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2Ba0.5SmxAlO4F oxyfluorides
Journal of Luminescence 132 (2012) 875–878
Contents lists available at SciVerse ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Luminescent properties of Sr2.5 3x/2Ba0.5SmxAlO4F oxyfluorides Sangmoon Park n Department of Engineering in Energy & Applied Chemistry, Silla University, Busan 617-736, Republic of Korea
a r t i c l e i n f o
abstract
Article history: Received 11 July 2011 Received in revised form 14 October 2011 Accepted 1 December 2011 Available online 13 December 2011
Effective orange Sm3 þ -doped Sr2.5Ba0.5AlO4F phosphors excited at 254 and 408 nm excitation were prepared by the solid-state method. The excitation and emission spectra of Sr2.5 3x/2Ba0.5SmxAlO4F and Sr2.5 3x/2Ba0.5SmxAlO4 aF1 d (x ¼0.001 0.1) based on photoluminescence spectroscopy are investigated. The defects in anion-deficient Sr2.5 3x/2Ba0.5SmxAlO4 aF1 d (x ¼ 0.001, 0.01) are monitored by broad-band photoluminescence emission centered near 480 nm along with the orange emission transitions of Sm3 þ . CIE values and relative luminescent intensities of Sr2.5 3x/2Ba0.5SmxAlO4F and Sr2.5 3x/2Ba0.5SmxAlO4 aF1 d by changing the Sm3 þ content (x ¼0.001 0.1) are discussed. & 2011 Elsevier B.V. All rights reserved.
Keywords: Photoluminescence Oxyfluoride hosts Sm3 þ ions
1. Introduction Remarkable properties of altering charge between oxide and fluoride within the oxyfluoride systems such as Sr2CuO2F2 þ d or Sr2LiSiO4F, are attractively considered for electrical and optical exploration [1,2]. The structure of Sr3AlO4F oxyfluoride host lattice, which is a tetragonal unit cell with space group I4/mcm, is also known as an unusual ordered anion arrangement material caused by spatial separation of ionic Sr–F and covalent Al–O bonds [3,4]. The staking of Sr2F3 þ and SrAlO34 layers in Sr3AlO4F oxyfluoride is along the c-axis as represented in Fig. 1. There are noticeably different two strontium sites, which are Sr(1) and Sr(2) coordinated by ten and eight anions, respectively. It was introduced that alkali earth ions or rare-earth ions can easily be accommodated in the Sr sites, and moreover, the replacement of Sr2 þ ion with larger Ba2 þ ion causing Sr(1)–O and Sr(1)–F distance change results in the stability of oxyfluoride structure. In previous study, trivalent Eu, Tb, Tm, Er into A(1)3 xA(2)xMO4F host lattice (A(1)/A(2)¼Sr, Ba, Ca; M¼Al, Ga) was introduced as a new family of UV-activated phosphors [4]. We also have recently reported that a family of anion-deficient oxyfluorides with the composition Sr3 xAxMO4 aF1 d (A¼Ca, Ba and M¼Al, Ga), general formula ((Sr3 xAx)1 a 2dMO4 aF1 d) show photoluminescence (PL) when excited by either far- and near-UV light [5,6]. The members of Sr3 xAxMO4F can be transformed into self-activating UV-excitable broad-band PL phosphors by controlling their defect structure using appropriate post-synthesis reduction conditions. Defects were created in this family of Sr3 xAxMO4F by exposing them to a 5% H2/95% Ar reducing gas atmosphere. When excited
n
Corresponding author. Tel.: þ82 51 999 5891; fax: þ 82 51 999 5335. E-mail address: [email protected]
0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2011.12.003
in the far-UV near 254 nm, the material of Sr3AlO4 aF1 d shows broad-band bluish PL emission centered near 500 nm. Moreover, Sr2.5Ba0.5Al0.9In0.1O4 aF1 d in the excited in the near-UV centered 365 nm shows broad-band orange PL emission centered near 605 nm with a spectral width of 90 nm (FWHM). Furthermore, the combination of broad and line emission by substituting Eu3 þ activator into defect-induced self-activating lattices [5,6], Sr2.85 Eu0.1Al0.9 In0.1O4 aF1 d, allowed attractive flexibility in tailoring the PL in this class of materials. In this report, the luminescent properties of Sm-doped in both Sr2.5Ba0.5AlO4F and Sr2.5Ba0.5 AlO4 aF1 d oxyfluorides will be scrutinized as phosphors for solid state lighting. Desired CIE values and relative luminescent intensities of Sr2.5 3x/2Ba0.5SmxAlO4F and Sr2.5 3x/2Ba0.5Smx Al O4 aF1 d by changing the Sm3 þ content (x¼0.001 0.1) are also attained.
