- Email: [email protected]
Low voltage cathode-luminescent properties of Zn co-doped Y2O3:Eu red phosphor
Journal of Luminescence 177 (2016) 249–253
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Full Length Article
Low voltage cathode-luminescent properties of Zn co-doped Y2O3:Eu red phosphor K. Lingadurai a, B. Sundarakannan b, E.R. Nagarajan a, H. Kominami c, Y. Nakanishi c, M. Kottaisamy b,n a
Department of Chemistry, Kalasalingam University, Krishnankoil, 626126, India Department of Chemistry, Thiagarajar College of Engineering, Madurai 625015,India c Graduate School of Electronic Science and Technology, Research Institute of Electronics, Shizuoka University, Hamamatsu 432-8011, Japan b
art ic l e i nf o
a b s t r a c t
Article history: Received 23 January 2015 Received in revised form 3 April 2016 Accepted 19 April 2016 Available online 27 April 2016
Zn co-doped and undoped Y2O3-Eu red phosphor was prepared by citric acid gel method and it was investigated to know its cathodoluminesecent properties at low voltage excitation ( o2 kVs). In this connection the cathodoluminescence (CL) spectra of Zn co-doped and undoped phosphors were taken and they showed various line emissions along with a sharp red emission at 611 nm, irrespective of Zn concentration, due to 5D0–7F2 transitions from Eu luminescent center. It was found that the incorporation of Zn could significantly enhance the CL emission intensity of Eu at about 33% when compared with Zn undoped phosphor. It was also found the Zn co-doping could not alter the crystal structure whereas it could change the morphology of the particles. The promising results revealed the possible application of this phosphor materials as red phosphor for field emission displays [FEDs]. & 2016 Elsevier B.V. All rights reserved.
Keywords: Phosphors Cathodoluminescence X-ray diffraction Excitation FEDs
1. Introduction In recent days, considerable interests are shown on the development of phosphors for low voltage display devices viz, field emission displays (FEDs) because its inherent advantages over the bulk cathode ray tube (CRT) and liquid crystal displays (LCD) and also for white light emitting diodes (WLEDs) due to energy saving lighting systems. In FEDs, excitation of phosphor involves at low energy electron (o 2 kV) and high current density irradiations (o 100 mA/cm2). Many sulfide phosphors, which yield efficient luminance at this excitation, unfortunately, decompose during operation due to electron-stimulated reaction. This decomposition leads to a sulfur deficient non-luminescent layer on the phosphor surface and reduces the phosphor efficiency. Moreover, the byproducts of the sulfur are known to poison the electron emitter [1–3]. The fast growing flat panel display market has boosted researches that focus on the improvement of oxide phosphor for FEDs due to its chemical stability and stable luminescence. Since the excitation involves at low voltage, wide band gap semiconducting oxide phosphors are gaining importance, due to reduction in surface charge up and generation of more e–h pairs in the host crystal that results in better luminous efficiency [4–7]. On n
Corresponding author. Tel.: þ 91 452 2482240x729. E-mail address: [email protected] (M. Kottaisamy).
http://dx.doi.org/10.1016/j.jlumin.2016.04.041 0022-2313/& 2016 Elsevier B.V. All rights reserved.
the other hand, efforts have been taken by coating wide band gap conductive oxides on the surface of the phosphors [8–10]. These oxides afford recombination centers like oxygen vacancies around the surface of the phosphor powder, remove the surface bound electrons and make incident electrons to easily excite in bulk crystal lattice of phosphors. In addition, attempts have been made to develop high efficient red phosphors by doping impurities that create oxygen vacancies in phosphor crystallites with the goal of developing high efficient phosphors at low voltage excitation [11– 15]. Among this, defective structure ZnO is widely studied for its peculiar stable luminescent properties that depend on the preparation condition, annealing atmosphere, deposition substrates, precursor, source, etc. [16,17]. In the present work, we executed researches with ZnO as a dopant in Y2O3 host lattice (along with Eu, as a luminescent center). The former is a chemically stable, wide band gap n-type semi-conductor (Eg, 3.3 eV) and transparent to visible radiation [18]. It can serve as high efficient low voltage phosphor as it has optonic applications due to its conductivity and excitonic emission in the UV region, respectively [19,20]. In addition it is well known for its strong defect broad band emission at the visible wavelength range at the variable excitation wavelength from near UV to blue light [17]. The later, Y2O3 is an insulating phosphor crystal (Eg, 6 eV), and it is an unsurpassed red emitting (Y2O3:Eu) component that emits a sharp emission line at 611 nm, in which larger part of intensity is concentrated. This phosphor is
250
K. Lingadurai et al. / Journal of Luminescence 177 (2016) 249–253
extensively applied in high efficiency fluorescent lamps, high definition television and FEDs because of its inherent advantages of stability towards the high energy irradiations [21–23]. By considering the above all concepts, in the present work, a new attempt has been made to increase the efficiency of phosphors at low voltage excitation by doping alieovalent conducting oxide in high resistance phosphor crystallites and an analysis has been carried out to know the luminescent properties for FED.
2. Experimental To prepare an uniformly Zn co-doped Y2O3-Eu red phosphor, citric acid gel method was adopted. This method was specifically selected for the synthesis of phase pure multicomponent oxide materials because of its tribasic character to chemically bind all the metal ions to facilitate uniform doping in the host matrix [24]. In this material, the zinc concentration was varied from 2.5 to 15 mole% with respect to 2 mol yttrium concentration in Y2O3 including the Europium (Eu) concentration. In a typical example, for a preparation of 5 mol% Zn doping, i.e., 0.1 mol Zn co-doped Y2O3:Eu red phosphor, required quantity of Y2O3 (0.93 mol), Eu2O3 (0.02 mol) and ZnO (0.1 mol) were dissolved in dilute nitric acid and then added with citric acid (2.2 mol). The addition of excess citric acid could help to grow the phosphor host materials without any secondary phase formation. The obtained solution was continuously stirred for three hours for a homogenous mixing of Y, Eu and Zn at molecular levels and heated at 90 °C to get a transparent gel. The gel was heated at 250 °C to obtain the precursor. Subsequently, the precursors were heat treated at 1100 °C for 4 h in alumina crucibles covered with a silica lid in open air atmosphere. In order to study the effect of Zn concentration on the cathodoluminecent properties of Y2O3:Eu, the Zn concentration was varied from 2.5 to 15 mol% with respect to total metal concentration [Y and Eu concentration] whereas the Eu concentration was fixed as 2 mol%. For a comparison purpose, Zn undoped Y2O3: Eu (2 mol %) was also prepared by the same method. Structural properties of the samples were characterized by Xray diffraction (XRD) analysis. For cathodoluminescent (CL) measurements, the powder phosphor samples (0.12 g) were deposited by sedimentation method with a dispersing agent in water on aluminum substrate (1 cmx1 cm). After the clear deposition of the powder, water was drawn off and the samples were dried in hot air oven at 60 °C. The CL properties of phosphors were studied using a continuous electron beam with a spot size of 2 10 6 m2 generated at various excitation voltages ( o2 kVs) at a current density of 60 mA/cm2 by employing TOPCON BM-5 luminance meter. Luminance and luminous efficiency of the best Zn co-doped Y2O3:Eu phosphor were compared with a commercial Y2O3:Eu red phosphor. The CL spectra were measured with a fiber optic coupled spectrophotometer. The spectral resolution of the spectra was determined to be 1 nm. The emission spectra were recorded from 400 to 800 nm and the excitation spectra were recorded from 150 to 400 nm on 200 HSC (JASCO) spectro-fluorimeter. The generated excitation spectra were corrected for a lamp output.
