- Email: [email protected]
Near-UV sensitized NIR emission in Nd3+ and Bi3+ co-doped GdVO4 phosphors
Optical Materials xxx (2017) 1e4
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Near-UV sensitized NIR emission in Nd3þ and Bi3þ co-doped GdVO4 phosphors K. Lenczewska*, R. Tomala, D. Hreniak** Institute of Low Temperature and Structural Research, Polish Academy of Sciences, 50-422 Wrocław, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 January 2017 Received in revised form 5 April 2017 Accepted 6 April 2017 Available online xxx
Near-infrared (NIR) luminescent nanophosphor of Nd3þ and Bi3þ co-doped GdVO4 was prepared by a microwave-assisted hydrothermal method. The phase purity and the structure of samples were characterized by an X-ray powder diffraction (XRD). The optical properties were investigated by analysis of the photoluminescence and excitation spectra and by the emission decay measurements. It was found that upon near-UV excitation of the charge transfer state or the Bi3þ ions absorption, a strong broad band yellow-green emission of Bi3þ ions accompanied with NIR emission of Nd3þ ions is observed. Enhancement of the Nd3þ emission through an energy transfer from the (O2 e V5þ) or the (Bi3þ e V5þ) charge transfer states to the excited states of Nd3þ ion was noticed and the possible energy transfer mechanisms were discussed. © 2017 Elsevier B.V. All rights reserved.
Keywords: Near-infrared phosphor Vanadates Bismuth Neodymium
1. Introduction Near-infrared (NIR) light-emitting optical materials attract attention because of wide applications as solid state lasers [1], fibre optical communications [2], chemical and biological sensing [3], infrared bioimaging [4]. Additionally, the NIR luminescent lanthanide materials can also help in enhancing of a silicon-based solar cell efficiency. The major issue with silicon-based solar cells is the spectral mismatching of incident photons especially the low energy photons (l > 1100 nm) as well as the high energy photons (l < 400 nm), which are not absorbed efficiently and cause thermalization losses [5]. Lanthanide ions (Ln3þ) like Nd3þ, Yb3þ or Er3þ are well-known NIR luminescence centres. It is well-known also that an emission intensity of Ln3þ is dependent on a host matrix, its crystal structure and type of a site symmetry. However, a forbidden nature of their luminescence leads not only to the longer emission lifetimes useful for laser applications and for designing up-converting phosphors, but unfortunately also to a lower absorption cross-section. Three efficient ways to increase the NIR fluorescence of Ln3þ ions may be distinguished: (a) choosing a self-activated host matrix that can
transfers its excitation energy to activators like Ln3þ ions (e.g. vanadates); (b) to incorporate a sensitizer (such as Bi3þ); (c) quantum cutting (QC) by suitable co-doping. Gadolinium orthovanadate (GdVO4) is a well-known host material with a lot of advantages like good thermal properties, chemical stability and high photoluminescence quantum yield [6]. GdVO4 is an attractive host lattice because of the efficient resonant energy transfer from the host to the lanthanide ions. In this work, Nd3þ has been chosen as the dopant ion because of rich energy levels structure, a strong NIR emission at around 1 mm and its low reduction potential. Bi3þ has been used as co-dopant ion because it may act both as an activator and a sensitizer for a luminescent material [7]. There are a few reports about the energy transfer between Bi3þ and Nd3þ ions in different hosts like Gd2O3: Bi3þ, Nd3þ [8]; GdNbO4: Bi3þ, Nd3þ [9] and YVO4: Bi3þ, Nd3þ [10]. In this work, luminescence properties of Nd3þ and Bi3þ codoped GdVO4 nanophosphors prepared via a microwave-assisted hydrothermal method are presented and discussed for the first time. 2. Experiment 2.1. Samples preparation
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (K. Lenczewska), D.Hreniak@ int.pan.wroc.pl (D. Hreniak).
