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Influence of annealing temperature on photoluminescence of CePO4 thin films on silicon substrates prepared by electron beam evaporation
Optik - International Journal for Light and Electron Optics 178 (2019) 944–949
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Influence of annealing temperature on photoluminescence of CePO4 thin films on silicon substrates prepared by electron beam evaporation Yu Wang, Shenwei Wang, Kai Ou, Yanwei Zhang, Liyuan Bai, Lixin Yi
T
⁎
Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, China
A R T IC LE I N F O
ABS TRA CT
Keywords: CePO4 Thin films Photoluminescence Annealing
Nanostructured CePO4 thin films were prepared on silicon substrates by electron beam evaporation technique. In this work, the effects of annealing temperature on photoluminescence (PL) of CePO4 thin films were investigated. CePO4 thin films were annealed at different temperatures between 700 and 1100 °C for 30 min in nitrogen. The annealed CePO4 films show a strong ultraviolet emission at room temperature. Besides, a violet/blue emission was observed when the annealing temperature reached 1000 °C. The X-ray diffraction results show the monoclinic structure of CePO4 thin films. The scanning electron microscopy images were taken to confirm different crystallinities. The concentration of different elements was determined by energy-dispersive X-ray spectroscopy. The excellent PL emission in the annealed films indicates that the CePO4 thin films could be useful for silicon-based light sources.
1. Introduction Rare-earth phosphates have been widely used in displays, lighting fields [1,2],ceramics [3] and catalytic [4] due to their advantages of high luminous efficiency, good thermal stability [5] and high maintenance of vacuum ultraviolet radiation [6]. Cerium is the most common of the lanthanides, the fluorescence of Ce3+-activated compounds is well known [7]. In recent years, luminescence of cerium doped ZnO nanoparticles [8], ZnO thin films [9], BaAlBO3F2 glass ceramics [10], CdS film [11], Li7La3Hf2O12 with tetragonal garnet structure [12] and Ca2+-Mg2+-Si4+ based garnet phosphors [13] have been studied. And cerium phosphate (CePO4) is well suited as a matrix for luminescence due to the high energy transfer efficiency between Ce3+ and rare-earth ions [14]. Therefore, it is significant to deeply investigate the luminescence of CePO4 materials. For instance, people have studied CePO4 and rare-earth ions doped CePO4 [15,16]. State to the art, there are many methods to prepare different CePO4 nanostructures, such as nanowires [17,18], nanocrystallines [3] and nanotubes [19]. And the methods include hydrothermal method [17], microemulsion method [18], aqueous sol–gel method [3], reflux method [20] and so on. However, to the best of our knowledge, the PL properties of pure CePO4 thin films have been rarely reported. Herein we report the fabrication of nanostructured CePO4 thin films on silicon substrates based on pure CePO4 powder by electron beam evaporation. The influence of annealing temperature was investigated in this article. The annealed films were characterized by X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS) and scanning electron microscopy (SEM). The annealed films showed excellent ultraviolet and violet/blue emission. It could be useful for preparing silicon-based light-emitting devices based on
⁎
Corresponding author. E-mail address: [email protected] (L. Yi).
https://doi.org/10.1016/j.ijleo.2018.10.033 Received 19 July 2018; Received in revised form 6 October 2018; Accepted 6 October 2018 0030-4026/ © 2018 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 178 (2019) 944–949
Y. Wang et al.
Fig. 1. XRD patterns of CePO4 films annealed at different temperatures for 30 min in N2.