Moreover, it was also noticed that the peaks were shifted to right side showed a possible reduction in the lattice parameter of Y2O3, due to the substitution of smaller size Zn (0.74 Å) in the Y (0.92 Å) site [25]. From these observations, it could be asserted that the Zn doped Y2O3 could form a solid solution. The presence of Zn in the Yttrium oxide lattice was also confirmed by EDAX spectrum.The amount of zinc that was added not retained in the phosphor and this could be correlated with the loss of zinc by evaporation at 1100 °C. Fig. 2 shows the CL emission spectra of various concentration of Zn (2.5–15 mol%) doped Y2O3:Eu red phosphor, from 350 to 800 nm, at the excitation of 2 kV and 60 mA/cm2. The emission spectra could depict a typical emission from Y2O3:Eu at 611 nm due to the 5D0–7F2 transition along with minor peaks originated from the other 5D0–7FJ transitions [26]. The most significant aspect of the CL spectrum revealed that there were no emissions due to the Zn centers (either from the excitonic emission or broad band defect emission) [19,20] even though the Zn concentration was much higher than the Eu concentration. This was quite interesting as far as the cathode ray excitation was concerned. Moreover, from the CL spectra, it was observed that addition of Zn did not shift the Eu emission at 611 nm. In a earlier report, it was observed that Zn co-doping in Gd2O3:Eu shifted the Eu dominant emission from 611 nm to 621 nm [13]. This shift was observed due to the modifications in the local crystal structure that would arise from the
Fig. 1. X-ray powder diffraction pattern of Y1.96 xZnxEu0.04O3.
3. Results and discussion Fig. 1 shows XRD pattern of Eu doped Y2O3 and co-doped with various concentration of Zn obtained at 1100 °C for 4 h. From the XRD pattern, it was observed that addition of Zn did not change the cubic crystal structure of Y2O3 (JCPDS-No.43-1036). At the same time it improved the crystallinity as observed from the peak narrowing and intensity. However, the addition of Zn at 10 mol% and above, it was noticed that the peak intensity decreased.
Fig. 2. Cathodoluminescent (CL) emission spectra of Y1.96 xZnxEu0.04O3 measured at 2 kV as a function of Zn content (x).
K. Lingadurai et al. / Journal of Luminescence 177 (2016) 249–253
introduction of oxygen vacancies by the incorporation of Zn in the Gd2O3 lattice. However, the addition of Zn to Y2O3 did not shift the emission but increased the emission intensity at 611 nm. Fig. 3, displays a relative peak intensity of the emission at 611 nm, at 2 kV and 60 mA/cm2 excitation. A significant enhancement of CL intensity was observed as a function of Zn concentration. The maximum improvement of about 33% was observed for Y1.86Zn0.1Eu0.04O2.95 and above the concentration of 5 mol%, the intensity decreased. For Zn doping Y2O3:Eu, the replacement of Y3 þ by Zn2 þ could give rise to the formation of more defects by creating oxygen vacancies in the host lattice. With the increase in Zn content, the oxygen vacancy increases which will destroy the crystallinity and lead to the luminescence quenching. It was interesting to note that there was no decrease of intensity below the Y2O3:Eu, prepared by the same method, at any Zn concentration. Hence it was believed that Zn could act as a sensitizer for Y2O3:Eu red phosphor at low voltage cathode ray excitation. For reference, the luminance and luminous efficiency of our best phosphor powder Y1.86Zn0.1Eu0.04O2.95 which was obtained at 1100 °C for 4 h were compared with those of the prepared Y2O3:Eu phosphor as a function of excitation voltage (250 V–2 kV) and current density of 60 mA/cm2. Fig. 4 shows that incorporation of Zn could significantly enhance this emission intensity. It was found that the highest cathodoluminance of 2218 cd/m2 with a luminous efficiency of 5.77 lm/W and 795 cd/m2 with a luminous efficiency of 4.14 lm/W excited at 2 and 1 kV excitation, respectively. In total, an improvement of about 33% was observed in luminance for Y1.86Zn0.1Eu0.04O2.95 as a function of excitation voltage when compared with a Zn undoped Y2O3:Eu phosphor. This phenomenon was very interesting as far as the phosphors for field emission was concerned. It was reported that during the electron beam irradiation of crystals, it created plasma electrons i.e. incident
Fig. 3. Relative peak intensity at 611 nm emission as a function of Zn content in Y1.96 xZnxEu0.04O3 at 2 kV and 60-mA/cm2 excitation.
Fig. 4. Luminance and luminescence efficiency of best phosphor Y1.86Zn0.1Eu0.04O3 with a commercial Y2O3:Eu as a function of excitation voltage.
