A new Eu2+-doped barium beryllium silicate, BaBeSiO4:xEu2+ (1% ≤ x ≤ 10%) phosphor was successfully synthesized by the high temperature solid-state reaction. The phosphor system was investigated as a novel candidate for phosphor-converted white light-emitting diode (LED) applications. The evolution of luminescence and structure of the phosphors induced by variation of sintering temperature are investigated using photoluminescence spectra and X-ray diffraction techniques. The concentration quenching and thermal quenching process of the phosphor were also investigated in detail. The BaBeSiO4:Eu2+ phosphor exhibits broad excitation spectra ranging from 250 to 400 nm, and a blue emission band centered at 460 nm under λex = 337 nm. The main parameters of the electron-vibrational interaction, such as Huang-Rhys factor, effective phonon energy, and zero-phonon line position, were all estimated. The optimal concentration of Eu2+ ions in BaBeSiO4 was determined to be 3 mol.%. The corresponding concentration quenching mechanism was verified to be the electric dipole−dipole interaction. The activation energy of BaBeSiO4:Eu2+ for thermal quenching was calculated to be 0.19 eV with the Arrhenius equation. The LED device exhibited an excellent color-rendering index Ra of 90.13 at a correlated color temperature of 4077 K with CIE color coordinates of (0.38, 0.39) under a 350 mA forward-bias current. Based on these results, we are currently evaluating the potential application of BaBeSiO4:Eu2+ as a white-emitting UV-convertible phosphor for high power LED applications.
© 2016 Optical Society of America
Nowadays, the lighting systems of White light-emitting diodes (w-LED) are rapidly replacing conventional energy-hungry lighting products like incandescent lamps and more recently environmentally hazardous fluorescent lamps, because w-LED are more advantageous if compared with their above-mentioned predecessors. The w-LED have high efficiency, long operation lifetime, good material stability and energy saving capabilities [1–3 ]. The majority of white LEDs use a blue InGaN chip pumped with yellow-emitting Y3Al5O12:Ce3+ (YAG:Ce3+) phosphors . However, this method has a number of disadvantages, such as cool white light with corresponding chromaticity coordinates of (0.292, 0.325), according to the Commission internationale de l’eclairage (CIE), poor color rendering indices (CRI) of 75 and a high correlated color temperature (CCT) of 7756 K . These disadvantages can be attributed to a lack of red-light contributions, which restricts the use of current white LEDs in more vivid applications. Recently, white LEDs fabricated using ultraviolet chips (350-380 nm) coupled with a blend of blue-, green- and red-emitting phosphors have exhibited favorable properties, including tunable CCTs, tunable CIE chromaticity coordinates, and excellent Ra values. Therefore, it is important to develop new phosphors for UV LED applications .
The luminescence of Eu2+-activated phosphors have been characterized by intense broad band absorption in the ultraviolet (350-380 nm) or near-ultraviolet (380-420 nm) regions and an emission wavelength that can be tuned from ultraviolet to red which was related to the crystal field splitting of the 5d levels and the covalent interaction with the surrounding oxygen anions . Many novel phosphors with Eu2+ activated for UV LED/near-UV LEDs have been reported, including KSrY(PO4)2:Eu2+ , Sr8MgLn(PO4)7:Eu2+ (Ln = Y, La) , CaSrSiO4:Eu2+ , BaLa2Si2S8:Eu2+ , Ba(Si, Al)5(O,N)8:Eu2+ , (Ba, Sr)Y2Si2Al2O2N5:Eu2+  and Ba9Y2Si6O24:Eu2+  All of these Eu2+-activated phosphors demonstrate intense broad band absorption and emission were assigned to the 4f-5d dipole-allowed electronic transitions of the Eu2+ ions .