CePO4 matrix. 2. Experimental CePO4 thin films were deposited onto Si substrates by electron beam evaporation (EVA450). High-purity CePO4 powders (99.99%) were used as initial materials. The powders were compressed with a tablet press machine (769YP-24B) to make the core materials. Then the CePO4 films were deposited at room temperature in a high-vacuum chamber with the pressure of 2.2 × 10 −3 Pa, the electron gun voltage was 6 kV, the operating current was 0.85 A. The thickness and deposition rate of the films were monitored by using a quartz crystal thickness monitor, and the rate of evaporation was controlled at 0.8–1.2 Å/s, and finally we got the asdeposited CePO4 films with a thickness of 160 nm. The films were annealed at different temperatures in nitrogen atmosphere (N2) for 30 min. The PL spectra and the PL excitation (PLE) spectra were measured by a fluorescence spectrometer (FLS920) using a 450 W xenon lamp as the excitation source. The crystal structures of the films were characterized by XRD using Cu Kα radiation (Bruker D8 ADVANCE). The surface morphologies of the films were studied by SEM (Hitachi, S-4800), and the contents of different elements in the films were measured by EDS (Hitachi, S-4800). All these measurements were made at room temperature. 3. Results and discussion Fig. 1 shows XRD patterns of pure CePO4 films annealed at 700, 800, 900, 1000 and 1100 °C in nitrogen atmosphere for 30 min. The diffraction peaks (1 0 1), (2 0 0), (1 2 0), (-2 2 0) of the annealed films are at 21.7°, 27.1°, 28.9°and 37.5°, respectively. All diffraction peaks can correspond to the monoclinic structure of CePO4, referring to JCPDF file 32-0199 [6]. With the increase of annealing temperature, the intensities of the diffraction peaks increase firstly. But once the annealing temperatures reached up to 1100 °C the diffraction peaks are different. Some diffraction peaks of CePO4 weaken while the position of (1 2 0) plane has changed a little, from 28.9° to 29.0°. These differences mean there have been some changes in the films when the annealing temperature was 1100 °C, and it may be attributed to the formation of new compounds of
Fig. 2. The normalized PL excitation (λem = 320 nm) and emission (λex = 273 nm) spectra of pure CePO4 films annealed at 900 °C for 30 min in N2. The inset shows the energy levels diagram of Ce3+. 945
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Fig. 3. PL spectra of pure CePO4 films annealed for 30 min at 900 °C in air and N2, respectively (λex = 273 nm).
cerium from the results of PL, SEM and EDS. We will discuss the changes again in the following discussion. Fig. 2 shows the PL and PLE spectra of CePO4 films annealed at 900 °C for 30 min in nitrogen atmosphere. The excitation peaks located at 258 and 273 nm can be assigned to the transitions from the ground state 2F5/2 (4f1) to the excited states 2D (5d1) of Ce3+ [6]. As we can see that the strongest intensity of the peak is at 273 nm. The emission spectrum shows a rather broad ultraviolet emission band between 300 and 370 nm. There are two emission peaks centered at 320 and 340 nm, respectively, corresponding to the 5d-4f transitions of Ce3+. Compared with the PL spectra of CePO4 reported by other papers [21], the nanostructured CePO4 films we got are monoclinic structure. Actually, the CePO4 films were also annealed in air. Fig. 3 shows the PL spectra of CePO4 films annealed in air and N2, respectively. We find that the intensity of ultraviolet emission weakens a lot when annealed in air. To get better emission of the CePO4 films, all the samples were annealed in N2 in the next experiments. In addition, the effect of annealing temperature on photoluminescence was investigated in Fig. 4. We find that there are some changes of emission spectra with different annealing temperatures. As shown in the insert Fig. 4(a), the intensity of emission at 320 nm increases firstly and then weakens with the annealing temperature, which reaches a maximum at 900 °C. This can be understood from the quality of the crystal, to a certain extent, better crystalline quality of the films is obtainedwith higher annealing temperature resulting in enhanced luminescence efficiency. However the crystalline quality can be destroyed by an excessively high temperature [22]. Besides, as shown in Fig. 4, when the annealing temperature reached 1000 °C or more, the intensity of the emission peaks at 320 and 340 nm weakens while a new emission peak occurs at about 400 nm, with a visible outstanding violet/blue light. As we mentioned above, the PL emission corresponds to the 5d-4f transitions of Ce3+ ions, while the 5d state of Ce3+ is strongly influenced by the matrix crystal field [23,24], the fluorescence spectra of Ce3+ are very different in different matrices [25]. So the new emission peak at about 400 nm indicates that there has been at least a new compound of Ce3+. Surface morphologies of the CePO4 thin films with different temperatures by SEM images are showed in Fig. 5. The surface of the as-deposited films is homogeneous. For the annealed films, it can be seen that the particles size is increased with increasing temperatures from the inset figures in Fig. 5(a–c). The changes in size of particles observed in the SEM images reveal different quality crystalline with the annealing temperature. The larger particles resulting in enhanced luminescence efficiency, therefore, the intensity of PL emission increases firstly when the annealing temperatures were lower than 900 °C. When the annealing temperature was higher than 1000 °C, crystalline quality is destroyed, and many particles are gathered together. Fig. 5(d) shows the cross-
Fig. 4. PL spectra of pure CePO4 films annealed at different temperatures for 30 min in N2, respectively (λex = 273 nm). The insert figure (a) shows the intensity trend at 320 nm of pure CePO4 thin films with annealing temperature. 946
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Fig. 5. SEM images of as-deposited and annealed films with various temperatures.
