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Upconversion emission of a novel glass ceramic containing Er3+, Yb3+:Sr1−xYxF2+x nano-crystals
Author’s Accepted Manuscript Upconversion emission of a novel glass ceramic containing Er3+, Yb3+: Sr1–xYxF2+x nano-crystals M.H. Imanieh, I.R. Martín, A. Nadarajah, J.G. Lawrence, V. Lavín, J. González-Platas www.elsevier.com/locate/jlumin
PII: DOI: Reference:
S0022-2313(15)30422-1 http://dx.doi.org/10.1016/j.jlumin.2015.12.026 LUMIN13770
To appear in: Journal of Luminescence Received date: 25 August 2015 Revised date: 18 November 2015 Accepted date: 13 December 2015 Cite this article as: M.H. Imanieh, I.R. Martín, A. Nadarajah, J.G. Lawrence, V. Lavín and J. González-Platas, Upconversion emission of a novel glass ceramic containing Er3+, Yb3+: Sr1–xYxF2+x nano-crystals, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2015.12.026 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.
Upconversion emission of a novel glass ceramic containing Er3+, Yb3+: Sr1–xYxF2+x nanocrystals M. H. Imanieh1,*, I. R. Martín2, A. Nadarajah1, J. G. Lawrence1, V. Lavín2, J. González-Platas2. 1. Department of Chemical and Environmental Engineering, University of Toledo, Toledo, 43606, OH, USA. 2. Departamento de Física, MALTA Consolider Team, Instituto Universitario de Materiales y Nanotecnología (IMN), and Instituto Universitario de Estudios Avanzados en Atómica, Molecular y Fotónica (IUdEA), Universidad de La Laguna, 38200 San Cristóbal de La Laguna, Santa Cruz de Tenerife, Spain
Abstract Transparent oxyfluoride glass ceramics with a novel composition of Er3+, Yb3+: Sr1–xYxF2+x nano-crystals were processed. When excited under infrared light, an intense green upconverted luminescence was observed for the glass ceramic obtained from the precursor glass after a heat treatment of 4 hours at 750 °C. X-ray diffraction and transmission electron microscopy analysis confirmed the precipitation of Sr1–xYxF2+x nano-crystals of sizes in the range of 9-45 nm in the heat treated samples. The position of Er3+, Yb3+ ions in the sample (either amorphous or crystalline phases) were studied at four different heat treatment temperatures of 600, 650, 700 and 750 °C. The measurements were based on analyzing the lifetimes of the 4S3/2 and 4I11/2 levels of erbium and the 2F5/2 level of ytterbium. Ytterbium and erbium ions started to incorporate into the crystalline phase when the heat treatment temperature was more than 650 °C and 600 °C, respectively. The sample that was heat treated at 750 °C for four hours showed the highest green upconversion intensity among the other heat treated samples. In this sample, majority of Er3+, Yb3+ ions were in the crystalline phase as the lifetime of 4S3/2 (Er3+) and 2F 5/2 (Yb3+) levels were increased to 500 μs and 2 ms, respectively, which are longer than their values in the precursor glass. High ion solubility of Er3+ and Yb3+ in the novel Sr1–xYxF2+x nano-crystals and their local environments in the nano-structure are believed to be the reasons for this high intense green upconversion. Keywords: Upconversion, oxyfluoride glass ceramic, SrF2-YF3 nano-crystals
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1. Introduction Luminescent materials doped with rare earth ions are used for efficient frequency conversion from infrared to visible radiation. A visible light source powered by a near infrared laser is beneficial for high-capacity data storage optical devices. In addition, it is useful for solar cells, eye safe lasers, radar range finders, temperature sensors, three-dimensional displays and fiber amplifiers [1, 2]. Glass ceramics have received greater attention as they have the potential to be used in different kind of technologies. Among them are oxyfluoride glass ceramics, which are nanocomposite materials that demonstrate optical properties of fluoride single-crystals when they are doped with rare-earth ions in the glassy oxides environment. Their simple preparation technique, which includes classical melting and quenching in air followed by an adapted thermal treatment during which fluoride phases are crystallized, make them a suitable candidate in optical materials [2, 3]. Glass ceramics of this kind usually contain small crystalline phases, such as nano sized PbxCd1-xF2 crystals in the host glass, which can improve optical properties without a loss of transparency [4]. Since lead and cadmium are toxic elements and could not be used extensively due to environmental concerns, the glass ceramics containing REF3 (RE=La,Y) or MF2 (M=Ca,Sr,Ba) nano-crystals were subsequently developed [5-8]. The alkaline-earth fluorides are important optical raw materials with high solubility for both sensitizer and activator rare earth (RE) ions, which have been successfully used as crystal laser hosts. The transparent glass ceramics (TGCs) containing MF2:RE3 (M= Ca or Ba) nano-crystals with low phonon energy and large transfer coefficient between RE3+ ions have been of interest
owing to their possible applications in optical materials [9]. On the other hand, YF3 is a promising candidate desired for host materials for its high solubility of RE ion [9]. Recently, in alumina silicate oxyfluoride glass system containing YF3 nano-crystals, the optical properties of different doping systems such as Tm3+/Yb3+ [10], Tm3+ /Er3+ /Yb3+ [11], Ho3+/Yb3+/Ce3+ [12], Ho3+ and Yb3+ [13], Tm3+/Yb3+[14], Eu3+ [15], Tm3+ [16], Tm3+/Yb3+/Nd3+[17] and Nd3+/Yb3+[15] have been investigated. Alternatively, in the SrF2 system there are reports about optical properties of doping systems such as Pr3+/Yb3+ [18], Yb3+ and Tm3+ [19], Tm3+ [20], Er3+/Yb3+ [21] and Eu2+ [22]. SrF2 and YF3 have solid solutions in a binary phase diagram [23]. This leads to an idea of using both components at the same time in a alumina silicate oxyfluoride glass ceramic system taking advantage of both the constituents. Researchers have selected 0.5 mol% ErF3 and 2 mol% YbF3 as the composition for the doping system due to its efficient energy transfer from Yb3+ to Er3+ ions, and the popularity of this combination of RE3+ ions in other fluoride phases which make the comparison of upconversion efficiency easier [5, 24-26]. In this study, and for the first time to our knowledge, a novel glass ceramic in SiO2-Al2O3-SrF2-YF3 system was fabricated. In order to improve Er3+ emission, oxyfluoride glass ceramics containing SrF2 and YF3 doped with a fixed amount of Er3+ and Yb3+ (0.5 mol% of ErF3 and 2 mol% of YbF3) and heat treated at 600, 650, 700 and 750 °C for 4 h were studied. Based on the emission spectra and luminescence lifetimes, the behavior of Er3+ and Yb3+ ions after heat treatment was evaluated.
