Optical properties of silica sol-gel materials singly- and doubly-doped with Eu3+and Gd3+ ions

JOURNAL OF RARE EARTHS, Vol. 34, No. 8, Aug. 2016, P. 786

Optical properties of silica sol-gel materials singly- and doubly-doped with Eu3+and Gd3+ ions Natalia Pawlik*, Barbara Szpikowska-Sroka, Marta Sołtys, Wojciech A. Pisarski (Institute of Chemistry, University of Silesia, 9 Szkolna Street, 40-007 Katowice, Poland) Received 21 December 2015; revised 29 March 2016

Abstract: In present work, the optical and structural properties of silica sol-gel glasses and glass-ceramic materials singly- and doubly-doped with Eu3+ and Gd3+ ions were investigated. The optical properties of studied systems were determined based on absorption, excitation and emission spectra as well as luminescence decay analysis. Conducted studies clearly indicated a significant enhancement of visible emission originated from Eu3+ ions as a result of changing the excitation mechanism, via Gd3+→Eu3+ energy transfer. The luminescence intensity R-ratio was analyzed before and after heat-treatment process upon excitation at λex=393 nm and λex=273 nm. Moreover, the influence of excitation wavelength on luminescence decay time of the 5D0 excited state was also analyzed. The Gd3+→Eu3+ energy transfer efficiencies for precursor and annealed samples were calculated based on luminescence lifetime of the 6 P7/2 level of Gd3+ ions. The X-ray diffraction measurements were conducted to verify the nature of obtained sol-gel materials. In result, the formation of orthorhombic GdF3 nanocrystal phase dispersed in amorphous silica glass host was identified after annealing. Obtained results clearly indicated an incorporation of Eu3+ activators into formed GdF3 nanocrystals. Thus, conducted heat-treatment process led to considerable changes in surrounding environment around Eu3+ ions. Actually, it was found that energy transfer phenomenon and heat-treatment process were responsible for significant improvement of Eu3+ luminescence in studied sol-gel samples. Keywords: energy transfer Gd3+→Eu3+; nanocrystals; sol-gel method; luminescence; rare earths

Over the last few years, much attention has been paid on researches in a field of luminescence materials activated by rare-earths (RE3+) due to their wide emission range from ultraviolet to infrared radiation. Thus, considered materials are commonly applied, e.g., in lighting, displays, optical communication and non-linear optics[1–3]. Particularly, white light-emitting diodes (WLEDs) with high energy efficiency have attracted great attention due to their potential replacement of conventional fluorescent lamps. In this case, phosphors doped with RE3+ ions have been wide used in the development of WLEDs[3,4]. It is well known that white fluorescence could be obtained as a result of simultaneous coincidence of three primary colors: red, green and blue[5–7]. Thus, the efficient red-emitting sources based on Eu3+ ions luminescence (λem=611 nm, 5D0→7F2 transition) are frequently used and further developed[8–11]. As de Moura et al.[2] and Guzik et al.[5] suggested, Eu3+ ion is one of the most studied rare-earth element. This tendency is closely related to the: (1) simplicity of Eu3+ emission spectra, (2) wide application of Eu3+-doped materials as biological sensors and red phosphors, e.g., in color screens, electroluminescent devices, fluorescent lamps and optical amplifiers. Generally, intense luminescence can be generated as a result of sensitizing Eu3+ by other RE3+ ions.

