Novel blue-emitting phosphors - BaBeSiO<sub>4</sub>:Eu<sup>2+</sup>: luminescence properties and its application for UV-light emitting diodes

Kai-Yuan Yeh; Chun-Chuen Yang; Wei-Ren Liu; M.G. Brik

doi:10.1364/OME.6.000416

1. Introduction

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 [4]. 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 [5]. 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 [6].

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 [32]. Many novel phosphors with Eu²⁺ activated for UV LED/near-UV LEDs have been reported, including KSrY(PO₄)₂:Eu²⁺ [8], Sr₈MgLn(PO₄)₇:Eu²⁺ (Ln = Y, La) [9], CaSrSiO₄:Eu²⁺ [10], BaLa₂Si₂S₈:Eu²⁺ [11], Ba(Si, Al)₅(O,N)₈:Eu²⁺ [12], (Ba, Sr)Y₂Si₂Al₂O₂N₅:Eu²⁺ [13] and Ba₉Y₂Si₆O₂₄:Eu²⁺ [14] 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 [7].

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 T_c = 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 [34]. Figure 1 presents the refinement results obtained at T_c = 900°C, 1000°C, 1200°C, and 1300°C. The formation process of BaBeSiO₄ at various T_cs 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 T_cs). Notably, the BaBeSiO₄ phase was firstly observed at a T_c of 1000°C, and the molar ratio of this phase increased with T_cs.

Fig. 1 Refined powder X-ray diffraction pattern of Eu²⁺-doped BaBeSiO₄ powders at T_a = (a) 1300°C, (a) 1200°C, (c) 1000°C, and (d) 900°C. Observed (crosses), calculated (solid line) and difference (bottom) values are shown.

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Figure 2(a) presents the results of the refinement of the T_c = 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 [35]. 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 T_c = 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 T_c, and suggests that the synthetic temperature must exceed 1400°C.

Fig. 2 (a) Rietveld refinement of BaBeSiO₄:Eu²⁺ phosphor XRD profiles with T_a = 1400°C. (b) Variations of molar ratios of intermediate and final products with T_a.

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Table 1. The refinement parameters of the T_a = 1400°C sample.

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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. [17] 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.

Fig. 3 (a) Emission spectra of BaBeSiO₄:Eu²⁺ phosphors processed at various temperatures; (b) PLE and PL spectra of BaBeSiO₄:xEu²⁺ phosphors 1%Eu, 2%Eu, 3%Eu, 5%Eu, 7%Eu and 10%Eu; (c) The CIE chromaticity diagram for BaBeSiO₄:Eu²⁺ phosphors with different Eu²⁺ contents. The inset shows the luminescence photograph of BaBeSiO₄:Eu²⁺ phosphor with different concentration of Eu²⁺ excited at 365 nm.

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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 [18]. The PLE spectrum showed broad absorption bands between 250 and 400 nm, which are attributed to the 4f⁷-4f⁶5d¹ transition of the Eu²⁺ ion [19]. 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⁺ [20]. The crystal field around Eu²⁺ can be estimated as [21]

D_{q} = \frac{Z e^{2} r^{4}}{6 R^{5}}

where D_q is the crystal field strength, R is the distance between the central ion and ligands, Z is the charge or valence of the anion, e stands for the charge of an electron, and r is the average radial coordinate of d-electron of the central ion. According to Eq. (1), D_q is inversely proportional to the 5th power of bond-length R: D_q ∝ R⁵, when the metal–ligand distance decreases, the crystal field splitting significantly increases. For the Ba_1–xEu_xBeSiO₄ 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 [23] pointed out that the critical transfer distance (R_c) is approximately equal to twice the radius of a sphere with the volume of the unit cell:

R_{c} \approx 2 {(\frac{3 V}{4 π x_{c} N})}^{\frac{1}{3}}

where x_c is the critical concentration, Z is the number of formula units per unit cell, and V is the volume of the unit cell. By taking the value of V = 191.18 Ǻ³, N = 2, and x_c = 0.03, the critical transfer distance R_c was found to be ~18.25 Ǻ. On the other hand, according to Dexter's theory on non-radiative energy transfer, the critical transfer distance (R_c) also can be calculated from the spectral experimental data [24]:

R_{c}^{6} = 0.63 \times 10^{28} Q_{A} / E^{4} \int f_{s} (E) F_{A} (E) d E

where Q_A = 4.8 × 10⁻¹⁶ f _d is the absorption coefficient, f _d is oscillator strength of the absorption transition of the Eu²⁺ ion. E is the energy of maximum spectral overlap, and ∫f_S(E)F_A(E)dE is the spectral overlap integral. The value of f _d is 10⁻² while considering the 4f⁶5d¹→4f⁷ absorption band. Based on 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 ∫f_S(E)F_A(E)dE between the emission and excitation was estimated to be 0.0154 eV⁻¹. A value of R_c = 16.22 Å is obtained, which agrees approximately with the data that were obtained by using the concentration quenching method.

