Sr1.94Tb0.06Si5N8 phosphors were prepared at different temperatures varying from 1300oC to 1600oC in a reducing atmosphere. Figure 2 presents the XRD patterns of Sr1.94Tb0.06Si5N8 phosphors synthesized at different temperatures. All the diffraction patterns matched well and indexed with the standard pattern of Sr2Si5N8 (ICDD No. 85-0101), and no impurity phase was found. The increase in the annealing temperature led to an increase of the diffraction intensity owing to the enhancement in the crystallinity of phosphors.
Figure 3 presents the XRD patterns and the refinement results of Sr1.94Tb0.06Si5N8 synthesized at 1600oC. The solid curve indicates the simulated diffraction data, the “×” marks represent the experimental diffraction data, the straight bars show the positions of simulated diffraction patterns, and the dotted line denotes the deviation between the simulated and experimental values. The calculated Rp and wRp parameters were converged to reliable values of 0.0482 and 0.0715, respectively. The refinement results confirmed that Sr1.94Tb0.06Si5N8 possessed an orthorhombic structure with the space group of Pmn21 (no. 31). Table 1 lists the as-calculated lattice parameters of Sr1.94Tb0.06Si5N8. The calculated lattice parameters were a = 5.7168 Å, b =6.8301 Å and c = 9.3371 Å. The inset of Fig. 3 shows the crystal structure of Sr1.94Tb0.06Si5N8. Figure 4 shows the elemental mapping of the Sr1.94Tb0.06Si5N8 phosphor. The figure reveals that the elements of Sr, Si, N, and Tb are uniformly distributed over the whole particle. Figure 5 (a) displays the TEM images and corresponding diffraction pattern. The diffraction pattern was indexed with the help of CrysTBox software 27 and is shown in Fig. 5 (b). The structure of the selected zone is orthorhombic, with a zone axis of 0 0 1. The indexing was carried out with the help of four vectors. The vectors and the corresponding angle are shown in Fig. 5 (c). Figure 5 (d) shows the high-resolution transmission electron microscopy pattern. The d spacing value was estimated as 0.68 nm which corresponded to the (0 1 0) plane of the synthesized Sr1.94Tb0.06Si5N8 phosphor. The HRTEM analysis indicates the formation of the pure phase of the present phosphor materials.
Figure 6 presents the photoluminescence emission spectra of Sr1.94Tb0.06Si5N8 phosphors under UV excitation at 276 nm. The emission spectrum of Sr1.94Tb0.06Si5N8 prepared at 1300oC displayed several sharp emission peaks at 488 nm, 545 nm, 587 nm, and 622 nm due to the 5D4?7FJ (J=6, 5, 4 and 3) transitions of Tb3+ ions, respectively 28. When the annealing temperature was increased from 1300oC to 1400oC, the emission intensity increased due to the enhanced crystallinity. With further increasing the annealing temperatures from 1400oC to 1600oC, the emission intensity of Tb3+ ions was observed to decrease significantly and an additional broad emission band at approximately 600 nm became visible. The reasons for the appearance of the broad emission band will be discussed in the later section.
The inset of Fig. 6 illustrates the excitation spectra of Sr1.94Tb0.06Si5N8 phosphors. The excitation spectra obtained at 544 nm revealed a broadband in the UV region from 200 nm to 300 nm due to the 4f8 ? 4f75d1 transition of Tb3+ ions 29. For phosphors synthesized at 1300oC, the position of the excitation band was centered at approximately 256 nm. Following an increase in the heating temperatures from 1300 to 1600oC, a red-shift of the excitation band to 276 nm was observed. The red-shift of the excitation band indicates the increase of nephelauxetic effect and crystal-field splitting in the host lattice 1. These phenomena are supposed to be due to the change of coordinate environment for Tb3+ ions from Tb-O to Tb-N at high temperatures. The nephelauxetic effect is owing to the highly covalent chemical bonding between rare earth ions and N3- ions, while the large crystal-field splitting results from the large electronic charge of N3- ions 1.
Figure 7 presents the emission spectra of Sr1.94Tb0.06Si5N8 phosphors under blue excitation at 420 nm. A broad emission band centered at 602 nm was observed for phosphors synthesized at 1500oC. Further increasing the annealing temperatures to 1600oC led to a significant increase in the emission intensity of the phosphors. In general, the emission of rare earth ions can be attributed to 4f-4f or 5d-4f electron transitions 30. The 4f levels of rare earth ions usually appear as parallel parabolas in configuration coordinate diagrams due to the shielding effects produced by the filled 5s2 and 5p6 orbitals 30. As a result, the emission spectra associated with the 4f-4f transition usually appear as sharp lines in the spectra. In contrast, the emission spectra associated with the 5d-4f transition tend to exhibit broad bands due to the crystal field splitting of the 5d configuration 30. Thus, the broad emission band of Sr1.94Tb0.06Si5N8 phosphors synthesized at high temperatures at Fig. 6 is proposed to be due to the 5d-4f transition of terbium ions. The inset of Fig. 6 shows the excitation spectra of Sr1.94Tb0.06Si5N8 monitored at 602 nm. The spectra included three broad excitation bands at 200-290 nm, 290-360 nm, and 360-500 nm, respectively. The broad excitation band from 200-290 nm can be attributed to the electronic transition from the valence band to the conduction band in the host lattice 31. The other two excitation bands are considered to result from the 4f-5d transition of terbium ions.
