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Photoluminescence and Raman studies of Sm3+ and Nd3+ ions in zirconia matrices: example of energy transfer and host–guest interactions
Spectrochimica Acta Part A 60 (2004) 89–95
Photoluminescence and Raman studies of Sm3+ and Nd3+ ions in zirconia matrices: example of energy transfer and host–guest interactions Zerihun Assefa a,∗ , R.G. Haire a , P.E. Raison b,1 b
a Oak Ridge National Laboratory, Chemical Sciences Division, MS 6375, Oak Ridge, TN 37831-6375, USA European Commission-Institute for Energy, Joint Research Center of Petten, Postbus Nr. 2 1755 ZG Petten, The Netherlands
Received 4 March 2003; accepted 31 March 2003
Abstract Photoluminescence and Raman studies on Sm3+ - and Nd3+ -doped zirconia are reported. The Raman studies indicate that the monoclinic (m) phase dominates up to a 10 at.% lanthanide level, while stabilization of the cubic phase is attained at ∼20 and ∼25 at.% of Sm3+ and Nd3+ , respectively. Both systems are strongly luminescent under photo-excitation. The emission spectrum at 77 K of the ZrO2 :Sm3+ system consists of a broad band at 505 nm, that corresponds to the zirconia matrix. At room temperature the band maximum blue-shifts to 490 nm. Sharper bands corresponding to f–f transitions within the Sm3+ ion are also exhibited in the longer wavelength region of the spectrum. Exclusive excitation of the zirconia matrix provides sensitized emission from the acceptor Sm3+ ion. The excitation profile is dominated by a broad band at 325 nm when monitored either at the zirconia or at one of the Sm3+ emissions. A spectral overlap between the 6 H5/2 →4 G7/2 absorption of the Sm3+ ion with the zirconia emission leads to an efficient energy transfer process in the systems. Multiple facets of the spectral behavior of the Sm3+ or Nd3+ in the zirconia matrices, as well as the effects of compositions on the emission and Raman properties of the materials, and the role of defect centers in photoluminescence and the energy transfer processes are discussed. Published by Elsevier B.V. Keywords: Photo-luminescence; Energy-transfer; Lanthanide; Sensitized emission
1. Introduction Zirconia-based oxide ceramics are attractive for a variety of applications, such as fuel cells, oxygen sensors, refractory materials, and optical transparency [1–4]. Several attributes influence the physical and chemical properties of these materials, which include crystal structures, type and level of dopant, as well as the temperature [4]. Pure ZrO2 exists in three polymorphic phases (monoclinic, tetragonal or cubic), with the more symmetrical phases being obtained with increasing temperatures. Doping zirconia with different cations [5] can stabilize the cubic and tetragonal phases at lower or even ambient temperatures. This stabilization improves important mechanical and electrical prop∗ Corresponding author. Tel.: +1-865-754-5013; fax: +1-865-574 4987. E-mail address: [email protected] (Z. Assefa). 1 On leave from the Commissariat a ` l’Energie Atomique, CEA-Cadarache DEN/DEC/SPUA/ LMPC 13108, France.
1386-1425/$ – see front matter. Published by Elsevier B.V. doi:10.1016/S1386-1425(03)00183-5
erties of ZrO2 . In this regard, lanthanide dopants in zirconia are known to stabilize the higher symmetry tetragonal and/or cubic phases [6–8] over a temperature range pertinent to catalytic reactions. Various metal ions incorporated into zirconia materials can achieve special optical properties. Placing f-electron elements in zirconia-based materials has the potential for solid-state photonic device applications [9]. The ability to stabilize each crystalline phase of zirconia at ambient temperature provides an opportunity for correlating the optical properties with the structure. In addition, structural modifications may impact the electronic structure of the lattice, and useful chemical and optical properties could emerge. As the main structural difference between these zirconia phases is due to displacements of the oxygen atoms in the lattices, it is of interest to follow the spectroscopic consequences accompany the structural modifications. Most of the luminescence work conducted on zirconia based materials to date has involved the cubic and/or tetragonal structures [10]. A number of cubic stabilized ZrO2 ma-
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terials are known to display emissions in the yellow-orange region at ∼590 nm, as well as in the blue–green region at ∼510 nm [11]. The type of dopant, it’s concentration, and the specific preparation methods are important factors that influence the emission properties. For example, cubic zirconia stabilized by MgO provides a blue green emission (at ∼510 nm) for an excitation with 254 nm radiation [12]. In contrast, a yellow–orange emission (at ∼590 nm) is dominant in a CaO stabilized zirconia, with only a minor contribution of the 510 nm emission [13,14]. Since zirconia is a low-phonon matrix, which minimizes non-radiative quenching, ‘optically active’ dopants have a higher probability for radiative transitions, and thus are attractive for investigations. We have recently embarked on investigating the dependence of various zirconia phases on lanthanide and actinide dopants and their spectral properties [15]. Of special interest has been the dependence of their photo-luminescence behavior as a function of the dopant concentration. Only a few luminescence studies have been conducted on these materials. Consequently, one of our goals has been to understand the effect of structural modifications on the photo-luminescence spectra of the zirconia system. As the highest phonon frequency in zirconia is <650 cm−1 , minimal non-radiative relaxation and/or quenching processes are expected. Hence, the efficiency of the radiative transition from optically-active ions should be increased. Herein we report the temperature dependent, photoluminescence behavior of different zirconia matrices doped with Sm3+ and Nd3+ ions, the nature of excited-state energy-transfer between the host matrix and the Sm3+ ion, and a Raman “fingerprint” technique for the assignment of the materials.
2. Experimental Samples were prepared by mixing appropriate quantities of aqueous solutions of zirconyl and the two lanthanide nitrates, evaporating to dryness and then calcining the resulting solids up to 1775 K in air for ∼20 h. The resulting oxide matrices were first characterized by x-ray diffraction using 114.6 mm Debye-Scherrer cameras (Mo λ␣1,2 radiation). The luminescence investigations were conducted using an Instrument SA’s optical system consisting of a monochromator (model 1000M) attached to CCD, PMT and IR detectors. A 450 W xenon lamp was used as the light source. Data analyses were performed with Grams/32 software (Galactic, version 5.1). The Raman studies were conducted on the same samples using the 457 nm argon-ion laser line (Coherent, model 306), and a double-meter spectrometer (Jobin-Yvon Ramanor model HG.2S) having a resolution of 0.5 cm−1 at 514.5 nm. Signal detection was by a photon counting system, that employed a photo-multiplier tube (Hamamatsu R 636) and a multi-channel analyzer (Nicolet 1170) interfaced with a personal computer. Lifetime measurements were con-
ducted using a pulsed nitrogen laser and an SR400 photon counting system.