2. Experimental Reagent-grade chemicals: 42 SiO2 (Alfa Aesar 88777), 23 Al2O3 (Alfa Aesar 42571), 20 SrF2 (Alfa Aesar 10878), 10 KF (Alfa Aesar 42217), 5 YF3 (Alfa Aesar 13650), 0.5 ErF3 (Alfa Aesar 13653) and 2 YbF3 (Alfa Aesar 13651) (in mol%) were used as raw materials. The batch compositions were mixed in an agate mortar while keeping humidity of the environment less than 10%. Glass samples were prepared by melting the mixtures in covered platinum crucibles at 1400 °C in an electric kiln for 15 min. The prepared glasses were annealed at 450 °C (close to the glass transition temperatures) for 8 h. The resulting samples were cut and polished to form 10x10x2 mm3 slides. Crystallization temperatures of glasses were determined by differential thermal analysis (DTA; Polymer Laboratories 1640, Amherst, MA). Glass frits with relatively coarse particle sizes (0.30 – 0.4 mm) and a heating rate of 10 K/min were used in each DTA run. This particle size was chosen in order to minimize the contribution of the surface area to the nucleation process and to obtain DTA results that were closer to the bulk glass. The reference material in these experiments was α-Al2O3 powder. The heat treatment of glasses were carried out in an electric kiln at 4 different temperatures (600, 650, 700 and 750°C) for 4h based on DTA measurements, carried out at a heating rate of 10K/min. The crystalline phases which were precipitated during the heat treatment were identified using a Rigaku Ultima III X-ray diffractometer (XRD) equipped with a primary monochromator and Cu Kα source. The XRD patterns were collected with a step size of 0.1° in the 2θ angular range from 10° to 90° and an acquisition time of 2 h. The microstructure of the
heat-treated samples was studied using a Hitachi HD2300A Scanning Transmission Electron Microscope (STEM) at an accelerating voltage of 200 KV. The transparency of the samples was determined by a UV–VIS spectrophotometer (Chromophor, Vrio model 2600). Transmission spectra were acquired between 190 nm and 1100 nm at a resolution of 2 nm and a scan speed of 240 nm/min. A tunable passive Ti:Sapphire laser source (Spectra-Physics 3900S) pumped by a Millenia laser (Spectra-Physics model 15SJSPG) was used as an excitation source for upconversion measurements. The laser power was 100 mW and the Gaussian beam was focused in the sample with a 500 mm lens. The waist spot size on the sample (defined as the 1/e2 radius of the intensity) was shown to be 50 x 50 μm2. The upconverted light was recorded by a spectrometer (Ocean Optics HR4000; equipped with optical fiber with 200 μm in diameter. In all measurements, the spectral resolution was about 0.5 nm. The luminescence decay curves of the 4I11/2 and 4S3/2 levels were measured by exciting the sample with an optical parametric oscillator OPO (EKSPLA/NT342/3UVE) with a power of around 4 mJ during a pulse of 10 ns. The signals were recorded and averaged using a digital storage oscilloscope (Tektronix 2430A). These emissions were collected with a converging lens, focused onto a monochromator with a resolution of 0.5 nm, and then detected either with a Hamamatsu 928 photomultiplier (for the visible) or a R406 photomultiplier (for the nearinfrared) and located at the output slit of the monochromator.
3. Results and Discussion 3.1 DTA Analysis DTA thermogram of the oxyfluoride glass specimen is shown in Fig.1. There are two exothermic peaks in the temperature range of 300–1000 oC. To identify theses peaks, XRD data was collected from the sample at different stages such as before and after each peak. XRD results confirmed that the first peak is attributed to the crystallization of Sr1–xYxF2+x phase, while the second is for Feldspar Sr(Al2Si2O8) structure. 3.2 X-ray diffraction analysis XRD patterns of the samples that were heat treated at four different temperatures (600, 650, 700 and 750 ◦C) for 4 h are shown in Fig.2-a. Crystallization of Sr1-xYxF2+x phase in the heat treated precursor glasses was confirmed by the identification of several its diffraction peaks. The lattice parameters, crystal size, crystallinity (wt. %) and strain in the samples were measured by Jade 2010 software (from Materials Data Inc.) and are shown in Fig. 2. Particle size and strain can cause broadening of the diffraction peaks [27]. Using the Williamson and Hall theory, Jade 2010 software carried out the calculations of the lattice strain and crystal size for the Sr1–xYxF2+x dispersed in the glass ceramics. As the heat treatment temperature was increased from 600 to 750 ºC, the crystal sizes increased from 6 to 37 nm. In addition, the crystallinity (wt. %) increased from 4.7 to 10.6 % (see Fig. 2-b). As seen from the results, crystallinity of sample that was heat treated at 750 ºC for 4 h did not change significantly with respect to the sample that was heat treated at 600 ºC for 4 h. However, their crystal sizes were six times larger. In this system it is observed that the relatively fast growth of crystallites from a limited number of nuclei can cause an early termination of the
crystallization process due their collision. After this, there will be no further crystallization and the growth process may continue from the existing crystals. Imanieh et al.[28] reported similar a phenomena in SiO2-Al2O3-CaF2 system. In addition, Marotta et al. [29] predicts that the rate of nucleation should be very sensitive to variations in temperature. The nucleation rate increases as the temperature falls until the kinetic barrier begins to exert a controlling influence; when this happens the nucleation rate decreases with decreasing temperature. For glass crystallized at temperatures well above the temperatures of high nucleation-rates (in this case 750 ºC), the number of nuclei already present cannot appreciably change during the crystallization process. As a result, the crystals grow from a nearly-fixed number of nuclei [29]. This can be a reason for the increase in crystal size without any or significant increase in the quantity of the crystalline phases. The XRD spectra obtained for the different glass ceramics did not show any evidence of a second phase. This suggests that SrF2 and YF3 formed a complete solid solution. The phase diagram [30] of SrF2-YF3 confirms the possibility of Sr1-xYxF2+x to form a solid solution in the current sample in the range of 0-25 mol%. The inset in Fig 2-a shows the shift in the SrF2 peaks by 0.15o. Shifting of the Bragg reflections to higher angles occurred by increasing the heat treatment temperature. According to this shift, the lattice parameters were decreased. Since the ionic radii of Sr2+ is larger than that of the Y3+ ions, replacing Sr2+ with Y3+ ions led to shrinking of the lattice. According to Bragg’s law, this contraction of the lattice will result in shifting of the diffraction peaks to a higher diffraction angle. In order to determine the solid solution composition, the unit cell parameters a of the crystallites was obtained from the XRD results and are presented in Fig.2-c. As observed in Fig 2-c, the contraction of cubic SrF2 lattice with increasing heat treatment temperature indicates the
formation of Sr1-xYxF2+x with different x values (x ranging from 0.23 to 0.46). By comparing the results of Mayakova et al. [31], we have tried to determine this x value. The circle points in the figure show the calculated unit cell parameter a from the present study, and the solid line with filled squares represent the unit cell parameter reported by Mayakova et al.[31]. As it can be seen from Fig.2-c, increasing the heat treatment temperature leads to raising the x value in the samples. Therefore, the composition of the nano-crystals changed during the heat treatment process. By increasing the temperature, the solubility of two phases in each other will increase and as a result the Y/Y+Sr ratio will increase. This increase was more significant at 750 °C whereas the x value remained constant for 650 and 700 °C. The calculated strain value for the samples that were heat treated at 600, 650, 700 and 750°C are shown in Fig.2-c. It is observed that the residual tensile strain started to decrease and changed to compressive stain for the sample that was heat treated at 750 °C. Tensile strain is defined as deformation along a line segment that increases in length when a load is applied along that line, whereas compressive strain is defined as deformation along a line segment that decreases. The lattice strain can be tuned by doping or making solid solution of two different atoms. As discussed earlier, during heat treatment more Y3+ ions migrate into SrF2 phase and the ‘x’ in the Sr1-xYxF2+x composition increased while increasing temperature. Hence, the size difference of Y3+ and Sr2+ will cause a compressive strain on the lattice as Y3+ is smaller than Sr2+, which is in good agreement with the increasing x value for the Sr1-xYxF2+x solid solution. It should be noted that there could be a slight error on calculated strain and x values since incorporation of Er3+ and Yb3+ ions, both are smaller than Sr2+ ion, also can lead to same results. There are reports [27, 32] that suggest the upconversion emission intensity increasing with decreasing tensile strain as nonradiative relaxation pathway increases with increasing lattice strain. Therefore, we are
expecting the improvement of upconversion intensity by increasing the temperature as this will decrease the tensile strain in the samples. 3.3 UV-VIS absorption analysis Absorption spectra of glass and glass ceramics in the range of 250-800 nm are shown in Fig.3. Each absorption peak was assigned to the corresponding energy level of Er3+ according to literature. The absorption spectra in the 250-800 nm range consisting of 10 bands were observed at 356, 364.5, 378, 405.5, 449.5, 487, 521, 540 and 651.5, nm corresponding to transitions form the 4I15/2 ground state to the excited levels 4G7/2, 4G9/2, 4G11/2, 4H9/2, 4F3/2, 4F5/2, 4F7/2, 4H11/2, 4S3/2, 4
F9/2, respectively. All the samples had absorbance less than 1, which indicates their high
transparency in the visible region. To have a better look regarding the color as well as the transparency (blur and haziest) of the samples a photograph of the fabricated samples is shown in the inset of Fig.3. 3.4 Transmission electron microscope analysis A bright field (TE) and Z contrast (ZC) transmission electron microscope (TEM) images of the glass
ceramics
heat-treated
at
750
°
C
for
4
h
are
shown
in
Fig.
