Photoluminescence properties and energy-level diagrams in (Ce3+, Tb3+)-codoped KCl green phosphor

Photoluminescence properties and energy-level diagrams in (Ce3+, Tb3+)-codoped KCl green phosphor

Journal of Luminescence 156 (2014) 157–163 Contents lists available at ScienceDirect Journal of Luminescence journal homepage: www.elsevier.com/loca...

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Journal of Luminescence 156 (2014) 157–163

Contents lists available at ScienceDirect

Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

Photoluminescence properties and energy-level diagrams in (Ce3 þ , Tb3 þ )-codoped KCl green phosphor Yuki Tosaka, Sadao Adachi n Division of Electronics and Informatics, Faculty of Science and Technology, Gunma University, Kiryu-shi, Gunma 376-8515, Japan

art ic l e i nf o

a b s t r a c t

Article history: Received 20 May 2014 Received in revised form 11 July 2014 Accepted 1 August 2014 Available online 9 August 2014

KCl:Ce3 þ , Tb3 þ green phosphor was synthesized from an aqueous solution of KCl–CeCl3–TbCl3. The synthesized phosphor was investigated using X-ray diffraction analysis, electron probe microanalysis, diffuse reflectance measurements, photoluminescence (PL) analysis, PL excitation spectroscopy, and PL decay measurements. The KCl:Ce3 þ , Tb3 þ phosphor showed a strong Tb3 þ -related emission in the 480–700 nm spectral region. The Tb3 þ -related emission intensity in the codoped phosphor was enhanced more than 300 times compared to that in the Tb3 þ singly doped phosphor. This enhancement in the PL intensity could be attributed to an efficient energy transfer from Ce3 þ to Tb3 þ in the KCl host. The maximum transfer efficiency was η  92% for the sample synthesized at a solution of KCl:CeCl3: TbCl3 ¼1:0.01:0.05 in molar ratio. The (Ce3 þ , Tb3 þ ) concentration dependences of the Ce3 þ - and Tb3 þ -emission decay times were determined. The temperature dependence of the Tb3 þ -related emission intensity was also measured and analyzed from T ¼20–450 K. & 2014 Elsevier B.V. All rights reserved.

Keywords: KCl Phosphor Ce3 þ Tb3 þ Photoluminescence Alkali halide

1. Introduction The large band-gap energy (  6–10 eV) of alkali halides [1] renders them optically transparent in the wide spectral range from the near-infrared to ultraviolet, making them suitable for excellent optical window materials and also for creating color centers in the band gap that could provide a study on the radiative and nonradiative recombination processes [2]. Potassium chloride is an alkali-halide compound with the chemical formula KCl, representing equal proportion of potassium and chlorine. These chemical elements are the eighth (K) and 19th most abundant elements (Cl) in the earth's crust. Thus, it is easily understood that KCl is a cheep compound material. Alkali halides that contain heavy-metal ions with an s2 configuration, including Ga þ , In þ , Tl þ , Sn2 þ , and Pb2 þ , have been widely investigated both theoretically and experimentally [3]. Rare-earth-doped alkali halides have also been of considerable interest for possible applications as efficient phosphors. Several aspects of KCl luminescence doped with the lanthanide ions have been reported in the past. For example, the luminescence properties of Eu2 þ -doped KCl have been extensively investigated (see, e.g., [4–8]). Studies on Sm2 þ (e.g., [5,9,10]) and Yb2 þ activators in KCl (e.g., [5,11]) have also been performed. Tivalent

n

Corresponding author. Tel.: þ 81 277 30 1710; fax: þ 81 277 30 1707. E-mail address: [email protected] (S. Adachi).

http://dx.doi.org/10.1016/j.jlumin.2014.08.010 0022-2313/& 2014 Elsevier B.V. All rights reserved.

lanthanide ions are known to act as efficient activators in various hosts. However, there have been only a few reports on the trivalent lanthanide activators in the KCl host. One reason for this comes from the difficulty of incorporating “trivalent” ions on a monovalent potassium site. Ce3 þ is a trivalent lanthanide ion showing 4f–5d transitions in the UV or visible spectral region of various phosphors. Recently, Bangaru et al. [12,13] studied the photoluminescence (PL) and thermoluminescence (TL) properties of KCl:Ce3 þ crystals before and after gamma irradiation. More recently, Tosaka and Adachi [14] performed details studies on the PL properties of KCl:Ce3 þ at temperatures between 20 and 450 K. TL properties of KCl:Tb3 þ irradiated by a γ ray were studied by Bangaru and Muralidharan [15]. They found that illumination at 560 nm leads to a drastic change in the PL glow curve. Lyoluminescence, TL, and mechanoluminescence studies on Dy3 þ -activated KCl were also performed by several authors [16,17]. To the best of our knowledge, however, no detailed PL properties of KCl:Tb3 þ have been published until now. No study on the codoping of Tb3 þ with other ions in KCl has been performed. The purpose of this paper is twofold: (i) to report the synthesis of a (Ce3 þ , Tb3 þ )-codoped KCl phosphor from an aqueous solution of KCl–CeCl3–TbCl3, and (ii) to study its PL properties in detail. The synthesis of KCl phosphor from aqueous solutions is more economical than the commonly used melt-growth method, namely, the Bridgman–Stockbarger method. Temperature dependence of the PL properties was studies from T ¼20 to 450 K in 10-K

