Fig. 4a shows the concentration dependent PL spectra of BC1-xZSO:xEu2+ phosphors (x = 0.02, 0.04, 0.06, 0.10, and 0.12 mol) under 340 nm excitation. This series of all samples produce a strong and broad green emission band with a peak around 502 nm, which corresponds to the 4f65d1’4f7 transition of the Eu2+ ion. The emission spectra have no obvious changes in the shape except for the emission intensity. It can be seen that the PL intensity of BC1-xZSO:xEu2+ phosphor varies with Eu2+ dopant concentrations. The effect of different concentrations of the Eu2+ ions on PL intensity of the obtained BCZSO:Eu2+ phosphor is depicted in Fig. 4b. The PL intensity increases with increasing Eu2+ concentration up to a maximum value at x = 0.06 mol. Then the PL intensity decreases as the Eu2+ concentration was further increased beyond x = 0.06 due to the concentration quenching effect. It is obvious that the optimum doping concentration of the Eu2+ ion is 0.06. The concentration quenching is mainly ascribed to the energy transfer occurs among the same type of Eu2+ ions, leading to non-radiative transitions [34].

3.4. Critical transfer distance (Rc) in BC1-xZSO:xEu2+ phosphors

The critical distance (Rc) between the Eu2+ ions for the energy transfer process can be calculated by utilizing the concentration quenching method suggested by Blasse [35].The Rc is approximately equal to twice the radius of a sphere with this volume:

Rc ‘ 2 (2)

Blasse has pointed out that if the activator is introduced solely on Z ion sites, where is the volume of the unit cell, is the critical concentration of the activator ion, and is the number of Z ions in the unit cell, then there is on the average of one activator ion per / . In the present case, BCZSO:Eu2+, we observed that quenching of emission intensity occurs when x exceeds 0.06 mol, namely the critical concentration is 0.06. Using = 1582.96 ï¿½ï¿½3, and N = 4, the Rc value was calculated to be about 23.2 ï¿½ï¿½ on the basis of the above equation. Thus, the energy transfer in the present case should be electric multipole – mulipole interaction.

In addition, the critical distance can be calculated from Dexter’s formula. Considering the non-radiative energy transfer, the critical distance of Eu2+ can be calculated from the spectral experimental data. The value of the critical transfer distance (Rc) can be found from the following equation [36]

= (3)

Where P is the oscillator strength of the Eu2+ ion. the energy of maximum spectral overlap and express the spectral overlap integral from the normalized excitation and emission spectrum of BC0.94ZSO: 0.06Eu2+ phosphor. For P, a value of 0.01 for an allowed 4f’5d transition of Eu2+ [37]. The values of and were calculated from the experimental data, are 3.1 eV and 0.021 eV-1 respectively. From Eq. (3), the value of for the energy transfer in BC0.94ZSO:0.06Eu2+ was calculated as 13.7 ï¿½ï¿½. The RC obtained using the critical concentration is larger than that obtained by using Dexter formula. This is because the value using the critical concentration data involves the entire possible interaction between the host lattice and the activator [38]. As a result, if the activating volume of luminescence in the phosphor is considered, the ratio of energy transfer possible by dipole interaction type to the whole possible interaction is (13.7/23.2)3 [38]. That is, the energy transfer rate by dipole interaction type is 20 % of total activating volume in the luminescence of BC0.94ZSO:0.06Eu2+ [38] phosphors. However, the value obtained using spectral overlap data considered dipole’dipole interaction [39]. This result additionally confirms that the energy transfer mechanism from Eu2+ to Eu2+ ion through the dipole’dipole interaction.

3.5. Mechanism of energy transfer in BCZSO:Eu2+ phosphor

In general, non-radiative energy transfer from a Eu2+ ion to another Eu2+ ion may occur by an exchange interaction, radiation reabsorption, or an electric multipole’multipole interaction [40]. Dexter [34] already reported that exchange interaction is responsible for the energy transfer of forbidden transition, and a typical critical distance is approximately 5ï¿½ï¿½. In the case of BCZSO:Eu2+ phosphor, the 4f7’4f65d1 transition of Eu2+ is allowed, and hence the mechanism of exchange interaction plays no role in the energy transfer between Eu2+ ions. Generally the mechanism of radiation reabsorption is only effective when the emission and excitation spectra are broadly overlapping. However, the radiation reabsorption is unlikely to occur in this case, due to the little overlapping between excitation and emission spectrum for BCZSO:Eu2+. Therefore, the process of energy transfer between Eu2+ ions could occur as an electric multipole’multipole interaction according to Dexter theory [34]. The multipole interaction occurs between the same sorts of activators, the strength of multipole’multipole interaction can be determined from the change in the emission intensity from the emitting level which has the multipole interaction, The emission intensity ( ) per activator ion can be expressed by the following equation [ 40,41]

