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Dy3+ co-doped zinc phosphate glasses
Journal of Luminescence 161 (2015) 142–146
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Down-shifting by energy transfer in Tb3 þ /Dy3 þ co-doped zinc phosphate glasses U. Caldiño a,n, G. Muñoz H. a, I. Camarillo a, A. Speghini b,c, M. Bettinelli b a
Departamento de Física, Universidad Autónoma Metropolitana-Iztapalapa, P.O. Box 55-534, México, D.F. 09340, Mexico Dipartimento di Biotecnologie, Università di Verona, and INSTM, UdR Verona, Strada Le Grazie 15, I-37314 Verona, Italy c IFAC CNR, Nello Carrara Institute of Applied Physics, MDF Lab, I-50019 Sesto Fiorentino, FI, Italy b
art ic l e i nf o
a b s t r a c t
Article history: Received 15 September 2014 Received in revised form 30 December 2014 Accepted 3 January 2015 Available online 12 January 2015
An optical spectroscopy investigation of zinc phosphate glasses activated with Tb3 þ /Dy3 þ ions is carried out through photoluminescence spectra and decay time measurements. The emission color can be adjusted from yellow–green light, with CIE1931 chromaticity coordinates (0.36,0.44), toward the white light region (0.35,0.39) by decreasing the Tb3 þ content from 1.0 to 0.1 mol% of Tb(PO3)3 upon Dy3 þ excitation at 423 nm. Such visible region luminescence is generated by 5D4-7F5 and 5D4-7F3 emissions of Tb3 þ in addition to 4I15/2-6H15/2, 4F9/2-6H15/2, 4F9/2-6H13/2 and 4F9/2-6H11/2 emissions of Dy3 þ , so that Tb3 þ emission is sensitized by Dy3 þ through a non-radiative resonant energy transfer. A dominant 5D4-7F5 green emission is observed in detriment of the 5D3-7FJ blue emissions upon 282 nm excitation, as well as an extended excitation range (280–500 nm), due to multiple Dy3 þ and Tb3 þ transitions, which might contribute to enhance the spectral response of solar photovoltaic cells by down-shifting of the incident solar spectrum. & 2015 Elsevier B.V. All rights reserved.
Keywords: Zinc phosphate glasses Photoluminescence Non-radiative energy transfer Dy3 þ Tb3 þ
1. Introduction Phosphate glasses are very versatile materials as hosts for luminescent trivalent lanthanide ions mainly by their isotropic refractive index, wide transparency range, low propagation losses and ease to prepare, so that they can be used for a great variety of optical devices. In particular, (Zn(PO3)2) zinc phosphate glasses have been used in solid state laser hosts, optical waveguides, novel glass– polymer composite materials, optical amplifiers, long lasting phosphors, optical displays, as solders and welds between metallic parts and glassy in electronic circuits and television tubes because their thermal expansion coefficients are similar to those of many metals [1]. All these applications are mainly possible to the zinc, since it acts as a network modifier/former, and it imparts good chemical durability, wide glass-forming compositional range, and especially low glass transition temperatures. Additionally, near UV–blue lightabsorbing activators can be incorporated in zinc phosphate glasses for applications in both liquid crystal monitor screens and white light-emitting diodes with near UV–blue LED chip considering that zinc phosphate glasses possess a high transparence in the near UV–blue region. Among such activators, Dy3 þ ions can be activated by near UV–blue light, acting as good sensitizers and transferring a part of their energy to other activator ions such as Tb3 þ [2–7].
n
Corresponding author. E-mail address: [email protected] (U. Caldiño).
http://dx.doi.org/10.1016/j.jlumin.2015.01.001 0022-2313/& 2015 Elsevier B.V. All rights reserved.
Specifically, part of the energy in the 4F9/2 level of Dy3 þ is transferred to the 5D4 level of Tb3 þ by resonance between the two energy levels, so that the population in the terbium 5D4 level is increased, resulting in enhancement of the Tb3 þ luminescence green. The population in the 5D4 level is also enhanced in detriment of that in the terbium 5D3 level through cross-relaxation between Dy3 þ and Tb3 þ [5–7], which is favored by increasing the dopant concentration. Such cross relaxation mechanism induces an enhancement of the luminescence green from the terbium 5D4 level at the expense of the luminescence violet–blue from the terbium 5D3 level, transforming the light violet– blue into green, which is more sensitive to human eyes. According to these perspectives and considering the importance of finding efficient luminescent materials for the design of optical devices based on zinc phosphate, in this manuscript we report a spectroscopic investigation of Zn(PO3)2 glasses activated with Dy3 þ /Tb3 þ ions. Non-radiative resonant energy transfer between Dy3 þ and Tb3 þ was observed in the glasses and spectroscopic analysis of this energy transfer has been complemented with emission and decay time data recorded for Tb3 þ -and Dy3 þ -singly doped zinc phosphate glasses.
2. Experimental The molar composition of the zinc phosphate glasses studied was 99.0 Zn(PO3)2–1.0 Tb(PO3)3, 99.5 Zn(PO3)2–0.5 Dy(PO3)3, 99.0 Zn(PO3)2–1.0 Dy(PO3)3, 95.0 Zn(PO3)2–5.0 Dy(PO3)3, 99.4 Zn(PO3)2– 0.1 Tb(PO3)3–0.5 Dy(PO3)3 and 98.5 Zn(PO3)2–1.0 Tb(PO3)3–0.5 Dy
U. Caldiño et al. / Journal of Luminescence 161 (2015) 142–146
Fig. 1. Emission spectra of the ZP1Tb0.5Dy and ZP0.1Tb0.5Dy glasses excited at 423 nm. Spectra have been normalized to the 4F9/2-6H13/2 emission intensity of Dy3 þ .
