J Sol-Gel Sci Technol (2011) 60:170–176 DOI 10.1007/s10971-011-2576-7
ORIGINAL PAPER
Structural and luminescent study in lanthanide doped sol–gel glass–ceramics comprising CeF3 nanocrystals J. del Castillo • A. C. Yanes • J. Me´ndez-Ramos J. J. Vela´zquez • V. D. Rodrı´guez
•
Received: 23 May 2011 / Accepted: 2 September 2011 / Published online: 10 September 2011 Ó Springer Science+Business Media, LLC 2011
Abstract Sol–gel derived glass–ceramics containing CeF3 nanocrystals have been developed for the first time, to the best of our knowledge, by adequate heat treatments of precursor bulk glasses with composition 95SiO2–5CeF3 doped with 0.1 Eu3? or 0.1 Sm3? and co-doped with 0.3 Yb3? and 0.1 Er3? ions (in mol%). X-Ray Diffraction and High Resolution Transmission Electron Microscopy confirm the precipitation of CeF3 nanocrystals. Moreover, this structural analysis is completed using Eu3? and Sm3? as probe ions of the different local environments for rare-earth ions in the nano-structured glass–ceramics. Luminescence measurements led us to discern the final environments for the ions, revealing the partition of a large fraction of these ions into like-crystalline environment of the precipitated CeF3 nanocrystals. Near infrared emission at 1.5 lm was observed after excitation at 980 nm in Yb3?–Er3? co-doped samples for potential applications in telecommunications. Keywords Sol–gel Rare-earth doped-materials CeF3 Luminescence
1 Introduction It is well-known that luminescent features of rare-earth (RE) ions, such as sharp absorption and emission bands from the UV to infrared range, lead the way for their J. del Castillo (&) A. C. Yanes Departamento de Fı´sica Ba´sica, Universidad La Laguna, 38206 La Laguna, Tenerife, Spain e-mail:
[email protected] J. Me´ndez-Ramos J. J. Vela´zquez V. D. Rodrı´guez Departamento de Fı´sica Fundamental y Experimental, Universidad La Laguna, 38206 La Laguna, Tenerife, Spain
123
application in optical devices. For that reason, the search of RE doped-materials continues being a challenge due to their potential applications such as high-performance luminescent displays, optical amplifiers for telecommunications, biochemical probes, laser materials and solar cells [1–5]. In that respect, oxyfluoride transparent nano-glass– ceramics (nGCs) have been investigated as hosts for these ions in order to obtain materials that present elaboration and manipulation advantages, with different sizes and shapes, chemical and mechanical stabilities, related to oxide glasses combined with high quantum luminescence efficiency of fluoride hosts [6, 7]. It is important to note that these materials remain transparent after the precipitation of crystalline phase due to the nanosized fluoride crystals precipitated into a silica glass matrix. Several studies indicate that CeF3 (cerium fluoride) is an important host material for these luminescent ions owing to its low vibrational energies and the subsequent minimization of the quenching of excited states [8]. Moreover, CeF3 has shown increasing technological importance as an inorganic scintillating crystal [9]. On the other hand, it is still a difficult task to fabricate structures of CeF3 with controlled morphologies and sizes. In this sense sol–gel method provides an alternative synthesis route to different methods including hydrothermal route [10], polyol methods [11], reverse micelles or microemulsions [12] and conventional melting technique [13] which has been developed to synthesize CeF3 nano- and microcrystals. Thus, authors have recently presented results on sol–gel derived oxyfluoride glass–ceramics systems based on silica matrices comprising fluoride nanocrystals, i.e. LaF3, NaYF4 or YF3 doped with different RE ions [14–16]. In this work we develop, for the first time to the best of our knowledge, sol–gel derived transparent silica based nGCs containing CeF3 nanoparticles. A complete structural
J Sol-Gel Sci Technol (2011) 60:170–176
analysis has been carried out in terms of X-Ray Diffraction and High Resolution Transmission Electron Microscopy confirming the successfully precipitation of CeF3 nanocrystals. With the aim to investigate the local structure around RE ions, Eu3? and Sm3? ions were selected as doping ions due to their properties as probe ions which are highly sensitive to the local environment [16], discerning between crystalline and glassy environments by means of site-selective spectroscopy. Moreover, their corresponding emissions in the reddish orange range of the visible spectrum can be efficiently obtained under UV-blue excitation, with applications in optoelectronics, medicine and so on [17, 18]. In addition we have also observed 1.5 micron infrared emission under 980 nm pumping in Yb3?–Er3? co-doped samples of special interest in the development for optical amplifiers.
