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Tb3+-codoped ZnO nanocrystals
Optical Materials xxx (2014) xxx–xxx
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Strong luminescence and efficient energy transfer in Eu3+/Tb3+-codoped ZnO nanocrystals L. Luo a,⇑, F.Y. Huang a, G.S. Dong a, H.H. Fan b, K.F. Li b, K.W. Cheah b, J. Chen c a
School of Physics and Optoelectronic Engineering, Guangdong University of Technology, Guangzhou 510006, China Center for Advanced Luminescence Materials, Department of Physics, Hong Kong Baptist University, Kowloon Tang, Hong Kong c Instrumental Analysis & Research Center, Sun Yat-sen University, Guangzhou 510275, China b
a r t i c l e
i n f o
Article history: Received 27 February 2014 Received in revised form 24 June 2014 Accepted 3 July 2014 Available online xxxx Keywords: Rare earth ZnO Efficient doping Energy transfer
a b s t r a c t Single crystalline Eu3+/Tb3+-codoped ZnO nanocrystals have been synthesized by using a simple coprecipitation method. Successful doping is realized so that strong green and red luminescence can be efficiently excited by ultraviolet and near ultraviolet radiation, demonstrating an efficient energy transfer from ZnO host to rare earth ions. The energy transfer from the ZnO host to Tb3+ in ZnO: Tb3+ samples and ZnO host to Eu3+ in the ZnO: Eu3+ samples under UV excitation are investigated. It is found that the red 5D0 ? 7F2 emission of Eu3+ ions decreases with increasing temperature but the green 5D4 ? 7F5 emission of Tb3+ ions increases with increasing temperature, implying a different energy transfer processes in the two samples. Moreover, energy transfer from Tb3+ ions to Eu3+ ions in ZnO nanocrystals is also observed by analyzing luminescence spectra and the decay curves. By adjusting the doping concentration, the Eu3+/Tb3+-codoped ZnO phosphors emit green and red luminescence with chromaticity coordinates near white light region, high color purity and high intensity, indicating that they are promising light-conversion materials and have potential in field emission display devices and liquid crystal display backlights. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Recently, there is an increasing interest in developing luminescent materials which can convert the near ultraviolet light to multi-color visible emissions due to rapidly growing demand in the advanced display and lighting applications [1–5]. It is expected that energy is transferred from the excited host to rare earth ions, resulting in sharp and intense emission peaks involving 4f–4f transitions [6–8]. ZnO is a direct gap semiconductor material with a wide band gap of 3.37 eV and a large exciton binding energy (60 meV) as well as excellent chemical and thermal stability [9– 11]. The wide band gap of ZnO can be combined with the luminescence of rare earth ions to develop blue, green or red emitting luminescent materials with the advantages such as strong near ultraviolet absorption, high color purity, low degradation and high efficiency [12–14]. They are promising light-conversion materials and have potential in field emission display devices and liquid crystal display (LCD) backlights [15]. Among the various rare earth (RE) ions, it is well known that Eu3+ ion emits bright red light, another important one is Tb3+ ion, ⇑ Corresponding author. E-mail address: [email protected] (L. Luo).
which can give strong green luminescence [16], hence it is interesting to study the photoluminescence properties and energy transfer in Eu3+ and Tb3+ ions codoped ZnO. The energy transfer from the ZnO host to the rare earth ions is desired in the display application, and the energy transfer between rare earth ions is beneficial to tune the emission color [17,18]. In the past few years, variety of synthetic methods, including solid-state reaction [8], hydrothermal [19], combustion [20], electrodeposition [21], sol–gel [22,23], pechini [24], microemulsion [25], have been used to dope rare earth ions into ZnO nanocrystals. However, it is still a challenge to dope impurity efficiently in semiconductor nanocrystals due to a self-purification effect [26], and the remarkable differences in ionic radius between Zn2+ and RE3+, as well as the charge imbalance [27]. Compared to the excitonic emission or defect emission from ZnO host, the UV-excited emission from rare earth ions was relatively weak. In most cases, emissions of Eu3+ ions are superimposed on a broadband defect-related emission or can only be observed at low temperature due to poor incorporation of rare earth ions into the ZnO lattice [28]. Although the luminescence properties of Eu3+ or Tb3+ single doped ZnO nanocrystals have been extensively investigated [29– 35], however, there are few reports on the Eu3+ and Tb3+ codoped ZnO nanocrystals and the dynamics study of the energy transfer
http://dx.doi.org/10.1016/j.optmat.2014.07.008 0925-3467/Ó 2014 Elsevier B.V. All rights reserved.
