J Mater Sci DOI 10.1007/s10853-014-8531-6 Environment segregation of Er3+ emission in bulk sol–gel-derived SiO2–SnO2 glass ceramics Tran T T Van • S Turrell • B Capoen • Le Van Hieu • M Ferrari • Davor Ristic • L Boussekey • C Kinowski Received: 25 April 2014 / Accepted: August 2014 Ó Springer Science+Business Media New York 2014 Abstract Er-doped (100-x) SiO2–x SnO2 glass–ceramic monoliths were prepared using a sol–gel method Raman spectroscopic measurements showed the structural evolution of the silica matrix caused by the formation and the growth of SnO2 nanocrystals Analysis of the photoluminescence properties shows that the quantity of Er3? ions embedded in the vicinity of SnO2 nanocrystals could be controlled by the SnO2 concentration We give spectroscopic evidence of energy transfer to erbium ions provided by SnO2 nanocrystals in the silica matrix The 4I13/2 level decay curves present a double-exponential profile with two lifetimes associated to rare-earth ions in two different environments Introduction For telecommunication application, the 1.55 lm wavelength range is of importance because of the minimum absorption and dispersion in optical fibers The geometry of T T T Van (&) Á L Van Hieu University of Science, Vietnam National University, Ho Chi Minh City, Vietnam e-mail: tttvan@hcmus.edu.vn S Turrell Á L Boussekey Á C Kinowski LASIR (CNRS, UMR 8516) and CERLA, Universite´ Lille 1, 59650 Villeneuve d’Ascq, France B Capoen Á C Kinowski PhLAM (CNRS, UMR 8523) and CERLA, Universite´ Lille 1, 59650 Villeneuve d’Ascq, France M Ferrari Á D Ristic CSMFO Lab., IFN-CNR, Via alla Cascata 56/c, 38050 Trento, Italy integrated optical components needs a large gain achieved over a short distance; therefore, a high erbium concentration is required Some studies have been carried out on the 1.55 lm luminescence of Er3?-doped glass systems such as phosphate-, silicate-, and tellurite-based glasses [1–5] However, the extremely low solubility of such active ions in pure silica matrices leads to quenching effects due to clustering of doping ions, even at low concentrations For example, significant Er3?–Er3? interactions have been found in silica at concentrations as low as 100 ppm [6] This grouping results in a reduction of luminescence efficiency due to energy transfers between ions, which then results in non-radiative relaxations A solution to this problem is the use of glass–ceramics because the incorporation of rare-earth ions in nanocrystals not only prevents the aggregation even at high concentrations but also allows crystal-ion energy transfers, thus enhancing the efficiency of ion luminescence, which compensates for the small absorption cross section of these ions [7] In this work, the well-known wide-band gap semiconductor SnO2 (Eg = 3.6 eV at 300 K) was chosen as the crystalline species Tin dioxide is transparent through the visible and infrared regions, which covers the emission range for active ions like erbium Moreover, with its very low cutoff phonon energy of 630 cm-1, SnO2 is prone to reduce the non-radiative decay of RE ion excited states Moreover, the tin oxide nanocrystals can be excited by a broad range of UV wavelengths, as compared with the narrow excitation peaks of the Er3? ions Therefore, these nanocrystals can be easily and efficiently excited by broadband arc lamps with UV emission Hence, the SnO2-doped silica glass–ceramic system should be an excellent host for active ions However, the low value of RE solubility in SnO2 is a well demonstrated matter of fact [8] An increase 123 J Mater Sci in the SnO2 concentration should serve to increase the solubility of RE-ions, which prevents the non-radiative processes due to ion–ion interactions In addition, this increase can enhance the emission efficiency of Er3? ions through energy transfer from SnO2 nanocrystals The bulk system 0.4 mol % SnO2 doped with 0.5 mol % Er3? in silica was prepared by N Chiodini et al [9] and under excitation at 514 nm, they obtained a spectrum of Er3? ions in an amorphous environment with a lifetime of the 4I13/2 level equal to 10 ms, and the decay curve presented a nonsingle-exponential behavior More recently, S Brovelli et al [10, 11] obtained bulk systems of Er-doped silica with mol % of SnO2 nanocrystals and Er3? ions concentration up to mol % They were the first to give evidence of energy transfer from the SnO2 nanocrystals to the Er3? ions in bulk systems However, these authors showed that an increase of Er3? concentration from 0.05 to mol % induces a decrease