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Luminescence and surface effects of closed-shell transition metal ions in sol-gel silica glasses
JOURNAL OF
LUMINESCENC ELWVIER
Journal
of Luminescence
72-74
(I 997) 4 17-418
Luminescence .and surface effects of closed-shell transition metal ions in sol-gel silica glasses Markus Herren, Kenji Yamanaka, Naoya Miyazaki, Makoto Morita” Department of Industrial Chemistv. Seikei Universit_v,Musashino-shi, To!go 180, Japan
Abstract Luminescence spectra of transition metal ions of closed-shell structures in sol-gel silica glasses are observed in the series: Ti4+, V5+, Crh+, Mn’+ at temperatures between 300 and 4 K. The luminescence properties are found to be very sensitive to the inhomogeneities, surface effects and preparation conditions. Keywords:
Sol-gel glass; Luminescence; Transition
metal ion; Mn’+
1. Introduction Chromium-doped silica glasses prepared by the sol-gel method show a very broad red and nearinfrared luminescence band at room temperature [ 11. Results from low-temperature measurements of luminescence spectra and lifetimes and from EPR experiments lead to the assignment of this luminescence to the ligand-to-metal-charge-transfer (3LMCT) transition of tetrahedrally oxo-coordinated Cr6+ ions with the closed-shell electronic structure 3d0 [2]. This luminescence is comparable to the reported luminescence of silica-supported chromate (VI) ions [3]. This paper describes the luminescence characteristics of a series of closedshell ions of titanium, vanadium and manganese in porous sol-gel Si02 glass. Ions with closed-shell structures are abundant in phosphors such as CaWO, and YV04 and were reported to be prom-
* Corresponding author. [email protected].
Fax:
+ 81 422 37 3871;
e-mail:
0022-2313/97/$17.00 cl 1997 Elsevier Science B.V. All rights reserved PII SOO22-23 13(97)00027-6
ising candidates for future laser materials [4]. The sol-gel glasses with transition metal ions are expected to substitute these phosphors in the future [S].
2. Experimental Preparation of sol--gel silica glasses doped with transition metal ions was described elsewhere [23. We have also prepared glasses by the sono-gel method (by the use of ultrasonic heat) and by the modified sol-gel method. In the latter method, colloidal silica solutions of Ludox HS 40 (Dupon Co.) were used as starting reagents to achieve preparation of glasses with 3 nm pore size [6]. Concentration of Ti, V, Cr, and Mn ions is in the range 0.05-5 mol%. Annealing process oxidizes most of the incorporated ions, irrespective of the composition and the valency of starting materials. Luminescence spectra and decay times were measured under N,-laser (337.1 nm) and dye-laser excitations at temperatures down to 4 K [2].
418
M. Hrrwn et cd.
.Joumal
of Luminrscmce72
3. Results and discussion
As the ionic valence and atomic number of the transition metal ions increase in the series Ti4+, V”+, CT’+, Mn’+, the luminescence band maximum shifts to lower energy and minority bands due to lower valence ions appear [S]. Position of the band maximum and associated splittings of the 3LMCT states are found, in general, to depend on concentration of dopant ions. Luminescence of Tidoped glass gives a very broad band composed of two components at 12 K. The broad band centered at 19 kK with a lifetime of 12 ms and the other band at 17 kK of 7 ms are ascribed to Ti4+ ions in Td and Oh symmetry, respectively. Luminescence spectra of V”+-doped silica glass are reported in Ref. [Z]. On the low-energy side of the Cr6+ band, a sharp peak at 7860 cm- ’ due to a minority center was observed at 4.2 K. This is assigned to CrS+ ion in tetrahedral (Td) symmetry with the following crystal field parameters: D, = 789. B = 480, C = 2105 (in cm ‘). The position and shape of luminescence band are consistent with those reported for Cr: Forsterite and tetrahedrally coordinated Cr4+ ions in glass [7]. In the case of manganese samples several luminescence centers were detected. Fig. 1 shows the luminescence of Mn-doped Si02 glass, measured at 4.2 K under different excitation wave-
74 (1997) 417-418
lengths. The dominant band at 12.5 kK is attributed to the sofar unknown permanganate (VII) ion. The luminescence decay of SiOZ : Mn’+ is nearly single exponential with a lifetime of 5.9 ms at 4.2 K. We also find a broad band at 17.5 kK due to a minority center of Mn2+ which is clearly enhanced in intensity under dye laser excitation of 500 nm. The luminescence properties of closed-shell ions in silica glasses were found to be very sensitive to the pore size, local structures and inhomogeneities of the glass samples and thus to the preparation conditions. Luminescence band positions of Si02 : Cr6+ are found to show blue shift in the following order: 15.5 kK (sol-gel method) < 16.0 kK (sono -gel method) < 19.5 kK (sol-gel method with colloidal silica) at 15 K. Accordingly, the lifetimes were found to decrease drastically in the same ms(sol-gel) > us(sono-gel) > ns(colloida1 order: silica).