The powder of GdVO4:1%Bi3þ, x%Nd3þ (x ¼ 1, 5, 7 mol%) and GdVO4:1%Nd3þ phosphors were prepared by the microwaveassisted hydrothermal method [11]. High purity (99.999%) oxides:
http://dx.doi.org/10.1016/j.optmat.2017.04.015 0925-3467/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: K. Lenczewska, et al., Near-UV sensitized NIR emission in Nd3þ and Bi3þ co-doped GdVO4 phosphors, Optical Materials (2017), http://dx.doi.org/10.1016/j.optmat.2017.04.015
2
K. Lenczewska et al. / Optical Materials xxx (2017) 1e4
Gd2O3, Nd2O3, Bi2O3 and a high purity (99.996%) ammonium metavanadate (NH4VO3) were used as starting reagents for a preparation of the samples. The stoichiometric amount of oxides were dissolved in ultrapure 65% nitric acid to obtain nitrates. At the same time, NH4VO3 was dissolved in warm water. Then, the solution of obtained nitrates was slowly added (one drop per second) to the NH4VO3 solution under vigorous stirring. After that, the reaction mixture was stirred for another 30 min. The homogeneous mixture was transferred to 100 ml Teflon tube and next placed into an autoclave for microwave hydrothermal treatment and allowed to heat for 30 min at 180 C. The obtained precipitate was filtered, washed several times with both distilled water and ethanol. Afterwards, the precipitate was dried at 60 C for 24 h. Finally, the product was calcined at 800 C for 3 h in air. 2.2. Measurements and apparatus Microwave hydrothermal treatment was carried out in a ERTEC e Magnum II microwave digestion system (microwave frequency e 2.45 GHz; power e 600 W). Structural studies of the samples were carried out by means of a X'Pert PRO PANalytical X-ray diffractometer using CuKa radiation (l ¼ 1.54056 Å). Photoluminescence excitation (PLE) measurements were performed with a spectrofluorimeter system consisting of a Dongwoo Optron System containing a DM158i excitation monochromator and a DM711 emission monochromator. A DL 80-Xe ozone-free xenon lamp was used as excitation source. Photoluminescence (PL) spectra were measured using a FLS980 Fluorescence Spectrometer from Edinburgh Instruments with an ozone-free xenon lamp. Fluorescence decay curves were recorded with a digital oscilloscope MD 04054B-3 (TEKTRONIX). An optical parametric oscillator (OPO) Opolette 355 (OPOTEK INC.) as the excitation source was used together with a photomultiplayer R298 as the detector and a monochromator GDM 1000 (CARLZEISS JENA). All measurements were performed at room temperature (300 K). 3. Results and discussion Fig. 1 shows the XRD patterns of the Bi3þ/Nd3þ co-doped GdVO4 samples annealed at 800 C. The patterns for all samples show a pure phase of the tetragonal zircon-type GdVO4 structure
corresponding to the PDF [00-017-0260] card. Due to their similar ionic radii (Gd3þ, r ¼ 1.053 Å; Nd3þ, r ¼ 1.109 Å; Bi3þ, r ¼ 1.17 Å) [12] the lattice site of Gd3þ ion in this structure may be substituted both with Bi3þ and Nd3þ ions. The average crystallite size D for the obtained samples was calculated from the XRD using Scherrer equation [13]:
D¼
kl bð2QÞcos Q
where k e constant (equal to 0.94), l e X-ray wavelength, b e full width at half-maximum, Q e angle, and shown in Table 1. In order to estimate energy levels of the Nd3þ ions, the PLE spectra of GdVO4:1% Nd3þand GdVO4:1% Bi3þ, x% Nd3þ (x ¼ 1, 5, 7) were measured at lem ¼ 1064 nm, that corresponds to the most intense band of the 4F3/2 / 4I11/2 electron transition (Fig. 2). Collected PLE spectra consist of typical absorption bands for intraconfigurational f-f electron transitions of Nd3þ ion [14]. On the other hand, a broad band absorption in the near-UV spectral region is assigned to the charge transfer (CT) band O2 e V5þ of the GdVO4 host centred at 310 nm. Co-doping with Bi3þ ions of GdVO4:Nd3þ has caused a shift of the edge of the CT band to a longer wavelength. This shift is associated with the presence of the Bi3þ e V5þ metal to metal CT band [7,15,16]. Additionally, the intensity of the f-f electron transitions of Nd3þ ions increases with concentration of the Nd3þ ions. The excitation spectra show that Nd3þ ions in this material can be excited directly in the f-f transition of the Nd3þ and also indirectly through the CT band. Both visible (VIS) and NIR emission can be observed upon the near-UV excitation in the GdVO4:Bi3þ, Nd3þ nanophosphor: 266 nm in the (O2 e V5þ) CT state or 330 nm in the (Bi3þ e V5þ) CT state. A yellow-green luminescence in the range of 450e800 nm (Fig. 3) is associated with the presence of Bi3þ dopant in the sample. Whereas, a NIR luminescence of Nd3þ ions is observed through an energy transfer between the (Bi3þ e V5þ) CT state and the Nd3þ ion. Fig. 3 shows a spectral overlap between the emission band of Bi3þ ion and the absorption bands of Nd3þ ion, which is responsible for an energy transfer process [17]. A similar energy transfer process but without the spectral overlap between Bi3þ emission and Ln3þ absorption bands has been observed in the same host matrix between Bi3þ and Yb3þ ions. It has been interpreted as a cooperative energy transfer (CET) process [18]. Additionally, in the emission spectrum of Bi3þ ions a reabsorption process is observed, which indicates the occurrence of the radiative energy transfer between the Bi3þ and Nd3þ ions. The NIR emission spectra of the GdVO4:1% Bi3þ, x% Nd3þ (x ¼ 1, 5, 7) samples were measured as a function of the Nd3þ dopant concentration as well as an excitation wavelength (see Figs. 4 and 5). Typically, the PL spectrum consists of characteristic bands in the NIR region ascribed to the specific electron transitions occurring within the f-f shell of Nd3þ ion, such as the 4F3/2 / 4I9/2 transition with maximum at 880 nm, the laser transition of the 4F3/ 4 4 4 2 / I11/2 at 1064 nm, and the lowest intensity F3/2 / I13/2 transition located at 1342 nm, respectively [1]. In order to discuss an effect of the dopant concentration, all PL spectra were
Table 1 Average crystallite size for GdVO4:x% Bi3þ, y% Nd3þ (x ¼ 0, 1; y ¼ 1, 5, 7) nanocrystals annealed at 800 C. Samples
Fig. 1. Powder X-ray diffraction patterns of the GdVO4:x% Bi3þ, y% Nd3þ (x ¼ 0, 1; y ¼ 1, 5, 7) nanocrystals annealed at 800 C.