4a
and
Fig. 4b, respectively. The two phases of the glass ceramic can be seen in the bright field image; the crystallites phase appears as black spots due to diffraction and the grey background corresponds to the glassy matrix. Several spherical Sr1-xYxF2+x (corresponding energy dispersive spectroscopy (EDS) analysis is shown in Fig.5) with average crystallite sizes of 50±10 nm are seen to be well distributed inside the glassy matrix. The EDS elemental mapping images in Fig. 5 show that a phase with high concentration of Sr, Y, F, Er and Yb ions has been precipitated inside the glass matrix. This confirms the XRD results and validates the existence of Sr1-xYxF2+x
solid solution. In Fig. 4c, the TEM image of the Sr1-xYxF2+x nanoparticles shows good crystallinity. This type of crystallinity was detected in almost all of the nano-crystalline particles inside the glass matrix. Similar results have been reported previously in SrF2 oxyfluoride glass ceramics [19, 33, 34]. 3.5 Upconversion The upconversion luminescence of the glass ceramics that were heat treated at 600, 650, 700 and 750 oC for 4 h are shown in Fig. 6. All the samples were excited at 975 nm. The emission bands assigned to Er3+ ions are 2H11/2→4I15/2 (520 nm), 4S3/2→4I15/2 (545 nm) and 4F9/2→4I15/2 (660 nm) transitions. From the data it is observed that the intensity of the upconverted luminescence increases with increase in heat treatment temperature. According to the literature, upconverted luminescence of rare earth ions is affected by multiphonon relaxation. The relationship between multiphonon decay rate and phonon energy is given by Miyakawa–Dexter theory [35]. According to this theory, the multiphonon decay rate Wp, is expressed by Wp=W0 e(-αΔE/ћω) where ΔE is the energy gap to the next lower level, α is defined as α=ћω-1[ln(p/g)-1] where p is the phonon number, g is the electron phonon coupling strength and ћω is the energy of the maximum lattice vibration of the surrounding host lattice. As seen in the equation, the multiphonon decay rate depends exponentially on E/ћω. In addition, the upconverted emission bands with well resolved structure (straight lines) in heattreated samples validate the incorporation of Er3+ ions into the nano-crystals. The approximate frequency of the highest energy lattice vibration in silicate oxide glass is 1100 cm −1 and this value decreased to 383 cm−1 for SrF2 and 450 cm−1 for YF3 crystal. Hence, increasing the heat treatment temperature leads to incorporation of Er3+ ions into Sr1-xYxF2+x crystalline phase.
Consequently, the upconversion intensity significantly increased due to the decrease of the multiphonon relaxation with the increase in heat treatment temperature. The mechanisms behind the green upconversion emission of Er3+ doped systems is very well known and has been investigated in several manuscripts’ [5, 11, 21, 25, 36, 37]. One of the most important aspects in the upconversion mechanism is the lifetimes of the involved levels. Increasing the lifetime of the particular levels will result in a high efficient upconversion process. Therefore, in the following sections the lifetimes of two important levels in the upconversion mechanism are analyzed. 3.6 Time-resolved luminescence The luminescence decay curves of Er3+ in the samples around 842 nm, which correspond to the 4
S3/2→4I13/2 transition are shown in Fig. 7. These decays were obtained under pulse excitation of
the 4S3/2 level at 550 nm. Imanieh et al. [5, 26] determined the lifetime of 4S3/2 level by monitoring the 840 nm luminescence, rather than using the more common 540 nm luminescence. They have shown the luminescence decay curves at 840 or 540 nm produced the same value for the lifetime, whereas excitation near the green and detection at 840 nm is more accurate. Therefore, the diffuse light will be eliminated and results will be more consistent. As it can be seen from Fig. 7, the lifetime of the Er3+ ions increase with increasing the heat treatment temperature. One possible reason for this is that, after crystallization of Sr1-xYxF2+x in the glass, the majority of Er3+ ions incorporate in the nano-crystals. As it was mentioned before, the multiphonon decay rate decreased in the ions inside the Sr1-xYxF2+x nano-crystals. As a result, the Er3+ ions inside the nano-crystal have longer lifetime. Increasing the temperature led to the increase in diffusion rate of ions in the glassy matrix. According to XRD results the composition
of the precipitates are similar when heating at 650 and 700 °C, which indicates that the phonon energy of the crystals are also similar. This can be a possible reason for the lifetime similarity of Er3+ ions at 650 and 700 °C. Further increasing the heat treatment temperature increases the crystal size and the Y3+ concentration in the SrF2 crystals, which allows more Er3+ or Yb3+ ions to incorporate inside the solid solution phase. According to the XRD and lattice data, there are three different compositions obtained when the temperature increased from 600 to 750 °C. Since increasing temperature changes the Sr1-xYxF2+x composition, the solubility of Er3+ and Yb3 ions also increases. The SrF2 structure consists of a cubic network comprising of cations M2+ with a coordinance coordination of 8 and twice as many F- anions with a coordination of 4. The incorporation of a trivalent doping Er3+ and Yb3+ ions require the substitution of one of the M2+ cations. However, the system is left with an excess of positive charge. In order to compensate it, several mechanisms have been proposed, such as the creation of cation vacancies, or the presence of an interstitial F- fluoride anion. This may decrease the solubility of RE3+ ions inside the nanocrystalline phase and greatly affect the luminescence of the material. However, Sr1-xYxF2+x could be promising materials that contain Sr heavy metal with high electronic density, and Y as site available for RE doping. In this way, the incorporation of a trivalent doping ion did not leave the system with an excess of positive charge and the solubility of trivalent doping ions will be high. This could also explain why β-NaYF4 host material is the most efficient upconversion host that has been reported to date [38]. Comparing the lifetime of Er3+ ions with other systems with similar concentrations of these ions, such as CaF2 [5] and Ca1−xLaxF2+x [26], it can be concluded that the lifetime of Er3+ ions in the present system is longer than the others. There could be two reasons to explain this phenomenon.
