doi:10.1006/jssc.2000.8949, available online at http://www.idealibrary.com on
A Study on the Luminescent Properties of Praseodymium-Activated
CaIn
2O
4Phosphors
F. S. Kao and Teng-Ming Chen
Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30050, Taiwan
Received June 20, 2000; in revised form August 1, 2000; accepted August 17, 2000; published online November 29, 2000
The photoluminescent (PL) properties and decay lifetime of a series of unprecedented praseodymium-activated calcium in-dates (CaIn2O4:xPr3ⴙ) have been investigated. The
CaIn2O4:xPr3ⴙ(xⴝ 6.25;10ⴚ4, 0.5%, 1%, and 2%) was found to exhibit orange emission under ultraviolet excitation. With di4erent levels of Pr3ⴙ doping, the X-ray di4raction pro5les, photoluminescence spectra and 6uorescence decay lifetime as a function of Pr3ⴙconcentration, and chromatic characteristics of CaIn2O4:xPr3ⴙ with di4erent x:s have been systematically
investigated. The analysis of PL emission and excitation spectra, decay lifetime, and comparison of chromaticity coordinates and hues for CaIn2O4:xPr3ⴙare presented and their implications are
discussed. 2000 Academic Press
Key Words: Luminescence; CaIn2O4:Pr3ⴙ; photoluminescence
spectra; decay lifetime; chromaticity diagram.
1. INTRODUCTION
The development of displays has always been accom-panied by improvements in the phosphors used. For example, the advent of color television depended on the development of e$cient red phosphors (1). Great e!ort has been made to discover host materials as well as activators with high performance for phosphor applications. The pro-gress in display technology will not be exploited to its full potential until the phosphors operating at required condi-tions have been synthesized. The requirements for various types of displays are di!erent. For instance, the cathode ray tube phosphors have been optimized to endure the bom-bardment of high-voltage electron beams. In particular, for "eld emission display (FED) applications, in order to over-come the impediment of the space charge at the surface of conventional phosphors, intrinsically conducting materials must be adopted (2). Therefore, there has been an urgent need for the investigation of new phosphors for practical use. In this work, we investigated the possible combination To whom correspondence should be addressed. E-mail: tmchen@ cc.nctu.edu.tw. Fax: 886#35723764.
of a new host lattice and an e$cient activator with high lumen equivalent and intrinsic conductivity.
The semiconducting CaInO with reported bandgap (E) of 3.9 eV was found to crystallize in an orthorhombic CaFeO-type structure with space group Pca2 or Pbcm (3). The lattice parameters have been reported to be
a"9.70 A> , b"11.30 A>, and c"3.21 A> for CaInO (3).
With the described properties it may possess the potential to serve as a candidate for new hosts in phosphor applications. The only disadvantage seems to be the high cost of source material InO.
On the other hand, rare earth ions have been widely used as the activators for di!erent host materials. Among these, trivalent praseodymium is known to exhibit very interesting prospects as an activator ion for luminescence and laser action, because its energy level contains several metastable multiplets such asP , D, and G (4). This provides many possibilities for pumping and lasing in several spectral regions, taking into account the large number of excitation and relaxation channels. It has been known that the emis-sion color of Pr> depends strongly on the type of host lattice, the concentration, and the pumping conditions and it was reported to cover the spectral range from blue to red (5). The advantage of Pr> as a luminescent activator lies in the fact that the emission of Pr>-activated phosphors is known to be similar to that of Eu> which has long been known as an e$cient red phosphor with great success in cathode ray tubes. For instance, in Pr>-activated CaTiO (CaTiO:Pr>), a single bright red emission peaking at 613 nm was observed (6). Diallo et al. reported that CaTiO: Pr> was excitable in a wide ultraviolet range and exhibited a unique red emission from the Pr> D level (7). Further-more, a long-decay Pr>-activated phosphor (Ca1!xMV)TiO: Pr> (M"Zn, Mg; 0(x40.1) was reported to exhibit red phosphorescence emission at a wavelength (j ) of 614 nm and with a persistent afterglow for 30 to 60 sec (8).
