Journal of Solid State Chemistry 156, 84} 87 (2001)
doi:10.1006/jssc.2000.8966, available online at http://www.idealibrary.com on
A Study on the Luminescent Properties of Red-Emitting
Praseodymium-Activated SrIn
2O
4Phosphors
Feng-Shih Kao and Teng-Ming Chen
Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30050, Taiwan
Received April 18, 2000; in revised form July 31, 2000; accepted September 15, 2000; published online December 21, 2000
The photoluminescent (PL) properties of new red-emitting praseodymium-activated strontium indates have been investi-gated. A series of SrIn2O4: x Pr3ⴙ (xⴝ 6.25 ⴛ 10ⴚ 4, 0.5%, 2%,
and 4%) phases were synthesized and found to exhibit red emission. With di4erent levels of Pr3ⴙ doping, the absolute brightness, CIE chromatic characteristics, PL and cathodo-luminescence spectra, and cathodo-luminescence decay lifetime (sR) for
SrIn2O4:xPr3ⴙ have been systematically investigated. The
re-sults are reported and their implications are discussed. 2001 Academic Press
Key Words: luminescence; SrIn2O4:xPr; photoluminescence
spectra; cathodoluminescence spectra; decay curve; chromaticity diagram.
1. INTRODUCTION
The development of displays has always been accom-panied by improvements in the phosphors used. For example, the advent of the present color television was dependent on the development of an e$cient red phosphor (1). Display technology will not be exploited to its full potential until the phosphors that operate under the re-quired conditions are synthesized. For instance, cathode ray phosphors have been optimized to endure the bombard-ment of high-voltage electron beams. In particular, for FED applications intrinsically conducting materials were gener-ally adopted as the host lattice of low-voltage phosphor (2) to prevent the buildup of space charges at the surface of phosphors at low-voltage excitation. Therefore, there has been an urgent need to investigate new phosphors with good high-voltage performance and decent intrinsic electri-cal conductivity for practielectri-cal uses. In this work, we investi-gated the possible combination of a new host lattice and a red-emitting activator with luminescence lying inside the eye sensitivity curve and having a high lumen equivalent.
To whom correspondence should be addressed. Fax: 886#35723764. E-mail: [email protected].
Semiconducting SrInO with a reported bandgap (E) of 3.6 eV was found to crystallize in an orthorhombic CaFeO-type structure with space group Pnam and the lattice parameters a"9.83 A> , b"11.50 A>, and c"3.27 A> (3). There exist a few unoccupied cation sites in the lattice of SrInO and this o!ers a great potential for cation substitu-tions. It is known that luminescence is related to the pres-ence of defects and, thus, the unoccupied sites in the lattice must have an in#uence on the luminescence properties of phosphors. As far as we are concerned, no previous work on phosphors with SrInO as a host lattice has been reported in the literature.
On the other hand, trivalent praseodymium has been known to exhibit a very interesting luminescence as an activator ion, since its energy level contains several meta-stable multipletsP , D, and G that o!er the possi-bility of e$cient emissions such as red (from theD level), green (from the P level), blue (from the S level), and ultraviolet (from the 4f 5d state) between the blue and the red spectral regions (4). Furthermore, the emission color of Pr> strongly depends on the structure of the host lattice, the concentration of activators, and the excitation condi-tions. The spectra of red-emitting Pr> phosphors were reported to be similar to those of Eu>-activated phosphors that have been used as red phosphors in cathode ray tubes with great success. For instance, Cho et al. (5) reported that in Pr>-activated CaTiO crystals, an intense emission peaking at 613 nm was observed in the photoluminescence spectra and characteristics of low-voltage excitation were also investigated. Diallo et al. reported that CaTiO:Pr> was excitable in a wide ultraviolet range and exhibited a unique red emission from the Pr> D level (6). Zhang
et al. presented the results of studies on the dynamic
spec-troscopic properties of the excited states and discussed the e!ect of the (SrVBa\V)NbO host lattice on the lumines-cence of Pr> (7).
Motivated by the investigations described above, we have begun investigations on the luminescent properties of Pr>-activated SrInO phases as potential phos-phors. In this work we report our results on the luminescent 84
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FIG. 1. PL emission spectra for the SrInO:xPr> phases with x" (a) 6.25;10\, (b) 0.5%, (c) 2%, and (d) 4%.
properties, apparent decay lifetime, and chromatic charac-teristics of SrInO:xPr> with di!erent activator concen-trations.
2. EXPERIMENTAL
Polycrystalline SrInO:xPr> phases with x"6.25; 10\, 0.5%, 2%, and 4% were synthesized by conventional solid state reactions. Stoichiometric amounts of source materials SrCO, InO, and PrO (all from Aldrich Chemicals Co., Milwaukee, WI) were thoroughly mixed and "nely ground together. The mixture was "rst calcined at 5503C in air for 6 h and then sintered at 12003C for 24 h also in the air; however, no #ux was used in this work.