Either the solubility of Er3+ and Yb3+ ions are higher than the other systems or the phonon energy of Sr1-xYxF2+x nano-structure is lower than the others systems. In order to identify the location of Yb3+ ions in the crystal and amorphous phases, the infrared luminescence decay curves of the samples were measured at around 975 nm. Fig. 8 shows the decays under pulse excitation of the 2F5/2 level of Yb3+ ions. At 975 nm the Er3+ ions have an excited level 4I11/2 which coincides with the 2F5/2 level of Yb3+ ions. If transfer and back-transfer processes between these ions are important it should be reflected in the experimental decay curves. As it can be seen from Fig.8 the decay curves of the samples that were heat-treated at 650, 700 and 750 °C for 4 h are different from the precursor glass. These processes were analyzed based on the fluorescence transfer function mode in fluoroindate glasses by Martin et al. [39]. This result indicates that the majority of Yb3+ ions transfer to crystal phase during the heat treatment process. The lifetime of the Er3+ ion excited to the level 4I11/2 (about 975 nm) is very long with a value of more than 300 µs. Therefore the decay time in co-doped samples was increased due to transfer and back-transfer processes between Er3+ and Yb3+ ions inside the fluoride phase and also due to the decrease of the energy phonon in this phase. In order to compare the green upconversion intensity of the most efficient sample (according to Fig. 6) with results presented in other published report [40], the intensity of green emission (540 nm) were measured using a passively tunable Ti:Sapphire laser at 973 nm as excitation source. Two different approaches could be taken: the first one is comparing the efficient sample (sample that was heat-treated at 750 ◦C for 4 h) with previous reported individual systems containing SrF2 or YF3, and the second approach is comparing with the heavy-metal fluoride glass[40] sample which had the same concentration of ErF3 and YbF3 in its composition. The first approach was not taken into account because the glass composition and optimum heat treatment condition
couldn’t match the present glass ceramic. This makes it very difficult to find the right match or composition for comparison. In the second approach, the heavy-metal fluoride glasses (fluoroindate glass) were chosen for comparison, since they are the most important fluoride materials used as optical components due to their premier upconversion efficiency. In addition, their phonon energy is lower than oxyfluoride glass ceramics [40]. According to the above explanation we have chosen the second approach to show the greater efficiency of our composition. The intensity in the sample that was heat-treated at 750 ◦C for 4 h was around 2.8 times higher than for a fluoroindate glass. High ion solubility of Er3+ and Yb3+ in the novel Sr1–xYxF2+x nano-crystals and their nano-crystalline structure are believed to be the primary reasons for this high intense green upconversion. In addition, strain analysis proved that increasing the temperature will decrease the tensile strain significantly that can also decrease non-radiative relaxation pathway. This also explains the significant increase in the overall luminescence intensity in this system. A photograph showing the upconverted green light is provided in the inset of Fig. 6. Despite the fact the laser power was only 15 mW and the photograph was taken under conventional fluorescent lamp an intense green light was recorded. 4. Conclusions A novel transparent oxyfluoride glass ceramic containing Er3+, Yb3+: Sr1-xYxF2+x.nano-crystals were prepared. Upconversion luminescence behavior of Yb3+ and Er3+ during heat treatment was investigated. The results showed that increasing the heat treatment temperature changed the solid solution composition of Sr1-xYxF2+x, x ranging from 0.23 to 0.46. Ytterbium and erbium ions started to incorporate into the crystal phase when the heat treatment temperature was more than 650 and 600°C respectively. The majority of Er3+ and Yb3+ ions were incorporated in the Sr1-
xYxF2+x phase
in the samples that were heat-treated at 750 oC for 4 h. As a result this composition
had the highest upconversion intensity among the other glass ceramics. Comparing the lifetimes of the 4S3/2 and 4I11/2 of erbium and 2F 5/2 of ytterbium levels of the synthesized sample with other reported sample, which had the same concentration of Er3+ and Yb3+ ions, it is observed that the solubility of these ions in the current system is high. This observation could explain the efficient upconversion process obtained from this composition.
Acknowledgments The authors thank the Ministerio de Economía y Competitividad of Spain (MINECO) within The National Program of Materials (MAT2013-46649-C4-4-P), and the Consolider-Ingenio 2010 Program (MALTA CSD2007-0045, www.malta-consolider.com) and the EU-FEDER for their financial support.
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FIGURES Fig. 1. DTA thermograph of the precursor glass at a heating rate of 10 K/min.
Fig. 2. (a) XRD patterns of glass and glass ceramics heat-treated at 600, 650, 700 and 750 °C for 4 h. The inset focuses on a subset of the data featuring the shift of (111) and (200) reflections. (b) Size and weight percent of Sr1-xYxF2+x crystallites in glass ceramics at different temperatures. (c) Contraction of cubic Sr1-xYxF2+x lattice with increasing heat treatment temperature. The dashed line with filled squares is from Mayakova et al. [31] (CS=crystal size). Strain data (ε) at different heat treating conditions are marked close to each point; errors in the Williamson–Hall analysis were calculated by using a method of least squares and were equal to 0.05 for all data points.
Fig. 3. Absorption spectra of glass and glass ceramics heat-treated at 600, 650, 700 and 750 °C for 4 h. Inset shows the photographs of samples for spectral experiments.
Fig. 4. TEM images: (a) bright field image; (b) Z contrast image; and (c) high magnification image of the sample heat-treated at 750 ºC for 4 h.
Fig. 5. EDS elemental mapping of the glass heat-treated at 750 ºC for 4 h: (a) TEM image; (b) strontium map; (c) yttrium map; (d) flourine map; (e) erbium map; and (f) ytterbium map. Fig. 6. Upconverted luminescence obtained by exciting at 973 nm the Yb3+ ions in the glass ceramics which were heat-treated at 600, 650, 700 and 750 °C for 4 h. (Inset shows the
luminescence green light from the sample treated at 750 ºC excited with 973 nm wavelength laser with 15 mW power. The photograph was taken under conventional fluorescent lamp.
Fig. 7. Luminescence decay curves of Er3+ ions monitoring the emission at around 842 nm (corresponding to the 4S3/2→4I11/2 emission) obtained in glasses ceramics heat-treated at 600, 650, 700 and 750 °C for 4 h.
