Luminescence investigation of visible light emitting Ho3+ doped tellurite glass

Author’s Accepted Manuscript Luminescence investigation of visible light emitting Ho3+ doped tellurite glass Anurag Pandey, Hendrik C. Swart

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S0022-2313(15)00493-7 http://dx.doi.org/10.1016/j.jlumin.2015.08.060 LUMIN13566

To appear in: Journal of Luminescence Received date: 17 March 2015 Revised date: 18 August 2015 Accepted date: 21 August 2015 Cite this article as: Anurag Pandey and Hendrik C. Swart, Luminescence investigation of visible light emitting Ho3+ doped tellurite glass, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2015.08.060 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Luminescence investigation of visible light emitting Ho3+ doped tellurite glass Anurag Pandey and Hendrik C. Swart Department of Physics, University of the Free State, Box 339, Bloemfontein 9300, South Africa Tel.: +27 514013852/ +27 58 718 5308; Fax: +27 58 718 444. Corresponding author E Mail: [email protected] (A. Pandey) [email protected] (H. C. Swart)

Abstract Dual mode (upconversion and downshifting) emissions from the holmium activated TeO2ZnO glass is the focus point of the present study. The glasses were prepared by the wellknown melt quenching technique and identified by X-ray diffraction analysis. The thermal stability and glass transition characteristics were detailed on the basis of differential thermal analysis. The luminescence behaviour of the prepared glasses was explained on the basis of optical absorption and emission spectra recorded at different excitations. Intense green and yellowish-green emissions were detected upon excitations with blue and near-infrared wavelength photons, respectively. The upconversion process and colour tuning emission properties were discussed in detail with the help of a power dependence study, energy level diagram and CIE calculations. Keywords: Visible emission, Tellurite glass, Power dependence, Colour tunability.

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1. Introduction Glasses are investigated due to their easiness of preparation, shaping ability, incorporation of various active ions, less expenditure, and wide range of application fields. Along the structural strength and thermal stability, optical emissions from the glassy materials have wide uses in the modern science and technological age. Multicolour lighting, infrared lasers, display devices, optical sensors, fiber amplifiers, bio probes, etc. are some of the popular application areas of optical glasses [1-5]. Among various glass systems, oxides are very functional for optical applications owing to their dominant thermal stability and chemical durability characteristics [6]. Tellurium dioxide is a promising glass former for optical investigations due to the fact of its low energy vibrating phonons (~700 cm-1) that suppress the non-radiative transitions [7]. It also displays the high refractive index (~2) large transparency region (mainly upto 5 µm) and high network polarizability which facilitates ionic mobility of cations and anions [8-10]. The ZnO is a good material to use as modifier due to its excitation binding energy of about 60 meV and tellurite zinc oxide glass is an auspicious host for photonic applications [11]. Furthermore, ZnO reduce the crystallization rate of tellurite network and improve the aptitude of glass formation [8]. Rare earth ions play a crucial role to achieve optical emission from solid materials due to their sharp intra 4f-transitions and abundant energy level structures. Holmium appears to be an important candidate among the rare earths because it allows multiple excited state absorptions which could activate a wide emission spectrum [12]. The Ho3+ doped glasses have been investigated vastly during the last two decades for different applications [13-18]. Lai et al. [13] have reported red-dominant upconversion luminescence, whereas Librantz et al. [14] explained pump excited state absorption, in holmium-doped fluoride glass. Upconversion emission from Ho3+ doped lithium tellurite Page 2 of 22