The phase purity and homogeneity of the as-prepared SrInO:xPr> samples were investigated by X-ray di!rac-tion (XRD). The XRD pro"les for Pr>-activated SrInO phases were collected by using a MAC Science MXP-3 automatic di!ractometer with a graphite-monochromatized and Ni-"ltered CuKa radiation. Special caution was taken to make sure that no starting material or any other allot-ropic form is present in the XRD pro"les. The ambient temperature photoluminescence (PL) spectra in the spectral region of 350}800 nm and excitation spectra were measured by using a Spex Fluorolog-3 spectro#uorometer equipped with a 450-W xenon lamp. An UV-35 cut-o! "lter was used to eliminate the second-order emission from the source. The cathodoluminescence (CL) spectra of polycrystalline SrInO:xPr> (x"6.25;10\) powder were measured with a Topcon ABT-150s type scanning electron micro-scope equipped with a Jobin Yvon H20UV mono-chromator, with the operation voltage ranging from 5 to 20 kV.
The measurements of decay lifetimes for Pr>-activated SrInO phases were carried out by exciting the samples using a Lamda Physik LPX150T excimer laser with a wavelength of 248 nm and a pulse duration of 0.1 sec, and the detector was equipped with a Hamamatsu R928 type photomultiplier. The absolute brightness of SrInO:xPr> was recorded by using a Topcon BM-7 luminance colorimeter.
3. RESULTS AND DISCUSSION
The XRD pro"les for SrInO:xPr (x"6.25;10\, 0.5%, 2%, and 4%) phases were found to be exactly the same as that of SrInO reported in the JCPDS card no. 33-1336. The XRD data indicate that the as-prepared sam-ples are essentially single-phased regardless the amount of Pr> doped. However, the presence of a second phase in the proportion of a few percents cannot be ruled out for the samples with the highest Pr> concentrations. Further-more, no systematic shifting in XRD pro"les was observed as x was increased and this observation indicated that the
lattice dimension is insensitive to the variation of dopant content for SrInO:xPr> phases.
The activator concentration-dependent PL spectra of SrInO:xPr> phosphors were measured under ultraviolet excitation withj of 254 nm at ambient temperature and the results are shown in Fig. 1. Although the excitation energy does not coincide with any energy di!erence between any two 4f levels of Pr>, it might correspond to a transition coupling the valence to the conduction band. However, typical Pr> emissions presumably induced by the excitation through the host were clearly observed. The emission spectra for as-prepared SrInO:xPr> phases exhibited the typically narrow line features of rare earth ions and consisted of two major groups in the blue and red spectral regions. The emission maxima peaking at 493 nm, 609 nm, and 621 nm were attributed to the transitions PPH, DPH, and PPH of Pr>, respective-ly (7).
For simplicity, only the excitation spectrum for SrInO:xPr> (x"6.25;10\) exhibiting the optimal luminescence intensity is represented in Fig. 2. The spec-trum reveals the luminescence output withj of 615 nm as a function of the exciting wavelength (j). The excitation spectrum for Pr>-activated SrInO consists of "ve groups of emission peaks corresponding to transitions from the fundamental multiplet H level to D (j"583 nm), P (j"492 nm), P (j"480 nm), I (j"473 nm) and P (j"452 nm), respectively.
To investigate the concentration quenching e!ect on the PL emission intensity and the lumen intensity attributed to activator concentration, we have doped di!erent levels of Pr> with composition ranging from 6.25;10\ to 4% into the SrInO host. The excitation wavelength and band slit of the spectro#uorometer were maintained consistent
FIG. 2. PL excitation spectra for the SrInO:xPr> (x"6.25;10\) phase withj "615 nm.
FIG. 3. Integrated PL intensity and absolute brightness as functions of Pr> content (x) for the SrInO:xPr> phases. The solid and dashed lines are guides only.
TABLE 1
Decay Lifetime (sR) for SrIn2O4:xPr Phases
x (atom %) q0 (ksec)
0.0625 49
0.5 63
2 42
4 23
FIG. 4. CL spectra for the SrInO:xPr> (x"6.25;10\) phase with excitation voltage of (a) 5 kV, (b) 10 kV, and (c) 20 kV.