Fig. 8. Infrared Luminescence decay curves of Yb3+ ions monitoring the emission at around 975 nm (corresponding to the 2F5/2→2F7/2 emission) in glass ceramics heat-treated at 600, 650, 700 and 750 °C for 4h.
PII: DOI: Reference:
S0022-2313(15)30422-1 http://dx.doi.org/10.1016/j.jlumin.2015.12.026 LUMIN13770
To appear in: Journal of Luminescence Received date: 25 August 2015 Revised date: 18 November 2015 Accepted date: 13 December 2015 Cite this article as: M.H. Imanieh, I.R. Martín, A. Nadarajah, J.G. Lawrence, V. Lavín and J. González-Platas, Upconversion emission of a novel glass ceramic containing Er3+, Yb3+: Sr1–xYxF2+x nano-crystals, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2015.12.026 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.
Upconversion emission of a novel glass ceramic containing Er3+, Yb3+: Sr1–xYxF2+x nanocrystals M. H. Imanieh1,*, I. R. Martín2, A. Nadarajah1, J. G. Lawrence1, V. Lavín2, J. González-Platas2. 1. Department of Chemical and Environmental Engineering, University of Toledo, Toledo, 43606, OH, USA. 2. Departamento de Física, MALTA Consolider Team, Instituto Universitario de Materiales y Nanotecnología (IMN), and Instituto Universitario de Estudios Avanzados en Atómica, Molecular y Fotónica (IUdEA), Universidad de La Laguna, 38200 San Cristóbal de La Laguna, Santa Cruz de Tenerife, Spain
Abstract Transparent oxyfluoride glass ceramics with a novel composition of Er3+, Yb3+: Sr1–xYxF2+x nano-crystals were processed. When excited under infrared light, an intense green upconverted luminescence was observed for the glass ceramic obtained from the precursor glass after a heat treatment of 4 hours at 750 °C. X-ray diffraction and transmission electron microscopy analysis confirmed the precipitation of Sr1–xYxF2+x nano-crystals of sizes in the range of 9-45 nm in the heat treated samples. The position of Er3+, Yb3+ ions in the sample (either amorphous or crystalline phases) were studied at four different heat treatment temperatures of 600, 650, 700 and 750 °C. The measurements were based on analyzing the lifetimes of the 4S3/2 and 4I11/2 levels of erbium and the 2F5/2 level of ytterbium. Ytterbium and erbium ions started to incorporate into the crystalline phase when the heat treatment temperature was more than 650 °C and 600 °C, respectively. The sample that was heat treated at 750 °C for four hours showed the highest green upconversion intensity among the other heat treated samples. In this sample, majority of Er3+, Yb3+ ions were in the crystalline phase as the lifetime of 4S3/2 (Er3+) and 2F 5/2 (Yb3+) levels were increased to 500 μs and 2 ms, respectively, which are longer than their values in the precursor glass. High ion solubility of Er3+ and Yb3+ in the novel Sr1–xYxF2+x nano-crystals and their local environments in the nano-structure are believed to be the reasons for this high intense green upconversion. Keywords: Upconversion, oxyfluoride glass ceramic, SrF2-YF3 nano-crystals
1 *
Corresponding author. Tel.: (+1) 419.530.8030 Fax: (+1) 419.530.8076
Email address: [email protected].
1. Introduction Luminescent materials doped with rare earth ions are used for efficient frequency conversion from infrared to visible radiation. A visible light source powered by a near infrared laser is beneficial for high-capacity data storage optical devices. In addition, it is useful for solar cells, eye safe lasers, radar range finders, temperature sensors, three-dimensional displays and fiber amplifiers [1, 2]. Glass ceramics have received greater attention as they have the potential to be used in different kind of technologies. Among them are oxyfluoride glass ceramics, which are nanocomposite materials that demonstrate optical properties of fluoride single-crystals when they are doped with rare-earth ions in the glassy oxides environment. Their simple preparation technique, which includes classical melting and quenching in air followed by an adapted thermal treatment during which fluoride phases are crystallized, make them a suitable candidate in optical materials [2, 3]. Glass ceramics of this kind usually contain small crystalline phases, such as nano sized PbxCd1-xF2 crystals in the host glass, which can improve optical properties without a loss of transparency [4]. Since lead and cadmium are toxic elements and could not be used extensively due to environmental concerns, the glass ceramics containing REF3 (RE=La,Y) or MF2 (M=Ca,Sr,Ba) nano-crystals were subsequently developed [5-8]. The alkaline-earth fluorides are important optical raw materials with high solubility for both sensitizer and activator rare earth (RE) ions, which have been successfully used as crystal laser hosts. The transparent glass ceramics (TGCs) containing MF2:RE3 (M= Ca or Ba) nano-crystals with low phonon energy and large transfer coefficient between RE3+ ions have been of interest
owing to their possible applications in optical materials [9]. On the other hand, YF3 is a promising candidate desired for host materials for its high solubility of RE ion [9]. Recently, in alumina silicate oxyfluoride glass system containing YF3 nano-crystals, the optical properties of different doping systems such as Tm3+/Yb3+ [10], Tm3+ /Er3+ /Yb3+ [11], Ho3+/Yb3+/Ce3+ [12], Ho3+ and Yb3+ [13], Tm3+/Yb3+[14], Eu3+ [15], Tm3+ [16], Tm3+/Yb3+/Nd3+[17] and Nd3+/Yb3+[15] have been investigated. Alternatively, in the SrF2 system there are reports about optical properties of doping systems such as Pr3+/Yb3+ [18], Yb3+ and Tm3+ [19], Tm3+ [20], Er3+/Yb3+ [21] and Eu2+ [22]. SrF2 and YF3 have solid solutions in a binary phase diagram [23]. This leads to an idea of using both components at the same time in a alumina silicate oxyfluoride glass ceramic system taking advantage of both the constituents. Researchers have selected 0.5 mol% ErF3 and 2 mol% YbF3 as the composition for the doping system due to its efficient energy transfer from Yb3+ to Er3+ ions, and the popularity of this combination of RE3+ ions in other fluoride phases which make the comparison of upconversion efficiency easier [5, 24-26]. In this study, and for the first time to our knowledge, a novel glass ceramic in SiO2-Al2O3-SrF2-YF3 system was fabricated. In order to improve Er3+ emission, oxyfluoride glass ceramics containing SrF2 and YF3 doped with a fixed amount of Er3+ and Yb3+ (0.5 mol% of ErF3 and 2 mol% of YbF3) and heat treated at 600, 650, 700 and 750 °C for 4 h were studied. Based on the emission spectra and luminescence lifetimes, the behavior of Er3+ and Yb3+ ions after heat treatment was evaluated.