251
electrons and internally generated secondary electrons and pairs of electrons and holes (e–hs) i.e. radiation induced carriers in the volume in which the incident electron penetrated [27]. The generation of plasma electron was possible, when the excitation occurs at high voltage (4 5 kV). Since the excitation involved at low voltage (o2 kV), the luminescence might be occured through the radiative recombination of e–h pairs at the luminescent centers. It was proposed that the penetration depth would be very shallow at low voltage excitation and effective luminescent region may be confined near the surface [28]. Applying this concept to our case, we believe that this increased luminance may be attributed to the presence of Zn, which enhance carrier (e–h) generation at low voltage excitation, because of its low energy band gap (3.3 eV) compared with Y2O3 (6 eV) and resulting the excitation of Eu through the recombination of cathode ray generated carrier confined in the Y–O–Zn matrix. It was very convincing evidence that the secondary electrons or electron–hole generation in Zn co-doped Y2O3 was much higher than those in the Y2O3 even though the excitation voltage was at and below 2 kV and current density at 60 mA/cm2. At room temperature, the excitons were mobile [29] and hence the energy was efficiently transferred to Eu. From these results, it could be asserted that the presence of Zn in Y2O3 could efficiently transfer the excitation energy to the Eu luminescent center. It has been well established that electrical conductivity of metal oxide powder was dominantly dependent on a deviation from the stoichiometry that was created by lattice defects or oxygen vacancies [15,30]. The incorporation of Zn2 þ into the Y2O3 host matrix in open air was as a subsitutional ion at a Y site with compensating oxygen vacancies as given below. 2ZnO þ 2YY 3 þ
-
2Zn2 þ Y þ Vo þ Y2 O3
where YY3 þ represents a yttrium cation in a yttrium site, Zn2Y þ represents a zinc ion in a yttrium site and Vo represents an oxygen vacancy. In-order to confirm an enhancement of the emission intensity of the Zn doped phosphor, studies were carried out by analysing the conductivity of the various Zn doped Y2O3 as shown in Fig. 5. The conductivity of 5 mol% Zn doped Y2O3 showed about one order higher and yielded about 33% improvement in luminance compared with the Zn undoped phosphor. However, the increase in the conductivity of 10 mol% doped phosphor was not showing improvement in luminance when compared with lower Zn doped Y2O3. Hence, it could be asserted that the increase in conductivity of the phosphor matrix had little effect on the luminescence efficiency of phosphor at low voltage excitations, however it could help to reduce the charging-up of the phosphor [31]. In order to have a further look in to the effect of Zn, CL spectrum of Zn (5 mol%) doped Y2O3 (without Eu) was monitored from
Fig. 5. Conductivity of Y2O3 and doped with various concentration of Zn.
252
K. Lingadurai et al. / Journal of Luminescence 177 (2016) 249–253
350 to 800 nm at 1 kV and 60 mA/cm2 excitation and the results were shown in Fig. 6. The figure depicted a weak shoulder emission at 360 nm and a strong narrow band emission at 392 nm with some sharp emission at 573 and 611 nm (due to the presence of rare earth impurities available in trace quantity in the host crystal). The shoulder band emission at 360 nm was originated from the self trapped excitons in Y2O3 [32] and the emission at 391 nm was thought to be originated from the Zn centers. It has been reported that ZnO emits a narrow UV band emission at 380 and 395 nm due to the radiative annihilation of excitons and bi-exciton, respectively [18,33,34]. However, Zn co-doped Y2O3–Eu was not showing any emission at 360 and 392 nm but only emission from the Eu luminescent center. By comparing the Figs. 2 and 6 it could be easily concluded that the excitation of Eu ions occured through the recombination of cathode generated carrier (e–h) confined in the Y–O–Zn matrix and subsequently transferred the energy to Eu excited level. Fig. 7 shows the PL emission spectrum of Y2O3:Eu (2 mol%) doped with various concentration of Zn at the excitation of 325 nm, in which Zn site getting excited (band gap of ZnO was 3.3 eV) instead of Y2O3 (6 eV). From the Fig. 7, it was observed that Eu doped Y2O3 had a typical sharp line emission at 611 nm due to the 5D0–7F2 electric dipole transition arising from the Eu luminescent center. In contrast to this results, Zn doped Y2O3:Eu had a broad green emission band with a band width of (320 nm), peaked at 525 nm and a sharp emission at 391 nm accompanied with the red emission at 611 nm. It clearly shows that there are two luminescent center arises due to the Zn and Eu. The broad bluish green emission band and sharp emission at 391 nm was thought to be originated from the zinc center due to stable defects and typical
Fig. 6. Cathodoluminescent emission spectrum of Y1.9Zn0.1O3 at 2 kV and 60 μA/ cm2 excitation.
Fig. 7. Photo luminescent emission spectra of Y2O3:Eu as a function of Zn concentration at the excitation of near UV at 325 nm.
excitonic emission, respectively [35,36]. The red emission at 611 nm was due to the Eu luminescent center. It was interestingly observed that the europium emission had an enhancement of about 1.5 times higher for 5 mol% doped Zn than Eu doped Y2O3. In order to further confirm the origin of the broad band emission, only Zn (5 mole%) doped Y2O3 was prepared and photoluminescent properties were studied at the excitation of 325 nm as shown in Fig. 8. From the Fig. 8, it was clear that the same broad band emission could be obtained due to the Zn doped in Y2O3. The PL excitation spectra of Zn co-doped (5–15 mol%) Y2O3:Eu was compared with the Y2O3:Eu from 150 nm to 400 nm by fixing the emission maximum at 611 nm (λem ¼611 nm) and the results have been depicted in Fig. 9(a–d). The Y2O3:Eu displayed a strong overlapped excitation of Y–O host at 208 nm with Eu–O charge transfer band (CT) at 255 nm and the CT band was showing higher intensity than that of the host excitation (Fig. 9a). However, for the Zn co-doped Y2O3, the host lattice excitation had markedly high intense than that of the charge transfer band (Fig. 9b–d). The charge transfer band was almost vanished for 15 mol% Zn codoped sample. It was reported that charge transfer (CT) excitation coupled very effectively to higher 4f states in Y2O3:Eu red phosphor and made this phosphor materials with high luminous efficiency in lighting industry [20]. For the Zn co-doped phosphors, the CT band is less intense, however, the CL intensity shows a significant improvement than the Zn free phosphor. This clearly showed that the energy of the excited Zn center in the Y2O3 matrix could be quantitatively transferred to the excited 4f states of Eu at low voltage electron excitation. It was well established that for efficient cathode ray phosphors, activator
Fig. 8. Photo luminescent emission spectrum of Zn doped Y2O3 at the excitation of 325 nm.
Fig. 9. Photo luminescent excitation spectra of Y2O3:Eu as a function of Zn concentration at the emission of 611 nm.
K. Lingadurai et al. / Journal of Luminescence 177 (2016) 249–253
should be excited through host excitation instead of direct activator excitation [37,38]. By considering this, the improvement in luminous efficiency for Zn co-doped Y2O3–Eu phosphor could be attributed to the more cathode ray generated carriers (e–hs) at low voltage excitation. It was pertinent to note that the charge transfer band (250 nm) and other f–f higher excited level of Eu 7F0–5L6 (394 nm) were localized below the band edge of Y2O3 matrix, which was around 6 eV (210 nm). In the sample co-doped with Zn, the energy transfer from the host lattice to the Eu ion was obviously more efficient than the Y2O3:Eu. From this observation, it was conceivable that in the zinc co-doped phosphor, there was an efficient energy transfer to the Eu luminescent center at low voltage electron excitation when compared with Zn un-doped phosphor.
