Photoluminescence (PL) of Sn2+ in oxide glasses is affected by the local coordination field because of the electrons in the outermost shell. Not only the emission energy varies depending on the local coordination, but also the transparency, and it is worth examining the local distribution of Sn2+ cations in oxide glasses, which have a tendency toward phase separation, especially in an ionic (phosphate)–covalent (borate) binary system. Here, we report the structural changes in zinc borophosphate glass and the PL properties of Sn2+ in oxide glasses. The building blocks of the main glass network vary depending on the chemical composition. We found that the Sn2+ species in B2O3-rich glasses differ from those in P2O5-rich glasses, as observed in the optical absorption, PL peaks, and PL decay constants. 119Sn Mössbauer spectra indicate that isomer shifts of Sn2+ also affect the local coordination change depending on the chemical composition. According to these results for the Sn2+ center, we conclude that Sn2+ centers are homogeneously dispersed in the borophosphate network without localization around the phosphate region.
© 2017 Optical Society of America
Oxide glasses have several advantages, such as high transparency, good formability, and good chemical durability. The wide transparent window from the UV to the near-infrared (NIR) makes them essential for use in optical devices . On the other hand, glass is a type of liquid whose structural ordering is much poorer than that of conventional stoichiometric crystals. This poor structural ordering without grain boundaries is the source of its good formability and also an important factor in its potential as a host material for various types of activators. Rare earth (RE)-doped glass is a typical example of an optical material containing activators [2–6]. Except for trivalent RE cations exhibiting the 4f–4f transition, the emission center of the glass is generally affected by the coordination field, which depends on the chemical composition of the glass . Because the dopant cation is an ionic species, the covalency or ionicity of the host glass network is crucial to the dispersion of activators. However, glass-forming oxides generally possess a covalent bonding network, which is why the dispersibility of dopant cations in oxide glasses is inferior to that in halide glasses. However, P2O5 differs from other network-forming oxides such as SiO2 or B2O3 because the P = O bond allows the inclusion of non-bridging [PO4]3− (ortho-) or [P2O7]2− (pyro-) phosphate units, which have delocalized electrons . Thus, P2O5 has been added to silica glass to improve the dispersibility of the dopant cation [8–10].
Among several emission centers, the ns2-type emission center is among the promising activators in practical devices, such as Sb3+-doped white fluorescent lamps . This emission center is characterized by the s-p parity-allowed transition and the electrons in the outermost shell, which are the origin of the wide and tunable emission wavelength [2, 11–24]. Recently, we have focused on oxide glasses containing the Sn2+ center, which is an ns2-type emission center [17–24]. It is notable that Sn2+-doped phosphate glasses exhibit high photoluminescence (PL) efficiency comparable to that of conventional phosphors. In recent papers, we also reported that the emission properties of the Sn2+ center depend on the network-forming oxide [17–19] and are little influenced by the network modifier cations . These findings suggest that the local coordination state of Sn2+ can be tailored by changing a glass's network-forming oxides. In other words, the emission of Sn2+-doped glasses consisting of two network-forming oxides is expected to vary depending on the network formation . In terms of the homogeneity of the host glass matrix, it is worth examining the optical and emission properties of Sn2+ in oxide glasses, which have a tendency toward phase separation.
Here, we examined the relationship between the glass network structure of ZnO–P2O5–B2O3 glasses [25–27] and the emission properties of the Sn2+ center. ZnO–P2O5–B2O3 glass reportedly exhibits phase separation behavior at an intermediate composition of B2O3 and P2O5 . On the other hand, it was reported that this glass is a host glass for activators [28-29]. However, these studies discussed either the structure [25–27]. or the emission properties [28-29], and there is no study on the correlation between the structure of these glasses and their emission properties as a luminescent material. In addition, no quantitative investigation of the network structure of these glasses based on NMR results has been reported. Considering these previous reports, we assume that the emission properties of the Sn2+ center should depend on the chemical composition of ZnO–P2O5–B2O3 glasses. We focus on whether the spatially neighboring units, i.e., the local coordination state, around the Sn2+ center depends on the P2O5–B2O3 ratio. To remove the interaction between Sn2+ centers, both the host glass network and the PL properties were examined using 0.1 mol% Sn-doped ZnO–P2O5–B2O3 glasses.
