Improvement of the energy transfer from Ca₃SnSi₂O₉ host to rare-earth ions with the assistance of oxygen vacancies

Abstract

The blue emission of Ca₃SnSi₂O₉ was identified to arise from the recombination of electrons and holes via intrinsic oxygen vacancy defects using photoluminescence spectra measurements, density functional theory calculations, and thermoluminescence analysis. A significant enhancement in the photoluminescence intensity of the blue emission was observed in samples of Ca₃SnSi₂O₉ co-doped with fluorine, which was attributed to the increased number of oxygen vacancy defects produced in Ca₃SnSi₂O₉. Most importantly, the improvement of the photoluminescence intensity of Dy³⁺ demonstrated that energy transfer from the Ca₃SnSi₂O₉ host to the activators became more effective when fluorine was co-doped. It indicates that fluorine could be introduced into the Ca₃SnSi₂O₉ host by this approach, which produced more oxygen vacancies and contributed to the improved photoluminescence performance of the activators.

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Introduction

Continuing improvement in the efficiency of light emitting diodes (LEDs) has provided the potential for tremendous energy saving in general lighting applications. LEDs can therefore play a significant role in combating global warming through reducing energy consumption.¹ Recent efforts to develop new LED phosphors include oxide,^2–7 oxyfluoride,^8–12 nitride,^12–15 oxynitride,^16–18 and sulfide hosts.¹⁹ Despite many successes, continued efforts are required to address issues of higher efficiencies, better color rendition, and better thermal quenching characteristics.²⁰ Although numerous phosphors have been proposed in the last few years, these studies have focused primarily on the synthesis of compositional variants, to optimize known properties within classes of materials with previously known structure types. Recently, Danielson et al. identified that a new oxide-based blue phosphor (Sr₂CeO₄) exhibited a bright blue-white emission due to the charge transfer (CT) mechanism, which possessed an unusual one-dimensional chain structure type.²⁰ These interesting results have encouraged us to exploit phosphors with unique crystal structures which could provide high chemical stability and excellent optical properties, which are essential to improve the performance of lighting and display devices.

Blasse et al. have reported that isolated d¹⁰ ion complexes were able to show self-excitation emission, such as Zn₄O compounds,²¹ Ba₂ZnF₆,²² Ca₄ZrGe₃O₁₂,²³ and Ca₃SnSi₂O₉.²⁴ Nowadays there also exists strong evidence for the observation of photoluminescence (PL) from transition-metal oxides. Examples are SrTiO₃,²⁵ SrZrO₃,^26,27 Sr₂V₂O₇,²⁸ and La_0.825Sr_0.175MnO₃ nanowires,²⁹ the Stokes shifts of which are very large. Several reports in the literature explain the conditions that favor PL emission in materials presenting a degree of order–disorder.^29–31 The authors attribute the radiative decay process to distorted octahedra,³⁰ self-trapped excitons, oxygen vacancies, surface states²⁹ and charge transfer via intrinsic defects inside an oxygen octahedron.³¹ Although there is no general consensus in the literature about why and how radiative decay takes place in these compounds and the nature of this emission is not yet completely clear, it is a very interesting phenomenon to study especially when these materials act as the host matrix of potential single-component phosphors, which could provide unique emission to be combined with activators to achieve white light. Single-component white-light phosphors can overcome many of the problems associated with multiple emitting components, such as intrinsic color balance, device complications and high cost.²⁸ Therefore, in this study, the focus of our research is on the rare earth doped Ca₃SnSi₂O₉ with d¹⁰ configuration to pursue excellent PL performance.

In addition, as is well known, energy transfer plays an important role in the optical properties of luminescent materials, and their illumination performance is related to the local structures around activated ion centers.²⁸ A new experimental design is therefore required to determine the origin of the emission transitions of Ca₃SnSi₂O₉, and to achieve a fundamental mechanistic understanding of the relationship between the crystal structures and the luminescent properties of these phosphors. Hence, the effect of the change in crystal environment on the PL performance was studied with a distorted Sn–O bond in this work. Halo-containing oxides are generally obtained by introducing halogen ions (F⁻, Cl⁻, Br⁻, I⁻) into some conventional inorganic oxides to partially substitute for O²⁻ ions, upon which the cation coordination environments are changed. These phosphors doped with rare earth activators usually exhibit high efficiencies, and other superior luminescent properties.^32,33 In this work, F⁻ ions were selected to substitute for O²⁻ ions in Ca₃SnSi₂O₉. Moreover, when the SnO₆ octahedron becomes distorted in such a way that one Sn–O bond becomes shorter and more covalent whereas the remaining Sn–O bonds become longer and more ionic, the luminescence properties could be expected to change drastically. Finally, the effects of oxygen vacancies on the optical properties of the Dy³⁺ activators in Ca₃SnSi₂O₉ were investigated.