Y. Tosaka, S. Adachi / Journal of Luminescence 156 (2014) 157–163

increments. PL excitation (PLE), diffuse reflectance, and PL decay measurements were also performed. Remarkably, the Tb3 þ related emission intensity in the (Ce3 þ , Tb3 þ )-codoped KCl phosphor was enhanced more than 300 times compared to that in the Tb3 þ singly doped phosphor, thereby suggesting an efficient resonant energy transfer occurring in KCl from Ce3 þ to Tb3 þ .

28.0

28.2

2θ (deg) 28.4

28.6

XRD (arb. units)

158

(200)

ASTM

2. Experimental procedure

3. Structural properties

XRD (arb. units)

K : Ce : Tb = 1 : 0.01 : 0.05

Ce = 0 Tb = 0 Ce = Tb = 0 (200)

20

(220)

30

40

ASTM

(222)

50 2θ (deg)

60

70

80

Fig. 1. XRD patterns of KCl:Ce3 þ , Tb3 þ phosphors measured in the θ–2θ scan mode. The phosphors were synthesized at (M¼ 0.01, N ¼ 0.05); (M ¼0, N ¼ 0.05) (Ce ¼ 0); (M ¼ 0.01, N ¼0) (Tb¼ 0); and (M¼ 0, N ¼0) (Ce ¼Tb ¼ 0; undoped KCl). The XRD pattern of KCl (No. 00-004-0587) taken from the ASTM card is also shown.

K

KCl:Ce3+, Tb3+ Intensity (counts)

A (Ce3 þ , Tb3 þ )-codoped KCl phosphor was synthesized by evaporating a mixture of KCl:CeCl3:TbCl3 ¼1:M:N in molar ratio dissolved in deionized water on an electric hot plate at 100 1C. No further thermal treatment was applied. Note that the evaporation method provided phosphor crystals in the platelet form. Therefore, the phosphor powder was prepared by grounding in an agate mortar for a few tens of minutes. The crystallinity of the synthesized KCl:Ce3 þ , Tb3 þ phosphor was examined by X-ray diffraction (XRD) analysis using a RAD-IIC X-ray diffractometer (Rigaku) provided with a Cu Kα radiation. Electron probe microanalysis (EPMA) measurements were performed with a SHIMADZU EPMA-1610 at a probe current of 100 nA and an accelerating voltage of 15 kV. Room-temperature PL measurements were performed in a single monochromator equipped with a charge-coupled device (PIXIS 100, Princeton Instruments) by exciting at the fourth harmonics (266 nm) of a Nd:YAG laser (MINILITE I, Continuum Electro-Optics, Inc.). Temperature dependence of the PL spectra was examined using a CryoMini cryostat (Iwatani Industrial Gases) at T¼ 20–300 K and a stainless-steel cryostat (Technolo Kogyo) at T ¼300–450 K. A 266 nm light from a Nd:YAG laser (MINILITE I, Continuum Electro-Optics, Inc.) was used as an excitation light source. PL excitation (PLE) measurements were performed at 300 K by using a fluorescence spectrometer (Hitachi F-4500). Diffuse reflectance spectra were measured at 300 K using a spectrometer (JASCO V-570). The luminescence decay curves in the milliseconds region were measured by exciting at the fourth harmonics (266 nm) of a Nd: YAG laser (MINILITE I, Continuum Electro-Optics, Inc.) at 300 K. The signal was detected with a Peltier-element-cooled photomultiplier tube (Hamamatsu R375), a multichannel scaler (SR 430, Stanford Research Systems, Inc.), and a preamplifier (SR 445A, Stanford Research Systems, Inc.). The luminescence decay curves in the nanoseconds region were measured using a lifetime fluorescence spectrometer (FluoroCube 3000U, Horiba Jobin Yvon) operating at an excitation light of 295 nm.

