Bulletin of Trends in Chemical Sciences

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1 Bulletin of Trends in Chemical Sciences Paper ISSN: ; Vol. 2 (4) Open access Structural and Photoluminescence properties of Rare Earth doped Luminescent material Shakul Muhammed 1, G. Shakil Muhammed 1*, A. Duke John David1 2, K. Indumathi 3, S. Tamilselvan 3 1 Department of Physics, Islamiah College, Vaniyambadi Tamil Nadu, India 2 Department of Physics, Voorhees College, Vellore Tamil Nadu, India 3 Department of Physics, Arignar Anna Government Arts College, Cheyyar Tamil Nadu, India ABSTRACT A new luminescent material Ca 9La (1-x)(VO 4) 7: xeu 3+ was synthesized by conventional high temperature solid state reaction method. From the measured emission profiles, we have noticed that Eu 3+ phosphor shows emission in the red region of the visible region and exhibits a high intense emission at around 616nm due to the transition of 5D 0 7F 2 along with a weak red emission at 650nm. Excitation spectra of these phosphors commonly show an intense andbroad excitation band in the wavelength region of nm which is a charge transfer band (CTB)aroused due to the interaction between the trivalent rare earth ion and the oxygen and V O bonds. Besides explaining the CTB, the photoluminescenceproperties of these phosphors were investigated under near-uv/blue light excitationwavelengths. In order to explore the structure and its phase formation, the XRD has been recorded. The particle size and the elements present in the luminescent materials were found using SEM and EDX. Key words Phase formation; Charge Transfer; Electronic transitions; Orthovanadate; Introduction The trivalent rare earth (RE) ion-doped vanadates have been paid more attractionfor applications in display devices, lights and detectors[1 2]. It is well recognized that the phosphor hosts are usually basedon vanadates, borates, phosphates, aluminates and silicates [3-4].Among these, vanadium (V 5+ ) oxide forms different complex vanadates,namely metavanadate, pyrovanadate and orthovanadatesdepending on the stoichiometry of the composition. Most of thecrystal structures of the vanadates are formed with VO 4 or VO 5polyhedra [5]. Calcium and Lanthanum-based vanadates are usefulphosphors in perceiving different visible colours with the appropriatedopant rare earth ions. Keeping in viewthe significance andimportance of these vanadate phosphors, we have recently investigatedthe emission spectra of Eu 3+ :Ca 9La[VO 4] 7 powderphosphors [6-8]. In the currentwork,we find the structural and optical properties of the luminescent phosphor material. Materials and Method The luminescent material Eu 3+ :Ca 9La[VO 4] 7was synthesized via the conventional high temperature solid state reaction. The high purity carbonates and oxides were used as precursors such as Ca 2CO 3- Calcium Carbonate,La 2O 3 Lanthanum Oxide, NH 4VO 3 - Ammonium metavanadate and Eu 2O 3 - Europium oxide. First the stoichiometric mixture was heated upto 700 C at a rate of 150 C per hour. After 4 hours of calcination, the temperature has reached a maximum of 700 C. Then it is

