- Email: [email protected]
Luminescence properties of Y2O3:Bi3+, Yb3+ co-doped phosphor for application in solar cells
Physica B xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Luminescence properties of Y2O3:Bi3+, Yb3+ co-doped phosphor for application in solar cells ⁎
⁎
E. Lee , R.E. Kroon, J.J. Terblans, H.C. Swart
Department of Physics, University of the Free State, Bloemfontein, South Africa
A R T I C L E I N F O
A BS T RAC T
Keywords: Downconversion Y2O3:Bi3+, Yb3+
Bismuth (Bi3+) and ytterbium (Yb3+) co-doped yttrium oxide (Y2O3) phosphor powder was successfully synthesised using the co-precipitation technique. The X-ray diffraction (XRD) patterns confirmed that a single phase cubic structure with a Ia-3 space group was formed. The visible emission confirmed the two symmetry sites, C2 and S6, found in the Y2O3 host material and revealed that Bi3+ ions preferred the S6 site as seen the stronger emission intensity. The near-infrared (NIR) emission of Yb3+ increased significantly by the presence of the Bi3+ ions when compared to the singly doped Y2O3:Yb3+ phosphor with the same Yb3+ concentration. An increase in the NIR emission intensity was also observed by simply increasing the Yb3+ concentration in the Y2O3:Bi3+, Yb3+ phosphor material where the intensity increased up to x = 5.0 mol% of Yb3+ before decreasing due to concentration quenching.
1. Introduction Silicon based solar cells are currently the most common form of photovoltaic cells used to convert solar energy to electrical energy. Unfortunately silicon solar cells suffer from low power conversion efficiencies, due to the mismatch between the solar spectrum and the maximum absorption spectrum of silicon [1]. In recent years, luminescent materials have been used to adapt the solar spectrum in order to reduce the spectral mismatch and improve the power conversion efficiency of the solar cells [2]. These luminescent materials have the ability of upconversion (absorbing two or more low energy photons and converting it into one with a higher energy) or down-conversion (absorbing a high energy photon and converting it into two or more lower energy photons), in order to utilise unabsorbed photons of the solar spectrum [3,4]. Downconversion (DC) phosphors materials doped with rare-earth elements have been used extensively in the lighting industry for potential application in mercury free fluorescent tube and plasma displays luminescent due to the wide range of possible luminescence from ultraviolet (UV) through the visible to the near-infrared (NIR) regions [5,6]. In past research, rare earth (RE3+) – Yb3+ (RE = Tb, Ce, Er and Pr) co-doped phosphors have been used to down-convert UV photons to NIR photons with hopes to improve the efficiency of solar cells [4]. Unfortunately lanthanides or rare-earth metals was found to be poor at absorbing photons in the UV to blue regions due to their parity forbidden 4f transition, which may cause Yb3+ to have a weak NIR emission limiting the potential for solar cell applications [7]. Metal
⁎
donors such as Bi3+ ions have shown to be promising alternative to rare-earth ions for enhancing the NIR emission of Yb3+ ions [6,8,9]. The 6s2 electron configuration of the Bi3+ ion consists of a 1S0 ground state and an excited state with a 6s6p configuration that splits into four levels, 3P0, 3P1, 3P2, and 1P1 (in order of increasing energy) [10]. Due to the ΔJ selection rule the 1S0 → 3P0 and 1S0 → 3P2 are strongly spin forbidden, while the 1S0 → 3P1 and 1S0 → 1P1 are allowed transition because of the 3P1 and 1P1 spin-orbital coupling [5,6,9]. In this research, only the 1S0 → 3P1 transition will be studied as the transition is in the region of interest, 300–400 nm [6,11]. Ytterbium (Yb3+) is a unique rare-earth as it contains only two energy states, the 2F7/2 ground state and 2F5/2 excited state, separated by around 10,000 cm−1 which translates to emission at approximately 1000 nm [12]. The NIR emission of Yb3+ makes an ideal acceptor ion for improving the efficiency of silicon solar cells when co-doped with Bi3+ in a Y2O3 host material due to the 3P1 → 1S0 of Bi3+ ion having twice the energy of the 2F7/2 → 2F5/2 transition of Yb3+ ions [13]. Y2O3 is an ideal host and well suited for coating solar cells due to its stable physical and chemical properties [14,15]. The host material is part of the cubic space group, 206 or Ia-3, where its cations are positioned in two non-equivalent Wyckoff sites, 24d with a C2 symmetry and 8b with symmetry S6 shown in Fig. 1 [11,14]. In this research, the luminescence properties of Y2O3:Bi3+, Yb3+ phosphor synthesised using the co-precipitation technique were studied and characterised for potential application in improving solar cell efficiency.
