Yellow–orange upconversion emission in Eu3+–Yb3+ codoped BaTiO3 phosphor

Yellow–orange upconversion emission in Eu3+–Yb3+ codoped BaTiO3 phosphor

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 98–101 Contents lists available at ScienceDirect Spectrochimica Acta ...

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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 98–101

Contents lists available at ScienceDirect

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa

Yellow–orange upconversion emission in Eu3+–Yb3+ codoped BaTiO3 phosphor Astha Kumari, Vineet Kumar Rai ⇑, Kaushal Kumar Laser and Spectroscopy Laboratory, Department of Applied Physics, Indian School of Mines, Dhanbad 826004, Jharkhand, India

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

codoped BaTiO3 phosphor is prepared via coprecipitation method.  Upconversion emission is studied by 980 nm diode laser excitation.  The colour tunability has been observed.  The observed upconversion transitions have been explained thoroughly.  The material can be used in making the colour tunable display devices.

Upconversion emission spectra of BaTiO3: Eu3+–Yb3+ phosphor at different concentrations.

a r t i c l e

a b s t r a c t

3+

 The Eu –Yb

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Article history: Received 11 November 2013 Received in revised form 30 January 2014 Accepted 9 February 2014 Available online 22 February 2014 Keywords: Upconversion emission Energy transfer Saturation effect

The Eu3+–Yb3+ codoped BaTiO3 phosphor is prepared via co-precipitation method and its upconversion emission is studied by 980 nm diode laser excitation. The X-ray diffraction pattern of the prepared sample showed the tetragonal BaTiO3 phase. The co-doped phosphor showed sharp upconversion emission bands peaking at 592, 614,  654, 704 and 796 nm due to the 5D0 ? 7F1, 5D0 ? 7F2, 5D0 ? 7F3, 5 D0 ? 7F4 and 5D0 ? 7F6 transitions, respectively of Eu3+ ions. The sharp band at 489 nm is assigned to the 2F5/2 ? 2F7/2 transition of Yb3+ ion while the broad band around 505 nm is assigned to the defect states present in the sample. Based on the available experimental data, the process involved in the UC emissions has been explored and elaborated. Ó 2014 Elsevier B.V. All rights reserved.

Introduction Phosphors are important materials of modern technology because of their ability to convert excitation energy (electrical, mechanical, optical, etc.) into the visible light [1]. Phosphors capable of converting the low frequency photons into the high frequency photons are known as upconversion phosphors and have ⇑ Corresponding author. Tel.: +91 326 223 5404. E-mail addresses: [email protected], [email protected] (V.K. Rai). http://dx.doi.org/10.1016/j.saa.2014.02.023 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

anticipated applications in the field of lasers, biomedical diagnostics, optical amplifiers, sensors, detection of infrared radiation, light emitting diodes, fingerprint detection, etc. [1–3]. The concept of frequency upconversion from infrared to visible light in rare earth (RE) ions doped materials was reported around forty years ago by Auzel [4] and since then upconversion emission is vastly studied in various rare earths doped/codoped materials [2–7]. Upconversion emission using the 980 nm excitation source is not possible in singly Eu3+ doped materials due to unavailability of energy levels corresponding to 10,200 cm1 energy (980 nm) [7]. But it is possible to achieve visible upconversion emission in