In this regard, phosphors based on gadolinium compounds play a particularly important role. This is due to the fact that Gd3+ ion has relatively high-energically the lowest excited states. Thus, it could be observed the efficiently transferring of absorbed excitation energy from the host lattice to the activator Eu3+ ions[11–22]. There are also some reports in available literatures on Tb3+→Eu3+ [10,23–27] and Dy3+→Eu3+ [28] energy transfer mechanisms. Many phosphors based on fluorides doped with RE3+ ions reveal excellent luminescent properties. However, considerable materials have poor mechanical and chemical stability in comparison to oxide matrices. To overcome this problem, there is a capability to achieve oxyfluoride materials. The advantage of such phosphors is the combination of mechanical and chemical resistance of oxide glasses with the low phonon energy of fluoride crystals. The preferred location of RE3+ dopant is in the crystalline phase instead of the glass host[29]. In result, RE3+ ions reveal the high emission quantum yields due to low probability of multiphonon relaxation processes which efficiently quench the luminescence[30,31]. The most commonly pathway for obtaining oxyfluoride glass-ceramic composites is heat-treatment of precursor glasses[13,32–34]. Actually, one of the most frequently used preparative methods is sol-gel technique. The signifi-

* Corresponding author: Natalia Pawlik (E-mail: [email protected]; Tel.: +48-32-359-1157) DOI: 10.1016/S1002-0721(16)60095-9

Natalia Pawlik et al., Optical properties of silica sol-gel materials singly- and doubly-doped with Eu3+and Gd3+ ions

cance of sol-gel method is still increasing due to several advantages, such as: simple preparation, easy control of composition, low processing temperature, high purity and chemical homogeneity of the final products even on a molecular scale[32–36]. Therefore, the sol-gel method offers inherent benefits compared with conventional synthesis route[33,36]. Moreover, according to the available literature, the weight fraction is calculated based on Rietveld analysis for quantitative description of formed crystalline phase dispersed in amorphous host. An extensive discussion in this field can be found in excellent works by de Pablos-Martín et al. for LaF3[37] and NaLaF4[38] systems. Generally, glass-ceramics doped with RE3+ ions are unique class of materials, which could be widely use in integrated optic devices, such as: optical amplifiers, liquid crystal displays, security markers, waveguide lasers and sensors[39–43]. Because of their special properties, glass-ceramic materials are of particular interest for photonic applications. This is possible due to low optical scattering[34,39]. Furthermore, the transparency of glassceramics is higher than expected from the Rayleigh’s theory. The properties of nanomaterials are significantly different from those of their bulk counterparts and strongly dependent on parameters such as their size and shape. Control of these parameters allows for modification of optical properties of nanomaterials and therefore it plays a crucial role in development of materials science[44,45]. Detailed discussion in this field can be found in an excellent review papers by Ferrari et al.[39], Gonçalves et al.[40] and de Pablos-Martín et al.[46]. Several interesting investigations on the optical properties of GdF3:Ln3+ nanocrystals are reported in Refs. [1,13,17,29–31,44,45,47–56]. Some of them are based on GdF3:Eu3+ nanocrystalline system[1,13,17,30,34,45,51–56]. Only in a few reports the comparison of emission properties of Eu3+ ions upon different excitation wavelengths was carried out. Moreover, the luminescence kinetics measurements and analysis of R-ratio values were conducted only in few systems. The calculations of energy-transfer efficiency are also uncommon in available literature. For aforementioned reasons a systematization and expanding knowledge on the optical properties and energy transfer in Gd3+/Eu3+ system is necessary. In this paper, the detailed characterization of sol-gel silica glasses and glass-ceramics doubly-doped with Gd3+ and Eu3+ ions and singly-doped with Gd3+ were presented based on absorption, excitation and emission spectra as well as luminescence decay analysis. The efficiencies of Gd3+→Eu3+ energy transfer before and after heat-treatment process were also presented.