2. Experimental details Samples of Sm-doped Sr2.5Ba0.5AlO4F were prepared by heating the appropriate stoichiometric amounts of SrCO3 (Aldrich 99.9%), BaCO3 (Alfa, 99.95%), SrF2 (Aldrich 99.99%), Al2O3 (Alfa 99.95%), and Sm2O3 (Alfa 99.9%) at temperature up to 1050 1C for 3 h in air. The as-made Sm-doped Sr2.5Ba0.5AlO4F samples were subsequently annealed for 1 h at 900 1C in reducing atmosphere (5% H2/95% Ar) to compose self-activating luminescent materials. Phase identification of phosphors was done using a Shimadzu XRD-6000 powder diffractometer using CuKa radiation and the unit cell parameters were determined using the least squares refinement program CELREF. Ultraviolet-visible spectroscopy to measure the excitation and emission spectra of the Sr2.5 3x/2 Ba0.5SmxAlO4F and Sr2.5 3x/2Ba0.5SmxAlO4 aF1 d (x ¼0.001 0.1) phosphor materials was done with a spectrofluorometer (Sinco
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S. Park / Journal of Luminescence 132 (2012) 875–878
Sr(1) Sr(2) c
F O
a
AlO4
Fig. 1. Crystallographic structure of Sr3AlO4F oxyfluoride host lattice.
Fig. 2. X-ray diffraction patterns of calculated (a) Sr3AlO4F and observed (b) Sr2.85Sm0.1AlO4F, (c) Sr2.35Ba0.5Sm0.1AlO4F phosphors.
Fluromate FS-2). Relative intensity and desired the Commission International de l’Eclairage (1931 CIE) values of samples were measured by Minolta CS-100A Chroma Meter under 254 nm.
3. Results and discussion Sr2.5Ba0.5SmxAlO4F phosphors were synthesized by solid state reaction at temperatures of up to 1050 1C. X-ray diffraction (XRD) patterns of Sm3 þ -doped oxyfluoride phosphors in Fig. 2 show single-phase host lattices without any obvious impurities indexed by a tetragonal unit cell. Fig. 1(a) indicates the calculated Sr3AlO4F (ICSD 50736) XRD pattern with lattice parameters a ¼6.78(1) and ˚ Partial substitution of Sr by Sm in oxyfluoride host c¼11.14(1) A. (Sr2.85Sm0.1AlO4F) resulted in a slight shift to higher angles as the ˚ Since smaller cell size decreased to a¼ 6.77(1) and c¼11.098(4) A. alkali-earth ions or rare-earth ions prefer the Sr(2) site, which is normally 8 coordinated as discussed above, Sm3 þ ions could suitably be proposed to occupy in the Sr(2) site. There is a gradual shift in the positions of the various Bragg reflections to higher ˚ angle for Sr2.35Ca0.5Sm0.1AlO4F (a ¼6.73(4) and c¼ 11.097(10) A) or lower angle for Sr2.35Ba0.5Sm0.1AlO4F (a ¼6.82(1) and c¼ ˚ upon addition of smaller Ca ions or larger Ba ions, 11.143(2) A) respectively. Fig. 3 compares the PL excitation and emission spectra of Sr2.5 3x/2 Ba0.5SmxAlO4F (A)–(D) and Sr2.5 3x/2Ba0.5SmxAlO4 aF1 d (E)–(H) (x¼ 0.001, 0.01, 0.05, 0.1). Sr2.5 3x/2Ba0.5SmxAlO4 aF1 d phosphors were prepared by exposing Sr2.5 3x/2Ba0.5SmxAlO4F to a 5% H2/95% Ar
Fig. 3. Excitation and emission (Black and red lines mean emission spectra excited at 254 and 408 nm, respectively) spectra of Sr2.5 3x/2Ba0.5SmxAlO4F (A) x¼ 0.001, (B) x¼ 0.01, (C) x¼ 0.05, (D) x¼ 0.1 and Sr2.5 3x/2Ba0.5SmxAlO4 aF1 d (E) x ¼0.001, (F) x¼ 0.01, (G) x ¼0.05, (H) x ¼0.1. Detail compositions and preparation atmosphere were summarized in Table 1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
