- Email: [email protected]
Luminescence of Dy3+ enhanced by sensitization
J•urnol *f
ALLOY5 AND COMPOUND5 ELSEVIER
Journal of Alloys and Compounds 225 (1995) 103-106
Luminescence of Dy 3+ enhanced by sensitization Qiang Su, Zhiwu Pei, Jun Lin, Feng Xue Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People's Republic of China
Abstract
Four types of sensitized luminescence of Dy3+ are reported: (1) by a host having a broad-band spectrum as in NaaYo.99Dyo.ol(VO4)2; (2) by a sensitizer having a broad-band spectrum as in Ca2BzOs:Dy3÷, Bi3+; (3) by a sensitizer having a narrow-band spectrum as in Mg2Gd7.9Dyoa(SiO4)602; (4) by a sensitizer having a broad-band spectrum and energy migration as in Gd compounds such as Cal.96Pbo.04Gd7.9Dyoa(SiO4)602. The luminescent intensity of Dy3+ can be enhanced in these ways. Keywords: Dysprosium; Rare earths; Luminescence
1. Introduction Two dominant bands appear in the emission spectrum of Dy 3+. The yellow band (575 nm) corresponds to the hypersensitive transition 4F9/2-->6H13/2 ( A L = 2 ; AJ = 2 ) and the blue band (485 nm) corresponds to the 4F9/z--->6H15/z transition. At a suitable yellow-toblue-intensity ratio, Dy 3+ will emit white light [1]; so luminescent materials doped with Dy 3+ may be used as potential two-band phosphors. The excitation spectrum of Dy 3+ consists of only narrow excitation bands of f - f transitions ranging from 300 to 500 nm (see the broken curve in Fig. l(a)); no broad excitation band such as a charge transfer band or an f--d transition band exists in the U V region 200-300 nm. Hence its luminescent efficiency is low when it is excited by the 254 nm U V radiation emitted from the mercury plasma. This is one of the drawbacks to its use as lamp phosphor. However, this can be overcome by sensitization [1-4]. In order to enhance the luminescence of Dy 3÷, four types of sensitization and some results obtained by us are reported.
293 357 (b
c)
t-4
II ItlliiN 216
200
I
,
300
1!I
:,, cJ
400
"-50Ohm
Wavelength Fig. 1. Excitation spectrum (Em=575 nm) (curve (a)) of Dy3+ in Ca2B2Os:Dy3+, K +. Excitation spectrum (Era=360 nm) (curve (b)) and emission spectrum (Ex=293 nm) (curve (c)) of Bi3÷ in Ca2B2Os:Bi3+, K + at room temperature.
2. Experimental details The vanadate Na3Yo.99Dyo.ol(VO4)2 was obtained from a stoichiometric mixture of Na2CO3, NH4VO3, Y 2 0 3 and D y 2 0 3. The reagents were carefully mixed and ground and heated to 600 °C for 12 h in an alumina 0925-8388/95/$09.50 © 1995 Elsevier Science S.A. Allrights rese~ed SSDI 0925-8388(94)07017-2
crucible, and then they were ground and heated to 820 °C for 12 h. Pale-yellow samples were obtained. The borate Ca2B2Os:Dy3+,Bi 3+ was prepared as described in [4].
104
Q. S u e t al. I Journal of Alloys and Compounds 225 (1995) 103-106
The oxyapatite Mg2Gd7.9Dyo.l(SiO4)602 and Cal.96Pbo.o4GdT.9Dy0.1(SiO4)602 were prepared by the sol-gel method as described in [5]. All samples were checked by X-ray powder diffraction with a Rigaku 2028 or Philips PW-1700 diffractometer (Cu Ka radiation). They were all single phase. The diffraction data obtained by us are consistent with those in literature. Excitation and emission spectra were recorded on a MPF-4 or Hitachi M-850 spectrofluorometer at room temperature.