(PO3)3, which will be referred as ZP1Tb, ZP0.5Dy, ZP1Dy, ZP5Dy ZP0.1Tb0.5Dy and ZP1Tb0.5Dy, respectively. These glasses were prepared by mixing appropriate quantities of ZnO (Aldrich 99þ %), NH4H2PO4 (Carlo Erba RPE), Tb4O7 (Aldrich 99.999%) and Dy2O3 (Aldrich 99.99þ %) in a sintered alumina crucible, and melting the composition for 2 h at 1250 1C. The melts were quenched onto a copper plate. The glass yield is around 99%, so that the loss of components during the heat treatment is negligible, and therefore the phosphor volatilization during melting can be neglected. The glasses resulted to be fully amorphous and transparent in bubble-shape of 7–8 mm in diameter with a maximum thickness of 4 mm, so that cut and polish were not necessary for photoluminescence measurements. However, they were annealed for 12 h at 350 1C to obtain thermal and structural stability. The photoluminescence spectra were recorded by means of a Horiba Jobin-Yvon Fluorolog 3-22 spectrofluorometer working with a 150 W ozone-free Xe lamp in the steady state mode or with a pulsed Xe lamp for decay time measurements. Decay time curves were recorded in the phosphorescence mode using a delay time of 0.01 ms after the excitation pulse (3 μs half-width) and a 5 ms sample window. All the measurements were carried out at room temperature.
3. Results and discussion Emission spectra of the ZP1Tb0.5Dy and ZP0.1Tb0.5Dy glasses excited into the Dy3 þ (6H15/2-4G11/2) band at 423 nm are displayed in Fig. 1. Both spectra have been normalized to the 4F9/2-6H13/2 emission intensity of Dy3 þ . Excitation at 423 nm is coupled well with the emission of InGaN-based LEDs [8]. At this wavelength (23,641 cm 1) the terbium cannot be excited [9], as it can be visualized from the approximated scheme of Tb3 þ energy levels portrayed in Fig. 2. However, 5D4-7F5 and 5D4-7F3 emissions of Tb3 þ are observed in addition to 4I15/2-6H15/2, 4F9/2-6H15/2, 4F9/ 6 4 6 4 6 6 3þ . 2- H13/2, F9/2- H11/2 and F9/2- H9/2, F11/2 emissions of Dy 4 6 The dysprosium I15/2- H15/2 emission is quite very weak because there exists a multiphonon relaxation from the 4I15/2 level to the next lower lying 4F9/2 level. Such multiphonon relaxation is entirely active since the energy gap between the 4I15/2 and 4F9/2 levels is only 1000 cm 1, as it can be visualized in Fig. 2, which shows an approximated scheme of Dy3 þ energy levels. This energy level scheme was determined from spectra of excitation (4K13/2, 4K15/2, 4 M15/2 þ 6P7/2, 4I11/2, 6P3/2,5/2, 4K17/2 þ 4M19/2,21/2 þ 4I13/2 þ 4F7/2, 4G11/2, 4 I15/2 and 4F9/2 levels), emission (6H13/2 and 6H15/2 levels) and
143
Fig. 2. Energy level scheme showing the transitions involved in: (i) Dy3 þ -Tb3 þ non-radiative resonant energy transfer (ET); (ii), (iii) and (iv) cross-relaxations of Dy3 þ ; (v) cross-relaxation of Tb3 þ ; (vi) cross-relaxation of Tb3 þ –Dy3 þ ; and (vii) Tb3 þ -Dy3 þ non-radiative energy back transfer (EBT).
Fig. 3. Emission and excitation spectra of the ZP0.5Dy glass. The excitation spectrum (thin solid line) was monitored at 571 nm. The emission spectrum (thick solid line) was recorded with 348 nm excitation. The inset shows the absorption spectrum of the ZP0.5Dy glass in the IR region.
absorption (6F3/2,1/2, 6F5/2, 6H5/2 þ 6F7/2, 6H7/2 þ 6F9/2, 6H9/2 þ 6F11/2 and 6H11/2 levels) recorded for the ZP0.5Dy glass (Fig. 3). The global emission generated by the ZP1Tb0.5Dy and ZP0.1Tb0.5Dy glasses (excited at 423 nm) was characterized by its chromaticity coordinates in a CIE1931 diagram. In the ZP1Tb0.5Dy glass the coordinates resulted to be x¼0.36 and y¼ 0.44, which are in the yellow–green light region (Fig. 4). Such coordinates are displaced toward the region of white light by decreasing the Tb3þ content, so that in the ZP0.1Tb0.5Dy glass the coordinates are x¼0.35 and y¼0.39 (Fig. 4). The two Tb3 þ /Dy3 þ ratios investigated are only a proof-ofprinciple to evidence the shift in the chromaticity coordinates, and therefore, the effect of other different Tb3þ /Dy3 þ ratios on such chromaticity coordinate shift could be studied in detail in the future to achieve white light emission. The ZP1Tb0.5Dy glass excitation spectrum monitored at 540 nm, within the terbium 5D4-7F5 emission and where Dy3 þ does not emit, is shown in Fig. 5. The excitation spectrum monitored at 568 nm, within the dysprosium 4F9/2-6H13/2 emission and where Tb3 þ does not emit, is also shown in Fig. 5 for easier identification of Dy3 þ excitation bands. The Tb3 þ excitation spectrum exhibits in 8 8 9 9 addition to terbium 4f -4f transitions, also dysprosium 4f -4f 3þ transitions. Thus, in the ZP1Tb0.5Dy glass the Tb excitation through Dy3þ (Fig. 1), and the presence of Dy3þ (excitation) absorption bands in the excitation spectrum of Tb3þ (Fig. 5), show clear evidence of an energy transfer from Dy3þ to Tb3þ . Such energy transfer is favoured by the overlap between the dysprosium 4F9/2-6H15/2 emission and terbium 7F6-5D4 (excitation) absorption, as it can be appreciated from
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Fig. 7. Decay time curves of the dysprosium 4F9/2-6H13/2 emission (λem ¼ 568 nm) in the ZP0.5Dy and ZP1Tb0.5Dy glasses excited at 423 nm. Fig. 4. Chromaticity coordinates of the emissions observed in the (a) ZP1Tb0.5Dy and (b) ZP0.1Tb0.5Dy glasses excited at 423 nm in CIE1931 diagram.
Fig. 5. Excitation spectra of the ZP1Tb0.5Dy glass monitored at 540 and 568 nm. Spectra have been normalized to the 6H15/2-4I15/2 transition intensity of Dy3 þ .