1 Experimental 1.1 Preparation of sol-gel samples The reagents used for sol-gel preparation were of ana-

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lytical purity from Aldrich Chemical Company and used without further purification. Deionized water was obtained from Elix 3 system (Millipore, Molsheim, France). The double-doped with Eu3+ and Gd3+ xerogel samples with following composition: 6.25 TEOS-25.10 C2H5OH62.75 H2O-3.15 CH3COOH-2.30 CF3COOH-0.43 Gd(CH3COO)3-0.02 Eu(CH3COO)3 (in mol.%) were prepared by low-temperature sol-gel route. To compare the luminescence properties, the singly-doped with Gd3+ ions samples with following composition: 6.25 TEOS25.10 C2H5OH-62.77 H2O-3.15 CH3COOH-2.30 CF3COOH-0.43 Gd(CH3COO)3 (in mol.%) were also prepared in the same way as doubly-doped materials. In presented procedure, a mixtures of TEOS, ethyl alcohol, water and acetic acid were stirred for 30 min to perform the hydrolysis process. After this time, the appropriate solutions of gadolinium and europium acetates dissolved in trifluoroacetic acid (TFA) and water, were introduced into previously prepared solutions with constantly stirring (60 min). Then, obtained wet-gels were dried at 35 ºC for 7 weeks. Finally, the precursor xerogel samples were annealed in a muffle furnace (FCF 5 5SHP produced by Czylok, Poland) at a heating rate of 10 ºC/min from room temperature to 350 ºC to obtain glass-ceramic samples. The samples were annealed for 10 h and after this time, they were cooled to room temperature in a closed furnace. 1.2 Characterization To examine the optical behavior of prepared sol-gel materials, the luminescence properties of singly- and doubly-doped samples were measured using a Horiba Jobin-Yvon FluoroMax-4 spectrofluorimeter with 150 W xenon lamp as a light source. The spectral resolution was ±0.1 nm. The excitation spectra were measured in 220–300 and 350–550 nm spectral ranges (λem=311 nm and λem=611 nm). The emission spectra were measured in 285–350 nm and 570–750 nm ranges upon excitation at λex=273 nm, λex=393 nm and λex=464 nm. Decay curves were detected with the accuracy of ±2 μs. The absorption spectra were recorded using the Varian 5000 UV-VIS-NIR spectrophotometer. To identify the crystal phase formed in glass-ceramic samples, the X-ray diffraction (XRD) analysis was carried out using an INEL diffractometer with Cu Kα radiation. All measurements were performed at room temperature.

2 Results and discussion The mechanism of Gd3+/Eu3+ energy transfer process is shown in Fig. 1 and described below. Generally, due to mutual similarity of excited states of Gd3+ and Eu3+ ions, the interaction between optically active dopants is possible. Conversion of ultraviolet radiation (UV) into visible

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Fig. 1 Energy level diagram for Eu3+-Gd3+ system

luminescence can also be successfully obtained. In the first step, the high-energy photon from ultraviolet range (λex=273 nm) is absorbed by Gd3+ ions in consequence of the 8S7/2→6IJ transition. Then, the excitation energy of Gd3+ can be directly transferred to Eu3+ ions as a result of the 6IJ (Gd3+)→5FJ, 5IJ (Eu3+) transition[14,57,58]. However, there is also another possible way to energy transfer in considered system. After the 8S7/2→6IJ transition, the nonradiative relaxation of excited Gd3+ ions from the 6IJ to 6PJ levels is followed and remaining excitation energy is transferred to Eu3+ due to the 6PJ (Gd3+)→5HJ (Eu3+) transition[13,34,58]. In the next step, the fast relaxation to the 5D0 state of Eu3+ ions is followed by nonradiative mechanism. Therefore, the characteristic emission originated from Eu3+ ions can be observed (5D0→7FJ). Obviously, the greatest practical importance could be assigned to orange (λem=590 nm, 5D0→7F1) and red (λem=611 nm, 5 D0→7F2) luminescence. Absorption spectrum for Gd3+/Eu3+ co-doped precursor silica sol-gel materials is presented in Fig. 2. Several absorption bands are located in spectral range between 265 and 475 nm. The spectrum consists of the lines within 4f6 (Eu3+) and 4f7 (Gd3+) electronic configurations.