Fig. 4 (a) The intensity and wavelength dependence of Eu²⁺ emission on the Eu²⁺ content x in samples BaBeSiO₄:xEu²⁺; (b) Log plot for the emission intensity of BaBeSiO₄: Eu²⁺ at 337 nm per activator ions as a function of the activator concentration.

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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 [25]. 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 [26]. The emission intensity (I) per activator concentration (x) can be expressed by the following equation:

\frac{I}{x} = k {[1 + β {(x)}^{\frac{θ}{3}}]}^{- 1}

where k and β are constants for each type of interaction for a given host lattice; θ = 6, 8, 10 for dipole–dipole, dipole–quadrupole, quadrupole–quadrupole interactions, respectively. The 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 [27]: 1) the Stokes shift ΔE_S, 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 ΔE_S, and 3) the effective phonon energy ħω. In the framework of the single configurational coordinate model [27] 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 [27], these parameters can be extracted from the following equations after a careful consideration of the experimental emission and absorption (or excitation) spectra [27]:

Δ E_{S} = (2 S - 1) ℏ ω,

Γ (T) = 2.35 ℏ ω \cdot {[S \cdot \coth (\frac{ℏ ω}{2 k T})]}^{1 / 2},

where 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.

With these values of Huang-Rhys factor and effective phonon energy, the emission band shape I was modelled using Eq. (7) [27, 28 ] and compared with the experimental spectrum:

I = \frac{e^{- S} S^{p}}{p!} (1 + S^{2} \frac{e^{- ℏ ω / k T}}{p + 1}), p = \frac{E_{0} - E}{ℏ ω},

where E ₀ is the zero-phonon line (ZPL) energy, p is the number of the effective phonons involved into the emission transition. This equation has been also obtained in the framework of the single configuration coordinate model [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

I = e^{- S} S^{p} / p!

. 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).

Fig. 5 Comparison of the calculated emission spectrum (blue dashed line) with the experimental emission spectrum (black solid line) for BaBeSiO₄: 1% Eu²⁺ phosphors. The experimental excitation spectrum is shown by the red solid line. All spectra were normalized to 1 for easiness of comparison.

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3.4. Luminescence decay times of BaBeSiO₄:Eu²⁺ phosphors

Figure 6 shows the decay curves of BaBeSiO₄:xEu²⁺ samples (λ_ex = 400 nm). The PL decay of 1%Eu²⁺ ions is almost singly-exponential, which can be fitted by the equation

I = I_{0} \exp (\frac{- t}{τ})

where I and I₀ are the luminescence intensities at time t and 0, respectively. This indicates that there are no any other channels to depopulate the 5d excited electrons of Eu²⁺ ions. However, when increasing the concentration of Eu²⁺ ions, the decay curve significantly deviates from the single exponential rule. The decay behavior may occur as a result of the nonradiative energy transfer among the Eu²⁺ ions, which indicating there are more than one relaxation processes because of concentration quenching on killer centers. As the concentration of Eu²⁺ increases, the distance between the Eu²⁺ ions decreases, leading to high probability of energy transfer among the Eu²⁺ ions which becomes more frequent, and therefore causing the non-monoexponential decay curves [29]. 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 [33]. 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.

Fig. 6 Decay curves of as-synthesized BaBeSiO₄: xEu²⁺ phosphors monitored at 400 nm (x = 1%, 3%, 5%, 7% and 10%). The inset shows the linear portion on a log axis with an exponential fit (Eq. (8)).

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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. [30], the activation energy (E_a) can be expressed using the following equation:

I_{T} = \frac{I_{0}}{1 + c \exp (- E_{a} / k_{B} T)}

where I₀ and I_T are the initial PL intensity and the PL intensity at a given temperature T (K), respectively; c is the frequency factor with the unit of radiative rate; k_B is the Boltzmann constant, 8.617 × 10⁻⁵ eV/K; and E_a is the activation energy for thermal quenching. This activation energy can be calculated from plotting of ln[(I₀/I_T)-1] against 1/k_BT, where a straight slope equals E_a. The activation energy for thermal quenching was found to be 0.1941 eV for BaBeSiO₄:Eu²⁺ as shown in 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.