of photoluminescence decay time at 1.5 lm from to 0.5 ms due to quenching effects Using visible photoluminescence data for the 95SiO2– 5SnO2 system doped with 0.4 mol % Er3?, J del-Castillo et al [12] showed that the Er3? ions are partially dispersed in the SnO2 nanocrystals and that the efficiency of energy transfer can be improved by changing both the SnO2 or Er3? concentrations, as well as the thermal treatment All these works have been focused first on increasing the ion solubility so as to avoid quenching effects and secondly on improving the efficiency of energy transfer between the SnO2 crystals and the Er3? ions It is necessary to increase the SnO2 concentration in order to enhance energy transfer However, the consequences on the form of the photoluminescence spectrum in the near-infrared region, particularly the emission bandwidth around 1500 nm, have not been discussed In the present work, these questions will be addressed by changing the SnO2 and Er3? concentrations and observing the effects on the form of the emission spectrum in the infrared and on the lifetimes of the 4I13/2 level of the erbium ion, both being consequences of the change in environment of the rare-earth ion Experimental Sample preparation (100-x)SiO2–xSnO2 (x = 4, 8, 12 mol %) glass–ceramics doped with 0.1, 0.5 and mol % Er3? were prepared using the sol–gel technique with a process similar to that of Hayakawa et al [13] The starting solution was obtained by mixing tetra-ethyl-orthosilicate (TEOS 99.9 %, SigmaAldrich), ethanol, and de-ionized water with hydrochloric 123 acid (0.1 mol/l) as a catalyst This solution, with a molar ratio TEOS: H2O:Ethanol equal to 1:4:8, was pre-hydrolyzed for hours at room temperature Separately, SnCl2.2H2O (98 %, Alfa Aesar) and Er(NO3)3.5H2O (99.9 %, Sigma-Aldrich) dissolved in ethanol were added to the solution containing TEOS After stirring for hours at room temperature (RT), the resulting solution was placed in sealed polypropylene containers, first at ambient temperatures for weeks and then at 55 °C for another weeks, so as to obtain monolithic gels To complete the hydrolysis and polymerization of terminal : Si–OH groups, the dried gels were heated in water vapor at 80 °C for days Finally, the resulting xerogels were annealed at temperatures ranging from 600 to 1100 °C for hour in air, with a ramp of 0.5 °C/min, thus forming a stiff glass network Crack-free and pinkish transparent cylindrical samples were obtained with dimensions of mm in diameter and 10 mm in height Characterization For high temperature X-ray diffraction (HTXRD) measurements, the diffractometer was equipped with an Anton Paar HTK1200 N high temperature chamber, which was coupled to a high speed Vantec1 detector After being placed in this chamber, the samples were subjected to a temperature increase of °C/min up to a desired temperature and then held at this temperature for the duration of the recording of the diffractogram The diffractograms were recorded at temperatures ranging from 650 to 1050 °C at intervals of 25 °C The crystal size and morphology were determined by transmission electron microscopy (TEM) using a Philips CM30 microscope For these measurements, the specimens were ground in ethanol A droplet of the resulting fine powder suspension was placed on a copper microscope grid The samples to be analyzed were annealed for hour at a desired temperature between 600 and 1100 °C UV–visible absorption spectra were recorded using a Perkin Elmer UV/Vis/Nir spectrophotometer Lambda 19 Raman scattering measurements were performed using the 488 nm line of an Ar? ion laser The scattered light was collected and analyzed using a T64000 JobinYvon spectrometer with a spectral resolution of cm-1 Room-temperature photoluminescence spectra were obtained with a specially designed Jobin–Yvon micro photoluminescence spectrometer using the 351 nm and 514 nm excitation lines of a CW Coherent Ar? laser The emission light was dispersed using a monochromator with a spectral resolution of nm and collected by a Peltiercooled InGaAs detector J Mater Sci For the lifetime measurements, experiments were performed by far-field excitation using the 514.5 nm line of an Ar? ion laser as source Si/InGaAs diode and a photomultiplier tube were used as detectors The excitation laser was modulated using a 70 Hz chopper, and the spectra were recorded using a standard lock-in technique A part of the exciting beam was deviated to a diode detector to use as the trigger for the lock-in Decay curves were