Fig. 2 Absorption spectrum for precursor silica sol-gel materials co-doped with Gd3+ and Eu3+ ions

The absorption bands of Eu3+ ions correspond to transitions originated from the 7F0 ground level to the 5L6 and the 5D2 excited states[59]. The bands originated from Gd3+ ions are related to electronic transitions from the 8S7/2 level to the 6IJ excited states[60]. However, some bands are not observed, because they lie on the absorption edge or they are masked by absorption of host matrix[59]. Fig. 3 presents the excitation spectrum of studied sol-gel xerogels doubly-doped with Eu3+ and Gd3+ ions monitored at λem=611 nm (5D0→7F2 transition of Eu3+). The spectrum recorded in range from 220 to 300 nm refers to excitation region of Gd3+ ions. The most intense peak was registered at λex=273 nm, related to the 8S7/2→6IJ transition. This effect clearly indicated the occurrence of effective energy transfer. Really weak line corresponds to the 8S7/2→6DJ transition of Gd3+ was also observed. The excitation spectra registered in range from 350 to 550 nm refer to excitation region of Eu3+ ions and indicate two peaks located at λex=393 nm and λex=464 nm, which can be attributed to the 7F0→5L6 and 7F0→5D2 transitions, respectively. The peak related to excitation of Eu3+ ions on the 5L6 state is more intense in comparison to excitation on the 5D2 level. To compare the luminescence intensity of the 5D0→7FJ bands originated from Eu3+ ions, the emission spectra were recorded upon excitation at λex=393 nm and λex=464 nm wavelengths. Results are presented in Fig. 4. Indeed, the characteristic emission bands of Eu3+ ions have grater intensities upon excitation at λex=393 nm line. Due to that fact, the λex=393 nm line was considered as favored parameter for excitation of Eu3+ ions in a direct way in all presented luminescence measurements. The emission spectra of Gd3+/Eu3+ doubly-doped silica sol-gel materials before and after heat-treatment are shown in Fig. 5. The most intense peaks are assigned to the 5D0→7F1 and the 5D0→7F2 transitions of Eu3+ ions. Other emission bands can be attributed to the following

Fig. 3 Excitation spectrum of studied silica glass sample double-doped with Eu3+/Gd3+ ions monitored at λem=611 nm

Natalia Pawlik et al., Optical properties of silica sol-gel materials singly- and doubly-doped with Eu3+and Gd3+ ions

Fig. 4 Emission spectra of Eu3+/Gd3+ system in studied sol-gel materials before heat-treatment process upon excitation at λex=393 nm and λex=464 nm

Fig. 5 Emission spectra of Eu3+/Gd3+ system in studied sol-gel materials before and after heat-treatment process upon excitation at λex=393 nm

transitions: 5D0→7F0, 5D0→7F3 and the 5D0→7F4. After heat-treatment the relative narrowing and sharpening of the emission bands were observed. Considered effect is closely related to transformation of precursor glasses to glass-ceramic materials[37]. Furthermore, the red emission (5D0→7F2) is dominant for xerogel samples, while the orange luminescence (5D0→7F1) has the greatest intensity for glass-ceramics. The orange emission of Eu3+ ions dominates in GdF3:Eu3+ systems upon 7F0→5L6 excitation wavelength in research papers by Cao et al.[30], Lorbeer et al.[52], Wong et al.[53], Zhang et al.[54] and Tian et al.[56]. It is worth nothing that Eu3+ ions are good probes due to their sensitivity to changes in the nearest environment[3,17,30,32,33,44,45,55,61]. The luminescence intensity R-ratio is related to red-to-orange emission and defined as I(5D0→7F2)/I(5D0→7F1). Therefore, the R-ratio can be used to establish how far from inversion center the Eu3+