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from Tb3+ ions to Eu3+ ions in ZnO host is rare. Moreover, intense emission is still lacking. In the present work, we report the synthesis of Eu3+/Tb3+ codoped ZnO nanocrystals and investigated their optical properties both by static and kinetic as well as temperature dependent photoluminescence spectra. The corresponding luminescence mechanisms have been discussed in detail. 2. Experiment The Eu3+/Tb3+ codoped ZnO nanocrystals were synthesized by a co-precipitation method by using zinc acetate dehydrate [Zn(OOCCH3)22H2O], terbium oxide (Tb2O3) and europium oxide (Eu2O3) as raw materials. Firstly, 4.4344 g zinc acetate dehydrate was dissolved in 40 ml ethanol and 80 ml deionized water mixed solution, 0.2112 g europium oxide and 0.1496 g terbium oxide were dissolved in dilute nitric acid respectively. Secondly, the three solutions were mixed, 0.6 g sodium acetate (CH3COONa) was added to the above mixed solution under continuous stirring to keep the solution pH value of 8–9, and 1 ml acetylacetone (C5H8O2) is added as a capping agent to protect the particle surface and prevent its growth, and the mixed solution were placed on magnetic stirrer, where the solutions were stirred at 50 °C. Highly transparent sols were obtained after stirring for about 1 h. Thirdly, a stoichiometric amount of sodium hydroxide (NaOH, 1.9167 g) solution is added dropwise to the transparent sols as precipitator, the resulting solution was stirred for 1.5 h and then concentrated for 10 h, and the precipitation was obtained by filtering the above solution. Finally, the obtained precipitate was dried in an oven at 50 °C and then annealed in air at 150 °C for 1 h. Thus, ZnO nanoparticles doped with 6 mol% Eu and 4 mol% Tb were synthesized as described by above method. The crystal structure was investigated by X-ray diffraction (XRD, D/max 2200 v, RIGAKU Corp., Cu Ka). Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were recorded with an F-7000 fluorescence spectroscopy at room temperature. For low temperature PL and lifetime measurement, the excitation source is a N2 laser (337 nm, 4 ns, 260 lJ, 10 Hz). 3. Results Fig. 1(a) shows the XRD patterns of Eu3+/Tb3+ codoped ZnO phosphors with different doping concentrations. It presents that the sample has good crystalline quality and all the diffraction peaks can be indexed to pure hexagonal wurtzite structure. There is not observable diffraction peaks related to Eu2O3 and Tb2O3, indicating that Eu3+ ions and Tb3+ ions may be doped into the ZnO crystal lattice via substitution of Zn2+ sites by Eu3+ ions and Tb3+ ions. Fig. 1(b) shows a low magnified transmission electron microscope (TEM) image of Eu3+-doped ZnO nanocrystals. The length of Eu3+-doped ZnO nanocrystals ranges from 100–200 nm with diameters of 50–70 nm. The high-resolution TEM image as shown in Fig. 1(c) indicates the single crystalline nature for Eu3+doped ZnO nanocrystals, with which a plane spacing of 0.26 nm can be indexed, corresponding to (0 0 0 1) plane. Inset in Fig. 1(c) shows the selective area electron diffraction pattern (SAED), clearly indicating growth direction of Eu3+-doped ZnO nanocrystals being along [0 0 0 1] direction. The photoluminescence excitation and emission spectra of Eu3+/Tb3+-codoped ZnO nanocrystals were measured using Xe lamp as a light source at room temperature. Typically, the UVexcited photoluminescence of ZnO consists of an excitonic emission band in the UV region and an intense and broad defect-related emission band in the visible range [36]. Under above bandgap excitation at 320 nm, very strong green and red emissions without
Fig. 1. (a) XRD patterns of Eu3+/Tb3+ codoped ZnO phosphors with different doping concentrations, (b) TEM images of ZnO: 3 mol% Eu nanocrystals and (c) highresolution TEM image, inset is the corresponding SAED pattern.
defect background are observed in ZnO: 6 mol% Eu 4 mol% Tb nanocrystals as shown in Fig. 2(a). The inset of Fig. 2(a) shows the digital optical image of the Eu3+/Tb3+-codoped ZnO nanocrystals excited by a 405 nm GaN laser diode (405 nm, 50 mW) with input power of 1 mW passing through three neutral filters. The red emissions are originated from 5D0 ? 7FJ (J = 1, 2) of Eu3+ ions inside the ZnO while the green emissions are originated from 5 D4 ? 7FJ (J = 3, 4, 5, 6) of Tb3+ ions. Note that background noise is quite small, indicating that the excited ZnO transfers the excited energy efficiently to the doped Eu3+ ions and Tb3+ ions, giving rise to efficient emission at red and green spectral region without defect background. It also indicates that successful doping of Eu3+ and Tb3+ ions into ZnO nanocrystals has been realized. Our previous paper reported the successful doping of Eu3+ ions into ZnO nanocrystals and the red emission quantum yield of ZnO: Eu sample was measured to be 31% at room temperature [13]. In this paper, emission spectra of Eu3+/Tb3+-codoped ZnO show that the green emission is much stronger than that of red emission, thus the green emission quantum yield is estimated to be higher than 31% at room temperature. It has been reported that some shadow levels below the conduction band induced by the incorporation of a trivalent ion in the ZnO matrix play a crucial role as temporary storage centers on the energy transfer process [8]. In our study, most of the excited electrons are captured by these traps and then transfer to the 5D0 levels of Eu3+ ions, consequently strong red emission from Eu3+ ions are observed. Moreover, a spectral overlap
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Fig. 2. The room temperature emission spectra of ZnO: 6 mol% Eu 4 mol% Tb nanocrystals under excitation at 320 nm, the inset shows the digital optical image of the Eu3+/ Tb3+-codoped ZnO nanocrystals excited by a 405 nm GaN laser diode; (b) the room temperature excitation spectra of ZnO: 6 mol% Eu 4 mol% Tb (curve i) and ZnO: 6 mol% Eu (curve ii) by monitoring the 5D0 ? 7F2 emission at 612 nm; ZnO: 6 mol% Eu 4 mol% Tb (curve iii) and ZnO: 4 mol% Tb (curve iv) by monitoring the 5D4 ? 7F5 emission at 549 nm.