Pap The 0.1SnO-60ZnO-(40-x)B2O3-xP2O5 (xSZBP) glasses were prepared according to a conventional melt-quenching method using a platinum crucible . Batches consisting of SnO (99.5%), ZnO (99.99%), (NH4)2HPO4 (99%), and B2O3 (99.9%) were mixed and melted at 1100°C for 30 min in air and Ar (99.999%) atmosphere. The glass melt was quenched on a stainless steel plate maintained at 200°C and then annealed at the glass transition temperature Tg, as measured by differential thermal analysis, for 1 h. After cutting (10 mm × 10 mm × 1 mm), the glass samples were polished with aqueous diamond slurries. Because the sample size for measurement was fixed, we can quantitatively compare the luminescence intensity among these samples.
The absorption spectra were measured using a U3500 UV–vis–NIR spectrophotometer (Hitachi, Japan). The PL and PL excitation (PLE) spectra were measured at room temperature using an F7000 fluorescence spectrophotometer (Hitachi, Japan). In the PL measurements, band pass filters were used for the excitation (2.5 nm) and emission (2.5 nm). The emission decay at room temperature was measured using a Quantaurus-Tau instrument (Hamamatsu Photonics, Japan). The excitation light source for emission decay measurement was a light-emitting diode operated at a photon energy of 4.43 eV and a frequency of 10 kHz. The internal quantum efficiency was evaluated using a Quantaurus-QY instrument (Hamamatsu Photonics, Japan). X-ray diffraction (XRD; RINT-2100, Rigaku) measurements were performed to assign the precipitated crystal phase. 11B magic-angle spinning (MAS) NMR spectra were measured (Delta, JEOL) using a magnetic field strength of 600 MHz (14.1 T), a frequency of 192.56 MHz, a spin rate of 16 kHz, a relaxation delay of 3 s, and a pulse width of 0.1 μs. The reference was a 1 M H3BO3 aqueous solution (19.6 ppm). Further, 31P NMR measurement was performed (CMX-400, JEOL) using a spin rate of 14 kHz and a pulse delay of 15 s. The frequency used in this study was 161.80 MHz, and the accumulation number for each measurement was 3600–4500. Each chemical shift is estimated in δ (ppm) with respect to the reference H3PO4 (in D2O) solution (0 ppm).
119Sn Mössbauer spectra, i.e., the spectra of γ-ray absorption by the 119Sn nuclei in the samples, were measured in a conventional transmission geometry using a Ca119mSnO3 source at room temperature. The energy of the γ-rays from the source was modulated by the Doppler effect using a velocity transducer with a constant acceleration mode, and the abscissa of the spectra represents values in units of the Doppler velocity, as in the literature. The valence states of the Sn atoms, which are sensitively reflected in the peak positions in the 119Sn Mössbauer spectra . were deduced by fitting the measured spectra using the standard software Normos (developed by R. A. Brand, commercially available from WissEl GmbH).
3. Results and discussion
3.1 Host glass network
The glass-forming region of the xSZBP glasses is shown in the photographs of several typical samples in Fig. 1(a). Although transparent glasses were obtained in the B2O3-rich and P2O5-rich regions, opaque or crystallized samples were obtained at intermediate compositions (x = 10–25). Figure 1(b) shows the powder XRD pattern of the 20SZBP glass, which appeared opaque. Among the prepared samples, the 20SZBP glass showed clear precipitation of the Zn3(PO4)2 phase [Joint Committee on Powder Diffraction Standards (JCPDS) card #29-1390] . On the other hand, because no diffraction peak was observed for the 10SZBP and 25SZBP glasses, the translucency is attributed to phase separation. The present tendency is similar to that of previously reported Sn-free glasses .
