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Photoluminescence spectroscopy of Cr3+ in ceramic Al2O3
Materials Science and Engineering B54 (1998) 33 – 37
Photoluminescence spectroscopy of Cr3 + in ceramic Al2O3 Taro Toyoda *, Takashi Obikawa, Takeshi Shigenari Department of Applied Physics and Chemistry, The Uni6ersity of Electro-Communications, 1 -5 -1 Chofugaoka, Chofu, Tokyo 182, Japan
Abstract Photoluminescence (PL) spectroscopy is applied to study the radiative processes of ceramic Al2 − x Crx O3 (0.002 x 0.15) with the excitation light of 295 nm wavelength together with the optical absorption characterization by photoacoustic (PA) spectroscopy. I (694 nm), the relative intensity of R line ( 694 nm) for ceramic Al2 − x Crx O3, decreases with increasing mole fraction x, indicating the concentration quenching by the increase of interaction between Cr3 + ions. I (704 nm) and I (775 nm), those of N line ( 704 nm) and broad band (775 nm), increase with increasing mole fraction x, indicating the increases of pair state of Cr3 + ions (N line) and the state by groups of more than two Cr3 + ions (broad band). The ratios, I (704 nm)/I (694 nm), increases linearly with increasing mole fraction x up to 0.15, indicating that it is different from the early results under the assumption that the states of pair and groups of more than two Cr3 + ions obtain some excitation by energy transfer from single ions. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Photoluminescence spectroscopy; Cr3 + ; Ceramic Al2O3
1. Introduction The energy levels of iron-group ions of the d3 configuration in nearly octahedral crystal sites have been the subject of intensive theoretical and experimental investigations. The greatest effort has focused on the Cr3 + ions in Al2O3 (ruby) [1]. The states are the ground state 4A2, metastable 2E lying at roughly 690 nm, and the excited states— 4T2 and 4T1 — which give rise to the broad absorption bands in ruby centered near 550 and 420 nm, respectively. Optical absorption spectra for ruby in the visible are dominated by two transitions, 4 A2 4T2 (U-band, 565 nm) and 4A2 4T1 (Y-band, 410 nm) (spin-allowed transitions). Transitions are also observed in other ion levels, but these transitions are sharp and relatively weak (spin-forbidden transitions) and will be ignored. Flourescence in ruby is demonstrated to excite the 4A2 4T2 transition or the 4A2 4 T1 transition. If a Cr3 + ion is raised to excited states, it decays nonradiatively by phonon emission until it ends on the lowest excited 2E state. The emission spectrum consists of a sharp doublet (2E 4A2) whose components at room temperature are at about 694 (R1) and 693 nm (R2) with the full width at half maximum * Corresponding author. Tel.: +81 424 832161 ext. 3911; fax: +81 424 847013; e-mail: [email protected] 0921-5107/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0921-5107(98)00122-6
of 0.4 and 0.3 nm, and they are used for the stimulated optical emission [2]. Although many investigations have been carried out to study optical properties of Al2 − x Crx O3 system, especially flourescence (radiative relaxation process) for dilute concentration of Cr3 + ions, there are a few investigations on the higher concentration of Cr3 + ions in Al2O3 [3]. In general, an externally excited solid can relax to thermal equilibrium by emission of either photons (radiative processes) or phonons (nonradiative processes). Highly sophisticated spectroscopic techniques to detect and analyze emitted photons have been well established. This paper represents an attempt at a qualititative investigation of the Cr3 + ion concentration dependence of the photoluminescence (PL) spectra in ceramic Al2 − x Crx O3 (x is extended to 0.15) together with the optical absorption measurements by photoacoustic (PA) spectroscopy. Since PA signals are produced only by the absorbed light actually converted to heat in the sample. PA spectroscopy is a useful tool to study the optical absorption for ceramics. PA spectroscopy is applicable to study for nonradiative processes [4]. Moreover, one of the principal advantages of PA spectroscopy is that it enables one to obtain not only signal intensity spectra similar to optical absorption but also signal phase spectra on any type of solid or semisolid material, whether they are crystalline, powder, and/or amorphous [5,6].
