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Journal of Alloys and Compound JALCOM - - [email protected] - 08-17-2017 ARTICLE IN PRESS +Model JALCOM-17427; No.of Pages4 Journal of Alloys and Compounds xx (2008) xx xx Mg 1.96-1.96x Zn 1.96x GeO 4 :Mn 0.04 phosphors for electroluminescent display applications G. Anoop, K. Mini Krishna, M.K. Jayaraj * Optoelectronic Devices Laboratory, Department of Physics, Cochin University of Science and Technology, Kochi 682022, Kerala, India Received 27 June 2007; received in revised form 7 January 2008; accepted 9 January 2008 Abstract Orthorhombic magnesium germenate (Mg 2 GeO 4 ) doped with manganese was synthesized by solid-state reaction technique. A broad red emission with a peak at 653 nm was observed independent of excitation wavelength. The effect of Zn incorporation on structural and optical properties is investigated, keeping the concentration of Mn fixed at 2 at.%. XRD and DRS analysis of the samples reveal the formation of a solid solution up to x = 0.10 beyond which phase segregation occurs. Formation of a sub-band gap is observed for Mn doped samples which decreased with Zn doping. Both green and red emission is observed for Zn doped samples above x = 0.10. Red emission is attributed to the 4 T 1 ? 6 A 1 transition of Mn 2+ at Mg 2+ site in Mg 2 GeO 4 and green emission is from 4 T 1 ? 6 A 1 transition of Mn 2+ at Zn 2+ site of Zn 2 GeO 4 . PLE was found to be red-shifted with Zn doping. 2008 Elsevier B.V. All rights reserved. Keywords: Mg 2 GeO 4 ; Luminescence; Phosphor 1. Introduction In the past few years, oxide phosphors have been widely investigated as potential candidates for electroluminescent dis- play applications [1 4]. They exhibit extreme stability in vacuum and emit fewer harmful gases under irradiation of electrons in comparison to conventional sulphide phosphors. Several oxide phosphor hosts like Y 2 O 3 [5], ZnGa 2 O 4 [6 14], Zn 2 GeO 4 [15 18], etc. have been investigated for full colour electroluminescent devices. One of the necessary criteria for phosphor hosts is its wide band gap. The band gap should be greater than 3 eV so that visible radiation emitted by the activa- tor/impurity is not absorbed by the host. Therefore, wide band gap oxides are of considerable interest. But there is a limit to band gap for electroluminescent display phosphors, above which no electroluminescence is observed, since in most cases luminescence arises due to resonant energy transfer from the host to activator. Manganese is an excellent activator for yellow (in ZnS) [19], green (ZnGa 2 O 4 , Zn 2 GeO 4 ) and red (ZnMgS) * Corresponding author. Tel.: +91 484 2577404; fax: +91 484 2577595. E-mail address: [email protected] (M.K. Jayaraj). [20], Mg 2 GeO 4 [21]) emissions. Eu and Cr doped Mg 2 GeO 4 also shows red emission, serves as a phosphor for plasma dis- play panels [22,23]. In orthorhombic Mg 2 GeO 4 (a = 10.29A, b = 6.023A, c = 4.905A) Mg 2+ ions occupy tetrahedral sites while Ge 3+ ions occupy octahedral sites in crystal lattice. The wide optical band gap of Mg 2 GeO 4 makes it as a suitable can- didate for wide band gap oxide phosphor. However, efficient high field electroluminescence from Mg 2 GeO 4 host material is not observed due to its high band gap and inefficient trans- fer of energy from the host to activator. Also Mg 2 GeO 4 : Mn phosphor has not been widely studied for luminescent appli- cations. Co-doping has been proved as an excellent technique for engineering the band gap [18,24 26]. Among the various ions that can replace Mg, Zn is better choice due to its sim- ilarity in ionic radii and valency. More over the band gap of ZnO has been engineered by alloying it with Mg for many opto- electronic applications [25]. Alloying up to x = 0.33 is observed in Zn 1-x Mg x O thin films [26]. Due to abundance and non- toxicity, compared to other ions like Cd, Zn is more appropriate co-dopant for engineering the band gap of Mg 2 GeO 4 . In the present work, the effect of zinc co-doping on the crystal struc- ture, band gap and photoluminescence of Mg 2 GeO 4 :Mn is studied. 