Corresponding authors. E-mail addresses: [email protected] (E. Lee), [email protected] (H.C. Swart).
http://dx.doi.org/10.1016/j.physb.2017.06.072 Received 10 April 2017; Received in revised form 17 June 2017; Accepted 26 June 2017 0921-4526/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Lee, E., Physica B (2017), http://dx.doi.org/10.1016/j.physb.2017.06.072
Physica B xxx (xxxx) xxx–xxx
E. Lee et al.
Fig. 1. (a) Crystal structure of Y2O3, the grey balls represent the yttrium (Y) ions situated at the 8b and 24d sites and the red balls are the oxygen (O) at the 48e ionic sites. (b) Schematic representation of the 8d and 24d sites with a polyhedral coordination in the Y2O3 host material [11]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
2. Experimental procedure Fig. 2. XRD patterns of the Y1.98−xO3:Bi0.02, Ybx (x = 0.01, 0.02, 0.03, 0.04, 0.05, 0.06) powder and the reference JCPDS for pure Y2O3.
2.1. Powder synthesis The powder samples Y1.97O3:Yb0.03 and Y1.98−xO3:Bi0.02, Ybx (x = 0.01, 0.02, 0.03, 0.04, 0.05 and 0.06) were synthesised using the coprecipitation technique. Y2O3 (99.99%), Bi2O3 (99.9%) and Yb2O3 (99.994%) were used as the starting materials. Stoichiometric amounts of Y2O3, Bi2O3 and Yb2O3 were placed in a beaker containing distilled water under heating and stirring. Concentrated HNO3 was added dropwise to the oxide solution until the solution became clear. A NH4OH solution was added to the reaction mixture, to a pH = 10, all at once to avoid preferential precipitation. The solution containing white precipitate was stirred for 2 h. The precipitate was separated and washed using distilled water to remove excess ammonia and ammonia nitrate salt from the precipitate. The solid was placed in a drying oven heated to 100 °C until the solid was completely dry. After the drying process the solid was ground and annealed at 450 °C for 1 h then at 1000 °C for 2 h both in air to produce the final product.
Using the XRD data structural parameters namely, crystallite size and lattice constants, were investigated. The crystallite size (D) and micro-strain (ε) within the sample were calculated using the Williamson-Hall equation [16]. The lattice parameter (a) was determined using the formula,
a=λ
h2 + k 2 + l 2 2 sin θ
(1)
where (h k l) are the Miller indexes, λ is the wavelength of the radiation source and θ is the Bragg angle. From the Williamson-Hall equation the crystallite size D was calculated by taking the inverse of the intercept gathered from the straight-line plot and similarly the micro-strain ε present in the samples were obtained from the slope shown in Fig. 3. The summary of all the structure parameters of the undoped Y2O3 host material and the co-doped samples are shown in Table 1. With an increase in the Yb3+ concentration, the lattice parameter of the prepared material decreases continuously due to the small ionic radius of Yb3+ as compared to Y3+ and Bi3+ (Yb3+, r = 0.087 nm; Y3+, r = 0.090 nm; Bi3+, r = 0.103 nm) [17]. The crystallite size was calculated to be between 27 nm and 52 nm using Williamson-Hall equation. The cause for much smaller crystallite size for the Y1.97O3:Bi0.02, Yb0.01 sample is currently still under investigation. From Williamson-Hall plot an apparent dependency of Yb3+ concentration on the micro-strain ε, was observed due to the changes in the lattice parameter with increasing Yb3+ concentration.