Figure 2(a) shows 31P MAS NMR spectra of the xSZBP glasses. These spectra are normalized using each P2O5 fraction. Several peaks are observed and assigned to Q0 (~2 ppm), Q1 (~−10 ppm), and Q2 (~−30 ppm), respectively [32–38]. The superscript n in Qn indicates the number of bridging oxygens of the phosphorus cation. Phosphorus cations exist as Q0 or Q1 units in the B2O3-rich composition, and Q2 units are generated in the P2O5-rich composition. The chemical shift of the Q0 peak is almost constant at P2O5 contents below 10 mol%, suggesting that the neighboring cation of the Q0 phosphate unit is boron. In contrast, the chemical shift of the Q1 and Q2 peaks change at higher magnetic fields with increasing P2O5 fraction. The chemical shift indicates that the neighboring cations of the Q1 and Q2 units change from boron to phosphorus. Figure 2(b) shows the peak area ratio of the Qn units after peak deconvolution using a Gaussian function as a function of the amount of substituted P2O5. The Q0 ratio decreases linearly, whereas the Q2 ratio increases with increasing P2O5 content. The decrease in Q0 means that instead of a borate glass network forming, the generated phosphate chains form the glass network. Figure 2(c) shows the number of each type of Qn unit, which is obtained by multiplying the ratio of borate units by the B2O3 fraction, (40 − x) mol%, as a function of the amount of substituted P2O5. It is notable that the Q0 and Q1 units are isolated phosphate units possessing delocalized electrons and are inherently unable to form the main network. The number of Q0 unit is maximum in the phase-separated composition (x = 10), suggesting that Q0 units are localized in the zinc borate network. The number of Q1 units is maximum around 33 mol%, which corresponds to the chemical composition Zn2P2O7. It is notable that the substitutional fraction of B2O3 in the P2O5-rich (P2O5 = 30–40 mol%) region corresponds to the increase in Q2 units in the same compositional region. BO3/2 and BO4/2, which are discussed later, are three-dimensional (3D) structural units, whereas the Q2 (chain) unit is two-dimensional. The increase in Q2 units instead of borate units follows a decrease in the network dimension of the SZBP glasses if the structural ZnO plays the same role in both B2O3- and P2O5-rich glasses. Because ZnO can act as an intermediate , i.e., it can serve as either a network former or a network modifier, it is expected that the role of structural ZnO in B2O3-rich glass is not equal to that in P2O5-rich glass .
The 11B MAS NMR spectra of the xSZBP glasses are shown in Fig. 3(a). The spectra are also normalized using each B2O3 fraction. The broad peak at around 15 ppm corresponds to three-coordinated boron (BO3/2), whereas the sharp peak at around 0–5 ppm is attributed to four-coordinated boron (BO4/2) [38-39]. The three-coordinated boron is further deconvoluted into BO3/2 ring and BO3/2 non-ring structures. Although both BO3/2 and BO4/2 exist in the B2O3-rich glasses, only BO4/2 is observed in the P2O5-rich glass. The BO4/2 peak shift moves towards higher magnetic field when P2O5 is substituted for B2O3 owing to a change in the average coordination state due to formation of B–O–P bonds. It is notable that the peaks around 0 ppm attributable to BO4/2 units are split. Because the peak at a higher magnetic field shifts with increasing P2O5 content, the observed change suggests the formation of BO4/2 units with different covalent bonds, i.e., B–O–B–O–B, B–O–B–O–P, and P–O–B–O–P. Figure 3(b) shows the unit ratios of BO4/2 and BO3/2 as a function of the amount of substituted P2O5. In the B2O3-rich region, the BO3/2 ratio is almost constant. BO4/2 increases continuously, and the ratio of BO4/2 exceeds that of BO3/2 around the phase-separated region (around x = 30). Figure 3(c) shows the number of BO3/2 and BO4/2 units as a function of the amount of substituted P2O5. The numbers of BO3/2 and BO4/2 units decrease linearly with increasing P2O5 fraction, and the slope of the number of BO4/2 units changes gradually. When BO4/2 units are formed, a counter cation is needed for charge compensation. Because Zn2+ cations act as a counter cation to the BO4/2 units, the number of residual ZnO units that can form a covalent bonding network is increased. We think that such ZnO can form a subnetwork by interaction with phosphate chains in P2O5-rich glass, as suggested above.