glass is reported by Singh et al. [15]. Xu et al. [16] have investigated visible emission from the Ho3+-Yb3+ codoped glass ceramics for optical thermometry while infrared emissions were achieved by Schweizer et al. [17], in Ho3+ doped gallium lanthanum sulphide glass. Kamma et al. [18] explained the energy upconversion in holmium doped lead-germanotellurite glass upon 532 and 762 nm excitations. These studies were mainly focused upon visible and infrared emission from different Ho3+ activated glass matrixes. However, no report could be found where TeO2-ZnO (TZO) was used as the host matrix. Multimodal emissions (downshifting, upconversion and quantum cutting) from singly Ho3+ doped glasses were also not found. Due to our interest in dual mode {downshifting (conversion of high energy photons into one or more lower-energy ones) and upconversion (conversion of IR light to visible light by simultaneous absorption of two or more photons)} optical emission, TeO2-ZnO: Ho3+ glass is investigated as a possible matrix in the present study. In this paper, we report on the visible upconversion (UC) and downshifting (DS) emissions from the Ho3+ doped TZO glass prepared by the melt quenching technique. Xray diffraction (XRD) and photoluminescence (PL) characterizations were performed to confirm glass formation and to obtain the optical emitting nature. Differential thermal analysis (DTA) was also performed to predict the thermal behaviour of prepared glasses. 2. Material and methods 2.1 Glass preparation The glasses were prepared via the conventional melt quenching technique [19] using ZnO as modifier and Ho3+ as activator. The raw materials TeO2, ZnO and Ho2O3 of highly pure (99.90-99.99%) analytical grade (Sigma Aldrich) were used to prepare the glasses. A number of samples have been prepared according to the following composition (80-x) TeO2+ 20 ZnO + x Ho2O3

………………… (i) Page 3 of 22

where x = 0.0, 0.5, 1.0, 1.5, 2.0 mol%. The oxide form of precursors in total 10 g for each concentration was grounded homogeneously about an hour using acetone as mixing medium. The crushed powders were transferred into alumina crucibles and melted for about 30 minutes in an electrical furnace at 750 °C in an air atmosphere. The melts were poured into the preheated brass mould and pressed by another brass plate to obtain uniform thickness and cooled to room temperature. Good transparent glasses of thickness about 0.4 cm were prepared as shown in inset of Fig. 1 (1.0 mol% holmium doped TZO). The prepared glasses were annealed at 300 °C for about 2 hour to remove the strain present in the matrix and to allow attaining room temperature gradually. Finally, the prepared glasses were well polished to get planer faces to record further structural and optical measurements. 2.2 Characterization performed The phase identification of the prepared glass has been done by XRD measurements using a Bruker-D8 Advance diffractometer with a Cu-target radiation (λ= 0.154 nm). DTA measurement was performed using a TGA/SDTA851e (METTLER TOLEDO) in an argon atmosphere at a heating rate of 10°C/min. The optical absorption spectrum was recorded by using a Perkin Elmer (Lambda-950) UV–VIS spectrophotometer and PL excitation and emission spectra by a Varian Cary eclipse fluorescence spectrophotometer. UC emission spectra were measured upon excitation with a fibre coupled 980 nm diode laser with beam of spot size 1.4 mm (3 Watt power limit) via a Horiba iHR320 monochromator. The colour coordinates were calculated by suitable Commission Internationale de L’Eclairage (CIE) software (GoCIE) to confirm the colour tuning behaviour of the emitted light.

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3. Results and discussion 3.1 XRD analysis The XRD patterns of undoped and 1 mol% Ho3+ doped TZO glasses are shown in Fig. 1. Both the patterns shows same nature and do not exhibit any detectable peak that supports the non-crystalline nature of the prepared glass samples. Thus, the XRD result confirms amorphous characteristics and hence the formation of a glass matrix. 3.2 Differential thermal analysis Fig. 2 presents the DTA profile of undoped and 1 mol% Ho3+ doped TZO glasses. Both the profiles are very similar and having exothermic and endothermic humps situated with very slight shift. From the figure we have estimated the glass transition (Tg) temperature about 352.31 0C and 351.85 0C for undoped and 1 mol% Ho3+ doped TZO glasses, respectively. The DTA profile shows exothermic humps correspond to the crystallization temperature and onset crystallizations (Tx) are about 485.27