sectional morphology of the CePO4 films annealed at 900 °C. We can also see from the image that the particles are gathered together. The results also correspond to the PL emission results in Fig. 4. Fig. 6 shows the PLE spectra of the films annealed at 1100 °C with detection wavelengths of 320 and 400 nm, respectively. The PLE spectra indicate that the 320 nm emission is still from CePO4. But for the 400 nm emission, the excitation peaks are located at
Fig. 6. The PLE (λem = 320 and 400 nm) spectra of pure CePO4 films annealed at 1100 °C for 30 min in N2. 947
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Table 1 Different elements concentration (at. %) of CePO4 films annealed at 900 °C, 1000 °C and 1100 °C.
900 °C 1000 °C 1100 °C
Cerium
Phosphorus
Oxygen
Total
8.53 8.55 9.44
11.91 9.50 8.00
79.56 81.95 82.56
100 100 100
about 283 and 325 nm. The results also indicate that the emission at about 400 nm is from a new compound of Ce3+. Compared to the previous work of our team [26], both the PLE peaks and violet/blue emission are similar, as we mentioned above, the fluorescence spectra of Ce3+ are very different in different matrices, so the violet/blue emission bands may be attributed to the formation of cerium silicate and correspond to the Ce3+ transitions from the relaxed lowest 5d excited state to 4f ground states. As reported in the previous work [27–29], when the annealing temperature was 1100 °C, the cerium silicate was produced from the interface between silicon substrate and cerium. It also can be seen from the SEM results in Fig. 5, that when the annealing temperature was 1100 °C the structures of the crystalline have changed. The changes may also be attributed to the formation of cerium silicate. In addition, as shown in Table 1, the concentration of different elements changed with annealing temperature through the analysis of EDS. With the increase of annealing temperature, Phosphorus decreases from 11.91 at% to 8 at%, while Cerium increases from 8.53 at% to 9.44 at%. The concentration of Cerium is higher than that of Phosphorus when the annealing temperature was 1100 °C. So, it means the compound of Ce3+ in the film is not only cerium phosphate. This also corresponds to the results of PL emission and SEM. And it could be an indirect evidence for the formation of cerium silicate. 4. Conclusion In this paper, nanostructured CePO4 thin films were successfully prepared by electron beam evaporation technique. The annealed monoclinic CePO4 films showed a strong ultraviolet emission. The influence of annealing temperature on PL properties of CePO4 thin films was investigated. It was confirmed that PL intensity of CePO4 films increased rapidly and then weakened with the annealing temperature, which reached a maximum at 900 °C. The trend of PL intensity with annealing temperatures was consistent with the analyses of XRD and SEM. The annealed condition at 900 °C in N2 was the optimal condition