Fig. 3. (a) The room temperature photoluminescence spectra of ZnO: 6 mol% Eu 4 mol% Tb and ZnO: 6 mol% Eu; (b) ZnO: 6 mol% Eu 4 mol% Tb and ZnO: 4 mol% Tb under excitation at 385 nm.
Fig. 4. (a) PL decay curves of red emission from Eu3+ ions at 612 nm in ZnO: 6% Eu3+ 4% Tb3+ (curve i) and ZnO: 6% Eu (curve ii) under excitation at 337 nm, (b) PL decay curves of green emission from Tb3+ ions at 549 nm in ZnO: 4% Tb3+ (curve i) and ZnO: 6% Eu 4% Tb (curve ii) under excitation at 337 nm, the curves were normalized. The black lines are measured decay profile, the red lines are exponential function fit curves. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Table 1 The lifetime of 5D4 level of Tb3+ ions and 5D0 level of Eu3+ ions in Eu3+/Tb3+ single doped ZnO and Eu3+/Tb3+ codoped ZnO nanocrystals with different doping concentration. Sample
Lifetime of 5D4 level (ms)
ZnO: 4%Tb ZnO:4%Tb6%Eu ZnO:2%Tb6%Eu ZnO:6%Eu
1 0.87 0.85
Lifetime 5D0 level (ms) 1.25 1.06 0.7
between the 5D4 ? 7F6 emission of Tb3+ ions and the 7F0 ? 5D2 absorption of Eu3+ ions are observed. It is reported that the energy transfer occurs when the emission band of the sensitizer (Tb3+ ions in this study) overlaps with the absorption band of the activator (Eu3+ ions in this study). In our study, as a result of spectral overlap between the 5D4 ? 7F6 emission of Tb3+ ion and the 7F0 ? 5D2 excitation spectrum of Eu3+ ion, efficient energy transfer from Tb3+ ions to the Eu3+ ions occurs, promoting the Eu3+ ions from the 7F0 ground state to the 5D2 excited state. Fig. 2(b) presents the excitation spectra by monitoring the 5 D0 ? 7F2 emission of Eu3+ ions at 612 nm and the 5D4 ? 7F5 emission of Tb3+ ions at 549 nm, respectively. The four absorption peaks centered at 392.6, 416.1, 462.8, and 535.8 nm can be attributed to the direct excitation of the Eu3+ ions from the 7F0 ground state to 5 L6, 5D3, 5D2 and 5D1 excited states, respectively. The absorption peaks at 485.0 and 489.9 nm can be attributed to the direct excitation of the Tb3+ ions from the 7F6 to 5D4. Besides the characteristic 4f absorption peaks of Eu3+ and Tb3+ ions, two broad ultraviolet (UV) absorption bands centered at 320 nm and 380 nm are also observed. They are band to band and band-edge excitation of ZnO, respectively. Excitation by a UV photon results in the creation of a large number of carriers, which then diffuse through the ZnO host and transfer their energy to Eu3+ and Tb3+ ions that subsequently emit light. Worth to note, monitoring the red emission of Eu3+ ions at 612 nm in Eu3+/Tb3+-codoped ZnO nanocrystals, an absorption peak at 485 nm attributed to Tb3+ ions is observed as shown in Fig. 2(b) (curve i), and the excitation peaks of Eu3+ ions are remarkably enhanced in the Eu3+/Tb3+ codoped ZnO nanoparticles [Fig. 2(b) (curve i)] compared with that in Eu3+ ions single doped ZnO nanocrystals [Fig. 2(b) (curve ii)]. The excitation peaks of Tb3+ ions are weaker in the Eu3+/Tb3+ codoped ZnO nanocrystals [Fig. 2(b) (curve iii)] compared with that in Tb3+ ions single doped ZnO nanocrystals [Fig. 2(b) (curve iv)]. The results demonstrate the energy transfer from Tb3+ ions to the Eu3+ ions Under near ultraviolet band edge excitation at 385 nm, similar sharp 4f–4f transition from Eu3+ ions and Tb3+ ions are also observed in Eu3+/Tb3+ codoped ZnO nanocrystals as shown in Fig. 3. It is found that the red emission of Eu3+ ions in Eu3+/Tb3+-codoped ZnO nanocrystals is enhanced compared to Eu3+ ions single doped ZnO nanocrystals and the intensity of green emission of Tb3+ ions is decreased in the Eu3+/Tb3+ codoped ZnO nanocrystals compared to Tb3+ ions single doped ZnO nanocrystals. In order to study the energy transfer process and the concentration dependence, the dynamics process of energy transfer is studied as shown in Fig. 4. It shows the lifetime of 5D4 level of Tb3+ ions in Eu3+/Tb3+ ions codoped ZnO nanocrystals is shorter than that of Tb3+ ions single doped ZnO nanocrystals. On the contrary, the lifetime of 5D0 level of Eu3+ ions in Eu3+/ Tb3+ codoped ZnO nanocrystals is longer than that of the Eu3+ ions single doped ZnO nanocrystals. Table 1 presents the lifetime of 5D4 level of Tb3+ ions and 5D0 level of Eu3+ ions in Eu3+/Tb3+ single doped and codoped ZnO with different doping concentration. When the doping concentration of Tb3+ ions is regular, the lifetime of the 5D4 level of Tb3+ ions became
Fig. 5. Photoluminescence spectra excited with a 337 nm nanosecond pulse from a N2 laser at 12 K and 300 K, respectively (a) ZnO: 6% Eu 4% Tb, (b) ZnO: 4% Eu and (c) ZnO: 4% Tb.