PET LiF

Tb Tb Tb

0.1

Cl

K Ce

0.2

Cl

0.3 0.4 Wavelength (nm)

0.5

0.6

Fig. 2. EPMA trace for the KCl:Ce3 þ , Tb3 þ phosphor synthesized at M ¼ 0.01 and N¼ 0.05. PET (pentaerythritol) and LiF were used as the radiation diffractors.

codoped KCl sample showed no clear shift in the diffraction angles from the undoped values. This may come from the effect of cancellation between expanded (Ce3 þ ) and contracted (Tb3 þ ) lattice distortions.

3.1. XRD measurements The XRD traces of the undoped and doped KCl samples, measured in the θ–2θ scan mode, are shown in Fig. 1. The XRD pattern of KCl, as reported by the American Society for Testing and Materials (ASTM), is also shown in Fig. 1. A shift in the diffraction peaks is observed by doping with Ce3 þ or Tb3 þ ions into the KCl host. In Fig. 1, the diffraction peaks shifted toward lower angle by the Ce3 þ doping. Their shifts were less than 0.11 at M ¼0.01. On the contrary, the Tb3 þ -doped sample showed a shift in the diffraction peaks toward higher angle (o0.051 at N ¼0.05; Fig. 1). It is known that the crystal lattice can expand or contract when impurity atoms are incorporated. The expansion or contraction of the KCl lattice observed in this study indicates that the Ce and Tb dopants are certainly incorporated in the KCl matrix. On the other hand, the (Ce3 þ , Tb3 þ )-

3.2. EPMA measurements The EPMA measurements were performed to examine the chemical species of our synthesized KCl:Ce3 þ , Tb3 þ phosphor. Fig. 2 shows the EPMA trace for the KCl:Ce3 þ , Tb3 þ phosphor synthesized at (M, N)¼(0.01, 0.05). Chemical species of potassium (K) and chlorine (Cl), together with cerium (Ce) and terbium (Tb), have been identified, thereby confirming the doping of Ce and Tb species in the KCl host. 4. PL and PLE spectra Fig. 3 shows the PL and PLE spectra of (a) KCl:Ce3 þ (M ¼0.01), (b) KCl:Tb3 þ (N ¼0.05), and (c) KCl:Ce3 þ , Tb3 þ (M¼ 0.01, N ¼0.05).

Y. Tosaka, S. Adachi / Journal of Luminescence 156 (2014) 157–163

PL

PLE ×10

×300

PLE

PL ×10 ×1

200

300

400 500 Wavelength (nm)

600

700

Fig. 3. PL and PLE spectra of (a) KCl:Ce3 þ (M ¼0.01), (b) KCl:Tb3 þ (N¼ 0.05), and (c) KCl:Ce3 þ , Tb3 þ (M ¼ 0.01, N¼ 0.05) phosphors measured at 300 K. The excitation wavelength was at λex ¼ 266 nm in (a)–(c) and monitoring wavelengths were at λem  370 nm in (a) and at λem  545 nm in (b) and (c). Note that the Tb3 þ greenemission intensity at  545 nm in (c) is about 300 times stronger than that in (b).

The PL spectra were measured by excitation at λex ¼266 nm, whereas the PLE spectra were measured by monitoring at λem 370 nm (Fig. 3(a)) and  545 nm (Fig. 3(b) and (c)). All these spectra were measured at room temperature. The PL emission band at  370 nm in Fig. 3(a) is attributed to the 5d-4f transition of Ce3 þ . This emission band can be deconvoluted into two Gaussian peaks with energy separation of 0.3 eV, corresponding to the spin–orbit split-off energy of the lowest 4f level (2F5/2 and 2F7/2). The PLE spectrum in Fig. 3 (a) exhibits three absorption peaks at  230,  260, and 320 nm, which can be assigned to the 4f-5d transitions in Ce3 þ . The fundamental absorption edge in KCl occurs at  9 eV (  140 nm) [1]. Thus, no direct excitation from the valence band (VB) to the conduction band (CB) in KCl occurs in our experimental spectral range (λ 4200 nm). The PL spectrum of KCl:Tb3 þ in Fig. 3(b) shows a series of the sharp emission lines at  480–620 nm. These emission lines correspond to the spin- and parity-forbidden 4f8-4f8 transitions in Tb3 þ . Thus, their intensities are very weak. The spin- and parity-forbidden 4f8-4f8 PLE peaks can also be observed as a weak peak series in the 300–480 nm spectral region. The spinforbidden, parity-allowed 4f8-4f75d PLE peaks are also observed at  250–300 nm. The intensities of these spin-forbidden, parityallowed 4f8-4f75d transitions are nearly the same as those of the parity- and spin-forbidden 4f8-4f8 transitions. In CaCO3:Tb3 þ [18], the former transitions were observed to be considerably stronger ( 3–10 times) than the latter ones. In Fig. 3(b), the remarkably strong PLE shoulder originating from the spin- and parity-allowed 4f8-4f75d transitions is observed at  230 nm. The PL spectrum of KCl:Ce3 þ , Tb3 þ in Fig. 3(c) shows a series of the strong Tb3 þ -related emission lines. The Tb3 þ emission intensity in the (Ce3 þ , Tb3 þ )-codoped KCl phosphor is enhanced more than 300 times compared to that in the Tb3 þ singly doped phosphor (see also Fig. 6 below). Moreover, the Ce3 þ blueemission band peaking at  370 nm is not clearly observed in Fig. 3(c). The PLE spectrum is dominated not only by the strong transitions at 310 nm (Ce3 þ ; 4f-5d) but also by the spinforbidden 4f8-4f75d transitions in Tb3 þ at  250–300 nm.