1% 1% 1% 1%
Nd3þ Bi3þ, 1% Nd3þ Bi3þ, 5% Nd3þ Bi3þ, 7% Nd3þ
Average crystallite size [nm] 77 80 80 56
Please cite this article in press as: K. Lenczewska, et al., Near-UV sensitized NIR emission in Nd3þ and Bi3þ co-doped GdVO4 phosphors, Optical Materials (2017), http://dx.doi.org/10.1016/j.optmat.2017.04.015
K. Lenczewska et al. / Optical Materials xxx (2017) 1e4
Fig. 2. Photoluminescence excitation spectra of GdVO4:Bi3þ, Nd3þ phosphors upon lem ¼ 1064 nm.
Fig. 3. The overlap between the emission spectrum of Bi3þ ions and the excitation spectrum of Nd3þ ions in the GdVO4:1%Bi3þ, 1%Nd3þ phosphors.
normalized to the intensity of the laser transition. Increasing of the Nd3þ dopant concentration causes a change of relative emission intensity of the Stark components (at 880 nm and 913 nm) in the 4 F3/2 / 4I9/2 resonant transition. It is due to a strong reabsorption process [19]. In Fig. 5 a change of relative emission intensity of Nd3þ electron transitions has been also observed. One can see, that upon 594 nm excitation directly in the Nd3þ ions the ratio of the intensity emission of the nonresonant 4F3/2 / 4I11/2 transition to the 4F3/ 4 2 / I13/2 transition is higher than upon 266 nm and 330 nm excitation. Fig. 6 shows the emission decay curves of GdVO4:1% Bi3þ, x% Nd3þ (x ¼ 1, 5, 7). The decay curves of Nd3þ emission were recorded at lem ¼ 1064 nm and upon 330 nm excitation. In the case of doping with 1% Nd3þ ions, the decay profile exhibit a single exponential character with the lifetime of 75 ms, which value is close to the decay time for YVO4:1% Nd3þ annealed at 800 C [19]. An increase of Nd3þ ions concentration results in more nonexponential behaviour of the decay curves and also the decay time shortening
3
Fig. 4. Impact of Nd3þ dopant concentration on emission spectra of the GdVO4: 1% Bi3þ, x% Nd3þ (x ¼ 1, 5, 7) upon lexc ¼ 330 nm.
Fig. 5. Impact of the excitation wavelength on emission spectra of the GdVO4: 1%Bi3þ, 1%Nd3þ upon lexc ¼ 266 nm, 330 nm and 594 nm, respectively.
from 14.6 ms to 8.9 ms (see Table 2). Observed behaviour is probably due to the presence of cross-relaxation (CR) processes leading to an emission concentration quenching [19,20]. Additionally, upon 266 nm excitation, the lifetime of Nd3þ ions is longer for GdVO4:1% Bi3þ, 1% Nd3þ than for GdVO4:1% Nd3þ. The elongation of the lifetime may be associated with the presence of an intermediate energy level, such as a ligand-to-metal charge transfer state (LMCT) that mediates in the energy transfer mechanism and affects the lifetime of Nd3þ ions strongly, mainly in a near-resonance. The influence of the presence of the intramolecular energy transfer on lifetime was described in the works of Ferreira et al. and Matos et al. [21,22]. It is known that silicon-based solar cells have a good response and conversion efficiency in the range of 500e1100 nm but the response of blue range and UV for wavelengths less than 500 nm is deteriorated obviously. The GdVO4:Bi3þ, Nd3þ nanophosphor displays a broad and intense absorption band in the range of 200e400 nm and can utilize a solar spectrum efficiently in the UV
Please cite this article in press as: K. Lenczewska, et al., Near-UV sensitized NIR emission in Nd3þ and Bi3þ co-doped GdVO4 phosphors, Optical Materials (2017), http://dx.doi.org/10.1016/j.optmat.2017.04.015
4
K. Lenczewska et al. / Optical Materials xxx (2017) 1e4
between Bi3þ - V5þ metal-to-metal CT and Nd3þ ions. The analysis of emission spectra has revealed the presence of reabsorption processes in case of both dopants, what may be explained by the spectral overlap between their absorption and emission bands. Furthermore, the lifetime shortening for the higher concentration of Nd3þ ions was due to the emission concentration quenching. The presence of an intermediate energy level, such as the ligand-tometal charge transfer (LMCT) state has affected the prolongation of the luminescence lifetime of Nd3þ ions upon the near-UV excitation. Additionally, the near-UV excitation has enhanced the 4F3/ 4 3þ ions. Taking into account all the results, 2 / I9/2 transition of Nd it was confirmed that the GdVO4 co-doped with Bi3þ and Nd3þ nanophosphor is a promising candidate for downconverting luminescent concentrators for the next generation silicon-based solar cells. Acknowledgements Fig. 6. The decay curves of the Nd3þ luminescence at lem ¼ 1064 nm and lexc ¼ 330 nm for the GdVO4: 1%Bi3þ, x%Nd3þ (x ¼ 1, 5, 7) annealed at 800 C.