To the best of our knowledge, no investigation regarding luminescence properties of earth ions-doped BaBeSiO4 phosphors has been reported yet. Herein, we have demonstrated a novel blue-emitting BaBeSiO4:Eu2+ phosphor. The crystal structure, luminescent properties, CIE coordinates, thermal stability and LED packages are discussed in detail. The results exhibit that BaBeSiO4:Eu2+ has good thermal stability, and the white LEDs based on BaBeSiO4:Eu2+ has good color rendering index and high luminous efficacy. These results demonstrate that BaBeSiO4:Eu2+ has great potential to be a blue-emitting phosphor for UV chip excited white LEDs.
2. Experimental section
2.1. Sample preparation
Blue-emitting BaBeSiO4:Eu2+ phosphors were synthesized by a solid-state reaction method at high temperature. The starting materials, BaCO3 (ACROS, 99.95%), BeO (ALDRICH, 99.99%), SiO2 (ALDRICH, 99.9%) and Eu2O3 (ALDRICH, 99.99%) were employed within a stoichiometric molar ratio of 1–x:1:1:x/2. After these powders were blended together and ground thoroughly in an agate mortar. The homogeneous mixture was divided into five parts, each of which was placed into an alumina crucible and calcined in a muffle furnace at Tc = 900 °C, 1000 °C, 1200 °C, 1300 °C and 1400 °C for 8 h under a reducing atmosphere of 15% H2/85% N2 in an alumina boat. The products were then cooled to room temperature in the furnace, ground, and pulverized for further measurements.
2.2. Sample characterization
Powder X-ray diffraction (XRD) measurements were performed on a D8 ECO diffractometer (Bruker) operating at 40 kV and 25 mA. The XRD profiles were collected in the range of 10° < 2θ < 80°. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra of the obtained powders were recorded with a Hitachi F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The luminescence decay curves were obtained from a Lecroy Wave Runner 6100 digital oscilloscope (1 GHz) using a tunable laser (pulse width, 4 ns; gate, 50 ns) as the excitation source (Continuum Sunlite OPO). The temperature-dependent properties of the phosphors were measured with a HORIBA Jobin Yvon Fluorolog-4 FL3-211 spectrometer equipped with a 450 W xenon lamp as the excitation source.
3. Results and discussion
3.1. Crystal structure of BaBeSiO4
The collected X-ray diffraction patterns were analyzed using the General Structure Analysis System (GSAS) program following the Rietveld refinement method . Figure 1 presents the refinement results obtained at Tc = 900°C, 1000°C, 1200°C, and 1300°C. The formation process of BaBeSiO4 at various Tcs was monitored. Rietveld refinement was performed using BaBeSiO4 as a starting model, which crystallizes with a monoclinic structure with space group C1m1. The 1% Eu2+ dopant is neglected in the refinement model owing to the resolution of XRD instrument. The phases of intermediate products are considered. The multiphase crystalline compounds were found. The patterns revealed a raw material SiO2 (at 900°C) and intermediate products of Ba2Be2O6 (at 900°C), Ba2SiO4 (below 1300°C), and BaSiO3 (at all Tcs). Notably, the BaBeSiO4 phase was firstly observed at a Tc of 1000°C, and the molar ratio of this phase increased with Tcs.
Figure 2(a) presents the results of the refinement of the Tc = 1400°C sample. All phases and refined parameters are listed at Table 1 . The major phase was the monoclinic BaBeSiO4 compound. All other impurity phases, such as SiO2, Ba2Be2O6, and Ba2SiO4 reacted further and were not observed. A minor impurity phase of BaSiO3 was reduced with a molar ratio of ~6.2%. Although the positions of Eu2+ ions cannot obtain from the refinement directly, but the possible locations in the crystal structure still can be estimated. The ionic radius (6-coordinate) of Ba2+, Be2+, and Eu2+ are 1.49Å, 0.59Å, and 1.31Å, respectively . The most possible cation that can be replaced by Eu2+ are the Ba2+ ions. The difference curve between the observed and calculated curves revealed good fitting. The refined lattice parameters of the Tc = 1400°C sample are a = 8.8290(9) Å, b = 5.1428(5) Å, c = 5.1046(5) Å, β = 124.428(4), V = 191.18(4) Å3, and Z = 2, consistent with the results of Ref [15, 16 ]. Figure 2(b) shows the variations of the molar ratios of the intermediate and final products with Tc, and suggests that the synthetic temperature must exceed 1400°C.