However, no remarkably strong Tb3 þ -related excitation peaks are observed in the  350–450 nm region. Nevertheless, an enhancement in the Tb3 þ -related emission lines can be observed in the 480–700 nm region. This enhancement in the PL intensity can be explained by an efficient energy transfer from Ce3 þ to Tb3 þ in the KCl host. When K þ is substituted for Ce3 þ or Tb3 þ or both ions, such trivalent ions may be associated with a charge-compensating cation vacancy. However, we could not observe any evidence of charge-compensating cation vacancies formed in the present phosphors from our PL measurements.

5. Diffuse reflectance spectra The optical transition properties of the Ce3 þ - and Tb3 þ -doped KCl crystals were investigated using the diffuse reflectance technique at 300 K. The phosphors examined were with the (Ce3 þ , Tb3 þ ) contents of (0.01, 0), (0, 0.05), and (0.01, 0.05), together with the undoped KCl crystal. As we will see latter, the (Ce3 þ , Tb3 þ ) contents of (0.01, 0.05) gives the strongest PL intensity. As mentioned before, the optical transitions in Tb3 þ for λ4 300 nm are both parity and spin forbidden. We can thus expect no strong Tb3 þ -related transition peaks in the optical spectra of the Tb3 þ -doped crystals. In fact, the diffuse reflectance spectrum obtained from the KCl:Tb3 þ crystal in Fig. 4 is essentially the same as that obtained from the undoped KCl crystal. The fine, but weak peak structures observed at the  350–400 nm region are due to the 4f8-4f8 transitions in Tb3 þ . The peak structures at  250–300 nm are also due to the spin-forbidden 4f8-4f75d transitions in Tb3 þ . A large dip in the diffuse reflectance spectrum of the KCl:Ce3 þ sample is observed at λ 200–400 nm. This structure is due to the dipole-allowed 4f1-5d1 transitions in Ce3 þ . The essentially same diffuse reflectance feature is observed in the (Ce3 þ , Tb3 þ )codoped sample; however, the large dip at 310 nm can be seen only in the (Ce3 þ , Tb3 þ )-codoped sample. This large dip corresponds to the absorption peak observed in the PLE spectrum of Fig. 3(c).

6. PL spectra: Ce3 þ and Tb3 þ concentration dependences 6.1. Ce3 þ concentration dependence The room-temperature PL spectra of the KCl:Ce3 þ , Tb3 þ phosphors synthesized with M varying from 0 to 0.1 at N ¼0.05

6 5 Diffuse reflectance (arb. units)

PL

PL intensity (arb. units)

PLE intensity (arb. units)

PLE

159

200

Photon energy (eV) 3 2

4

Pure KCl Tb3+

Ce3+ (Ce3+, Tb3+)

300

400 500 600 Wavelength (nm)

700

800

Fig. 4. (a) Diffuse reflectance spectra for the undoped KCl, KCl:Ce3 þ (M ¼ 0.01), KCl: Tb3 þ (N¼ 0.05), and KCl:Ce3 þ , Tb3 þ (M ¼0.01, N ¼0.05) measured at 300 K.

160

Y. Tosaka, S. Adachi / Journal of Luminescence 156 (2014) 157–163

Photon energy (eV)

3.0

2.5

100

2.0

K:Ce:T b = 1 : M : 0.05 M = 0.1

∝ M 1.0

10-1

IPL (normal.)

4.0 3.5

10-2 ∝ M 2.0

0.05

10-3

0.02

10-4

PL intensity (arb. units)

K:Ce:Tb = 1 : M : 0.05

×20

0

10-4

10-3

10-2

10-1

M

0.01 0.005

Fig. 6. Integrated PL intensity (IPL) vs Ce3 þ concentration (M) for the Ce3 þ -related (solid circles) and Tb3 þ -related emissions (open circles) in the KCl:Ce3 þ , Tb3 þ (M ¼ 0–0.1, N ¼ 0.05) phosphors at 300 K. The solid lines represent the relations of IPL p Mα with α¼ 2.0 and 1.0 for the Ce3 þ - and Tb3 þ -related emissions, respectively.