shorter with increasing doping concentration of Eu3+ ions. It indicates that there is an additional excitation energy loss of Tb3+ ions in the sensitizing process of Eu3+ ions by Tb3+ ions, resulting in shorter lifetime of Tb3+ ions. On the contrary, when the doping concentration of Eu3+ ions is regular, the lifetime of the 5D0 level of Eu3+ ions became longer with increasing doping concentration of Tb3+ ions. It indicates that additional excitation energy is transferred to Eu3+ ions from Tb3+ ions, resulting in longer lifetime of 5D0 level of Eu3+ ions. In order to further understand the energy transfer from ZnO host to RE3+ ions in Eu3+/Tb3+-codoped ZnO nanocrystals, the temperature dependence of emission spectra is studied. Fig. 5(a) shows the emission spectra of Eu3+ and Tb3+ codoped ZnO nanocrystals
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Table 2 The CIE chromaticity coordinates of Eu3+/Tb3+-codoped ZnO nanocrystals with different doping concentration under 385 nm excitation.
Fig. 6. Schematic illustration showing the energy transfer process in Eu3+/Tb3+codoped ZnO nanocrystals. where 1 corresponds to near band edge exciton emission, 2 is self-activated yellow emission, 3 is the energy transfer from the trap level to Eu3+ ions, 4 is the energy transfer from free electron hole pair to Tb3+ ions, and 5 is the energy transfer from Tb3+ ions to Eu3+ ions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
excited with a 337 nm nanosecond pulse from a N2 laser at 12 K and 290 K, respectively. It can be seen clearly by the naked eyes that the sample emits green luminescence at room temperature, but it emits very strong red luminescence at low temperature. The intensity of red emission is enhanced about 6 times while the intensity of green is reduced 3 times when the temperature decreased from 290 K to 12 K. Fig. 5(b) shows the emission spectra of Eu3+ ion single doped ZnO nanocrystals, the inset of Fig. 5(b) compares the temperature dependence of the UV near-band-edge emission from ZnO and red emission from Eu3+, respectively. It is found that the red emission increases greatly at low temperature, but the UV near band-edge emission from ZnO increases only slightly at low temperature. Fig. 5(c) shows the opposite temperature dependence of green emission in Tb3+ ion single doped ZnO nanocrystals, the green emission of Tb3+ ions decreases with decreasing temperature. The above results indicate that for the Eu3+ ions doped ZnO sample, energy is mainly transferred from the bound electron hole pair to the Eu3+ ions, while for the Tb3+ ions doped ZnO sample, energy is mainly transferred from the free electron hole pair to the Tb3+ ions, so that it is beneficial for Eu3+ ions to emit red light in low temperature. The schematic mechanism of UV-excited Eu3+ and Tb3+ emission is illustrated in Fig. 6, where 1 corresponds to near band edge exciton emission, 2 is
Sample
(x, y)
ZnO: ZnO: ZnO: ZnO:
(0.3064, (0.2932, (0.2899, (0.2815,
4% Tb 6% Eu 2% Tb 6% Eu 1.5% Tb 4.5% Eu 1% Tb 3% Eu
0.4420) 0.3734) 0.3499) 0.3424)
self-activated yellow emission, 3 is the energy transfer from the trap level to Eu3+ ions, 4 is the energy transfer from free electron hole pair to Tb3+ ions, and 5 is the energy transfer from Tb3+ ions to Eu3+ ions. Finally, Fig. 7(a) shows the room temperature emission spectra of Eu3+/Tb3+ codoped ZnO phosphors with different doping concentrations under excitation at 385 nm, Fig. 7(b) shows the corresponding CIE chromaticity diagram. Their CIE chromaticity coordinates are summarized in Table 2. It can be seen from Fig. 7(b), by adjusting the doping concentration, CIE chromaticity coordinates of Eu3+/Tb3+-codoped ZnO under 385 nm excitation are close to the white light region.
4. Conclusions In conclusion, we studied the structural and optical properties of Eu3+/Tb3+-codoped ZnO nanocrystals prepared by using a simple co-precipitation method. Strong green and red luminescence without defect background under ultraviolet or near-ultraviolet excitation was observed in the Eu3+/Tb3+-codoped ZnO nanocrystals, demonstrating an efficient energy transfer from ZnO host to rare earth ions. In particular, it is found that red luminescence is greatly enhanced and green luminescence is remarkably quenched at low temperature. The different temperature dependent characteristics of red and green emission imply that energy is mainly transferred from the free electron hole pair to the Tb3+ ions, but from the bound electron hole pair to the Eu3+ ions. Energy transfer from Tb3+ ions to Eu3+ ions is also observed, by adjusting the doping concentration, CIE chromaticity coordinates of Eu3+/Tb3+-codoped ZnO under 385 nm excitation are very close to the white light region. The results indicate Eu3+/Tb3+-codoped ZnO nanocrystals are promising light-conversion materials and have potential in field emission display devices and LCD backlights.
Fig. 7. (a) The room temperature emission spectra of Eu3+/Tb3+ codoped ZnO phosphors with different doping concentrations under excitation at 385 nm and (b) the CIE chromaticity diagram.