3. Results and discussion 3.1. Raman studies Several zirconia samples containing Sm3+ and Nd3+ ions from 0.5 to 50 at.% were prepared in this study. The structure of the different materials was assigned using a Raman “fingerprint” technique, and established in conjunction with X-ray diffraction analyses [16,17]. It was determined that the monoclinic (m) phase of the ZrO2 based material is prevalent for up to 10 at.% Ln3+ substitution. The m-ZrO2 phase has a P21 /c space group (z=4), where the Zr atom is located in a sevenfold coordination [18] environment. The four zirconia nuclei occupy crystallographically independent e-sites and have C1 point symmetry. Based on nuclear site group analysis [19], 18 Raman vibrational modes of 9Ag +9Bg symmetry are predicted for this structure. In Fig. 1 the Raman spectrum of m-ZrO2 is shown along with that of a 0.5 at.% Sm3+ -doped ZrO2 samples. The spectrum of neat m-Sm2 O3 is also shown in the figure in order to facilitate the comparison. As seen in Fig. 1a, 16 of the expected 18 bands are visible with the most intense at 176, 190, 311, 332, 345, 379, 475, 612 and 639 cm−1 . As comparison of this spectrum with that in Fig. 1b indicates, little difference occurs upon the 0.5 at.% incorporation of Sm3+ ions. The absence of local modes assignable to pure m-Sm2 O3 (see Fig. 1c) is consistent with the absence of clustering and/or segregation of sesquioxide in the matrix. Hence it is conclude that the Sm3+ ions are uniformly distributed at crystallograpic sites of the Zr4+ ions. Up to 10 at.% doping, the Raman features associated with the monoclinic phase are dominant [15], albeit slight broadening is evident. Higher dopings result in broadening of
Fig. 1. Raman spectra of: (a) m-ZrO2 ; (b) m-ZrO2 doped with 0.5 at.% Sm3+ ; (c) m-Sm2 O3 .
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Table 1 Stark components observed at liquid nitrogen temperature for different f–f transitions for Sm3+ in m-ZrO2 (at 0.1 at.% Sm3+ doping) Transitions →6 H
4F
a
3/2 7/2 4G 6 5/2 → H5/2 4G 6 7/2 → H9/2 4G 6H → 5/2 7/2 4F 6 3/2 → H9/2 4G 6 7/2 → H11/2 4G 6 a 5/2 → H9/2 4F 6H → 3/2 11/2 4G 6 5/2 → H11/2 4F 6 3/2 → H13/2 a
Fig. 2. Effect of temperature on the PL spectra of m-ZrO2 doped with 0.5 at.% Sm3+ . Excitation wavelength was 330 nm: (a) recorded at 77 K; (b) room temperature.
the bands and disappearance of features associated with the monoclinic phase [15]. At 20 –30 at.% of Sm3+ , a broad band at ∼620 cm−1 corresponding to the cubic phase is the dominant feature, while those arising from the monoclinic phase are absent. Similar results are obtained for the ZrO2 :Nd3+ system [17], where stabilization of the cubic phase was obtained with 25 at.% neodymium. 3.2. Photo-luminescence studies 3.2.1. ZrO2 :Sm3+ system Luminescence studies with Sm3+ in zirconia were conducted at 0.5–50 at.% levels, to provide monoclinic, cubic, and pyrochlore phases. In Fig. 2 is shown a comparison of the emission spectra from a 0.5 at.% sample at both liquid N2 and room temperature. The spectra exhibit two distinct regions. The first region is associated with a broad band centered at 505 nm and having a full-width at half-height (FWHH) of 60 nm. The intensity of this band decreases dramatically as the temperature changes to room temperature (Fig. 2b), and its center blue-shifts to 490 nm. The second region involves sharp emission bands at longer wavelengths that are characteristic of f–f transitions originating from excited levels of the Sm3+ ion. In the zirconia matrix, three of the lowest, excited levels of samarium are emitting. The highest of these emitting levels is the 4 G7/2 state which produces weak bands corresponding to transitions to the 6 H9/2 , and 6 H11/2 ground levels (observed at 562 and 607 nm, respectively). The 4 F3/2 state is the second, highest emitting level, and yields the strongest emissions that correspond to the 4F 6 6 6 6 3/2 → H9/2 , H7/2 , H11/2 , H13/2 transitions. All of the transitions originating from this 4 F3/2 excited level exhibit Stark components due to ground-state splitting. As shown in Table 1, the 4 F3/2 →6 H9/2 transition exhibits five Stark components. The lowest excited state (4 G5/2 ) also provides four bands corresponding to the 4 G5/2 →6 H5/2 , 6 H7/2 ,
Stark components (cm−1 )
Average
18 130, 17 557, 17 290, 16 810, 16 470, 16 100, 15 470, 15 230, 14 720, 13 950,
17 965 17 535 17 216 16 780 16 348 15 926 15 435 15 066 14 586 13 794
17 800 17 530, 17 240, 16 750 16 410, 16 040, 15 400 15 170, 14 580, 13 850,
17 520 17 120 16 400, 16 290, 16 170 16 000, 15 820, 15 670 15 060, 14 970, 14 900 14 460 13 800, 13 720, 13 650
The transitions were observed only at room temperature.
and 6 H11/2 transitions. The 4 G5/2 →6 H5/2 transition is observed at ∼570 nm, and shows three Stark components with a total splitting of ∼40 cm−1 . The dependence of the emission spectra on samarium concentration is shown in Fig. 3. The spectra were collected upon direct f–f excitation of the Sm3+ ion to the 4 G7/2 level with the 457 nm argon laser line. As the concentration of the Sm3+ increases, the emission intensity decreases significantly and the bands appear broader. For example, when compared with the 0.5 mol% sample (Fig. 3a), the emission intensities of the samples with 10 and 20 at.% Sm3+ (Fig. 3b and c) decrease by 6- and 33-fold, respectively. The FWHH of the band at 17560 cm−1 (569.5 nm) changes from 38 to 344 cm−1 when the samarium content changes from 0.5 to 50 at.%. The excitation spectra for of the ZrO2 :Sm3+ samples are shown in Fig. 4. The profiles shown in figures a–d were collected by monitoring the Sm3+ emission at 569.5 nm. The
6H
9/2
Fig. 3. Dependence of emission spectra on Sm3+ concentration. The spectra are collected by direct f –f excitation using the 457 nm argon ion laser line: (a) 0.5; (b) 10; (c) 20; (d) 30; (e) 40; (f) 50 at.% Sm3+ in zirconia. The spectra are separated for visual clarity. Note that the monoclinic phase is dominant in samples (a), (b) and (c). In figures (d) and (e) cubic stabilization has been attained and the emission bands are broader while the intensity is quenched significantly.