Em=573 nm
"i
3.1. Sensitization of Dy 3+ by the host in Na 3Yo.ogDyo.01( V0 4) 2
The VO4 3- radical in some vanadates has a charge transfer band in the range 250-350 nm (assigned to ta-+ 2e) [6]; so it can absorb energy in this UV region and fluoresce. The positions of its absorption and emission bands depend on the host lattice. When its spectra match the excitation spectrum of Dy 3+, it may sensitize the luminescence of Dy 3+ as is observed in the hosts of Ba2LaV3On [1], Y V O 4 [2] o r YPxVI_xO4
[3]. Na3Y(VO4)2 is isomorphous with Na3Er(VO4)2, which is a very slightly distorted version of the glaserite lattice [7]. Its low temperature phase possesses a monoclinic crystal lattice with the symmetry of the P21/n space group. The Y atom occupies a slightly distorted octahedral site and its coordination number equals 6. When Eu 3÷ is doped and used as a luminescent structural probe, we observed that its red emission intensity corresponding to the SDo--+TF2 transition is stronger than the orange emission intensity corresponding to the SDo--+7F1 transition, and the 5D0--+7Fo transition does not appear. This implies that the site occupied by Eu 3+ ( y 3 + o r D y 3+) has deviated from centrosymmetry. Na3Y(VO4)2 has broad excitation and emission bands at room temperature with maxima at 313 nm and 457 nm respectively. The blue emission of the vanadate overlaps several Dy 3+ absorption lines. For VO43Dy 3+ transfer, the resonance condition (spectral overlap) is satisfied. In this case, Dy ~+ can be excited by 254 nm radiation and emits characteristic blue light which peaks at 481 nm and yellow light which peaks at 572.5 nm; the yellow-to-blue-intensity ratio is 2.19 (Fig. 2).
3.2. Sensitization of Dy 3÷ by the sensitizer Bi 3+ which has a broad-band spectrum in CazBzOs:Dy a÷, Bi3+
CaeB205 is a monoclinic crystal lattice. Since the ionic radii of Dy 3÷, Bi 3÷ and Eu 3+ are about as large
(a)
(b)
I--4
200 3. Results and discussion
Ex=254 nm
400
400
600
nm
Wavelength Fig. 2. Excitation spectrum (Era= 573 nm) (curve (a)) and emission spectrum (E,= 254 nm) (curve (b)) of Na3Y0.99Dyo.m(VO4)2at room temperature.
as that of the C a 2+ ion, it is assumed that they occupy t h e C a 2+ site. In order to understand the site symmetry of C a 2+, E u 3+ is doped and used as a luminescent structural probe. The aDo--+ 7F o transition peaking at 578 nm, the 5Do-+7F 1 transition around 592 nm and the 5Do-+7F 2 transition peaking at 611.5 and 623 nm were observed. Furthermore, the intensity of the SDo ~ 7F2 transition is stronger than that of the 5Do --+ 7F 1 transition; this implies that the site symmetry of Ca 2+ (or Dy3+ and B ? +) lacks an inverse centre. When Bi3+ is doped in Ca2B2Os, Fig. 1, curve (b) shows its excitation spectrum with maxima at 216 and 293 nm, which correspond to the transitions from the ground state ~So of Bi 3+ to the ~P~ and 3P 1 excited states respectively. Under 293 nm excitation, a broad emission band with a maximum at 357 nm and a width at half-height of 50 nm is observed. The Stokes shift is 6118 cm -~ (see Fig. 1, curve (c)). Curves (c) and (a) of Fig. 1 show that the broad emission band of Bi 3+ and the excitation spectrum of Dy 3+ overlap each other. The presence of the strong Bi3÷ absorption band at 293 nm for the Dy 3+ emission at 575 nm in Ca2B2Os:Dy3+, Bi 3+ indicates the B i 3 + ~ Dy 3+ energy transfer (Fig. 3(a)). The characteristic emission of Dy 3+ is clearly observed when Bi3+ is excited by 293 nm radiation (Fig. 3(b)). 3.3. Sensitization of Dy 3+ by the sensitizer Gd 3+ which has a narrow-band spectrum in Mg2 Gd7 9Dyo.1(Si04) 602
For this case, spectral overlap between the sensitizer and activator is unnecessary. The conditions required are that the energy of the excited state of the sensitizer must be higher than that of the excited state 4Fg/E of Dy 3+ which is located at about 20 833 cm -1 and can be transferred non-radiatively to 4F9a of Dya +. Then, 4Fg/2--+6HJ transitions take place and result in the
105
Q. Su et aL / Journal of Alloys and Compounds 225 (1995) 103-106
Em--575 nm
(o)
(a)
i~/~57Ex =290 nm
Em=577 nm Gd3+
Ex =275 nm
293 Dy31" DY3+
I_a,/ JBi-'
I
200
I
300
I
i
•
I
300
400
i
400
i
i
500
4
6 0 0 nm
Wavelength
,% It}
Fig. 3. (a) Excitation spectrum (Em=575 nm) and (b) emission spectrum (E~=290 nm) of Ca2B2Os:Dy3+, Bi3+, K + at room temperature.