Table 2 Concentration and excitation wavelength dependence of GdVO4: Bi3þ, Nd3þ lifetime (ms). Samples
1% 1% 1% 1%
Nd3þ Bi3þ, 1% Nd3þ Bi3þ, 5% Nd3þ Bi3þ, 7% Nd3þ
Excitation wavelength 266 nm
330 nm
594 nm
70.9 81.5 14.1 9
e 75 14.6 8.9
79.9 76.3 14.2 8.5
part and converts efficiently UV into NIR due to the effective energy transfer from the (O2 e V5þ) CT and the (Bi3þ e V5þ) charge transfer states to Nd3þ ions. Therefore, this material may have a feasibility of potential application in enhancing the efficiency of the Si-based solar cells. 4. Conclusions Near-UV sensitized NIR emission has been demonstrated in the nanocrystals of GdVO4 co-doped with Nd3þ and Bi3þ ions. The nanocrystals in various co-dopant concentrations were prepared by a microwave-assisted hydrothermal method. In the GdVO4:Bi3þ, Nd3þ, the visible and the NIR emission upon the excitation of O2 e V5þ or V5þ e Bi3þ charge transfer state have been observed. Yellowgreen luminescence of the phosphors due to doping with Bi3þ ions upon the near-UV excitation was recorded as a result of the Bi3þ V5þ metal-to-metal CT state and the near-infrared luminescence of Nd3þ ions (activator) was observed through the energy transfer
The autors are thankful to PhD Adam Strze˛ p for help in luminescence measurements. References [1] T. Jensen, V.G. Ostroumov, J.-P. Meyn, G. Huber, A.I. Zagumennyi, I.A. Shcherbakov, Appl. Phys. B 58 (1994) 373e379. [2] J.C.G. Bunzli, S.V. Eliseeva, J. Rare Earths 28 (2010) 824e842. [3] C.A. Primmerman, Linc. Lab. J. 12 (2000) 3e32. z-Delgado, M. Misiak, A. Bednarkiewicz, A.S. Vanetsev, [4] B. del Rosal, A. Pere Y. Orlovskii, D.J. Jovanovi c, M.D. Drami canin, U. Rocha, K. Upendra Kumar, C. Jacinto, E. Navarro, E.M. Rodríguez, M. Pedroni, A. Speghini, G.A. Hirata, I.R. Martín, D. Jaque, J. Appl. Phys. 118 (2015) 143104. [5] X. Huang, S. Han, W. Huang, X. Liu, Chem. Soc. Rev. 42 (2013) 173e201. [6] T. Minami, T. Miyata, Y. Suzuki, Y. Mochizuki, Thin Solid Films 469 (2004) 65e69. [7] G. Blasse, A. Bril, J. Chem. Phys. 48 (1968) 217e222. [8] G.X. Liu, R. Zhang, Q.L. Xiao, S.Y. Zou, W.F. Peng, L.W. Cao, J.X. Meng, Opt. Mater 34 (2011) 313e316. [9] P.K. Shahi, P. Singh, S.B. Rai, A. Bahadur, Inorg. Chem. 55 (2016) 1535e1541. [10] Q.L. Xiao, J.X. Meng, J.R. Qiu, J. Alloys Compd. 574 (2013) 600e603. [11] A.S. Vanetsev, O.M. Gaitko, I.G. Chuvashova, M.N. Sokolov, Yu.D. Tret’yakov, Dokl. Chem. 441 (2011) 325e329. [12] R.D. Shannon, Acta Crystallogr. A32 (1976) 751e767. [13] P. Scherrer, Nachrichten von der gesellschaft der Wissenschaften zu Gottingen, 1918, pp. 98e100. [14] A.A. Kaminskii, Crystalline Lasers: Physical Processes and Operating Schemes, CRC Press, Boca Raton, 1996. [15] R.K. Datta, J. Electrochem. Soc. 114 (1967) 1057e1063. [16] J. Ghamri, H. Baussart, M. Le Bras, Jean-Marie Leroy, J. Phys. Chem. Solids 50 (1989) 1237e1244. [17] R. Reisfeld, Struct. Bond. 30 (1976) 65e97. [18] K. Lenczewska, M. Stefanski, D. Hreniak, J. Rare Earths 34 (2016) 837e842. [19] R.J. Wiglusz, L. Marciniak, R. Pazik, W. Strek, Cryst. Growth Des. 14 (2014) 5512e5520. [20] V. Ostroumov, T. Jensen, J.-P. Meyn, G. Huber, M.A. Noginov, J. Opt. Soc. Am. B 15 (1998) 1052e1060. [21] R.A.S. Ferreira, M. Nolasco, A.C. Roma, R.L. Longo, O.L. Malta, L.D. Carlos, Chem. Eur. J. 18 (2012) 12130e12139. [22] M.G. Matos, L.A. Rocha, E.J. Nassar, M. Verelst, Opt. Mater 62 (2016) 12e18.