reducing gas atmosphere at 900 1C. In Fig. 3(A)–(D), the excitation spectra of Sr2.5 3x/2Ba0.5SmxAlO4F (x¼0.001 0.1) can be divided into two regions: the first region reveals a broad excitation band around 240 nm, which is caused by O2 –Sm3 þ charge-transfer transition and the other region is attributed to high intensity f–f transitions of Sm3 þ , which broadly ranges from 290 nm to 480 nm. The strongest excitation peak in the Sr2.5 3x/2Ba0.5SmxAlO4F spectra located near 408 nm, which is assigned to 6H5/2-4L13/2 transition of Sm3 þ , was observed. Under the excitation of 252 nm and 408 nm, there is the major sharp orange emission peak of S2.5 3x/2Ba0.5Sm0xAlO4F corresponded to the 4G5/2-6H7/2 transition of Sm3 þ at 604 nm alongside three relatively weak peaks assigned to the 4G5/2-6H5/2 (565 nm), 4 G5/2-6H9/2 (644 nm), 4G5/2-6H11/2 (707 nm) in Fig. 3(A)–(D) [7,8]. This effective excitation centered at 408 nm causing intense orange emission could be adopted in a near-ultraviolet LED, which has excitation peaks between 365 nm and 410 nm. In Fig. 3(A) the relatively weak intensity in the excitation and emission spectrum due to insufficient concentration of Sm3þ as an activator was attained. PL intensity steadily increased with increasing Sm3þ concentration to x¼0.05 in the Sr2.5 3x/2Ba0.5SmxAlO4F shown in Fig. 3(C). Fig. 3(E) and (F) show the broad-band emitting spectra centered near 480 nm along with small typical f–f transition of Sm3 þ in Sr2.5 3x/2Ba0.5SmxAlO4 aF1 d (x¼0.001, 0.01) when excited at 254 nm, whereas strong emission line near 603 nm ascribed to 4G5/2-6H7/2 transitions of Sm3 þ under the excitation of 408 nm was achieved. As we reported before, there was no emission on Sr2.5Ba0.5AlO4F oxyfluoride host; however, nonstoichiometric self-activating bluish phosphors with the general formulas
S. Park / Journal of Luminescence 132 (2012) 875–878
877
Table 1 Corresponding chromaticity Commission International de l’Eclairage (CIE) coordinates and preparation atmosphere of phosphors (A–H) detailed in text and Figs. 3–5. Phosphors
A B C D E F G H
Fig. 4. Corresponding chromaticity Commission International de l’Eclairage (CIE) coordinates and photographs of phosphors (A–G) taken under a handheld UV lamp exciting 254 nm light. CIE coordinates were summarized in Table 1. Sr2.5 3x/2 Ba0.5SmxAlO4F (A) x¼ 0.001, (B) x ¼0.01, (C) x¼ 0.05, (D) x ¼0.1 and Sr2.5 3x/2 Ba0.5SmxAlO4 aF1 d (E) x ¼0.001, (F) x¼ 0.01, (G) x¼ 0.05, (H) x¼ 0.1.
20
E
Relative Intensity (arb. unit)
F 15
G
10
C 5 B
H
A
D
0 0
0.05
0.1
x, Sm 3+ con n centration Fig. 5. Relative emission intensity as a function of Sm3 þ concentration under a handheld UV lamp at 254 nm excitation light; Sr2.5 3x/2Ba0.5SmxAlO4F (A) x¼0.001, (B) x¼0.01, (C) x¼0.05, (D) x¼ 0.1 and Sr2.5 3x/2Ba0.5SmxAlO4 aF1 d (E) x¼0.001, (F) x¼0.01, (G) x¼0.05, (H) x¼ 0.1.
Sr2.5Ba0.5AlO4 aF1 d evolving the creation of defects were obtained by mild reducing conditions [6]. In this study, it can be considered that the excitation spectra of Sr2.5 3x/2Ba0.5Smx AlO4 aF1 d (x ¼0.001, 0.01) are composed of charge transfer centered 240 nm and f–f transitions of Sm3 þ as well as a clear absorption (arrow) around 250 nm, which correspond to the creation of defects in the oxyfluoride hosts. At the high Sm3 þ concentrations in Sr2.5 3x/2Ba0.5SmxAl O4 aF1 d (x ¼0.05, 0.1) there was no obvious change in emission spectra compared to Sr2.5 3x/2Ba0.5SmxAl O4F (x ¼0.05, 0.1). The chromaticity coordinates x and y are shown in Fig. 4 in accordance with CIE values of the new phosphors in the Sr2.5 3x/2 Ba0.5Smx AlO4 F (x ¼0.001 0.1) (A–D) and Sr 2.5 3x/2Ba0.5 Sm x
Sr2.4985Ba0.5Sm0.001AlO4F Sr2.485Ba0.5Sm0.01AlO4F Sr2.42Ba0.5Sm0.05AlO4F Sr2.35Ba0.5Sm0.1AlO4F Sr2.4985Ba0.5Sm0.001AlO4 aF1 d Sr2.485Ba0.5Sm0.01AlO4 aF1 d Sr2.42Ba0.5Sm0.05AlO4 aF1 d Sr2.35Ba0.5Sm0.1AlO4 aF1 d