-r tD
I -%
6 emission of Dy 3+ . Gd 3+ belongs to this kind of narrowband sensitizer; the excitation bands of its SST/2---~6D, 6i, 6p transitions are located at 250 nm (40 000 cm-~), 275 nm (36364 cm -a) and 310 nm (32258 cm -a) respectively, which are higher than the excited state 4F9/2 of Dy 3+. U n d e r excitation with these excitation bands of Gd 3+, sensitization of Dy 3+ was observed in
It. Gd 3+
I-t
Dy 3+
MgeGd7.9Dyo.a ( 8 i 0 4 ) 6 0 2 .
M2REs(SiO4)60 2 (M-=Mg or Ca; R E - G d or Y) crystallizes in the hexagonal system with space group Pb3/m. Rare earth ions occupy two kinds of site. One is the 4f site (2M 2+, 2 R E 3+) with symmetry C3; its coordination n u m b e r is 9. The other is the 6h site (6RE 3+) with symmetry Cs; its coordination number is 7. The distance between 4f(Gd 3+) and 4f(Gd 3+) is about 0.35 nm, and those between 6h(Gd 3+) and 6h(Gd 3+) or 6h(Gd 3+) and 4f(Gd 3+) are about 0.4
200
300
400
400
500
600 nm
Wavelength
Fig. 4. Excitation spectrum (Era= 577 nm) (curve (a)) and emission spectrum (Ex=275 nm) (curve (b)) of Mg2GdT.9Dyo3(SiO4)602 at room temperature.
rim.
Fig. 4, curve (a) shows that the excitation spectrum for Dy 3+ emission in Mg2Gd79Dyo.a(SiO4)6Oz consists of f - f transitions of Dy 3+ followed by Gd 3 + 8S7a--* 6D, 6i, 6p transitions peaking at 250 nm, 275 nm and 312 nm respectively. The presence of both Gd 3 + and Dy 3÷ excitation confirms the sensitization of Dy 3+ by Gd 3+. Efficient Dy 3+ emission is observed after excitation into the Gd 3+ ion with the 275 nm radiation. The Dy 3+ emission spectrum consists of the usual blue line at 485 nm and yellow line at 577 nm; the yellow-to-blueintensity ratio is 1.37 (Fig. 4, curve (b)). The emission intensity of Dy 3 ÷ in the gadolinium compound is 4.3 times that in the yttrium compound; the latter is excited into the Dy 3+ ion with the 348 nm radiation. This implies that the luminescence of Dy 3+ can be enhanced by Gd 3+ which has a narrow-band spectrum. It can be seen that, when the Gd 3+ ion is used as the sensitizer, the excitation wavelength is confined and cannot be varied. In this case, 275 nm radiation is usually used for excitation, but this does not match the 254 nm radiation emitted from the mercury plasma
for lamp applications. This disadvantage of Gd 3+ for use as sensitizer can be overcome if a sensitizer which has a broad excitation band around 254 nm is codoped with Gd 3÷ ion. For example, Pb 2+ can be codoped with Gd 3+ to sensitize Dy 3+. This is discussed below.
3.4. Sensitization of Dy s+ by a sensitizer Pb2+ having a broad band spectrum and via energy migration of Gd 3+ in Cal.96Pbo.04GdT.gDyo.1(Si04)602 It is of interest to compare the excitation and emission spectra of the gadolinium compound with those of the yttrium compound in Cal.96Pbo.04RET.9Dyo.1(SiO4)602 where R E - G d or Y. When R E = Y , the excitation spectrum of the Dy 3+ emission consists of some narrow bands of Dy 3+ due to f-f transitions and a broad band with a maximum at 265 nm which is ascribed to transition from the ground state 1S0 of Pb 2+ to 3P 1 (see the broken curve in Fig. 5(a)). This indicates that energy transfer from Pb 2+ to Dy 3+ occurs. When Pb 2+ is
106
Q. Su et al. /Journal of Alloys and Compounds 225 (1995) 103-106
transition and shifts the excitation band to a shorter wavelength matching the mercury emission lines. When R E = Gd, the emission intensity of Pb 2+ becomes weak; its integrated intensity is only a seventh of that when R E = Y. Furthermore, all energy of Gd 3+ was trapped by Dy3+; no Gd 3+ luminescence was observed. The emission intensity of Dy 3+ in the Gd compound is six times that in the Y compound (see the full curve in Fig. 5(b)).