Our work was motivated by the desire to understand the #uorescence behavior of Pr>-activated phosphors and, we hoped, to develop potentially e$cient phosphors for practi-cal applications.
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FIG. 1. XRD pro"les for CaInO:xPr> phases with x"(a) 6.25;10\, (b) 0.5%, (c) 1%, and (d) 2%, respectively.
During the past few years, several investigations on the luminescence of Pr> have been reported in the literature (5}10). However, to the best of our knowledge, no photo-luminescence characterization of Pr>-activated CaInO (CaInO:xPr>) has been reported so far. In the present work we report the results of our investigation on the luminescence spectra, possible radiative luminescent decay lifetime (q0), and chromatic characteristics of the unprece-dented CaInO:xPr> as a function of Pr> activator concentrations.
2. EXPERIMENTAL
Polycrystalline CaInO:xPr> phases with x"0.0625%, 0.5%, 1%, and 2%, respectively, were synthesized by con-ventional solid-state reactions. The starting materials used for the phosphor preparation were of the highest purity commercially available. The stoichiometric constituent components of CaO, InO (both from Aldrich Chemicals, U.S.A.) and PrO (from CERAC Inc., U.S.A.) were thor-oughly mixed and "nely ground together. The mixtures were "rst calcined at 5503C in the air for 6 h and then sintered at 14003C for 24 h also in the air; however, no #ux was used in the synthesis.
The phase purity and homogeneity of the as-prepared CaInO:xPr> samples were investigated by X-ray di!rac-tion (XRD). The XRD pro"les for Pr>-activated CaInO phases were collected by using a MAC Science MXP-3 automatic di!ractometer using a graphite-monochroma-tized and Ni-"ltered CuKa (j"1.5418 A>) radiation. Special caution was taken to make sure that none of the starting material nor any other allotropic form was present in the samples that could be identi"ed in the XRD pro"les. Only single-phase samples were used for this investigation.
The ambient temperature photoluminescence (PL) spectra (with j at 254 nm) in the spectral region of 350 to 800 nm and excitation spectra (with j set at 613 nm) were measured at room temperature with use of a Spex Fluorolog-3 (Instruments S.A., Inc., U.S.A.) spectro#uorometer equipped with a 450-W xenon lamp as the excitation source. To eliminate the second-order emission of the source radiation, a UV-35 cuto! "lter was used.
The measurements of decay lifetime (q0) for CaInO:
xPr> phases were carried out by exciting the samples by
using a Lamda Physik LPX150T excimer laser with ultra-violet wavelength of 248 nm and pulse duration of 0.1 sec; a Hamamatsu R928 type photomultiplier was used as a detector. The CIE chromaticity coordinates of CaInO:
xPr> phases were measured to an accuracy of $0.001 in
chromaticity coordinates (x, y) by using a Minolta CS-100 chromameter.
3. RESULTS AND DISCUSSION
The XRD pro"les for CaInO:xPr> (x"0.0625%, 0.5%, 1%, and 2%) phases are shown in Fig. 1, respectively. The di!raction patterns for CaInO:xPr> phases were found to be exactly the same as that of CaInO reported in JCPDS Card No. 17-643. This observation indicates that pure crystalline CaInO was found in the as-prepared samples and no starting materials nor any other allotropic phase exists. However, no systematic shifting of di!raction peaks in the XRD pro"les was observed as x increases and this observation can be attributed to the small amount of Pr> doped into the CaInO host lattice.
The Pr> concentration-dependent PL emission spectra for CaInO:xPr> phases shown in Fig. 2 were measured under 254-nm ultraviolet excitation at ambient temper-ature. A bright #uorescent orange color attributed to an
fPf transition of praseodymium(III) was observed. In
gen-eral, the photoluminescent properties of the four CaInO:xPr> samples with di!erent x's do not di!er greatly from one another and no obvious red or blue shift was observed. Essentially, as indicated in Fig. 2, the PL spectra for the four CaInO:xPr> phases essentially con-sist of three groups of emissions, namely, blue, green, and red, peaking at 492.5 nm, 537 nm, and 605 nm, that were attributed to PPH, PPH, and DPH transitions of Pr>, respectively. The emission spectra for CaInO:xPr> were found to be very similar to those of SrInO:xPr> reported by Kao et al. (9); the only di!er-ence is the relative luminescdi!er-ence intensity for peaks at three spectral regions (i.e., 492 nm, 537 nm, and 605}621 nm). In the previously reported SrInO:xPr> phases (9), the ratio of (integrated red band intensity)/(integrated blue band intensity#integrated green band intensity) is larger than that for CaInO:xPr> and, therefore, the SrInO:xPr> samples show red emission color but CaInO:xPr>
FIG. 2. PL emission spectra for Pr>-activated CaInO:xPr> phases with x"(a) 6.25;10\, (b) 0.5%, (c) 1%, and (d) 2%, respectively.