2.2. Characterisation of Y2O3:Bi3+, Yb3+ Phase characterisation was obtained from the X-ray powder diffraction (XRD) patterns recorded on a Bruker D8 Advance diffractometer. The diffraction data was gathered using CuKα (1.5406 Å) Xrays at 40 kV and 40 mA, performed in air at room temperature. The ultra-violet (UV) excitation and visible emission spectra were recorded using a Varian Cary Eclipse Fluorescence spectrophotometer equipped with a xenon lamp excitation source. The near-infrared (NIR) luminescence spectrum was generated by using a Kimmon IK Series He-Cd Laser (325 nm) as the excitation source, the NIR emission was dispersed by a Horiba iHR 320 monochromator and detected by a solid state DSS-IGA020T detector.
3.2. Photoluminescence (PL) analysis Fig. 4(a) shows the PL excitation and emission spectrum of Y1.98−xO3:Bi0.02, Ybx phosphor due to the Bi3+ dopant. Using an excitation wavelength λex = 330 nm a spectrum showing a broad green emission centred at 489 nm was observed corresponding to the C2 symmetry site [14]. With an excitation wavelength λex = 380 nm an emission band centred at 409 nm was obtained for the S6 symmetry site [14]. As discussed in the literature, the excitation and emission are attributed to the 1S0 → 3P1 and 3P1 → 1S0 transitions of the Bi3+ ions [8,11]. The results also revealed a variation in the luminescent intensity of the two sites, the 409 nm emission of the S6 site was significantly stronger than the C2 site suggesting that the Bi3+ prefers to occupy the S6 site. The preferred S6 site for the Bi3+ ions is due to its larger ionic size as compared to the Y3+ ions and is therefore more likely to occupy
3. Results and discussion 3.1. XRD analysis Fig. 2 shows the XRD patterns of Y1.98−xO3:Bi0.02, Ybx (x = 0.01, 0.02, 0.03, 0.04, 0.05, 0.06) powder phosphors, synthesised at pH 10 using the co-precipitation technique. The experimental XRD patterns indicated a simple cubic C-type R2O3 structure with the Ia-3 space group which correlate well with the referenced Y2O3 structure using the JCPDS file no.: 83-0927, which is also presented in Fig. 2. The absence of any additional diffraction patterns indicated that the addition of Bi3+ and Yb3+ ions did not alter the crystal structure of the Y2O3 host. 2
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Fig. 3. Williamson-Hall plot of undoped Y2O3 and Y1.98−xO3:Bi0.02, Ybx (x = 0.01, 0.02, 0.03, 0.04, 0.05, 0.06) powder.
then decreased due to concentration quenching. The quenching of the NIR emission is associated with the higher Yb3+ concentration, as cross relaxation between the Yb3+ ions decreases the luminescence yield.
Table 1 The structural parameters of both undoped and doped phosphors (JCPDS lattice parameter a = 10.60 Å). Sample
Lattice parameter a (Å)
Crystallite size D (nm)
Micro-strain ε (× 10−4)
Undoped Y2O3 Y1.97O3:Bi0.02, Yb0.01 Y1.96O3:Bi0.02, Yb0.02 Y1.95O3:Bi0.02, Yb0.03 Y1.94O3:Bi0.02, Yb0.04 Y1.93O3:Bi0.02, Yb0.05 Y1.92O3:Bi0.02, Yb0.06
10.613 10.639 10.637 10.634 10.629 10.623 10.621
49.0 27.2 43.9 46.1 51.5 49.5 50.7
0.688 4.445 3.129 2.154 1.997 0.704 0.594
4. Conclusion The Y2O3:Bi3+, Yb3+ phosphor powders with varying Yb3+ ion concentrations have been successfully synthesised using the co-precipitation technique. XRD patterns confirmed that the samples crystallised to a single phase cubic structure corresponding to the Y2O3 with a Ia-3 space group. The visible emission confirmed the C2 and S6 symmetry sites found within the host matrix and revealed that Bi3+ ions prefer the S6 site due to a stronger emission intensity when compared to the emission from the C2 site. The NIR emission of Yb3+ was observed using a 325 nm He-Cd laser and revealed that Bi3+ ions do enhance the Yb3+ emission intensity and that increasing the Yb3+ concentration leads to an increase in the NIR emission intensity until x = 5.0 mol% where the intensity decreased due to concentration quenching.