7
reveals that the Dy3 þ -Tb3 þ energy transfer is occurring predominantly through the mechanism (i). Decay time curve measurements of the dysprosium 4F9/2 level in the ZP0.5Dy and ZP1Tb0.5Dy glasses upon 423 nm excitation were performed monitoring the dysprosium 4F9/2-6H13/2 emission at 568 nm. In both glasses the 4F9/2 level emission decay curve is not exponential, so that the effective decay time, τf , was obtained through the following relation: Z Z τf ¼ tIðtÞdt= IðtÞdt: ð1Þ A τf value of 1.28 ms is obtained for the 4F9/2 level decay time in the ZP0.5Dy glass, while a faster decay of 1.13 ms is obtained for the ZP1Tb0.5Dy glass. Such faster decay time of Dy3 þ in the presence of Tb3 þ can be explained with the non-radiative resonant energy transfer from Dy3 þ to Tb3 þ described by the mechanism (i). The 4 F9/2-6H13/2 emission decay time curves recorded for the two glasses are shown in Fig. 7. The non-exponential decay for the 4F9/2 emission has been attributed, in the case of the Dy3 þ singly-doped glass, to cross-relaxation processes from Dy3 þ ions excited up to the 4 F9/2 level to nearby Dy3 þ ions in the 6H15/2 ground state [10]. Additionally, the lifetime of the 4F9/2 level was found to decrease with increase of dysprosium content: 1.13 ms (ZP0.5Dy), 1.01 ms (ZP1Dy) and 0.69 ms (ZP5Dy). Decay time curves of the 4F9/2-6H13/ 2 emission (λem ¼ 568 nm) recorded for the ZP0.5Dy, ZP1Dy and ZP5Dy glasses excited at 423 nm are shown in Fig. 8. Thus, the nonexponential nature of the 4F9/2 level luminescence decay along with a decay time decreasing with increase of dysprosium concentration could be due to the enhancement of energy transfer between Dy3 þ ions via cross-relaxations [11]. Such cross-relaxation channels could be (see energy level diagram in Fig. 2) [10,11]: 4 F9=2 þ 6 H15=2 - 6 H9=2 ; 6 F11=2 Þ þ 6 F3=2 ; 6 F1=2 Þ; ðiiÞ 4
F9=2 þ 6 H15=2 - 6 H5=2 ; 6 F7=2 Þ þ 6 H7=2 ; 6 F9=2 Þ;
ðiiiÞ
4
F9=2 þ 6 H15=2 - 6 F3=2 ; 6 F1=2 Þ þ ð6 H9=2 ; 6 F11=2 Þ:
ðivÞ
5
Fig. 6. Overlap region between terbium F6- D4 (excitation) absorption [6] (dotted curve) and dysprosium 4F9/2-6H15/2 emission (solid curve).
the spectra shown in Fig. 6. Thus, the Dy3þ -Tb3þ energy transfer could be (see energy level diagram in Fig. 2): 4
h
i
h
3þ
F9=2 Dy3 þ þ 7 F6 Tb
i
h
i
h
3þ
-6 H15=2 Dy3 þ þ 5 D4 Tb
i
:
ðiÞ
From the emission spectra displayed in Fig. 1 an enhancement of the terbium 5D4-7F5 emission at the expense of a weakening of the dysprosium 4F9/2-6H15/2 emission by increasing the Tb3 þ content from 0.1 to 1.0 mol% of Tb(PO3)3 can be noticed. This fact
The available data do not allow discrimination among these three possible mechanisms, which leaves open future work on Dy3 þ ions in oxide glasses. The η energy transfer efficiency from Dy3 þ to Tb3 þ was estimated from the dysprosium 4F9/2 level lifetime data in the presence ðτDy Þ and absence ðτoDy Þ of Tb3 þ through the following expression [12]:
η ¼ 1
τDy : τoDy
ð2Þ
U. Caldiño et al. / Journal of Luminescence 161 (2015) 142–146
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Such enhancement of the 5D4-7FJ emissions could be attributed to a cross-relaxation process between Tb3 þ and Dy3 þ given by [4]: h i h i h i h i 3þ 3þ 5 þ 6 H15=2 Dy3 þ -6 D4 Tb þ 6 H11=2 Dy3 þ ; ðviÞ D3 Tb
Fig. 8. Decay time curves of the dysprosium 4F9/2-6H13/2 emission (λem ¼568 nm) recorded for the ZP0.5Dy, ZP1Dy and ZP5Dy glasses excited at 423 nm.
Fig. 9. Emission spectra of the ZP1Tb0.5Dy and ZP1Tb glasses excited at 282 nm. Spectra have been normalized to the 5D3-7F4 emission intensity of Tb3 þ .