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T. Toyoda et al. / Materials Science and Engineering B54 (1998) 33–37
2. Experimental
x is the increase of nonradiative processes at the 2E A2 transition according to the discussion by Murphy and coworker using a rate-equation model, indicating concentration quenching [4]. The values of the ratio, RI agree with those obtained by Murphy and coworker within the experimental accuracy. Fig. 2 shows the examples of PL spectra for x=0.01, 0.05, and 0.15 in ceramic Al2 − x Crx O3. They show the 4
Ceramic Al2 − x Crx O3 (0.002 x 0.15) samples were prepared from mixtures of Al2O3 (99.99%) and Cr2O3 (99.9%) powders. They were pressed to be plates with the thickness of 1.3 mm. Then they were sintered at 1400°C for 1 h in air. The excitation light used for the PL measurements was the second harmonic generation (SHG) of the dye laser and was fixed at 295 nm with average power of less than 0.5 mW. A dye laser at 590 nm excited by the second harmonics of a mode-locked YAG laser was used as the excitation source. The normal of the sample was about 50° to the incident light and the fluorescence light was measured by a spectrometer with an ICCD detector [7]. The body of the PA cell was an aluminium cylinder with a small channel in its periphery in which a microphone was inserted. The window of the cell was silica glass. The inside volume of the cell was approximately 0.5 cm3. The cell was suspended by four rubber wires in order to avoid outside vibrations. The light source was a 300 W xenon arc lamp and the light beam was focused on the sample through a monochromator with an impinging area of 0.15 cm2. Modulation frequency for the spectroscopy measurements was 133 Hz using a mechanical chopper. The measurements were carried out at room temperature in the wavelength range of 330–800 nm. The PA signal intensities were monitored by passing the microphone signal first through a preamplifier and then into a two-phase lock-in amplifier. The spectra were normarized by the PA signal of carbon black sheet.
3. Results and discussion X-ray diffraction patterns showed that the crystal structures of Al2 − x Crx O3 (0.002 x 0.15) samples were a -aliminum oxide structures. We could not find the lattice constant changes by adding Cr2O3 in Al2O3 because of the small changes of them (a = 0.476 nm and c = 1.299 nm for Al2O3; a = 0.4959 nm and c = 1.360 nm for Cr2O3). Fig. 1 shows the examples of PA intensity spectra for x = 0.01, 0.05. and 0.15 in ceramic Al2 − x Crx O3. Qualitatively, those spectra are similar to that obtained by optical absorption measurements in single crystal of ruby reported [2]. The peaks which correspond to Uband ( 565 nm) and Y-band ( 410 nm) can be observed in all the spectra. The shapes of the spectra are similar to each other except that the change of intensity ratio, RI =S (Y-band)/S (U-band) (S denotes the PA intensity for each band), shows decrease with increasing mole fraction x. One of the possibility that the values of RI decrease with increasing mole fraction
Fig. 1. Photoacoustic signal intensity spectra of ceramic Al2 − x Crx O3 for x=0.01, 0.05, and 0.15 in the wavelength range of 330–800 nm with a modulation frequency of 133 Hz.
T. Toyoda et al. / Materials Science and Engineering B54 (1998) 33–37
35
Fig. 2. Photoluminescence spectra of ceramic Al2 − x Crx O3 for x = 0.01, 0.05, and 0.15 with the excitation light of 295 nm wavelength.
intense R line ( 694 nm) originating from isolated single Cr3 + ions. In order to compare the relative changes of PL intensity with mole fraction x. PL mea-
surements of R line ( 694 nm) for the single crystal of ruby at the same experimental condition have been carried out as standard. The concentration of Cr3 +
36
T. Toyoda et al. / Materials Science and Engineering B54 (1998) 33–37
Fig. 3. Relative intensity of I (694nm) to that of the standard single crystal of ruby for ceramic Al2 − x Crx 03 as a function of mole fraction x.
