1. Introduction

In recent years, white light-emitting-diode (LED) technology has attracted much attention in lighting industry due to the advantages of white-LEDs including low power consumption, reliability, long lifetime, safety, and environmentally friendly characteristics [1, 2]. The most dominant way to produce commercial white light-emitting diodes (WLEDs) is a combination of a blue emitting InGaN chip with a yellow emitting Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺) phosphor. However, that combination shows several disadvantages including high correlated color temperature (CCT~7750 K) and low color-rendering index (CRI~70-80) because of a lack of sufficient red emission [3–5]. Therefore, as an alternative, a combination of UV LED with red, green and blue emitting phosphors has been developed to improve the CRI, color stability and to tune CCT value [6, 7]. However, the luminescence efficiency is low in this system because the blue emission is reabsorbed by green and red phosphors [8]. Therefore, a single composition white-emitting phosphor which can be effectively excited by UV light ought to receive much attention to enhance the luminous efficiency. More importantly, a single- phased white light emitting phosphor with UV-LED chip system show better performances in stability, color reproducibility, and fabrication process than those of white LEDs with multiple emitting components phosphor system [9, 10]. The method of co-doping sensitizer and activator into one host lattice is one way to create a single–phased white-light-emitting phosphor using the principle of energy transfer from sensitizer to activator, such as Eu²⁺ and Mn²⁺ co-doping system, which has been introduced and demonstrated in several previous works. The examples are follows, Ca_6-x-yMg_x-z(PO₄)₄:yEu²⁺,zMn²⁺, CaAl₂Si₂O₈:Eu²⁺,Mn²⁺, Ca₉Gd(PO₄)₇: Eu²⁺,Mn²⁺, BaMg₂Si₂O₇: Eu²⁺,Mn²⁺, SrMg₂(PO₄)₂: Eu²⁺,Mn²⁺, Ca₂Gd₈(SiO₄)₆O₂:Ce³⁺,Mn²⁺, Ba₂Ca(BO₃)₂:Ce³⁺,Mn²⁺, Sr₃B₂O₆:Ce³⁺,Eu²⁺ phosphors [11–18].

In this work, we prepared a novel single-phased emission-tunable Na_0.34Ca_0.66Al_1.66Si_2.34O₈:Eu²⁺,Mn²⁺ (NCASO:Eu²⁺,Mn²⁺) phosphor suitable for UV-pumped white-emitting LED applications. Photoluminescence excitation (PLE) of NCASO:Eu²⁺,Mn²⁺ phosphor ranging from 250 to 420 nm fits well for the UV-LED application. Photoluminescence (PL) spectrum shows that the phosphor covers the full range of visible region which can create warm white emission resulted from the energy transfer from Eu²⁺ to Mn²⁺ ions. Furthermore, we have also investigated its luminescence properties as well as the energy transfer mechanism which was confirmed by observing the change in the luminescence spectra, energy transfer efficiency, and by decay profile analysis.

2. Experimental

2.1. Synthesis

Powder samples Na_0.34Ca_(0.66-x-y)Al_1.66Si_2.34O₈:xEu²⁺,yMn²⁺ (x = 0.07, y = Mn dopant concentration) were synthesized by a wet chemical synthesis method based on hydrolysis of tetraethylorthosilicate (TEOS) [19]. As raw materials, sodium nitrate (NaNO₃ ≥ 99.99%, Aldrich), europium (III) chloride hexahydrate (EuCl₃·6H₂O ≥ 99.99%, Aldrich), calcium nitrate nonahydrate (Ca(NO₃)₂·9H₂O ≥ 99.99%, Aldrich), aluminum nitrate nonahydrate (Al(NO₃)₃·9H₂O ≥ 98%, Aldrich), Manganese (II) chloride tertahydrate (MnCl₂·4H₂O ≥ 98%, Aldrich), tetraethyl orthosilicate (TEOS, 99.999%, Aldrich) were used. All materials except for TEOS were dissolved in deionized water, and then the TEOS was dissolved in ethanol. These two solutions were thoroughly mixed together. The mixture was dehydrated in an oven at 120°C for about 24 h until the solvent was completely dried. The dried powders were fired at 1400°C in a reducing atmosphere of a mixture of H₂ (5%) and N₂ (95%) for 3 h.