2 maintained for 20hours constantly at 700 C. Then after this duration of sintering, the synthesized luminescent compound is made to cool and the reach room temperature. Then it is grinded again well to reach a homogeneous mixture. Now, the sample is subjected to various structural and optical characterization techniques. Characterization methods The Phase purity was checked by powder Xray diffraction (XRD) analysis collected on a Bruker's X-ray Diffraction D8-Discover instrumentusing Cu-Kα radiation ( = Å). The morphology of the samples were seen through the high resolution SEM instrument TESCAN. The UV-DRS measurement is made with UV-DRS model UV2600. The photoluminescence excitation and emission measurement were measured by Fluoromax 4C [JobinYvon spectrofluorometer] with Xenon lamp as the source for all the synthesized phosphor materials. The colour coordinates CIE is calculated using CIE calculator along with MATLAB software is employed, using the photoluminescence emission intensity, the CIE x and y values were found. Results and Discussion XRD - X- Ray Diffraction crystal Phase Formation The XRD profiles of the synthesized luminescent materials were shown in figure 1. The XRD patterns are in good agreement with the Rhombohederalcrystal system and also it is found that it is well matched with JCPDS card no This clearly indicates that the luminescent material was successfully synthesized in a perfect crystal system via the high temperature solid state reaction method. The unit cell parameters are a = 10.89Å, c= 38.13Å and the interfacial angles are α = β = 90, = 120. The XRD analysis shows that it comes under the hexagonal structure with space group R3c(161). There is no other phase is identified, specifying that the obtained samples are single phase and Eu 3+ ions have been successfully incorporated in the host lattice in the place of La3+ ions due to their similar ionic radii and charge. Therefore, we can conclude that Eu3+ ions can be doped easily to replace the La3+ ions in the host lattice without any structural changes. The XRD patternsconfirms a Figure 1 Shows XRD patterns of Undoped and Eu3+ doped luminescent materials single phase phosphor material for both the undoped luminescent materials and the doped trivalent rare earth ion Eu3+ ion. Morphology and Elemental Analysis SEM with EDX analysis The SEM and the EDX spectra of the 0.05Eu3+ doped Ca9La0.95(VO4)7luminescent materials are shown in the figure 3.2. To realize the morphology of this luminescent material, wehave found that the particles are agglomerated, which is shown clearly in the SEM image. The particlesize cannot be measured exactly from the SEM micrographs asshown in Fig. 2. It is evident that the particles areof 70 m. 2

3 Figure - 2 SEM and EDX spectra of 0.05Eu 3+ doped Ca 9La 0.95(VO 4) 7 It may be mentioned that crystalline powder and micrometer dimension of the powder with a high strength could be useful for applications, as these micro crystalline luminescent materials can result in high luminescent intensities [10]. To verify the chemical contents qualitatively, EDX profiles have also been measured, the element presents in the compound along with its weights in percentage are shown in the table-1. The presence of the Europium confirms that the dopant has been successfully incorporated in the host lattice. Table 1 shows the EDX composition of the synthesized compound Element Weight Atomic % % O Ca V C La Eu Total UV DRS Diffuse Reflectance Spectra To explore the energy absorption and diffuse reflectance as a function of wavelength were measured at room temperature. Figure 3 (a,b & c) shows the UV- Visible diffuse reflectance spectrum of the synthesized luminescent material Ca 9La (1-x)[VO 4] 7: xeu 3+ recorded in the wavelength from 200 to 800nm. Figure 3(a) shows the DRS of undoped luminescent material Ca 9La(VO 4) 7 In all the three figures a wide absorption band starting from 200nm to 450nm which corresponds to ligand to metal (O V) charge transfer transition in the VO 4 tetrahedron. The absorptions at 390nm are ascribed to the f-f electronic transitions of Eu 3+ ion, which is consistent with the conclusion of PL excitation spectra. As a representative result, the optical band gap value Eg of undoped and Eu 3+ doped compounds are estimated to be 3.2 ev. All the three figures also infers that there is a highest percentage of reflectance from 500nm to 800nm in common. 3

4 Figure-3(b) shows the DRS of 0.05Eu 3+ doped Ca 9La 0.95(VO 4) 7 Figure-3(e) Band gap of 0.05Eu 3+ dopedca 9La 0.95(VO 4) 7 Figure-3(f) Band gap of 0.1Eu 3+ dopedca 9La 0.9(VO 4) 7 Figure-3(c) shows the DRS of 0.1Eu 3+ doped Ca 9La 0.9(VO 4) 7 Figure-3(d) Band gap of UndopedCa 9La(VO 4) 7 The energy band gap value Egof synthesized luminescent material can be found using the fundamental absorption, which corresponds to the transition from valance band to the band. The general relation which correlates the absorption coefficient (α) and the incident photon energy (h ) can be written as αh = A(h - E g) n (1) Where A is constant, α is the absorption coefficient and n depends on the type of transition having values ½, 2, 3/2 and 3 corresponding to the allowed direct, allowed indirect, forbidden direct and forbidden indirectly respectively [9]. The value of the band gap hasbeen determined by extra-plotting the straight line portion of the (αh ) 1/n versus h graph, which is shown in the Figure-3 (d), (e) & (f). 3

