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Luminescence of the Cr3+ ions doped in PLZT ceramics
Chemical Physics Letters 506 (2011) 201–204
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Luminescence of the Cr3+ ions doped in PLZT ceramics T.P.J. Han a,⇑, M.T. Jardiel b, M. Villegas b, A.C. Caballero b, D. Bravo c, F. Jaque a a
Department of Physics, University of Strathclyde, Glasgow, G4 ONG, Scotland, UK Department of Electroceramics, Instituto de Cerámica y Vidrio, CSIC Kelsen 5, Madrid 28049, Spain c Departamento de Física de Materiales, Universidad Autónoma de Madrid, Madrid 28049, Spain b
a r t i c l e
i n f o
Article history: Received 16 December 2010 In final form 9 March 2011 Available online 12 March 2011
a b s t r a c t The photoluminescence properties of Cr-doped PLZT-9/65/35 (Pb0.95La0.09Zr0.65Ti0.35O3) have been studied. The fluorescence spectra consist of two inhomogeneously broadened narrow bands centred at 734 and 740 nm and a broadband emission with a maximum at 1000 nm. The two high energy bands have been assigned to the 2 Eð2 T1 Þ ! 4 A2 transition of two Cr3+ defect centres in octahedral site with high/ intermediate crystal field strength whereas the near infrared emission is tentatively assigned to the overlapping of the broadband emission of the Cr3+ ions defect centre’s 4 T2 ! 4 A2 transition and the emission of the Cr5+ ions’ 2 E ! 2 T2 transition. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Lead lanthanum zirconate titanate (PLZT) is a typical ferroelectric relaxor material with very interesting piezoelectric properties for applications in optoelectronics devices, particularly in ferroelectric memories applications [1]. In general, the properties of PLZT materials are very sensitive to the processing method because it is very difficult to maintain the stoichiometric composition during the various different thermal treatments. Composition variation within a sample or from sample to sample results in inconsistency and degrading of the piezoelectric properties [2]. It is therefore important to have detail knowledge of a number of properties that have direct effect on the ferroelectric properties, such as defects formation, changes in composition and/or grain sizes, due to the fabrication processes in PLZT ceramics. In other ferroelectric materials with similar stoichiometric problems, such as Lithium Niobate (LiNbO3) and Strontium Barium Niobate (SBN), the use of Cr3+ ions as an optical and paramagnetic probe has been very useful in the characterisation of defects formation during the crystal growth procedure [3]. For this purpose, different optical and luminescence spectroscopic techniques such as absorption, photoluminescence, fluorescence decaytime, timeresolved spectroscopy and electron paramagnetic resonance (EPR) techniques were utilised. These studies allow the determination of the presence of Cr ions in different valence states and defect centres they formed as consequence of changes in the stoichiometric compositions. However, to-dates little work has been reported on applying the Cr ions as probe to the PLZT ferroelectrics families. In this work a photoluminescence study of Cr-doped PLZT-9/65/35 (Pb0.95La0.09Zr0.65Ti0.35O3) ceramic samples is presented. This study ⇑ Corresponding author. E-mail address: [email protected] (T.P.J. Han). 0009-2614/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2011.03.034
involved the excitation at several wavelengths between 450 and 650 nm and at temperatures range from 10 to 100 K. 2. Experimental The precursor of PLZT was prepared from stoichiometric quantities of the solutions of lead acetate in acetic acid and tetrabutyl titanium, zirconium nitrate and lanthanum nitrate in 2-methoxyetanol. In order to adjust the concentration, the 2-methoxyetanol and acetic acid solvents with a volume ratio of 5/1.3 were then mixed together under agitation to give a 0.3 M PLZT sol–gel solution. The stoichiometric amount of chromium nitrate (corresponding to a 1 wt.% of chromium oxide) was added to this solution. In this condition the sol obtained has a green colour. This sol was kept under agitation at 313 K for 24 h and then the temperature is increased to 353 K to obtain a viscose gel. The resin was heated at 383 K to produce a brown colour powder. This powder was calcined for 2 h at 1023 K to form the Cr doped PLZT phase. Compacted discs were prepared from this powder by uniaxial pressing at 200 MPa. Dense polycrystalline samples (95–96% of the theoretical density) were obtained after sintering at 1323 K for 4 h. In order to control the PbO volatilisation, a PZT commercial buffer inside the crucible was used during the thermal treatment. Photoluminescence spectra were obtained in the 10–100 K temperature range excited using different wavelengths of a continuous wave (cw) Ar-ion laser (457.9, 488.0 and 514.5 nm), a continuous wave frequency doubled output of a Nd:YAG laser at 532 and at 610 nm from a continuous wave dye laser. The emission light was dispersed by a SPEX 500 M monochromator and detected with a Photomultiplier tube (Hamamatsu R928) in the region from 400 to 800 nm and a solid state InGaAs detector for the region beyond 800 nm. The geometry of the experimental is such that excitation laser beam is focused with a 10 cm focal length lens at a glancing