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Fig. 4. Excitation spectra as a function of Sm3+ concentration (at.%): (a) 0.5; (b) 1; (c) 10. The spectra were collected at liquid N2 temperature while monitoring the acceptor emission at 569.5 nm; (d) 0.5 at.% at room temperature; (e) 0.5 at.% at 77 K and monitored at the donor emission band at 505 nm.
Fig. 5. Photoluminescence spectra of 10 at.% Nd3+ in zirconia. The 488 nm argon laser line was used for excitation. The band on the extreme left in the 750–840 nm region was collected using a PMT detector, while the rest are collected using a Ge solid-state detector.
broad band at ∼325 nm is dominant in the spectra but its maximum blue shifts slightly with the Sm3+ concentration. Samples with 0.5, 1 and 10 at.% Sm3+ exhibited excitation maxima at 325, 321 and 319 nm, respectively. In addition to this broad band, the excitation spectra consist of several sharp bands corresponding to f–f transitions within the Sm3+ electronic envelope. These bands are observed at 378.4, 406.6 and 467.6 nm, and assigned to transitions between the ground state (6 H5/2 ) and the excited 4 K11/2 , 4L 4 13/2 and G7/2 levels, respectively.
The emission intensity decreases drastically, albeit with little shift in band position, as the composition approaches that associated with the cubic phase. When compared to the sample with a 10 mol% Nd3+ , the emission intensities for the 20 and 25 at.% samples decrease by a factor of 6 and 20, respectively. Although, the most intense band for Nd3+ is commonly observed at 1064 nm [20], a significant red-shift is found in the zirconia matrix. The most intense band appears at 1080 nm (Fig. 6) in all of the samples studied, reflecting a red shift by ∼16 nm due to the matrix effect. However, a much weaker band is evident at ∼1064 nm, and its intensity increases with the neodymium concentration. The maximum number of splittings expected [21] in a non-cubic environment for both Nd3+ and Sm3+ is J+1/2, for an odd number of f-electrons. As mentioned above, several of the f–f transitions provided more splittings than
3.2.2. The ZrO2 :Nd3+ system The incorporation of neodymium in zirconia produces similar structural features [16,17] to those described for samarium. As with samarium, a monoclinic phase is dominant up to 10 at.% substitution of neodymium. However, stabilization of cubic zirconia was attained at 25 at.% substitution, slightly higher value than found for samarium (20 at.%). Upon excitation with the 488 nm argon line to the (4 I9/2 →4 S9/2 ) emission bands originating from two of the lowest excited levels are observed in the 800–1350 nm spectral region. The spectrum of a 10 at.% Nd3+ sample collected at 77 K is shown in Fig. 5. The band observed in the 800–840 nm region corresponds to the 4 F5/2 →4 I9/2 transition and consists of nine well-resolved Stark splittings. The other three bands originate from the next lower 4 F3/2 excited state. The 4 F3/2 →4 I9/2 transition covers the 890-976 nm region, and has six well resolved components. The most intense band corresponds to the 4 F3/2 →4 I11/2 transition, where six well resolved bands are observed at 1062, 1080, 1088, 1110, 1129, and 1130 nm at room temperature. At 77 K the bands appear sharper while the overall emission intensity increases by more than fourfold. Weak bands corresponding to the 4F 4 3/2 → I13/2 transitions also appear at 1360 and 1375 nm. The dependence of the emission profile on the lanthanide content is shown in Fig. 6, for the 4 F3/2 →4 I11/2 transition.
Fig. 6. of the photoluminescence spectra of Nd3+ in the 4 F3/2 →4 I11/2 transition region for samples containing: (a) 10; (b) 20; (c) 25 at.% Nd3+ in zirconia. Sample (a) is mainly monoclinic, while (c) is cubic. Raman and structural studies indicated that sample (b) is biphasic consisting of both the monoclinic and cubic. Note that the normally strong band at 1064 nm is red-shifted by ∼16 nm and maximizes at ∼1080 nm.
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allowed for a single site occupation. The 4 F3/2 →4 I9/2 transition in the 10 at.% Nd3+ sample shows six Stark components, although the maximum number of splittings for a single-site occupation is five. At the 25 at.% of Nd3+ , which possesses cubic symmetry, nine well-separated splittings are observed for the same transition. Similarly, the eight-Stark components observed for the 4 F3/2 →4 I11/2 transition (10 at.%) is in contrast with the expected maximum of six. Hence, the spectroscopic results suggest that the Ln3+ dopants occupy more than one site in the zirconia matrices. The results are consistent with previous studies of Er3+ in cubic zirconia, where two erbium sites were identified in eight and sevenfold coordination sites [22]. Similar coordination environments are anticipated in our samples also. 3.2.3. The energy-transfer process Although, evidence for energy transfer between the host matrix and the Nd3+ ion has not been obtained, the situation with Sm3+ is different in that efficient energy-transfer has been noted. The main argument for energy-transfer in the ZrO2 :Sm3+ system is based on the observance of the donor excitation while monitoring the acceptor emission. The 325 nm excitation band is dominant when the spectrum is monitored either at the zirconia matrix (505 nm) or at the samarium emission bands. Although Sm3+ has negligible absorbance [21] in the 300–340 nm spectral regions, the prominence of the 325 nm band in its excitation spectrum provides direct evidence for the existence of energy-transfer from the matrix, and proceeds efficiently when compared to direct f–f transitions. A spectral overlap region exists between the donor emission in the 455–500 nm spectral region and the Sm3+ absorption involving the 6 H5/2 →4 G7/2 transition. The system, therefore, satisfies one of the essential requirements for energy transfer processes. As shown in Fig. 7a, the energy transfer persists at the 20 at.% Sm3+ level, even when the monoclinic phase is absent. Complete transformation to the cubic phase at the 20 at.% doping level has been confirmed
Fig. 7. Emission spectra of the cubic ZrO2 matrix with different lanthanide concentrations: (a) 20; (b) 30 at.% of Sm3+ ; (c) 25 at.% Nd3+ .