4. Summary We report on broad and visible luminescence and decay profiles of a series of Ti4+, V5+, Cr6+ and Mn’ + ions incorporated in sol-gel SiOz glass. Spectroscopic properties of these closed-shell ions are found to be very sensitive to the preparation methods and surface conditions of the glasses. Luminescence from minority centers due to Cr4+ and Mn” was also observed with very fragile properties.
References Ill
I
20,000
I
16,000 18,000 Wavenumbedcm’
I
I
14,000
12,000
Fig. 1. Luminescence of Mn-doped sol-gel silica glass at 4.2 K, measured under excitations of (a) ( -) pulsed N,-laser (337.1 nm) and (b) (------) pulsed N,-dye laser (500nm).
M. Herren, H. Nishiuchi and M. Morita, J. Chem. Phys. 101 (1994) 4461. c21 M. Herren. K. Yamanaka and M. Morita, Tech. Report, Seikei University 32 (1995) 61. and G. Blasse, J. Phys. Chem. 96 (1992) [31 M.F. Hazenkamp 3442. and A. Lempicki, IEEE J. M Cz. Koepke, A.J. Woitowicz Quant. Electron. 31 (1995) 1554. I51 M. Morita and M. Herren, J. Sot. Info. Disp. (1996) in print. 161 M.W. Shafer. D.D. Awschalom, J. Warnock and G. Ruben, J. Appl. Phys. 61 (1987) 5438. H. Eilers, W.M. Yen, J.S. Hayden and c71 U. Hommerlich. M.K. Aston, J. Lumin. 60.61 (1994) 119.
LUMINESCENC ELWVIER
Journal
of Luminescence
72-74
(I 997) 4 17-418
Luminescence .and surface effects of closed-shell transition metal ions in sol-gel silica glasses Markus Herren, Kenji Yamanaka, Naoya Miyazaki, Makoto Morita” Department of Industrial Chemistv. Seikei Universit_v,Musashino-shi, To!go 180, Japan
Abstract Luminescence spectra of transition metal ions of closed-shell structures in sol-gel silica glasses are observed in the series: Ti4+, V5+, Crh+, Mn’+ at temperatures between 300 and 4 K. The luminescence properties are found to be very sensitive to the inhomogeneities, surface effects and preparation conditions. Keywords:
Sol-gel glass; Luminescence; Transition
metal ion; Mn’+
1. Introduction Chromium-doped silica glasses prepared by the sol-gel method show a very broad red and nearinfrared luminescence band at room temperature [ 11. Results from low-temperature measurements of luminescence spectra and lifetimes and from EPR experiments lead to the assignment of this luminescence to the ligand-to-metal-charge-transfer (3LMCT) transition of tetrahedrally oxo-coordinated Cr6+ ions with the closed-shell electronic structure 3d0 [2]. This luminescence is comparable to the reported luminescence of silica-supported chromate (VI) ions [3]. This paper describes the luminescence characteristics of a series of closedshell ions of titanium, vanadium and manganese in porous sol-gel Si02 glass. Ions with closed-shell structures are abundant in phosphors such as CaWO, and YV04 and were reported to be prom-
* Corresponding author. [email protected].
Fax:
+ 81 422 37 3871;
e-mail:
0022-2313/97/$17.00 cl 1997 Elsevier Science B.V. All rights reserved PII SOO22-23 13(97)00027-6
ising candidates for future laser materials [4]. The sol-gel glasses with transition metal ions are expected to substitute these phosphors in the future [S].
