Spectroscopic and structural properties of polycrystalline Y2Si2O7 doped with Er3+

Spectroscopic and structural properties of polycrystalline Y2Si2O7 doped with Er3+

Author's Accepted Manuscript Spectroscopic and structural Properties of polycrystalline Y2Si2O7 doped with Er3 þ L. Marciniak, D. Hreniak, W. Strek, ...

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Author's Accepted Manuscript

Spectroscopic and structural Properties of polycrystalline Y2Si2O7 doped with Er3 þ L. Marciniak, D. Hreniak, W. Strek, F. Piccinelli, A Speghini, M. Bettinelli, M. Miritello, R. Lo Savio, P. Cardile, F. Priolo

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S0022-2313(15)00086-1 http://dx.doi.org/10.1016/j.jlumin.2015.02.015 LUMIN13200

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Journal of Luminescence

Received date: 18 December 2014 Revised date: 6 February 2015 Accepted date: 9 February 2015 Cite this article as: L. Marciniak, D. Hreniak, W. Strek, F. Piccinelli, A Speghini, M. Bettinelli, M. Miritello, R. Lo Savio, P. Cardile, F. Priolo, Spectroscopic and structural Properties of polycrystalline Y2Si2O7 doped with Er3 þ , Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2015.02.015 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Spectroscopic and Structural Properties of Polycrystalline Y2Si2O7 Doped with Er3+ L. Marciniak1,*, D. Hreniak1, W. Strek1 1

Institute for Low Temperature and Structure Research, Polish Academy of Sciences, Wroclaw, Poland F. Piccinelli2,*, A Speghini2, M. Bettinelli2

2

Laboratorio di Chimica dello Stato Solido, DB, Università di Verona and INSTM, UdR Verona, Strada Le Grazie 15, 37134 Verona, Italy M. Miritello3,*, R. Lo Savio3, P. Cardile3, F. Priolo3

CNR-IMM MATIS and Dipartimento di Fisica e Astronomia, Università di Catania, via S. Sofia 64, 95123 Catania, Italy * corresponding authors: [email protected] [email protected] [email protected] Keywords: silicates, nanocrystals, Rare Earths, size effect

Abstract Powders of yttrium disilicate (Y2Si2O7)doped with Er3+ have been prepared by the sol-gel method. The structure of the obtained powders has been determined. Room temperature emission spectra have been recorded and excited state decay profiles have been analyzed. Differences between the spectroscopic properties of Er3+ in monoclinic α-Y2Si2O7 (space group P −1 ) and β-Y2Si2O7 (space group C2/m) polymorphs have been investigated and shown. The significant broadening of the emission spectra recorded for the Į phase compared to the one for the ȕ phase was discussed in terms of higher number of Y3+ sites (4) present in the Į phase with respect to only one Y3+ site in the case of ȕ phase. The higher value of the luminescence decay time of ȕ phase (11.2 ms) compared to the Į phase (8.5 ms) is associated with the higher site symmetry of ȕ -Y2Si2O7. Moreover it was found that Er3+ concentration affects the shape of the 4I13/2 ĺ 4I15/2 emission band. It results in changes of the relative emission intensities of peaks localized at 1527 nm and 1532 nm; this indicates changes of the Y3+ sites occupation on increasing the Er3+ concentration. The luminescence lifetime was observed to decrease with the increase of Er3+ concentration. The spectroscopic results have been compared with the ones relative to thin films of Y2Si2O7:Er3+ with a similar composition. The lower value of the luminescence decay time observed for thin films compared to the powder of Į phase was explained with the changes of the particles packing resulting in the change of the effective refractive index.