(b)
;d 3+
Dy3+ +i
v
Em=575nm
Ex:254 nm /
6
I-4
./11
/\
I:i
l li' i .
200
4. Conclusion
Dy3+ [Gd3+!
300
400
: 500
I
300
400
I
I
500
I
I
600 nm
Wovelenglh Fig. 5. (a) Excitation spectrum (Er,=575 nm) and (b) emission spectrum (Ex=254 nm) of Cal.96Pbo.o4RET.9Dyo.l(SiO4)602at room temperature: - - , RE---Gd; . . . . , RE---Y.
excited with 254 nm radiation, a broad emission band was observed with a maximum at 370 nm and a width at half-height of 100 nm which overlaps the excitation bands of the Dy 3÷ ion. This satisfies the criterion for sensitization of Dy 3+ by Pb 2+, but the efficiency is not very high, because the emission intensity of Pb 2÷ is still intense (see the broken curve in Fig. 5(b)). This leads to incomplete Pb2+-Dy 3+ energy transfer, and weak luminescence of Dy 3÷ with maxima at 480 and 570 nm was observed. W h e n R E = Gd, the excitation spectrum of the Dy 3 ÷ emission shows narrow bands of Gd 3+ with maxima at 274 and 312 nm, which correspond to the 887/2-->617/2 and 6p7/z transitions respectively together with a strong and broad band of Pb 2+ with a maximum at 260 nm and narrow bands of Dy 3+ due to f - f transitions. The presence of both Pb 2+ and Gd 3+ excitations confirms the transfer process Pb 2+ ~ Gd 3+ -~ (Gd 3+). ~ Gd 3 + Dy 3+ (see the full curve in Fig. 5(a)). When the sample containing Gd~+ is excited with the same 254 nm radiation as the sample without Gd 3+, because the emission spectrum of Pb 2 + overlaps the excitation spectra of Gd 3+ and Dy 3+, this satisfies the criterion for sensitization of Dy 3+ by Pb 2+ and via Gd 3+ migration. T h e sensitization effect of Pb 2+ is enhanced by the presence of Gd 3+ ion, in which the energy migrates rapidly over the Gd 3÷ ions. At the same time, the sensitization effect of Gd 3+ is also enhanced by the presence of Pb 2+ which has the allowed 1So~3P 1
When hosts or sensitizers having a broad-band spectrum are used for sensitization, overlap between their excitation and emission spectra as well as the Stokes shift and overlap between their emission spectrum and the excitation spectrum of Dy 3+ play a crucial role. The excitation wavelength of this kind of phosphor can be tunable within a certain range and can be shifted to 254 nm to match the mercury emission lines for lamp use by choosing a suitable host or sensitizer. Nevertheless, when sensitizers having a narrow-band spectrum such as Gd 3 + are used, the excitation wavelength is confined and limited. When a broad-band sensitizer and narrow-band Gd 3+ are codoped in the phosphor activated by Dy 3+, excitation is into the broadband sensitizer via the allowed transition; transfer occurs into the Gd 3+ sublattice in which the energy migrates rapidly over the Gd 3+ ions until it is trapped by the Dy 3÷ ion from which emission occurs. This p h e n o m e n o n has attracted considerable interest. All four types of sensitization mentioned above are of benefit to absorb U V radiation in the range 200-300 nm for Dy 3 + and to enhance the Dy 3+ luminescence. Sensitized luminescence of Dy 3 + may lead to the development of new and efficient photoluminescent materials.
Acknowledgement This work was supported by the National Committee of Science and Technology of China.
References [1] Q. Su, Z. Pei, L. Chi, H. Zhang, Z. Zhang and F. Zou, J. Alloys Comp., 192 (1993) 25. [2] J.L. Sommerdijk, A. Bril and F.M.J.H. Hoex-Strik, Philips Res. Rep., 32 (1977) 149. [31 L. Chi and Q. Su, Chin. J. Appl. Chem., 10 (1993) 27. [4] Z. Pei, Q. Su and J. Zhang, Solid State Commun., 86 (1993) 377. [5] J. Lin and Q. Su, Mater. Chem. Phys., 38 (1994) 98. [6] H. Ronde and J.G. Snijders, Chem. Phys. Lett., 50 (1977) 282. [7] M. Vlasse, C. Parent, R. Salmon, G. Le Flem and P. Hagenmuller, 3". Solid State Chem., 35 (1980) 318.