XPS analysis was used to investigate the coordinate environment and chemical states of terbium ions in Sr1.94Tb0.06Si5N8. Figure 8 presents the XPS spectra of Sr1.94Tb0.06Si5N8 synthesized at various temperatures ranging from 1300oC to 1600oC. At the calcination temperature of 1300oC, one peak at 1276 eV due to 3d3/2 photoelectrons of terbium ions was observed. The spectrum indicates the existence of Tb3+ ions and Tb-O chemical bonds. After raising the annealing temperatures, another peak at 1284 eV due to the shake-up satellite of terbium ions was observed 32. The ratios of the satellite to parent photoelectron peaks were found to increase with the heating temperatures, indicating the enhancement of covalency for chemical bonds surrounding terbium ions 33- 34. Most metal nitrides exhibit higher covalency than metal oxides due to the fact that the electronegativity of N3- ions are lower than that of O2- ions 35. Thus, the increase of satellite to parent photoelectron peaks ratios at high temperatures is considered to be due to the formation of Tb-N bonds. The change of the coordinate environment for terbium ions from Tb-O to Tb-N may be owing to the increased reductive ability of H2 at high temperatures. These results supported the red-shift of Tb3+ excitation peaks with the annealing temperatures, as shown in the inset of Fig. 6.
A further increase in synthesis temperatures to 1600oC resulted in a shift of the photoelectron peak for terbium ions to a low binding energy. The binding energy of photoelectrons for metal ions was usually found to increase with oxidation states. Therefore, it can be proposed that Tb3+ ions were reduced to low charge terbium ions such as Tb2+ ions at high annealing temperatures owing to the enhanced reductive ability of H2. The broad emission band as observed in Fig. 6 and Fig. 7 is considered to be the emission of such terbium ions with a low oxidation state 36- 38.
Photoluminescence properties of Sr2-xTbxSi5N8 phosphors and electroluminescence properties of phosphors-converted WLEDs.
To investigate the effects of concentration for terbium ions, Sr1.94Tb0.06Si5N8 (x = 0.02-0.10) phosphors were prepared at 1600oC in a reducing atmosphere. Figures 9 and 10 present the excitation and emission spectra of Sr2-xTbxSi5N8 (x = 0.02-0.10). The excitation spectra monitored at 602 nm included three broad excitation bands at 200-290 nm, 290-360 nm, and 360-500 nm, respectively, while the emission spectra presented a broad emission band from 500 to 700 nm for all samples. The relationship between the concentration of terbium ions and the relative emission intensity of Sr2-xTbxSi5N8 is illustrated in the inset of Fig. 10. Increasing the doping amount of terbium ions to x = 0.06 led to an increase in the emission intensity of Sr2-xTbxSi5N8. However, further increasing the concentration of terbium ions reduced the emission intensity owing to the concentration quenching effects 39. Concentration quenching effects are usually caused by the transfer of energy from one activator to another until an energy sink in the lattice is reached 40. The critical distance for energy transfer, Rc, can be calculated using the following equation 40:
Rc ? 2(3V/4?xcN)1/3 (1)
where xc is the critical concentration, N is the number of metal ions in the unit cell and V is the unit cell volume. From the appropriate V (363.99 Å3), N (2) and xc values (0.03), the value of Rc for Sr2-xTbxSi5N8 was calculated to be 22.63 Å.
Figure 11 shows the electroluminescent (EL) spectra of LEDs driven via a current of 280 mA. When Sr1.94Tb0.06Si5N8 phosphor was coated on a blue LED chip, the emission spectrum shows a blue emission peak at 460 nm as well as a red emission band of Sr1.94Tb0.06Si5N8 at around 600 nm. The resultant CIE coordinate was (0.42, 0.30) with a near white correlated color temperature (CCT) value of 2355 K and a Ra value of 49. For improving the values of CCT and Ra for WLEDs, commercial YAG: Ce3+ was added into the phosphor mixtures of Sr1.94Tb0.06Si5N8 and coated on another LED chip. The emission spectrum exhibits a combination of blue, yellow and red emission bands. The corresponding CIE coordinate shifted to (0.33, 0.33) with a pure white CCT value of 5474 K and a Ra value of 81. The full set of CRI and average CRI (Ra) values are listed in Table 2. The images of packaged LEDs are presented in the insets of Fig. 8.