T.P.J. Han et al. / Chemical Physics Letters 506 (2011) 201–204
Figure 1a shows the emission spectrum in the 710–760 nm wavelength range excited by the 488 nm laser output at 10 K. The spectrum consists of an asymmetric and complex band with a prominent peak centred at 740.5 nm (13 504 cm1). There is also an underlying background emission with intensity increasing towards the longer wavelength region indicating the presence of another broader band extending from the near infrared (IR) region. Figure 1b and c show the 10 K emission spectra obtained under cw 532 and 610 nm excitation respectively. It is clear that both emission bands are more symmetrical in appearance than that the on excited using 488 nm, even though the background emission is still presence in both cases. The emission bands in Figure 1b and c have their maximum at 741.7 nm (13 483 cm1) and 739.6 nm (13 521 cm1) respectively. It is evidenced that there is a shift of the peak position of the band excitation and the overall emission intensity decreases from 610, 532 to 488 nm. This emission band is analysed by fitting to it using two Gaussian bands (see insets in Figure 1). The fitted parameters are presented in Table 1, showing that this complex emission spectrum consists of at least 2 bands (band-1 and band-2). The lower energy band (band-2) is 3–9 times more intense than the high energy band (band-1) depending on the excitation wavelength. The bandwidth of these fitted bands shows that they are inhomogeneously broadened, which is not surprising for ceramic samples. The peaks of the two fitted bands also shifted slightly with excitation wavelength and the changes of the relatively intensity of the two fitted bands suggest the possibility of two different defect centres. This observation is in accordance with EPR data presented in Figure 2 recorded at 80 K and also in agreement with the Bykov work [4] on PLZT samples of nominally the same composition. The EPR spectrum can be understood as the superposition of two spectra ascribed to two Cr3+ centres (S = 3/2). One of them would be located in a nearly regular octahedral environment, which gives rise to overlapping of all the three allowed transitions around 330 mT as was ascribed in [4]. The second one could be due to Cr3+ ions in a distorted environment, which would allow us to explain the low field broad line as a |±3/2i !|±1/2i transition. The |±1/2i !|1/2i transition overlaps with the 330 mT broad intense line, whereas the other |±3/2i !|±1/2i transition should appear at high field values, but is not observed here as it is expected to be broader and weaker. Since the Cr3+ ions are in a lattice position with octahedral symmetry, one has also to consider the possibility of the two bands belonging to only one single defect centre as in the case of Ruby [5] and Cr-doped double oxides crystals [3]. Using the Sugano–Tanabe diagram [6] for a d3 ion in a relatively high crystal field strength centre of octahedral symmetry, as is the present case, the 2E and 2 T1 energy levels are the lowest of the high energy states. Hence there is the possibility of two emission lines from the 2 E ! 4 A2 and 2 T1 ! 4 A2 transitions from a single defect centre being thermally linked by Fermi thermal distribution. Figure 3 shows this emission band taken at different temperatures in the range 10–100 K under 488 nm excitation. It is evidenced that apart
Fluorescence Intensity (a.u.)
3. Results and discussion
a
730
740
750
Wavelength (nm)
710
720
730
740
750
760
Wavelength (nm)
b Fluorescence Intensity (a.u.)
angle to the front surface of the ceramic samples and the fluorescence is collected at right-angle to the excitation beam. Fluorescence decaytime measurements were performed using the 532 nm output of a Q-switched Nd:YAG laser coupled with a 300 MHz digital storage oscilloscope. EPR spectra were recorded at room temperature and 90 K using a Bruker ESP 300E X-band spectrometer with field modulation of 100 kHz. Samples were cooled with a nitrogen gas flow and the temperature controlled with a Bruker 4121 variable temperature unit.
730
740
750
760
Wavelength (nm)
710
720
730
740
750
760
Wavelength (nm)
c Fluorescence Intensity (a.u.)
202
720
730
740
750
760
Wavelength (nm)
40 cm-1 710
720
730
740
750
760
Wavelength (nm) Figure 1. Emission spectra in the 720–760 nm range at 10 K of Cr-doped PLZT-9/ 65/35 samples. Wavelengths excitation (a) 488 (b) 532 (c) 610 nm. The insets show the best-fit curves considering two Gaussian bands using fitting parameters listed in Table 1.
from the decrease of the overall intensity, there is no significant changes to the band shape in the temperature range from 10 to 100 K. A plot of the intensity ratio at the wavelengths of 733 and 740 nm shows relatively constant values over the temperature range considered, suggesting that the two bands do not belong to the same defect centres, i.e. there is no thermal link between the two emission bands.
203
T.P.J. Han et al. / Chemical Physics Letters 506 (2011) 201–204 Table 1 Fitting parameters used for the best-fit of the emission bands presented in Figures 1 and 4.