Preparation atmosphere
In In In In In In In In
air air air air air air air air
þ þ þ þ
5%H2/95%Ar 5%H2/95%Ar 5%H2/95%Ar 5%H2/95%Ar
CIE coordinates x
y
0.561 0.608 0.614 0.603 0.297 0.363 0.486 0.548
0.356 0.376 0.377 0.369 0.354 0.362 0.371 0.351
AlO4 aF1 d (x¼ 0.001 0.1) (E–H) family under a handheld UV lamp emitting 254 nm light. The CIE values are summarized inset in Fig. 4 with those of the Sm3 þ -doped members in oxyfluoride host. Orange emissions of the resulting Sr2.5 3x/2Ba0.5SmxAlO4F (x ¼0.001 0.1) and Sr2.5 3x/2Ba0.5 SmxAlO4 aF1 d (x ¼0.05, 0.1) phosphors (A–D, G, H) were monitored. As depicted in Fig. 4, the CIE coordinate x ¼0.614, y¼0.377 of Sr2.35Ba0.5Sm0.1AlO4F (C) is close to the edge of CIE diagram. The CIE coordinates of defected Sr2.5 3x/2Ba0.5SmxAlO4 aF1 d phosphors at low Sm3 þ concentration (x ¼0.001, 0.01) (E, F) are very different from the distinct orange phosphors. When the Sm3 þ content (x) is decreased, it is worth to compare their CIE coordinates of Sr2.5 3x/2Ba0.5Smx AlO4 aF1 d, which are shifted from orange region x¼0.486, y¼0.371 (G), x ¼0.548, y¼0.351 (H) to white region x¼0.297, y¼0. 354 (E), x¼0.363, y¼ 0.362 (F). Photographs of A–H phosphors showing orange, white, and bluish-white PL emissions under 254 nm were observed in accordance with the characteristics of Sm3 þ transitions and oxyfluoride host defects. Fig. 5 shows the relative luminescent intensity of Sr2.5 3x/2 Ba0.5SmxAlO4F (A–D) and Sr2.5 3x/2Ba0.5SmxAlO4 aF1 d (E–H) phosphors by changing the Sm3 þ concentration (x ¼0.001 0.1). The orange PL emission of Sr2.5 3x/2Ba0.5SmxAl O4F (A–D) phosphors clearly increases with Sm3 þ content from x value of 0.001 (0.1 mol%) to the amount of x¼0.05 (5 mol%), which reaches a maximum emission intensity. When the Sm3 þ content exceeded 5 mol%, the concentration quenching was manifestly observed. As already known, the decrease of the emission intensity occurs due to the nonradiative energy transfer between activators caused by electronic dipole–dipole interaction [9]. The relative emission intensity of Sr2.5 3x/2Ba0.5 SmxAlO4 aF1 d phosphors (E–H) linearly decreases with increasing Sm3 þ concentration from 0.1 to 10 mol%. It is indicated that the relative PL intensity of Sr2.4985 Ba0.5Sm0.001AlO4 aF1 d (E) phosphor was about 20 times greater than that of as-made Sr2.4985Ba0.5Sm0.001AlO4F (A) phosphor. Table 1 summarized the corresponding chromaticity Commission International de l’Eclairage (CIE) coordinates and preparation atmosphere of all prepared phosphors (A–H) shown in Figs. 3–5. The extensively enhanced luminescent intensity of the phosphor was as a result of Sm3 þ transitions in cooperation with defectinduced self-activating oxyfluoride phosphors.
4. Conclusion Under the excitation of 408 nm competent orange emitting Sr2.5 3x/2Ba0.5SmxAlO4F (x¼0.001 0.1) phosphors, which are quite effective to prepare white-emitting light for near-UV LED applications, were initiated. Defects could be visibly created in the Sr2.5 3x/2Ba0.5SmxAl O4F host lattices when Sm3 þ ions are doped less than 5 mol%. The gradual substitution of Sm3 þ
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S. Park / Journal of Luminescence 132 (2012) 875–878
contents in oxyfluoride hosts is amenable to change CIE values and desired emitting intensity.
Acknowledgment I would like to thank for the support from the SANHAKFUND 2010 Fellowship. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0010756).