3.2. Luminescence properties of BaBeSiO4:Eu2+ phosphors
Photoluminescence (PL) spectra of BaBeSiO4:Eu2+ phosphors processed at various temperatures were recorded using 337 nm laser excitation (Fig. 3(a) ). At 1400 °C, it is the typical emission spectra of the Eu2+-doped BaBeSiO4, and the emission intensity is the highest. The emission spectrum at 1300 °C is similar to that one at 1400 °C, but the peak at 460 nm is much lower. When the sintering was conducted at the temperature lower than 1300 °C, the intensity and wavelength of the emission band were obviously changed. At 1200 °C and 1000 °C a wider band with a maximum at 507 nm is observed. According to Fig. 2(b), there are different compositions with varying temperature. Zhang et al.  has reported that the emission of Ba2SiO4:Eu2+ exhibits a broad band centered at 505 nm. Therefore, the green emission band around 507 nm in the emission spectra may be originated from Ba2SiO4 phase. Because the Ba2SiO4 content was very low at 900 °C, the luminescence was almost not detected.
The concentration dependence of the relative PL/PLE intensity of BaBeSiO4:xEu2+ (x = 1%–10%) under 337 nm excitation is demonstrated in Fig. 3(b). The PL spectrum exhibited a blue broad emission band from 400 to 650 nm, centered at 460 nm, which was attributed to the 4f65d1-4f7 transitions of the Eu2+ ion . The PLE spectrum showed broad absorption bands between 250 and 400 nm, which are attributed to the 4f7-4f65d1 transition of the Eu2+ ion . Although the excitation maximum occurs at 337 nm, the samples could be efficiently excited in the range of 350–400 nm. Consequently, a UV chip (365 nm) was selected for use in the subsequent fabrication of the white LEDs. The optimal concentration of the Eu2+-doped content was found to be 3 mol.%, and the PL intensity was observed to increase when x is increased up to x < 3 mol.%. Quenching was observed for samples with concentrations of Eu2+-doped content higher than 3 mol.%, and the PL intensity was found to decrease with an increase in Eu2+-doped content. The CIE chromaticity diagram for BaBeSiO4:Eu2+ phosphors with different Eu2+ content is shown in Fig. 3(c). The CIE chromaticity coordinates for these phosphors were located in the blue region at 1 (x = 0.19, y = 0.22), 2 (x = 0.18, y = 0.21), 3 (x = 0.18, y = 0.22), 4 (x = 0.18, y = 0.24), 5 (x = 0.19, y = 0.25), 6 (x = 0.18, y = 0.25), corresponding to 1%, 2%, 3%, 5%, 7% and 10% Eu2+, respectively, and the CIE coordinates shift slightly from left to right with different Eu2+ contents. The inset of the CIE chromaticity diagram shows the luminescence photograph of BaBeSiO4:Eu2+ phosphor with different concentrations Eu2+ excited at 365 nm. Under UV excitation, BaBeSiO4:Eu2+ phosphor emitted blue light. Under excitation at 337 nm, the quantum efficiency of BaBeSiO4:0.03Eu2+ phosphor was determined to be 53.35%. The lower quantum efficiency of BaBeSiO4:Eu2+ could be further enhanced by reducing the content of impurity in the future via process optimization.