2. Experimental Reagent-grade chemicals: 42 SiO2 (Alfa Aesar 88777), 23 Al2O3 (Alfa Aesar 42571), 20 SrF2 (Alfa Aesar 10878), 10 KF (Alfa Aesar 42217), 5 YF3 (Alfa Aesar 13650), 0.5 ErF3 (Alfa Aesar 13653) and 2 YbF3 (Alfa Aesar 13651) (in mol%) were used as raw materials. The batch compositions were mixed in an agate mortar while keeping humidity of the environment less than 10%. Glass samples were prepared by melting the mixtures in covered platinum crucibles at 1400 °C in an electric kiln for 15 min. The prepared glasses were annealed at 450 °C (close to the glass transition temperatures) for 8 h. The resulting samples were cut and polished to form 10x10x2 mm3 slides. Crystallization temperatures of glasses were determined by differential thermal analysis (DTA; Polymer Laboratories 1640, Amherst, MA). Glass frits with relatively coarse particle sizes (0.30 – 0.4 mm) and a heating rate of 10 K/min were used in each DTA run. This particle size was chosen in order to minimize the contribution of the surface area to the nucleation process and to obtain DTA results that were closer to the bulk glass. The reference material in these experiments was α-Al2O3 powder. The heat treatment of glasses were carried out in an electric kiln at 4 different temperatures (600, 650, 700 and 750°C) for 4h based on DTA measurements, carried out at a heating rate of 10K/min. The crystalline phases which were precipitated during the heat treatment were identified using a Rigaku Ultima III X-ray diffractometer (XRD) equipped with a primary monochromator and Cu Kα source. The XRD patterns were collected with a step size of 0.1° in the 2θ angular range from 10° to 90° and an acquisition time of 2 h. The microstructure of the
heat-treated samples was studied using a Hitachi HD2300A Scanning Transmission Electron Microscope (STEM) at an accelerating voltage of 200 KV. The transparency of the samples was determined by a UV–VIS spectrophotometer (Chromophor, Vrio model 2600). Transmission spectra were acquired between 190 nm and 1100 nm at a resolution of 2 nm and a scan speed of 240 nm/min. A tunable passive Ti:Sapphire laser source (Spectra-Physics 3900S) pumped by a Millenia laser (Spectra-Physics model 15SJSPG) was used as an excitation source for upconversion measurements. The laser power was 100 mW and the Gaussian beam was focused in the sample with a 500 mm lens. The waist spot size on the sample (defined as the 1/e2 radius of the intensity) was shown to be 50 x 50 μm2. The upconverted light was recorded by a spectrometer (Ocean Optics HR4000; equipped with optical fiber with 200 μm in diameter. In all measurements, the spectral resolution was about 0.5 nm. The luminescence decay curves of the 4I11/2 and 4S3/2 levels were measured by exciting the sample with an optical parametric oscillator OPO (EKSPLA/NT342/3UVE) with a power of around 4 mJ during a pulse of 10 ns. The signals were recorded and averaged using a digital storage oscilloscope (Tektronix 2430A). These emissions were collected with a converging lens, focused onto a monochromator with a resolution of 0.5 nm, and then detected either with a Hamamatsu 928 photomultiplier (for the visible) or a R406 photomultiplier (for the nearinfrared) and located at the output slit of the monochromator.
3. Results and Discussion 3.1 DTA Analysis DTA thermogram of the oxyfluoride glass specimen is shown in Fig.1. There are two exothermic peaks in the temperature range of 300–1000 oC. To identify theses peaks, XRD data was collected from the sample at different stages such as before and after each peak. XRD results confirmed that the first peak is attributed to the crystallization of Sr1–xYxF2+x phase, while the second is for Feldspar Sr(Al2Si2O8) structure. 3.2 X-ray diffraction analysis XRD patterns of the samples that were heat treated at four different temperatures (600, 650, 700 and 750 ◦C) for 4 h are shown in Fig.2-a. Crystallization of Sr1-xYxF2+x phase in the heat treated precursor glasses was confirmed by the identification of several its diffraction peaks. The lattice parameters, crystal size, crystallinity (wt. %) and strain in the samples were measured by Jade 2010 software (from Materials Data Inc.) and are shown in Fig. 2. Particle size and strain can cause broadening of the diffraction peaks [27]. Using the Williamson and Hall theory, Jade 2010 software carried out the calculations of the lattice strain and crystal size for the Sr1–xYxF2+x dispersed in the glass ceramics. As the heat treatment temperature was increased from 600 to 750 ºC, the crystal sizes increased from 6 to 37 nm. In addition, the crystallinity (wt. %) increased from 4.7 to 10.6 % (see Fig. 2-b). As seen from the results, crystallinity of sample that was heat treated at 750 ºC for 4 h did not change significantly with respect to the sample that was heat treated at 600 ºC for 4 h. However, their crystal sizes were six times larger. In this system it is observed that the relatively fast growth of crystallites from a limited number of nuclei can cause an early termination of the
crystallization process due their collision. After this, there will be no further crystallization and the growth process may continue from the existing crystals. Imanieh et al.[28] reported similar a phenomena in SiO2-Al2O3-CaF2 system. In addition, Marotta et al. [29] predicts that the rate of nucleation should be very sensitive to variations in temperature. The nucleation rate increases as the temperature falls until the kinetic barrier begins to exert a controlling influence; when this happens the nucleation rate decreases with decreasing temperature. For glass crystallized at temperatures well above the temperatures of high nucleation-rates (in this case 750 ºC), the number of nuclei already present cannot appreciably change during the crystallization process. As a result, the crystals grow from a nearly-fixed number of nuclei [29]. This can be a reason for the increase in crystal size without any or significant increase in the quantity of the crystalline phases. The XRD spectra obtained for the different glass ceramics did not show any evidence of a second phase. This suggests that SrF2 and YF3 formed a complete solid solution. The phase diagram [30] of SrF2-YF3 confirms the possibility of Sr1-xYxF2+x to form a solid solution in the current sample in the range of 0-25 mol%. The inset in Fig 2-a shows the shift in the SrF2 peaks by 0.15o. Shifting of the Bragg reflections to higher angles occurred by increasing the heat treatment temperature. According to this shift, the lattice parameters were decreased. Since the ionic radii of Sr2+ is larger than that of the Y3+ ions, replacing Sr2+ with Y3+ ions led to shrinking of the lattice. According to Bragg’s law, this contraction of the lattice will result in shifting of the diffraction peaks to a higher diffraction angle. In order to determine the solid solution composition, the unit cell parameters a of the crystallites was obtained from the XRD results and are presented in Fig.2-c. As observed in Fig 2-c, the contraction of cubic SrF2 lattice with increasing heat treatment temperature indicates the
formation of Sr1-xYxF2+x with different x values (x ranging from 0.23 to 0.46). By comparing the results of Mayakova et al. [31], we have tried to determine this x value. The circle points in the figure show the calculated unit cell parameter a from the present study, and the solid line with filled squares represent the unit cell parameter reported by Mayakova et al.[31]. As it can be seen from Fig.2-c, increasing the heat treatment temperature leads to raising the x value in the samples. Therefore, the composition of the nano-crystals changed during the heat treatment process. By increasing the temperature, the solubility of two phases in each other will increase and as a result the Y/Y+Sr ratio will increase. This increase was more significant at 750 °C whereas the x value remained constant for 650 and 700 °C. The calculated strain value for the samples that were heat treated at 600, 650, 700 and 750°C are shown in Fig.2-c. It is observed that the residual tensile strain started to decrease and changed to compressive stain for the sample that was heat treated at 750 °C. Tensile strain is defined as deformation along a line segment that increases in length when a load is applied along that line, whereas compressive strain is defined as deformation along a line segment that decreases. The lattice strain can be tuned by doping or making solid solution of two different atoms. As discussed earlier, during heat treatment more Y3+ ions migrate into SrF2 phase and the ‘x’ in the Sr1-xYxF2+x composition increased while increasing temperature. Hence, the size difference of Y3+ and Sr2+ will cause a compressive strain on the lattice as Y3+ is smaller than Sr2+, which is in good agreement with the increasing x value for the Sr1-xYxF2+x solid solution. It should be noted that there could be a slight error on calculated strain and x values since incorporation of Er3+ and Yb3+ ions, both are smaller than Sr2+ ion, also can lead to same results. There are reports [27, 32] that suggest the upconversion emission intensity increasing with decreasing tensile strain as nonradiative relaxation pathway increases with increasing lattice strain. Therefore, we are
expecting the improvement of upconversion intensity by increasing the temperature as this will decrease the tensile strain in the samples. 3.3 UV-VIS absorption analysis Absorption spectra of glass and glass ceramics in the range of 250-800 nm are shown in Fig.3. Each absorption peak was assigned to the corresponding energy level of Er3+ according to literature. The absorption spectra in the 250-800 nm range consisting of 10 bands were observed at 356, 364.5, 378, 405.5, 449.5, 487, 521, 540 and 651.5, nm corresponding to transitions form the 4I15/2 ground state to the excited levels 4G7/2, 4G9/2, 4G11/2, 4H9/2, 4F3/2, 4F5/2, 4F7/2, 4H11/2, 4S3/2, 4
F9/2, respectively. All the samples had absorbance less than 1, which indicates their high
transparency in the visible region. To have a better look regarding the color as well as the transparency (blur and haziest) of the samples a photograph of the fabricated samples is shown in the inset of Fig.3. 3.4 Transmission electron microscope analysis A bright field (TE) and Z contrast (ZC) transmission electron microscope (TEM) images of the glass
ceramics
heat-treated
at
750
°
C
for
4
h
are
shown
in
Fig.