2.2. Characterizations

The luminescent properties and quantum efficiency of NCASO:Eu²⁺,Mn²⁺ phosphors were analyzed by using a F-7000 Hitachi fluorescence spectrophotometer PL system equipped with a xenon lamp (500 W) as an excitation source. X-ray diffraction (XRD) data was obtained over a range of 15° ≤ 2θ ≤ 60° at step of 3°/min with Cu-Kα radiation using a diffractometer (D/MAX-RB(12KW), RIGAKU, Japan). Decay characteristics are probed by a Fluorescence Lifetime Spectrometer (FL920, Edinburgh Instruments, Wales) which is operated based upon time correlated single photon counting scheme.

3. Result and discussion

3.1 phase identification of NCASO:0.07Eu²⁺,yMn²⁺ phosphor

Phase purity of all samples was analyzed by XRD analysis. Figure 1 illustrates the X-ray diffraction (XRD) patterns of NCASO:0.07Eu²⁺,yMn²⁺ phosphors with various Mn²⁺doping concentration (y = 0, 0.05, 0.1, 0.2, 0.3). All of the profiles were found to match well with that reported in JCPDS file 86-1650 regardless of the content of dopants and this observation indicates that no impurity phase is present in the prepared samples. It clearly suggests that the activator and sensitizer have been incorporated in the lattice. The crystal structure of NCASO host, which is included in the plagioclase feldspars group, was reported in our previous report [20]. In the crystal structure of NCASO phosphor, Na⁺ and Ca²⁺ ions share the same sites. The Na⁺/Ca²⁺ ions have four different coordination numbers (Na⁺/Ca²⁺-1, Na⁺/Ca²⁺-2, and Na⁺/Ca²⁺-4 are 8-coordinated; Na⁺/Ca²⁺-3 is 10-coordinated). Typically, effective radii (r) of cations change depending on the coordination number (CN) [21]. The ionic radii of Eu²⁺ ions (r = 1.25 Å when CN = 8, r = 1.35 Å when CN = 10) and those of Mn²⁺ ions (r = 0.96 Å when CN = 8) are similar to those of Na⁺ ions (r = 1.18 Å when CN = 8, 1.24 Å when CN = 9, 1.39 Å when CN = 12) and Ca²⁺ ions (r = 1.12 Å when CN = 8, 1.23 Å when CN = 10). Since both 4-coordinated Al³⁺ (r = 0.39 Å) and Si⁴⁺ (r = 0.26 Å) sites are too small for Eu²⁺ and Mn²⁺ ions to occupy, we have suggested that Eu²⁺ and Mn²⁺ ions are expected to occupy the Na⁺/Ca²⁺ shared sites in the host lattice.

 

Fig. 1 XRD patterns of NCASO:0.07Eu²⁺,yMn²⁺ phosphors.

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3.2 Photoluminescence properties

PLE and PL spectra of NCASO:0.07Eu²⁺ phosphor are presented in Fig. 2 . PLE spectrum shows a broad band from 250 to 420 nm which corresponds to the 4f⁷ → 4f⁶5d¹ transition of Eu²⁺ ions. Under excitation at 365 nm, the NCASO:Eu²⁺ phosphor produces broad band blue emission centered at around 440 nm which is attributed to the 4f⁶5d¹ →4f⁷ of the Eu²⁺ ion. As represented in Fig. 3 , PL spectrum of NCASO:Eu²⁺ shows a very broad blue emission band peaking at around 440 nm, which is assigned to the typical 4f⁶5d¹ → 4f⁷ transition of Eu²⁺ ion and the PLE spectrum of NCASO:Mn²⁺ consist of several bands centered at around 342, 405, 420, and 460 nm, corresponding to the transition of the Mn²⁺ ion from the ground level ⁶A₁(⁶S) to ⁴T₂(⁴D), [⁴A₁(⁴G), ⁴E(⁴G)], ⁴T₂(⁴G), and ⁴T₁(⁴G) levels, respectively [13]. We have observed a spectral overlap between the emission band of the Eu²⁺ ions and the excitation band of the Mn²⁺ ions at around 410 nm which implies that a part of energy transfers between Eu²⁺ and Mn²⁺ ions. Therefore, effective energy transfer from Eu to Mn (ET_Eu→Mn) was expected.

 

Fig. 2 PLE and PL spectra of NCASO:0.07Eu²⁺ phosphor.