The intensity and wavelength dependence of Eu2+ emission on the Eu2+ content x in samples BaBeSiO4:xEu2+ are shown in Fig. 4(a) . The emission band shifts toward longer wavelength as the concentration of the Eu2+ dopant increases. This bathochromic shift is ascribed to the change in the crystal-field splitting of Eu2+ . The crystal field around Eu2+ can be estimated as Eq. (1), Dq is inversely proportional to the 5th power of bond-length R: Dq ∝ R5, when the metal–ligand distance decreases, the crystal field splitting significantly increases. For the Ba1–xEuxBeSiO4 lattice, the progressive replacement of Ba2+ by the smaller Eu2+ ions is expected to be followed by a shorter metal–ligand distance, resulting in a stronger crystal field environment, thereby causing a red shift in the emission [21, 22 ]. Fig. 4(a) (red line) shows the PL intensity of BaBeSiO4:Eu2+ as a function of the Eu2+ content. The optimal doping concentration was found to be at 3 mol.%, the PL intensity was found to decline dramatically as the content of Eu2+ exceeds 3 mol.% due to concentration quenching. Concentration quenching is mainly caused by the energy transfer among Eu2+ ions, the possibility of which increases as the concentration of Eu2+ increases. Blasse  pointed out that the critical transfer distance (Rc) is approximately equal to twice the radius of a sphere with the volume of the unit cell:24]:Fig. 3(b), the wavelength at maximum spectral overlap is 406 nm; the value of E is calculated as around 3.0538 eV. The spectral overlap ∫fS(E)FA(E)dE between the emission and excitation was estimated to be 0.0154 eV−1. A value of Rc = 16.22 Å is obtained, which agrees approximately with the data that were obtained by using the concentration quenching method.
The non-radiative energy transfer from one Eu2+ ion to another Eu2+ ion may take place via an exchange interaction, radiation reabsorption, or an electric multipolar interaction . The exchange interaction requires a large direct or indirect overlap of the wavefunctions of the donor and acceptor, and this mechanism is responsible for energy transfer in the case of forbidden transitions. The critical distance for the exchange interaction is approximately 5 Ǻ. The 4f7-4f65d1 transition of Eu2+ is parity allowed; hence, the exchange mechanism plays no role in the energy transfer within the BaBeSiO4:Eu2+ phosphors.
The mechanism of radiation reabsorption is only effective when the spectra are broadly overlapping of the emission spectrum of the sensitizer and the excitation spectrum of the activator. In the present case, there is a small overlap between the excitation and emission spectra shown in Fig. 3(b), which probably influenced the luminescence intensity. The observation of little overlapping of PLE and PL for BaBeSiO4:Eu2+ phosphors indicate that reabsorption could not be a main mechanism of non-radiative energy transfer in this case. Therefore, the process of energy transfer between Eu2+ ions in the BaBeSiO4:Eu2+ phosphor was attributed to the electric multipolar interaction, as suggested by Dexter . The emission intensity (I) per activator concentration (x) can be expressed by the following equation:Fig. 4(b) illustrates the I/x dependence on x on a logarithmic scale. The dependence of log (I/x) on log (x) was found to be relatively linear, and the slope was determined to be −1.431. The value of θ was found to be approximately 6, indicating that the concentration quenching mechanism of Eu2+ emission was dominated by the dipole–dipole interaction.
