A study on the luminescent properties of new green-emitting terbium-activated CaIn2O4 : xTb phosphors

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A study on the luminescent properties of new green-emitting

terbium-activated CaIn

₂

O

₄

: xTb phosphors

F.S. Kao, Teng-Ming Chen*

Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30050, Taiwan Received 30 August 2000; received in revised form 17 May 2001; accepted 18 June 2001

Abstract

Calcium indate phosphors have been prepared by solid-state reactions at 14001C. This indate matrix permits photoluminescence when doped with terbium. It was found that under ultraviolet excitation with a wavelength of 254 nm the phosphors revealed yellowish green luminescence that was attributed to the transitions from 5D4excited

state to7FJmultiplet ground states of Tb3+ions. The powder X-ray diﬀraction, photoluminescence spectra, brightness,

ﬂuorescence decay lifetime, and chromatic characteristics of the Tb3+-activated CaIn2O4phosphors were systematically

Keywords: CaIn2O4: xTb phosphor; Photoluminescence spectra; CIE coordinates decay lifetime

1. Introduction

In recent years, great efforts have been made to develop efficient phosphor systems. Among these, a number of works have been carried out on investigations of the luminescent properties for borates [1], aluminates [2] and gallates [3]. From the fact that indium belongs to the same group as boron, aluminum and gallium in the periodic table, it was suggested that metal indates might also act as a potential host lattice for lumines-cence. This consideration prompted us to extend our research to compounds with different compo-sitions in the ternary Ca–In–O system. We have been working with a semiconducting compound

CaIn2O4 as a potential host matrix for phos-phors since its band gap of 3.9 eV [4] is wide enough to incorporate visible luminescence centers such as rare-earth ions. Moreover, no self-acti-vated photoluminescence was observed for the un-doped CaIn2O4 samples measured at ambient temperature.

Recently, we have synthesized and characterized a series of Pr3+-doped CaIn2O4that emits orange ﬂuorescence under ultraviolet excitation. Some results obtained from the spectroscopic and luminescence studies have already been repor-ted elsewhere [5]. Since Tb3+-activated green phosphors have been reported in three-band ﬂuorescent lamps (e.g., (Ce,Gd)MgB5O10: Tb3+ [6]), projection television tubes (e.g., Y3Al5O12: Tb3+ [7]), and X-ray intensifying screens (e.g., Gd2O2S : Tb3+ [8]), our work is essentially moti-vated by the quest to acquire a deeper insight into the materials nature of CaIn2O4 as a potentially

*Corresponding author. Tel: +886–35731695; fax: +886– 5723764.

E-mail addresses: [email protected] (F.S. Kao), [email protected] (T.-M. Chen).

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in detail the investigation results of the photo-luminescent properties, possible radiative relaxation mechanisms and ﬂuorescence decay lifetimes for a series of Tb3+-activated CaIn2O4 phosphors.

2. Experimental procedure

Polycrystalline (Ca1xTbx)In2O4 (as CaIn2O4: xTb) phases with x ¼ 0:125%; 0.5% and 2% were investigated, respectively. The series of samples were synthesized by conventional solid-state reactions. Stoichiometric amounts of constituent materials CaO, In2O3, and Tb4O7 with high purity (99.9%, all from Aldrich Chemicals Co., Milwaukee, WI, USA) were ﬁnely ground to-gether. Without addition of any ﬂux, the mixtures were then calcined at 5501C for 6 h to avoid the inclusion of carbonate impurities and, subse-quently, sintered at 14001C for 24 h in air.

The chemical purity and phase homogeneity of the as-prepared polycrystalline CaIn2O4: xTb phosphors were examined by powder X-ray diffraction (XRD) profiles. The XRD data were collected in the range of 51o 2yo 801 by using a MAC Science MXP-3 automatic diffractometer (MAC Science Co., Tokyo, Japan) equipped with graphite-monochromatized and Ni-filtered CuKa radiation operating at 40 kV and 20 mA. Only single-phased samples were used for further studies.

The measurements of photoluminescence (PL) emission and excitation (PLE) spectra were carried out at room temperature by means of a Spex Fluorolog-3 spectroﬂuorometer (Instruments S.A., Inc., Edison, NJ, USA) equipped with a 450 W Xe lamp as an excitation source. The spectral

with a Shimadzu UV-2501PC double-beam UV-VIS spectrometer (Shimadzu Co., Tokyo, Japan) with Al2O3used as a standard.

The ﬂuorescence decay lifetimes ðtÞ were mea-sured by exciting the samples with UV of 248 nm by using a Lamda Physik LPX150 T excimer laser (Spectra-Physics Co., Mountain View, CA, USA) and pulse duration of 0.1 s. The CIE chromaticity coordinates and relative brightness were deter-mined by using a Minolta CS-100 chromameter (Minolta Co., Tokyo, Japan).