4. Conclusion Zinc co-doped Y2O3:Eu was prepared by employing citric acid gel method and its CL properties were studied. The additions of Zn in Y2O3 could increase crystallinity, enhance conductivity and show a strong UV emission at 391 nm at low voltage cathode ray excitation. However, the CL emission spectra of Zn co-doped Y2O3: Eu showed emission only from Eu center with a sharp emission at 611 nm due to 5D0–7F2 transition, irrespective of Zn concentration. The incorporation of 5 mol% Zn in the phosphor significantly enhances the cathode-luminance and efficiency which was about 33% higher than the Zn undoped phosphor. The PL emission spectra of Zn co-doped Y2O3:Eu at the near UV excitation showed broad band emission along with a red emission at 611 nm. On the basis of above observations, it could be concluded that Zn codoped Y2O3:Eu phosphor would be employed as a red phosphor component to the FEDs, operated at low voltage cathode ray excitations.
Acknowledgment One of the authors M.K sincerely thanks University Grant Commission, New Delhi, and Government of India for providing financial support through the UGC-Major Research Project (F.no. 41-904/2012 (SR)).
253
References [1] A. Vecht, D.W. Smith, S.S. Chada, C.S. Gibbons, J. Koh, D. Mortan, J. Vac. Sci. Technol. B12 (1997) 781. [2] S. Itoh, T. Kimizuka, T. Tonegawa, J. Electrochem. Soc. 136 (1989) 1819. [3] P.H. Holloway, J. Sebastian, T. Trottier, H. Swart, Solid State Technol. 38 (1995) 47. [4] S. Itoh, H. Toki, Y. Sata, K. Morinoto, T. Kishino, J. Electrochem. Soc. 138 (1995) 1509. [5] H. Yamamoto, K. Urabe, J. Electrochem. Soc. 129 (1984) 2069. [6] S.S. Chada, D.W. Smith, A. Vecht, C.S.C. Gibbons, in: Proceedings of SID Dig 51 (1994). [7] S. Itoh, H. Toki, K. Tamura, F. Kataoka, Jpn. J. Appl. Phys. Part 1 (38) (1999) 6387. [8] H. Kominami, T. Nakamura, Y. Nakanishi, Y. Hatanaka, Jpn. J. Appl. Phys. 35 (1996). [9] S.H. Cho, J.S. Yoo, J.D. Lee, in: Proceedings of J. SID 6/3 167 (1998). [10] T. Igarashi, T. Kusunoki, K. Ohno, T. Isobe, M. Senna, Mater. Res. Bull. 36 (2001) 1317. [11] X.D. Sun, X.D. Xiang, Appl. Phys. Lett. 72 (1998) 525. [12] J.C. Park, H.K. Moon, D.K. Kim, S.H. Byeon, B.C. Kim, K.S. Suh, Appl. Phys. Lett. 77 (2000) 2162. [13] O.A. Lopez, J. Mckittrick, L.E. Shea, J. Lumin. 71 (1997) 1. [14] M. Sychov, Y. Nakanishi1, H. Nakajima, H. Kominami, Y. Hatanaka, Jpn. J. Appl. Phys. 45 (2006) 141. [15] S.H. Shin, J.H. Kang, D.Y. Jeon, S.H. Choi, S.H. Lee, Y.C. You, D.S. Zang, Solid State Commun. 135 (2005) 30–33. [16] K. Vanheusden, C.H. Seager, W.L. Warren, D.R. Tallart, J.A. Voigt, Appl. Phys. Lett. 68 (1996) 403. [17] P. Thiyagarajan, M. Kottaisamy, N. Rama, M.S. Ramachandra Rao, Scr. Mater. 59 (2008) 722–725. [18] V. Srikant, D.R. Clarke, J. Appl. Phys. 83 (1998) 5447. [19] H.J. Egelhaaf, D. Oelkrug, J. Cryst. Growth 161 (1996) 190. [20] S. Bathke, H. Pan, B.W. Wessels, Appl. Phys. Lett. 52 (1988) 138. [21] H. Kominmai, C. Eguchi, M. Kottaisamy, Y. Nakanshi, Y. Hatanaka, J. Vac. Sci. Technol. B-22 (3) (2004) 1386–1389. [22] B.M.J. Smets, Mater. Chem. Phys. 16 (1987) 283. [23] G. Blasse, B.C. Grabmaier, Luminescent Materials, first ed., Springer Verlag, Berlin, 1994. [24] M. Kottaisamy, P. Thiyagarajan, J. Mishra, M.S. Ramachandra Rao, Mater. Res. Bull. 43 (2008) 1657. [25] R.D. Shanon, Acta Crystallogr. 32 (1976) 751. [26] G.H. Dieke, Spectra and Energy Levels of Rare Earth Ions in Crystal, Interscience Publishers, Wiley, New York, 1968. [27] L. Ozawa, Cathodoluminescence: Theory and Application, VCH, Germany, 1990. [28] L.E. Shea, J. McKittrick, M.F. Philips, J. Electrochem. Soc. 145 (1998) 3165. [29] J.S. Yoo, J.D. Lee, J. Appl. Phys. 81 (1997) 2810. [30] J. Douglas, Illett, M. Salful Islam, J. Chem. Soc. Faraday Trans. 89 (1993) 3833. [31] K. Katayama, H. Osawa, J. Akiba, J. Eur. Ceram. Soc. 6 (1990) 39. [32] G. Blasse, L.H. Brixner, Eur. J. Solid State Inorg. Chem. 28 (1991) 767. [33] P.D. Rack, M.D. Potter, S. Kurinec, W. Park, J. Penczek, B.T. Wagner, C. J. Summers, J. Appl. Phys. 84 (1998) 4466. [34] B. Yao, H. Shi, H. Bi, L. Zhang, J. Phys.: Condens. Matter 12 (2000) 6265. [35] M. Anpo, Y. Kubokawa, J. Phys. Chem. 88 (1984) 5556. [36] Y. Kayanuma, H. Momiji, Phys. Rev. B 41 (1990) 10261. [37] M. Bouguerra, M. Samah, M.A. Belkhir, A. Chergui, L. Gerbous, G. Novet, D. Chateigner, R. Madelon, Chem. Phys. Lett. 425 (2006) 77. [38] D.B.M. Klaassen, H. Mulder, C.R. Ronda, Phys. Rev. B 39 (1989) 42.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Full Length Article
Low voltage cathode-luminescent properties of Zn co-doped Y2O3:Eu red phosphor K. Lingadurai a, B. Sundarakannan b, E.R. Nagarajan a, H. Kominami c, Y. Nakanishi c, M. Kottaisamy b,n a
Department of Chemistry, Kalasalingam University, Krishnankoil, 626126, India Department of Chemistry, Thiagarajar College of Engineering, Madurai 625015,India c Graduate School of Electronic Science and Technology, Research Institute of Electronics, Shizuoka University, Hamamatsu 432-8011, Japan b
art ic l e i nf o
a b s t r a c t
Article history: Received 23 January 2015 Received in revised form 3 April 2016 Accepted 19 April 2016 Available online 27 April 2016
Zn co-doped and undoped Y2O3-Eu red phosphor was prepared by citric acid gel method and it was investigated to know its cathodoluminesecent properties at low voltage excitation ( o2 kVs). In this connection the cathodoluminescence (CL) spectra of Zn co-doped and undoped phosphors were taken and they showed various line emissions along with a sharp red emission at 611 nm, irrespective of Zn concentration, due to 5D0–7F2 transitions from Eu luminescent center. It was found that the incorporation of Zn could significantly enhance the CL emission intensity of Eu at about 33% when compared with Zn undoped phosphor. It was also found the Zn co-doping could not alter the crystal structure whereas it could change the morphology of the particles. The promising results revealed the possible application of this phosphor materials as red phosphor for field emission displays [FEDs]. & 2016 Elsevier B.V. All rights reserved.