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based on the Raman and structural studies [15]. To the best of our knowledge, this is the first reported example of energy transfer from cubic zirconia, although similar results have been reported recently for monoclinic zirconia [23]. When compared to the 77 K profile the donor emission is quenched significantly at room temperature. The acceptor samarium emission also decreases concomitantly, although the quenching is less when compared to that of the donor. As a result, the ratio of the acceptor to donor intensities (based on peak heights) increases by more than twofold (1.6 vs. 3.5) in going from 77 K to room temperature, respectively. It is reasonable to assume correlation between the increased ratio and blue-shifting of the donor emission exhibited at room temperature. Blue-shifting creates a better match-up between the donor emission and the acceptor 6H 4 5/2 → G7/2 transition. Hence, the donor/acceptor spectral overlap and by inference the energy transfer process increases at room temperature. However, the absolute intensity of the acceptor emission decreases with temperature, suggesting a non-radiative de-excitation process competing with the radiative pathway at higher temperatures. Between 77 K and room temperatures, the emission lifetime of the Sm3+ ion (measured at 569.5 nm) decreased by more than tenfold (8.2 vs. 0.6 s, respectively). A decrease in the acceptor lifetime with a temperature increase is consistent with a non-radiative process being responsible for the quenching of the emission at room temperature. Similarly, the lifetime of the donor emission (measured at 500 nm) at 77 K is 600 s and decreases to 2 s at room temperature. As suggested above, the decrease in the donor lifetime is also consistent with an increased donor–acceptor interaction at room temperature. 3.3. Defect centers and photo-luminescence One consequence of replacing a tetravalent Zr4+ by a trivalent Ln3+ ion is formation of defect centers, where charge neutrality requires each substitution creates a 0.5 anion vacancy [10]. As a result, lattices with different short-range coordination sites will be available either for dopant ions or defect centers. Understanding the nature of defect centers in doped zirconia has generated extensive interest in recent years due to the influence of intrinsic defects on the optical properties of the materials [10,24]. For example, zirconia materials doped with alkaline-earth metals display broad emission at 600 nm, while emission in Y3+ -doped products maximizes at ∼510 nm [13,24,25]. The luminescence behavior displayed by the Sm3+ – and Nd3+ –zirconia systems is similar to that reported for the Y3+ -doped material. In addition, the excitation maximum (325 nm, 3.8 eV) appeared at a lower energy than the band-gap reported for the zirconia matrix (∼5.0 eV) [10]. Besides, a large Stokes shift (>11,000 cm−1 ) exists between the excitation wavelength and the emission maximum at 505 nm. Although the origin and detailed mechanism of the blue–green emission has been a subject of controversy for
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a number of years [26–28], it is now associated with anion vacancies involving sevenfold coordination sites [12]. The studies also indicated that substitution of Zr4+ by trivalent ions increase the concentration of oxygen vacancies in the matrix, and induce the creation of cation lattice sites with lower point symmetries. One of the major defects created during the substitution processes is the F-center [29]. The center involves anion vacancy surrounded by Zr4+ ions as its near neighbors. Another type of defect commonly exhibited in these materials involves the FA and FAA type-center, where one- and two- trivalent Ln3+ cations, respectively, are located as near neighbors. Both types of defect centers are luminescent, and display characteristic broad emissions upon UV excitation. Emission originating from the former defect center (F+ ) is usually exhibited at ∼450 nm. The latter sites have energy levels lying within the band gap of the zirconia matrix and display characteristic emission at longer wavelengths (>500 nm) [30]. Based on the luminescence spectral profile, the latter-type defects, in particular the FA centers appear the important components in the Sm3+ and Nd3+ -doped zirconia matrices. In Fig. 7, the emission behavior of the zirconia matrix is shown for 20 and 30 at.% Sm3+ , and 25 at.% Nd3+ . The zirconia matrix has somewhat stronger emission at the 20 at.% Sm3+ doping. Both the X-ray and Raman data indicated a cubic structure for all three samples shown in the figure. The energy transfer process persists at 20 at.% Sm3+ , even though the intensities of both the donor and acceptor emissions are quenched significantly when compared to the monoclinic phase. This is also consistent with previous results where a stronger luminescence has been reported [31] for the monoclinic phase than for tetragonal or cubic phases. At 30 at.% Sm3+ , emission corresponding to the acceptor (upon donor excitation) is no longer evident (Fig. 7b), even at 77 K. However, direct f–f excitation (Fig. 3) does provide weak, but characteristics emissions of the Sm3+ ion. It is likely that changes in defect center types and/or environments may influence the efficiency of the excite-state interactions in the cubic and pyrochlore structures.
4. Conclusion Photoluminescence and Raman studies were conducted on Sm3+ - and Nd3+ -ions doped in zirconia matrices. In monoclinic zirconia, strong emissions originating from both the matrix and dopant ions were noted. Exclusive excitation of the zirconia matrix leads to sensitized luminescence from the acceptor Sm3+ ion. The excitation profile consists of a broad band at 325 nm, when monitored either at the donor or acceptor emission bands. A spectral overlap region exists between the zirconia emission and the Sm3+ absorption, which leads to efficient energy transfer between them. The ratio of the acceptor/donor emission intensity increases by twofold upon changing the temperature from liquid nitrogen
to room temperature. The energy-transfer persists even at room temperature, albeit a competing thermal non-radiative process decreases the overall emission. In contrast to the zirconia–Sm3+ system, the zirconia–Nd3+ system does not display the energy-transfer interaction, and the sensitized emissions. Only direct f–f excitation provided strong emission in the infrared region.
Acknowledgements This research was sponsored by the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences, US Department of Energy, under contract DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed by UT-Battelle, LLC. P.E.R also acknowledges the support from Commissariat à l’Energie Atomique and Electricit de France.