4a
and
Fig. 4b, respectively. The two phases of the glass ceramic can be seen in the bright field image; the crystallites phase appears as black spots due to diffraction and the grey background corresponds to the glassy matrix. Several spherical Sr1-xYxF2+x (corresponding energy dispersive spectroscopy (EDS) analysis is shown in Fig.5) with average crystallite sizes of 50±10 nm are seen to be well distributed inside the glassy matrix. The EDS elemental mapping images in Fig. 5 show that a phase with high concentration of Sr, Y, F, Er and Yb ions has been precipitated inside the glass matrix. This confirms the XRD results and validates the existence of Sr1-xYxF2+x
solid solution. In Fig. 4c, the TEM image of the Sr1-xYxF2+x nanoparticles shows good crystallinity. This type of crystallinity was detected in almost all of the nano-crystalline particles inside the glass matrix. Similar results have been reported previously in SrF2 oxyfluoride glass ceramics [19, 33, 34]. 3.5 Upconversion The upconversion luminescence of the glass ceramics that were heat treated at 600, 650, 700 and 750 oC for 4 h are shown in Fig. 6. All the samples were excited at 975 nm. The emission bands assigned to Er3+ ions are 2H11/2→4I15/2 (520 nm), 4S3/2→4I15/2 (545 nm) and 4F9/2→4I15/2 (660 nm) transitions. From the data it is observed that the intensity of the upconverted luminescence increases with increase in heat treatment temperature. According to the literature, upconverted luminescence of rare earth ions is affected by multiphonon relaxation. The relationship between multiphonon decay rate and phonon energy is given by Miyakawa–Dexter theory [35]. According to this theory, the multiphonon decay rate Wp, is expressed by Wp=W0 e(-αΔE/ћω) where ΔE is the energy gap to the next lower level, α is defined as α=ћω-1[ln(p/g)-1] where p is the phonon number, g is the electron phonon coupling strength and ћω is the energy of the maximum lattice vibration of the surrounding host lattice. As seen in the equation, the multiphonon decay rate depends exponentially on E/ћω. In addition, the upconverted emission bands with well resolved structure (straight lines) in heattreated samples validate the incorporation of Er3+ ions into the nano-crystals. The approximate frequency of the highest energy lattice vibration in silicate oxide glass is 1100 cm −1 and this value decreased to 383 cm−1 for SrF2 and 450 cm−1 for YF3 crystal. Hence, increasing the heat treatment temperature leads to incorporation of Er3+ ions into Sr1-xYxF2+x crystalline phase.
Consequently, the upconversion intensity significantly increased due to the decrease of the multiphonon relaxation with the increase in heat treatment temperature. The mechanisms behind the green upconversion emission of Er3+ doped systems is very well known and has been investigated in several manuscripts’ [5, 11, 21, 25, 36, 37]. One of the most important aspects in the upconversion mechanism is the lifetimes of the involved levels. Increasing the lifetime of the particular levels will result in a high efficient upconversion process. Therefore, in the following sections the lifetimes of two important levels in the upconversion mechanism are analyzed. 3.6 Time-resolved luminescence The luminescence decay curves of Er3+ in the samples around 842 nm, which correspond to the 4
S3/2→4I13/2 transition are shown in Fig. 7. These decays were obtained under pulse excitation of
the 4S3/2 level at 550 nm. Imanieh et al. [5, 26] determined the lifetime of 4S3/2 level by monitoring the 840 nm luminescence, rather than using the more common 540 nm luminescence. They have shown the luminescence decay curves at 840 or 540 nm produced the same value for the lifetime, whereas excitation near the green and detection at 840 nm is more accurate. Therefore, the diffuse light will be eliminated and results will be more consistent. As it can be seen from Fig. 7, the lifetime of the Er3+ ions increase with increasing the heat treatment temperature. One possible reason for this is that, after crystallization of Sr1-xYxF2+x in the glass, the majority of Er3+ ions incorporate in the nano-crystals. As it was mentioned before, the multiphonon decay rate decreased in the ions inside the Sr1-xYxF2+x nano-crystals. As a result, the Er3+ ions inside the nano-crystal have longer lifetime. Increasing the temperature led to the increase in diffusion rate of ions in the glassy matrix. According to XRD results the composition
of the precipitates are similar when heating at 650 and 700 °C, which indicates that the phonon energy of the crystals are also similar. This can be a possible reason for the lifetime similarity of Er3+ ions at 650 and 700 °C. Further increasing the heat treatment temperature increases the crystal size and the Y3+ concentration in the SrF2 crystals, which allows more Er3+ or Yb3+ ions to incorporate inside the solid solution phase. According to the XRD and lattice data, there are three different compositions obtained when the temperature increased from 600 to 750 °C. Since increasing temperature changes the Sr1-xYxF2+x composition, the solubility of Er3+ and Yb3 ions also increases. The SrF2 structure consists of a cubic network comprising of cations M2+ with a coordinance coordination of 8 and twice as many F- anions with a coordination of 4. The incorporation of a trivalent doping Er3+ and Yb3+ ions require the substitution of one of the M2+ cations. However, the system is left with an excess of positive charge. In order to compensate it, several mechanisms have been proposed, such as the creation of cation vacancies, or the presence of an interstitial F- fluoride anion. This may decrease the solubility of RE3+ ions inside the nanocrystalline phase and greatly affect the luminescence of the material. However, Sr1-xYxF2+x could be promising materials that contain Sr heavy metal with high electronic density, and Y as site available for RE doping. In this way, the incorporation of a trivalent doping ion did not leave the system with an excess of positive charge and the solubility of trivalent doping ions will be high. This could also explain why β-NaYF4 host material is the most efficient upconversion host that has been reported to date [38]. Comparing the lifetime of Er3+ ions with other systems with similar concentrations of these ions, such as CaF2 [5] and Ca1−xLaxF2+x [26], it can be concluded that the lifetime of Er3+ ions in the present system is longer than the others. There could be two reasons to explain this phenomenon.