Keywords: Phosphors Cathodoluminescence X-ray diffraction Excitation FEDs
1. Introduction In recent days, considerable interests are shown on the development of phosphors for low voltage display devices viz, field emission displays (FEDs) because its inherent advantages over the bulk cathode ray tube (CRT) and liquid crystal displays (LCD) and also for white light emitting diodes (WLEDs) due to energy saving lighting systems. In FEDs, excitation of phosphor involves at low energy electron (o 2 kV) and high current density irradiations (o 100 mA/cm2). Many sulfide phosphors, which yield efficient luminance at this excitation, unfortunately, decompose during operation due to electron-stimulated reaction. This decomposition leads to a sulfur deficient non-luminescent layer on the phosphor surface and reduces the phosphor efficiency. Moreover, the byproducts of the sulfur are known to poison the electron emitter [1–3]. The fast growing flat panel display market has boosted researches that focus on the improvement of oxide phosphor for FEDs due to its chemical stability and stable luminescence. Since the excitation involves at low voltage, wide band gap semiconducting oxide phosphors are gaining importance, due to reduction in surface charge up and generation of more e–h pairs in the host crystal that results in better luminous efficiency [4–7]. On n
Corresponding author. Tel.: þ 91 452 2482240x729. E-mail address: [email protected] (M. Kottaisamy).
http://dx.doi.org/10.1016/j.jlumin.2016.04.041 0022-2313/& 2016 Elsevier B.V. All rights reserved.
the other hand, efforts have been taken by coating wide band gap conductive oxides on the surface of the phosphors [8–10]. These oxides afford recombination centers like oxygen vacancies around the surface of the phosphor powder, remove the surface bound electrons and make incident electrons to easily excite in bulk crystal lattice of phosphors. In addition, attempts have been made to develop high efficient red phosphors by doping impurities that create oxygen vacancies in phosphor crystallites with the goal of developing high efficient phosphors at low voltage excitation [11– 15]. Among this, defective structure ZnO is widely studied for its peculiar stable luminescent properties that depend on the preparation condition, annealing atmosphere, deposition substrates, precursor, source, etc. [16,17]. In the present work, we executed researches with ZnO as a dopant in Y2O3 host lattice (along with Eu, as a luminescent center). The former is a chemically stable, wide band gap n-type semi-conductor (Eg, 3.3 eV) and transparent to visible radiation [18]. It can serve as high efficient low voltage phosphor as it has optonic applications due to its conductivity and excitonic emission in the UV region, respectively [19,20]. In addition it is well known for its strong defect broad band emission at the visible wavelength range at the variable excitation wavelength from near UV to blue light [17]. The later, Y2O3 is an insulating phosphor crystal (Eg, 6 eV), and it is an unsurpassed red emitting (Y2O3:Eu) component that emits a sharp emission line at 611 nm, in which larger part of intensity is concentrated. This phosphor is
250
K. Lingadurai et al. / Journal of Luminescence 177 (2016) 249–253
extensively applied in high efficiency fluorescent lamps, high definition television and FEDs because of its inherent advantages of stability towards the high energy irradiations [21–23]. By considering the above all concepts, in the present work, a new attempt has been made to increase the efficiency of phosphors at low voltage excitation by doping alieovalent conducting oxide in high resistance phosphor crystallites and an analysis has been carried out to know the luminescent properties for FED.
2. Experimental To prepare an uniformly Zn co-doped Y2O3-Eu red phosphor, citric acid gel method was adopted. This method was specifically selected for the synthesis of phase pure multicomponent oxide materials because of its tribasic character to chemically bind all the metal ions to facilitate uniform doping in the host matrix [24]. In this material, the zinc concentration was varied from 2.5 to 15 mole% with respect to 2 mol yttrium concentration in Y2O3 including the Europium (Eu) concentration. In a typical example, for a preparation of 5 mol% Zn doping, i.e., 0.1 mol Zn co-doped Y2O3:Eu red phosphor, required quantity of Y2O3 (0.93 mol), Eu2O3 (0.02 mol) and ZnO (0.1 mol) were dissolved in dilute nitric acid and then added with citric acid (2.2 mol). The addition of excess citric acid could help to grow the phosphor host materials without any secondary phase formation. The obtained solution was continuously stirred for three hours for a homogenous mixing of Y, Eu and Zn at molecular levels and heated at 90 °C to get a transparent gel. The gel was heated at 250 °C to obtain the precursor. Subsequently, the precursors were heat treated at 1100 °C for 4 h in alumina crucibles covered with a silica lid in open air atmosphere. In order to study the effect of Zn concentration on the cathodoluminecent properties of Y2O3:Eu, the Zn concentration was varied from 2.5 to 15 mol% with respect to total metal concentration [Y and Eu concentration] whereas the Eu concentration was fixed as 2 mol%. For a comparison purpose, Zn undoped Y2O3: Eu (2 mol %) was also prepared by the same method. Structural properties of the samples were characterized by Xray diffraction (XRD) analysis. For cathodoluminescent (CL) measurements, the powder phosphor samples (0.12 g) were deposited by sedimentation method with a dispersing agent in water on aluminum substrate (1 cmx1 cm). After the clear deposition of the powder, water was drawn off and the samples were dried in hot air oven at 60 °C. The CL properties of phosphors were studied using a continuous electron beam with a spot size of 2 10 6 m2 generated at various excitation voltages ( o2 kVs) at a current density of 60 mA/cm2 by employing TOPCON BM-5 luminance meter. Luminance and luminous efficiency of the best Zn co-doped Y2O3:Eu phosphor were compared with a commercial Y2O3:Eu red phosphor. The CL spectra were measured with a fiber optic coupled spectrophotometer. The spectral resolution of the spectra was determined to be 1 nm. The emission spectra were recorded from 400 to 800 nm and the excitation spectra were recorded from 150 to 400 nm on 200 HSC (JASCO) spectro-fluorimeter. The generated excitation spectra were corrected for a lamp output.