From the 11B and 31P MAS NMR data, it is expected that PO4 units form phosphate chains in P2O5-rich glass, in which BO4/2 units are chemically bonded with the phosphate units. In contrast, PO4 units are isolated without a long P–O–P chain structure in B2O3-rich glasses, in which the network consists of both BO3/2 and BO4/2 units.
3.2 Local coordination of Sn2+
In this section, we determine whether local coordination in the Sn2+-P2O5 region is stable in ZnO–B2O3 glass so that Sn2+ exhibits the same PL properties as it does in ZnO–P2O5 glass, considering the result that adding P2O5 to silica glass improves the dispersibility of the dopant cation [8–10].
First, we examined the valence state of the Sn cation using 119Sn Mössbauer spectroscopy. Figure 4(a) shows 119Sn Mössbauer spectra of the xSZBP glasses (x = 0, 4, 8, 10, and 40). The peaks around 0 mm s−1 correspond to the Sn4+ species, whereas those around 2 and 4 mm s−1 correspond to the Sn2+ species . These spectra show that the Sn4+ species exists in the 0SZBP glass; that is, Sn2+ is oxidized to Sn4+ even though the samples were melted in an inert atmosphere (99.999% Ar) . However, with a small increase in the P2O5 fraction, the oxidation reaction was effectively prevented as the 4SZBP in Fig. 4(a). It is known that the Sn2+/Sn4+ ratio is also dependent on the redox-behavior of the starting material. Since the starting material of P2O5 was (NH4)2HPO4, it is expected that (NH4)2HPO4 worked as a reducing agent. Although calcination before melting was performed in order to prevent damage to Pt crucible during the melting , we assume that the residual species act to increase the Sn2+ ratio. If the difference in recoilless fraction between the Sn4+ and Sn2+ sites is assumed to be the same at room temperature, the Sn2+/Sn4+ ratio can be estimated. Figure 4(b) shows the Sn2+ ratio, isomer shift (IS), and quadrupole splitting of the Sn2+ species in the xSZBP glasses as a function of P2O5 content. To discuss the IS, the position of the peak splitting is used as a standard by averaging the two peaks. Although the IS depends on the P2O5 fraction, the slope of the increase in Sn2+ in the B2O3-rich region differs from that in the P2O5-rich region. Because the IS is affected by the surrounding Coulombic interaction, this result clearly suggests that the local coordination state depends on the chemical composition of the glass [42–46].
Figure 5(a) shows optical absorption spectra of the xSZBP glasses. Since the observed absorption edge is due to the s-p transition of Sn2+, the values of (hνα)2 is plotted as the left axis. The optical absorption edge of Sn2+ is red-shifted with increasing P2O5 content. In zinc phosphate or zinc borate glasses, a red-shift in the absorption edge of Sn2+ is observed when the amount of added Sn2+ increases . On the other hand, optical absorption due to Sn4+ exists in the UV region . If the shift in the optical absorption edges is correlated only with the valence state of tin, the change in the spectra should be nonlinear, similar to that in Fig. 4(b). Considering that the IS of Sn2+ depends on the chemical composition, it is natural that the optical absorption is also affected by the chemical composition. To discuss the optical properties, the optical absorption edge of tin is determined by extrapolation of these absorption curves to the 3 × 104 to 6 × 104 eV2 cm−2 region. Figure 5(b) shows the optical band gap Egopt of the xSZBP glasses as a function of P2O5 content. Like the IS, the slope of Egopt in the B2O3-rich glasses differs from that in the P2O5-rich glasses. The absorption shift, therefore, is due mainly to a change in the coordination state of the Sn2+ centers, which was observed in previous studies .