Band-2
Broadband
Band-3
Band-4
Peak (nm) Height (a.u.) Width (nm) Peak (nm) Height (a.u.) Width (nm) Peak (nm) Height (a.u.) Width (nm) Peak (nm) Height (a.u.) Width (nm)
733.8 ± 0.1
733.6 ± 0.5
732.8 ± 0.3
104
180
382
5.0 ± 0.1
43 ± 2
28 ± 2
740.6 ± 0.1
741.4 ± 0.2
739.2 ± 0.1
446
1510
5827
10.0 ± 0.1
80 ± 1
103 ± 1
973.0 ± 0.9
982.6 ± 3.1
971.6 ± 0.3
65
982
1162
614 ± 54
973 ± 40
870 ± 17
1046.4 ± 1.9
1068.8 ± 3.4
1047.9 ± 1.1
130
1211
1638
1534 ± 60
1750 ± 80
1740 ± 40
EPR signal (arb. units)
90 K Cr3+ Cr3+
100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 Magnetic Field (mT)
6
1.9
80K
1.7
1.8 1.6
Ratio
610 nm
Fluorescence Intensity (a.u)
Band-1
532 nm
7
60K 40K
5
1.5 1.4 1.3 1.2
20K
1.1 1.0
10K
0
20
40
60
80
100
Temperature (K)
4 3 2 1 0 730
740
750
760
Wavelength (nm) Figure 3. Temperature dependence of the broadbands emission under 488 nm excitation. The inset shows the intensity ratio at the wavelengths of 733 and 740 nm.
Normalised Fluorescence Intensity (a.u.)
R-lines
488 nm
2.0
100K
610nm 488nm 532nm
1.0
10K
0.8 0.6 0.4 0.2 0.0 800
900
1000 1100 Wavelength (nm)
1200
Figure 2. EPR spectrum of Cr-doped PLZT-9/65/35 at 80 K.
Figure 4. Emission spectra of Cr-doped PLZT-9/65/35 samples in the near-infrared range from 800 to 1300 nm recorded under excitation wavelengths at 610, 532 and 488 nm at 10 K.
The higher intensity of this emission band when excited by the 610 nm wavelength enables higher resolution spectra to be recorded (Figure 1c). It reveals a periodic structure with separation of 40 cm1. The origin of this structure could be associated with electronic energy levels of the Cr3+ ions coupling to the PLZT phonons. Similar effect has been observed in cation doped alkali halides crystals [7] and in codoped ZnO:Co ceramics [8] where this has been attributed to coupling between one of the phonon modes of the host matrix to a specific excited level of the optical active ions. Raman phonons frequencies have a slight dependence on the composition of the sample and for PLZT-9/65/35 ceramic composition values of 50, 210, 290, and 525 cm1 have been reported [9]. It strongly suggests that the vibronic structure observed on the R-line emission spectrum in this work is associated to the coupling of the R-line excited electronic level of the Cr3+ ions with the low lying phonon energies of the PLZT. Figure 4 shows the normalised and spectrally corrected emission spectra in the near-infrared range from 800 to 1300 nm
recorded under excitation at 610, 532 and 488 nm taken at 10 K. In all the three cases the emission spectrum consists of an asymmetric broad emission band centred at around 1000 nm (10 000 cm1) with a band-width of 2000 cm1. It should be noted that the excessive noise introduced to the spectra is due to the correction made for the detection system responds. This is mainly noticeable on the high energy side of the spectra profile reflecting the poor spectral responds (i.e. low signal/noise) of the InGaAs detector. Hence the values stated above have relatively large uncertainty associated with them however, it can be seen that the overall intensity of this broadband follows that of the R-line emission. Also the emission is strongest for the 610 nm excitation and weakest for the 488 nm. Similar to the R-line emission, there is a weak but noticeable shift of the peak position of the broadband depending on the excitation wavelength. Hence, following the R-line emission band analysis, this broadband has been fitted using two Gaussian bands (band-3 and band-4), where the fitted parameters are also presented in Table 1.