0.002

×2

Photon energy (eV)

0.001

×8

500 600 Wavelength (nm)

2.5

2.0

N = 0.1

700

Fig. 5. PL spectra of the KCl:Ce3 þ , Tb3 þ phosphors at 300 K. The Ce3 þ concentrations were varied from M ¼ 0 to 0.1 with N ¼0.05 (Tb3 þ ).

are shown in Fig. 5. The KCl phosphor synthesized without codoping with Ce3 þ (M ¼0) exhibits a very weak Tb3 þ -relatied emission peak at 545 nm. Increasing M leads to an appearance of many sharp emission peaks in the 475–635 nm region. Such emission peaks correspond to the intra-f-shell transitions in Tb3 þ , and have been observed in various host materials, such as Zn2SiO4, Lu2O3, GdOBr, CaIn2O4, ZnGa2O4, ZnAl2O4, SiO2, NaGd (WO4)2, YAG, and YTaO4 (see [18]). As expected from the intrashell transition nature, each emission wavelength in Fig. 5 gets no influence from the Ce3 þ concentration variation. For the phosphors synthesized with MZ0.02, a blue emission band centered at 370 nm becomes detectable. As seen in Fig. 3(a), this emission band comes from the Ce3 þ 5d-4f transitions in the KCl host. It is also understood from Fig. 5 that the Tb3 þ -related emission intensity greatly increases with increasing M from 0 to 0.01. The Ce3 þ - and Tb3 þ -related emission intensities vs Ce3 þ concentration (M) plots for the KCl:Ce3 þ , Tb3 þ phosphors with N ¼0.05 are shown in Fig. 6. The emission intensity data IPL were obtained by integrating the PL spectra in Fig. 5. The solid lines in Fig. 6 show the Ce3 þ - and Tb3 þ -related emission intensities, IPL, vs Ce3 þ concentration, M, given by IPL p Mα with α¼ 2.0 and 1.0, respectively. The Tb3 þ -related emission intensity shows maximum at M 0.01. Further increase of M results in the gradual decrease of IPL (Tb3 þ ). 6.2. Tb3 þ concentration dependence Fig. 7 shows the room-temperature PL spectra of the KCl:Ce3 þ , Tb3 þ phosphors synthesized with N varying from 0 to 0.1 at M¼ 0.01. As expected, the KCl phosphor synthesized without adding Tb3 þ (N ¼0) shows no any Tb3 þ -related emission peaks. Increasing N leads to many sharp Tb3 þ -related emission peaks at 475–635 nm. For N r0.01, the Ce3 þ blue emission at  370 nm can be observed in the PL spectra (Fig. 7). Observation of the Ce3 þ blue emission at such lower N values corresponds to an insufficient

0.05 PL intensity (arb. units)

400

3.0

K:Ce:Tb = 1 : 0.01 : N

0

×200

300

4.0 3.5

0.02 ×2

0.01

×4

0.005

×8

0.002

×6

0.001

×4

0

300

400

500 600 Wavelength (nm)

700

Fig. 7. PL spectra of the KCl:Ce3 þ , Tb3 þ phosphors at 300 K. The Tb3 þ concentrations were varied from N ¼0 to 0.1 with M¼ 0.01 (Ce3 þ ).

Tb3 þ acceptor concentration against Ce3 þ donor concentration, conversely, an excess Ce3 þ donor concentration against Tb3 þ acceptor concentration, in the Ce3 þ - Tb3 þ energy transfer process. Fig. 8 shows the Ce3 þ - and Tb3 þ -related emission intensities (IPL) vs Tb3 þ concentration (N) plots for the KCl:Ce3 þ , Tb3 þ phosphors with M ¼0.01. The emission intensity data IPL were obtained by integrating the PL spectra in Fig. 7. The solid line in Fig. 8 represents the Tb3 þ -related emission intensity, IPL (Tb3 þ ), vs Tb3 þ concentration, N, given by IPL (Tb3 þ ) p N1.0. The Tb3 þ -related emission intensity shows maximum at N  0.05. Further increase of N results in the gradually decreased emission intensity. The decreased PL intensity with increasing acceptor concentration (N) is known as the concentration quenching. From Figs. 5–8, we obtain the values of M  0.01 (Ce3 þ ) and N  0.05 (Tb3 þ ), which promise the strongest Tb3 þ emission intensity.

Y. Tosaka, S. Adachi / Journal of Luminescence 156 (2014) 157–163

161

CB

IPL (normal.)