References [1] M. Al-Mamour, P.P. Edwards, C. Greaves, M. Slaski, Nature 369 (1994) 382. [2] A. Akella, D.A. Keszler, Chem. Mater. 7 (1995) 1299. [3] A.K. Produosantsos, B.J. Kennedy, T. Vogt, P.M. Woodward, J. Solid State Chem. 172 (2003) 89. [4] S. Park, T. Vogt, J. Lumin. 129 (2009) 952. [5] S. Park, T. Vogt, J. Am. Chem. Soc. 132 (2010) 4516. [6] S. Park, T. Vogt, J. Phys. Chem. C 114 (2010) 11576. [7] X. He, J. Zhou, N. Lian, J. Sun, M. Guan, J. Lumin. 130 (2010) 743. [8] P. Li, Z. Wabg, Z. Yang, Q. Guo, X. Li, J. Lumin. 130 (2010) 222. [9] D.L. Dexter, J. Chem. Phys. 21 (1953) 836.
Contents lists available at SciVerse ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Luminescent properties of Sr2.5 3x/2Ba0.5SmxAlO4F oxyfluorides Sangmoon Park n Department of Engineering in Energy & Applied Chemistry, Silla University, Busan 617-736, Republic of Korea
a r t i c l e i n f o
abstract
Article history: Received 11 July 2011 Received in revised form 14 October 2011 Accepted 1 December 2011 Available online 13 December 2011
Effective orange Sm3 þ -doped Sr2.5Ba0.5AlO4F phosphors excited at 254 and 408 nm excitation were prepared by the solid-state method. The excitation and emission spectra of Sr2.5 3x/2Ba0.5SmxAlO4F and Sr2.5 3x/2Ba0.5SmxAlO4 aF1 d (x ¼0.001 0.1) based on photoluminescence spectroscopy are investigated. The defects in anion-deficient Sr2.5 3x/2Ba0.5SmxAlO4 aF1 d (x ¼ 0.001, 0.01) are monitored by broad-band photoluminescence emission centered near 480 nm along with the orange emission transitions of Sm3 þ . CIE values and relative luminescent intensities of Sr2.5 3x/2Ba0.5SmxAlO4F and Sr2.5 3x/2Ba0.5SmxAlO4 aF1 d by changing the Sm3 þ content (x ¼0.001 0.1) are discussed. & 2011 Elsevier B.V. All rights reserved.
Keywords: Photoluminescence Oxyfluoride hosts Sm3 þ ions
1. Introduction Remarkable properties of altering charge between oxide and fluoride within the oxyfluoride systems such as Sr2CuO2F2 þ d or Sr2LiSiO4F, are attractively considered for electrical and optical exploration [1,2]. The structure of Sr3AlO4F oxyfluoride host lattice, which is a tetragonal unit cell with space group I4/mcm, is also known as an unusual ordered anion arrangement material caused by spatial separation of ionic Sr–F and covalent Al–O bonds [3,4]. The staking of Sr2F3 þ and SrAlO34 layers in Sr3AlO4F oxyfluoride is along the c-axis as represented in Fig. 1. There are noticeably different two strontium sites, which are Sr(1) and Sr(2) coordinated by ten and eight anions, respectively. It was introduced that alkali earth ions or rare-earth ions can easily be accommodated in the Sr sites, and moreover, the replacement of Sr2 þ ion with larger Ba2 þ ion causing Sr(1)–O and Sr(1)–F distance change results in the stability of oxyfluoride structure. In previous study, trivalent Eu, Tb, Tm, Er into A(1)3 xA(2)xMO4F host lattice (A(1)/A(2)¼Sr, Ba, Ca; M¼Al, Ga) was introduced as a new family of UV-activated phosphors [4]. We also have recently reported that a family of anion-deficient oxyfluorides with the composition Sr3 xAxMO4 aF1 d (A¼Ca, Ba and M¼Al, Ga), general formula ((Sr3 xAx)1 a 2dMO4 aF1 d) show photoluminescence (PL) when excited by either far- and near-UV light [5,6]. The members of Sr3 xAxMO4F can be transformed into self-activating UV-excitable broad-band PL phosphors by controlling their defect structure using appropriate post-synthesis reduction conditions. Defects were created in this family of Sr3 xAxMO4F by exposing them to a 5% H2/95% Ar reducing gas atmosphere. When excited
n
Corresponding author. Tel.: þ82 51 999 5891; fax: þ 82 51 999 5335. E-mail address: [email protected]
0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2011.12.003
in the far-UV near 254 nm, the material of Sr3AlO4 aF1 d shows broad-band bluish PL emission centered near 500 nm. Moreover, Sr2.5Ba0.5Al0.9In0.1O4 aF1 d in the excited in the near-UV centered 365 nm shows broad-band orange PL emission centered near 605 nm with a spectral width of 90 nm (FWHM). Furthermore, the combination of broad and line emission by substituting Eu3 þ activator into defect-induced self-activating lattices [5,6], Sr2.85 Eu0.1Al0.9 In0.1O4 aF1 d, allowed attractive flexibility in tailoring the PL in this class of materials. In this report, the luminescent properties of Sm-doped in both Sr2.5Ba0.5AlO4F and Sr2.5Ba0.5 AlO4 aF1 d oxyfluorides will be scrutinized as phosphors for solid state lighting. Desired CIE values and relative luminescent intensities of Sr2.5 3x/2Ba0.5SmxAlO4F and Sr2.5 3x/2Ba0.5Smx Al O4 aF1 d by changing the Sm3 þ content (x¼0.001 0.1) are also attained.