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T.P.J. Han et al. / Chemical Physics Letters 506 (2011) 201–204
The fluorescence decaytime of the R-lines (measured at 740 nm) and the broadband (measured at 800 nm) were measured at 10 K using the 532 nm output of a Q-switched Nd:YAG laser coupled with a PMT (R928) and a fast digital storage oscilloscope. The decaytime of the broadband was measured at 800 nm (tail end of the broadband on the high energy side) because of the spectral response of the PMT (the temporal response of the InGaAs detector was too slow for this purpose). The measured decaytimes are 20 and 10 ls for 740 and 800 nm respectively with an experimental uncertainty of 30%. The similarity and the shortness of the two decaytimes suggest: (i) the defect centre(s) responsible for these fluorescence has crystal field strength very close to the ‘cross-over’ point in the Sugano–Tanabe diagram, i.e. in an intermediate crystal field strength site; (ii) it also indicates strong non-radiative decay as a result of strong coupling with the lattice matrix. An inspection of the fitted parameters reveals that the shift of the peak position and the peak height of the fitted bands for the broadband, band-3 and band-4, with respect to the excitation wavelengths are similar to band-1 for the R-lines, whereas the characteristics of band-2 are quite different. This indicates that the absorption profile of band-2 is of different nature to band-1 whereas, band-1 is similar in nature to band-3 and band-4 of the broadband. First, we will analyse band-4. In a sol–gel derived Crdoped PLZT-9/65/35 ceramic sample [10] a broad emission band centred at 9000 cm1 (1100 nm) with a bandwidth of 2000 cm1 has been reported at 10 K and was tentatively assigned belonging to the Cr5+ (3d1) ions. However, in the Letter by Trepakov et al. [2] the Cr5+ emission band was referenced to be in the region of 714–833 nm. This assignment would then place the absorption band of the Cr5+ ions in the blue/UV region implying a defect centre of high crystal field strength. A review of the general nature of the 3d1 ions in other host matrix such as Li3PO4 and Li3VO4 single crystals [11] doped with CrO3, shows that a broad emission band centred at 9000 cm1 is observed with a bandwidth of 2000 cm1 and a fluorescence decaytime of the order of hundreds of nanoseconds at 10 K. This band was assigned to the 2 E ! 2 T2 transition of the 3d1 ions. Taking all these facts into account, it seems that the assignment made in Trepakov et al. Letter [2] is incorrect and that the band-4 revealed in the present work could be that observed by Murakami et al. tentatively associated with the Cr5+ ions. However, the EPR data presented in Figure 2 do not clearly identify or dispel the presence of Cr5+ ions due to the un-resolvable overlapping broad EPR peaks. Hence based on the presented optical data alone one cannot ignore the fact that this broad emission could also be due to Cr3+ ions in a low crystal field strength centre. In this case the transition responsible is 4 T2 ! 4 A2 of the 3d3 ions. Further work is needed to clarify the true nature of this emission band. Band-3 from the fitting of the broadband has similar nature to that of band-1 from the fitting of the R-lines. The similarity of their fluorescence decaytime suggests that they belong together and are in an intermediate crystal field strength centre. The thermal behaviour of band-1 and band-2 suggests that they belong to different centres, which leads to the following possible scenarios:
(i) There are two different defect centres, both are in intermediate crystal field strength site, and band-3 is an unresolved band composed of two broadbands belonging to the two centres. The transition responsible is therefore 4 T2 ! 4 A2 of the Cr3+ ions. (ii) There are two different defect centres one is in intermediate crystal field strength site (band-1 and band-3) and the other is in high crystal field strength site (band-2). The observed broadband (band-3) is only associated with the intermediate crystal field site. The EPR data seems to support the second scenario as it confirmed the existence of two Cr3+ defect centres with one in octahedral symmetry centre and the other in a more distorted site. If this is the case, one can only explain the fluorescence decaytime measurements if there is a strong electron–phonon coupling between the Cr3+ ions and the host matrix. In summary the experimental data found in this work suggest the presence of two Cr3+ defect centres with octahedral symmetry of high and intermediate crystal field strength. The inhomogeneously broadened emission band observed at 740 nm is due to the 2 E ! 4 A2 (R-line) transition whereas the inhomogeneously broadened broadband emission at 1000 nm consists of overlapping broadbands from the 4 T2 ! 4 A2 transition from the Cr3+ defect centres and possibly the 2 E ! 2 T2 transition of the Cr5+ defect centre. Fluorescence decaytime measurements also indicate strong electron–phonon coupling for this material. Acknowledgements One of the authors, T.P.J. Han, would like to acknowledge the support of EPSRC, UK. F. Jaque would like to acknowledge the support of MICINN and MICROSERES Projects. References [1] J.F. Scott, Integr. Ferroelectr. 