3. Results and discussions

The XRD profiles for CaIn2O4: xTb phases with x ¼ 0:125%; 0.5%, and 2% are shown in Fig. 1. As a result, a typical CaIn2O4pattern [12] was observed in the XRD profiles for all the samples investigated. No peak corresponding to any of the source materials or allotropic forms was found, suggesting that a pure crystalline compound with the same structure as CaIn2O4 exists. As expected, no obvious shifting of the diffraction peaks was observed as x increases, since the variation of unit cell dimension as a result of a small amount of replacement of Tb3+ is beyond the detection limit of the XRD technique.

The ambient temperature PL emission spectra of CaIn2O4: xTb (x ¼ 0:125%; 0.5% and 2%) phosphors excited with 254 nm UV excitation are shown in Fig. 2. These phosphors exhibited a typical, characteristic Tb3+emission. The photo-luminescent properties of the three phosphors with diﬀerent levels of Tb3+ doping do not diﬀer greatly from each other and no obvious red or blue shift in wavelength was observed.

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The ﬂuorescence of Tb3+ mainly originated from the transitions from5D3to

7

FJand/or 5

D4to 7

FJ (where J ¼ 1; 2; 3; 4; 5; 6; respectively). When the Tb3+-activated phosphors are excited by ultraviolet exication of 254 nm, the Tb3+ ions (4f8) would be excited to higher 4f75d1levels and feed afterward, nonradiatively, the5D3or/and5D4 excited states. Due to the relatively wide energy gap (i.e., at least 13,000 cm1) between these excited states and the 7FJ multiplet ground state, the relaxation process occurs only radiatively,

namely, through the emissions in the visible spectral region.

In general, for systems with homogeneously distributed Tb3+activator, a blue emission attrib-uted to the transition from5D3excited state to7FJ ground states occurring at wavelength below 485 nm was observed at low Tb3+concentrations [13]. This blue emission was generally found to vanish while the Tb3+concentration is increased beyond the critical concentration for cross-relaxa-tion to occur [14]. On the other hand, there exists another category of Tb3+-activated phosphors such as CsCdBr3: Tb [15] and LnBO : Tb [16], exhibiting no blue emissions even at very low Tb3+ concentration. The Tb3+ ions doped in the CsCdBr3 matrix were reported to incorporate in pairs or clusters by Lammers et al. [15] and the formation of pairs or clusters of Tb3+implies that the energy transfer between Tb3+ions may occur even for very low concentrations. Therefore, the so-called cross-relaxation can be dominant and, thus, nonradiative relaxation from 5D3 state to lower 5D4 level occurs, as proposed by Fordster [17] and Blasse [18] and, consequently, the absence of 5D3 emission in the PL spectra for CaI-n2O4: xTb3+ is expected and only the 5D4 emis-sion with relatively high intensity was observed.

Fig. 2. The PL emission spectra for CaIn2O4: xTb phosphors

with x ¼ 0:125%; 0.5% and 2%, respectively.

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concentration of Tb [19] and is induced by the resonance between the excited states and the ground states of two Tb3+ ions described in the following equation [17,18]:

Tb3þð5D3Þ þ Tb3þð7F6Þ- Tb3þð5D4Þ þ Tb3þð7F0Þ:

The occurrence of this process depends on the interaction between two adjacent Tb3+ ions. For the very low concentration of Tb3+ in CaI-n2O4: xTb ðx ¼ 0:125%Þ; the Tb

3+

ions are postulated to incorporate in pairs or clusters as in CsCdBr3[15] for the relaxation to occur.

On the other hand, the multiphonon mechanism is intrinsically a thermal relaxation process, i.e., the relaxation of energy into lattice vibration [20]. For Tb3+, the relaxation starts from the pure electronic state A of 5D3to the electron–phonon-coupled state of5D4with phonons occurring at A and followed by the instantaneous transfer to C and, subsequently, relaxation to B [20], as indi-cated in the following conﬁgurational coordinate diagram. A C C B 5 D3 5 D4 R E

Hence, the green to red emissions at 486, 543, 585, and 618 nm that correspond to transitions from5D4to7F6,7F5,7F4and7F3multiplet ground states of Tb3+, respectively, are remarkably enhanced. Since the J values of the 7FJ ground states are high, the crystal ﬁeld splits the levels into many sublevels and gives the spectrum its compli-cated appearance and ﬁne structures. The phos-phor of CaIn2O4: xTb exhibiting no blue emissions belongs to the same category as CsCdBr3: Tb and LnBO3: Tb phosphors.

The excitation spectrum for CaIn2O4: xTb ðx ¼ 0:125%Þ measured with an emission wavelength ðlemÞ of 543 nm is shown in Fig. 3. The spectrum

was characterized by an asymmetrical broad band with wavelength ranging from 250 to 350 nm. This broad band could be deconvoluted into two Gaussian-shaped bands centered at 294 and 318 nm, represented as a dashed line and a thin solid line in Fig 3, respectively. As indicated by the comparison of Figs. 2 and 3, the excitation spectrum is comparable to the absorption spec-trum and the two bands can be reasonably referred to two diﬀerent absorption mechanisms.