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Fig. 3 Spectral overlap between the normalized PL spectrum of NCASO:Eu²⁺ and the PLE spectrum of NCASO:Mn²⁺.

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In addition, another evidence for the energy transfer in NCASO:Eu²⁺,yMn²⁺ is shown in Fig. 4 . PLE spectra monitored at 440 nm and 570 nm of Eu²⁺ and Mn²⁺, respectively, appears very similar to each other. Moreover, the absence of characteristic Mn²⁺ excitation band rules out direct excitation of Mn²⁺ ions which indicates efficient energy transfer form Eu²⁺ ions to Mn²⁺ ions. The PLE spectrum shows a broad band ranged at 250-420 nm which means that the NCASO:Eu²⁺,Mn²⁺ phosphor has a potential for UV LED application. After co-doping Eu²⁺ and Mn²⁺ ion in NCASO host lattice, the sample exhibits two broad emission bands centered at around 440 and 570 nm under the 365 nm excitation which consist of a blue band and orange one originating from the f-d transition of the Eu²⁺ ion and the ⁴T₁-⁶A₁ transition of the Mn²⁺ ion, respectively. The emission spectrum nearly covers the entire visible region. Thus, white-light can be obtained by combining the emission of Eu²⁺ and Mn²⁺ ions present in a single host lattice by simply adjusting the relative ratio of Eu²⁺ and Mn²⁺ activators via the principle of energy transfer.

 

Fig. 4 PLE and PL spectra of NCASO:0.07Eu²⁺,0.20Mn²⁺ phosphor.

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In order to further investigate the energy transfer mechanism between the Eu²⁺ and Mn²⁺ ions on NCASO phosphor, a series of samples were synthesized and their luminescent properties are demonstrated. Figure 5 shows that the PL spectra of NCASO:0.07Eu²⁺,yMn²⁺ phosphors with different Mn²⁺ contents (y = 0, 0.03, 0.07, 0.10, 0.15, 0.20, 0.30) under 365 nm excitation. The PL emission intensity of Eu²⁺ at around 440 nm was found to decrease with increasement of Mn²⁺ content due to the enhancement of the energy transfer from Eu²⁺ to Mn²⁺ ions. On the other hand, the emission intensity of Mn²⁺ at 570 nm reaches a maximum when y = 0.20 and decreases on account of the Mn²⁺-Mn²⁺ internal concentration quenching [22]. This result further supports the energy transfer process from Eu²⁺ to Mn²⁺ ions.

 

Fig. 5 PL spectra for NCASO:0.07Eu²⁺,yMn²⁺ phosphors on Mn²⁺ doping content (y)

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The decay curves of NCASO:0.07Eu²⁺,yMn²⁺ phosphor samples excited at 375 nm and monitored at 440 nm are shown in Fig. 6 . The corresponding luminescence decay curves can be fitted by a second-order exponential decay mode by following equation [23, 24]:

 

Fig. 6 Decay curves of Eu²⁺ emission for NCASO:0.07Eu²⁺,yMn²⁺ monitored at 440nm.

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where I is the luminescence intensity; A₁ and A₂ are constant; t is the time, and τ₁ and τ₂ are decay time for exponential components. Using this parameters, the average decay time (τ) can be obtained by the formula given in the following [25];

The value of τ₁, τ₂, A₁, and A₂ are analyzed and summarized in Table 1 . The average decay times were determined to be 0.637, 0.578, 0.540, 0.492, 0.468, and 0.435 μs for NCASO:0.07Eu²⁺,yMn²⁺ with y = 0, 0.05, 0.10, 0.15, 0.20, and 0.30, respectively. The results show that the average decay time for the Eu²⁺ ions decreases with increase in the Mn²⁺ doping content y, which is a strong evidence for the energy transfer from Eu²⁺ to Mn²⁺ ions in the NCASO:0.07Eu²⁺,yMn²⁺ phosphor.

Table 1. Decay times of NCASO:0.07Eu²⁺,yMn²⁺ phosphors excited at 375 nm with emission monitored at 440nm.