in our experiment. A new broad violet/ blue emission peak at about 400 nm was observed when the annealing temperature was higher than 1000 °C. And the PL, PLE, XRD, SEM and EDS results indicate that the new broad violet/blue emission peak was attributed to cerium silicates when the annealing temperature was 1100 °C. Acknowledgments This work was financially supported by the National Science Foundation of China (Grant no. 61275058, 51772019). It was also supported by the Key Laboratory of Luminescence and Optical Information of China in Beijing Jiaotong University. References [1] U. Rambabu, S. Buddhudu, Optical properties of LnPO 4 :Eu 3+ (Ln=Y, La and Gd) powder phosphors, Opt. Mater. 17 (2001) 401–408. [2] U. Rambabu, D.P. Amalnerkar, B.B. Kale, S. Buddhudu, Optical properties of LnPO 4 :Tb 3+ (Ln = Y, La and Gd) powder phosphors, Mater. Chem. Phys. 70 (2001) 1–6. [3] K. Rajesh, P. Mukundan, P. Krishna Pillai, A.V.R. Nair, K.G.K. Warrier, High-surface-area nanocrystalline cerium phosphate through aqueous sol−gel route, Chem. Mater. 16 (2004) 2700–2705. [4] H. Onoda, H. Nariai, M. Ai, H. Maki, I. Motooka, Formation and catalytic characterization of various rare earth phosphates, J. Mater. Chem. 12 (2002) 1754–1760. [5] Y. Hikichi, T. Nomura, Melting temperatures of Monazite and Xenotime, J. Am. Ceram. Soc. 70 (1987) C-252–C-253. [6] Y.P. Fang, A.W. Xu, R.Q. Song, H.X. Zhang, L.P. You, J.C. Yu, H.Q. Liu, Systematic synthesis and characterization of single-crystal lanthanide orthophosphate nanowires, J. Am. Chem. Soc. 125 (2003) 16025–16034. [7] G. Blasse, A. Bril, Investigation of some Ce3+‐Activated phosphors, J. Chem. Phys. 47 (1967) 5139–5145. [8] P. Pandey, R. Kurchania, F.Z. Haque, Structural, diffused reflectance and photoluminescence study of cerium doped ZnO nanoparticles synthesized through simple sol–gel method, Optik Int. J. Light Electron Opt. 126 (2015) 3310–3315. [9] A. Chelouche, T. Touam, M. Tazerout, F. Boudjouan, D. Djouadi, A. Doghmane, Low cerium doping investigation on structural and photoluminescence properties of sol-gel ZnO thin films, J. Lumin. 181 (2017) 448–454. [10] P. Muralimanohar, R. Parasuraman, J.R. Gandhi, M. Rathnakumari, P. Sureshkumar, Photoluminescence in cerium doped barium aluminium borate difluoride—BaAlBO 3 F 2 glass ceramics, Optik Int. J. Light Electron Opt. 127 (2016) 8956–8962. [11] T. Hurma, Effect of cerium incorporation on the structural and optical properties of CdS film, Optik Int. J. Light Electron Opt. 127 (2016) 10670–10675. [12] Y.V. Baklanova, A.V. Ishchenko, M.A. Melkozerova, L.G. Maksimova, T.A. Denisova, A.P. Tyutyunnik, V.G. Zubkov, B.V. Shulgin, Synthesis and optical properties of cerium doped Li 7 La 3 Hf 2 O 12 with tetragonal garnet structure, J. Lumin. (2018) 194. [13] V. Gorbenko, T. Zorenko, S. Witkiewicz, K. Paprocki, A. Iskaliyeva, A.M. Kaczmarek, R.V. Deun, M.N. Khaidukov, M. Batentschuk, Y. Zorenko, Luminescence of Ce 3+ multicenters