Moreover, it was also noticed that the peaks were shifted to right side showed a possible reduction in the lattice parameter of Y2O3, due to the substitution of smaller size Zn (0.74 Å) in the Y (0.92 Å) site [25]. From these observations, it could be asserted that the Zn doped Y2O3 could form a solid solution. The presence of Zn in the Yttrium oxide lattice was also confirmed by EDAX spectrum.The amount of zinc that was added not retained in the phosphor and this could be correlated with the loss of zinc by evaporation at 1100 °C. Fig. 2 shows the CL emission spectra of various concentration of Zn (2.5–15 mol%) doped Y2O3:Eu red phosphor, from 350 to 800 nm, at the excitation of 2 kV and 60 mA/cm2. The emission spectra could depict a typical emission from Y2O3:Eu at 611 nm due to the 5D0–7F2 transition along with minor peaks originated from the other 5D0–7FJ transitions [26]. The most significant aspect of the CL spectrum revealed that there were no emissions due to the Zn centers (either from the excitonic emission or broad band defect emission) [19,20] even though the Zn concentration was much higher than the Eu concentration. This was quite interesting as far as the cathode ray excitation was concerned. Moreover, from the CL spectra, it was observed that addition of Zn did not shift the Eu emission at 611 nm. In a earlier report, it was observed that Zn co-doping in Gd2O3:Eu shifted the Eu dominant emission from 611 nm to 621 nm [13]. This shift was observed due to the modifications in the local crystal structure that would arise from the
Fig. 1. X-ray powder diffraction pattern of Y1.96 xZnxEu0.04O3.
3. Results and discussion Fig. 1 shows XRD pattern of Eu doped Y2O3 and co-doped with various concentration of Zn obtained at 1100 °C for 4 h. From the XRD pattern, it was observed that addition of Zn did not change the cubic crystal structure of Y2O3 (JCPDS-No.43-1036). At the same time it improved the crystallinity as observed from the peak narrowing and intensity. However, the addition of Zn at 10 mol% and above, it was noticed that the peak intensity decreased.
Fig. 2. Cathodoluminescent (CL) emission spectra of Y1.96 xZnxEu0.04O3 measured at 2 kV as a function of Zn content (x).
K. Lingadurai et al. / Journal of Luminescence 177 (2016) 249–253
introduction of oxygen vacancies by the incorporation of Zn in the Gd2O3 lattice. However, the addition of Zn to Y2O3 did not shift the emission but increased the emission intensity at 611 nm. Fig. 3, displays a relative peak intensity of the emission at 611 nm, at 2 kV and 60 mA/cm2 excitation. A significant enhancement of CL intensity was observed as a function of Zn concentration. The maximum improvement of about 33% was observed for Y1.86Zn0.1Eu0.04O2.95 and above the concentration of 5 mol%, the intensity decreased. For Zn doping Y2O3:Eu, the replacement of Y3 þ by Zn2 þ could give rise to the formation of more defects by creating oxygen vacancies in the host lattice. With the increase in Zn content, the oxygen vacancy increases which will destroy the crystallinity and lead to the luminescence quenching. It was interesting to note that there was no decrease of intensity below the Y2O3:Eu, prepared by the same method, at any Zn concentration. Hence it was believed that Zn could act as a sensitizer for Y2O3:Eu red phosphor at low voltage cathode ray excitation. For reference, the luminance and luminous efficiency of our best phosphor powder Y1.86Zn0.1Eu0.04O2.95 which was obtained at 1100 °C for 4 h were compared with those of the prepared Y2O3:Eu phosphor as a function of excitation voltage (250 V–2 kV) and current density of 60 mA/cm2. Fig. 4 shows that incorporation of Zn could significantly enhance this emission intensity. It was found that the highest cathodoluminance of 2218 cd/m2 with a luminous efficiency of 5.77 lm/W and 795 cd/m2 with a luminous efficiency of 4.14 lm/W excited at 2 and 1 kV excitation, respectively. In total, an improvement of about 33% was observed in luminance for Y1.86Zn0.1Eu0.04O2.95 as a function of excitation voltage when compared with a Zn undoped Y2O3:Eu phosphor. This phenomenon was very interesting as far as the phosphors for field emission was concerned. It was reported that during the electron beam irradiation of crystals, it created plasma electrons i.e. incident
Fig. 3. Relative peak intensity at 611 nm emission as a function of Zn content in Y1.96 xZnxEu0.04O3 at 2 kV and 60-mA/cm2 excitation.
Fig. 4. Luminance and luminescence efficiency of best phosphor Y1.86Zn0.1Eu0.04O3 with a commercial Y2O3:Eu as a function of excitation voltage.