Introduction Polycrystalline yttrium silicates have attracted great attention, due to many advantages such as a high resistance to radiation damages [1, 2, 3] and thermal and chemical stability [4, 5]. Yttrium silicates are among the most interesting hosts for rare-earth (RE) ions, which substitute easily Y3+ ions [1-3, 6-19]. These hosts exhibit also a minimal effect of nuclear spins of the constituent elements which results in narrow homogeneous emission lines of the dopant RE ions [7, 8, 9]. Y2SiO5 (YSO) doped with Ce-ions (YSO:Ce3+) is one of the best low-voltage phosphors for field emission displays due to its high efficiency, colour purity and stability [2, 3, 9-11, 18], YSO:Tb3+ is considered one of the best green emitting cathodoluminescent phosphors [1] and YSO:Eu3+ as a promising phosphor for high resolution applications [13-15, 19]. Moreover in more recent years, erbium compounds [20-24] and erbium doped yttrium silicates [25-27] have attracted considerable attention also as active medium of small-size on-chip amplifiers and lasers around the telecom wavelength in silicon photonics. Indeed the performances of the planar waveguide amplifiers based on Er-doped SiO2 amorphous (EDFA)are limited by the number of optically active Er ions since its low solubility, about 1020 cm-3, instead the introduction of yttrium silicates can permit to incorporate higher Er density up to 1022 cm-3 and all the Er ions have been demonstrated to be optically active [21, 25]. However the maximum Er optical efficiency strongly depends on Er content and host preparation, since the specific sensitivity of Er3+ on the host material and the strongly concentration depending co-operative processes occurring between Er3+ ions, dramatically change the spectroscopic properties. But very few detailed works have been reported for Y2Si2O7 doped with RE ions (rare-earth pyrosilicates or disilicates) [17-21, 25, 28, 29]. The main reason for this is that its different polymorph structures naturally lead to relatively broad emissions originating from ions placed in different symmetry sites. Unfortunately, Y2Si2O7 has been reported to form as many as five polymorphs:y, α, β, Ȗ, and į [30-33], in many cases co-existing ones, stabilized by different impurities and strongly depending on the method of synthesis. Therefore, it is interesting to carry out the preparation of the Y2Si2O7:Er3+composition as a single phase material and at different Er contents. In this work, we investigated single phase powders of two different polymorphs of Y2Si2O7 doped with three Er3+ concentrations. The main aim of this work is to show, by a high resolution spectroscopy, how strongly the spectroscopic features of Er3+ ions in this compound are affected by the nature of the host. The evolution of the Y2Si2O7 structure and its influence on the spectroscopic properties of Er3+ is discussed in the cases of monoclinic ß-

Y2Si2O7 and triclinic Į- Y2Si2O7. The comparison between the optical properties of Er3+ doped α-Y2Si2O7 powder and thin films is also discussed in details.  2. Experimental 2.1. Synthesis Polycrystalline powders of yttrium disilicates doped with different content of Er3+ ions ( 0.2%, 1% and 3%, mol.%) were prepared by the sol-gel method, details of which have been published elsewhere [16]. Briefly, tetraethoxysilane (Si(OC2H5)4, TEOS), yttrium oxide (Y2O3) and erbium oxide (Er2O3) were used as starting reagents. Stoichiometric amounts of Y2O3 and Er2O3 were digested with ultrapure nitric acid (HNO3) to obtain yttrium and erbium nitrates, respectively. At the same time, TEOS was hydrolyzed in water (3:4 ratio by volume) by stirring at 50°C for 2 h. To produce powders the sols were left for one week for gelation. After this time gels were dried in air at 90°C for another week. Next, dry gels (xerogels) were annealed at different temperatures in the range of 1100-1400°C for 4h. Erbium-yttrium disilicate thin film, about 130 nm thick, has been grown on (100) c-Si substrates heated at 400 °C by radiofrequency (rf) cosputtering from Er2O3, Y2O3 and SiO2 targets. Further details on the apparatus and on the deposition procedure can be found elsewhere [25]. Stoichiometry of the thin film was controlled by the rf-powers applied to the targets. The Er content was fixed to 0.3% in erbium-yttrium disilicate film. Deposited film is amorphous and after rapid thermal annealing at 1200 °C for 30 s in O2 atmosphere it crystallizes in a mixture of the y and Į phases typical of rare earth disilicates, as already observed for Er2Si2O7 thin films [20]. The crystalline structure is independent of the Er content, as verified by x-ray diffraction.

2.2 Equipment X-ray experimental section X-ray-diffraction (XRD) data were taken with a Thermo ARL X´TRA powder diffractometer, operating in Bragg-Brentano geometry equipped with a Cu-anode X-ray source (KĮ, Ȝ =1.5418 Å) and using a Peltier Si(Li) cooled solid state detector. The spectra were collected with a scan rate of 0.02°/sec, time of exposure 1.5 sec/step and 2ș range of 10°-90°. The structural model exploited for the Rietveld refinement in the case of α-Y2Si2O7 is the one proposed by Dolan et al. [4], and the one proposed by Redhammer et al. [34] in the case of βY2Si2O7. The powder samples were ground in a mortar suspended in a few drops of ethanol

and deposited in a low-background sample stage. The data collection was started after the complete evaporation of ethanol.

Spectroscopic experimental section Room-temperature photoluminescence measurements were performed by exciting with the 488 nm line of an Ar laser. The light was chopped through an acousto-optic modulator at a frequency of 11 Hz, analyzed by a single grating monochromator, and detected by a Hamamatsu infrared extended photomultiplier tube cooled at liquid-nitrogen temperature coupled with a lock-in amplifier having the modulation frequency as a reference. All the spectra have been corrected for the spectral system response. Time resolved PL measurements were performed analyzing the modulated luminescence signal with a photon counting multichannel scaler. The overall time resolution of the system is 30 ns.