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Generally, energy transfer efficiency (η_T) can be described by using the following formula [26];

where I_S0 is the luminescence intensities of the Eu²⁺ of the samples in the absence of the Mn²⁺ ions and I_S is the luminescence intensities of the Eu²⁺ ions in the presence of the Mn²⁺ ions. The dependance of the energy transfer efficiency (η_T) as a function of Mn²⁺ content is displayed in Fig. 7 . It is shown that the energy transfer efficiency (η_T) of NCASO:0.07Eu²⁺,yMn²⁺ phosphor increase gradually with increasing the Mn²⁺ doping concentration. Thus, we confirm that the energy transfer process from Eu²⁺ to Mn²⁺ ions is efficient in NCASO host.

 

Fig. 7 Dependence of the energy transfer efficiency η_T on the Mn²⁺ content (y)

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According to Dexter and Schulman, the concentration quenching of luminescence occurs due to the energy transfer from one activator to another until the energy is consumed in the lattice [27]. The critical distance R_Eu-Mn for energy transfer from Eu²⁺ to Mn²⁺ ions was suggested by Blasse [28]. The critical distance can be expressed according to the following equation:

where V is the volume of the unit cell; x is the critical concentration; and N is the number of available cation site for dopant in the unit cell. For the NCASO host, N = 8 and V = 1345.28 ų [20]. The critical concentration is 0.13 at which the luminescence intensity of Eu²⁺ becomes only half of that of the sample with no Mn²⁺ included (when the energy transfer efficiency is 0.5). From the above equation, the critical distance R_Eu-Mn was calculated to be about 13.52 Å. There are two types of energy transfer: one is exchange interaction and the other is multi-polar interaction [29, 30]. In the case of the exchange interaction, the critical distance between the sensitizer and activator should be shorter than 3-4 Å [31]. However, the calculated critical distance for NCASO:Eu²⁺,Mn²⁺ phosphor is much longer than 3-4 Å so that the energy transfer between the Eu²⁺ and Mn²⁺ ion in NCASO phosphor is probably the electric multi-polar interaction type.

On the basis of Dexter’s energy transfer formula for exchange interaction and the Reisfeld’s approximation, the following relation can be described [27, 32]:

where η₀ and η are the luminescence quantum efficiency of Eu²⁺ without and with Mn²⁺; C is the sum of the content of Eu²⁺ and Mn²⁺; n = 6, 8, and 10 correspond to dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole interactions, respectively. The value of η₀/η can be approximately calculated by the ratio of relative luminescence intensities [27, 28, 32]:

where I₀ is intrinsic luminescence intensity of Eu²⁺ and I is the luminescence intensity of Eu²⁺ with the presence of Mn²⁺. The results of I₀ /I-C^n/3 plots are illustrated in Fig. 8 and a linear relationship was found when n = 6. This clearly implies that the energy transfer mechanism from the Eu²⁺ to Mn²⁺ ions in NCASO:Eu²⁺,Mn²⁺ phosphor is an electric dipole-dipole interaction.

 

Fig. 8 Dependence of I₀ /I of Eu²⁺ on (a) C^6/3,(b) C^8/3, and (c) C^10/3

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For high power LED applications, thermal stability of phosphor is one of the important parameter. Temperature-dependent emission spectra of NCASO:0.07Eu²⁺,0.20Mn²⁺ phosphor under 365 nm excitation are indicated in Fig. 9 . The inset displays the thermal quenching of Eu²⁺ and Mn²⁺ emission intensity in NCASO and that of commercial BaMgAl₁₀O₁₇:Eu²⁺ phosphor (KEMK63/F-P1, Phosphor Technology Ltd). It can be seen that normalized PL peak intensity of Eu²⁺ and Mn²⁺ ions decreased to 74% and 79% of the initial value at 100 °C, respectively (integrated intensity of NCASO: 0.07Eu²⁺,0.20Mn²⁺ phosphor decreased to 76%). Above 100 °C, the thermal quenching of NCASO:0.07Eu²⁺,0.20Mn²⁺ phosphor is much significant. Also, compared to BaMgAl₁₀O₁₇:Eu²⁺ phosphor, NCASO:0.07Eu²⁺,0.20Mn²⁺ phosphor shows lower thermal stability. These thermal quenching phenomena can be explained by the help of configurational coordinate diagram. With increasing temperature, electron-phonon interaction is enhanced. Through phonon interaction, the excited luminescent center is thermally activated and subsequently released through cross-over between the excited state and ground state. As a result, the emission intensity decreases due to enhanced population density of phonon [33].