Please cite this article in press as: K. Lenczewska, et al., Near-UV sensitized NIR emission in Nd3þ and Bi3þ co-doped GdVO4 phosphors, Optical Materials (2017), http://dx.doi.org/10.1016/j.optmat.2017.04.015
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Near-UV sensitized NIR emission in Nd3þ and Bi3þ co-doped GdVO4 phosphors K. Lenczewska*, R. Tomala, D. Hreniak** Institute of Low Temperature and Structural Research, Polish Academy of Sciences, 50-422 Wrocław, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 January 2017 Received in revised form 5 April 2017 Accepted 6 April 2017 Available online xxx
Near-infrared (NIR) luminescent nanophosphor of Nd3þ and Bi3þ co-doped GdVO4 was prepared by a microwave-assisted hydrothermal method. The phase purity and the structure of samples were characterized by an X-ray powder diffraction (XRD). The optical properties were investigated by analysis of the photoluminescence and excitation spectra and by the emission decay measurements. It was found that upon near-UV excitation of the charge transfer state or the Bi3þ ions absorption, a strong broad band yellow-green emission of Bi3þ ions accompanied with NIR emission of Nd3þ ions is observed. Enhancement of the Nd3þ emission through an energy transfer from the (O2 e V5þ) or the (Bi3þ e V5þ) charge transfer states to the excited states of Nd3þ ion was noticed and the possible energy transfer mechanisms were discussed. © 2017 Elsevier B.V. All rights reserved.
Keywords: Near-infrared phosphor Vanadates Bismuth Neodymium
1. Introduction Near-infrared (NIR) light-emitting optical materials attract attention because of wide applications as solid state lasers [1], fibre optical communications [2], chemical and biological sensing [3], infrared bioimaging [4]. Additionally, the NIR luminescent lanthanide materials can also help in enhancing of a silicon-based solar cell efficiency. The major issue with silicon-based solar cells is the spectral mismatching of incident photons especially the low energy photons (l > 1100 nm) as well as the high energy photons (l < 400 nm), which are not absorbed efficiently and cause thermalization losses [5]. Lanthanide ions (Ln3þ) like Nd3þ, Yb3þ or Er3þ are well-known NIR luminescence centres. It is well-known also that an emission intensity of Ln3þ is dependent on a host matrix, its crystal structure and type of a site symmetry. However, a forbidden nature of their luminescence leads not only to the longer emission lifetimes useful for laser applications and for designing up-converting phosphors, but unfortunately also to a lower absorption cross-section. Three efficient ways to increase the NIR fluorescence of Ln3þ ions may be distinguished: (a) choosing a self-activated host matrix that can
transfers its excitation energy to activators like Ln3þ ions (e.g. vanadates); (b) to incorporate a sensitizer (such as Bi3þ); (c) quantum cutting (QC) by suitable co-doping. Gadolinium orthovanadate (GdVO4) is a well-known host material with a lot of advantages like good thermal properties, chemical stability and high photoluminescence quantum yield [6]. GdVO4 is an attractive host lattice because of the efficient resonant energy transfer from the host to the lanthanide ions. In this work, Nd3þ has been chosen as the dopant ion because of rich energy levels structure, a strong NIR emission at around 1 mm and its low reduction potential. Bi3þ has been used as co-dopant ion because it may act both as an activator and a sensitizer for a luminescent material [7]. There are a few reports about the energy transfer between Bi3þ and Nd3þ ions in different hosts like Gd2O3: Bi3þ, Nd3þ [8]; GdNbO4: Bi3þ, Nd3þ [9] and YVO4: Bi3þ, Nd3þ [10]. In this work, luminescence properties of Nd3þ and Bi3þ codoped GdVO4 nanophosphors prepared via a microwave-assisted hydrothermal method are presented and discussed for the first time. 2. Experiment 2.1. Samples preparation
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (K. Lenczewska), D.Hreniak@ int.pan.wroc.pl (D. Hreniak).