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Photoluminescence and Raman studies of Sm3+ and Nd3+ ions in zirconia matrices: example of energy transfer and host–guest interactions Zerihun Assefa a,∗ , R.G. Haire a , P.E. Raison b,1 b
a Oak Ridge National Laboratory, Chemical Sciences Division, MS 6375, Oak Ridge, TN 37831-6375, USA European Commission-Institute for Energy, Joint Research Center of Petten, Postbus Nr. 2 1755 ZG Petten, The Netherlands
Received 4 March 2003; accepted 31 March 2003
Abstract Photoluminescence and Raman studies on Sm3+ - and Nd3+ -doped zirconia are reported. The Raman studies indicate that the monoclinic (m) phase dominates up to a 10 at.% lanthanide level, while stabilization of the cubic phase is attained at ∼20 and ∼25 at.% of Sm3+ and Nd3+ , respectively. Both systems are strongly luminescent under photo-excitation. The emission spectrum at 77 K of the ZrO2 :Sm3+ system consists of a broad band at 505 nm, that corresponds to the zirconia matrix. At room temperature the band maximum blue-shifts to 490 nm. Sharper bands corresponding to f–f transitions within the Sm3+ ion are also exhibited in the longer wavelength region of the spectrum. Exclusive excitation of the zirconia matrix provides sensitized emission from the acceptor Sm3+ ion. The excitation profile is dominated by a broad band at 325 nm when monitored either at the zirconia or at one of the Sm3+ emissions. A spectral overlap between the 6 H5/2 →4 G7/2 absorption of the Sm3+ ion with the zirconia emission leads to an efficient energy transfer process in the systems. Multiple facets of the spectral behavior of the Sm3+ or Nd3+ in the zirconia matrices, as well as the effects of compositions on the emission and Raman properties of the materials, and the role of defect centers in photoluminescence and the energy transfer processes are discussed. Published by Elsevier B.V. Keywords: Photo-luminescence; Energy-transfer; Lanthanide; Sensitized emission
1. Introduction Zirconia-based oxide ceramics are attractive for a variety of applications, such as fuel cells, oxygen sensors, refractory materials, and optical transparency [1–4]. Several attributes influence the physical and chemical properties of these materials, which include crystal structures, type and level of dopant, as well as the temperature [4]. Pure ZrO2 exists in three polymorphic phases (monoclinic, tetragonal or cubic), with the more symmetrical phases being obtained with increasing temperatures. Doping zirconia with different cations [5] can stabilize the cubic and tetragonal phases at lower or even ambient temperatures. This stabilization improves important mechanical and electrical prop∗ Corresponding author. Tel.: +1-865-754-5013; fax: +1-865-574 4987. E-mail address: [email protected] (Z. Assefa). 1 On leave from the Commissariat a ` l’Energie Atomique, CEA-Cadarache DEN/DEC/SPUA/ LMPC 13108, France.
1386-1425/$ – see front matter. Published by Elsevier B.V. doi:10.1016/S1386-1425(03)00183-5
erties of ZrO2 . In this regard, lanthanide dopants in zirconia are known to stabilize the higher symmetry tetragonal and/or cubic phases [6–8] over a temperature range pertinent to catalytic reactions. Various metal ions incorporated into zirconia materials can achieve special optical properties. Placing f-electron elements in zirconia-based materials has the potential for solid-state photonic device applications [9]. The ability to stabilize each crystalline phase of zirconia at ambient temperature provides an opportunity for correlating the optical properties with the structure. In addition, structural modifications may impact the electronic structure of the lattice, and useful chemical and optical properties could emerge. As the main structural difference between these zirconia phases is due to displacements of the oxygen atoms in the lattices, it is of interest to follow the spectroscopic consequences accompany the structural modifications. Most of the luminescence work conducted on zirconia based materials to date has involved the cubic and/or tetragonal structures [10]. A number of cubic stabilized ZrO2 ma-
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terials are known to display emissions in the yellow-orange region at ∼590 nm, as well as in the blue–green region at ∼510 nm [11]. The type of dopant, it’s concentration, and the specific preparation methods are important factors that influence the emission properties. For example, cubic zirconia stabilized by MgO provides a blue green emission (at ∼510 nm) for an excitation with 254 nm radiation [12]. In contrast, a yellow–orange emission (at ∼590 nm) is dominant in a CaO stabilized zirconia, with only a minor contribution of the 510 nm emission [13,14]. Since zirconia is a low-phonon matrix, which minimizes non-radiative quenching, ‘optically active’ dopants have a higher probability for radiative transitions, and thus are attractive for investigations. We have recently embarked on investigating the dependence of various zirconia phases on lanthanide and actinide dopants and their spectral properties [15]. Of special interest has been the dependence of their photo-luminescence behavior as a function of the dopant concentration. Only a few luminescence studies have been conducted on these materials. Consequently, one of our goals has been to understand the effect of structural modifications on the photo-luminescence spectra of the zirconia system. As the highest phonon frequency in zirconia is <650 cm−1 , minimal non-radiative relaxation and/or quenching processes are expected. Hence, the efficiency of the radiative transition from optically-active ions should be increased. Herein we report the temperature dependent, photoluminescence behavior of different zirconia matrices doped with Sm3+ and Nd3+ ions, the nature of excited-state energy-transfer between the host matrix and the Sm3+ ion, and a Raman “fingerprint” technique for the assignment of the materials.
2. Experimental Samples were prepared by mixing appropriate quantities of aqueous solutions of zirconyl and the two lanthanide nitrates, evaporating to dryness and then calcining the resulting solids up to 1775 K in air for ∼20 h. The resulting oxide matrices were first characterized by x-ray diffraction using 114.6 mm Debye-Scherrer cameras (Mo λ␣1,2 radiation). The luminescence investigations were conducted using an Instrument SA’s optical system consisting of a monochromator (model 1000M) attached to CCD, PMT and IR detectors. A 450 W xenon lamp was used as the light source. Data analyses were performed with Grams/32 software (Galactic, version 5.1). The Raman studies were conducted on the same samples using the 457 nm argon-ion laser line (Coherent, model 306), and a double-meter spectrometer (Jobin-Yvon Ramanor model HG.2S) having a resolution of 0.5 cm−1 at 514.5 nm. Signal detection was by a photon counting system, that employed a photo-multiplier tube (Hamamatsu R 636) and a multi-channel analyzer (Nicolet 1170) interfaced with a personal computer. Lifetime measurements were con-
ducted using a pulsed nitrogen laser and an SR400 photon counting system.