0

C and 486.81 0C in two cases

respectively. The glass thermal stability factor (∆T), which is the difference of Tx and Tg (i.e. Tx-Tg), are found 132.96 and 134.96 0C for undoped and 1 mol% Ho3+ doped TZO respectively. Higher stability factor display greater working ability during operations of fiber drawing [20]. Present result suggests a small improvement in thermal stability factor on codoping of holmium ions. Endothermic peaks around 600 0C were also observed which correspond to the melting temperature (Tm) of the glasses. 3.3 Absorption study Fig. 3 shows the room temperature absorption spectra of undoped and 1 mol% Ho3+ doped TZO glasses recorded in 300-1600 nm range with the assignments of observed absorption peaks. In case of undoped system, no absorption peak was found excluding band absorption below 400 nm whereas eight prominent peaks were observed on incorporation Page 5 of 22

of holmium in glass matrix. The absorption peaks detected around 419 nm, 453 nm, 471 nm, 486 nm, 540 nm, 643 nm, 901 nm and 1172 nm were assigned to the transitions from the ground state (5I8) to different excited states 5G5, 5G6, 3K8+5F2, 5F3, 5F4+5S2, 5F5, 5I5 and 5

I6 of the Ho3+ ion, respectively [18, 21]. From the figure it is also confirmed that the

strongest absorptions corresponds to the 5I8→5G6 transition of the Ho3+ ion. 3.4 Photoluminescence study 3.4.1 Photon downshifting The room temperature PL excitation and emission spectra of TZO glasses embedded with different Ho3+ concentration are shown in Fig. 4. The excitation spectra {Fig. 4(a)}, recorded by keeping the emission line at 550 nm, gave five peaks around 403, 420, 453, 475 and 488 nm due to the 5I8→5G4, 5I8→(5G/ 3G)5, 5I8→5G6, 5I8→5F2 and 5I8→5F3 transitions, respectively [21-23]. Maximum excitation intensity was detected in case of 1 mol% Ho3+ concentration with dominant peak around 453 nm (5I8→5G6 transition). The PL emission spectra of the prepared glass recorded at strongest excitation around 453 nm have given strong green emission band along with weak red and NIR emission bands at each concentration {Fig. 4(b)}. The emission peaks were centred on 528, 550, 662 and 755 nm due to the 5F4→5I8, 5S2→5I8, 5F5→5I8 and 5S2→5I7 transitions of the Ho3+ ion, respectively [24]. The PL emission intensity of all the emission bands are increased up to 1 mol% of Ho3+ concentration and then after decreased on increasing the concentration further due to effect of concentration quenching [25]. Thus 1 mol% of holmium concentration is optimum one in present study. The green emission band observed around 550 nm was ~12.1 times and ~10.4 times greater than the red and NIR emissions around 662 and 755 nm, respectively. Therefore, intense green light emitting from the prepared glass via a downshifting process could be seen with the naked eyes and identified by calculating the Page 6 of 22

colour coordinates correspondingly, as shown in inset of Fig 4(b). The values of the colour coordinates were found to be (0.33, 0.63), (0.32, 0.65), (0.32, 0.64) and (0.31, 0.61) on increasing the holmium concentration from 0.5 to 2.0 mol% respectively, which has approached the pure green wavelength region. Thus, bright green emission was found from the prepared glass upon blue excitation that might be applicable in lighting technology in near future. 3.4.2 Photon upconversion The room temperature UC emission spectra of the optimized Ho3+ doped TZO glass recorded upon a 980 nm pump wavelength in the 400-800 nm range at different excitation powers are shown in Fig. 5. Three UC emission bands were observed around 547, 660 and 760 nm in the green, red and NIR regions, assigned by the 5F4/5S2→5I8, 5F5→5I8 and 5