2 Experimental We report silica bulk glasses with composition 95SiO2– 5CeF3 doped with 0.1Eu3?, 0.1Sm3? or co-doped with 0.3Yb3? and 0.1Er3? (mol %) obtained by sol–gel method as described by Fujihara et al. [19]. Tetraethoxysilane (TEOS) Si(OCH2CH3)4, used as a source of SiO2, was hydrolyzed for 1 h at room temperature with a mixed solution of ethanol and H2O, using acetic acid as a catalyst. The molar ratio of TEOS:ethanol:H2O:CH3COOH was 1:4:10:0.5. As a source of Ce, Ce(CH3COO)3xH2O was used. The required quantities of Ce(CH3COO)3xH2O, Eu(CH3COO)3xH2O, Sm(CH3COO)3xH2O, Er(CH3COO)3 xH2O, and Yb(CH3COO)3xH2O were dissolved in a CF3COOH and H2O solution, which was slowly mixed with the initial solution. The molar ratio of metal ions to CF3COOH was 1:4. In order to obtain a homogeneous solution, the resultant one was stirred vigorously for 1 h at room temperature. A highly transparent gel was obtained by leaving the resultant homogeneous solution in a sealed container at 35 °C for several days. Then, the gels were dried by slow evaporation of residual water and solvent. Finally, these sol–gel bulk glasses were heat-treated in air from 600 to 900 °C in order to achieve controlled precipitation of CeF3 nanocrystallites, giving rise to transparent nGCs crack-free. X-ray powder diffraction (XRD) patterns of the samples were recorded with a Philips X’Pert Pro diffractometer equipped with a primary monochromator, Cu Ka,1,2 radiation, and an X’Celerator detector. The XRD patterns were collected with a step of 0.0168 in the 2h angular range from 15 to 808 and acquisition time of 2 h. Furthermore, the patterns were corrected by using LaB6. High resolution transmission electron microscopy (HRTEM) images were obtained by using a JEOL 3010F microscope operating at
171
300 kV, equipped with a Field Emission Gun, which allowed us to achieve a point-to-point resolution of 0.17 nm. Samples were prepared by dispersing the fine powder, obtaining by grinding the sample, in ethanol and dropping onto carbon-coated copper grids. Selected areas of the HRTEM images were mathematically filtered by means of Fast Fourier Transform (FFT) analysis resulting in Power Spectra patterns, corresponding to the eigen-frequencies of the observed nanocrystals. Further, the relevant frequencies were selected to filter the noise in the zoomed areas of the HRTEM images and to produce higher contrast images of the atomic planes of the observed nanocrystals. Luminescence measurements 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. A commercial laser diode was used as excitation source at 980 nm with a pump power of 300 mW. All spectra were collected at room temperature and corrected by the instrumental response.