20 (1998) 15. [2] V. Trepakov, V. Dimza, L. Jastrabik, A. Savinov, Z. Bryknar, Phy. Status Solid BBasic Res. 183 (1994) 299. [3] E. Camarillo, J. Tocho, I. Vergara, E. Dieguez, J. G Sole, F. Jaque, Phys. Rev. B 45 (1992) 4600. [4] I.P. Bykov, M.D. Glinchuk, V.V. Laguta, Y.L. Maximenko, L. Jastrabik, V.A. Trpakov Dimza, M. Hrabovsky, J. Phys. Chem. Solids 56 (1995) 919. [5] J. Garcia Sole, L.E. Bausá, D. Jaque, An Introduction to the Optical Spectroscopy of Inorganic Solids. John Wiley& Sons Ltd. (2005). [6] B. Henderson, G.F. Imbusch, Optical Spectroscopy of Inorganic Crystals. Oxford University Press, Oxford (1989). [7] F. Jaque, F.J. Lopez, F. Cusso, M. Moreno, F. Agullo-Lopez, Solid State Commun. 47 (1983) 103. [8] T.P.J. Han, M. Villegas, M. Peiteado, A.C. Caballero, F. Rodríguez, F. Jaque, Chem. Phys. Lett. 488 (2010) 173. [9] E. Buixaderas, I. Gregora, S. Kamca, J. Petzelt, M. Kosec, J. Phys. Condens. Matter 20 (2008) 345229. [10] S. Murakami, M. Herrren, D. Rau, T. Sakurai, M. Morita, J. Lumin. 83 (1999) 215. [11] M.F. Hazenkamp, H.U. Gudel, J. Lumin. 69 (1996) 235.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Luminescence of the Cr3+ ions doped in PLZT ceramics T.P.J. Han a,⇑, M.T. Jardiel b, M. Villegas b, A.C. Caballero b, D. Bravo c, F. Jaque a a
Department of Physics, University of Strathclyde, Glasgow, G4 ONG, Scotland, UK Department of Electroceramics, Instituto de Cerámica y Vidrio, CSIC Kelsen 5, Madrid 28049, Spain c Departamento de Física de Materiales, Universidad Autónoma de Madrid, Madrid 28049, Spain b
a r t i c l e
i n f o
Article history: Received 16 December 2010 In final form 9 March 2011 Available online 12 March 2011
a b s t r a c t The photoluminescence properties of Cr-doped PLZT-9/65/35 (Pb0.95La0.09Zr0.65Ti0.35O3) have been studied. The fluorescence spectra consist of two inhomogeneously broadened narrow bands centred at 734 and 740 nm and a broadband emission with a maximum at 1000 nm. The two high energy bands have been assigned to the 2 Eð2 T1 Þ ! 4 A2 transition of two Cr3+ defect centres in octahedral site with high/ intermediate crystal field strength whereas the near infrared emission is tentatively assigned to the overlapping of the broadband emission of the Cr3+ ions defect centre’s 4 T2 ! 4 A2 transition and the emission of the Cr5+ ions’ 2 E ! 2 T2 transition. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Lead lanthanum zirconate titanate (PLZT) is a typical ferroelectric relaxor material with very interesting piezoelectric properties for applications in optoelectronics devices, particularly in ferroelectric memories applications [1]. In general, the properties of PLZT materials are very sensitive to the processing method because it is very difficult to maintain the stoichiometric composition during the various different thermal treatments. Composition variation within a sample or from sample to sample results in inconsistency and degrading of the piezoelectric properties [2]. It is therefore important to have detail knowledge of a number of properties that have direct effect on the ferroelectric properties, such as defects formation, changes in composition and/or grain sizes, due to the fabrication processes in PLZT ceramics. In other ferroelectric materials with similar stoichiometric problems, such as Lithium Niobate (LiNbO3) and Strontium Barium Niobate (SBN), the use of Cr3+ ions as an optical and paramagnetic probe has been very useful in the characterisation of defects formation during the crystal growth procedure [3]. For this purpose, different optical and luminescence spectroscopic techniques such as absorption, photoluminescence, fluorescence decaytime, timeresolved spectroscopy and electron paramagnetic resonance (EPR) techniques were utilised. These studies allow the determination of the presence of Cr ions in different valence states and defect centres they formed as consequence of changes in the stoichiometric compositions. However, to-dates little work has been reported on applying the Cr ions as probe to the PLZT ferroelectrics families. In this work a photoluminescence study of Cr-doped PLZT-9/65/35 (Pb0.95La0.09Zr0.65Ti0.35O3) ceramic samples is presented. This study ⇑ Corresponding author. E-mail address: [email protected] (T.P.J. Han). 0009-2614/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2011.03.034
involved the excitation at several wavelengths between 450 and 650 nm and at temperatures range from 10 to 100 K. 2. Experimental The precursor of PLZT was prepared from stoichiometric quantities of the solutions of lead acetate in acetic acid and tetrabutyl titanium, zirconium nitrate and lanthanum nitrate in 2-methoxyetanol. In order to adjust the concentration, the 2-methoxyetanol and acetic acid solvents with a volume ratio of 5/1.3 were then mixed together under agitation to give a 0.3 M PLZT sol–gel solution. The stoichiometric amount of chromium nitrate (corresponding to a 1 wt.% of chromium oxide) was added to this solution. In this condition the sol obtained has a green colour. This sol was kept under agitation at 313 K for 24 h and then the temperature is increased to 353 K to obtain a viscose gel. The resin was heated at 383 K to produce a brown colour powder. This powder was calcined for 2 h at 1023 K to form the Cr doped PLZT phase. Compacted discs were prepared from this powder by uniaxial pressing at 200 MPa. Dense polycrystalline samples (95–96% of the theoretical density) were obtained after sintering at 1323 K for 4 h. In order to control the PbO volatilisation, a PZT commercial buffer inside the crucible was used during the thermal treatment. Photoluminescence spectra were obtained in the 10–100 K temperature range excited using different wavelengths of a continuous wave (cw) Ar-ion laser (457.9, 488.0 and 514.5 nm), a continuous wave frequency doubled output of a Nd:YAG laser at 532 and at 610 nm from a continuous wave dye laser. The emission light was dispersed by a SPEX 500 M monochromator and detected with a Photomultiplier tube (Hamamatsu R928) in the region from 400 to 800 nm and a solid state InGaAs detector for the region beyond 800 nm. The geometry of the experimental is such that excitation laser beam is focused with a 10 cm focal length lens at a glancing