S2→5I7 transitions of the Ho3+ ion [23, 24]. These UC emission bands are detected due to

multiphoton absorption of pump photons. Remarkably, the UC emission intensity has increased with increasing the pump power and hence the brightness of the light emitted. This is happening due to the increment of the populations in the excited states of the Ho3+ ions on increasing the excitation power [26]. Furthermore, the intensity of the green and red UC emission bands seems to be changed corresponding to the excitation power i.e. the ratio of the green to red emission band has increased with pump power upto 1620 mW as shown in inset Fig. 5. This variation in UC emission intensity was attributed to the change in their excited energy level population. The increment in green to red emission intensity ratio tuned the colour of light emitted from the sample on increasing the excitation power as confirmed by the CIE diagram {Fig. 6(a)}. Fig. 6(a) displays that the colour coordinates have shifted towards the green region from 217 mW (0.47, 0.51) to 1620 mW (0.39, 0.59) and then return back to the yellow region at 1901 mW (0.40, 0.58). Hence, a power tunable Page 7 of 22

yellowish-green light was emitted from the glass upon 980 nm excitation. Also, the UC emission intensity enhancement rate was reduced at higher power due to the saturation of the excited levels population at higher powers [27]. The values of green to red ratio and colour coordinates were also listed in Table 1. 3.5 Pump power dependence In order to confirm the number of pump photons (n) involved in the UC emission from the developed glass, it is necessary to see the variation of the pump power (P) as a function of UC emission intensity (I) which is related by the following equation [26] Iα Pn

………………… (ii)

Fig. 6(b) shows the ln-ln plot of power versus UC emission intensity for the green and red emission bands. Linear fittings of the experimental data resulted into slopes with values of 1.77 and 1.52 for the UC emission bands observed through the 5F4/5S2→5I8 and 5F5→5I8 transitions, respectively. Thus a two pump photons are responsible for the UC emission form present glass system. 3.6 Energy transition property The dual mode excitation and emission pathways are shown in Fig. 7, in order to illustrate the PL properties. The 5G6 level of the Ho3+ ion is populated by direct absorption of 453 nm photon in the case of the DS process. This populated level relax down to the 5F4 and 5S2 levels and further to the 5F5 level via a nonradiative relaxation process, then after radiative relaxations to the ground state by emitting green and red photons around 528, 550 and 662 nm. The red emission is very weak compared to the green emission because of a lower probability of populating the 5F5 level through multiphonon relaxation and a weaker radiative transition probability of the 5F5→5I8 than the 5F4/ 5S2→5I8 [13]. Also, a weak NIR emission around 755 nm was detected through the 5S2→5I7 transitions. Page 8 of 22

On the other hand, three possible steps could occur to achieve the observed UC emissions:(a) Multiphonon relaxation assisted two photon absorption, (b) Phonon assisted two photon absorption, and (c) Two photon absorption succeed by cross relaxation (CR) and excited state absorption (ESA). Sequential absorption of two 980 nm photons (as confirmed from the pump power dependence study) populated the thermally coupled levels 5

F4/5S2 and 3K8/5F3 via phonon relaxed and phonon assisted ground state absorption (GSA)

and ESA processes, respectively. Excited atoms in the 3K8/5F3 states nonradiatively relax down to the 5F4/5S2 levels and hence de-excited to ground level via direct and phonons assist emissions in the green, red and NIR regions through the 5F4/5S2→5I8, 5F5→5I8 and the 5S2→5I7 transitions, respectively [28, 29]. As lower excitation power results dominant red emission over green which is possible due to the resonant cross-relaxations between holmium ion pair [30]. Though, the efficiency of cross-relaxation mechanism depends upon the interionic distance of the host matrix and dopant concentrations both. The crossrelaxation process involved 3K8; 5I8 → 5F5; 5I7 transitions populated the first excited state 5