a site with a larger volume (shown in Fig. 1b). From Fig. 4(b) a decrease in the Bi3+ emission intensity was observed with an increase in Yb3+ ion concentration. The quenching of the Bi3+ emission is attributed to the energy transfer from the Bi3+ activator to the Yb3+ ions. The NIR emission spectrum of Y1.99O3:Yb0.01 and Y1.98−xO3:Bi0.02, Ybx obtained through 325 nm excitation using a He-Cd laser is presented in Fig. 5(a). The spectrum showed a broad near-infrared emission band with a sharp maximum situated at 976 nm due to the Yb3+: 2F5/2 → 2F7/2 transition, accompanied by several other weaker emission peaks centred at 950 nm, 1030 nm and 1074 nm due to the crystal field Stark splitting of the 2F5/2 and 2F7/2 levels. The strong increase in the NIR emission with the addition of Bi3+ ions indicated that the Bi3+ transferred primary excitation energy to the Yb3+ ion. Fig. 5(b) represents the NIR emission of Y1.98−xO3:Bi0.02, Ybx it also showed that the 976 nm emission intensity increased with an increase in Yb3+ concentration until x = 5.0 mol% where the intensity
Acknowledgements This research paper is made possible thanks to Prof HC Swart for his guidance through this project, to Prof. RE Kroon and LJB Erasmus for helping with PL measurements, and Dr. S Cronjé for his help in XRD measurements. Funding support is acknowledged to the South African Research Chairs Initiative of the Department of Science and Technology (DST) and the National Research Fund (NRF) (Grants 88415 and 93214). 3
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Fig. 4. (a) Excitation and emission spectra of Y1.96O3:Bi0.02, Yb0.02 measured with the Cary Eclipse Xe lamp and (b) PL spectra of the Y1.98−xO3:Bi0.02, Ybx (x = 0.00, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06) powder measured using a 325 nm He-Cd laser.
Fig. 5. (a) NIR emission of Y1.99O3:Yb0.01 and Y1.97O3:Bi0.02, Yb0.01 using a 325 nm He-Cd laser. (b) NIR emission of Y1.98−xO3:Bi0.02, Ybx and the 976 nm emission intensity as a function of Yb3+ concentration, using a 325 nm He-Cd laser. [10] L.G. Jacobsohn, M.W. Blair, S.C. Tornga, L.O. Brown, B.L. Bennett, R.E. Muenchausen, J. Appl. Phys. 104 (2008) 124303. [11] R.M. Jafer, E. Coetsee, A. Yousif, R.E. Kroon, O.M. Ntwaeaborwa, H.C. Swart, Appl. Surf. Sci. 332 (2015) 198–204. [12] T. Patrick, E. Cebisa, H.C. Swart, V. Kumar, O. Martin, Ceram. Int. 43 (2016) 174–181. [13] W. Xian-Tao, Z. Jiang-Bo, C. Yong-Hu, Y. Min, L. Yong, Chin. Phys. B 19 (2010) 77804. [14] D. Avram, B. Cojocaru, M. Florea, C. Tiseanu, Opt. Mater. Express 6 (2016) 1635. [15] S. Som, S.K. Sharma, T. Shripathi, J. Fluoresc. 23 (2013) 439–450. [16] V.D. Mote, Y. Purushotham, B.N. Dole, J. Theor. Appl. Phys. 6 (2012) 6. [17] R.D. Shannon, Acta Crystallogr. Sect. A 32 (1976) 751–767.