0925-8388/$ see front matter 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.01.043Page 2 Please cite this article in press as: G. Anoop, et al., J. Alloys Compd. (2008), doi:10.1016/j.jallcom.2008.01.043 ARTICLE IN PRESS +Model JALCOM-17427; No.of Pages4 2 G. Anoop et al. / Journal of Alloys and Compounds xx (2008) xx xx 2. Experiment The samples were synthesised by conventional high temperature solid-state reaction of constituent oxides, namely MgO, ZnO, GeO 2 . Manganese was added in the form of manganous acetate [Mn(CH 3 COO) 2 ]. The stoichiometric powders were mixed in ethanol medium and calcined in air at 1200 ? C for 12 h in a tubular furnace to obtain Mg 1.96-1.96x Zn 1.96x GeO 4 :Mn 0.04 (x was varied from 0 to 0.5). The concentration of Mn was fixed at 2 at.% in all the samples. The concentration of Zn was varied from 0 to 50 at.% of Mg. The crystal structure of the powder phosphors were analyzed using X-ray powder diffraction method using Cu K radiation (Rigaku, Japan). The diffuse reflectance spectra were recorded to analyze the band gap using Jasco V-570 spectrophotometer with an integrating sphere attachment. BaSO 4 was used as the reference. The room temperature photoluminescent emission and excitation spectra were recorded using Spex Fluoromax-3 Spectro-fluorimeter in the range 200 800 nm. 3. Results and discussion Fig. 1 shows XRD patterns of pure Mg 2 GeO 4 :Mn, pure Zn 2 GeO 4 :Mn and zinc doped Mg 1.96-1.96x GeO 4 :Mn 0.04 at dif- ferent concentrations (x = 0.1, 0.15, 0.20, 0.25, 0.5) of zinc. The pattern clearly shows the formation of solid solution up to x = 0.10. But above x = 0.10, additional peaks are observed indicating phase segregation. These additional peaks are iden- tified as that of Zn 2 GeO 4 . No traces of constituent oxides like ZnO and MgO, were found in the XRD pattern as the pattern matches well with the JCPDS data of Zn 2 GeO 4 and Mg 2 GeO 4 . Normally Magnesium germenate crystallizes in orthorhombic structure and zinc germenate crystallizes in rhombohedral struc- ture, thereby limiting simple incorporation of Zn in to Mg 2 GeO 4 lattice. Also the ionic radii of Zn and Mg differ; substituting Mg with Zn will not be possible for all concentrations and at a par- ticular concentration phase will start to segregate. The phase segregation occurs due to inbuilt strain due to the difference in ionic radii and strain reaches a maximum at particular Zn concentration. However, in Zn 1.96-1.96x Mg 1.96x GeO 4 Mg forms solid solution up to x = 0.3 clearly indicating that rhombohedral Zn 2 GeO 4 have more stability than Mg 2 GeO 4 naturally favour- ingtheformationofZn 2 GeO 4 [18].Therefore,asmoreandmore ZnaddstotheMg 2 GeO 4 :Mnsystem,formationofZn 2 GeO 4 :Mn Fig. 1. XRD patterns of Mg 1.96-1.96x Zn 1.96x GeO 4 :Mn 0.04 (0 = x = 1). Fig. 2. Variation of Cell volume and band gap with Zn concentration. Straight (dashed) line shows apparent linear fit to data. is favoured and when x = 0.5, Zn 2 GeO 4 phase got enhanced thereby forming Zn 1.96-1.96x Mg 1.96x GeO 4 :Mn. The cell volume [27] is found to increase with x which is expected as the cell volume of Zn 2 GeO 4 is larger than that of Mg 2 GeO 4 (Fig. 2). The band gap of Mg 2 GeO 4 :Mn 2+ and Zn 2 GeO 4 :Mn 2+ , cal- culated from the spectra are 5 eV and 3.45 eV, respectively. In both cases strong sub-band absorption is observed. This sub- band state lie nearly 1 eV below the conduction band and formed due to intrinsic oxygen vacancies during the compound forma- tion. The schematic energy level scheme is shown in Fig. 3. The variation of band gap of Mg 2 GeO 4 :Mn with Zn concentration (x) is shown in Fig. 4. As x increases band gap is found to be red-shifted. Fig. 4 clearly indicates mixed phase for doped sam- ples (x = 0.15, 0.2, 0.25 and 0.5). So from the DRS spectra it is concluded that formation of solid solution is favoured for the zinc doping up to x = 0.10 and beyond that phase segregation occurs due to inbuilt strain in the lattice. The shift in band gap can be attributed to difference in ionic radii of Zn 2+ and Mg 2+ ions. Moreover, when Zn is co-doped in to Mg 2 GeO 4 system, Zn 2+ will create states below the sub-band levels created due to oxygen vacancy. So as more and more Zn gets added in to the Mg 2 GeO 4 matrix, the band gap reduces. Fig. 5 shows PL emission spectra of Mg 1.96-1.96x Zn 1.96x GeO 4 :Mn 0.04 (0 = x = 0.25) samples. All samples show emis- sion in the red region and has intensity greater than