3. Results and discussion All the synthesized powders were analyzed by X-ray diffraction, in particular we performed a QPA-Rietveld analysis (Quantitative phase analysis using Rietveld refinement) using the computer program MAUD [35]. For all the samples treated below 1250°C the main crystal phase is α-Y2Si2O7 [Triclinic (P-1) space group], in contrast with the samples treated at 1400°C, for which the main phase is β- Y2Si2O7 [monoclinic (C2/m) space group]. In all synthesized samples we observed the presence of the apatite like phase Y4.67(SiO4)3O [hexagonal (P63/m) space group] and of X2-Y2SiO5 [monoclinic (C2/c) space group] as the main impurity phases. For the samples treated below 1250°C the powder showing the highest level of phase purity is the one treated at 1100°C containing 1% of dopant Er3+ ion [98.5% αY2Si2O7, 1.5% Y4.67(SiO4)3O (volume fraction)]. The α-Y2Si2O7 samples doped with 0.2% and 3% of Er3+ show a degree of purity slightly lower than the sample doped with 1% (around 90% of purity for both samples, data not shown). Concerning the samples treated at 1400°C, the best quality powder is that one containing 1% of Er3+ ion [93% β-Y2Si2O7, 2.5% Y4.67(SiO4)3O, 4% X2-Y2SiO5 and traces of Er2O3 (volume fraction)]. The final results of the QPA-Rietveld analysis performed on these samples are shown in Fig. 1. From the crystallographic point of view in the α-Y2Si2O7 phase there are four possible sites for the Y3+ ions (or Er3+ ions assuming the Er/Y substitution) possessing C1 point symmetry. On the other hand, in the β-Y2Si2O7 there is only one site for the Y3+ ions possessing C2 point symmetry.

Finally, the value of the average particle size estimated by Rietveld calculation falls in the range of nanoparticles [58(1) nm] for α-Y2Si2O7. On the contrary the ß-phase shows an average particle size of 140(1) nm.  Figure 1(a) and (b) around here

Photoluminescence (PL) emissions have been recorded upon 488 nm excitation in the 4F7/2 state from all the powders at room temperature (Fig 2). It is evident that in the examined spectral range the only recorded emission is centered around 1.54 µm, corresponding to the typical 4I13/2 ĺ 4I15/2 transition of Er3+ ions. But if we compare the high resolution PL spectra obtained from the two powders containing different phase, α and β, and the same Er content (1 Er%), reported in Fig. 2 (a) and Fig. 2(b), the PL shape appears strongly dependent on the crystalline phase in which Er ions are embedded. PL emission from the α powder, shown in figure 2(a), presents two main peaks at 1527 nm and 1532 nm and two weaker peaks at higher wavelengths, respectively at 1543 nm and at 1556 nm. On the other hand, when Er is in the β crystalline phase of yttrium disilicate powder the spectrum, shown in figure 2(b), is characterized by one main peak at around 1537 nm and other weaker peaks for lower and higher wavelengths. Moreover the emission band recorded for the Į-phase reveals much broader features. These differences are associated with the different number of Y3+(Er3+) sites in these two structures. As mentioned before, in higher temperature ȕ-phase there is only one Y3+ site of C2 symmetry, while in Į-phase there are four different Y3+ sites of C1 symmetry. In the latter case, the coordination of the three of Y3+ ions is bicapped trigonal prism, while the fourth Y3+ ion is located in distorted octahedron. Due to the fact that Er3+ ions can be localized in four different crystallographic sites of Y3+ ions in Į-phase and emission from all of these sites can take place results in the broadening of the emission band. Thus the PL shape represents a real fingerprint of the silicate crystalline phase in which Er is placed. 

Figure 2 around here 

The PL spectrum associated to Er in α−powder is also compared in Fig. 2(a) to the PL emission in the Y2Si2O7 thin film doped with Er3+ (0.3%), whose XRD pattern reveals that it crystallizes mainly in α-phase [25]. The shape of the emission spectra is comparable for the powder and thin film and in particular it is very similar to that one observed from Er3+ in αEr2Si2O7 thin films [20]. This results additionally confirms that the shape of the emission band is specific for the crystallographic phase and is independent on the material form (powder vs. thin film). Also the luminescence decay profiles of Er3+-doped Y2Si2O7 in the two crystalline phases, have been investigated. Fig. 3 presents the comparison of luminescent decay profiles recorded at the respective main peaks, 1527 nm for α phase and 1537 nm for β phase corresponding to the 4I13/2 ĺ 4I15/2 electronic transition. Both curves are fitted by simple exponential, with lifetime values of respectively 8.5 ms and 11.2 ms for α and β phase. The fact that lifetime calculated for β phase is longer is probably associated with the higher symmetry of the Er3+ site (C2). It is worth to notice that despite the fact that there are four sites of Er3+ ions in the Į phase the luminescence decay profile reveals exponential decay. Presumably all four C1 sites are occupied, and the sums of their decay profiles results to be exponential. Figure 3 around here