in Ca 2+ -Mg 2+ -Si 4+ based garnet phosphors, J. Lumin. (2018) 199. [14] G. Blasse, A. Bril, Study of energy transfer from Sb3+, Bi3+, Ce3+ to Sm3+, Eu3+, Tb3+, Dy3+, J. Chem. Phys. 47 (1967) 1920–1926. [15] M. Guan, J. Sun, M. Han, Z. Xu, F. Tao, G. Yin, X. Wei, J. Zhu, X. Jiang, Synthesis and luminescence of CePO4 and CePO4:Tb hollow and core–shell microspheres composed of single-crystal nanorods, Nanotechnology 18 (2007) 415602. [16] A.K. Gulnar, V. Sudarsan, R.K. Vatsa, R.C. Hubli, U.K. Gautam, A. Vinu, A.K. Tyagi, CePO4:Ln (Ln = Tb3+ and Dy3+) nanoleaves incorporated in silica sols, Cryst. Growth Des. 9 (2009) 2451–2456. [17] W.B. Bu, Z.L. Hua, H.R. Chen, L.X. Zhang, J.L. Shi, Hydrothermal synthesis of ultraviolet-emitting cerium phosphate single-crystal nanowires, Cheminform 35
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(2004) 612–613. [18] X. Yan, L. Mei, S.A.D. And, S. Mann, Synthesis and characterization of cerium phosphate nanowires in microemulsion reaction media, J. Phys. Chem. B 110 (2006) 1111. [19] C. Tang, Y. Bando, D. Golberg, R. Ma, Cerium phosphate nanotubes: synthesis, valence state, and optical properties, Angew. Chem. Int. Ed. 44 (2010) 576–579. [20] M. Kirubanithy, A.A. Irudayaraj, A.D. Raj, S. Manikandan, Synthesis, characterization and photoluminescence behaviours of CePO 4 and Tb-doped CePO 4 nanostructures, Mater. Today Proc. 2 (2015) 4344–4347. [21] J. Bao, R. Yu, J. Zhang, X. Yang, D. Wang, J. Deng, J. Chen, X. Xing, Low-temperature hydrothermal synthesis and structure control of nano-sized CePO4, CrystEngComm 11 (2009) 1630. [22] K. Ou, S. Wang, M. Huang, Y. Zhang, Y. Wang, X. Duan, L. Yi, Influence of thickness and annealing on photoluminescence of nanostructured ZnSe/ZnS multilayer thin films prepared by electron beam evaporation, J. Lumin. 199 (2018) 34–38. [23] L.G. van Uitert, An empirical relation fitting the position in energy of the lower d-band edge for Eu 2+ OR Ce 3+ in various compounds, J. Lumin. 29 (1984) 1–9. [24] P. Dorenbos, 5d-level energies of Ce3+ and the crystalline environment. III. Oxides containing ionic complexes, Phys. Rev. B 64 (2001) 125117. [25] P. Dorenbos, L. Pierron, L. Dinca, C.W.E. van Eijk, A. Kahn-Harari, B. Viana, 4f–5d spectroscopy of Ce3+ in CaBPO5, LiCaPO4 and Li2CaSiO4, J. Phys. Condensed Matter 15 (2003) 511. [26] L. Li, S. Wang, G. Mu, Y. Xue, K. Ou, L. Yi, A novel violet/blue light-emitting device based on Ce2Si2O7, Sci. Rep. 5 (2015) 16659. [27] W.C. Choi, H.N. Lee, E.K. Kim, Y. Kim, C.Y. Park, H.S. Kim, J.Y. Lee, Violet/blue light-emitting cerium silicates, Appl. Phys. Lett. 75 (1999) 2389–2391. [28] J. Li, O. Zalloum, T. Roschuk, C. Heng, J. Wojcik, P. Mascher, The formation of light emitting cerium silicates in cerium-doped silicon oxides, Appl. Phys. Lett. 94 (2009) 225. [29] F. Pagliuca, P. Luches, S. Valeri, Interfacial interaction between cerium oxide and silicon surfaces, Surf. Sci. 607 (2013) 164–169.