2. Experimental details Samples of Sm-doped Sr2.5Ba0.5AlO4F were prepared by heating the appropriate stoichiometric amounts of SrCO3 (Aldrich 99.9%), BaCO3 (Alfa, 99.95%), SrF2 (Aldrich 99.99%), Al2O3 (Alfa 99.95%), and Sm2O3 (Alfa 99.9%) at temperature up to 1050 1C for 3 h in air. The as-made Sm-doped Sr2.5Ba0.5AlO4F samples were subsequently annealed for 1 h at 900 1C in reducing atmosphere (5% H2/95% Ar) to compose self-activating luminescent materials. Phase identification of phosphors was done using a Shimadzu XRD-6000 powder diffractometer using CuKa radiation and the unit cell parameters were determined using the least squares refinement program CELREF. Ultraviolet-visible spectroscopy to measure the excitation and emission spectra of the Sr2.5 3x/2 Ba0.5SmxAlO4F and Sr2.5 3x/2Ba0.5SmxAlO4 aF1 d (x ¼0.001 0.1) phosphor materials was done with a spectrofluorometer (Sinco
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S. Park / Journal of Luminescence 132 (2012) 875–878
Sr(1) Sr(2) c
F O
a
AlO4
Fig. 1. Crystallographic structure of Sr3AlO4F oxyfluoride host lattice.
Fig. 2. X-ray diffraction patterns of calculated (a) Sr3AlO4F and observed (b) Sr2.85Sm0.1AlO4F, (c) Sr2.35Ba0.5Sm0.1AlO4F phosphors.
Fluromate FS-2). Relative intensity and desired the Commission International de l’Eclairage (1931 CIE) values of samples were measured by Minolta CS-100A Chroma Meter under 254 nm.
3. Results and discussion Sr2.5Ba0.5SmxAlO4F phosphors were synthesized by solid state reaction at temperatures of up to 1050 1C. X-ray diffraction (XRD) patterns of Sm3 þ -doped oxyfluoride phosphors in Fig. 2 show single-phase host lattices without any obvious impurities indexed by a tetragonal unit cell. Fig. 1(a) indicates the calculated Sr3AlO4F (ICSD 50736) XRD pattern with lattice parameters a ¼6.78(1) and ˚ Partial substitution of Sr by Sm in oxyfluoride host c¼11.14(1) A. (Sr2.85Sm0.1AlO4F) resulted in a slight shift to higher angles as the ˚ Since smaller cell size decreased to a¼ 6.77(1) and c¼11.098(4) A. alkali-earth ions or rare-earth ions prefer the Sr(2) site, which is normally 8 coordinated as discussed above, Sm3 þ ions could suitably be proposed to occupy in the Sr(2) site. There is a gradual shift in the positions of the various Bragg reflections to higher ˚ angle for Sr2.35Ca0.5Sm0.1AlO4F (a ¼6.73(4) and c¼ 11.097(10) A) or lower angle for Sr2.35Ba0.5Sm0.1AlO4F (a ¼6.82(1) and c¼ ˚ upon addition of smaller Ca ions or larger Ba ions, 11.143(2) A) respectively. Fig. 3 compares the PL excitation and emission spectra of Sr2.5 3x/2 Ba0.5SmxAlO4F (A)–(D) and Sr2.5 3x/2Ba0.5SmxAlO4 aF1 d (E)–(H) (x¼ 0.001, 0.01, 0.05, 0.1). Sr2.5 3x/2Ba0.5SmxAlO4 aF1 d phosphors were prepared by exposing Sr2.5 3x/2Ba0.5SmxAlO4F to a 5% H2/95% Ar
Fig. 3. Excitation and emission (Black and red lines mean emission spectra excited at 254 and 408 nm, respectively) spectra of Sr2.5 3x/2Ba0.5SmxAlO4F (A) x¼ 0.001, (B) x¼ 0.01, (C) x¼ 0.05, (D) x¼ 0.1 and Sr2.5 3x/2Ba0.5SmxAlO4 aF1 d (E) x ¼0.001, (F) x¼ 0.01, (G) x ¼0.05, (H) x ¼0.1. Detail compositions and preparation atmosphere were summarized in Table 1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