ALLOY5 AND COMPOUND5 ELSEVIER
Journal of Alloys and Compounds 225 (1995) 103-106
Luminescence of Dy 3+ enhanced by sensitization Qiang Su, Zhiwu Pei, Jun Lin, Feng Xue Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People's Republic of China
Abstract
Four types of sensitized luminescence of Dy3+ are reported: (1) by a host having a broad-band spectrum as in NaaYo.99Dyo.ol(VO4)2; (2) by a sensitizer having a broad-band spectrum as in Ca2BzOs:Dy3÷, Bi3+; (3) by a sensitizer having a narrow-band spectrum as in Mg2Gd7.9Dyoa(SiO4)602; (4) by a sensitizer having a broad-band spectrum and energy migration as in Gd compounds such as Cal.96Pbo.04Gd7.9Dyoa(SiO4)602. The luminescent intensity of Dy3+ can be enhanced in these ways. Keywords: Dysprosium; Rare earths; Luminescence
1. Introduction Two dominant bands appear in the emission spectrum of Dy 3+. The yellow band (575 nm) corresponds to the hypersensitive transition 4F9/2-->6H13/2 ( A L = 2 ; AJ = 2 ) and the blue band (485 nm) corresponds to the 4F9/z--->6H15/z transition. At a suitable yellow-toblue-intensity ratio, Dy 3+ will emit white light [1]; so luminescent materials doped with Dy 3+ may be used as potential two-band phosphors. The excitation spectrum of Dy 3+ consists of only narrow excitation bands of f - f transitions ranging from 300 to 500 nm (see the broken curve in Fig. l(a)); no broad excitation band such as a charge transfer band or an f--d transition band exists in the U V region 200-300 nm. Hence its luminescent efficiency is low when it is excited by the 254 nm U V radiation emitted from the mercury plasma. This is one of the drawbacks to its use as lamp phosphor. However, this can be overcome by sensitization [1-4]. In order to enhance the luminescence of Dy 3÷, four types of sensitization and some results obtained by us are reported.
293 357 (b
c)
t-4
II ItlliiN 216
200
I
,
300
1!I
:,, cJ
400
"-50Ohm
Wavelength Fig. 1. Excitation spectrum (Em=575 nm) (curve (a)) of Dy3+ in Ca2B2Os:Dy3+, K +. Excitation spectrum (Era=360 nm) (curve (b)) and emission spectrum (Ex=293 nm) (curve (c)) of Bi3÷ in Ca2B2Os:Bi3+, K + at room temperature.
2. Experimental details The vanadate Na3Yo.99Dyo.ol(VO4)2 was obtained from a stoichiometric mixture of Na2CO3, NH4VO3, Y 2 0 3 and D y 2 0 3. The reagents were carefully mixed and ground and heated to 600 °C for 12 h in an alumina 0925-8388/95/$09.50 © 1995 Elsevier Science S.A. Allrights rese~ed SSDI 0925-8388(94)07017-2
crucible, and then they were ground and heated to 820 °C for 12 h. Pale-yellow samples were obtained. The borate Ca2B2Os:Dy3+,Bi 3+ was prepared as described in [4].
104
Q. S u e t al. I Journal of Alloys and Compounds 225 (1995) 103-106
The oxyapatite Mg2Gd7.9Dyo.l(SiO4)602 and Cal.96Pbo.o4GdT.9Dy0.1(SiO4)602 were prepared by the sol-gel method as described in [5]. All samples were checked by X-ray powder diffraction with a Rigaku 2028 or Philips PW-1700 diffractometer (Cu Ka radiation). They were all single phase. The diffraction data obtained by us are consistent with those in literature. Excitation and emission spectra were recorded on a MPF-4 or Hitachi M-850 spectrofluorometer at room temperature.