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Acknowledgments This work was supported by the National Natural Science Foundation of China No. 10774031 and No. 21271048. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Strong luminescence and efficient energy transfer in Eu3+/Tb3+-codoped ZnO nanocrystals L. Luo a,⇑, F.Y. Huang a, G.S. Dong a, H.H. Fan b, K.F. Li b, K.W. Cheah b, J. Chen c a
School of Physics and Optoelectronic Engineering, Guangdong University of Technology, Guangzhou 510006, China Center for Advanced Luminescence Materials, Department of Physics, Hong Kong Baptist University, Kowloon Tang, Hong Kong c Instrumental Analysis & Research Center, Sun Yat-sen University, Guangzhou 510275, China b
a r t i c l e
i n f o
Article history: Received 27 February 2014 Received in revised form 24 June 2014 Accepted 3 July 2014 Available online xxxx Keywords: Rare earth ZnO Efficient doping Energy transfer
a b s t r a c t Single crystalline Eu3+/Tb3+-codoped ZnO nanocrystals have been synthesized by using a simple coprecipitation method. Successful doping is realized so that strong green and red luminescence can be efficiently excited by ultraviolet and near ultraviolet radiation, demonstrating an efficient energy transfer from ZnO host to rare earth ions. The energy transfer from the ZnO host to Tb3+ in ZnO: Tb3+ samples and ZnO host to Eu3+ in the ZnO: Eu3+ samples under UV excitation are investigated. It is found that the red 5D0 ? 7F2 emission of Eu3+ ions decreases with increasing temperature but the green 5D4 ? 7F5 emission of Tb3+ ions increases with increasing temperature, implying a different energy transfer processes in the two samples. Moreover, energy transfer from Tb3+ ions to Eu3+ ions in ZnO nanocrystals is also observed by analyzing luminescence spectra and the decay curves. By adjusting the doping concentration, the Eu3+/Tb3+-codoped ZnO phosphors emit green and red luminescence with chromaticity coordinates near white light region, high color purity and high intensity, indicating that they are promising light-conversion materials and have potential in field emission display devices and liquid crystal display backlights. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Recently, there is an increasing interest in developing luminescent materials which can convert the near ultraviolet light to multi-color visible emissions due to rapidly growing demand in the advanced display and lighting applications [1–5]. It is expected that energy is transferred from the excited host to rare earth ions, resulting in sharp and intense emission peaks involving 4f–4f transitions [6–8]. ZnO is a direct gap semiconductor material with a wide band gap of 3.37 eV and a large exciton binding energy (60 meV) as well as excellent chemical and thermal stability [9– 11]. The wide band gap of ZnO can be combined with the luminescence of rare earth ions to develop blue, green or red emitting luminescent materials with the advantages such as strong near ultraviolet absorption, high color purity, low degradation and high efficiency [12–14]. They are promising light-conversion materials and have potential in field emission display devices and liquid crystal display (LCD) backlights [15]. Among the various rare earth (RE) ions, it is well known that Eu3+ ion emits bright red light, another important one is Tb3+ ion, ⇑ Corresponding author. E-mail address: [email protected] (L. Luo).
which can give strong green luminescence [16], hence it is interesting to study the photoluminescence properties and energy transfer in Eu3+ and Tb3+ ions codoped ZnO. The energy transfer from the ZnO host to the rare earth ions is desired in the display application, and the energy transfer between rare earth ions is beneficial to tune the emission color [17,18]. In the past few years, variety of synthetic methods, including solid-state reaction [8], hydrothermal [19], combustion [20], electrodeposition [21], sol–gel [22,23], pechini [24], microemulsion [25], have been used to dope rare earth ions into ZnO nanocrystals. However, it is still a challenge to dope impurity efficiently in semiconductor nanocrystals due to a self-purification effect [26], and the remarkable differences in ionic radius between Zn2+ and RE3+, as well as the charge imbalance [27]. Compared to the excitonic emission or defect emission from ZnO host, the UV-excited emission from rare earth ions was relatively weak. In most cases, emissions of Eu3+ ions are superimposed on a broadband defect-related emission or can only be observed at low temperature due to poor incorporation of rare earth ions into the ZnO lattice [28]. Although the luminescence properties of Eu3+ or Tb3+ single doped ZnO nanocrystals have been extensively investigated [29– 35], however, there are few reports on the Eu3+ and Tb3+ codoped ZnO nanocrystals and the dynamics study of the energy transfer
http://dx.doi.org/10.1016/j.optmat.2014.07.008 0925-3467/Ó 2014 Elsevier B.V. All rights reserved.