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Eu3+ doped materials with the use of Yb3+ ions. In a double codoped system one ion acts as an activator and the other ion acts as a sensitizer which sensitizes the process of upconversion. Among lanthanide elements, Europium (Eu) is considered as one of the best known red emitting lanthanide ion having quantum efficiency (QE) up to 90% in the red region [8,9] and for this reason the Eu3+ ion, a well-known activator, is widely used as red phosphor in television screens and fluorescent lamps [10]. The emission from Eu3+ is mostly due to the 5D0 ? 7F1,2,3,4,5,6 transitions [11–14,16]. Many works have been reported in the Eu3+ and Eu3+–Yb3+ doped/codoped systems in different host materials [11–13]. Dwivedi et al. [11] have prepared Y8V2O17: Eu3+/Yb3+ nanophosphor and analysed its structural and optical properties including upconversion and downconversion luminescence. Using 976 nm laser excitation authors have observed emission peaks at 511.7, 538, 556.3, 580, 587, 594.5, 609, 613.4, 615, 619, 622, 651, 698, 704, 750, 791 and 813 nm. Lu et al. [12] have prepared Eu3+ doped BaMgAl10O17 phosphor via sol–gel and conventional solid-state routes and studied the effect of various preparation conditions on the optical properties of BaMgAl10O17 based phosphors. Kaur et al. [13] have reported strong red emission (5D0 ? 7F2) from cooperative upconversion process (Yb3++Yb3+ ? Eu3+ on 976 nm excitation) as well as through downconversion process (on 355 nm excitation) in Gd2O3: Eu3+/Yb3+ nanophosphor. The unusual and rarely reported emissions from the higher lying 5DJ (J = 2, 3, and 4) to 7FJ (J = 0, 1, 2, 3, and 4) levels are also observed with 976 nm excitation in Gd2O3: Eu3+/Yb3+ nanophosphor [13]. Here authors have chosen BaTiO3 as a host material because of its applications in ferroelectric devices, second harmonic generation devices and optoelectronics viz. in high-density optical data storage, piezoelectric transducer, etc. [15]. The optical functionality in BaTiO3 material can be introduced by the addition of small amount of rare earth ions. Small doping of rare earth ions does not alter the ferro- or piezo-properties of BaTiO3 but it adds an additional functionality. The demand of multifunctional materials is ever increasing [17,18]. In this view, authors have selected the BaTiO3 as a suitable host material for the present study.

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ple was annealed for 3 h at 800 °C to get the required phosphor sample. The annealed sample was then taken for the optical and structural analysis. The XRD study of the sample was done using Bruker D8 advanced X-ray diffractometer. The UC emission spectra of the phosphor powders were recorded on a Princeton triple turret grating monochromator (Acton SP-2300) attached with a photomultiplier tube (PMT) upon excitation with 980 nm continuous wave (CW) diode laser (CNI, China). The colour coordinates of the samples were calculated using the ‘‘GoCIE’’ software [20]. All the measurements were performed at room temperature. Results and discussions Structural analysis X-ray diffraction (XRD) patterns of the as-synthesized and heat treated samples were recorded. As-synthesized BaTiO3: Eu3+–Yb3+ phosphor was found amorphous in nature. The XRD pattern of the sample annealed at 800 °C is shown in Fig. 1. The diffraction peaks observed are well indexed to the tetragonal BaTiO3 as major phase (JCPDS file No. 05-0626). A minor phase is also observed and assigned to orthorhombic BaCO3 (JCPDS file No. 44-1487). The minor orthorhombic BaCO3 phase may reduce the upconversion emission intensity by a small amount due to the high frequency vibrations (856–1450 cm1) of BaCO3, however peak position may not change due to presence of low amount of BaCO3 phase. The crystallite size of the phosphor was calculated using famous Debye–Scherrer equation [2],



0:89k b cos h

ð2Þ

where ‘D’ is the crystallite size, ‘k’ is the wavelength of X-ray radiation, ‘b’ is the full-width half maxima (FWHM) and ‘h’ is the diffraction angle. The crystallite size was calculated corresponding to three most intense peaks and is found in 15 ± 3 nm range. Upconversion emission

Experimental 3+

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BaTiO3: Eu –Yb phosphor was prepared by co-precipitation method. This method provides us a material with size in the nanometre range and also the upconversion efficiency observed in a phosphor prepared by this method is better than the other preparation methods [19]. To prepare the BaTiO3: Eu3+–Yb3+ phosphor, the concentration of ytterbium was fixed at 2.0 wt.% whereas the concentration of europium was varied from 0.5 to 1.5 wt.%. The compositions of the compounds taken are shown below:

ð100  x  yÞBaTiO3 þ xEu2 O3 þ yYb2 O3

The upconversion emission spectra of the BaTiO3: Eu3+–Yb3+ phosphors at different concentrations of Eu3+ are shown in Fig. 2. The concentration of Yb3+ was fixed at 2.0 wt.% because in Eu3+– Yb3+ system the optimum intensity is observed for 2.0 wt.% of Yb3+ ion concentration [14]. The sample was excited with 980 nm diode laser. Since Eu3+ ion does not excite with this wave-