Fig. 5. Ratio, I (704 nm)/I (694 nm), for ceramic Al2 − x Crx O3 as a function of mole fraction x.
ions of single crystal of ruby used is x = 0.002 which was utilized as a laser material. Fig. 3 shows that I (694 nm). the relative intensity ratio of R line for ceramic Al2 − x Crx O3 to that of the standard single crystal of ruby, decreases with increasing mole fraction x. indicating the increase of interaction between Cr3 + ions. The weaker sharp lines which are called N line ( 704 nm) can also be observed and have been originated on closely coupled pairs of Cr3 + ions. Fig. 4 shows that I (704 nm). the relative intensity ratio of N line for ceramic Al2 − x Crx O3 to that of the standard single crystal of ruby, increases with increasing mole fraction x, indicating the increase of pair interaction. Fig. 5 shows that the ratio, I (704 nm)/I (694 nm), increases linearly with increasing mole fraction x up to x =0.15, indicating that it was different from the early results
under the assumption that the pairs of Cr3 + obtain some excitation energy by transfer from single ions (the ratio shows nonlinear by the early results reported with the assumption) [8–10]. The long wavelength part of the broad PL spectra (775 nm) can be seen in Fig. 2. Fig. 6 shows that I (775nm), the relative intensity ratio at 775 nm for ceramic Al2 − x Crx O3 to that of the standard single crystal of ruby, increases with increasing mole fraction x, indicating the increase of groups of more than two Cr3 + ions [9]. Quantitative clarification of the electronic state of Cr3 + ions in ceramic Al2O3 is left to future experimental and theoretical investigation. Photoluminescence excitation (PLE) spectroscopy and the detailed analysis of quantum efficincy of ceramic Al2 − x Crx O3 are necessary to clarify the nature of the radiative and nonradiative processes [11–14].
Fig. 4. Relative intensity of I (704 nm) to that of the standard single crystal of ruby for ceramic Al2 − x Crx O3 as a function of mole fraction x.
Fig. 6. Relative intensity of I (775 nm) to that of the standard single crystal of ruby for ceramic Al2 − x Crx O3 as a function of mole fraction x.
T. Toyoda et al. / Materials Science and Engineering B54 (1998) 33–37
Acknowledgements The authors are grateful to Dr Q. Shen, Mr K. Yokomizo and Mr H. Zhang for valuable discussions. Mr K. Shinoyama is acknowledged for help in the photoacoustic measurements and technical discussion. The authors thank Mr T. Takeuchi for helpful photoluminescence measurements.
References [1] P. Kisliuk, C.A. Moore, Phys.Rev. 160 (1967) 307.
.
37
[2] T.H. Maiman, R.H. Hoskins, I.J. D’Haenens, C.K. Asaka, V. Evtuhov, Phys. Rev. 123 (1961) 1151. [3] J. Bohandy, J. Solid State Chem. 3 (1971) 467. [4] J.C. Murphy, L.C. Aamodt, J. Appl. Phys. 48 (1977) 3502. [5] A. Rosencwaig, A. Gersho, J. Appl. Phys. 47 (1976) 64. [6] E.M. Monahan Jr., A.W. Nolle, J. Appl. Phys. 48 (1977) 3519. [7] K. Morita, K. Shinjo, Qi Ping, K. Abe, T. Shigenari, Ferroelectrics 159 (1994) 203. [8] A.L. Schawlow, D.L. Wood, A.M. Clogston, Phys. Rev. Lett. 3 (1959) 271. [9] N.A. Tolstoi, Liu Shun’-fu, M.E. Lapidus, Opt. Spectros. 13 (1961) 133. [10] G.F. Imbusch, Phys. Rev. 153 (1967) 326. [11] R.S. Quimby, W.M. Yen, J. Appl. Phys. 51 (1980) 1780. [12] Y. Li, I. Duncan, T. Morrow, J. Luminescence 50 (1991) 69. [13] A. Mandelis, Z.H. Chen, R. Bleiss, J. Phys. IV Suppl. C7 (4) (1994) C7 – 401. [14] T. Toyoda, T. Obikawa, Mater. Sci. Eng. B41 (1996) 72.