3.3. Study of electron-vibrational interaction of BaBeSiO4:Eu2+ phosphors
The electron-vibrational interaction (EVI) between the electronic states of an impurity ion (Eu2+ ions in this particular case) and vibrations of surrounding crystal lattice ions can be effectively described by three main parameters : 1) the Stokes shift ΔES, which is the energy difference between the absorption/excitation and emission bands corresponding to the same electronic transition; 2) the Huang-Rhys factor S, which is proportional to the Stokes shift ΔES, and 3) the effective phonon energy ħω. In the framework of the single configurational coordinate model  interaction of an impurity ion with the complete set of the normal vibrational modes existing in a particular impurity center (the number of these normal modes and their frequencies are determined by the point group symmetry and masses of the ions involved) is reduced to an interaction with one effective breathing mode only, whose frequency is supposed to be the same in the ground and excited electron states. The frequency of such effective mode is not necessarily exactly equal to one of the vibrational frequencies of a particular crystal (which can be determined from the Raman spectra, for example), although it should be reasonably close to those. Although this model is based on such an approximation, it has been shown to be quite successful in interpretation and analysis of manifestation of the EVI in the optical spectra of impurity ions in crystals. The physical reason of the EVI is the shift of the equilibrium positions of the potential energy surfaces in the ground and excited states. In the framework of the considered here model , these parameters can be extracted from the following equations after a careful consideration of the experimental emission and absorption (or excitation) spectra :Eq. (6) describes the full width Γ(T) at half maximum (FWHM) of the emission band determined at the absolute temperature T. Taking the maximum of the lowest excitation transition at 26396 cm−1 and the maximum of the emission transition at 19279 cm−1, the Stokes shift is then 4509 cm−1. The FWHM of the emission band is 4247 cm−1 (Fig. 3(b)). It is also easy to see from Fig. 3(b), that the main characteristics of the Eu2+ excitation/emission bands, such as maxima and FWHM, are almost independent of concentration that is why for our analysis of the EVI we have chosen the excitation/emission spectra with 1% of Eu impurities. The system of Eqs. (5)-(6) was solved graphically to yield the following values of the Huang-Rhys factor S and effective phonon energy ħω: 2.46 and 1150 cm−1, respectively.27] that assumes interaction of the impurity center with a single vibrational mode only. When T = 0, only the ground vibrational level is populated, only the first term survives in Eq. (7) and the intensity of the emission line is described by well-known Poisson-type distribution . When temperature is not zero, higher excited levels are getting involved into the emission transitions, and the contribution of the second term in Eq. (7) becomes important. The only variable parameter E0 in the last Eq. (7) can be varied until the best agreement with the experimental emission band profile is reached. With the ZPL energy E 0 = 24000 cm−1, the modelled emission band shape is compared with the experimental spectrum as shown in Fig. 5 . Good agreement between the modelled/experimental emission spectra in Fig. 5 serves as a good test for validity of the obtained Huang-Rhys factor, effective phonon energy, and ZPL position. It also can be mentioned that the value of the ZPL energy determined from the emission band shape fit is reasonably close to that one which can be determined from the point of intersection of the excitation and emission spectra (24700 cm−1).
3.4. Luminescence decay times of BaBeSiO4:Eu2+ phosphors
Figure 6 shows the decay curves of BaBeSiO4:xEu2+ samples (λex = 400 nm). The PL decay of 1%Eu2+ ions is almost singly-exponential, which can be fitted by the equation29]. The lifetime (τ) of Eu2+ ions defined by the PL intensity decay to 1/e are 423, 371, 339, 348 and 296 ns, corresponding to Eu2+ concentration x = 1%, 3%, 5%, 7% and 10%, respectively. There seems to be a slight increase in the experimentally determined luminescence life time upon increasing the Eu-concentration (5% to 7%Eu). Most probably this is caused by reabsorption of the emission at the higher Eu-concentrations. It is well known that reabsorption of emission gives rise to a longer decay time . Mostly, we can observed that with increasing Eu2+ doping, the lifetime of Eu2+ emission at 460 nm drops quickly. Furthermore, the lifetime is short enough as Eu2+-doped blue phosphors for potential applications in displays and lightings.
3.5. Thermal quenching properties of BaBeSiO4:Eu2+ phosphors
Thermal stability is very important for phosphor applications in white LEDs. The temperature dependence of the PL intensity of BaBeSiO4:Eu2+ under excitation at 337 nm and above room temperature is shown in Fig. 7(a) and comparison between the thermal quenching properties of BaBeSiO4:Eu2+ and BaMgAl10O17:Eu2+ (Fig. 7(a), inset). As can be seen in the inset, relative emission intensity decreases with an increase in temperature. We observed decays of 15% and 30% at 100°C and 200°C, respectively, for BaBeSiO4:Eu2+, and decays of 6% and 11% for BaMgAl10O17:Eu2+. These results indicate that BaBeSiO4:Eu2+ exhibits lower thermal stability than BaMgAl10O17:Eu2+. The thermal quenching originates from the temperature dependence of the electron-phonon interaction for both of the ground state and excited states. This nonradiative transition probability is strongly favored at high temperature, leading to the decrease in emission intensity. Such a high quenching temperature indicates weaker electron-phonon interaction in BaBeSiO4 system, which is supported by the rigid lattice with a more extended network of BeO tetrahedral. According to Lin et al. , the activation energy (Ea) can be expressed using the following equation:Fig. 7(b). The emission spectra of the phosphor do not shift as the temperature increases, indicative of its high stability of chromaticity against temperature.