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ions are located[30,32,33,37,44,45,54,61]. With respect to other RE3+ ions, the Eu3+ is a probe for the chemical environment in different matrices, which is caused by the relative high polarizability and the positions of energy levels[30]. The 5D0→7F1 is a magnetic dipole transition, in which intensity is largely independent of the environment. In contrary, the 5D0→7F2 line, which is commonly known as electric dipole hypersensitive transition is forbidden in inversion center of environment[62,63]. Therefore, the probability of electric dipole transition increases for Eu3+ residing in asymmetric localization and decreases for ions entrapped in centrosymmetric framework[30,33,45,54,55,61]. In consequence, the 5D0→7F1 band dominates after heat-treatment, whereas the 5D0→7F2 line is the greatest for precursor xerogels samples. The R-ratio values before and after heat-treatment were varied and changed from 3.09 to 0.89, respectively. Generally, decrease in R-ratio value is associated with the presence of Eu3+ in a local symmetry more close of an inversion center. It suggests that Eu3+ ions go from amorphous host to crystal phase during heat-treatment process. Similar R-ratio value for glass-ceramic samples was obtained by Chen et al.[34] and equaled to 0.75. However, the R-ratio value calculated for precursor sample was significantly different from ours and equaled to 1.60. Despite the R-ratio analysis, the splitting of characteristic emission bands of Eu3+ ions was also taken into account. A strong split has been observed after heat-treatment: the 5D0→7F1 band is peaking at 586 and 592 nm, and the 5D0→7F2 band is peaking at 613 and 618 nm. Strong splitting of the 5D0→7F1 and the 5D0→7F2 emission bands after heat-treatment implying a low local symmetry around Eu3+ ions[37,62,63]. This is an expected effect due to formation of orthorhombic GdF3 phase. Rhombic crystallographic class is a group with relatively low symmetry, similarly as in the case of monoclinic or triclinic groups. Similar effects have been observed in works published by Cao et al.[30] and Zhao et al.[45]. Moreover, the relative intensities and splitting of other characteristic emission bands: 5D0→7F0, 5D0→7F3 and 5 D0→7F4 are also considerable related to the local site symmetry of Eu3+ ions due to their sensitivity to local framework[62,64]. The appearance of the non-degenerate 5 D0→7F0 transition indicates that Eu3+ ions are located in an environment of low symmetry[37,65]. Thus, according to the literature[37], the appearance of that band suggests that part of Eu3+ ions are still incorporated in amorphous glassy framework. The 5D0 → 7F3 line is usually very weak and is a forbidden transition. The 5D0→7F4 line is a transition, in which intensity is dependent on local environment, however, it is not classified as hypersensitive transition as in the case of the 5D0→7F2 one. According to the literature[63], the intensity of the 5D0→7F4 transition is also de-

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termined by the chemical composition of the host matrix. As was proved, observation of broad 5D0→7FJ (J=0, 2, 3, 4) electric-dipole transitions could be related to the distorted sites or other defects in studied glass-ceramic samples[66,67]. The luminescence decay curves of the 5D0 excited state of Eu3+ ions in studied sol-gel materials doubly-doped with Eu3+ and Gd3+ are shown in Fig. 6. Measurements of luminescence decay kinetics were carried out by monitoring the λem=611 nm emission line (5D0→7F2) upon excitation at λex=393 nm (7F0→5L6). The decay curve measured for precursor glass samples before heat-treatment is well-fitted to single-exponential function and the lifetime is equal to 0.22 ms. Short luminescence lifetime of the 5D0 state determined for precursor xerogel samples is caused by relatively high phonon energy of the host and presence of hydroxyl OH– groups. The OH– groups effectively suppress the characteristic luminescence of RE3+ ions, which results in decrease of the emission efficiency. O–H bonds contribute non-radiative deactivation channels for excited RE3+ ions. Thus, the elimination of hydroxyl groups from xerogels network is necessary in order to avoid quenching luminescence[33,68]. On the other hand, the luminescence decay curve for glass-ceramic samples is well-fitted to double-exponential function with two significantly different times of decay (τ1=0.39 ms, τ2=1.38 ms). Long luminescence lifetime is closely related to incorporation of optically active Eu3+ ions in GdF3 nanocrystals. When the luminescent centers exhibit different local frameworks, the ions will relax at different rates[69]. Indeed, doubleexponential fitting behavior corresponds to inhomogeneous distribution of active ions (Eu3+) in prepared glassceramic samples. First luminescence decay time (shorter time) is related to Eu3+ ions that remained in amorphous glassy framework with relatively high-vibrational energy.