References [1] B.M. van der Ende, L. Aarts, A. Meijerink, Phys. Chem. Chem. Phys. 11 (2009) 11081–11095. [2] Q.Y. Zhang, X.Y. Huang, Prog. Mater. Sci. 55 (2010) 353–427. [3] T. Trupke, M.A. Green, P. Würfel, J. Appl. Phys. 92 (2002) 4117–4122. [4] T. Trupke, M.A. Green, P. Würfel, J. Appl. Phys. 92 (2002) 1668–1674. [5] X. Wei, S. Huang, Y. Chen, C. Guo, M. Yin, W. Xu, J. Appl. Phys. 107 (2010) 2–7. [6] X.Y. Huang, X.H. Ji, Q.Y. Zhang, J. Am. Ceram. Soc. 94 (2011) 833–837. [7] J.-L. Yuan, X.-Y. Zeng, J.-T. Zhao, Z.-J. Zhang, H.-H. Chen, X.-X. Yang, J. Phys. D Appl. Phys. 41 (2008) 105406. [8] M.H. Qu, R.Z. Wang, Y. Zhang, K.Y. Li, H. Yan, J. Appl. Phys. 111 (2012) 93108. [9] U. Rambabu, S. Do Han, Ceram. Int. 39 (2013) 1603–1612.
4
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Luminescence properties of Y2O3:Bi3+, Yb3+ co-doped phosphor for application in solar cells ⁎
⁎
E. Lee , R.E. Kroon, J.J. Terblans, H.C. Swart
Department of Physics, University of the Free State, Bloemfontein, South Africa
A R T I C L E I N F O
A BS T RAC T
Keywords: Downconversion Y2O3:Bi3+, Yb3+
Bismuth (Bi3+) and ytterbium (Yb3+) co-doped yttrium oxide (Y2O3) phosphor powder was successfully synthesised using the co-precipitation technique. The X-ray diffraction (XRD) patterns confirmed that a single phase cubic structure with a Ia-3 space group was formed. The visible emission confirmed the two symmetry sites, C2 and S6, found in the Y2O3 host material and revealed that Bi3+ ions preferred the S6 site as seen the stronger emission intensity. The near-infrared (NIR) emission of Yb3+ increased significantly by the presence of the Bi3+ ions when compared to the singly doped Y2O3:Yb3+ phosphor with the same Yb3+ concentration. An increase in the NIR emission intensity was also observed by simply increasing the Yb3+ concentration in the Y2O3:Bi3+, Yb3+ phosphor material where the intensity increased up to x = 5.0 mol% of Yb3+ before decreasing due to concentration quenching.
1. Introduction Silicon based solar cells are currently the most common form of photovoltaic cells used to convert solar energy to electrical energy. Unfortunately silicon solar cells suffer from low power conversion efficiencies, due to the mismatch between the solar spectrum and the maximum absorption spectrum of silicon [1]. In recent years, luminescent materials have been used to adapt the solar spectrum in order to reduce the spectral mismatch and improve the power conversion efficiency of the solar cells [2]. These luminescent materials have the ability of upconversion (absorbing two or more low energy photons and converting it into one with a higher energy) or down-conversion (absorbing a high energy photon and converting it into two or more lower energy photons), in order to utilise unabsorbed photons of the solar spectrum [3,4]. Downconversion (DC) phosphors materials doped with rare-earth elements have been used extensively in the lighting industry for potential application in mercury free fluorescent tube and plasma displays luminescent due to the wide range of possible luminescence from ultraviolet (UV) through the visible to the near-infrared (NIR) regions [5,6]. In past research, rare earth (RE3+) – Yb3+ (RE = Tb, Ce, Er and Pr) co-doped phosphors have been used to down-convert UV photons to NIR photons with hopes to improve the efficiency of solar cells [4]. Unfortunately lanthanides or rare-earth metals was found to be poor at absorbing photons in the UV to blue regions due to their parity forbidden 4f transition, which may cause Yb3+ to have a weak NIR emission limiting the potential for solar cell applications [7]. Metal
⁎
donors such as Bi3+ ions have shown to be promising alternative to rare-earth ions for enhancing the NIR emission of Yb3+ ions [6,8,9]. The 6s2 electron configuration of the Bi3+ ion consists of a 1S0 ground state and an excited state with a 6s6p configuration that splits into four levels, 3P0, 3P1, 3P2, and 1P1 (in order of increasing energy) [10]. Due to the ΔJ selection rule the 1S0 → 3P0 and 1S0 → 3P2 are strongly spin forbidden, while the 1S0 → 3P1 and 1S0 → 1P1 are allowed transition because of the 3P1 and 1P1 spin-orbital coupling [5,6,9]. In this research, only the 1S0 → 3P1 transition will be studied as the transition is in the region of interest, 300–400 nm [6,11]. Ytterbium (Yb3+) is a unique rare-earth as it contains only two energy states, the 2F7/2 ground state and 2F5/2 excited state, separated by around 10,000 cm−1 which translates to emission at approximately 1000 nm [12]. The NIR emission of Yb3+ makes an ideal acceptor ion for improving the efficiency of silicon solar cells when co-doped with Bi3+ in a Y2O3 host material due to the 3P1 → 1S0 of Bi3+ ion having twice the energy of the 2F7/2 → 2F5/2 transition of Yb3+ ions [13]. Y2O3 is an ideal host and well suited for coating solar cells due to its stable physical and chemical properties [14,15]. The host material is part of the cubic space group, 206 or Ia-3, where its cations are positioned in two non-equivalent Wyckoff sites, 24d with a C2 symmetry and 8b with symmetry S6 shown in Fig. 1 [11,14]. In this research, the luminescence properties of Y2O3:Bi3+, Yb3+ phosphor synthesised using the co-precipitation technique were studied and characterised for potential application in improving solar cell efficiency.