ing measurements for accurate comparison of intensity. A drastic decrease of more than 1 order of magnitude in the PL emission intensity was observed while the Pr> content was increased from 6.25;10\ to 4%, as indicated by the data shown in Fig. 3. For the composition range investi-gated, the maximal PL emission intensity was observed for the sample with x"6.25;10\. On the other hand, the absolute brightness for SrInO:xPr> phases under ultra-violet excitation (j"365 nm) is measured with the power of excitation source of 2.3 mW and is also represented in Fig. 3. The absolute brightness was found to be 0.305, 0.153, 0.115 and 0.001 cd/m for SrInO:xPr> phases with
x"6.25;10\, 0.5%, 2%, and 4%, respectively. The
strongest brightness was observed for the sample with
a Pr> concentration of 6.25;10\, which is considered to be the optimum activator concentration. Therefore, it is concluded that for SrInO:xPr> phases the luminescence quenching due to Pr> occurs at a relatively low concentra-tion (i.e., below 6.25;10\), much lower than that observed by Diallo et al. in CaTiO:Pr> (6). This observation indi-cates that the concentration quenching due to Pr> is very sensitive to the type of host into which Pr> is doped. With such a low quenching concentration a technique to disperse Pr> ions e!ectively in the host lattice of SrInO may have to be developed to gain some emission e$ciency.
The observed apparent luminescence decay lifetimesq0 (8) for SrInO:xPr> phases are summarized in Table 1. The experimental decay lifetimes for Pr> in SrInO: xPr> phases with di!erent activator contents were found to be of the same order of magnitude but signi"cantly shorter than that (i.e., 132ksec) observed in CaTiO:Pr> (6). The ob-served q0 was also found to meet the application require-ment for CRT color televisions. As a matter of fact, not only #uorescence but also phosphorescent afterglow lasting for few seconds were observed by the naked eye under ultra-violet excitation for the SrInO:xPr> (x"6.25;10\) phase. Investigations of the phosphorescent properties of SrInO:xPr> phases are currently in progress.
FIG. 5. Chromaticity diagram indicating the di!erence of hues for red-emitting phosphors of SrInO:xPr>, YO:Eu>, and YOS:Eu>. NTSC represents National Television Standard Committee.
To investigate the potential of Pr>-activated indates as a cathodoluminescent material, we have also studied the CL spectra of SrInO:xPr> (x"6.25;10\). The lumines-cence intensity as a function of excitation voltage (from 5 to 20 kV) was measured and is represented in Fig. 4. An analy-sis of the CL spectra for SrInO:xPr> indicated that the Pr> emission tends to be broadened, as compared to that observed in the PL spectra. Due to the thermal e!ect that originates from heating induced by electron beam bombard-ment, the full width at half-maximum (FWHM) of the emission bands was found to be much wider than that of the PL spectra for SrInO:xPr> phases. We have also ob-served an apparent red shift in the emission wavelength for SrInO:xPr> as the excitation voltage increases, as in-dicated by the CL spectra shown in Fig. 4. The rationaliza-tion for this observarationaliza-tion is uncertain and requires further investigations.
On the other hand, the Commission Internationale de l'Eclairage (CIE) chromaticity coordinates of SrInO:
xPr> (x"6.25;10\) were calculated from the
corre-sponding PL spectrum represented in Fig. 1a and found to be (0.587, 0.354) with an accuracy of $0.001 in (x, y). The chromaticity coordinates for SrInO:xPr> (x"6.25; 10\) were then compared with those of two well-known red phosphors, YO:Eu> with (0.666,0.332) and YOS: Eu> with (0.630, 0.365), respectively (9), and the results are summarized in the CIE chromaticity diagram shown in Fig. 5.
We found that the hue of SrInO:xPr> (x"6.25; 10\) is apparently more orange than that of YO:Eu> or YOS:Eu>. This observed hue may be attributed to the contribution from the blue emission band centered atj of
493 nm of SrInO:xPr> (x"6.25;10\). Furthermore, the observed brightness decrease in green and red in SrInO:xPr> is also worthy of investigation. The purity of as-prepared samples and/or whether they are free of Pr> (since samples of SrInO:xPr> were sintered in the air at 12003C) may be the key factor responsible for the above observations. Special techniques capable of detecting these impurities may have to be adopted.
4. CONCLUSIONS
We have synthesized a series of new red-emitting SrInO:xPr> phases and investigated their luminescent properties. The PL emission spectra of SrInO:xPr> phases exhibited two groups of emission withj peaking at 493 nm, 609 nm, and 621 nm which are attributed to the transitions of PPH, DPH, and PPH for Pr>, respectively. The optimal Pr> content was deter-mined to be 6.25;10\ for SrInO:xPr> phases, as indicated by studies on the quenching e!ect of activator concentration. On the other hand, the Pr> emission tends to be broadened and to shift to a slightly longer wavelength, as indicated by the CL spectra. Analysis of the chromaticity diagram indicated that the emitting hue was orange-red for SrInO:xPr> phases, as compared to a purely red-emit-ting YSO:Eu> phosphor.
ACKNOWLEDGMENTS
This research is supported by National Science Council of Taiwan, R.O.C., under Contract No. NSC89-2113-M-009-014. We are indebted to Dr. Spring Yeh of the Center of Measurement Standards, ITRI, for assist-ance with the brightness measurements and to the Regional Instruments Center of the National Science Council in Hsinchu for the decay lifetime measurements.
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