Either the solubility of Er3+ and Yb3+ ions are higher than the other systems or the phonon energy of Sr1-xYxF2+x nano-structure is lower than the others systems. In order to identify the location of Yb3+ ions in the crystal and amorphous phases, the infrared luminescence decay curves of the samples were measured at around 975 nm. Fig. 8 shows the decays under pulse excitation of the 2F5/2 level of Yb3+ ions. At 975 nm the Er3+ ions have an excited level 4I11/2 which coincides with the 2F5/2 level of Yb3+ ions. If transfer and back-transfer processes between these ions are important it should be reflected in the experimental decay curves. As it can be seen from Fig.8 the decay curves of the samples that were heat-treated at 650, 700 and 750 °C for 4 h are different from the precursor glass. These processes were analyzed based on the fluorescence transfer function mode in fluoroindate glasses by Martin et al. [39]. This result indicates that the majority of Yb3+ ions transfer to crystal phase during the heat treatment process. The lifetime of the Er3+ ion excited to the level 4I11/2 (about 975 nm) is very long with a value of more than 300 µs. Therefore the decay time in co-doped samples was increased due to transfer and back-transfer processes between Er3+ and Yb3+ ions inside the fluoride phase and also due to the decrease of the energy phonon in this phase. In order to compare the green upconversion intensity of the most efficient sample (according to Fig. 6) with results presented in other published report [40], the intensity of green emission (540 nm) were measured using a passively tunable Ti:Sapphire laser at 973 nm as excitation source. Two different approaches could be taken: the first one is comparing the efficient sample (sample that was heat-treated at 750 ◦C for 4 h) with previous reported individual systems containing SrF2 or YF3, and the second approach is comparing with the heavy-metal fluoride glass[40] sample which had the same concentration of ErF3 and YbF3 in its composition. The first approach was not taken into account because the glass composition and optimum heat treatment condition
couldn’t match the present glass ceramic. This makes it very difficult to find the right match or composition for comparison. In the second approach, the heavy-metal fluoride glasses (fluoroindate glass) were chosen for comparison, since they are the most important fluoride materials used as optical components due to their premier upconversion efficiency. In addition, their phonon energy is lower than oxyfluoride glass ceramics [40]. According to the above explanation we have chosen the second approach to show the greater efficiency of our composition. The intensity in the sample that was heat-treated at 750 ◦C for 4 h was around 2.8 times higher than for a fluoroindate glass. High ion solubility of Er3+ and Yb3+ in the novel Sr1–xYxF2+x nano-crystals and their nano-crystalline structure are believed to be the primary reasons for this high intense green upconversion. In addition, strain analysis proved that increasing the temperature will decrease the tensile strain significantly that can also decrease non-radiative relaxation pathway. This also explains the significant increase in the overall luminescence intensity in this system. A photograph showing the upconverted green light is provided in the inset of Fig. 6. Despite the fact the laser power was only 15 mW and the photograph was taken under conventional fluorescent lamp an intense green light was recorded. 4. Conclusions A novel transparent oxyfluoride glass ceramic containing Er3+, Yb3+: Sr1-xYxF2+x.nano-crystals were prepared. Upconversion luminescence behavior of Yb3+ and Er3+ during heat treatment was investigated. The results showed that increasing the heat treatment temperature changed the solid solution composition of Sr1-xYxF2+x, x ranging from 0.23 to 0.46. Ytterbium and erbium ions started to incorporate into the crystal phase when the heat treatment temperature was more than 650 and 600°C respectively. The majority of Er3+ and Yb3+ ions were incorporated in the Sr1-
xYxF2+x phase
in the samples that were heat-treated at 750 oC for 4 h. As a result this composition
had the highest upconversion intensity among the other glass ceramics. Comparing the lifetimes of the 4S3/2 and 4I11/2 of erbium and 2F 5/2 of ytterbium levels of the synthesized sample with other reported sample, which had the same concentration of Er3+ and Yb3+ ions, it is observed that the solubility of these ions in the current system is high. This observation could explain the efficient upconversion process obtained from this composition.
Acknowledgments The authors thank the Ministerio de Economía y Competitividad of Spain (MINECO) within The National Program of Materials (MAT2013-46649-C4-4-P), and the Consolider-Ingenio 2010 Program (MALTA CSD2007-0045, www.malta-consolider.com) and the EU-FEDER for their financial support.
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FIGURES Fig. 1. DTA thermograph of the precursor glass at a heating rate of 10 K/min.
Fig. 2. (a) XRD patterns of glass and glass ceramics heat-treated at 600, 650, 700 and 750 °C for 4 h. The inset focuses on a subset of the data featuring the shift of (111) and (200) reflections. (b) Size and weight percent of Sr1-xYxF2+x crystallites in glass ceramics at different temperatures. (c) Contraction of cubic Sr1-xYxF2+x lattice with increasing heat treatment temperature. The dashed line with filled squares is from Mayakova et al. [31] (CS=crystal size). Strain data (ε) at different heat treating conditions are marked close to each point; errors in the Williamson–Hall analysis were calculated by using a method of least squares and were equal to 0.05 for all data points.
Fig. 3. Absorption spectra of glass and glass ceramics heat-treated at 600, 650, 700 and 750 °C for 4 h. Inset shows the photographs of samples for spectral experiments.
Fig. 4. TEM images: (a) bright field image; (b) Z contrast image; and (c) high magnification image of the sample heat-treated at 750 ºC for 4 h.
Fig. 5. EDS elemental mapping of the glass heat-treated at 750 ºC for 4 h: (a) TEM image; (b) strontium map; (c) yttrium map; (d) flourine map; (e) erbium map; and (f) ytterbium map. Fig. 6. Upconverted luminescence obtained by exciting at 973 nm the Yb3+ ions in the glass ceramics which were heat-treated at 600, 650, 700 and 750 °C for 4 h. (Inset shows the
luminescence green light from the sample treated at 750 ºC excited with 973 nm wavelength laser with 15 mW power. The photograph was taken under conventional fluorescent lamp.
Fig. 7. Luminescence decay curves of Er3+ ions monitoring the emission at around 842 nm (corresponding to the 4S3/2→4I11/2 emission) obtained in glasses ceramics heat-treated at 600, 650, 700 and 750 °C for 4 h.
Fig. 8. Infrared Luminescence decay curves of Yb3+ ions monitoring the emission at around 975 nm (corresponding to the 2F5/2→2F7/2 emission) in glass ceramics heat-treated at 600, 650, 700 and 750 °C for 4h.