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from Tb3+ ions to Eu3+ ions in ZnO host is rare. Moreover, intense emission is still lacking. In the present work, we report the synthesis of Eu3+/Tb3+ codoped ZnO nanocrystals and investigated their optical properties both by static and kinetic as well as temperature dependent photoluminescence spectra. The corresponding luminescence mechanisms have been discussed in detail. 2. Experiment The Eu3+/Tb3+ codoped ZnO nanocrystals were synthesized by a co-precipitation method by using zinc acetate dehydrate [Zn(OOCCH3)22H2O], terbium oxide (Tb2O3) and europium oxide (Eu2O3) as raw materials. Firstly, 4.4344 g zinc acetate dehydrate was dissolved in 40 ml ethanol and 80 ml deionized water mixed solution, 0.2112 g europium oxide and 0.1496 g terbium oxide were dissolved in dilute nitric acid respectively. Secondly, the three solutions were mixed, 0.6 g sodium acetate (CH3COONa) was added to the above mixed solution under continuous stirring to keep the solution pH value of 8–9, and 1 ml acetylacetone (C5H8O2) is added as a capping agent to protect the particle surface and prevent its growth, and the mixed solution were placed on magnetic stirrer, where the solutions were stirred at 50 °C. Highly transparent sols were obtained after stirring for about 1 h. Thirdly, a stoichiometric amount of sodium hydroxide (NaOH, 1.9167 g) solution is added dropwise to the transparent sols as precipitator, the resulting solution was stirred for 1.5 h and then concentrated for 10 h, and the precipitation was obtained by filtering the above solution. Finally, the obtained precipitate was dried in an oven at 50 °C and then annealed in air at 150 °C for 1 h. Thus, ZnO nanoparticles doped with 6 mol% Eu and 4 mol% Tb were synthesized as described by above method. The crystal structure was investigated by X-ray diffraction (XRD, D/max 2200 v, RIGAKU Corp., Cu Ka). Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were recorded with an F-7000 fluorescence spectroscopy at room temperature. For low temperature PL and lifetime measurement, the excitation source is a N2 laser (337 nm, 4 ns, 260 lJ, 10 Hz). 3. Results Fig. 1(a) shows the XRD patterns of Eu3+/Tb3+ codoped ZnO phosphors with different doping concentrations. It presents that the sample has good crystalline quality and all the diffraction peaks can be indexed to pure hexagonal wurtzite structure. There is not observable diffraction peaks related to Eu2O3 and Tb2O3, indicating that Eu3+ ions and Tb3+ ions may be doped into the ZnO crystal lattice via substitution of Zn2+ sites by Eu3+ ions and Tb3+ ions. Fig. 1(b) shows a low magnified transmission electron microscope (TEM) image of Eu3+-doped ZnO nanocrystals. The length of Eu3+-doped ZnO nanocrystals ranges from 100–200 nm with diameters of 50–70 nm. The high-resolution TEM image as shown in Fig. 1(c) indicates the single crystalline nature for Eu3+doped ZnO nanocrystals, with which a plane spacing of 0.26 nm can be indexed, corresponding to (0 0 0 1) plane. Inset in Fig. 1(c) shows the selective area electron diffraction pattern (SAED), clearly indicating growth direction of Eu3+-doped ZnO nanocrystals being along [0 0 0 1] direction. The photoluminescence excitation and emission spectra of Eu3+/Tb3+-codoped ZnO nanocrystals were measured using Xe lamp as a light source at room temperature. Typically, the UVexcited photoluminescence of ZnO consists of an excitonic emission band in the UV region and an intense and broad defect-related emission band in the visible range [36]. Under above bandgap excitation at 320 nm, very strong green and red emissions without
Fig. 1. (a) XRD patterns of Eu3+/Tb3+ codoped ZnO phosphors with different doping concentrations, (b) TEM images of ZnO: 3 mol% Eu nanocrystals and (c) highresolution TEM image, inset is the corresponding SAED pattern.
defect background are observed in ZnO: 6 mol% Eu 4 mol% Tb nanocrystals as shown in Fig. 2(a). The inset of Fig. 2(a) shows the digital optical image of the Eu3+/Tb3+-codoped ZnO nanocrystals excited by a 405 nm GaN laser diode (405 nm, 50 mW) with input power of 1 mW passing through three neutral filters. The red emissions are originated from 5D0 ? 7FJ (J = 1, 2) of Eu3+ ions inside the ZnO while the green emissions are originated from 5 D4 ? 7FJ (J = 3, 4, 5, 6) of Tb3+ ions. Note that background noise is quite small, indicating that the excited ZnO transfers the excited energy efficiently to the doped Eu3+ ions and Tb3+ ions, giving rise to efficient emission at red and green spectral region without defect background. It also indicates that successful doping of Eu3+ and Tb3+ ions into ZnO nanocrystals has been realized. Our previous paper reported the successful doping of Eu3+ ions into ZnO nanocrystals and the red emission quantum yield of ZnO: Eu sample was measured to be 31% at room temperature [13]. In this paper, emission spectra of Eu3+/Tb3+-codoped ZnO show that the green emission is much stronger than that of red emission, thus the green emission quantum yield is estimated to be higher than 31% at room temperature. It has been reported that some shadow levels below the conduction band induced by the incorporation of a trivalent ion in the ZnO matrix play a crucial role as temporary storage centers on the energy transfer process [8]. In our study, most of the excited electrons are captured by these traps and then transfer to the 5D0 levels of Eu3+ ions, consequently strong red emission from Eu3+ ions are observed. Moreover, a spectral overlap
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Fig. 2. The room temperature emission spectra of ZnO: 6 mol% Eu 4 mol% Tb nanocrystals under excitation at 320 nm, the inset shows the digital optical image of the Eu3+/ Tb3+-codoped ZnO nanocrystals excited by a 405 nm GaN laser diode; (b) the room temperature excitation spectra of ZnO: 6 mol% Eu 4 mol% Tb (curve i) and ZnO: 6 mol% Eu (curve ii) by monitoring the 5D0 ? 7F2 emission at 612 nm; ZnO: 6 mol% Eu 4 mol% Tb (curve iii) and ZnO: 4 mol% Tb (curve iv) by monitoring the 5D4 ? 7F5 emission at 549 nm.