3. Results and discussion 3.1. Raman studies Several zirconia samples containing Sm3+ and Nd3+ ions from 0.5 to 50 at.% were prepared in this study. The structure of the different materials was assigned using a Raman “fingerprint” technique, and established in conjunction with X-ray diffraction analyses [16,17]. It was determined that the monoclinic (m) phase of the ZrO2 based material is prevalent for up to 10 at.% Ln3+ substitution. The m-ZrO2 phase has a P21 /c space group (z=4), where the Zr atom is located in a sevenfold coordination [18] environment. The four zirconia nuclei occupy crystallographically independent e-sites and have C1 point symmetry. Based on nuclear site group analysis [19], 18 Raman vibrational modes of 9Ag +9Bg symmetry are predicted for this structure. In Fig. 1 the Raman spectrum of m-ZrO2 is shown along with that of a 0.5 at.% Sm3+ -doped ZrO2 samples. The spectrum of neat m-Sm2 O3 is also shown in the figure in order to facilitate the comparison. As seen in Fig. 1a, 16 of the expected 18 bands are visible with the most intense at 176, 190, 311, 332, 345, 379, 475, 612 and 639 cm−1 . As comparison of this spectrum with that in Fig. 1b indicates, little difference occurs upon the 0.5 at.% incorporation of Sm3+ ions. The absence of local modes assignable to pure m-Sm2 O3 (see Fig. 1c) is consistent with the absence of clustering and/or segregation of sesquioxide in the matrix. Hence it is conclude that the Sm3+ ions are uniformly distributed at crystallograpic sites of the Zr4+ ions. Up to 10 at.% doping, the Raman features associated with the monoclinic phase are dominant [15], albeit slight broadening is evident. Higher dopings result in broadening of
Fig. 1. Raman spectra of: (a) m-ZrO2 ; (b) m-ZrO2 doped with 0.5 at.% Sm3+ ; (c) m-Sm2 O3 .
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Table 1 Stark components observed at liquid nitrogen temperature for different f–f transitions for Sm3+ in m-ZrO2 (at 0.1 at.% Sm3+ doping) Transitions →6 H
4F
a
3/2 7/2 4G 6 5/2 → H5/2 4G 6 7/2 → H9/2 4G 6H → 5/2 7/2 4F 6 3/2 → H9/2 4G 6 7/2 → H11/2 4G 6 a 5/2 → H9/2 4F 6H → 3/2 11/2 4G 6 5/2 → H11/2 4F 6 3/2 → H13/2 a
Fig. 2. Effect of temperature on the PL spectra of m-ZrO2 doped with 0.5 at.% Sm3+ . Excitation wavelength was 330 nm: (a) recorded at 77 K; (b) room temperature.
the bands and disappearance of features associated with the monoclinic phase [15]. At 20 –30 at.% of Sm3+ , a broad band at ∼620 cm−1 corresponding to the cubic phase is the dominant feature, while those arising from the monoclinic phase are absent. Similar results are obtained for the ZrO2 :Nd3+ system [17], where stabilization of the cubic phase was obtained with 25 at.% neodymium. 3.2. Photo-luminescence studies 3.2.1. ZrO2 :Sm3+ system Luminescence studies with Sm3+ in zirconia were conducted at 0.5–50 at.% levels, to provide monoclinic, cubic, and pyrochlore phases. In Fig. 2 is shown a comparison of the emission spectra from a 0.5 at.% sample at both liquid N2 and room temperature. The spectra exhibit two distinct regions. The first region is associated with a broad band centered at 505 nm and having a full-width at half-height (FWHH) of 60 nm. The intensity of this band decreases dramatically as the temperature changes to room temperature (Fig. 2b), and its center blue-shifts to 490 nm. The second region involves sharp emission bands at longer wavelengths that are characteristic of f–f transitions originating from excited levels of the Sm3+ ion. In the zirconia matrix, three of the lowest, excited levels of samarium are emitting. The highest of these emitting levels is the 4 G7/2 state which produces weak bands corresponding to transitions to the 6 H9/2 , and 6 H11/2 ground levels (observed at 562 and 607 nm, respectively). The 4 F3/2 state is the second, highest emitting level, and yields the strongest emissions that correspond to the 4F 6 6 6 6 3/2 → H9/2 , H7/2 , H11/2 , H13/2 transitions. All of the transitions originating from this 4 F3/2 excited level exhibit Stark components due to ground-state splitting. As shown in Table 1, the 4 F3/2 →6 H9/2 transition exhibits five Stark components. The lowest excited state (4 G5/2 ) also provides four bands corresponding to the 4 G5/2 →6 H5/2 , 6 H7/2 ,
Stark components (cm−1 )
Average
18 130, 17 557, 17 290, 16 810, 16 470, 16 100, 15 470, 15 230, 14 720, 13 950,
17 965 17 535 17 216 16 780 16 348 15 926 15 435 15 066 14 586 13 794
17 800 17 530, 17 240, 16 750 16 410, 16 040, 15 400 15 170, 14 580, 13 850,
17 520 17 120 16 400, 16 290, 16 170 16 000, 15 820, 15 670 15 060, 14 970, 14 900 14 460 13 800, 13 720, 13 650
The transitions were observed only at room temperature.
and 6 H11/2 transitions. The 4 G5/2 →6 H5/2 transition is observed at ∼570 nm, and shows three Stark components with a total splitting of ∼40 cm−1 . The dependence of the emission spectra on samarium concentration is shown in Fig. 3. The spectra were collected upon direct f–f excitation of the Sm3+ ion to the 4 G7/2 level with the 457 nm argon laser line. As the concentration of the Sm3+ increases, the emission intensity decreases significantly and the bands appear broader. For example, when compared with the 0.5 mol% sample (Fig. 3a), the emission intensities of the samples with 10 and 20 at.% Sm3+ (Fig. 3b and c) decrease by 6- and 33-fold, respectively. The FWHH of the band at 17560 cm−1 (569.5 nm) changes from 38 to 344 cm−1 when the samarium content changes from 0.5 to 50 at.%. The excitation spectra for of the ZrO2 :Sm3+ samples are shown in Fig. 4. The profiles shown in figures a–d were collected by monitoring the Sm3+ emission at 569.5 nm. The
6H
9/2
Fig. 3. Dependence of emission spectra on Sm3+ concentration. The spectra are collected by direct f –f excitation using the 457 nm argon ion laser line: (a) 0.5; (b) 10; (c) 20; (d) 30; (e) 40; (f) 50 at.% Sm3+ in zirconia. The spectra are separated for visual clarity. Note that the monoclinic phase is dominant in samples (a), (b) and (c). In figures (d) and (e) cubic stabilization has been attained and the emission bands are broader while the intensity is quenched significantly.