Second decay time (longer time) is related to this part of Eu3+, which are already incorporated into GdF3 crystal phase with relatively low vibrational-energy. It is worth mentioning that after heat-treatment the concentration of OH– groups is significantly reduced. Therefore, the luminescence lifetime of the 5D0 state of Eu3+ ions, which are embeded into amorphous phase in heat-treated samples, is longer in comparison to lifetime value obtained for Eu3+ before heat-treatment. According to the literature[70], other authors also interpret the distribution of RE3+ ions between glassy matrix and crystal phase based on luminescence decay kinetics. However, the distribution of RE3+ ions in glass-ceramics could be investigated based on electron energy loss spectroscopy. Such researches have been conducted and presented in available Refs. [71,72]. The luminescence properties of Gd3+ ions in studied sol-gel precursor xerogels singly-doped with Gd3+ and doubly-doped with Gd3+/Eu3+ ions were also investigated and are presented in Fig. 7. The excitation spectra were obtained by monitoring the 6PJ→8S7/2 transition of Gd3+ ions (λem=311 nm). The excitation broad band with maximum at λem=273 nm is assigned to the 8S7/2→6IJ transition. Considered band registered for xerogels singly-doped with Gd3+ ions is more intense in comparison to samples doubly-doped with Gd3+/Eu3+. The characteristic sharp peak registered in emission spectra and located at λem=311 nm is assigned to the 6 PJ→8S7/2 transition of Gd3+ and was registered for singly- and doubly-doped samples. The emission band of Gd3+ is significantly weaker for silica sol-gel materials doubly-doped with Gd3+ and Eu3+ in comparison to singly-doped samples. Both of observed effects suggest that excitation energy of Gd3+ was efficiently transferred to Eu3+ ions in result of energy migration process. The emission spectrum for Gd3+/Eu3+ co-doped glass

Fig. 6 Luminescence decay curves from the 5D0 excited state of Eu3+ ions in sol-gel materials before and after heattreatment (λex=393 nm, λem=611 nm)

Fig. 7 Excitation (λem=311 nm) and emission (λex=273 nm) spectra of Gd3+ ions in studied Gd3+ and Eu3+/Gd3+ systems

Natalia Pawlik et al., Optical properties of silica sol-gel materials singly- and doubly-doped with Eu3+and Gd3+ ions

systems is shown in Fig. 8. In spectral scope from 570 to 750 nm the characteristic bands of Eu3+ ions were registered and assigned to the 5D0→7FJ (J=1–4) transitions. Occurrence of characteristic reddish-orange luminescence originated from Eu3+ ions upon excitation at λex=273 nm is a direct proof of the Gd3+→Eu3+ energy transfer. The spectra registered in range from 285 to 350 nm revealed characteristic for Gd3+ ions emission band located at λem=311 nm (6PJ→8S7/2 transition). Also, the UV emission has been obtained by other authors, who conducted the studies on the Gd3+/Eu3+ energy transfer[30,34,55]. It is worth noting that the shift of red luminescence band of Eu3+ ions to λem=616 nm was also observed (Fig. 8, Inset). For explaining considered effect, the emission spectra were also registered for samples singly-doped with Gd3+ ions upon excitation at λex=273 nm. The origin of peak located at 622 nm wavelength is very interesting. The explanation for this line can be done in three different ways, and it was discussed by us in the previously published works[73,74]. The peak located at 622 nm wavelength could be described as 2nd order light as a subsequent line of the λem=311 nm emission. Similar phenomenon was observed by Kondo et al.[58] and Lo et al.[75]. On the other hand, it is known from literatures[14,64,76,77] that Gd3+ ions are able to emit in visible light scope. Nevertheless, the spectral progression should be observed due to the possibility of combination of the transitions between individual 6GJ (J=11/2–5/2) and 6PJ (J=7/2–3/2) levels of Gd3+ (e.g. 6G11/2→6P7/2, 6G7/2→6P5/2, 6 G 5/2 → 6 P 3/2 , etc.) as was presented in following