reducing gas atmosphere at 900 1C. In Fig. 3(A)–(D), the excitation spectra of Sr2.5 3x/2Ba0.5SmxAlO4F (x¼0.001 0.1) can be divided into two regions: the first region reveals a broad excitation band around 240 nm, which is caused by O2 –Sm3 þ charge-transfer transition and the other region is attributed to high intensity f–f transitions of Sm3 þ , which broadly ranges from 290 nm to 480 nm. The strongest excitation peak in the Sr2.5 3x/2Ba0.5SmxAlO4F spectra located near 408 nm, which is assigned to 6H5/2-4L13/2 transition of Sm3 þ , was observed. Under the excitation of 252 nm and 408 nm, there is the major sharp orange emission peak of S2.5 3x/2Ba0.5Sm0xAlO4F corresponded to the 4G5/2-6H7/2 transition of Sm3 þ at 604 nm alongside three relatively weak peaks assigned to the 4G5/2-6H5/2 (565 nm), 4 G5/2-6H9/2 (644 nm), 4G5/2-6H11/2 (707 nm) in Fig. 3(A)–(D) [7,8]. This effective excitation centered at 408 nm causing intense orange emission could be adopted in a near-ultraviolet LED, which has excitation peaks between 365 nm and 410 nm. In Fig. 3(A) the relatively weak intensity in the excitation and emission spectrum due to insufficient concentration of Sm3þ as an activator was attained. PL intensity steadily increased with increasing Sm3þ concentration to x¼0.05 in the Sr2.5 3x/2Ba0.5SmxAlO4F shown in Fig. 3(C). Fig. 3(E) and (F) show the broad-band emitting spectra centered near 480 nm along with small typical f–f transition of Sm3 þ in Sr2.5 3x/2Ba0.5SmxAlO4 aF1 d (x¼0.001, 0.01) when excited at 254 nm, whereas strong emission line near 603 nm ascribed to 4G5/2-6H7/2 transitions of Sm3 þ under the excitation of 408 nm was achieved. As we reported before, there was no emission on Sr2.5Ba0.5AlO4F oxyfluoride host; however, nonstoichiometric self-activating bluish phosphors with the general formulas
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Table 1 Corresponding chromaticity Commission International de l’Eclairage (CIE) coordinates and preparation atmosphere of phosphors (A–H) detailed in text and Figs. 3–5. Phosphors
A B C D E F G H
Fig. 4. Corresponding chromaticity Commission International de l’Eclairage (CIE) coordinates and photographs of phosphors (A–G) taken under a handheld UV lamp exciting 254 nm light. CIE coordinates were summarized in Table 1. Sr2.5 3x/2 Ba0.5SmxAlO4F (A) x¼ 0.001, (B) x ¼0.01, (C) x¼ 0.05, (D) x ¼0.1 and Sr2.5 3x/2 Ba0.5SmxAlO4 aF1 d (E) x ¼0.001, (F) x¼ 0.01, (G) x¼ 0.05, (H) x¼ 0.1.
20
E
Relative Intensity (arb. unit)
F 15
G
10
C 5 B
H
A
D
0 0
0.05
0.1
x, Sm 3+ con n centration Fig. 5. Relative emission intensity as a function of Sm3 þ concentration under a handheld UV lamp at 254 nm excitation light; Sr2.5 3x/2Ba0.5SmxAlO4F (A) x¼0.001, (B) x¼0.01, (C) x¼0.05, (D) x¼ 0.1 and Sr2.5 3x/2Ba0.5SmxAlO4 aF1 d (E) x¼0.001, (F) x¼0.01, (G) x¼0.05, (H) x¼ 0.1.