3 Results and discussion 3.1 Structural characterization The structural and morphologic characterization was carried out by using XRD patterns and HRTEM images in transparent sol–gel derived nGCs with composition 95SiO2–5CeF3–0.1Eu3? in mol%. Fig. 1 shows the XRD patterns of those nGCs as function of temperature of heat treatment ranging from 600 to 900 °C. The diffraction peaks pointed out at 24.8, 27.9, 35.1, 43.9, 45.1, 50.8, 52.9, 64.7, 69.0 and 71.2° and their relative intensities match well with results for CeF3 hexagonal crystals (JCPDS Card 08-0045). No extra peaks were detected, so no other complex products were formed. These nanocrystals present weak diffraction peaks in the sample with 600 °C heat treatment temperature, meanwhile they can be easier observed in samples heat treated at higher temperatures. Thus, when temperature of the treatment is increased, peaks appear strongly and their shapes results sharper, which can be related with a higher degree of crystallinity. The average nanocrystal size in these samples can be calculated from the position and width of the XRD peaks, by Scherrer’s equation varying from 9.4 to 15.8 nm, see Table 1. Moreover, in order to study the morphology and structure, TEM and HRTEM images are presented next. TEM bright-field micrograph for the nGC heat treated at 800 °C, see Fig. 2a, shows ‘‘cylindrical-shaped’’ nanocrystals homogeneously dispersed and clearly visible as dark tubes in the amorphous silica network. The average length and
123
214 115 410 411
302 221
Intensity (arb. units)
112
300 113
111
J Sol-Gel Sci Technol (2011) 60:170–176
002 110
172
900 °C 800 °C 700 °C 600 °C
JCPDS 08-0045
20
30
40
50
60
70
80
2θ (degree) Fig. 1 XRD patterns of 95SiO2–5CeF3–0.1Eu (mol%), heat-treated at indicated temperatures. Standard peaks of CeF3 (JCPDS 08-0045) are also included for comparison Table 1 Sizes calculated by using Scherrer0 s equation in the 95SiO2– 5CeF3–0.1Eu (mol%) glass–ceramics heat treated from 600 to 900 °C Temp. (°C)
Size (nm)
600
9.4
700
11.2
800 900
14.4 15.8
width of these nanoparticles are 18.5 and 5.7 nm, comparable to the calculated by Scherrer’s equation from XRD patterns. Furthermore, the structure of a single nanocrystal was analysed in depth by using HRTEM images, see Fig. 2b, (the one selected in white squared region). The power spectrum obtained from this squared nanocrystal is presented in the right-top inset of Fig. 2b. The indexation of the spectrum spots, corresponds to the CeF3 crystallized in the hexagonal phase. In addition, higher contrast magnified detail of the white-squared region is shown in the right-bottom inset of Fig. 2b where resolved lattice fringes ˚, are clearly observed with constant spacing of 3.3 A ascribed to (1,1,1) plane of CeF3, revealing their high degree of crystallinity. 3.2 Visible luminescence The optical properties of 95SiO2–5CeF3–0.1Eu (mol%) transparent nGCs for different heat treatment temperatures have been studied by using the luminescent features of Eu3? ions, taking advantage of their properties to discern between different surroundings in the studied samples. Thus, in order to understand energy transfer processes among RE ions, the energy level diagrams of Ce3? and
123
Eu3? ions along with their main transitions are presented in Fig. 3. Excitation spectra of nGCs heat treated at 900 °C and detecting at different wavelengths (590, 612 and 617 nm) are presented in Fig. 4, along with excitation spectrum of precursor glass Eu3?