The powder of GdVO4:1%Bi3þ, x%Nd3þ (x ¼ 1, 5, 7 mol%) and GdVO4:1%Nd3þ phosphors were prepared by the microwaveassisted hydrothermal method [11]. High purity (99.999%) oxides:
http://dx.doi.org/10.1016/j.optmat.2017.04.015 0925-3467/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: K. Lenczewska, et al., Near-UV sensitized NIR emission in Nd3þ and Bi3þ co-doped GdVO4 phosphors, Optical Materials (2017), http://dx.doi.org/10.1016/j.optmat.2017.04.015
2
K. Lenczewska et al. / Optical Materials xxx (2017) 1e4
Gd2O3, Nd2O3, Bi2O3 and a high purity (99.996%) ammonium metavanadate (NH4VO3) were used as starting reagents for a preparation of the samples. The stoichiometric amount of oxides were dissolved in ultrapure 65% nitric acid to obtain nitrates. At the same time, NH4VO3 was dissolved in warm water. Then, the solution of obtained nitrates was slowly added (one drop per second) to the NH4VO3 solution under vigorous stirring. After that, the reaction mixture was stirred for another 30 min. The homogeneous mixture was transferred to 100 ml Teflon tube and next placed into an autoclave for microwave hydrothermal treatment and allowed to heat for 30 min at 180 C. The obtained precipitate was filtered, washed several times with both distilled water and ethanol. Afterwards, the precipitate was dried at 60 C for 24 h. Finally, the product was calcined at 800 C for 3 h in air. 2.2. Measurements and apparatus Microwave hydrothermal treatment was carried out in a ERTEC e Magnum II microwave digestion system (microwave frequency e 2.45 GHz; power e 600 W). Structural studies of the samples were carried out by means of a X'Pert PRO PANalytical X-ray diffractometer using CuKa radiation (l ¼ 1.54056 Å). Photoluminescence excitation (PLE) measurements were performed with a spectrofluorimeter system consisting of a Dongwoo Optron System containing a DM158i excitation monochromator and a DM711 emission monochromator. A DL 80-Xe ozone-free xenon lamp was used as excitation source. Photoluminescence (PL) spectra were measured using a FLS980 Fluorescence Spectrometer from Edinburgh Instruments with an ozone-free xenon lamp. Fluorescence decay curves were recorded with a digital oscilloscope MD 04054B-3 (TEKTRONIX). An optical parametric oscillator (OPO) Opolette 355 (OPOTEK INC.) as the excitation source was used together with a photomultiplayer R298 as the detector and a monochromator GDM 1000 (CARLZEISS JENA). All measurements were performed at room temperature (300 K). 3. Results and discussion Fig. 1 shows the XRD patterns of the Bi3þ/Nd3þ co-doped GdVO4 samples annealed at 800 C. The patterns for all samples show a pure phase of the tetragonal zircon-type GdVO4 structure
corresponding to the PDF [00-017-0260] card. Due to their similar ionic radii (Gd3þ, r ¼ 1.053 Å; Nd3þ, r ¼ 1.109 Å; Bi3þ, r ¼ 1.17 Å) [12] the lattice site of Gd3þ ion in this structure may be substituted both with Bi3þ and Nd3þ ions. The average crystallite size D for the obtained samples was calculated from the XRD using Scherrer equation [13]:
D¼
kl bð2QÞcos Q
where k e constant (equal to 0.94), l e X-ray wavelength, b e full width at half-maximum, Q e angle, and shown in Table 1. In order to estimate energy levels of the Nd3þ ions, the PLE spectra of GdVO4:1% Nd3þand GdVO4:1% Bi3þ, x% Nd3þ (x ¼ 1, 5, 7) were measured at lem ¼ 1064 nm, that corresponds to the most intense band of the 4F3/2 / 4I11/2 electron transition (Fig. 2). Collected PLE spectra consist of typical absorption bands for intraconfigurational f-f electron transitions of Nd3þ ion [14]. On the other hand, a broad band absorption in the near-UV spectral region is assigned to the charge transfer (CT) band O2 e V5þ of the GdVO4 host centred at 310 nm. Co-doping with Bi3þ ions of GdVO4:Nd3þ has caused a shift of the edge of the CT band to a longer wavelength. This shift is associated with the presence of the Bi3þ e V5þ metal to metal CT band [7,15,16]. Additionally, the intensity of the f-f electron transitions of Nd3þ ions increases with concentration of the Nd3þ ions. The excitation spectra show that Nd3þ ions in this material can be excited directly in the f-f transition of the Nd3þ and also indirectly through the CT band. Both visible (VIS) and NIR emission can be observed upon the near-UV excitation in the GdVO4:Bi3þ, Nd3þ nanophosphor: 266 nm in the (O2 e V5þ) CT state or 330 nm in the (Bi3þ e V5þ) CT state. A yellow-green luminescence in the range of 450e800 nm (Fig. 3) is associated with the presence of Bi3þ dopant in the sample. Whereas, a NIR luminescence of Nd3þ ions is observed through an energy transfer between the (Bi3þ e V5þ) CT state and the Nd3þ ion. Fig. 3 shows a spectral overlap between the emission band of Bi3þ ion and the absorption bands of Nd3þ ion, which is responsible for an energy transfer process [17]. A similar energy transfer process but without the spectral overlap between Bi3þ emission and Ln3þ absorption bands has been observed in the same host matrix between Bi3þ and Yb3þ ions. It has been interpreted as a cooperative energy transfer (CET) process [18]. Additionally, in the emission spectrum of Bi3þ ions a reabsorption process is observed, which indicates the occurrence of the radiative energy transfer between the Bi3þ and Nd3þ ions. The NIR emission spectra of the GdVO4:1% Bi3þ, x% Nd3þ (x ¼ 1, 5, 7) samples were measured as a function of the Nd3þ dopant concentration as well as an excitation wavelength (see Figs. 4 and 5). Typically, the PL spectrum consists of characteristic bands in the NIR region ascribed to the specific electron transitions occurring within the f-f shell of Nd3þ ion, such as the 4F3/2 / 4I9/2 transition with maximum at 880 nm, the laser transition of the 4F3/ 4 4 4 2 / I11/2 at 1064 nm, and the lowest intensity F3/2 / I13/2 transition located at 1342 nm, respectively [1]. In order to discuss an effect of the dopant concentration, all PL spectra were
Table 1 Average crystallite size for GdVO4:x% Bi3þ, y% Nd3þ (x ¼ 0, 1; y ¼ 1, 5, 7) nanocrystals annealed at 800 C. Samples
Fig. 1. Powder X-ray diffraction patterns of the GdVO4:x% Bi3þ, y% Nd3þ (x ¼ 0, 1; y ¼ 1, 5, 7) nanocrystals annealed at 800 C.