Sr2.5Ba0.5AlO4 aF1 d evolving the creation of defects were obtained by mild reducing conditions [6]. In this study, it can be considered that the excitation spectra of Sr2.5 3x/2Ba0.5Smx AlO4 aF1 d (x ¼0.001, 0.01) are composed of charge transfer centered 240 nm and f–f transitions of Sm3 þ as well as a clear absorption (arrow) around 250 nm, which correspond to the creation of defects in the oxyfluoride hosts. At the high Sm3 þ concentrations in Sr2.5 3x/2Ba0.5SmxAl O4 aF1 d (x ¼0.05, 0.1) there was no obvious change in emission spectra compared to Sr2.5 3x/2Ba0.5SmxAl O4F (x ¼0.05, 0.1). The chromaticity coordinates x and y are shown in Fig. 4 in accordance with CIE values of the new phosphors in the Sr2.5 3x/2 Ba0.5Smx AlO4 F (x ¼0.001 0.1) (A–D) and Sr 2.5 3x/2Ba0.5 Sm x
Sr2.4985Ba0.5Sm0.001AlO4F Sr2.485Ba0.5Sm0.01AlO4F Sr2.42Ba0.5Sm0.05AlO4F Sr2.35Ba0.5Sm0.1AlO4F Sr2.4985Ba0.5Sm0.001AlO4 aF1 d Sr2.485Ba0.5Sm0.01AlO4 aF1 d Sr2.42Ba0.5Sm0.05AlO4 aF1 d Sr2.35Ba0.5Sm0.1AlO4 aF1 d
Preparation atmosphere
In In In In In In In In
air air air air air air air air
þ þ þ þ
5%H2/95%Ar 5%H2/95%Ar 5%H2/95%Ar 5%H2/95%Ar
CIE coordinates x
y
0.561 0.608 0.614 0.603 0.297 0.363 0.486 0.548
0.356 0.376 0.377 0.369 0.354 0.362 0.371 0.351
AlO4 aF1 d (x¼ 0.001 0.1) (E–H) family under a handheld UV lamp emitting 254 nm light. The CIE values are summarized inset in Fig. 4 with those of the Sm3 þ -doped members in oxyfluoride host. Orange emissions of the resulting Sr2.5 3x/2Ba0.5SmxAlO4F (x ¼0.001 0.1) and Sr2.5 3x/2Ba0.5 SmxAlO4 aF1 d (x ¼0.05, 0.1) phosphors (A–D, G, H) were monitored. As depicted in Fig. 4, the CIE coordinate x ¼0.614, y¼0.377 of Sr2.35Ba0.5Sm0.1AlO4F (C) is close to the edge of CIE diagram. The CIE coordinates of defected Sr2.5 3x/2Ba0.5SmxAlO4 aF1 d phosphors at low Sm3 þ concentration (x ¼0.001, 0.01) (E, F) are very different from the distinct orange phosphors. When the Sm3 þ content (x) is decreased, it is worth to compare their CIE coordinates of Sr2.5 3x/2Ba0.5Smx AlO4 aF1 d, which are shifted from orange region x¼0.486, y¼0.371 (G), x ¼0.548, y¼0.351 (H) to white region x¼0.297, y¼0. 354 (E), x¼0.363, y¼ 0.362 (F). Photographs of A–H phosphors showing orange, white, and bluish-white PL emissions under 254 nm were observed in accordance with the characteristics of Sm3 þ transitions and oxyfluoride host defects. Fig. 5 shows the relative luminescent intensity of Sr2.5 3x/2 Ba0.5SmxAlO4F (A–D) and Sr2.5 3x/2Ba0.5SmxAlO4 aF1 d (E–H) phosphors by changing the Sm3 þ concentration (x ¼0.001 0.1). The orange PL emission of Sr2.5 3x/2Ba0.5SmxAl O4F (A–D) phosphors clearly increases with Sm3 þ content from x value of 0.001 (0.1 mol%) to the amount of x¼0.05 (5 mol%), which reaches a maximum emission intensity. When the Sm3 þ content exceeded 5 mol%, the concentration quenching was manifestly observed. As already known, the decrease of the emission intensity occurs due to the nonradiative energy transfer between activators caused by electronic dipole–dipole interaction [9]. The relative emission intensity of Sr2.5 3x/2Ba0.5 SmxAlO4 aF1 d phosphors (E–H) linearly decreases with increasing Sm3 þ concentration from 0.1 to 10 mol%. It is indicated that the relative PL intensity of Sr2.4985 Ba0.5Sm0.001AlO4 aF1 d (E) phosphor was about 20 times greater than that of as-made Sr2.4985Ba0.5Sm0.001AlO4F (A) phosphor. Table 1 summarized the corresponding chromaticity Commission International de l’Eclairage (CIE) coordinates and preparation atmosphere of all prepared phosphors (A–H) shown in Figs. 3–5. The extensively enhanced luminescent intensity of the phosphor was as a result of Sm3 þ transitions in cooperation with defectinduced self-activating oxyfluoride phosphors.
4. Conclusion Under the excitation of 408 nm competent orange emitting Sr2.5 3x/2Ba0.5SmxAlO4F (x¼0.001 0.1) phosphors, which are quite effective to prepare white-emitting light for near-UV LED applications, were initiated. Defects could be visibly created in the Sr2.5 3x/2Ba0.5SmxAl O4F host lattices when Sm3 þ ions are doped less than 5 mol%. The gradual substitution of Sm3 þ
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contents in oxyfluoride hosts is amenable to change CIE values and desired emitting intensity.
Acknowledgment I would like to thank for the support from the SANHAKFUND 2010 Fellowship. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0010756).
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