-doped sample detecting at 612 nm. In the precursor glass sample, excitation peaks at around 360, 380, 393 and 464 nm are observed, which correspond to Eu3? transitions from 7F0 to 5D4, 5GJ, 5L7, 5L6 and 5D2 levels, respectively. However, peaks at wavelengths shorter than 370 nm are completely quenched in the nGCs samples. This fact is related with the absorption of the host matrix. It is important to notice that in the precursor glass there is a main excitation peak at 393 nm characteristic of Eu3? ions in a glassy matrix. However, in the nGCs samples this most intense peak splits into two components clearly observed, located at 393 and 396 nm. The left one at 393 nm would be associated to Eu3? ions in a noncrystalline environment by comparison with the above commented precursor sample. On the other hand, the right one at 396 nm, would be related with a nano-crystalline environment. This crystalline component at 396 nm, was also observed by the authors in Eu3? ions partitioned into hexagonal LaF3 nanocrystals [14]. In this previous work, authors adopted that similar intensities observed at 393 and 396 nm correspond to roughly comparable fractions of Eu3? ions in the LaF3 nanocrystals and in the glassy phase. In that sense, in the CeF3 nGCs the relative intensity of the crystalline component at 396 nm is increased when detecting either at 590 or 617 nm, versus detecting at 612 nm that would correspond to glassy environment. In particular when detecting at 590 nm, the intensity at 396 nm doubles the 393 nm ones, indicating that about two-thirds of Eu3? ions are partitioned into the CeF3 nanocrystals while the rest are remaining in a glassy environment. Next, the emission spectra of nGCs samples heat treated at different temperatures varying excitation wavelengths are presented in Fig. 5, where the main emissions at about 578, 590, 612-617, 650 and 690 nm, corresponding to the 5 D0 ? 7F0-4 transitions of Eu3? ions respectively, can be clearly observed. Moreover emission spectrum of precursor glass exciting at 393 nm is also presented (lower curve in Fig. 5). It is well-known that the 5D0 ? 7F1 transition has magnetic dipole character and does not depend appreciably on the environment of the Eu3? ions, while the 5D0 ? 7F2 hypersensitive transition has electric dipole character and strongly depends on the local symmetry of the Eu3? ions, being forbidden in centro-symmetric environments. In that respect, the asymmetry ratio (R) of the electric dipole (ED) to magnetic dipole transitions (MD), 5D0 ? 7F2/5D0 ? 7F1, is used as a probe of the local environment symmetry of these
J Sol-Gel Sci Technol (2011) 60:170–176
173
(a)
(b)
[111] CeF3
3.3Å 5 nm
20 nm
Fig. 2 a TEM bright-field image of 95SiO2–5CeF3–0.1Eu (mol%), heat treated at 800 °C where ‘‘cylindrical-shaped’’ CeF3 nanoparticles are observed (dark contrast). b HRTEM micrograph with a larger magnification. Right-top inset corresponds to the power spectrum of
the region squared in where CeF3 nanocrystals are observed. Rightbottom inset corresponds to magnified detail of the same whitesquared region showing the crystalline pattern of a CeF3 nanoparticle
35
7
F0→
H4
5
5
D4 5 G , L7 L 5 6 D3 5 D2 5 D 5 1 D0
5
20
690 nm
650 nm
15
10
Intensity (arb. units)
5 J
590 nm 612-617 nm
Energy (×103 cm-1)
5
25
0 3+
Eu
5
D2
5
D3
G 612 nm
GC 612 nm GC 617 nm
F6
GC 590 nm 2
F7/2
2
3+
F5/2
Ce
Fig. 3 Eu3? and Ce3? ions energy level diagrams with main transitions indicated. Dotted lines stand for the cross-relaxation processes occurring between Eu3? and Ce3? ions 3?
D4
7
5 4 3 2 1 0
5
L6
5
30
5
GJ, L7
5
Eu ions [20, 21]. Accordingly, the emission spectrum of the precursor glass sample shows an intense emission peak observed at around 612 nm, corresponding to the 5D0 ? 7F2 transition, whereas the 5D0 ? 7F1 transition at 590 nm is weaker, with an R value of 4.9, also with non-resolved Stark components characteristic of amorphous surrounding for the Eu3? ions. On the other hand, corresponding emission spectra of the nGCs samples present sharp peaks with better resolved Stark components, characteristics of Eu3? ions in likecrystalline environments. It is also important to note that emissions coming from upper-laying, 5D1 and 5D2 levels, which were previously observed by authors in Eu3?-doped LaF3 based nGCs [14], are inhibited in this case by
350
400
450
500
Wavelength (nm) Fig. 4 Excitation spectra of 95SiO2–5CeF3:0.1Eu (mol%) nGCs heat treated at 900 °C (labelled as GC), detecting at indicated wavelengths. Excitation spectrum of the precursor glass Eu3?-doped sample (labelled as G) detecting at 612 nm is also included