949
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Influence of annealing temperature on photoluminescence of CePO4 thin films on silicon substrates prepared by electron beam evaporation Yu Wang, Shenwei Wang, Kai Ou, Yanwei Zhang, Liyuan Bai, Lixin Yi
T
⁎
Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, China
A R T IC LE I N F O
ABS TRA CT
Keywords: CePO4 Thin films Photoluminescence Annealing
Nanostructured CePO4 thin films were prepared on silicon substrates by electron beam evaporation technique. In this work, the effects of annealing temperature on photoluminescence (PL) of CePO4 thin films were investigated. CePO4 thin films were annealed at different temperatures between 700 and 1100 °C for 30 min in nitrogen. The annealed CePO4 films show a strong ultraviolet emission at room temperature. Besides, a violet/blue emission was observed when the annealing temperature reached 1000 °C. The X-ray diffraction results show the monoclinic structure of CePO4 thin films. The scanning electron microscopy images were taken to confirm different crystallinities. The concentration of different elements was determined by energy-dispersive X-ray spectroscopy. The excellent PL emission in the annealed films indicates that the CePO4 thin films could be useful for silicon-based light sources.
1. Introduction Rare-earth phosphates have been widely used in displays, lighting fields [1,2],ceramics [3] and catalytic [4] due to their advantages of high luminous efficiency, good thermal stability [5] and high maintenance of vacuum ultraviolet radiation [6]. Cerium is the most common of the lanthanides, the fluorescence of Ce3+-activated compounds is well known [7]. In recent years, luminescence of cerium doped ZnO nanoparticles [8], ZnO thin films [9], BaAlBO3F2 glass ceramics [10], CdS film [11], Li7La3Hf2O12 with tetragonal garnet structure [12] and Ca2+-Mg2+-Si4+ based garnet phosphors [13] have been studied. And cerium phosphate (CePO4) is well suited as a matrix for luminescence due to the high energy transfer efficiency between Ce3+ and rare-earth ions [14]. Therefore, it is significant to deeply investigate the luminescence of CePO4 materials. For instance, people have studied CePO4 and rare-earth ions doped CePO4 [15,16]. State to the art, there are many methods to prepare different CePO4 nanostructures, such as nanowires [17,18], nanocrystallines [3] and nanotubes [19]. And the methods include hydrothermal method [17], microemulsion method [18], aqueous sol–gel method [3], reflux method [20] and so on. However, to the best of our knowledge, the PL properties of pure CePO4 thin films have been rarely reported. Herein we report the fabrication of nanostructured CePO4 thin films on silicon substrates based on pure CePO4 powder by electron beam evaporation. The influence of annealing temperature was investigated in this article. The annealed films were characterized by X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS) and scanning electron microscopy (SEM). The annealed films showed excellent ultraviolet and violet/blue emission. It could be useful for preparing silicon-based light-emitting devices based on
⁎
Corresponding author. E-mail address: [email protected] (L. Yi).
https://doi.org/10.1016/j.ijleo.2018.10.033 Received 19 July 2018; Received in revised form 6 October 2018; Accepted 6 October 2018 0030-4026/ © 2018 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 178 (2019) 944–949
Y. Wang et al.
Fig. 1. XRD patterns of CePO4 films annealed at different temperatures for 30 min in N2.