Photoluminescence spectroscopy of Cr3 + in ceramic Al2O3 Taro Toyoda *, Takashi Obikawa, Takeshi Shigenari Department of Applied Physics and Chemistry, The Uni6ersity of Electro-Communications, 1 -5 -1 Chofugaoka, Chofu, Tokyo 182, Japan
Abstract Photoluminescence (PL) spectroscopy is applied to study the radiative processes of ceramic Al2 − x Crx O3 (0.002 x 0.15) with the excitation light of 295 nm wavelength together with the optical absorption characterization by photoacoustic (PA) spectroscopy. I (694 nm), the relative intensity of R line ( 694 nm) for ceramic Al2 − x Crx O3, decreases with increasing mole fraction x, indicating the concentration quenching by the increase of interaction between Cr3 + ions. I (704 nm) and I (775 nm), those of N line ( 704 nm) and broad band (775 nm), increase with increasing mole fraction x, indicating the increases of pair state of Cr3 + ions (N line) and the state by groups of more than two Cr3 + ions (broad band). The ratios, I (704 nm)/I (694 nm), increases linearly with increasing mole fraction x up to 0.15, indicating that it is different from the early results under the assumption that the states of pair and groups of more than two Cr3 + ions obtain some excitation by energy transfer from single ions. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Photoluminescence spectroscopy; Cr3 + ; Ceramic Al2O3
1. Introduction The energy levels of iron-group ions of the d3 configuration in nearly octahedral crystal sites have been the subject of intensive theoretical and experimental investigations. The greatest effort has focused on the Cr3 + ions in Al2O3 (ruby) [1]. The states are the ground state 4A2, metastable 2E lying at roughly 690 nm, and the excited states— 4T2 and 4T1 — which give rise to the broad absorption bands in ruby centered near 550 and 420 nm, respectively. Optical absorption spectra for ruby in the visible are dominated by two transitions, 4 A2 4T2 (U-band, 565 nm) and 4A2 4T1 (Y-band, 410 nm) (spin-allowed transitions). Transitions are also observed in other ion levels, but these transitions are sharp and relatively weak (spin-forbidden transitions) and will be ignored. Flourescence in ruby is demonstrated to excite the 4A2 4T2 transition or the 4A2 4 T1 transition. If a Cr3 + ion is raised to excited states, it decays nonradiatively by phonon emission until it ends on the lowest excited 2E state. The emission spectrum consists of a sharp doublet (2E 4A2) whose components at room temperature are at about 694 (R1) and 693 nm (R2) with the full width at half maximum * Corresponding author. Tel.: +81 424 832161 ext. 3911; fax: +81 424 847013; e-mail: [email protected] 0921-5107/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0921-5107(98)00122-6
of 0.4 and 0.3 nm, and they are used for the stimulated optical emission [2]. Although many investigations have been carried out to study optical properties of Al2 − x Crx O3 system, especially flourescence (radiative relaxation process) for dilute concentration of Cr3 + ions, there are a few investigations on the higher concentration of Cr3 + ions in Al2O3 [3]. In general, an externally excited solid can relax to thermal equilibrium by emission of either photons (radiative processes) or phonons (nonradiative processes). Highly sophisticated spectroscopic techniques to detect and analyze emitted photons have been well established. This paper represents an attempt at a qualititative investigation of the Cr3 + ion concentration dependence of the photoluminescence (PL) spectra in ceramic Al2 − x Crx O3 (x is extended to 0.15) together with the optical absorption measurements by photoacoustic (PA) spectroscopy. Since PA signals are produced only by the absorbed light actually converted to heat in the sample. PA spectroscopy is a useful tool to study the optical absorption for ceramics. PA spectroscopy is applicable to study for nonradiative processes [4]. Moreover, one of the principal advantages of PA spectroscopy is that it enables one to obtain not only signal intensity spectra similar to optical absorption but also signal phase spectra on any type of solid or semisolid material, whether they are crystalline, powder, and/or amorphous [5,6].