Fig. 3. (a) The room temperature photoluminescence spectra of ZnO: 6 mol% Eu 4 mol% Tb and ZnO: 6 mol% Eu; (b) ZnO: 6 mol% Eu 4 mol% Tb and ZnO: 4 mol% Tb under excitation at 385 nm.
Fig. 4. (a) PL decay curves of red emission from Eu3+ ions at 612 nm in ZnO: 6% Eu3+ 4% Tb3+ (curve i) and ZnO: 6% Eu (curve ii) under excitation at 337 nm, (b) PL decay curves of green emission from Tb3+ ions at 549 nm in ZnO: 4% Tb3+ (curve i) and ZnO: 6% Eu 4% Tb (curve ii) under excitation at 337 nm, the curves were normalized. The black lines are measured decay profile, the red lines are exponential function fit curves. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Table 1 The lifetime of 5D4 level of Tb3+ ions and 5D0 level of Eu3+ ions in Eu3+/Tb3+ single doped ZnO and Eu3+/Tb3+ codoped ZnO nanocrystals with different doping concentration. Sample
Lifetime of 5D4 level (ms)
ZnO: 4%Tb ZnO:4%Tb6%Eu ZnO:2%Tb6%Eu ZnO:6%Eu
1 0.87 0.85
Lifetime 5D0 level (ms) 1.25 1.06 0.7
between the 5D4 ? 7F6 emission of Tb3+ ions and the 7F0 ? 5D2 absorption of Eu3+ ions are observed. It is reported that the energy transfer occurs when the emission band of the sensitizer (Tb3+ ions in this study) overlaps with the absorption band of the activator (Eu3+ ions in this study). In our study, as a result of spectral overlap between the 5D4 ? 7F6 emission of Tb3+ ion and the 7F0 ? 5D2 excitation spectrum of Eu3+ ion, efficient energy transfer from Tb3+ ions to the Eu3+ ions occurs, promoting the Eu3+ ions from the 7F0 ground state to the 5D2 excited state. Fig. 2(b) presents the excitation spectra by monitoring the 5 D0 ? 7F2 emission of Eu3+ ions at 612 nm and the 5D4 ? 7F5 emission of Tb3+ ions at 549 nm, respectively. The four absorption peaks centered at 392.6, 416.1, 462.8, and 535.8 nm can be attributed to the direct excitation of the Eu3+ ions from the 7F0 ground state to 5 L6, 5D3, 5D2 and 5D1 excited states, respectively. The absorption peaks at 485.0 and 489.9 nm can be attributed to the direct excitation of the Tb3+ ions from the 7F6 to 5D4. Besides the characteristic 4f absorption peaks of Eu3+ and Tb3+ ions, two broad ultraviolet (UV) absorption bands centered at 320 nm and 380 nm are also observed. They are band to band and band-edge excitation of ZnO, respectively. Excitation by a UV photon results in the creation of a large number of carriers, which then diffuse through the ZnO host and transfer their energy to Eu3+ and Tb3+ ions that subsequently emit light. Worth to note, monitoring the red emission of Eu3+ ions at 612 nm in Eu3+/Tb3+-codoped ZnO nanocrystals, an absorption peak at 485 nm attributed to Tb3+ ions is observed as shown in Fig. 2(b) (curve i), and the excitation peaks of Eu3+ ions are remarkably enhanced in the Eu3+/Tb3+ codoped ZnO nanoparticles [Fig. 2(b) (curve i)] compared with that in Eu3+ ions single doped ZnO nanocrystals [Fig. 2(b) (curve ii)]. The excitation peaks of Tb3+ ions are weaker in the Eu3+/Tb3+ codoped ZnO nanocrystals [Fig. 2(b) (curve iii)] compared with that in Tb3+ ions single doped ZnO nanocrystals [Fig. 2(b) (curve iv)]. The results demonstrate the energy transfer from Tb3+ ions to the Eu3+ ions Under near ultraviolet band edge excitation at 385 nm, similar sharp 4f–4f transition from Eu3+ ions and Tb3+ ions are also observed in Eu3+/Tb3+ codoped ZnO nanocrystals as shown in Fig. 3. It is found that the red emission of Eu3+ ions in Eu3+/Tb3+-codoped ZnO nanocrystals is enhanced compared to Eu3+ ions single doped ZnO nanocrystals and the intensity of green emission of Tb3+ ions is decreased in the Eu3+/Tb3+ codoped ZnO nanocrystals compared to Tb3+ ions single doped ZnO nanocrystals. In order to study the energy transfer process and the concentration dependence, the dynamics process of energy transfer is studied as shown in Fig. 4. It shows the lifetime of 5D4 level of Tb3+ ions in Eu3+/Tb3+ ions codoped ZnO nanocrystals is shorter than that of Tb3+ ions single doped ZnO nanocrystals. On the contrary, the lifetime of 5D0 level of Eu3+ ions in Eu3+/ Tb3+ codoped ZnO nanocrystals is longer than that of the Eu3+ ions single doped ZnO nanocrystals. Table 1 presents the lifetime of 5D4 level of Tb3+ ions and 5D0 level of Eu3+ ions in Eu3+/Tb3+ single doped and codoped ZnO with different doping concentration. When the doping concentration of Tb3+ ions is regular, the lifetime of the 5D4 level of Tb3+ ions became
Fig. 5. Photoluminescence spectra excited with a 337 nm nanosecond pulse from a N2 laser at 12 K and 300 K, respectively (a) ZnO: 6% Eu 4% Tb, (b) ZnO: 4% Eu and (c) ZnO: 4% Tb.