5 emission due to the parity-forbidden transitions of the Eu 3+ ion borrow intensity from the lowest strongabsorption band (CTB). In addition to the CTB, a sharp excitation at 7F 0 5L 6 (393 nm) corresponding to the inner 4fshellexcitations of the Eu 3+ ion have also been identified. One ofthe interesting results of this work is that these phosphors couldbe strongly excited both by the near-uv light at 393nm ( 7F 0 5L 6 )which mightfound applications in SSL technology. Figure 4(a) show Photoluminescence excitation spectra of Ca 9La 0.95[VO 4] 7: 0.05Eu 3+ Figure 4(b) show Photoluminescence excitation spectra of Ca 9La 0.9[VO 4] 7: 0.1Eu 3+ Photoluminescence The excitation and emission spectrum of Ca 9La 0.95(VO 4) 7: 0.05Eu 3+ luminescent material is shown in figure 4(a) by monitoring a red emission (616 nm) wavelength.the excitation spectrum consists of an intense and broadexcitation band centered at 316 nm, which is a CTB of Eu 3+ O 2interaction and also due to the V O components of the host matricesstudied in the ultraviolet region. The CTB of Eu 3+ plays animportant role in red The PL spectra consist of emission peaks at 585, 596, 616, 649 and 700nm and these areassigned to the electronic transitions from the level 5D 0 to thelower lying levels of 7F 0, 7F 1, 7F 2, 7F 3 and 7F 4, respectively. Dueto the shielding effect of 4f electrons by 5s and 5p electrons inthe outermost shells of the Europium ion, narrow emission peaksare expected, consistent with the sharp and intense peak around618nm and is due to the 5D 0 7F 2 transition, based on selectionrules. Due to non-radiative relaxation from excited states ofenergy higher than 5D 0 state, intense emission bands in the rangeof nm are caused by the 5D 0 7F J transitions. The intensityof emission transition is proportional to the radiative decayof these transitions. The above fig 4(b) shows similar kind of excitation and emission but there is an increase in the intensity of the emission, which is due to the increase in the dopant level of the trivalent rare earth ion Eu 3+. Conclusion A new luminescent material Ca 9La (1-x)[VO 4] 7doped with Eu 3+ for two different concentrations has been successfully synthesized by conventional high temperature solid state reaction. The Phase formation has been verified by powder XRD, the undoped and the trivalent rare earth ion doped shows no structural changes, which confirms a uniform phase between them. The SEM shows the micrometer range with particles are agglomerated and the EDX confirms the elements which has been synthesized. Also it infers that it is oxygen rich. From DRS the reflectance, absorption and the bandgap has been found, which is highly essential for the display applications. The Photoluminescence confirms the presence of Eu 3+ ions in the host lattice by showing the emission in the red region of the visible region. Hence, 4

6 this luminescent material is a promising candidate to be used for the solid state lighting display systems. References [1] C.Wu, Y.Wang, Mater. Lett. 61 (2007) [2] X. Li, H. Liu, J. Wang, H. Lui, S. Liang Yang, J. Phys. Chem. Solids66 (2005) [3] J.E. Van Haecke, P.F. Smet, D. Poleman, J. Lumin. 126 (2007) [4] J.A. Jhonson, S. Scwuzer, B. Henke, G. Chen,J. Appl. Phys. 100 (2006) [5] E. Nakazawa, Y. Murazaki, S. Saito, J. Appl. Phys. 100 (2006) [6] F. Shen, D. He, H. Lin, J. Xu, J. Lumin. 122 (2007) 973. [7] P.H. Holloway, T.A. Trottier, J.S. Sebastain,J. Vac. Sci. Technol., B 17 (1999) 758. [8] A.F. Wells, Structural In. org. Chem, University Press, Oxford, 1962, p.686. [9] Abeles FOptical Properties of Metals, North-Holland Pub.Co.Amst dam 97(1972). [10] G. Liu, G. Hong, J.Wang, X. Dong, Nano Technol. 17 (2006)