The comparison of emission spectra measured for different concentration of Er3+ ions in the Į phase powders is shown in Fig. 4. One can see that with the increase of the dopant concentration the integral emission intensity increases. Moreover it is worth to notice that the relative emission intensity between two the most intense peaks localized at 1527 and 1532 nm changes with the Er3+ content. In case of low concentration the peak localized at 1527 nm is dominant, while for 1.0% of Er3+ the peaks are of comparable intensity and the peak localized at 1532 nm becomes dominant for 3.0% of Er3+ ions. This comparison indicates the relative occupation of the four available sites changes with dopant concentration. Figure 4 around here The luminescence decay profiles measured for different dopant concentrations in powders are presented in Fig. 5. One can see that the lifetimes decrease from 8.9 ms for 0.2 % Er3+ to 6.8 ms for 3.0% of Er3+. However in case of all dopant concentration the shape of the profile

remains exponential. This phenomenon is associated with the change of the Er3+ amounts in the different Y3+ sites observed in emission spectra (Fig. 4). It could be that for higher Er3+ concentration the emission from the Er3+ sites of the lower symmetry becomes dominant manifesting in the decrease of the decay constant. Alternatively, energy transfer to killer centers could become more probable for higher dopant concentrations. Additionally the decay profile recorded for the thin film of Į phase is presented in Fig. 5. Significantly shorter decay constant measured for thin film containing 0.3% of Er3+, is probably associated with the difference in the particles packing which is much dense in case of the thin films comparing to the nanocrystalline powders indicating the differences in the refractive indexes of the phosphors. According to the Meltzer model [36], the decrease of the grain size results in the decrease of the effective refractive index and hence increase of the decay constant. Therefore the luminescent lifetime is much longer in case of nanocrystalline powders. Figure 5 around here

4. Conclusions The comparison of the spectroscopic properties of the nanocrystalline powders of Į- and ȕY2Si2O7 doped Er3+ ions was presented. The powder and film of the Į phase containing about the same Er3+ content are compared. It was shown that the shape of the emission band associated with the 4I13/2 ĺ 4I15/2 transition strongly depends on the crystal structure and hence the shape of the emission bands can be used as a fingerprint of the structure of the host. The differences in the width of the emission band is associated with the different number of the Y3+ sites in which the Er3+ ions are localized (one site in case of ȕ-phase and four sites in Į phase). Moreover the luminescent decay time for ȕ-phase is longer comparing to the one of Įphase what is associated with the higher site symmetry of the Y3+ in the ȕ-phase. It was shown that in the Į-phase the integral emission intensity increases with the Er3+ amount. Moreover with increase of the Er3+ amount the relative emission intensities of peaks localized at 1527 nm and 1532 nm changes. This is probably associated with the nonequivalent occupation of the Y3+ crystallographic sites in the host. In case of the Į phase thin film the luminescence decay profile was significantly shorter comparing to the nanocrystalline powders. This observation was discussed in terms of differences in the particles packing for nanocrystalline powders and thin films, resulting in the differences in the effective refractive index of the sample material.

Acknowledgments The work was supported by Wroclaw Research Centre EIT+ within the project "The Application of Nanotechnology in Advanced Materials” - NanoMat (POIG.01.01.02-02002/08) co-financed by the European Regional Development Fund (Operational Programme Innovative Economy, 1.1.2).

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Figure captions Fig. 1. (a) XRD pattern of Y2Si2O7: 1%Er3+ treated at 1100°C (black dots), calculated pattern of a 98.5% α-Y2Si2O7, 1.5% Y4.67(SiO4)3O mixture (red solid line) and line of residuals (lower black line). (b) XRD pattern of Y2Si2O7: 1%Er3+ treated at 1400°C (black dots), calculated pattern of a mixture of 93% β-Y2Si2O7, 2.5% Y4.67(SiO4)3O, 4% X2-Y2SiO5 and traces of Er2O3 (red solid line) and line of residuals (lower black line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article). Fig. 2. Room temperature photoluminescence spectra obtained for α-Y2Si2O7 film and powder (a) and β-Y2Si2O7 powder (b) doped with the similar content of erbium ions. Fig. 3. Room temperature photoluminescence decays obtained for α-Y2Si2O7 (a) and βY2Si2O7 powders (b) doped with the same content of erbium ions (1% Er3+). Fig. 4. Room temperature photoluminescence spectra obtained for α-Y2Si2O7 doped with different content of erbium ions (x% Er3+ vs. Y3+). Fig. 5. Room temperature photoluminescence decays recorded for α-Y2Si2O7 powders doped with different content of erbium ions and an α-Y2Si2O7 thin film doped with 0.3 Er% .



Highlights

silicates, nanocrystals, Rare Earths, size effect

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