34
T. Toyoda et al. / Materials Science and Engineering B54 (1998) 33–37
2. Experimental
x is the increase of nonradiative processes at the 2E A2 transition according to the discussion by Murphy and coworker using a rate-equation model, indicating concentration quenching [4]. The values of the ratio, RI agree with those obtained by Murphy and coworker within the experimental accuracy. Fig. 2 shows the examples of PL spectra for x=0.01, 0.05, and 0.15 in ceramic Al2 − x Crx O3. They show the 4
Ceramic Al2 − x Crx O3 (0.002 x 0.15) samples were prepared from mixtures of Al2O3 (99.99%) and Cr2O3 (99.9%) powders. They were pressed to be plates with the thickness of 1.3 mm. Then they were sintered at 1400°C for 1 h in air. The excitation light used for the PL measurements was the second harmonic generation (SHG) of the dye laser and was fixed at 295 nm with average power of less than 0.5 mW. A dye laser at 590 nm excited by the second harmonics of a mode-locked YAG laser was used as the excitation source. The normal of the sample was about 50° to the incident light and the fluorescence light was measured by a spectrometer with an ICCD detector [7]. The body of the PA cell was an aluminium cylinder with a small channel in its periphery in which a microphone was inserted. The window of the cell was silica glass. The inside volume of the cell was approximately 0.5 cm3. The cell was suspended by four rubber wires in order to avoid outside vibrations. The light source was a 300 W xenon arc lamp and the light beam was focused on the sample through a monochromator with an impinging area of 0.15 cm2. Modulation frequency for the spectroscopy measurements was 133 Hz using a mechanical chopper. The measurements were carried out at room temperature in the wavelength range of 330–800 nm. The PA signal intensities were monitored by passing the microphone signal first through a preamplifier and then into a two-phase lock-in amplifier. The spectra were normarized by the PA signal of carbon black sheet.
3. Results and discussion X-ray diffraction patterns showed that the crystal structures of Al2 − x Crx O3 (0.002 x 0.15) samples were a -aliminum oxide structures. We could not find the lattice constant changes by adding Cr2O3 in Al2O3 because of the small changes of them (a = 0.476 nm and c = 1.299 nm for Al2O3; a = 0.4959 nm and c = 1.360 nm for Cr2O3). Fig. 1 shows the examples of PA intensity spectra for x = 0.01, 0.05. and 0.15 in ceramic Al2 − x Crx O3. Qualitatively, those spectra are similar to that obtained by optical absorption measurements in single crystal of ruby reported [2]. The peaks which correspond to Uband ( 565 nm) and Y-band ( 410 nm) can be observed in all the spectra. The shapes of the spectra are similar to each other except that the change of intensity ratio, RI =S (Y-band)/S (U-band) (S denotes the PA intensity for each band), shows decrease with increasing mole fraction x. One of the possibility that the values of RI decrease with increasing mole fraction
Fig. 1. Photoacoustic signal intensity spectra of ceramic Al2 − x Crx O3 for x=0.01, 0.05, and 0.15 in the wavelength range of 330–800 nm with a modulation frequency of 133 Hz.
T. Toyoda et al. / Materials Science and Engineering B54 (1998) 33–37
35
Fig. 2. Photoluminescence spectra of ceramic Al2 − x Crx O3 for x = 0.01, 0.05, and 0.15 with the excitation light of 295 nm wavelength.
intense R line ( 694 nm) originating from isolated single Cr3 + ions. In order to compare the relative changes of PL intensity with mole fraction x. PL mea-
surements of R line ( 694 nm) for the single crystal of ruby at the same experimental condition have been carried out as standard. The concentration of Cr3 +
36
T. Toyoda et al. / Materials Science and Engineering B54 (1998) 33–37
Fig. 3. Relative intensity of I (694nm) to that of the standard single crystal of ruby for ceramic Al2 − x Crx 03 as a function of mole fraction x.