Em=573 nm
"i
3.1. Sensitization of Dy 3+ by the host in Na 3Yo.ogDyo.01( V0 4) 2
The VO4 3- radical in some vanadates has a charge transfer band in the range 250-350 nm (assigned to ta-+ 2e) [6]; so it can absorb energy in this UV region and fluoresce. The positions of its absorption and emission bands depend on the host lattice. When its spectra match the excitation spectrum of Dy 3+, it may sensitize the luminescence of Dy 3+ as is observed in the hosts of Ba2LaV3On [1], Y V O 4 [2] o r YPxVI_xO4
[3]. Na3Y(VO4)2 is isomorphous with Na3Er(VO4)2, which is a very slightly distorted version of the glaserite lattice [7]. Its low temperature phase possesses a monoclinic crystal lattice with the symmetry of the P21/n space group. The Y atom occupies a slightly distorted octahedral site and its coordination number equals 6. When Eu 3÷ is doped and used as a luminescent structural probe, we observed that its red emission intensity corresponding to the SDo--+TF2 transition is stronger than the orange emission intensity corresponding to the SDo--+7F1 transition, and the 5D0--+7Fo transition does not appear. This implies that the site occupied by Eu 3+ ( y 3 + o r D y 3+) has deviated from centrosymmetry. Na3Y(VO4)2 has broad excitation and emission bands at room temperature with maxima at 313 nm and 457 nm respectively. The blue emission of the vanadate overlaps several Dy 3+ absorption lines. For VO43Dy 3+ transfer, the resonance condition (spectral overlap) is satisfied. In this case, Dy ~+ can be excited by 254 nm radiation and emits characteristic blue light which peaks at 481 nm and yellow light which peaks at 572.5 nm; the yellow-to-blue-intensity ratio is 2.19 (Fig. 2).
3.2. Sensitization of Dy 3÷ by the sensitizer Bi 3+ which has a broad-band spectrum in CazBzOs:Dy a÷, Bi3+
CaeB205 is a monoclinic crystal lattice. Since the ionic radii of Dy 3÷, Bi 3÷ and Eu 3+ are about as large
(a)
(b)
I--4
200 3. Results and discussion
Ex=254 nm
400
400
600
nm
Wavelength Fig. 2. Excitation spectrum (Era= 573 nm) (curve (a)) and emission spectrum (E,= 254 nm) (curve (b)) of Na3Y0.99Dyo.m(VO4)2at room temperature.
as that of the C a 2+ ion, it is assumed that they occupy t h e C a 2+ site. In order to understand the site symmetry of C a 2+, E u 3+ is doped and used as a luminescent structural probe. The aDo--+ 7F o transition peaking at 578 nm, the 5Do-+7F 1 transition around 592 nm and the 5Do-+7F 2 transition peaking at 611.5 and 623 nm were observed. Furthermore, the intensity of the SDo ~ 7F2 transition is stronger than that of the 5Do --+ 7F 1 transition; this implies that the site symmetry of Ca 2+ (or Dy3+ and B ? +) lacks an inverse centre. When Bi3+ is doped in Ca2B2Os, Fig. 1, curve (b) shows its excitation spectrum with maxima at 216 and 293 nm, which correspond to the transitions from the ground state ~So of Bi 3+ to the ~P~ and 3P 1 excited states respectively. Under 293 nm excitation, a broad emission band with a maximum at 357 nm and a width at half-height of 50 nm is observed. The Stokes shift is 6118 cm -~ (see Fig. 1, curve (c)). Curves (c) and (a) of Fig. 1 show that the broad emission band of Bi 3+ and the excitation spectrum of Dy 3+ overlap each other. The presence of the strong Bi3÷ absorption band at 293 nm for the Dy 3+ emission at 575 nm in Ca2B2Os:Dy3+, Bi 3+ indicates the B i 3 + ~ Dy 3+ energy transfer (Fig. 3(a)). The characteristic emission of Dy 3+ is clearly observed when Bi3+ is excited by 293 nm radiation (Fig. 3(b)). 3.3. Sensitization of Dy 3+ by the sensitizer Gd 3+ which has a narrow-band spectrum in Mg2 Gd7 9Dyo.1(Si04) 602
For this case, spectral overlap between the sensitizer and activator is unnecessary. The conditions required are that the energy of the excited state of the sensitizer must be higher than that of the excited state 4Fg/E of Dy 3+ which is located at about 20 833 cm -1 and can be transferred non-radiatively to 4F9a of Dya +. Then, 4Fg/2--+6HJ transitions take place and result in the
105
Q. Su et aL / Journal of Alloys and Compounds 225 (1995) 103-106
Em--575 nm
(o)
(a)
i~/~57Ex =290 nm
Em=577 nm Gd3+
Ex =275 nm
293 Dy31" DY3+
I_a,/ JBi-'
I
200
I
300
I
i
•
I
300
400
i
400
i
i
500
4
6 0 0 nm
Wavelength
,% It}
Fig. 3. (a) Excitation spectrum (Em=575 nm) and (b) emission spectrum (E~=290 nm) of Ca2B2Os:Dy3+, Bi3+, K + at room temperature.