100

7

E E

∝ N 1.0

4f 8 ⇔ 4f 75d

9

I=0 1 2 5DI 3 4

10-1 Tb3+ 8

4f ⇔ 4f

8

10-2

J=0 7

K:Ce:Tb = 1 : 0.01 : N 10-4

10-3

10-2

10-1

N Fig. 8. Integrated PL intensity (IPL) vs Tb3 þ concentration (N) for the Ce3 þ -related (solid circles) and Tb3 þ -related emissions (open circles) in the KCl:Ce3 þ , Tb3 þ (M¼ 0.01, N ¼0–0.1) phosphors at 300 K. The solid line represents the relation of IPL pN1.0 (Tb3 þ emission).

PL, PLE

10-3 0

PLE ×10

PL

×50

200

100

300

400 500 600 Wavelength (nm)

700

Fig. 10. (a) Electronic energy-level scheme for Tb3 þ in KCl. (b) PL and PLE spectra for the KCl:Tb3 þ (N ¼ 0.05) phosphor measured at 300 K with λem  545 nm (PLE) and λex ¼266 nm (PL).

80

η (%)

4 FJ 5 6

VB

60 40

CB 7

E

K:Ce:Tb = 1 : 0.01 : N

20

2

T2

RET

2

10-4

10-3

10-2 N

10-1

⇒ ⇒

0

E

RET

3 4

η ¼ 1

to Tb

5

DI

J=0 5 6

7

FJ

in KCl can be

Id I d0

ð1Þ

where Id and Id0 are the donor (Ce3 þ ) emission intensities with and without the acceptor (Tb3 þ ) doping at the same donor concentration, respectively. The solid circles in Fig. 9 show the results calculated using Eq. (1). The energy transfer efficiency at (M, N)¼(0.01, 0.05) is 92%.

PL, PLE

The energy transfer efficiency from Ce calculate using the following expression:

Tb3+

2

F7/2 F5/2

2

VB 3þ

E

I=0

Ce3+

Fig. 9. Energy transfer efficiency η from Ce3 þ to Tb3 þ in the KCl:Ce3 þ , Tb3 þ (M¼ 0.01, N ¼0–0.1) phosphors at 300 K obtained from Eq. (1). 3þ

9

PLE

×10

PL ×10 ×50 ×10

200

300

400 500 600 Wavelength (nm)

700

7. Energy-level scheme and energy transfer process in KCl: Ce3 þ , Tb3 þ

Fig. 11. (a) Electronic energy-level scheme for Ce3 þ and Tb3 þ in KCl:Ce3 þ , Tb3 þ . (b) PL and PLE spectra for the KCl:Ce3 þ , Tb3 þ (M ¼0.01, N¼ 0.05) phosphor measured at 300 K. The PLE and PL spectra were measured with λem  545 nm (Tb3 þ green emission) and λex ¼266 nm, respectively. The resonant energy transfer (RET) processes from Ce3 þ to Tb3 þ are schematically shown in (a).

The energy-level scheme for the light emission (PL) and absorption (PLE) processes in the Tb3 þ singly doped KCl phosphor is shown in Fig. 10. The sharp PL lines in Fig. 10(b) can be attributed to the 4f 8 (5D4)-4f 8 (7FJ) transitions of Tb3 þ . Because of their parity- and spin-forbidden nature, the Tb3 þ -related emission intensity is very weak and its decay time is very slow (see Figs. 12 and 13 below). Thus, we observed the blue-emission (5D4-7F6) and red-emission peaks (5D4-7F3–0) in the Tb3 þ singly doped KCl phosphor as the very weak peaks. The PLE peaks observed in the 300–400 nm region of Fig. 10 (b) may be due to the 7FJ (4f8)-5DI (4f8) transitions (Ir3). The PLE peaks in the 250–300 nm region and that at  230 nm (shoulder) can be assigned to the spin-forbidden 4f 8 (7F6)-4f 75d (9E) and spinallowed 4f 8 (7F6)-4f 75d (7E) transitions in Tb3 þ , respectively. The energy-level scheme for the PL and PLE processes in (Ce3 þ , 3þ Tb )-codoped KCl is shown in Fig. 11(a). No strong blue emission

at  370 nm can be observed in the (Ce3 þ , Tb3þ )-codoped KCl phosphor. The PLE spectrum for the Tb3 þ emission in the (Ce3 þ , Tb3 þ )-codoped KCl is also clearly different from that in the Tb3þ singly doped sample (cf. Figs. 10(b) and 11(b)). On the other hand, the PLE spectrum for the Tb3 þ emission in the (Ce3þ , Tb3þ )-codoped sample is essentially the same as that for the Ce3þ blue emission in the Ce3þ singly doped sample (cf. Fig. 3(a) and (c)). The clearly different PLE spectra for the Tb3 þ emission between in the Tb3þ singly doped and (Ce3þ , Tb3þ )-codoped samples are understood to come from the resonant energy transfer (RET) between Ce3þ and Tb3þ ions, as schematically shown in Fig. 11(a). As a result, the radiative Tb3þ transitions of 5D4-7FJ (J¼ 0–6) can be greatly enhanced in the (Ce3 þ , Tb3 þ )-codoped KCl phosphor. The most important feature found in Fig. 6 is that the Tb3 þ related emission intensity in the (Ce3 þ , Tb3 þ )-codoped KCl phosphor is more than 300 times larger than that in the Tb3 þ singly