3.6. White LED-lamp fabrication and electroluminescence (EL) spectra of BaBeSiO4:Eu2+ phosphors
To demonstrate the potential of BaBeSiO4:Eu2+ for pc-WLEDs application, the BaBeSiO4:3%Eu2+ phosphor was utilized to fabricate a white LED device driven by 350 mA current with red-emitting CaAlSiN3:Eu2+ phosphor, green-emitting (Ba,Sr)2SiO4:Eu2+ phosphor and a 365 nm LED chip. The mixing ratio of blue:green:red is 2:1.5:1.2 in wt%. The corresponding EL spectrum is shown in Fig. 8 , and the inset shows the photographs of the fabricated w-LED lamp and its practical emission under the same forward bias. An emitter-type LED package was chosen for the fabrication of the LED device on the basis of its high light extraction efficiency. The CIE color coordinates of the white LED was found to be (0.38, 0.39) at a CCT of 4077 K. The full set of 14 CRIs with the color rendering index Ra is listed in Table 2 . The average color-rendering index, Ra, was determined to be 90.13, which was considered to be fitted for lighting applications. In comparison with the blue InGaN chip pumped with YAG:Ce3+ phosphor (Ra = 75, CCT = 7756 K), the white LEDs in this study show higher Ra values and lower CCT values . These results show that BaBeSiO4:Eu2+ could be a potential blue-emitting phosphor for applications of display and illumination.
In summary, a blue-emitting BaBeSiO4:Eu2+ phosphor has been reported in this study. The excitation and emission spectra of this phosphor are broadband due to the 4f7 – 4f65d1 transitions of Eu2+ ions. It was also found that the PL emission spectrum is broadband with peak at 460 nm. The emission band shows a redshift (from 457 to 471 nm) with increasing Eu2+ concentration, mainly because of the change in the crystal-field splitting of 5d states. From the comparison between the experimental emission and excitation spectra we could estimate the main parameters of the electron-vibrational interaction for the studied system. The optimal concentration for Eu2+ in BaBeSiO4 was found to be about 3 mol.% and the critical transfer distance of Eu2+ 18.25 Å and the quenching mechanism is dominated by the dipole–dipole interaction. The energy barrier for thermal quenching was calculated to be 0.19 eV with the Arrhenius equation. Moreover, white UV LEDs are fabricated by integrating a 365 nm UV chip and a mixture of green-emitting (Ba,Sr)2SiO4:Eu2+ and red-emitting CaAlSiN3:Eu2+ phosphors into a single package. Under 350 mA forward-bias current, the package shows an excellent Ra of 90.13 with CCT and CIE chromaticity coordinates of 4077 and (0.38, 0.39), respectively. These results indicate the as-synthesized blue-emitting BaBeSiO4:Eu2+ phosphor can serve as the key material for phosphor-converted white UV LEDs.
This research was supported by the National Science Council of Taiwan under contract No. NSC-102-2221-E-033-050-MY2 and Ministry of Science and Technology under contract No. MOST 104-2623-E-033-002-ET and MOST 104-2623-E-033-003-ET. M.G. Brik acknowledges the Recruitment Program of High-end Foreign Experts (grant No. GDW20145200225), the Program for the Foreign Experts offered by Chongqing University of Posts and Telecommunications, European Regional Development Fund (Center of Excellence ‘Mesosystems: Theory and Applications’, TK114), Marie Curie Initial Training Network LUMINET, grant agreement No. 316906, the Ministry of Education and Research of Estonia, Project PUT430 and Visiting Professorship at the Institute of Physics, Polish Academy of Sciences.
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