coming from the upper-lying levels 5D1,2 in the glass–ceramics spectra, in addition to the emission from the 5D0 level, indicates the presence of Eu3+ ions in low phonon energy environments. The LaF3 crystals present in the glass–ceramics have maximum vibration energy of 350 cm1, which is low as compared with the value for silica. These results suggest that some Eu3+ ions in the glass–ceramics are incorporated in the LaF3 nanocrystals. The emission spectra give information about the local environment of the Eu3+ ions. The relative intensities and splitting of the Eu3+ emission peaks are ruled by the local symmetry. It is known that the intensity ratio 5D0 !7F2/ 5 D0 ! 7F1, called asymmetry ratio, is an effective spectroscopic probe of the Eu3+ ion site symmetry [17]. From these considerations, the spectra in Fig. 5 obtained by exciting at 396 nm correspond to Eu3+ ions in more symmetric environments than those excited at 393 nm.
300 K
100 K
13 K
393 nm
(c)
SiO2:Eu3+ as made
500
550
600
650
700
Wavelength (nm) Fig. 5. Low (100 and 13 K) and room (300 K) temperature emission spectra of glass–ceramic heat treated at 800 C exciting at (a) 396 and (b) 393 nm. Room temperature emission spectra of as made 89.9SiO2– 10LaF3–0.1EuF3 glassy sample and SiO2:Eu3+ sol–gel glass, heat treated at 800 C, exciting at 393 nm (c). Corresponding transitions from 5D0,1,2 to 7 FJ levels are labelled. The spectra have been normalized to the maximum of the 5D0 ! 7F1 emission.
Additional information about the Eu3+ ion environments was obtained by comparing excitation spectra with different detection wavelengths. Different relative intensities of the excitation at 393 and 396 nm can be observed in Fig. 6. The excitation at 393 nm is favoured by detecting at 613 nm (electric dipole transition), meanwhile the excitation at 396 nm is favoured by detecting emissions coming from upper-lying levels (as 553 nm). On the other hand, the excitation spectra of the as made glassy sample and the heat treated SiO2:Eu3+ glass, see Fig. 6, show the 393 nm peak but not the 396 nm component. This fact is in agreement with the lack of LaF3 nanocrystals in these glasses. These results confirm the assignation of the
A.C. Yanes et al. / Optical Materials 29 (2007) 999–1003
613 nm
the increase of the nanocrystal size with the temperature of the treatment, an increase of the relative intensity of the high-energy emissions is also observed.
590 nm
4. Conclusions
553 nm
Nano-structured Eu3+-doped LaF3–SiO2 transparent glass–ceramics have been synthesized by sol–gel method and characterized by using X-ray diffraction and HRTEM, showing the presence of LaF3 hexagonal nanocrystals. In addition, a spectroscopic study has also been carried out. The presence of high-energy emissions, coming from upper-lying levels 5D1,2 in these samples, indicates the low phonon energy of the Eu3+ ion environments. Moreover, from site selective excitation it is concluded that about the half the Eu3+ ions is partitioned into fluoride nanocrystals meanwhile the rest remains in the glassy phase.
Intensity (arb. units)
393 nm 396 nm
as made 3+
SiO2:Eu
360
380
400
420
440
Wavelength (nm) Fig. 6. Room temperature excitation spectra of glass–ceramic heat treated at 800 C, detecting at 553, 590 and 613 nm. Excitation spectra of as made 89.9SiO2–10LaF3–0.1EuF3 glassy sample and SiO2:Eu3+ sol–gel glass, heat treated at 800 C, detecting at 590 nm are also included. Peaks corresponding to different environments of Eu3+ ions, at 393 and 396 nm, are arrowed.
396 nm
5
Intensity (arb. units)
D0
7
F0
5 5
7
D 2 F0 - 3
7
F1
7
F2
7
F3
7
F4
7
The authors would like to thank to the Gobierno Auto´nomo de Canarias (PI 2002/201), Comisio´n Interministerial Ciencia y Tecnologı´a (MAT 2001-3363) and Universidad de La Laguna (Beca SEGAI) for financial support. References
800 ºC
500
Acknowledgements
D1 F0 - 4
1000 ºC
450
1003
550
600
650
700
Wavelength (nm) Fig. 7. Room temperature emission spectra of glass–ceramics heat treated at 800 and 1000 C exciting at 396 nm. Transitions from 5D0,1,2 to 7FJ levels are labelled.
393 nm excitation peak to the Eu3+ ions in the silica glassy phase, meanwhile the 396 nm peak would correspond to Eu3+ ions partitioned into LaF3 nanocrystals. Furthermore, Pi et al. [18] observed an excitation peak at about 396 nm in LaF3 nanoparticles with Eu3+ ions substituting La3+ sites, supporting our assumption. In the spectrum obtained by detecting at 590 nm, magnetic dipole transition with low dependence on the environment shows the two components of 393 and 396 nm with similar intensities, indicating that roughly similar fractions of Eu3+ ions are in the nanocrystals and in the glassy phase. Finally, Fig. 7 shows emission spectra corresponding to glass–ceramics heat treated at 800 and 1000 C under excitation at 396 nm, i.e., exciting those Eu3+ ions in like-crystalline LaF3 environments. It was found that, together with
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