CePO4 matrix. 2. Experimental CePO4 thin films were deposited onto Si substrates by electron beam evaporation (EVA450). High-purity CePO4 powders (99.99%) were used as initial materials. The powders were compressed with a tablet press machine (769YP-24B) to make the core materials. Then the CePO4 films were deposited at room temperature in a high-vacuum chamber with the pressure of 2.2 × 10 −3 Pa, the electron gun voltage was 6 kV, the operating current was 0.85 A. The thickness and deposition rate of the films were monitored by using a quartz crystal thickness monitor, and the rate of evaporation was controlled at 0.8–1.2 Å/s, and finally we got the asdeposited CePO4 films with a thickness of 160 nm. The films were annealed at different temperatures in nitrogen atmosphere (N2) for 30 min. The PL spectra and the PL excitation (PLE) spectra were measured by a fluorescence spectrometer (FLS920) using a 450 W xenon lamp as the excitation source. The crystal structures of the films were characterized by XRD using Cu Kα radiation (Bruker D8 ADVANCE). The surface morphologies of the films were studied by SEM (Hitachi, S-4800), and the contents of different elements in the films were measured by EDS (Hitachi, S-4800). All these measurements were made at room temperature. 3. Results and discussion Fig. 1 shows XRD patterns of pure CePO4 films annealed at 700, 800, 900, 1000 and 1100 °C in nitrogen atmosphere for 30 min. The diffraction peaks (1 0 1), (2 0 0), (1 2 0), (-2 2 0) of the annealed films are at 21.7°, 27.1°, 28.9°and 37.5°, respectively. All diffraction peaks can correspond to the monoclinic structure of CePO4, referring to JCPDF file 32-0199 [6]. With the increase of annealing temperature, the intensities of the diffraction peaks increase firstly. But once the annealing temperatures reached up to 1100 °C the diffraction peaks are different. Some diffraction peaks of CePO4 weaken while the position of (1 2 0) plane has changed a little, from 28.9° to 29.0°. These differences mean there have been some changes in the films when the annealing temperature was 1100 °C, and it may be attributed to the formation of new compounds of
Fig. 2. The normalized PL excitation (λem = 320 nm) and emission (λex = 273 nm) spectra of pure CePO4 films annealed at 900 °C for 30 min in N2. The inset shows the energy levels diagram of Ce3+. 945
Optik - International Journal for Light and Electron Optics 178 (2019) 944–949
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Fig. 3. PL spectra of pure CePO4 films annealed for 30 min at 900 °C in air and N2, respectively (λex = 273 nm).
cerium from the results of PL, SEM and EDS. We will discuss the changes again in the following discussion. Fig. 2 shows the PL and PLE spectra of CePO4 films annealed at 900 °C for 30 min in nitrogen atmosphere. The excitation peaks located at 258 and 273 nm can be assigned to the transitions from the ground state 2F5/2 (4f1) to the excited states 2D (5d1) of Ce3+ [6]. As we can see that the strongest intensity of the peak is at 273 nm. The emission spectrum shows a rather broad ultraviolet emission band between 300 and 370 nm. There are two emission peaks centered at 320 and 340 nm, respectively, corresponding to the 5d-4f transitions of Ce3+. Compared with the PL spectra of CePO4 reported by other papers [21], the nanostructured CePO4 films we got are monoclinic structure. Actually, the CePO4 films were also annealed in air. Fig. 3 shows the PL spectra of CePO4 films annealed in air and N2, respectively. We find that the intensity of ultraviolet emission weakens a lot when annealed in air. To get better emission of the CePO4 films, all the samples were annealed in N2 in the next experiments. In addition, the effect of annealing temperature on photoluminescence was investigated in Fig. 4. We find that there are some changes of emission spectra with different annealing temperatures. As shown in the insert Fig. 4(a), the intensity of emission at 320 nm increases firstly and then weakens with the annealing temperature, which reaches a maximum at 900 °C. This can be understood from the quality of the crystal, to a certain extent, better crystalline quality of the films is obtainedwith higher annealing temperature resulting in enhanced luminescence efficiency. However the crystalline quality can be destroyed by an excessively high temperature [22]. Besides, as shown in Fig. 4, when the annealing temperature reached 1000 °C or more, the intensity of the emission peaks at 320 and 340 nm weakens while a new emission peak occurs at about 400 nm, with a visible outstanding violet/blue light. As we mentioned above, the PL emission corresponds to the 5d-4f transitions of Ce3+ ions, while the 5d state of Ce3+ is strongly influenced by the matrix crystal field [23,24], the fluorescence spectra of Ce3+ are very different in different matrices [25]. So the new emission peak at about 400 nm indicates that there has been at least a new compound of Ce3+. Surface morphologies of the CePO4 thin films with different temperatures by SEM images are showed in Fig. 5. The surface of the as-deposited films is homogeneous. For the annealed films, it can be seen that the particles size is increased with increasing temperatures from the inset figures in Fig. 5(a–c). The changes in size of particles observed in the SEM images reveal different quality crystalline with the annealing temperature. The larger particles resulting in enhanced luminescence efficiency, therefore, the intensity of PL emission increases firstly when the annealing temperatures were lower than 900 °C. When the annealing temperature was higher than 1000 °C, crystalline quality is destroyed, and many particles are gathered together. Fig. 5(d) shows the cross-
Fig. 4. PL spectra of pure CePO4 films annealed at different temperatures for 30 min in N2, respectively (λex = 273 nm). The insert figure (a) shows the intensity trend at 320 nm of pure CePO4 thin films with annealing temperature. 946
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Y. Wang et al.