obtained using a standard oscilloscope, the same chopper used for the modulation of the signal, and the lock-in technique being used to chop the excitation beam Results and discussions was also investigated by XRD measurement Fig displays the XRD patterns of glass–ceramic monoliths doped with 0.5 mol % Er3? annealed at 1100 °C for h in air The mean crystal size estimated using the Scherrer equation ranges from 4.6 to 5.4 nm for % and 12 mol % SnO2, respectively A high-resolution TEM (HRTEM) image of the 88 % SiO2–12 % SnO2 doped with mol % Er3? sample heat treated at 1100 °C for h is presented in Fig 3, showing both spherical crystallites and others, which are slightly oblong The average size of crystals is found to be around nm Measurements yield interplanar spacings of 0.34 nm, which correspond to the (110) planes of rutilelike SnO2 Structural properties High temperature X-ray diffraction (HTXRD) and TEM In order to study the evolution of the structure of the SnO2 nanocrystals upon heat treatment, in situ HTXRD measurement were performed The diffractograms were recorded at temperatures varying from 650 to 1050 °C Fig presents the HTXRD patterns of the sample 88 % SiO2–12 % SnO2 doped with mol % Er3? pre-heated at 600 °C The appearance of peaks at 2h = 26.4, 33.5, 37.7, 51.5, 54.6, 57.5, and 64.9° can be assigned to the (110), (101), (200), (211), (220), (002), and (112) planes of the tetragonal rutile-type SnO2 crystal (International Centre of Difraction Data (JPCD) file 41–1445) The width of the diffraction peaks is virtually independent of annealing temperature indicating that the heat treatment has a little effect on the growth of crystals In addition, the effect of percentage of tin dioxide on the size of SnO2 nanocrystals Fig In situ HTXRD patterns of the 88SiO2–12SnO2 doped %Er3? samples with different annealing temperatures Fig XRD patterns of 4, 8, and 12 % SnO2 doped with 0.5 % Er3? samples heat treated at 1100 °C for h Fig HRTEM image of an 88 SiO2–12 SnO2 doped % Er3? glass–ceramic sample heat treated at 1100 °C for h 123 J Mater Sci Raman spectroscopy The evolutions of a given silica matrix structure with heat treatment and doping concentrations were studied by Raman spectroscopy An example is given in Fig for the sample 92SiO2–8SnO2 doped with 0.5 mol %-Er3? and for annealing temperatures ranging from 600 °C to 1100 °C For comparison, the top spectrum is that of an mol % SnO2 sample without erbium and heat treated at 1100 °C It can be noted that this latter spectrum is essentially identical to that of the Er3?-doped sample heat treated at the same temperature Both spectra are basically characteristic of amorphous silica with additional bands due to SnO2 but with no bands which can be related to erbium oxide or erbium mixed tin oxide phases When the erbium concentration is increased to mol %, there are still no changes in the Raman spectrum, thus indicating that the presence of Er3? has very little effect on the final structure of the silica matrix The gradual broadening of the T-O-T band (attributed to d(Si–O-Si) bending mode) around 440 cm-1 with increased temperature is a well-known characteristic of the densification process of a silica matrix The ratio of the intensities of the bands at 490 and 603 cm-1, assigned to D1 and D2 rings, to that of the T-O-T band decreases with increasing temperature This behavior is to be expected as these two types of rings are associated with the pore surfaces, and the calcination processes decrease the porosity of the systems The profile of the band at 800 cm-1 in the spectra of the samples treated at 1100 °C is characteristic of densified silica Finally, the band at 980 cm-1, which is assigned to vibrations of Si–OH groups, decreases in intensity with increasing annealing temperatures, indicating the gradual removal of solvent and precursor molecules [9, 14–16] Fig Raman spectra of 4, 8, and 12 % SnO2 doped with 0.5 % Er3? samples heat treated at 1100 °C for h The decrease in intensity of the surface phonon mode of SnO2 at 348 cm-1 for annealing temperatures above 600 °C demonstrates the increase in size of the nanocrystals and the resulting reduction in the surface to volume (S/V) ratio [16, 17] The band at 632 cm-1 is due to the A1g volume phonon mode of SnO2 in its rutile structure [14, 18, 19] The increase in intensity of this band with increasing heat treatment temperatures from 600 to 1100 °C is consistent with an increase in crystalline volume Finally, for systems annealed at 1100 °C, the relative