Fig. 7 (a) Temperature dependence of PL spectra of BaBeSiO₄: 3%Eu²⁺ phosphor under 337 nm excitation. The inset shows thermal quenching for the relative emission intensity of BaBeSiO₄: 3%Eu²⁺ and commercial silicate phosphor; (b) A ln[(I₀/I_T)-1] vs. 1/k_BT activation energy graph for thermal quenching of BaBeSiO₄: 3%Eu²⁺ phosphor.

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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 [31]. These results show that BaBeSiO₄:Eu²⁺ could be a potential blue-emitting phosphor for applications of display and illumination.

Fig. 8 EL spectrum of white LED from blue-emitting BaBeSiO₄: Eu²⁺, green-emitting (Ba,Sr)₂SiO₄:Eu²⁺, and red-emitting CaAlSiN₃:Eu²⁺ phosphors using a 365 nm NUV chip. The inset shows a photograph of a LED lamp package driven by 350 mA current.

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Table 2. Full of 14 CRIs and Ra of a 365 nm near-UV chip with BaBeSiO₄:Eu²⁺, (Ba,Sr)₂SiO₄:Eu²⁺ and CaAlSiN₃:Eu²⁺

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4. Conclusions

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.

Acknowledgments

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.

References and links

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Phase 1 (93.8%):BaBeSiO_4, monoclinic C 1 m 1 a = 8.8290(9) Å, b = 5.1428(5) Å, β = 124.428(4)˚
Atom	x	y	z	U_iso×100	Wyckoff Position	Fraction
Ba	0.053(7)	0	0.154(16)	2.84	2a	1
Be	-0.026(32)	0	-0.10(7)	6.85	2a	1
Si	0.581(10)	0	0.670(22)	1.88	2a	1
O(1)	0.365(13)	0	0.198(26)	11.59	2a	1
O(2)	0.456(15)	0	0.883(25)	5.33	2a	1
O(3)	0.182(7)	0.246(12)	0.759(21)	5.12	4b	1
Phase 2(6.2%):BaBeSiO_3, Orthorhombic P 2₁2₁2₁ a = 4.5801(26) Å, b = 5.608(4) Å, c = 12.462(8) Å
Atom	x	y	z	U_iso×100	Wyckoff Position	Fraction
Ba	0.2461	0.2995	0.3517	2.61	4a	1
Si	0.1595	0.3726	0.0663	3.03	4a	1
O(1)	0.0289	0.6230	0.0618	0.69	4a	1
O(2)	0.2259	-0.1876	0.2421	0.17	4a	1
O(3)	1.0821	0.3751	0.0429	0.29	4a	1
χ²=1.509		R_p=15.32%			R_wp=21.96%

Phase 1 (93.8%):BaBeSiO_4, monoclinic C 1 m 1 a = 8.8290(9) Å, b = 5.1428(5) Å, β = 124.428(4)˚
Atom	x	y	z	U_iso×100	Wyckoff Position	Fraction
Ba	0.053(7)	0	0.154(16)	2.84	2a	1
Be	-0.026(32)	0	-0.10(7)	6.85	2a	1
Si	0.581(10)	0	0.670(22)	1.88	2a	1
O(1)	0.365(13)	0	0.198(26)	11.59	2a	1
O(2)	0.456(15)	0	0.883(25)	5.33	2a	1
O(3)	0.182(7)	0.246(12)	0.759(21)	5.12	4b	1
Phase 2(6.2%):BaBeSiO_3, Orthorhombic P 2₁2₁2₁ a = 4.5801(26) Å, b = 5.608(4) Å, c = 12.462(8) Å
Atom	x	y	z	U_iso×100	Wyckoff Position	Fraction
Ba	0.2461	0.2995	0.3517	2.61	4a	1
Si	0.1595	0.3726	0.0663	3.03	4a	1
O(1)	0.0289	0.6230	0.0618	0.69	4a	1
O(2)	0.2259	-0.1876	0.2421	0.17	4a	1
O(3)	1.0821	0.3751	0.0429	0.29	4a	1
χ²=1.509		R_p=15.32%			R_wp=21.96%

Novel blue-emitting phosphors - BaBeSiO₄:Eu²⁺: luminescence properties and its application for UV-light emitting diodes

Abstract

1. Introduction