1% 1% 1% 1%
Nd3þ Bi3þ, 1% Nd3þ Bi3þ, 5% Nd3þ Bi3þ, 7% Nd3þ
Average crystallite size [nm] 77 80 80 56
Please cite this article in press as: K. Lenczewska, et al., Near-UV sensitized NIR emission in Nd3þ and Bi3þ co-doped GdVO4 phosphors, Optical Materials (2017), http://dx.doi.org/10.1016/j.optmat.2017.04.015
K. Lenczewska et al. / Optical Materials xxx (2017) 1e4
Fig. 2. Photoluminescence excitation spectra of GdVO4:Bi3þ, Nd3þ phosphors upon lem ¼ 1064 nm.
Fig. 3. The overlap between the emission spectrum of Bi3þ ions and the excitation spectrum of Nd3þ ions in the GdVO4:1%Bi3þ, 1%Nd3þ phosphors.
normalized to the intensity of the laser transition. Increasing of the Nd3þ dopant concentration causes a change of relative emission intensity of the Stark components (at 880 nm and 913 nm) in the 4 F3/2 / 4I9/2 resonant transition. It is due to a strong reabsorption process [19]. In Fig. 5 a change of relative emission intensity of Nd3þ electron transitions has been also observed. One can see, that upon 594 nm excitation directly in the Nd3þ ions the ratio of the intensity emission of the nonresonant 4F3/2 / 4I11/2 transition to the 4F3/ 4 2 / I13/2 transition is higher than upon 266 nm and 330 nm excitation. Fig. 6 shows the emission decay curves of GdVO4:1% Bi3þ, x% Nd3þ (x ¼ 1, 5, 7). The decay curves of Nd3þ emission were recorded at lem ¼ 1064 nm and upon 330 nm excitation. In the case of doping with 1% Nd3þ ions, the decay profile exhibit a single exponential character with the lifetime of 75 ms, which value is close to the decay time for YVO4:1% Nd3þ annealed at 800 C [19]. An increase of Nd3þ ions concentration results in more nonexponential behaviour of the decay curves and also the decay time shortening
3
Fig. 4. Impact of Nd3þ dopant concentration on emission spectra of the GdVO4: 1% Bi3þ, x% Nd3þ (x ¼ 1, 5, 7) upon lexc ¼ 330 nm.
Fig. 5. Impact of the excitation wavelength on emission spectra of the GdVO4: 1%Bi3þ, 1%Nd3þ upon lexc ¼ 266 nm, 330 nm and 594 nm, respectively.
from 14.6 ms to 8.9 ms (see Table 2). Observed behaviour is probably due to the presence of cross-relaxation (CR) processes leading to an emission concentration quenching [19,20]. Additionally, upon 266 nm excitation, the lifetime of Nd3þ ions is longer for GdVO4:1% Bi3þ, 1% Nd3þ than for GdVO4:1% Nd3þ. The elongation of the lifetime may be associated with the presence of an intermediate energy level, such as a ligand-to-metal charge transfer state (LMCT) that mediates in the energy transfer mechanism and affects the lifetime of Nd3þ ions strongly, mainly in a near-resonance. The influence of the presence of the intramolecular energy transfer on lifetime was described in the works of Ferreira et al. and Matos et al. [21,22]. It is known that silicon-based solar cells have a good response and conversion efficiency in the range of 500e1100 nm but the response of blue range and UV for wavelengths less than 500 nm is deteriorated obviously. The GdVO4:Bi3þ, Nd3þ nanophosphor displays a broad and intense absorption band in the range of 200e400 nm and can utilize a solar spectrum efficiently in the UV
Please cite this article in press as: K. Lenczewska, et al., Near-UV sensitized NIR emission in Nd3þ and Bi3þ co-doped GdVO4 phosphors, Optical Materials (2017), http://dx.doi.org/10.1016/j.optmat.2017.04.015
4
K. Lenczewska et al. / Optical Materials xxx (2017) 1e4
between Bi3þ - V5þ metal-to-metal CT and Nd3þ ions. The analysis of emission spectra has revealed the presence of reabsorption processes in case of both dopants, what may be explained by the spectral overlap between their absorption and emission bands. Furthermore, the lifetime shortening for the higher concentration of Nd3þ ions was due to the emission concentration quenching. The presence of an intermediate energy level, such as the ligand-tometal charge transfer (LMCT) state has affected the prolongation of the luminescence lifetime of Nd3þ ions upon the near-UV excitation. Additionally, the near-UV excitation has enhanced the 4F3/ 4 3þ ions. Taking into account all the results, 2 / I9/2 transition of Nd it was confirmed that the GdVO4 co-doped with Bi3þ and Nd3þ nanophosphor is a promising candidate for downconverting luminescent concentrators for the next generation silicon-based solar cells. Acknowledgements Fig. 6. The decay curves of the Nd3þ luminescence at lem ¼ 1064 nm and lexc ¼ 330 nm for the GdVO4: 1%Bi3þ, x%Nd3þ (x ¼ 1, 5, 7) annealed at 800 C.