Experimental section

A high-temperature solid-state synthesis procedure was employed to synthesize the Eu³⁺ doped Ca₃SnSi₂O₉ phosphors. Analytical reagent grade CaCO₃ (99.9%), CaF₂ (99.9%), SnO₂ (99.9%), SiO₂ (99.9%), Eu₂O₃ (99.99%), and Dy₂O₃ (99.99%) were employed as raw materials. Small quantities of H₃BO₃ (5 mol%) were added as a flux. Stoichiometric amounts of the raw materials were mixed together with a small amount of ethanol (analytical reagent) homogeneously in an agate mortar for 30 min, and then the mixtures were first preheated at 900 °C in air for 2 h. After cooling to room temperature, the sample was reground and then sintered at 1400 °C for 4 h in an ambient atmosphere. After that, the as-obtained powders were cooled to room temperature naturally to yield the final product.

The phases of samples were identified by X-ray powder diffraction (XRD) with Ni-filtered Cu Kα radiation at a scanning step of 0.02 in the 2θ range from 10° to 80°. A Hitachi F-7000 fluorescence spectrophotometer was used to record excitation and emission spectra. The weight of all powder samples was kept constant (0.5 g). The powder samples were compacted and excited under 45° incidence and emitted fluorescence was detected perpendicular to the excitation beam. All the measurements were carried out at room temperature. We take advantage of density functional theory (DFT) calculations that provide a detailed picture of the local atomistic structure and the electronic structure. The local-density approximations based on density functional theory were chosen for the theoretical basis of the density function. First, the crystallographic data from the Inorganic Crystal Structure Database no. 80466 were used to optimize the crystal structure. The second step was to calculate the density of states for the optimized structure. The convergences were set as 5 × 10⁻⁴ for maximum displacement tolerances, 0.1 eV nm⁻¹ for maximum force, 0.02 GPa for maximum stress and 5 × 10⁻⁶ eV per atom for total energy change in the geometry optimization. The convergence criteria for the electronic wave function and for the geometry were 10⁻⁵ and 10⁻⁴ eV, respectively. The plane-wave energy cutoff was 380 eV.

Results and discussion

Fig. 1 shows the XRD pattern of the synthesized Ca₃SnSi₂O₉ host matrix. The XRD pattern was found to be in good agreement with the literature (JCPDS no. 46-0812).²⁴ This observation implies that the as-synthesized Ca₃SnSi₂O₉ is chemically and structurally single-phase Ca₃SnSi₂O₉, and agrees well with results from previous literature.²⁴ The crystal structure of Ca₃SnSi₂O₉ contains ribbons of edge-sharing CaO₆ and SnO₆ octahedra as shown in the inset of Fig. 1. The long direction of the ribbons is parallel to [100] and the width spans four octahedra. The SnO₆ octahedra are in the middle of the ribbons and alternate with CaO₆ octahedra parallel to [100]. The SnO₆ octahedra have a common edge almost parallel to (010) and are related by an inversion center. A given ribbon touches (and shares oxygen with) two other ribbons at either end of its width, and the touching ribbons diverge at an angle of approximately 48°.

Fig. 1 XRD patterns of Ca₃SnSi₂O₉ host matrix. Inset: Crystal structure of Ca₃SnSi₂O₉.