FIG. 3. PLE spectra for CaInO:xPr> (x"6.25;10\) phase with j "605 nm. orange. This observation may be attributed to the di!erence
in crystal "eld strength that Pr> experiences in the two alkaline earth indate host lattices.
As compared to the broadband feature of Pr> in some hosts such as SrVBa\VNbO (10), the #uorescence emis-sion spectra attributed to Pr> for CaInO:xPr> exhibit line features. The spectra shown in Fig. 2 also reveal that the width of emission peaks exhibits a considerable dependence on the activator content x. Hence, this observation indicates the strong e!ect of local crystal "eld on the energy level structure of Pr>. The splitting of the energy levels in the
crystal "eld was proposed to be responsible for the broaden-ing of emission peaks (11).
In order to understand the energy levels involved in the energy absorption process, we have measured the photo-luminescence excitation (PLE) spectrum of CaInO:xPr> (x"0.0625%) and the results are represented in Fig. 3. The spectrum yields the luminescence output withj of 605 nm (the strongest emission) as a function of the exciting wavelength (j) and it allows us to construct an energy level scheme for Pr> in CaInO. The excitation spectrum consists of six peaks corresponding to the transitions from
FIG. 4. Integrated PL emission intensity (solid circles) as a function of Pr> content for CaInO:xPr> phases. The solid line represents the best-"t curve for experimental data.
TABLE 1
Observed Luminescence Decay Lifetime (sR) for
CaIn2O4: xPr3ⴙPhases x (%) q0 (lsec) 0.0625 210 0.5 180 1 100 2 50
the fundamental multiplet H level to G (j"985 nm), D (j"600 nm), D (j"584.5 nm), P (j" 492.5 nm),P (j"481 nm), I (j"475 nm), and P (j" 454 nm) in the far-infrared and visible spectral regions, respectively.
To examine the e!ect of luminescence quenching at-tributed to activator concentration on the PL emission intensity, we have tested di!erent activator Pr> concentra-tions and optimized from 0.0625 atom% to 2 atom%. For accurate comparison of intensity, care was taken to main-tain the excitation wavelength (j) and band slits for both excitation and emission monochromators of the spectro-#uorometer strictly identical. The integrated #uorescence intensity is represented as a function of nominal Pr> con-centration x and shown in Fig. 4. The correlation of PL intensity and x can be best "t according to the following equation,
I"!a ln x#b,
where I is the PL intensity, a and b are scalar constants, and
x is Pr> content (in atom %). The "tting parameters of
a and b were found to be 22.4 and 20.0, respectively. A
dras-tic decrease in the PL intensity by more than 1 order of magnitude was observed while Pr> concentration in-creased from 0.0625% to 2%. For the composition range investigated, the maximal PL emission intensity was ob-served in the sample of CaInO:xPr> with x of 0.0625%. The lifetime or radiative decay time (q0) reported here is conventionally de"ned as the time required for the lumines-cence intensity to decay down to ca. 36.8% (or 1/e) of its initial value (12). The observed luminescence decay lifetimes
for CaInO:xPr> phases are summarized in Table 1. The magnitude of q0 for Pr>-activated CaInO phases with di!erent Pr> contents was found to be in the range of 50}210lsec which is comparable with that (i.e., 132 lsec) reported for CaTiO:Pr> (7). Furthermore, a shortening of the lifetime with increasing Pr> content, presumably due to a concentration quenching e!ect, was also observed. The observedq0 and the decay rate appear to meet the applica-tion requirement for CRT color televisions. As a matter of fact, not only #uorescence but also phosphorescent after-glow (for several seconds) of CaInO:xPr> (x"0.0625%) was actually observed by the naked eye under ultraviolet excitation. Investigations of phosphorescence properties for CaInO:xPr> phases are currently in progress.