Fig. 5. Ratio, I (704 nm)/I (694 nm), for ceramic Al2 − x Crx O3 as a function of mole fraction x.
ions of single crystal of ruby used is x = 0.002 which was utilized as a laser material. Fig. 3 shows that I (694 nm). the relative intensity ratio of R line for ceramic Al2 − x Crx O3 to that of the standard single crystal of ruby, decreases with increasing mole fraction x. indicating the increase of interaction between Cr3 + ions. The weaker sharp lines which are called N line ( 704 nm) can also be observed and have been originated on closely coupled pairs of Cr3 + ions. Fig. 4 shows that I (704 nm). the relative intensity ratio of N line for ceramic Al2 − x Crx O3 to that of the standard single crystal of ruby, increases with increasing mole fraction x, indicating the increase of pair interaction. Fig. 5 shows that the ratio, I (704 nm)/I (694 nm), increases linearly with increasing mole fraction x up to x =0.15, indicating that it was different from the early results
under the assumption that the pairs of Cr3 + obtain some excitation energy by transfer from single ions (the ratio shows nonlinear by the early results reported with the assumption) [8–10]. The long wavelength part of the broad PL spectra (775 nm) can be seen in Fig. 2. Fig. 6 shows that I (775nm), the relative intensity ratio at 775 nm for ceramic Al2 − x Crx O3 to that of the standard single crystal of ruby, increases with increasing mole fraction x, indicating the increase of groups of more than two Cr3 + ions [9]. Quantitative clarification of the electronic state of Cr3 + ions in ceramic Al2O3 is left to future experimental and theoretical investigation. Photoluminescence excitation (PLE) spectroscopy and the detailed analysis of quantum efficincy of ceramic Al2 − x Crx O3 are necessary to clarify the nature of the radiative and nonradiative processes [11–14].
Fig. 4. Relative intensity of I (704 nm) to that of the standard single crystal of ruby for ceramic Al2 − x Crx O3 as a function of mole fraction x.
Fig. 6. Relative intensity of I (775 nm) to that of the standard single crystal of ruby for ceramic Al2 − x Crx O3 as a function of mole fraction x.
T. Toyoda et al. / Materials Science and Engineering B54 (1998) 33–37
Acknowledgements The authors are grateful to Dr Q. Shen, Mr K. Yokomizo and Mr H. Zhang for valuable discussions. Mr K. Shinoyama is acknowledged for help in the photoacoustic measurements and technical discussion. The authors thank Mr T. Takeuchi for helpful photoluminescence measurements.
References [1] P. Kisliuk, C.A. Moore, Phys.Rev. 160 (1967) 307.
.
37
[2] T.H. Maiman, R.H. Hoskins, I.J. D’Haenens, C.K. Asaka, V. Evtuhov, Phys. Rev. 123 (1961) 1151. [3] J. Bohandy, J. Solid State Chem. 3 (1971) 467. [4] J.C. Murphy, L.C. Aamodt, J. Appl. Phys. 48 (1977) 3502. [5] A. Rosencwaig, A. Gersho, J. Appl. Phys. 47 (1976) 64. [6] E.M. Monahan Jr., A.W. Nolle, J. Appl. Phys. 48 (1977) 3519. [7] K. Morita, K. Shinjo, Qi Ping, K. Abe, T. Shigenari, Ferroelectrics 159 (1994) 203. [8] A.L. Schawlow, D.L. Wood, A.M. Clogston, Phys. Rev. Lett. 3 (1959) 271. [9] N.A. Tolstoi, Liu Shun’-fu, M.E. Lapidus, Opt. Spectros. 13 (1961) 133. [10] G.F. Imbusch, Phys. Rev. 153 (1967) 326. [11] R.S. Quimby, W.M. Yen, J. Appl. Phys. 51 (1980) 1780. [12] Y. Li, I. Duncan, T. Morrow, J. Luminescence 50 (1991) 69. [13] A. Mandelis, Z.H. Chen, R. Bleiss, J. Phys. IV Suppl. C7 (4) (1994) C7 – 401. [14] T. Toyoda, T. Obikawa, Mater. Sci. Eng. B41 (1996) 72.