The excitation and emission spectra of Ca₃SnSi₂O₉ with different fluorine concentrations are depicted in Fig. 2. Although it has been reported that Ca₃SnSi₂O₉ is one of the very few stannates which shows luminescence of reasonable intensity at room temperature under 220 nm excitation,²⁴ the PL spectra yielded some interesting results which are presented in this work. The emission spectrum of Ca₃SnSi₂O₉ displays a broad band extending from 350 to 550 nm, and the excitation spectrum consists of three broad bands between 200 and 350 nm which has not yet been reported. The emission was described in the literature as a charge transfer transition involving Sn(IV) and oxygen in the Sn₂O₁₀ group, which will be discussed subsequently. A particularly impressive result exhibited in Fig. 2 is that the intensity of the charge transfer transition located at 417 nm was significantly improved with increasing concentrations of fluorine. It is clearly shown that the emission intensity located at 417 nm is greatly enhanced when the concentration of fluorine increased from 1% to 2.2%. Meanwhile, the shape and the central frequency of the PL spectrum do not show appreciable changes between the Ca₃SnSi₂O₉ host matrix and the fluorine doped samples, which indicates that the radiative processes giving rise to blue-light luminescence at room temperature have the same mechanism. The excitation spectrum corresponding to this emission consists of three bands, a strong one with a maximum at 311 nm and two weak ones at 220 and 244 nm. Although the central frequencies remain unchanged between the Ca₃SnSi₂O₉ host matrix and Ca₃SnSi₂O₉ doped with fluorine samples in the PL and PLE spectra, the great enhancement of the PL intensity indicates that the charge transfer transition of the host could be improved by co-doping with fluorine.

Fig. 2 Excitation and emission spectra of Ca₃SnSi₂O₉ co-doped with different concentrations of fluorine.

The density functional theory calculations of Ca₃SnSi₂O₉ based on crystal structure refinement were employed and are shown in Fig. 3. The local density approximation (LDA) was chosen for the theoretical basis of the density function. Ca₃SnSi₂O₉ possessed an indirect band-gap of about 3.936 eV (315 nm) with the valence band (VB) maximum at the B point and the conduction band (CB) minimum at the G point of the Brillouin zone. The electronic structure of the VB originates predominantly from Ca, O 2p states, whereas the CB is composed mostly of Sn 4s and Ca 3d states. It is expected that the value of the calculated band-gap of Ca₃SnSi₂O₉, about 3.936 eV, will be smaller than the experimental one as the LDA underestimates the size of the band-gap.³⁴ The calculation results provide evidence that the broad band peaks from 200–350 nm originate from the absorption of the host crystal Ca₃SnSi₂O₉, which is predominantly ascribed to the charge transfer of Sn(IV)–O and Ca–O. The 417 nm (2.973 eV) emission band can be assigned to the recombination of the excited electrons with holes in an in-gap defect state, which results in the blue emission. In this work, oxygen vacancy defects are speculated to be electron traps contributing to the blue emission, which were produced during the high temperature synthesis processes.

Fig. 3 Calculated band structure and electronic structure of the Ca₃SnSi₂O₉ host matrix.

To further illustrate the origin of the blue emission, the thermoluminescence (TL) technique was employed in this study. TL measurement is a main research tool as it has been proven to be a very useful means to reveal valuable information about the traps.^35,36 Although the vast majority of the created free electrons and holes (electron deficient centers) in the crystalline materials recombine at room temperature, a few electrons diffuse away from their point of formation and become stabilized by being trapped at some defect in the structure. When the material is heated, the trapped electrons escape from the traps and a proportion of them recombine with holes at luminescence centers, emitting their excess energy in the form of light.³⁷ Therefore, the TL measurements provide evidence for the existence of trapping centers. The TL glow curve of the Ca₃SnSi₂O₉ was shown in Fig. 4; three TL peaks located at 326.3, 429.5, and 486.1 K were detected according to Gaussian fitting. The three TL peaks are labeled as T_A, T_B, and T_C corresponding to traps with different depths. On the basis of TL results the trapping parameters were calculated by Chen's half width method. The activation energy (E) can be calculated by eqn (1) and (2).^38,39