I7 and then absorption of second 980 nm photon through ESA process boost the

population of 5F5 level. Then a direct transition to ground level results red luminescence around 660 nm. 4. Conclusion Non-crystalline Ho3+ activated TeO2-ZnO glass prepared by a melt-quenching technique, gave bright green and yellowish-green emission via downshifting and upconversion processes, respectively. Differential thermal analysis resulted glass transition, onset crystallization and melting temperatures around 352, 486 and 600 0C, respectively. Thermal stability factor was also found about 133 0C and not affected much on incorporation of holmium ions. Optical absorption study suggested strongest absorption Page 9 of 22

around 453 nm through the 5I8→5G6 transition while intense green emission arose due to the 5F4/5S2→5I8 transition upon this excitation. Visible upconversion emissions were visualized upon a 980 nm laser excitation and tuning in colour from yellow to green was noticed on increasing the pump power from 217 to 1620 mW, verified by calculating the colour coordinates correspondingly. Acknowledgements This research is supported by the South African Research Chairs Initiative of the Department of Science and Technology (84415) and National Research Foundation of South Africa. The financial support from the University of the Free State is also acknowledged. References [1] L. H. C. Andrade, S. M. Lima, M. L. Baesso, A. Novatski, J. H. Rohling, Y. Guyot, G. Boulon, J. Alloys Compd., 510 (2012) 54-59. [2] R. Xu, Y. Tian, L. Hu, J. Zhang, Appl. Phys. B, 108 (2012) 597-602. [3] A. Pandey, S. Som, V. Kumar, V. Kumar, K. Kumar, V. K. Rai, H. C. Swart, Sens. Actuators B, 202 (2014) 1305-1312. [4] N. G. Boetti, J. Lousteau, D. Negro, E. Mura, G. Scarpignato, S. Abrate, D. Milanese, Opt. Express, 20 (2012) 5409-5418. [5] D. L. Yang, H. Gong, E. Y. B. Pun, X. Zhao, H. Lin, Opt. Express 18 (2010) 18997– 19008. [6] S. A. Lourenco, N. O. Dantas, E. O. Serqueira, W. E. F. Ayta, A. A. Andrade, M. C. Filadelpho, J. A. Sampaio, M. J. V. Bell, M. A. Pereira-da-Silva, J. Lumin., 131 (2011) 850-855.

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[7] R. A.H. El-Mallawany, Tellurite Glasses Handbook: Physical Properties and Data, CRC Press, 2011, pp. 532 [ISBN 9781439849835]. [8] Z. A. S. Mahraz, M. R. Sahar, S. K. Ghoshal, M. R. Dousti, J. Lumin., 144 (2013) 139-145. [9] D. Larink, M. T. Rinke, H. Eckert, J. Phys. Chem. C, 119 (2015) 17539-17551. [10] R. L. Thomas, Synthesis and characterization of Tellurium oxide glasses for photonic applications, Ph.D. Thesis, Cochin University of Science and Technology, April 2013. [11] V. K. Rai, D. K. Mohanty, Appl. Phys. B, 109 (2012) 599-603. [12] Y. Yu, Y. D. Zheng, F. Qin, Z. M. Cheng, C. B. Zheng, Z. G. Zhang, W. W. Cao, J. Lumin., 131 (2011) 190-193. [13] B. Lai, L. Feng, J. Zhang, J. Wang, Q. Su, Appl. Phys. B, 110 (2013) 101-110. [14] A. F. H. Librantz, S. D. Jackson, L. Gomes, S. J. L. Ribeiro, Y. Messaddeq, J. Appl. Phys., 103 (2008) 023105. [15] A. K. Singh, S. B. Rai, V. B. Singh, J. Alloys Compd., 403 (2005) 97-103. [16] W. Xu, X. Gao, L. Zheng, Z. Zhang, W. Cao, Opt. Express, 20 (2012) 18127-18137. [17] T. Schweizer, B. N. Samson, J. R. Hector, W. S. Brocklesby, D. W. Hewak, D. N. Payne, Infrared Phys. Techn., 40 (1999) 329–335. [18] I. Kamma, B. R. Reddy, J. Appl. Phys., 107 (2010) 113102. [19] M. Bettinelli, A. Speghini, M. Ferrari, M. Montagna, J. Non-Cryst. Solids, 201 (1996) 211-221. [20] N. Vijaya, K. Upendra Kumar, C. K. Jayasankar, Spectrochim. Acta A, 113 (2013) 145153. [21] N. K. Giri, D. K. Rai, S. B. Rai, Appl. Phys. B, 91 (2008) 437-441.