T.P.J. Han et al. / Chemical Physics Letters 506 (2011) 201–204
Figure 1a shows the emission spectrum in the 710–760 nm wavelength range excited by the 488 nm laser output at 10 K. The spectrum consists of an asymmetric and complex band with a prominent peak centred at 740.5 nm (13 504 cm1). There is also an underlying background emission with intensity increasing towards the longer wavelength region indicating the presence of another broader band extending from the near infrared (IR) region. Figure 1b and c show the 10 K emission spectra obtained under cw 532 and 610 nm excitation respectively. It is clear that both emission bands are more symmetrical in appearance than that the on excited using 488 nm, even though the background emission is still presence in both cases. The emission bands in Figure 1b and c have their maximum at 741.7 nm (13 483 cm1) and 739.6 nm (13 521 cm1) respectively. It is evidenced that there is a shift of the peak position of the band excitation and the overall emission intensity decreases from 610, 532 to 488 nm. This emission band is analysed by fitting to it using two Gaussian bands (see insets in Figure 1). The fitted parameters are presented in Table 1, showing that this complex emission spectrum consists of at least 2 bands (band-1 and band-2). The lower energy band (band-2) is 3–9 times more intense than the high energy band (band-1) depending on the excitation wavelength. The bandwidth of these fitted bands shows that they are inhomogeneously broadened, which is not surprising for ceramic samples. The peaks of the two fitted bands also shifted slightly with excitation wavelength and the changes of the relatively intensity of the two fitted bands suggest the possibility of two different defect centres. This observation is in accordance with EPR data presented in Figure 2 recorded at 80 K and also in agreement with the Bykov work [4] on PLZT samples of nominally the same composition. The EPR spectrum can be understood as the superposition of two spectra ascribed to two Cr3+ centres (S = 3/2). One of them would be located in a nearly regular octahedral environment, which gives rise to overlapping of all the three allowed transitions around 330 mT as was ascribed in [4]. The second one could be due to Cr3+ ions in a distorted environment, which would allow us to explain the low field broad line as a |±3/2i !|±1/2i transition. The |±1/2i !|1/2i transition overlaps with the 330 mT broad intense line, whereas the other |±3/2i !|±1/2i transition should appear at high field values, but is not observed here as it is expected to be broader and weaker. Since the Cr3+ ions are in a lattice position with octahedral symmetry, one has also to consider the possibility of the two bands belonging to only one single defect centre as in the case of Ruby [5] and Cr-doped double oxides crystals [3]. Using the Sugano–Tanabe diagram [6] for a d3 ion in a relatively high crystal field strength centre of octahedral symmetry, as is the present case, the 2E and 2 T1 energy levels are the lowest of the high energy states. Hence there is the possibility of two emission lines from the 2 E ! 4 A2 and 2 T1 ! 4 A2 transitions from a single defect centre being thermally linked by Fermi thermal distribution. Figure 3 shows this emission band taken at different temperatures in the range 10–100 K under 488 nm excitation. It is evidenced that apart
Fluorescence Intensity (a.u.)
3. Results and discussion
a
730
740
750
Wavelength (nm)
710
720
730
740
750
760
Wavelength (nm)
b Fluorescence Intensity (a.u.)
angle to the front surface of the ceramic samples and the fluorescence is collected at right-angle to the excitation beam. Fluorescence decaytime measurements were performed using the 532 nm output of a Q-switched Nd:YAG laser coupled with a 300 MHz digital storage oscilloscope. EPR spectra were recorded at room temperature and 90 K using a Bruker ESP 300E X-band spectrometer with field modulation of 100 kHz. Samples were cooled with a nitrogen gas flow and the temperature controlled with a Bruker 4121 variable temperature unit.
730
740
750
760
Wavelength (nm)
710
720
730
740
750
760
Wavelength (nm)
c Fluorescence Intensity (a.u.)
202
720
730
740
750
760
Wavelength (nm)
40 cm-1 710
720
730
740
750
760
Wavelength (nm) Figure 1. Emission spectra in the 720–760 nm range at 10 K of Cr-doped PLZT-9/ 65/35 samples. Wavelengths excitation (a) 488 (b) 532 (c) 610 nm. The insets show the best-fit curves considering two Gaussian bands using fitting parameters listed in Table 1.
from the decrease of the overall intensity, there is no significant changes to the band shape in the temperature range from 10 to 100 K. A plot of the intensity ratio at the wavelengths of 733 and 740 nm shows relatively constant values over the temperature range considered, suggesting that the two bands do not belong to the same defect centres, i.e. there is no thermal link between the two emission bands.
203
T.P.J. Han et al. / Chemical Physics Letters 506 (2011) 201–204 Table 1 Fitting parameters used for the best-fit of the emission bands presented in Figures 1 and 4.