Fig. 5. SEM images of as-deposited and annealed films with various temperatures.
sectional morphology of the CePO4 films annealed at 900 °C. We can also see from the image that the particles are gathered together. The results also correspond to the PL emission results in Fig. 4. Fig. 6 shows the PLE spectra of the films annealed at 1100 °C with detection wavelengths of 320 and 400 nm, respectively. The PLE spectra indicate that the 320 nm emission is still from CePO4. But for the 400 nm emission, the excitation peaks are located at
Fig. 6. The PLE (λem = 320 and 400 nm) spectra of pure CePO4 films annealed at 1100 °C for 30 min in N2. 947
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Y. Wang et al.
Table 1 Different elements concentration (at. %) of CePO4 films annealed at 900 °C, 1000 °C and 1100 °C.
900 °C 1000 °C 1100 °C
Cerium
Phosphorus
Oxygen
Total
8.53 8.55 9.44
11.91 9.50 8.00
79.56 81.95 82.56
100 100 100
about 283 and 325 nm. The results also indicate that the emission at about 400 nm is from a new compound of Ce3+. Compared to the previous work of our team [26], both the PLE peaks and violet/blue emission are similar, as we mentioned above, the fluorescence spectra of Ce3+ are very different in different matrices, so the violet/blue emission bands may be attributed to the formation of cerium silicate and correspond to the Ce3+ transitions from the relaxed lowest 5d excited state to 4f ground states. As reported in the previous work [27–29], when the annealing temperature was 1100 °C, the cerium silicate was produced from the interface between silicon substrate and cerium. It also can be seen from the SEM results in Fig. 5, that when the annealing temperature was 1100 °C the structures of the crystalline have changed. The changes may also be attributed to the formation of cerium silicate. In addition, as shown in Table 1, the concentration of different elements changed with annealing temperature through the analysis of EDS. With the increase of annealing temperature, Phosphorus decreases from 11.91 at% to 8 at%, while Cerium increases from 8.53 at% to 9.44 at%. The concentration of Cerium is higher than that of Phosphorus when the annealing temperature was 1100 °C. So, it means the compound of Ce3+ in the film is not only cerium phosphate. This also corresponds to the results of PL emission and SEM. And it could be an indirect evidence for the formation of cerium silicate. 4. Conclusion In this paper, nanostructured CePO4 thin films were successfully prepared by electron beam evaporation technique. The annealed monoclinic CePO4 films showed a strong ultraviolet emission. The influence of annealing temperature on PL properties of CePO4 thin films was investigated. It was confirmed that PL intensity of CePO4 films increased rapidly and then weakened with the annealing temperature, which reached a maximum at 900 °C. The trend of PL intensity with annealing temperatures was consistent with the analyses of XRD and SEM. The annealed condition at 900 °C in N2 was the optimal condition in our experiment. A new broad violet/ blue emission peak at about 400 nm was observed when the annealing temperature was higher than 1000 °C. And the PL, PLE, XRD, SEM and EDS results indicate that the new broad violet/blue emission peak was attributed to cerium silicates when the annealing temperature was 1100 °C. Acknowledgments This work was financially supported by the National Science Foundation of China (Grant no. 61275058, 51772019). It was also supported by the Key Laboratory of Luminescence and Optical Information of China in Beijing Jiaotong University. References [1] U. Rambabu, S. 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