 

Fig. 9 PL spectra of NCASO:0.07Eu²⁺,0.20Mn²⁺ phosphor excited at 365 nm with different temperatures. The inset shows the normalized PL intensity as a function of temperatures.

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To better understand the thermal quenching phenomena, the thermal quenching data were fitted using the Arrhenius equation [34],

where I₀ and I(T) are the luminescence intensity of NCASO:0.07Eu²⁺,0.20Mn²⁺ (by integrating the area of each spectrum) at room temperature and a given temperature, respectively, A is constant, E_a is the activation energy for thermal quenching, and k_B is Boltzmann’s constant (8.617 x 10⁻⁵ eV K⁻¹). From the equation, we have obtained E_a of NCASO:0.07Eu²⁺,0.20Mn²⁺ to be 0.066 eV.

Table 2 reports the Commission Internationale de L’Eclairage (CIE) chromaticity coordinates, Internal Quantum Efficiency (IQE), and External Quantum Efficiency (EQE) of NCASO:Eu²⁺,yMn²⁺ phosphors excited at 365nm, which were calculated based on the corresponding PL spectrum. Figure 10 represents the data in the CIE chromaticity diagram. The result shows that the emission color can be gradually modulated from blue to white by increasing the Mn²⁺ concentration from 0 to 0.30. In other words, it is possible to obtain both blue emission from the Eu²⁺ ions and the orange emission from the Mn²⁺ ions in a single host based on the energy transfer mechanism under 365nm excitation. In particular, the sample composition of NCASO:0.07Eu²⁺,0.30Mn²⁺ with CIE coordinates of (0.333, 0.317) shows the warm white light with correlated color temperatures of 5469 K and is very close to ideal white point (0.333, 0.333). Thus, we can simply control the color of the phosphor and make the white light with a preferred CCT value through changing the Mn²⁺ doping concentration so as to meet the requirements of practical lighting application.

Table 2. Comparison of the CIE chromaticity coordinates (x, y), IQE and EQE for NCASO:0.07Eu²⁺,yMn²⁺ phosphors excited at 365nm

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Fig. 10 CIE chromaticity diagram for NCASO:Eu²⁺,yMn²⁺ phosphors (point A to H) excited at 365nm and the ideal white point (0.33, 0.33) depending on the different y value.

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In addition, the quantum efficiency (QE) of a phosphor is an important parameter for LED application. In order to determine the absolute quantum efficiency of photo conversion for phosphor, the internal quantum efficiencies (IQE, η_i) and external quantum efficiencies (EQE, η_o) were calculated by using the following equations [35];

where E(λ)/hν, R(λ)/hν, and P(λ)/hν are the number of photons in the spectrum of excitation, reflectance, and emission of the phosphor, respectively. The reflection spectrum of spectralon diffusive white standards is used for calibration (the reflectivity is nearly 100% in the range of 200-900 nm). Under 365 nm excitation, IQE and EQE of NCASO:Eu²⁺,yMn²⁺ phosphors are displayed in Table 2. The IQE and EQE of NCASO:Eu²⁺,yMn²⁺ phosphor decrease from 89% to 57% and from 68% to 46%, respectively for an increase in y content from 0 to 0.3. The tendency shows that the QE decreases with increase of Mn²⁺ content. The overall QEs can be enhanced through further optimization of the experimental condition and composition of phosphors.

4. Conclusion

In summary, we have successfully synthesized a new single-phased emission-tunable NCASO:Eu²⁺,Mn²⁺ phosphor. The phase formation of NCASO:Eu²⁺,Mn²⁺ was identified by the XRD analysis. Under 365 nm excitation, the emission color can be easily tuned from blue to white by simply adjusting the Mn²⁺ content in the host lattice due to energy transfer from Eu²⁺ to Mn²⁺ ions. An efficient energy transfer was inferred from the changes in relative intensity of blue and orange emission from Eu²⁺ and Mn²⁺ respectively and was identified as electric dipole-dipole mechanism. Finally, a warm-white-light emission with CIE coordinates of (0.333, 0.317) and CCT of 5469 K was realized in NCASO:0.07Eu²⁺,0.30Mn²⁺ phosphor under 365 nm excitation. Thus, the obtained phosphor has been proven to be potentially useful as a single-phased white- emitting phosphor for UV-LEDs.