The observed PL emission spectra (Fig. 2) con"rm the fact that blue and green emissions actually dominate the red for all polycrystalline CaInO:xPr> phosphors investi-gated. Therefore, the #uorescence for CaInO:xPr> ob-served is orange but not purely red, as compared to that for
TABLE 2
Chromaticity Coordinates and Relative Integrated Intensity Ratios for CaIn2O4:xPr3ⴙPhases
x (%) Chromaticity coordinates Ratio of integrated intensity?
0.0625 (0.544, 0.297) 0.69
0.5 (0.523, 0.295) 0.96
1 (0.447, 0.299) 1.24
2 (0.442, 0.301) 1.39
?(Integrated intensity of blue band#integrated intensity of green band)/integrated of red band intensity.
FIG. 5. Chromaticity diagram indicating the di!erence of hues of CaInO:xPr>, NTSC green, NTSC red, and NTSC blue phosphors (NTSC"National Television Standard Color).
SrInO:xPr> (9). The orange color emission was analyzed and con"rmed with the help of CIE (Commission Interna-tionale de l'Eclairage) chromaticity coordinates and integ-rated intensity ratios, de"ned as (integinteg-rated blue band intensity#integrated green band intensity)/(integrated red band intensity). The chromaticity coordinates for CaInO:xPr> denoted by (x, y) and measured with a chromameter are summarized in Table 2. The coordinates for CaInO:xPr> phases were found to fall in the orange spectral region of the CIE chromaticity diagram represented in Fig. 5. It seems that the color coordinate x decreases linearly with increasing composition x. In general, the coor-dinate x is very sensitive to the relative ratio of intensity of the emissions in the blue (492 nm) and red (605 nm) regions observed in the PL spectra for CaInO:xPr> phases with di!erent x's. We have noticed that concentration quenching of Pr> luminescence occurs at relatively low x and was already observed in the CaInO:xPr> phase with x as low
as 6.25;10\, as compared to that (i.e., 0.2%) found in CaTiO: Pr> reported by Diallo et al. (7). Since we are not sure whether the sample with x"6.25;10\ is really opti-mally doped in terms of emission intensity, it is di$cult to predict the color trend for samples with x greater than 2%. However, for those with x less than 6.25;10\ the linear correlation between coordinate x and composition x may still be valid.
In particular, the emissive color of CaInO:xPr> (x"6.25;10\) phase was found to be appreciably redder than that of samples with other compositions; this observa-tion may be raobserva-tionalized by the smaller integrated intensity ratios de"ned and represented in Table 2.
4. CONCLUSIONS
We have synthesized a series of unprecedented orange-emitting and Pr>-activated calcium indates (CaInO) and investigated their photoluminescent properties. The PL emission spectra of CaInO:xPr> phases essentially ex-hibited three groups of emissions with j peaking at 492.5 nm, 537 nm, and 605 nm, which are attributed to PPH, PPH, and DPH transitions of Pr>, respectively. The activator concentration quenching e!ect has been investigated and the optimal Pr> content was determined to be 6.25;10\ for CaInO:xPr> phases with 6.25;10\4x42%. The decay lifetime was found to be in the range of 50}210lsec for CaInO:xPr> phases, depending on x. On the other hand, the hue of orange-emitting CaInO:xPr> phases with di!erent x's was com-pared to NTSC red, green, and blue, as indicated by the coordinates in the CIE chromaticity diagram.
ACKNOWLEDGMENTS
This research is supported by the National Science Council of Taiwan (R.O.C.) under Contract No. NSC89-2113-M-009-024. We are indebted to Dr. Fred C. Chen of Microelectronics and Information System Research Center of NCTU for the assistance with the measurements of chromaticity coordinates. The Regional Instruments Center of the National Science Council in Hsinchu is acknowledged for the luminescence decay lifetime measurements.
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