251
electrons and internally generated secondary electrons and pairs of electrons and holes (e–hs) i.e. radiation induced carriers in the volume in which the incident electron penetrated [27]. The generation of plasma electron was possible, when the excitation occurs at high voltage (4 5 kV). Since the excitation involved at low voltage (o2 kV), the luminescence might be occured through the radiative recombination of e–h pairs at the luminescent centers. It was proposed that the penetration depth would be very shallow at low voltage excitation and effective luminescent region may be confined near the surface [28]. Applying this concept to our case, we believe that this increased luminance may be attributed to the presence of Zn, which enhance carrier (e–h) generation at low voltage excitation, because of its low energy band gap (3.3 eV) compared with Y2O3 (6 eV) and resulting the excitation of Eu through the recombination of cathode ray generated carrier confined in the Y–O–Zn matrix. It was very convincing evidence that the secondary electrons or electron–hole generation in Zn co-doped Y2O3 was much higher than those in the Y2O3 even though the excitation voltage was at and below 2 kV and current density at 60 mA/cm2. At room temperature, the excitons were mobile [29] and hence the energy was efficiently transferred to Eu. From these results, it could be asserted that the presence of Zn in Y2O3 could efficiently transfer the excitation energy to the Eu luminescent center. It has been well established that electrical conductivity of metal oxide powder was dominantly dependent on a deviation from the stoichiometry that was created by lattice defects or oxygen vacancies [15,30]. The incorporation of Zn2 þ into the Y2O3 host matrix in open air was as a subsitutional ion at a Y site with compensating oxygen vacancies as given below. 2ZnO þ 2YY 3 þ
-
2Zn2 þ Y þ Vo þ Y2 O3
where YY3 þ represents a yttrium cation in a yttrium site, Zn2Y þ represents a zinc ion in a yttrium site and Vo represents an oxygen vacancy. In-order to confirm an enhancement of the emission intensity of the Zn doped phosphor, studies were carried out by analysing the conductivity of the various Zn doped Y2O3 as shown in Fig. 5. The conductivity of 5 mol% Zn doped Y2O3 showed about one order higher and yielded about 33% improvement in luminance compared with the Zn undoped phosphor. However, the increase in the conductivity of 10 mol% doped phosphor was not showing improvement in luminance when compared with lower Zn doped Y2O3. Hence, it could be asserted that the increase in conductivity of the phosphor matrix had little effect on the luminescence efficiency of phosphor at low voltage excitations, however it could help to reduce the charging-up of the phosphor [31]. In order to have a further look in to the effect of Zn, CL spectrum of Zn (5 mol%) doped Y2O3 (without Eu) was monitored from
Fig. 5. Conductivity of Y2O3 and doped with various concentration of Zn.
252
K. Lingadurai et al. / Journal of Luminescence 177 (2016) 249–253
350 to 800 nm at 1 kV and 60 mA/cm2 excitation and the results were shown in Fig. 6. The figure depicted a weak shoulder emission at 360 nm and a strong narrow band emission at 392 nm with some sharp emission at 573 and 611 nm (due to the presence of rare earth impurities available in trace quantity in the host crystal). The shoulder band emission at 360 nm was originated from the self trapped excitons in Y2O3 [32] and the emission at 391 nm was thought to be originated from the Zn centers. It has been reported that ZnO emits a narrow UV band emission at 380 and 395 nm due to the radiative annihilation of excitons and bi-exciton, respectively [18,33,34]. However, Zn co-doped Y2O3–Eu was not showing any emission at 360 and 392 nm but only emission from the Eu luminescent center. By comparing the Figs. 2 and 6 it could be easily concluded that the excitation of Eu ions occured through the recombination of cathode generated carrier (e–h) confined in the Y–O–Zn matrix and subsequently transferred the energy to Eu excited level. Fig. 7 shows the PL emission spectrum of Y2O3:Eu (2 mol%) doped with various concentration of Zn at the excitation of 325 nm, in which Zn site getting excited (band gap of ZnO was 3.3 eV) instead of Y2O3 (6 eV). From the Fig. 7, it was observed that Eu doped Y2O3 had a typical sharp line emission at 611 nm due to the 5D0–7F2 electric dipole transition arising from the Eu luminescent center. In contrast to this results, Zn doped Y2O3:Eu had a broad green emission band with a band width of (320 nm), peaked at 525 nm and a sharp emission at 391 nm accompanied with the red emission at 611 nm. It clearly shows that there are two luminescent center arises due to the Zn and Eu. The broad bluish green emission band and sharp emission at 391 nm was thought to be originated from the zinc center due to stable defects and typical
Fig. 6. Cathodoluminescent emission spectrum of Y1.9Zn0.1O3 at 2 kV and 60 μA/ cm2 excitation.
Fig. 7. Photo luminescent emission spectra of Y2O3:Eu as a function of Zn concentration at the excitation of near UV at 325 nm.
excitonic emission, respectively [35,36]. The red emission at 611 nm was due to the Eu luminescent center. It was interestingly observed that the europium emission had an enhancement of about 1.5 times higher for 5 mol% doped Zn than Eu doped Y2O3. In order to further confirm the origin of the broad band emission, only Zn (5 mole%) doped Y2O3 was prepared and photoluminescent properties were studied at the excitation of 325 nm as shown in Fig. 8. From the Fig. 8, it was clear that the same broad band emission could be obtained due to the Zn doped in Y2O3. The PL excitation spectra of Zn co-doped (5–15 mol%) Y2O3:Eu was compared with the Y2O3:Eu from 150 nm to 400 nm by fixing the emission maximum at 611 nm (λem ¼611 nm) and the results have been depicted in Fig. 9(a–d). The Y2O3:Eu displayed a strong overlapped excitation of Y–O host at 208 nm with Eu–O charge transfer band (CT) at 255 nm and the CT band was showing higher intensity than that of the host excitation (Fig. 9a). However, for the Zn co-doped Y2O3, the host lattice excitation had markedly high intense than that of the charge transfer band (Fig. 9b–d). The charge transfer band was almost vanished for 15 mol% Zn codoped sample. It was reported that charge transfer (CT) excitation coupled very effectively to higher 4f states in Y2O3:Eu red phosphor and made this phosphor materials with high luminous efficiency in lighting industry [20]. For the Zn co-doped phosphors, the CT band is less intense, however, the CL intensity shows a significant improvement than the Zn free phosphor. This clearly showed that the energy of the excited Zn center in the Y2O3 matrix could be quantitatively transferred to the excited 4f states of Eu at low voltage electron excitation. It was well established that for efficient cathode ray phosphors, activator
Fig. 8. Photo luminescent emission spectrum of Zn doped Y2O3 at the excitation of 325 nm.
Fig. 9. Photo luminescent excitation spectra of Y2O3:Eu as a function of Zn concentration at the emission of 611 nm.
K. Lingadurai et al. / Journal of Luminescence 177 (2016) 249–253
should be excited through host excitation instead of direct activator excitation [37,38]. By considering this, the improvement in luminous efficiency for Zn co-doped Y2O3–Eu phosphor could be attributed to the more cathode ray generated carriers (e–hs) at low voltage excitation. It was pertinent to note that the charge transfer band (250 nm) and other f–f higher excited level of Eu 7F0–5L6 (394 nm) were localized below the band edge of Y2O3 matrix, which was around 6 eV (210 nm). In the sample co-doped with Zn, the energy transfer from the host lattice to the Eu ion was obviously more efficient than the Y2O3:Eu. From this observation, it was conceivable that in the zinc co-doped phosphor, there was an efficient energy transfer to the Eu luminescent center at low voltage electron excitation when compared with Zn un-doped phosphor.