Using the lifetime data obtained for τDy (1.28 ms) and τ oDy (1.13 ms), the energy transfer efficiency resulted to be 0.12. It could be possible that the efficiency of Dy3 þ -Tb3 þ energy transfer is affected by the cross-relaxation processes (ii), (iii) or (iv) occurring between Dy3 þ ions. This energy transfer efficiency is consistent with that (0.16) reported for lead borate glass containing 0.5Dy2O3 and1.5Tb2O3 [4,13]. Emission spectra of the ZP1Tb and ZP1Tb0.5Dy glasses excited into the Tb3 þ (7F6-5I8) band at 282 nm are shown in Fig. 9. Excitation at 282 nm fits to the requirements of AlGaN-based LEDs [14]. At this wavelength (35,461 cm 1) the Dy3 þ cannot be excited, as it can be appreciated from the approximated scheme of Dy3 þ energy levels portrayed in Fig. 2. After excitation of Tb3 þ at 282 nm, the 5D3 level is populated by fast non-radiative decays from the 5I8 level (Fig. 2), and then emissions 5D3-7FJ (J¼2, 3, 4, 5, and 6) and 5 D4-7FJ (J¼ 3, 4, 5, and 6) of Tb3 þ are observed. Emissions from the 5 D3 level are due to a slow non-radiative relaxation from the 5D3 to the 5D4 level, which is promoted by excitation from the 7F6 to the 7 F0 level through a cross-relaxation process [15] (Fig. 2): 5
D3 þ 7 F6 -5 D4 þ 7 F0 ;
since the energy difference between terbium 5D3 and 5D4 levels (5927 cm 1) is almost the same as that between dysprosium 6H11/2 and 6H15/2 levels (6000 cm 1), see energy level diagram displayed in Fig. 2. Therefore, the ZP1Tb0.5Dy glass exhibits a dominant green emission at 545 nm in detriment of the 5D3-7FJ blue emissions upon 282 nm excitation, in addition to an extended excitation range from 280 to 500 nm of the Dy3 þ yellow and Tb3 þ green luminescence (Fig. 5), due to multiple electronic transitions of Dy3 þ and Tb3 þ . These spectroscopic features might contribute to enhance the spectral response of solar photovoltaic cells by down-shifting of the incident solar spectrum. The present UV-green conversion phosphor could thereby be placed as a layer on the front side of a Si or Si/Ge solar cell, allowing down-converted photons to be absorbed without interfering with the active material, which could lead to a great promise for enhancement of solar cell efficiency. Improved solar cells UV protected through a phosphor converting the UV radiation to visible can have a wide range of applications that require harvesting of UV photons [16]. It can be noticed that the energy level of Dy3 þ (4F9/2) is a little higher ( 400 cm 1) than that of Tb3 þ (5D4), which makes the energy back transfer from Tb3 þ to Dy3 þ inefficient (Fig. 2). However, the emission spectrum recorded for the codoped glass, upon excitation of Tb3 þ at 282 nm, shows a small shoulder at 571 nm (Fig. 9), which is due to the dysprosium 4F9/2-6H13/2 emission. Thus, excitation to the dysprosium 4F9/2 level could be through energy transfer from Tb3 þ (5D3) to Dy3 þ (4F7/2), followed by fast non-radiative decays to the dysprosium 4F9/2 emitting level, as indicated in Fig. 2. The mechanism involved in this energy transfer could then be: h i h i h i h i 3þ 3þ 5 þ 6 H15=2 Dy3 þ -6 F6 Tb þ 6 F7=2 Dy3 þ : ðviiÞ D3 Tb In fact, such energy transfer is expected to occur considering that there exists a overlapping between the terbium 5D3-7F6 emission and the dysprosium 6H15/2-4F7/2 (excitation) absorption (Fig. 10). In order to obtain a clear evidence on the energy back transfer taking place from Tb3 þ to Dy3 þ , the decay time curves of the terbium 5D3 level in the ZP1Tb and ZP1Tb0.5Dy glasses excited at 282 nm were recorded monitoring the (5D3-7F4) 435 nm emission of Tb3 þ (Fig. 11). In both glasses the 5D3 level emission decay has a non-exponential shape, so that the τf lifetime was obtained through Eq. (1). Thus, a τf value of 0.95 ms for the 5D3 level emission decay in the Tb3 þ singly-doped glass, and a faster decay of 0.72 ms in the Tb3 þ and Dy3 þ codoped glass are obtained. Such shortening of the Tb3 þ lifetime in the presence of Dy3 þ reveals the Tb3 þ -Dy3 þ non-radiative energy back transfer described by the mechanism (vii). The η energy transfer efficiency from Tb3 þ to Dy3 þ was estimated from Eq. (2) using the terbium 5D3 level lifetime data in the presence ðτTb Þ and absence ðτoTb Þ of Dy3 þ . The energy transfer efficiency resulted to be 0.24.
ðvÞ 5
5
1
since the energy gap between D3 and D4 levels (5927 cm ) is similar to that between 7F0 and 7F6 levels (5700 cm 1). In Fig. 2 the Tb3 þ energy level scheme was obtained from emission spectrum shown in Fig. 9 (7F6, 7F5, 7F4, 7F3 and 7F2 levels), excitation spectrum previously reported [12] (5D4, 5L10 þ 5G6 þ 5D3, 5L9 þ 5D2 þ 5G5, 5 L7,8 þ 5G3, 5H7 þ 5D1, 5H5,6 and 5I8 þ 5F4,5 þ 5H4 levels) and levels involved in the (v) cross-relaxation process (7F0 level). From Fig. 9 it can be noticed that the emission intensity from the 5D4 level in the codoped glass increases with respect to that from the 5D3 level.
4. Conclusions An optical spectroscopy investigation of down-shifting by energy transfer in Tb3 þ /Dy3 þ co-doped zinc phosphate glasses is performed through photoluminescence spectra and decay time measurements. The visible region luminescence is generated by 5D4-7F5 and 5 D4-7F3 emissions of Tb3 þ in addition to 4I15/2-6H15/2, 4F9/2-6H15/2, 4 F9/2-6H13/2 and 4F9/2-6H11/2 emissions of Dy3þ , Tb3þ emission being sensitized by Dy3þ through a non-radiative resonant energy
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light region (0.35,0.39) by decreasing the Tb3þ content from 1.0 to 0.1 mol% of Tb(PO3)3 after Dy3þ excitation at 423 nm. The glass codoped with 1.0 mol% of Tb(PO3)3 and 0.5 mol% of Dy(PO3)3 exhibits a dominant terbium 5D4-7F5 green emission at the expense of the terbium 5D3-7FJ blue emissions upon 282 nm excitation, as well as an extended excitation range from 280 to 500 nm, due to multiple electronic transitions of Dy3 þ and Tb3þ . These spectroscopic findings might contribute to enhance the spectral response of solar photovoltaic cells by down-shifting of the incident solar spectrum.
Acknowledgments This work was supported by the CONACYT-CNR bilateral agreement under Project contract 173855. Fig. 10. Overlap region between terbium 5D3-7F6 emission (dotted curve) and dysprosium 6H15/2-4F7/2 (excitation) absorption (solid curve).