Table 2 Concentration and excitation wavelength dependence of GdVO4: Bi3þ, Nd3þ lifetime (ms). Samples
1% 1% 1% 1%
Nd3þ Bi3þ, 1% Nd3þ Bi3þ, 5% Nd3þ Bi3þ, 7% Nd3þ
Excitation wavelength 266 nm
330 nm
594 nm
70.9 81.5 14.1 9
e 75 14.6 8.9
79.9 76.3 14.2 8.5
part and converts efficiently UV into NIR due to the effective energy transfer from the (O2 e V5þ) CT and the (Bi3þ e V5þ) charge transfer states to Nd3þ ions. Therefore, this material may have a feasibility of potential application in enhancing the efficiency of the Si-based solar cells. 4. Conclusions Near-UV sensitized NIR emission has been demonstrated in the nanocrystals of GdVO4 co-doped with Nd3þ and Bi3þ ions. The nanocrystals in various co-dopant concentrations were prepared by a microwave-assisted hydrothermal method. In the GdVO4:Bi3þ, Nd3þ, the visible and the NIR emission upon the excitation of O2 e V5þ or V5þ e Bi3þ charge transfer state have been observed. Yellowgreen luminescence of the phosphors due to doping with Bi3þ ions upon the near-UV excitation was recorded as a result of the Bi3þ V5þ metal-to-metal CT state and the near-infrared luminescence of Nd3þ ions (activator) was observed through the energy transfer
The autors are thankful to PhD Adam Strze˛ p for help in luminescence measurements. References [1] T. Jensen, V.G. Ostroumov, J.-P. Meyn, G. Huber, A.I. Zagumennyi, I.A. Shcherbakov, Appl. Phys. B 58 (1994) 373e379. [2] J.C.G. Bunzli, S.V. Eliseeva, J. Rare Earths 28 (2010) 824e842. [3] C.A. Primmerman, Linc. Lab. J. 12 (2000) 3e32. z-Delgado, M. Misiak, A. Bednarkiewicz, A.S. Vanetsev, [4] B. del Rosal, A. Pere Y. Orlovskii, D.J. Jovanovi c, M.D. Drami canin, U. Rocha, K. Upendra Kumar, C. Jacinto, E. Navarro, E.M. Rodríguez, M. Pedroni, A. Speghini, G.A. Hirata, I.R. Martín, D. Jaque, J. Appl. Phys. 118 (2015) 143104. [5] X. Huang, S. Han, W. Huang, X. Liu, Chem. Soc. Rev. 42 (2013) 173e201. [6] T. Minami, T. Miyata, Y. Suzuki, Y. Mochizuki, Thin Solid Films 469 (2004) 65e69. [7] G. Blasse, A. Bril, J. Chem. Phys. 48 (1968) 217e222. [8] G.X. Liu, R. Zhang, Q.L. Xiao, S.Y. Zou, W.F. Peng, L.W. Cao, J.X. Meng, Opt. Mater 34 (2011) 313e316. [9] P.K. Shahi, P. Singh, S.B. Rai, A. Bahadur, Inorg. Chem. 55 (2016) 1535e1541. [10] Q.L. Xiao, J.X. Meng, J.R. Qiu, J. Alloys Compd. 574 (2013) 600e603. [11] A.S. Vanetsev, O.M. Gaitko, I.G. Chuvashova, M.N. Sokolov, Yu.D. Tret’yakov, Dokl. Chem. 441 (2011) 325e329. [12] R.D. Shannon, Acta Crystallogr. A32 (1976) 751e767. [13] P. Scherrer, Nachrichten von der gesellschaft der Wissenschaften zu Gottingen, 1918, pp. 98e100. [14] A.A. Kaminskii, Crystalline Lasers: Physical Processes and Operating Schemes, CRC Press, Boca Raton, 1996. [15] R.K. Datta, J. Electrochem. Soc. 114 (1967) 1057e1063. [16] J. Ghamri, H. Baussart, M. Le Bras, Jean-Marie Leroy, J. Phys. Chem. Solids 50 (1989) 1237e1244. [17] R. Reisfeld, Struct. Bond. 30 (1976) 65e97. [18] K. Lenczewska, M. Stefanski, D. Hreniak, J. Rare Earths 34 (2016) 837e842. [19] R.J. Wiglusz, L. Marciniak, R. Pazik, W. Strek, Cryst. Growth Des. 14 (2014) 5512e5520. [20] V. Ostroumov, T. Jensen, J.-P. Meyn, G. Huber, M.A. Noginov, J. Opt. Soc. Am. B 15 (1998) 1052e1060. [21] R.A.S. Ferreira, M. Nolasco, A.C. Roma, R.L. Longo, O.L. Malta, L.D. Carlos, Chem. Eur. J. 18 (2012) 12130e12139. [22] M.G. Matos, L.A. Rocha, E.J. Nassar, M. Verelst, Opt. Mater 62 (2016) 12e18.
Please cite this article in press as: K. Lenczewska, et al., Near-UV sensitized NIR emission in Nd3þ and Bi3þ co-doped GdVO4 phosphors, Optical Materials (2017), http://dx.doi.org/10.1016/j.optmat.2017.04.015