Corresponding authors. E-mail addresses: [email protected] (E. Lee), [email protected] (H.C. Swart).
http://dx.doi.org/10.1016/j.physb.2017.06.072 Received 10 April 2017; Received in revised form 17 June 2017; Accepted 26 June 2017 0921-4526/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Lee, E., Physica B (2017), http://dx.doi.org/10.1016/j.physb.2017.06.072
Physica B xxx (xxxx) xxx–xxx
E. Lee et al.
Fig. 1. (a) Crystal structure of Y2O3, the grey balls represent the yttrium (Y) ions situated at the 8b and 24d sites and the red balls are the oxygen (O) at the 48e ionic sites. (b) Schematic representation of the 8d and 24d sites with a polyhedral coordination in the Y2O3 host material [11]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
2. Experimental procedure Fig. 2. XRD patterns of the Y1.98−xO3:Bi0.02, Ybx (x = 0.01, 0.02, 0.03, 0.04, 0.05, 0.06) powder and the reference JCPDS for pure Y2O3.
2.1. Powder synthesis The powder samples Y1.97O3:Yb0.03 and Y1.98−xO3:Bi0.02, Ybx (x = 0.01, 0.02, 0.03, 0.04, 0.05 and 0.06) were synthesised using the coprecipitation technique. Y2O3 (99.99%), Bi2O3 (99.9%) and Yb2O3 (99.994%) were used as the starting materials. Stoichiometric amounts of Y2O3, Bi2O3 and Yb2O3 were placed in a beaker containing distilled water under heating and stirring. Concentrated HNO3 was added dropwise to the oxide solution until the solution became clear. A NH4OH solution was added to the reaction mixture, to a pH = 10, all at once to avoid preferential precipitation. The solution containing white precipitate was stirred for 2 h. The precipitate was separated and washed using distilled water to remove excess ammonia and ammonia nitrate salt from the precipitate. The solid was placed in a drying oven heated to 100 °C until the solid was completely dry. After the drying process the solid was ground and annealed at 450 °C for 1 h then at 1000 °C for 2 h both in air to produce the final product.
Using the XRD data structural parameters namely, crystallite size and lattice constants, were investigated. The crystallite size (D) and micro-strain (ε) within the sample were calculated using the Williamson-Hall equation [16]. The lattice parameter (a) was determined using the formula,
a=λ
h2 + k 2 + l 2 2 sin θ
(1)
where (h k l) are the Miller indexes, λ is the wavelength of the radiation source and θ is the Bragg angle. From the Williamson-Hall equation the crystallite size D was calculated by taking the inverse of the intercept gathered from the straight-line plot and similarly the micro-strain ε present in the samples were obtained from the slope shown in Fig. 3. The summary of all the structure parameters of the undoped Y2O3 host material and the co-doped samples are shown in Table 1. With an increase in the Yb3+ concentration, the lattice parameter of the prepared material decreases continuously due to the small ionic radius of Yb3+ as compared to Y3+ and Bi3+ (Yb3+, r = 0.087 nm; Y3+, r = 0.090 nm; Bi3+, r = 0.103 nm) [17]. The crystallite size was calculated to be between 27 nm and 52 nm using Williamson-Hall equation. The cause for much smaller crystallite size for the Y1.97O3:Bi0.02, Yb0.01 sample is currently still under investigation. From Williamson-Hall plot an apparent dependency of Yb3+ concentration on the micro-strain ε, was observed due to the changes in the lattice parameter with increasing Yb3+ concentration.