4. Conclusion Zinc co-doped Y2O3:Eu was prepared by employing citric acid gel method and its CL properties were studied. The additions of Zn in Y2O3 could increase crystallinity, enhance conductivity and show a strong UV emission at 391 nm at low voltage cathode ray excitation. However, the CL emission spectra of Zn co-doped Y2O3: Eu showed emission only from Eu center with a sharp emission at 611 nm due to 5D0–7F2 transition, irrespective of Zn concentration. The incorporation of 5 mol% Zn in the phosphor significantly enhances the cathode-luminance and efficiency which was about 33% higher than the Zn undoped phosphor. The PL emission spectra of Zn co-doped Y2O3:Eu at the near UV excitation showed broad band emission along with a red emission at 611 nm. On the basis of above observations, it could be concluded that Zn codoped Y2O3:Eu phosphor would be employed as a red phosphor component to the FEDs, operated at low voltage cathode ray excitations.
Acknowledgment One of the authors M.K sincerely thanks University Grant Commission, New Delhi, and Government of India for providing financial support through the UGC-Major Research Project (F.no. 41-904/2012 (SR)).
253
References [1] A. Vecht, D.W. Smith, S.S. Chada, C.S. Gibbons, J. Koh, D. Mortan, J. Vac. Sci. Technol. B12 (1997) 781. [2] S. Itoh, T. Kimizuka, T. Tonegawa, J. Electrochem. Soc. 136 (1989) 1819. [3] P.H. Holloway, J. Sebastian, T. Trottier, H. Swart, Solid State Technol. 38 (1995) 47. [4] S. Itoh, H. Toki, Y. Sata, K. Morinoto, T. Kishino, J. Electrochem. Soc. 138 (1995) 1509. [5] H. Yamamoto, K. Urabe, J. Electrochem. Soc. 129 (1984) 2069. [6] S.S. Chada, D.W. Smith, A. Vecht, C.S.C. Gibbons, in: Proceedings of SID Dig 51 (1994). [7] S. Itoh, H. Toki, K. Tamura, F. Kataoka, Jpn. J. Appl. Phys. Part 1 (38) (1999) 6387. [8] H. Kominami, T. Nakamura, Y. Nakanishi, Y. Hatanaka, Jpn. J. Appl. Phys. 35 (1996). [9] S.H. Cho, J.S. Yoo, J.D. Lee, in: Proceedings of J. SID 6/3 167 (1998). [10] T. Igarashi, T. Kusunoki, K. Ohno, T. Isobe, M. Senna, Mater. Res. Bull. 36 (2001) 1317. [11] X.D. Sun, X.D. Xiang, Appl. Phys. Lett. 72 (1998) 525. [12] J.C. Park, H.K. Moon, D.K. Kim, S.H. Byeon, B.C. Kim, K.S. Suh, Appl. Phys. Lett. 77 (2000) 2162. [13] O.A. Lopez, J. Mckittrick, L.E. Shea, J. Lumin. 71 (1997) 1. [14] M. Sychov, Y. Nakanishi1, H. Nakajima, H. Kominami, Y. Hatanaka, Jpn. J. Appl. Phys. 45 (2006) 141. [15] S.H. Shin, J.H. Kang, D.Y. Jeon, S.H. Choi, S.H. Lee, Y.C. You, D.S. Zang, Solid State Commun. 135 (2005) 30–33. [16] K. Vanheusden, C.H. Seager, W.L. Warren, D.R. Tallart, J.A. Voigt, Appl. Phys. Lett. 68 (1996) 403. [17] P. Thiyagarajan, M. Kottaisamy, N. Rama, M.S. Ramachandra Rao, Scr. Mater. 59 (2008) 722–725. [18] V. Srikant, D.R. Clarke, J. Appl. Phys. 83 (1998) 5447. [19] H.J. Egelhaaf, D. Oelkrug, J. Cryst. Growth 161 (1996) 190. [20] S. Bathke, H. Pan, B.W. Wessels, Appl. Phys. Lett. 52 (1988) 138. [21] H. Kominmai, C. Eguchi, M. Kottaisamy, Y. Nakanshi, Y. Hatanaka, J. Vac. Sci. Technol. B-22 (3) (2004) 1386–1389. [22] B.M.J. Smets, Mater. Chem. Phys. 16 (1987) 283. [23] G. Blasse, B.C. Grabmaier, Luminescent Materials, first ed., Springer Verlag, Berlin, 1994. [24] M. Kottaisamy, P. Thiyagarajan, J. Mishra, M.S. Ramachandra Rao, Mater. Res. Bull. 43 (2008) 1657. [25] R.D. Shanon, Acta Crystallogr. 32 (1976) 751. [26] G.H. Dieke, Spectra and Energy Levels of Rare Earth Ions in Crystal, Interscience Publishers, Wiley, New York, 1968. [27] L. Ozawa, Cathodoluminescence: Theory and Application, VCH, Germany, 1990. [28] L.E. Shea, J. McKittrick, M.F. Philips, J. Electrochem. Soc. 145 (1998) 3165. [29] J.S. Yoo, J.D. Lee, J. Appl. Phys. 81 (1997) 2810. [30] J. Douglas, Illett, M. Salful Islam, J. Chem. Soc. Faraday Trans. 89 (1993) 3833. [31] K. Katayama, H. Osawa, J. Akiba, J. Eur. Ceram. Soc. 6 (1990) 39. [32] G. Blasse, L.H. Brixner, Eur. J. Solid State Inorg. Chem. 28 (1991) 767. [33] P.D. Rack, M.D. Potter, S. Kurinec, W. Park, J. Penczek, B.T. Wagner, C. J. Summers, J. Appl. Phys. 84 (1998) 4466. [34] B. Yao, H. Shi, H. Bi, L. Zhang, J. Phys.: Condens. Matter 12 (2000) 6265. [35] M. Anpo, Y. Kubokawa, J. Phys. Chem. 88 (1984) 5556. [36] Y. Kayanuma, H. Momiji, Phys. Rev. B 41 (1990) 10261. [37] M. Bouguerra, M. Samah, M.A. Belkhir, A. Chergui, L. Gerbous, G. Novet, D. Chateigner, R. Madelon, Chem. Phys. Lett. 425 (2006) 77. [38] D.B.M. Klaassen, H. Mulder, C.R. Ronda, Phys. Rev. B 39 (1989) 42.