2. Experimental Preparation of sol--gel silica glasses doped with transition metal ions was described elsewhere [23. We have also prepared glasses by the sono-gel method (by the use of ultrasonic heat) and by the modified sol-gel method. In the latter method, colloidal silica solutions of Ludox HS 40 (Dupon Co.) were used as starting reagents to achieve preparation of glasses with 3 nm pore size [6]. Concentration of Ti, V, Cr, and Mn ions is in the range 0.05-5 mol%. Annealing process oxidizes most of the incorporated ions, irrespective of the composition and the valency of starting materials. Luminescence spectra and decay times were measured under N,-laser (337.1 nm) and dye-laser excitations at temperatures down to 4 K [2].
418
M. Hrrwn et cd.
.Joumal
of Luminrscmce72
3. Results and discussion
As the ionic valence and atomic number of the transition metal ions increase in the series Ti4+, V”+, CT’+, Mn’+, the luminescence band maximum shifts to lower energy and minority bands due to lower valence ions appear [S]. Position of the band maximum and associated splittings of the 3LMCT states are found, in general, to depend on concentration of dopant ions. Luminescence of Tidoped glass gives a very broad band composed of two components at 12 K. The broad band centered at 19 kK with a lifetime of 12 ms and the other band at 17 kK of 7 ms are ascribed to Ti4+ ions in Td and Oh symmetry, respectively. Luminescence spectra of V”+-doped silica glass are reported in Ref. [Z]. On the low-energy side of the Cr6+ band, a sharp peak at 7860 cm- ’ due to a minority center was observed at 4.2 K. This is assigned to CrS+ ion in tetrahedral (Td) symmetry with the following crystal field parameters: D, = 789. B = 480, C = 2105 (in cm ‘). The position and shape of luminescence band are consistent with those reported for Cr: Forsterite and tetrahedrally coordinated Cr4+ ions in glass [7]. In the case of manganese samples several luminescence centers were detected. Fig. 1 shows the luminescence of Mn-doped Si02 glass, measured at 4.2 K under different excitation wave-
74 (1997) 417-418
lengths. The dominant band at 12.5 kK is attributed to the sofar unknown permanganate (VII) ion. The luminescence decay of SiOZ : Mn’+ is nearly single exponential with a lifetime of 5.9 ms at 4.2 K. We also find a broad band at 17.5 kK due to a minority center of Mn2+ which is clearly enhanced in intensity under dye laser excitation of 500 nm. The luminescence properties of closed-shell ions in silica glasses were found to be very sensitive to the pore size, local structures and inhomogeneities of the glass samples and thus to the preparation conditions. Luminescence band positions of Si02 : Cr6+ are found to show blue shift in the following order: 15.5 kK (sol-gel method) < 16.0 kK (sono -gel method) < 19.5 kK (sol-gel method with colloidal silica) at 15 K. Accordingly, the lifetimes were found to decrease drastically in the same ms(sol-gel) > us(sono-gel) > ns(colloida1 order: silica).
4. Summary We report on broad and visible luminescence and decay profiles of a series of Ti4+, V5+, Cr6+ and Mn’ + ions incorporated in sol-gel SiOz glass. Spectroscopic properties of these closed-shell ions are found to be very sensitive to the preparation methods and surface conditions of the glasses. Luminescence from minority centers due to Cr4+ and Mn” was also observed with very fragile properties.
References Ill
I
20,000
I
16,000 18,000 Wavenumbedcm’
I
I
14,000
12,000
Fig. 1. Luminescence of Mn-doped sol-gel silica glass at 4.2 K, measured under excitations of (a) ( -) pulsed N,-laser (337.1 nm) and (b) (------) pulsed N,-dye laser (500nm).
M. Herren, H. Nishiuchi and M. Morita, J. Chem. Phys. 101 (1994) 4461. c21 M. Herren. K. Yamanaka and M. Morita, Tech. Report, Seikei University 32 (1995) 61. and G. Blasse, J. Phys. Chem. 96 (1992) [31 M.F. Hazenkamp 3442. and A. Lempicki, IEEE J. M Cz. Koepke, A.J. Woitowicz Quant. Electron. 31 (1995) 1554. I51 M. Morita and M. Herren, J. Sot. Info. Disp. (1996) in print. 161 M.W. Shafer. D.D. Awschalom, J. Warnock and G. Ruben, J. Appl. Phys. 61 (1987) 5438. H. Eilers, W.M. Yen, J.S. Hayden and c71 U. Hommerlich. M.K. Aston, J. Lumin. 60.61 (1994) 119.