intensity between D1 and D2 bands and Si–O-Si vibration is much greater than would be expected for a densified silica system This observation supports the proposition that the presence of SnO2 nanocrystals induces a residual porosity in the matrix [20] The influence of the concentration of SnO2 on the structural evolution of the matrix and on the formation of the particles for systems annealed at 1100 °C has also been examined (See Fig 5) At this temperature, a slight increase of the relative intensities of the D1 and D2 bands to the Si–O-Si vibration with the percentage of SnO2 indicates that an increase of SnO2 concentrations from % to 12 mol % has very little effect on the matrix structure of silica The increase in intensity of the A1g band with SnO2 concentration reflects either an increase in nanocrystals volume or in their number Optical properties Absorption spectroscopy Fig Evolution of the Raman spectra of the 92SiO2–8SnO2 doped 0.5 %Er3? samples as a function of increasing annealing temperature The Raman spectrum of an undoped sample heat treated at 1100 °C is added as a reference for comparison 123 Figure 6a presents absorption spectra for the %SnO2 system doped with % Er3?, in which the transition from the 4I15/2 fundamental level to excited levels of Er3? ions can be observed In addition, the band around 1365 nm is J Mater Sci Fig Photoluminescence upon different excitations of %SnO2 samples doped with different erbium concentrations, annealed at 1100 °C : 0.5 mol % Er3? (a) and mol % Er3? (b) Fig Absorption spectra of 96 %SiO2–4 %SnO2 doped with % Er3?, heat treated at 600° (a) and 1000 °C (b) Inset Absorption spectra of 4, 8, and 12 % SnO2 doped with 0.5 % Er3? samples heat treated at 1000 °C attributed to the second harmonic vibrations of isolated Si– OH groups, while the band at 1400 nm is associated to hydrogen-bonded Si–OH silanol groups Finally, the band at 1900 nm is assigned to hydrogen-bonded water The appearance of these two features is due to the adsorption of residual Si–OH groups on the pore surface of the sample [21, 22] Obviously, the presence of these OH groups has detrimental effects on optical properties However, with higher heat treatment temperatures (at 1000 °C in Fig 6b), the disappearance of the band at 1365 nm reflects the more efficient removal of isolated silanols, while the downshift of the band 1400–1380 nm suggests a lengthening of the Si–OH bonds, which correlates with their progressive destruction A decrease in intensity of the band at 1900 nm correlates with the loss of water with annealing However, an increase of SnO2 concentration causes a residual OH groups in higher SnO2 percentage samples despite a heat treatment at 1000 °C as presented in the inset of Fig 6b Photoluminescence measurements In order to study the environment of the Er3? ions, infrared photoluminescence measurements were undertaken using 351 and 514 nm as excitation wavelengths These two lines correspond to the band gap of SnO2 and to the 4I15/2–2H11/2 transition of Er3? ions, respectively Figure shows emission spectra for the samples containing mol % SnO2 doped with 0.5 and mol % Er3? For systems doped with mol % Er3? (Fig 7b), upon excitation at 514 nm, one obtains an emission spectrum characteristic of Er3? ions in an amorphous medium with a broad band (full width at half maximum: FWHM equal to 33 nm) centered around 1535 nm However, excitation at 351 nm results in a completely different spectrum, in which the presence of narrow bands at 1521, 1531, 1549, and 1571 nm can be attributed to the Stark effect, a splitting of Er3? -ion energy levels caused by the SnO2 crystal field Hence, this emission results from an efficient energy transfer between SnO2 nanoparticles and the rare-earth ions Therefore, this spectrum corresponds to that of Er3? ions located within or in the close vicinity of SnO2 123 J Mater Sci Fig Decay curves of emission at 1535 nm (kex = 514 nm) for an %SnO2 sample annealed at 1100 °C and for different Er3? concentrations The solid lines represent double-exponential fits to the decay data (correlation coefficient R [0.99, for all the fittings) Decay-time measurements Fig Photoluminescence upon different excitations of %SnO2 samples doped with different erbium concentrations, annealed at 1100 °C : 0.5 mol % Er3? (a) and mol % Er3? (b) nanocrystals In fact, Er3? can substitute for Sn4? in the rutile crystal structure of SnO2, but it could also be within the crystal without substitution or even on the nanocrystal surface