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Fig. 4. Excitation spectra as a function of Sm3+ concentration (at.%): (a) 0.5; (b) 1; (c) 10. The spectra were collected at liquid N2 temperature while monitoring the acceptor emission at 569.5 nm; (d) 0.5 at.% at room temperature; (e) 0.5 at.% at 77 K and monitored at the donor emission band at 505 nm.
Fig. 5. Photoluminescence spectra of 10 at.% Nd3+ in zirconia. The 488 nm argon laser line was used for excitation. The band on the extreme left in the 750–840 nm region was collected using a PMT detector, while the rest are collected using a Ge solid-state detector.
broad band at ∼325 nm is dominant in the spectra but its maximum blue shifts slightly with the Sm3+ concentration. Samples with 0.5, 1 and 10 at.% Sm3+ exhibited excitation maxima at 325, 321 and 319 nm, respectively. In addition to this broad band, the excitation spectra consist of several sharp bands corresponding to f–f transitions within the Sm3+ electronic envelope. These bands are observed at 378.4, 406.6 and 467.6 nm, and assigned to transitions between the ground state (6 H5/2 ) and the excited 4 K11/2 , 4L 4 13/2 and G7/2 levels, respectively.
The emission intensity decreases drastically, albeit with little shift in band position, as the composition approaches that associated with the cubic phase. When compared to the sample with a 10 mol% Nd3+ , the emission intensities for the 20 and 25 at.% samples decrease by a factor of 6 and 20, respectively. Although, the most intense band for Nd3+ is commonly observed at 1064 nm [20], a significant red-shift is found in the zirconia matrix. The most intense band appears at 1080 nm (Fig. 6) in all of the samples studied, reflecting a red shift by ∼16 nm due to the matrix effect. However, a much weaker band is evident at ∼1064 nm, and its intensity increases with the neodymium concentration. The maximum number of splittings expected [21] in a non-cubic environment for both Nd3+ and Sm3+ is J+1/2, for an odd number of f-electrons. As mentioned above, several of the f–f transitions provided more splittings than
3.2.2. The ZrO2 :Nd3+ system The incorporation of neodymium in zirconia produces similar structural features [16,17] to those described for samarium. As with samarium, a monoclinic phase is dominant up to 10 at.% substitution of neodymium. However, stabilization of cubic zirconia was attained at 25 at.% substitution, slightly higher value than found for samarium (20 at.%). Upon excitation with the 488 nm argon line to the (4 I9/2 →4 S9/2 ) emission bands originating from two of the lowest excited levels are observed in the 800–1350 nm spectral region. The spectrum of a 10 at.% Nd3+ sample collected at 77 K is shown in Fig. 5. The band observed in the 800–840 nm region corresponds to the 4 F5/2 →4 I9/2 transition and consists of nine well-resolved Stark splittings. The other three bands originate from the next lower 4 F3/2 excited state. The 4 F3/2 →4 I9/2 transition covers the 890-976 nm region, and has six well resolved components. The most intense band corresponds to the 4 F3/2 →4 I11/2 transition, where six well resolved bands are observed at 1062, 1080, 1088, 1110, 1129, and 1130 nm at room temperature. At 77 K the bands appear sharper while the overall emission intensity increases by more than fourfold. Weak bands corresponding to the 4F 4 3/2 → I13/2 transitions also appear at 1360 and 1375 nm. The dependence of the emission profile on the lanthanide content is shown in Fig. 6, for the 4 F3/2 →4 I11/2 transition.
Fig. 6. of the photoluminescence spectra of Nd3+ in the 4 F3/2 →4 I11/2 transition region for samples containing: (a) 10; (b) 20; (c) 25 at.% Nd3+ in zirconia. Sample (a) is mainly monoclinic, while (c) is cubic. Raman and structural studies indicated that sample (b) is biphasic consisting of both the monoclinic and cubic. Note that the normally strong band at 1064 nm is red-shifted by ∼16 nm and maximizes at ∼1080 nm.
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allowed for a single site occupation. The 4 F3/2 →4 I9/2 transition in the 10 at.% Nd3+ sample shows six Stark components, although the maximum number of splittings for a single-site occupation is five. At the 25 at.% of Nd3+ , which possesses cubic symmetry, nine well-separated splittings are observed for the same transition. Similarly, the eight-Stark components observed for the 4 F3/2 →4 I11/2 transition (10 at.%) is in contrast with the expected maximum of six. Hence, the spectroscopic results suggest that the Ln3+ dopants occupy more than one site in the zirconia matrices. The results are consistent with previous studies of Er3+ in cubic zirconia, where two erbium sites were identified in eight and sevenfold coordination sites [22]. Similar coordination environments are anticipated in our samples also. 3.2.3. The energy-transfer process Although, evidence for energy transfer between the host matrix and the Nd3+ ion has not been obtained, the situation with Sm3+ is different in that efficient energy-transfer has been noted. The main argument for energy-transfer in the ZrO2 :Sm3+ system is based on the observance of the donor excitation while monitoring the acceptor emission. The 325 nm excitation band is dominant when the spectrum is monitored either at the zirconia matrix (505 nm) or at the samarium emission bands. Although Sm3+ has negligible absorbance [21] in the 300–340 nm spectral regions, the prominence of the 325 nm band in its excitation spectrum provides direct evidence for the existence of energy-transfer from the matrix, and proceeds efficiently when compared to direct f–f transitions. A spectral overlap region exists between the donor emission in the 455–500 nm spectral region and the Sm3+ absorption involving the 6 H5/2 →4 G7/2 transition. The system, therefore, satisfies one of the essential requirements for energy transfer processes. As shown in Fig. 7a, the energy transfer persists at the 20 at.% Sm3+ level, even when the monoclinic phase is absent. Complete transformation to the cubic phase at the 20 at.% doping level has been confirmed
Fig. 7. Emission spectra of the cubic ZrO2 matrix with different lanthanide concentrations: (a) 20; (b) 30 at.% of Sm3+ ; (c) 25 at.% Nd3+ .