= (4)

where is the concentration of the activator ions which is not less than the critical concentration (Xc), and are constants for each interaction in the same excitation condition for a given host lattice. For ‘ Xc, the non-radiative losses are attributable to multipolar transfer, and Eq. (4) can be simplified as given below

= (5)

where is a constant [40,41] and ï¿½ï¿½ is an indication of electric multipolar character. According to the equation (5), ï¿½ï¿½ = 3 stands for energy transfer among the nearest neighbor ions, while ï¿½ï¿½ = 6, 8, 10 corresponds to dipole dipole (d-d), dipole ‘ quadrupole (d-q), quadrupole ‘ quadrupole (q-q) interactions, respectively. Fig. 5 shows the dependence of log (I/ ) on log( ) was found to be relatively linear, and it yields a straight line with a slope equal to ‘ï¿½ï¿½/3. The slope was determined to be -1.98. Therefore, the value of ï¿½ï¿½ can be calculated to be 5.94, which is close to 6. The result indicates that the concentration quenching is caused by the energy transfer mechanism among the Eu2+ ions should be dominated by d-d interaction in BCZSO:Eu2+ phosphor.

3.6. Photoluminescence lifetime analysis

Fig. 6 depicts the decay curve of BC0.94ZSO:0.06Eu2+ phosphors at room temperature. The experimental decay curve was obtained under the excitation at 340 nm and monitored at 502 nm. The corresponding luminescence decay times can be fitted well by double exponential equation as [42]

(6)

where I and I0 is the luminescence intensity at times t and 0, and are fitting constants, t is the time, and are the decay time for the two exponential components, respectively. In addition, the effective average lifetime ( ) can be calculated as the following equation [43]

(7)

of Eu2+ (4f65d1’4f7 transition) was calculated to be 1.27ï¿½ï¿½s. The double exponential decay dynamics is in agreement with the fact that two types of cation sites could exist in the host lattice. In order to avoid the superimposition of images and signals, it is well known that the lifetime of phosphors applied in the field of displays and lights is a main issue [42]. The results show that the lifetime is reasonable for the 5d’4f allowed transition of Eu2+, and it is also suitable for WLEDs.

3.7. Commission International del’Eclairage (CIE) chromaticity coordinates

Fig. 7 shows the 1931 CIE chromaticity diagram of as’synthesized BC1-xZSO:xEu2+ (x = 0.06) phosphor under 340 nm UV excitation. The color coordinates were calculated to be (x = 0.259, y = 0.463) which is indicated by a cross mark. The obtained values are located in the green region and hence it can be used as a green phosphor for potential applications in pc-WLEDs. In order to examine the quality of light, correlated color temperature (CCT) values have been calculated using McCamy empirical formula [44] which is expressed as:

(8)

where n = ( x – xe)/(y – ye) is the inverse slope line, x = 0.259, y = 0.463) and xe = 0.332, ye = 0.186 is the epicentre. Generally, a CCT value greater than 5000 K indicates the cold white light used for commercial lighting purpose.The estimated CCT values are found to be ~7571 K (cold white light) at 340 nm UV excitation.

4. Conclusions

A novel green emitting BCZSO:Eu2+ phosphor has been synthesized by conventional solid state reaction method and their photoluminescence properties were studied. The phosphor BCZSO: Eu2+ showed a strong and broad excitation band ranging from 200- 450 nm, which matches well with the emission of UV-LED chip. Under 340 nm excitation, all the samples exhibits broad green emission band centered at 502 nm. The optimum concentrations of Eu2+ ions in BCZSO are found to be 0.06 mol and the concentration quenching mechanism has been investigated to be dipole-dipole interaction. The critical distance calculated by critical concentration method and spectral overlap method are 23.2 ï¿½ï¿½ and 13.7 ï¿½ï¿½, respectively. The CIE coordinates of (0.259, 0.463), which corresponds to green emission and CCT of 7571 K was measured. All the characteristics suggest that BCZSO:Eu2+ was a promising green-emitting phosphor candidate for WLEDs.

Acknowledgements

The authors thank the management of SSN College of Engineering for their constant support and encouragement. The author G. Annadurai extends his gratitude to the management of SSN for the JRF provided. We thank Dr. A. R. Lakshmanan for the measurement of Photoluminescence spectra. We thank Dr. Paulraj Arunkumar and Dr. Roman Kubrin for helpful discussions.

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