2.2. Characterisation of Y2O3:Bi3+, Yb3+ Phase characterisation was obtained from the X-ray powder diffraction (XRD) patterns recorded on a Bruker D8 Advance diffractometer. The diffraction data was gathered using CuKα (1.5406 Å) Xrays at 40 kV and 40 mA, performed in air at room temperature. The ultra-violet (UV) excitation and visible emission spectra were recorded using a Varian Cary Eclipse Fluorescence spectrophotometer equipped with a xenon lamp excitation source. The near-infrared (NIR) luminescence spectrum was generated by using a Kimmon IK Series He-Cd Laser (325 nm) as the excitation source, the NIR emission was dispersed by a Horiba iHR 320 monochromator and detected by a solid state DSS-IGA020T detector.
3.2. Photoluminescence (PL) analysis Fig. 4(a) shows the PL excitation and emission spectrum of Y1.98−xO3:Bi0.02, Ybx phosphor due to the Bi3+ dopant. Using an excitation wavelength λex = 330 nm a spectrum showing a broad green emission centred at 489 nm was observed corresponding to the C2 symmetry site [14]. With an excitation wavelength λex = 380 nm an emission band centred at 409 nm was obtained for the S6 symmetry site [14]. As discussed in the literature, the excitation and emission are attributed to the 1S0 → 3P1 and 3P1 → 1S0 transitions of the Bi3+ ions [8,11]. The results also revealed a variation in the luminescent intensity of the two sites, the 409 nm emission of the S6 site was significantly stronger than the C2 site suggesting that the Bi3+ prefers to occupy the S6 site. The preferred S6 site for the Bi3+ ions is due to its larger ionic size as compared to the Y3+ ions and is therefore more likely to occupy
3. Results and discussion 3.1. XRD analysis Fig. 2 shows the XRD patterns of Y1.98−xO3:Bi0.02, Ybx (x = 0.01, 0.02, 0.03, 0.04, 0.05, 0.06) powder phosphors, synthesised at pH 10 using the co-precipitation technique. The experimental XRD patterns indicated a simple cubic C-type R2O3 structure with the Ia-3 space group which correlate well with the referenced Y2O3 structure using the JCPDS file no.: 83-0927, which is also presented in Fig. 2. The absence of any additional diffraction patterns indicated that the addition of Bi3+ and Yb3+ ions did not alter the crystal structure of the Y2O3 host. 2
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Fig. 3. Williamson-Hall plot of undoped Y2O3 and Y1.98−xO3:Bi0.02, Ybx (x = 0.01, 0.02, 0.03, 0.04, 0.05, 0.06) powder.
then decreased due to concentration quenching. The quenching of the NIR emission is associated with the higher Yb3+ concentration, as cross relaxation between the Yb3+ ions decreases the luminescence yield.
Table 1 The structural parameters of both undoped and doped phosphors (JCPDS lattice parameter a = 10.60 Å). Sample
Lattice parameter a (Å)
Crystallite size D (nm)
Micro-strain ε (× 10−4)
Undoped Y2O3 Y1.97O3:Bi0.02, Yb0.01 Y1.96O3:Bi0.02, Yb0.02 Y1.95O3:Bi0.02, Yb0.03 Y1.94O3:Bi0.02, Yb0.04 Y1.93O3:Bi0.02, Yb0.05 Y1.92O3:Bi0.02, Yb0.06
10.613 10.639 10.637 10.634 10.629 10.623 10.621
49.0 27.2 43.9 46.1 51.5 49.5 50.7
0.688 4.445 3.129 2.154 1.997 0.704 0.594
4. Conclusion The Y2O3:Bi3+, Yb3+ phosphor powders with varying Yb3+ ion concentrations have been successfully synthesised using the co-precipitation technique. XRD patterns confirmed that the samples crystallised to a single phase cubic structure corresponding to the Y2O3 with a Ia-3 space group. The visible emission confirmed the C2 and S6 symmetry sites found within the host matrix and revealed that Bi3+ ions prefer the S6 site due to a stronger emission intensity when compared to the emission from the C2 site. The NIR emission of Yb3+ was observed using a 325 nm He-Cd laser and revealed that Bi3+ ions do enhance the Yb3+ emission intensity and that increasing the Yb3+ concentration leads to an increase in the NIR emission intensity until x = 5.0 mol% where the intensity decreased due to concentration quenching.