ð1Þ

where x = 0.5, 0.75, 1, 1.25 and 1.5 wt.% and y = 0, 2.0 wt.%. Firstly, the appropriate weighted amount, i.e.; 2.5 g of BaCO3 and 1.5 ml of CH3COOH were mixed with a little heat treatment to make the transparent solution of barium acetate. This barium acetate solution was mixed with 3.7 ml of titanium isopropoxide (C12H28O4Ti) solution to get the required BaTiO3 solution. Then appropriate amounts of nitrates of Eu2O3 and Yb2O3 were prepared and mixed with the barium titanate solution. Some amount of distilled water and 2.0 g oxalic acid was added to the final solution to get precipitate. The solution was left for 10–12 h to get the required precipitate. After that the precipitate was filtered with the help of filter paper and then washed with distilled water and ethanol for 4–6 times. The filtrate was then left for drying for at least one day. The dried filtrate was then heated in furnace at 100 °C for 30 min to get the as-synthesized sample. The as-synthesized sam-

Fig. 1. X-ray diffraction pattern of BaTiO3: Eu3+–Yb3+ phosphor annealed at 800 °C.

100

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length, hence the spectra of co-doped system (Eu3+–Yb3+) are discussed here. It is observed from the upconversion emission spectra that the prominent peaks appears at 489, 592, 614, 654, 704, 796 nm wavelengths. These peaks are assigned to the 2F5/2 ? 2 F7/2 (Yb3+), 5D0 ? 7F1 (Eu3+), 5D0 ? 7F2 (Eu3+), 5D0 ? 7F3 (Eu3+), 5 D0 ? 7F4 (Eu3+), 5D0 ? 7F6 (Eu3+) transitions, respectively. The most intense peak due to Eu3+ ion appears at 614 nm (red region). The intensity variation of this peak with Eu3+ ions concentration is given in the inset of Fig. 2. The optimum emission intensity is found for the 0.75 wt.% concentration of Eu3+. After this concentration the intensity of all the emission bands is found to decrease with increase in Eu3+ ions concentration. This decrease in intensity happens due to the concentration quenching [21]. The 5D0 ? 7F1 band originates from magnetic-dipole transition and, in this case, the change of the crystal field strength has very little influence on it [16]. The dominant peak observed around 614 nm (5D0 ? 7F2) is attributed to the forced electric-dipole transition which is allowed only at low symmetries with no inversion centre [16]. The band at 505 nm is broad compared to the other bands, so it is thought that this emission may occur due to two reasons. First is the creation of defect states in the host and second is the emission from Yb2+ ions. The ionic radius of Yb3+ is 0.101 nm which does not match with the ionic radii of host ions viz. Ba2+ (0.134 nm) and Ti4+ (0.068 nm). However, the ionic radius of Yb2+ is around 0.116 nm which is much closer to the radii of Ba2+ ions; hence during the crystal formation some Ba2+ sites are replaced by ytterbium ions in form of Yb2+ state. Because of this replacement charge imbalance occurs and to overcome it some oxygen vacancies are created in the sample. These vacancies form F-centres and its emission is supposed to occur at 505 nm. These F-centres are supposed to excite by the absorption of radiation from the cooperative emission of Yb3+ ions. Another reason for the broad emission is the presence of Yb2+ ion state which is supposed to be luminescent and giving emission in blue–green region [22]. The similar broad emission is reported by several authors and explained by the presence of luminescent Yb2+ state [22,23]. Nikl et al. [23] reported well-defined excitation peaks at 326, 291 and 237 nm and a weak one around 350 nm as characteristic of 4f–5d transitions of Yb2+ ion. The band intensity at 505 nm is maximum for 0.5 wt.% Eu3+ ions concentration and above this concentration the intensity is found to decrease drastically (Fig. 2). It is supposed that at 0.5 wt.% Eu3+ ions concentration the probability of energy transfer

Fig. 2. Upconversion emission spectra of BaTiO3: Eu3+–Yb3+ phosphor (annealed at 800 °C) at different Eu3+ concentrations.

from Yb3+ to Eu3+ is low and hence the emission at 505 nm is large. Above this concentration an efficient energy transfer from Yb3+ to Eu3+ may decrease the intensity at 505 nm. Power dependence As we know that the UC emission intensity is directly proportional to the pump power as shown by Eq. (3) [13],