where μ_g is a symmetry factor, and T_m, T₁, T₂ are the peak temperatures at the maximum and on either side of the maximum respectively, corresponding to half intensity. k is Boltzmann's constant. The following parameters can be defined: τ = T_m − T₁ is the half width at the low temperature side of the peak, δ = T₂ − T_m is the half width towards the fall-off of the glow peak, ω = T₂ − T₁ is the total half width, μ_g = δ/ω is the symmetrical geometrical factor. γ stands for τ, δ or ω. The trapping parameters were calculated by Chen's half width method as shown in Table 1. The depths for traps T_A, T_B, and T_C are 0.347, 1.175, and 2.764 eV for Ca₃SnSi₂O₉, respectively. From the above results, we can affirm that there are some shallow traps (T_A and T_B), of which the TL peaks close to (or above) room temperature are expected to be essential to the recombination of the electrons and holes. Trap T_C, responsible for TL peak T_C, is not considered to be thermally released at room temperature and can hardly contribute to the blue light emission, because peak T_C occurs at relatively higher temperature. Accordingly, the traps T_A and T_B arising from oxygen defects (oxygen vacancies) are responsible for the blue emission performance of the Ca₃SnSi₂O₉ phosphor, which is also consistent with the results derived from the density functional theory calculations. Furthermore, the decay curves monitored at the blue emission of 417 nm of Ca₃SnSi₂O₉ and the Ca₃SnSi₂O₉ sample co-doped with fluorine were recorded in the inset of Fig. 4; the decay times of which were 1.516 and 1.406 μs, respectively. The decay time does not show an appreciable change, which indicates that the origin of the blue emission of the Ca₃SnSi₂O₉ sample does not change with the co-doping of fluorine. From the above analysis, it is preliminarily concluded that the excited electrons recombine with holes through the defect level created by the oxygen deficiency (oxygen vacancies) in the band-gap, and this contributes to the blue-light luminescence at room temperature. Therefore, the enhancement in the PL intensity in the Ca₃SnSi₂O₉ co-doped with fluorine is related to the distortion of the SnO₆ octahedra and the creation of the oxygen vacancies. As the oxygen deficiencies are increased, the conduction electron density increases, thus leading to higher luminescence intensity.

Fig. 4 TL glow curve of the Ca₃SnSi₂O₉ host matrix. Inset: the decay curves monitored at the blue emission of 417 nm of Ca₃SnSi₂O₉ and the Ca₃SnSi₂O₉ sample co-doped with fluorine.

Table 1 Experimental peak parameters and calculated trap depth of Ca₃SnSi₂O₉ host matrix

	T1 (K)	Tm (K)	T2 (K)	τ (K)	δ (K)	ω (K)	μg	Trap depth
								Eτ (eV)	Eδ (eV)	Eω (eV)	E (eV) mean value
TA	288.4	326.3	364.3	37.9	38.0	75.9	0.501	0.316	0.378	0.348	0.347
TB	407.1	429.5	452.3	22.4	22.8	45.2	0.504	1.107	1.108	1.114	1.110
TC	474.2	486.1	498.1	11.9	12.0	23.9	0.502	2.843	2.672	2.776	2.764

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Fig. 5 exhibited the refinement of XRD patterns of Ca₃SnSi₂O₉ when the fluorine doping concentration reached 2.2%. The black line and red crosses depict the observed and calculated patterns, respectively. The as-obtained fit parameter χ² = 1.05 and R_wp = 9.1% ensure phase purity in the sample of Ca₃SnSi₂O₉ with 2.2% fluorine doping. From the XRD refinement of Fig. 5, we could see that all the diffraction peaks could be indexed to Ca₃SnSi₂O₉, which indicates that the pure phase could be derived when fluorine was doped. Samples were well crystallized, and the diffraction patterns could all be recognized as single phase in line with JCPDF 46-0812. No impurity phase was observed when fluorine was doped into Ca₃SnSi₂O₉, which clearly indicates that the increase of PL emission in Ca₃SnSi₂O₉ could be ascribed to the improved crystallization or the change in crystal environmental with the doping of fluorine.

Fig. 5 Refinement of XRD patterns of Ca₃SnSi₂O₉ co-doped with fluorine at a concentration of 2.2%.