Fig. 8 Emission spectrum of Eu3+/Gd3+ studied system before annealing process upon excitation at λex=273 nm (Inset shows the spectral shift of red-luminescence band to λem=616 nm) (1) Eu3+/Gd3+ co-doped system upon excitation at 273 nm; (2) Eu3+/Gd3+ co-doped system upon excitation at 393 nm; (3) Gd3+-doped system upon excitation at 273 nm (For comparison the spectra were normalized)

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works[78–80]. Actually, in the case of registered spectra the progression was not observed. Moreover, the 6GJ→6PJ emission intensity is much smaller than the total Eu3+ luminescence intensity[77]. Presented arguments suggest that 622 nm red line peak should be described as 2nd order light. The emission spectra of synthesized materials before and after heat-treatment upon λex=273 nm excitation are shown in Fig. 9. Obviously, the electric dipole transition band dominates in emission spectrum registered for xerogel samples, while the magnetic dipole transition line dominates for glass-ceramics. After heat-treatment, the spectral shift of red emission band has not been observed. Calculated R-ratio values upon excitation at λex=273 nm are equaled to 4.99 and 0.82 for samples before and after heat-treatment, respectively. Furthermore, the R-ratios obtained for xerogel samples upon λex=393 nm and λex=273 nm excitation were significantly different and increased from 3.09 to 4.99. Described phenomenon also suggests that the 5 D0→7F2 emission of Eu3+ ions and 2nd order line at 622 nm overlap to each other. The luminescence decay curves of Eu3+ ions (λex= 273 nm, λem=611 nm) were carried out and are shown in Fig. 10. The luminescence lifetime for the 5D0 excited state obtained upon λex=273 nm (8S7/2→6IJ, Gd3+) excitation is longer in comparison to λex=393 nm (7F0→5L6, Eu3+) excitation. Calculated lifetime value equals to 0.34 ms for xerogel samples and decay curve is well-fitted to single-exponential function. The decay curve registered for glass-ceramic samples after heat-treatment is well-fitted to double-exponential function and the lifetime values equal to τ1=0.43 ms and τ2=2.15 ms. As was mentioned earlier, photoluminescence measurements clearly indicate that Eu3+ ions are distributed between the amorphous host and GdF3 crystals. Generally, the beneficial effect of energy transfer on luminescence properties of

Fig. 9 Emission spectra of Eu3+/Gd3+ system before and after annealing process upon excitation at λex=273 nm

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Fig. 10 Luminescence decay curves from the 5D0 excited state of Eu3+ ions in sol-gel materials before and after heat-treatment (λex=273 nm, λem=611 nm)

prepared samples was observed. As was proved, the change of excitation parameter has a significant influence on the 5D0 luminescence lifetime. The discussion in this field can be found in review paper by Ferreira et al.[81]. As was proved in cited work, the luminescence decay time strongly depends on the energy transfer channels, such as: ion-to-ion, intra- and intermolecular energy transfer. It is noteworthy that the luminescence lifetime could be prolonged through excitation different rare-earth ion, as in the case of Gd3+→Eu3+ system. This is expected due to the fact that donor states (Gd3+) are located above the 4f emitting levels (Eu3+). In this case, a balance between transfer and back-transfer leads to a longer transient of the average population in the 4f emitting level. In consequence, the luminescence lifetime is prolonged. Furthermore, the authors establish that sometimes there are some intermediate states resulted from ligand-to-metal charge transfers, defects or trapping states. It should be noticed that the intermediate states do not necessarily represent any specific state and are certainly at quasi-resonance with the 5D0 level of Eu3+. In such cases mentioned above, the 5D0 decay curves depend strongly on the excitation wavelength. Moreover, the energy transfer efficiency could be calculated based on the following formula:

⎛τ ⎞ ⎟ ⎝ τ0 ⎠

ηGd→Eu=1– ⎜

wavelength. The values are depicted in Table 1. The X-ray diffraction patterns of Eu3+-doped sol-gel materials before and after heat-treatment are shown in Fig. 11. XRD measurements were carried out to confirm the structure of examined sol-gel samples. Obtained effects proved amorphous nature of precursor sol-gel materials (Fig. 11(1)). Registered diffraction peaks indicated that the crystallization of orthorhombic GdF3 phase has been successfully carried out during performed heattreatment process (Fig. 11(3)). Obtained results clearly indicated that applied annealing conditions allow for efficient decomposition of Gd(CF3COO)3 into gadolinium fluoride. Nanocrystal size has been estimated by using Scherrer’s equation (D=Kλ/βcosθ, where D is the crystal size, K is a constant value depending on crystal shape, λ is the X-ray wavelength, β is a half width of the diffraction peak and θ is the diffraction angle). The average diameter value of particles equals to about 6 nm. GdF3:Eu3+ nanocrystals with similar size were obtained by Chen et al.[34] and Wong et al.[53]. The achievement of low optical absorption and scattering is essential for designing transparent glass-ceramic materials. According to the Rayleigh-Gans theory, crystal sizes should be smaller than the light wavelength in order to minimize light scattering. For visible light the particle size should be lower Table 1 Luminescence lifetime obtained for singly- (τ0) and doubly-doped (τ) sol-gel samples and energy transfer efficiencies ηGd→Eu Excitation Emission wavelength/ wavelength/ Transition nm nm Before heattreatment After heattreatment

τ0/ ms

τ/ ms

PJ → 8S7/2 0.75 0.14

ηGd→Eu/ %

273

311

6

82.7

273

311

6

99.9

PJ → 8S7/2 0.90 0.001

(1)

where ηGd→Eu is the energy transfer efficiency, τ is a fluorescence lifetime of Gd3+ with Eu3+ in host matrix and τ0 is the Gd3+ donor lifetime in the absence of Eu3+ acceptor[82,83]. The values of energy transfer efficiency were estimated for xerogels and heat-treated samples upon excitation at λex=273 nm (8S7/2→6IJ transition of Gd3+) and monitoring λem=311 nm (6P7/2→8S7/2 emission line of Gd3+)

Fig. 11 X-ray diffraction patterns for GdF3:Eu3+ nanocrystals formed in studied silica glass-ceramic samples

Natalia Pawlik et al., Optical properties of silica sol-gel materials singly- and doubly-doped with Eu3+and Gd3+ ions

than about 15 nm[68–70,73–75]. Thus, studied sol-gel glassceramics with average particle diameter about 6 nm could be interesting materials dedicated for photonic applications.

3 Conclusions In summary, optical properties of silicate sol-gel materials doped with Eu3+ and Gd3+ ions were investigated. X-ray diffraction analysis for heat-treated samples revealed that the formation of GdF3 nanocrystals dispersed in amorphous host was successful. After annealing process the enhancement of characteristic emission of Eu3+ was observed due to embedding of the optically active ions in crystal phase with relatively low phonon energy. It was clearly observed that the photoluminescence properties of Eu3+ ions were strongly dependent on excitation parameter. The ultraviolet excitation via Gd3+→ Eu3+ energy transfer process (λex=273 nm) resulted in considerable prolongation of the luminescence lifetime compared to direct excitation of Eu3+ ions (λex=393 nm). The energy transfer efficiencies for precursor and glass-ceramic samples were also calculated based on luminescence lifetime of the 6P7/2 level of Gd3+. Studied materials can provide a good point for further researches in a field of efficiently-emitted materials, that may find potential use in photonic applications.

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