PL-PLE 3D contour plots of the xSZBP glasses (x = 0, 8, 30, and 40) are shown in Fig. 6(a). In terms of the excitation energy, the PLE peak energy of SZP glass is higher than that of SZB glass. With increasing P2O5 content, a higher excitation band emerges, and the emission intensity increases. The PLE spectra consisting of at least two components change in a complex manner. Figure 6(b) shows PL and PLE spectra of the 0SZBP and 40SZBP glasses. When the P2O5 fraction increases, the excitation band becomes higher with increasing emission intensity, i.e. the 40SZBP glass exhibited the highest emission intensity. Figure 6(c) shows the peak energies of the PL and PLE after peak deconvolution. Both peak energies are blue-shifted with increasing P2O5 fraction. This behavior may be correlated with the basicity of the glass , which decreases with increasing P2O5 fraction. Thus, the PL energies are affected by the chemical composition of the glass with different B2O3/P2O5 ratios.
Figure 7 shows the PL decay curves of the xSZBP glasses (x = 0, 2, 4, 6, 8, 10, 30, 35, and 40) at an excitation energy of 4.43 eV (280 nm). The decay curves are monitored at the photon energy of each PL spectrum. In the B2O3-containing glasses, the decay curves consist of two components: one is faster (the lifetime, τ′1/e, is of nanosecond order), and the other is slower (τ′′1/e is of microsecond order). As P2O5 is substituted for B2O3, the faster component, whose decay constant is several tens of nanoseconds, disappears. The observed decay in the B2O3-containing glasses, which was also observed in another borate glass system , is thought to be associated with the borate glass network. The PL decay constant is on the order of microseconds owing to relaxation from the triplet state and singlet state. It may seem that the fast decay component of nanosecond order is associated with Sn4+, because only Sn2+ centers exist in ZnO–P2O5 glass exhibiting no fast decay component. However, there is no relationship between the Sn2+ content (see Fig. 4(b)) and the fast (ns)/slow (μs) component ratio. Therefore, it is natural that the Sn2+ species in the B2O3 region are the main influence on the decay profile in Fig. 7.
Here, we consider the relationship between the emission properties and the glass network structure. With increasing P2O5 content, the main glass network changed from three- and four-coordinated borates to a chain phosphate network. This structural change causes changes in the IS, Egopt, the PL peak, and the PL decay constant of Sn2+. However, the degree of change in the P2O5-rich glasses differs from that in the B2O3-rich glasses, although tin takes the divalent state in most of the glasses. Because the local coordination state of tin changes continuously depending on the chemical composition, we conclude that no localized tin exists around the phosphorus units in the glass; i.e., tin species are homogenously dispersed in the glass network. It is notable that the present glasses were prepared by melt-quenching method, although quenched glass samples were annealed at the Tg. Spatial distribution of Sn2+ centers might be changed depending on the cooling condition from the melt . Since homogeneity of activators governs the performance of the phosphor, such transient state of glass material will be important for future functionalization.
We examined the Sn2+ emission center in zinc borophosphate glasses possessing different random networks. The glass network changed continuously depending on the chemical composition. Although the Sn2+ center is an ionic species, a localized distribution of Sn2+ around the phosphate region was not found. With increasing P2O5 content, the glass structure consisting of BO3/2 and BO4/2 changed into a phosphate chain structure via a phase separation region. The optical properties, such as the Egopt value of Sn2+, PL-PLE, and PL decay constant, varied continuously depending on the chemical composition. The emergence of a faster decay component of Sn2+ observable in the B2O3-rich glass is associated with this structural change. The homogenous dispersion of the Sn2+ center in the zinc borophosphate glass is also confirmed by 119Sn Mössbauer spectra, in which the IS of Sn2+ is affected by the chemical composition.
JSPS Grant-in-Aid for Young Scientists (A) (No.26709048), Izumi Science and Technology Foundation, Collaborative research program of I.C.R. Kyoto University (No. 2015-69).
This work was partially supported by a Grant-in-Aid for Young Scientists (A), by the Izumi Science and Technology Foundation, and by the Collaborative Research Program of the Institute for Chemical Research, Kyoto University. The authors also thank Professor Masahide Takahashi (Osaka Prefecture University) for allowing them to use the prism coupler. The 119Sn Mössbauer measurement was performed by Prof. K. Mibu at Nagoya Institute of Technology under the Nanotechnology Platform Program of MEXT, Japan.
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