phonon-assisted energy transfer between Eu3? and Ce3? ions. This fact is related with the energy gap between 2F7/2 and 2F5/2 of Ce3? ions (about 2,300 cm-1), see energy level in Fig. 3, that enables cross-relaxation process with phonon assistance between Eu3? and Ce3? ions [22]. Therefore non-radiative de-excitation from upper-laying levels to the emitting 5D0 level are induced, as indicated by dotted arrows in the left-hand side of Eu3? diagram, see Fig. 3. Remarkably, the asymmetry ratio R of the electric dipole to magnetic dipole transitions substantially changes in the nGCs samples with respect to the precursor glass sample, see Fig. 5, being the 5D0 ? 7F1 transition the most intense peak with a drastic reduction of the hypersensitive electric dipole 5D0 ? 7F2 transition, with R values around
123
174
J Sol-Gel Sci Technol (2011) 60:170–176
F2
7
F4
7
F3
F0
Intensity (arb. units)
393 nm
800 °C 396 nm 393 nm
700 °C 396 nm 393 nm Glass 393 nm 550
600
650
700
Wavelength (nm) Fig. 5 Emission spectra of 95SiO2–5CeF3:0.1Eu (mol%) nGCs heat treated from 700 to 900 °C and exciting at 393 and 396 nm. All spectra of the nGCs samples have been normalized to the maximum of the 5D0 ? 7F1 transition at 590 nm. Emission spectrum of the precursor glass, exciting at 393 nm is also included
0.7, supporting the assumption of the partition of the majority of the Eu3? ions into the precipitated CeF3 nanocrystals during the ceramming process. Moreover, a double peak is observed at about 612–617 nm. The left component at 612 nm, can be related with a amorphous environment of Eu3? ions by comparison with precursor glass, while the longer wavelength component at 617 nm would be associated to crystallinelike environment. The evolution of the relative intensity of these two components is analyzed by changing heat treatment temperature and excitation wavelength. Thus, a diminishment of the glassy component at 612 nm is observed with raising heat treatment temperature from 700 to 900 °C, revealing a more-crystalline environment for the Eu3? ions, according to the observed increase of the nanocrystal size by XRD measurements in Fig. 1. Furthermore, the emission component at 617 nm is favoured when exciting at the nano-crystalline environment excitation peak at 396 nm compared to the 393 excitation, as above discussed in Fig. 4. Next, luminescence features of the 95SiO2–5CeF3– 0.1Sm (mol%) nGCs heat treated at 900 °C was studied, by using the ED character of the 4G5/2 ? 6H9/2 hypersensitive transition of Sm3? ions, which intensity decreases as the
123
environment symmetry of the luminescent site increases [23]. Moreover, high efficient emissions of Sm3? ions in the reddish-orange range, assigned to 4G5/2 ? 6HJ = 5/2, 7/2, 9/2, 11/2 transitions, emerge as suitable sources for illuminating appliances and displays [18]. Figure 6 shows the emission spectra exciting at 400 nm of the Sm3?-doped glass–ceramics and of Sm3? ions in a silica glassy environment observed previously by the authors [23], both showing main red–orange emissions, as indicated in the energy level diagram also depicted. This excitation wavelength correspond to the maximum of the excitation spectrum for the glass–ceramic, see inset in Fig. 6, where the observed narrow bands correspond to excitations from the ground state 6H5/2 to excited levels. It is important to notice the shift to shorter wavelength of the emission bands in the glass–ceramic when comparing with the glassy environment, due to crystal field effect. In this case for Sm3? ions, the asymmetry ratio R between ED (4G5/2 ? 6H9/2) and MD (4G5/2 ? 6H5/2) transitions is also used as a measurement of the local symmetry around trivalent 4f ions. From Fig. 6, an R value of 3.7 is obtained for glassy environment. On the other hand, from the emission spectrum of the glass–ceramics, an R value of 0.9 is obtained, in agreement with the observed decrease of the hypersensitive transition, associated to nanocrystalline CeF3 environment around Sm3? ions. It should be remarked that the suggested crystalline environment for both Eu3? and Sm3? ions could be supported by the fact that they can be easily partitioned into the CeF3 lattices by substituting Ce3? ones due to their same valence and similar ionic radius may be favorable ˚ vs. Eu3?:0.95 A ˚ , Sm3?:0.96 A ˚ ) [24]. (Ce3?:1.15 A 35