a site with a larger volume (shown in Fig. 1b). From Fig. 4(b) a decrease in the Bi3+ emission intensity was observed with an increase in Yb3+ ion concentration. The quenching of the Bi3+ emission is attributed to the energy transfer from the Bi3+ activator to the Yb3+ ions. The NIR emission spectrum of Y1.99O3:Yb0.01 and Y1.98−xO3:Bi0.02, Ybx obtained through 325 nm excitation using a He-Cd laser is presented in Fig. 5(a). The spectrum showed a broad near-infrared emission band with a sharp maximum situated at 976 nm due to the Yb3+: 2F5/2 → 2F7/2 transition, accompanied by several other weaker emission peaks centred at 950 nm, 1030 nm and 1074 nm due to the crystal field Stark splitting of the 2F5/2 and 2F7/2 levels. The strong increase in the NIR emission with the addition of Bi3+ ions indicated that the Bi3+ transferred primary excitation energy to the Yb3+ ion. Fig. 5(b) represents the NIR emission of Y1.98−xO3:Bi0.02, Ybx it also showed that the 976 nm emission intensity increased with an increase in Yb3+ concentration until x = 5.0 mol% where the intensity
Acknowledgements This research paper is made possible thanks to Prof HC Swart for his guidance through this project, to Prof. RE Kroon and LJB Erasmus for helping with PL measurements, and Dr. S Cronjé for his help in XRD measurements. Funding support is acknowledged to the South African Research Chairs Initiative of the Department of Science and Technology (DST) and the National Research Fund (NRF) (Grants 88415 and 93214). 3
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Fig. 4. (a) Excitation and emission spectra of Y1.96O3:Bi0.02, Yb0.02 measured with the Cary Eclipse Xe lamp and (b) PL spectra of the Y1.98−xO3:Bi0.02, Ybx (x = 0.00, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06) powder measured using a 325 nm He-Cd laser.
Fig. 5. (a) NIR emission of Y1.99O3:Yb0.01 and Y1.97O3:Bi0.02, Yb0.01 using a 325 nm He-Cd laser. (b) NIR emission of Y1.98−xO3:Bi0.02, Ybx and the 976 nm emission intensity as a function of Yb3+ concentration, using a 325 nm He-Cd laser. [10] L.G. Jacobsohn, M.W. Blair, S.C. Tornga, L.O. Brown, B.L. Bennett, R.E. Muenchausen, J. Appl. Phys. 104 (2008) 124303. [11] R.M. Jafer, E. Coetsee, A. Yousif, R.E. Kroon, O.M. Ntwaeaborwa, H.C. Swart, Appl. Surf. Sci. 332 (2015) 198–204. [12] T. Patrick, E. Cebisa, H.C. Swart, V. Kumar, O. Martin, Ceram. Int. 43 (2016) 174–181. [13] W. Xian-Tao, Z. Jiang-Bo, C. Yong-Hu, Y. Min, L. Yong, Chin. Phys. B 19 (2010) 77804. [14] D. Avram, B. Cojocaru, M. Florea, C. Tiseanu, Opt. Mater. Express 6 (2016) 1635. [15] S. Som, S.K. Sharma, T. Shripathi, J. Fluoresc. 23 (2013) 439–450. [16] V.D. Mote, Y. Purushotham, B.N. Dole, J. Theor. Appl. Phys. 6 (2012) 6. [17] R.D. Shannon, Acta Crystallogr. Sect. A 32 (1976) 751–767.
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