Consideration of these spectra suggests the existence of two types of sites for Er3? ions: those within the close vicinity of tin oxide nanocrystals and those within the amorphous silica matrix Nevertheless, the spectra of the systems doped with 0.5 mol % under two excited wavelengths are similar The observed narrow bands can be attributed to Er3? ion located in the Sn4? sites of the cassiterite structure For higher SnO2 concentrations, for example, %SnO2 (Fig 8), in both cases regardless of the excitation wavelength, the emission spectra are characteristic of Er3? ions under the influence of the crystal field of SnO2 nanocrystals Comparison of Fig 5a, b suggests that % of SnO2 is not enough to contain mol % Er3? ions Thus, low concentrations of SnO2 would appear to ease the dispersion of Er3? ions in a silica matrix On the other hand, with high SnO2 concentrations, the majority of Er3? ions are incorporated in or in the vicinity of SnO2 nanocrystals, reflecting the affinity of rare-earth ions for SnO2, and their low solubility in SiO2 123 The lifetime of the metastable level 4I13/2 was measured at 1535 nm upon 514.5 nm excitation As seen in Fig 9, the decay curves of 4I13/2-4I15/2 were not single exponential In effect, these curves can be fitted using the double-exponential function: Itị t t ẳ A1 exp ỵ A2 exp ; It ẳ 0ị sf ss where sf is the decay time of the fast component, ss is the decay time of the slow component, A1 and A2 are the amplitudes of the fast and slow components, respectively Such a behavior constitutes additional evidence for the existence of two kinds of sites for the Er3? ions [23–26] These ions can be located in SnO2 crystals or in the glassy phase The value of A1, A2 permits to roughly assign the population ratio of erbium ions between the two sites In the present work, the fast decay component of glass– ceramic monoliths is attributed to Er3? in the nanocrystals An increase in SnO2 concentration makes a reduction of Er3? ions clustering of nanocrystals as displayed in Table 1, thus leading to longer luminescence lifetimes from 0.61 to 1.17 ms The slow decay rate is thus related to Er3? ions in the glass environment As shown in Fig 6b, the residual OH groups in the higher SnO2 percentage samples (8 % and 12 mol % SnO2) are more than those of mol % sample These OH groups, which are mainly associated with the silica matrix, are known to quench the erbium luminescence at 1.5 lm This effect results a reduction of long lifetime values when the SnO2 concentration increases from % to mol % Moreover, the lengthening of the lifetime of the metastable level 4I13/2 J Mater Sci Table Lifetime s for the 4I13/2 erbium level of 4, 8, and 12 % SnO2 doped with 0.1 and 0.5 mol % Er3? samples heat treated at 1100 °C for h 0.1 % Er3? % SnO2 0.5 % Er3? 9.8 ms (41 %) 7.85 ms (35 %) 0.61 ms (59 %) 0.14 ms (65 %) % SnO2 6.9 ms (43 %) 0.93 ms (57 %) 3.57 ms (42 %) 0.49 ms (58 %) 12 % SnO2 7.84 ms (46 %) 4.88 ms (52 %) 1.17 ms (54 %) 0.78 ms (48 %) within a silica matrix makes it possible to limit their growth, even at temperatures as high as 1100 °C Photoluminescence features have shown that an increase in SnO2 concentration promotes the incorporation of Er3? ions in SnO2 nanocrystals Energy transfer has been evidenced between these nanocrystals and the rare-earth ions This transfer may serve wide-band pumping applications of lasers Nevertheless, for applications in telecommunications, a compromise between SnO2 and Er3? concentrations must be found in order to obtain a long luminescence lifetime at 1.5 lm and broad emission spectra in the infrared region The population ratio of each erbium site is given in brackets Acknowledgement The authors would like to thank P Russell (UCCS-Lille1) for his help with HTXRD measurements This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under Grant Number 103.06-2012.16 References Fig 10 Luminescence lifetime of the 4I13/2 level as a function of SnO2 concentration and for two different Er3? concentrations in samples annealed at 1100 °C between the % and 12 mol % SnO2 samples is due to the better solubility of Er3? ions in glassy matrix for the higher SnO2 concentration [8] (Table 1) The shortening of the longer lifetime with an increase of the erbium concentration, as presented in Fig 10, suggests a significant luminescence quenching Finally, even the fast component shows a lifetime increase with the SnO2 concentration This observation reflects the fact that Er3? 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