-r tD
I -%
6 emission of Dy 3+ . Gd 3+ belongs to this kind of narrowband sensitizer; the excitation bands of its SST/2---~6D, 6i, 6p transitions are located at 250 nm (40 000 cm-~), 275 nm (36364 cm -a) and 310 nm (32258 cm -a) respectively, which are higher than the excited state 4F9/2 of Dy 3+. U n d e r excitation with these excitation bands of Gd 3+, sensitization of Dy 3+ was observed in
It. Gd 3+
I-t
Dy 3+
MgeGd7.9Dyo.a ( 8 i 0 4 ) 6 0 2 .
M2REs(SiO4)60 2 (M-=Mg or Ca; R E - G d or Y) crystallizes in the hexagonal system with space group Pb3/m. Rare earth ions occupy two kinds of site. One is the 4f site (2M 2+, 2 R E 3+) with symmetry C3; its coordination n u m b e r is 9. The other is the 6h site (6RE 3+) with symmetry Cs; its coordination number is 7. The distance between 4f(Gd 3+) and 4f(Gd 3+) is about 0.35 nm, and those between 6h(Gd 3+) and 6h(Gd 3+) or 6h(Gd 3+) and 4f(Gd 3+) are about 0.4
200
300
400
400
500
600 nm
Wavelength
Fig. 4. Excitation spectrum (Era= 577 nm) (curve (a)) and emission spectrum (Ex=275 nm) (curve (b)) of Mg2GdT.9Dyo3(SiO4)602 at room temperature.
rim.
Fig. 4, curve (a) shows that the excitation spectrum for Dy 3+ emission in Mg2Gd79Dyo.a(SiO4)6Oz consists of f - f transitions of Dy 3+ followed by Gd 3 + 8S7a--* 6D, 6i, 6p transitions peaking at 250 nm, 275 nm and 312 nm respectively. The presence of both Gd 3 + and Dy 3÷ excitation confirms the sensitization of Dy 3+ by Gd 3+. Efficient Dy 3+ emission is observed after excitation into the Gd 3+ ion with the 275 nm radiation. The Dy 3+ emission spectrum consists of the usual blue line at 485 nm and yellow line at 577 nm; the yellow-to-blueintensity ratio is 1.37 (Fig. 4, curve (b)). The emission intensity of Dy 3 ÷ in the gadolinium compound is 4.3 times that in the yttrium compound; the latter is excited into the Dy 3+ ion with the 348 nm radiation. This implies that the luminescence of Dy 3+ can be enhanced by Gd 3+ which has a narrow-band spectrum. It can be seen that, when the Gd 3+ ion is used as the sensitizer, the excitation wavelength is confined and cannot be varied. In this case, 275 nm radiation is usually used for excitation, but this does not match the 254 nm radiation emitted from the mercury plasma
for lamp applications. This disadvantage of Gd 3+ for use as sensitizer can be overcome if a sensitizer which has a broad excitation band around 254 nm is codoped with Gd 3÷ ion. For example, Pb 2+ can be codoped with Gd 3+ to sensitize Dy 3+. This is discussed below.
3.4. Sensitization of Dy s+ by a sensitizer Pb2+ having a broad band spectrum and via energy migration of Gd 3+ in Cal.96Pbo.04GdT.gDyo.1(Si04)602 It is of interest to compare the excitation and emission spectra of the gadolinium compound with those of the yttrium compound in Cal.96Pbo.04RET.9Dyo.1(SiO4)602 where R E - G d or Y. When R E = Y , the excitation spectrum of the Dy 3+ emission consists of some narrow bands of Dy 3+ due to f-f transitions and a broad band with a maximum at 265 nm which is ascribed to transition from the ground state 1S0 of Pb 2+ to 3P 1 (see the broken curve in Fig. 5(a)). This indicates that energy transfer from Pb 2+ to Dy 3+ occurs. When Pb 2+ is
106
Q. Su et al. /Journal of Alloys and Compounds 225 (1995) 103-106
transition and shifts the excitation band to a shorter wavelength matching the mercury emission lines. When R E = Gd, the emission intensity of Pb 2+ becomes weak; its integrated intensity is only a seventh of that when R E = Y. Furthermore, all energy of Gd 3+ was trapped by Dy3+; no Gd 3+ luminescence was observed. The emission intensity of Dy 3+ in the Gd compound is six times that in the Y compound (see the full curve in Fig. 5(b)).