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LIfetime (ns)

doped phosphor. The energy transfer from Ce3 þ to Tb3 þ observed here may be of the Förster–Dexter type [19,20]. This type of transfer requires an overlap of the emission (donor) and excitation (acceptor) spectra (see Fig. 3). The transfer efficiency is obtained to be η  92% for M¼ 0.01 and N ¼0.05 (Fig. 9).

8. Luminescence decay characteristics

LIfetime (ms)

The decay time measurements were performed at λem 370 nm (Ce3 þ emission) and at λem  545 nm (Tb3 þ emission). The observed decay curves were fitted to a double-exponential expression as follows:   t IðtÞ ¼ ∑ ai exp  þb ð2Þ τi i ¼ 1;2 The effective decay time τeff was then calculated from τeff ¼

∑i ¼ 1;2 ai τ2i ∑i ¼ 1;2 ai τi

ð3Þ

LIfetime (ms)

LIfetime (ns)

Fig. 12 shows the Ce3 þ concentration (M) dependences of the (a) Ce3 þ blue-emission and (b) Tb3 þ green-emission decay times τeff in KCl:Ce3 þ , Tb3 þ with N ¼0.05. Our observed τeff values for the Ce3 þ blue emission are  20 ns for M¼ 0.001–0.1. These values fall within the range of the commonly observed Ce3 þ emission values (10–70 ns) [21–27]. Such a small Ce3 þ concentration dependence of the Ce3 þ blue-emission decay time (  20 ns) has also been observed in CaCO3 [18,23,26]. A very fast decay time of a few or a few tens of nanoseconds of Ce3 þ ions comes from the parityallowed (4f25d) transition nature. The Tb3 þ -related emission peaks at 480–700 nm are due to the parity-forbidden (4f8-4f8) transitions. Thus, their decay times are slow, falling in the milliseconds range. The experimental data in Fig. 12(b) show a slight increase of τeff with increasing M. Fig. 13 shows the Tb3 þ concentration (N) dependences of the 3þ Ce blue-emission and Tb3 þ green-emission decay times in KCl: Ce3 þ , Tb3 þ with M ¼0.01. The experimental data show a gradual increase and a decrease of τeff with increasing N for the Ce3 þ blue and Tb3 þ green emissions, respectively. The luminescent decay times in various (Ce3 þ , Tb3 þ )-codoped phosphors have been reported [18,21–23,27–31]. The blueemission decay times in (Ce3 þ , Tb3 þ )-codoped phosphors were

30 25 20 15 10 5 0 1.2 1.0 0.8 0.6 0.4 0.2 0

30 25 20 15 10 5 0 1.2 1.0 0.8 0.6 0.4 0.2 0

K:Ce:Tb = 1 : 0.01 : N

Ce3+ emission

Tb3+ emission

0.001

0.01 N

0.1

Fig. 13. Tb3 þ concentration (N) dependence of (a) Ce3 þ blue-emission and (b) Tb3 þ green-emission decay times in the KCl:Ce3 þ , Tb3 þ phosphor at 300 K. The Tb3 þ concentrations were varied from N ¼0.001 to 0.1 with M¼ 0.01 (Ce3 þ ).

reported to be shorter than the Ce3 þ singly doped ones [22,23,27]. On the contrary, the green-emission decay times in (Ce3 þ , Tb3 þ )codoped phosphors were longer than the Tb3 þ singly doped phosphors [28–30]. However, our measured data in Figs. 12 and 13 show no remarkable Ce3 þ /Tb3 þ concentration dependence of the blue- and green-emission decay times in (Ce3 þ , Tb3 þ )codoped KCl phosphor. Ce3 þ and Tb3 þ in KCl may occupy a K þ site. An excessive positive charge derived from Ce3 þ and/or Tb3 þ in the KCl host should be compensated by generating a charge-compensating cation vacancy randomly distributed in the crystal volume. Luminescent lifetime is in inverse proportion to the sum of the rates for various radiative and non-radiative decay pathways. The radiative decay rate is strongly dependent on the electronic transitions but is insensitive to the environment, whereas the non-radiative decay rate is dependent on the environment. We can suppose that the energy transfer process of Ce3 þ -Tb3 þ affects not only on the Tb3 þ -related emission process itself but also on its decay time via a trivalent-ion-induced non-radiative decay pathway. This may result in no strong donor/acceptor concentration dependence of the radiative decay times (Figs. 12 and 13). Further study needs to give more detailed explanation on this problem.