shorter with increasing doping concentration of Eu3+ ions. It indicates that there is an additional excitation energy loss of Tb3+ ions in the sensitizing process of Eu3+ ions by Tb3+ ions, resulting in shorter lifetime of Tb3+ ions. On the contrary, when the doping concentration of Eu3+ ions is regular, the lifetime of the 5D0 level of Eu3+ ions became longer with increasing doping concentration of Tb3+ ions. It indicates that additional excitation energy is transferred to Eu3+ ions from Tb3+ ions, resulting in longer lifetime of 5D0 level of Eu3+ ions. In order to further understand the energy transfer from ZnO host to RE3+ ions in Eu3+/Tb3+-codoped ZnO nanocrystals, the temperature dependence of emission spectra is studied. Fig. 5(a) shows the emission spectra of Eu3+ and Tb3+ codoped ZnO nanocrystals
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Table 2 The CIE chromaticity coordinates of Eu3+/Tb3+-codoped ZnO nanocrystals with different doping concentration under 385 nm excitation.
Fig. 6. Schematic illustration showing the energy transfer process in Eu3+/Tb3+codoped ZnO nanocrystals. where 1 corresponds to near band edge exciton emission, 2 is self-activated yellow emission, 3 is the energy transfer from the trap level to Eu3+ ions, 4 is the energy transfer from free electron hole pair to Tb3+ ions, and 5 is the energy transfer from Tb3+ ions to Eu3+ ions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
excited with a 337 nm nanosecond pulse from a N2 laser at 12 K and 290 K, respectively. It can be seen clearly by the naked eyes that the sample emits green luminescence at room temperature, but it emits very strong red luminescence at low temperature. The intensity of red emission is enhanced about 6 times while the intensity of green is reduced 3 times when the temperature decreased from 290 K to 12 K. Fig. 5(b) shows the emission spectra of Eu3+ ion single doped ZnO nanocrystals, the inset of Fig. 5(b) compares the temperature dependence of the UV near-band-edge emission from ZnO and red emission from Eu3+, respectively. It is found that the red emission increases greatly at low temperature, but the UV near band-edge emission from ZnO increases only slightly at low temperature. Fig. 5(c) shows the opposite temperature dependence of green emission in Tb3+ ion single doped ZnO nanocrystals, the green emission of Tb3+ ions decreases with decreasing temperature. The above results indicate that for the Eu3+ ions doped ZnO sample, energy is mainly transferred from the bound electron hole pair to the Eu3+ ions, while for the Tb3+ ions doped ZnO sample, energy is mainly transferred from the free electron hole pair to the Tb3+ ions, so that it is beneficial for Eu3+ ions to emit red light in low temperature. The schematic mechanism of UV-excited Eu3+ and Tb3+ emission is illustrated in Fig. 6, where 1 corresponds to near band edge exciton emission, 2 is
Sample
(x, y)
ZnO: ZnO: ZnO: ZnO:
(0.3064, (0.2932, (0.2899, (0.2815,
4% Tb 6% Eu 2% Tb 6% Eu 1.5% Tb 4.5% Eu 1% Tb 3% Eu
0.4420) 0.3734) 0.3499) 0.3424)
self-activated yellow emission, 3 is the energy transfer from the trap level to Eu3+ ions, 4 is the energy transfer from free electron hole pair to Tb3+ ions, and 5 is the energy transfer from Tb3+ ions to Eu3+ ions. Finally, Fig. 7(a) shows the room temperature emission spectra of Eu3+/Tb3+ codoped ZnO phosphors with different doping concentrations under excitation at 385 nm, Fig. 7(b) shows the corresponding CIE chromaticity diagram. Their CIE chromaticity coordinates are summarized in Table 2. It can be seen from Fig. 7(b), by adjusting the doping concentration, CIE chromaticity coordinates of Eu3+/Tb3+-codoped ZnO under 385 nm excitation are close to the white light region.
4. Conclusions In conclusion, we studied the structural and optical properties of Eu3+/Tb3+-codoped ZnO nanocrystals prepared by using a simple co-precipitation method. Strong green and red luminescence without defect background under ultraviolet or near-ultraviolet excitation was observed in the Eu3+/Tb3+-codoped ZnO nanocrystals, demonstrating an efficient energy transfer from ZnO host to rare earth ions. In particular, it is found that red luminescence is greatly enhanced and green luminescence is remarkably quenched at low temperature. The different temperature dependent characteristics of red and green emission imply that energy is mainly transferred from the free electron hole pair to the Tb3+ ions, but from the bound electron hole pair to the Eu3+ ions. Energy transfer from Tb3+ ions to Eu3+ ions is also observed, by adjusting the doping concentration, CIE chromaticity coordinates of Eu3+/Tb3+-codoped ZnO under 385 nm excitation are very close to the white light region. The results indicate Eu3+/Tb3+-codoped ZnO nanocrystals are promising light-conversion materials and have potential in field emission display devices and LCD backlights.
Fig. 7. (a) The room temperature emission spectra of Eu3+/Tb3+ codoped ZnO phosphors with different doping concentrations under excitation at 385 nm and (b) the CIE chromaticity diagram.
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