Fig. 11. Decay time curves of the terbium 5D3-7F4 emission (λem ¼ 435 nm) in the ZP1Tb and ZP1Tb0.5Dy glasses excited at 282 nm.
transfer. Such luminescence can be adjusted from yellow–green light, with CIE1931 chromaticity coordinates (0.36,0.44), toward the white
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Down-shifting by energy transfer in Tb3 þ /Dy3 þ co-doped zinc phosphate glasses U. Caldiño a,n, G. Muñoz H. a, I. Camarillo a, A. Speghini b,c, M. Bettinelli b a
Departamento de Física, Universidad Autónoma Metropolitana-Iztapalapa, P.O. Box 55-534, México, D.F. 09340, Mexico Dipartimento di Biotecnologie, Università di Verona, and INSTM, UdR Verona, Strada Le Grazie 15, I-37314 Verona, Italy c IFAC CNR, Nello Carrara Institute of Applied Physics, MDF Lab, I-50019 Sesto Fiorentino, FI, Italy b
art ic l e i nf o
a b s t r a c t
Article history: Received 15 September 2014 Received in revised form 30 December 2014 Accepted 3 January 2015 Available online 12 January 2015
An optical spectroscopy investigation of zinc phosphate glasses activated with Tb3 þ /Dy3 þ ions is carried out through photoluminescence spectra and decay time measurements. The emission color can be adjusted from yellow–green light, with CIE1931 chromaticity coordinates (0.36,0.44), toward the white light region (0.35,0.39) by decreasing the Tb3 þ content from 1.0 to 0.1 mol% of Tb(PO3)3 upon Dy3 þ excitation at 423 nm. Such visible region luminescence is generated by 5D4-7F5 and 5D4-7F3 emissions of Tb3 þ in addition to 4I15/2-6H15/2, 4F9/2-6H15/2, 4F9/2-6H13/2 and 4F9/2-6H11/2 emissions of Dy3 þ , so that Tb3 þ emission is sensitized by Dy3 þ through a non-radiative resonant energy transfer. A dominant 5D4-7F5 green emission is observed in detriment of the 5D3-7FJ blue emissions upon 282 nm excitation, as well as an extended excitation range (280–500 nm), due to multiple Dy3 þ and Tb3 þ transitions, which might contribute to enhance the spectral response of solar photovoltaic cells by down-shifting of the incident solar spectrum. & 2015 Elsevier B.V. All rights reserved.
Keywords: Zinc phosphate glasses Photoluminescence Non-radiative energy transfer Dy3 þ Tb3 þ
1. Introduction Phosphate glasses are very versatile materials as hosts for luminescent trivalent lanthanide ions mainly by their isotropic refractive index, wide transparency range, low propagation losses and ease to prepare, so that they can be used for a great variety of optical devices. In particular, (Zn(PO3)2) zinc phosphate glasses have been used in solid state laser hosts, optical waveguides, novel glass– polymer composite materials, optical amplifiers, long lasting phosphors, optical displays, as solders and welds between metallic parts and glassy in electronic circuits and television tubes because their thermal expansion coefficients are similar to those of many metals [1]. All these applications are mainly possible to the zinc, since it acts as a network modifier/former, and it imparts good chemical durability, wide glass-forming compositional range, and especially low glass transition temperatures. Additionally, near UV–blue lightabsorbing activators can be incorporated in zinc phosphate glasses for applications in both liquid crystal monitor screens and white light-emitting diodes with near UV–blue LED chip considering that zinc phosphate glasses possess a high transparence in the near UV–blue region. Among such activators, Dy3 þ ions can be activated by near UV–blue light, acting as good sensitizers and transferring a part of their energy to other activator ions such as Tb3 þ [2–7].
n
Corresponding author. E-mail address: [email protected] (U. Caldiño).
http://dx.doi.org/10.1016/j.jlumin.2015.01.001 0022-2313/& 2015 Elsevier B.V. All rights reserved.
Specifically, part of the energy in the 4F9/2 level of Dy3 þ is transferred to the 5D4 level of Tb3 þ by resonance between the two energy levels, so that the population in the terbium 5D4 level is increased, resulting in enhancement of the Tb3 þ luminescence green. The population in the 5D4 level is also enhanced in detriment of that in the terbium 5D3 level through cross-relaxation between Dy3 þ and Tb3 þ [5–7], which is favored by increasing the dopant concentration. Such cross relaxation mechanism induces an enhancement of the luminescence green from the terbium 5D4 level at the expense of the luminescence violet–blue from the terbium 5D3 level, transforming the light violet– blue into green, which is more sensitive to human eyes. According to these perspectives and considering the importance of finding efficient luminescent materials for the design of optical devices based on zinc phosphate, in this manuscript we report a spectroscopic investigation of Zn(PO3)2 glasses activated with Dy3 þ /Tb3 þ ions. Non-radiative resonant energy transfer between Dy3 þ and Tb3 þ was observed in the glasses and spectroscopic analysis of this energy transfer has been complemented with emission and decay time data recorded for Tb3 þ -and Dy3 þ -singly doped zinc phosphate glasses.
2. Experimental The molar composition of the zinc phosphate glasses studied was 99.0 Zn(PO3)2–1.0 Tb(PO3)3, 99.5 Zn(PO3)2–0.5 Dy(PO3)3, 99.0 Zn(PO3)2–1.0 Dy(PO3)3, 95.0 Zn(PO3)2–5.0 Dy(PO3)3, 99.4 Zn(PO3)2– 0.1 Tb(PO3)3–0.5 Dy(PO3)3 and 98.5 Zn(PO3)2–1.0 Tb(PO3)3–0.5 Dy
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Fig. 1. Emission spectra of the ZP1Tb0.5Dy and ZP0.1Tb0.5Dy glasses excited at 423 nm. Spectra have been normalized to the 4F9/2-6H13/2 emission intensity of Dy3 þ .