Band-2
Broadband
Band-3
Band-4
Peak (nm) Height (a.u.) Width (nm) Peak (nm) Height (a.u.) Width (nm) Peak (nm) Height (a.u.) Width (nm) Peak (nm) Height (a.u.) Width (nm)
733.8 ± 0.1
733.6 ± 0.5
732.8 ± 0.3
104
180
382
5.0 ± 0.1
43 ± 2
28 ± 2
740.6 ± 0.1
741.4 ± 0.2
739.2 ± 0.1
446
1510
5827
10.0 ± 0.1
80 ± 1
103 ± 1
973.0 ± 0.9
982.6 ± 3.1
971.6 ± 0.3
65
982
1162
614 ± 54
973 ± 40
870 ± 17
1046.4 ± 1.9
1068.8 ± 3.4
1047.9 ± 1.1
130
1211
1638
1534 ± 60
1750 ± 80
1740 ± 40
EPR signal (arb. units)
90 K Cr3+ Cr3+
100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 Magnetic Field (mT)
6
1.9
80K
1.7
1.8 1.6
Ratio
610 nm
Fluorescence Intensity (a.u)
Band-1
532 nm
7
60K 40K
5
1.5 1.4 1.3 1.2
20K
1.1 1.0
10K
0
20
40
60
80
100
Temperature (K)
4 3 2 1 0 730
740
750
760
Wavelength (nm) Figure 3. Temperature dependence of the broadbands emission under 488 nm excitation. The inset shows the intensity ratio at the wavelengths of 733 and 740 nm.
Normalised Fluorescence Intensity (a.u.)
R-lines
488 nm
2.0
100K
610nm 488nm 532nm
1.0
10K
0.8 0.6 0.4 0.2 0.0 800
900
1000 1100 Wavelength (nm)
1200
Figure 2. EPR spectrum of Cr-doped PLZT-9/65/35 at 80 K.
Figure 4. Emission spectra of Cr-doped PLZT-9/65/35 samples in the near-infrared range from 800 to 1300 nm recorded under excitation wavelengths at 610, 532 and 488 nm at 10 K.
The higher intensity of this emission band when excited by the 610 nm wavelength enables higher resolution spectra to be recorded (Figure 1c). It reveals a periodic structure with separation of 40 cm1. The origin of this structure could be associated with electronic energy levels of the Cr3+ ions coupling to the PLZT phonons. Similar effect has been observed in cation doped alkali halides crystals [7] and in codoped ZnO:Co ceramics [8] where this has been attributed to coupling between one of the phonon modes of the host matrix to a specific excited level of the optical active ions. Raman phonons frequencies have a slight dependence on the composition of the sample and for PLZT-9/65/35 ceramic composition values of 50, 210, 290, and 525 cm1 have been reported [9]. It strongly suggests that the vibronic structure observed on the R-line emission spectrum in this work is associated to the coupling of the R-line excited electronic level of the Cr3+ ions with the low lying phonon energies of the PLZT. Figure 4 shows the normalised and spectrally corrected emission spectra in the near-infrared range from 800 to 1300 nm
recorded under excitation at 610, 532 and 488 nm taken at 10 K. In all the three cases the emission spectrum consists of an asymmetric broad emission band centred at around 1000 nm (10 000 cm1) with a band-width of 2000 cm1. It should be noted that the excessive noise introduced to the spectra is due to the correction made for the detection system responds. This is mainly noticeable on the high energy side of the spectra profile reflecting the poor spectral responds (i.e. low signal/noise) of the InGaAs detector. Hence the values stated above have relatively large uncertainty associated with them however, it can be seen that the overall intensity of this broadband follows that of the R-line emission. Also the emission is strongest for the 610 nm excitation and weakest for the 488 nm. Similar to the R-line emission, there is a weak but noticeable shift of the peak position of the broadband depending on the excitation wavelength. Hence, following the R-line emission band analysis, this broadband has been fitted using two Gaussian bands (band-3 and band-4), where the fitted parameters are also presented in Table 1.