Det 560 nnm 6
4
6
H9/2
4
H5/2→ F7/2, G11/2
30
350
400
450
500
6
4
G5/2→
H7/2
4 4
25
D 3/2
6
P 7/2 4 F 7/2 , G 11/2 5 G 9/2 ,4 I 15/2 I 11/2 , I 13/2 4 4 G 7/2 F 3/2 4
P 5/2
4
4
4
20 4
I 9/2
G 5/2
15
6
10
7/2 3/2
5
H5/2
0
F 11/2
9/2 5/2
6
H 9/2
6
400 nm
396 nm
900 °C
-1-1
7
7
33
F1
Energy (x (10 10 cm Energy cm ))
7
D0 →
Intensity (arb. units)
5
6
H 11/2
9/2
7/2
5/2
Sm
3+
6
H11/2
550
600
650
700
750
Wavelength (nm) Fig. 6 Emission spectra of 95SiO2–5CeF3:0.1Sm (mol%) nGCs heat treated at 900 °C (thick line) and of Sm3? ions in a silica glass environment included for comparison [23] (thin line), both exciting at 400 nm along with energy level diagram of Sm3? ions. Inset shows excitation spectrum of glass–ceramic detecting at 560 nm
J Sol-Gel Sci Technol (2011) 60:170–176
175
-1
Energy (10 cm )
1400
2
0
1450
1500
1550
2
Ce
3+
I13/2
1.5 μm
Intensity (arb. units)
3
4
5
2
I11/2
F 5/2
Pump : 980 nm
4
10
F 7/2
F 5/2
1600
4
Er
3+
I15/2
2
Yb
3+
F 7/2
for the first time to our knowledge. The precipitation of CeF3 nanoparticles during the thermal treatment of precursor glasses is confirmed by means of XRD and HRTEM measurements. Emission and excitation spectra have been analysed as a function of excitation wavelength and heat treatment temperature, confirming the incorporation of a large fraction of these ions into the precipitated CeF3 nanoparticles. Intense red–orange emissions, due to Eu3? and Sm3? transitions, allow us to consider these materials as potential candidates for illuminating devices. In addition, the observed 1.5 lm infrared emission under 980 nm pumping in Yb3?–Er3? co-doped samples can be consider of special interest in the development for optical amplifiers.
1650
Wavelength (nm) Fig. 7 Infrared emission spectrum of 95SiO2–5CeF3:0.3Yb–0.1Er (mol%) nGCs heat treated at 900 °C exciting at 980 nm with 300 mW of pump power. Inset shows energy level diagram of Yb3?, Er3? and Ce3? ions
Finally, a candidate for a potential application in optical amplifier would be a CeF3 based nano-glass–ceramic doped with Er3? and Yb3? ions. In particular a glass– ceramic with composition 95SiO2–5CeF3:0.3Yb–0.1Er (mol%) and heat treated at 900 °C was excited with a commercial laser diode at 980 nm, showing an intense near infrared (IR) emission at 1.55 lm, minimum loss telecommunication window, see Fig. 7. It should be mentioned that visible up-conversion emission, previously observed by the authors in different fluoride matrices [15, 16, 25], is completely quenched in this case of CeF3 based nanoglass–ceramic. The presence of Ce3? ions favours the nonradiative relaxation from 4I11/2 to 4I13/2 of Er3? ions due to phonon-assisted cross-relaxation channel associated with the energy mismatch with the Ce3? gap as indicated in the energy levels diagram, see inset in Fig. 7 [8]. Thus, this energy mismatch benefits a large population of 4I13/2 level, giving rise to emission at 1.5 lm, and also to a drastic inhibition of the up-conversion emission due to the depletion of the population of the 4I11/2 and upper-laying levels [22]. The full width half-maximum (FWHM) of this emission band is about 64 nm, comparable with the values obtained by authors in a previous work [26], which also supports the interest of these materials for wavelengthdivision multiplexing (WDM).
4 Conclusions Glass–ceramics containing Eu3? or Sm3? doped and Yb3?–Er3? co-doped CeF3 nanocrystals were successfully synthesized by thermal treatment of precursor sol–gel derived glasses with composition 95SiO2–5CeF3 (in mol%)
Acknowledgments The authors would like to thank Ministerio de Ciencia e Innovacio´n (MAT2009-12079) and Gobierno Auto´nomo de Canarias: Agencia Canaria de Investigacio´n, Innovacio´n y Sociedad de la Informacio´n (SolSubC200801000286 and FPI grant PI97718301).