(PO3)3, which will be referred as ZP1Tb, ZP0.5Dy, ZP1Dy, ZP5Dy ZP0.1Tb0.5Dy and ZP1Tb0.5Dy, respectively. These glasses were prepared by mixing appropriate quantities of ZnO (Aldrich 99þ %), NH4H2PO4 (Carlo Erba RPE), Tb4O7 (Aldrich 99.999%) and Dy2O3 (Aldrich 99.99þ %) in a sintered alumina crucible, and melting the composition for 2 h at 1250 1C. The melts were quenched onto a copper plate. The glass yield is around 99%, so that the loss of components during the heat treatment is negligible, and therefore the phosphor volatilization during melting can be neglected. The glasses resulted to be fully amorphous and transparent in bubble-shape of 7–8 mm in diameter with a maximum thickness of 4 mm, so that cut and polish were not necessary for photoluminescence measurements. However, they were annealed for 12 h at 350 1C to obtain thermal and structural stability. The photoluminescence spectra were recorded by means of a Horiba Jobin-Yvon Fluorolog 3-22 spectrofluorometer working with a 150 W ozone-free Xe lamp in the steady state mode or with a pulsed Xe lamp for decay time measurements. Decay time curves were recorded in the phosphorescence mode using a delay time of 0.01 ms after the excitation pulse (3 μs half-width) and a 5 ms sample window. All the measurements were carried out at room temperature.
3. Results and discussion Emission spectra of the ZP1Tb0.5Dy and ZP0.1Tb0.5Dy glasses excited into the Dy3 þ (6H15/2-4G11/2) band at 423 nm are displayed in Fig. 1. Both spectra have been normalized to the 4F9/2-6H13/2 emission intensity of Dy3 þ . Excitation at 423 nm is coupled well with the emission of InGaN-based LEDs [8]. At this wavelength (23,641 cm 1) the terbium cannot be excited [9], as it can be visualized from the approximated scheme of Tb3 þ energy levels portrayed in Fig. 2. However, 5D4-7F5 and 5D4-7F3 emissions of Tb3 þ are observed in addition to 4I15/2-6H15/2, 4F9/2-6H15/2, 4F9/ 6 4 6 4 6 6 3þ . 2- H13/2, F9/2- H11/2 and F9/2- H9/2, F11/2 emissions of Dy 4 6 The dysprosium I15/2- H15/2 emission is quite very weak because there exists a multiphonon relaxation from the 4I15/2 level to the next lower lying 4F9/2 level. Such multiphonon relaxation is entirely active since the energy gap between the 4I15/2 and 4F9/2 levels is only 1000 cm 1, as it can be visualized in Fig. 2, which shows an approximated scheme of Dy3 þ energy levels. This energy level scheme was determined from spectra of excitation (4K13/2, 4K15/2, 4 M15/2 þ 6P7/2, 4I11/2, 6P3/2,5/2, 4K17/2 þ 4M19/2,21/2 þ 4I13/2 þ 4F7/2, 4G11/2, 4 I15/2 and 4F9/2 levels), emission (6H13/2 and 6H15/2 levels) and
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Fig. 2. Energy level scheme showing the transitions involved in: (i) Dy3 þ -Tb3 þ non-radiative resonant energy transfer (ET); (ii), (iii) and (iv) cross-relaxations of Dy3 þ ; (v) cross-relaxation of Tb3 þ ; (vi) cross-relaxation of Tb3 þ –Dy3 þ ; and (vii) Tb3 þ -Dy3 þ non-radiative energy back transfer (EBT).
Fig. 3. Emission and excitation spectra of the ZP0.5Dy glass. The excitation spectrum (thin solid line) was monitored at 571 nm. The emission spectrum (thick solid line) was recorded with 348 nm excitation. The inset shows the absorption spectrum of the ZP0.5Dy glass in the IR region.
absorption (6F3/2,1/2, 6F5/2, 6H5/2 þ 6F7/2, 6H7/2 þ 6F9/2, 6H9/2 þ 6F11/2 and 6H11/2 levels) recorded for the ZP0.5Dy glass (Fig. 3). The global emission generated by the ZP1Tb0.5Dy and ZP0.1Tb0.5Dy glasses (excited at 423 nm) was characterized by its chromaticity coordinates in a CIE1931 diagram. In the ZP1Tb0.5Dy glass the coordinates resulted to be x¼0.36 and y¼ 0.44, which are in the yellow–green light region (Fig. 4). Such coordinates are displaced toward the region of white light by decreasing the Tb3þ content, so that in the ZP0.1Tb0.5Dy glass the coordinates are x¼0.35 and y¼0.39 (Fig. 4). The two Tb3 þ /Dy3 þ ratios investigated are only a proof-ofprinciple to evidence the shift in the chromaticity coordinates, and therefore, the effect of other different Tb3þ /Dy3 þ ratios on such chromaticity coordinate shift could be studied in detail in the future to achieve white light emission. The ZP1Tb0.5Dy glass excitation spectrum monitored at 540 nm, within the terbium 5D4-7F5 emission and where Dy3 þ does not emit, is shown in Fig. 5. The excitation spectrum monitored at 568 nm, within the dysprosium 4F9/2-6H13/2 emission and where Tb3 þ does not emit, is also shown in Fig. 5 for easier identification of Dy3 þ excitation bands. The Tb3 þ excitation spectrum exhibits in 8 8 9 9 addition to terbium 4f -4f transitions, also dysprosium 4f -4f 3þ transitions. Thus, in the ZP1Tb0.5Dy glass the Tb excitation through Dy3þ (Fig. 1), and the presence of Dy3þ (excitation) absorption bands in the excitation spectrum of Tb3þ (Fig. 5), show clear evidence of an energy transfer from Dy3þ to Tb3þ . Such energy transfer is favoured by the overlap between the dysprosium 4F9/2-6H15/2 emission and terbium 7F6-5D4 (excitation) absorption, as it can be appreciated from
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Fig. 7. Decay time curves of the dysprosium 4F9/2-6H13/2 emission (λem ¼ 568 nm) in the ZP0.5Dy and ZP1Tb0.5Dy glasses excited at 423 nm. Fig. 4. Chromaticity coordinates of the emissions observed in the (a) ZP1Tb0.5Dy and (b) ZP0.1Tb0.5Dy glasses excited at 423 nm in CIE1931 diagram.
Fig. 5. Excitation spectra of the ZP1Tb0.5Dy glass monitored at 540 and 568 nm. Spectra have been normalized to the 6H15/2-4I15/2 transition intensity of Dy3 þ .