Long persistent luminescence properties of Sr2-xTbxSi5N8 phosphors
For investigating the trap level of Sr2-xTbxSi5N8 phosphors, Fig. 12 shows the variation of the afterglow intensity for Sr1.94Tb0.06Si5N8 monitored at 602 nm as a function of different excitation wavelengths. The afterglow intensity of phosphors decreased sharply as the excitation wavelength increased from 265 nm to 420 nm. Following UVC excitation (<280 nm), Sr1.94Tb0.06Si5N8 exhibited pronounced afterglow properties. However, after blue excitation, no afterglow properties were observed for phosphors. The observed persistent luminescence in the present phosphors can be attributed due to the difference in ionic radius between strontium and terbium ions. Because of the ionic radius mismatch, different trap levels are created during the substitution of terbium ions into the strontium ionic sites in Sr2Si5N8 and results in the afterglow phenomena 18- 19. The decay curves of Sr2-xTbxSi5N8 (x = 0.02-0.10) phosphors were monitored for 602 nm emission band under UV excitation of 276 nm, and the results are depicted in Fig. 13. The received decay curves were fitted into two exponential components according to the following equation 41 I (t) = A1 exp(?t/?1) + A2 exp(?t/?2) (2) where I is the luminescence intensity, A1 and A2 are constants, t is time, and ?1 and ?2 are the decay times of the exponential components. The average decay time (?av) can be calculated from Eq. 3 as mentioned below 42 ?av = (A1?12 + A2?22) /(A1?1 + A2?2) (3) According to Eq. 2 and Eq. 3, the relation between the average decay time (?av) and the concentration of terbium ions is plotted in the inset of Fig. 10. At x = 0.2, ?av was calculated to be approximately 1.5 s. Raising the concentration of terbium ions to x = 0.4 increased ?av to a maximum of 2.4 s. A further increase in the concentration of terbium ions decreased the decay time of phosphors due to the non-radiative decay between the activator ions. Figure 14 shows the afterglow spectra of Sr1.94Tb0.06Si5N8 after closing the excitation shutter from 3 to 30 sec. The upper inset of Fig. 11 depicts a comparison of the fluorescence and afterglow spectra for Sr1.94Tb0.06Si5N8. Upon UV excitation at 276 nm, the emitting color of Sr1.94Tb0.06Si5N8 was yellow, and the associated CIE coordinate was (0.48, 0.47). After turning off the excitation shutter, the associated CIE coordinate shifted to (0.58, 0.41) in the red region. The emitting light of the phosphors was still visible after 30 sec without any obvious change in emitting color. The lower inset of Fig. 14 depicts the energy level diagram of Sr2-xTbxSi5N8 phosphors. Under UV excitation, two kinds of terbium ions with different oxidation states can be excited. Therefore, an overlap of emission spectra was observed in the upper inset of Fig. 14. After turning off the excitation shutter, only the red emission band can be observed. These results indicated that terbium ions with low oxidation state were responsible for the persistent luminescence properties. Such emission properties revealed the suitability of the present phosphors as red-emitting afterglow phosphors. As a consequence of the present work, Sr2-xTbxSi5N8 phosphors with a broad emission band in the red region were prepared. The potential of the present phosphors for both WLEDs and persistent luminescence devices was demonstrated. Conclusions Terbium ions-activated Sr2Si5N8 phosphors with a broad emission band in the red region were developed in this study. The oxidation states of terbium ions and the emission colors of phosphors were controlled via the variation of synthesized temperatures. The Rietveld refinement indicated that the prepared phosphors exhibited a hexagonal structure with the space group of Pmn21 (no. 31). Under blue excitation, Sr2-xTbxSi5N8 phosphors prepared at temperatures higher than 1500oC showed a broad emission band centered at approximately 602 nm. This emission band is proposed to be the emission of terbium ions with an oxidation state lower than Tb3+ ions due to the reduction of Tb3+ ions at high temperatures. The emission intensity of phosphors increased with the concentration of terbium ions to a maximum at x = 0.06. Through the combination of a blue LED chip with Sr1.94Tb0.06Si5N8 and YAG: Ce3+ phosphors, a white LED with a pure white CCT of 5474 K and CIE coordinate of (0.33, 0.33) was successfully fabricated. On the other hand, the present phosphors exhibited long afterglow properties after UVC excitation. The decay time (?av) of phosphors increased from 1.5 s to 2.4 s when x raised from 0.2 to 0.4 and then decreased as further increasing the concentration of terbium ions. The results indicated that the present phosphors are suitable for the application in both WLEDs and long-persistent luminescence devices.