ðUC IntensityÞ / ðPump PowerÞn

ð3Þ

where, ‘n’ is the number of pump photons involved in UC emission process. Room temperature pump power dependence of the 489 nm, 505 nm, and 614 nm emission bands under 980 nm excitation at Eu3+ concentrations of 0.5 wt.% and 1.5 wt.% were examined and the results are presented in a ln–ln plot in Figs. 3 and 4. For the Eu3+ concentration at 0.5 wt.%, the slopes of 489 nm, 505 nm and 614 nm are found to be 1.09, 1.04 and 1.03, respectively. It is expected that these bands must arise from two photon absorption and slope should come around 2.0. But here slope values are far from the integer 2 because of the low emission efficiency of this phosphor. Further, obtained slope value for 614 nm emission (from Eu3+) is found to decrease as Eu3+ ions concentration increases from 0.5 wt.% to 1.5 wt.%. This decrease in slope is due to the increase in energy transfer rate which is expected at higher concentration. These results can be correlated with the upconversion emission spectra in which the peak at 614 nm has more intensity in the case of 0.5 wt.% than in the case of 1.5 wt.%. One thing also to be noticed in Fig. 4 is that at high pump power the slopes follow a linear trend which is due to the effect of saturation and thermal equilibrium taking place [24]. This occurs because at high pump power non-radiative relaxation rate increases and the internal temperature of the sample also increases which causes a thermalization effect and hence the upconversion luminescence intensity decreases and a decrease in the slope at high pump power is observed [24]. The possible upconversion emission pathways for Eu3+–Yb3+ system in this phosphor are shown in Fig. 5. In Eu3+–Yb3+ system the transfer of energy is through cooperative sensitization. Co-operative sensitization is the process which involves energy transfer from two excited Yb3+ ions going through cooperative emission [25]. Firstly, the energy of two ytterbium ions combine to give an emission at 489 nm which is shown in the energy level diagram by virtual state, then the energy of the Yb3+ excited states is transferred to the 5D1 excited level of europium ion which has the comparable energy with the virtual excited level of Yb3+, and then the 5D1 excited states of Eu3+ ion relax non-radi-

Fig. 3. ln–ln plot of BaTiO3: Eu3+–Yb3+ phosphor at 0.5 wt.% Eu3+ concentration.

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CIE diagram The CIE emission colour coordinates of developed phosphor at various Eu3+ ions concentrations has been calculated using the CIE software and are shown in the CIE chromaticity diagram Fig. 6. From Fig. 6 it can be seen that at 0.5 wt.% europium the emission colour appears green (CIE = 0.25, 0.49) which is due to the higher intensity at 505 nm compared to the intensity at 614 nm. When the Eu3+ ions concentration increases to 0.75 wt.% the colour of the emission comes in the yellowish-orange region. Further increase in Eu3+ ions concentration causes the CIE coordinates to shift towards the orange region. Hence, wide colour tunability from green to orange is obtained in this codoped phosphor.

Conclusions

Fig. 4. ln–ln plot of BaTiO3: Eu3+–Yb3+ phosphor at 1.5 wt.% Eu3+ concentration.

BaTiO3: Eu3+–Yb3+ phosphor was prepared from co-precipitation method and its tetragonal phase is confirmed by XRD study. The minor BaCO3 phases found in the XRD peaks may alter the upconversion properties of the phosphor by a very little amount. The efficient energy transfer from Yb3+ to Eu3+ ions is observed on 980 nm excitation to the Yb3+ ions. The emission at 614 nm from Eu3+ ions is optimized by changing the Eu3+ ions concentration. A part of Yb3+ ions is found to convert into Yb2+ state and due to this conversion defect centres are formed in the host. These defects are found to give broad emission around 505 nm. The CIE colour coordinates is found tunable in a wide range from green to orange colour for this codoped phosphor showing its applicability to be used in making the colour tunable display devices. Acknowledgements Authors are thankful to the University Grants Commission (UGC), New Delhi, India and Indian School of Mines, Dhanbad, India for providing financial support. References

3+

3+

Fig. 5. Energy level diagram and energy transfer channels in Eu –Yb

phosphor.

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atively to the D0 level which is the metastable level from which luminescence is observed. This process involves the simultaneous absorption of two photons [14].

Fig. 6. CIE chromaticity diagram for different wt.% Eu3+ ions concentration.

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