Zn free Fig. 3. Energy level scheme describing the excitation and emission mechanism of Mg 2 GeO 4 :Mn phosphor.Page 3 Please cite this article in press as: G. Anoop, et al., J. Alloys Compd. (2008), doi:10.1016/j.jallcom.2008.01.043 ARTICLE IN PRESS +Model JALCOM-17427; No.of Pages4 G. Anoop et al. / Journal of Alloys and Compounds xx (2008) xx xx 3 Fig. 4. The band gap of Mg 1.96-1.96x Zn 1.96x GeO 4 :Mn 0.04 (0 = x = 0.5). sample. But the peak emission wavelength changes as x varies. In Mg 2 GeO 4 :Mn the emission is observed from tetrahedrally coordinated Mn 2+ at Mg sites [21]. The 3d 5 electrons of Mn 2+ is highly influenced by crystal field environment. The crystal field environment determines the colour or the emission wavelength of the activator. In Mg 2 GeO 4 , since we are getting red emis- sion, crystal field has considerable effect on 3d 5 electrons. Also energy transfer mechanism is not so efficient as in the case of Zn 2 GeO 4 :Mn. In Zn 2 GeO 4 the mechanism of PL is identified as resonant energy transfer from a sub-band gap level, due to intrin- sic defects, to Mn 2+ levels [18,28].InMg 2 GeO 4 also a sub-band gap level is observed as the Mn 2+ emission is triggered from a level which is nearly 1 eV low from its conduction band. But this level is not in the vicinity of excited levels of Mn 2+ for efficient resonant energy transfer to take place. But as more Zn is added to the system, transfer will take place to Mn 2+ levels through Zn 2+ levels formed near Mn 2+ levels. This is also observed in DRS measurements as reduction in band gap is observed as more Zn adds to the system. However, no red emission can be detected from x = 0.5 sample indicating the complete replace- ment of Mn 2+ at Zn 2+ sites in Zn 2 GeO 4 . Therefore, similar to the Fig. 5. PL emission spectra (red) of Mg 1.96-1.96x Zn 1.96x GeO 4 (0 = x = 0.25), ? exc = 300 nm. Fig. 6. Room temperature Photoluminescent emission spectra (green) of Mg 1.96-1.96x GeO 4 :Mn 0.04 (0 = x = 0.5), ? exc = 300 nm. formation of Zn 2 GeO 4 , which is more favourable, Mn 2+ is more likely to replace Zn 2+ rather than Mg 2+ . Green emission (Fig. 6) is also detected from the samples at and above x = 0.15 clearly showing the limit of solid solubility of Zn in Mg 2 GeO 4 :Mn. But in Zn 2 GeO 4 :Mn 2+ , Mg is found to be soluble up to x = 0.3 [18]. This is due to greater stability of rhombohedral Zn 2 GeO 4 struc- ture compared to orthorhombic Mg 2 GeO 4 . The green emission at 535 nm can be attributed to 4 T 1 ? 6 A 1 transition of Mn 2+ in Zn 2 GeO 4 and gets enhanced when Zn concentration increases. The photoluminescence excitation (PLE) spectra for red emission (653 nm) are shown in Fig. 7. A broad excitation peak- ing at 268 nm is obtained for Mg 2 GeO 4 :Mn which is found to be red-shifted as more Zn 2+ replaces Mg 2+ . In Zn 2 GeO 4 :Mn PLE at 332 nm is obtained for emission at 535 nm showing the formation of sub-band gap [18]. Since increased Zn con- centration results in phase segregation, the excitation for red emission shifts to higher wavelength region indicating the pres- Fig. 7. PLE spectra of Mg 1.96 GeO 4 :Mn 0.04 and inset shows its variation with Zn concentration, ? em = 653 nm.Page 4 Please cite this article in press as: G. Anoop, et al., J. Alloys Compd. (2008), doi:10.1016/j.jallcom.2008.01.043 ARTICLE IN PRESS +Model JALCOM-17427; No.of Pages4 4 G. Anoop et al. / Journal of Alloys and Compounds xx (2008) xx xx enceofZn 2 GeO 4 .Butwhenexcitedforgreenemission(535 nm) the PLE spectra show a peak at 300 nm, for x = 0.50 sample, indicating the formation of Zn 1.96-1.96x Mg 1.96x GeO 4 , Mg being substituted for Zn, there by emphasizing the fact that formation of Zn 2 GeO 4 is more favored rather than Mg 2 GeO 4 . 4. Conclusion Manganese doped Mg 2 GeO 4 is synthesized by solid-state reaction. The phosphor shows emission in red region. The effect of Zn doping on structural and optical properties have been explored in detail. XRD patterns show solid solution forma- tion up to 10 at% Zn doping and beyond that phase segregation occurs. When Zn is co-doped in to Mg 2 GeO 4 :Mn, PL emis- sion intensity increases compared to Zn free sample and PL peak excitation wavelength red-shifted with Zn addition. 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