K:Ce:Tb = 1 : M : 0.05 9. PL spectra: temperature dependence Fig. 14 shows the temperature dependence of the PL spectra for the KCl:Ce3 þ , Tb3þ phosphors with M¼0.01 and N¼ 0.05 measured at T¼20–440 K in 20-K increments. Because of the intra-f-shell transition nature, the Tb3þ -related emission peaks exhibited no remarkable dependence on T below about 300 K; however, their intensities rapidly decreased at high temperatures (T4300 K). Fig. 15 shows the integrated PL intensity (IPL) vs 1/T plots for the Tb3 þ -related emission in the KCl:Ce3 þ , Tb3 þ (M ¼0.01, N ¼0.05) phosphor. These plots are rationalized using the following equation:

3+

Ce emission

Tb3+ emission

0.001

0.01 M

0.1

Fig. 12. Ce3 þ concentration (M) dependence of (a) Ce3 þ blue-emission and (b) Tb3 þ green-emission decay times in the KCl:Ce3 þ , Tb3 þ phosphors at 300 K. The Ce3 þ concentrations were varied from M ¼0.001 to 0.1 with N¼ 0.05 (Tb3 þ ).

I PL ðTÞ ¼

I0 1 þ ∑i ai expð  Eai =kB TÞ

ð4Þ

where Eai is the quenching (activation) energy and kB is the Boltzmann constant. The solid line in Fig. 15 represents the result calculated using Eq. (4). The fit-determined parameters are I0 ¼ 1.0, a1 ¼8.0  107, Ea1 ¼ 0.60 eV, a2 ¼2.0  103, and Ea2 ¼0.25 eV. The

Y. Tosaka, S. Adachi / Journal of Luminescence 156 (2014) 157–163

10. Conclusions

Photon energy (eV) 3.0 2.8 2.6 2.4

2.2

2.0

1.8

×20

We synthesized (Ce3 þ , Tb3 þ )-codoped KCl phosphor from an aqueous solution of KCl–CeCl3–TbCl3. The synthesized phosphor was investigated using XRD analysis, EPMA study, diffuse reflectance measurements, PL analysis, PLE spectroscopy, and PL decay measurements. The Tb3 þ -related emission intensity in the (Ce3 þ , Tb3 þ )-codoped phosphor was enhanced more than 300 times compared to that in the Tb3 þ singly doped phosphor, thereby indicating the occurrence of an efficient energy transfer from Ce3 þ to Tb3 þ . The schematic energy-level diagrams for the KC:Tb3 þ and KCl:Ce3 þ , Tb3 þ phosphors were proposed for the sake of a better understanding of the PL and PLE processes in these phosphors.

T 20 K

PL intensity (arb. units)

100 K

200 K

Acknowledgments

300 K

The authors would like to thank T. Miyazaki, T. Nakamura, and H. Oike for their experimental support and useful discussion. This work was supported by a Grant-in-Aid for Scientific Research (B) (26289085) and a Grant-in-Aid for Exploratory Research (25630120) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

400 K 440 K

400

450

500 550 600 Wavelength (nm)

650

700

Fig. 14. PL spectra of the KCl:Ce3 þ , Tb3 þ (M ¼0.01, N ¼0.05) phosphor measured between T ¼ 20 and 440 K in 20-K increments.

300

T (K) 50 40

100

30

20

IPL(T)/IPL(0)

400 300

[5] [6] [7] [8]

200

0.5

[9] [10] [11] [12] [13] [14] [15] [16] [17]

1 0.5

0.1

0.1 0.05

0.05

0

0.01

0

0.002

0.02

0.03

0.004

0.04

0.006

0.05

1/T (K–1) 3þ

References [1] [2] [3] [4]

1 1000

163



Fig. 15. Integrated PL intensity (IPL) for the Tb -related emission in the KCl:Ce , Tb3 þ (M ¼0.01, N ¼0.05) phosphor measured between T ¼ 20 and 450 K in 10-K increments. The solid line represents the result calculated using Eq. (4). The solid and dashed lines in the inset also show the results calculated using Eq. (4) with and without taking into account the i¼ 2 term (Eq2 ¼ 0.25 eV), respectively.

observed decrease in IPL with increasing T above 300 K is well explained by the thermal quenching energy of Ea1 ¼0.60 eV. The solid and dashed lines in the inset of Fig. 15 show the results calculated using Eq. (4) with and without taking into account the i¼2 term (Eq2 ¼0.25 eV), respectively. The fit between experimental and calculated data can be greatly improved by considering this second term.

[18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]

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