As noted in the diffuse reflectance spectrum for the undoped CaIn2O4sample shown in Fig. 4, the optical band gap of the compound was roughly estimated to be 3.86 eV that amounts to ca. 318 nm observed in the reflectance spectrum (similar to that reported in Ref. [4]). The absorption band observed at 318 nm in the excitation spectrum was then inferred to be the absorption of the host

Fig. 3. The PL excitation spectra for CaIn2O4: xTb phosphors

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lattice itself. On the other hand, the absorption band at 294 nm was essentially attributed to the absorption of the Tb3+ions.

The allowed transitions of rare-earth ions are interconﬁgurational and consist of two diﬀerent types, viz., charge–transfer transitions and 4fn-4fn1 5d transitions. The charge–transfer transitions were found for rare-earth ions, which are likely to be reduced, such as Eu3+and Sm3+. For trivalent ions having a tendency to become tetravalent, the 4f-5d absorption band in the ultraviolet was often observed, for instance, Pr3+ and Tb3+ [21]. From these arguments, the absorption band at 294 nm, being well above the host absorption edge, can be ascribed to the 4f-5d transitions of Tb3+in CaIn2O4matrix.

Considering the large area ratio of band at 318 nm/band at 294 nm (i.e., 0.53), the band centered at 318 nm at the absorption edge of the host that gives rise to signiﬁcant emission indicates that the energy transfer from the host to the activator ions is eﬃcient in this condition. Furthermore, no obvious 4f-4f absorption features were observed in the excitation spec-trum, since the parity-selection rule forbids these transitions.

To examine the quenching eﬀect on the bright-ness due to activator concentration, a number of samples were prepared with terbium content varying from 0.125 to 2 at%. The relative bright-ness is displayed as a function of Tb3+ concentra-tion and represented in Fig. 5. The activator concentration dependence of brightness can be

well ﬁtted with a solid line in Fig. 5 according to the following equation:

I ¼ ax2þ bx þ c;

where I is the relative brightness, a; b and c are constants and x is Tb3+concentration (at%). The values of a; b; c and deviation ðR2_{Þ were found to}

be 0.152, 0.175, 1.024 and 1, respectively. From these results, it was concluded that the maximal brightness was obtained for the sample with a terbium concentration of 0.125% and the bright-ness decreased by more than one order of magnitude while Tb3+ concentration was in-creased from 0.125% up to 2%. The decrease in brightness with increasing activator concentration was presumably due to the well-established theory of concentration quenching [22]. Moreover, CaIn2O4: xTb ðx ¼ 0:125%Þ was found to be much brighter than (Y2.95Tb0.05)Al5O12 under 254 nm UV excitation, as indicated by brightness measurements.

The fluorescence decay lifetime ðtÞ for CaI-n2O4: xTb is defined as the time required for the fluorescence intensity to decay down to 1/e of its initial value [23] and the measured lifetimes are summarized in Table 1. The fluorescence decay lifetimes for the phosphors were found to be in the range of microseconds and appear to meet the application requirements for CRT color televisions.

Fig. 4. The diﬀuse reﬂectance spectrum of CaIn2O4.

Fig. 5. The relative brightness as a function of Tb3+_contents

for CaIn2O4: xTb phosphors. The solid line indicates the

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The PL emission spectra shown in Fig. 2 conﬁrm the fact that the CaIn2O4: xTb phosphors emit in both the green and red spectral regions and the green emission was found to dominate the red. Consequently, the emission color for CaIn2O4: xTb would shift slightly from pure green toward red and results in yellowish green. The emission color was analyzed and conﬁrmed with the help of CIE chromaticity coordinates. The measured color coordinates for CaIn2O4: xTb denoted by ðx; yÞ are shown in Table 2. The coordinates were found to fall into the yellowish green region of the CIE chromaticity diagram and the hue of the phosphor becomes a deeper green with increasing terbium concentration, as shown in Fig. 6. Furthermore, as comparing to (Y2.95Tb0.05)Al5O12, (Y2.95Tb0.05)Al5O12 exhibits deeper green emission than CaIn2O4: xTb ðx ¼ 0:125%Þ under the UV excitation of 254 nm.

4. Conclusions

A series of unprecedented yellowish green emitting CaIn2O4: xTb (x ¼ 0:125%; 0.5% and 2%) phosphors were synthesized at 14001C. The PL emission spectra of these phosphors are

composed of four sharp bands with maxima at 486, 543, 585 and 618 nm which resulted from the 5

D4 transitions to 7F6, 7F5, 7F4 and 7F3 ground states of Tb3+, respectively. The brightness was found to decrease with increasing activator con-centration and the ﬂuorescence decay lifetimes were determined in microsecond range and meet the requirements for CRT applications.

Acknowledgements

This research was supported by the National Science Council of ROC under Contract No. NSC89-2113-M-009-024. We thank Miss Shu-Chi Lin of the Regional Instruments Center of NSC in Hsinchu for the measurements of ﬂuorescence decay lifetimes and Dr. Fred Chen of National Chiao Tung University for his assistance with the brightness measurements.

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