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[22] S. Balaji, A. D. Sontakke, R. Sen, A. Kalyandurg, Opt. Mater. Express, 1 (2011) 138-150. [23] H. M. Crosswhite, H. Crosswhite, N. Edelstein, K. Rajnak, J. Chem. Phys., 67 (1977) 3002-3010. [24] G. H. Dieke, Spectra and Energy Levels of Rare Earth Ions in Crystals, Interscience Publishers, USA, 1968, 253-261 [SBN 470 21390 6]. [25] B. P. Singh, A. K. Parchur, R. S. Ningthoujam, A. A. Ansari, P. Singh, S. B. Rai, Dalton Trans., 43 (2014) 4779-4789. [26] A. Pandey, V. K. Rai, V. Kumar, V. Kumar, H. C. Swart, Sens. Actuators B, 209 (2015) 352-358. [27] M. Pollnau, D. R. Gamelin, S. R. Luthi, H. U. Gudel, M. P. Hehlen, Phys. Rev. B, 61 (2000) 3337-3346. [28] A. Pandey, and V. K. Rai, Dalton Trans., 42 (2013) 11005-11011. [29] Y. Dwivedi, A. Bahadur, S. B. Rai, J. Appl. Physics, 110 (2011) 043103. [30] A. S. Gouveia-Neto, E. B. da Costa, L. A. Bueno, S. J. L. Ribeiro, J. Lumin., 110 (2004) 79-84.

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Table 1: Green to red ratio and corresponding colour coordinates at different pump powers of Ho3+ doped TZO glass.

Figure Caption Fig. 1: XRD patterns of undoped and 1 mol% Ho3+ doped TZO glasses. Fig. 2: DTA profiles of undoped and 1 mol% Ho3+ doped TZO glasses. Fig. 3: Optical absorption spectrum of undoped and 1 mol% Ho3+ doped TZO glasses. Fig. 4: (a) PL Excitation spectra and (b) Emission spectra [inset-corresponding colour coordinates], of Ho3+ doped TZO glasses at different holmium concentration. Fig. 5: UC emission spectra of 1 mol% Ho3+ doped TZO glass on increasing excitation power [inset-green to red emission intensity ratio plot]. Fig. 6: (a) Colour coordinates variation on increasing pump power and (b) Logarithmic dependence of pump power versus integrated UC intensity, of 1 mol% Ho3+ doped TZO glass. Fig. 7: Schematic Energy level diagram of the holmium ion with possible excitation and emission pathways.

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Table 1 S. No.

Power (mW)

Green/Red (a.u.)

CIE coordinates (x, y)

1

217

0.7647

0.47, 0.51

2

459

1.0058

0.45, 0.53

3

794

1.1538

0.44, 0.54

4

1087

1.4375

0.42, 0.57

5

1315

1.6473

0.40, 0.59

6

1620

1.7609

0.39, 0.59

7

1901

1.5827

0.40, 0.58

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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Figure 7

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Highlights  Dual mode (downshifting and upconversion) emissions were detected from the Ho3+ doped TZO glasses.  Intense green and power tunable yellowish-green lights were emitting upon 453 and 980 nm excitations.  Two photons upconversion process is responsible for UC emissions by involving

cross-relaxations between the Ho3+ ions.

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