To further illustrate the effect of co-doping fluorine on the Ca₃SnSi₂O₉ crystal, hypersensitive PL spectra of Ca₃SnSi₂O₉: Eu with different fluorine doping concentrations were measured and are shown in Fig. 6. Emission lines of the Eu³⁺ ion correspond to transitions from the excited ⁵D₀ level to ⁷F_J (J = 1, 2, 3). We observe three main emissions: first, the emission in the vicinity of 590–600 nm is due to the magnetic dipole (MD) transition ⁵D₀–⁷F₁; second, the red emission around 610–630 nm is due to the hypersensitive electric dipole (ED) transition ⁵D₀–⁷F₂. The change in ratio of I_ED/I_MD is a good measure of the symmetry of the Eu³⁺ site and is related to the Judd–Ofelt parameter Ω₂.⁴⁰ As the ratio of I_ED/I_MD becomes larger with increasing fluorine concentration, the electric dipole transition is enhanced and quickly increases the crystal field strength. This increase could be related to an increase in the covalence or the distortion of the bonds surrounding the active ion. The ⁵D₀–⁷F₁ transition dominates in sites with inversion symmetry, whereas the ⁵D₀–⁷F₂ transition is the strongest in sites without inversion symmetry.⁴¹ Thus, it could be understood that the crystal field is significantly higher with increasing fluorine concentration. This ratio of intensities is related to the short average distance between Eu–O in the C₂ sites and to the high distortion in the short order range (the degree of distortion in the CaO₆ octahedra) with fluorine co-doping. It produces a strong crystal field around the Eu³⁺ ion with a high covalence in the Eu–O/Eu–F bond. This demonstrates that the crystal environment indeed changed with fluorine co-doping. The location of fluorine in Ca₃SnSi₂O₉ is crucial to understand the optical properties of the phosphors. However, O²⁻ and F⁻ have no contrast in X-ray scattering, and crystallographic studies of O/F ordering are difficult to undertake. Because oxygen and fluorine are neighboring atoms in the periodic table, we propose that F⁻ substitutes O²⁻ in this study. This could be consistent with the fact that no impurity was observed in the X-ray Rietveld refinement.

Fig. 6 PL spectra of Ca₃SnSi₂O₉: Eu³⁺ with different fluorine doping concentrations.

In principle, the Ca₃SnSi₂O₉ contains ribbons of edge-sharing CaO₆ and SnO₆ octahedra, which formed a low dimensional structure as discussed above. In this kind of low dimensional structure, it is very easy to implant other ions into the host lattice and create an energy transfer process between them if these ions show an absorption peak in the range of 400–500 nm. Here, we adopted Dy³⁺ as the activator in Ca₃SnSi₂O₉, co-doping of which with different concentrations of fluorine then further increased the oxygen vacancies and enhanced the blue emission. As we expected, a significant improvement of the PL performance was observed in the Ca₃SnSi₂O₉: Dy³⁺ samples. Fig. 7 shows the PL spectra of Ca₃SnSi₂O₉: Dy³⁺ with different fluorine doping concentrations. The characteristic emission of Dy³⁺ (482, 574 nm) was observed under 310 nm excitation, which is ascribed to the ⁴F_9/2–⁶H_15/2, ⁴F_9/2–⁶H_13/2 transitions of Dy³⁺ respectively.⁴² The red emission (⁴F_9/2–⁶H_11/2) is too weak to be checked. Both the PL emission intensity of Ca₃SnSi₂O₉ and Dy³⁺ was drastically improved with increasing fluorine concentration, although the doping concentration of Dy³⁺ is fixed. On one hand, the electric dipole transition was enhanced and the crystal field strength was quickly increased as well. On the other hand, more oxygen vacancies were formed. It was established that energy transfer from Ca₃SnSi₂O₉ host to Dy³⁺ ions became more effective upon introducing fluorine ions, which was the main reason for the enhancement in the characteristic emission of the activators.

Fig. 7 PL spectra of Ca₃SnSi₂O₉: Dy³⁺ with different fluorine doping concentrations.

Conclusions

In summary, a Ca₃SnSi₂O₉ host matrix was prepared by a solid state reaction. Ca₃SnSi₂O₉ exhibited a blue emission which was ascribed to the recombination of electrons and holes via the intrinsic defects of oxygen vacancies under UV excitation. The emission intensity could be greatly improved by co-doping with fluorine due to the increase in the oxygen vacancies and the crystal field strength. The PL properties of Dy³⁺ as an activator could be improved by the enhancement of energy transfer efficiency from the host to the activators with the assistance of oxygen vacancies.

Acknowledgements

We are thankful for financial support from the National Nature Science Foundation of China (11204113, 61308091, 61265004 and 51272097), Specialized Research Fund for the Doctoral Program of Higher Education of China (20115314120001), and the Foundation of Natural Science of Yunnan Province (2011FB022).

Notes and references

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