7
reveals that the Dy3 þ -Tb3 þ energy transfer is occurring predominantly through the mechanism (i). Decay time curve measurements of the dysprosium 4F9/2 level in the ZP0.5Dy and ZP1Tb0.5Dy glasses upon 423 nm excitation were performed monitoring the dysprosium 4F9/2-6H13/2 emission at 568 nm. In both glasses the 4F9/2 level emission decay curve is not exponential, so that the effective decay time, τf , was obtained through the following relation: Z Z τf ¼ tIðtÞdt= IðtÞdt: ð1Þ A τf value of 1.28 ms is obtained for the 4F9/2 level decay time in the ZP0.5Dy glass, while a faster decay of 1.13 ms is obtained for the ZP1Tb0.5Dy glass. Such faster decay time of Dy3 þ in the presence of Tb3 þ can be explained with the non-radiative resonant energy transfer from Dy3 þ to Tb3 þ described by the mechanism (i). The 4 F9/2-6H13/2 emission decay time curves recorded for the two glasses are shown in Fig. 7. The non-exponential decay for the 4F9/2 emission has been attributed, in the case of the Dy3 þ singly-doped glass, to cross-relaxation processes from Dy3 þ ions excited up to the 4 F9/2 level to nearby Dy3 þ ions in the 6H15/2 ground state [10]. Additionally, the lifetime of the 4F9/2 level was found to decrease with increase of dysprosium content: 1.13 ms (ZP0.5Dy), 1.01 ms (ZP1Dy) and 0.69 ms (ZP5Dy). Decay time curves of the 4F9/2-6H13/ 2 emission (λem ¼ 568 nm) recorded for the ZP0.5Dy, ZP1Dy and ZP5Dy glasses excited at 423 nm are shown in Fig. 8. Thus, the nonexponential nature of the 4F9/2 level luminescence decay along with a decay time decreasing with increase of dysprosium concentration could be due to the enhancement of energy transfer between Dy3 þ ions via cross-relaxations [11]. Such cross-relaxation channels could be (see energy level diagram in Fig. 2) [10,11]: 4 F9=2 þ 6 H15=2 - 6 H9=2 ; 6 F11=2 Þ þ 6 F3=2 ; 6 F1=2 Þ; ðiiÞ 4
F9=2 þ 6 H15=2 - 6 H5=2 ; 6 F7=2 Þ þ 6 H7=2 ; 6 F9=2 Þ;
ðiiiÞ
4
F9=2 þ 6 H15=2 - 6 F3=2 ; 6 F1=2 Þ þ ð6 H9=2 ; 6 F11=2 Þ:
ðivÞ
5
Fig. 6. Overlap region between terbium F6- D4 (excitation) absorption [6] (dotted curve) and dysprosium 4F9/2-6H15/2 emission (solid curve).
the spectra shown in Fig. 6. Thus, the Dy3þ -Tb3þ energy transfer could be (see energy level diagram in Fig. 2): 4
h
i
h
3þ
F9=2 Dy3 þ þ 7 F6 Tb
i
h
i
h
3þ
-6 H15=2 Dy3 þ þ 5 D4 Tb
i
:
ðiÞ
From the emission spectra displayed in Fig. 1 an enhancement of the terbium 5D4-7F5 emission at the expense of a weakening of the dysprosium 4F9/2-6H15/2 emission by increasing the Tb3 þ content from 0.1 to 1.0 mol% of Tb(PO3)3 can be noticed. This fact
The available data do not allow discrimination among these three possible mechanisms, which leaves open future work on Dy3 þ ions in oxide glasses. The η energy transfer efficiency from Dy3 þ to Tb3 þ was estimated from the dysprosium 4F9/2 level lifetime data in the presence ðτDy Þ and absence ðτoDy Þ of Tb3 þ through the following expression [12]:
η ¼ 1
τDy : τoDy
ð2Þ
U. Caldiño et al. / Journal of Luminescence 161 (2015) 142–146
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Such enhancement of the 5D4-7FJ emissions could be attributed to a cross-relaxation process between Tb3 þ and Dy3 þ given by [4]: h i h i h i h i 3þ 3þ 5 þ 6 H15=2 Dy3 þ -6 D4 Tb þ 6 H11=2 Dy3 þ ; ðviÞ D3 Tb
Fig. 8. Decay time curves of the dysprosium 4F9/2-6H13/2 emission (λem ¼568 nm) recorded for the ZP0.5Dy, ZP1Dy and ZP5Dy glasses excited at 423 nm.
Fig. 9. Emission spectra of the ZP1Tb0.5Dy and ZP1Tb glasses excited at 282 nm. Spectra have been normalized to the 5D3-7F4 emission intensity of Tb3 þ .