References 1. Ying Y, Alivisatos AP (2005) Nature 437:664 2. Trupke T, Shalav A, Richards BS, Wu¨rfel P, Green MA (2006) Sol Energ Mater Sol C 90:3327 3. Becker PC, Olsson NA, Simpson JR (1999) Erbium-doped fibre amplifiers: fundamental and technologies. Academic Press, San Diego 4. Richards BS (2006) Sol Energ Mater Sol C 90:1189 5. Stru¨mpel C, McCann M, Beaucarne G, Arkhipov V, Slaoui A, Svrcek V, del Can˜izo C (2007) Sol Energ Mater Sol C 91:238 6. Leo´n-Luis SF, Abreu-Afonso J, Pen˜a-Martı´nez J, Me´ndez-Ramos J, Yanes AC, del-Castillo J, Rodrı´guez VD (2009) J Alloys Compds 479:557 7. Tick PA, Borrelli NF, Cornelius LK, Newhouse MA (1995) J Appl Phys 78:6367 8. Yu Y, Shan Z, Cheng D, Huang P, Lin H, Wang Y (2010) J App Phys 108:123523 9. Shimamura K, Vı´llora EG, Nakakita S, Nikl M, Ichinose N (2004) J Cryst Growth 264:208 10. Wang X, Zhuang J, Peng Q, Li YD (2006) Inorg Chem 45:6661 11. Wang ZL, Quan ZW, Jia PY, Lin CK, Luo Y, Chen Y, Fang J, Zhou W, O’Connor CJ, Lin J (2006) Chem Mater 18:2030 12. Lian H, Zhang M, Liu J, Ye Z, Yan J, Shi C (2004) Chem Phys Lett 395:362 13. Cheng D, Yu Y, Li H, Huang P, Weng F, Shan Z, Wang Y (2009) Opt Lett 34:2882 14. Yanes AC, Del-Castillo J, Me´ndez-Ramos J, Rodrı´guez VD, Torres ME (2007) J Arbiol Opt Mater 29:999 15. Me´ndez-Ramos J, Yanes AC, Santana-Alonso A, del-Castillo J, Rodrı´guez VD (2010) J Nanosci Nanotech 10:1273 16. Yanes AC, Santana-Alonso A, Me´ndez-Ramos J, del-Castillo J, Rodrı´guez VD (2011) Adv Funct Mater (in press) 17. Jia M, Zhang J, Lu S, Sun J, Luo Y, Ren X, Song H, Wang X (2004) Chem Phys Lett 384:193 18. Mu J, Liu L, Kang S (2007) Nanoscale Res Lett 2:100 19. Fujihara S, Mochizuki C, Kimura T (1999) J Non Cristal Solids 244:267 20. Me´ndez-Ramos J, Lavı´n V, Martı´n IR, Rodrı´guez-Mendoza UR, Rodrı´guez VD, Lozano-Gorrı´n AD, Nu´n˜ez P (2001) J Appl Phys 89:5307
123
176 21. Reisfeld R (1973) Struct Bonding Berlin 13:53 22. Dantelle G, Mortier M, Vivien D, Patriarche G (2005) Chem Mater 17(8):2216 23. Yanes AC, Vela´zquez JJ, del-Castillo J, Me´ndez-Ramos J, Rodrı´guez VD (2008) Nanotechnology 19:295707 24. Goldschmidt ZB (1978) Handbook on physics and chemistry of rare earths, vol 1. North-Holland Publishing Company, Amsterdam
123
J Sol-Gel Sci Technol (2011) 60:170–176 25. del-Castillo J, Yanes AC, Me´ndez-Ramos J, Tikhomirov VK, Moshchalkov VV, Rodrı´guez VD (2010) J Sol-Gel Sci Tech 53:509 26. Rodrı´guez VD, Del-Castillo J, Yanes AC, Me´ndez-Ramos J, Torres ME, Peraza J (2007) Opt Mat 29:1557