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based on the Raman and structural studies [15]. To the best of our knowledge, this is the first reported example of energy transfer from cubic zirconia, although similar results have been reported recently for monoclinic zirconia [23]. When compared to the 77 K profile the donor emission is quenched significantly at room temperature. The acceptor samarium emission also decreases concomitantly, although the quenching is less when compared to that of the donor. As a result, the ratio of the acceptor to donor intensities (based on peak heights) increases by more than twofold (1.6 vs. 3.5) in going from 77 K to room temperature, respectively. It is reasonable to assume correlation between the increased ratio and blue-shifting of the donor emission exhibited at room temperature. Blue-shifting creates a better match-up between the donor emission and the acceptor 6H 4 5/2 → G7/2 transition. Hence, the donor/acceptor spectral overlap and by inference the energy transfer process increases at room temperature. However, the absolute intensity of the acceptor emission decreases with temperature, suggesting a non-radiative de-excitation process competing with the radiative pathway at higher temperatures. Between 77 K and room temperatures, the emission lifetime of the Sm3+ ion (measured at 569.5 nm) decreased by more than tenfold (8.2 vs. 0.6 s, respectively). A decrease in the acceptor lifetime with a temperature increase is consistent with a non-radiative process being responsible for the quenching of the emission at room temperature. Similarly, the lifetime of the donor emission (measured at 500 nm) at 77 K is 600 s and decreases to 2 s at room temperature. As suggested above, the decrease in the donor lifetime is also consistent with an increased donor–acceptor interaction at room temperature. 3.3. Defect centers and photo-luminescence One consequence of replacing a tetravalent Zr4+ by a trivalent Ln3+ ion is formation of defect centers, where charge neutrality requires each substitution creates a 0.5 anion vacancy [10]. As a result, lattices with different short-range coordination sites will be available either for dopant ions or defect centers. Understanding the nature of defect centers in doped zirconia has generated extensive interest in recent years due to the influence of intrinsic defects on the optical properties of the materials [10,24]. For example, zirconia materials doped with alkaline-earth metals display broad emission at 600 nm, while emission in Y3+ -doped products maximizes at ∼510 nm [13,24,25]. The luminescence behavior displayed by the Sm3+ – and Nd3+ –zirconia systems is similar to that reported for the Y3+ -doped material. In addition, the excitation maximum (325 nm, 3.8 eV) appeared at a lower energy than the band-gap reported for the zirconia matrix (∼5.0 eV) [10]. Besides, a large Stokes shift (>11,000 cm−1 ) exists between the excitation wavelength and the emission maximum at 505 nm. Although the origin and detailed mechanism of the blue–green emission has been a subject of controversy for
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a number of years [26–28], it is now associated with anion vacancies involving sevenfold coordination sites [12]. The studies also indicated that substitution of Zr4+ by trivalent ions increase the concentration of oxygen vacancies in the matrix, and induce the creation of cation lattice sites with lower point symmetries. One of the major defects created during the substitution processes is the F-center [29]. The center involves anion vacancy surrounded by Zr4+ ions as its near neighbors. Another type of defect commonly exhibited in these materials involves the FA and FAA type-center, where one- and two- trivalent Ln3+ cations, respectively, are located as near neighbors. Both types of defect centers are luminescent, and display characteristic broad emissions upon UV excitation. Emission originating from the former defect center (F+ ) is usually exhibited at ∼450 nm. The latter sites have energy levels lying within the band gap of the zirconia matrix and display characteristic emission at longer wavelengths (>500 nm) [30]. Based on the luminescence spectral profile, the latter-type defects, in particular the FA centers appear the important components in the Sm3+ and Nd3+ -doped zirconia matrices. In Fig. 7, the emission behavior of the zirconia matrix is shown for 20 and 30 at.% Sm3+ , and 25 at.% Nd3+ . The zirconia matrix has somewhat stronger emission at the 20 at.% Sm3+ doping. Both the X-ray and Raman data indicated a cubic structure for all three samples shown in the figure. The energy transfer process persists at 20 at.% Sm3+ , even though the intensities of both the donor and acceptor emissions are quenched significantly when compared to the monoclinic phase. This is also consistent with previous results where a stronger luminescence has been reported [31] for the monoclinic phase than for tetragonal or cubic phases. At 30 at.% Sm3+ , emission corresponding to the acceptor (upon donor excitation) is no longer evident (Fig. 7b), even at 77 K. However, direct f–f excitation (Fig. 3) does provide weak, but characteristics emissions of the Sm3+ ion. It is likely that changes in defect center types and/or environments may influence the efficiency of the excite-state interactions in the cubic and pyrochlore structures.
4. Conclusion Photoluminescence and Raman studies were conducted on Sm3+ - and Nd3+ -ions doped in zirconia matrices. In monoclinic zirconia, strong emissions originating from both the matrix and dopant ions were noted. Exclusive excitation of the zirconia matrix leads to sensitized luminescence from the acceptor Sm3+ ion. The excitation profile consists of a broad band at 325 nm, when monitored either at the donor or acceptor emission bands. A spectral overlap region exists between the zirconia emission and the Sm3+ absorption, which leads to efficient energy transfer between them. The ratio of the acceptor/donor emission intensity increases by twofold upon changing the temperature from liquid nitrogen
to room temperature. The energy-transfer persists even at room temperature, albeit a competing thermal non-radiative process decreases the overall emission. In contrast to the zirconia–Sm3+ system, the zirconia–Nd3+ system does not display the energy-transfer interaction, and the sensitized emissions. Only direct f–f excitation provided strong emission in the infrared region.
Acknowledgements This research was sponsored by the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences, US Department of Energy, under contract DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed by UT-Battelle, LLC. P.E.R also acknowledges the support from Commissariat à l’Energie Atomique and Electricit de France.
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