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The fluorescence decaytime of the R-lines (measured at 740 nm) and the broadband (measured at 800 nm) were measured at 10 K using the 532 nm output of a Q-switched Nd:YAG laser coupled with a PMT (R928) and a fast digital storage oscilloscope. The decaytime of the broadband was measured at 800 nm (tail end of the broadband on the high energy side) because of the spectral response of the PMT (the temporal response of the InGaAs detector was too slow for this purpose). The measured decaytimes are 20 and 10 ls for 740 and 800 nm respectively with an experimental uncertainty of 30%. The similarity and the shortness of the two decaytimes suggest: (i) the defect centre(s) responsible for these fluorescence has crystal field strength very close to the ‘cross-over’ point in the Sugano–Tanabe diagram, i.e. in an intermediate crystal field strength site; (ii) it also indicates strong non-radiative decay as a result of strong coupling with the lattice matrix. An inspection of the fitted parameters reveals that the shift of the peak position and the peak height of the fitted bands for the broadband, band-3 and band-4, with respect to the excitation wavelengths are similar to band-1 for the R-lines, whereas the characteristics of band-2 are quite different. This indicates that the absorption profile of band-2 is of different nature to band-1 whereas, band-1 is similar in nature to band-3 and band-4 of the broadband. First, we will analyse band-4. In a sol–gel derived Crdoped PLZT-9/65/35 ceramic sample [10] a broad emission band centred at 9000 cm1 (1100 nm) with a bandwidth of 2000 cm1 has been reported at 10 K and was tentatively assigned belonging to the Cr5+ (3d1) ions. However, in the Letter by Trepakov et al. [2] the Cr5+ emission band was referenced to be in the region of 714–833 nm. This assignment would then place the absorption band of the Cr5+ ions in the blue/UV region implying a defect centre of high crystal field strength. A review of the general nature of the 3d1 ions in other host matrix such as Li3PO4 and Li3VO4 single crystals [11] doped with CrO3, shows that a broad emission band centred at 9000 cm1 is observed with a bandwidth of 2000 cm1 and a fluorescence decaytime of the order of hundreds of nanoseconds at 10 K. This band was assigned to the 2 E ! 2 T2 transition of the 3d1 ions. Taking all these facts into account, it seems that the assignment made in Trepakov et al. Letter [2] is incorrect and that the band-4 revealed in the present work could be that observed by Murakami et al. tentatively associated with the Cr5+ ions. However, the EPR data presented in Figure 2 do not clearly identify or dispel the presence of Cr5+ ions due to the un-resolvable overlapping broad EPR peaks. Hence based on the presented optical data alone one cannot ignore the fact that this broad emission could also be due to Cr3+ ions in a low crystal field strength centre. In this case the transition responsible is 4 T2 ! 4 A2 of the 3d3 ions. Further work is needed to clarify the true nature of this emission band. Band-3 from the fitting of the broadband has similar nature to that of band-1 from the fitting of the R-lines. The similarity of their fluorescence decaytime suggests that they belong together and are in an intermediate crystal field strength centre. The thermal behaviour of band-1 and band-2 suggests that they belong to different centres, which leads to the following possible scenarios:
(i) There are two different defect centres, both are in intermediate crystal field strength site, and band-3 is an unresolved band composed of two broadbands belonging to the two centres. The transition responsible is therefore 4 T2 ! 4 A2 of the Cr3+ ions. (ii) There are two different defect centres one is in intermediate crystal field strength site (band-1 and band-3) and the other is in high crystal field strength site (band-2). The observed broadband (band-3) is only associated with the intermediate crystal field site. The EPR data seems to support the second scenario as it confirmed the existence of two Cr3+ defect centres with one in octahedral symmetry centre and the other in a more distorted site. If this is the case, one can only explain the fluorescence decaytime measurements if there is a strong electron–phonon coupling between the Cr3+ ions and the host matrix. In summary the experimental data found in this work suggest the presence of two Cr3+ defect centres with octahedral symmetry of high and intermediate crystal field strength. The inhomogeneously broadened emission band observed at 740 nm is due to the 2 E ! 4 A2 (R-line) transition whereas the inhomogeneously broadened broadband emission at 1000 nm consists of overlapping broadbands from the 4 T2 ! 4 A2 transition from the Cr3+ defect centres and possibly the 2 E ! 2 T2 transition of the Cr5+ defect centre. Fluorescence decaytime measurements also indicate strong electron–phonon coupling for this material. Acknowledgements One of the authors, T.P.J. Han, would like to acknowledge the support of EPSRC, UK. F. Jaque would like to acknowledge the support of MICINN and MICROSERES Projects. References [1] J.F. Scott, Integr. Ferroelectr. 20 (1998) 15. [2] V. Trepakov, V. Dimza, L. Jastrabik, A. Savinov, Z. Bryknar, Phy. Status Solid BBasic Res. 183 (1994) 299. [3] E. Camarillo, J. Tocho, I. Vergara, E. Dieguez, J. G Sole, F. Jaque, Phys. Rev. B 45 (1992) 4600. [4] I.P. Bykov, M.D. Glinchuk, V.V. Laguta, Y.L. Maximenko, L. Jastrabik, V.A. Trpakov Dimza, M. Hrabovsky, J. Phys. Chem. Solids 56 (1995) 919. [5] J. Garcia Sole, L.E. Bausá, D. Jaque, An Introduction to the Optical Spectroscopy of Inorganic Solids. John Wiley& Sons Ltd. (2005). [6] B. Henderson, G.F. Imbusch, Optical Spectroscopy of Inorganic Crystals. Oxford University Press, Oxford (1989). [7] F. Jaque, F.J. Lopez, F. Cusso, M. Moreno, F. Agullo-Lopez, Solid State Commun. 47 (1983) 103. [8] T.P.J. Han, M. Villegas, M. Peiteado, A.C. Caballero, F. Rodríguez, F. Jaque, Chem. Phys. Lett. 488 (2010) 173. [9] E. Buixaderas, I. Gregora, S. Kamca, J. Petzelt, M. Kosec, J. Phys. Condens. Matter 20 (2008) 345229. [10] S. Murakami, M. Herrren, D. Rau, T. Sakurai, M. Morita, J. Lumin. 83 (1999) 215. [11] M.F. Hazenkamp, H.U. Gudel, J. Lumin. 69 (1996) 235.