(b)
;d 3+
Dy3+ +i
v
Em=575nm
Ex:254 nm /
6
I-4
./11
/\
I:i
l li' i .
200
4. Conclusion
Dy3+ [Gd3+!
300
400
: 500
I
300
400
I
I
500
I
I
600 nm
Wovelenglh Fig. 5. (a) Excitation spectrum (Er,=575 nm) and (b) emission spectrum (Ex=254 nm) of Cal.96Pbo.o4RET.9Dyo.l(SiO4)602at room temperature: - - , RE---Gd; . . . . , RE---Y.
excited with 254 nm radiation, a broad emission band was observed with a maximum at 370 nm and a width at half-height of 100 nm which overlaps the excitation bands of the Dy 3÷ ion. This satisfies the criterion for sensitization of Dy 3+ by Pb 2+, but the efficiency is not very high, because the emission intensity of Pb 2÷ is still intense (see the broken curve in Fig. 5(b)). This leads to incomplete Pb2+-Dy 3+ energy transfer, and weak luminescence of Dy 3÷ with maxima at 480 and 570 nm was observed. W h e n R E = Gd, the excitation spectrum of the Dy 3 ÷ emission shows narrow bands of Gd 3+ with maxima at 274 and 312 nm, which correspond to the 887/2-->617/2 and 6p7/z transitions respectively together with a strong and broad band of Pb 2+ with a maximum at 260 nm and narrow bands of Dy 3+ due to f - f transitions. The presence of both Pb 2+ and Gd 3+ excitations confirms the transfer process Pb 2+ ~ Gd 3+ -~ (Gd 3+). ~ Gd 3 + Dy 3+ (see the full curve in Fig. 5(a)). When the sample containing Gd~+ is excited with the same 254 nm radiation as the sample without Gd 3+, because the emission spectrum of Pb 2 + overlaps the excitation spectra of Gd 3+ and Dy 3+, this satisfies the criterion for sensitization of Dy 3+ by Pb 2+ and via Gd 3+ migration. T h e sensitization effect of Pb 2+ is enhanced by the presence of Gd 3+ ion, in which the energy migrates rapidly over the Gd 3÷ ions. At the same time, the sensitization effect of Gd 3+ is also enhanced by the presence of Pb 2+ which has the allowed 1So~3P 1
When hosts or sensitizers having a broad-band spectrum are used for sensitization, overlap between their excitation and emission spectra as well as the Stokes shift and overlap between their emission spectrum and the excitation spectrum of Dy 3+ play a crucial role. The excitation wavelength of this kind of phosphor can be tunable within a certain range and can be shifted to 254 nm to match the mercury emission lines for lamp use by choosing a suitable host or sensitizer. Nevertheless, when sensitizers having a narrow-band spectrum such as Gd 3 + are used, the excitation wavelength is confined and limited. When a broad-band sensitizer and narrow-band Gd 3+ are codoped in the phosphor activated by Dy 3+, excitation is into the broadband sensitizer via the allowed transition; transfer occurs into the Gd 3+ sublattice in which the energy migrates rapidly over the Gd 3+ ions until it is trapped by the Dy 3÷ ion from which emission occurs. This p h e n o m e n o n has attracted considerable interest. All four types of sensitization mentioned above are of benefit to absorb U V radiation in the range 200-300 nm for Dy 3 + and to enhance the Dy 3+ luminescence. Sensitized luminescence of Dy 3 + may lead to the development of new and efficient photoluminescent materials.
Acknowledgement This work was supported by the National Committee of Science and Technology of China.
References [1] Q. Su, Z. Pei, L. Chi, H. Zhang, Z. Zhang and F. Zou, J. Alloys Comp., 192 (1993) 25. [2] J.L. Sommerdijk, A. Bril and F.M.J.H. Hoex-Strik, Philips Res. Rep., 32 (1977) 149. [31 L. Chi and Q. Su, Chin. J. Appl. Chem., 10 (1993) 27. [4] Z. Pei, Q. Su and J. Zhang, Solid State Commun., 86 (1993) 377. [5] J. Lin and Q. Su, Mater. Chem. Phys., 38 (1994) 98. [6] H. Ronde and J.G. Snijders, Chem. Phys. Lett., 50 (1977) 282. [7] M. Vlasse, C. Parent, R. Salmon, G. Le Flem and P. Hagenmuller, 3". Solid State Chem., 35 (1980) 318.