Using the lifetime data obtained for τDy (1.28 ms) and τ oDy (1.13 ms), the energy transfer efficiency resulted to be 0.12. It could be possible that the efficiency of Dy3 þ -Tb3 þ energy transfer is affected by the cross-relaxation processes (ii), (iii) or (iv) occurring between Dy3 þ ions. This energy transfer efficiency is consistent with that (0.16) reported for lead borate glass containing 0.5Dy2O3 and1.5Tb2O3 [4,13]. Emission spectra of the ZP1Tb and ZP1Tb0.5Dy glasses excited into the Tb3 þ (7F6-5I8) band at 282 nm are shown in Fig. 9. Excitation at 282 nm fits to the requirements of AlGaN-based LEDs [14]. At this wavelength (35,461 cm 1) the Dy3 þ cannot be excited, as it can be appreciated from the approximated scheme of Dy3 þ energy levels portrayed in Fig. 2. After excitation of Tb3 þ at 282 nm, the 5D3 level is populated by fast non-radiative decays from the 5I8 level (Fig. 2), and then emissions 5D3-7FJ (J¼2, 3, 4, 5, and 6) and 5 D4-7FJ (J¼ 3, 4, 5, and 6) of Tb3 þ are observed. Emissions from the 5 D3 level are due to a slow non-radiative relaxation from the 5D3 to the 5D4 level, which is promoted by excitation from the 7F6 to the 7 F0 level through a cross-relaxation process [15] (Fig. 2): 5
D3 þ 7 F6 -5 D4 þ 7 F0 ;
since the energy difference between terbium 5D3 and 5D4 levels (5927 cm 1) is almost the same as that between dysprosium 6H11/2 and 6H15/2 levels (6000 cm 1), see energy level diagram displayed in Fig. 2. Therefore, the ZP1Tb0.5Dy glass exhibits a dominant green emission at 545 nm in detriment of the 5D3-7FJ blue emissions upon 282 nm excitation, in addition to an extended excitation range from 280 to 500 nm of the Dy3 þ yellow and Tb3 þ green luminescence (Fig. 5), due to multiple electronic transitions of Dy3 þ and Tb3 þ . These spectroscopic features might contribute to enhance the spectral response of solar photovoltaic cells by down-shifting of the incident solar spectrum. The present UV-green conversion phosphor could thereby be placed as a layer on the front side of a Si or Si/Ge solar cell, allowing down-converted photons to be absorbed without interfering with the active material, which could lead to a great promise for enhancement of solar cell efficiency. Improved solar cells UV protected through a phosphor converting the UV radiation to visible can have a wide range of applications that require harvesting of UV photons [16]. It can be noticed that the energy level of Dy3 þ (4F9/2) is a little higher ( 400 cm 1) than that of Tb3 þ (5D4), which makes the energy back transfer from Tb3 þ to Dy3 þ inefficient (Fig. 2). However, the emission spectrum recorded for the codoped glass, upon excitation of Tb3 þ at 282 nm, shows a small shoulder at 571 nm (Fig. 9), which is due to the dysprosium 4F9/2-6H13/2 emission. Thus, excitation to the dysprosium 4F9/2 level could be through energy transfer from Tb3 þ (5D3) to Dy3 þ (4F7/2), followed by fast non-radiative decays to the dysprosium 4F9/2 emitting level, as indicated in Fig. 2. The mechanism involved in this energy transfer could then be: h i h i h i h i 3þ 3þ 5 þ 6 H15=2 Dy3 þ -6 F6 Tb þ 6 F7=2 Dy3 þ : ðviiÞ D3 Tb In fact, such energy transfer is expected to occur considering that there exists a overlapping between the terbium 5D3-7F6 emission and the dysprosium 6H15/2-4F7/2 (excitation) absorption (Fig. 10). In order to obtain a clear evidence on the energy back transfer taking place from Tb3 þ to Dy3 þ , the decay time curves of the terbium 5D3 level in the ZP1Tb and ZP1Tb0.5Dy glasses excited at 282 nm were recorded monitoring the (5D3-7F4) 435 nm emission of Tb3 þ (Fig. 11). In both glasses the 5D3 level emission decay has a non-exponential shape, so that the τf lifetime was obtained through Eq. (1). Thus, a τf value of 0.95 ms for the 5D3 level emission decay in the Tb3 þ singly-doped glass, and a faster decay of 0.72 ms in the Tb3 þ and Dy3 þ codoped glass are obtained. Such shortening of the Tb3 þ lifetime in the presence of Dy3 þ reveals the Tb3 þ -Dy3 þ non-radiative energy back transfer described by the mechanism (vii). The η energy transfer efficiency from Tb3 þ to Dy3 þ was estimated from Eq. (2) using the terbium 5D3 level lifetime data in the presence ðτTb Þ and absence ðτoTb Þ of Dy3 þ . The energy transfer efficiency resulted to be 0.24.
ðvÞ 5
5
1
since the energy gap between D3 and D4 levels (5927 cm ) is similar to that between 7F0 and 7F6 levels (5700 cm 1). In Fig. 2 the Tb3 þ energy level scheme was obtained from emission spectrum shown in Fig. 9 (7F6, 7F5, 7F4, 7F3 and 7F2 levels), excitation spectrum previously reported [12] (5D4, 5L10 þ 5G6 þ 5D3, 5L9 þ 5D2 þ 5G5, 5 L7,8 þ 5G3, 5H7 þ 5D1, 5H5,6 and 5I8 þ 5F4,5 þ 5H4 levels) and levels involved in the (v) cross-relaxation process (7F0 level). From Fig. 9 it can be noticed that the emission intensity from the 5D4 level in the codoped glass increases with respect to that from the 5D3 level.
4. Conclusions An optical spectroscopy investigation of down-shifting by energy transfer in Tb3 þ /Dy3 þ co-doped zinc phosphate glasses is performed through photoluminescence spectra and decay time measurements. The visible region luminescence is generated by 5D4-7F5 and 5 D4-7F3 emissions of Tb3 þ in addition to 4I15/2-6H15/2, 4F9/2-6H15/2, 4 F9/2-6H13/2 and 4F9/2-6H11/2 emissions of Dy3þ , Tb3þ emission being sensitized by Dy3þ through a non-radiative resonant energy
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light region (0.35,0.39) by decreasing the Tb3þ content from 1.0 to 0.1 mol% of Tb(PO3)3 after Dy3þ excitation at 423 nm. The glass codoped with 1.0 mol% of Tb(PO3)3 and 0.5 mol% of Dy(PO3)3 exhibits a dominant terbium 5D4-7F5 green emission at the expense of the terbium 5D3-7FJ blue emissions upon 282 nm excitation, as well as an extended excitation range from 280 to 500 nm, due to multiple electronic transitions of Dy3 þ and Tb3þ . These spectroscopic findings might contribute to enhance the spectral response of solar photovoltaic cells by down-shifting of the incident solar spectrum.
Acknowledgments This work was supported by the CONACYT-CNR bilateral agreement under Project contract 173855. Fig. 10. Overlap region between terbium 5D3-7F6 emission (dotted curve) and dysprosium 6H15/2-4F7/2 (excitation) absorption (solid curve).
Fig. 11. Decay time curves of the terbium 5D3-7F4 emission (λem ¼ 435 nm) in the ZP1Tb and ZP1Tb0.5Dy glasses excited at 282 nm.
transfer. Such luminescence can be adjusted from yellow–green light, with CIE1931 chromaticity coordinates (0.36,0.44), toward the white
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